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Industrial Engineering

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Course

BBE 4505
Omar Espinoza
University Of Minnesota
NATURAL RESOURCES

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Copyright 2012 by The McGraw-Hill Companies, Inc. All rights reserved. Printed in the United States of America. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without prior written permission of the publisher.
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ISBN-10: 1121789048

ISBN-13: 9781121789043

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Contents
1. Preface 1
2. Methods, Standards, and Work Design: Introduction 7

Problem-Solving Tools

27

3. Tex 29
4. Operation Analysis 79
5. Manual Work Design 133
6. Workplace, Equipment, and Tool Design 185
7. Work Environment Design 239
8. Design of Cognitive Work 281
9. Workplace and Systems Safety 327
10. Proposed Method Implementation 379
11. Time Study 413
12. Performance Rating and Allowances 447
13. Standard Data and Formulas 485
14. Predetermined Time Systems 507
15. Work Sampling 553
16. Indirect and Expense Labor Standards 585
17. Standards Follow-Up and Uses 611
18. Wage Payment 631
19. Training and Other Management Practices 655
20. Appendix 1: Glossary 685
21. Appendix 2: Helpful Formulas 704
22. Appendix 3: Special Tables 706
23. Index 719

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Credits
1. Preface: Chapter from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds, 2009 1
2. Methods, Standards, and Work Design: Introduction: Chapter 1 from Niebel's Methods, Standards, and Work
Design, 12th Edition by Freivalds, 2009 7

Problem-Solving Tools

27

3. Tex: Chapter from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds, 2009 29
4. Operation Analysis: Chapter 3 from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds,
2009 79

5. Manual Work Design: Chapter 4 from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds,
2009 133

6. Workplace, Equipment, and Tool Design: Chapter 5 from Niebel's Methods, Standards, and Work Design, 12th
Edition by Freivalds, 2009 185

7. Work Environment Design: Chapter 6 from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds,
2009 239

8. Design of Cognitive Work: Chapter 7 from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds,
2009 281

9. Workplace and Systems Safety: Chapter 8 from Niebel's Methods, Standards, and Work Design, 12th Edition by
Freivalds, 2009 327

10. Proposed Method Implementation: Chapter 9 from Niebel's Methods, Standards, and Work Design, 12th Edition by
Freivalds, 2009 379

11. Time Study: Chapter 10 from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds, 2009 413
12. Performance Rating and Allowances: Chapter 11 from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds, 2009 447

13. Standard Data and Formulas: Chapter 12 from Niebel's Methods, Standards, and Work Design, 12th Edition by
Freivalds, 2009 485

14. Predetermined Time Systems: Chapter 13 from Niebel's Methods, Standards, and Work Design, 12th Edition by
Freivalds, 2009 507

15. Work Sampling: Chapter 14 from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds, 2009 553
16. Indirect and Expense Labor Standards: Chapter 15 from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds, 2009 585

17. Standards Follow-Up and Uses: Chapter 16 from Niebel's Methods, Standards, and Work Design, 12th Edition by
Freivalds, 2009 611

18. Wage Payment: Chapter 17 from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds, 2009 631
19. Training and Other Management Practices: Chapter 18 from Niebel's Methods, Standards, and Work Design, 12th
Edition by Freivalds, 2009 655

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20. Appendix 1: Glossary: Chapter from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds, 2009 685
21. Appendix 2: Helpful Formulas: Chapter from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds,
2009 704

22. Appendix 3: Special Tables: Chapter from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds,
2009 706

23. Index: Chapter from Niebel's Methods, Standards, and Work Design, 12th Edition by Freivalds, 2009 719

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Freivalds: Niebel’s
Methods, Standards, and
Work Design, 12th Edition

Front Matter

Preface

1

© The McGraw−Hill
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Niebel's Methods, Standards, and Work Design, 12th Edition

Preface
BACKGROUND
Faced with increasing competition from all parts of the world, almost every industry, business, and service organization is restructuring itself to operate more effectively. As downsizing and outsourcing become more common, these organizations must increase the intensity of cost reduction and quality improvement efforts while working with reduced labor forces. Cost-effectiveness and product reliability without excess capacity are the keys to successful activity in all areas of business, industry, and government and are the end result of methods engineering, equitable time standards, and efficient work design.
Also, as machines and equipment grow increasingly complex and semiautomated if not fully automated, it is increasingly important to study both the manual components and the cognitive aspects of work as well as the safety of the operations. The operator must perceive and interpret large amounts of information, make critical decisions, and control these machines both quickly and accurately. In recent years, jobs have shifted gradually from manufacturing to the service sector. In both sectors, there is increasingly less emphasis on gross physical activity and a greater emphasis on information processing and decision making, especially via computers and associated modern technology. The same efficiency and work design tools are the keys to productivity improvement in any industry, business, or service organization, whether in a bank, a hospital, a department store, a railroad, or the postal system. Furthermore, success in a given product line or service leads to new products and innovations. It is this accumulation of successes that drives hiring and the growth of an economy.
The reader should be careful not to be swayed or intimidated by the latest jargon offered as a cure-all for an enterprise’s lack of competitiveness. Often these fads sideline the sound engineering and management procedures that, when properly utilized, represent the key to continued success. Today we hear a good deal about reengineering and use of cross-functional teams as business leaders reduce cost, inventory, cycle time, and nonvalue activities. However, experience in the past few years has proved that cutting people from the payroll just for the sake of automating their jobs is not always the wise procedure. The authors, with many years of experience in over 100 industries, strongly recommend sound methods engineering, realistic standards, and good work design as the keys to success in both manufacturing and service industries. x McGraw-Hill Create™ Review Copy for Instructor Espinoza. Not for distribution.

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Freivalds: Niebel’s
BBE 4505
Methods, Standards, and
Work Design, 12th Edition

Front Matter

Preface

© The McGraw−Hill
Companies, 2009

P R E FA C E

WHY THIS BOOK WAS WRITTEN
The objectives of the twelfth edition have remained the same as for the eleventh: to provide a practical, up-to-date college textbook describing engineering methods to measure, analyze, and design manual work. The importance of ergonomics and work design as part of methods engineering is emphasized, not only to increase productivity, but also to improve worker health and safety and thus company bottom-line costs. Far too often, industrial engineers have focused solely on increasing productivity through methods changes and job simplification, resulting in overly repetitive jobs for the operators and increased incidence rates of musculoskeletal injuries. Any cost reductions obtained are more than offset by the increased medical and worker’s compensation costs, especially considering today’s ever-escalating health care costs.

WHAT’S NEW IN THE TWELFTH EDITION
A new Chapter 8 on workplace and systems safety has been added that includes material on accident causation models, accident prevention, quantitative analyses, and general hazard control. This then completes the knowledge that a basic industrial engineer should have for managing a production line or a service center.
Old Chapters 10 and 11 on ratings and allowances were combined as support materials to the new Chapter 10 on time study. Chapter 13 was expanded to include more material on BasicMOST.
Approximately 10 to 15 percent more examples, problems, and case studies have been added. The twelfth edition still provides a continued reliance on work design, work measurement, facilities layout, and various flow process charts for students entering the industrial engineering profession and serves as a practical, up-to-date source of reference material for the practicing engineer and manager.

HOW THIS BOOK DIFFERS FROM OTHERS
Most textbooks on the market deal strictly either with the traditional elements of motion and time study or with human factors and ergonomics. Few textbooks integrate both topics into one book or, for that matter, one course. In this day and age, the industrial engineer needs to consider both productivity issues and their effects on the health and safety of the worker simultaneously. Few of the books on the market are formatted for use in the classroom setting. This text includes additional questions, problems, and sample laboratory exercises to assist the educator. Finally, no text provides the extensive amount of online student and instructor resources, electronic forms, current information, and changes as this edition does.

ORGANIZATION OF THE TEXT AND COURSE
MATERIAL
The twelfth edition is laid out to provide roughly one chapter of material per week of a semester-long introductory course. Although there are a total of 18 chapters,

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Front Matter

Preface

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P R E FA C E

Chapter 1 is short and introductory, much of Chapter 7 on cognitive work design and Chapter 8 on safety may be covered in other courses, and Chapter 15 on standards for indirect and expense work may not need to be covered in an introductory course, all of which leaves only 15 chapters to be covered in the semester.
A typical semester plan, chapter by chapter, using the first lecture number, might be as follows:

Chapter

Lectures

1

1

2

3–6

3
4

4
4

5
6

4
3–4

7

0–4

8

0–5

9

3–5

10
11

3
3–5

12

1–3

13

4–7

14
15

2–3
0–3

16
17
18

2–3
3–4
3–4

Coverage
Quick introduction on the importance of productivity and work design, with a bit of historical perspective.
A few tools from each area (Pareto analysis, job analysis/worksite guide, flow process charts, worker– machine charts) with some quantitative analysis on worker–machine interactions. Line balancing and
PERT may be covered in other courses.
Operation analysis with an example for each step.
Full, but can gloss over basic muscle physiology and energy expenditure.
Full.
Basics on illumination, noise, temperature; other topics as desired may be covered in another course.
Coverage depends on instructor’s interest; may be covered in another course.
Coverage depends on instructor’s interest; may be covered in another course.
Three tools: value engineering, cost-benefit analysis, and crossover charts; job analysis and evaluation, and interaction with workers. Other tools may be covered in other classes.
Basics of time study.
One form of rating; first half of the allowances that are well established.
Coverage of standard data and formulas depends on instructor’s interest.
Only one predetermined time system in depth; the second may be covered in another course.
Work sampling.
Coverage of indirect and expense labor standards depends on instructor’s interest.
Overview and costing.
Day work and standard hour plan.
Learning curves, motivation, and people skills.

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Methods, Standards, and
Work Design, 12th Edition

Front Matter

© The McGraw−Hill
Companies, 2009

Preface

P R E FA C E

The recommended plan covers 43 lectures, with two periods for examinations. Some instructors may wish to spend more time on any given chapter, for which additional material is supplied, for example, work design (Chapters 4 to 7), and less time on traditional work measurement (Chapters 8 to 16), or vice versa.
The text allows for this flexibility.
Similarly, if all the material is used (the second lecture number), there is enough material for one lecture course and one course with a lab, as is done at
Penn State University. Both courses have been developed with appropriate materials such that they can be presented completely online. For an example of an online course using this text, go to www.engr.psu.edu/cde/courses/ie327/ index.html SUPPLEMENTARY MATERIAL AND ONLINE SUPPORT
The twelfth edition of this text continues to focus on the ubiquitous use of PCs as well as the Internet to establish standards, conceptualize possibilities, evaluate costs, and disseminate information. A website, hosted by the publisher at www.mhhe.com/niebel-freivalds, furthers that objective by providing the educator with various online resources, such as an updated instructor’s manual. DesignTools version 4.1.1, a ready-to-use software program for ergonomics analysis and work measurement, appears on the site as well. A special new feature of DesignTools is the addition of QuikTS, a time study data collection program, and QuikSamp, a work sampling program. The program may be downloaded via hot synch to a Palm device (m105 or higher) and used to collect time study data. The data are then uploaded directly to the time study form on DesignTools for easy and accurate calculation of standard time.
The book’s website also links to a website hosted by the author at www2.ie.psu.edu/Freivalds/courses/ie327new/index.html which provides instructors with online background material, including electronic versions of the forms used in the textbook. Student resources include practice exams and solutions.
Up-to-date information on any errors found or corrections needed in this new edition appear on this site as well. Suggestions received from individuals at universities, colleges, technical institutes, industries, and labor organizations that regularly use this text have helped materially in the preparation of this twelfth edition. Further suggestions are welcome, especially if any errors are noticed. Please simply respond to the OOPS! button on the website or by email to axf@psu.edu

ACKNOWLEDGMENTS
I wish to acknowledge the late Ben Niebel for providing me with the opportunity to contribute to his well-respected textbook. I hope the additions and modifications will match his standards and continue to serve future industrial engineers as they enter their careers. Thanks to Dr. Dongjoon Kong, University of Tennessee,

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Front Matter

Preface

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P R E FA C E

for devoting so much of his time at Penn State to programming DesignTools.
Thanks also to the following reviewers for their invaluable input:
David R. Clark, Kettering University
Luis Rene Contreras, University of Texas, El Paso
Jerry Davis, Auburn University
Corinne MacDonald, Dalhousie University
Gary Mirka, Iowa State University
Durward K. Sobek, Montana State University
Harvey Wolfe, University of Pittsburgh
Finally, I wish to express my gratitude to Dace for her patience and support.
Andris Freivalds

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Methods, Standards, and
Work Design, 12th Edition

1. Methods, Standards, and
Work Design: Introduction

Text

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Methods,
Standards, and
Work Design:
Introduction

CHAPTER

1

KEY POINTS







Increasing productivity drives U.S. industry.
Worker health and safety are just as important as productivity.
Methods engineering simplifies work.
Work design fits work to the operator.
Time study measures work and sets standards.

1.1

PRODUCTIVITY IMPORTANCE

Certain changes continually taking place in the industrial and business environment must be considered both economically and practically. These include the globalization of both the market and the producer, the growth of the service sector, the computerization of all facets of an enterprise, and the ever-expanding applications of the
Internet and Web. The only way a business or enterprise can grow and increase its profitability is by increasing its productivity. Productivity improvement refers to the increase in output per work-hour or time expended. The United States has long enjoyed the world’s highest productivity. Over the last 100 years, productivity in the
United States has increased approximately 4 percent per year. However, in the last decade, the U.S. rate of productivity improvement has been exceeded by that of
Japan, Korea, and Germany, and it may soon be challenged by China.
The fundamental tools that result in increased productivity include methods, time study standards (frequently referred to as work measurement), and work design. Of the total cost of the typical metal products manufacturing enterprise,
12 percent is direct labor, 45 percent is direct material, and 43 percent is overhead.
All aspects of a business or industry—sales, finance, production, engineering, cost, maintenance, and management—provide fertile areas for the application of methods, standards, and work design. Too often, people consider only production, when other aspects of the enterprise could also profit from the application of productivity
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Work Design, 12th Edition

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1. Methods, Standards, and
Work Design: Introduction

Text

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CHAPTER 1

tools. In sales, for example, modern information retrieval methods usually result in more reliable information and greater sales at less cost.
Today, most U.S. businesses and industries are, by necessity, restructuring themselves by downsizing, to operate more effectively in an increasingly competitive world. With greater intensity than ever before, they are addressing cost reduction and quality improvement through productivity improvement. They are also critically examining all nonvalue business components, those that do not contribute to their profitability.
Since the production area within manufacturing industries utilizes the greatest number of engineers in methods, standards, and work design efforts, this text will treat that field in greater detail than any other. However, examples from other areas of the manufacturing industry, such as maintenance, transportation, sales, and management, as well as the service industry, will be provided.
Traditional areas of opportunity for students enrolled in engineering, industrial management, business administration, industrial psychology, and labor– management relations are (1) work measurement, (2) work methods and design,
(3) production engineering, (4) manufacturing analysis and control, (5) facilities planning, (6) wage administration, (7) ergonomics and safety, (8) production and inventory control, and (9) quality control. However, these areas of opportunity are not confined to manufacturing industries. They exist, and are equally important, in such enterprises as department stores, hotels, educational institutions, hospitals, banks, airlines, insurance offices, military service centers, government agencies, and retirement complexes. Today, in the United States, only about 10 percent of the total labor force is employed in manufacturing industries. The remaining 90 percent is engaged in service industries or staff-related positions. As the United
States becomes ever more service-industry-oriented, the philosophies and techniques of methods, standards, and work design must also be utilized in the service sector. Wherever people, materials, and facilities interact to obtain some objective, productivity can be improved through the intelligent application of methods, standards, and work design.
The production area of an industry is key to success. Here materials are requisitioned and controlled; the sequence of operations, inspections, and methods is determined; tools are ordered; time values are assigned; work is scheduled, dispatched, and followed up; and customers are kept satisfied with quality products delivered on time.
Similarly, the methods, standards, and work design activity is the key part of the production group. Here more than in any other place, people determine whether a product is going to be produced on a competitive basis, through efficient workstations, tooling, and worker and machine relationships. Here is where they are creative in improving existing methods and products and maintaining good labor relations through fair labor standards.
The objective of the manufacturing manager is to produce a quality product, on schedule, at the lowest possible cost, with a minimum of capital investment and a maximum of employee satisfaction. The focus of the reliability and quality control manager is to maintain engineering specifications and satisfy

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Work Design, 12th Edition

1. Methods, Standards, and
Work Design: Introduction

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Text

Niebel's Methods, Standards, and Work Design, 12th Edition

Methods, Standards, and Work Design: Introduction

customers with the product’s quality level and reliability over its expected life.
The production control manager is principally interested in establishing and maintaining production schedules with due regard for both customer needs and the favorable economics obtainable with careful scheduling. The maintenance manager is primarily concerned with minimizing facility downtime due to unscheduled breakdowns and repairs. Figure 1.1 illustrates the relationship of all these areas and the influence of methods, standards, and work design on overall production.

General
Manager

Sales
Manager

A

Maintenance
Manager

Manufacturing
Manager

Controller

B

C

Reliability and
Quality
Control
Manager

H

Industrial
Relations
Manager

Purchasing
Agent

D

Manager
Methods
Standards, and
Work design

G

E

I

Chief
Engineer

F

Production
Control
Manager

J
Manufacturing
Departments

A— Cost is largely determined by manufacturing methods.
B— Time standards are the bases of standard costs.
C— Standards (direct and indirect) provide the bases for measuring the performance of production departments. D— Time is a common denominator for comparing competitive equipment and supplies.
E— Good labor relations are maintained with equitable standards and a safe work environment.
F— Methods work design and processes strongly influence product designs.
G— Standards provide the bases for preventive maintenance.
H— Standards enforce quality.
I— Scheduling is based on time standards.
J— Methods, standards, and work design provide how the work is to be done and how long it will take.

Figure 1.1

Typical organization chart showing the influence of methods, standards, and work design on the operation of the enterprise.

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Text

CHAPTER 1

1.2

METHODS AND STANDARDS SCOPE

Methods engineering includes designing, creating, and selecting the best manufacturing methods, processes, tools, equipment, and skills to manufacture a product based on the specifications that have been developed by the product engineering section. When the best method interfaces with the best skills available, an efficient worker–machine relationship exists. Once the complete method has been established, a standard time for the product must be determined.
Furthermore there is the responsibility to see that (1) predetermined standards are met; (2) workers are adequately compensated for their output, skills, responsibilities, and experience; and (3) workers have a feeling of satisfaction from the work that they do.
The overall procedure includes defining the problem; breaking the job down into operations; analyzing each operation to determine the most economical manufacturing procedures for the quantity involved, with due regard for operator safety and job interest; applying proper time values; and then following through to ensure that the prescribed method is put into operation. Figure 1.2 illustrates

Minimum work content of product
Total
time of operation under existing conditions or under future conditions when methods engineering, standards, and work design are not practiced 1

Work content added by defects in design or specification of product, including material specification, geometry specification, tolerance and finish specification

2

Work content added by inefficient work design and methods of manufacture or operation, including setup, tools, working conditions, workplace layout, and motion economy

3

Time added due to shortcomings of the management, including poor planning, poor material and tool inventory control, poor scheduling, and weak supervision, instruction, and training

4

Time added due to shortcomings of the worker, including working at less than normal pace, taking excessive allowances

Goal of methods, standards, and work design

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Opportunities for savings through methods engineering, standards, and work design

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Figure 1.2

Opportunities for savings through the applications of methods engineering and time study.

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Work Design, 12th Edition

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Work Design: Introduction

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Methods, Standards, and Work Design: Introduction

the opportunities for reducing the standard manufacturing time through the application of methods engineering and time study.

METHODS ENGINEERING
The terms operation analysis, work design, work simplification, and methods engineering and corporate reengineering are frequently used synonymously. In most cases, the person is referring to a technique for increasing the production per unit of time or decreasing the cost per unit output—in other words, productivity improvement. However, methods engineering, as defined in this text, entails analyses at two different times during the history of a product. First, the methods engineer is responsible for designing and developing the various work centers where the product will be produced. Second, that engineer must continually restudy the work centers to find a better way to produce the product and/or improve its quality.
In recent years, this second analysis has been called corporate reengineering.
In this regard, we recognize that a business must introduce changes if it is to continue profitable operation. Thus, it may be desirable to introduce changes outside the manufacturing area. Often, profit margins may be enhanced through positive changes in such areas as accounting, inventory management, materials requirements planning, logistics, and human resource management. Information automation can provide dramatic rewards in all these areas. The more thorough the methods study during the planning stages, the less the necessity for additional methods studies during the life of the product.
Methods engineering implies the utilization of technological capability.
Primarily because of methods engineering, improvements in productivity are never-ending. The productivity differential resulting from technological innovation can be of such magnitude that developed countries will always be able to maintain competitiveness with low-wage developing countries. Research and development (R&D) leading to new technology is therefore essential to methods engineering. The 10 countries with the highest R&D expenditures per worker, as reported by the United Nations Industrial Development Organization (1985), are the United States, Switzerland, Sweden, Netherlands, Germany, Norway, France,
Israel, Belgium, and Japan. These countries are among the leaders in productivity. As long as they continue to emphasize research and development, methods engineering through technological innovation will be instrumental in their ability to provide high-level goods and services.
Methods engineers use a systematic procedure to develop a work center, produce a product, or provide a service (see Figure 1.3). This procedure is outlined here, and it summarizes the flow of the text. Each step is detailed in a later chapter.
Note that steps 6 and 7 are not strictly part of a methods study, but are necessary in a fully functioning work center.
1. Select the project. Typically, the projects selected represent either new products or existing products that have a high cost of manufacture and a low profit. Also, products that are currently experiencing difficulties in

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Work Design, 12th Edition

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Work Design: Introduction

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1. Select Project
New plants and plant expansion
New products, new methods
Products with high cost/low profit
Products unable to meet competition
Manufacturing difficulties
Bottleneck operations/exploratory tools
2. Get and Present Data
Obtain production requirements
Procure engineering data
Procure manufacturing and cost data
Develop description and sketches of workstation and tools
Construct operation process charts
Construct flow process chart for individual items
3. Analyze Data
Systematic Procedure for Methods and Work Measurement

12

Use 9 primary approaches to operation analysis
Question every detail
Use: why, where, what, who, when, how
4. Develop Ideal Method
Worker and machine process charts
Mathematical techniques
Eliminate, combine, simplify, rearrange steps
Principles of work design with respect to:
Motion economy, manual work, workplace
Equipment, tools, work environment, safety
5. Present and Install Method
Use decision-making tools
Develop written and oral presentation
Overcome resistance
Sell method to operator, supervisor, and management
Put method to work
6. Develop Job Analysis
Job analysis
Job descriptions
Accommodation of differently abled workers
7. Establish Time Standards
Stopwatch time study
Work sampling
Standard data
Formulas
Predetermined time systems
8. Follow Up
Verify savings
Assure that installation is correct
Keep everyone sold
Repeat methods procedure

Figure 1.3

The principal steps in a methods engineering program.

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Work Design, 12th Edition

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Work Design: Introduction

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2.

3.

4.

5.

6.

7.
8.

Text

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Methods, Standards, and Work Design: Introduction

maintaining quality and are having problems meeting competition are logical projects for methods engineering. (See Chapter 2 for more details.)
Get and present the data. Assemble all the important facts relating to the product or service. These include drawings and specifications, quantity requirements, delivery requirements, and projections of the anticipated life of the product or service. Once all important information has been acquired, record it in an orderly form for study and analysis. The development of process charts at this point is very helpful. (See Chapter 2 for more details.)
Analyze the data. Utilize the primary approaches to operations analysis to decide which alternative will result in the best product or service. These primary approaches include purpose of operation, design of part, tolerances and specifications, materials, process of manufacture, setup and tools, working conditions, material handling, plant layout, and work design.
(See Chapter 3 for more details.)
Develop the ideal method. Select the best procedure for each operation, inspection, and transportation by considering the various constraints associated with each alternative, including productivity, ergonomics, and health and safety implications. (See Chapters 3 to 7 for more details.)
Present and install the method. Explain the proposed method in detail to those responsible for its operation and maintenance. Consider all details of the work center, to ensure that the proposed method will provide the results anticipated. (See Chapter 8 for more details.)
Develop a job analysis. Conduct a job analysis of the installed method to ensure that the operators are adequately selected, trained, and rewarded.
(See Chapter 8 for more details.)
Establish time standards. Establish a fair and equitable standard for the installed method. (See Chapters 9 to 15 for more details.)
Follow up the method. At regular intervals, audit the installed method to determine if the anticipated productivity and quality are being realized, whether costs were correctly projected, and whether further improvements can be made. (See Chapter 16 for more details.)

In summary, methods engineering is the systematic close scrutiny of all direct and indirect operations to find improvements that make work easier to perform, in terms of worker health and safety, and also allow work to be done in less time with less investment per unit (i.e., greater profitability).

WORK DESIGN
As part of developing or maintaining the new method, the principles of work design must be used to fit the task and workstation ergonomically to the human operator.
Unfortunately, work design is typically forgotten in the quest for increased productivity. Far too often, overly simplified procedures result in machinelike repetitive jobs for the operators, leading to increased rates of work-related musculoskeletal

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disorders. Any productivity increases and reduced costs are more than offset by the increased medical and workers’ compensation costs, especially considering today’s ever-escalating health-care trends. Thus, it is necessary for the methods engineer to incorporate the principles of work design into any new method, so that it not only will be more productive but also will be safe and injury-free for the operator. (Refer to Chapters 4 to 7.)

STANDARDS
Standards are the end result of time study or work measurement. This technique establishes a time standard allowed to perform a given task, based on measurements of the work content of the prescribed method, with due consideration for fatigue and for personal and unavoidable delays. Time study analysts use several techniques to establish a standard: a stopwatch time study, computerized data collection, standard data, predetermined time systems, work sampling, and estimates based on historical data. Each technique is applicable to certain conditions.
Time study analysts must know when to use a given technique and must then use that technique judiciously and correctly.
The resulting standards are used to implement a wage payment scheme. In many companies, particularly in smaller enterprises, the wage payment activity is performed by the same group responsible for the methods and standards work.
Also, the wage payment activity is performed in concert with those responsible for conducting job analyses and job evaluations, so that these closely related activities function smoothly.
Production control, plant layout, purchasing, cost accounting and control, and process and product design are additional areas closely related to both the methods and standards functions. To operate effectively, all these areas depend on time and cost data, facts, and operational procedures from the methods and standards department. These relationships are briefly discussed in Chapter 16.

OBJECTIVES OF METHODS, STANDARDS,
AND WORK DESIGN
The principal objectives of methods, standards, and work design are (1) to increase productivity and product reliability safely and (2) to lower unit cost, thus allowing more quality goods and services to be produced for more people. The ability to produce more for less will result in more jobs for more people for a greater number of hours per year. Only through the intelligent application of the principles of methods, standards, and work design can producers of goods and services increase, while, at the same time, the purchasing potential of all consumers grows. Through these principles, unemployment and relief rolls can be minimized, thus reducing the spiraling cost of economic support to nonproducers.
Corollaries to the principal objectives are as follows:
1. Minimize the time required to perform tasks.
2. Continually improve the quality and reliability of products and services.

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3. Conserve resources and minimize cost by specifying the most appropriate direct and indirect materials for the production of goods and services.
4. Consider the cost and availability of power.
5. Maximize the safety, health, and well-being of all employees.
6. Produce with an increasing concern for protecting the environment.
7. Follow a humane program of management that results in job interest and satisfaction for each employee.

1.3 HISTORICAL DEVELOPMENTS
THE WORK OF TAYLOR
Frederick W. Taylor is generally conceded to be the founder of modern time study in this country. However, time studies were conducted in Europe many years before Taylor’s time. In 1760, Jean Rodolphe Perronet, a French engineer, made extensive time studies on the manufacture of No. 6 common pins, while 60 years later, an English economist, Charles W. Babbage, conducted time studies on the manufacture of No. 11 common pins.
Taylor began his time study work in 1881 while associated with the Midvale
Steel Company in Philadelphia. Although born in a wealthy family, he disdained his upbringing and started out serving as an apprentice. After 12 years’ work, he evolved a system based on the “task.” Taylor proposed that the work of each employee be planned out by the management at least one day in advance. Workers were to receive complete written instructions describing their tasks in detail and noting the means to accomplish them. Each job was to have a standard time, determined by time studies made by experts. In the timing process, Taylor advocated breaking up the work assignment into small divisions of effort known as “elements.” Experts were to time these individually and use their collective values to determine the allowed time for the task.
Taylor’s early presentations of his findings were received without enthusiasm, because many of the engineers interpreted his findings to be a new piece-rate system rather than a technique for analyzing work and improving methods. Both management and employees were skeptical of piece rates, because many standards were either typically based on the supervisor’s guess or inflated by bosses to protect the performance of their departments.
In June 1903, at the Saratoga meeting of the American Society of Mechanical Engineers (ASME), Taylor presented his famous paper “Shop Management,” which included the elements of scientific management: time study, standardization of all tools and tasks, use of a planning department, use of slide rules and similar timesaving implements, instruction cards for workers, bonuses for successful performance, differential rates, mnemonic systems for classifying products, routing systems, and modern cost systems. Taylor’s techniques were well received by many factory managers, and by 1917, of 113 plants that had installed
“scientific management,” 59 considered their installations completely successful,
20 partly successful, and 34 failures (Thompson, 1917).

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In 1898, while at the Bethlehem Steel Company (he had resigned his post at
Midvale), Taylor carried out the pig-iron experiment that came to be one of the most celebrated demonstrations of his principles. He established the correct method, along with financial incentives, and workers carrying 92-lb pigs of iron up a ramp onto a freight car were able to increase their productivity from an average of 12.5 tons/day to between 47 and 48 tons/day. This work was performed with an increase in the daily rate of $1.15 to $1.85. Taylor claimed that workmen performed at the higher rate “without bringing on a strike among the men, without any quarrel with the men and were happier and better contented.”
Another of Taylor’s Bethlehem Steel studies that gained fame was the shoveling experiment. Workers who shoveled at Bethlehem owned their own shovels and would use the same one for any job—lifting heavy iron ore to lifting light rice coal. After considerable study, Taylor designed shovels to fit the different loads: short-handled shovels for iron ore, long-handled scoops for light rice coal.
As a result, productivity increased, and the cost of handling materials decreased from 8 cents/ton to 3 cents/ton.
Another of Taylor’s well-known contributions was the discovery of the
Taylor–White process of heat treatment for tool steel. Studying self-hardening steels, he developed a means of hardening a chrome–tungsten steel alloy without rendering it brittle, by heating it close to its melting point. The resulting “highspeed steel” more than doubled machine cutting productivity and remains in use today all over the world. Later, he developed the Taylor equation for cutting metal.
Not as well known as his engineering contributions is the fact that in 1881, he was a U.S. tennis doubles champion. Here he used an odd-looking racket he had designed with a spoon curved handle. Taylor died of pneumonia in 1915, at the age of 59. For more information on this multitalented individual, the authors recommend Kanigel’s biography (1997).
In the early 1900s, the country was going through an unprecedented inflationary period. The word efficiency became passé, and most businesses and industries were looking for new ideas that would improve their performance. The railroad industry also felt the need to increase shipping rates substantially to cover general cost increases. Louis Brandeis, who at that time represented the eastern business associations, contended that the railroads did not deserve, or in fact need, the increase because they had been remiss in not introducing the new
“science of management” into their industry. Brandeis claimed that the railroad companies could save $1 million/day by introducing the techniques advocated by Taylor. Thus, Brandeis and the Eastern Rate Case (as the hearing came to be known) first introduced Taylor’s concepts as “scientific management.”
At this time, many people without the qualifications of Taylor, Barth, Merrick, and other early pioneers, were eager to make names for themselves in this new field. They established themselves as “efficiency experts” and endeavored to install scientific management programs in industry. They soon encountered a natural resistance to change from employees, and since they were not equipped to handle problems of human relations, they met with great difficulty. Anxious to make a good showing and equipped with only a pseudoscientific knowledge,

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they generally established rates that were too difficult to meet. Situations became so acute that some managers were obliged to discontinue the whole program in order to continue operation.
In other instances, factory managers would allow the establishment of time standards by the supervisors, but this was seldom satisfactory. Once standards were established, many factory managers of that time, interested primarily in the reduction of labor costs, would unscrupulously cut rates if some employee made what the employer felt was too much money. The result was harder work at the same, and sometimes less, take-home pay. Naturally, violent worker reaction resulted.
These developments spread in spite of the many favorable installations started by Taylor. At the Watertown Arsenal, labor objected to such an extent to the new time study system that in 1910 the Interstate Commerce Commission (ICC) started an investigation of time study. Several derogatory reports on the subject influenced
Congress to add a rider to the government appropriations bill in 1913, stipulating that no part of the appropriation should be made available for the pay of any person engaged in time study work. This restriction applied to the government-operated plants where government funds were used to pay the employees.
It wasn’t until 1947 that the House of Representatives passed a bill that rescinded the prohibition against using stopwatches and the use of time study. It is of interest that even today the use of the stopwatch is still prohibited by unions in some railroad repair facilities. It is also interesting to note that Taylorism is very much alive today, in contemporary assembly lines, in lawyer’s bills that are calculated in fractional hours, and in documentation of hospital costs for patients.

MOTION STUDY AND THE WORK OF THE GILBRETHS
Frank and Lilian Gilbreth were the founders of the modern motion-study technique, which may be defined as the study of the body motions used in performing an operation, to improve the operation by eliminating unnecessary motions, simplifying necessary motions, and then establishing the most favorable motion sequence for maximum efficiency. Frank Gilbreth originally introduced his ideas and philosophies into the bricklayer’s trade in which he was employed.
After introducing methods improvements through motion study, including an adjustable scaffold that he had invented, as well as operator training, he was able to increase the average number of bricks laid to 350 per worker per hour. Prior to
Gilbreth’s studies, 120 bricks per hour was considered a satisfactory rate of performance for a bricklayer.
More than anyone else, the Gilbreths were responsible for industry’s recognition of the importance of a detailed study of body motions to increase production, reduce fatigue, and instruct operators in the best method of performing an operation. They developed the technique of filming motions to study them, in a technique known as micromotion study. The study of movements through the aid of the slow-motion moving picture is by no means confined to industrial applications.
In addition, the Gilbreths developed the cyclegraphic and chronocyclegraphic analysis techniques for studying the motion paths made by an operator.

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The cyclegraphic method involves attaching a small electric lightbulb to the finger or hand or part of the body being studied and then photographing the motion while the operator is performing the operation. The resulting picture gives a permanent record of the motion pattern employed and can be analyzed for possible improvement. The chronocyclegraph is similar to the cyclegraph, but its electric circuit is interrupted regularly, causing the light to flash. Thus, instead of showing solid lines of the motion patterns, the resulting photograph shows short dashes of light spaced in proportion to the speed of the body motion being photographed. Consequently, with the chronocyclegraph it is possible to compute velocity, acceleration, and deceleration, as well as to study body motions. The world of sports has found this analysis tool, updated to video, invaluable as a training tool to show the development of form and skill.
As an interesting side note, the reader may wish to read about the extreme lengths to which Frank Gilbreth went to achieve maximum efficiency even in his personal life. His eldest son and daughter recount vignettes of their father shaving with razors simultaneously in both hands or using various communication signals to assemble all the children, of which there were 12. Hence the title of their book Cheaper by the Dozen (Gilbreth and Gilbreth, 1948)! After Frank’s relatively early death at the age of 55, Lillian, who had received a Ph.D. in psychology and had been a more than equal collaborator, continued on her own, advancing the concept of work simplification especially for the physically handicapped. She passed away in 1972 at the distinguished age of 93 (Gilbreth, 1988).

EARLY CONTEMPORARIES
Carl G. Barth, an associate of Frederick W. Taylor, developed a production slide rule for determining the most efficient combinations of speeds and feeds for cutting metals of various hardnesses, considering the depth of cut, size of tool, and life of the tool. He is also noted for his work in determining allowances. He investigated the number of foot-pounds of work a worker could do in a day. He then developed a rule that equated a certain push or pull on a worker’s arms with the amount of weight that worker could handle for a certain percentage of the day.
Harrington Emerson applied scientific methods to work on the Santa Fe
Railroad and wrote a book, Twelve Principles of Efficiency, in which he made an effort to inform management of procedures for efficient operation. He reorganized the company, integrated its shop procedures, installed standard costs and a bonus plan, and transferred its accounting work to Hollerith tabulating machines. This effort resulted in annual savings in excess of $1.5 million and the recognition of his approach, termed efficiency engineering.
In 1917, Henry Laurence Gantt developed simple graphs that would measure performance while visually showing projected schedules. This production control tool was enthusiastically adopted by the shipbuilding industry during World
War I. For the first time, this tool made it possible to compare actual performance against the original plan, and to adjust daily schedules in accordance with capacity, backlog, and customer requirements. Gantt is also known for his invention of a

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wage payment system that rewarded workers for above-standard performance, eliminated any penalty for failure, and offered the boss a bonus for every worker who performed above standard. Gantt emphasized human relations and promoted scientific management as more than an inhuman “speedup” of labor.
Motion and time study received added stimulus during World War II when
Franklin D. Roosevelt, through the U.S. Department of Labor, advocated establishing standards for increasing production. The stated policy advocated greater pay for greater output but without an increase in unit labor costs, incentive schemes to be collectively bargained between labor and management, and the use of time study or past records to set production standards.

EMERGENCE OF WORK DESIGN
Work design is a relatively new science that deals with designing the task, workstation, and working environment to fit the human operator better. In the United
States, it is more typically known as human factors, while internationally it is better known as ergonomics, which is derived from the Greek words for work
(erg) and laws (nomos).
In the United States, after the initial work of Taylor and the Gilbreths, the selection and training of military personnel during World War I and the industrial psychology experiments of the Harvard Graduate School at Western Electric (see the Hawthorne studies in Chapter 9) were important contributions to the work design area. In Europe, during and after World War I, the British Industrial Fatigue
Board performed numerous studies on human performance under various conditions. These were later extended to heat stress and other conditions by the British
Admiralty and Medical Research Council.
World War II, with the complexity of military equipment and aircraft, led to the development of the U.S. military engineering psychology laboratories and a real growth of the profession. The start of the race to space with the launch of
Sputnik in 1957 only accelerated the growth of human factors, especially in the aerospace and military sectors. From the 1970s on, the growth has shifted to the industrial sector and, more recently, into computer equipment, user-friendly software, and the office environment. Other driving forces for the growth in human factors are the rise in product liability and personal injury litigation cases and also, unfortunately, tragic, large-scale technological disasters, such as the nuclear incident at Three-Mile Island and the gas leak at the Union Carbide Plant in
Bhopal, India. Obviously, the growth of computers and technology will keep human factors specialists and ergonomists busy designing better workplaces and products and improving the quality of life and work for many years to come.

ORGANIZATIONS
Since 1911, there has been an organized effort to keep industry abreast of the latest developments in the techniques inaugurated by Taylor and Gilbreth. Technical organizations have contributed much toward bringing the science of time study, work design, and methods engineering up to present-day standards. In 1915, the

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Taylor Society was founded to promote the science of management, while in 1917 the Society of Industrial Engineers was organized by those interested in production methods. The American Management Association (AMA) traces its origins to
1913, when a group of training managers formed the National Association of Corporate Schools. Its various divisions sponsor courses and publications on productivity improvement, work measurement, wage incentives, work simplification, and clerical standards. Together with the American Society of Mechanical Engineers
(ASME), AMA annually presents the Gantt Memorial Medal for the most distinguished contribution to industrial management as a service to the community.
The Society for the Advancement of Management (SAM) was formed in
1936 by the merger of the Society of Industrial Engineers and the Taylor Society.
This organization emphasized the importance of time study and methods and wage payment. Industry has used SAM’s time study rating films over a long period of years. SAM annually offers the Taylor key for the outstanding contribution to the advancement of the science of management and the Gilbreth medal for noteworthy achievement in the field of motion, skill, and fatigue study. In
1972, SAM combined forces with AMA.
The Institute of Industrial Engineers (IIE) was founded in 1948 with the purposes of maintaining the practice of industrial engineering on a professional level; fostering a high degree of integrity among the members of the industrial engineering profession; encouraging and assisting education and research in areas of interest to industrial engineers; promoting the interchange of ideas and information among members of the industrial engineering profession (e.g., publishing the journal IIE Transactions); serving the public interest by identifying persons qualified to practice as industrial engineers; and promoting the professional registration of industrial engineers. IIE’s Society of Work Science (the result of merging the Work Measurement and Ergonomics Divisions in 1994) keeps the membership up to date on all facets of this area of work. This society annually gives the Phil Carroll Award and M. M. Ayoub Award for achievement in work measurement and ergonomics, respectively.
In the area of work design, the first professional organization, the Ergonomics Research Society, was founded in the United Kingdom in 1949. It started the first professional journal, Ergonomics, in 1957. The U.S. professional organization
The Human Factors and Ergonomics Society was founded in 1957. In the 1960s, there was rapid growth in the society, with membership increasing from 500 to
3,000. Currently, there are well over 5,000 members organized in 20 different technical groups. Their primary goals are to (1) define and support human factors/ergonomics as a scientific discipline and in practice, with the exchange of technical information among members; (2) educate and inform business, industry, and government about human factors/ergonomics; and (3) promote human factors/ergonomics as a means for bettering the quality of life. The society also publishes an archival journal, Human Factors, and holds annual conferences where members can meet and exchange ideas.
With the proliferation of national professional societies, an umbrella organization, the International Ergonomics Association, was founded in 1959 to coordinate

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ergonomic activities at an international level. At present, there are 42 individual societies encompassing over 15,000 members worldwide.

PRESENT TRENDS
Practitioners of methods, standards, and work design have come to realize that such factors as gender, age, health and well-being, physical size and strength, aptitude, training attitudes, job satisfaction, and motivation response have a direct bearing on productivity. Furthermore, present-day analysts recognize that workers object, and rightfully so, to being treated as machines. Workers dislike and fear a purely scientific approach and inherently dislike any change from their present way of operation. Even management frequently rejects worthwhile methods innovations because of a reluctance to change.
Workers tend to fear methods and time study, for they see that the results are an increase in productivity. To them, this means less work and consequently less pay. They must be sold on the fact that they, as consumers, benefit from lower costs, and that broader markets result from lower costs, meaning more work for more people for more weeks of the year.
Some fears of time study today are due to unpleasant experiences with efficiency experts. To many workers, motion and time study is synonymous with the speedup of work and the use of incentives to spur employees to higher levels of output. If the new levels established were normal production, the workers were forced to still greater exertions to maintain their previous earning power. In the past, shortsighted and unscrupulous managers did resort to this practice. Even today, some unions oppose the establishment of standards by measurement, the development of hourly base rates by job evaluation, and the application of incentive wage payment. These unions believe that the time allowed to perform a task and the amount that an employee should be paid represent issues that should be resolved by collective bargaining arrangements.
Today’s practitioners must use the “humane” approach. They must be well versed in the study of human behavior and accomplished in the art of communication. They must also be good listeners, respecting the ideas and thinking of others, particularly the worker at the bench. They must give credit where credit is due. In fact, they should habitually give the other person credit, even if there is some question of that person deserving it. Also, practitioners of motion and time study should always remember to use the questioning attitude emphasized by the Gilbreths, Taylor, and the other pioneers in the field. The idea that there is
“always a better way” needs to be continually pursued in the development of new methods that improve productivity, quality, delivery, worker safety, and worker well-being. Today, there is a greater intrusion by the government in the regulation of methods, standards, and work design. For example, military equipment contractors and subcontractors are under increased pressure to document direct labor standards as a result of MIL-STD 1567A (released 1975; revised 1983 and

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1987). Any firm awarded a contract exceeding $1 million is subject to MIL-STD
1567A, which requires a work measurement plan and procedures, a plan to establish and maintain engineered standards of known accuracy and traceability, a plan for methods improvement in conjunction with standards, a plan for the use of the standards as an input to budgeting, estimating, planning, and performance evaluation, and detailed documentation for all these plans. However, this requirement was cancelled in 1995.
Similarly, in the area of work design, Congress passed the OSHAct establishing the National Institute for Occupational Safety and Health (NIOSH), a research agency for developing guidelines and standards for worker health and safety, and the Occupational Safety and Health Administration (OSHA), an enforcement agency to maintain these standards. With the sudden increase in repetitive-motion injuries in the food processing industry, OSHA established the
Ergonomics Program Management Guidelines for Meatpacking Plants in 1990.
Similar guidelines for general industry slowly evolved into a final OSHA Ergonomics Standard, signed into law by President Clinton in 2001. However, the measure was rescinded soon afterward by Congress.
With increasing numbers of individuals with different abilities, Congress passed the Americans with Disabilities Act (ADA) in 1990. This regulation has a major impact on all employers with 15 or more employees, affecting such employment practices as recruiting, hiring, promotions, training, laying off, firing, allowing leaves, and assigning jobs.
While work measurement once concentrated on direct labor, methods and standard development have increasingly been used for indirect labor. This trend will continue as the number of traditional manufacturing jobs decreases and the number of service jobs increases in the United States. The use of computerized techniques will also continue to grow, for example, predetermined time systems such as MOST. Many companies have also developed time study and work sampling software, using electronic data collectors for compiling the information required.
Table 1.1 illustrates the progress made in methods, standards, and work design. SUMMARY
Industry, business, and government are in agreement that the untapped potential for increasing productivity is the best hope for dealing with inflation and competition. The principal key to increased productivity is a continuing application of the principles of methods, standards, and work design. Only in this way can greater output from people and machines be realized. The U.S. government has pledged itself to an increasingly paternalistic philosophy of providing for the disadvantaged—housing for the poor, medical care for the aged, jobs for minorities, and so on. To accommodate the spiraling costs of labor and government taxes and still stay in business, we must get more from our productive elements— people and machines.

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Table 1.1

Year
1760
1820
1832
1881
1901
1903
1906
1910
1911
1912
1913

1915
1917
1923
1927
1933
1936
1945
1947
1948
1949
1957
1959
1970
1972
1975
1981
1986
1988

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Progress Made in Connection with Methods, Standards, and Work Design

Event
Perronet makes time studies on No. 6 common pins.
Charles W. Babbage makes time studies on No. 11 common pins.
Charles W. Babbage publishes On the Economy of Machinery and Manufactures.
Frederick W. Taylor begins his time study work.
Henry L. Gantt develops the task and bonus wage system.
Taylor presents paper on shop management to ASME.
Taylor publishes paper On the Art of Cutting Metals.
Interstate Commerce Commission starts an investigation of time study.
Gilbreth publishes Motion Study.
Gantt publishes Work, Wages, and Profits.
Taylor publishes text The Principles of Scientific Management.
Society to Promote the Science of Management is organized.
Emerson estimates $1 million per day can be saved if eastern railroads apply scientific management.
Emerson publishes The Twelve Principles of Efficiency.
Congress adds rider to government appropriation bill stipulating that no part of this appropriation should be made available for the pay of any person engaged in time study work.
Henry Ford unveils the first moving assembly line in Detroit.
Taylor Society is formed to replace the Society to Promote the Science of
Management.
Frank B. and Lillian M. Gilbreth publish Applied Motion Study.
American Management Association is formed.
Elton Mayo begins Hawthorne study at Western Electric Company’s plant in
Hawthorne, IL.
Ralph M. Barnes receives the first Ph.D. granted in the United States in the field of industrial engineering from Cornell University. His thesis leads to the publication of “Motion and Time Study.”
Society for the Advancement of Management is organized.
Department of Labor advocates establishing standards to improve productivity of supplies for the war effort.
Bill is passed allowing the War Department to use time study.
The Institute of Industrial Engineers is founded in Columbus, Ohio.
Eiji Toyoda and Taichi Ohno at Toyota Motor Company pioneer the concept of lean production.
Prohibition against using stopwatches is dropped from appropriation language.
The Ergonomics Research Society (now The Ergonomics Society) is founded in the United Kingdom.
The Human Factors and Ergonomics Society is founded in the United States.
E. J. McCormick publishes Human Factors Engineering.
International Ergonomics Association is founded to coordinate ergonomics activities worldwide.
Congress passes the OSHAct, establishing the Occupational Safety and Health
Administration.
Society for the Advancement of Management combines with the American
Management Association.
MIL-STD 1567 (USAF), Work Measurement, is released.
NIOSH lifting guidelines are first introduced.
MIL-STD 1567A, Work Measurement Guidance Appendix, is finalized.
ANSI/HFS Standard 100-1988 for Human Factors Engineering of Visual
Display Terminal Workstations is released.

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Table 1.1 (continued)

Year
1990

1991
1995
1995
2001
2006

Event
Americans with Disabilities Act (ADA) is passed by Congress.
Ergonomics Program Management Guidelines for Meatpacking Plants are established by OSHA. This serves as a model for developing an OSHA ergonomics standard.
NIOSH lifting guidelines are revised.
Draft ANSI Z-365 Standard for Control of Work-Related Cumulative Trauma
Disorders is released.
MIL-STD 1567A Work Measurement is canceled.
OSHA Ergonomics Standard signed into law but rescinded soon afterward by
Congress.
50th Anniversary of the Human Factors and Ergonomics Society.

QUESTIONS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.

What is another name for time study?
What is the principal objective of methods engineering?
List the eight steps in applying methods engineering.
Where were time studies originally made and who conducted them?
Explain Frederick W. Taylor’s principles of scientific management.
What is meant by motion study, and who are the founders of the motion-study technique? Was the skepticism of management and labor toward rates established by
“efficiency experts” understandable? Why?
Which organizations are concerned with advancing the ideas of Taylor and the
Gilbreths?
What psychological reaction is characteristic of workers when methods changes are suggested? Explain the importance of the humanistic approach in methods and time study work. How are time study and methods engineering related?
Why is work design an important element of methods study?
What important events have contributed to the need for ergonomics?

REFERENCES
Barnes, Ralph M. Motion and Time Study: Design and Measurement of Work. 7th ed.
New York: John Wiley & Sons, 1980.
Eastman Kodak Co., Human Factors Section. Ergonomic Design for People at Work.
New York: Van Nostrand Reinhold, 1983.
Gilbreth, F., and L. Gilbreth. Cheaper by the Dozen. New York: T. W. Crowell, 1948.
Gilbreth, L. M. As I Remember: An Autobiography. Norcross, GA: Engineering &
Management Press, 1988.

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Kanigel, R. One Best Way. New York: Viking, 1997.
Konz, S., and S. Johnson. Work Design. 5th ed. Scottsdale, AZ: Holcomb Hathaway,
2000.
Mundell, Marvin E. Motion and Time Study: Improving Productivity. 5th ed. Englewood
Cliffs, NJ: Prentice-Hall, 1978.
Nadler, Gerald. “The Role and Scope of Industrial Engineering.” In Handbook of
Industrial Engineering, 2d ed. Ed. Gavriel Salvendy. New York: John Wiley &
Sons, 1992.
Niebel, Benjamin W. A History of Industrial Engineering at Penn State. University
Park, PA: University Press, 1992.
Salvendy, G., ed. Handbook of Human Factors. New York: John Wiley & Sons, 1987.
Saunders, Byron W. “The Industrial Engineering Profession.” In Handbook of Industrial
Engineering. Ed. Gavriel Salvendy. New York: John Wiley & Sons, 1982.
Taylor, F. W. The Principles of Scientific Management. New York: Harper, 1911.
Thompson, C. Bertrand. The Taylor System of Scientific Management. Chicago: A. W.
Shaw, 1917.
United Nations Industrial Development Organization. Industry in the 1980s: Structural
Change and Interdependence. New York: United Nations, 1985.

WEBSITES
The Ergonomics Society—http://www.ergonomics.org.uk/
Human Factors and Ergonomics Society—http://hfes.org/
Institute of Industrial Engineers—http://www.iienet.org/
International Ergonomics Association—http://www.iea.cc/
OSHA—http://www.osha.gov/

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Tools

CHAPTER

2

KEY POINTS

• Select the project with the exploratory tools: Pareto analyses, fish diagrams,
Gantt charts, PERT charts, and job/worksite analysis guides.

• Get and present data with the recording tools: operation, flow, worker/machine, and gang process charts, and flow diagrams.

• Develop the ideal method with quantitative tools: worker/machine

relationships with synchronous and random servicing and line balancing calculations. A

good methods engineering program will follow an orderly process, starting from the selection of the project and ending with the implementation of the project (see Figure 1.3). The first, and perhaps most crucial, step—whether designing a new work center or improving an existing operation—is the identification of the problem in a clear and logical form. Just as the machinist uses tools such as micrometers and calipers to facilitate performance, so the methods engineer uses appropriate tools to do a better job in a shorter time. A variety of such problem-solving tools are available, and each tool has specific applications.
The first five tools are primarily used in the first step of methods analysis, select the project. Pareto analysis and fish diagrams evolved from Japanese quality circles of the early 1960s (see Chapter 18) and were quite successful in improving quality and reducing costs in their manufacturing processes. Gantt and PERT charts emerged during the 1940s in response to a need for better project planning and control of complex military projects. They can also be very useful in identifying problems in an industrial setting.
Typically, project selection is based on three considerations: economic (probably the most important), technical, and human. Economic considerations may involve new products, for which standards have not been implemented, or existing products that have a high cost of manufacturing. Problems could be large amounts of scrap or rework, excessive material handling, in terms of either cost or distance, or simply “bottleneck” operations. Technical considerations may include processing techniques that need to be improved, quality control problems due to the method,

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or product performance problems compared to the competition. Human considerations may involve highly repetitive jobs, leading to work-related musculoskeletal injuries, high-accident-rate jobs, excessively fatiguing jobs, or jobs about which workers constantly complain.
The first four exploratory tools are most typically used in the analyst’s office.
The fifth tool, job/worksite analysis guide, helps identify problems within a particular area, department, or worksite and is best developed as part of a physical walk-through and on-site observations. The guide provides a subjective identification of key worker, task, environmental, or administrative factors that may cause potential problems. It also indicates appropriate tools for further, more quantitative evaluations. Use of the job/worksite analysis guide should be a necessary first step before extensive quantitative data are collected on the present method. The next five tools are used to record the present method, and they comprise the second step of methods analysis, get and present the data. Pertinent factual information—such as the production quantity, delivery schedules, operational times, facilities, machine capacities, special materials, and special tools—may have an important bearing on the solution of the problem, and such information needs to be recorded. (The data are also useful in the third step of methods analysis, analyze the data.)
The final three tools are more useful as a quantitative approach in the fourth step of methods analysis, develop the ideal method. Once the facts are presented clearly and accurately, they are examined critically, so that the most practical, economical, and effective method can be defined and installed. They should therefore be used in conjunction with the operational analysis techniques described in Chapter 3. Note that most of the tools from all four groupings can easily be utilized in the operational analysis phase of development.

2.1

EXPLORATORY TOOLS

PARETO ANALYSIS
Problem areas can be defined by a technique developed by the economist
Vilfredo Pareto to explain the concentration of wealth. In Pareto analysis, items of interest are identified and measured on a common scale and then are ordered in descending order, as a cumulative distribution. Typically 20 percent of the ranked items account for 80 percent or more of the total activity; consequently, the technique is sometimes called the 80-20 rule. For example, 80 percent of the total inventory is found in only 20 percent of the inventory items, or 20 percent of the jobs account for approximately 80 percent of the accidents (Figure 2.1), or 20 percent of the jobs account for 80 percent of the workers’ compensation costs. Conceptually, the methods analyst concentrates the greatest effort on the few jobs that produce most of the problems. In many cases, the Pareto distribution can be transformed to a straight line using a lognormal transformation, from which further quantitative analyses can be performed (Herron, 1976).

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Figure 2.1 Pareto distribution of industrial accidents.

Twenty percent of job codes (CUP and ABY) cause approximately 80 percent of accidents.

FISH DIAGRAMS
Fish diagrams, also known as cause-and-effect diagrams, were developed by
Ishikawa in the early 1950s while he was working on a quality control project for
Kawasaki Steel Company. The method consists of defining an occurrence of a typically undesirable event or problem, that is, the effect, as the “fish head” and then identifying contributing factors, that is, the causes, as “fish bones” attached to a backbone and the fish head. The principal causes are typically subdivided into five or six major categories—the human, machines, methods, materials, environmental, administrative—each of which is further subdivided into subcauses.
The process is continued until all possible causes are listed. A good diagram will have several levels of bones and will provide a very good overview of a problem and its contributing factors. The factors are then critically analyzed in terms of their probable contribution to the overall problem. Hopefully, this process will also tend to identify potential solutions. An example of a fish diagram used to identify operator health complaints in a cutoff operation is shown in Figure 2.2.
Fish diagrams have worked quite successfully in Japanese quality circles, where input is expected from all levels of workers and managers. Such diagrams may prove to be less successful in U.S. industry, where the cooperation between labor and management may be less effective in producing the desired solutions and outcomes (Cole, 1979).

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CHAPTER 2

Environment

Materials

Methods
Housekeeping

Intensity
Noise
Frequency

Contrast
Visibility
Lighting
Job enlargement
Job organization
Job rotation
Rest
Self–paced

Chucking
Fixture
Setup
Humidity
Temperature
Ventilation

Reach
Layout
Interference

Pretreatment

Type of Alloy
Impurities

Experience
Training
Heads-up video
Skill
Visibility
Work Hardening
Magnification
Stereoscope
Orientation
Indexing
Age
Alignment
Precision

Employee
Complaints

Capacity
Foot pedal
Activation
Palm button
Pay
Incentives
Vibration

Administrative

Tweezers
Parts feed
Gravity

Stiffness
Sheet gage
Thickness

Machine

Morale
Boredom

Human

Figure 2.2 Fish diagram for operator health complaints in a cutoff operation.

GANTT CHART
The Gantt chart was probably the first project planning and control technique to emerge during the 1940s in response to the need to manage complex defense projects and systems better. A Gantt chart simply shows the anticipated completion times for various project activities as bars plotted against time on the horizontal axis
(Figure 2.3a). Actual completion times are shown by shading the bars appropriately.
If a vertical line is drawn through a given date, you can easily determine which project components are ahead of or behind schedule. For example, in Figure 2.3a, by the end of the third month, mock-up work is behind schedule. A Gantt chart forces the project planner to develop a plan ahead of time and provides a quick snapshot of the progress of the project at any given time. Unfortunately, it does not always completely describe the interaction between different project activities. More analytical techniques, such as PERT charts, are required for that purpose.
The Gantt chart can also be utilized for sequencing machine activity on the plant floor. The machine-based chart can include repair or maintenance activity by crossing out the time period in which the planned downtime will occur.
For example, in the job shop in Figure 2.3b, in the middle of the month, lathe work is behind schedule, while production on the punch press is ahead of schedule. PERT CHARTING
PERT stands for Program Evaluation and Review Technique. A PERT chart, also referred to as a network diagram or critical path method, is a planning and control tool that graphically portrays the optimum way to attain some predetermined objective, generally in terms of time. This technique was employed by the U.S.

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(a)

(b)
Figure 2.3 Example of (a) project-based Gantt chart and (b) machine- or process-based Gantt chart.

military in the design of such processes as the development of the Polaris missile and the operation of control systems in nuclear-powered submarines. Methods analysts usually use PERT charting to improve scheduling through cost reduction or customer satisfaction.

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In using PERT for scheduling, analysts generally provide two or three time estimates for each activity. For example, if three time estimates are used, they are based on the following questions:
1. How much time is required to complete a specific activity if everything works out ideally (optimistic estimate)?
2. Under average conditions, what would be the most likely duration of this activity? 3. What is the time required to complete this activity if almost everything goes wrong (pessimistic estimate)?
With these estimates, the analyst can develop a probability distribution of the time required to perform the activity.
On a PERT chart, events (represented by nodes) are positions in time that show the start and completion of a particular operation or group of operations.
Each operation or group of operations in a department is defined as an activity and is called an arc. Each arc has an attached number representing the time
(days, weeks, months) needed to complete the activity. Activities that utilize no time or cost yet are necessary to maintain a correct sequence are called dummy activities and are shown as dotted lines (activity H in Fig 2.4.).
Dummy activities are typically used to indicate precedence or dependencies, because, under the rules, no two activities can be identified by the same nodes; that is, each activity has a unique set of nodes.

D-1

G-3

5

7

5

2

I-6

EH

10

-0

N-1

L3

3

6

C-

11
P-2

B-2

J-2

1

8
12

M-4

3
F-4

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A-

34

27 Weeks

5
K-

4

O-

4
9

Figure 2.4 Network showing critical path (heavy line).

Circled numbers are nodes that represent the beginning and ending of activities which are represented as lines. The values above each line represent the normal duration of that activity in weeks.

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The minimum time needed to complete the entire project corresponds to the longest path from the initial node to the final node. Termed the critical path in Figure
2.4, the minimum time needed to complete the project is the longest path from node
1 to node 12. While there is always one such path through any project, more than one path can reflect the minimum time needed to complete the project.
Activities not on the critical path have a certain time flexibility. This time flexibility, or freedom, is referred to as float and is defined as the amount of time that a noncritical activity can be lengthened without delaying the project’s completion date. This implies that when the intent is to reduce the project completion time, termed crashing, it is better to concentrate on activities that lie on the critical path than on those on other pathways.
Although the critical path can be found through trial and error there is a formal procedure for uniquely finding the critical path using various time concepts.
These are (1) the earliest starting (ES) for each activity such that all precedence relationships are upheld and (2) the earliest finish (EF) for that activity, which is the earliest start plus the estimated time for that activity, or
EFij ϭESij ϩ tij where i and j are the nodes.
These times are typically found by a forward pass through the network, as shown in the network table in Table 2.1. Note that for an activity that has two predecessor activities, the earliest start is computed as the maximum of the previous earliest furnishes
ESij ϭmax 1 EFij 2

Similar to the earliest start and finish times are the latest start (LS) and latest finish (LF) times which are found through a backward pass through the
Table 2.1 Network Diagram

Activity
A
B
C
D
E
F
G
H (Dummy)
I
J
K
L
M
N
O
P

Nodes

ES

EF

LS

LF

Float

(1, 2)
(1, 3)
(1, 4)
(2, 5)
(3, 5)
(4, 6)
(5, 7)
(3, 6)
(6, 7)
(7, 8)
(8, 9)
(8, 10)
(9, 10)
(10, 11)
(9, 12)
(11, 12)

0
0
0
4
2
3
7
2
7
13
15
15
20
24
20
25

4
2
3
5
7
7
10
2
13
15
20
18
24
25
24
27

5
3
0
9
5
3
10
7
7
13
15
21
20
24
23
25

9
5
3
10
10
7
13
7
13
15
20
24
24
25
27
27

4
3
0
5
3
0
3
0
0
0
0
3
0
0
3
0

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network. The latest start time is the latest time an activity can start without delaying the project. It is found by subtracting the activity time from the latest finish time.
LSij ϭLFij Ϫ tij
Where two or more activities emanate from one node, the latest finish time is the minimum of the latest start times of the emanating activities.
LFij ϭ min 1 LSij 2
The network table for the network diagram in Fig 2.5 is given in Table 2.1.
Float is formally defined as
Float ϭLS ϪES or Float ϭ LF Ϫ EF
Note that all activities with float equal to zero define the critical path, which for this example, is 27 weeks.
Several methods can be used to shorten a project’s duration, and the cost of various alternatives can be estimated. For example, Table 2.2 identifies the normal times and costs as well as crash times and crash costs that would occur in the project shown in Fig 2.4 were shortened. Using this table, and the network diagram, and assuming that a linear relationship exists between time and the cost per week, various alternatives shown in Table 2.3 can be computed.
Note that at 19 weeks, a second critical path is developed through nodes
1, 3, 5, and 7, and any further crashing would need to consider both paths.

JOB/WORKSITE ANALYSIS GUIDE
The job/worksite analysis guide (see Figure 2.5) identifies problems within a particular area, department, or worksite. Before collecting quantitative data, the
Table 2.2 Cost and Time Values to Perform a Variety of Activities under Normal and
Crash Conditions

Normal
Activities
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P

Crash

Cost per Week

Nodes

Weeks

$

Weeks

$

$

(1, 2)
(1, 3)
(1, 4)
(2, 5)
(3, 5)
(4, 6)
(5, 7)
(3, 6)
(6, 7)
(7, 8)
(8, 9)
(8, 10)
(9, 10)
(10, 11)
(9, 12)
(11, 12)

4
2
3
1
5
4
3
0
6
2
5
3
4
1
4
2

4,000
1,200
3,600
1,000
6,000
3,200
3,000
0
7,200
1,600
3,000
3,000
1,600
700
4,400
1,600

2
1
2
0.5
3
3
2
0
4
1
3
2
3
1
2
1

6,000
2,500
4,800
1,800
8,000
5,000
5,000
0
8,400
2,000
4,000
4,000
2,000
700
6,000
2,400

1,000
1,300
1,200
1,600
1,000
1,800
2,000

600
400
500
1,000
400

800
800

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Table 2.3 Times and Costs for Various Alternatives for the Network Shown in Figure 2.4 and Data Given in Table 2-2

Schedule (weeks) Least Expensive Alternative
27
26
25
24
23
22
21
20
19

Normal duration of project
Crash activity M (or J) by one week for an added cost of $400
Crash activity J (or M) by one week for an additional cost of $400
Crash activity K by one week for an additional cost of $500
Crash activity K by another week for an additional cost of $500
Crash activity I by one week for an additional cost of $600
Crash activity I by another week for an additional cost of $600
Crash activity P by one week for an additional cost of $800
Crash activity C by one week for an additional cost of $1,200

Total Weeks
Gained

Total Added
Cost ($)

0

0

1

400

2

800

3

1,300

4

1,800

5

2,400

5

3,000

7

3,800

8

5,000

analyst first walks through the area and observes the worker, the task, the workplace, and the surrounding working environment. In addition, the analyst identifies any administrative factors that may affect the worker’s behavior or performance. All these factors provide an overall perspective of the situation and help guide the analyst in using other, more quantitative tools for collecting and analyzing data. The example in Figure 2.5 shows the application of the job/worksite analysis guide to a hot-end operation in a television manufacturing facility. Key concerns include the lifting of heavy loads, heat stress, and noise exposure.

2.2

RECORDING AND ANALYSIS TOOLS

OPERATION PROCESS CHART
The operation process chart shows the chronological sequence of all operations, inspections, time allowances, and materials used in a manufacturing or business process, from the arrival of raw material to the packaging of the finished product.
The chart depicts the entrance of all components and subassemblies to the main assembly. Just as a blueprint displays such design details as fits, tolerances, and specifications, the operation process chart gives manufacturing and business details at a glance.
Two symbols are used in constructing the operation process chart: a small circle denotes an operation, and a small square denotes an inspection. An operation

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Figure 2.5 Job/worksite analysis guide for a hot-end job in a television manufacturing facility.

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takes place when a part being studied is intentionally transformed or when it is being studied or planned prior to productive work being performed on it. An inspection takes place when the part is being examined to determine its conformity to a standard. Note that some analysts prefer to outline only the operations, calling the result an outline process chart.
Before beginning the actual construction of the operation process chart, analysts identify the chart with a title, Operation Process Chart, and other information, such as the part number, drawing number, process description, present or proposed method, date, and name of the person doing the charting. Additional information may include such items as the chart number, plant, building, and department. Vertical lines indicate the general flow of the process as work is accomplished, while horizontal lines feeding into the vertical flow lines indicate material, either purchased or worked on during the process. Parts are shown as entering a vertical line for assembly or leaving a vertical line for disassembly.
Materials that are disassembled or extracted are represented by horizontal material lines drawn to the right of the vertical flow line, while assembly materials are shown as horizontal lines drawn to the left of the vertical flow line.
In general, the operation process chart is constructed such that vertical flow lines and horizontal material lines do not cross. If it becomes necessary to cross a vertical and a horizontal line, use conventional practice to show that no juncture occurs; that is, draw a small semicircle in the horizontal line at the point where the vertical line crosses it (see Figure 2.6).
Junction

No Junction

Vertical flow line

Horizontal connecting line

Alternate Paths

Vertical flow line

Horizontal connecting line

Rework

01

Figure 2.6 Flowcharting conventions.

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Operation Process Chart
Manufacturing Type 2834421 Telephone Stands--Present Method
Part 2834421 Dwg. No. SK2834421
Charted By B.W.N. 4-12Legs (4 Reg'd) Dwg. 2834421-3
2 1/2"x2 1/2"x16" White Maple
.09 Min. 0-12

Sills (4 Reg'd) Dwg. 2834421-2
1 1/2"x3"x12" Yellow Pine

Saw to Rough
Length

.08 Min. 0-6

Top Dwg. 2834421-1
1 1/2"x14"x14" White Maple

Saw to Rough
Length

.13 Min. 0-1

Saw to Rough
Length

.30 "

0-3

Joint Two
Edges

.15 "

0-7

Joint Two
Edges

.23 "

0-2

Joint Two
Edges

.32 "

0-4

Plane to
Size

.30 "

0-8

Plane to
Size

.32 "

0-3

Plane to
Size

.11 "

0-5

Saw to Finished
Length

.10 "

0-9

Saw to Finished
Length

.18 "

0-4

Saw to Finished
Length

D.W.

Ins. Check Over-All
3 Dimensions

D.W.

Ins. Check Over-All
2 Dimensions

D.W.

Ins. Check Over-All
1 Dimensions

.28 Min. 0-16

Sand All
Over

.25 Min. 0-10

Sand All
Over

.50 Min. 0-5

Sand All
Over

2.00 Min. 0-11

Assemble Four
Sills to Top

3.25 Min. 0-17

Assemble Legs
Complete

8 Slotted Head 1 1/2" Wood Screws
Pc. 416412

D.W.

Ins. Inspect
4 Complete

Clear Shellac #173-111
1.15 Min. 0-18
.75 "

Spray One Coat
Clear Shellac

0-19 Sand Complete

Gun Lacquer #115-309
1.15 Min. 0-20
D.W.

Spray One Coat
Lacquer

Ins. Inspect Finish
5

Summary:
Event
Operations
Inspections

Number
20
5

Time
17.58 minutes
Day work

Figure 2.7 Operation process chart illustrating manufacture of telephone stands.

Time values, based on either estimates or actual measurements, may be assigned to each operation and inspection. A typical completed operation process chart illustrating the manufacture of telephone stands is shown in Figure 2.7.
The completed operation process chart helps analysts visualize the present method, with all its details, so that new and better procedures may be devised. It shows analysts what effect a change on a given operation will have on the preceding and subsequent operations. It is not unusual to realize a 30 percent reduction in performance time by utilizing the principles of operations analysis
(see Chapter 3) in conjunction with the operation process chart, which inevitably suggests possibilities for improvement. Also since each step is shown

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in its proper chronological sequence, the chart in itself is an ideal plant layout.
Consequently, methods analysts find this tool extremely helpful in developing new layouts and improving existing ones.

FLOW PROCESS CHART
In general, the flow process chart contains considerably greater detail than the operation process chart. Consequently, it is not usually applied to entire assemblies but rather for each component of an assembly. The flow process chart is especially valuable in recording nonproduction hidden costs, such as distances traveled, delays, and temporary storages. Once these nonproduction periods are highlighted, analysts can take steps to minimize them and hence their costs.
In addition to recording operations and inspections, flow process charts show all the moves and storage delays encountered by an item as it goes through the plant. Flow process charts therefore need several symbols in addition to the operation and inspection symbols used in operation process charts. A small arrow signifies transportation, which can be defined as moving an object from one place to another, except when the movement takes place during the normal course of an operation or inspection. A large capital D indicates a delay, which occurs when a part is not immediately permitted to be processed at the next workstation. An equilateral triangle standing on its vertex signifies a storage, which occurs when a part is held and protected against unauthorized removal.
These five symbols (see Figure 2.8) are the standard set of process chart symbols
(ASME, 1974). Several other nonstandard symbols may sometimes be utilized for clerical or paperwork operations and for combined operations, as shown in
Figure 2.9.
Two types of flowcharts are currently in general use: product or material
(see Figure 2.10, preparation of direct mail advertising) and operative or person
(see Figure 2.11, service personnel inspecting LUX field units). The product chart provides the details of the events involving a product or a material, and the operative flowchart details how a person performs an operational sequence.
Like the operation process chart, the flow process chart is identified by a title, Flow Process Chart, and accompanying information that usually includes the part number, drawing number, process description, present or proposed method, date, and name of the person doing the charting. Additional data that may be valuable for completely identifying the job being charted include the plant, building, or department; chart number; quantity; and cost.
For each event in the process, the analyst writes a description of the event, circles the appropriate process chart symbol, and indicates the times for processes or delays and distances for transportations. The analyst then connects succeeding event symbols with a vertical line. The right-hand column provides space for the analyst to enter comments or make recommendations for potential changes.
To determine the distance moved, the analyst need not measure each move accurately with a tape or a 6-ft rule. A sufficiently correct figure usually results by counting the number of columns that the material moves past and then multiplying

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Operation

A large circle indicates an operation such as

Drive nail

Mix

Drill hole

Move material by conveyor

Move material by carrying (messenger)

Raw material in bulk storage

Finished stock stacked on pallets

Protective filing of documents

Wait for elevator

Material in truck or on floor at bench waiting to be processed

Papers waiting to be filed

Examine material for quality or quantity

Read steam gauge on boiler

Examine printed form for information

Transportation

An arrow indicates a transportation, such as Move material by truck
Storage

A triangle indicates a storage, such as
Delay

A large capital D indicates a delay, such as
Inspection

A square indicates an inspection such as

Figure 2.8 The ASME standard set of process chart symbols.

this number, less 1, by the span. Moves of 5 ft or less are usually not recorded; however, they may be if the analyst feels that they materially affect the overall cost of the method being plotted.
All delay and storage times must be included on the chart. The longer a part stays in storage or is delayed, the more cost it accumulates and the longer the customer must wait for delivery. It is therefore important to know how much time a part spends at each delay or storage. The most economical method of determining the duration of delays and storages is to mark several parts with chalk, indicating the exact time they went into a storage or were delayed. Then check the section periodically to see when the marked parts are brought back into production. By taking a number of cases, recording the elapsed time, and then averaging the results, analysts can obtain sufficiently accurate time values.

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A record was created.

Information was added to a record.

A decision was made.

An inspection was performed in conjunction with an operation.

An operation and transportation took place simultaneously.

Figure 2.9 Nonstandard process chart symbols.

The flow process chart, like the operation process chart, is not an end in and of itself; it is merely a means to an end. This tool facilitates the elimination or reduction of the hidden costs of a component. Since the flowchart clearly shows all transportations, delays, and storages, the information it provides can lead to a reduction of both the quantity and duration of these elements. Also, since distances are recorded on the flow process chart, the chart is exceptionally valuable in showing how the layout of a plant can be improved. These techniques are described in greater detail in Chapter 3.

FLOW DIAGRAM
Although the flow process chart gives most of the pertinent information related to a manufacturing process, it does not show a pictorial plan of the flow of work.
Sometimes this information is helpful in developing a new method. For example,

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Figure 2.10 Flow process chart (material) for preparation of direct mail advertising.

before a transportation can be shortened, the analyst needs to see or visualize where room can be made to add a facility so that the transportation distance can be shortened. Likewise, it is helpful to visualize potential temporary and permanent storage areas, inspection stations, and work points.
The best way to provide this information is to take an existing drawing of the plant areas involved and then sketch in the flow lines, indicating the movement of the material from one activity to the next. A pictorial representation of the layout of floors and buildings, showing the locations of all activities on the flow

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Figure 2.11 Flow process chart (worker) for field inspection of LUX.

process chart, is a flow diagram. When constructing a flow diagram, analysts identify each activity by symbols and numbers corresponding to those appearing on the flow process chart. The direction of flow is indicated by placing small arrows periodically along the flow lines. Different colors can be used to indicate flow lines for more than one part.

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Inspection bench A

Burr
Drill bench press Polishing bench
Plating
tank

7

2
2

Wiping bench B

4

3
Drain

8
Oil
tank

Barrel storage

Barrel storage

Lift

Shipping platform 38

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C
Barrel storage
Barrel
storage

46

Platform

5

6
Barrel storage

Parkerizing tank Rinse tank

1

Polishing bench Bench

Figure 2.12 Flow diagram of the old layout of a group of operations on the
Garand rifle.

(Shaded section of plant represents the total floor space needed for the revised layout
[Figure 2.13]. This represented a 40 percent savings in floor space.)

Figure 2.12 illustrates a flow diagram made in conjunction with a flow process chart to improve the production of the Garand (M1) rifle at Springfield
Armory. This pictorial representation, together with the flow process chart, resulted in savings that increased production from 500 rifle barrels per shift to
3,600—with the same number of employees. Figure 2.13 illustrates the flow diagram of the revised layout.
The flow diagram is a helpful supplement to the flow process chart because it indicates backtracking and possible traffic congestion areas, and it facilitates developing an ideal plant layout.

WORKER AND MACHINE PROCESS CHARTS
The worker and machine process chart is used to study, analyze, and improve one workstation at a time. The chart shows the exact time relationship between the working cycle of the person and the operating cycle of the machine. These facts can lead to a fuller utilization of both worker and machine time, and a better balance of the work cycle.
Many machine tools are either completely automatic (the automatic screw machine) or semiautomatic (the turret lathe). With these types of facilities, the

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Drill press 2

Platform

Parkerizing tank Bench inspection 3 A

Problem-Solving Tools

Stamping machine B

Speed head
4

Roller conveyor
Hot
rinse tank 1

Tank
Drain

Polishing machines Glue pot table 5

6

7

Emery wheel drying oven

Figure 2.13 Flow diagram of the revised layout of a group of operations on the M1
Garand rifle.

operator is often idle for a portion of the cycle. The utilization of this idle time can increase operator earnings and improve production efficiency.
The practice of having one employee operate more than one machine is known as machine coupling. Because organized labor may resist this concept, the best way to sell machine coupling is to demonstrate the opportunity for added earnings. Since machine coupling increases the percentage of “effort time” during the operating cycle, greater incentive earnings are possible if a company is on an incentive wage payment plan. Also, higher base rates result when machine coupling is practiced, since the operator has greater responsibility and can exercise greater mental and physical effort.
When constructing the worker and machine process chart, the analyst must first identify the chart with a title such as Worker and Machine Process Chart.
Additional identifying information would include the part number, drawing number, operation description, present or proposed method, date, and name of the person doing the charting.
Since workers and machine charts are always drawn to scale, the analyst selects a distance in inches to conform with a unit of time such that the chart can be neatly arranged. The longer the cycle time of the operation being charted, the shorter the distance per decimal minute of time. Once exact values have been established for the distance, in inches per unit of time, the chart is begun. The left side shows the operations and time for the worker, and the right side shows the working time and the idle time of the machine or machines. A solid line drawn vertically represents the employee’s working time. A break in the vertical work–time line signifies idle time. Likewise, a solid vertical line under each machine heading indicates machine operating time, and a break in the vertical machine line designates idle machine time. A dotted line under the machine column indicates loading and unloading machine time, during which the machine is neither idle nor productive (see Figure 2.14).
The analyst charts all elements of occupied and idle time for both the worker and the machine through the termination of the cycle. The bottom of the chart

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CHAPTER 2

Worker and Machine Process Chart
Subject Charted Milling slot in regulator clamp
J-1492
Drawing No.
Part No. J-1492-1
Chart Begins Loading mchs. for milling
Chart Ends Unloading milled clamps

807
Chart No.
Chart of Method Proposed
Charted by C.A. Anderson
Date 8-27
Sheet 1 of 1
B.&S. Hor. Mill

Element description
Stop machine #1

.0010

Loosen vise remove part and lay aside (mch. #1)

Machine 2

.0004

Return table mch #1
5 inches

B.&S. Hor. Mill

Machine 1

Operator

.0010

Pickup part and tighten vise mch. #1

.0010

Walk to machine #2

.0011

Stop machine #2

.0004

Return table mch #2
5 inches

.0010

Loosen vise remove part and lay aside (mch. #2)

.0040

.0004

Advance table and engage feed mch. #1

Mill slot

.0018

Start machine #1

Unloading .0024

.0010

Loading

.0032
Idle

Mill slot

.0040
Unloading .0024

Pick up part and tighten vise mch. #2

.0018

Start machine #2

.0004

Advance table and engage feed mch. #2

.0010

Walk to machine #1

.0011

Idle man time per cycle
Working man time per cycle
Man-hours per cycle

.0000
.0134
.0134

Loading

Idle hours machine #2
Productive hours mch. #2
Machine #2 cycle time

.0032

Idle

Idle hours machine #1
Productive hours mch. #1
Machine #1 cycle time

.0038
.0096
.0134

.0038
.0096
.0134

Figure 2.14 Worker and machine process chart for milling machine operation.

shows the employee’s total working time and total idle time, as well as the total working time and idle time of each machine. The productive time plus the idle time of the worker must equal the productive time plus the idle time of each machine that the worker operates.
Accurate elemental time values are necessary before the worker and machine chart can be constructed. These time values should represent standard times that include an acceptable allowance for fatigue, unavoidable delays, and personal delays (see Chapter 11 for more details). The analyst should never use overall stopwatch readings in the construction of the chart.

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The completed worker and machine process chart clearly shows the areas in which both idle machine time and idle worker time occur. These areas are generally a good place to start in effecting improvements. However, the analyst must also compare the cost of the idle machine with that of the idle worker. It is only when total cost is considered that the analyst can safely recommend one method over another. Economical considerations are presented in the next section.

GANG PROCESS CHARTS
The gang process chart is, in a sense, an adaptation of the worker and machine chart. A worker and machine process chart helps determine the most economical number of machines one worker can operate. However, several processes and facilities are of such magnitude that instead of one worker operating several machines, several workers are needed to operate one machine effectively. The gang process chart shows the exact relationship between the idle and operating cycles of the machine and the idle and operating times per cycle of the workers who service that machine. This chart reveals the possibilities for improvement by reducing both idle operator time and idle machine time.
Figure 2.15 illustrates a gang process chart for a process in which a large number of idle work-hours exist, up to 18.4 h per 8-h shift. The chart also shows that the company is employing two more operators than are needed. By relocating some of the controls of the process, the company was able to reassign the elements of work so that four, rather than six, workers could effectively operate the extrusion press. A better operation of the same process is shown on the gang process chart in Figure 2.16. The savings of 16 h per shift was easily developed through the use of this chart.

2.3 QUANTITATIVE TOOLS, WORKER
AND MACHINE RELATIONSHIPS
Although the worker and machine process chart can illustrate the number of facilities that can be assigned to an operator, this can often be computed in much less time through the development of a mathematical model. A worker and machine relationship is usually one of three types: (1) synchronous servicing, (2) completely random servicing, and (3) a combination of synchronous and random servicing.

SYNCHRONOUS SERVICING
Assigning more than one machine to an operator seldom results in the ideal case where both the worker and the machine are occupied during the whole cycle.
Such ideal cases are referred to as synchronous servicing, and the number of machines to be assigned can be computed as nϭ lϩm l 49

41

.06

.10

.15

Unlock die

Loosen & push out shell

Withdraw ram
& lock die in head Withdraw ram
& lock die in head Loosen & push out shell

Unlock die

1.00 Min.
0
"

.15

.10

.06

.45

Extrude

Run head & shell out
Shear rod from shell Pull die off end of rod

Idle time

Grease die & position back in die head

OPERATION

Figure 2.15

.32 Min.
.68 "

.04
.05

.11

.68

.12

TIME

Ram billet from furnace & close furnace door

Open furnace door & remove billet Idle time

Rearrange billets in furnace

OPERATION

.49 Min.
.51 "

.10

.19

.51

.20

TIME

.12

.10

TIME

.12

Dispose of shell

.57 Min.
.43 "

.05

Idle

Grab tongs & move to position

.43

Dispose of dummy and lay aside tongs .18

Press dummy out of shell

Position shell on small press

OPERATION

DUMMY KNOCKER

Gang process chart of the present method of operation of a hydraulic extrusion process.

Idle time = 2.30 Man-minutes per cycle = 18.4 man-hours per eight-hour day

1.00 Min.
0
"

.45

Extrude

.08
.04
.05

.07

TIME

Position billet
Position dummy
Build pressure

Elevate billet

OPERATION

FURNACE MAN

Guide shell from shear to small press

Idle time

Move away from small press and aside tongs

OPERATION

.32 Min.
.68 "

.20

.68

.12

TIME

ASSISTANT
DUMMY KNOCKER

Hold rod while die removed at press

Straighten rod end with mallet

Grab rod with tongs and pull out Walk back toward press Pull rod toward cooling rack

1.00 Min.
0
"

.09

.11

.45

.15

.20

TIME

PULL-OUT MAN
OPERATION

Tex

Working time
Idle time

.08
.04
.05

.07

TIME

ASSISTANT
PRESS OPERATOR

2. Problem−Solving Tools

Position billet
Position dummy
Build pressure

Elevate billet

OPERATION

PRESS OPERATOR

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GANG PROCESS CHART OF PRESENT METHOD
Hydraulic Extrusion
Press Dept. II
Bellefonte Pa. Plant
Charted by B.W.N.
4-15Chart No. G-85

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GANG PROCESS CHART—PROPOSED METHOD
Hydraulic Extrusion Press Dept. II
Bellefonte, Pa. Plant
Charted by B.W.N
4-15
Chart G-85

MACHINE
OPERATION

TIME OPERATION

TIME OPERATION

Elevate billet

.07 Elevate billet

Position billet
Position dummy
Build pressure

.08 Position billet
.04 Position dummy
.05 Build pressure

Extrude

ASSISTANT
PRESS OPERATOR

MACHINE

TIME OPERATION

.07 Grease die & position back in die head
.08 Walk to furnace
.04
.05 Rearrange billets in furnace .05 Dispose of dummy and lay
.09 aside tongs

Open furnace door & remove billet Unlock die

.06 Unlock die

Loosen & push out shell

.10 Loosen & push out shell

Withdraw ram
& lock die in head Withdraw ram
.15 & lock die in head Ram billet from furnace & close
.06 furnace door
.10 Run head & shell Shear rod from
.15 shell
Pull die off end of rod
1.00 Min.
0

.05 Press dummy out of shell
.20

PULL-OUT MAN

TIME OPERATION

Position shell
.12 on small press

.45 Return to press
Idle time

.45 Extrude

Working time 1.00 Min.
Idle time
0

DUMMY
KNOCKER

Dispose of shell

.10 Pull rod toward cooling rack .12
Walk back toward press
.18

.12 Grab rod with tongs and pull out

TIME
.20
.15

.45

.19
Idle time
.10

.23

Grab tongs & move to position

.05

.11 Guide shell from shear to small
.04 press
.05

Straighten rod
.20 end with mallet

.11

Hold rod while die removed at press

.09

.91 Min.
.09 Min.

.77 Min.
.23 Min.

1.00 Min.
0

Figure 2.16 Gang process chart of the proposed method of operation of a hydraulic extrusion process.

where n ϭ number of machines the operator is assigned l ϭ total operator loading and unloading (servicing) time per machine m ϭ total machine running time (automatic power feed)
For example, assume a total cycle time of 4 min to produce a product, as measured from the start of the unloading of the previously completed product to the end of the machine cycle time. Operator servicing, which includes both the unloading of the completed product and the loading of the raw materials, is 1 min, while the cycle time of the automatic machine cycle is 3 min. Synchronous servicing would result in the assignment of
1ϩ3
ϭ 4 machines
1
Graphically, this assignment would appear as shown in Figure 2.17, with the operator moving to the second machine once the first machine is serviced. By the time the fourth machine is serviced, the operator would need to return to the first machine to service it, since that machine’s automatic cycle would have just ended.
If the number of machines in this example is increased, machine interference takes place, and we have a situation in which one or more of the facilities sit idle for a portion of the work cycle. If the number of machines is reduced to nϭ 51

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Figure 2.17

Synchronous servicing assignment for one operator and four machines.

some figure less than 4, then the operator is idle for a portion of the cycle. In such cases, the minimum total cost per piece usually represents the criterion for optimum operation.
An additional complication occurs because of less than ideal conditions. The operator may need to walk between machines or clean and adjust the machines.
This worker time also needs to be accounted for based on the cost of each idle machine and the hourly rate of the operator.
The number of machines that the operator should be assigned under realistic conditions can be reestimated by the lowest whole number from the revised equation: n1 Յ

lϩm lϩw where n1ϭ lowest whole number w ϭ total worker time (not directly interacting with the machine, typically walking time to the next machine)
The cycle time with the operator servicing n1 machines is l ϩ m, since in this case, the operator is not busy the whole cycle, yet the facilities are occupied during the entire cycle.

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45

Using n1, we can compute the total expected cost (TEC) as follows:
TECn1 ϭ ϭ where

K1 1l ϩ m2 ϩ n1K2 1l ϩ m 2 n1 (1)

1l ϩ m 2 1K1 ϩ n1K2 2 n1 TEC ϭ total expected cost in dollars per unit of production from one machine K1 ϭ operator rate, in dollars per unit of time
K2 ϭ cost of machine, in dollars per unit of time

After this cost is computed, a cost should be calculated with n1 ϩ 1 machines assigned to the operator. In this case, the cycle time is governed by the working cycle of the operator, since there is some idle machine time. The cycle time is now
(n1 ϩ 1)(l ϩ w). Let n2 ϭ n1 ϩ 1. Then the total expected cost with n2 facilities is
TECn2 ϭ

1K1 2 1n2 2 1l ϩ w2 ϩ 1K2 2 1n2 2 1n2 2 1l ϩ w2 n2 (2)

ϭ 1l ϩ w2 1K1 ϩ n2K2 2

The number of machines assigned depends on whether n1 or n2 gives the lowest total expected cost per piece.
EXAMPLE 2.1

Synchronous Servicing
It takes an operator 1 min to service a machine and 0.1 min to walk to the next machine.
Each machine runs automatically for 3 min, the operator earns $10.00/h and the machines cost $20.00/h to run. How many machines can the operator service?
The optimum number of machines that the operator can service is n ϭ 1l ϩ m2> 1l ϩ w2 ϭ 11 ϩ 3 2>11 ϩ 0.1 2 ϭ 3.6
The number being fractional, leaves two choices. The operator can be assigned 3 machines (option 1), in which case the operator will be idle some portion of the time. Or the operator can be assigned 4 machines (option 2), in which case the machines will be idle some portion of the time. The best choice may be based on the economics of the situation, that is, the lowest cost per unit.
In option 1, the expected production cost from Equation (1) is (divided by 60 so as to convert hours to minutes)
TEC3 ϭ 1l ϩ m 2 1K1 ϩ n1K2 2>n1 ϭ 11 ϩ 3 2 110 ϩ 3 ϫ 20 2 13>60 2 ϭ $1.556>unit
An alternate approach is to calculate the production rate R per hour:


60 ϫ n1 lϩm The production rate is based on the machines being the limiting factor (i.e., the worker is idle at times) and the machines producing on one unit per machine per 4.0-min total

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cycle (1.0-min service time, 3.0. min of machine time). With 3 machines running 60 min/h, the production rate is


60 ϫ 3 ϭ 45 units>h
1ϩ3

The expected cost is then the cost of the labor and machines divided by the production rate: TEC3 ϭ 1K1 ϩ n1K2 2>R ϭ 110 ϩ 3 ϫ 20 2 >45 ϭ $1.556>unit
In option 2, the expected production cost from Equation (2) is
TEC4 ϭ 11 ϩ w2 1K1 ϩ n2K2 2 ϭ 11 ϩ 0.1 2 110 ϩ 4 ϫ 202>60 ϭ $1.65>unit
In the alternate approach, the production rate is based on the worker being the limiting factor (i.e., the machines are idle at times). Since the worker can produce one unit per
1.1-min cycle (1.0. min service time and 0.1-min walk time), the production rate (R) per hour for the alternate approach is


60
60
ϭ ϭ 54.54 units>h
1ϩw
1.1

The expected cost is then the cost of the labor and machines divided by the production rate: TEC4 ϭ 1K1 ϩ n1K2 2 >R ϭ 110 ϩ 4 ϫ 20 2>54.54 ϭ $1.65>unit
Based on lowest cost, the setup with three machines is best. However, if there is market demand at a good sales price, profits can be maximized using a four-machine setup. Note also that in this example, with a walk time of 0.1 min, production decreases from the ideal of 60 units/min (see figure on page 48).
Note the effect of reducing loading/unloading time from 1. min to 0.9 min, a relatively small amount. The optimum number of machines that the operator can service is now n ϭ 1l ϩ m 2>1l ϩ w 2 ϭ 10.9 ϩ 3 2 > 10.9 ϩ 0.1 2 ϭ 3.9
Although the number is still fractional, it is very close to 4, the realistic amount. If the operator is assigned three machines (option 1), operator will be idle a greater portion of the time, increasing from 0.7 to 0.9 min or to almost 25% of the time. The expected production cost from Equation (1) is (with 60 needed to convert hours to minutes) TEC3 ϭ 1l ϩ m 2 1K1 ϩ n1K2 2 >n1 ϭ 10.9 ϩ 32 110 ϩ 3 ϫ 20 2 13>60 2 ϭ $1.517>unit
The alternate approach yields a production rate


60
60
ϫ n1 ϭ ϫ 3 ϭ 46.15 units>h lϩm 3.9

The expected cost is the cost of the labor and machines divided by the production rate:
TEC3 ϭ1 K1 ϩn1K2 2 > R ϭ1 10 ϩ3 ϫ 20 2 > 46.5 ϭ$1.517> unit

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If the worker is assigned the more realistic number of 4 machines (option 2), the more costly machine idle time decreases from 0.4 to 0.1 min. The expected production cost from Equation (2) is
TEC4 ϭ 1 l ϩ w2 1K1 ϩ n2K2 2 ϭ 10.9 ϩ 0.1 2 110 ϩ 4 ϫ 202 >60 ϭ $1.50>unit
The alternate approach yields a production rate R per hour of


60
60
ϭ ϭ 60 units>h lϩw 1.0

The expected cost is the cost of the labor and machines divided by the production rate:
TEC4 ϭ 1K1 ϩ n1K2 2>R ϭ 110 ϩ 4 ϫ 20 2 >60 ϭ $1.50>unit
Based on lowest cost and minimum idle time, the setup with 4 machines is now best. Note that a 10% decrease in loading/unloading time (from 1 to 0.9 min) yielded several positive improvements:





A 10% increase in production (60 compared to 54.54 units/h)
A reduction of idle time from 0.7 min for the operator (17.5% of cycle time) in the first scenario to 0.1 min for the machines in the second scenario
A 3.6% decrease in unit costs from $1.556 to $1.50 per unit

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This demonstrates the importance of decreasing loading or setup time, to be discussed in greater detail in Chapter 3. Note also that decreasing the walk time by a comparable amount (0.1 min which in this case eliminates it completely) results in the ideal case shown below or in Figure 2.17 with the same unit cost of $1.50.

RANDOM SERVICING
Completely random servicing situations are those cases in which it is not known when a facility needs to be serviced or how long servicing takes. Mean values are usually known or can be determined; with these averages, the laws of probability can provide a useful tool in determining the number of machines to assign a single operator.
The successive terms of the binomial expansion give a useful approximation of the probability of 0, 1, 2, 3, . . ., n machines down (where n is relatively small), assuming that each machine is down at random times during the day and that the probability of downtime is p and the probability of runtime is q ϭ 1 Ϫ p. Each term of the binomial expansion can be expressed as a probability of m (out of n) machines down:
P1m of n2 ϭ

n! pmqnϪm m!1n Ϫ m2!

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As an example, let us determine the minimum proportion of machine time lost for various numbers of turret lathes assigned to an operator where the average machine runs unattended 60 percent of the time. Operator attention time (machine is down or requires servicing) at irregular intervals is 40 percent on average. The analyst estimates that three turret lathes should be assigned per operator on this class of work. Under this arrangement, the probabilities of m (out of n) machines down would be as follows:
Machines down m

Probability

0

3!
10.40 2 10.63 2 ϭ 11 2 11 2 10.216 2 ϭ 0.216
0!13 Ϫ 0 2 !

1

3!
1 0.41 2 1 0.62 2 ϭ1 3 2 1 0.4 2 1 0.36 2 ϭ0.432
1!1 3 Ϫ1 2 !

2

3!
10.42 2 10.61 2 ϭ 132 10.16 2 10.6 2 ϭ 0.288
2!13 Ϫ 2 2!

3

3!
1 0.43 2 1 0.60 2 ϭ1 1 2 1 0.064 2 1 1 2 ϭ0.064
3!1 3 Ϫ3 2 !

By using this approach, the proportion of time that some machines are down may be determined, and the resulting lost time of one operator per three machines may be readily computed. In this example, we have the following:
No. of machines down
0
1
2
3

Probability

Machine hours lost per 8-h day

0.216
0.432
0.288
0.064
_____
1.000

0
0*
(0.288)(8) ϭ 2.304
(2)(0.064)(8) ϭ 1.024
_____
3.328

*

Since only one machine is down at a time, the operator can be attending the down machine.

3.328 ϭ 13.9 percent
24.0
Similar computations can be made for more or less machine assignments to determine the assignment resulting in the least machine downtime. The most satisfactory assignment is usually the arrangement showing the least total expected cost per piece, while the total expected cost per piece for a given arrangement is computed by the expression
Proportion of machine time lost ϭ

TEC ϭ

K1 ϩ nK2
R

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K1 ϭ hourly rate of the operator
K2 ϭ hourly rate of the machine n ϭ number of machines assigned
R ϭ rate of production, pieces from n machines per hour

The rate of production, in pieces per hour, from n machines is computed with the mean machine time required per piece, the average machine servicing time per piece, and the expected downtime or lost time per hour.
For example, under a five-machine assignment to one operator, an analyst determined that the machining time per piece was 0.82 h, the machine servicing time per piece was 0.17 h, and the machine downtime was an average of 0.11 h per machine per hour. Thus, each machine was available for production work only 0.89h each hour. The average time required to produce one piece per machine would be
0.82 ϩ 0.17 ϭ 1.11
0.89
Therefore, the five machines would produce 4.5 pieces per hour. With an operator hourly rate of $12 and a machine hourly rate of $22, we have a total expected cost per piece of
$12.00 ϩ 51$22.00 2
4.5

ϭ $27.11

EXAMPLE 2.2

Random servicing
An operator is assigned to service three machines that have an expected downtime of
40 percent. When running, each machine can produce 60 units/h. The operator is paid
$10.00/h, and a machine costs $60.00/h to run. Is it worth hiring another operator to keep the machines running?
Case A - One operator

Machines down m

Probability

Machine hours lost per 8-h day

0

3!
10.4 2 0 10.6 2 3 ϭ 0.216
0! 3!

0

1

3!
10.4 2 1 10.6 2 2 ϭ 0.432
1! 2!

0

2

3!
10.4 2 2 10.6 2 1 ϭ 0.288
2! 1!

0.288 ϫ 8 ϭ 2.304

3

3!
10.4 2 3 10.6 2 0 ϭ 0.064
3! 0!

0.064 ϫ 16 ϭ 1.024

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Considering that a total of 3.328 production hours (2.304 ϩ 1.024) are lost in an 8-h day, only 1,240.3 units (20.672 ϫ 60) can be produced at an hourly average of 155.04.
The unit cost is
TEC ϭ 110 ϩ 3 ϫ 60 2 >155.04 ϭ $1.23>unit

Case B - Two operators

Machines down m

Probability

Machine hours lost per 8-h day

0

3!
10.4 2 0 10.6 2 3 ϭ 0.216
0! 3!

0

1

3!
10.4 2 1 10.6 2 2 ϭ 0.432
1! 2!

0

2

3!
10.4 2 2 10.6 2 1 ϭ 0.288
2! 1!

0

3

3!
10.4 2 3 10.6 2 0 ϭ 0.064
3! 0!

0.064 ϫ 8 ϭ 0.512

There is a considerable improvement from case A. Since only 0.512 production hour is lost in an 8-h day, production increases to 1,409.28 units (23.488 ϫ 60) or an hourly average of 176.16. The unit cost is
TEC ϭ 12 ϫ 10 ϩ 3 ϫ 60 2>176.16 ϭ $1.14>unit
Therefore it is more cost-efficient to hire another operator and keep the machines running.
Note that hiring a third operator to keep all three machines running all the time would not be cost-efficient. Although the total production increases marginally, the total cost increases more, and the unit cost becomes
TEC ϭ 13 ϫ 10 ϩ 3 ϫ 60 2 >180 ϭ $1.17>unit

COMPLEX RELATIONSHIPS
Combinations of synchronous and random servicing are perhaps the most common type of worker and machine relationships. Here the servicing time is relatively constant, but the machines are serviced randomly. Furthermore, the time between breakdowns is assumed to have a particular distribution. As the number of machines increases and the relationship between the operator and machines becomes more complex, machine interference and consequent delay times increase. In practice, machine interference predominantly occurs from 10 to 30 percent of the total working time, with extremes of up to 50 percent. Various approaches have been developed to deal with such situations.
One such approach assumes an expected workload for the operator based on the number of machines assigned and mean machine running times and mean

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53

N

100
80
60

=6
=5

N

N
=
4

40

N
=
3

Machine interference as percentage of servicing time (I)

200

20

N

=

2

10
8
6
4
2
1

1

2

4

6

8 10

20

40

60 100

Ratio of machine time to servicing time (X)

Figure 2.18

Machine interference as a percentage of servicing time when the number of machines assigned to one operator is six or less.

servicing times. For up to six machines, the use of the empirical curves illustrated in Figure 2.18 is recommended.
For seven or more machines, Wright’s formula (Wright, Duvall, and Freeman, 1936) can be used:
I ϭ 505 2΄11 ϩ X Ϫ N2 2 ϩ 2N΅ Ϫ 11 ϩ X Ϫ N 2 6 where I ϭ interference, expressed as a percentage of the mean servicing time
X ϭ ratio of mean machine running time to mean machine servicing time
N ϭ number of machine units assigned to one operator

An application of this formula is shown in Example 2.3.

Calculation of Machine Interference Time
In a quilling production, an operator is assigned 60 spindles. The mean machine running time per package, determined by stopwatch study, is 150 min. The standard mean servicing time per package, also developed by time study, is 3 min. The computation of the machine interference, expressed as a percentage of the mean operator attention time, is

EXAMPLE 2.3

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I ϭ 505 2΄11 ϩ X Ϫ N 2 2 ϩ 2N΅ Ϫ 11 ϩ X Ϫ N2 6
2
150
150
Ϫ 60 b ϩ 120 Ϫ a 1 ϩ
Ϫ 60 b d ϭ 50 c a 1 ϩ ϩ
3.00
3.00
B

2
I ϭ 50΄ 211 ϩ 50 Ϫ 60 2 ϩ 120 Ϫ 11 ϩ 50 Ϫ 60 2΅

I ϭ 1.159%
Thus, we would have
Machine running time
Servicing time
Machine interference time

150.0 min
3.0 min
11.6 ϫ 3.0 ϭ 34.8 min

Using queuing theory with the time between breakdowns assumed to have an exponential distribution, Ashcroft (1950) extended the above approach and developed tables to determine machine interference times. These are shown in
Table A3-13 (Appendix 3) and provide values of machine running time and machine interference time for values of the service ratio k: k ϭ l>m where l ϭ servicing time m ϭ machine running time

The total cycle time to produce one piece is cϭmϩlϩi where

c ϭ total cycle time i ϭ machine interference time

Note that the values of machine running time and machine interference time in
Table A3-13 are given as a percentage of total cycle time. Also, any walking or worker time w should be included as part of servicing time. Example 2.4 demonstrates Ashcroft’s method for calculating machine interference time.
EXAMPLE 2.4

Calculation of Machine Interference, Using Ashcroft’s Method
With reference to Example 2.3: k ϭ l>m ϭ 3>150 ϭ 0.02
N ϭ 60
From Table A3–13, Appendix 3, with exponential service time and k ϭ 0.02 and
N ϭ 60, we have a machine interference time of 16.8 percent of the cycle time. We have Ti ϭ 0.168c, where c is the cycle time to produce one unit per spindle. Then

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cϭmϩlϩi c ϭ 150 ϩ 3.00 ϩ 0.168c
0.832c ϭ 153 c ϭ 184 min and Ti ϭ 0.168c ϭ 30.9 min
The interference time computed by the equation (34.8 min, Example 2.3) closely agrees with that developed here by the queuing model. However, as n (the number of machines assigned) becomes smaller, the proportional difference between the two techniques increases.

LINE BALANCING
The problem of determining the ideal number of workers to be assigned to a production line is analogous to that of determining the number of workers to be assigned to a workstation; the gang process chart solves both problems. Perhaps the most elementary line balancing situation, yet one that is very often encountered, is one in which several operators, each performing consecutive operations, work as a unit. In such a situation, the rate of production is dependent on the slowest operator. For example, we may have a line of five operators assembling bonded rubber mountings prior to the curing process. The specific work assignments might be as follows: Operator 1, 0.52 min; operator 2, 0.48 min; operator 3, 0.65 min; operator 4, 0.41 min; operator 5, 0.55 min. Operator 3 establishes the pace, as is evidenced by the following:
Operator

Standard minutes to perform operation

Wait time based on slowest operator

Standard time
(min)

1
2
3
4
5
Totals

0.52
0.48
0.65
0.41
0.55
____
2.61

0.13
0.17

0.24
0.10

0.65
0.65
0.65
0.65
0.65
____
3.25

The efficiency of this line can be computed as the ratio of the total actual standard minutes to the total allowed standard minutes, or
5



a SM
1
5

a AM
1

ϫ 100 ϭ

2.61 ϫ 100 ϭ 80%
3.25

63

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where E ϭ efficiency
SM ϭ standard minutes per operation
AM ϭ allowed standard minutes per operation
Details on standard times will be covered later in Chapter 9.
Some analysts prefer to consider percent idle time (%Idle):
%Idle ϭ 100 Ϫ E ϭ 20%
In a real-life situation similar to this example, the opportunity for significant savings exists. If an analyst can save 0.10 min on operator 3, the net savings per cycle is not 0.10 min, but 0.10 ϫ 5, or 0.50, min.
Only in the most unusual situations would a line be perfectly balanced; that is, the standard minutes to perform an operation would be identical for each member of the team. The “standard minutes to perform an operation” is not really a standard. It is only a standard to the individual who established it. Thus, in our example, where operator 3 has a standard time of 0.65 min to perform the first operation, a different work measurement analyst might have allowed as little as 0.61 min, or as much as 0.69 min. The range of standards established by different work measurement analysts on the same operation might be even greater than the range suggested. The point is that whether the issued standard is
0.61, 0.65, or 0.69, the typical conscientious operator should have little difficulty in meeting the standard. In fact, the operator will probably better the standard in view of the performance of the operators on the line with less work content in their assignments. Those operators who have a wait time based on the output of the slowest operator are seldom observed as actually waiting. Instead, they reduce the tempo of their movements to utilize the number of standard minutes established by the slowest operator.
The number of operators needed for the required rate of production can be estimated by
N ϭ R ϫ © AM ϭ R ϫ

© SM
E

where N ϭ number of operators needed in the line
R ϭ desired rate of production
For example, assume that we have a new design for which we are establishing an assembly line. Eight distinct operations are involved. The line must produce 700 units per day (or 700/480 ϭ 1.458 units/min), and since it is desirable to minimize storage, we do not want to produce many more than 700 units/day.
The eight operations involve the following standard minutes based on existing standard data: Operation 1, 1.25 min; operation 2, 1.38 min; operation 3, 2.58 min; operation 4, 3.84 min; operation 5, 1.27 min; operation 6, 1.29 min; operation 7, 2.48 min; and operation 8, 1.28 min. To plan this assembly line for the

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most economical setup, we estimate the number of operators required for a given level of efficiency (ideally, 100 percent), as follows:
N ϭ 1.458 ϫ 11.25 ϩ 1.38 ϩ 2.58 ϩ 3.84 ϩ 1.27 ϩ 1.29 ϩ 2.48 ϩ 1.28 2>1.00 ϭ 22.4

For a more realistic 95 percent efficiency, the number of operators becomes
22.4/0.95 ϭ 23.6.
Since it is impossible to have six-tenths of an operator, you would endeavor to set up the line utilizing 24 operators. An alternate approach would be to utilize part-time hourly workers.
Next, we estimate the number of operators to be utilized at each of the eight specific operations. Since 700 units of work are required a day, it will be necessary to produce 1 unit in about 0.685 min (480/700). We estimate the number of operators needed on each operation by dividing the number of minutes allowable to produce one piece into the standard minutes for each operation, as follows:
Operation

Standard minutes

Standard minutes Min/unit

No. of operators

Operation 1
Operation 2
Operation 3
Operation 4
Operation 5
Operation 6
Operation 7
Operation 8
Total

1.25
1.38
2.58
3.84
1.27
1.29
2.48
1.28
____
15.37

1.83
2.02
3.77
5.62
1.86
1.88
3.62
1.87

2
2
4
6
2
2
4
2
____
24

To identify the slowest operation, we divide the estimated number of operators into the standard minutes for each of the eight operations. The results are shown in the following table.
Operation 1

1.25>2 ϭ 0.625

Operation 2

1.38>2 ϭ 0.690

Operation 3

2.58>4 ϭ 0.645

Operation 4

3.84>6 ϭ 0.640

Operation 5

1.27>2 ϭ 0.635

Operation 6

1.29>2 ϭ 0.645

Operation 7

2.48>4 ϭ 0.620

Operation 8

1.28>2 ϭ 0.640

Thus, operation 2 determines the output from the line. In this case, it is
2 workers ϫ 60 min ϭ 87 pieces, or 696 pieces>day
1.38 standard min

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If this rate of production is inadequate, we would need to increase the rate of production of operator 2. This can be accomplished by
1. Working one or both of the operators at the second operation overtime, thus accumulating a small inventory at this workstation.
2. Utilizing the services of a third part-time worker at the workstation of operation 2.
3. Reassigning some of the work of operation 2 to operation 1 or operation 3.
(It would be preferable to assign more work to operation 1.)
4. Improving the method at operation 2 to diminish the cycle time of this operation. In the preceding example, given a cycle time and operation times, an analyst can determine the number of operators needed for each operation to meet a desired production schedule. The production line work assignment problem can also be to minimize the number of workstations, given the desired cycle time; or, given the number of workstations, assign work elements to the workstations, within the restrictions established, to minimize the cycle time.
An important strategy in assembly line balancing is work element sharing.
Two or more operators whose work cycle includes some idle time might share the work of another station, to make the entire line more efficient. For example,
Figure 2.19 shows an assembly line involving six workstations. Station 1 has three work elements—A, B, and C—for a total of 45 seconds (s). Note that work elements B, D, and E cannot begin until A is completed and that B, D, and E can occur in any order. It may be possible to share element H between stations 2 and

F
16
15
20

I

G
15

40

C

B
10
A
H

20
D

Station 1 Station 2

27

30

J

K
16

E
10

Station 3

Station 4

Figure 2.19 Assembly line involving six workstations.

Station 5

Station 6

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4, with only a 1-s increase in cycle time (from 45 to 46 s), while saving 30 s per assembled unit. We should note that element sharing may result in an increase in material handling, since parts may have to be delivered to more than one location. In addition, element sharing may necessitate added costs for duplicate tooling. A second possibility for improving the balance of an assembly line involves dividing a work element. Referring again to Figure 2.19, it may be possible to divide element H, rather than have one-half of the parts go to station 2 and the other half to station 4.
Many times, it is not economical to divide an element. An example would be driving home eight machine screws with a power screwdriver. Once the operator has located the part in a fixture, gained control of the power tool, and brought the tool to the work, it would usually be more advantageous to drive home all eight screws, rather than only a portion of them, leaving the rest for a different operator. Whenever elements can be divided, workstations may be better balanced as a result of the division.
The following procedure for solving an assembly line balancing problem is based on Helgeson and Birnie (1961). The method assumes the following:
1. Operators are not able to move from one workstation to another to help maintain a uniform workload.
2. The work elements that have been established are of such magnitude that further division would substantially decrease the efficiency of performing the work element. (Once established, the work elements should be identified with a code.)
The first step in the solution of the problem is to determine the sequence of individual work elements. The fewer the restrictions on the order in which the work elements can be done, the greater the probability that a favorable balance in the work assignments will be achieved. To determine the sequence of the work elements, the analyst determines the answer to the following question: What other work elements, if any, must be completed before this work element can be started? This question is applied to each element to establish a precedence chart for the production line under study (see Figure 2.20). Functional design, available production methods, floor space, and so on can all introduce constraints with respect to work element sequence.
A second consideration in the production line work assignment problem is zoning restraints. A zone represents a subdivision that may or may not be physically separated or identified from other zones in the system. Confining certain work elements to a given zone may be justified, to congregate similar jobs, working conditions, or pay rates. Or zoning restraints may help to identify physically specific stages of a component, such as keeping it in a certain position while performing certain work elements. As an example, all work elements related to one side of a component might be performed in a certain zone before the component is allowed to be turned over.

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.258
033

1.111
017

.181
001

.420
011

.81
021

1.418
002

.172
005

.119
006

2.011
032

.092
004
.142
007
.761
003

.178
008

.180
009

Figure 2.20 Partially completed precedence chart.

Note that work elements 002 and 003 may be done in any sequence with respect to any of the other work elements and that 032 cannot be started until 005, 006, 008, and 009 have been completed. Note also that after 004 has been finished, we may start 033, 017,
021, 005, 011, 006, 007, 008, or 009.

Obviously, the more zoning restraints placed on the system, the fewer the combinational possibilities available for investigation. The analyst begins by making a sketch of the system and coding the applicable zones. Within each zone, the work elements that may be done in that area are shown. The analyst then estimates the production rate, using the expression
Production per day ϭ

working min>day

cycle time of system 1min>unit2

where the cycle time of the system is the standard time of the limiting zone or station. Consider another assembly line with the following, the precedence chart:
(00)

(02)

(05)

(06)

(01)

(03)

(04)

(07)

(08)

(09)

(10)

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Estimated work unit time (minutes)

Work unit 0.46

00

0.35

01

0.25

02

0.22

03

1.10

04

0.87

05

0.28

06

0.72

Problem-Solving Tools

Work unit
02

03

04

05

06

07

08

09

10

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

07

1

1

1.32

08

1

1

0.49

09

0.55

10

00

01

1

1

1

1

1

6.61

Figure 2.21 A precedence matrix used for a line balancing problem.

This precedence graph shows that work unit (00) must be completed before
(02), (03), (05), (06), (04), (07), (08), (09), and (10); and work unit (01) must be completed before (03), (04), (07), (08), (09), and (10). Either (00) or (01) can be done first, or they can be done concurrently. Also, work unit (03) cannot be started until work units (00) and (01) are completed, and so on.
To describe these relationships, the precedence matrix illustrated in Figure 2.21 is established. Here the numeral 1 signifies a “must precede” relationship. For example, work unit (00) must precede work units (02), (03), (04), (05), (06), (07),
(08), (09), and (10). Also, work unit (09) must precede only work unit (10).
Now, a positional weight must be computed for each work unit. This is done by computing the summation of the given work unit and all the work units that must follow it. Thus, the positional weight for work unit (00) would be
© 00, 02, 03, 04, 05, 06, 07, 08, 09, 10 ϭ 0.46 ϩ 0.25 ϩ 0.22 ϩ 1.10 ϩ 0.87 ϩ 0.28 ϩ 0.72 ϩ 1.32 ϩ 0.49 ϩ 0.55 ϭ 6.26

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Listing the positional weights in decreasing order of magnitude gives the following: Unsorted work elements

Sorted work elements

Positional weight Immediate predecessors 00
01
02
03
04
05
06
07
08
09
10

00
01
03
04
02
05
06
08
07
09
10

6.26
4.75
4.40
4.18
3.76
3.51
2.64
2.36
1.76
1.04
0.55



(00), (01)
(03)
(00)
(02)
(05)
(04), (06)
(04)
(07), (08)
(09)

Work elements must then be assigned to various workstations. This process is based on the positional weights (i.e., those work elements with the highest positional weights are assigned first) and the cycle time of the system. The work element with the highest positional weight is assigned to the first workstation. The unassigned time for this workstation is determined by subtracting the sum of the assigned work element times from the estimated cycle time. If there is adequate unassigned time, the work element with the next highest-positional weight is assigned, provided that the work elements in the “immediate predecessors” column have already been assigned. Once a workstation’s allotted time has been filled, the analyst moves on to the next workstation, and the procedure continues until all the work elements have been assigned.
As an example, assume that the required production per 450-min shift is 300 units. The cycle time of the system is 450/300 ϭ 1.50 min, and the final balanced line is shown in Table 2.4.
Under the arrangement illustrated, with six workstations, we have a cycle time of 1.32 min (workstation 4). This arrangement produces 450/1.32 ϭ 341 units, which more than meets the daily requirement of 300.
However, with six workstations, we also have considerable idle time. The idle time per cycle is
6

a 0.04 ϩ0.22 ϩ0.17 ϩ0 ϩ0.11 ϩ0.77 ϭ1.31 min
1

For more favorable balancing, the problem can be solved for cycle times of less than 1.50 min. This may result in more operators and greater production per day, which may have to be stored. Another possibility involves operating the line under a more efficient balancing for a limited number of hours per day.
A variety of commercially available software packages, as well as Design
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Balanced Assembly Line

Work
Station
Element

Positional weight Immediate predecessors Work element time

Station time
Cumulative
Unassigned

Remarks*

1
1
1
1
1
1

00
01
03
04
02
05

6.26
4.75
4.40
4.18
3.76
3.56



(00), (01)
(03)
(00)
(02)

0.46
0.35
0.22
1.10
0.25
0.87

.46
.81
1.03
2.13
1.28
2.05

1.04
0.69
0.47

0.22





N.A.

N.A.

2
2

04
05

4.18
3.56

(03)
(02)

1.10
0.87

1.10
1.97

0.40



N.A.

3
3
3

05
06
08

3.56
2.64
2.36

(02)
(05)
(04), (06)

0.87
0.28
1.32

0.87
1.15
2.47

0.63
0.35




N.A.

4
4

08
07

2.36
1.76

(04), (06)
(04)

1.32
0.72

1.32
2.04

0.18



N.A.

5
5
5

07
09
10

1.76
1.04
0.55

(04)
(07), (08)
(09)

0.72
0.49
0.55

0.72
1.21
1.76

0.78
0.29




N.A.

6

10

0.55

(09)

0.55

0.55

0.95



*

N.A. means not acceptable.

SUMMARY
The various charts presented in this chapter are valuable tools for presenting and solving problems. Just as several types of tools are available for a particular job, so several chart designs can help solve an engineering problem. Analysts should understand the specific functions of each process chart and choose the appropriate one for solving a specific problem and improving operations.
Pareto analyses and fish diagrams are used to select a critical operation and to identify the root causes and contributing factors leading to the problem. Gantt and PERT charts are project scheduling tools. The Gantt chart provides only a good overview, and the PERT chart quantifies the interactions between different activities. The job/worksite analysis guide is primarily used on a physical walkthrough to identify key worker, task, environmental, and administrative factors that may cause potential problems. The operation process chart provides a good overview of the relationships between different operations and inspections on assemblies involving several components. The flow process chart provides more details for the analysis of manufacturing operations, to find hidden or indirect costs, such as delay time, storage costs, and material handling costs. The flow diagram is a useful supplement to the flow process chart in developing plant layouts. The worker/machine and gang process charts show machines or facilities in conjunction with the operator or operators, and are used to analyze idle operator

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time and idle machine time. Synchronous and random servicing calculations and line balancing techniques are used to develop more efficient operations through quantitative methods.
These 13 tools are very important for methods analysts. The charts are valuable descriptive and communicative aids for understanding a process and its related activities. Their correct use can aid in presenting and solving the problem, and in selling and installing the solution. Quantitative techniques can determine the optimum arrangement of operators and machines. Analysts should be acquainted with sufficient algebra and probability theory to develop a mathematical model that provides the best solution to the machine or facility problem. Thus, they are effective in presenting improved methods to management, training employees in the prescribed method, and focusing pertinent details, in conjunction with plant layout work.

QUESTIONS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.

What does the operation process chart show?
What symbols are used in constructing the operation process chart?
How does the operation process chart show materials introduced into the general flow? How does the flow process chart differ from the operation process chart?
What is the principal purpose of the flow process chart?
What symbols are used in constructing the flow process chart?
Why is it necessary to construct process charts from direct observation, as opposed to information obtained from the foreman?
In the construction of the flow process chart, what method can be used to estimate distances moved?
How can delay times be determined in the construction of the flow process chart?
Storage times?
When would you advocate using the flow diagram?
How can the flow of several different products be shown on the flow diagram?
What two flowchart symbols are used exclusively in the study of paperwork?
What are the limitations of the operation and flow process charts and the flow diagram? Explain how PERT charting can save a company money.
What is the purpose of crashing?
When is it advisable to construct a worker and machine process chart?
What is machine coupling?
In what way does an operator benefit through machine coupling?
How does the gang process chart differ from the worker and machine process chart?
In a process plant, which of the following process charts has the greatest application: worker and machine, gang, operation, flow? Why?
What is the difference between synchronous and random servicing?
Reducing which of the three times—worker, machine, or loading—would have the greatest effect on increasing productivity? Why?

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PROBLEMS
1.

Based on the following crash cost table, what would be the minimum time to complete the project described by Figure 2.4, whose normal costs are shown in
Table 2.2 What would be the added cost to complete the project within this time period? Crash Schedule
Weeks
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P

2.

3.

4.

$

2
1
2
0.5
4
3
2
0
4
1
4
2
3
1
2
1

7,000
2,500
5,000
2,000
6,000
5,000
6,000
0
7,600
2,200
4,500
2,200
3,000
700
6,000
3,000

The machining time per piece is 0.164 h, and the machine loading time is 0.038 h.
With an operator rate of $12.80/h and a machine rate of $14/h, calculate the optimum number of machines for lowest cost per unit of output.
At Dorben Company, a worker is assigned to operate several machines. Each of these machines is down at random times during the day. A work sampling study indicates that, on average, the machines operate unattended 60 percent of the time.
Operator attention time at irregular intervals averages 40 percent. If the machine rate is $20/h and the operator rate is $12/h, what would be the most favorable number of machines (from an economic standpoint) that should be operated by one operator?
The analyst in the Dorben Company wishes to assign a number of similar facilities to an operator, based on minimizing the cost per unit of output. A detailed study of the facilities reveals the following:
Loading machine standard time ϭ 0.34 min
Unloading machine standard time ϭ 0.26 min
Walk time between two machines ϭ 0.06 min
Operator rate ϭ $12.00> h

Machine rate 1both idle and working 2 ϭ $18.00> h
Power feed time ϭ 1.48 min
How many of these machines should be assigned to each operator?

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5.

6.

A study reveals that a group of three semiautomatic machines assigned to one operator operates unattended 80 percent of the time. Operator service time at irregular intervals averages 20 percent of the time on these three machines. What would be the estimated machine hours lost per 8-h day because of lack of an operator? Based upon the following data, develop your recommended allocation of work and the number of workstations.

work unit

Estimated Work unit time (min)

0
1
2
3
4
5
6
7
8
9
10

0.76
1.24
0.84
2.07
1.47
2.40
0.62
2.16
4.75
0.65
1.45

The minimum required production per day is 90 assemblies. The following precedence matrix was developed by the analyst.
(0 )

8.

9.

(4)

(5)

(1 )

7.

(3)

(2)

(7)

(6)

(9)

(10)

(8)

How many machines should be assigned to an operator for lowest-cost operations when a. Loading and unloading time on one machine is 1.41 min.
b. Walking time to the next facility is 0.08 min.
c. Machine time (power feed) is 4.34 min.
d. Operator rate is $13.20/h.
e. Machine rate is $18.00/h.
What proportion of machine time would be lost in operating four machines when each machine operates unattended 70 percent of the time and the operator attention time at irregular intervals averages 30 percent? Is this the best arrangement for minimizing the proportion of machine time lost?
In an assembly process involving six distinct operations, it is necessary to produce
250 units per 8-h day. The measured operation times are as follows:
a. 7.56 min.
b. 4.25 min.

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10.

11.

12.

13.

Problem-Solving Tools

c. 12.11 min.
d. 1.58 min.
e. 3.72 min.
f. 8.44 min.
How many operators would be required at 80 percent efficiency? How many operators will be utilized at each of the six operations?
A study reveals the following steps in the assembly of a truss (small triangle of three small pieces within a large triangle of three larger pieces):
Forklift delivers 2 ϫ 4 pieces of pine from outside storage area (20 min).
Bandsaw operator cuts six pieces to appropriate length (10 min).
Assembler #1 gets three short pieces, bolts small triangle (5 min).
Assembler #2 gets three long pieces, bolts large triangle (10 min).
Assembler #3 gets one of each triangle and fastens into truss (20 min).
Supervisor inspects complete truss and prepares for delivery (5 min).
a. Complete a flow process chart of the operation.
b. What are the %Idle time and production for an unbalanced, linear assembly line? c. Balance the assembly line using appropriate workstations. What are the %Idle time and production now?
The current operation consists of the following elements:
Operator removes pressed unit (0.2 min).
Operator walks to inspection area, checks for defects (0.1 min).
Operator files rough edges (0.2 min).
Operator places unit on conveyor for further processing and returns to press (0.1 min).
Operator cleans press die element with compressed air (0.3 min).
Operator sprays lubricant into die (0.1 min).
Operator places sheet metal into press, pushes START (0.2 min).
Press cycles automatically for 1.2 min.
Given that the operator is paid $10/h and that presses cost $15/hr to run, find and draw the worker-machine chart for the lowest-cost operation. What is the production?
What is the unit cost?
Given the OSHA recordable (i.e., those that must be recorded on the OSHA 300 log and open to inspection) injuries shown on the next page, what can you conclude about the injuries? Which job code would you study first? If you had limited resources, where would you put them?
Exploratory analysis has identified the following job as a problem area. Complete a flow process chart (material type) for the following engine stripping, cleaning, and degreasing operation.
Engines are stored in the old-engine storeroom. When needed, an engine is picked up by an electric hoist on a monorail, transported to the stripping bay, and unloaded onto an engine stand. There the operator strips the engine, putting the engine parts into the degreasing basket. The basket is transported to the degreaser, loaded into the degreaser, degreased, and then unloaded from the degreaser. The basket with degreased engine parts is then transported to the cleaning area, where the parts are simply dumped on the ground for drying. After several minutes of drying, the parts are lifted to the cleaning benches and cleaned. The cleaned parts are collected in special trays to await transport. The parts are loaded onto a trolley and transported to the inspection station. There they are slid from the trays onto the inspection benches.

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Type of injury
Job Code Strain/sprain CTD Other
AM9
BTR
CUE
CUP
DAW
EST
FAO
FAR
FFB
FGL
FPY
FQT
FQ9
GFC
IPM
IPY
IP9
MPL
MST
MXM
MYB
WCU

1
1
2
4
0
0
3
3
1
1
1
0
2
0
4
1
1
1
0
1
1
1

0
2
0
4
0
0
1
1
0
0
2
0
0
0
1
0
0
0
0
0
1
0

0
0
1
19
2
2
1
3
1
1
0
3
3
1
16
0
0
0
0
2
3
1

14. Given the following operations and unit times in minutes (#1 ϭ 1.5, #2 ϭ 3, #3 ϭ 1,
#4 ϭ 2, #5 ϭ 4), balance the production line with the goal of producing 30 units/h.

1
2

3

4

5

15. The following activities and times (in minutes) were recorded for a mold operator:
■ Removes molded piece from die

0.6
■ Walks 10 ft to a workbench
0.2
■ Boxes widget and places on conveyor 1.0
■ Walks back to molder
0.2
■ Blows out dirt from mold
0.4
■ Sprays oil into mold, pushes “GO”
0.2
■ Mold cycles automatically
3.0
The cycle then repeats itself. The operator is paid $10.00/h, and it costs $15.00/h to run the molder. What is the optimum number of machines that can be assigned to the operator to produce the widgets at lowest cost? Draw a worker-machine chart.

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16. TOYCO produces toy shovels on a 20-ton press. The steps taken by the press operator to produce one shovel are:
■ Remove finished shovel and put on conveyor 0.1 min
■ Remove debris from the dies
0.2 min
■ Spray dies with oil
0.1 min
■ Check raw material (flat sheet) for defects
0.3 min
■ Place flat sheet into press
0.1 min
■ Press cycles automatically
1.0 min
The operator is paid $10/h, and the press costs $100/h to run. Raw material in shovel costs $1.00, and it sells for $4.00. What is the optimum number of presses for one operator for lowest unit cost? Draw the worker-machine chart for this situation.
17. In the project shown below, activities are represented by arrows, and the number for each activity also indicates its normal duration (in days).
a. Determine the critical path and the length of this project.
b. Assume that each activity, except 1 and 2, can be crashed up to 2 days at a cost equal to the activity number. E.g., activity 6 normally takes 6 days, but could be crashed to 5 days for a cost of $6, or to 4 days for a total of $12. Determine the least-cost 26-day schedule. Show the activities that are crashed and the total crash costs.
4
2

5

8

1

10
3

6

9

7

REFERENCES
Ashcroft, H. “The Productivity of Several Machines under the Care of One Operator.”
Journal of the Royal Statistical Society B, 12, no. 1 (1950), pp. 145–51.
ASME. ASME Standard—Operation and Flow Process Charts, ANSI Y15.3-1974. New
York: American Society of Mechanical Engineers, 1974.
Baker, Kenneth R. Elements of Sequencing and Scheduling. Hannover, NJ: K. R. Baker,
1995.
Cole, R. Work, Mobility, and Participation: A Comparative Study of American and
Japanese Industry. Berkeley, CA: University of California Press, 1979.
Helgeson, W. B., and D. P. Birnie, “Assembly Line Balancing Using Ranked Positional
Weight Technique,” Journal of Industrial Engineering, 12, no. 6, (1961) pp. 394–398.
Herron, D. “Industrial Engineering Applications of ABC Curves.” AIIE Transactions 8, no. 2 (June 1976), pp. 210–18.
Moodie, C. “Assembly Line Balancing.” In Handbook of Industrial Engineering, 2d ed.,
Ed. Gavriel Salvendy. New York: John Wiley & Sons, 1992, pp. 1446–1459.

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Stecke, K. “Machine Interference.” In Handbook of Industrial Engineering, 2d ed., Ed.
Gavriel Salvendy. New York: John Wiley & Sons, 1992, pp. 1460–1494.
Takeji, K., and S. Sakamato, “Charting Techniques.” In Handbook of Industrial
Engineering, 2d ed., Ed. Gavriel Salvendy. New York: John Wiley & Sons, 1992, pp. 1415–1445.
Wright, W. R., W. G. Duvall, and H. A. Freeman. “Machine Interference.” Mechanical
Engineering, 58, no. 8 (August 1936), pp. 510–14.

SELECTED SOFTWARE
Design Tools (available from the McGraw-Hill text website at www.mhhe.com/niebelfreivalds). New York: McGraw-Hill, 2002.

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CHAPTER

3

KEY POINTS









Use operation analysis to improve the method by asking what.
Focus on the purpose of operation by asking why.
Focus on design, materials, tolerances, processes, and tools by asking how.
Focus on the operator and work design by asking who.
Focus on the layout of the work by asking where.
Focus on the sequence of manufacture by asking when.
Always try to simplify by eliminating, combining, and rearranging operations. M

ethods analysts use operation analysis to study all productive and nonproductive elements of an operation, to increase productivity per unit of time, and to reduce unit costs while maintaining or improving quality.
When properly utilized, methods analysis develops a better method of doing the work by simplifying operational procedures and material handling and by utilizing equipment more effectively. Thus, firms are able to increase output and reduce unit cost; ensure quality and reduce defective workmanship; and facilitate operator enthusiasm by improving working conditions, minimizing operator fatigue, and permitting higher operator earnings.
Operation analysis is the third methods step, the one in which analysis takes place and the various components of the proposed method crystallize. It immediately follows obtaining and presenting facts using a variety of flow process charting tools presented in Chapter 2. The analyst should review each operation and inspection presented graphically on these charts and should ask a number of questions, the most important of which is why:
1.
2.
3.
4.
5.

Why is this operation necessary?
Why is this operation performed in this manner?
Why are these tolerances this close?
Why has this material been specified?
Why has this class of operator been assigned to do the work?

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The question why immediately suggests other questions, including how, who, where, and when. Thus, analysts might ask,
1. How can the operation be performed better?
2. Who can best perform the operation?
3. Where could the operation be performed at a lower cost or improved quality? 4. When should the operation be performed to yield the least amount of material handling?
For example, in the operation process chart shown in Figure 2.7, analysts might ask the questions listed in Table 3.1 to determine the practicability of the methods improvements indicated. Answering these questions helps initiate the elimination, combination, and simplification of the operations. Also in obtaining the answers to such questions, analysts become aware of other questions that may lead to improvement. Ideas seem to generate more ideas, and experienced analysts usually arrive at several improvement possibilities. Analysts must keep an open mind, so that previous disappointments do not discourage trying new ideas. Such opportunities for methods improvement usually appear in every plant, with consequent beneficial results.
Note that much of the information presented in Chapter 3 is currently used in a repackaged format termed lean manufacturing. Lean manufacturing originated with the Toyota Motor Corporation as a means of eliminating waste in the aftermath of the 1973 oil embargo and followed the footsteps of the Taylor system of scientific management but in much broader approach, targeting not only manufacturing costs, but also sales, administrative, and capital costs. Highlights of the Toyota Production System (TPS) included seven types of muda or waste
(Shingo, 1987): (1) overproduction, (2) waiting for the next step, (3) unnecessary transportation, (4) inappropriate processing, (5) excess inventory, (6) unnecessary motion, and (7) defective products. The overlaps with traditional approaches are exemplified by the following: (1) waiting and transportation wastes are elements to be examined and eliminated within flow process charts analyses, (2) waste of motion summarizes the Gilbreths’ lifelong work in motion study culminating the principles of work design and motion economy, (3) waste of overproduction and excess inventory are based on the additional storage requirements and material handling requirements to move items into and out of storage, and (4) waste in defective products is an obvious waste producing scrap or requiring rework.
A corollary to the seven mudas is the 5S system to reduce waste and optimize productivity by maintaining an orderly workplace and consistent methods. The 5S pillars are (1) sort (seiri), (2) set in order (seiton), (3) shine (seiso), (4) standardize (seiketsu) and (5) sustain (shitsuke). Sort focuses on removing all unnecessary items from the workplace and leaving only the bare essentials. Set in order arranges needed items so that they are easy to find and use. Once the clutter is removed, shine ensures further cleanliness and tidiness. Once the first three pillars

22.

20.
21.

19.

18.

17.

16.

14.
15.

13.

12.

Text

11.

10.

3. Operation Analysis

8.
9.

7.

6.

Eliminate planing to size.

Eliminate waste ends from lengths that are 14Љ.
Eliminate jointing of ends (operation 2).

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Reduce time of 0.18 (operation 4).
If the percentage is low, perhaps this inspection can be eliminated.
Why should the top of the table be sanded all over? . . . . . . . . . . . . . . . . . . . . . . . . Eliminate sanding of one side of top and reduce time
(operation 5).
Can fixed lengths of 11⁄2Љ ϫ 3Љ yellow pine be purchased at no extra square . . . . . . Eliminate waste ends from lengths that are not footage cost? multiples of 12"
Can purchased yellow pine boards be secured with edges smooth and parallel? . . . Eliminate jointing of one edge.
Can sill boards be purchased to thickness size and have one side planed smooth? If so, how much extra will this cost? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eliminate planing to size.
Why cannot two or more boards be stacked and sawed into 14Љ sections simultaneously? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reduce time of 0.10 (operation 9).
What percentage of rejects do we have at the first inspection of the sills?. . . . . . . . If the percentage is low, perhaps this inspection can be eliminated.
Why is it necessary to sand the sills all over? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eliminate some sanding and reduce time
(operation 10).
Can fixed lengths of 21⁄2 Љ ϫ 21⁄2Љ white maple be purchased at no . . . . . . . . . . . . . . Eliminate waste ends from lengths that are not extra square footage cost? multiples of 16"
Can a smaller size than 21⁄2Љ ϫ 21⁄2Љ be used? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reduce material cost.
Can purchased white maple boards be secured with edges smooth and parallel? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eliminate jointing of edges.
Can leg boards be purchased to thickness size and have sides planed smooth? If so, how much extra will this cost? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eliminate planing to size.
Why cannot two or more boards be stacked and sawed into 14" sections simultaneously? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reduce time (operation 15).
What percentage of rejects do we have at the first inspection of the legs? . . . . . . . If the percentage is low, perhaps this inspection can be eliminated. Why is it necessary to sand the legs all over?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eliminate some sanding and reduce time
(operation 16).
Could a fixture facilitate assembly of the sills to the top? . . . . . . . . . . . . . . . . . . . . Reduce assembly time (operation 11).
Can a sampling inspection be used on the first inspection of the assembly? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reduce inspection time (operation 4).
Is it necessary to sand after one coat of shellac? . . . . . . . . . . . . . . . . . . . . . . . . . . . Eliminate operation 19.

1. Can fixed lengths of 11⁄2Љ ϫ 14Љ white maple be purchased at no extra square footage cost? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2. Can purchased maple boards be secured with edges smooth and parallel? . . . . . . .
3. Can boards be purchased to thickness size and have at least one side planed smooth? If so, how much extra will this cost? . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4. Why cannot two boards be stacked and sawed into 14” sections simultaneously? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5. What percentage of rejects do we have at the first inspection station? . . . . . . . . . . .

Question

Table 3.1 Questions to Ask in the Manufacture of Telephone Stands

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have been implemented, standardize serves to maintain the order and consistent approach to housekeeping and the methods. Finally, sustain maintains the full 5S process on a regular basis.

3.1

OPERATION PURPOSE

This is probably the most important of the nine points of operation analysis. The best way to simplify an operation is to devise some way to get the same or better results at no additional cost. An analyst’s cardinal rule is to try to eliminate or combine an operation before trying to improve it. In our experience, as much as
25 percent of the operations being performed can be eliminated if sufficient study is given to the design and process. This also corresponds closely to eliminating the muda of inappropriate processing.
Far too much unnecessary work is done today. In many instances, the task or the process should not be simplified or improved, but eliminated entirely.
Eliminating an activity saves money on the installation of an improved method, and there is no interruption or delay because no improved method is being developed, tested, and installed. Operators need not be trained on the new method, and resistance to change is minimized when an unnecessary task or activity is eliminated. With respect to paperwork, before a form is developed for information transfer, analysts should ask, Is the form really needed?
Today’s computer-controlled systems should reduce the generation of forms and paperwork.
Unnecessary operations frequently result from improper planning when the job is first set up. Once a standard routine is established, it is difficult to change, even if such a change would eliminate a portion of the work and make the job easier. When new jobs are planned, the planner may include an extra operation if there is any possibility that the product would be rejected without that extra work. For example, in turning a steel shaft, if there is some question whether to take two or three cuts to maintain a 40-microinch finish, the planner invariably specifies three cuts, even though proper maintenance of the cutting tools, supplemented by ideal feeds and speeds, would allow the job to be done with two cuts.
Unnecessary operations often develop because of the improper performance of a previous operation. A second operation must be done to “touch up” or make acceptable the work done by the first operation. In one plant, for example, armatures were previously spray painted in a fixture, making it impossible to cover the bottom of the armature with paint because the fixture shielded the bottom from the spray blast. It was therefore necessary to touch up the armature bottoms after spray painting. A study of the job resulted in a redesigned fixture that held the armature and still allowed complete coverage. In addition, the new fixture permitted seven armatures to be spray painted simultaneously, while the old method called for spray painting one at a time. Thus, by considering that an unnecessary operation may have developed because of

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(a)
(b)
Figure 3.1 (a) Painted armature as removed from the old fixture (left) and as removed from the improved fixture (right). (b) Armature in spray-painting fixture allowing complete coverage of the armature bottom.

the improper performance of a previous operation, the analyst was able to eliminate the touch-up operation (see Figure 3.1).
As another example, in the manufacture of large gears, it was necessary to introduce a hand-scraping and lapping operation to remove waves in the teeth after they had been hobbed. An investigation disclosed that contraction and expansion, brought about by temperature changes in the course of the day, were responsible for the waviness in the teeth’s surfaces. By enclosing the whole unit and installing an air-conditioning system within the enclosure, the company was able to maintain the proper temperature during the whole day. The waviness disappeared immediately, and it was no longer necessary to continue the hand-scraping and lapping operations.
To eliminate an operation, analysts should ask and answer the following question: Can an outside supplier perform the operation more economically? In one example, ball bearings purchased from an outside vendor had to be packed in grease prior to assembly. A study of bearing vendors revealed that “sealed-forlife” bearings could be purchased from another supplier at lower cost.
The examples given in this section highlight the need to establish the purpose of each operation before endeavoring to improve the operation. Once the necessity of the operation has been determined, the remaining nine steps to operation analysis should help to determine how it can be improved.

3.2

PART DESIGN

Methods engineers are often inclined to feel that once a design has been accepted, their only recourse is to plan its economical manufacture. While introducing even a slight design change may be difficult, a good methods analyst

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should still review every design for possible improvements. Designs can be changed, and if improvement is the result and the activity of the job is significant, then the change should be made.
To improve the design, analysts should keep in mind the following pointers for lower-cost designs on each component and each subassembly:
1. Reduce the number of parts by simplifying the design.
2. Reduce the number of operations and the length of travel in manufacturing by joining the parts better and by making the machining and assembly easier. 3. Utilize a better material.
4. Liberalize tolerances and rely on key operations for accuracy, rather than on series of closely held limits.
5. Design for manufacturability and assembly.
Note that the first two will help in reducing muda in unappropriate processing, unnecessary transportation, and excess inventory.
The General Electric Company summarized the ideas for developing minimum cost designs in Table 3.2.
The following examples of methods improvement resulted from considering a better material or process in an effort to improve the design. Conduit boxes were originally built of cast iron. The improved design, making a stronger, neater, lighter, and less expensive conduit box, was fabricated from sheet steel.
A four-step process was used to bend a part into the desired shape (see Figure
3.2). This was inefficient and stressed the metal at the bends. The design was slightly altered so that the less expensive process of extruding could be utilized.
The extruded sections were then cut to the desired length. In the redesigned process, three steps were eliminated.
Design simplification through the better joining of parts was used in assembling terminal clips to their mating conductors. The original practice required turning up the end of the clip to form a socket. The socket was filled with solder, and the wire conductor was then tinned, inserted into the solder-filled socket, and held there until the solder solidified. The altered design called for resistance welding the clip to the wire conductor, eliminating both the forming and dipping operations. The original part was designed with three components that had to be assembled (see Figure 3.3). A significantly less costly approach utilized a onepiece design which could be machined as a solid piece, eliminating two components and several operations.
Just as opportunities exist to improve productivity through better product design, similar opportunities exist to improve the design of forms (whether hard copy or electronic) used throughout an industry or business. Once a form is proved necessary, it should be studied to improve both the collection and flow of information. The following criteria apply to the development of forms:
1. Maintain simplicity in the form design, keeping the amount of necessary input information at a minimum.

Screw machine parts
1. Eliminate second operation.
2. Use cold-rolled stock.
3. Design for header instead of screw machine.
4. Use rolled threads instead of cut threads.

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Punchings
1. Use punched parts instead of molded, cast, machined, or fabricated parts.
2. “Nestable” punchings economize on material.
3. Holes requiring accurate relation to each other should be made by the same die.
4. Design to use coil stock.
5. Design punchings to have minimum sheared length and maximum die strength with fewest die moves.

Assemblies
1. Make assemblies simple.
2. Make assemblies progressive.
3. Make only one assembly and eliminate trial assemblies.
4. Make component parts right in the first place so that fitting and adjusting will not be required in assembly.
This means that drawings must be correct, with proper tolerances, and that parts must be made according to drawings.
General
1. Reduce number of parts.
2. Reduce number of operations.
3. Reduce length of travel in manufacturing.

Text

Treatments and finishes
1. Reduce baking time to minimum.
2. Use air drying instead of baking.
3. Use fewer or thinner coats.
4. Eliminate treatments and finishes entirely. Welded parts
1. Fabricated construction instead of castings or forgings.
2. Minimum sizes of welds.
3. Welds made in flat position rather than vertical or overhead.
4. Eliminate chamfering edges before welding. 5. Use “burnouts” (torch-cut contours) instead of machined contours.
6. Lay out parts to cut to best advantage from standard rectangular plates and avoid scrap.
7. Use intermittent instead of continuous weld. 8. Design for circular or straight-line welding to use automatic machines.

3. Operation Analysis

Machined parts
1. Use rotary machining processes instead of shaping methods.
2. Use automatic or semiautomatic machining instead of hand-operated.
3. Reduce the number of shoulders.
4. Omit finishes where possible.
5. Use rough finish when satisfactory.
6. Dimension drawings from same point as used by factory in measuring and inspecting.
7. Use centerless grinding instead of betweencenter grinding.
8. Avoid tapers and formed contours.
9. Allow a radius or undercut at shoulders.

Fabricated parts
1. Self-tapping screws instead of standard screws. 2. Drive pins instead of standard screws.
3. Rivets instead of screws.
4. Hollow rivets instead of solid rivets.
5. Spot or projection welding instead of riveting.
6. Welding instead of brazing or soldering.
7. Use die castings or molded parts instead of fabricated construction requiring several parts. Formed parts
1. Drawn parts instead of spun, welded, or forged parts.
2. Shallow draws if possible.
3. Liberal radii on corners.
4. Bent parts instead of drawn.
5. Parts formed of strip or wire instead of punched from sheet.

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Moldings
1. Eliminate inserts from parts.
2. Design molds with smallest number of parts. 3. Use simple shapes.
4. Locate flash lines so that the flash does not need to be filed and polished.
5. Minimize weight.

Castings
1. Eliminate dry sand (baked-sand) cores.
2. Minimize depth to obtain flatter castings.
3. Use minimum weight consistent with sufficient thickness to cast without chilling.
4. Choose simple forms.
5. Symmetrical forms produce uniform shrinkage. 6. Liberal radii—no sharp corners.
7. If surfaces are to be accurate with relation to each other, they should be in the same part of the pattern, if possible.
8. Locate parting lines so that they will not affect looks and utility, and need not be ground smooth.
9. Specify multiple patterns instead of single ones. 10. Metal patterns are preferable to wood.
11. Use permanent molds instead of metal patterns.

Table 3.2 Methods for Minimum Cost Design

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(a)

(b)

Figure 3.2 Part redesign to eliminate three steps

(a) A four-step process was used to bend this piece into the desired shape. This is inefficient and stresses the metal at the bends. (Courtesy of Alexandria Extrusion
Company.) (b) This piece was extruded in one step and will later be cut to appropriate lengths. (Courtesy of Alexandria Extrusion Company.)

(a)

(b)

Figure 3.3 Part redesign to eliminate multiple pieces

(a) Original part was designed in three pieces and had to be assembled. (Courtesy of
Alexandria Extrusion Company.) (b) Improved one-piece design can be machined as a solid piece. (Courtesy of the Minister Machine Company.)

2. Provide ample space for each bit of information, allowing for different input methods (writing, typewriter, word processor).
3. Sequence the information input in a logical pattern.
4. Color-code the form to facilitate distribution and routing.
5. Confine computer forms to one page.

3.3

TOLERANCES AND SPECIFICATIONS

The third of the nine points of operation analysis concerns tolerances and specifications that relate to the quality of the product, that is, its ability to satisfy given

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needs. While tolerances and specifications are always considered when reviewing the design, this is usually not sufficient; they should be considered independently of the other approaches to operation analysis.
Designers may have a tendency to incorporate specifications that are more rigid than necessary when developing a product. This can be due to a lack of knowledge about cost and the thought that it is necessary to specify closer tolerances and specifications than are actually needed to have the manufacturing departments produce to the actual required tolerance range.
Methods analysts should be well versed in the details of cost and should be fully aware of what unnecessarily close tolerances and/or rejects can do to the selling price. Figure 3.4 illustrates the pronounced relationship between the increased cost of tighter machining tolerances. If designers are being needlessly

Approximate relative cost

3

2

1

0
0.010

0.010

0.010

Tolerance inches
(plus or minus)

Fine machining
(± 0.001)

Standard machining
(± 0.005)

Rough machining
(± 0.030)

Figure 3.4 Approximate relationship between cost and machining tolerance.

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tight in establishing tolerances and specifications, management should embark on a training program clearly presenting the economies of specifications. Developing quality products in a manner that actually reduces costs is a major tenet of the approach to quality instituted by Taguchi (1986). This approach involves combining engineering and statistical methods to achieve improvements in cost and quality by optimizing product design and manufacturing methods. This step corresponds to reducing the muda of inappropriate processing.
One manufacturer’s drawings called for a 0.0005-in tolerance on a shoulder ring for a DC motor shaft. The original specifications called for a 1.8105 to
1.8110-in tolerance on the inside diameter. This close tolerance was deemed necessary because the shoulder ring was shrunk onto the motor shaft. Investigation revealed that a 0.003-in tolerance was adequate for the shrink fit. The drawing was immediately changed to specify a 1.809 to 1.812-in inside diameter. This change meant that a reaming operation was eliminated because someone questioned the absolute necessity of a close tolerance.
Analysts should also take into consideration the ideal inspection procedure.
Inspection is a verification of quantity, quality, dimensions, and performance.
Such inspections can usually be performed by a variety of techniques: spot inspection, lot-by-lot inspection, or 100 percent inspection. Spot inspection is a periodic check to ensure that established standards are being realized. For example, a nonprecision blanking and piercing operation set up on a punch press should have a spot inspection to ensure the maintenance of size and the absence of burrs. As the die begins to wear or as deficiencies in the material being worked begin to show up, the spot inspection would catch the trouble in time to make the necessary changes, without generating an appreciable number of rejects.
Lot-by-lot inspection is a sampling procedure in which a sample is examined to determine the quality of the production run or lot. The size of the sample depends on the allowable percentage of defective unity and the size of the production lot being checked. A 100 percent inspection involves inspecting every unit of production and rejecting the defective units. However, experience has shown that this type of inspection does not ensure a perfect product. The monotony of screening tends to create fatigue, thus lowering operator attention. The inspector may pass some defective parts, or reject good parts. Because a perfect product is not ensured under 100 percent inspection, acceptable quality may be realized by the considerably more economical methods of lot-by-lot or spot inspection.
For example, in one shop, a certain automatic polishing operation had a normal rejection quantity of 1 percent. Subjecting each lot of polished goods to 100 percent inspection would have been quite expensive. Management therefore decided, at an appreciable saving, to consider 1 percent the allowable percentage defective, even though this quantity of defective material would go through to plating and finishing, only to be thrown out in the final inspection before shipment.
By investigating tolerances and specifications and taking action when desirable, the company can reduce the costs of inspection, minimize scrap, diminish repair costs, and keep quality high. Also the company is addressing the muda of defective products.

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3.4

Operation Analysis

MATERIAL

One of the first questions an engineer considers when designing a new product is, What material shall be used? Since choosing the correct material may be difficult because of the great variety available, it is often more practical to incorporate a better and more economical material into an existing design.
Methods analysts should consider the following possibilities for the direct and indirect materials utilized in a process:
1.
2.
3.
4.
5.
6.
7.

Finding a less expensive and lighter material.
Finding materials that are easier to process.
Using materials more economically.
Using salvage materials.
Using supplies and tools more economically.
Standardizing materials.
Finding the best vendor from the standpoint of price and vendor stocking.

FINDING A LESS EXPENSIVE AND LIGHTER
MATERIAL
Industry is continually developing new processes for producing and refining materials. Monthly publications summarize the approximate cost per pound of steel sheets, bars, and plates, and the cost of cast iron, cast steel, cast aluminum, cast bronze, thermoplastic and thermosetting resins, and other basic materials. These costs can be used as anchor points from which to judge the application of new materials. A material that was not competitive in price yesterday may be very competitive today.
One company used Micarta spacer bars between the windings of transformer coils. Separating the windings permitted the circulation of air between the windings. An investigation revealed that glass tubing could be substituted for the Micarta bars at a considerable savings. The glass tubing was less expensive, and it met service requirements better because the glass could withstand higher temperatures. Furthermore, the hollow tubing permitted greater air circulation than did the solid Micarta bars.
Another company also used a less expensive material that still met service requirements in the production of distribution transformers. Originally, a porcelain plate separated and held the wire leads coming out of the transformers. The company found that a fullerboard plate stood up just as well in service, yet was considerably less expensive. Today, one of the many types of plastic available would provide an even cheaper solution.
Another concern for manufacturers, especially today with high transportation costs due to continual increases in crude oil prices, is the weight of the product itself. Finding a lighter material or decreasing the amount of raw material used is of prime concern. A good example is shown by the changing nature of beverage cans (see Figure 3.5). All steel cans in the early 1970’s weighed 1.94 oz (55 gr)

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Figure 3.5 Decreasing weight of beverage cans

(a) All steel can from 1970 weighing 1.94 oz (55 gr) (b) Steel can with aluminum top and bottom from 1975 weighing 1.69 oz (48 gr) (c) All aluminum can from
1980 weighing 0.6 oz (17 gr) (d) All aluminum ribbed can from 1992 weighing
0.56 (16 gr) (Courtesy R. Voigt, Penn State)

(see Figure 3.5a). By replacing the top and bottom with aluminum discs, approximately 0.25 oz (7 gr) in weight savings could be achieved (see Figure 3.5b).
Going to an all aluminum can decreased the total weight to 0.6 oz (17 gr) for considerable weight savings (see Figure 3.5c). However, the walls of the can became so thin that the walls easily crumpled. This was solved by creating ribbing in the walls (see Figure 3.5d).
Methods analysts should remember that items such as valves, relays, air cylinders, transformers, pipe fittings, bearings, couplings, chains, hinges, hardware, and motors can usually be purchased at less cost than they can be manufactured.

FINDING A MATERIAL THAT IS EASIER
TO PROCESS
Some materials are usually more readily processed than others. Referring to handbook data on the physical properties usually helps analysts discern which material will react most favorably to the processes to which it must be subjected in its conversion from raw material to finished product. For example,

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machinability varies inversely with hardness, and hardness usually varies directly with strength.
Today the most versatile material is reinforced composites. Resin transfer molding can produce more complex parts advantageously from the standpoint of quality and production rate than most other metal and plastic forming procedures.
Thus, by specifying a plastic made of reinforcing carbon fibers and epoxy, the analyst can substitute a composite for a metal part, at both a quality and a cost advantage. This step is also addressing the muda of inappropriate processing.

USING MATERIAL MORE ECONOMICALLY
The possibility of using material more economically is a fertile field for analysis.
If the ratio of scrap material to that actually going into the product is high, then greater utilization should be examined. For example, if the material put into a plastic compression mold is preweighed, it may be possible to use only the exact amount required to fill the cavity; excessive flash can also be eliminated.
In another example, the production of stampings from sheet metal should utilize multiple dies carefully arranged to assure maximum use of material.
Given consistent raw material and standard-sized dies, this typically is done through the use of CAD-assisted layout, yielding efficiencies exceeding 95%
(i.e., less than 5% scrap). Similar approaches are utilized in the garment industry in the layout of patterns on cloth and the glass industry for the cutting of different sized windows. However, if the material is not consistent, then problems arise and the layout may still need to be performed by a human operator.
The production of leather seats for automobiles requires layout of cutting dies on a tanned hide before entering a rolling press, which applies pressure on the dies to cut the leather in appropriate patterns. The operator needs to be highly skilled in handling variably sized cow hides full of imperfections from brands and barbed wire, especially to maximize the usage of quite expensive leather
(see Figure 3.6).
Many world-class manufacturers are finding it not only desirable, but absolutely necessary, to take weight out of existing designs. For example, Ford engineers all looking a 40 percent weight reduction to achieve an 80 mi/gal fuel efficiency for the Taurus. This will require the cladding of stainless steel to highstrength aluminum to replace chrome-plated steel bumpers, as well as a much greater use of plastics and structural composites to replace ferrous components.
Similar weight reduction is taking place on many other well-known products, such as washing machines, video cameras, VCRs, suitcases, and TV sets.
Today, powder coating is a proven technology that is replacing many other methods of metal finishing. Coating powders are finely divided particles of organic polymers (acrylic, epoxy, polyester, or blends) that usually contain pigments, fillers, and additives. Powder coating is the application of a suitable formulation to a substrate, which are then fused into a continuous film by the application of heat, forming a protective and decorative finish. In view of current environmental regulations affecting traditional metal finishing operations, such

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Figure 3.6 Layout of cutting dies on a tanned hide before entering a rolling press
(note the careful layout of the dies to maximize the use of the expensive leather).

as electroplating and wet painting, powder coating offers a safer and cleaner environment. The methodology can also provide a durable, attractive, cost-effective finish for metal surfaces used in many commercial products, such as wire shelving, control boxes, trailer hitches, water meters, handrails, boat racks, office partitions, and snow shovels.

USING SALVAGE MATERIALS
Materials can often be salvaged, rather than sold as scrap. By-products from an unworked portion or scrap section can sometimes offer real possibilities for savings. For example, one manufacturer of stainless steel cooling cabinets had 4- to
8-in-wide sections left as cuttings on the shear. An analysis identified electric light switchplate covers as a possible by-product. Another manufacturer, after salvaging the steel insert from defective bonded rubber ringer rolls, was able to utilize the hollow, cylindrical rubber rolls as bumpers for protecting moored motorboats and sailboats.
If it is not possible to develop a by-product, then scrap materials should be separated to obtain top scrap prices. Separate bins should be provided for tool steel, steel, brass, copper, and aluminum. Chip-haulers and floor sweepers should specifically be instructed to keep the scrap segregated. For electric lightbulbs, for example, the brass socket would be stored in one area, and after the glass bulb is broken and disposed of, the tungsten filament is removed and stored separately for greatest residual value. Many companies save wooden boxes from incoming shipments, and

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then saw the boards to standard lengths for use in making smaller boxes for outgoing shipments. This practice is usually economical, and it is now being followed by many large industries, as well as by service maintenance centers.
There are also a few interesting examples from the food industry. A manufacturer of tofu processes the beans, centrifuges out the edible protein material, and leaves behind tons of waste fiber. Rather than paying to haul it away to a landfill, the manufacturer gives it away to local farmers for hog feed, as long as they come and pick it up. Similarly, meatpackers utilize everything from a cow: hides, bones, even blood, all except the “moo.”

USING SUPPLIES AND TOOLS FULLY
Management should encourage full use of all shop supplies. One manufacturer of dairy equipment introduced the policy that no new welding rod was to be distributed to workers without the return of old tips under 2 in long. The cost of welding rods was reduced immediately by more than 15 percent. Brazing or welding is usually the most economical way to repair expensive cutting tools, such as broaches, special form tools, and milling cutters. If it has been company practice to discard broken tools of this nature, the analyst should investigate the potential savings of a tool salvage program.
Analysts can also find a use for the unworn portions of grinding wheels, emery disks, and so forth. Also, items such as gloves and rags should not be discarded simply because they are soiled. Storing dirty items and then laundering them is less expensive than replacing them. Methods analysts can make a real contribution to a company by simply minimizing waste, one of the mudas in the TPS system.

STANDARDIZING MATERIALS
Methods analysts should always be alert to the possibility of standardizing materials. They must minimize the sizes, shapes, grades, and so on of each material utilized in the production and assembly processes. The typical economies resulting from reductions in the sizes and grades of the materials employed include the following: ■







Purchase orders are used for larger amounts, which are almost always less expensive per unit.
Inventories are smaller, since less material must be maintained as a reserve.
Fewer entries need to be made in storage records.
Fewer invoices need to be paid.
Fewer spaces are needed to house materials in the storeroom.
Sampling inspection reduces the total number of parts inspected.
Fewer price quotations and purchase orders are needed.

The standardization of materials, like other methods improvement techniques, is a continuing process. It requires the continual cooperation of the design, production planning, and purchasing departments and fits in nicely with the 5S system.

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FINDING THE BEST VENDOR
For the vast majority of materials, supplies, and parts, numerous suppliers will quote different prices, quality levels, delivery times, and willingness to hold inventories. It is usually the responsibility of the purchasing department to locate the most favorable supplier. However, the best supplier last year may not be the best one now. The methods analyst should encourage the purchasing department to rebid the highest-cost materials, supplies, and parts to obtain better prices and superior quality and to increase vendor stocking, where the vendors agree to hold inventories for their customers. It is not unusual for methods analysts to achieve a 10 percent reduction in the cost of materials and a 15 percent reduction in inventories by regularly pursuing this approach through their purchasing departments. Perhaps the most important reason for continued Japanese success in the manufacturing sector is the keiretsu. This is a form of business and manufacturing organization that links businesses together. It can be thought of as a web of interlocking relationships among manufacturers—often between a large manufacturer and its principal suppliers. Thus, in Japan such companies as Hitachi and Toyota and other international competitors are able to acquire parts for their products from regular suppliers who produce to the quality called for and are continually looking for improvement so as to provide better prices for the firms in their network. Alert purchasing departments are often able to create relationships with suppliers comparable to the so-called production keiretsu.

3.5 MANUFACTURE SEQUENCE AND PROCESS
As manufacturing technology in the twenty-first century eliminates laborintensive manufacturing in favor of capital-intensive procedures, the methods engineer will focus on multiaxis and multifunctioning machining and assembly.
Modern equipment is capable of cutting at higher speeds on more accurate, rigid and flexible machines that utilize both advanced controls and tool materials. Programming functions permit in-process and postprocess gaging for tool sensing and compensation, resulting in dependable quality control.
The methods engineer must understand that the time utilized by the manufacturing process is divided into three steps: inventory control and planning, setup operations, and in-process manufacturing. Furthermore, it is not unusual to find that these procedures, in aggregate, are only about 30 percent efficient from the standpoint of process improvement.
To improve the manufacturing process, the analyst should consider (1) rearranging the operations; (2) mechanizing manual operations; (3) utilizing more efficient facilities on mechanical operations; (4) operating mechanical facilities more efficiently; (5) manufacturing near the net shape; and (6) using robots, all which address the muda of inappropriate processing.

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REARRANGING OPERATIONS
Rearranging operations often results in savings. As an example, the flange of a motor conduit box required four drilled holes, one in each corner. Also, the base had to be smooth and flat. Originally, the operator began by grinding the base, then drilling the four holes using a drill jig. The drilling operation threw up burrs, which then had to be removed in another step. By rearranging the operation so that the holes were drilled first and the base then ground, analysts eliminated the deburring operation. The base-grinding operation automatically removed the burrs.
Combining operations usually reduces costs. For example, a manufacturer fabricated the fan motor support and the outlet box of its electric fans. After painting the parts separately, operators then riveted them together. By having the outlet box riveted to the fan motor support prior to painting, analysts effected an appreciable time savings for the painting operation. Similarly, using a more complex machine that combines several operations can reduce the time to produce the finished piece and increase productivity (see Figure 3.7). Although the machine may be more expensive, considerable savings are incurred by reduced labor costs.
In another example, the market for aluminum cylinder head castings is growing, and foundries are finding it cost-effective to go from the steel-mold casting process to the lost foam process. Lost foam is an investment casting

(a)

(b)
Figure 3.7 Combining operations to eliminate steps. Stock material shown in
(b) is cut to size and threaded in one step on the Citizen CNC lathe shown in (a) to yield the finished piece shown in (c).

(a) Citizen CNC lathe. (Courtesy of
Jergens, Inc.) (b) Stock material. (Courtesy of Jergens, Inc.) (c) Finished piece.
(Courtesy of Jergens, Inc.)

(c)

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procedure that uses an expendable pattern of polystyrene foam surrounded by a thin ceramic shell. Steel-mold castings require considerable subsequent machining. In comparison, the lost foam process reduces the amount of machining and also eliminates the sand disposal costs usually associated with investment casting. Before changing any operation, however, the analyst must consider possible detrimental effects on subsequent operations down the line. Reducing the cost of one operation could result in higher costs for other operations. For example, a change recommended in the manufacture of AC field coils resulted in higher costs and was therefore not practical. The field coils were made of heavy copper bands, which were formed and then insulated with mica tape. The mica tape was hand-wrapped on the already coiled parts. The company decided to machine wrap the copper bands prior to coiling. This did not prove practical, as the forming of the coils cracked the mica tape, necessitating time-consuming repairs prior to product acceptance.

MECHANIZING MANUAL OPERATIONS
Today, any practicing methods analyst should consider using special-purpose and automatic equipment and tooling, especially if production quantities are large.
Notable among industry’s latest offerings are program controlled, numerically controlled (NC), and computer controlled (CNC) machining and other equipment. These afford substantial savings in labor cost as well as the following advantages: reduced work-in-process inventory, less parts damage due to handling, less scrap, reduced floor space, and reduced production throughput time. For example (see Figure 3.8), whereas two operators are required for a manually operated machine tool, only one operator is required for a computer-controlled machine tool. Use of a robotic arm operating a fully automated machine tool would not even require the one operator, considerably reducing labor costs
(albeit with higher initial capital costs).
Other automatic equipment includes automatic screw machines; multiplespindle drilling, boring, and tapping machines; index-table machine tools; automatic casting equipment combining automatic sand-mold making, pouring, shakeout, and grinding; and automatic painting and plating finishing equipment. The use of power assembly tools, such as power nut- and screwdrivers, electric or air hammers, and mechanical feeders, is often more economical than the use of hand tools.
To illustrate, a company that produces specialty windows was using manual methods to press rails over both ends of plate window glass that had been covered with a synthetic rubber wrap. The plates of glass were held in position by two pads that were pneumatically squeezed together. The operator would pick up a rail and position it over the end of the window glass and then pick up a mallet and hammer the rail into position over the glass. The operation was slow and it resulted in considerable operator work-related musculoskeletal disorders. McGraw-Hill Create™ Review Copy for Instructor Espinoza. Not for distribution.
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(a)

(c)

Operation Analysis

(b)

(d)

Figure 3.8 Mechanizing manual operations can reduce labor costs.

(a) Two operators are required for a manually operated machine tool. (b) A computercontrolled machine tool requires only one operator. (c) A state-of-the-art computercontrolled machine tool still requires one operator but performs more operations.
(d) A robotic arm operating a fully automated machine tool requires no operators.
[(a) © Yogi, Inc./CORBIS; (b) © Molly O’Bryon Welpott; (c) and (d) Courtesy of Okuma.]

Furthermore, scrap was high because of glass breakage due to pounding the rails over the glass. A new facility was designed that pneumatically squeezed rails onto the window glass over the synthetic rubber wrap. Operators enthusiastically accepted the new facility because the work was much easier to perform; health problems disappeared, productivity increased, and glass breakage dropped to near zero.
The application of mechanization applies not only to process operations, but also to paperwork. For example, bar coding applications can be invaluable to the operations analyst. Bar coding can rapidly and accurately enter a variety of data.
Computers can then manipulate the data for some desired objective, such as counting and controlling inventory, routing specific items to or through a process, or identifying the state of completion and the operator currently working on each item in a work-in-process.

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UTILIZING MORE EFFICIENT MECHANICAL
FACILITIES
If an operation is done mechanically, there is always the possibility of a more efficient means of mechanization. At one company, for example, turbine blade roots were machined by using three separate milling operations. Both the cycle time and the costs were high. When external broaching was introduced, all three surfaces could be finished at once, for considerable time and cost savings. Another company overlooked the possibility of utilizing a press operation. This process is one of the fastest for forming and sizing processes. A stamped bracket had four holes that were drilled after the bracket was formed. By using a die designed to pierce the holes, the work could be performed in a fraction of the drilling time.
Work mechanization applies to more than just manual work. For example, one company in the food industry was checking the weight of various product lines with a balance. This equipment required the operator to note the weight visually, record the weight on a form, and subsequently perform several calculations. A methods engineering study resulted in the introduction of a statistical weight control system. Under the improved method, the operator weighs the product on a digital scale programmed to accept the product within a certain weight range. As the product is weighed, the weight information is transferred to a personal computer that compiles the information and prints the desired report.

OPERATING MECHANICAL FACILITIES
MORE EFFICIENTLY
A good slogan for methods analysts is, “Design for two at a time.” Usually multiple-die operation in presswork is more economical than single-stage operation. Again, multiple cavities in diecasting, molding, and similar processes are viable options when there is sufficient volume. On machine operations, analysts should be sure that proper feeds and speeds are used. They should investigate the grinding of cutting tools for maximum performance. They should check to see whether the cutting tools are properly mounted, whether the right lubricant is being used, and whether the machine tool is in good condition and is adequately maintained. Many machine tools are operated at a fraction of their possible output. Endeavoring to operate mechanical facilities more efficiently nearly always pays dividends.

MANUFACTURING NEAR THE NET SHAPE
Using a manufacturing process that produces components closer to the final shape can maximize material use, reduce scrap, minimize secondary processing such as final machining and finishing, and permit manufacturing with more

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environmentally friendly materials. For example, forming parts with powder metals (PM) instead of conventional casting or forging often provides the manufacture of near-net shapes for many components, resulting in dramatic economic savings as well as functional advantages. In the case of forged PM connecting rods, it has been reported that they have reduced the reciprocating mass of competing alternatives, resulting in less noise and vibration as well as major cost economies.

CONSIDERING THE USE OF ROBOTS
For cost and productivity reasons, it is advantageous today to consider the use of robots in many manufacturing areas (see Figure 3.9). For example, assembly areas include work that typically has a high direct labor cost, in some cases accounting for as much as one-half of the manufacturing cost of a product. The principal advantage of integrating a modern robot in the assembly process is its inherent flexibility. It can assemble multiple products on a single system and can be reprogrammed to handle various tasks with part variations. In addition, robotic assembly can provide consistently repeatable quality with predictable product output.
A robot’s typical life is approximately 10 years. If it is well maintained and if it is used for moving small payloads, the life can be extended to up to 15 years.
Consequently, a robot’s depreciation cost can be relatively low. Also, if a given robot’s size and configuration are appropriate, it can be used in a variety of operations. For example, a robot could be used to load a die-casting facility, load a quenching tank, load and unload a board drop-hammer forging operation, load a plate glass washing operation, and so on. In theory, a robot of the correct size and configuration can be programmed to do any job.
In addition to productivity advantages, robots also offer safety advantages.
They can be used in work centers where there is danger to the worker because of the nature of the process. For example, in the die-casting process, there can be considerable danger due to hot metal splashing when the molten metal is injected into the die cavity. One of the original applications for robots was die casting. In one company, a five-axis robot developed by Unimation, Inc., serves a 600-ton microprocessor-controlled die-casting machine. In the operation, the robot moves into position when the die opens, grasps the casting by its slug, and clears it from the cavity. At the same time, it initiates automatic die-lubrication sprays. The robot displays the casting to infrared scanners, then signals the die-casting machine to accept another shot. The casting is deposited by the robot on an output station for trimming. Here an operator, remote from the die-casting machine, safely trims the casting preparatory to subsequent operations.
Automobile manufacturers have placed particular emphasis on the use of robots in welding. For example, at Nissan Motors, 95 percent of the welds on vehicles are made by robots; and Mitsubishi Motors reported that about 70 percent

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Flow line

(a) Welding

(b) Die casting and press feeding

Transfer conveyor Input conveyor

Supervisory control Washer
Output
conveyor

Silo

Elevator
Flow
Milling casting Inspection station Storage and drilling input pallet machine

Deburring station Robot
Drilling
machine

(c) Machining center

(d) Assembly

Figure 3.9 Illustration of a few common industrial robot applications.

(a) One welding robot is shown, but typically a number of robots would be used along an automotive assembly line. (b) In a die-casting application, a robot unloads die-casting machines, performs quench operations, and loads material into a press. (c) The production machining line is used for producing cam housings. (d) The assembly line uses a combination of robots, parts feeders, and human operators.

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of its welding is performed by robots. In these companies, robot downtime averages less than 1 percent.

3.6

SETUP AND TOOLS

One of the most important elements of all forms of work holders, tools, and setups is economics. The amount of tooling up that proves most advantageous depends on (1) the production quantity, (2) repeat business, (3) labor, (4) delivery requirements, and (5) required capital.
The most prevalent mistake of planners and toolmakers is to tie up money in fixtures that may show a large savings when in use, but are seldom used. For example, a savings of 10 percent in direct labor cost on a job in constant use would probably justify greater expense in tools than an 80 or 90 percent savings on a small job that appears on the production schedule only a few times a year. (This is an example of Pareto analysis, from Chapter 2.) The economic advantage of lower labor costs is the controlling factor in determining the tooling; consequently, jigs and fixtures may be desirable, even when only small quantities are involved. Other considerations, such as improved interchangeability, increased accuracy, or labor trouble reduction, may provide the dominant reasons for elaborate tooling, although this is usually not the case. An example of the trade-off between fixturing and tooling costs is discussed in Chapter 9 in the section on break-even charts.
Once the needed amount of tooling has been determined (or if tooling already exists, once the ideal amount needed has been determined), specific considerations for producing the most favorable designs should be evaluated.
These are outlined in the Setup and Tooling Evaluation Checklist shown in
Figure 3.10.
Setup ties in very closely with tooling, because tooling invariably determines the setup and teardown time. When we speak of setup time, we usually include such items as arriving on the job; procuring instructions, drawings, tools, and material; preparing workstations so that production can begin in the prescribed manner (setting up tools; adjusting stops; setting feeds, speeds, and cut depth; and so on); tearing down the setup; and returning tools to the crib.
Setup operations are especially important in the job shop where production runs tend to be small. Even if this type of shop has modern facilities and puts forth a high effort, it may still have difficulty meeting the competition if setups are too long because of poor planning and inefficient tooling. When the ratio of setup time to production runtime is high, a methods analyst can usually develop several possibilities for setup and tool improvement. One notable option is a group technology system.
The essence of group technology is the classification of the various components of a company’s products, so that parts similar in shape and processing sequence are identified numerically. Parts belonging to the same family group, such as rings, sleeves, discs, and collars, are scheduled for production over the same time interval on a general-purpose line arranged in the optimal operational

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Figure 3.10 Setup and tooling evaluation checklist.

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20

30

40

50

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90

0 Without subforms
1 Set-off or shoulder on one side
2 Set-offs or shoulder on two sides
3 With flanges, protuberances 4 With open or closed forking or slotting
5 With hole
6 With hole and threads
7 With slots or knurling
8 With supplementary extensions Figure 3.11 Subdivision of a system grouping for group technology.

sequence. Since both the size and shape of the parts in a given family vary considerably, the line is usually equipped with universal-type, quick-acting jigs and fixtures. This approach also fits in with eliminating the muda of excess inventory and the 5S pillar of standardization.
As an example, Figure 3.11 illustrates a system grouping subdivided into nine classes of parts. Note the similarity of parts within each vertical column. If we were machining a shaft with external threads and a partial bore at one end, the part would be identified as Class 206.

REDUCE SETUP TIME
Just-in-time (JIT) techniques, which have become popular in recent years, emphasize decreasing the setup times to the minimum by simplifying or eliminating them. The SMED (single minute exchange of die) System of the Toyota
Production System (Shingo, 1981) is a good example of this approach. A significant portion of setup time can often be eliminated by ensuring that raw materials are within specifications, tools are sharp, and fixtures are available and in good condition. Producing in smaller lots can often prove cost-effective.
Smaller lot sizes can lead to smaller inventories, with reduced carrying costs and shelf-life problems, such as contamination, corrosion, deterioration, obsolescence, and theft. The analyst must understand that decreasing the lot size will result in an increase in total setup costs for the same total production quantity over a given period. Several points should be considered in reducing setup time:
1. Work that can be done while the equipment is running should be done at that time. For example, presetting tools for numerical control (NC) equipment can be done while the machine is running.
2. Use the most efficient clamping. Usually, quick-acting clamps that employ cam action, levers, wedges, and so on are much faster, provide adequate force, and are usually a good alternative to threaded fasteners. When

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threaded fasteners must be used (for clamping force), C washers or slotted holes can be used so that nuts and bolts do not have to be removed from the machine and can be reused, reducing the setup time on the next job.
3. Eliminate machine base adjustment. Redesigning part fixtures and using preset tooling may eliminate the need for spacers or guide-block adjustments to the table position.
4. Use templates or block gages to make quick adjustments to machine stops. The time spent in requisitioning tools and materials, preparing the workstation for actual production, cleaning up the workstation, and returning the tools to the tool crib is usually included in setup time. This time is often difficult to control, and the work usually is performed least efficiently. Effective production control can often reduce this time. Making the dispatch section responsible for seeing that the tools, gages, instructions, and materials are provided at the correct time, and that the tools are returned to their respective cribs after the job has been completed, eliminates the need for the operator to leave the work area. The operator then only has to perform the actual setting up and tearing down of the machine. The clerical and routine function of providing drawings, instructions, and tools can be performed by those more familiar with this type of work. Thus, large numbers of requisitions for these requirements can be performed simultaneously, and setup time can be minimized. Here again, group technology can be advantageous. Duplicate cutting tools should be available, rather than having the operators sharpen their tools. When the operators get new tools, the dull ones are turned in to the tool crib attendant and replaced with sharp ones. Tool sharpening becomes a separate function, and the tools can be standardized more readily.
To minimize downtime, each operator should have a constant backlog of work. The operators should always know what the next work assignment is.
A technique frequently used to keep the workload apparent to the operator, supervisor, and superintendent is a board over each production facility, with three wire clips or pockets to receive work orders. The first clip contains all work orders scheduled ahead; the second clip holds the orders currently being worked on; and the last clip holds the completed orders. When issuing work orders, the dispatcher places them in the work-ahead station. At the same time, the dispatcher picks up all completed job tickets from the work-completed station and delivers them to the scheduling department for recording. This system ensures the operators of continuous loads and makes it unnecessary for them to go to the supervisor for their next work assignments.
Making a record of difficult, recurring setups can save considerable setup time when repeat business is received. Perhaps the simplest and yet most effective way to compile a record of a setup is to take a photograph of the setup once it is complete. The photograph should either be stapled to and filed with the production operation card, or placed in a plastic envelope and attached to the tooling prior to storage in the tool crib.

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UTILIZE THE FULL CAPACITY OF THE MACHINE
A careful review of many jobs often reveals possibilities for utilizing a greater share of the machine’s capacity. For example, a milling setup for a toggle lever was changed so that the six faces were milled simultaneously by five cutters. The old setup required that the job be done in three steps, which meant that the part had to be placed in a separate fixture three different times. The new setup reduced the total machining time and increased the accuracy of the relationship between the six machined faces.
Analysts should also consider positioning one part while another is being machined. This opportunity exists on many milling machine jobs where it is possible to conventional mill on one stroke of the table and climb mill on the return stroke. While the operator is loading a fixture at one end of the machine table, a similar fixture is holding a piece being machined by power feed. As the table of the machine returns, the operator removes the first piece from the machine and reloads the fixture. While this internal work is taking place, the machine is cutting the piece in the second fixture.
In view of the ever-increasing cost of energy, it is important to utilize the most economical equipment to do the job. Several years ago, the cost of energy was such an insignificant proportion of total cost that little attention was given to utilizing the full capacity of machines. There are literally thousands of operations where only a fraction of machine capacity is utilized, with a resulting waste of electric power. In the metal trades industry today, the cost of power is over 2.5 percent of total cost, with strong indications that the present cost of power will increase by at least 50 percent in the next decade. It is highly probable that careful planning to utilize a larger proportion of the capacity of a machine to do the work can effect a 50 percent savings in power usage in many of our plants. Typically, for most motors, if the percent of the rated full load is increased from
25 percent to 50 percent, as much as an 11 percent increase in efficiency could be realized. INTRODUCE MORE EFFICIENT TOOLING
Just as new processing techniques are continually being developed, new and more efficient tooling should be considered. Coated cutting tools have dramatically improved the critical wear-resistance/breakage-resistance combination. For example, TiC-coated tools have provided a 50 to 100 percent increase in speed over uncoated carbide where each has the same breakage resistance. Advantages include harder surfaces, thus reducing abrasive wear; excellent adhesion to the substrates; low coefficient of friction with most workpiece materials; chemical inertness; and resistance to elevated temperatures.
Carbide tools are usually more cost-effective than high-speed steel tools on many jobs. For example, one company realized a 60 percent savings by changing the milling operation of a magnesium casting. Originally, the base was milled complete in two operations, using high-speed steel milling cutters. An analysis resulted in the employment of three carbide-tipped fly cutters mounted in a special

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Figure 3.12 More efficient fixturing and tooling.

(Courtesy of Jergens, Inc.)

holder to mill parts complete. Faster feeds and speeds were possible, and surface finish was not impaired.
Savings can often be achieved by altering tool geometries. Each setup has different requirements that can be achieved only by designing an engineered system that optimizes the feed range for chip control, cutting forces, and edge strength. For example, single-sided low-force geometries may be designed to provide both good chip control and force reduction. In this case, high positive rake angles are grouped to reduce the chip thickness ratio, providing a low cutting force and cutting temperature.
While introducing more efficient tooling, the analyst should develop better methods for holding the work. The work must be held so that it can be positioned and removed quickly (see Figure 3.12). Although the loading of parts is still a manual operation, productivity, as well as equality, will be increased.

3.7

MATERIAL HANDLING

Material handling includes motion, time, place, quantity, and space constraints.
First, material handling must ensure that parts, raw materials, in-process materials, finished products, and supplies are moved periodically from location to location.
Second, since each operation requires materials and supplies at a particular time,

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material handling ensures that no production process or customer is hampered by either the early or late arrival of materials. Third, material handling must ensure that materials are delivered to the correct place. Fourth, material handling must ensure that materials are delivered at each location without damage and in the proper quantity. Finally, material handling must consider storage space, both temporary and dormant.
A study conducted by the Material Handling Institute revealed that between
30 and 85 percent of the cost of bringing a product to market is associated with material handling. Axiomatically, the best handled part is the least manually handled part. Whether the distances of the moves are large or small, these moves should be scrutinized. The following five points should be considered for reducing the time spent in handling material: (1) reduce the time spent in picking up material; (2) use mechanized or automated equipment; (3) make better use of existing handling facilities; (4) handle material with greater care; and (5) consider the application of bar coding for inventory and related applications.
A good example of the application of these five points is the evolution of warehousing; the former storage center has become an automated distribution center. Today, the automated warehouse uses computer control for material movement, as well as information flow through data processing. In this type of automated warehouse, receiving, transporting, storing, retrieving, and controlling inventory are treated as an integrated function.

REDUCE THE TIME SPENT IN PICKING
UP MATERIAL
Material handling is often thought of as only transportation, neglecting consideration of positioning at the workstation, which is equally important. Since it is often overlooked, workstation positioning of material may offer even greater opportunities for savings than does transportation. Reducing the time spent in picking up material minimizes tiring, costly manual handling at the machine or the workplace. It gives the operator a chance to do the job faster with less fatigue and greater safety.
For example, consider eliminating loose piling on the floor. Perhaps the material can be stacked directly on pallets or skids after being processed at the workstation. This can result in a substantial reduction of terminal transportation time (the time that material handling equipment stands idle while loading and unloading take place). Usually some type of conveyor or mechanical fingers can bring material to the workstation, thus reducing or eliminating the time needed to pick up the material. Plants can also install gravity conveyors, in conjunction with the automatic removal of finished parts, thus minimizing material handling at the workstation. Figure 3.13 shows examples of typical handling equipment.
Interfaces between different types of handling and storage equipment should be studied to develop more efficient arrangements. For example, the sketch in Figure 3.14 shows the order picking arrangements, depicting how materials can be removed from the reserve or staging storage either by a worker-aboard order picking vehicle (left), or manually (right). A lift truck can be used to replenish pallet racks.

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Flat steel strapping tool

Pallets—
Four-way entry

Pallets—Box Type

Skids—Box types

Skid platform

Platform truck

Hand truck—
2 wheel

Screw conveyor

Wheel conveyor

Bridge plate

Gravity chute

Conveyor—Portable, belt type

Platform lift

Hydraulic table elevating Low lift platform truck

Low lift pallet truck

Figure 3.13 Typical handling equipment used in industry today.

(Source: The Material Handling Institute.)

After the required items are removed from the flow rack, they are sent by conveyor to order accumulation and packaging operations.

USE MECHANICAL EQUIPMENT
Mechanizing the handling of material usually reduces labor costs, reduces materials damage, improves safety, alleviates fatigue, and increases production. However, care must be exercised in selecting the proper equipment and methods.
Equipment standardization is important because it simplifies operator training, allows equipment interchangeability, and requires fewer repair parts.
The savings possible through the mechanization of material handling equipment are typified by the following examples. In the original design of a circuit board assembly task, the operator would go to the storage crib, select the proper electronic components required for a specific board based on its “plug” list, return to the workbench, and then proceed to insert the components into the board in

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Powerized hand lift truck

Tractor—4 wheel

Portable elevator

High lift platform truck Chain trolley

Portable gooseneck crane Bracket jib crane

Monorail electric hoist Casters— swivel plate

Roller conveyor

Fork truck— telescopic type

Automatic grabs

Industrial crane truck Traveling crane

Straddle truck

Motor truck mounted crane

Figure 3.13 (continued)

Pallet rack Order picking vehicle

Flow rack

Pallet rack Mezzanine deck Lift truck Take-away conveyor Figure 3.14 Schematic of efficient warehousing operations.

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Figure 3.15 Work area of vertical storage machine used in the assembly of computer panels.

accordance with the plug list. The improved method utilizes two automated, vertical storage machines, each with 10 carriers and four pullout drawers per carrier
(see Figure 3.15). The carriers move up and around in a system that is a compressed version of a Ferris wheel. With 20 possible stop positions on call, the unit always selects the closest route—either forward or backward—to bring the proper drawers to the opening in the shortest possible time. From a seated position, the operator dials the correct stop, pulls open the drawer to expose the needed components, one withdraws the proper one, and places it in the board. The improved method has reduced the required storage area by approximately 50 percent, improved workstation layout, and substantially reduced populating errors by minimizing operator handling, decision making, and fatigue.
Often, an automated guided vehicle (AGV) can replace a driver. AGVs are successfully used in a variety of applications, such as mail delivery. Typically, these vehicles are not programmed; rather, they follow a magnetic or optical guide for a planned route. Stops are made at specific locations for a predetermined

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Figure 3.16 Hydraulic lift table used to minimize manual lifting.

(Courtesy of Bishamon.)

period, giving an employee adequate time for unloading and loading. By pressing a “hold” button and then pressing a “start” button at the conclusion of the loading/unloading operation, the operator can lengthen the dwell period at each stop.
AGVs can be programmed to go to any location over more than one path. They are equipped with sensing and control instrumentation to avoid collisions with other vehicles. Also, when such guide path equipment is used, material handling costs vary little with distance.
Mechanization is also useful for manual materials handling, such as palletizing. There are a variety of devices under the generic label of lift tables, which eliminate most of the lifting required of the operator. Some lift tables are springloaded, which, when set with a proper spring stiffness, will adjust automatically to the optimal height for the operator as boxes are placed on a pallet on top of the lift.
(See Chapter 4 for a discussion on the determination of optimal lifting heights.)
Others are pneumatic (see Figure 3.16) and can be easily adjusted with a control, so that lifting is eliminated and material can be slid from one surface to another.
Some tilt for easier access into bins, while others rotate, facilitating palletizing. In general, lift tables are probably the least expensive engineering control measure used in conjunction with the NIOSH1 lifting guidelines (see Chapter 4).
1

NIOSH is the National Institute of Occupational Safety and Health.

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Maximum Net Load That Can Safely Be Handled by Fork Trucks

Figure 3.17 Typical forklift truck.

Start by computing the torque rating by multiplying the distance from the center of the front axle to the center of the load (see Figure 3.17):
Load ϭ torque rating>B where B is distance C ϩ D, with D ϭ A/2.
If the distance C from the center of the front axle to the front end of the fork truck is 18 in and the length of pallet A is 60 in, then the maximum gross weight that a
200,000. in.lb fork truck should handle would be


200,00
18 ϩ 60>2

ϭ 4,167 lb

By planning the pallet size to make full use of the equipment, the company can realize a greater return from the material handling equipment.

MAKE BETTER USE OF EXISTING
HANDLING FACILITIES
To ensure the greatest return from material handling equipment, that equipment must be used effectively. Thus, both the methods and the equipment should be sufficiently flexible that a variety of material handling tasks can be accomplished under variable conditions. Palletizing material in temporary and permanent storage allows greater quantities to be transported faster than storing material without the

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use of pallets, saving up to 65 percent in labor costs. Sometimes, material can be handled in larger or more convenient units by designing special racks. When this is done, the compartments, hooks, pins, or supports for holding the work should be in multiples of 10 for ease of counting during processing and final inspection.
If any material handling equipment is used only part of the time, consider the possibility of putting it to use a greater share of the time. By relocating production facilities or adapting material handling equipment to diversified areas of work, companies may achieve greater utilization.

HANDLE MATERIAL WITH GREATER CARE
Industrial surveys indicate that approximately 40 percent of plant accidents happen during material handling operations. Of these, 25 percent are caused by lifting and shifting material. By exercising greater care in handling material, and using mechanical mechanisms wherever possible for material handling, employees can reduce fatigue and accidents. Records prove that the safe factory is also an efficient factory. Safety guards at points of power transmission, safe operating practices, good lighting, and good housekeeping are essential to making material handling equipment safer. Workers should install and operate all material handling equipment in a manner compatible with existing safety codes.
Better handling also reduces product damaged. If the number of reject parts is at all significant in the handling of parts between workstations, then this area should be investigated. Usually, parts damaged during handling can be minimized if specially designed racks or trays are fabricated to hold the parts immediately after processing. For example, one manufacturer of aircraft engine parts incurred a sizable number of damaged external threads on one component that was stored in metal tote pans after the completion of each operation. When twowheeled hand trucks moved the filled tote pans to the next workstation, the machined forgings bumped against one another and against the sides of the metal pan to such an extent that they became badly damaged. Someone investigated the cause of the rejects and suggested making wooden racks with individual compartments to support the machined forgings. This prevented the parts from bumping against one another or the metal tote pan, thereby significantly reducing the number of damaged parts. Production runs were also more easily controlled because of the faster counting of parts and rejects.
Similar considerations apply to service industries and the health care sector, not only from the standpoint of the “product,” which in many cases is a person, but also with respect to the material handler. For example, patient handling in hospitals and personal care facilities is a major factor in low back and shoulder injuries of nurses. Traditionally, relatively immobile patients are moved from a bed to a wheelchair or vice versa with the use of a walking belt (see Figure
3.18a). However, these maneuvers require considerable amounts of strength and generate very high levels of low back compressive forces (see Section 4.4). Assist devices, such as the Williamson Turn Stand, require much less strength from

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Figure 3.18 Patient handling using three different assists.

(a) Traditional walking belt requires considerable strength and generates very high low-back compressive forces. (b) The Williamson Turn
Stand requires much less strength and is less stressful for the low back. (c) A Hoyer-type lift requires even less strength, but is considerably more expensive and cumbersome in small spaces.

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the nurse and generate much less stress on the low back (see Figure 3.18b). However, the patient must have the leg strength to maintain body weight, with some support. Finally, a Hoyer-type lift requires even less strength, but is considerably more expensive and cumbersome in small areas (see Figure 3.18c).

CONSIDER BAR CODING FOR INVENTORY
AND RELATED APPLICATIONS
The majority of technical people have some familiarity with bar coding and bar code scanning. Bar coding has shortened queues at grocery and department store checkout lines. The black bars and white spaces represent digits that uniquely identify both the item and the manufacturer. Once this Universal Product Code
(UPC) is scanned by a reader at the checkout counter, the decoded data are sent to a computer that records timely information on labor productivity, inventory status, and sales. The following five reasons justify the use of bar coding for inventory and related applications:
1. Accuracy. Typically representative performance is less than 1 error in 3.4 million characters. This compares favorably with the 2 to 5 percent error that is characteristic of keyboard data entry.
2. Performance. A bar code scanner enters data three to four times faster than typical keyboard entry.
3. Acceptance. Most employees enjoy using the scanning wand. Inevitably, they prefer using a wand to keyboard entry.
4. Low cost. Since bar codes are printed on packages and containers, the cost of adding this identification is extremely low.
5. Portability. An operator can carry a bar code scanner into any area of the plant to determine such things as inventories and order status, etc.
Bar coding is useful for receiving, warehousing, job tracking, labor reporting, tool crib control, shipping, failure reporting, quality assurance, tracking, production control, and scheduling. For example, the typical storage bin label provides the following information: part description, size, packing quantity, department number, storage number, basic stock level, and order point. Considerable time can be saved by using a scanning wand to gather these data for inventory reordering.
Some practical applications reported by Accu-Sort Systems, Inc., include automatically controlling conveyor systems; diverting material to the location where it is needed; and providing material handlers with clear, concise instructions about where to take materials, automatically verifying that the proper material is handled. If bar coding is incorporated into programmable controllers and automatic packaging equipment, online real-time verification of packing labels with container contents can be used to avoid costly product recalls.

SUMMARY: MATERIAL HANDLING
Analysts should always be looking for ways to eliminate inefficient material handling without sacrificing safety. To assist the methods analyst in this endeavor,

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the Materials Handling Institute (1998) has developed 10 principles of material handling. 1. Planning principle. All material handling should be the result of a deliberate plan in which the needs, performance objectives, and functional specifications of the proposed methods are completely defined at the outset. 2. Standardization principle. Material handling methods, equipment, controls, and software should be standardized within the limits of achieving overall performance objectives and without sacrificing needed flexibility, modularity, and throughput.
3. Work principle. Material handling work should be minimized without sacrificing productivity or the level of service required of the operation.
4. Ergonomic principle. Human capabilities and limitations must be recognized and respected in the design of material handling tasks and equipment, to ensure safe and effective operations.
5. Unit load principle. Unit loads shall be appropriately sized and configured in a way that achieves the material flow and inventory objectives at each stage in the supply chain.
6. Space utilization principle. Effective and efficient use must be made of all available space.
7. System principle. Material movement and storage activities should be fully integrated to form a coordinated, operational system that spans receiving, inspection, storage, production, assembly, packaging, unitizing, order selection, shipping, transportation, and returns handling.
8. Automation principle. Material handling operations should be mechanized and/or automated where feasible, to improve operational efficiency, increase responsiveness, improve consistency and predictability, decrease operating costs, and eliminate repetitive or potentially unsafe manual labor.
9. Environmental principle. Environmental impacts and energy consumption are criteria to be considered when designing or selecting alternative equipment and material handling systems.
10. Life-cycle-cost principle. A thorough economic analysis should account for the entire life cycle of all material handling equipment and resulting systems.
To reiterate, the predominant principle is that the less a material is handled, the better it is handled, which fits in nicely with eliminating the mudas of unnecessary transportation and unnecessary motions.

3.8 PLANT LAYOUT
The principal objective of effective plant layout is to develop a production system that permits the manufacture of the desired number of products with the desired quality at the least cost. Physical layout is an important element of an entire

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production system that embraces operation cards, inventory control, material handling, scheduling, routing, and dispatching. All these elements must be carefully integrated to fulfill the stated objective. Poor plant layouts result in major costs. The indirect labor expense of long moves, backtracking, delays, and work stoppages due to bottlenecks in the transportation muda are characteristic of a plant with an antiquated and costly layout.

LAYOUT TYPES
Is there one type of layout that tends to be the best? The answer is no. A given layout can be best in one set of conditions and yet poor in a different set of conditions. In general, all plant layouts represent one or a combination of two basic layouts: product or straight-line layouts and process or functional layouts. In the straight-line layout, the machinery is located such that the flow from one operation to the next is minimized for any product class. In an organization that utilizes this technique, it would not be unusual to see a surface grinder located between a milling machine and a turret lathe, with an assembly bench and plating tanks in the immediate area. This type of layout is quite popular for certain mass-production manufacture, because material handling costs are lower than for process grouping.
Product layout has some distinct disadvantages. Since a broad variety of occupations are represented in a relatively small area, employee dissatisfaction can escalate. This is especially true when different opportunities carry a significant money rate differential. Because unlike facilities are grouped together, operator training can be more cumbersome, especially if an experienced employee is not available in the immediate area to train a new operator. The problem of finding competent supervisors is also exacerbated, due to the variety of facilities and jobs that must be supervised. Then, too, this type of layout invariably necessitates a larger initial investment because duplicate service lines are required, such as air, water, gas, oil, and power. Another disadvantage of product grouping is the fact that this arrangement tends to appear disorderly and chaotic. With these conditions, it is often difficult to promote good housekeeping. In general, however, the disadvantages of product grouping are more than offset by the advantages, if production requirements are substantial.
Process layout is the grouping of similar facilities. Thus, all turret lathes would be grouped in one section, department, or building. Milling machines, drill presses, and punch presses would also be grouped in their respective sections.
This type of arrangement gives a general appearance of neatness and orderliness, and tends to promote good housekeeping. Another advantage of functional layout is the ease with which a new operator can be trained. Surrounded by experienced employees operating similar machines, the new worker has a greater opportunity to learn from them. The problem of finding competent supervisors is lessened, because the job demands are not as great. Since these supervisors need only be familiar with one general type or class of facilities, their backgrounds do not have to be as extensive as those of supervisors in shops using product grouping.

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Also, if production quantities of similar products are limited and there are frequent “job” or special orders, a process layout is more satisfactory.
The disadvantage of process grouping is the possibility that long moves and backtracking will be needed on jobs that require a series of operations on diversified machines. For example, if the operation card of a job specifies a sequence of drill, turn, mill, ream, and grind, the movement of the material from one section to the next could prove extremely costly. Another major disadvantage of process grouping is the large volume of paperwork required to issue orders and control production between sections.

TRAVEL CHARTS
Before designing a new layout or correcting an old one, analysts must accumulate the facts that may influence that layout. Travel or from-to charts can be helpful in diagnosing problems related to the arrangement of departments and service areas, as well as the location of equipment within a given sector of the plant. The travel chart is a matrix that presents the magnitude of material handling that takes place between two facilities per time period. The unit identifying the amount of handling may be whatever seems most appropriate to the analyst. It can be pounds, tons, handling frequency, and so on. Figure 3.19 illustrates a very elementary travel chart from which the analyst can deduce that of the all machines, No. 4 W&S turret lathe and No. 2 Cincinnati Horizontal mill should be next to each other because of the high number of items (200) passing between the two machines.

MUTHER’S SYSTEMATIC LAYOUT PLANNING
A systematic approach to plant layout developed by Muther (1973) is termed systematic layout planning (SLP). The goal of SLP is to locate two areas with high frequency and logical relationships close to one another using a straightforward six-step procedure:
1. Chart relationships. In the first step, the relationships between different areas are established and then charted on a special form called the relationship chart (or rel chart for short; see Figure 3.20). A relationship is the relative degree of closeness, desired or required, among different activities, areas, departments, rooms, etc., as determined from quantitative flow information (volume, time, cost, routing) from a from-to chart, or more qualitatively from functional interactions or subjective information. For example, although painting may be the logical step between finishing and final inspection and packing, the toxic materials and hazardous or flammable conditions may require that the paint area be completely separated from the other areas. The relationship ratings range in value from 4 to –1, based on the vowels that semantically define the relationship, as shown in Table 3.3.
2. Establish space requirements. In the second step, space requirements are established in terms of square footage. These values can be calculated based on production requirements, extrapolated from existing areas,

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2-Spindle
L. & G. Drill

No. 2 Cinn.
Hor. Mill

No. 3B. & S.
Verticle Mill

Niagara 100Ton
Press

No. 2 Cinn.
Centerless

No. 3 Excello
Thd. Grinder

45

80

32

4

6

2

6

8

4

22

2

3

22

No. 4 W. & S.
Turret Lathe

20

No. 4 W. & S.
Turret Lathe

Delta 17"
Drill Press

To

14

18

4

4

10

5

4

2

6

3

1

0

1

Delta 17"
Drill Press

From

2-Spindle
L. & G. Drill
No. 2 Cinn.
Hor. Mill

120

No. 3B. & S.
Verticle Mill
Niagara 100Ton
Press

60

No. 2 Cinn.
Centerless

15

No. 3 Excello
Thd. Grinder

12

2

15
15

8

Figure 3.19 The travel chart is a useful tool in solving material handling and plant layout problems related to process-type layouts. The chart enumerates the number of items (per given time period) or the volume (e.g., tons per shift) transported between the different machines.

projected for future expansion, or fixed by legal standards, such as the
ADA or architectural standards. In addition to square footage, the kind and shape of the area being laid out, or the location with respect to required utilities, may be very important.
3. Activity relationships diagram. In the third step, a visual representation of the different activities is drawn. The analyst starts with the absolutely important relationships (A’s), using four short, parallel lines to join the two areas. The analyst then proceeds to the E’s, using three parallel lines approximately double the length of the A lines. The analyst continues this procedure for the I’s, O’s, etc., progressively increasing the length of the lines, while attempting to avoid crossing or tangling the lines. For undesirable relationships, the two areas are placed as far apart as possible, and a squiggly line (representing a spring) is drawn between them. (Some

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Relationship Chart

Page 1 of 1

Project: Construction of new office
Plant: Dorben Consulting
Date: 6-9-97
Charted By: AF
Reference:
Activity

Remarks:

Area (ft2)

M. Dorben Office (DOR)

125

Engineering Office (ENG)

120

Secretary (SEC)
Foyer (FOY)
Files (FIL)
Copy Area (COP)
Storeroom (STO)

65
50
40
20
80

O
A
O

O
X

I

I
U

I
E

U

U

O
U

U

U

U
O
E

U
E

Figure 3.20 Relationship chart for Dorben Consulting.

analysts may also define an extremely undesirable relationship with a
Ϫ2 value and a double squiggly line.)
4. Layout space relationships. Next, a spatial representation is created by scaling the areas in terms of relative size. Once the analyst is satisfied with the layout, the areas are compressed into a floor plan. This is typically not as easy as it sounds, and the analyst may want to utilize templates. In addition, modifications may be made to layout based on material handling

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113

Table 3.3 SLP Relationship Ratings

Relationship
Absolutely necessary
Especially important
Important
Ordinary
Unimportant
Not desirable

Closeness Rating
A
E
I
O
U
X

Value

Diagram Lines

4
3
2
1
0
Ϫ1

Color
Red
Yellow
Green
Blue
Brown

requirements (e.g., a shipping or receiving department would necessarily be located on an exterior wall), storage facilities (perhaps similar exterior access requirements), personnel requirements (a cafeteria or restroom located close by), building features (crane activities in a high bay area; forklift operations on the ground floor), and utilities.
5. Evaluate alternative arrangements. With numerous possible layouts, it would not be unusual to find that several appear to be equally likely possibilities. In that case, the analyst will need to evaluate the different alternatives to determine the best solution. First, the analyst will need to identify factors deemed important: for example, future expansion capability, flexibility, flow efficiency, material handling effectiveness, safety, supervision ease, appearance or aesthetics, etc. Second, the relative importance of these factors will need to be established through a system of weights, such as a 0-to-10 basis. Next, each alternative is rated for satisfying each factor. Muther (1973) suggests the same 4 to Ϫ1 scale: 4 is almost perfect; 3, especially good; 2, important; 1, ordinary result;
0, unimportant; and Ϫ1, not acceptable. Each rating is then multiplied by the weight. The products for each alternative are summed, with the largest value indicating the best solution.
6. Select layout and install. The final step is to implement the new method.

Plant Layout of Dorben Consulting Using SLP
The Dorben Consulting group would like to lay out a new office area. There are seven activity areas: M. Dorben’s office, engineering office (occupied by two engineers), secretarial area, foyer and waiting area for visitors, file area, copy area, and storeroom. The activity relationships are subjectively assessed by M. Dorben to be as shown in the rel chart in Figure 3.20. The chart also indicates space allotments for each area, ranging from a low of 20 ft2 for the copy area to 125 ft2 for M. Dorben’s office. For example, the relationship between M. Dorben and the secretary is deemed absolutely important (A), while the relationship between the engineering area and the foyer is deemed not desirable (X), so that the engineers are not disrupted in their work by visitors.

EXAMPLE 3.2

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A relatively good first attempt at an activity relationship diagram yields Figure 3.21.
Adding in the relative size of each area yields the space relationship chart in Figure 3.22.
Compressing the areas yields the final floor plan in Figure 3.23.
Since Dorben’s office and the engineering area are practically the same size, they could easily be interchanged, leaving two alternative layouts. These are evaluated
(Figure 3.24) on the basis of personnel isolation (which is very important to M. Dorben, yielding a high weight of 8), supplies movement, visitor reception, and flexibility. The big difference in the layouts is the closeness of the engineering area to the foyer. Thus, alternative B (shown in Figure 3.23) at 68 points, compared to 60 points for alternative A, turns out to be the preferred layout.

FOY

STO
FIL

ENG
COP
DOR

DOR
SEC

SEC

COP
ENG

STO

FOY

FIL

Figure 3.21 Activity relationship diagram for Dorben
Consulting.

Figure 3.22 Space relationship layout for Dorben
Consulting.

N
ENG

SEC

FIL

COP

STO

DOR

FOY

Figure 3.23 Floor plan for Dorben Consulting.

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Alternatives

Evaluating Alternatives
Plant: Dorben Consulting
Project:
New Office Construction

Operation Analysis

A
Dorben
office facing west

B
Dorben
office facing east

C

D

E

Analyst: AF
Factor/Consideration

Wt.

Ratings and Weighted Ratings
B
C
D
E
8 3 24

A

Isolation of personnel

8

1

Movement of supplies

4

3

12

3

12

Visitor reception

4

4

16

4

16

Flexibility

8

3

24

2

Comments

16

Totals

60

68

Remarks:
Alternative B, with Dorben's office facing east and engineers' office facing west, lessens disruptions of the engineers' work due to visitors.

Figure 3.24 Evaluating alternatives for Dorben Consulting.

COMPUTER-AIDED LAYOUT
Commercially available software can help analysts develop realistic layouts rapidly and inexpensively. The Computerized Relative Allocation Facilities
(CRAFT) program is one that has been used extensively. An activity center could be a department or work center within a department. Any one activity center can be identified as fixed, freezing it and allowing freedom of movement in those that

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can be readily moved. For example, it is often desirable to freeze such activity centers as elevators, restrooms, and stairways. Input data include fixed work center numbers and locations, material handling costs, interactivity center flow, and a block layout representation. The governing heuristic algorithm asks, What change in material handling costs would result if work centers were exchanged?
Once the answer is stored, the computer proceeds in an iterative manner until it converges on a good solution. CRAFT calculates the distance matrix as the rectangular distances from the department centroids.
Another layout program is CORELAP. The input requirements for CORELAP are the number of departments, the departmental areas, the departmental relationships, and the weights for these relationships. CORELAP constructs layouts by locating the departments, using rectangular areas. The objective is to provide a layout with “high-ranking” departments close together.
ALDEP, still another layout program, constructs plant layouts by randomly selecting a department and locating it in a given layout. The relationship chart is then scanned, and a department that has a high closeness rating is introduced into the layout. This process continues until the program places all departments.
ALDEP then computes a score for the layout, and repeats the process a specific number of times. The program also has the ability to provide multifloor layouts.
All these plant layout programs were originally developed for large mainframe computers. With the advent of personal computers, the algorithms have been incorporated into PC programs, as have other algorithms. One such program, SPIRAL, attempts to optimize the adjacency relationship by summing the positive relationships and deducting the negative relationships for adjacent areas.
This is essentially a quantified Muther’s approach and is described in greater detail in Goetschalckx (1992). For example, entering the data for the Dorben
Consulting example yields a slightly different layout, as shown in Figure 3.25.
Note that there is a tendency to generate long, narrow rooms, to minimize the distance between room centers. This is an especially big problem with CRAFT,
ALDEP, etc. SPIRAL at least attempts to modify this tendency by adding a shape penalty. Also, there is a tendency for many of these programs (i.e., those that are improvement programs, such as CRAFT, that build upon an initial layout) to reach a local minimum and not attain the optimum layout. This problem can be circumvented by starting with alternate layouts. This is less a problem with construction programs, such as SPIRAL, which generate a solution from scratch.
A more powerful and perhaps more useful set of programs are FactoryPLAN,
FactoryFLOW, and FactoryCAD, which input existing AutoCAD files of floor plans and create very detailed layouts suitable for architectural planning.

3.9 WORK DESIGN
Because of the recent regulatory (i.e., OSHA) and health (i.e., rising medical and workers’ compensation costs) concerns, work design techniques will be covered in detail in separate chapters. Chapter 4 addresses manual work and the principles of motion economy; Chapter 5 addresses ergonomic principles of workplace and

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Figure 3.25
SPIRAL input files:
(a) DORBEN.DAT,
(b) DORBEN.DEP, and
(c) resulting layout for the DORBEN Consulting example. Operation Analysis

a)
[project_name]
[number_of_departments]
[department_file_name]
[building_width]
[building_depth]
[seed]
[tolerance]
[time_limit]
[number_of_iterations]
[report_level]
[max_shape_ratio]
[shape_penalty]

DORBEN
7
DORBEN.DEP
25
20
12345
0.00010
120
20
2
2.50
500.00

DOR
ENG
SEC
FOY
FIL
COP
STO
DOR
DOR
DOR
DOR
ENG
ENG
ENG
SEC
SEC
SEC
SEC
FOY
COP
OUT

0
0
0
0
0
0
0

b)
0 0
0 0
0 0
0 0
0 0
0 0
0 0
ENG
SEC
FOY
FIL
SEC
FOY
FIL
FOY
FIL
COP
STO
STO
STO
OUT

125
120
65
50
40
20
80

0
0
0
0
0
0
0

GREEN
BLUE
RED
YELLOW
BROWN
GRAY
BLACK

1
4
1
2
1
-1
2
2
3
1
1
3
3
0

c)
STO

COP

3
1

1
SEC

3
FOY

2

3

1

4

FIL

DOR

2
1

2
ENG

Dorben
Engineers
Secretary
Foyer
Files
Copy
Storeroom

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tool design; Chapter 6 covers working and environmental conditions; Chapter 7 presents cognitive work with respect to informational input from displays, information processing, and interaction with computers; and Chapter 8 addresses workplace and systems safety.

SUMMARY
The nine primary approaches to operation analysis represent a systematic approach to analyzing the facts presented on the operation and flow process charts.
These principles are just as applicable to the planning of new work as to the improvement of work already in production. While decreased waste, increased output, and improved quality, consistent with lean manufacturing principles, are the primary outcomes of operation analysis, it also provides benefits to all workers with better working conditions and methods.
A systematic method for remembering and applying the nine operation analyses is offered by a checklist of pertinent questions, as shown in Figure
3.26. In the figure, the checklist demonstrates how its use resulted in a cost reduction on an electric blanket control knob shaft. Redesigning the shaft so that it could be economically produced as a die casting rather than a screw machine part reduced factory cost from $68.75 per 1,000 pieces to $17.19 per 1,000 pieces. This check sheet is also useful as an outline in providing methods training to factory foremen and superintendents. Thought-provoking questions, when intelligently used, help factory supervisors to develop constructive ideas and assist in operations analysis.

QUESTIONS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.

Explain how design simplification can be applied to the manufacturing process.
How is operation analysis related to methods engineering?
How do unnecessary operations develop in an industry?
Compare and contrast operations analysis with the lean manufacturing approach.
What are the seven mudas?
What are the 5S pillars?
What is meant by “tight” tolerances?
Explain why it may be desirable to “tighten up” tolerances and specifications.
What is meant by lot-by-lot inspection?
When is an elaborate quality control procedure not justified?
What six points should be considered when endeavoring to reduce material cost?
How does a changing labor and equipment situation affect the cost of purchased components? 13. Explain how rearranging operations can result in savings.
14. What process is usually considered the fastest for forming and sizing operations?
15. How should the analyst investigate the setup and tools to develop better methods?

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Figure 3.26 knob shaft.

Operation Analysis

Operations analysis checklist for manufacture of blanket control

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Figure 3.26 (continued)

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16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
31.

Operation Analysis

Give some applications of bar coding for the improvement of productivity.
What are the two general types of plant layout? Explain each in detail.
What is the best way to test a proposed layout?
Which questions should the analyst ask when studying work performed at a specific workstation?
Explain the advantages of using a checklist.
In connection with automated guided vehicles, why do costs vary little with distance? On what does the extent of tooling depend?
How can planning and production control affect setup time?
How can a material best be handled?
How is the travel chart related to Muther’s SLP?
Why does the travel chart have greater application in process layout than in product layout? Explain the fundamental purpose of group technology.
Explain how the conservation of welding rods can result in 20 percent material savings. Identify several automobile components that have been converted from metal to plastic in recent years.
Where would you find application for a hydraulic elevating table?
What is the difference between a skid and a pallet?

PROBLEMS
1.
2.

3.

4.

5.

The finish tolerance on the shaft in Figure 3.4 was changed from 0.004 in to 0.008 in. How much cost improvement resulted from this change?
The Dorben Company is designing a cast-iron part whose strength T is a known function of the carbon content C, where T ϭ 2C2 ϩ 3>4C Ϫ C3 ϩ k . To maximize strength, what carbon content should be specified?
To make a given part interchangeable, it was necessary to reduce the tolerance on the outside diameter from Ϯ0.010 to Ϯ0.005 at a resulting cost increase of 50 percent of the turning operation. The turning operation represented 20 percent of the total cost. Making the part interchangeable meant that the volume of this part could be increased by 30 percent. The increase in volume would permit production at 90 percent of the former cost. Should the methods engineer proceed with the tolerance change? Explain.
The Dorben Group suite consists of five rooms, with areas and relationships as shown in Figure 3.27. Obtain an optimal layout, using Muther’s SLP and SPIRAL.
Compare and contrast the resulting layouts.
Using the from-to chart on p122 showing the number of units handled from one area to another per hour and the desired size of each area (in square feet), develop an optimal layout using Muther’s SLP and SPIRAL. Note that you will need to devise a relationship scheme for the given flows. Also, * means an undesirable relationship. 129

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Activity

Area (sq. ft.)

A

160

B

160

C

240

O
I
I

I
U

U
D

160

U
U

U
U

E

80

Figure 3.27 Information for Problem 4.

Size

Area

A

B

C

D

E

150

A



1

20

8

1

50

B

0



30

0

8

90

C

20

5



40

20

90

D

0

1

2



*

40

E

0

0

11

0



REFERENCES
Bralla, James G. Handbook of Product Design for Manufacturing. New York: McGrawHill, 1986.
Buffa, Elwood S. Modern Production Operations Management. 6th ed. New York: John
Wiley & Sons, 1980.
Chang, Tien-Chien, Richard A. Wysk, and Wang Hsu-Pin. Computer Aided
Manufacturing. Englewood Cliffs, NJ: Prentice-Hall, 1991.
Drury, Colin G. “Inspection Performance.” In Handbook of Industrial Engineering, 2d ed. Ed. Gavriel Salvendy. New York: John Wiley & Sons, 1992.
Francis, Richard L., and John A. White. Facility Layout and Location: An Analytical
Approach. Englewood Cliffs, NJ: Prentice-Hall, 1974.
Goetschalckx, M. “An Interactive Layout Heuristic Based on Hexagonal Adjacency
Graphs.” European Journal of Operations Research, 63, no. 2 (December 1992), pp. 304–321.

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Operation Analysis

Konz, Stephan. Facility Design. New York: John Wiley & Sons, 1985.
Material Handling Institute. The Ten Principles of Material Handling. Charlotte, NC,
1998.
Muther, R. Systematic Layout Planning, 2d ed. New York: Van Nostrand Reinhold, 1973.
Niebel, Benjamin W., and C. Richard Liu. “Designing for Manufacturing.” In Handbook of Industrial Engineering, 2d ed. Ed. Gavriel Salvendy. New York: John Wiley &
Sons, 1992.
Nof, Shimon Y. “Industrial Robotics.” In Handbook of Industrial Engineering, 2d ed.
Ed. Gavriel Salvendy. New York: John Wiley & Sons, 1992.
Shingo, S. Study of Toyota Production System. Tokyo, Japan: Japan Management Assoc.
(1981), pp. 167–182.
Sims, Ralph E. “Material Handling Systems.” In Handbook of Industrial Engineering,
2d ed. Ed. Gavriel Salvendy. New York: John Wiley & Sons, 1992.
Spur, Gunter. “Numerical Control Machines.” In Handbook of Industrial Engineering,
2d ed. Ed. Gavriel Salvendy. New York: John Wiley & Sons, 1992.
Taguchi, Genichi. Introduction to Quality Engineering. Tokyo, Japan: Asian
Productivity Organization, 1986.
Wemmerlov, Urban, and Nancy Lea Hyer. “Group Technology.” In Handbook of
Industrial Engineering, 2d ed. Ed. Gavriel Salvendy. New York: John Wiley &
Sons, 1992.
Wick, Charles, and Raymond F. Veilleux. Quality Control and Assembly, 4. Detroit, MI:
Society of Manufacturing Engineers, 1987.

SELECTED SOFTWARE
ALDEP, IBM Corporation, program order no. 360D-15.0.004.
CORRELAP, Engineering Management Associates, Boston, MA.
CRAFT, IBM share library No. SDA 3391.
Design Tools (available from the McGraw-Hill text website at www.mhhe.com/niebelfreivalds. New York: McGraw-Hill, 2002.
FactoryPLAN, FactoryFLOW, and FactoryCAD, (vol. 3) EDS PLM Solutions, 2321
North Loop Dr. ISU Research Park, Ames, IA, 50010, 2001. (http://www.eds.com/)
SPIRAL, User’s Manual, 4031 Bradbury Dr., Marietta, GA, 30062, 1994.

SELECTED VIDEOTAPES/DVDS
Design for Manufacture and Assembly. DV05PUB2. Dearborn, MI: Society of
Manufacturing Engineers, 2005.
Flexible Material Handling. DV03PUB104. Dearborn, MI: Society of Manufacturing
Engineers, 2003.
Flexible Small Lot Production for Just-In-Time. DV03PUB107. Dearborn, MI: Society of Manufacturing Engineers, 2003.
Introduction to Lean Manufacturing. DV03PUB46. Dearborn, MI: Society of
Manufacturing Engineers, 2003.

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Layout Improvements for Just-In-Time. Manufacturing Insights Videotape Series. 1/29
VHS VT393–1368 & 3/49 U-Matic VT393U-1368. Dearborn, MI: Society of
Manufacturing Engineers,1990.
Quick Changeover for Lean Manufacturing. DV03PUB33. Dearborn, MI: Society of
Manufacturing Engineers, 2003.
Total Quality Management. Manufacturing Insights Videotape Series. VT91PUB1.
Dearborn, MI: Society of Manufacturing Engineers, 1991.

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Design

133

CHAPTER

4

KEY POINTS



Design work according to human capabilities and limitations.

• For manipulative tasks:
• Use dynamic motions rather than static holds.
• Keep the strength requirement below 15 percent of maximum.
• Avoid extreme ranges of motion.
• Use the smallest muscles for speed and precision.
• Use the largest muscles for strength.


For lifting and other heavy manual work:

• Keep workloads below one-third of the maximum work capacity.
• Minimize horizontal load distances.
• Avoid twisting.
• Use frequent, short work/rest cycles.

T

he design of manual work was introduced by the Gilbreths through motion study and the principles of motion economy, and later scientifically developed by human factor specialists for military applications. The principles have traditionally been broken down into three basic subdivisions: (1) the use of the human body, (2) the arrangement and conditions of the workplace, and (3) the design of tools and equipment. More important, although developed empirically, the principles are in fact based on established anatomical, biomechanical, and physiological principles of the human body. They form the scientific basis for ergonomics and work design. Accordingly, some theoretical background will be presented so that the principles of motion economy can be understood better rather than merely being accepted as memorized rules. Furthermore, the traditional principles of motion economy have been considerably expanded and are now called

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the principles of and guidelines for work design. This chapter presents the principles related to the human body and the guidelines for the design of work as related to physical activity. Chapter 5 covers those principles related to the design of workstations, tools, and equipment. Chapter 6 presents guidelines for the design of the work environment. Chapter 7 presents cognitive work design. Although not traditionally included as part of methods engineering, it is becoming an increasingly important aspect of work design. Chapter 8 covers workplace and systems safety.

4.1

THE MUSCULOSKELETAL SYSTEM

The human body is able to produce movements because of a complex system of muscles and bones, termed the musculoskeletal system. The muscles are attached to the bones on either side of a joint (see Figure 4.1), so that one or several muscles, termed agonists, act as the prime activators of motion. Other muscles, termed antagonists, counteract the agonists and oppose the motion.
For elbow flexion, which is a decrease in the internal joint angle, the biceps, the brachioradialis, and the brachialis form the agonists, while the triceps forms the antagonist. However, on elbow extension, which is an increase in the joint angle, the triceps becomes the agonist, while the other three muscles become the antagonists. There are three types of muscles in the human body: skeletal or striated muscles, attached to the bones; cardiac muscle, found in the heart; and smooth muscle, found in the internal organs and the walls of the blood vessels. Only the skeletal muscles (of which there are approximately 500 in the body) will be discussed here, because of their relevance to motion.
Each muscle is made up of a large number of muscle fibers, approximately
0.004 in (0.1 mm) in diameter and ranging in length from 0.2 to 5.5 in (5 to 140 mm), depending on the size of the muscle. These fibers are typically bound together in bundles by connective tissue, which extends to the end of the muscle and assists in

Biceps
Brachialis
Brachioradialis
Triceps

Figure 4.1 The musculoskeletal system of the arm.

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Manual Work Design

Figure 4.2 The structure of muscle.

Muscle

(From: Gray’s Anatomy, 1973, by permission of
W. B. Saunders Co., London.)

Fasciculus

Group of muscle fibres

Myofibril

Myofilaments

Myosin

Actin

firmly attaching the muscle and muscle fibers to the bone (see Figure 4.2). These bundles are penetrated by tiny blood vessels that carry oxygen and nutrients to the muscle fibers, as well as by small nerve endings that carry electrical impulses from the spinal cord and brain.
Each muscle fiber is further subdivided into smaller myofibrils and ultimately into the protein filaments that provide the contractile mechanism. There are two types of filaments: thick filaments, comprised of long proteins with molecular heads, called myosin; and thin filaments, comprised of globular proteins, called actin. The two types of filaments are interlaced, giving rise to the striated appearance and alternate name, as shown in Figure 4.3. This allows the muscle to contract as the filaments slide over one another, which occurs as molecular bridges or bonds are formed, broken, and reformed between the myosin heads and actin globules. This sliding filament theory explains how the muscle length can change from approximately 50 percent of its resting length (the neutral

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Z
Z
Z
Z
Z

Z

Force
(% of maximum)

136

Z

Z

Z

Z

100

50

50

100
Length of contractile element (% of resting length)

150

Figure 4.3 Force–length relationship of skeletal muscle.

(From: Winter, 1979, p. 114. Reprinted by permission of John Wiley & Sons, Inc.)

uncontracted length at approximately the midpoint in the normal range of motion) at complete contraction to 180 percent of its resting length at complete extension (see Figure 4.3).

4.2 PRINCIPLES OF WORK DESIGN: HUMAN
CAPABILITIES AND MOTION ECONOMY
ACHIEVE THE MAXIMUM MUSCLE STRENGTH AT
THE MIDRANGE OF MOTION
The first principle of human capability derives from the inverted-U-shaped property of muscle contraction shown in Figure 4.3. At the resting length, optimal bonding occurs between the thick and thin filaments. In the stretched state, there is minimal overlap or bonding between the thick and thin filaments, resulting in considerably decreased (almost zero) muscle force. Similarly, in the completely contracted state, interference occurs between the opposing thin filaments, again preventing optimum bonding and decreasing muscle force. This muscle property is typically termed the force–length relationship. Therefore, a task requiring considerable muscle force should be performed at the optimum position. For

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Vertical reference Manual Work Design

Figure 4.4 Typical relaxed posture assumed by people in weightless conditions.

24° ± 5°

90°
Horizontal reference
One Ϫ g line o
Zer
f sight 10° oϪ g li ne of s igh 15° ± 2° t 122° ± 24°

(From: Thornton, 1978,
Fig. 16.)

36° ± 19°

128° ± 7°

133° ± 8°

111° ± 6°
Horizontal reference

example, the neutral or straight position will provide the strongest grip strength for wrist motions. For elbow flexion, the strongest position would be with the elbow bent somewhat beyond the 90° position. For plantar flexion (i.e., depressing a pedal), again the optimum position is slightly beyond 90°. A rough rule of thumb for finding the midrange of motion is to consider the posture assumed by an astronaut in weightless conditions when both agonist and antagonist muscles surrounding the joint are most relaxed and the limb attains a neutral position
(see Figure 4.4).

ACHIEVE THE MAXIMUM MUSCLE STRENGTH WITH
SLOW MOVEMENTS
The second principle of human capability is based on another property of the sliding filament theory and muscle contraction. The faster the molecular bonds are formed, broken, and reformed, the less effective is the bonding and the less muscular force is produced. This is a pronounced nonlinear effect (see Figure 4.5) with maximum muscle force being produced with no externally measurable shortening

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Figure 4.5

100

Force–velocity relationship of skeletal muscle. Force
(% of maximum)

138

50

0
0

100
50
Velocity
(% of maximum no-load velocity)

(i.e., zero velocity or a static contraction), and minimal muscle force being produced at the maximum velocity of muscle shortening. The force is only sufficient to move the mass of that body segment. This muscle property is known as the force–velocity relationship and is especially important with respect to heavy manual work.

USE MOMENTUM TO ASSIST WORKERS WHEREVER
POSSIBLE; MINIMIZE IT IF IT IS COUNTERACTED BY
MUSCULAR EFFORT
There is a trade-off between the second and third principles. Faster movements produce higher momentum and higher impact forces in the case of blows.
Downward motions are more effective than upward motions, because of the assistance from gravity. To make full use of the momentum built up, workstations should allow operators to release a finished part into a delivery area while their hands are on their way to get component parts or tools to begin the next work cycle. DESIGN TASKS TO OPTIMIZE HUMAN
STRENGTH CAPABILITY
Human strength capability depends on three major task factors: (1) the type of strength, (2) the muscle or joint motion being utilized, and (3) posture. There are three types of muscle exertions, defined primarily by the way the strength of the exertion is measured. Muscular exertions resulting in body motions result from dynamic strength. These are sometimes termed isotonic contractions, because the load and body segments lifted nominally maintain a constant external force on the muscle. (However, the internal force produced by the muscle varies due to the geometry of the effective moment arms.) Because of the many variables involved

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Arm strength distribution
Females
Males x = 44.9 x = 85.8 s = 17.6 s = 28.6

Long handle Frequency

90˚

100

200
Lbs.

Arm lifting strength test position

Short handle Frequency

Leg strength distribution
Females
x = 93.8 s = 44.4

Males x = 211.8 s = 76.5

100

200

300

Lbs.
Leg lifting strength test position

Long handle Frequency

Torso strength distribution
Females
x = 59.9 s = 31.0

100
15

Torso lifting strength test position

Males x = 122.4 s = 54.8

200

300

Lbs.

Figure 4.6 Static strength positions and results for 443 males, 108 females.
(Chaffin et al., 1977.)

in such contractions, some variables necessarily need to be constrained to obtain a measurable strength. Thus, dynamic strength measurements have typically been made using constant-velocity (isokinetic) dynamometers, such as the Cybex or the
Mini-Gym (Freivalds and Fotouhi, 1987). In the case where the body motion is restrained, an isometric or static strength is obtained. As seen in Figure 4.5, an isometric strength is necessarily greater than a dynamic strength because of the

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Table 4.1 A. Static Muscle Strength Moment Data (ft·lb) for 25 Men and 22 Women Employed in Manual
Jobs in Industry

Male (%ile)
Muscle Function
Elbow flexion
Elbow extension
Medial humeral
(shoulder) rotation
Lateral humeral
(shoulder) rotation
Shoulder horizontal flexion
Shoulder horizontal extension Shoulder vertical adduction
Shoulder vertical abduction
Ankle extension
(plantar flexion)
Knee extension
Knee flexion
Hip extension
Hip flexion
Torso extension
Torso flexion
Torso lateral flexion

Joint Angles

Female (%ile)

5

50

95

5

50

95

90° Included to arm (arm at side)
70° Included to arm (arm at side)

31
23

57
34

82
49

12
7

30
20

41
28

90° Vertical shoulder (abducted)

21

38

61

7

15

24

5° Vertical shoulder (at side)
90° Vertical shoulder (abducted)

17
32

24
68

38
89

10
9

14
30

21
44

90° Vertical shoulder (abducted)
90° Vertical shoulder (abducted)
90° Vertical shoulder (abducted)

32
26
32

49
49
52

76
85
75

14
10
11

24
22
27

42
40
42

90° Included to shank
120° Included to thigh (seated)
135° Included to thigh (seated)
100° Included to torso (seated)
110° Included to torso (seated)
100° Included to thigh (seated)
100° Included to thigh (seated)
Sitting erect

51
62
43
69
87
121
66
70

93
124
74
140
137
173
106
117

175
235
116
309
252
371
159
193

29
38
16
28
42
52
36
37

60
78
46
72
93
136
55
69

97
162
77
133
131
257
119
120

B. Static Muscle Strength Moment Data (N·m) for 25 Men and 22 Women Employed in Manual Jobs in Industry

Male (%ile)
Muscle Function
Elbow flexion
Elbow extension
Medial humeral
(shoulder) rotation
Lateral humeral
(shoulder) rotation
Shoulder horizontal flexion
Shoulder horizontal extension Shoulder vertical adduction
Shoulder vertical abduction
Ankle extension
(plantar flexion)
Knee extension
Knee flexion
Hip extension
Hip flexion
Torso extension
Torso flexion
Torso lateral flexion

Joint Angles

Female (%ile)

5

50

95

5

50

95

90° Included to arm (arm at side)
70° Included to arm (arm at side)

42
31

77
46

111
67

16
9

41
27

55
39

90° Vertical shoulder (abducted)

28

52

83

9

21

33

5° Vertical shoulder (at side)
90° Vertical shoulder (abducted)

23
44

33
92

51
119

13
12

19
40

28
60

90° Vertical shoulder (abducted)
90° Vertical shoulder (abducted)
90° Vertical shoulder (abducted)

43
35
43

67
67
71

103
115
101

19
13
15

33
30
37

57
54
57

90° Included to shank
120° Included to thigh (seated)
135° Included to thigh (seated)
100° Included to torso (seated)
110° Included to torso (seated)
100° Included to thigh (seated)
100° Included to thigh (seated)
Sitting erect

69
84
58
94
118
164
89
95

126
168
100
190
185
234
143
159

237
318
157
419
342
503
216
261

31
52
22
38
57
71
49
50

81
106
62
97
126
184
75
94

131
219
104
180
177
348
161
162

Source: Chaffin and Anderson, 1991. Reprinted by permission of John Wiley & Sons, Inc.

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EXAMPLE 4.1

Elbow Flexion Torque
Consider the free-body diagram of the upper limb with the elbow at 90° in Figure 4.7.
There are three muscles involved in elbow flexion: biceps brachii, brachioradialis, and brachialis (see Figure 4.1). However, the biceps is the primary flexor, and for the purpose of this example, it is the only muscle depicted. It can be also considered to be an equivalent muscle combining the characteristics of all three muscles. (Note that a solution of all three muscles independently is not possible because of a condition termed static indeterminancy.) The equivalent muscle inserts approximately 2 in forward of the elbow point of rotation. The lower arm weighs approximately 3 lb for an average male, and the weight can be considered to act at the lower arm center of gravity, approximately 4 in (0.33 ft) forward of the elbow. The hand bolds an unknown load L at a distance of 11 in (0.92 ft) from the elbow. The maximum load that can be held is determined by maximum voluntary elbow flexion torque, which for a 50th percentile male is 57 ft·lb (see Table 4.1). In the static equilibrium position as shown in Figure 4.7, the 57 ft·lb counterclockwise torque is balanced by two clockwise torques, one for the weight of the lower limb and the other for the load:
57 ϭ 0.33 ϫ 3 ϩ 0.92 ϫ L
Solving the equation yields L ϭ 60.9 lb. Therefore the maximum load that an average male could lift through elbow flexion is approximately 61 lb.
It might be of interest to calculate how much force must be exerted by the equivalent muscle to lift this load. The maximum voluntary torque is produced by an unknown muscle force Fbiceps acting through a 2-in (0.167-ft) moment arm.
57 ϭ 0.167 ϫ Fbiceps
Then Fbiceps equals 57/0.167, or 342, lb which means that the muscle must exert close to 6 (342/61) times as much force as the load lifted. One can conclude from this that the human body is built not for strength, but for range of movement.

Fbiceps

2

2

7

3

Figure 4.7

L

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Table 4.2 Maximum Weights (in lb and kg) Acceptable to Average Males and Females Lifting Compact Boxes
[14 in (34 cm) wide] with Handles

1 lift per 0.5 min
Males
Task

Females

1 lift per 1 min
Males

Females

1 lift per 30 min
Males

Females

lb

Floor to knuckle height
Knuckle to shoulder height
Shoulder to arm reach

kg

lb

kg

lb

kg

lb

kg

lb

kg

lb

kg

42
42
37

19
19
17

26
20
18

12
9
8

66
55
51

30
25
23

31
29
24

14
13
11

84
64
59

38
29
27

37
33
29

17
15
13

Note: For lowering, increase values by 6%. For boxes without handles, decrease values by 15%. Increasing the box size (away from body) to 30 in (75 cm) decreases values by 16%.
Source: Adapted from Snook and Ciriello, 1991.

more efficient bonding in the slower sliding muscle filaments. Some representative isometric muscle strengths for various postures are given in Table 4.1, and representative lifting strengths for 551 industrial workers in different postures are shown in Figure 4.6.
Most industrial tasks typically involve some movement; therefore, completely isometric contractions are relatively rare. Most typically, the movement range is somewhat limited, and the dynamic contraction is not a true isokinetic contraction, but a set of quasi-static contractions. Thus, dynamic strengths are very much task- and condition-dependent, and little is published regarding dynamic strength data.
Finally, a third type of muscle strength capability, psychophysical strength, has been defined for those situations in which the strength demands are required for an extended time. A static strength capability is not necessarily representative of what would be repetitively possible over an 8-h shift. Typically, the maximum acceptable load (determined by adjusting the load lifted or force exerted until the subject feels that the load or force would be acceptable on a repetitive basis for the given time period) is 40 to 50 percent less than a one-time static exertion.
Extensive tables have been compiled for psychophysical strengths of various frequencies and postures (Snook and Ciriello, 1991). A summary of these values is provided in Tables 4.2, 4.3, and 4.4.

USE LARGE MUSCLES FOR TASKS
REQUIRING STRENGTH
Muscle strength is directly proportional to the size of the muscle, as defined by the cross-sectional area [specifically, 87 psi (60 N/cm2) for both males and females] (Ikai and Fukunaga, 1968). For example, leg and trunk muscles should

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Table 4.3 Push Forces (in lbs and kg) at Waist Height Acceptable to Average Males and Females
(I ϭ Initial, S ϭ Sustained)

1 lift/min

1 lift per 30 min

Males
Distance
Pushed, ft (m)
150 (45)
50 (15)
7 (2)

I

Females
S

I

Males
S

I

Females
S

I

S

lb

kg

lb

kg

lb

kg

lb

kg

lb

kg

lb

kg

lb

kg

lb kg

51
77
95

23
35
43

26
42
62

12
19
28

40
44
55

18
20
25

22
29
40

10
13
18

66
84
99

30
38
45

42
51
75

19
23
34

51
53
66

23
24
30

26 12
33 15
46 21

Note: For push forces at shoulder heights or knuckle/knee heights, decrease values by 11%.
Source: Adapted from Snook and Ciriello, 1991.

Table 4.4 Pull Forces (in lbs and kg) at Waist Height Acceptable to Average Males and Females
(I ϭ Initial, S ϭ Sustained)

1 pull/min

1 pull per 30 min

Males
Distance
Pulled, ft (m)
150 (45)
50 (15)
7 (2)

I

Females
S

I

Males
S

I

Females
S

I

S

lb

kg

lb

kg

lb

kg

lb

kg

lb

kg

lb

kg

lb

kg

lb kg

37
57
68

17
26
31

26
42
57

12
19
26

40
42
55

18
19
25

24
26
35

11
12
16

48
62
73

22
28
33

42
51
70

19
23
32

48
51
66

22
23
30

26 12
33 15
44 20

Note: For pull forces at knuckle/knee heights, increases values by 75%. For pull forces at shoulder heights, decreases values by 15%.
Source: Adapted from Snook and Ciriello, 1991

be used in heavy load lifting, rather than weaker arm muscles. The posture factor, although somewhat confounded by geometric changes in the muscle moment or lever arm, is related to the resting length of the muscle fibers being roughly midrange of motion for most joints as stated in the first principle of motion economy.

STAY BELOW 15 PERCENT OF MAXIMUM
VOLUNTARY FORCE
Muscle fatigue is a very important but little utilized criterion in designing tasks appropriately for the human operator. The human body and muscle tissue rely primarily on two types of energy sources, aerobic and anaerobic (see later section on
Manual Work). Since the anaerobic metabolism can supply energy for only a very small time, the oxygen supplied to the muscle fibers via peripheral blood flow becomes critical in determining how long the muscle contractions will last. Unfortunately, the harder the muscle fibers contract, the more the interlaced arterioles

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10
9
8
7
Endurance time (min)

144

6
5
4
3
2
1
0
0

20

40

60

80

100 %

Exertion level (% maximum muscle force)

Figure 4.8 Static muscle endurance–exertion level relationship with Ϯ1 SD ranges depicted. (From: Chaffin and Anderson, 1991). Reprinted by permission of John Wiley & Sons, Inc.

and capillaries are compressed (see Figure 4.2), and the more the blood flow and oxygen supplies are restricted, the faster the muscle fatigues. The result is the endurance curve in Figure 4.8. The relationship is very nonlinear, ranging from a very short endurance time of approximately 6 s at a maximal contraction, at which point the muscle force rapidly drops off, to a rather indefinite endurance time at approximately 15 percent of a maximal contraction.
This relationship can be modeled by
T ϭ 1.2/(f Ϫ 0.15)0.618 Ϫ 1.21 where T ϭ endurance time, min f ϭ required force, expressed as a fraction of maximum isometric strength
For example, a worker would be able to sustain a force level of 50 percent of maximum strength for only about 1 min:
T ϭ 1.2/(0.5 Ϫ 0.15)0.618 Ϫ 1.21 ϭ 1.09 min

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The indefinite asymptote is due to early researchers stopping their experimentation without reaching complete muscle fatigue. Later researchers suggested reducing this level of acceptable static force levels from 15 percent to below
10 percent, perhaps even 5 percent (Jonsson, 1978). The amount of rest needed to recover from a static hold will be presented as a set of relaxation allowances that depend on the force exerted and the holding time (see Chapter 11).

USE SHORT, FREQUENT, INTERMITTENT
WORK/REST CYCLES
Whether performing repeated static contractions (such as holding a load in elbow flexion) or a series of dynamic work elements (such as cranking with the arms or legs), work and recovery should be apportioned in short, frequent cycles. This is due primarily to a fast initial recovery period, which then tends to level off with increasing time. Thus, most of the benefit is gained in a relatively short period. A much higher percentage of maximum strength can be maintained if the strength is exerted as a series of repetitive contractions rather than one sustained static contraction (see Figure 4.9). However, if the person is driven to complete muscle (or whole body) fatigue, full recovery will take a fairly long time, perhaps several hours.

DESIGN TASKS SO THAT MOST
WORKERS CAN DO THEM
As can be seen in Figure 4.6, for a given muscle group, there is a considerable range of strength in the normal, healthy adult population, with the strongest being five to eight times stronger than the weakest. These large ranges are due to
Figure 4.9 Percentage of maximum isometric strength that can be maintained in a steady state during rhythmic contractions. 90

Percent of maximum strength

85
80

Points are averages for finger muscles, hand muscles, arm muscles, and leg muscles, combined. Vertical lines denote Ϯ standard error.
(From: Åstrand and
Rodahl, 1986.)

75
70
65
60

0

5

10

15

Contractions/min

20

25

30

145

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individual factors that affect strength performance: gender, age, handedness, and fitness/training. Gender accounts for the largest variation in muscle strength, with average female strength ranging from 35 to 85 percent of average male strength, with an average effect of 66 percent (see Figure 4.10). The difference is greatest for upper extremity strengths and smallest for lower extremity strengths. However, this effect is primarily due to average body size (i.e., total muscle mass) and not strictly to gender; the average female is considerably smaller and lighter than the average male. Furthermore, with the wide distribution for a given muscle strength, there are many females who are stronger than many males.
In terms of age, muscle strength appears to peak in the mid-20s and then decreases linearly by 20 to 25 percent by the mid-60s (see Figure 4.10). The decrease in strength is due to reduced muscle mass and a loss of muscle fibers.
However, whether this loss is due to physiological changes of aging or just a gradual reduction in activity levels is not well known. It has definitely been shown that by starting a strength training program, a person can increase strength by 30 percent in the first several weeks, with maximum increases approaching
100 percent (Åstrand and Rodahl, 1986). In terms of handedness, the nondominant hand typically produces about 90 percent of the dominant hand’s grip strength, with the effect being less pronounced in lefties, probably because they have been forced to adapt to a right-handed world (Miller and Freivalds, 1987).
In any case, it is best to design tools and machines such that they can be used by either hand, to avoid placing any individual at a strength disadvantage.

USE LOW FORCE FOR PRECISE MOVEMENTS
OR FINE MOTOR CONTROL
Muscle contractions are initiated by neural innervation from the brain and spinal cord, which together comprise the central nervous system. A typical motor

100

Percent variation in muscle strength

146

80
60
40
20
0
0

10

20

30
40
Age years

50

60

70

Figure 4.10 Changes in maximal isometric strength with age in women and men.

(From: Åstrand and Rodahl, 1986.)

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neuron, or nerve cell leading to the muscle from the central nervous system, may innervate or have connections with several hundred muscle fibers. The innervation ratio of the number of fibers per neuron ranges from less than 10 in the small muscles of the eye to over 1,000 for the large calf muscles, and can vary considerably even within the same muscle. Such functional arrangement is called a motor unit and has important implications in movement control. Once a neuron is stimulated, the electrical potential is transferred simultaneously to all the muscle fibers innervated by that neuron, and the motor unit acts as one contractile or motor control unit. Also, the central nervous system tends to recruit these motor units selectively by increasing size as higher muscle forces are needed (Figure 4.11). The initial motor units recruited are small in size, with few muscle fibers and low produced forces. However, since these are small and low in tension, the change in force production from one to two or more motor units recruited is very gradual, and very fine precision in motor control can be produced. Near the end of motor recruitment, the total muscle force is high, and each additional motor unit recruited becomes a large increment in force, with little sensitivity in terms of precision or control. This muscle property is sometimes termed the size principle.
The electrical activity of muscles, termed electromyograms (EMGs), is a useful measure of local muscle activity. Such activity is measured by placing recording electrodes on the skin surface over the muscles of interest, then modifying and processing to the amplitude and frequency of the signal. For amplitude analysis, the signal is typically rectified and smoothed (with a resistor-capacitor circuit). The result has a reasonably linear relationship to the muscle force exerted (Bouisset, 1973). The frequency approach involves digitizing the signal and performing a fast Fourier transform analysis to yield a frequency spectrum. As the muscle begins to fatigue, muscle activity shifts from high frequency (Ͼ60 Hz) to lower frequencies (Ͻ60 Hz) (Chaffin, 1969). Also, the EMG amplitude tends to increase with fatigue, for a given level of exertion.
100

Force (% maximum)

90
80
70
60
50
40
30
20
10
0
0

25

50

75

Motor units recruited (% of maximum)

Figure 4.11 Muscle recruitment demonstrating size principle.

100

147

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DO NOT ATTEMPT PRECISE MOVEMENTS OR FINE
CONTROL IMMEDIATELY AFTER HEAVY WORK
This is a corollary to the previous principle of human capability. The small motor units tend to be used continually during normal motion, and although more resistant to fatigue than the large motor units, they can still experience fatigue.
A typical example where this principle is violated occurs when operators load their workstations before their shift or replenish parts during a shift. Lifting heavy parts containers requires the recruitment of the small motor units, as well as the later larger motor units, to generate the necessary muscle forces. During lifting and restocking, some of the motor units will fatigue and others will be recruited to compensate for the fatigued ones. Once the operator has restocked the bins and returned to more precise assembly work, some of the motor units, including the smaller precision ones, will not be available for use. The larger motor units recruited to replace the fatigued ones will provide larger increments in force and less-precise motor control. After several minutes, the motor units will have recovered and will be available, but in the meantime, the quality and speed of the assembly work may suffer. One solution would be to provide lessskilled laborers to restock the bins on a regular basis.

USE BALLISTIC MOVEMENTS FOR SPEED
Through spinal reflexes, cross innervation of agonists and antagonists always occurs. This minimizes any unnecessary conflict between the muscles as well as the consequent excess energy expenditure. Typically, in a short (less than 200 ms), gross, voluntary motion, the agonist is activated and the antagonist is inhibited
(termed reciprocal inhibition), to reduce counterproductive muscle contractions.
On the other hand, for precise movements, feedback control from both sets of muscles is utilized, increasing motion time. This is sometimes referred to as the speed–accuracy trade-off.

BEGIN AND END MOTIONS WITH BOTH
HANDS SIMULTANEOUSLY
When the right hand is working in the normal area to the right of the body and the left hand is working in the normal area to the left of the body, a feeling of balance tends to induce a rhythm in the operator’s performance, which leads to maximum productivity. The left hand, in right-handed people, can be just as effective as the right hand, and it should be used. A right-handed boxer learns to jab just as effectively with the left hand as with the right hand. A speed typist is just as proficient with one hand as the other. In a large number of instances, workstations can be designed to do “two at a time.” Using dual fixtures to hold two components, both hands can work at the same time, making symmetric moves in opposite directions. A corollary to this principle is that both hands should not be idle at the same time, except during rest periods. (This principle was the one followed by
Frank Gilbreth in shaving with both hands simultaneously.)

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MOVE THE HANDS SYMMETRICALLY AND
SIMULTANEOUSLY TO AND FROM THE CENTER
OF THE BODY
It is natural for the hands to move in symmetric patterns. Deviations from symmetry in a two-handed workstation result in slow, awkward movements of the operator. The difficulty of patting the stomach with the left hand while rubbing the top of the head with the right hand is familiar to many. Another experiment that can readily illustrate the difficulty of performing nonsymmetric operations is to try to draw a circle with the left hand while trying to draw a square with the right hand. Figure 4.12 illustrates an ideal workstation that allows the operator to assemble a product by going through a series of symmetric, simultaneous motions away from and toward the center of the body.

USE THE NATURAL RHYTHMS OF THE BODY
The spinal reflexes that excite or inhibit muscles also lead to natural rhythms in the motion of body segments. These can be logically compared to second-order mass–spring–dashpot systems, with the body segments providing mass and the muscle having internal resistance and damping. The natural frequency of the system will depend on all three parameters, but the segment mass will have the greatest effect. This natural frequency is essential to the smooth and automatic performance of a task. Drillis (1963) has studied a variety of very common manual tasks and has suggested optimum work tempos, as follows:
Filing metal
Chiseling
Arm cranking
Leg cranking
Shoveling

60–78 strokes per minute
60 strokes per minute
35 rpm
60–72 rpm
14–17 tosses per minute
Figure 4.12 An ideal workstation that permits the operator to assemble a product by going through a series of symmetric motions made simultaneously away from and toward the center of the body.

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USE CONTINUOUS CURVED MOTIONS
Because of the nature of body segment linkages (typically approximating pin joints), it is easier for the human to produce curved motions, that is, to pivot around a joint. Straight-line motions involving sudden and sharp changes in direction require more time and are less accurate. This law is very easily demonstrated by moving either hand in a rectangular pattern and then moving it in a circular pattern of about the same magnitude. The greater amount of time required to make the abrupt 90° directional changes is quite apparent. To make a directional change, the hand must decelerate, change direction, and accelerate until it is time to decelerate again for the next directional change. Continuous curved motions do not require deceleration and are consequently performed faster per unit of distance. This is demonstrated very nicely in Figure 4.13, with subjects who made positioning movements with the right hand in eight directions in a horizontal plane from a center starting point. Motion from the lower left to the upper right (pivoting about the elbow) required 20 percent less time than the perpendicular motion from the lower right to upper left (additional awkward shoulder and arm line movements).

USE THE LOWEST PRACTICAL
CLASSIFICATION OF MOVEMENT
Understanding the classifications of motions plays a major role in using this fundamental law of motion economy appropriately in methods studies. The classifications are as follows:
1. Finger motions are made by moving the finger or fingers while the remainder of the arm is kept stationary. They are first-class motions and
90˚
145˚

135˚

180˚

Concentric circles represent equal time intervals.

Figure 4.13 Forearm motion is best while pivoting on elbow.

55˚
45˚

(Source: Adapted from Schmidtke and
Stier 1960.)



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2.

3.

4.

5.

Manual Work Design

the fastest of the five motion classes. Typical finger motions are running a nut down on a stud, depressing the keys of a typewriter, or grasping a small part. There is usually a significant difference in the time required to perform finger motions with the various fingers with the index finger being the fastest. Because repetitive finger motions can result in cumulative trauma disorders (see Chapter 5), finger forces should be kept low by using bar switches in place of trigger switches.
Finger and wrist motions are made while the forearm and upper arm are stationary and are referred to as second-class motions. In the majority of cases, finger and wrist motions consume more time than strictly finger motions. Typical finger and wrist motions occur when a part is positioned in a jig or fixture, or when two mating parts are assembled.
Finger, wrist, and lower arm motions are commonly referred to as forearm or third-class motions and include those movements made by the arm below the elbow while the upper arm is stationary. Since the forearm includes relatively strong and nonfatiguing muscles, workstations should be designed to utilize these third-class motions, rather than fourth-class motions. However, repetitive work involving force with the arms extended can induce injury and the workstation should be designed so that the elbows can be kept at 90° while work is being done.
Finger, wrist, lower arm, and upper arm motions, commonly known as fourth-class or shoulder motions, require considerably more time for a given distance than the three classes previously described. Fourth-class motions are required to perform transport motions for parts that cannot be reached without extending the arm. To reduce static loading of shoulder motions, tools should be designed so that the elbow is not elevated while the work is being performed.
Fifth-class motions include body motions such as of the trunk, which are the most time-consuming and should generally be avoided.

First-class motions require the least amount of effort and time, while fifthclass motions are considered the least efficient. Therefore, always utilize the lowest practicable motion classification to perform the work properly. This will involve careful consideration of the location of tools and materials, so that ideal motion patterns can be arranged.
This classification of movement was shown experimentally by Langolf et al.
(1976), in a series of positioning movements to and from targets, known as Fitts’ tapping task (Fitts, 1954), discussed in greater detail in Chapter 7. The movement time increases with the difficulty of the task (see Figure 4.14), but also increases with higher levels of classification; that is, the slope for the arm (105 ms) is steeper than for the wrist (45 ms), which in turn is steeper than for the finger
(26 ms). The effect is due simply to the added time required for the central nervous system to process additional joints, motor units, and receptors.

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Figure 4.14 Classifications of movements 800

(Source: Data from Langolf et al., 1976. Reproduced with permission of the McGraw-Hill
Companies.)

ϭ Arm ϭ Wrist ϭ Finger

600
Movement time (msec)

152

400

200

0
0

2

4
6
Log2 (2D/W)
(Difficulty of task)

8

WORK WITH BOTH HANDS AND
FEET SIMULTANEOUSLY
Since the major part of work cycles is performed by the hands, it is economical to relieve the hands of work that can be done by the feet, but only if this work is performed while the hands are occupied. Since the hands are more skillful than the feet, it would be foolish to have the feet perform elements while the hands are idle. Foot pedal devices that allow clamping, parts ejection, or feeding can often be arranged, freeing the hands for other, more useful work and consequently reducing the cycle time (see Figure 4.15). When the hands are moving, the feet should not be moving, since the simultaneous movement of the hands and feet is difficult. However, the feet can be applying pressure to something, such as a foot pedal. Also, the operator should be seated, as it is difficult to operate a foot pedal while standing, which would mean maintaining the full body weight on the other foot.

MINIMIZE EYE FIXATIONS
Although eye fixations or eye movements cannot be eliminated for most work, the location of the primary visual targets should be optimized with respect to the human operator. The normal line of sight is roughly about 15° below the horizontal (see Figure 5.5), and the primary visual field is roughly defined as a cone
Ϯ15° in arc centered on the line of sight. The implication is that within this area, no head movements are needed and eye fatigue is minimized.

SUMMARY
The principles of human capabilities and motion economy are based on an elementary understanding of human physiology and should be very useful in

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Figure 4.15 Foot-operated machine tool.

(Courtesy of Okuma.)

applying methods analysis with the human operator in mind. However, the analyst need not be an expert in human anatomy and physiology to be able to apply these principles. In fact, for most task analysis purposes, it may be sufficient to use the Motion Economy Checklist, which summarizes most of the principles in a questionnaire format (see Figure 4.16).

4.3

MOTION STUDY

Motion study is the careful analysis of body motions employed in doing a job.
The purpose of motion study is to eliminate or reduce ineffective movements, and facilitate and speed effective movements. Through motion study, in conjunction with the principles of motion economy, the job is redesigned to be more effective and to produce a higher rate of output. The Gilbreths pioneered the study of manual motion and developed basic laws of motion economy that are still considered fundamental. They were also responsible for the development of detailed motion picture studies, known as micromotion studies, which have proved invaluable in studying highly repetitive manual operations. Motion study, in the

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Figure 4.16 Motion Economy Checklist.

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broad sense, covers both studies that are performed as a simple visual analysis and studies that utilize more expensive equipment. Traditionally, motion picture film cameras were utilized, but today the videotape camera is used exclusively, because of the easy ability to rewind and replay sections, the freeze-frame capability in four-head videotape cassette recorders (VCRs), and the elimination of the need for film development. In view of its much higher cost, micromotion is usually practical only on extremely active jobs with high repetitiveness.
The two types of studies may be compared to viewing a part under a magnifying glass versus viewing it under a microscope. The added detail revealed by the microscope is needed only on the most productive jobs. Traditionally, micromotion studies were recorded on a simultaneous motion (simo) chart, while motion studies are recorded on a two-hand process chart. A true simo chart is hardly used today, but the term is sometimes applied to a two-hand process chart.

BASIC MOTIONS
As part of motion analysis, the Gilbreths concluded that all work, whether productive or nonproductive, is done by using combinations of 17 basic motions that they called therbligs (Gilbreth spelled backward). The therbligs can be either effective or ineffective. Effective therbligs directly advance the progress of the work. They can frequently be shortened, but typically cannot be completely eliminated. Ineffective therbligs do not advance the progress of the work and should be eliminated by applying the principles of motion economy. The 17 therbligs, along with their symbols and definitions, are shown in Table 4.5.

THE TWO-HAND PROCESS CHART
The two-hand process chart, sometimes referred to as an operator process chart, is a motion study tool. This chart shows all movements and delays made by the right and left hands, and the relationships between them. The purpose of the twohand process chart is to identify inefficient motion patterns and observe violations of the principles of motion economy. This chart facilitates changing a method so that a balanced two-handed operation can be achieved and a smoother, more rhythmic cycle that keeps both delays and operator fatigue to a minimum.
As usual, the analyst heads the chart Two-Hand Process Chart and adds all necessary identifying information, including the part number, drawing number, operation or process description, present or proposed method, date, and name of the person doing the charting. Immediately below the identifying information, the analyst sketches the workstation, drawn to scale. The sketch materially aids in presenting the method under study. Figure 4.17 shows a typical two-hand process chart for a cable-clamp assembly, with the times for each therblig obtained from stopwatch timing.
Next the analyst begins constructing the two-hand process chart by observing the duration of each element and determining the amount of time to be represented on the chart drawn to scale. For example, in Figure 4.17, the first element, Get U-bolt, has a time of 1.00 min and one large or five small vertical

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Table 4.5 Gilbreths’ Therbligs

Effective Therbligs
(Directly advance progress of work. May be shortened but difficult to eliminate completely.)
Therblig

Symbol

Description

Reach

RE

Move

M

Grasp

G

Release

RL

Preposition

PP

Use

U

Assemble

A

Disassemble

DA

Motion of empty hand to or from object; time depends on distance moved; usually preceded by Release and followed by Grasp.
Movement of loaded hand; time depends on distance, weight, and type of move; usually preceded by Grasp and followed by Release or Position.
Closing fingers around an object; begins as the fingers contact the object and ends when control has been gained; depends on type of grasp; usually preceded by
Reach and followed by Move.
Relinquishing control of object, typically the shortest of the therbligs.
Positioning object in predetermined location for later use; usually occurs in conjunction with Move, as in orienting a pen for writing.
Manipulating tool for intended use; easily detected, as it advances the progress of work.
Bringing two mating parts together; usually preceded by Position or Move; followed by Release.
Opposite of Assemble, separating mating parts; usually preceded by Grasp and followed by Move or Release.

Ineffective Therbligs
(Do not advance progress of work. Should be eliminated if possible.)
Therblig

Description

Search

S

Select

SE

Position

P

Inspect

I

Plan

PL

Unavoidable
Delay

UD

Avoidable Delay
Rest to Overcome
Fatigue
Hold
148

Symbol

AD
R

Eyes or hands groping for object; begins as the eyes move in to locate an object.
Choosing one item from several; usually follows
Search.
Orienting object during work, usually preceded by
Move and followed by Release (as opposed to during for Preposition).
Comparing object with standard, typically with sight, but could also be with the other senses.
Pausing to determine next action; usually detected as a hesitation preceding Motion.
Beyond the operator’s control due to the nature of the operation, e.g., left hand waiting while right hand completes a longer Reach.
Operator solely responsible for idle time, e.g., coughing.
Appears periodically, not every cycle, depends on the physical workload.
One hand supports object while other does useful work.

H

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Two-hand process chart

Page 1 of 1

Operation: Assemble cable clamps
Operator name and no.: J.B. #1157
Analyst: G. Thuering
Method (circle choice) Present
Proposed
Sketch:
U bolt

Summary
Effective time:

Date: 6-11-98

2.7
11.6
14.30 sec.

Ineffective time:
Cycle time =

Nuts

Left hand Right hand

11.6
2.7

Clamp Note: Gravity feed chutes for assembly parts 14

14"

"

Assembled units Part: SK-112

Man

Left hand description

Sym- Time bol Right hand description

1.20

Get cable clamp (10")

1.20

M
P
RL

Place cable clamp (10")

RE
G

Get first nut (9")

M
P

Place first nut (9")

3.40

M
P

RE
G

1.20

Place U-bolt (10")

1.00

1.00

1.00

Get U-bolt (10")

RE
G

Time Symbol

U

Run down first nut

RL

Hold U-bolt

H

11.00
1.00

RE
G

Get second nut (9")

1.20

M
P

Place second nut (9")

3.40

U

Run down second nut

RL

Dispose of assembly

M
RL

1.10

0.90

UD

Wait

Figure 4.17

Two-hand process chart for assembly of cable clamps.

spaces are marked. Under the “Symbols” column RE (for reach) is written, indicating that an effective motion has been accomplished. Note also that a grasp (G) is involved, but is not measured separately, since it is not possible in most instances to time individual therbligs. Next, the analyst charts “Place U-bolt” and continuing on with the left hand. Usually it is less confusing to chart the activities of one hand completely before examining the other hand.

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After the activities of both the right and the left hand have been charted, the analyst creates a summary at the bottom of the sheet, indicating the cycle time, pieces per cycle, and time per piece. Once the two-hand process chart has been completed for an existing method, the analyst can determine what improvements can be introduced. Several important corollaries to the principles of motion economy should be applied at this point:
1. Establish the best sequences of therbligs.
2. Investigate any substantial variation in the time required for a given therblig and determine the cause.
3. Examine and analyze hesitations, to determine and then eliminate their causes. 4. As a goal, aim for cycles and portions of cycles completed in the least amount of time. Study deviations from these minimum times to determine the causes.
In the example, the “delays” and “holds” are good places to begin. For example, in Figure 4.17, the left hand acted as a holding device for almost the entire cycle. This would suggest the development of a fixture to hold the U-bolt.
Further considerations to achieve balanced motions of both hands would suggest that when the fixture holds the U-bolts, the left hand and the right hand can be used simultaneously so that each completely assembles a cable clamp. Additional study of this chart might result in the introduction of an automatic ejector and gravity chute, to eliminate the final cycle element “dispose of assembly.” The use of the Therblig Analysis Checklist (see Figure 4.18) may also be helpful in this analysis. 4.4

MANUAL WORK AND DESIGN GUIDELINES

Although automation has significantly reduced the demands for human power in the modern industrial environment, muscular strength still remains an essential part of many occupations, particularly those involving manual materials handling
(MMH) or manual work. In these activities, overexertion from moving heavy loads can highly stress the musculoskeletal system, resulting in nearly one-third of all occupational injuries. The low back alone accounts for almost one-quarter of these injuries and one-quarter of the annual workers’ compensation costs
(National Safety Council, 2003). Back injuries are especially detrimental because they often result in permanent disorders, with considerable discomfort and limitations for the employee as well as a large expense for the employer (an average case involving surgery may exceed $60,000 in direct costs).

ENERGY EXPENDITURE AND WORKLOAD GUIDELINES
Energy is required for the muscle contraction process. The molecule called ATP
(adenosine triphosphate) is the immediate energy source, which physically interacts with the protein cross bridging as one of ATP’s high-energy phosphate bonds

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Figure 4.18 Therblig Analysis Checklist.

is broken. This source is very limited, lasting only several seconds, and the
ATP must immediately be replenished from another molecule termed CP
(creatine phosphate). The CP source is also limited, less than 1 min of duration
(see Figure 4.19), and must ultimately be regenerated from the metabolism of the basic foods we eat: carbohydrates, fats, and proteins. This metabolism can occur

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Figure 4.18 (continued )

in two different modes: aerobic, requiring oxygen, and anaerobic, not using oxygen. Aerobic metabolism is much more efficient, generating 38 ATPs for each glucose molecule (basic unit of carbohydrates), but it is relatively slow. Anaerobic metabolism is very inefficient, producing only 2 ATPs for each glucose molecule, but it is much quicker. Also, the glucose molecule is only partially broken down into two lactate molecules, which in the watery environment of the body

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%
2 (700)
CP
Energy supply

Aerobic glycolysis

50

1 (350)
ATP

0

Anaerobic glycolysis
(lactic acid formation)

1

2
Time (minutes)

3

O2 intake liters/min (watts)

100

4

Figure 4.19 Sources of energy during the first few minutes of moderately heavy work.

High-energy phosphate stores (ATP and CP) provide most of the energy during the first seconds of work. Anaerobic glycolysis supplies less and less of the energy required as the duration of work increases, and aerobic metabolism takes over.
(Source: Jones, Moran-Campbell, Edwards and Robertson, 1975.)

forms lactic acid, a direct correlate of fatigue. Thus, during the first few minutes of heavy work, the ATP and CP energy sources are used up very quickly, and anaerobic metabolism must be utilized to regenerate the ATP stores. Eventually, as the worker reaches steady state, the aerobic metabolism catches up and maintains the energy output, as the anaerobic metabolism slows down. By warming up and starting heavy work slowly, the worker can minimize the amount of anaerobic metabolism and the concurrent buildup of lactic acid associated with feelings of fatigue. This delay of full aerobic metabolism is termed oxygen deficit and must eventually be repaid by the oxygen debt of a cooling down period, which is always larger than the oxygen deficit.
The energy expended on a task can be estimated by assuming that most of the energy is produced through aerobic metabolism and measuring the amount of oxygen consumed by the worker. The amount of inspired air is measured with a flowmeter and assumed to contain 21 percent oxygen. However, not all this oxygen is utilized by the body; therefore, the expired oxygen must also be measured.
Typically, the volume of air inspired and expired is the same, and only the percentage of expired oxygen must be found, using an oxygen meter. A conversion factor is included for a typical diet in which 4.9 kcal (19.6 Btu) of energy is produced for each liter of oxygen used in metabolism.
.
E (kcal/min) ϭ 4.9 * V(0.21 Ϫ EO2 ) where E ϭ energy expenditure, kcal/min
.
V ϭ volume of air inspired, L/min
EO2 ϭ fraction of oxygen (O2) in expired air (roughly 0.17)

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The energy expended on a task varies by the type of task being performed, the posture maintained during the task, and the type of load carriage. Energy expenditure data on several hundred different types of tasks have been collected, with the most common summarized in Figure 4.20. Alternatively, one may also estimate energy expenditure by using Garg’s (1978) metabolic prediction model. For manual materials handling, the manner in which the load is carried is most critical, with lowest energy costs for balanced loads held closest to the center of gravity of the body, which has the largest muscle groups. For example, a backpack supported by the trunk muscles is less demanding than holding an equal weight in two suitcases, one in each arm. Although balanced, the latter situation places the load far from the center of gravity and on the smaller arm muscles. Posture also plays an important role, with the least amount of energy expenditure for supported postures. Thus, a posture with the trunk bent over, with no arm support, will expend 20% more energy than a standing posture.
A 5.33 kcal/min (21.3 Btu/min) limit for acceptable energy expenditure for an 8-h workday has been proposed by Bink (1962). This number corresponds to one-third the maximum energy expenditure of the average U.S. male [for females, it would be 1/3 ϫ 12 ϭ 4 kcal/min (16 Btu/min)]. If the overall workload is exceedingly high (i.e., exceeds the recommended limits), aerobic metabolism may

1.6
2.7

2.2

3.0

4.0

6.8

7.7

115 lb

2.5 mph

4.2

5.0

22 lb 27

ft/

mi

16lb

8.0

8.5

n

34

17 lb

9.0

ft/

mi

10.2

n

16.2

Figure 4.20 Examples of energy costs of various types of human activity. Energy costs are given in kilocalories per minute.

(Source: Passmore and Dumin, 1955, as adapted and presented by Gordon, 1957.)

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not be sufficient to provide all the energy requirements, and the worker may rely on greater amounts of anaerobic metabolism, resulting in fatigue and the buildup of lactic acid. Sufficient recovery must then be provided to allow the body to recover from fatigue and recycle the lactic acid. One guideline for rest allocation was developed by Murrell (1965):
R ϭ (W Ϫ 5.33)/(W Ϫ 1.33) where R ϭ time required for rest, as percent of total time
W ϭ average energy expenditure during work, kcal/min

The value of 1.33 kcal/min (5.3 Btu/min) is the energy expenditure during rest.
Consider a strenuous task of shoveling coal into a hopper, which has an energy expenditure of 9.33 kcal/min (37.3 Btu/min). Entering W ϭ 9.33 into the equation yields R ϭ 0.5. Therefore, to provide adequate time for recovery from fatigue, the worker would need to spend roughly one-half of an 8-h shift, or 4-h, resting.
The manner in which rest is also allocated is important. It serves no purpose to have the laborer work for 4-h straight at a rate of 9.33 kcal/min (37.3 Btu/min), suffer from extreme fatigue, and then rest for 4 h. In general, the duration of the work cycle is the primary determinant of fatigue buildup. With heavy work, blood flow tends to be occluded, further accelerating the use of anaerobic pathways. In addition, the recovery process tends to be exponential, with later times providing minimal incremental benefits. Therefore, short bursts (approximately
1/2 to 1 min) of heavy work interspersed with short rest periods provide maximum benefit. During the 1/2 to 1 min periods, the immediate energy sources of
ATP and CP get used up, but they can also be quickly replenished. Once lactic acid builds up during longer work periods, it becomes more difficult to remove.
Micropauses of 1 to 3s are also useful for flushing any occluded blood vessels, and active breaks, during which the worker alternates hands or uses other muscles, serve to relieve the fatigued muscles. Also, it is best for workers to decide when to take the breaks, whenever they feel the need for rest (self-paced), as opposed to prescribed (or machine-paced) breaks. In summary, the use of frequent, short work/rest cycles is highly recommended.

HEART RATE GUIDELINES
Unfortunately, the measurement of oxygen consumption and the computation of energy expenditure are both costly and cumbersome in an industrial work situation. The equipment costs several thousand dollars and interferes with the worker performing the job. An alternative indirect measure of energy expenditure is the heart rate level. Since the heart pumps the blood carrying oxygen to the working muscles, the higher the required energy expenditure, the higher the corresponding heart rate (Figure 4.21). The instrumentation needed to measure heart rate is inexpensive (less than $100 for a visual readout, several hundred dollars for a
PC interface) and relatively nonintrusive (worn regularly by athletes to monitor performance). On the other hand, the analyst must be careful, since heart rate

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Heart rate (beats/min)
200

150

100

50

0
0

5
10
Energy expenditure (kcal/min)

15

Figure 4.21 Linear increase in heart rate with physical workload, as measured by energy expenditure.

measurement is most appropriate for dynamic work involving the large muscles of the body at fairly high levels (40 percent of maximum) and can vary considerably between individuals, depending on their fitness levels and age. In addition, heart rate can be confounded by other stressors including heat, humidity, emotional levels, and mental stress. Limiting these external influences will result in a better estimate of physical workload. However, if the desired goal is to obtain the overall stress on the worker on the job, this may not be necessary.
A methodology for interpreting heart rate has been proposed by various
German researchers (cited in Grandjean, 1988). The average working heart rate is compared to the resting prework heart rate, with 40 beats/min being proposed as an acceptable increase. This increase corresponds very nicely with the recommended working energy expenditure limits. The average increase in heart rate per increase in energy expenditure for dynamic work (i.e., the slope in Figure 4.21) is 10 beats/min per 1 kcal/min. Thus, a 5.33 kcal/min workload (4 kcal/min above the resting level of 1.33 kcal/min) produces a 40 beat/min increase in heart rate, which is the limit for an acceptable workload. This value also corresponds closely to the heart rate recovery index presented by Brouha (1967).
The average heart rate is measured in two time periods (cross hatched areas) during recovery after the cessation of work (see Figure 4.22): (1) between 1/2 and 1 min after cessation and (2) between 21/2 and 3 min after cessation. Acceptable heart rate recovery (and therefore acceptable workload) occurs if the first reading does not exceed 110 beats/min and the difference between the two readings is at least 20 beats. Given a typical resting heart rate of 72 beats/min, the addition of the acceptable increment of 40 beats/min yields a working heart rate of
112 beats/min, which corresponds closely to Brouha’s first criterion.
As a final note on heart rate, it is very important to observe the course of heart rate during the working hours. An increase in heart rate during steady-state work
(see upper curve in Figure 4.22), termed heart rate creep, indicates an increasing

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Work rate
8 kcal/min

140
130
120
110
100
90
80
70
60

Work rate
4 kcal/min
Resting
pulse

0
0

6

2
Rest

Recovery pulse

Working pulse

Work

7

8 9 10
Recovery

min

Figure 4.22 Heart rate for two different workloads.

The work rate of 8 kcal/min exhibits heart rate creep. The two marked time periods are used in Brouha’s criteria.

buildup of fatigue and insufficient recovery during rest pauses (Brouha, 1967).
This fatigue most likely results from the physical workload, but could also result from heat and mental stress, and a greater proportion of static rather than dynamic work. In any case, heart rate creep should be avoided by providing additional rest.

SUBJECTIVE RATINGS OF PERCEIVED EXERTION
An even simpler approach to estimating workload and the stress on the worker is the use of subjective ratings of perceived exertion. These can replace the expensive and relatively cumbersome equipment required for physiological measurements with the simplicity of verbal ratings. Borg (1967) developed the most popular scale for assessing the perceived exertion during dynamic whole-body activities—the Borg Rating of Perceived Exertion (RPE) scale. The scale is constructed such that the ratings 6 through 20 correspond directly to the heart rate
(divided by 10) expected for that level of exertion (Table 4.6). Verbal anchors are provided to assist the worker in performing the ratings. Therefore, to ensure an acceptable heart rate recovery, based on the previous heart rate guidelines, the
Borg scale should probably not exceed a rating of 11.
Note that the ratings, being subjective, can be affected by previous experience and the individual’s level of motivation. Therefore, the ratings should be used with caution and should perhaps be normalized to each individual’s maximum rating.

LOW BACK COMPRESSIVE FORCES
The adult human spine, or vertebral column, is an S-shaped assembly of 25 separate bones (vertebrae) divided into four major regions: 7 cervical vertebrae in the neck, 12 thoracic vertebrae in the upper back, 5 lumbar vertebrae in the low back, and the sacrum in the pelvic area (Figure 4.23). The bones have a roughly cylindrical body, with several bony processes emanating from the rear, which

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Table 4.6

Borg’s (1967) RPE Scale with Verbal Anchors

Rating
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

Verbal Anchor
No exertion at all
Extremely light
Very light
Light
Somewhat hard
Hard
Very hard
Extremely hard
Maximal exertion

Cervical region

Thoracic region

Lumbar area

Sacrum

40˚

Figure 4.23 Anatomy of the human spine.

(From: Rowe, 1983.)

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4

1
2
5

3

(a)

(b)

(c)

Figure 4.24 Anatomy of a vertebra and the process of disk degeneration.

(a) Normal state: (1) body of vertebra; (2) spinous process, serves as muscle attachment point; (3) intervertebral disk; (4) spinal cord; (5) nerve root. (b) Narrowing of the disk space, allowing the nerve root to be pinched. (c) Herniated disk, allowing the gel material to extrude and impinge upon the nerve root. (Adapted from Rowe, 1983.)

serve as attachments for the back muscles, the erector spinae. Through the center of each vertebra is an opening that contains and protects the spinal cord as it travels from the brain to the end of the vertebral column (Figure 4.24). At various points along the way, spinal nerve roots separate from the spinal cord and pass between the vertebral bones out to the extremities, heart, organs, and other parts of the body.
The vertebral bones are separated by softer tissue, the intervertebral disks.
These serve as joints, allowing a large range of motion in the spine, although most trunk flexion occurs in the two lowest joints, the one on the border between the lowest lumbar vertebra and the sacrum (termed the L5/S1 disk, where the numbering of vertebrae is top down by region), and the next one up (L4/L5 disk).
The disks also act as cushions between the vertebral bones, and along with the
S-shaped spine, they help to protect the head and brain from the jarring impacts of walking, running, and jumping. The disks are composed of a gellike center surrounded by onionlike layers of fibers, separated from the bone by a cartilage end plate. Considerable movement of fluid occurs between the gel center and the

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surrounding tissue, depending on the pressure on the disk. Consequently, the length of the vertebral column (measured by change in overall stature) can change by as much as 1/2 to 1 in (1.3 to 2.5 cm) over the course of a workday and is sometimes used as an independent measure of an individual’s physical workload. (Interestingly, astronauts in space, removed from the effects of gravity, can be as much as 2 in taller.)
Unfortunately, due to the combined effects of aging and heavy manual work exposure (these effects are hard to separate), the disks can weaken with time.
Some of the enclosing fiber can become frayed, or the cartilage end plate can suffer microfractures, releasing some of the gelatinous material, reducing inner pressures, and allowing the center to start drying up. Correspondingly, the disk space narrows, allowing the vertebral bones to come closer together and eventually even touch, causing irritation and pain. Even worse, the nerve roots are impinged upon, leading to pain and sensory and motor impairments. As the fibers lose integrity, the vertebral bones may shift, causing uneven pressure on the disks and even more pain. In more catastrophic cases, termed disk herniation, or more commonly, a slipped disk, the fiber casings can actually rupture, allowing large amounts of the gel substance to extrude and impinge upon the nerve roots even more (Figure 4.24c).
The causes for low-back problems are not always easy to identify. As with most occupational diseases, both job and individual factors are at play. The latter may include a genetic predisposition toward weaker connective tissues, disks, and ligaments, and personal lifestyle conditions, such as smoking and obesity, over which the industrial engineer has very little control. Changes can only be made with the job factors. Although epidemiological data are easily confounded with survivor population effects or individual compensatory mechanisms, it can be shown statistically that heavy work leads to an increase in low back problems.
Heavy work includes more than just the frequent lifting of large loads; it also encompasses the static maintenance of forward-bending trunk postures for long periods. Long periods of immobility, even in sitting postures, and whole-body vibration are also contributing factors. Therefore, scientists have associated the buildup of high disk pressures with eventual disk failures and have resorted to biomechanical calculations or estimations of disk compressive forces from intraabdominal or direct intradisk pressure measurements, neither of which are practical for industry.
A crude but useful analogy (Figure 4.25) considers a free-body diagram of the L5/S1 disk (where most of trunk flexion and disk herniation occurs) and models the components as a first-class lever, with the center of the disk acting as the fulcrum. The load acting through a moment arm determined by the distance from the center of the hands to the center of the disk creates a clockwise moment, while erector spinae muscle is modeled as a force acting downward through a very small moment arm [approximately 2 in (5 cm)], creating a counterclockwise moment barely sufficient to maintain equilibrium. Thus, the two moments must be equal, allowing calculation of the internal force of the erector spinae muscle:

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2"

2"
30"
L5/S1
L5/S1

50

Manual Work Design

30"
F ϭ Fcomp

50

FM

Figure 4.25 Back compressive forces modeled as a first-class lever.

2 ϫ FM ϭ 30 ϫ 50 where FM ϭ muscle force. Then FM ϭ 1,500/2, or 750, lb (341 kg). Solving for the total compressive force Fcomp exerted on the disk yields
Fcomp ϭ FM ϩ 50 ϭ 800
This disk compressive force of 800 lb (364 kg) is a considerable load, which may cause injury in certain individuals.
Note that this simple analogy neglects the offset alignment of the disks, weights of the body segments, multiple action points of erector spinae components, and other factors, and probably underpredicts the extremely high compressive forces typically obtained in the low back area. More accurate values for various loads and horizontal distances are presented in Figure 4.26. Due to the considerable individual variation in force levels resulting in disk failures,
Waters (1994) recommended that a compressive force of 770 lb (350 kg) be considered the danger threshold.
The hand calculation of such compressive forces through biomechanical modeling is exceedingly time-consuming and has led to the development of various computerized biomechanical models, the best known of which is the 3D
Static Strength Prediction Program.
Note that although disk herniation may be the most severe of low back injuries, there are other problems, such as soft tissue injuries involving ligaments, muscles, and tendons. These are probably more common, resulting in the backache that most people associate with manual work. Such pain, although uncomfortable, will probably recede over the course of several days with moderate rest. Physicians are currently recommending moderate daily activity to accelerate recovery, rather than the traditional complete bed rest. In addition, researchers are incorporating the soft tissue components in ever more complex back models.

NIOSH LIFTING GUIDELINES
Recognizing and attempting to control the growing problem of work-related back injuries, the National Institute for Occupational Safety and Health (NIOSH) issued what is commonly referred to as the NIOSH lifting guidelines (Waters et al., 1994).

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2,000
Predicted L5 / S1 compressive force, lb

170

Load lb (kg)
132 (60)

1,750
1,500
1,250

Fcomp

88

(40)

44

(20)

H

1,000
750
500
250
0

5

10

15
20
Horizontal distance H in

25

30

35

Figure 4.26

Effect of weight of load and horizontal distance between the load center of gravity and the L5/S1 disk on the predicted compressive force on the L5/S1 disk.
(Source: Adapted from NIOSH, 1981, Figs 3.4 and 3.5.)

Although these are only guidelines, OSHA uses them extensively in its workplace inspections and will issue citations based on these through the General Duty Clause.
The key output is the recommended weight limit (RWL), which is based on the concept of an optimum weight, with adjustments for various factors related to task variables. The RWL is meant to be a load that can be handled by most workers: 1. The 770-lb (350-kg) compression force on the L5/S1 disk, created by the
RWL, can be tolerated by most young, healthy workers.
2. Over 75 percent of women and over 99 percent of men have the strength capability to lift a load described by the RWL.
3. Maximum resulting energy expenditures of 4.7 kcal/min (18.8 Btu/min) will not exceed recommended limits.
Once the RWL is exceeded, musculoskeletal injury incidences and severity rates increase considerably. The formulation for RWL is based on a maximum load that can be handled in an optimum posture. As the posture deviates from the optimum, adjustments for various task factors, in the form of multipliers, decrease the acceptable load.
RWL ϭ LC * HM * VM * DM * AM * FM * CM where LC ϭ load constant ϭ 51 lb
HM ϭ horizontal multiplier ϭ 10/H
VM ϭ vertical multiplier ϭ 1 Ϫ 0.0075 | V Ϫ 30 |

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DM ϭ distance multiplier ϭ 0.82 ϩ 1.8/D
AM ϭ asymmetry multiplier ϭ 1 Ϫ 0.0032*A
FM ϭ frequency multiplier from Table 4.7
CM ϭ coupling multiplier from Table 4.8
H ϭ horizontal location of the load cg forward of the midpoint between the ankles, 10 Յ H Յ 25 in
V ϭ vertical location of the load cg, 0 Յ V Յ 70 in
D ϭ vertical travel distance between origin and destination of lift,
10 Յ D Յ 70 in
A ϭ angle of asymmetry between the hands and feet (degrees),
0° Յ A Յ 135°
Table 4.7

Frequency Multiplier (FM) Table

Work Duration
Յ1h
Frequency
Lifts/min (F)‡
Յ 0.2
0.5
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
Ͼ15



Ͼ1 but Յ 2 h

Ͼ2 but Ն 8 h

V Ͻ 30†

V Ն 30

V Ͻ 30

V Ն 30

V Ͻ 30

V Ն 30

1.00
0.97
0.94
0.91
0.88
0.84
0.80
0.75
0.70
0.60
0.52
0.45
0.41
0.37
0.00
0.00
0.00
0.00

1.00
0.97
0.94
0.91
0.88
0.84
0.80
0.75
0.70
0.60
0.52
0.45
0.41
0.37
0.34
0.31
0.28
0.00

0.95
0.92
0.88
0.84
0.79
0.72
0.60
0.50
0.42
0.35
0.30
0.26
0.00
0.00
0.00
0.00
0.00
0.00

0.95
0.92
0.88
0.84
0.79
0.72
0.60
0.50
0.42
0.35
0.30
0.26
0.23
0.21
0.00
0.00
0.00
0.00

0.85
0.81
0.75
0.65
0.55
0.45
0.35
0.27
0.22
0.18
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00

0.85
0.81
0.75
0.65
0.55
0.45
0.35
0.27
0.22
0.18
0.15
0.13
0.00
0.00
0.00
0.00
0.00
0.00

Values of V are in inches.
For lifting less frequently than once per 5 min, set F ϭ 0.2 lift/min.
Table 4.8 Coupling Multiplier

Coupling Multiplier
Coupling Type
Good
Fair
Poor

V Ͻ 30 in
(75 cm)

V Ն 30 in
(75 cm)

1.00
0.95
0.90

1.00
1.00
0.90

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More simply,
RWL (lb) ϭ 51(10/H) (1 Ϫ 0.0075 | V Ϫ 30 |) (0.82 ϩ 1.8/D)
(1 Ϫ 0.0032A) ϫ FM ϫ CM
Note that these multipliers range from a minimum value of 0 for extreme postures to a maximum value of 1 for an optimal posture or condition. Table 4.7 provides frequency multipliers for three different work durations and for frequencies varying from 0.2/min to 15/min. Work duration is divided into three categories:
1. Short duration. One hour or less followed by a recovery time equal to
1.2 times the work time. (Thus, even though an individual works for three
1-h periods, as long as these work periods are interspersed with recovery times of 1.2 h, the overall work will still be considered of short duration.)
2. Moderate duration. Between 1 and 2 h of work, followed by a recovery period of at least 0.3 times the work time.
3. Long duration. Anything longer than 2 h but less than 8 h.
The coupling multiplier depends on the nature of the hand-to-object interface.
In general, a good interface or grip will reduce the maximum grasp forces required and increase the acceptable weight for lifting. On the other hand, a poor interface will require large grasp forces and decrease the acceptable weight. For the revised
NIOSH guidelines, three classes of couplings are used: good, fair, and poor.
A good coupling is obtained if the container is of optimal design, such as boxes and crates with well-defined handles or hand-hold cutouts. An optimal container has a smooth, nonslip texture, is no greater than 16 in (40 cm) in the horizontal direction, and is no greater than 12 in (30 cm) high. An optimal handle is cylindrical, with a smooth, nonslip surface, 0.75 to 1.5 in (1.9 to 3.8 cm) in diameter, more than 4.5 in (11.3 cm) long, and with 2-in (5-cm) clearance. For loose parts or irregular objects that are not found in containers, a good coupling would consist of a comfortable grip in which the hand can comfortably wrap around the object without any large wrist deviations (typically, small parts in a power grip).
A fair coupling results from less than optimal interfaces due to less than optimal handles or hand-hold cutouts. For containers of optimal design but with no handles or cutouts, or for loose parts, a fair coupling results if the hand cannot wrap all the way around but is flexed to only 90°. This would typically apply to most industrial packaging boxes.
A poor coupling results from containers of less than optimal design with no handles or hand-hold cutouts, or from loose parts that are bulky or hard to handle. Any container with rough or slippery surfaces, sharp edges, an asymmetric center of gravity, or unstable contents, or one that requires gloves would result in a poor coupling, by definition. To assist in the classification of couplings, the decision tree shown in Figure 4.27 may be useful.
The multipliers for each variable act as simple design tools for fairly straightforward job redesign. For example, if HM ϭ 0.4, 60 percent of the potential lifting capability is lost due to a large horizontal distance. Therefore, the horizontal distance should be reduced as much as possible.

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Object Lifted

Container

Loose object

NO

Optimal container? YES

Bulky object? NO

YES
POOR
Optimal handles? YES

Optimal grip? NO

NO

NO

YES

Fingers flexed 90 degrees?
YES
FAIR

GOOD

Figure 4.27 Decision tree for coupling quality

NIOSH also devised a lifting index (LI) to provide a simple estimate of the hazard level of lifting a given load, with values exceeding 1.0 deemed to be hazardous. Also, the LI is useful in prioritizing jobs for ergonomic redesign.
LI ϭ load weight/RWL
In terms of controlling the hazard, NIOSH recommends engineering controls, physical changes, or a job and workplace redesign rather than administrative controls consisting of specialized selection and training of workers. Most common changes include avoiding high and low locations, using lift and tilt tables, using handles or specialized containers for handling loads, and reducing the horizontal distance by cutting out work surfaces and bringing loads closer to the body.

MULTITASK LIFTING GUIDELINES
For jobs with a variety of lifting tasks, the overall physical/metabolic load is increased compared to the single lifting task. This is reflected in a decreased RWL

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and an increased LI, and there is a special procedure to handle such situations.
The concept is a composite lifting index (CLI), which represents the collective demands of the job. The CLI equals the largest single-task lifting index (STLI) and increases incrementally for each subsequent task. The multitask procedure is as follows:
1. Compute a single-task RWL (STRWL) for each task.
2. Compute a frequency-independent RWL (FIRWL) for each task by setting
FM ϭ 1.
3. Compute a single-task LI (STLI) by dividing the load by STRWL.
4. Compute a frequency-independent LI (FILI) by dividing the load by FIRWL.
5. Compute the CLI for the overall job by rank-ordering the tasks according to decreasing physical stress, that is, the STLI for each task. The CLI is then
CLI ϭ STLI1 ϩ ⌺⌬LI where ⌺⌬LI ϭ FILI2 (1/FM1,2 Ϫ 1/FM1) ϩ FILI3 (1/FM1,2,3 Ϫ 1/FM1,2) ϩ …
Consider the three-task lifting job shown in Table 4.9. The multitask lifting analysis is as follows:
1. The task with the greatest lifting index is new task 1 (old task 2) with
STLI ϭ 1.6.
2. The sum of the frequencies for new tasks 1 and 2 is 1 ϩ 2 ϭ 3.
3. The sum of the frequencies for new tasks 1, 2, and 3 is 1 ϩ 2 ϩ 4 ϭ 7.
4. From Table 4.7, the new frequency multipliers are FM1 ϭ 0.94, FM1,2 ϭ
0.88, and FM1,2,3 ϭ 0.70.
5. The combined lifting index is therefore
CLI ϭ1.6 ϩ 1 (1/0.88 Ϫ 1/0.94) ϩ 0.67 (1/0.7 Ϫ 1/0.88) ϭ1.60 ϩ 0.07 ϩ 0.20 ϭ 1.90
EXAMPLE 4.2

NIOSH Analysis of Lifting a Box into the Trunk of a car
Before recent automotive design changes, it was not unusual to have to lean forward and extend the arms while placing an object into the trunk of a car (Figure 4.28). Assume the occupant lifts a 30-lb box from the ground into a trunk. Being lazy, the occupant simply twists 90° to pick up the box from the ground level (V ϭ 0) at a short horizontal distance (H ~ 10 in). The vertical travel distance is the difference between the vertical location of the box at the destination (assume the bottom of the trunk is 25 in from the ground) and the vertical location of the box at the origin (V ϭ 0), yielding
D ϭ 25. Assume that this is a one-time lift; therefore, FM ϭ 1. Also assume that the box is fairly small and compact, but has no handles. Thus, coupling is fair with CM ϭ
0.95. This yields the following calculation for the origin:

Figure 4.28

Posture for trunk loading example

RWLORG ϭ 51(10/10) (1 Ϫ 0.0075 | 0 Ϫ 30 |) (0.82 ϩ 1.8/25) ϫ (1 Ϫ 0.0032 * 90) (1) (0.95)

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ϭ 51(1)(0.775) (0.892) (0.712) (1) (0.95) ϭ 23.8
Assuming a larger reach (H ϭ 25 in) into the trunk because of the bumper and high trunk lip, no twisting, the distance traveled remaining the same, and the coupling remaining fair, the calculation at the destination is
RWLDEST ϭ 51(10/25) (1 Ϫ 0.0075 | 25 Ϫ 30 |)(0.82 ϩ 1.8/25) ϫ (1 Ϫ 0.0032 * 0) (1) (0.95) ϭ 51(0.4)(0.963) (0.892) (1) (1) (0.95) ϭ 16.6 and LI ϭ 30/16.6 ϭ 1.8
Thus, in the worst-case approach, only 16.6 lb could be lifted safely by most individuals, and the 30-lb box would create a hazard almost twice the acceptable level. The biggest reduction in capability is the horizontal distance at the destination, due to the trunk design. Decreasing the horizontal distance to 10 in would increase the H factor to 10/10 ϭ 1 and increase RWL to 41.5 lb. For most newer cars, this has been accomplished by the auto manufacturers by opening the front part of the trunk such that once the load is lifted to the lower lip, minimum horizontal lifting is needed; the load can be simply pushed forward. However, the limiting case is now the origin, which can be improved by moving the feet and eliminating the twist, increasing RWL to 33.4 lb. Note that a two-step analysis is necessary if the occupant lifts the load from ground level to the lip of the trunk and then lowers the box into the trunk. This lift is also improved in newer-model cars because of a decrease in the vertical height of the lip, which also decreases the distance lifted.

Table 4.9 A Sample Three-Task Lifting Job’s Characteristics

Task Number
Load weight L
Task frequency F
FIRWL
FM
STRWL
FILI
STLI
New task number

1
20
2
20
0.91
18.2
1.0
1.1
2

2

3

30
1
20
0.94
18.8
1.5
1.6
1

10
4
15
0.84
12.6
0.67
0.8
3

This procedure is facilitated by the NIOSH Multitask Job Analysis Worksheet
(see Figure 4.29). However, once the number of tasks exceeds three or four, it becomes very time-consuming to calculate CLI by hand. A variety of software programs and websites are now available to assist the user in this effort, including
Design Tools. Of course, the best solution overall is to avoid manual materials handling and use mechanical assist devices or completely automated material handling systems (see Chapter 3).

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Multitask job analysis worksheet
Department
Job title
Analyst's name
Date

Job description

Step 1. Measure and record task variable data
Object
Hand location (in)
Vertical
Task no.
Origin
Dest. weight (lbs)
L (avg.) L (max.) H

V

H

V

distance (in)
D

Asymmetry angle (degs) Frequency rate Duration Coupling
Lifts/min
Hrs
Origin
Dest.
A
A
F
C

Step 2. Compute multipliers and FIRWL, STRWL, FILI, and STLI for each task
Task
no.

LC x HM x VM x DM x AM x CM

FILI =
STLI =
FIRWL x FM STRWL L/FIRWL L/STRWL

New task no.

F

51
51
51
51
51
Step 3. Compute the composite lifting index for the job (after renumbering tasks)
CLI = STLI1, + Δ FILI2 + Δ FILI3 + Δ FILI4 + Δ FILI5
FILI2 (1/FM1,2 – 1/FM1) FILI3 (1/FM1,2,3 – 1/FM1,2)

FILI4 (1/FM1,2,3,4 – 1/FM1,2,3)

FILI5 (1/FM1,2,3,4,5 – 1/FM1,2,3,4)

CLI =

Figure 4.29 Multitask Job Analysis Worksheet.

GENERAL GUIDELINES: MANUAL LIFTING
Although no one optimal lifting technique is suitable for all individuals or task conditions, several guidelines are generally appropriate overall (see Figure 4.30).
First, plan the lift by evaluating the size and shape of the load, determining whether assistance is needed, and ascertaining what worksite conditions may interfere with the lift. Second, determine the best lifting technique. In general, a squat lift, keeping the back relatively straight and lifting with the knees bent, is the safest in terms of lowback compressive forces. However, bulky loads may interfere with the knees, and a stoop lift, in which the individual bends over and then extends the back, may be required. Third, spread the feet apart, both side ways and fore–aft, to maintain a good balance and stable posture. Fourth, secure a good grip on the load. These last two guidelines are especially important in avoiding sudden twisting and jerking movements, both of which are extremely detrimental to the low back. Fifth, hold the load close to the body to minimize the horizontal moment arm created by the load and the resulting moment on the low back.
Avoiding twisting and jerky motions is critical. The first produces an asymmetric orientation of the disks, leading to increased disk pressures, while the

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a.

Plan the lift.

b.

Determine the best lifting technique. c.

Get a secure grip.

d.

Pull the load in close to your body.

e.

Alternate lifting and light work tasks.

Figure 4.30 Safe lifting procedure.

(Available through S. H. Rodgers, Ph.D. P.O. Box 23446, Rochester, NY 14692.)

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Figure 4.31 General Posture and Task Evaluation Checklist.

second generates additional accelerative forces on the back. One nonintuitive method for discouraging twisting in workers is actually to increase the travel distance between the origin and the destination. This will force the worker to take a step and, in so doing, turn the whole body, rather than twisting the trunk. Carrying uneven loads in both arms or an entire load in only one arm generates similar asymmetric disk orientations and should also be avoided.
The General Posture and Task Evaluation Checklist (see Figure 4.31) can be very useful in reminding the analyst of the basic principles of good work design.

BACK BELTS
A cautionary note should be given regarding back belts. Although commonly found on many workers and automatically prescribed in some companies, back belts are not the ultimate panacea and must be regarded with caution. Back belts originated from early studies of weightlifting, showing that for extreme loads, belts relieved 15 to 30 percent of low back compressive forces, as estimated from back electromyograms (Morris et al., 1961). However, these studies were performed on trained weightlifters, lifting much larger loads, in a completely sagittal plane. Industrial workers lift much lighter loads, producing a much lower effect. Twisting because of misaligned muscles probably reduces this effect even further. There are also anecdotal data of the “superman” effect—industrial workers with back belts selecting heavier loads than those without back belts—and some workers having coronary incidents from the 10 to 15 mmHg increase in blood pressure due to the abdominal compression.

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Finally, a longitudinal study of airline baggage handlers (Ridell et al., 1992) concluded there was no significant difference in back injuries between workers using back belts and “control” workers without back belts. Surprisingly, a smaller group of workers, who for one reason or another (e.g., discomfort, heat) quit wearing the belts but continued in the study, had significantly higher injuries. This may be attributed to atrophy of the abdominal muscles, which should naturally provide an internal back belt but were weakened due to decreased stress. A positive approach may be to encourage workers to strengthen these abdominal muscles through abdominal crunches (modified sit-ups), regular exercise, and body weight reduction. Back belts should only be used with proper training and only after engineering controls have been attempted.

SUMMARY
Chapter 4 introduces some of the theoretical concepts of the human musculoskeletal and physiological systems as a means of providing a framework for a better understanding of the principles of motion economy and work design.
These principles are presented as a set of rules to be utilized in redesigning manual assembly work as part of the motion study. Hopefully, with a better understanding of the functioning of the human body, the analyst will view these rules as less arbitrary. These same concepts will be elaborated on in Chapter 5, for the discussions on the design of the workplace, tools, and equipment.

QUESTIONS
1. What structural components are found in muscles? What do these components have to do with muscle performance?
2. Explain the elements of static and dynamic muscle performance with the sliding filament theory.
3. Describe the different types of muscle fibers and relate their properties to muscle performance. 4. Why does a change in the number of active motor units not result in a proportional change in muscle tension?
5. What does EMG measure? How is EMG interpreted?
6. Explain why workstation designers should endeavor to have operators perform work elements without lifting their elbows.
7. What viewing distance would you recommend for a seated operator working at a computer terminal?
8. Define and give examples of the 17 fundamental motions, or therbligs.
9. How may the basic motion Search be eliminated from the work cycle?
10. What basic motion generally precedes Reach?
11. What three variables affect the time for the basic motion Move?
12. How does the analyst determine when the operator is performing the element
Inspect?
13. Explain the difference between avoidable and unavoidable delays.

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14. Which of the 17 therbligs are classed as effective and usually cannot be removed from the work cycle?
15. Why should fixed locations be provided at the workstation for all tools and materials? 16. Which of the five classes of motions is preferred for industrial workers? Why?
17. Why is it desirable to have the feet working only when the hands are occupied?
18. In a motion study, why is it inadvisable to analyze both hands simultaneously?
19. What task factors increase the index of difficulty in a Fitts’ tapping task?
20. What factors affect back compressive forces during lifting?
21. What factors influence the measurement of isometric muscle strength?
22. Why do psychophysical, dynamic, and static strength capabilities differ?
23. What methods can be used to estimate the energy requirements of a job?
24. What factors change the energy expended for a given job?
25. How does work capability vary with gender and age?
26. What limits endurance in a whole-body manual task?

PROBLEMS
1.
2.

3.

4.

5.
6.

What is the maximum load that can be raised by an outstretched stiff arm by a
50th-percentile female? (For estimating anthropometry use Table 5.1.)
In the packing department, a worker stands sideways between the end of a conveyor and a pallet. The surface of the conveyor is 40 in from the floor, and the top of the pallet is 6 in from the floor. As a box moves to the end of the conveyor, the worker twists 90° to pick up the box, then twists 180° in the opposite direction and sets the box down on the pallet. Each box is 12 in on a side and weighs 25 lb. Assume the worker moves five boxes per minute for an 8-h shift and a horizontal distance of
12 in. Using the NIOSH lifting equation, calculate RWL and LI. Redesign the task to improve it. What are the RWL and LI now?
For Problem 2 calculate the low back compressive forces incurred in the performance of this job, using the University of Michigan 3D Static Strength
Prediction Program.
A 95th percentile male is holding a 20-lb load in his outstretched arm in 90° abduction. What is the voluntary torque required at the shoulder to be able to hold this load?
A worker is shoveling sand at a rate of 8 kcal/min. How much rest does he need during an 8-h shift? How should the rest be allocated?
A current problem in the U.S. Army is the neck/shoulder fatigue experienced by helicopter pilots. To be able to fly missions at night, the pilots wear night vision goggles, which are attached to the front of the helmet. Unfortunately, these are fairly heavy, causing a large downward torque of the head. This torque must be counteracted by the neck muscles, which then fatigue. To alleviate this problem, many pilots have started attaching random lead weights to the back of the helmet.
Find the appropriate weight that would best balance the head and minimize neck fatigue. Assumptions: (a) cg of goggles is 8 in in front of neck pivot point;
(b) goggles weigh 2 lb; (c) maximum volitional neck torque is 480 in.lb; (d) cg of

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lead weight is 5 in behind neck pivot point; (e) bare helmet weighs 4 lb; and (f) cg of helmet is 0.5 in in front of neck pivot.
The laborer on a palletizing operation has been complaining about fatigue and the lack of rest. You measure his heart rate and find it to be 130 beats/min and slowly increasing during work. When he sits down, his heart rates dropped to 125 beats/min by the end of the first minute of rest and 120 beats/min by the end of the third minute. What do you conclude?
A grievance has been filed by the union at Dorben Co. regarding the final inspection station, in which the operator slightly lifts a 20-lb assembly, examines all sides, and, if acceptable, sets it back down on the conveyor that takes the assembly to the packing station. On average, the inspector examines five assemblies per minute, at an energy expenditure level of 6 kcal/min. The conveyor is 40 in off the floor, and the assembly is roughly 20 in from the inspector while being inspected. Evaluate the job with respect to the NIOSH lifting guidelines and metabolic energy expenditure considerations. Indicate whether the job exceeds allowable limits. If it does, calculate how many assemblies the inspector may inspect per minute without exceeding acceptable guidelines.
A relatively unfit worker with resting heart rate of 80 beat/min starts his job of palletizing boxes. At the morning break, an industrial engineer quickly measures the worker’s heart rate and finds a peak value of 110 beats/min, a value of
105 beats/min 1 min after stopping work, and a value of 95 beat/min 3 min after stopping work. What can you conclude about the workload for this worker?

REFERENCES
Åstrand, P. O., and K. Rodahl. Textbook of Work Physiology. 3d ed. New York:
McGraw-Hill, 1986.
Bink, B. “The Physical Working Capacity in Relation to Working Time and Age.”
Ergonomics, 5, no.1 (January 1962), pp. 25–28.
Borg, G., and H. Linderholm. “Perceived Exertion and Pulse Rate During Graded
Exercise in Various Age Groups.” Acta Medica Scandinavica, Suppl. 472 (1967), pp. 194–206.
Bouisset, S. “EMG and Muscle Force in Normal Motor Activities.” In New
Developments in EMG and Clinical Neurophysiology. Ed. J. E. Desmedt. Basel,
Switzerland: S. Karger, 1973.
Brouha, L. Physiology in Industry. New York: Pergamon Press, 1967.
Chaffin, D. B. “Electromyography—A Method of Measuring Local Muscle Fatigue.”
The Journal of Methods-Time Measurement, 14 (1969), pp. 29–36.
Chaffin, D. B., and G. B. J. Andersson. Occupational Biomechanics. New York: John
Wiley & Sons, 1991.
Chaffin, D. B., G. D. Herrin, W. M. Keyserling, and J. A. Foulke. Preemployment
Strength Testing. NIOSH Publication 77-163. Cincinnati, OH: National Institute for
Occupational Safety and Health, 1977.
Drillis, R. “Folk Norms and Biomechanics.” Human Factors, 5 (October 1963), pp. 427–441.
Dul, J., and B. Weerdmeester. Ergonomics for Beginners. London: Taylor & Francis,
1993.

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Eastman Kodak Co., Human Factors Section. Ergonomic Design for People at Work.
New York: Van Nostrand Reinhold, 1983.
Fitts, P. “The Information Capacity of the Human Motor System in Controlling the
Amplitude of Movement.” Journal of Experimental Psychology, 47, no. 6 (June
1954), pp. 381–391.
Freivalds, A., and D. M. Fotouhi. “Comparison of Dynamic Strength as Measured by the Cybex and Mini-Gym Isokinetic Dynamometers.” International Journal of
Industrial Ergonomics, 1, no. 3 (May 1987), pp. 189–208.
Garg, A. “Prediction of Metabolic Rates for Manual Materials Handling Jobs,”
American Industrial Hygiene Association Journal, 39, pp.661–674.
Gordon, E. (1957). The use of energy costs in regulating physical activity in chronic disease. A.M.A. Archives of Industrial Health, 16, 437–441.
Grandjean, E. Fitting the Task to the Man. New York: Taylor & Francis, 1988.
Gray, H. Gray’s Anatomy. 35th ed. Eds. R. Warrick and P. Williams. Philadelphia: W.B.
Saunders, 1973.
Ikai, M., and T. Fukunaga. “Calculation of Muscle Strength per Unit Cross-Sectional
Area of Human Muscle by Means of Ultrasonic Measurement.” Internationale
Zeitschrift für angewandte Physiologie einschließlich Arbeitsphysiologie, 26 (1968), pp. 26–32.
Jonsson, B. “Kinesiology.” In Contemporary Clinical Neurophysiology (EEG Sup. 34).
New York: Elsevier-North-Holland, 1978.
Langolf, G., D. G. Chaffin, and J. A. Foulke. “An Investigation of Fitt’s Law Using a
Wide Range of Movement Amplitudes.” Journal of Motor Behavior, 8, no.2 (June
1976), pp. 113–128.
Miller, G. D., and A. Freivalds. “Gender and Handedness in Grip Strength—A Double
Whammy for Females.” Proceedings of the Human Factors Society, 31 (1987), pp.
906–910.
Morris, J. M., D. B. Lucas, and B. Bressler. “Role of the Trunk in Stability of the
Spine.” Journal of Bone and Joint Surgery, 43-A, no. 3 (April 1961), pp. 327–351.
Mundel, M. E., and D. L. Danner. Motion and Time Study. 7th ed. Englewood Cliffs,
NJ: Prentice-Hall, 1994.
Murrell, K. F. H. Human Performance in Industry. New York: Reinhold Publishing, 1965.
National Safety Council. Accident Facts. Chicago: National Safety Council, 2003.
Passmore, R., and J. Durnin, (1955). Human energy expenditure. Physiological
Reviews, 35, 801–875.
Ridell, C. R., J. J. Congleton, R. D. Huchingson, and J. T. Montgomery. “An Evaluation of a Weightlifting Belt and Back Injury Prevention Training Class for Airline
Baggage Handlers.” Applied Ergonomics, 23, no. 5 (October 1992), pp. 319–329.
Rodgers, S. H. Working with Backache. Fairport, NY: Perinton Press, 1983.
Rowe, M. L. Backache at Work. Fairport, NY: Perinton Press, 1983.
Sanders, M. S., and E. J. McCormick. Human Factors in Engineering and Design. New
York: McGraw-Hill, 1993.
Snook, S. H., and V. M. Ciriello. “The Design of Manual Handling Tasks: Revised
Tables of Maximum Acceptable Weights and Forces.” Ergonomics, 34, no. 9
(September 1991), pp. 1197–1213.

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Thorton, W. “Anthropometric Changes in Weightlessness.” In Anthropometric Source
Book, 1, ed. Anthropology Research Project, Webb Associates. NASA RP1024.
Houston, TX: National Aeronautics and Space Administration, 1978.
Waters, T. R., V. Putz-Anderson, and A. Garg. Revised NIOSH Lifting Equation, Pub.
No. 94-110, Cincinnati, OH: National Institute for Occupational Safety and Health,
1994.
Winter, D. A. Biomechanics of Human Movement. New York: John Wiley & Sons, 1979.

SELECTED SOFTWARE
3D Static Strength Prediction Program. University of Michigan Software, 475 E.
Jefferson, Room 2354, Ann Arbor, MI 48109. (http://www.umichergo.org)
Design Tools (available from the McGraw-Hill website at www.mhhe.com/neibelfreivalds), New York: McGraw-Hill, 2002.
Energy Expenditure Prediction Program. University of Michigan Software, 475 E.
Jefferson, Room 2354, Ann Arbor, MI 48109. (http://www.umichergo.org)
Ergointelligence (Manual Material Handling). Nexgen Ergonomics, 3400 de
Maisonneuve Blvd. West, Suite 1430, Montreal, Quebec, Canada H3Z 3B8.
(http://www.nexgenergo.com/)
ErgoTRACK (NIOSH Lifting Equation). ErgoTrack.com, P.O. Box 787, Carrboro,
NC 27510.

WEBSITES
NIOSH Homepage–http://www.cdc.gov/niosh/homepage.html
NIOSH Lifting Guidelines–http://www.cdc.gov/niosh/94-110.html
NIOSH Lifting Calculator–http://www.industrialhygiene.com/calc/lift.html
NIOSH Lifting Calculator–http://tis.eh.doe.gov/others/ergoeaser/download.html

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CHAPTER

5

KEY POINTS








Fit the workplace to the operator.
Provide adjustability.
Maintain neutral postures (joints in midrange).
Minimize repetitions.
Use power grips when force is required.
Use pinch grips for precision and not force.

D

esigning the workplace, tools, equipment, and work environment to fit the human operator is called ergonomics. Rather than devoting a lot of space to the underlying theory of the physiology, capabilities, and limitations of the human, this chapter presents the principles of work design and appropriate checklists to facilitate the use of these design principles. With each design principle, a brief explanation of its origin or relationship to the human is provided. This approach will better assist the methods analyst in designing the workplace, equipment, and tools to meet the simultaneous goals of (1) increased production and efficiency of the operation and (2) decreased injury rates for the human operator.

5.1

ANTHROPOMETRY AND DESIGN

The primary guideline is to design the workplace to accommodate most individuals with regard to structural size of the human body. The science of measuring the human body is termed anthropometry and typically utilizes a variety of caliperlike devices to measure structural dimensions, for example, stature and forearm length.
Practically speaking, however, few ergonomists or engineers collect their own data, because of the wealth of data that has already been collected and tabulated. Close to
1,000 different body dimensions, for close to 100 mostly military population types, are available in the somewhat dated Anthropometric Source Book (Webb Associates,
1978). More recently the CAESAR (Civilian American and European Surface

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Stature height (1)
Eye height (2)
Shoulder
height (3)
Chest
depth
(13)

Elbow height (4)

Sitting height, erect (6)

Elbow rest height
(8)

Eye height, sitting (7)
Elbow-to-elbow
breadth (14)

Thigh clearance height (9)
Knee height
(10)
Buttock knee length (11)

Knuckle height (5)

Hip breadth (15)

Popliteal height (12)

Table 5.1 Selected Body Dimensions and Weights of U.S. Adult Civilians

Dimension (in)
Body dimension
1. Stature (height)
2. Eye height
3. Shoulder height
4. Elbow height
5. Knuckle height
6. Height, sitting
7. Eye height, sitting 8. Elbow rest height, sitting
9. Thigh clearance height 10. Knee height, sitting 11. Buttock-knee distance, sitting
12. Popliteal height, sitting 13. Chest depth
14. Elbow-elbow breadth 15. Hip breadth, sitting X. Weight
(lb and kg)
Source: Kroemer, 1989.

Sex

Dimension (cm)

5th

50th

95th

5th

50th

95th

Male
63.7
Female 58.9
Male
59.5
Female 54.4
Male
52.1
Female 47.7
Male
39.4
Female 36.9
Male
27.5
Female 25.3
Male
33.1
Female 30.9
Male
28.6
Female 26.6
Male
7.5
Female
7.1
Male
4.5
Female
4.2
Male
19.4
Female 17.8
Male
21.3
Female 20.4
Male
15.4
Female 14.0
Male
8.4
Female
8.4
Male
13.8
Female 12.4
Male
12.1
Female 12.3
Male
123.6
Female 101.6

68.3
63.2
63.9
58.6
56.2
51.6
43.3
39.8
29.7
27.6
35.7
33.5
30.9
28.9
9.6
9.2
5.7
5.4
21.4
19.6
23.4
22.4
17.4
15.7
9.5
9.5
16.4
15.1
13.9
14.3
162.8
134.4

72.6
67.4
68.0
62.7
60.0
55.9
46.9
42.8
31.7
29.9
38.1
35.7
33.2
30.9
11.6
11.1
7.0
6.9
23.3
21.5
25.3
24.6
19.2
17.4
10.9
11.7
19.9
19.3
16.0
17.2
213.6
197.8

161.8
149.5
151.1
138.3
132.3
121.1
100.0
93.6
69.8
64.3
84.2
78.6
72.6
67.5
19.0
18.1
11.4
10.6
49.3
45.2
54.0
51.8
39.2
35.5
21.4
21.4
35.0
31.5
30.8
31.2
56.2
46.2

173.6
160.5
162.4
148.9
142.8
131.1
109.9
101.2
75.4
70.2
90.6
85.0
78.6
73.3
24.3
23.3
14.4
13.7
54.3
49.8
59.4
56.9
44.2
39.8
24.2
24.2
41.7
38.4
35.4
36.4
74.0
61.1

184.4
171.3
172.7
159.3
152.4
141.9
119.0
108.8
80.4
75.9
96.7
90.7
84.4
78.5
29.4
28.1
17.7
17.5
59.3
54.5
64.2
62.5
48.8
44.3
27.6
29.7
50.6
49.1
40.6
43.7
97.1
89.9

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A kth percentile is defined as a value such that k percent of the data values (plotted in ascending order) are at or below this value and 100 – k percent of the data values are at or above this value. A histogram plot of U.S. adult male statures shows a bellshaped curve, termed a normal distribution, with a median value of 68.3 in (see
Figure 5.1). This is also the 50th percentile value; for example, one-half of all males are shorter than 68.3 in, while one-half are taller. The 5th percentile male is only 63.7 in tall, while a 95th percentile male is 72.6 in tall. The proof is as follows.
Typically, in a statistical approach, the approximately bell-shaped curve is normalized by the transformation z ϭ (xϪ␮)/␴
␮ϭ mean

␴ ϭ standard deviation (measure of dispersion)

to form a standard normal distribution (also termed a z distribution; see Figure 5.2).
Once normalized, any approximately bell-shaped population distribution will have the same statistical properties. This allows easy calculation of any percentile value desired, using the appropriate k and z values, as follows:

␬th percentile z value

10 or 90
Ϯ1.28

5 or 95
Ϯ1.645

2.5 or 97.5
Ϯ1.96

1 or 99
Ϯ2.33

kth percentile ϭ ␮ Ϯ z␴
Given that the mean stature for males in the United States is 68.3 in (173.6 cm), while the standard deviation is 2.71 in (6.9 cm) (Webb Associates, 1978), the 95th percentile male stature is calculated as
68.3 ϩ 1.645(2.71) ϭ 72.76 in while the 5th percentile male stature is
68.3 Ϫ 1.645(2.71) ϭ 63.84 in
Shaded area = 5%

68.3
63.7

179

EXAMPLE 5.1

Probability Distributions and Percentiles

where

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72.6

Figure 5.1 Normal distribution of U.S. adult male statures.

Note that the calculated values of 72.76 and 63.84 in are not exactly equal to the actual values of 72.6 and 63.7 in. This is so because the U.S. male height distribution is not a completely normal distribution.
(continued)

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z curve
Shaded area ϭ 5%

Shaded area ϭ 5%

0
Ϫ1.645 ϭ 5th percentile ϭ Ϫz

.05

z ϭ 95th percentile ϭ 1.645
.05

Figure 5.2 Standard normal distribution of male weights.

Anthropometry Resource) project collected over 100 dimensions on 5,000 civilians using three-dimensional body scans. A summary of useful dimensions that apply to the particular postures needed for workplace design for U.S. males and females is given in Table 5.1. Much of this anthropometric data is included in computerized human models such as COMBIMAN, Jack, MannequinPro, and
Safeworks that provide easy size adjustments and limitations in ranges of motion or visibility as part of the computer-aided design process.

DESIGN FOR EXTREMES
Designing for most individuals is an approach that involves the use of one of three different specific design principles, as determined by the type of design problem.
Design for extremes implies that a specific design feature is a limiting factor in determining either the maximum or minimum value of a population variable that will be accommodated. For example, clearances, such as a doorway or an entry opening into a storage tank, should be designed for the maximum individual, that is, a
95th percentile male stature or shoulder width. Then 95 percent of all males and almost all females will be able to enter the opening. Obviously, for doorways, space is usually not at a premium, and the opening can be designed to accommodate even larger individuals. On the other hand, added space in military aircraft or submarines is expensive, and these areas are therefore designed to accommodate only a certain
(smaller) range of individuals. Reaches, for such things as a brake pedal or control knob, are designed for the minimum individual, that is, a 5th percentile female leg or arm length. Then 95 percent of all females and practically all males will have a longer reach and will be able to activate the pedal or control.

DESIGN FOR ADJUSTABILITY
Design for adjustability is typically used for equipment or facilities that can be adjusted to fit a wider range of individuals. Chairs, tables, desks, vehicle seats, steering columns, and tool supports are devices that are typically adjusted to accommodate the worker population ranging from 5th percentile females to 95th percentile males. Obviously, designing for adjustability is the preferred method of design, but there is a trade-off with the cost of implementation. (Specific adjustment ranges for seat design are given later in Table 5.2)

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DESIGN FOR THE AVERAGE
Design for the average is the cheapest but least preferred approach. Even though there is no individual with all average dimensions, there are certain situations where it would be impractical or too costly to include adjustability for all features. For example, most industrial machine tools are too large and too heavy to include height adjustability for the operator. Designing the operating height at the
50th percentile of the elbow height for the combined female and male populations (roughly the average of the male and female 50th percentile values) means that most individuals will not be unduly inconvenienced. However, the exceptionally tall male or very short female may experience some postural discomfort.

PRACTICAL CONSIDERATIONS
Finally, the industrial designer should also consider the legal ramifications of design work. Due to the passage of the Americans with Disabilities Act of 1990
(see Section 9.6), reasonable effort must be made to accommodate individuals with all abilities. Special accessibility guidelines (U.S. Department of Justice,
1991) have been issued regarding parking lots, entryways into buildings, assembly areas, hallways, ramps, elevators, doors, water fountains, lavatories, restaurant or cafeteria facilities, alarms, and telephones.
It is also very useful, if practical and cost-effective, to build a full-scale mock-up of the equipment or facility being designed and then have the users evaluate the mock-up. Anthropometric measurements are typically made in standardized postures. In real life, people slouch or have relaxed postures, changing the effective dimensions and the ultimate design. Many costly errors have occurred during production, because of the lack of mock-up evaluations. In Example 5.2, the final design actually accommodates more than 95 percent of the population, yielding a rise height larger than necessary. The true design should have used the body dimensions for a combined male and female population. However, such combined data are rarely available. The data can be created through statistical techniques, but the general design approach is sufficient for most industrial applications. 5.2 PRINCIPLES OF WORK DESIGN:
THE WORKPLACE
DETERMINE WORK SURFACE HEIGHT
BY ELBOW HEIGHT
The work surface height (whether the worker is seated or standing) should be determined by a comfortable working posture for the operator. Typically, this means that the upper arms are hanging down naturally and the elbows are flexed at 90°so that the forearms are parallel to the ground (see Figure 5.4). The elbow height becomes the proper operation or work surface height. If the work surface

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is too high, the upper arms are abducted, leading to shoulder fatigue. If the work surface is too low, the neck or back is flexed forward, leading to back fatigue.

ADJUST THE WORK SURFACE HEIGHT BASED
ON THE TASK BEING PERFORMED
There are modifications to the first principle. For rough assembly involving the lifting of heavy parts, it is more advantageous to lower the work surface by as much as 8 in (20 cm) to take advantage of the stronger trunk muscles (see Figure 5.5). For fine assembly involving minute visual details, it is more advantageous to raise the work surface by up to 8 in (20 cm) to bring the details closer to the optimum line of sight of 15°(principle from Chapter 4). Another, perhaps better, alternative is to slant the work surface approximately 15° then both principles can be satisfied. However, rounded parts then have a tendency to roll off the surface.
These principles also apply to a seated workstation. A majority of tasks, such as writing or light assembly, are best performed at the resting-elbow height. If the job requires the perception of fine detail, it may be necessary to raise the work to bring it closer to the eyes. Seated workstations should be provided with adjustable chairs and adjustable footrests (see Figure 5.6). Ideally, after the operator is comfortably seated with both feet on the floor, the work surface is positioned at the appropriate elbow height to accommodate the operation. Thus, the workstation also needs to be adjustable. Short operators whose feet do not reach the floor, even after adjusting the chair, should utilize a footrest to provide support for the feet.

PROVIDE A COMFORTABLE CHAIR FOR
THE SEATED OPERATOR
The seated posture is important from the standpoint of reducing both the stress on the feet and the overall energy expenditure. Because comfort is a very individual response, strict principles for good seating are somewhat difficult to define. Furthermore, few chairs will comfortably adapt to the many possible seating postures (see Figure 5.7). However, several general principles hold true for all seats. When a person is standing erect, the lumbar portion of the spine (the small of the back, approximately at the belt level) curves naturally inward, which is termed lordosis. However, as a person sits down, the pelvis rotates backward, flattening the lordotic curve and increasing the pressure on the disks in the vertebral column (see Figure 5.8). Therefore, it is very important to provide lumbar support in the form of an outward bulge in the seat back, or even a simple lumbar pad placed at the belt level.
Another approach to preventing flattening of the lordotic curve is to reduce the pelvic rotation by maintaining a large angle between the torso and thighs, via a forward-tilting seat (kneeling posture in Figure 5.7). The theory is that this is a shape maintained by astronauts in the weightless environment of space (see
Figure 4.4). The disadvantage of this type of seat is that it may put additional

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Designing Seating in a Large Training Room
This example will show the step-by-step procedures utilized in a typical design problem—arranging seating in an industrial training room such that most individuals will have an unobstructed view of the speaker and screen (see Figure 5.3).
1. Determine the body dimensions critical to the design—sitting height, erect; and eye height, sitting.
2. Define the population being served—U.S. adult males and females.
3. Select a design principle and the percentage of the population to be accommodated—Designing for extremes and accommodating 95 percent of the population. The key principle is to allow a 5th percentile female sitting behind a
95th percentile male to have an unimpeded line of sight.
4. Find appropriate anthropometric values from Table 5.1. The 5th percentile female seated eye height is 26.6 in (67.5 cm), while the 95th percentile male erect sitting height is 38.1 in (96.7 cm). Thus, for the small female to see over the large male, a rise height of 11.5 in (29.2 cm) is necessary between the two rows. This would be a very large rise height, which would create a very steep slope. Typically, therefore, the seats are staggered, so that the individual in the back is looking over the head of an individual two rows in front, decreasing the rise height by one-half.
5. Add allowances and test. Many anthropometric measurements have been made on nude human bodies. Therefore, allowances for heavy clothing, hats, or shoes may be necessary. For example, if all the trainees will be wearing hard hats, an additional 2 to 3 in might be needed for the rise height. It would be much more practical to remove the hard hats in the training room.
5th percentile female

Rise height 191

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95th percentile male

{

Figure 5.3 Seating design in a large training room.

stress on the knees. The addition of a pommel to the forward-sloping seat, forming a saddlelike seat, may be a better overall approach, as it eliminates the need for knee supports and still allows for back support (see Figure 5.9).

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Work height

90؇

Elbow height from floor surface

Tool height

Figure 5.4 Graphic aid for determining correct work surface height.

(From: Putz-Anderson, 1988.)

15؇
8
Height of objects being handled

(a)
(b)

15؇
Overhead
clearance
203 cm
(80 in)

Knee clearance 10 cm (4 in)
Optimal
working height of the hands 4
0

(c)

4
8

Height of work surface

Floor level
Foot height clearance 10 cm (4 in)

Foot depth clearance 13 cm (5 in)

Figure 5.5 Recommended standing workplace dimensions.

(a) For precision work with armrest, (b) for light assembly, (c) for heavy work.

In. from elbow rest height

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F
M
K
G
L

B

C

N

E

D
J
A
I
H

Side view

Figure 5.6 Adjustable chair (specific seat parameter values found in Table 5.2).

Table 5.2

Recommended Seat Adjustment Ranges

Seat parameter

Design Value
[in (cm) unless specified] A–Seat height

16–20.5 (40–52)

B–Seat depth

15–17 (38–43)

C–Seat width

Ն18.2 (Ն46.2)

D–Seat pan angle
E–Seat back to pan angle

Ϫ10°– ϩ10°
Ͼ90°

F–Seat back width
G–Lumbar support

Ͼ12 (Ͼ30.5)
6–9 (15–23)

H–Footrest height
I–Footrest depth
J–Footrest distance
K–Leg clearance
L–Work surface height
M–Work surface thickness
N–Thigh clearance

1–9 (2.5–23)
12 (30.5)
16.5 (42)
26 (66)
~32 (~81)
Ͻ2 (Ͻ5)
Ͼ8 (Ͼ20)

Comments
Too high—compresses thighs; too low—disk pressure increases
Too long—cuts popliteal region, use waterfall contour
Wider seats recommended for heavy individuals Downward tilting requires greater friction in the fabric
Ͼ105°preferred, but requires workstation modifications
Measured in the lumbar region
Vertical height from seat pan to center of lumbar support

Determined by elbow rest height
Maximum value
Minimum value

Note: A–G from ANSI (1988); H–M from Eastman Kodak (1983).

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Figure 5.7 Six basic seating postures. Front
Support

Reclining

Kneeling

(From: Serber, 1990. Reprinted with permission of the Human
Factors and Ergonomics
Society. All rights reserved.)

Re-balance
Rebalance

Stool

Traditional

Figure 5.8 Posture of the spine when standing and sitting. Lumbar portion of spine is lordotic when standing (a) and kyphotic when sitting (b).
The shaded vertebrae are the lumbar portion of the spine.
(Source: Grandjean 1988,
Fig.47.)
Lordotic inward arch

(a) Standing

Kyphotic outward arch

(b) Sitting

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Figure 5.9 Saddle chair.

(Courtesy: This version of the Nottingham Chair, called the checkmate, is made by the
Osmond Group: Photograph by Nigel Corlett. For further details of the Nottingham chair see Nottinghamchair.com; for the Checkmate see ergonomics.co.uk and search for the Checkmate chair.)

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PROVIDE ADJUSTABILITY IN THE SEAT
A second consideration is the reduction of disk pressure, which can increase considerably with a forward tilt of the trunk. Reclining the backrest from the vertical also has a dramatic effect in reducing disk pressures (Andersson et al., 1974).
Unfortunately, there is a trade-off. With increasing angles, it becomes more difficult to look down and perform productive work.
Another factor is the need to provide easy adjustability for specific seat parameters. Seat height is most critical, with ideal height being determined by the person’s popliteal height, which is defined in the figure accompanying Table 5.1.
A seat that is too high will uncomfortably compress the underside of the thighs.
A seat that is too low will raise the knees uncomfortably high and decrease trunk angle, again increasing disk pressure. Specific recommendations for seat height and other seat parameters (shown in Figure 5.6) are given in Table 5.2.
In addition, armrests for shoulder and arm support and footrests for shorter individuals are recommended. Casters assist in movement and ingress/egress from workstations. However, there may be situations where a stationary chair is desired. In general, the chair should be slightly contoured, slightly cushioned, and covered in a breathable fabric to prevent moisture buildup. Overly soft cushioning restricts posture and may restrict circulation in the legs. An overall optimum working posture and workstation is shown in Figure 5.10.

ENCOURAGE POSTURAL FLEXIBILITY
The workstation height should be adjustable so that the work can be performed efficiently either standing or sitting. The human body is not designed for long periods of sitting. The disks between the vertebrae do not have a separate blood supply, and they rely on pressure changes resulting from movement to receive nutrients and remove wastes. Postural rigidity also reduces blood flow to the muscles and induces muscle fatigue and cramping. An alternate compromise is to provide a sit/stand stool so that the operator can change postures easily. Two key features for a sit/stand stool are height adjustability and a large base of support so that the stool does not tip, preferably long enough that the feet can rest on and counterbalance it (see Figure 5.11).

PROVIDE ANTIFATIGUE MATS FOR
A STANDING OPERATOR
Standing for extended periods on a cement floor is fatiguing. The operators should be provided with resilient antifatigue mats. The mats allow small muscle contractions in the legs, forcing the blood to move and keeping it from tending to pool in the lower extremities.

LOCATE ALL TOOLS AND MATERIALS WITHIN
THE NORMAL WORKING AREA
In every motion, a distance is involved. The greater the distance, the larger the muscular effort, control, and time. It is therefore important to minimize distances.

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Arms: When operator's hands are on keyboard, upper arm and forearm should form right angle; hands should be lined up with forearm; if hands are angled up from the wrist, try lowering or downward tilting the keyboard; optional arm rests should be adjustable.
Backrest:
Adjustable for occasional variations; shape should match contour of lower back, providing even pressure and support.

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Telephone: Cradling telephone receiver between head and shoulder can cause muscle strain; headset allows head, neck to remain straight while keeping hands free.
Screen:
Positioned so that midscreen is 150 down from eye level. Keyboard:
Positioned to allow hands, forearms to remain straight, level.

Posture: Sit all the way back into chair for proper back support; back, neck should be comfortably erect; knees should be slightly lower than hips; do not cross legs or shift weight one side; give joints, muscles a chance to relax; periodically, get up and walk around.
Desk: Thin work surface to allow leg room and posture adjustments; adjustable surface height preferable; table should be large enough for books, files, telephone while permitting different positions of screen, keyboard, mouse pad.

Document holder: Same height and distance from user as the screen, so eyes can remain focused as they look from one to

Seat: Adjustable height, angle; firm cushion; "waterfall" front helps circulation to legs.
Feet: Entire sole should rest comfortably on floor or foot rest.
Avoiding eye strain:
1. Get glasses that improve focus on screen; measure distance before visiting eye doctor.
2. Try to position screen or lamps so that lighting is indirect; do not have light shining directly at screen or into eyes.

3. Use a glare-reducing screen. 4. Periodically rest eyes by looking into the distance.

Figure 5.10 Properly adjusted workstation.

The normal working area in the horizontal plane of the right hand includes the area circumscribed by the arm below the elbow when it is moved in an arc pivoted at the elbow (see Figure 5.12). This area represents the most convenient zone within which motions may be made by that hand with a normal expenditure of energy. The normal area of the left hand may be similarly established. Since movements are made in the third dimension, as well as in the horizontal plane, the normal working area also applies to the vertical plane. The normal area relative to

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Figure 5.11 Industrial sit/stand stools.

(Courtesy: BioFit, Waterville, OH)

Assembly
Normal

Maximum

.8"
15

24.

3"

Length of arm
Length of forearm
Length of upper arm
Length of hand
Length of end joint
(2nd finger)

28
10
12
6.7
0.9

1.3"

3.5"
2"
12.4"

Figure 5.12 Normal and maximum working areas in the horizontal plane for women
(for men, multiply by 1.09).

height for the right hand includes the area circumscribed by the lower arm in an upright position hinged at the elbow moving in an arc. There is a similar normal area in the vertical plane (see Figure 5.13).

FIX LOCATIONS FOR ALL TOOLS AND MATERIALS
TO PERMIT THE BEST SEQUENCE
In driving an automobile, we are all familiar with the short time required to apply the foot brake. The reason is obvious: since the brake pedal is in a fixed cation, no time is required to decide where the brake is located. The body responds instinctively and applies pressure to the area where the driver knows the foot pedal is located. If the location of the brake foot pedal varied, the driver would need considerably more time to brake the car. Similarly, providing fixed locations for all tools and materials at the workstation eliminates, or at least

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Maximum for extended arm

28

"

Normal for forearm 16.7"

Figure 5.13 Normal and maximum working areas in the vertical plane for women
(for men, multiply by 1.09).

minimizes, the short hesitations required to search for and select the objects needed to do the work. These are the ineffective Search and Select therbligs discussed in
Chapter 4 (see Figure 5.14).

USE GRAVITY BINS AND DROP DELIVERY TO REDUCE
REACH AND MOVE TIMES
The time required to perform both of the transport therbligs Reach and Move is directly proportional to the distance that the hands must move in performing these therbligs. Utilizing gravity bins, components can be continuously brought to the normal work area, thus eliminating long reaches to get these supplies (see
Figure 5.15). Likewise, gravity chutes allow the disposal of completed parts within the normal area, eliminating the necessity for long moves to do so. Sometimes, ejectors can remove finished products automatically. Gravity chutes make a clean work area possible, as finished material is carried away from the work area, rather than stacked up all around it. A bin raised off the work surface (so that the hand can partially slide underneath) will also decrease the time required to perform this task by approximately 10 to 15 percent.

ARRANGE TOOLS, CONTROLS, AND OTHER
COMPONENTS OPTIMALLY TO MINIMIZE MOTIONS
The optimum arrangement depends on many characteristics, both human (strength, reach, sensory) and task (loads, repetition, orientation). Obviously, not all factors can be optimized. The designer must set priorities and make trade-offs in the layout

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Figure 5.14 Tool balancers provide fixed locations for tools.

(Courtesy of Packers Kromer.)

of the workplace. However, certain basic principles should be followed. First, the designer must consider the general location of components relative to other components, using the importance and frequency-of-use principles. The most important, as determined by overall goals or objectives, or most frequently used components, should be placed in the most convenient locations. For example, an emergency stop button should be placed in a readily visible, reachable, or convenient position. Similarly, a regularly used activation button, or the most often used fasteners, should be within easy reach of the operator.
Once the general location has been determined for a group of components, that is, the most frequently used parts for assembly, the principles of functionality and sequence of use must be considered. Functionality refers to the grouping of components by similar function, for example, all fasteners in one area, all

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Figure 5.15 A workstation utilizing gravity bins and a belt conveyor to reduce reach and move times.

The conveyor in the background carries other parts past this particular workstation.
The operator is feeding the conveyor from under the platform by merely dropping assembled parts onto the feeder belt. (Source: Alden Systems Co.)

gaskets and rubber components in another area. Since many products are assembled in a strict sequence, cycle after cycle, it is very important to place the components or subassemblies in the order that they are assembled, since this will have a very large effect on reducing wasteful motions. The designer should also consider using Muther’s Systematic Layout Planning (see Chapter 3) or other types of adjacency layout diagramming techniques, to develop a quantitative or relative comparison of the various layouts of components on a work surface. The relationships between components can be modified from traditional data on the flow from one area to another, and should include visual links (eye movements), auditory links (voice communications or signals), and tactile and control motions.
These principles of work design for workstations are summarized in the
Workstation Evaluation Checklist (see Figure 5.16). The analyst may find this useful in evaluating existing workstations or implementing new workstations.

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Sitting Workstation
1. Is the chair easily adjustable according to the following features:
a. ls the seat height adjustable from 15 to 22 inches?
b. Is the seat width a minimum of 18 inches?
c. Is the seat depth 15 to 16 inches?
d. Can the seat be sloped Ϯ 10° from horizontal?
e. Is a back rest with lumbar support provided?
f. Is the back rest a minimum of 8 x 12 inches in size?
g. Can the back rest be moved 7 to 10 inches above the seat?
h. Can the back rest be moved 12 to 17 inches from the front of the seat?
i. Does the chair have five legs for support?
j. Are casters and swivel capability provided for mobile tasks?
k. Is the chair covering breathable?
I. Is a footrest (large, stable, and adjustable in height and slope) provided?
2. Has the chair been adjusted properly?
a. ls the seat height adjusted to the popliteal height with the feet flat on the floor?
b. Is there approximately a 90° angle between the trunk and thigh?
c. ls the lumbar area of the back support in the small of the back (~ belt line)?
d. Is there sufficient legroom (i.e., to the back of the workstation)?
3. Is the workstation surface adjustable?
a. Is the workstation surface roughly at elbow rest height?
b. Is the surface lowered 2 to 4 inches for heavy assembly?
c. ls the surface raised 2 to 4 inches (or tilted) for detailed assembly or visually intensive tasks?
d. Is there sufficient thigh room (i.e., from the bottom of the worksurface)?
4. Is sitting alternated with standing or walking?

Yes

























No

























Computer Workstation
1. Has the chair been adjusted first, then keyboard and mouse, finally the monitor?
2. Is the keyboard as low as possible (without hitting the legs)?
a. Are the shoulders relaxed, upper arms hanging down comfortably, and forearms below horizontal
(i.e., elbow angle >90°)?
b. Is a keyboard shelf utilized (i.e., lower than a normal 28-inch writing surface)?
c. Is the keyboard sloped downward so as to maintain a neutral wrist position?
d. Is the mouse positioned next to the keyboard at the same height?
e. Are armrests (adjustable in height at least 5 inches) provided?
f. If no armrest, are wrist rests provided?
3. Is the monitor positioned 16 to 30 inches (roughly arm's length) from the eyes?
a. ls the top of the screen slightly below eye level?
b. Is the bottom of the screen roughly 30° down from horizontal eye level?
c. ls the monitor positioned at a 90° angle to windows to minimize glare?
d. Can the windows be covered with curtains or blinds to reduce bright light?
e. ls the monitor tilted to minimize ceiling light reflections?
f. If glare still exists, is an antiglare filter utilized?
g. Is a document holder utilized for data transfer from papers?
h. Is the main visual task (monitor or documents) placed directly in front?

Yes



No



































Standing Workstation
1. Is the workstation surface adjustable?
a. Is the workstation surface roughly at elbow rest height?
b. Is the surface lowered 4 to 8 inches for heavy assembly?
c. Is the surface raised 4 to 8 inches (or tilted) for detailed assembly or visually intensive tasks?
2. Is there sufficient legroom?
3. Is a sit/stand stool (adjustable in height) provided?
4. Is standing alternated with sitting?

Yes








No








Figure 5.16 Workstation Evaluation Checklist.

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5.3 PRINCIPLES OF WORK DESIGN:
MACHINES AND EQUIPMENT
TAKE MULTIPLE CUTS WHENEVER POSSIBLE BY
COMBINING TWO OR MORE TOOLS IN ONE, OR BY
ARRANGING SIMULTANEOUS CUTS FROM BOTH
FEEDING DEVICES
Advanced production planning for the most efficient manufacture includes taking multiple cuts with combination tools and simultaneous cuts with different tools. Of course, the type of work to be processed and the number of parts to be produced determine the desirability of combining cuts, such as cuts from both the square turret and the hexagonal turret.

USE A FIXTURE INSTEAD OF THE HAND
AS A HOLDING DEVICE
If either hand is used as a holding device during the processing of a part, then the hand is not performing useful work. Invariably, a fixture can be designed to hold the work satisfactorily, thus allowing both hands to do useful work. Fixtures not only save time in processing parts, but also permit better quality in that the work can be held more accurately and firmly. Many times, foot-operated mechanisms allow both hands to perform productive work.
An example will help clarify the principle of using a fixture, as opposed to the hands, for holding work. A company that produced specialty windows needed to remove a 0.75-in-wide strip of protective paper from around all four edges of both sides of Lexan panels. An operator would pick up a single sheet of Lexan and bring it to the work area. The operator would then pick up a pencil and square and mark the four corners of the Lexan panel. The pencil and square would be laid aside and a template would be picked up and located on the pencil marks. The operator would then strip the protective paper from around the periphery of the Lexan panels. The standard time developed by MTM-1 was
1.063 min per piece.
A simple wood fixture was developed to hold three Lexan panels while each was stripped of the 0.75-in-wide protection paper around the periphery. With the fixture method, the worker picked up three Lexan sheets and located them in the fixture (see Figure 5.17). The protective paper was stripped, the sheets were turned 180°, and the protective paper was removed from the remaining two sides.
The improved method resulted in a standard of 0.46 min per panel, or a savings of 0.603 min of direct labor per panel.

LOCATE ALL CONTROL DEVICES FOR BEST OPERATOR
ACCESSIBILITY AND STRENGTH CAPABILITY
Many of our machine tools and other devices are mechanically perfect, yet incapable of effective operation, because the facility designer overlooked various

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Lexan sheets
Hinge
3/4"

3/4"

Lid

Figure 5.17 Fixture for stripping 3⁄4 in-wide protection paper around periphery of
Lexan sheets.

human factors. Handwheels, cranks, and levers should be of such a size and placed in such positions that operators can manipulate them with maximum proficiency and minimum fatigue. Frequently used controls should be positioned between elbow and shoulder height. Seated operators can apply maximum force to levers located at elbow level; standing operators, to levers located at shoulder height. Handwheel and crank diameters depend on the torque to be expended and the mounting position. The maximum diameters of handgrips depend on the forces to be exerted. For example, for a 10- to 15-lb (4.5- to 6-kg) force, the diameter should be no less than 0.25 in (0.6 cm) and preferably larger; for 15 to
25 lb (6.8 to 11.4 kg) , a minimum of 0.5 in (1.3 cm) should be used; and for
25 lb or more (11.4 kg), a minimum of 0.75 in (1.9 cm). However, diameters should not exceed 1.5 in (3.8 cm), and the grip length should be at least 4 in (10 cm), to accommodate the breadth of the hand.
Guidelines for crank and handwheel radii are as follows: light loads, radii of 3 to 5 in (7.6 to 12.7 cm); medium to heavy loads, radii of 4 to 7 in (10.2 to
17.8 cm); very heavy loads, radii of more than 8 in (20 cm) but not in excess of
20 in (51 cm). Knob diameters of 0.5 to 2 in (1.3 to 5.1 cm) are usually satisfactory. The diameters of knobs should be increased as greater torques are needed.

USE SHAPE, TEXTURE, AND SIZE CODING
FOR CONTROLS
Shape coding, using two- or three-dimensional geometric configurations, permits both tactual and visual identification. It is especially useful under low-light conditions or in situations where redundant or double-quality identification is desirable, thus helping to minimize errors. Shape coding permits a relatively large number of discriminable shapes. An especially useful set of known shapes that are seldom confused is shown in Figure 5.18. Multiple rotation knobs are used for continuous controls in which the adjustment range is more than one full turn.
Fractional rotation knobs are used for continuous controls with a range less than a full turn, while detent positioning knobs are for discrete settings. In addition to shape, the surface texture can provide cues for discrimination by touch. Typically,

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Class A
Multiple rotation

Class B
Fractional rotation

197

Class C
Detent positioning

(a) Types of rotation

(b) Shape of knobs

A
Smooth

B

C
Fluted

D

E

F

G

H

I

J

Knurled

(c) Texture of knobs

Figure 5.18 Examples of knob designs for three classes of use that are seldom confused by touch.

The diameter or length of these controls should be between 0.5 and 4.0 in (1.3 and 10 cm), except for class C, where 0.75 in (1.9 cm) is the minimum suggested. The height should be between 0.5 and 1 in (1.3 and 2.5 cm).
(a&b) Adapted from Hunt, 1953 (c) Source: Bradley, 1967.

three textures are rarely confused: smooth, fluted, and knurled. However, as the number of shapes and textures increases, discrimination can be difficult and slow if the operator must identify controls without vision. If the operator is obliged to wear gloves, then shape coding is only desirable for visual discrimination, or for the tactual discrimination of only two to four shapes.
Size coding, analogous to shape coding, permits both tactual and visual identification of controls. Size coding is used principally where the controls cannot be seen by the operators. Of course, as is the case with shape coding, size coding permits redundant coding, since controls can be discriminated both tactually and visually. In general, try to limit the size categories to three or four, with at least a
0.5-in size difference between controls. Operational coding requiring a unique

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movement (e.g., putting the gearshift into reverse) is especially useful for critical controls that shouldn’t be activated inadvertently.

USE PROPER CONTROL SIZE, DISPLACEMENT,
AND RESISTANCE
In their work assignments, workers continually use various types and designs of controls. The three parameters that have a major impact on performance are control size, control-response ratio, and control resistance when engaged. A control that is either too small or too large cannot be activated efficiently. Tables 5.3,
5.4, and 5.5 provide helpful design information about minimum and maximum dimensions for various control mechanisms.
The control-response (C/R) ratio is defined as the amount of movement in a control divided by the amount of movement in the response (see Figure 5.19).
Table 5.3 Control Size Criteria

Control size
Control
Pushbutton

Fingertip
Thumb/palm
Foot

Diameter
Diameter
Diameter
Tip diameter
Lever arm length
Length
Width
Depth

13
19
8
3
13
25
*
16

*
*
*
25
50
*
25
*

Finger/thumb

Depth
Diameter
Depth
Diameter
Radius
Radius
Diameter
Rim thickness
Diameter
Width
Protrusion from surface Diameter
Diameter
Grasp area
Length
Width
Diameter 75 in per inch of valve size

13
10
19
38
13
13
175
19
38
*

25
100
*
75
113
500
525
50
*
*

3
13
38
75
88
25

*
75
75
*



Toggle switch
Rotary selector
Continuous adjustment knob Hand/palm
Cranks

For rate
For force

Handwheel
Thumbwheel

Lever handle

Minimum Maximum
(mm)
(mm)

Dimension

Finger
Hand

Crank handle
Pedal
Valve handle
* No limit set by operator performance.
† Dependent on space available.

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Table 5.4 Control Displacement Criteria

Displacement
Control
Pushbutton

Toggle switch
Rotary selector

Condition

Minimum

Thumb/fingertip operation
Foot normal
Heavy boot
Ankle flexion only
Leg movement
Between adjacent positions
Total
Between adjacent detents: Visual
Nonvisual
For facilitating performance
When special engineering is required

Maximum

3 mm
13 mm
25 mm


30°

15°
30°



25 mm


63 mm
100 mm

120°


40°
90°

Continuous adjustment knob

Determined by desired control/display ratio
(mm of control movement for each mm of display movement)

Crank

Determined by desired control/display ratio

Handwheel
Thumbwheel
Lever handle
Pedals

Determined by desired control/display ratio 90°–120°†
Determined by number of positions
Fore-aft movement
Lateral movement
Normal
Heavy boot
Ankle flexion (raising)
Leg movement

*
*
13 mm
25 mm



350 mm
950 mm


63 mm
175 mm

*None established.
†Provided optimum control/display ratio is not hindered.

A low C/R ratio indicates high sensitivity, such as in the coarse adjustment of a micrometer. A high C/R ratio means low sensitivity, such as the fine adjustment on a micrometer. Overall control movement depends on the combination of the primary travel time to reach the approximate target setting and the secondary adjust time to reach the exact target setting accurately. The optimum C/R ratio that minimizes this total movement time depends on the type of control and task conditions (see Figure 5.20). Note that there is also a range effect—the tendency to overshoot short distances and undershoot long distances.
Control resistance is important in terms of providing feedback to the operator.
Ideally, it can be of two types: pure displacement with no resistance or pure force with no displacement. The first has the advantage of being less fatiguing, while the second is a deadman’s control, that is, the control returns to zero upon release. Reallife controls are typically spring-loaded, incorporating the features of both. Faulty

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Table 5.5 Control Resistance Criteria

Resistance
Control

Toggle switch
Rotary selector
Continuous
adjustment knob

Handwheel†

Lever handle

Pedal

1444444444442444444444443 14243

Thumbwheel

Fingertip
Foot: Normally off control
Rested on control
Finger operation
Torque
Torque: Fingertip Ͻ1-in dia
Fingertip Ͼ 1-in dia
Rapid, steady turning: Ͻ3-in radius
5–8 in radius
Precise settings
Precision operation: Ͻ3-in radius
5–8 in radius
Resistance at rim: One-hand
Two-hand
Torque
Finger grasp
Hand grasp: One-hand
Two-hand
Fore-aft: Along median plane:
One-hand—10 in forward SRP§
—16–24 in forward SRP
Two-hand—10–19 in forward SRP
Lateral:
One-hand—10–19 in forward SRP
Two-hand—10–19 in forward SRP
Foot: Normally off control
Rested on control
Ankle flexion only
Leg movement

123 123

Crank

123

Pushbutton

Condition

Minimum Maximum
(kg)
(kg)
0.17
1.82
4.55
0.17
1 cm·kg
*
*
0.91
2.28
1.14
*
1.14
2.28
2.28
1 cm·kg
0.34
0.91
1.82

1.14
9.10
9.10
1.14
7 cm·kg
0.3 cm·kg
0.4 cm·kg
2.28
4.55
3.64
*
3.64
13.64
22.73
3 cm·kg
1.14







13.64
22.73
45



1.82
4.55



9.09
22.73


4.55
80

* Not established.
† For valve handles/wheels: 25 Ϯ cm·kg of torque/cm of valve size (8 cm·kg of torque/cm of handle diameter).
§ SRP ϭ Seat reference point.

control aspects include high initial static friction, excessive viscous damping, and deadspace, that is, control movement with no response. All three impair tracking and use performance. However, the first two are sometimes incorporated purposely to prevent inadvertent activation of the control (Sanders and McCormick, 1993).

ENSURE PROPER COMPATIBILITY BETWEEN
CONTROLS AND DISPLAYS
Compatibility is defined as the relationship between controls and displays that is consistent with human expectations. Basic principles include affordance, the

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Large display movement Small lever movement Small display movement Limited movement or limited rotation

Large lever movement Low C/R ratio
(high sensitivity)
(high gain)

Large movement or several rotations

High C/R ratio
(low sensitivity)
(low gain)

Figure 5.19 Generalized illustrations of low and high control–response ratios (C/R ratios) for lever and rotary controls. The C/R ratio is a function of the linkage between the control and the display.

7

4
3
2

Optimum
C/R ratio

us t tim

Trav el 5

j
Ad

Time (seconds)

6

e

1
0

Low
High
(High sensitivity)
(Low sensitivity)
Control - response ratio (C/R ratio)

Figure 5.20 Relationship between C/R ratio and movement time (travel time and adjust time).

The specific C/R ratios are not meaningful out of their original context, so are omitted here. These data, however, very typically depict the nature of the relationships, especially for knob controls.
(Source: Jenkins and Connor, 1949.)

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perceived property results in the desired action; mapping, the clear relationship between controls and responses; and feedback, so that the operator knows that the function has been accomplished. For example, good affordance is a door with a handle that pulls open or a door with a plate that pushes open. Spatial mapping is provided on well-designed stoves. Movement compatibility is provided by direct drive action, scale readings that increase from left to right, and clockwise movements that increase settings. For circular displays, the best compatibility is accomplished with a fixed scale and moving pointer display (see Section 7.4).
For vertical or horizontal displays, Warrick’s principle, which says that points closest on the display and control move in the same direction, provides the best compatibility (see Figure 5.19). For controls and displays in different planes, a clockwise movement for increases and the right-hand screw rule (the display advances in the direction of motion of a right-handed screw or control) are most compatible. For stick controls of a direct drive, the best approach is up results in up movement (Sanders and McCormick, 1993).
The principles of work design for machines and equipment are summarized in the Machine Evaluation Checklist (Figure 5.21). The analyst may find this useful in evaluating and designing machines or other equipment.

5.4 CUMULATIVE TRAUMA DISORDERS
The cost of work-related musculoskeletal disorders such as cumulative trauma disorders (CTDs) in U.S. industry, although not all due to improper work design, is quite high. Data from the National Safety Council (2003) suggest that 15 to 20 percent of workers in key industries (meatpacking, poultry processing, auto assembly, and garment manufacturing) are at potential risk for CTD, and 61 percent of all occupational illnesses are associated with repetitive motions. The worst industry is manufacturing, while the worst occupational title is butchering, with 222 CTD claims per 100,000 workers. With such high rates, and with average medical costs of $30,000 per case, NIOSH and OSHA have focused on the reduction of incidence rates for work-related musculoskeletal disorders as one of their main objectives.
Cumulative trauma disorders (sometimes called repetitive motion injuries, or work-related musculoskeletal disorders) are injuries to the musculoskeletal system that develop gradually as a result of repeated microtrauma due to poor design and the excessive use of hand tools and other equipment. Because of the slow onset and relatively mild nature of the trauma, the condition is often ignored until the symptoms become chronic and more severe injury occurs. These problems are a collection of a variety of problems, including repetitive motion disorders, carpal tunnel syndrome, tendinitis, ganglionitis, tenosynovitis, and bursitis, with these terms sometimes being used interchangeably.
Four major work-related factors seem to lead to the development of CTD: (1) excessive force, (2) awkward or extreme joint motions, (3) high repetition, and (4) duration of work. The most common symptoms associated with CTD include: pain, joint movement restriction, and soft tissue swelling. In the early stages, there may be few visible signs; however, if the nerves are affected, sensory

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Machine Efficiency and Safety

203

Yes

No



















Yes

No

















Design of Emergency Controls
1. Are power-on controls designed to prevent accidental activation?
2. Do activation controls require a unique or dual-action motion?
3. Are power-on buttons recessed?
4. Are activation controls colored green?
5. Are deadman controls utilized for continually activated controls?
6. Are emergency controls designed for quick activation?
7. Are stop buttons protruding?
8. Are emergency controls large and easy to activate?
9. Are emergency controls easily reachable?
10. Are emergency controls visible and colored red?
11. Are emergency controls placed away from other normally used controls?

Yes












No












Control Placement
1. Are primary controls placed in front of the operator at elbow height?
a. Are frequency-of-use and importance principles used to identify primary controls?
2. Are secondary controls placed next to primary controls, but still within reach?
3. Is twisting avoided in reaching for controls?
4. Are controls located in the proper sequence of operation?
5. Are mutually related controls grouped together?
6. Are hand-operated controls separated by at least 2 inches?
7. Are three or less foot pedals utilized?
8. Are foot pedals located at floor level to avoid raising the leg?
9. Is a sit/stand stool provided for extended foot pedal operation?

Yes











No











Display Design
1. Are displays located on the visual cone of sight (horizontal to 30° down)?
2. Are indicator lights used to attract the operator's attention?
3. Are acoustic signals used for critical warnings?
4. Are movable pointers used to indicate trends?
5. Are counters provided for accurate readings?
6. Are displays grouped so as to accentuate an abnormal display?
7. Are mutually related displays grouped together?

Yes








No








Figure 5.21 Machine Evaluation Checklist

(continued)

1. Are multiple or simultaneous cuts possible?
2. Are handles, wheels, and levers readily accessible?
3. Are handles, wheels, and levers designed for best mechanical advantage?
a. Are knobs at least 0.5–2 inches in diameter, with larger sizes for greater torque?
b. Are cranks and handwheels a minimum of 3–5 inches in diameter for low loads?
c. Are cranks and handwheels more than 8 inches in diameter for heavy loads?
4. Are fixtures used to avoid holding with the hand?
5. Are guards or interlocks used to prevent unintended entry?
Design of General Controls
1. Are different colors used for different controls?
2. Are controls clearly labeled?
3. Are shape and texture coding used for tactual identification?
a. Are no more than seven unique codes being utilized?
4. Is size coding used for tactual identification?
a. Are no more than three unique codes being utilized?
b. Are size differences greater than 0.5 inch?

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Control-Display Compatibility
1.
2.
3.
4.
5.
6.
7.
8.

Yes

1.
2.
3.
4.
5.

No







Is clear and concise wording used?
Do the letters subtend at least 12 arcminutes of visual angle?
Are dark letters used on a white background?
Are uppercase letters used for only a few words?
Are symbols (preferably simple) used only if clearly understood?










Yes

Label Design

No










Is affordance (perceived property results in desired action) used?
Is feedback utilized to indicate completion of action?
Does the control and display have a direct-drive relationship?
Does the display reading increase from left to right?
Do clockwise motions increase settings?
Do clockwise motions close valves?
For stick controls, does upward or backward motion produce upward motion?
For controls out of plane, does the right-hand rule apply?







Figure 5.21 Machine Evaluation Checklist
Figure 5.22 A pictorial view of the carpal tunnel

(From: Putz-Anderson, 1988.)

Median nerve Ulnar nerve Tendons
Ligament

Bones

responses and motor control may be impaired. If left untreated, CTD can result in permanent disability.
The human hand is a complex structure of bones, arteries, nerves, ligaments, and tendons. The fingers are controlled by the extensor carpi and flexor carpi muscles in the forearm. The muscles are connected to the fingers by tendons, which pass through a channel in the wrist, formed by the bones of the back of the hand on one side and the transverse carpal ligament on the other. Through this channel, called the carpal tunnel, also pass various arteries and nerves

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(see Figure 5.22). The bones of the wrist connect to two long bones in the forearm, the ulna and the radius. The radius connects to the thumb side of the wrist, and the ulna connects to the little-finger side of the wrist. The orientation of the wrist joint allows movement in two planes, at 90°to each other (see Figure 5.23).
The first permits flexion and extension. The second movement plane permits ulnar and radial deviation. Also, rotation of the forearm can result in pronation with the palm down or supination with the palm up.
Tenosynovitis, one of the more common CTDs, is the inflammation of the tendon sheaths due to overuse or unaccustomed use of improperly designed

Ulnar deviation Neutral

Radial deviation Abduction

Pronation

Supination

Extension

Neutral

Neutral

Figure 5.23 Positions of the hand and arm.

(From: Putz-Anderson, 1988.)

Flexion

Adduction

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tools. If the inflammation spreads to the tendons, it becomes tendinitis. It is often experienced by trainees exposed to large ulnar deviations, coupled with supination of the wrist. Repetitive motions and impact shocks may further aggravate the condition. Carpal tunnel syndrome is a disorder of the hand caused by injury of the median nerve inside the wrist. Repetitive flexion and extension of the wrist under stress may cause inflammation of the tendon sheaths. The sheaths, sensing increased friction, secrete more fluid to lubricate the sheaths and facilitate tendon movement. The resulting buildup of fluid in the carpal tunnel increases pressure, which in turn compresses the median nerve. Symptoms include impaired or lost nervous function in the first 31/2 fingers, manifesting as numbness, tingling, pain, and loss of dexterity. Again, proper tool design is very important for avoiding these extreme wrist positions. Extreme radial deviations of the wrist result in pressure between the head of the radius and the adjoining part of the humerus, resulting in tennis elbow, a form of tendinitis. Similarly, simultaneous extension of the wrist, concurrent with full pronation, is equally stressful on the elbow.
Trigger finger is a form of tendinitis resulting from a work situation in which the distal phalanx of the index finger must be bent and flexed against resistance before more proximal phalanges are flexed. Excessive isometric forces impress a groove on the bone, or the tendon enlarges due to inflammation.
When the tendon moves within the sheath, it may jerk or snap with an audible click. White finger results from excessive vibration from power tools, inducing the constriction of arterioles within the digits. The resulting lack of blood flow appears as a blanching of the skin, with a corresponding loss of motor control.
A similar effect can occur as a result of exposure to cold and is termed Raynaud’s syndrome. A very good introduction to these and other CTDs can be found in Putz-Anderson (1988).
Not all incidences are traumatic. Short-term fatigue and discomfort have also been shown to result from poor handle and work orientation in hammering, and improper tool shape and work height in work with screwdrivers. Typically, a poor tool grip design leads to the exertion of higher grip forces and to extreme wrist deviations, resulting in greater fatigue (Freivalds, 1996).
To evaluate the level of CTD problems in a plant, the methods analyst or ergonomist typically starts out by surveying the workers to determine their health and discomfort at work. One common tool utilized for this purpose is the body discomfort chart (Corlett and Bishop, 1976; see Figure 5.24) in which the worker rates the level of pain or body discomfort for various parts of the body, on a scale from
0 (nothing at all) to 10 (almost maximum). The rating scale is based on Borg’s
(1990) category ratio scale (CR-10) with verbal anchors shown in Figure 5.24.
A more quantitative approach is the novel CTD risk analysis procedure that sums risk values for all three major causative factors into one risk score (see Figure 5.25; Seth, et al., 1999). A frequency factor is determined by the number of damaging wrist motions and then scaled by a threshold value of 10,000. A posture factor is determined from the degree of deviation from the neutral posture for major upper extremity motions. A force factor is determined from the relative percentage of maximum muscle exertion required for the task, and then scaled by

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H

Workplace, Equipment, and Tool Design

H
H
G F

G
F G
A A
B B
C C
D D
E

E

0
.5
1
2
3
4
5
6
7
8
9
10



Nothing at all
Extremely weak (just noticeable)
Very weak
Weak
(light)
Moderate
Strong

(heavy)

Very strong
Extremely strong (almost max)
Maximal

Figure 5.24 Body discomfort chart.

(Adapted from Corlett and Bishop, 1976.)

15 percent, the maximum allowed for extended static contractions (see Chapter 4).
A final miscellaneous factor incorporates a variety of conditions that may have a role in CTD causation, such as vibration and temperature. They are weighted appropriately and then summed to yield a final CTD risk index. For relatively safe conditions, the index should be less than 1 (similar to the NIOSH lifting index,
Chapter 4).
One example (see Figure 5.25) analyzes the CTD stress incurred on a highly repetitive cutoff operation described in greater detail in Example 8-1. Both the frequency factor of 1.55 and the force factor of 2.00 exceed the safety threshold of 1.0, leading to a total risk value of 1.34, which also exceeds 1.0. Thus, the most cost-effective approach is to decrease the frequency by eliminating or combining unnecessary motions (which may or may not be possible) and decrease the force component by modifying the grasp utilized (the basis for methods change in Example 8.1).
The CTD index has proved to be quite successful at identifying injurious jobs, but it works much better on a relative basis, rather than an absolute basis, for example, rank-ordering critical jobs. Note that the CTD risk index also serves as both a useful checklist for identifying poor postures and a design tool for selecting key conditions to redesign.

5.5 PRINCIPLES OF WORK DESIGN: TOOLS
USE A POWER GRIP FOR TASKS REQUIRING FORCE
AND PINCH GRIPS FOR TASKS REQUIRING PRECISION
Prehension of the hand can basically be defined as variations of grip between two extremes: a power grip and a pinch grip. In a power grip, the cylindrical handle

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Figure 5.25

CTD risk index.

of the tool, whose axis is more or less perpendicular to the forearm, is held in a clamp formed by the partly flexed fingers and the palm. Opposing pressure is applied by the thumb, which slightly overlaps the middle finger (see Figure 5.26).
The line of action of the force can vary with (1) the force parallel to the forearm,

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Two-point pulp pinch

Lateral pinch

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Internal precision

External precision

Hook grip

Palm pinch

Finger press

Figure 5.26 Types of grip.

as in sawing; (2) the force at an angle to the forearm, as in hammering; and
(3) the force acting on a moment arm, creating torque about the forearm, as in using a screwdriver. As the name implies, the power grip is used for power or for holding heavy objects. However, the more the fingers or the thumb deviate from the cylindrical grip, the less force is produced and the greater the precision that can be provided. For example, in holding a light hammer as in tacking, the thumb may deviate from opposing the fingers to aligning with the handle. If the index finger also deviates to the tool axis, as in holding a knife for a precise cut, then a pinch grip is approached, with the blade being pinched between the thumb and index finger. This grip is sometimes called an internal precision grip (Konz and
Johnson, 2000). A hook grip, used for holding a box or a handle, is an incomplete power grip in which the thumb counterforce is not applied, thereby considerably reducing the available grip force.
The pinch grip is used for control or precision. In a pinch grip, the item is held between the distal ends of one or more fingers and the opposing thumb (the thumb is sometimes omitted). The relative position of the thumb and fingers determines how much force can be applied and provides a sensory surface for receiving the feedback necessary to give the precision needed. There are four basic types of pinch grips, with many variations (see Figure 5.26): (1) lateral pinch, thumb opposes the side of the index finger; (2) two- and three-point tip (or pulp)

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pinches, in which the tip (or palmar pad) of the thumb opposes to the tips (or palmar pads) of one or more fingers (for a relatively small cylindrical object, the three digits act as a chuck, resulting in a chuck grip); (3) palm pinch, the fingers oppose the palm of the hand without the thumb participating, as in glass windshield handling; and (4) finger press, the thumbs and fingers press against a surface, as in garment workers pushing cloth into a sewing machine. One specialized grip is an external precision or writing grip, which is a combination of a lateral pinch with the middle finger and a two-point pinch to hold the writing implement
(Konz and Johnson, 2000).
Complete gradation and naming of grips can be found in Kroemer (1986).
Note the significantly decreased strength capability of the various pinch grips as compared to the power grip (see Table 5.6). Large forces should never be applied with pinch grips.

AVOID PROLONGED STATIC MUSCLE LOADING
When tools are used in situations in which the arms must be elevated or the tools must be held for extended periods, muscles of the shoulders, arms, and hands may be statically loaded, resulting in fatigue, reduced work capacity, and soreness. Abduction of the shoulder, with corresponding elevation of the elbow, will occur if work must be done with a pistol-grip tool on a horizontal workplace.
An in-line or straight tool reduces the need to raise the arm and also permits a neutral wrist posture. Prolonged work with arms extended, as in assembly tasks done with force, can produce soreness in the forearm. Rearranging the workplace so as to keep the elbows at 90° eliminates most of the problem (see Figure 5.4).
Similarly, continuously holding an activation switch can result in fatigue of the fingers and reduced flexibility.

PERFORM TWISTING MOTIONS WITH
THE ELBOWS BENT
When the elbow is extended, tendons and muscles in the arm stretch out and provide low force capability. When the elbow is bent 90° or less, the biceps brachii has a good mechanical advantage and can contribute to forearm rotation.

Table 5.6 Relative Strengths for Different Types of Grips

Male
Grip
Power
Tip pinch
Pulp pinch
Lateral pinch

Female

lb

kg

lb

kg

89.9
14.6
13.7
24.5

40.9
6.6
6.2
11.1

51.2
10.1
9.7
17.1

23.3
4.6
4.4
7.8

Source: Adapted from An et al., 1986.

Mean % of
Power Grip
100
17.5
16.6
29.5

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MAINTAIN A STRAIGHT WRIST
As the wrist is moved from its neutral position, a loss of grip strength occurs.
Starting from a neutral wrist position, pronation decreases grip strength by 12 percent, flexion/extension by 25 percent, and radial/ulnar deviation by 15 percent
(see Figure 5.27). Furthermore, awkward hand positions may result in soreness of the wrist, loss of grip, and, if sustained for extended periods, the occurrence of carpal tunnel syndrome. To reduce this problem, the workplace or tools should be redesigned to allow for a straight wrist; for example, lower work surface and edges of containers, and tilt jigs toward the user. Similarly, the tool handle should reflect the axis of grasp, which is about 78° from horizontal, and should be oriented such that the eventual tool axis is in line with the index finger; examples are bent plier handles and a pistol-grip knife (see Figure 5.28).

AVOID TISSUE COMPRESSION
Often, in the operation of hand tools, considerable force is applied by the hand.
Such actions can concentrate considerable compressive force on the palm of the hand or the fingers, resulting in ischemia, which is the obstruction of blood flow to the tissues and eventual numbness and tingling of the fingers. Handles should
Forearm position:
100

Supination (palm up)
Midposition
Pronation (palm down)

Grip strength as percentage of maximum

90
80
70
60
50
40
30
20
10

Neutral

Flexion

Extension

Radial deviation Wrist position

Figure 5.27 Grip strength as a function of wrist and forearm position.

(Source: Based on data form Terrell and Purswell, 1976, Table 1.)

Ulnar deviation 219

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Figure 5.28 (a)
Conventional in-line knife. (b) Modified pistol grip knife.

(From: Putz-Anderson,
1988.)

(a)

(b)

(a) Conventional handle

Figure 5.29

(b) Modified handle

Handle design.

Shown here is a conventional paint scraper (a) that presses on the ulnar artery and a modified handle (b) that rests on the tough tissues between thumb and index finger and prevents pressure on the critical areas of the hand. Note that the handle extends beyond the base of the palm. (Source: Tichauer, 1967.)

be designed with large contact surfaces, to distribute the force over a larger area
(see Figure 5.29) or to direct it to less sensitive areas, such as the tissue between the thumb and index finger. Similarly, finger grooves or recesses in tool handles should be avoided. Since hands vary considerably in size, such grooves would accommodate only a fraction of the population.

DESIGN TOOLS SO THAT THEY CAN BE USED BY
EITHER HAND AND BY MOST INDIVIDUALS
Alternating hands allows the reduction of local muscle fatigue. However, in many situations, this is not possible, as the tool use is one-handed. Furthermore, if the tool is designated for the user’s preferred hand, which for 90 percent of the population is the right hand, then 10 percent are left out. Good examples of right-handed

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tools that cannot be used by a left-handed person include a power drill with side handle on the left side only, a circular saw, and a serrated knife leveled on one side only. Typically, right-handed males show a 12 percent strength decrement in the left hand, while right-handed females show a 7 percent strength decrement.
Surprisingly, both left-handed males and females had nearly equal strengths in both hands. One conclusion is that left-handed subjects are forced to adapt to a right-handed world (Miller and Freivalds, 1987).
Female grip strength typically ranges from 50 to 67 percent of male strength
(Pheasant and Scriven, 1983); for example, the average male can be expected to exert approximately 110 lb (50 kg), while the average female can be expected to exert approximately 60 lb (27.3 kg). Females have a twofold disadvantage: an average lower grip strength and an average smaller grip span. The best solution is to provide a variety of tool sizes.

AVOID REPETITIVE FINGER ACTION
If the index finger is used excessively for operating triggers, symptoms of trigger finger develop. Trigger forces should be kept low, preferably below 2 lb (0.9 kg)
(Eastman Kodak, 1983), to reduce the load on the index finger. Two or three finger-operated controls are preferable (see Figure 5.30); finger strip controls or a power grip bar is even better, because they require the use of more and stronger fingers. Absolute finger flexion strengths and their relative contributions to grip are shown in Table 5.7.
For a two-handled tool, a spring-loaded return saves the fingers from having to return the tool to its starting position. In addition, the high number of repetitions must be reduced. Although critical levels of repetitions are not known,
NIOSH (1989) found high rates of muscle–tendon disorders in workers exceeding 10,000 motions per day.

USE THE STRONGEST WORKING FINGERS:
THE MIDDLE FINGER AND THE THUMB
Although the index finger is usually the finger that is capable of moving the fastest, it is not the strongest finger (see Table 5.7). Where a relatively heavy load is involved, it is usually more efficient to use the middle finger, or a combination of the middle finger and the index finger.

DESIGN 1.5-IN HANDLE DIAMETERS FOR POWER GRIPS
Power grips around a cylindrical object should entirely surround the circumference of the cylinder, with the fingers and thumb barely touching. For most individuals, this would entail a handle diameter of approximately 1.5 in (3.8 cm), resulting in minimum EMG activity, minimum grip endurance deterioration, and maximum thrust forces. In general, the upper end of the range is best for maximum torque, and the lower end is best for dexterity and speed. The handle diameter for precision grips should be approximately 0.5 in (1.3 cm) (Freivalds, 1996).

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Figure 5.30 Thumb-operated and finger-strip-operated pneumatic tool.

Thumb operation (a) results in overextension of the thumb.
Finger-strip control (b,c) allows all the fingers to share the load and the thumb to grip and guide the tool.

(b) Recessed finger strip
(a) Thumb switch

(c) Three-finger trigger for power tools

Table 5.7 Maximal Static Finger Flexion Forces

Max Force
Digit

lb

kg

% Force (of thumb)

% Contribution to
Power Grip

Thumb
Index
Middle
Ring
Little

16
13
14
11
7

7.3
5.9
6.4
5.0
3.2

100
81
88
69
44


29
31
24
16

Source: Adapted from Hertzberg, 1973.

DESIGN HANDLE LENGTHS TO BE A MINIMUM OF 4 IN
For both handles and cutouts, there should be enough space to allow for all four fingers. Hand breadth across the metacarpals ranges from 2.8 in (7.1 cm) for a
5th percentile female to 3.8 in (9.7 cm) for a 95th percentile male (Garrett, 1971).

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Thus, 4 in (10 cm) may be a reasonable minimum, but 5 in (12.5 cm) may be more comfortable. If the grip is enclosed, or if gloves are used, even larger openings are recommended. For an external precision grip, the tool shaft must be long enough to be supported at the base of the first finger or thumb. For an internal precision grip, the tool should extend past the palm, but not far enough to hit the wrist (Konz and Johnson, 2000).

DESIGN A 3-IN GRIP SPAN FOR TWO-HANDLED TOOLS
Grip strength and the resulting stress on finger flexor tendons vary with the size of the object being grasped. On a dynamometer with handles angled inward, a maximum grip strength is achieved at about 3 to 3.2 in (7.68.1 cm) (Chaffin and
Andersson, 1991). At distances different from the optimum, the percent grip strength decreases (see Figure 5.31), as defined by
%Grip strength ϭ 100 Ϫ 0.28*S Ϫ 65.8*S2 where S is the given grip span minus the optimum grip span (3 in for females and
3.2 in for males). For dynamometers with parallel sides, this optimum span decreases to 1.8 to 2 in (4.5 to 5 cm) (Pheasant and Scriven, 1983). Because of the large variation in individual strength capacities, and the need to accommodate most of the working population (i.e., the 5th percentile female), maximal grip requirements should be limited to less than 20 lb. A similar effect is found for pinch

120

50% Males

Maximum grip forces (lb)

100
Grip axis
80
60

95% Males

40
50% Females
20

95% Females

0
0

Figure 5.31 of grip span.

1

2
3
Handle opening (at grip axis)

4

5 inches

Grip strength capability for various population distributions as a function

(From: Greenberg and Chaffin, 1976)

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25

Force (lb)

224

Male

Female

15

5
0

1

2

3
Span (in)

4

5

6

Figure 5.32 Pulp pinch strength capability for various spans.

(From Heffernan and Freivalds, 2000.)

strength (see Figure 5.32). However, the overall pinch force is at a much more reduced force level (approximately 20 percent of power grip) and the optimum pinch span (for a 4-point pulp pinch) ranges from 0.5 to 2 in (1.3 to 5.1 cm) and then drops sharply for larger spans (Heffernan and Freivalds, 2000).

DESIGN APPROPRIATELY SHAPED HANDLES
For a power grip, design for maximum surface contact to minimize unit pressure of the hand. Typically, a tool with a circular cross section is thought to give the largest torque. However, the shape may be dependent on the type of task and the motions involved (Cochran and Riley, 1986). For example, the maximum pull force and the best thrusting actions are actually obtained with a triangular cross section. For a rolling type of manipulation, the triangular shape is slowest. A rectangular shape (with corners rounded) with width to height ratios from 1:1.25 to
1:1.5 appears to be a good compromise. A further advantage of a rectangular cross section is that the tool does not roll when placed on a table. Also, the handles should not have the shape of a true cylinder, except for a hook grip. For screwdriver-type tools, the handle end should be rounded to prevent undue pressure at the palm; for hammer-type tools, the handle may have some flattening curving, to indicate the end of the handle.
In a departure from the circular, cylindrically shaped handles, Bullinger and
Solf (1979) proposed a more radical design using a hexagonal cross section, shaped as two truncated cones joined at the largest ends. Such a shape fits the contours of the palm and thumb best, in both precision and power grips, and it yielded the highest torques in comparison with more conventional handles. A similar dual-truncated conical shape was also developed for a file handle. In this

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case, the heavily rounded square-shaped cross section was found to be markedly superior to more conventional shapes.
A final note on shape is that T-handles yield a much higher torque (up to 50 percent more) than straight screwdriver handles. The slanting of the T-handle generates even larger torques by allowing the wrist to remain straight (Saran, 1973).

DESIGN GRIP SURFACE TO BE COMPRESSIBLE
AND NONCONDUCTIVE
For centuries, wood was the material of choice for tool handles. Wood is readily available and easily worked. It has good resistance to shock and thermal and electrical conductivity, and it has good frictional qualities, even when wet. Since wooden handles can break and stain with grease and oil, there has recently been a shift to plastic and even metal. However, metal should be covered with rubber or leather to reduce shock and electrical conductivity and increase friction
(Fraser, 1980). Such compressible materials also dampen vibration and allow a better distribution of pressure, reducing fatigue and hand tenderness (Fellows and
Freivalds, 1991). However, the grip material should not be too soft; otherwise sharp objects, such as metal chips, will get embedded in the grip and make it difficult to use. The grip surface area should be maximized to ensure pressure distribution over as large an area as possible. Excessive localized pressure may cause pain sufficient to interrupt the work.
The frictional characteristics of the tool surface vary with the pressure exerted by the hand, the smoothness and porosity of the surface, and the type of contamination (Bobjer et al., 1993). Sweat increases the coefficient of friction, while oil and fat reduce it. Adhesive tape and suede provide good friction when moisture is present. The type of surface pattern, as defined by the ratio of ridge area to groove area, shows some interesting characteristics. When the hand is clean or sweaty, the maximum frictions are obtained with high ratios (maximizing the hand–surface contact area); when the hand is contaminated, maximum frictions are obtained with low ratios (maximizing the capacity to channel contaminants away).

KEEP THE WEIGHT OF THE TOOL BELOW 5 LB
The weight of the hand tool will determine how long it can be held or used and how precisely it can be manipulated. For tools held in one hand with the elbow at
90° for extended periods, Greenberg and Chaffin (1976) recommend loads of no more than 5 lb (2.3 kg). In addition, the tool should be well balanced, with the center of gravity as close as possible to the center of gravity of the hand (unless the purpose of the tool is to transfer force, as in a hammer). Thus, the hand or arm muscles do not need to oppose any torque development by an unbalanced tool.
Heavy tools used to absorb impact or vibration should be mounted on telescoping arms or tool balancers to reduce the effort required by the operator. For precision operations, tool weights greater than 1 lb are not recommended, unless a counterbalanced system is used.

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USE GLOVES JUDICIOUSLY
Gloves are often used with hand tools for safety and comfort. Safety gloves are seldom bulky, but gloves worn in subfreezing climates can be very heavy and can interfere with grasping ability. Wearing woolen or leather gloves may add 0.2 in
(0.5 cm) to the hand thickness and 0.3 in (0.8 cm) to the hand breadth at the thumb, while heavy mittens add 1 in (2.5 cm) and 1.6 in (4.0 cm), respectively
(Damon et al., 1966). More important, gloves reduce grip strength 10 to 20 percent (Hertzberg, 1973), torque production, and manual dexterity performance times. Neoprene gloves slow performance times by 12.5 percent over bare-handed performance, terry cloth by 36 percent, leather by 45 percent, and PVC by 64 percent (Weidman, 1970). A trade-off between increased safety and decreased performance with gloves must be considered.

USE POWER TOOLS SUCH AS NUT AND
SCREWDRIVERS INSTEAD OF MANUAL TOOLS
Power hand tools not only perform work faster than manual tools, but also do the work with considerably less operator fatigue. Greater uniformity of product can be expected when power hand tools are used. For example, a power nut driver can drive nuts consistently to a predetermined tightness in inch-pounds, while a manual nut driver cannot be expected to maintain a constant driving pressure due to operator fatigue.
There is, however another trade-off. Powered hand tools produce vibration, which can induce white finger syndrome, the primary symptom of which is a reduction in blood flow to the fingers and hand due to vasoconstriction of the blood vessels. As a result, there is a sensory feedback loss and decreased performance, and the condition may contribute to the development of carpal tunnel syndrome, especially in jobs with a combination of forceful and repetitive exertions. It is generally recommended that vibrations in the critical range of 40 to
130 Hz or a slightly larger (but safer) range of 2 to 200 Hz (Lundstrom and Johansson, 1986) be avoided. The exposure to vibration can be minimized through a reduction in the driving force, the use of specially designed vibration damping handles (Andersson, 1990) or vibration-absorbing gloves, and better maintenance to decrease misalignments or unbalanced shafts.

USE THE PROPER CONFIGURATION AND
ORIENTATION OF POWER TOOLS
In a power drill or other power tools, the major function of the operator is to hold, stabilize, and monitor the tool against a workpiece, while the tools perform the main effort of the job. Although the operator may at times need to shift or orient the tool, the main function for the operator is effectively to grasp and hold the tool. A hand drill is comprised of a head, body, and handle, with all three ideally being in line. The line of action is the line from the extended index finger, which means that in the ideal drill, the head is off-center with respect to the central axis of the body.

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Figure 5.33 Proper orientation of power tools in the workplace.

(From: Armstrong, 1983)

Good
Poor

Poor

Good

Poor
Good

Handle configuration is also important, with the choices being pistol grip, in-line, or right-angle. As a rule of thumb, in-line and right-angle are best for tightening downward on a horizontal surface, while pistol grips are best for tightening on a vertical surface, with the aim being to obtain a standing posture with a straight back, upper arms hanging down, and the wrist straight (see Figure
5.33). For the pistol grip, this results in the handle being at an angle of approximately 78° with the horizontal (Fraser, 1980).
Another important factor is the center of gravity. If it is too far forward in the body of the tool, a turning moment is created, which must be overcome by the muscles of the hand and forearm. This requires muscular effort additional to that required for holding, positioning, and pushing the drill into the workpiece. The primary handle is placed directly under the center of gravity, so that the body juts out behind the handle, as well as in front. For heavy drills, a secondary supportive handle may be needed, either to the side or preferably below the tool, so that the supporting arm can be tucked in against the body, rather than being abducted.

CHOOSE A POWER TOOL WITH THE
PROPER CHARACTERISTICS
Power tools, such as nut-runners used to tighten nuts, are commercially available in a variety of handle configurations, spindle diameters, speeds, weights, shutoff

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mechanisms, and torque outputs. The torque is transferred from the motor to the spindle through a variety of mechanisms, so that the power (often compressed air) can be shut off quickly once the nut or other fastener is tight. The simplest and cheapest mechanism is a direct drive, which is under the operator’s control, but because of the long time to release the trigger once the nut is tightened, direct drive transfers a very large reaction torque to the operator’s arm. Mechanical friction clutches will allow the spindle to slip, reducing some of this reaction torque.
A better mechanism for reducing the reaction torque is the airflow shutoff, which automatically senses when to cut off the air supply as the nut is tightened. A still faster mechanism is an automatic mechanical clutch shutoff. Recent mechanisms include the hydraulic pulse system, in which the rotational energy from the motor is transferred over a pulse unit containing an oil cushion (filtering off the highfrequency pulses, as well as noise), and a similar electrical pulse system, both of which reduce the reaction torque to a large extent (Freivalds and Eklund, 1993).
Variations of torque delivered to the nut depend on several conditions including: properties of the tool; the operator; properties of the joint, for example, the combination of the fastener and the material being fastened (ranging from soft, in which the materials have elastic properties, such as body panels, to hard, in which there are two stiff surfaces, such as pulleys on a crankshaft); and stability of the air supply. The torque experienced by the user (the reaction torque) depends on these factors plus the torque shutoff system. In general, using electrical tools at lower than normal rpm levels, or underpowering pneumatic tools, results in larger reaction torques and more stressful ratings. Pulse-type tools produce the lowest reaction torques, perhaps because the short pulses “chop up” the reaction torque. Other potential problems include noise from the pneumatic mechanism reaching levels as high as 95 dB(A), vibration levels exceeding 132 dB(V), and dust or oil fumes emanating from the exhaust (Freivalds and Eklund, 1993).

USE REACTION BARS AND TOOL BALANCERS
FOR POWER TOOLS
Reaction torque bars should be provided if the torque exceeds 53 in·lb (6 N·m) for in-line tools used for a downward action, 106 in·lb (12 N·m) for pistol grip tools used in a horizontal mode, and 444 in·lb (50 N·m) for right-angled tools used in a downward or upward motion (Mital and Kilbom, 1992).
This information is summarized in an evaluative checklist for tools
(see Figure 5.34). If the tool does not conform to the recommendations and desired features, it should be redesigned or replaced.

SUMMARY
Many factors significantly impact both the productivity and the well-being of the operator at the workstation. Sound ergonomics technology applies to both the equipment being used and the general conditions surrounding the work area. For the equipment point and the workstation environment, adequate flexibility should

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Basic Principles

Yes

No













Yes

No

















Handles and Grips
1. For power uses, is the tool grip 1.5–2 inches in diameter?
a. Can the handle be grasped with the thumb and fingers slightly overlapped? 2. For precision tasks, is the tool grip 5⁄16 – 5⁄8 inches in diameter?
3. Is the grip cross section circular?
4. Is the grip length at least 4 inches (5 inches if gloves are worn)?
5. Is the grip surface finely textured and slightly compressible?
6. Is the handle nonconductive and stain free?
7. For power uses, does the tool have a pistol grip angled at 78°?
8. Can a two-handled tool be operated with less than 20 pounds grip force?
9. Is the span of the tool handles between 2 3/4 – 3 1/4 inches?

Yes


No






















Power Tool Considerations
1. Are trigger activation forces less than 1 pound?
2. For repetitive use, is a finger strip trigger present?
3. Are less than 10,000 triggering actions required per shift?
4. Is a reaction bar provided for torques exceeding:
a. 50 inch-pounds for in-line tools?
b. 100 inch-pounds for pistol-grip tools?
c. 400 inch-pounds for right-angled tools?
5. Does the tool create less than 85 dBA for a full day of noise exposure?
6. Does the tool vibrate?
a. Are the vibrations outside the 2–200 Hz range?

Yes











No











Miscellaneous and General Considerations
1. For general use, is the weight of the tool less than 5 pounds?
2. For precision tasks, is the weight of the tool less than 1 pound?
3. For extended use, is the tool suspended?
4. Is the tool balanced (i.e., center of gravity on the grip axis)?
5. Can the tool be used without gloves?
6. Does the tool have stops to limit closure and prevent pinching?
7. Does the tool have smooth and rounded edges?

Yes








No








1.
2.
3.
4.
5.

Does the tool perform the desired function effectively?
Does the tool match the size and strength of the operator?
Can the tool be used without undue fatigue?
Does the tool provide sensory feedback?
Are the tool capital and maintenance costs reasonable?

Anatomical Concerns
1. If force is required, can the tool be grasped in a power grip
(i.e., handshake)?
2. Can the tool be used without shoulder abduction?
3. Can the tool be used with a 90° elbow angle (i.e., forearms horizontal)?
4. Can the tool be used with the wrist straight?
5. Does the tool handle have large contact surfaces to distribute forces?
6. Can the tool be used comfortably by a 5th percentile female operator?
7. Can the tool be used in either hand?

Figure 5.34 Tool Evaluation Checklist

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be provided so that variations in employee height, reach, strength, reflex time, and so on can be accommodated. A workbench that is 32 in (81 cm) high may be just right for a 75-in (191-cm) tall worker, but would definitely be too high for a
66-in (167.6-cm) tall employee. Adjustable-height workstations and chairs are desirable to accommodate the full range of workers, based on plus or minus two standard deviations from the norm. The better able we are to provide a flexible work center to accommodate the total range of the workforce, the better will be the productivity results and worker satisfaction.
Just as there are significant variations in height and size in the workforce, there are equal or greater variations in visual capacity, hearing ability, feeling ability, and manual dexterity. The vast majority of workstations can be improved.
Applying ergonomic considerations along with methods engineering will lead to more efficient competitive work environments that will improve the well-being of the workers, the quality of the product, the labor turnover of the business, and the prestige of the organization.

QUESTIONS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.

What seat width would accommodate 90 percent of adults?
Compare and contrast the three different design strategies.
Explain how a proper work surface height would be determined.
What are the most critical features in a good ergonomic chair? Which should be adjustable? What is the principle behind the design of a saddle seat?
What is lordosis and how does it relate to a lumbar pad?
What is the principle behind antifatigue mats?
What is the principle behind the proper layout of bins, parts, and tools on a work surface? Why is a fixture so important in workplace? List as many reasons as possible.
What does Warrick’s principle refer to in designing controls and displays?
What is the optimum line of sight?
List three principles for arranging components on a panel.
What is the range effect?
List the three principles for effective control–display compatibility.
What is operational coding?
What is the main disadvantage of tactile controls?
What is “control movement without system response” known as?
If the C/R ratio is increased from 1.0 to 4.0, what happens to travel time, adjust time, and total time?
What are the three most important task factors leading to cumulative trauma disorders? What is the most important factor leading to white finger?
What is trigger finger?

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22. Describe the progression of the disease state for carpal tunnel syndrome.
23. Design an ergonomic handle, indicating all the principles used in the design.
24. What are the key concerns in the design of a power tool?

PROBLEMS
1.

Because of the Challenger disaster, NASA has decided to include a personal escape capability (i.e., a launch compartment) for each space shuttle astronaut. Because space is at a premium, proper anthropometric design is crucial. Also, because of budget restrictions, the design is to be nonadjustable; for example, the same design must fit all present and future astronauts, both males and females. For each launch compartment feature, indicate the body feature used in the design, the design principle used, and the actual value (in inches) to be used in its construction.

Parachute
7
Controls
CONTROLS

4

2
Oxygen 1
Propulsion
fuel

6

Batteries
Heat
Shield

3

5

Launch Compartment
Feature
1.
2.
3.
4.
5.
6.
7.
8.
9.

2.

Body Feature

Design Principle

Actual Value

Height of seat
Seat depth
Height of joystick
Height of compartment
Depth of foot area
Depth of leg area
Depth of chamber
Width of compartment
Weight limit

You are asked to design a control/display panel for the NASA escape launch. After the initial escape, propulsion is to be used to decelerate against the earth’s

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gravitational field. The parachute can be released only within a given, narrow altitude range. Arrange the seven displays/controls, using the same-sized dials as shown on the following control panel. Explain the logic for your arrangement.
30؇ above
20؇
10؇
Eye level
10؇
20؇
30؇
Left Shoulder up A

B

Right Shoulder up C

D

E

% of Viewing
Time

Control/Display

F

G

Importance

A
B
C
D
E
F
G

Launch release
Propulsion fuel level
Air speed indicator
Oxygen pressure
Electric power level
Altitude indicator
Parachute release

3.

The Dorben Foundry uses an overhead crane with a magnetic head to load scrap iron into the blast furnace. The crane operator uses various levers to control the three degrees of freedom needed for the crane and its magnetic head. A control is used to activate/deactivate the magnetic pull of the head. The operator is above the operation and looking down much of the time. Operators frequently complain of

Lever
A
B
C
D

1
20
15
1
2
60
1

# Times Used

Critical
Very important
Important
Unimportant
Important
Critical
Critical

1
10
5
2
3
50
1

Throw Distance (in)

Crane Movement (ft)

Time to Target(s)

20
20
20
20

20
10
80
40

1.2
2.2
1.8
1.2

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back pain. Information on various commercially available lever controls is as follows: a. Design an appropriate control system for the crane operator. Indicate the number of controls needed, their location (especially in reference to the operator’s line of sight), their direction of movement, and their type of feedback.
b. Indicate an appropriate control-response ratio for these controls.
c. What other factors may be important in designing these controls?
The following are data on two different control-response setups. Based on these data, what is the optimum C/R ratio for each setup? Which setup would you recommend as best?

Setup

A

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C/R Ratio

Travel Time (s)

Adjust Time (s)

1
5
10
20

0.1
1
5
10

5
2
0.5
0.1

2
10
15
20

1
3
5
10

6
5
4
3

5.

In a small manufacturing plant, the soldering iron shown here is used to solder connections on a large vertical panel. Several musculoskeletal injuries have been reported on this job over the last year in addition to many operator complaints. In general, it seems that
a. It is difficult to see the point of application when using this tool.
b. The operators are gripping the tool unnecessarily tightly.
c. The power cord tends to get entangled.
d. Operators complain about wrist pain.
Redesign the soldering iron to eliminate the problems outlined above. Point out the ergonomic or other special features that have been incorporated into the design.

6.

Use the CTD risk index to calculate the potential risk of injury for the right hand on the following jobs shown on the Web page:
a. Stamping extrusions—assume a grip force of 30% MVC.
b. Stamping end couplings—assume a grip force of 15% MVC.
c. Flashlight assembly—assume a grip force of 15% MVC.

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d.
e.
f.
g.
h.
7.

Union assembly—assume a grip force of 15% MVC.
Hospital bed rail assembly—assume a grip force of 30% MVC.
Stitching (garments)—assume a grip force of 30% MVC.
Labeling (garments)—assume a grip force of 15% MVC.
Cut and tack (garments)—assume a grip force of 30% MVC.

The worker shown below is driving 4 screws into a panel with a powered driver.
His production quota is 2,300 panes per 8-h shift. What specific ergonomic problems are found on this job? For each problem: (a) specify an ergonomics improvement that would correct the problem and (b) provide a specific work design principle to support this methods change.

REFERENCES
An, K., L. Askew, and E. Chao. “Biomechanics and Functional Assessment of Upper
Extremities.” In Trends in Ergonomics/Human Factors III. Ed. W. Karwowski.
Amsterdam: Elsevier, 1986, pp. 573–580.
Andersson, E. R. “Design and Testing of a Vibration Attenuating Handle.” International
Journal of Industrial Ergonomics, 6, no. 2 (September 1990), pp. 119–125.
Andersson, G. B. J., R. Örtengren, A. Nachemson, and G. Elfström. “Lumbar Disc Pressure and Myoelectric Back Muscle Activity During Sitting.” I, Studies on an Experimental
Chair. Scandinavian Journal of Rehabilitation Medicine, 6 (1974), pp. 104–114.
Armstrong, T. J. Ergonomics Guide to Carpal Tunnel Syndrome. Fairfax, VA: American
Industrial Hygiene Association, 1983.
Bobjer, O., S. E. Johansson, and S. Piguet. “Friction Between Hand and Handle. Effects of Oil and Lard on Textured and Non-textured Surfaces; Perception of Discomfort.”
Applied Ergonomics, 24, no.3 (June 1993), pp. 190–202.
Borg, G. “Psychophysical Scaling with Applications in Physical Work and the
Perception of Exertion.” Scandinavian Journal of Work Environment and Health,
16, Supplement 1 (1990), pp. 55–58.
Bradley, J. V. (1967). Tactual coding of cylindrical knobs. Human Factors, 9(5),
483–496.

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Bullinger, H. J., and J. J. Solf. Ergononomische Arbeitsmittel-gestaltung, II Handgeführte Werkzeuge - Fallstudien. Dortmund, Germany: Bundesanstalt fr
Arbeitsschutz und Unfallforschung, 1979.
Chaffin, D. B., and G. Andersson. Occupational Biomechanics. New York: John Wiley
& Sons, 1991, pp. 355–368.
Cochran, D. J., and M. W. Riley. “An Evaluation of Knife Handle Guarding.” Human
Factors, 28, no. 3 (June 1986), pp. 295–301.
Congleton, J. J. The Design and Evaluation of the Neutral Posture Chair. Doctoral dissertation. Lubbock, TX: Texas Tech University, 1983.
Corlett, E. N., and R. A. Bishop. “A Technique for Assessing Postural Discomfort.”
Ergonomics, 19, no. 2 (March 1976), pp. 175–182.
Damon, A., H. W. Stoudt, and R. A. McFarland. The Human Body in Equipment Design.
Cambridge, MA: Harvard University Press, 1966.
Eastman Kodak Co. Ergonomic Design for People at Work. Belmont, CA: Lifetime
Learning Pub., 1983.
Fellows, G. L., and A. Freivalds. “Ergonomics Evaluation of a Foam Rubber Grip for
Tool Handles.” Applied Ergonomics, 22, no. 4 (August 1991), pp. 225–230.
Fraser, T. M. Ergonomic Principles in the Design of Hand Tools. Geneva, Switzerland:
International Labor Office, 1980.
Freivalds, A. “Tool Evaluation and Design.” In Occupational Ergonomics. Eds. A.
Bhattacharya and J. D. McGlothlin. New York: Marcel Dekker, 1996, pp. 303–327.
Freivalds, A., and J. Eklund. “Reaction Torques and Operator Stress While Using
Powered Nutrunners.” Applied Ergonomics, 24, no. 3 (June 1993), pp. 158–164.
Garrett, J. “The Adult Human Hand: Some Anthropometric and Biomechanical
Considerations.” Human Factors, 13, no. 2 (April 1971), pp. 117–131.
Grandjean, E. (1998). Fitting the task to the man (4th ed.). London: Taylor & Francis.
Greenberg, L., and D. B. Chaffin. Workers and Their Tools. Midland, MI: Pendell Press,
1976.
Heffernan, C., and A. Freivalds. “Optimum Pinch Grips in the Handling of Dies.”
Applied Ergonomics, 31 (2000), pp. 409–414.
Hertzberg, H. “Engineering Anthropometry.” In Human Engineering Guide to
Equipment Design. Eds. H. Van Cott, and R. Kincaid. Washington, DC: U.S.
Government Printing Office, 1973, pp. 467–584.
Hunt, D. P. (1953). The coding of aircraft controls (Tech. Rept. 53–221). U. S. Air
Force, Wright Air Development Center.
Jenkins, W., and Conner, M. B. (1949). Some design factors in making settings on a linear scale. Journal of Applied Psychology, 33, 395–409.
Konz, S., and S. Johnson. Work Design, 5th ed. Scottsdale, AZ: Holcomb Hathaway
Publishers, 2000.
Kroemer, K. H. E. “Coupling the Hand with the Handle: An Improved Notation of
Touch, Grip and Grasp.” Human Factors, 28, no. 3 (June 1986), pp. 337–339.
Kroemer, K. (1989). Engineering anthropometry. Ergonomics, 32(7), 767–784.
Lundstrom, R., and R. S. Johansson. “Acute Impairment of the Sensitivity of Skin
Mechanoreceptive Units Caused by Vibration Exposure of the Hand.” Ergonomics,
29, no. 5 (May 1986), pp. 687–698.

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Miller, G., and A. Freivalds. “Gender and Handedness in Grip Strength.” Proceedings of the Human Factors Society 31st Annual Meeting. Santa Monica, CA, 1987, pp.
906–909.
Mital, A., and Å. Kilbom. “Design, Selection and Use of Hand Tools to Alleviate
Trauma of the Upper Extremities.” International Journal of Industrial Ergonomics,
10, no. 1 (January 1992), pp. 1–21.
National Safety Council. Accident Facts. Chicago: National Safety Council, 2003.
NIOSH, Health Hazard Evaluation-Eagle Convex Glass, Co. HETA-89-137-2005.
Cincinnati, OH: National Institute for Occupational Safety and Health, 1989.
Pheasant, S. T., and S. J. Scriven. “Sex Differences in Strength, Some Implications for the Design of Handtools.” In Proceedings of the Ergonomics Society. Ed. K.
Coombes. London: Taylor & Francis, 1983, pp. 9–13.
Putz-Anderson, V. Cumulative Trauma Disorders. London: Taylor & Francis, 1988.
Sanders, M. S., and E. J. McCormick. Human Factors in Engineering and Design. New
York: McGraw-Hill, 1993.
Saran, C. “Biomechanical Evaluation of T-handles for a Pronation Supination Task.”
Journal of Occupational Medicine, 15, no. 9 (September 1973), pp. 712–716.
Serber, H. “New Developments in the Science of Seating.” Human Factors Bulletin, 33, no. 2 (February 1990), pp. 1–3.
Seth, V., R. Weston, and A. Freivalds. “Development of a Cumulative Trauma Disorder
Risk Assessment Model.” International Journal of Industrial Ergonomics, 23, no. 4
(March 1999), pp. 281–291.
Terrell, R., and J. Purswell. “The Influence of Forearm and Wrist Orientation on Static
Grip Strength as a Design Criterion for Hand Tools.” Proceedings of the Human
Factors Society 20th Annual Meeting. Santa Monica, CA, 1976, pp. 28–32.
Tichauer, E. (1967). Ergonomics: The state of the Art. American Industrial Hygiene
Association Journal, 28, 105–116.
U.S. Department of Justice. Americans with Disabilities Act Handbook. EEOC-BK-19.
Washington, DC: U.S. Government Printing Office, 1991.
Webb Associates. Anthropometric Source Book. II, Pub. 1024. Washington, DC:
National Aeronautics and Space Administration, 1978.
Weidman, B. Effect of Safety Gloves on Simulated Work Tasks. AD 738981. Springfield,
VA: National Technical Information Service, 1970.

SELECTED SOFTWARE
COMBIMAN, User’s Guide for COMBIMAN, CSERIAC. Dayton, OH: WrightPatterson AFB. (http://dtica.dtic.mil/hsi/srch/hsi5.html)
Design Tools (available from the McGraw-Hill text website at www.mhhe.com/niebelfreivalds), New York: McGraw-Hill, 2002.
Ergointelligence (Upper Extremity Analysis). 3400 de Maisonneuve Blvd. West, Suite
1430, Montreal, Quebec, Canada H3Z 3B8.
Jack®. Engineering Animation, Inc., 2321 North Loop Dr., Ames, IA 50010.
(http://www.eai.com/)
Job Evaluator ToolBox™. ErgoWeb, Inc., P.O. Box 1089, 93 Main St., Midway, UT
84032.

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ManneQuinPRO. Nexgen Ergonomics, 3400 de Maisonneuve Blvd. West, Suite 1430,
Montreal, Quebec, Canada H3Z 3B8. (http://www.nexgenergo.com/)
Multimedia Video Task Analysis. Nexgen Ergonomics, 3400 de Maisonneuve Blvd.
West, Suite 1430, Montreal, Quebec, Canada H3Z 3B8.
Safework. Safework (2000) Inc., 3400 de Maisonneuve Blvd. West, Suite 1430,
Montreal, Quebec, Canada H3Z 3B8.

WEBSITES
CTD News— http://ctdnews.com/
CTD Resource Network—http://www.ctdrn.org/
ErgoWeb—http://www.ergoweb.com/
Examples of bad ergonomic design—http://www.baddesigns.com/
CAESAR—http://store.sae.org/caesar

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CHAPTER

6

KEY POINTS








Provide both general and task lighting—avoid glare.
Control noise at the source.
Control heat stress with radiation shielding and ventilation.
Provide both overall air movement and local ventilation for hot areas.
Dampen tool handles and seats to reduce vibration exposure.
Use rapid, forward-rotating shifts, if shiftwork can’t be avoided.

M

ethods analysts should provide good, safe, comfortable working conditions for the operator. Experience has conclusively proved that plants with good working conditions outproduce those with poor conditions.
The economic return from investment in an improved working environment is usually significant. In addition to increasing production, ideal working conditions improve the safety record; reduce absenteeism, tardiness, and labor turnover; raise employee morale; and improve public relations. The acceptable levels for working conditions and the recommended control measures for problem areas are presented in greater detail in this chapter.

6.1 ILLUMINATION
THEORY
Light is captured by the human eye (see Figure 6.1) and processed into an image by the brain. It is a fairly complicated process with the light rays passing through the pupil, an opening in the eye, and through the cornea and lens, which focus the light rays on the retina at the back of the eyeball. The retina is composed of photosensitive receptors, the rods, which are sensitive to black and white, especially at night, but have poor visual acuity, and the cones, which are sensitive to colors in daylight and have good visual acuity. The cones are concentrated in the fovea, while the rods are spread out over the retina. Electrical signals from the photoreceptors are
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lris
Lens

Retina

Vitreous humor Fovea

Pupil
Cornea
Aqueous humor

Optic nerve

Figure 6.1

The human eye.

collected and passed by the optic nerve to the brain where the light from external illumination is processed and interpreted.
The basic theory of illumination applies to a point source of light (such as a candle) of a given luminous intensity, measured in candelas (cd) (see Figure 6.2).
Light emanates spherically in all directions from the source with 1-cd sources emitting 12.57 lumens (lm) (as determined from the surface area of a sphere,
4pr2). The amount of light striking a surface, or a section of this sphere, is termed illumination or illuminance and is measured in footcandles (fc). The amount of illumination striking a surface drops off as the square of the distance d in feet from the source to the surface:
Illuminance ϭ intensity>d2
Some of that light is absorbed and some of it is reflected (for translucent materials, some is also transmitted), which allows humans to “see” that object and provides a perception of brightness. The amount reflected is termed luminance and is measured in foot-lamberts (fL). It is determined by the reflective properties of the surface, known as reflectance:
Luminance ϭ illuminance ϫ reflectance
Reflectance is a unitless proportion and ranges from 0 to 100 percent. Highquality white paper has a reflectance of about 90 percent, newsprint and concrete around 55 percent, cardboard 30 percent, and matte black paint 5 percent.
The reflectances for various color paints or finishes are presented in Table 6.1.

VISIBILITY
The clarity with which the human sees something is usually referred to as visibility. The three critical factors of visibility are visual angle, contrast, and most

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Spherical surface
1 m2 in area

1m radius 1 ft radius Spherical surface
1 ft2 in area

I
I

I

IIII
I II I I I I I I I I

I

I

I II
I II I I I I I I I I

II
I II I I I I I II I

II
I II I I I I II I

Light source
1 cd or 12.57 lumens output

Illumination
1 fc or
1 lumen/ft2 or 10.76 lux

Illumination
1 lux or
1 lumen/m2 or
0.0926 fc

Figure 6.2 Illustration of the distribution of light from a light source following the inverse-square law.

(Source: General Electric Company, 1965, p. 5.)
Table 6.1

Reflectances of Typical Paint and Wood Finishes

Color or finish

Percent of reflected light

White
Light cream
Light gray
Light yellow
Light buff
Light green
Light blue
Medium yellow
Medium buff
Medium gray
Medium green

85
75
75
75
70
65
55
65
63
55
52

Color or finish

Percent of reflected light

Medium blue
Dark gray
Dark red
Dark brown
Dark blue
Dark green
Maple
Satinwood
Walnut
Mahogany

35
30
13
10
8
7
42
34
16
12

important, illuminance. Visual angle is the angle subtended at the eye by the target, and contrast is the difference in luminance between a visual target and its background. Visual angle is usually defined in arc minutes (1/60 of a degree) for small targtets by
Visual angle 1arc min2 ϭ 3,438 ϫ h>d

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arc d (minutes)
1

1/ second exposure
3
50% Accuracy

1
2
Log target contrast

242

0

4
40
60

؊1

؊2
؊2

Figure 6.3

؊1

0
1
2
Log background luminance (f L)

3

Smoothed threshold contrast curves for disks of diameter d.

(Adapted from: Blackwell, 1959)

where h is the height of the target or critical detail (or stroke width for printed matter) and d is the distance from the target to the eye (in the same units as h).
Contrast can be defined in several ways. A typical one is
Contrast ϭ 1Lmax Ϫ Lmin 2 >Lmax where L is luminance. Contrast, then, is related to the difference in maximum and minimum luminances of the target and background. Note that contrast is unitless.
Other less important factors for visibility are exposure time, target motion, age, known location, and training, which will not be included here.
The relationship between these three critical factors was quantified by Blackwell
(1959) in a series of experiments that led to the development of the Illuminating
Engineering Society of North America (IESNA, 1995) standards for illumination.
Although the Blackwell curves (see Figure 6.3) as such are not often used today, they show the trade-off between the size of the object, the amount of illumination (in this case, measured as luminance reflected from the target), and the contrast between the target and background. Thus, although increasing the amount of illumination is the simplest approach to improving task visibility, it can also be improved by increasing the contrast or increasing the size of the target.

ILLUMINANCE
Recognizing the complexity of extending the point source theory to real light sources (which can be anything but a point source) and some of the uncertainties and constraints of Blackwell’s (1959) laboratory setting, the IESNA adopted a

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Recommended Illumination Levels for Use in Interior Lighting Design

Range of illuminance (fc)

Type of activity

A
B

2-3-5
5-7.5-10

Public areas with dark surroundings.
Simple orientation for short temporary visits.

C

10-15-20

Working spaces where visual tasks are performed only occasionally.

D

20-30-50

E

50-75-100

F

100-150-200

Performance of visual tasks of high contrast or large size, e.g., reading printed material, typed originals, handwriting in ink and xerography; rough bench and machine work; ordinary inspection; rough assembly.
Performance of visual tasks of medium contrast or small size, e.g., reading mediumpencil handwriting, poorly printed or reproduced material; medium bench and machine work; difficult inspection; medium assembly. Performance of visual tasks of low contrast or very small size, e.g., reading handwriting in hard pencil on poor-quality paper and very poorly reproduced material; highly difficult inspection, difficult assembly.

G

200-300-500

H

500-750-1,000

I

1,000-1,500-2,000

Category

Performance of visual tasks of low contrast and very small size over a prolonged period,
e.g., fine assembly; very difficult inspection; fine bench and machine work; extra fine assembly. Performance of very prolonged and exacting visual tasks, e.g., the most difficult inspection; extra fine bench and machine work; extra fine assembly.
Performance of very special visual tasks of extremely low contrast and small size, e.g., surgical procedures.

Reference area
General lighting throughout room or area.

Illuminance on task.

Illuminance on task via a combination of general and supplementary local lighting. Source: Adapted from IESNA, 1995.

much simpler approach for determining minimum levels of illumination (IESNA,
1995). The first step is to identify the general type of activity to be performed and classify it into one of nine categories, shown in Table 6.2. A more extensive list of specific tasks for this process can be found in IESNA (1995). Note that categories A, B, and C do not involve specific visual tasks. For each category, there is a range of illuminances (low, middle, high). The appropriate value is selected by calculating a weighting factor (Ϫ1, 0, ϩ1) based on three task and worker characteristics, shown in Table 6.3. These weights are then summed to obtain the total weighing factor. Note that since categories A, B, and C do not involve visual

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Table 6.3 Weighting Factors to Be Considered in Selecting Specific Illumination
Levels Within Each Category of Table 6.2

Weight
Task and worker characteristics
Age
Reflectance of task/surface background
Speed and accuracy (only for categories D – I)

Ϫ1

0

ϩ1

Ͻ40
Ͼ70%
Not important

40–55
30–70%
Important

Ͼ55
Ͻ30%
Critical

(Adapted from IESNA, 1995)

tasks, the speed/accuracy characteristic is not utilized for these categories, and overall room surfaces are utilized in place of task background. If the total sum of the two or three weighting factors is Ϫ2 or Ϫ3, the low value of the three illuminances is used; if Ϫ1, 0, or ϩ1, the middle value is used; and if ϩ2 or ϩ3, the high value is used.
In practice, illumination is typically measured with a light meter (similar to one found on cameras, but in different units), while luminance is measured with a photometer (typically, a separate attachment to the light meter). Reflectance is usually calculated as the ratio between the luminance of the target surface and the luminance of a standard surface of known reflectance (e.g., a Kodak neutral test card of reflectance ϭ 0.9) placed at the same position on the target surface. The reflectance of the target is then
Reflectance ϭ 0 .9 ϫ Ltarget>Lstandard

LIGHT SOURCES AND DISTRIBUTION
After determining the illumination requirements for the area under study, analysts select appropriate artificial light sources. Two important parameters related to artificial lighting are efficiency [light output per unit energy; typically, lumens per watt (lm/W)] and color rendering. Efficiency is particularly important, since it is related to cost; efficient light sources reduce energy consumption. Color rendering relates to the closeness with which the perceived colors of the object being observed match the perceived colors of the same object when illuminated by standard light sources. The more efficient light sources (high- and low-pressure sodium) have only fair to poor color rendering characteristics and consequently may not be suitable for certain inspection operations where color discrimination is necessary. Table 6.4 provides efficiency and color rendering information for the principal types of artificial light. Typical industrial lighting sources, that is, luminaires, are shown in Figure 6.4.
Luminaires for general lighting are classified in accordance with the percentage of total light output emitted above and below the horizontal (see
Figure 6.5). Indirect lighting illuminates the ceiling, which in turn reflects light downward. Thus, the ceilings should be the brightest surface in the room

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Artificial Light Sources

Type

Efficiency
(lm/W)

Color rendering Incandescent

17–23

Good

Fluorescent

50–80

Fair to good

Mercury

50–55

Very poor to fair

Metal halide

80–90

Fair to moderate

High-pressure sodium 85–125

Fair

Low-pressure sodium 100–180

Poor

Comments
A commonly used light source, but the least efficient. Lamp cost is low.
Lamp life is typically less than
1 year.
Efficiency and color rendering vary considerably with type of lamp: cool white, warm white, deluxe cool white. Significant energy cost reductions are possible with new energy-saving lamps and ballasts.
Lamp life is typically 5–8 years.
A very long lamp life (9–12 years), but efficiency drops off substantially with age.
Color rendering is adequate for many applications. Lamp life is typically
1–3 years.
Very efficient light source. Lamp life is 3–6 years at average burning rates, up to 12 h/day.
The most efficient light source. Lamp life is 4–5 years at average burning rate of 12 h/day. Mainly used for roadways and warehouse lighting.

Source: Courtesy Human Factors Section, Eastman Kodak Co.
Note: The efficiency (column 2), in lumens per watt (lm/W), and color rendering (column 3) of six frequently used light sources (column 1) are indicated. Lamp life and other features are given in column 4. Color rendering is a measure of how colors appear under any of these artificial light sources compared with their color under a standard light source. Higher values for efficiency indicate better energy conservation.

(see Figure 6.6), with reflectances above 80 percent. The other areas of the room should reflect lower and lower percentages of the light as one moves downward from the ceiling until the floor is reached, which should reflect no more than 20 to 40 percent of the light, to avoid glare. To avoid excessive luminance, the luminaires should be evenly distributed across the ceiling.

Calculation of Required Illumination
Consider workers of all ages performing an important, medium-difficulty assembly on a dingy metal workstation with a reflectance of 35 percent. The appropriate weights would be age ϭ ϩ1, reflectance ϭ 0, and accuracy ϭ 0. The total weight of ϩ1 implies that the middle value of category E is utilized with a required illumination of 75 fc.

EXAMPLE 6.1

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(d)

(a)
(b)

(c)

(g)

(e)
(f)

Figure 6.4

Types of industrial ceiling-mounted luminaires: (a,c) downlighting, (b,d) diffuse, (e) damp location, (f) high bay, (g) low bay. (From: IESNA, 1995)

(a)

(b)

(c)

Figure 6.5

Luminaires for general lighting are classified in accordance with the percentage of total light output emitted above and below the horizontal. Three of the classifications are
(a) direct lighting, (b) indirect lighting, and (c) direct–indirect lighting. (From:
IESNA, 1995)

Direct lighting deemphasizes the ceiling surface and places more of the light on the work surfaces and the floor. Direct–indirect lighting is a combination of both. This distribution of lighting is important, as IESNA (1995) recommends that the ratio of luminances of any adjacent areas in the visual field not exceed
3/1. The purpose of this is to avoid glare and problems in adaptation.

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Ceilings: 80% or more

Partitions:
40 – 70%
Furniture:
25 – 45%
Walls:
50 – 70%
Floors: 20 – 40%

Figure 6.6

Reflectances recommended for room and furniture surfaces in offices.

(From: IESNA, 1995)

GLARE
Glare is the excessive brightness in the field of vision. This excessive light is scattered in the cornea, lens, and even corrective lenses (Freivalds, Harpster, and
Heckman, 1983), decreasing visibility so that additional time is required for the eyes to adapt from light to darker conditions. Also, unfortunately, the eyes tend to be drawn directly to the brightest light source, which is known as phototropism. Glare can be either direct, as caused by light sources directly in the field of view, or indirect, as reflected from a surface in the field of view. Direct glare can be reduced by using more luminaires with lower intensities, using baffles or diffusers on luminaires, placing the work surface perpendicular to the light source, and increasing overall background lighting so as to decrease the contrast.
Reflected glare can be reduced by using nonglossy or matte surfaces and reorienting the work surface or task, in addition to the modifications recommended for direct glare. Also, polarizing filters can be used at the light source as part of glasses worn by the operator. A special problem is the stroboscopic effect caused by the reflections from moving parts or machinery. Avoiding polished mirrorlike surfaces is important here. For example, the mirrorlike qualities of the glass screen on computer monitors is a problem in office areas. Repositioning the monitor or using a screen filter is helpful. Typically, most jobs will require supplementary task lighting. This can be provided in a variety of forms, depending on the nature of the task (see Figure 6.7).

COLOR
Both color and texture have psychological effects on people. For example, yellow is the accepted color of butter; therefore, margarine must be made yellow to appeal to the appetite. Steak is another example. Cooked in 45 s on an electronic grill, it does not appeal to customers because it lacks a seared, brown, “appetizing”

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(a)

(b)

(c)

(d)

(e)

Figure 6.7 Examples of placement of supplementary luminaires.

(a) Luminaire located to prevent veiling reflections and reflected glare; reflected light does not coincide with angle of view. (b) Reflected light coincides with angle of view.
(c) Low-angle (grazing) lighting to emphasize surface irregularities. (d) Large-area surface source and pattern are reflected toward the eye. (e) Transillumination from diffuse source. (From: IESNA, 1995)

surface. A special attachment had to be designed to sear the steak. In a third example, employees in an air-conditioned Midwestern plant complained of feeling cold, although the temperature was maintained at 72°F (22.2°C). When the white walls of the plant were repainted in a warm coral color, complaints ceased.
Perhaps the most important use of color is to improve the environmental conditions of the workers by providing more visual comfort. Analysts use colors to reduce sharp contrasts, increase reflectance, highlight hazards, and call attention to features of the work environment.
Sales are also affected or conditioned by colors. People recognize a company’s products instantly by the pattern of colors used on packages, trademarks, letterheads, trucks, and buildings. Some research has indicated that color preferences are influenced by nationality, location, and climate. Sales of a product formerly made in one color increased when several colors suited to the differences in customer demands were supplied. Table 6.5 illustrates the typical emotional effects and psychological significances of the principal colors.

6.2

NOISE

THEORY
From the analyst’s point of view, noise is any unwanted sound. Sound waves originate from the vibration of some object, which in turns sets up a succession of compression and expansion waves through the transporting medium (air, water, and so on). Thus, sound can be transmitted not only through air and liquids, but also through solids, such as machine tool structures. We know that the velocity of sound waves in air is approximately 1,100 ft/s (340 m/s). In viscoelastic materials, such as lead and putty, sound energy is dissipated rapidly as viscous friction. McGraw-Hill Create™ Review Copy for Instructor Espinoza. Not for distribution.
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Table 6.5

Color

Work Environment Design

The Emotional and Psychological Significance of the Principal Colors

Characteristics

Yellow

Has the highest visibility of any color under practically all lighting conditions. It tends to instill a feeling of freshness and dryness. It can give the sensation of wealth and glory, yet can also suggest cowardice and sickness.
Orange
Tends to combine the high visibility of yellow and the vitality and intensity characteristic of red. It attracts more attention than any other color in the spectrum. It gives a feeling of warmth and frequently has a stimulating or cheering effect.
Red
A high-visibility color having intensity and vitality. It is the physical color associated with blood. It suggests heat, stimulation, and action. Blue
A low-visibility color. It tends to lead the mind to thoughtfulness and deliberation. It tends to be a soothing color, although it can promote a depressed mood.
Green
A low-visibility color. It imparts a feeling of restfulness, coolness, and stability.
Purple and violet Low-visibility colors. They are associated with pain, passion, suffering, heroism, and so on. They tend to bring a feeling of fragility, limpness, and dullness.

Sound can be defined in terms of the frequencies that determine its tone and quality, along with the amplitudes that determine its intensity. Frequencies audible to the human ear range from approximately 20 to 20,000 cycles per second, commonly called hertz and abbreviated Hz. The fundamental equation of wave propagation is c ϭ fl c ϭ sound velocity (1,100 ft/s) f ϭ frequency, (Hz) l ϭ wavelength (ft)
Note that as the wavelength increases, the frequency decreases.
The sound pressure waves are captured by the human ear (see Figure 6.8) through a complex process. The outer ear funnels the pressure waves onto the eardrum or tympanic membrane, which starts vibrating. The membrane is attached to three little bones (malleus, incus, and stapes), which transmit the vibrations to the oval window of the cochlea. The cochlea is a coiled, fluid-filled structure split lengthwise by the basilar membrane containing hair cells with nerve endings. The vibrations from the bones set the fluid into a wavelike motion, which then causes the hair cells to vibrate, activating these-nerve endings, which transmit the impulses via the auditory nerve to the brain for further processing. Note the series of energy transformations: the original pneumatic pressure waves are converted to mechanical vibrations, then to hydraulic waves, back to mechanical vibrations, and finally to electrical impulses.

where

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Inner ear
Incus
Malleus

Auditory nerve Middle ear

Outer ear Cochlea
Oval
Window

Auditory canal Stapes
Pinna

Eardrum
(tympanic
membrane)

Basilar membrane
Round
Window

Figure 6.8 The human ear.

MEASUREMENT
Because of the very large range of sound intensities encountered in the normal human environment, the decibel (dB) scale has been chosen. In effect, it is the logarithmic ratio of the actual sound intensity to the sound intensity at the threshold of hearing of a young person. Thus, the sound pressure level L in decibels is given by
L ϭ 20 log10 Prms>Pref where Prms ϭ root-mean-square sound pressure [microbars (mbar)]
Pref ϭ sound pressure at the threshold of hearing of a young person at
1,000 Hz (0.0002 mbar)
Since sound pressure levels are logarithmic quantities, the effect of the coexistence of two or more sound sources in one location requires that a logarithmic addition be performed as follows:
LTOT ϭ 10 log10 110L1>10 ϩ 10L2>10 ϩ p 2 where LTOT ϭ total noise
L1 and L2 ϭ two noise sources
The A-weighted sound level used in Figure 6.9 is the most widely accepted measure of environmental noise. The A weighting recognizes that from both the psychological and physiological points of view, the low frequencies (50 to 500 Hz) are far less annoying and harmful than sounds in the critical frequency range of

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Common sounds 0

Whisper

10

20

Train

80

Grinder
Grinder

90

Air
Quiet
Quiet conditioning Neighborhood Air conditioning Traffic unit neighborhood
Unit

Bedroom

30

40

50

100

60

Quiet

Very quiet Threshold Just of hearing audible

Intrusive

120

70

Large
Large
propeller aircraft
Propeller Aircraft

Riveting machine
Riveting Machine

110

Work Environment Design

130

Very annoying

140

150

Jet
Jet
aircraft
Aircraft

160

Painfully loud Figure 6.9 Decibel values of typical sounds (dBA).

1,000 to 4,000 Hz. Above frequencies of 10,000 Hz, hearing acuity (and therefore noise effects) again drops off (see Figure 6.10). The appropriate electronic network is built into sound level meters to attenuate low and high frequencies, so that the sound level meter can read in dBA units directly, to correspond to the effect on the average human ear.

HEARING LOSS
The chances of damage to the ear, resulting in “nerve” deafness, increase as the frequency approaches the 2,400- to 4,800-Hz range. This loss of hearing is a result of a loss of receptors in the inner ear, which then fail to transmit the sound waves further to the brain. Also, as the exposure time increases, especially where higher intensities are involved, there will eventually be an impairment in hearing.
Nerve deafness is due most commonly to excessive exposure to occupational noise. Individuals vary widely in their susceptibility to noise-induced deafness.
In general, noise is classified as either broadband noise or meaningful noise.
Broadband noise is made up of frequencies covering a significant part of the sound spectrum. This type of noise can be either continuous or intermittent. Meaningful noise represents distracting information that impacts the worker’s efficiency. In long-term situations, broadband noise can result in deafness; in day-to-day operations, it can result in reduced worker efficiency and ineffective communication.
Continuous broadband noise is typical of such industries as the textile industry and an automatic screw machine shop, where the noise level does not

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140

130
Octave band sound pressure level in decibels

252

12

5

120

12

11

0

110

10

0

0

11

5

10

5

100

90
90

95

A – Weighted
Sound Level

80
100

200
500
1,000
2,000
4,000
Band center frequency in cycles per second

8,000

Figure 6.10 Equivalent sound level contours.

deviate significantly during the entire working day. Intermittent broadband noise is characteristic of a drop forge plant and a lumber mill. When a person is exposed to noise that exceeds the damage level, the initial effect is likely to be a temporary hearing loss from which there is complete recovery within a few hours after leaving the work environment. If repeated exposure continues over a long period, irreversible hearing damage can result. The effects of excessive noise depend on the total energy that the ear has received during the work period. Thus, reducing the time of exposure to excessive noise during the work shift reduces the probability of permanent hearing impairment.
Both broadband and meaningful noise have proved to be sufficiently distracting and annoying to result in decreased productivity and increased employee fatigue. However, federal legislation was enacted primarily because of the possibility of permanent hearing damage due to occupational noise exposure. The
OSHA (1997) limits for permissible noise exposure are contained in Table 6.6.
When noise levels are determined by octave-band analysis (a special filter attachment to the sound level meter that decomposes the noise into component frequencies), the equivalent A-weighted sound level may be determined as follows:
Plot the octave-band sound pressure levels on the graph in Figure 6.10, and note the A-weighted sound level corresponding to the point of highest penetration into the sound level contours. This is the dBA value to be used in further calculations.

NOISE DOSE
OSHA uses the concept of noise dose, with the exposure to any sound level above
80 dBA causing the listener to incur a partial dose. If the total daily exposure

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245

Table 6.6 Permissible Noise Exposures

Duration per day (h)

Sound level (dBA)

8
6
4
3
2
1.5
1
0.5
0.25 or less

90
92
95
97
100
102
105
110
115

Note: When the daily noise exposure is composed of two or more periods of noise exposure of different levels, their combined effect should be considered rather than the individual effects of each. If the sum of the following fractions C1/T1 ϩ C2/T2 ϩ . . . ϩ Cn/Tn exceeds unity, then the mixed exposure should be considered to exceed the limit value. Cn indicates the total time of exposure at a specified noise level, and Tn equals the total time of exposure permitted during the workday.
Exposure to impulsive or impact noise should not exceed 140-dB peak sound pressure level.

consists of several partial exposures to different noise levels, then the several partial doses are added to obtain a combined exposure:
D ϭ 100 ϫ 1C1>T1 ϩ C2>T2 ϩ p ϩ Cn>Tn 2 Յ 100

where D ϭ noise dose
C ϭ time spent at specified noise level (h)
T ϭ time permitted at specified noise level (h) (see Table 6.6)
The total exposure to various noise levels cannot exceed a 100-percent dose.

Calculation of OSHA Noise Dose
A worker is exposed to 95 dBA for 3 h and to 90 dBA for 5 h. Although each partial dose is separately permissible, the combined dose is not:
3
5
D ϭ 100 ϫ a ϩ b ϭ 137.5 7 100
4
8

Thus, 90 dBA is the maximum permissible level for an 8-h day, and any sound level above 90 dBA will require some noise abatement. All sound levels between 80 and 130 dBA must be included in the noise dose computations (although continuous levels above 115 dBA are not allowed at all). Since Table 6.6 provides only certain key times, a computational formula can be used for intermediate noise levels:
T ϭ 8>21LϪ902>5 where L ϭ noise level (dBA).

EXAMPLE 6.2

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The noise dose can also be converted to an 8-h time-weighted average
(TWA) sound level. This is the sound level that would produce a given noise dose if a worker were exposed to that sound level continuously over an 8-h workday.
The TWA is defined by
TWA ϭ 16 .61 ϫ log10 1D>1002 ϩ 90
Thus, in the last example, a 139.3 percent dose would yield a TWA of
TWA ϭ 16 .61 ϫ log10 1139 .3>1002 ϩ 90 ϭ 92 .39 dB
Today, OSHA also requires a mandatory hearing conservation program, including exposure monitoring, audiometric testing, and training, for all employees who have occupational noise exposures equal to or exceeding TWA of 85 dB.
Although noise levels below 85 dB may not cause hearing loss, they contribute to distraction and annoyance, resulting in poor worker performance. For example, typical office noises, although not loud, can make it difficult to concentrate, resulting in low productivity in design and other creative work. Also, the effectiveness of telephone and face-to-face communications can be considerably distracted by noise levels less than 85 dB.

PERFORMANCE EFFECTS
Generally, performance decrements are most often observed in difficult tasks that place high demands on perceptual, information processing, and short-term memory capacities. Surprisingly, noise may have no effect, or may even improve performance, on simple routine tasks. Without the noise source, the person’s attention may wander due to boredom.
EXAMPLE 6.3

Calculation of OSHA Noise Dose with Additional Exposures
A worker is exposed to 1 h at 80 dBA, 4 h at 90 dBA, and 3 h at 96 dBA. The worker is permitted 32 h for the first exposure, 8 h for the second exposure, and

T ϭ 8>2196Ϫ902>5 ϭ 3.48 hours for the third exposure. The total noise dose becomes

D ϭ 100 ϫ 11>32 ϩ 4>8 ϩ 3>3.48 2 ϭ 139.3

Thus, for this worker, the 8-h noise exposure dosage exceeds OSHA requirements, and either the noise must be abated or the worker must be provided with a rest allowance
(see Chapter 11) to comply with OSHA requirements.

Annoyance is even more complicated and is fraught with emotional issues.
Acoustic factors, such as intensity, frequency, duration, fluctuations in level, and spectral composition, play a major role, as do nonacoustic factors, such as past noise experience, activity, personality, noise occurrence predictability, time of

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day and year, and type of locale. There are approximately a dozen different methods for evaluating annoyance aspects (Sanders and McCormick, 1993). However, most of these measures involve community-type issues with noise levels in the
60- to 70-dBA range, much lower than could reasonably be applied in an industrial situation.

NOISE CONTROL
Management can control the noise level in three ways. The best, and usually the most difficult, is to reduce the noise level at its source. However, it would be very difficult to redesign such equipment as pneumatic hammers, steam forging presses, board drop hammers, and woodworking planers and joiners so that the efficiency of the equipment would be maintained while the noise level was being brought into a tolerable range. In some instances, however, more quietly operating facilities may be substituted for those operating at a high noise level. For example, a hydraulic riveter may be substituted for a pneumatic riveter, an electrically operated apparatus for a steam-operated apparatus, and an elastomerlined tumble barrel for an unlined barrel. Low-frequency noise at the source is effectively controlled at the source by using rubber mounts and better alignment and maintenance of the equipment.
If the noise cannot be controlled at its source, then analysts should investigate the opportunity to isolate the equipment responsible for the noise; that is, control the noise that emanates from a machine by housing all or a substantial portion of the facility in an insulating enclosure. This has frequently been done in connection with power presses having automatic feeds. Ambient noise can frequently be reduced by isolating the noise source from the remainder of the structure, thus preventing a sounding board effect. This can be done by mounting the facility on a shear-type elastomer, thus damping the telegraphing of noise.
In situations where enclosing the facility would not interfere with operation and accessibility, the following steps can ensure the most satisfactory enclosure design: 1. Clearly establish design goals and the acoustical performance required of the enclosure.
2. Measure the octave-band noise levels of the equipment to be enclosed, at 3 ft (1 m) from the major machine surfaces.
3. Determine the spectral attenuation of each enclosure. This is the difference between the design criteria determined in step 1 and the noise level determined in step 2.
4. Select the materials from Table 6.7 that are popular for relatively small enclosures and will provide the protection needed. A viscoelastic damping material should be applied if any of these materials (with the exception of lead) is used. This can provide an additional attenuation of 3 to 5 dB.
Figure 6.11 illustrates the amount of noise reduction typically possible through various acoustical treatments and enclosures.

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Table 6.7 Octave-Band Noise Reduction of Single-Layer Materials Commonly Used for Enclosures

Octave-band center frequency
16-gage steel
7-mm steel
7-mm plywood
0.32 kg/0.1 m2
3/4-in plywood
0.9 kg/0.1 m2
14-mm gypsum board
1 kg/0.1 m2
7-mm fiberglass
0.23 kg/0.1 m2
0.2-mm lead
0.45 kg/0.1 m2
0.4-mm lead
0.9 kg/0.1 m2

125

250

500

1,000

2,000

4,000

15
25

23
38

31
41

31
45

35
41

41
48

11

15

20

24

29

30

19

24

27

30

33

35

14

20

30

35

38

37

5

15

23

24

32

33

19

19

24

28

33

38

23

24

29

33

40

43

Octave-band analysis of noise

(c)
100
(b)

(a)

Band level, decibels

256

Original

90
80 (a)

(b)

70

(c)

60

(a+b+c)

50
(a+2b+2c)

40
30

20
75

75
150

150 300 600 1200 2400 4800
300 600 1200 2400 4800 9600
Frequency band

Figure 6.11 Illustrations of the possible effects of some noise control measures.

The lines on the graph show the possible reductions in noise (from the original level) that might be expected by vibration insulation, a; an enclosure of acoustic absorbing material, b; a rigid, sealed enclosure, c; a single combined enclosure plus vibration insulation, a ϩ b ϩ c; and a double combined enclosure plus vibration insulation, a ϩ 2b ϩ 2c. (Adapted from: Peterson and Gross, 1978)

Note that some sounds are desirable in a work environment. For example, background music has been used in factories for many years to improve the work environment, especially where voice communications aren’t critical. The majority of production and indirect workers (maintenance, shipping, receiving, etc.) enjoy listening to music while they work. However, first consult the employees on the type of music to be played.
The third level of noise control is with hearing protection, though in most cases
OSHA accepts this as only a temporary solution. Personal protective equipment

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can include various types of earplugs, some of which are able to attenuate noises in all frequencies up to sound pressure levels of 110 dB or more. Also available are earmuffs that attenuate noises to 125 dB above 600 Hz and up to 115 dB below this frequency. Earplug effectiveness is measured quantitatively by a noise reduction rating (NRR), which is marked on the packaging. The equivalent noise exposure for the listener is equal to the TWA plus 7 minus the NRR (NIOSH,
1998). In general, insert-type (e.g., expandable foam) devices provide better protection than muff-type devices. A combination of an insert device and a muff device can yield NRR values as high as 30. Note that this is a laboratory value obtained under ideal conditions. Typically, in a real-world setting, with hair, beards, eyeglasses, and improper fit, the NRR value is going to be considerably lower, perhaps by as much as 10 (Sanders and McCormick, 1993).

6.3 TEMPERATURE
Most workers are exposed to excessive heat at one time or another. In many situations, artificially hot climates are created by the demands of the particular industry.
Miners are subjected to hot working conditions due to the increase of temperature with depth, as well as a lack of ventilation. Textile workers are subjected to the hot, humid conditions needed for weaving cloth. Steel, coke, and aluminum workers are subject to intense radiative loads from open-hearth furnaces and refractory ovens.
Such conditions, while present for only a limited part of the day, may exceed the climatic stress found in the most extreme, naturally occurring climates.

THEORY
The human is typically modeled as a cylinder with a shell, corresponding to the skin, surface tissues, and limbs, and with a core, corresponding to the deeper tissues of the trunk and head. Core temperatures exhibit a narrow range around a normal value of 98.6°F (37°C). At values between 100 and 102°F (37.8 and
38.9°C), physiological performance drops sharply. At temperatures above 105°F
(40.6°C), the sweating mechanism may fail, resulting in a rapid rise in core temperature and eventual death. The shell tissues of the body, on the other hand, can vary over a much wider range of temperatures without serious loss of efficiency, and can act as a buffer to protect core temperatures. Clothing, if worn, acts as a second shell to insulate the core temperature further.
The heat exchanges between the body and its environment can be represented by the following heat balance equation:
SϭM ;C ;RϪE where M ϭ heat gain of metabolism
C ϭ heat gained (or lost) due to convection
R ϭ heat gained (or lost) due to radiation
E ϭ heat lost through evaporation of sweat
S ϭ heat storage (or loss) of body

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For thermal neutrality, S must be zero. If the summation of the various heat exchanges across the body results in a heat gain, the resulting heat will be stored in the tissues of the body, with a concomitant increase in core temperature and a potential heat stress problem.
A thermal comfort zone, for areas where 8 h of sedentary or light work is done, has been defined as the range of temperatures of from 66 to 79°F (18.9 to
26.1°C), with a relative humidity ranging from 20 to 80 percent (see Figure 6.12).
Of course, the workload, clothing, and radiant heat load all affect the individual’s sense of comfort within the comfort zone.

in mm
Hg Hg
1.3

30

85

Air velocity = 0.2 m/sec (50 ft/min)

؇F

؇C

Work = sedentary or light assembly,
70–100 W (60–90 kcal/hr)

1.2

30

90%

Clothing = 0.6 clo heat, 1.25 clo cold
No radiant heat

%

We t-bu lb

per

%

atu

re

25

0.9

75

70

1.0 tem 0.8

%

60

0.7
50%

A. Dry-bulb temperature

65

15

60
80%
60%

0.6 15
40%

0.5

30%

0.4 10
0.3

20%

40%

0.2 y e humidit 10%
. Relativ

20%

؇F

60

؇C 15

D

65

70
20

20

75

80

25
Dry-bulb temperature

Figure 6.12 The thermal comfort zone.

(Courtesy: Eastman Kodak Co.)

85
30

0.1
90

95
35

5

B. Water vapor pressure

80

C.

25

= Thermal Comfort Zone

80

1.1

70

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HEAT STRESS: WBGT
Many attempts have been made to combine into one index the physiological manifestations of these heat exchanges with environmental measurements. Such attempts have centered on designing instruments intended to simulate the human body, or devising formulas and models based on theoretical or empirical data to estimate the environmental stresses or the resulting physiological strains. In the simplest form, an index consists of the dominant factor, such as the dry-bulb temperature, which is used by most people in temperate zones.
Probably the most commonly used index in industry today establishes heat exposure limits and work/rest cycles based on the wet-bulb globe temperature, or
WBGT (Yaglou and Minard, 1957), and the metabolic load. In slightly different forms it is recommended by ACGIH (1985), NIOSH (1986), and ASHRAE
(1991). For outdoors with a solar load, the WBGT is defined as
WBGT ϭ 0 .7 NWB ϩ 0 .2 GT ϩ 0 .1 DB and indoors or outdoors with no solar load, the WBGT is
WBGT ϭ 0 .7 NWB ϩ 0 .3 GT where NWB ϭ natural wet-bulb temperature (measure of evaporative cooling, using a thermometer with a wet wick and natural air movement)
GT ϭ globe temperature (measure of radiative load, using a thermometer in a 6-in-diameter black copper sphere)
DB ϭ Dry-bulb temperature (basic ambient temperature; thermometer shielded from radiation)
Note that NWB is different from a psychometric wet bulb, which uses maximum air velocity and is used in conjunction with DB to establish relative humidity and thermal comfort zones.
Once the WBGT is measured (commercially available instruments provide instantaneous weighted readings), it is used along with the metabolic load of the workers to establish the amount of time an unacclimatized worker and an acclimatized worker are allowed to work under the given conditions (see Figure 6.13).
These limits are based on the individual’s core temperature having increased by approximately 1.8°F (1°C) as calculated by the heat balance equation. The 1.8°F increase has been established by NIOSH (1986) as the upper acceptable limit for heat storage in the body. The appropriate amount of rest is assumed to be under the same conditions. Obviously, if the worker rests in a more comfortable area, less rest time will be needed.

CONTROL METHODS
Heat stress can be reduced by implementing either engineering controls, that is, modifying the environment, or administrative controls. Modifying the environment follows directly from the heat balance equation. If the metabolic load is a significant contributor to heat storage, the workload should be reduced by mechanization of the operation. Working more slowly will also decrease the workload, but will

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؇F ؇C

Environmental heat–WBGT

104 40

95 35
Ceiling

86 30
15 min/h REL
30 min/h REL
15 min/h RAL
45 min/h REL
30 min/h RAL
60 min/h REL

77 25
RAL for unacclimatized workers REL for acclimatized workers 68 20
100
400
116

200
800
233

300
1200
349

45 min/h RAL
60 min/h RAL
400
1600
465

500 kcal/h
2000 Btu/h
580 Watts

Metabolic heat

Figure 6.13 Recommended heat stress levels based on metabolic heat (1-h timeweighted average), acclimatization, and workrest cycle.

A rough approximation for metabolic heat from Figure 4.21 is W ϭ (HR Ϫ 50) ϫ 6.
Temperature limits are 1-h time-weighted average WGBT. RAL ϭ recommended alert limit for unacclimatized workers. REL ϭ recommended exposure limit for acclimatized workers. (From: NIOSH, 1986, Figs. 1 and 2)

have the negative effect of decreasing productivity. The radiative load can be decreased by controlling the heat at the source: insulating hot equipment, providing drains for hot water, maintaining tight joints where steam may escape, and using local exhaust ventilation to disperse heated air rising from a hot process.
Radiation can also be intercepted before it reaches the operator, via radiation shielding: sheets of reflective material, such as aluminum or foil-covered plasterboard, or metal chain curtains, wire mesh screens, or tempered glass, if visibility is required. Reflective garments, protective clothing, or even long-sleeved clothing will also help in reducing the radiative load.
EXAMPLE 6.4

Calculation of WBGT and Heat Stress Level
Consider an unacclimatized worker palletizing a skid at 400 kcal/h (1,600 BU/h) with a thermal load of WBGT ϭ 77°F (25°C). This individual would be able to work for 45 min and would then need to rest. At this point, the worker must rest for at least 15 min in the same environment or a shorter time in a less stressful environment.

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Convective heat loss from the operator can be increased by increasing air movement through ventilation, as long as the dry-bulb temperature is less than skin temperature, which is typically around 95°F (35°C) in such environments. Convection is more effective over bare skin; however, bare skin also absorbs more radiation. Thus, there is a trade-off between convection and radiation. Evaporative heat loss from the operator can be improved by again increasing air movement and decreasing the ambient water vapor pressure, using dehumidifiers or air-conditioning.
Unfortunately, the latter approach, although creating a very pleasant environment, is quite costly and is often not practical for the typical production facility.
Administrative measures, though less effective, include modifying work schedules to decrease the metabolic load, using work/rest schedules per Figure
6.13, acclimatizing workers (this may take close to two weeks, and the effect is lost over a similar time period), rotating workers into and out of the hot environment, and using cooling vests. The cheapest vests utilize ice frozen in small, plastic packets placed into numerous pockets in the vest (Kamon et al., 1986).

COLD STRESS
The most commonly used cold stress index is the wind chill index. It describes the rate of heat loss by radiation and convection as a function of ambient temperature and wind velocity. Typically, the wind chill index is not used directly, but is converted to an equivalent wind chill temperature. This is the ambient temperature that, in calm conditions, would produce the same wind chill index as the actual combination of air temperature and wind velocity (Table 6.8). For the operator to maintain thermal balance under such low-temperature conditions, there must be a close relationship between the worker’s physical activity (heat production) and the insulation provided by protective clothing (see Figure 6.14). Here, clo represents the insulation needed to maintain comfort for a person sitting where the relative humidity is
50 percent, the air movement is 20 ft/min, and the dry-bulb temperature is 70°F
(21.1°C). A light business suit is approximately equivalent to 1 clo of insulation.
Table 6.8 Equivalent Wind Chill Temperatures (°F) of Cold Environments Under Calm
Conditions

Wind speed
(mi/h)

Actual thermometer reading (°F)
40

5
10
15
20
30
40

30

20

10

0

Ϫ10

Ϫ20

Ϫ30

36
34
32
30
28
27

25
21
19
17
15
13

13
9
6
4
1
Ϫ1

1
Ϫ4
Ϫ7
Ϫ9
Ϫ12
Ϫ15

Ϫ11
Ϫ16
Ϫ19
Ϫ22
Ϫ26
Ϫ29

Ϫ22
Ϫ28
Ϫ32
Ϫ35
Ϫ39
Ϫ43

Ϫ34
Ϫ41
Ϫ45
Ϫ48
Ϫ53
Ϫ57

Ϫ46
Ϫ53
Ϫ58
Ϫ61
Ϫ67
Ϫ71

Little Danger:
Exposed, dry flesh won’t freeze for 5 h.
Source: National Weather Service

Increasing Danger:
Frostbite occurs within 30 mins.

Great Danger:
Frostbite occurs within 5 min.

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11
10
9
8

qu

=

1.

6

5

(s

M

itt

in

=1

g

.3

6

ie

(sl

ee

tly

)

pin

g)

7

M

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Insulation required for comfort (CLO)

262

4 in t ga 3

.5

ork
(w

sk)

de

)

ork tw gh g li

rk) e wo

— mph t to

ligh

rat ode m

) min work
5
for g 2. erage ad) per lkin
— av
(
a
20 kg lo ph 3.2
.8 (w h with
3.5 m p 4
=
g 3.5 m
M
alking
M=
.3 (w = 9.5 (walkin
M=6
M

M

2
1
0
100

80

60

=2

40
20
0
Ambient temperature (؇F)

–20

–40

Figure 6.14 Prediction of the total insulation required as a function of the ambient temperature for a 50th percentile male (M ϭ heat production in kcal/min).

(From: Redrawn from Belding and Hatch, 1955.)

Probably the most critical effects for industrial workers exposed to outdoor conditions are decreased tactile sensitivity and manual dexterity due to vasodilation and decreased blood flow to the hands. Manual performance may decrease as much as 50 percent as the hand skin temperature drops from 65 to 45°F
(18.3 to 7.2°C) (Lockhart, Kiess, and Clegg, 1975). Auxiliary heaters, hand warmers, and gloves are potential solutions to the problem. Unfortunately, as indicated in Chapter 5, gloves can impair manual performance and decrease grip strength. A compromise that protects the hands and minimally affects performance may be fingerless gloves (Riley and Cochran, 1984).

6.4

VENTILATION

If a room has people, machinery, or activities in it, the air in the room will deteriorate due to the release of odors, the release of heat, the formation of water

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3

Figure 6.15 Guidelines for ventilation requirements for sedentary workers given the available volume of room air.

4

Strong
Intensity of odors

Very strong 5f

t 3/m

10

(Flow rates are per person.)
(Adapted from: Yaglou, Riley, and Coggins, 1936)

in

ft 3/

min

Limit
2
(moderate)

Work Environment Design

20 f

t 3/m

30 f

in in t 3/m

Definite

None

1

0
100

200

300

500 700 1,000

Volume of air per person (ft3)

vapor, the production of carbon dioxide, and the production of toxic vapors.
Ventilation must be provided to dilute these contaminants, exhaust the stale air, and supply fresh air. This can be done in one or more of three approaches: general, local, or spot. General or displacement ventilation is delivered at the 8- to
12-ft (2.4- to 3.6-m) level and displaces the warm air rising from the equipment, lights, and workers. Recommended guidelines for fresh air requirements, based on the room volume per person, are shown in Figure 6.15 (Yaglou, Riley, and Coggins, 1936). A rough rule of thumb is 300 ft3 (8.5 m3) of fresh air per person per hour.
In a building with only a few work areas, it would be impractical to ventilate the whole building. In that case, local ventilation can be provided at a lower level, or perhaps in an enclosed area, such as a ventilated control booth or crane cab. Note that fan velocity drops rapidly with increasing distance from the fan (see Figure 6.16), and directionality of airflow is very critical. Acceptable air velocities at the worker are specified in Table 6.9 (ASHRAE, 1991). A rough rule of thumb is that at a distance of 30 fan diameters, the fan velocity drops to less than 10 percent of its face velocity (Konz, 1995). Finally, in areas with localized heat sources, such as refractory ovens, spot cooling with a direct high-velocity airstream at the worker will increase convective and evaporative cooling. 6.5

VIBRATION

Vibration can cause detrimental effects on human performance. Vibrations of high amplitude and low frequency have especially undesirable effects on body organs and tissue. The parameters of vibration are frequency, amplitude, velocity,

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8.0

7.0

6.0

5.0
Air velocity

264

4.0

3.0

2.0

1.0

0
0

1.0

2.0

3.0

4.0
Distance

5.0

6.0

7.0

Figure 6.16 Air velocity versus distance for fan placement.

(From: Konz and Johnson, 2000)

Table 6.9 Acceptable Air Motion at the Worker

Exposure
Continuous
Air-conditioned space
Fixed workstation, general ventilation, or spot cooling
Sitting
Standing
Intermittent, spot cooling, or relief stations
Light heat loads and activity
Moderate heat loads and activity
High heat loads and activity
Source: Reprinted with permission from ASHRAE, 1991.

Air Velocity (ft/min)
50 to 75
75 to 125
100 to 200
1,000 to 2,000
2,000 to 3,000
3,000 to 4,000

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Table 6.10

Resonant Frequencies for Different Body Parts

Frequency (Hz)
3–4
4
5
20–30
Ͼ30
60–90

Work Environment Design

Body part affected
Cervical vertebrae
Lumbar vertebrae (key for forklift and truck operators)
Shoulder girdle
Between head and shoulder
Fingers, hands, and arms (key for power tool operators)
Eyeballs (key for pilots and astronauts)

acceleration, and jerk. For sinusoidal vibrations, amplitude and its derivations with respect to time are
Amplitude s ϭ maximum displacement from static position (in) ds Maximum velocity ϭ 2p 1s2 1f2 in>s dt d2 s
Maximum acceleration 2 ϭ 4p2 1s 2 1f 2 2 in>s2 dt d3s
Maximum jerk 3 ϭ 8p3 1s2 1f 3 2 in>s3 dt where f ϭ frequency s ϭ displacement amplitude
Displacement and maximum acceleration are the principal parameters used to characterize the intensity of vibration.
There are three classifications of vibration exposure:
1. Circumstances in which the whole or a major portion of the body surface is affected, for example, when high-intensity sound in air or water excites vibration. 2. Cases in which vibrations are transmitted to the body through a supporting area, for example, through the buttocks of a person driving a truck, or through the feet of a person standing by a shakeout facility in a foundry.
3. Instances in which vibrations are applied to a localized body area, for example, to the hand when holding and operating a power tool.
Every mechanical system can be modeled using a mass, spring, and dashpot, which, in combination, result in the system having its own natural frequency.
The nearer the vibration comes to this frequency, the greater the effect on that system. In fact, if the forced vibrations induce larger-amplitude vibrations in the system, then the system is in resonance. This can have dramatic effects, for example, large winds causing the Tacoma Narrows bridge in Washington to oscillate and eventually collapse, or soldiers breaking step in crossing bridges. For a sitting person, the critical resonant frequencies are given in Table 6.10.
On the other hand, oscillations in the body, or any system, tend to be dampened.
Thus, in a standing posture, the muscles of the legs heavily dampen vibrations.

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20
12.5
8.0

3.15

6 dB

5.0

1 min
16 min
25 min

2.0
1.25

1h

0.8
0.5

10 dB

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Root mean square (rms) acceleration (m/sec2)

266

2.5 h
4h

0.315

0.63

8h

1.0

1.6

2.5

4.0
8.0 10
Frequency (Hz)

16

25

40

63

Figure 6.17 The fatigue-decreased proficiency boundary for vertical vibration contained in ISO 2631 and ANSI S3.18-1979.

To obtain the boundary for reduced comfort, subtract 10 dB (i.e., divide each value by
3.15); to obtain the boundary for safe physiological exposure, add 6 dB (i.e., multiply each value by 2.0). (Source: Acoustical Society of America, 1980)

Frequencies above 35 Hz are especially dampened. Amplitudes of oscillations induced in the fingers will reduce 50 percent in the hands, 66 percent in the elbows, and 90 percent in the shoulders.
The human tolerance for vibration decreases as the exposure time increases.
Thus, the tolerable acceleration level increases with decreasing exposure time.
The limits for whole-body vibration have been developed by both the International Standards Organization (ISO) and the American National Standards Institute (ANSI) (ASA, 1980) for transportation and industrial applications. The standards specify limits in terms of acceleration, frequency, and time duration
(Figure 6.17). The plotted lines show fatigue/performance limits. For comfort limits, the acceleration values are divided by 3.15; for safety limits, the values are multiplied by 2. Unfortunately, no limits have been developed for the hands and upper extremities.
Low-frequency (0.2- to 0.7-Hz), high-amplitude vibrations are the principal cause of motion sickness in sea and air travel. Workers experience fatigue much more rapidly when they are exposed to vibrations in the range of 1 to 250 Hz.
Early symptoms of vibration fatigue are headache, vision problems, loss of

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appetite, and loss of interest. Later problems include motor control impairments, disk degeneration, bone atrophy, and arthritis. Vibrations experienced in this range are often characteristic of the trucking industry. The vertical vibrations of many rubber-tired trucks when traveling at typical speeds over ordinary roads range from 3 to about 7 Hz, which are exactly in the critical range for resonances in the human trunk.
Power tools with frequencies of 40 to 300 Hz tend to occlude blood flow and affect nerves, resulting in the white fingers syndrome. The problem is exacerbated in cold conditions, with the additional problem of cold-induced occlusion of blood flow, or Raynaud’s syndrome. Better dampened tools, the exchange of detachable handles with special vibration-absorbing handles, and the wearing of gloves, especially those padded with vibration-absorbing gel, will help reduce the problem.
Management can protect employees against vibration in several ways. The applied forces responsible for initiating the vibration may be reduced by modifying the speed, feed, or motion, and by properly maintaining the equipment, balancing and/or replacing worn parts. Analysts can place equipment on antivibration mountings (springs, shear-type elastomers, compression pads) or alter workers’ body positions to lessen the disturbing vibratory forces. They can also reduce the time workers are exposed to the vibration by alternating work assignments within a group of employees. Last, they can introduce supports that cushion the body and thus dampen higher-amplitude vibrations. Seat suspension systems involving hydraulic shock absorbers, coil or leaf springs, rubber shear-type mountings, or torsion bars may be used. In standing operations, a soft, elastomer floor mat usually proves helpful.

6.6

RADIATION

Although all types of ionizing radiation can damage tissue, beta and alpha radiation are so easy to shield that most attention today is given to gamma ray, X-ray, and neutron radiation. High-energy electron beams impinging on metal in vacuum equipment can produce very penetrating X-rays that may require much more shielding than the electron beam itself.
The absorbed dose is the amount of energy imparted by ionizing radiation to a given mass of material. The unit of absorbed dose is the rad, which is equivalent to the absorption of 0.01 joule per kilogram (J/kg) [100 ergs per gram
(erg/g)]. The dose equivalent is a way of correcting for the differences in the biological effect of various types of ionizing radiation on humans. The unit of dose equivalent is the rem, which produces a biological effect essentially the same as that of 1 rad of absorbed dose of X or gamma radiation. The roentgen (R) is a unit of exposure that measures the amount of ionization produced in air by X or gamma radiation. Tissue located at a point where the exposure is 1 R receives an absorbed dose of approximately 1 rad.
Very large doses of ionizing radiation—100 rads or more—received over a short time span by the entire body can cause radiation sickness. An absorbed

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dose of about 400 rads to the whole body would be fatal to approximately onehalf of adults. Small doses received over a longer period may increase the probability of contracting various types of cancers or other diseases. The overall risk of a fatal cancer from a radiation dose equivalent of 1 rem is about 10Ϫ4; that is, a person receiving a dose equivalent of 1 rem has about 1 chance in 10,000 of dying from a cancer produced by the radiation. The risk can also be expressed as the expectation of one fatal cancer in a group of 10,000 persons, if each person receives a dose equivalent of 1 rem.
Persons working in areas where access is controlled for the purpose of radiation protection are generally limited to a dose equivalent of 5 rem/yr. The limit in uncontrolled areas is usually the same. Working within these limits should have no significant effect on the health of the individuals involved. All persons are exposed to radiation from naturally occurring radioisotopes in the body, cosmic radiation, and radiation emitted from the earth and building materials. The dose equivalent from natural background sources is about 0.1 rem/yr (100 mrem/yr).

6.7

SHIFTWORK AND WORKING HOURS

SHIFTWORK
Shiftwork, defined as working other than daytime hours, is becoming an everincreasing problem for industry. Traditionally, the need for continuous services from police, fire, and medical personnel, or for continuous operations in the chemical or pharmaceutical industries, has required the use of shiftwork. More recently, however, the economics of manufacturing, that is, the capitalization or payback of ever more expensive automated machinery, are also increasing the demand for shiftwork. Similarly, just-in-time production and seasonal demands for products (i.e., decreased inventory space) have also required more shiftwork.
The problem with shiftwork is the stress on circadian rhythms, which are the roughly 24-h variations in bodily functions in humans (as well as other organisms).
The length of the cycle varies from 22 to 25 h, but is kept synchronized into a
24-h cycle by various timekeepers, such as the daily light–dark changes, social contacts, work, and clock time. The most marked cyclic changes occur in sleep, core temperature, heart rate, blood pressure, and task performance, such as critical tracking capability (see Figure 6.18). Typically, bodily functions and performance start increasing upon awakening, peak in midafternoon, then steadily decline to a low point in the middle of the night. There may also be a dip after midday, typically known as the postlunch dip. Thus, individuals who are asked to work on night shift will exhibit a marked degradation in performance, from truck drivers falling asleep at the wheel to gas inspectors reading meters (Grandjean, 1988).
It could be assumed that night workers would adapt to night work because of the change in work patterns. Unfortunately, the other social interactions still play a very important role, and the circadian rhythm never truly shifts (as it would for individuals traveling to the other side of the globe for extensive periods) but rather flattens, which some researchers consider to be a worse scenario. Thus, night workers

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AVERAGE 3 SUBJECTS,
3 DAYS EACH
␮ = 36.3 ؇C

Rectal temperature
3

% Change — amplitude

2
1
0
؊1
؊2

12

24

36

؊3

Dinner
Time (hours)

Critical tracking capability
20

AVERAGE OF ALL SUBJECTS,
ALL DAYS
␮ = 819

% Change — amplitude

10

0

؊10

؊20

12

24
Time (hours)

Figure 6.18

36
Dinner

Examples of circadian rhythms.

(From: Freivalds, Chaffin, and Langolf, 1983)

also experience health problems, such as appetite loss, digestive problems, ulcers, and increased sickness rates. The problems become even worse as the worker ages.
There are many ways to organize shiftwork. Typically, a three-shift system has an early (E) shift from 8 A.M. to 4 P.M., a late afternoon shift (L) from 4 P.M.

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to 12 P.M. (midnight), and a night (N) shift from 12 P.M. to 8 A.M.. In the simplest case, because of short-term increased production demands, a company may go from just an early shift to both an early and a late afternoon shift. Usually, because of seniority, the early shift is requested by older, established workers, while new hires start on the late afternoon shift. The rotation of the two shifts on a weekly basis does not cause major physiological problems, since the sleep pattern is not disrupted. However, the social patterns can be considerably disrupted.
Progressing to a third, night shift becomes more problematic. Since there is difficulty adjusting to a new circadian rhythm, even over the course of several weeks, most researchers advocate a rapid rotation, with shift changes every two or three days. This maintains the quality of sleep as well as possible and does not disrupt family life and social contacts for extended periods. The weekly rotation typically found in the United States is perhaps the worst scenario, because the workers never truly adjust to any one shift.
One rapid-rotation shiftwork system for a 5-day production system (i.e., weekends off) is given in Table 6.11. However, in many companies, the night shift is mainly a maintenance shift with limited production. In that case, a full crew is not needed, and it may be simpler to rotate only the early and late afternoon shifts and to operate a smaller, fixed night shift, which can be staffed primarily by volunteers who can better adapt to that shift.
For continuous round-the-clock operations, a rapid-rotation seven-day shift system is needed. Two plans commonly used in Europe are the 2-2-2 system, with no more than two days on any one shift (see Table 6.12), and the 2-2-3 system, with no more than three days on any one shift (see Table 6.13). There are trade-offs with each of these systems. The 2-2-2 system provides a free weekend
Table 6.11

Eight-Hour Shift Rotation (Weekends Off)

Week

M

T

W

Th

F

S

Su

1
2
3

E
N
L

E
N
L

L
E
N

L
E
N

L
E
N









Note: E ϭ early, L ϭ late afternoon, N ϭ night.
Table 6.12

The 2-2-2 Shift Rotation (8-h Continuous)

Week

M

T

W

Th

F

S

Su

1
2
3
4
5
6
7
8

E


N
N
L
L
E

E
E


N
N
L
L

L
E
E


N
N
L

L
L
E
E


N
N

N
L
L
E
E


N

N
N
L
L
E
E




N
N
L
L
E
E


Note: E ϭ early, L ϭ late afternoon, N ϭ night.

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Table 6.13

Work Environment Design

The 2-2-3 Shift Rotation (8-h Continuous)

Week

M

T

W

Th

F

S

Su

1
2
3
4

E

N
L

E

N
L

L
E

N

L
E

N

N
L
E


N
L
E


N
L
E


Note: E ϭ early, L ϭ late afternoon, N ϭ night.
Table 6.14

A 12-h Shift Rotation (3 Days On, 3 Days Off)

Week

M

T

W

Th

F

S

Su

1
2
3
4
5
6

D
N
D




D
N



D

D



N
D




D
N
D



N
D
N



D
N
D



N
D
N




Note: D ϭ day, N ϭ night.
Table 6.15

A 12-h Shift Rotation (Every Other Weekend Off)

Week

M

T

W

Th

F

S

Su

1
2
3
4

D

N



D

N


D

N

N

D


N

D



N

D


N

D

Note: D ϭ day, N ϭ night.

only once in eight weeks. The 2-2-3 system provides a free three-day weekend once in four weeks, but requires workers to work seven days straight, which is not appealing. A basic problem in both systems is that with 8-h shifts, a total of
42 h/week is worked. Alternative systems with more crews and shorter hours may be required (Eastman Kodak, 1986).
Another possible approach is to schedule 12-h shifts. Under these systems, workers either work 12-h day (D) shifts or 12-h night (N) shifts, on either a regular three days on and three days off schedule (see Table 6.14), or a more complicated two or three days on or off, with every other weekend free (see Table 6.15).
There are several advantages in that there are longer rest periods between workdays, and at least one-half of the rest days coincide with a weekend. Of course, the obvious disadvantage is having to work extended days or essentially regular overtime (see next section).
More complicated systems exist with reduced hours (40 or less) per week.
These can be studied in further detail in Eastman Kodak (1986) or Schwarzenau et al. (1986).

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In summary, definite health and accident risks are associated with shiftwork.
However, if shiftwork is unavoidable, due to manufacturing process considerations, the following recommendations should be considered:
1.
2.
3.
4.
5.
6.
7.
8.

Avoid shiftwork for workers older than 50.
Use rapid rotations as opposed to weekly or monthly cycles.
Schedule as few night shifts (three or less) in succession as possible.
Use forward rotation of shifts if possible (e.g., E-L-N or D-N).
Limit the total number of working shifts in succession to seven or less.
Include some free weekends, with at least two successive full days off.
Schedule rest days after night shifts.
Keep the plans simple, predictable, and equitable for all workers.

OVERTIME
Many studies have shown that changes in the length of the workday or workweek have a direct effect on work output. Unfortunately, the result is not typically the direct proportionality expected. Note that in Figure 6.19, the theoretical daily performance is linear (line 1), but, in practice, is more S-shaped (curve 2). For example, there is an initial setup or preparatory period with little productivity
(area A), a gradual warming up, a steeper section with greater than the theoretical productivity (area B), and graduate leveling off as the end of the shift approaches. In an 8-h shift, the two areas, subpar (area A) and excess productivity (area B), are equal, whereas in longer than 8-h shifts for heavy manual work
(curve 3), the subpar productivity is greater than the excess productivity,
%
1

120

C
3

100

Sub par production
Productivity (%)

272

80

Excess production

B

1 = Theoretical productivity
2 = Actual 8-hour productivity
3 = 12-hour productivity for heavy manual work

60
40
2
20

A

0
1

Figure 6.19

2

3

4
5
6
7
8
Working hours on shift

9

10

Productivity as a function of working hours.

(Adapted from: Lehmann, 1953)

11

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especially with additional subpar performance (area C) in the last few hours
(Lehmann, 1953).
The results of an old British survey (cited in Grandjean, 1988) found that shortening the working day resulted in a higher hourly output, with fewer rest pauses taken. This change in the working performance required at least several days (sometimes longer) before steady state was reached. Conversely, making the working day longer, that is, assigning overtime, causes productivity to fall, sometimes to the point that the total output over the course of the shift actually drops, even though the total hours worked are longer (see curve 3 in Figure 6.19).
Therefore, any expected benefit from increased hours is typically offset by decreased productivity. This effect depends on the level of the physical workload: the more strenuous the work, the greater the decrease in productivity, with the worker using more rest to pace himself or herself.
More recent data (cited in Eastman Kodak, 1986) indicate that the expected increase in output is approximately 10 percent for each 25 percent increase in hours worked. This definitely does not justify the time-and-a-half pay expended for overtime work. This discussion presumes a day work pay scale (see Chapter 17).
With an incentive scheme throughout the extended hours, the fall in productivity may not be so great. Similarly, if the work is machine-paced, productivity is tied to that machine-pace. However, the operator may reach unacceptable fatigue levels, and additional rest per appropriate allowances (see Chapter 11) may be needed. A secondary effect of overtime is that excessive or continuous overtime is accompanied by increased accident rates and sick leave (Grandjean, 1988).
Scheduling overtime on a regular basis cannot be recommended. However, overtime may be necessary for transient short periods, to maintain production or alleviate temporary labor shortages. In such cases, the following guidelines should be followed:
1. Avoid overtime for heavy manual work.
2. Reevaluate machine-paced work for appropriate rest periods or lowered rates.
3. For continuous or long periods of overtime, rotate the work among several workers, or examine alternate shift systems.
4. In choosing between extending a series of workdays by 1 or 2 h versus extending the workweek by 1 day, most workers will opt for the former, to avoid losing a weekend day with the family (Eastman Kodak, 1986).

COMPRESSED WORKWEEK
A compressed workweek implies that 40 h is performed in fewer than 5 days. Typically, this occurs in the form of four 10-h days, three 12-h days, or four 9-h days with a half-day on Friday. From the management perspective, this concept offers several advantages: reduced absenteeism, relatively less time spent on coffee or lunch breaks, and reduced start-up and shutdown costs (relative to operating time).
For example, heat treating, forging, and melting facilities require a significant amount of time, up to 15 percent of the 8-h workday or more, to bring the facility

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and material up to the required temperature before production can begin. By going to a 10-h day, the operation can gain an additional 2 h of production time with no additional setup time. Here, the economic savings from the longer workday can be significant. Workers also gain from increased leisure time, reduced commuter time (relative to working time), and lower commuting costs.
On the other hand, based on the discussions on overtime, a compressed workweek essentially amounts to continuous overtime. Although the total hours worked are less, the hours worked on a given day are more. Therefore, many of the same disadvantages of overtime would apply to a compressed workweek
(Eastman Kodak, 1986). Other objections to the 10-h day, 4-day week stem from members of management who state that they are obliged to be on the job not only
10 h for 4 days, but at least 8 h on the fifth day.

ALTERNATIVE WORK SCHEDULES
With the greater influx of women, especially mothers with school-age children, single parents, older workers, and dual-career-family workers into the workforce, and with the increased concerns for the cost and time of commuting and the value of quality of life, alternate work schedules are needed. One such schedule is flextime, where the starting and stopping times are established by the workers, within limits set up by management. Various plans of this nature currently exist.
Some require employees to work at least 8 h/day, others require a specified number of hours in a week or a month, while still others require all workers to be on site four or five middle hours of the shift.
There are many advantages to these plans for both employees and management. Employees can work the morning or evening hours most conducive to their circadian rhythms, they can better handle family needs or emergencies, and they can take care of personal business during business hours without requiring special leave from work. Management gains from reduced tardiness and sick leave.
Even the surrounding community gains from decreased traffic congestion and better use of recreational and service facilities. On the other hand, flextime may have limited use in manufacturing, machine-paced, and continuous-process operations, because of problems in scheduling and coordinating the labor force.
However, in situations where work groups (see Chapter 18) are utilized, flextime may still be possible (Eastman Kodak, 1986).
Part-time employment and job sharing may be especially useful to single parents with children or to retirees seeking to supplement their retirement incomes. Both groups can provide considerable talent and services to a company, but may be limited by circumstances from performing traditional 8-h shifts.
While there may be problems regarding benefits or other fixed employee costs, these may be handled on a prorated or other creative basis.

SUMMARY
A proper working environment is important not only from the standpoint of increasing productivity and improving the physical health and safety of the workers,

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but also for promoting worker morale and consequent reductions in worker absenteeism and labor turnover. Although many of these factors may seem intangible or of marginal effect, controlled scientific studies have shown the positive benefits of improved illumination, decreased noise and heat stress, and better ventilation. Visibility is directly dependent on the illumination provided, but is also affected by the visual angle of the target viewed and the contrast of the target with the background. Consequently, improvement in task visibility can be accomplished through various means and does not always depend on increasing the light source.
Extended exposures to loud noise, although not directly affecting productivity, can cause hearing loss and are definitely annoying. The control of noise (and vibration) is simplest at the source and typically becomes more costly farther away. Although using hearing protection may seem the simplest approach, it requires the expense of continuous motivation and enforcement.
Similarly, the effect of climate on productivity is quite variable, depending on individual motivation. A comfortable climate is a function of the amount and velocity of air exchange, the temperature, and the humidity. For hot areas, the climate is controlled most easily through adequate ventilation to remove pollutants and improve the evaporation of sweat. (Air-conditioning is more effective, but is also more expensive.) For cold climates, adequate clothing is the primary control. Shiftwork should utilize short, rapid, forward-rotating schedules with limited overtime.
To assist the methods analyst in utilizing the various factors discussed in this chapter, they have been summarized in the Work Environment Checklist shown in Figure 6.20.

QUESTIONS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.

What factors affect the quantity of light needed to perform a task satisfactorily?
Explain the color rendering effect of low-pressure sodium lamps.
What is the relationship between contrast and visibility?
What footcandle intensity would you recommend 30 in above the floor in the company washroom?
Explain how sales may be influenced by colors.
What color has the highest visibility?
How is sound energy dissipated in viscoelastic materials?
A frequency of 2,000 Hz would have approximately what wavelength in meters?
What would be the approximate decibel value of a grinder being used to grind a high-carbon steel?
Distinguish between broadband noise and meaningful noise.
Would you advocate background music at the workstation? What results would you anticipate? According to the present OSHA law, how many continuous hours per day of a
100-dBA sound level would be permissible?

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Illumination

Yes

No























Thermal conditions—Heat
1. Is the worker within the thermal comfort zone?
a. If not within the thermal comfort zone, has the WBGT of the working environment been measured?
2. Are the thermal conditions within ASHRAE guidelines?
a. If not within guidelines, is sufficient recovery time provided?
3. Are procedures in place for control of potential heat stress conditions?
a. Is the escape of heat controlled at the source?
b. Are radiation shields in place?
c. Is ventilation provided?
d. Is the air dehumidified?
e. Is air-conditioning provided?

Yes


No






















Thermal conditions—Cold
1. Is the worker adequately clothed for the equivalent wind chill temperature?
2. Are auxiliary heaters provided?
3. Are gloves provided?

Yes




No




Ventilation
1. Are ventilation levels acceptable per guidelines?
a. Is a minimum of 300 ft3/h/per person provided?
2. If necessary, are local fans provided for workers?
a. Are these tans within a distance of 30 times the fan diameter?
3. For local heat sources, is spot cooling provided?

Yes






No






Noise Levels
1. Are noise levels below 90 dBA?
a. If the noise levels exceed 90 dBA, is there sufficient rest such that the 8-h dose is less than 100%?
2. Are noise control measures in place?
a. Is the noise controlled at the source with better maintenance, mufflers, and rubber mounts?
b. Is the noise source isolated?
c. Are acoustical treatments being utilized?
d. As a last resort, are earplugs (or earmuffs) being used properly?

Yes


No


















Vibration
1. Are vibration levels within acceptable ANSI standards?
2. If there is vibration, can the vibration-causing sources be eliminated?
3. Have specially dampened seats been installed on vehicles?
4. Have vibration-absorbing handles been attached to power tools?
5. Have resilient, fatigue-resistant mats been supplied to standing operators?

Yes






No






1. ls the illumination sufficient for the task, per IESNA recommendations?
a. To increase illumination, are more luminaires provided, rather than increasing the wattage of existing ones?
2. Is there general lighting as well as supplementary lighting?
3. Are the workplace and lighting arranged so as to avoid glare?
a. Are direct luminaires placed away from the field of vision?
b. Do the luminaires have baffles or diffusers?
c. Are work surfaces laid out perpendicular to the luminaires?
d. Are surfaces matted or nonglossy?
4. If necessary, are screen filters available for computer monitors?

Figure 6.20 Work Environment Checklist.

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13. What three classifications have been identified from the standpoint of exposure to vibration? 14. In what ways can workers be protected from vibration?
15. What is meant by the environmental temperature?
16. Explain what is meant by the thermal comfort zone.
17. What is the maximum rise in body temperature that analysts should allow?
18. How would you go about estimating the maximum length of time that a worker should be exposed to a particular heat environment?
19. What is WBGT?
20. What is the WBGT with a dry-bulb temperature of 80°F, a wet-bulb temperature of
70°F, and a globe temperature of 100°F?
21. Which type of radiation is given the most attention by the safety engineer?
22. What is meant by absorbed dose of radiation? What is the unit of absorbed dose?
23. What is meant by rem?
24. What steps would you take to increase the amount of light in the following assembly department by about 15 percent? The department currently uses fluorescent fixtures, and the walls and ceiling are painted a medium green. The assembly benches are a dark brown.
25. What color combination would you use to attract attention to a new product being displayed? 26. When would you advocate that the company purchase aluminized clothing?
27. Are possible health hazards associated with electron beam machining? With laser beam machining? Explain.
28. Explain the impact of noise levels below 85 dBA on office work.
29. What environmental factors affect heat stress? How can each be measured?
30. How would you determine if a job places an excessive heat load on the worker?

PROBLEMS
1. A work area has a reflectivity of 60 percent, based on the color combinations of the workstations and the immediate environment. The seeing task of the assembly work could be classified as difficult. What would be your recommended illumination?
2. What is the combined noise level of two sounds of 86 and 96 dB?
3. In the Dorben Company, an industrial engineer designed a workstation where the seeing task was difficult because of the size of the components going into the assembly. The desired brightness was 100 fL, and the workstation was painted a medium green with a reflectance of 50 percent. What illumination in footcandles would be required at this workstation to provide the desired brightness? Estimate the required illumination if you repainted the workstation with a light cream paint.
4. In the Dorben Company, an industrial engineer (IE) was assigned to alter the work methods in the press department to meet OSHA standards relative to permissible noise exposures. The IE found a time-weighted average sound level of 100 dBA.
The 20 operators in this department wore earplugs provided by Dorben with an
NRR value of 20 dB. What improvement resulted? Do you feel that this department is now in compliance with the law? Explain.

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5. In the Dorben Company, an all-day study revealed the following noise sources:
0.5 h, 100 dBA; 1 h, less than 80 dBA; 3.5 h, 90 dBA; 3 h, 92 dBA. Is this company in compliance? What is the dose exposure? What is the TWA noise level? 6. In Problem 5, consider that the last exposure is in the press room, which currently has five presses operating. Assuming that Dorben Company can eliminate some of the presses and transfer production to the remaining presses, how many presses should Dorben eliminate so as not to exceed 100 percent dose exposure for the workers? 7. What is the illumination on a surface 6 in from a 2-cd source?
8. What is the luminance of a surface having a 50 percent reflectance and 4-fc illumination? 9. What is the contrast created by black text (reflectance ϭ 10 percent) on white paper
(reflectance ϭ 90 percent)?
10. How much louder is an 80-dB noise than a 60-dB noise?
11. What is the increase in decibels of a noise that doubles in intensity?
12. A supervisor is sitting at her desk illuminated by a 180-cd source 3 ft above it. She is writing with green ink (reflectance ϭ 30 percent) on a yellow notepad
(reflectance ϭ 60 percent). What is the illumination on the notepad? Is that sufficient? If not, what amount of illumination is needed? What is the contrast of the writing task? What is the luminance of the notepad?
13. How much ventilation would be recommended for a classroom of area 1,000 ft2 with 12-ft ceilings? Assume that the class size may reach 40 students.

REFERENCES
ACGIH. Threshold Limit Values for Chemical Substances and Physical Agents in the
Work Environment, Cincinnati, OH: American Conference of Government
Industrial Hygienists, 1985.
ASA. American National Standard: Guide for the Evaluation of Human Exposure to
Whole Body Vibration (ANSI S3.18-1979). New York: Acoustical Society of
America, 1980.
ASHRAE. Handbook, Fundamentals. Chapter 8. Atlanta, GA: American Society of
Heating, Refrigeration and Air Conditioning Engineers, 1993.
ASHRAE. Handbook, Heating, Ventilation and Air Conditioning Applications. Chapter
25. Atlanta, GA: American Society of Heating, Refrigeration and Air Conditioning
Engineers, 1991.
Belding, H. S., and T. F. Hatch. “Index for Evaluating Heat Stress in Terms of
Physiological Strains.” Heating, Piping and Air Conditioning, 27(August 1955), pp. 129–136.
Blackwell, H. R. “Development and Use of a Quantitative Method for Specification of
Interior Illumination Levels on the Basis of Performance Data.” Illuminating
Engineer, 54 (June 1959), pp. 317–353.
Eastman Kodak Co. Ergonomic Design for People at Work, vol. 1. New York: Van
Nostrand Reinhold, 1983.
Eastman Kodak Co. Ergonomic Design for People at Work, vol. 2. New York: Van
Nostrand Reinhold, 1986.

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Freivalds, A., D. B. Chaffin, and G. D. Langolf. “Quantification of Human Performance
Circadian Rhythms.” Journal of the American Industrial Hygiene Association, 44, no. 9 (September 1983), pp. 643–648.
Freivalds, A., J. L. Harpster, and L. S. Heckman. “Glare and Night Vision Impairment in
Corrective Lens Wearers,” Proceedings of the Human Factor Society, (27th Annual
Meeting, 1983), pp. 324–328.
General Electric Company. Light Measurement and Control (TP-118). Nela Park,
Cleveland, OH: Large Lamp Department, G.E., March 1965.
Grandjean, E. Fitting the Task to Man. 4th ed. London: Taylor & Francis, 1988.
IESNA. Lighting Handbook, 8th ed. Ed. M. S. Rea. New York: Illuminating
Engineering Society of North America, 1995, pp. 459–478.
Kamon, E., W. L. Kenney, N. S. Deno, K. J. Soto, and A. J. Carpenter. “Readdressing
Personal Cooling with Ice.” Journal of the American Industrial Hygiene
Association, 47, no. 5 (May 1986), pp. 293–298.
Konz, S., and S. Johnson. Work Design, 5th ed. Scottsdale, AZ: Holcomb Hathaway
Publishers, 2000.
Lehmann, G. Praktische Arbeitsphysiologie. Stuttgart: G. Thieme, 1953.
Lockhart, J. M., H. O. Kiess, and T. J. Clegg. “Effect of Rate and Level of Lowered
Finger-Surface Temperature on Manual Performance.” Journal of Applied
Psychology, 60, no. 1 (February 1975), pp. 106–113.
NIOSH. Criteria for a Recommended Standard ... Occupational Exposure to Hot
Environments, Revised Criteria. Washington, DC: National Institute for
Occupational Safety and Health, Superintendent of Documents, 1986.
NIOSH. Occupational Noise Exposure, Revised Criteria 1998. DHHS Publication No.
98-126. Cincinnati, OH: National Institute for Occupational Safety and Health,
1998.
OSHA. Code of Federal Regulations—Labor. (29 CFR 1910). Washington, DC: Office of the Federal Register, 1997.
OSHA. Ergonomics Program Management Guidelines for Meatpacking Plants. OSHA
3123. Washington, DC: The Bureau of National Affairs, Inc., 1990.
Peterson, A., and E. Gross, Jr. Handbook of Noise Measurement, 8th ed. New Concord,
MA: General Radio Co., 1978.
Riley, M. W., and D. J. Cochran. “Partial Gloves and Reduced Temperature.” In
Proceedings of the Human Factors Society 28th Annual Meeting. Santa Monica,
CA: Human Factors and Ergonomics Society, 1984, pp. 179–182.
Sanders, M. S., and E. J. McCormick. Human Factors in Engineering and Design,
7th ed. New York: McGraw-Hill, 1993.
Schwarzenau, P., P. Knauth, E. Kiessvetter, W. Brockmann, and J. Rutenfranz.
“Algorithms for the Computerized Construction of Shift Systems Which Meet
Ergonomic Criteria.” Applied Ergonomics, 17, no. 3 (September 1986), pp. 169–176.
Yaglou, C. P., and D. Minard. “Control of Heat Casualties at Military Training Centers.”
AMA Archives of Industrial Health, 16 (1957), pp. 302–316.
Yaglou, C. P., E. C. Riley, and D. I. Coggins. “Ventilation Requirements.” American
Society of Heating, Refrigeration and Air Conditioning Engineers Transactions, 42
(1936), pp. 133–158.

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SELECTED SOFTWARE
DesignTools (available from the McGraw-Hill text website at www.mhhe.com/niebelfreivalds), New York: McGraw-Hill, 2002.

WEBSITES
American Society for Safety Engineers—http://www.ASSE.org/
CalOSHA Standard—http://www.ergoweb.com/Pub/Info/Std/calstd.html
National Safety Council— http://www.nsc.org/
NIOSH homepage—http://www.cdc.gov/niosh/homepage.html
OSHA homepage—http://www.osha.gov/
OSHA Proposed Ergonomics Standard—http://www.oshaslc.gov/FedReg_osha_data/FED20001114.html

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CHAPTER

7

KEY POINTS








Minimize informational workload.
Limit absolute judgments to 7 Ϯ 2 items.
Use visual displays for long, complex messages in noise areas.
Use auditory displays for warnings and short, simple messages.
Use color, symbols, and alphanumerics in visual displays.
Use color and flashing lights to get attention.

T

he design of cognitive work has not been traditionally included as part of methods engineering. However, with ongoing changes in jobs and the working environment, it is becoming increasingly important to study not only the manual components of work but also the cognitive aspects of work. Machines and equipment are becoming increasingly complex and semiautomated, if not fully automated. The operator must be able to perceive and interpret large amounts of information, make critical decisions, and control these machines quickly and accurately. Furthermore, there has been a gradual shift of jobs from manufacturing to the service sector. In either case, there typically will be less emphasis on gross physical activity and a greater emphasis on information processing and decision making, especially via computers and associated modern technology. Thus, this chapter explains information theory, presents a basic conceptual model of the human as an information processor, and details how best to code and display information for maximum effectiveness, especially with auditory and visual displays.
Also, a final section outlines both software and hardware considerations of the human interacting with computers.

7.1

INFORMATION THEORY

Information, in the everyday sense of the word, is knowledge received regarding a particular fact. In the technical sense, information is the reduction of uncertainty about that fact. For example, the fact that the engine (oil) light comes on when a
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car is started provides very little information (other than the fact that the lightbulb is functioning) because it is expected. On the other hand, when that same light comes on when you are driving down a road, it conveys considerable information about the status of the engine because it is unexpected and a very unlikely event.
Thus, there is a relationship between the likelihood of an event and the amount of information it conveys, which can be quantified through the mathematical definition of information. Note that this concept is irrespective of the importance of the information; that is, the status of the engine is quite a bit more important than whether the windshield washer container is empty.
Information theory measures information in bits, where a bit is the amount of information required to decide between two equally likely alternatives. The term bit came from the first and last part of the words binary digit used in computer and communication theory to express the on/off state of a chip or the polarized/reverse-polarized position of small pieces of ferromagnetic core used in archaic computer memory. Mathematically this can be expressed as
H ϭ log2n where H ϭ the amount of information n ϭ the number of equally likely alternatives
With only two alternatives, such as the on/off state of a chip or the toss of an unweighted coin, there is 1 bit of information presented. With 10 equally likely alternatives, such as the numbers from 0 to 9, 3.322 bits of information can be conveyed (log2 10 ϭ 3.322). An easy way of calculating log2 is to use the following formula: log2 n ϭ 1 .4427 ϫ ln n
When the alternatives are not equally likely, the information conveyed is determined by
H ϭ ©pi ϫ log2 11>pi 2

pi ϭ probability of ith event i ϭ alternatives from 1 to n
As an example, consider a coin weighted so that heads comes up 90 percent of the time and tails only 10 percent of time. The amount of information conveyed in a coin toss becomes

where

H ϭ 0 .9 ϫ log2 11>0 .9 2 ϩ 0 .1 ϫ log2 11>0 .1 2 ϭ 0 .9 ϫ 0 .152 ϩ 0 .1 ϫ 3 .32 ϭ 0.469 bit

Note that the amount of information (0.469) conveyed by a weighted coin is less than the amount of information conveyed by an unweighted coin (1.0). The maximum amount of information is always obtained when the probabilities are equally likely. This is so because the more likely an alternative becomes, the less information is being conveyed (i.e., consider the engine light upon starting a car).
This leads to the concept of redundancy and the reduction of information from

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the maximum possible due to unequal probabilities of occurrence. Redundancy can be expressed as
% redundancy ϭ 11 Ϫ H>Hmax 2 ϫ 100

For the case of the weighted coin, the redundancy is

% redundancy ϭ 11 Ϫ 0.469>1 2 ϫ 100 ϭ 53.1%

An interesting example relates to the use of the English language. There are
26 letters in the alphabet (A through Z) with a theoretical informational content for a randomly chosen letter of 4.7 bits (log2 26 ϭ 4.7). Obviously, with the combinations of letters into words, considerably more information can be presented.
However, there is a considerable reduction in the amount of information that can be actually presented due to the unequal probabilities of occurrence. For example, letters s, t, and e are much more common than q, x, and z. It has been estimated that the redundancy in the English language amounts to 68 percent
(Sanders and McCormick, 1993). On the other hand, redundancy has some important advantages that will be discussed later with respect to designing displays and presenting information to users.
One final related concept is the bandwidth or channel capacity, the maximum information processing speed of a given communication channel. In terms of the human operator, the bandwidth for motor-processing tasks could be as low as 6 to 7 bits/s or as high as 50 bits/s for speech communication. For purely sensory storage of the ear (i.e., information not reaching the decision-making stage), the bandwidth approaches 10,000 bits/s (Sanders and McCormick, 1993). The latter value is much higher than the actual amount of information that is processed by the brain in that time because most of the information received by our senses is filtered out before it reaches the brain.

7.2 HUMAN INFORMATION PROCESSING
MODEL
Numerous models have been put forward to explain how people process information. Most of these models consist of black boxes (because of relatively incomplete information) representing various processing stages. Figure 7.1 presents one such generic model consisting of four major stages or components: perception, decision and response selection, response execution, memory, and attentional resources distributed over various stages. The decision-making component, when combined with working memory and long-term memory, can be considered as the central processing unit while the sensory store is a very transient memory, located at the input stage (Wickens, Gordon, and Liu, 1997).

PERCEPTION AND SIGNAL DETECTION THEORY
Perception is the comparison of incoming stimulus information with stored knowledge to categorize the information. The most basic form of perception is

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Attention resources Short-term sensory store

Decision and response selection

Stimuli
Perception

Response execution Responses

Working memory Long-term memory Memory
Feedback

Figure 7.1

A model of human information processing.

(Source: Wickens, 1984, Fig. 1.1. Reprinted by permission of the publisher.)

simple detection, that is, determining whether the stimulus is actually present. It becomes more complicated if the person is asked to indicate the type of stimulus or the stimulus class to which it belongs and then gets into the realm of identification and recognition with the use of prior experiences and learned associations.
The consequent linkage between long-term memory and perceptual encoding is shown in Figure 7.1. This latter more complex perception can be explained in terms of feature analysis, breaking down objects into component geometric shapes or text into words and character strings, and, simultaneously, of top-down or bottom-up processing to reduce the amount of information entering central processing. Top-down processing is conceptually driven using high-level concepts to process low-level perceptual features, while bottom-up processing is data-driven and guided by sensory features.
The detection part of perceptual encoding can be modeled or, in fairly simple tasks, even quantified through signal detection theory (SDT). The basic concept

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of SDT is that in any situation, an observer needs to identify a signal (i.e., whether it is present or absent) from confounding noise. For example, a quality inspector in an electronics operation must identify and remove defective chip capacitors from the good capacitors being used in the assembly of printed-circuit boards. The defective chip capacitor is the signal, which could be identified by excessive solder on the capacitor that shorts out the capacitor. The good capacitors, in this case, would be considered noise. Note that one could just as easily reverse the decision process and consider good capacitors as the signal and defective capacitors as noise. This would probably depend on the relative proportions of each.
Given that the observer must identify whether the signal is present and that only two possible states exist (i.e., the signal is either there or not there), there are a total of four possible outcomes:
1.
2.
3.
4.

Hit—saying there is a signal when the signal is present
Correction rejection—saying there is no signal when no signal is present
False alarm—saying there is a signal when no signal is present
Miss—saying there is no signal when the signal is present

Both the signal and noise can vary over time, as is the case with most industrial processes. For example, the soldering machine may warm up and initially expel a larger drop of solder on the capacitors, or there may be simply “random” variation in the capacitors with no cause yet determined. Therefore both the signal and noise form distributions of varying solder quantity from low to high, which typically are modeled as overlapping normal distributions (Figure 7.2). Note that the distributions overlap because excessive solder on the body of the capacitor would cause it to short out, causing a defective product (in this case a signal).
However, if there is excessive solder, but primarily on the leads, the capacitor may not short out and thus is still a good capacitor (in this case noise). With evershrinking electronic products, chip capacitors are smaller than pinheads, and the visual inspection of these is not a trivial task.
When a capacitor appears, the inspector needs to decide if the quantity of solder is excessive and whether to reject the capacitor. Either through instructions and/or sufficient practice, the inspector has made a mental standard of judgment, which is depicted as the vertical line in Figure 7.2 and termed the response criterion. If the detected quantity of the solder, which enters the visual system as a high level of sensory stimulation, exceeds the criterion, the inspector will say there is a signal. On the other hand, if the detected quantity is small, a smaller level of sensory stimulation is received, landing below the criterion, and the inspector will say there is no signal.
Related to the response criterion is the quantity beta. Numerically beta is the ratio of the height of the two curves (signal to noise) in Figure 7.2 at the given criterion point. If the criterion shifts to the left, beta decreases with an increase of hits but at the cost of a corresponding increase of false alarms. This behavior on the part of the observer is termed risky. If the criterion were at the point where the two curves intersect, beta would be 1.0. On the other hand, if the criterion

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Beta = b/a

d'
Say "signal"

Say "no signal"
Probability of occurrence

286

b

Signal + noise distribution Noise only distribution HIT
CORRECT
REJECTION

a

MISS
FALSE ALARM low criterion

high

Intensity of "sensory activity"

Figure 7.2 Conceptual illustration of signal detection theory.

shifts to the right, beta increases with a decrease of both hits and false alarms.
This behavior on the part of the observer would be termed conservative.
The response criterion (and beta) can easily change depending on the mood or fatigue of the visual inspector. It would not be unexpected for the criterion to shift to the right and the miss rate to increase dramatically late Friday afternoons shortly before quitting times. Note that there will be a corresponding decrease in the hit rate because the two probabilities sum to 1. Similarly, the probabilities of a correct rejection and false alarms also sum to 1. The change in the response criterion is termed response bias and could also change with prior knowledge or changes in expectancy. If it were known that the soldering machine was malfunctioning, the inspector would most likely shift the criterion to the left, increasing the number of hits. The criterion could also change due to the costs or benefits associated with the four outcomes. If a particular batch of capacitors were being sent to NASA for use in the space shuttle, the costs of having a defect would be very high, and the inspector would set a very low criterion, producing many hits but also many false alarms with corresponding increased costs (e.g., losing good products). On the other hand, if the capacitors were being used in cheap give-away cell phones, the inspector might set a very high criterion, allowing many defective capacitors to pass through the checkpoint as misses.
A second important concept in SDT is that of sensitivity or the resolution of the sensory system. In SDT, sensitivity is measured as the separation between the two distributions shown in Figure 7.2 and labeled as dЈ. The greater the separation, the greater the observer’s sensitivity and the more correct responses (more

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hits and more correct rejections) and fewer errors (fewer false alarms and misses) are made. Usually sensitivity will improve with greater training and alertness
(e.g., through more frequent rest breaks) on the part of the inspector, better illumination at the workstation, and slowing the rate of signal presentation (which has the trade-off of decreasing productivity). Other factors that may help increase sensitivity involve supplying visual templates of the defective parts, providing redundant representations or clues for the defective parts, and providing knowledge of results. Note that providing incentives will help increase hit rates. However, this is typically due to a shift of the response bias (not an increase in sensitivity) with a corresponding increase in false-alarm rates. Similarly introducing “false signals” to increase alertness will again have a greater tendency to shift the response bias. More information on signal detection theory can be found in Green and Swets (1988).

Signal Detection Theory as Applied to the Inspection of Glass
A good application of signal detection theory was detailed by Drury and Addison
(1973) in the visual inspection of glass. The inspection was in two stages: (1) 100 percent general inspection in which each item was either accepted or rejected and (2) a sample inspection by special examiners who reexamined the previous results and provided feedback to the general inspectors. Considering the quality of the items being inspected, a proportion was good and the rest was bad. The general inspector could make only two decisions: accept or reject. The proper responses would be the acceptance of a good item (hit) and the rejection of a bad item (correct rejection). However, some good items could be rejected (misses), and some bad items could be accepted
(false alarms). Consider four different cases of varying conditions.
Case 1—Conservative Inspector A conservative inspector sets the criterion far to the right (Figure 7.3a). In such a situation, the probability of hits (saying yes to the signal of good glass) is low (e.g., 0.30). The probability of false alarms (saying yes to the noise of bad glass) is even lower (e.g., 0.05). Beta is determined by the ratio of the ordinates of the signal curve over the noise curve at the criterion. The ordinate for a standard normal curve is eϪz >2
2



22p

For the signal curve, a probability of 0.30 yields a z of 0.524 and an ordinate of 0.348.
For the noise curve, a probability of 0.05 yields a z of 1.645 and an ordinate of 0.103.
Beta then becomes 3.38 (0.348/0.103). Note that the probability of hits and misses equals 1.0 (i.e., 0.30 ϩ 0.70 ϭ 1.0). The same is true for false alarms and correct rejections. Case 2—Average Inspector If the inspector is average—neither conservative nor risky—the probability of hits roughly equals the probability of correct rejections
(Figure 7.3b). The curves intersect symmetrically, resulting in the same ordinate values and a value of 1.0 for beta.
Case 3—Risky Inspector A risky inspector (Figure 7.3c) sets the criterion far to the left, increasing the probability of hits (e.g., 0.95) at the cost of a high probability of false alarms (e.g., 0.70). In this case, for the signal curve, a probability of 0.95 yields a

EXAMPLE 7.1

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Signal
Yes

0.30
0.70

0.70

0.30

No

0.95

Noise

Yes

0.05

No

Signal

0.30

0.70

Noise

β=1

High β

(a)

(b)

Signal

Noise

Yes

0.95

0.70

No

0.05

Signal

0.30

Noise

Yes

0.90

0.10

No

0.10

0.90

Low β

(c)

β=1

(d)

Figure 7.3 Signal detection theory as applied to inspection: (a) conservative inspector, (b) average inspector,
(c) risky inspector, and (d) increased sensitivity.

z of Ϫ1.645 and an ordinate of 0.103. For the noise curve, a probability of 0.70 yields a z of Ϫ0.524 and an ordinate of 0.348. Beta then becomes 0.296 (0.103/0.348).
Case 4—Increased Sensitivity Sensitivity can be calculated as the difference of the z value for the same abscissa for both signal and noise curves (Figure 7.3): d¿ ϭ z 1false alarms 2 Ϫ z 1hits2
Using the criterion of case 1, d¿ ϭ 1.645 Ϫ 0.524 ϭ 1.121

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The same is found from the criterion in case 3: d¿ ϭ Ϫ 0 .524 Ϫ 1 Ϫ 1 .645 2 ϭ 1 .121
If the signal can be better separated from the noise, the probability of hits will increase (e.g., up to 0.90), while the probability of false alarms will remain fairly low
(e.g., 0.10). Using the criterion as the comparison point, the probability of hits is
0.90 with a corresponding z value of Ϫ1.283 and an ordinate of 0.175. The probability of false alarms is 0.10, with a corresponding z value of 1.283 and an ordinate of 0.175. The sensitivity then becomes d¿ ϭ 1 .283 Ϫ 1 Ϫ 1 .283 2 ϭ 2 .566
With increased sensitivity there is better performance in identifying defective parts.
Sometimes, the hit rate is plotted against the false alarm rate to yield a receiver operator characteristic curve in which the deviation of the curve from the 45 degree slope indicates sensitivity.
In the Drury and Addison (1973) case study, weekly data on glass inspection were collected from which the value dЈ was calculated. A change in inspection policy of providing more rapid feedback to the general inspector resulted in dЈ increasing from a mean value of 2.5 to a mean value of 3.16, a 26 percent increase in sensitivity over the course of 10 weeks (see Figure 7.4). This represented a 60 percent increase in the signal-to-noise ratio (i.e., beta) and a 50 percent decrease in the probability of missing a defect.

MEMORY
Once the stimulus has been perceptually encoded, it goes into working memory, one of the three components of the human memory system. The other two are sensory store and long-term memory. Each sensory channel has a temporary storage mechanism that prolongs the stimulus for it to be properly encoded. This storage is very short, on the order of 1 or 2 s depending on the sensory channel, before the stimulus representation disappears. It is also fairly automatic, in that it doesn’t require much attention to maintain it. On the other hand, there is very little that can be done to maintain this storage or increase the length of the time period. Note also from Figure 7.1 that although there may be a vast amount of stimuli, which could be represented on the order of millions of bits of information entering the sensory storage, only a very small portion of that information is actually encoded and sent on to working memory.
As opposed to long-term memory, working memory is a means of temporarily storing information or keeping it active while it is being processed for a response. Thus, it sometimes is termed short-term memory. Looking up a phone number and retaining it until the number has been dialed and finding a processing code from a list and entering it into the control pad of a machine are good examples of working memory. Working memory is limited in both the amount of information and the length of time that information can be maintained.
The upper limit for the capacity of working memory is approximately 7 Ϯ 2 items, sometimes known as Miller’s rule after the psychologist who defined it
(Miller, 1956). For example, the 11 digits 12125551212 would be very difficult,

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4

3
Detectability (d')

290

2

1

Change

0
0

5

10

15

20

25

30

35

40

45

Week number

Figure 7.4

Change in sensitivity with time for glass inspection.

(From: Drury and Addison, 1973; Taylor & Francis, Philadelphia, PA.)

if not impossible, to recall. Recall can be improved by the use of chunking, or the grouping of similar items. The above numbers, when properly grouped as 1-212555-1212, are much more easily remembered as three chunks (the 1 is the obvious standard for long distance). Similarly, rehearsal, or mentally repeating the numbers, which shifts additional attentional resources (see Figure 7.1) to working memory can improve recall.
Working memory also decays quickly, in spite of rehearsing or serially cycling through the items being actively maintained. The more items in working memory, the longer it takes to cycle through and the greater likelihood of one or more of the items being lost. It is estimated that the half-life for a memory store of three items is 7 s. This can be easily demonstrated by presenting a subject three random numbers (e.g., 5 3 6). After 7 s counting backward to prevent rehearsal, most individuals would have forgotten at least one number, if not two numbers. Some recommendations for minimizing errors on tasks requiring the use of working memory are to (Wickens, Gordon, and Liu, 1997)





Minimize the memory load, in terms of both capacity and time to maintain recall Utilize chunking, especially in terms of meaningful sequences and use of letters over numbers (e.g., the use of words or acronyms instead of numbers for toll-free telephone numbers, such as 1-800-CTD-HELP)
Keep chunks small, no more than three or four items of arbitrary nature

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Keep numbers separate from letters (e.g., chunks should contain similar entities) Minimize confusion of similar-sounding items (e.g., letters D, P, and T are easily confused, as opposed to letters J, F, and R)

Information from working memory may be transferred to long-term memory if it will be needed for later use. This could be information related to general knowledge in semantic memory or information on specific events in one’s life in the form of event memory. This transfer must be done in an orderly manner so as to be able to easily retrieve the data at a later time in a process we know as learning. The process of retrieving the information is the weak link and can be facilitated by frequent activation of that memory trace (e.g., a social security or phone number used every day) and the use of associations with previous knowledge.
These associations should be concrete rather than abstract and meaningful to the user, utilizing the user’s expectations and stereotypes. For example, the name
John Brown can be associated with the image of a brown house.
If there is the lack of clear or organized associations, the process can be done artificially in the form of mnemonics—an acronym or phrase—the letters of which represent a series of items. For example, the resistor color code (black, brown, red, orange, yellow, green, blue, violet, gray, white) can be remembered from the first letters of each word in the expression “big brown rabbits often yield great big vocal groans when gingerly slapped.” Standardization of procedures or use of memory aids (signs or notes) for complicated procedures also helps by decreasing the load on long-term memory. Unfortunately, long-term memory decays exponentially, with the most rapid decline in the first few days. Because of this, the effectiveness of training programs should not be evaluated immediately after the program.

DECISION MAKING AND RESPONSE SELECTION
Decision making is really the core of information processing, in which people evaluate alternatives and select an appropriate response. This is a relatively longterm process and should be distinguished from short-term processing as in choice reaction time. Unfortunately people are not optimal decision makers and frequently do not make rational decisions based on objective numbers or hard information. The rational approach in classical decision theory would be to calculate an expected value based on the sum of products of each outcome multiplied by its expected probability:
E ϭ ©pivi where E ϭ expected value pi ϭ probability of ith outcome vi ϭ value of the ith outcome
Unfortunately, people typically use a variety of heuristics to make decisions, in which case a variety of biases may influence the way they seek information,

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attach values to outcomes, and make overall decisions. A short list of such biases is derived from Wickens, Gordon, and Liu (1997):











A limited number of cues or pieces of information are used.
Undue weight is given to early cues.
Inattention is given to later cues.
Prominent cues are given greater weight.
All information is weighted equally regardless of true weight.
A limited number of hypotheses are generated.
Once a hypothesis has been selected, later cues are omitted.
Only confirming information is sought for the chosen hypothesis.
Only a small number of responses are chosen.
Potential losses are weighted more heavily than comparable potential gains.

By understanding these biases, the industrial engineer may be able to better present information and better set up the overall process to improve the quality of decision making and minimize errors.
In addition, current theories on decision making center around situational awareness, which is an evaluation of all the cues received from the surrounding environment. It requires the integration of cues or information into mental representations, ranging from simple schemata to complex mental models. To improve situational awareness, operators need to be trained to recognize and consider appropriate cues, to check the situation for consistency within the cues, and to analyze and resolve any conflicts in the cues or the situation. Decision aides, such as simple decision tables (discussed in Chapter 9) or more complex, expert systems, may assist in the decision-making process. Also, the display of key cues, the filtering out of undesirable cues, and the use of spatial techniques and display integration can assist in this process. Some of these will be discussed further in the section below on display modality.
The speed and difficulty of decision making and response selection, as discussed above, are influenced by many factors. One attempt at quantifying this process is typically performed through a choice–reaction time experiment, in which the operator will respond to several stimuli with several appropriate responses (see Figure 7.5a). This can be considered as simple decision making, and based on the human information processing system, the response time should increase as the number of alternative stimuli increases. The response is nonlinear (see Figure 7.5b), but when decision complexity is quantified in terms of the amount of information conveyed in bits, then the response becomes linear and is referred to as the Hick-Hyman law (Hick, 1952; Hyman,
1953; see Figure 7.5c)
RT ϭ a ϩ bH where RT ϭ response time (s)
H ϭ amount of information (bits) a ϭ intercept b ϭ slope, sometimes referred to as information processing rate

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Note that when there is only one choice (e.g., when the light appears, press the button), H ϭ 0 and the response time is equal to the intercept. This is known as simple reaction time. It can vary depending on the type of stimulus (auditory reaction times are about 40 ms faster than visual reaction times), the intensity of the stimulus, and preparedness for the signal.
Overall choice reaction times also vary considerably due to a variety of factors. The greater the compatibility (see also Section 5.3) between the stimulus and the response, the faster the response. The more practice, the faster the response. However, the faster the operator tries to respond, the greater the number of errors. Similarly, if there is a requirement for very high accuracy (e.g., air-traffic control), the response time will become slower. This inverse relationship is known as the speed-accuracy trade-off.
The use of multiple dimensions, in another form of redundancy, can also decrease response time in decision making; or, conversely, if there is conflicting information, response time will be slowed. A classic example is the Stroop Color-Word
Task (Stroop, 1935), in which the subject is asked to read a series of words expressing colors as rapidly as possible. In the control redundant case, having red ink spell out the word red will result in a fast response. In the conflict case, red ink letters spelling out blue will slow response time due to the semantic and visual conflicts.

(a)
Figure 7.5

Hick-Hyman law illustrated in a choice–reaction time experiment: (a) choice–reaction time experiment from Design Tools, (b) raw data, (c) information expressed in bits.

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700

Reaction time (msec)

600
500
400
300
200
100
0
0

2

4

6

8

10

Number of alternatives

(b)

700

Reaction time (msec)

600
500
400 b = 140 msec/bit
300
200 a = 190

100
0
0

0.5

1

1.5
2
2.5
Amount of information (bits)

3

3.5

(c)
Figure 7.5 (continued)

EXAMPLE 7.2

Human Information Processing in a Wiring Task
A good example of quantifying the amount of information processed in an industrial task was presented by Bishu and Drury (1988). In a simulated wiring task, industrial operators moved a stylus to the proper terminal or location on a control panel, which consisted of four different plates, each having eight possible components. Each component area was divided into 128 terminals in an array of eight columns and 16 rows.
The most complex task involved all four plates (log2 4 ϭ 2 bits of information), all eight components (3 bits), eight columns (3 bits), and 16 rows (4 bits) for a total

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complexity of 12 bits (sum of 2, 3, 3, and 4). From this control panel, others of less complexity could be produced by reducing the number of plates, components, columns, and rows. A low-complexity task involved only two plates (1 bit), four components (2 bits), four columns (2 bits), and 8 rows (3 bits) for a total complexity of 8 bits (sum of 1, 2, 2, and 3). Other intermediate-complexity tasks were also utilized.
The final results showed a linear relationship between information processing
(simulated wiring or placement) time and the information complexity of the input (see
Figure 7.6). Using the Hick-Hyman law, this relationship can be expressed as
IP ϭ Ϫ 2.328 ϩ 0.7325H where IP ϭ information processing time (s)
H ϭ amount of information (bits)
Thus, as the number of alternatives in performing the task increased, so also increased the informational loading on the central processing unit of the human operator, and the corresponding time for task performance. Note that in this relatively real-world case of a complex task, the intercept may not always be a positive value corresponding to simple reaction time.

Fitts’ Law and Information Processing of Movement
Information theory was applied to the modeling of human movement by Fitts (1954) who developed the index of difficulty to predict movement time. The index of difficulty was defined as a function of the distance of movement and target size in a series of positioning movements to and from identical targets:
ID ϭ log2 12D>W2
ID ϭ index of difficulty (bits)
D ϭ distance between target centers
W ϭ target width
Movement time was found to follow the Hick-Hyman law, now termed Fitts’ law: where MT ϭ a ϩ b 1ID2 where MT ϭ movement time (s) a ϭ intercept b ϭ slope
In a particularly successful application of Fitts’ law, Langolf, Chaffin, and Foulke
(1976) modeled human movement by different limbs across a wide range of distances, including very small targets visible only with the assistance of a microscope. Their results (see Figure 4.14) yielded slopes of 105 ms/bit for the arm, 45 ms/bit for the wrist, and 26 ms/bit for the finger. The inverse of the slope is interpreted, according to information theory, as the motor system bandwidth. In this case, the bandwidths were
38 bits/s for the finger, 23 bits/s for the wrist, and 10 bits/s for the arm. This decrease in information processing rates was explained as the result of added processing for the additional joints, muscles, and motor units. Interestingly, these results are identical to
Gilbreth’s classification of movements (see Section 4.2).

EXAMPLE 7.3

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7.0
HHHH

6.5
LHHH
6.0
I.P. time (s)

296

5.5
5.0
LLHL

LHLH
HLLH
LLHH
HLHL
LHHL

HHLH
HLHH
HHHL

LLLH

4.5

HLLL
4.0

HHLL

LLLL
LHLL

3.5
3.0
7

Figure 7.6

8

9
10
Information input (bits)

11

12

Hick-Hyman law illustrated in a wiring task.

(Reprinted from Applied Ergonomics, Vol. 19, Bishu and Drury, Information Processing in Assembly Tasks—A Case Study, pp. 90–98, 1988, with permission from Elsevier
Science.)

RESPONSE EXECUTION
Response execution depends primarily on human movement. More details on the musculoskeletal system, motor control, and manual work can be found in
Chapter 4. Note that the Fitts’Tapping Task (see Figure 7.7) is a simple extension of the Hick-Hyman law with respect to movement and also an example of a speedaccuracy trade-off with respect to the size of the target and movement time. Specific applications of responses with respect to controls and to the operation of machines and other equipment are discussed in Chapter 5.

ATTENTION RESOURCES
Attention resources, or more simply attention, refer to the amount of cognitive capacity devoted to a particular task or processing stage. This amount could vary considerably from very routine, well-practiced assembly tasks with low attentional demands to air-traffic controllers with very high attentional demands. Furthermore, this cognitive capacity can be applied in a very directed manner, such as a spotlight on a particular part of the human information processing system, termed focused attention. Or it can be applied in a much more diffuse manner to various parts or even all the human information processing system, termed

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Figure 7.7 Fitts’ Tapping Task from DesignTools.

divided attention. An example of focused attention on working memory would occur while an operator was trying to remember a looked-up processing code while entering it into a computer-controlled machine tool. Focused attention can be improved by reducing the number of competing sources of information or demands on the human information processing system or separating these sources in as distinct a manner as possible.
On the other hand, an inspector, sorting apples on a conveyor line, divides his attention between visual perception of blemishes and apple sizes, decision making on the nature of the blemish and the size of the apple, with reference to memory and the images stored there from training, and hand movements to remove the damaged apples and sort the good ones into appropriate bins by size.
This latter case of performing various tasks simultaneously is also termed multitasking or timesharing. Because our cognitive resources for attention are relatively limited, timesharing between several tasks will probably result in a decrease in performance for one or more of these tasks as compared to a single task baseline. Again, it can be difficult to improve task performance in such situations, but similar strategies as discussed for focused attention are also utilized.
The number and difficulty of tasks should be minimized. The tasks should be made as dissimilar as possible in terms of the demands placed on a processing stage of Figure 7.1. Whereas a purely manual assembly task with auditory instructions can be managed, a musician tuning an instrument would have a much more difficult time listening to verbal comments. One approach that is fairly

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successful in explaining timesharing performance with multiple tasks is the multiple-resource model of Wickens (1984).
An extension of multiple-resource modes of attention relates to the measurement of mental workload or the demands placed on the human information processor. One definition uses the ratio of resources required to the resources available, where time is one of the more important of a number of required resources. In the examples mentioned above, simple assembly may be timeconsuming, but is not especially demanding of cognitive resources. On the other hand, air-traffic control, at peak times, can be very demanding. It can be very difficult to actually quantify these demands placed on the operator. Some of the approaches used to do so include these:








Primary task measures may be time required to perform the task divided by total time available, or the number of items completed per unit time. The problem with this approach is that some tasks are better timeshared than others. Sondary task measures utilize the concept of a reserve capacity that, if not being directed to the performance of the primary task, will be used by a secondary task (choice reaction time), which can be controlled and more easily measured. The problem with this approach is that the secondary task typically seems artificial and intrusive, and it is difficult to identify how the operator prioritizes the performance of both tasks.
Physiological measures (e.g., heart rate variability, eye movement, pupil diameter, electroencephalograms) are thought to respond to the stress imposed by mental workload; although they typically don’t interfere with the primary task performance, the equipment needed to measure them may do so.
Subjective measures are thought to aggregate all aspects of mental workload in a simple overall rating (or a weighted average of several scales). Unfortunately subjective reports don’t always accurately reflect true performance; also, motivation may strongly affect the ratings.

For a more detailed discussion of mental workload and the advantages and disadvantages of various means to measure it, see Wickens (1984), Eggemeier
(1988), and Sanders and McCormick (1993).
A final example of attentional resources relates to the ability of an operator
(e.g., a visual inspector) to maintain attention and remain alert over prolonged periods of time. Termed sustained attention or vigilance, the concern is how to minimize the vigilance decrement that occurs after as little as 30 min and increases up to 50 percent with increasing time (Giambra and Quilter, 1987; see
Figure 7.8). Unfortunately, there are few documented countermeasures that work well for industrial tasks. The basic approach is to try to maintain a high level of arousal, which then maintains performance, following the Yerkes-Dodson (1908) inverted-U curve (see Figure 7.9). This can be done by providing more frequent rest breaks, providing task variation, providing operators with more feedback on

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1.000

Hit likelihood

0.875

0.750

0.625

0.500
0

8

16

24
32
40
48
Time into task (min)

56

64

Figure 7.8 Vigilance decrement over time.

(From: Giambra and Quilter, 1987. Reprinted with permission from Human Factors,
Vol. 29, No. 6, 1987. Copyright 1987 by the Human Factors and Ergonomics Society.
All rights reserved.)

High

Performance

Low
Low

Optimum

High

Arousal

Figure 7.9 The Yerkes-Dodson law showing the inverted-U relationship between performance and the level of arousal.

their detection performance, and using appropriate levels of stimulation, either internal (e.g., caffeine) or external (e.g., music or white noise), or even through the introduction of false signals. However, the latter change of detection criteria will also increase the rate of false alarms (see the discussion of signal detection

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Perception Considerations

1. Are key signals enhanced?
2. Are overlays, special patterns, or grazing light used to enhance defects?
Are both top-down and bottom-up processing used simultaneously?
a. Are high-level concepts used to process low-level features?
b. Is data-driven information used to identify sensory features?
4. Is better training used to increase sensitivity of signal detection?
5. Are incentives provided to change the response bias and increase hits?
Memory Considerations

1.
2.
3.
4.
5.
6.

Is short-term memory load limited to 7±2 items?
Is chunking utilized to decrease memory load?
Is rehearsal utilized to enhance recall?
Are numbers separated from letters in lists or chunks?
Are similar-sounding items separated?
Are mnemonics and associations used to enhance long-term memory?

Decision and Response Selection

1.
2.
3.
4.
5.
6.
7.
8.
9.

Are a sufficient number of hypotheses examined?
Are a sufficient number of cues utilized?
Are later cues given equal weight to early cues?
Are undesirable cues filtered out?
Are decision aids utilized to assist in the process?
Are a sufficient number of responses evaluated?
Are potential losses and gains weighted appropriately?
Are speed-accuracy trade-offs considered?
Are the stimuli and responses compatible?

Attentional Resource Considerations

1.
2.
3.
4.
5.

ls there task variety?
Is performance feedback provided to the operator?
Does the operator have internal stimulation (e.g., caffeine)?
Does the operator have external stimulation (e.g., music, incentives)?
Are rest breaks provided?

Yes

No

















Yes

No















Yes

No





















Yes

No













Figure 7.10 Cognitive Work Evaluation Checklist.

theory) with consequent costs. Increasing the prominence of the signal will assist in the detection performance (e.g., making the signal brighter, larger, or with greater contrast through special illumination). Overlays that act as special patterns to enhance differences between the defective part and the rest of the object may also be useful. Finally, selecting inspectors with faster eye fixation time and better peripheral vision will also help in inspection performance (Drury, 1982).
To assist the industrial engineer in evaluating and redesigning cognitive tasks, the above details on the human information processing system have been summarized in the Cognitive Work Evaluation Checklist (see Figure 7.10).

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7.3 CODING OF INFORMATION:
GENERAL DESIGN PRINCIPLES
As mentioned in the introduction to Chapter 4, many, if not most, industrial functions or operations will be performed by machines, because of the greater force, accuracy, and repeatability considerations. However, to ensure that these machines are performing satisfactorily at the desired specifications, there will always be the need for a human monitor. This operator will then receive a variety of information input (e.g., pressure, speed, temperature, etc.) that has to be presented in a manner or form that will be both readily interpretable and unlikely to result in an error. Therefore, there are a number of design principles that will assist the industrial engineer in providing the appropriate information to the operator.

TYPE OF INFORMATION TO BE PRESENTED
Information to be presented can be either static or dynamic, depending on whether it changes over time. The former includes any printed text (even scrolling text on a computer screen), plots, charts, labels, or diagrams that are unchanging. The latter is any information that is continually updated such as pressure, speed, temperature, or status lights. Either of the two categories can also be classified as








Quantitative—presenting specific numerical values (e.g., 50°F, 60 rpm)
Qualitative—indicating general values or trends (e.g., up, down, hot, cold)
Status—reflecting one of a limited number of conditions (e.g., on/off, stop/caution/go) Warnings—indicating emergencies or unsafe conditions (e.g., fire alarm)
Alphanumeric—using letters and numbers (e.g., signs, labels)
Representational—using pictures, symbols, and color to code information
(e.g., “wastebasket” for deleted files)
Time-phased—using pulsed signals, varying in duration and intersignal interval (e.g., Morse code or blinking lights)

Note that one informational display may incorporate several of these types of information simultaneously. For example, a stop sign is a static warning using alphanumeric letters, an octagonal shape, and the red color as representations of information. DISPLAY MODALITY
Since there are five different senses (vision, hearing, touch, taste, smell), there could be five different display modalities for information to be perceived by the human operator. However, given that vision and hearing are by far the most developed senses and most used for receiving information, the choice is generally limited to those two. The choice of which of the two to use depends on a variety of factors, with each sense having certain advantages as well as certain disadvantages. The

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Table 7.1 When to Use Visual or Auditory Forms of Presentation

Use visual forms if:

Use auditory forms if:

The message is long and complex
The message deals with spatial references
The message needs to be referred to later
No immediate action is needed
Hearing is difficult (noise) or overburdened
The operator can be stationary

The message is short and simple
The message deals with events in time
The message is transient
Immediate action is needed
Vision is difficult or overburdened
The operator is moving about

Adapted from Deatherage, 1972.

detailed comparisons given in Table 7.1 may aid the industrial engineer in selecting the appropriate modality for the given circumstances.
Touch or tactile stimulation is useful primarily in the design of controls, which is discussed further in Section 5.3. Taste has been used in a very limited range of circumstances, primarily added to give medicine a “bad” taste and prevent children from accidentally swallowing it. Similarly, odors have been used in the ventilation system of mines to warn miners of emergencies or in natural gas to warn the homeowner of a leaking stove.

SELECTION OF APPROPRIATE DIMENSION
Information can be coded in a variety of dimensions. Select a dimension appropriate for the given conditions. For example, if lights are to be used, one can then select brightness, color, and frequency of pulsing as dimensions to code information. Similarly, if sound is to be used, one can select dimensions such as loudness, pitch, and modulation.

LIMITING ABSOLUTE JUDGMENTS
The task of differentiating between two stimuli along a particular dimension depends on either a relative judgment, if a direct comparison can be made of the two stimuli, or an absolute judgment, if no direct comparison can be made. In the latter case the operator must utilize working memory to hold one stimulus and make the comparison. As discussed previously, the capacity of working memory is limited to approximately 7 Ϯ 2 items by Miller’s rule. Therefore, an individual will be able to identify, at most, five to nine items based on absolute judgment. Research has shown that this holds true for a variety of dimensions: five levels for pure tones, five levels for loudness, seven levels for size of the object, five levels for brightness, and up to a dozen colors. On the other hand, individuals have been able to identify up to
300,000 different colors on relative basis, when comparing them two at a time.
If multiple dimensions are used (e.g., brightness and color), then the range can be increased to some degree, but less than would be expected from the combination
(direct product) of the two coding dimensions (Sanders and McCormick, 1993).

INCREASING DISCRIMINABILITY OF CODES
When selecting a coding scheme, one must consider that with regards to relative judgment, it is apparent that there has to be some minimum difference between

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20

30

Just noticeable difference (oz)

0.8

0.6

0.4

0.2

0
0

10
Standard weight (oz)

Figure 7.11 Weber’s law showing the relationship between just noticeable difference and the level of stimuli which, for this example, is standard weight.

the two codes or stimuli before they can be distinguished as different. This difference is termed the just noticeable difference (JND) and is found to vary depending on the level of the stimuli. For example (see Figure 7.11), if an individual is given a 10-oz (0.283-kg) weight, the JND is roughly 0.2 oz (0.056 kg). If the weight is increased to 20 oz (0.566 kg), the JND increases to 0.4 oz
(0.113 kg), and so on. This relationship is termed Weber’s law and can be expressed as k ϭ JND>S where k ϭ Weber’s fraction or slope
S ϭ standard stimulus
The application of this law is quite evident in an industrial setting. Consider a lighting source with a three-way bulb (100-200-300 W). The brightness change from 100 to 200 W is quite noticeable, whereas the change from 200 to 300 W is much less noticeable. Thus, for a change in a high-intensity signal to be noticeable, the change must be quite large. Although, Weber’s law was formulated for relative thresholds, Fechner (1860) extended it to develop psychological scales for measuring a wide range of sensory experiences to form the basis for the science of psychophysics.

COMPATIBILITY OF CODING SCHEMES
Compatibility refers to the relationship of stimuli and responses that are consistent with human expectations, resulting in decreased errors and faster response time. This can occur at several levels: conceptual, movement, spatial, and modality. Conceptual compatibility refers to how meaningful the codes are to the individuals using them. Red is an almost universal code for danger or stop. Similarly pictorial realism is very useful, for example, a symbol for a female on a door

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Figure 7.12 Spatial compatibility of controls and burners on a stove top.
A
B

C
D

A

B

C

D

indicating women’s toilets. Movement compatibility refers to the relationship between the movement of controls and displays and is discussed further in Section 5.3. Spatial compatibility refers to the physical arrangement of controls and displays. The classic example (see Figure 7.12) is the optimum arrangement of knobs to control the burners on a stove from Chapanis and Lindenbaum (1959).
Modality compatibility refers to using the same stimulus modality for both the signal and response. For example, verbal tasks (responses to verbal commands) are performed best with auditory signals and spoken responses. Spatial tasks
(moving a cursor to a target) are performed best with a visual display and a manual response.

REDUNDANCY FOR CRITICAL SITUATIONS
When several dimensions are combined in a redundant manner, the stimulus or code will be more likely to be interpreted correctly and fewer errors will be made. The stop sign is a good example with three redundant codes: the word stop, the universal red color, and the unique (among traffic signs) octagonal shape. Note that these dimensions are all in the visual modality. Using two different modalities will improve the response as compared to two different dimensions within a modality. Thus, for an emergency evacuation of a plant in case of a fire, using both an auditory signal (a siren) and a visual signal (flashing red lights) will be more effective than either modality alone. The trade-off, as discussed earlier in this chapter, is a reduction in the number of potential codes available and a reduction in the amount of information that can be presented.

MAINTAINING CONSISTENCY
When coding systems have been developed by different people in different situations, it is important to maintain consistency. Otherwise, especially under times of stress, it is likely that operators will respond instinctively to previous habits and errors will occur. Therefore, as new warnings are added to a factory having an existing warning system, it is important that the coding system duplicate the existing scheme when the same information is being transmitted, even though the old

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system might not have had the optimum design in the first place. For example, the color yellow, typically meaning proceed with caution, should have the same meaning for all displays.

7.4 DISPLAY OF VISUAL INFORMATION:
SPECIFIC DESIGN PRINCIPLES
FIXED SCALE, MOVING POINTER DESIGN
There are two major alternative analog display designs: a fixed scale with a moving pointer and a moving scale with a fixed pointer (see Figure 7.13). The first is the preferred design because all major compatibility principles are maintained: increasing values on the scale go from left to right, and a clockwise (or left to right) movement of the pointer indicates increasing values. With a moving scale and fixed pointer, one of these two compatibility principles will always be violated. Note that the display itself can be circular, semicircular, a vertical bar, a horizontal bar, or an open window. The only situation in which the moving scale and fixed pointer design has an advantage is for very large scales, which cannot be fully shown on the fixed scale display. In that case an open window display can accommodate a very large scale in back of the display with only the relevant portion showing. Note that the fixed scale and moving pointer design can display very nicely quantitative information as well as general trends in the readings.
Also, the same displays can be generated with computer graphics or electronics without a need for traditional mechanical scales.

DIGITAL DISPLAYS FOR PRECISION
When precise numeric values are required and the values remain relatively static
(at least long enough to be able to be read), then a digital display or counter should be utilized (see Figure 7.13). However, once the display changes rapidly, then counters become difficult to use. Also, counters are not good for identifying trends. Thus, digital counters were never very successful as a “high-tech” feature for automobile speedometers. A more detailed comparison in Table 7.2 shows the advantages and disadvantages of using moving pointers, moving scales and counters. DISPLAY BASIC FEATURES
Figure 7.14 illustrates some of the basic features utilized in a dial design. The scale range is clearly depicted with an orderly numerical progression with numbered major markers at 0, 10, 20, and so on, and minor markers at every unit. An intermediate marker at 5, 15, 25, and so on, helps identify readings better. A progression by 5s is less good but still satisfactory. The pointer has a pointed tip just meeting, but not overlapping, the smallest scale markers. Also, the pointer should be close to the surface of the scale to avoid parallax and erroneous readings.

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Increase

30

Decrease
60

50

40
20

70

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Text

Fixed scale, moving pointer
0
90
10
30
20

10
0

+20

–20
–30

+30

3

2

40

Increase

4 5 6

7

1
0

Increase

Decrease

Decrease

Positive
+10

80
Increase

50

0

70

100

(a) Circular scales
Negative
–10

60

90
0

50

20

80

50
Decrease

40

30
Increase

40

60

Decrease

70

10

30

8
9
10

20

40

50

60

Decrease

70

Increase

10

–40

+40
–50

0

+50

(b) Circular scale with positive and negative values

(c) Semicircular or curved scale

(d) Vertical scale (e) Horizontal scale Moving scale, fixed pointer

65 7
0

60

60 7
0

80 9 0 1

30
20

50

0

10

(f) Circular scale Decrease

Increase

70

50

Increase

40

65

60

4

5

6

Increase

Decrease

00

306

7

(g) Open-window scale 40

(i) Horizontal scale Digital display
(j) Digital 2 7 9 4 3 display Electronic display ok Cold

0

20

30

40

50

0

(k) Circular

10

10

20

30

40

50

(l) Horizontal

50

30

(h) Vertical scale Hot

Decrease

(m) Digital

Figure 7.13 Types of displays for presenting quantitative information.

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Table 7.2

Design of Cognitive Work

307

299

Comparison of Pointers, Scales, and Counters

Service rendered
Quantitative
reading

Indicator

Qualitative reading

Setting

Tracking

Moving pointer…Fair

Good (changes are easily detected)

Moving scale . . . Fair

Poor (may be difficult to identify direction and magnitude) Good (easily discernible relation between setting knob and pointer)
Fair (may be difficult to identify relation between setting and motion)

Counter . . . . . . . . Good (minimum time to read and results in minimum error) Poor (position change may not indicate qualitative change) Good (pointer position is easily controlled and monitored) Fair (may have ambiguous relationship to manual-control motion)
Poor (not readily monitored) 50
60

40

70

30

20

80

lbs
10

100

0
Graduation
interval
(1 lb)

90

Scale unit
(nearest 1 lb)

Scale range
(0–100 lb)

Figure 7.14 Basic features in the design of a dial.

Numbered interval (10 lb)

Good (accurate method to monitor numerical setting)

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PATTERNS FOR A PANEL OF DIALS
Usually a panel of dials is used to indicate the status of a series of pressure lines or valves as in the control room of a power plant. In such cases, the operator is primarily a monitor, performing check readings to ascertain whether readings
(and the status of the system) are normal. Although the actual dials may be quantitative, the operator is primarily determining whether any one dial is indicating a condition out of a normal range. Thus, the key design aspect is to align all normal states and all dial pointers in the same direction, such that any change or deviant reading will stand out from the others. This pattern is accentuated by having extended lines between dials (see Figure 7.15).

MINIMIZING INFORMATIONAL LOADING
ON THE WORKER
Human error in reading display information increases as the amount of information per unit area increases. Always consider Miller’s rule in limiting the amount of information presented. Proper coding of the information improves readability of the displays and decreases the number of errors. In general, color, symbols or geometric figures, and alphanumerics are the best coding methods. These require little space and allow easy identification of information.

INDICATOR LIGHTS TO GET THE ATTENTION
OF THE WORKER
Indicator or warning lights are especially good in attracting attention to potentially dangerous situations. Several basic requirements should be incorporated into their use. They should indicate to the operator what is wrong and what action should be taken. In poor background conditions with poor contrasts, a red
(typically a serious situation), a green, and a yellow (less-serious condition) light have advantages over a white light. In terms of size and intensity, a good rule is

Figure 7.15 Panel of dials designed for check reading.

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Table 7.3

Design of Cognitive Work

301

Recommended Coding of Indicator Lights

Color
Diameter

State

Red

Yellow

Green

12.5 mm. . . . . . . . . . . Steady

Failure; Stop action; Malfunction

Delay;
Inspect

25 mm or larger . . . . . Steady

System or subsystem in stop action
Emergency condition

Caution

Circuit energized;
Functional;
Go ahead: Ready;
In position;
Producing
Normal (on)
System or subsystem in go-ahead state

25 mm or larger . . . . . Flashing

to make it twice the size (at least 1 degree of visual arc) and brightness of other panel indicators, and place it not more than 30 degrees off the operator’s line of sight. A flashing light, with a flash rate between 1 to 10 per second, will especially attract attention. Immediately after the operator takes action, the flashing should stop, but the light should remain on until the improper condition has been completely remedied. Further details on the coding of indicator lights are given in
Table 7.3.

PROPER SIZE ALPHANUMERIC CHARACTERS
FOR LABELS
For the most effective alphanumeric coding use recommended illumination levels (see Section 6.1) and consider the following information on letter height, stroke width, width-height ratio, and font. Based on a viewing distance of 20 in
(51 cm), the letter or numeral height should be at least 0.13 in (0.325 cm) and the stroke width at least 0.02 in (0.055 cm), to give a stroke width-height ratio of 1:6.
This creates a preferred visual angle of 22 arc minutes as recommended by
ANSI/HFS 100 (1988) or a point size of 10 as typically found in newspapers
(one point equals 1/72 in or 0.035 mm). For distances other than 20 in (51 cm), the value can be scaled so as to yield a comparable visual angle (e.g., for a distance of 40 in, sizes would be doubled).
The above recommendations refer to dark letters on a white background, the preferred format for reading for well-illuminated areas (e.g., typical work areas with windows or good lighting). White letters on a dark background are more appropriate for dark areas (e.g., nighttime conditions) and have a narrower stroke width (1:8 to 1:10 ratio) due to a spreading effect of the white letters. The font refers to the style of type such as Roman, with serifs or the special embellishments on the ends of the strokes, or Gothic, without serifs. In general, a mix of uppercase and lowercase letters is easier for extended reading. However, for special emphasis and attracting attention, as in labels, boldface or uppercase letters with a width-height ratio of about 3:5 are useful
(Sanders and McCormick, 1993).

309

White

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7.5 DISPLAY OF AUDITORY INFORMATION:
SPECIFIC DESIGN PRINCIPLES
AUDITORY SIGNALS FOR WARNINGS
As discussed previously, there are special characteristics of the auditory system that warrant using an auditory signal for warnings. Simple reaction time is considerably quicker to auditory than visual signals (e.g., consider the starter pistol to start races). An auditory signal places much higher attentional demands on the worker than a visual signal. Since hearing is omnidirectional and sound waves penetrate barriers (to some degree, depending on thickness and material properties), auditory signals are especially useful if workers are at an unknown location and moving about.

TWO-STAGE AUDITORY SIGNALS
Since the auditory system is limited to short and simple messages, a two-stage signal should be considered when complex information is to be presented. The first stage should be an attention-demanding signal to attract attention, while the second stage would be used to present more precise information.

HUMAN ABILITIES AND LIMITATIONS
Since human auditory sensitivity is best at approximately 1,000 Hz, use auditory signals with frequencies in the range of 500 and 3,000 Hz. Increasing signal intensity will serve two purposes. First it will increase the attention-demanding quality of the signal and decrease response time. Second, it will tend to better differentiate the signal from background noise. On the other hand, one should avoid excessive levels (e.g., well above 100 dB) as these will tend to cause a startle response and perhaps disrupt performance. Where feasible, avoid steady-state signals so as to avoid adaption to the signal. Thus, modulation of the signal (i.e., turning the signal on and off in a regular cycle) in the frequency range of 1 to 3
Hz will tend to increase the attention-demanding quality of the signal.

ENVIRONMENTAL FACTORS
Since sound waves can be dispersed or attenuated by the working environment, it is important to take environmental factors into account. Use signal frequencies below 1,000 Hz when the signals need to travel long distances (i.e., more than 1,000 ft), because higher frequencies tend to be absorbed or dispersed more easily. Use frequencies below 500 Hz when signals need to bend around obstacles or pass through partitions. The lower the signal frequency, the more similar sound waves become to vibrations in solid objects, again with lower absorption characteristics.

DISSOCIATING THE SIGNAL FROM NOISE
Auditory signals should be as separate as possible from other sounds, whether useful auditory signals or unneeded noise. This means the desired signal should

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be as different as possible from other signals in terms of frequency, intensity, and modulation. If possible, warnings should be placed on a separate communication channel to increase the sense of dissociability and increase the attention-demanding qualities of the warning.
The above principles for designing displays, both auditory and visual, are summarized as an evaluative checklist in Figure 7.16. If purchased equipment has dials or other displays that don’t correspond to these design guidelines, then there is the possibility for operator error and potential loss. If at all possible, those problems should be corrected or the displays replaced.

7.6 HUMAN-COMPUTER INTERACTION:
HARDWARE CONSIDERATIONS
KEYBOARDS
The standard computer keyboard used today is based on the typewriter key layout patented by C. L. Sholes in 1878. Termed a QWERTY keyboard, because of the sequence of the first six leftmost keys in the third row, it has the distinction of allocating some of the most common English letters to the weakest fingers. One potential explanation is that the most commonly used keys were separated from each other so that they would not jam upon rapid sequential activation. Other more scientifically based alternative layouts, which allocate the letters more proportionately, have been developed, with the 1936 Dvorak keyboard being the most notable. However, scientific studies have shown the Dvorak layout to achieve at most a 5 percent improvement over the QWERTY layout, which is probably not a large enough improvement to justify switching and retraining millions of operators.
In certain special circumstances such as stenotyping and mail sorting, a chord keyboard may be more appropriate. Whereas in a standard keyboard, individual characters are keyed in sequentially, a chord keyboard requires the simultaneous activation of two or more keys. The basic trade-off is that with such activation, fewer keys are required and considerably more (estimates of 50 percent to 100 percent) information can be entered. It also has the distinct advantage of small size and one-handed operation, allowing the other hand to perform other tasks. However, for general use, the standard keyboard is more than sufficient without the need for additional specialized training.
Many keyboards will have a separate numeric keypad. Early research on telephones established that users preferred that numerals increase from left to right and then from top to bottom, which resulted in the standard layout for telephones. An alternate layout was developed for calculators, in which the numbers increase from the bottom to the top. Most research has shown that the telephone layout is slightly but significantly faster and more accurate than the calculator layout. Unfortunately, the difference was not large enough for the ANSI/HFS 100 standard (Human Factors Society, 1988) to favor one over the other, resulting in, perhaps, the worst situation in performance, with an operator having to alternate

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General Principles

1.
2.
3.
4.
5.

Is the number of absolute judgments limited to 7±2 items?
Is the difference between coding levels well above the JND?
Is the coding scheme compatible with human expectations?
Is the coding scheme consistent with existing schemes?
Is redundancy utilized for critical situations?

Yes

No





















































































Yes

No







Visual Displays

1.
2.
3.
4.
5.
6.

7.
8.
9.
10.

11.

Is the message long and complex?
Does the message deal with spatial information?
Does one need to refer to the message later?
Is hearing overburdened or is noise present?
Is the operator in a stationary location?
For general purpose and trends, is a fixed-scale, moving-pointer display being used?
a. Do scale values increase from left to right?
b. Does a clockwise movement indicate increasing values?
c. Is there an orderly numerical progression with major markers at 0, 10, 20, etc.?
d. Are there intermediate markers at 5, 15, 25, etc. and minor markers at each unit?
e. Does the pointer have a pointed tip just meeting the smallest scale markers?
f. Is the pointer close to the surface of the scale to avoid parallax?
For a very large scale, is an open-window display being used?
For precise readings, is a digital counter being used?
For check reading of a panel of dials, are pointers aligned and a pattern utilized?
For attentional purposes, are indicator lights being used?
a. Do the lights flash (1 to 10/sec) to attract attention?
b. Are the lights large (1 degree of visual arc) and bright?
c. Does the light remain on until the improper condition has been remedied? Are alphanumeric characters of proper size?
a. Are they at least a 10 point font at a distance of 20 inches (22 min of visual arc)?
b. In a well-illuminated area, are the letters dark on a light background?
Do they have a stroke width-to-height ratio of approximately 1:6?
c. In a dark area, are the letters white on a dark background?
Do they have a stroke width-to-height ratio of approximately 1:8 for nighttime use?
d. Are both uppercase and lowercase letters utilized?
e. For special emphasis, are capitals or boldface utilized?

Auditory Displays

1. Is the message short and simple?
2. Does the message deal with events in time?
Figure 7.16 Display design checklist.

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3.
4.
5.
6.
7.
8.
9.
10.
11.

Design of Cognitive Work

Is the message a warning or is immediate action required?
Is vision difficult or overburdened?
Is the operator moving about?
Is a two-stage signal being utilized?
Is the frequency of the signal in the range of 500 to 3,000 Hz for best auditory sensitivity?
Is the sound level of the signal well above background noise?
Is the signal being modulated (1 to 3 Hz) to attract attention?
If the signal is traveling over 1,000 ft or around obstacles, is the frequency below 500 Hz?
If a warning, is a separate communication channel being used?

























Figure 7.16 (continued)

between both layouts in an office environment (e.g., a telephone next to a computer keyboard).
For any keyboard the keys should be relatively large, spaced center to center
0.71 to 0.75 in (18 to 19 mm) horizontally and 0.71 to 0.82 in (18 to 21 mm) vertically. Smaller keys, which are becoming more common as PCs become smaller and smaller, become a disadvantage to individuals with large fingers with a definite decrease in speed and an increase in errors. The keys will have a preferred displacement between 0.08 and 0.16 in (2.0 and 4.0 mm) with the key force not to exceed 0.33 lb (1.5 N). Actuation of a key should be accompanied by either tactile or auditory feedback. Traditionally, a slight upward slope (0 to 15°) has been advocated. However, more recent research shows that a slight downward slope of Ϫ10° may actually provide a more neutral and favorable wrist posture.
Also split keyboards have been shown to reduce the ulnar deviation commonly found while using traditional one-piece keyboards. As mentioned in Chapter 5, armrests provide shoulder/arm support and reduce the shoulder muscle activity.
These are recommended in place of the more commonly seen wrist rest, which may actually increase the pressure in the carpal tunnel and increase operator discomfort. POINTING DEVICES
The primary device for data entry is the keyboard. However, with the growing ubiquitousness of graphical user interfaces and depending on the task performed, the operator may actually spend less than half the time using the keyboard. Especially for window and menu-based systems, some type of a cursor-positioning or a pointing device better than the cursor keys on a keyboard is needed. A wide variety of devices have been developed and tested. The touch screen either uses a touch-sensitive overlay on the screen or senses the interruption of an infrared beam across the screen as the finger approaches the screen. This approach is quite natural with the user simply touching the target directly on the screen.
However, the finger can obscure the target and, in spite of the fairly large targets required, accuracy can be poor. The light pen is a special stylus linked to the

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computer by an electrical cable that senses the electron scanning beam at the particular location on the screen. The user has a similar natural pointing response as with a touch screen, but usually with greater accuracy.
A digitizing tablet is a flat pad placed on the desktop, again linked to the computer. Movement of a stylus is sensed at the appropriate position on the tablet, which can either be absolute (i.e., the tablet is a representation of the screen) or relative (i.e., only direct movement across the tablet is shown). Further complications are tablet size versus accuracy trade-offs and optimum controlresponse ratios. Also, the user needs to look back at the screen to receive feedback. Both displacement and force joysticks (currently termed track sticks or track points) can be used to control the cursor and have a considerable background of research on types of control systems, types of displays, controlresponse ratios, and tracking performance. The mouse is a handheld device with a roller ball in the base to control position and one or more buttons for other inputs. It is a relative positioning device and requires a clear space next to the keyboard for operation. The trackball, an upside-down mouse without the mouse pad, is a good alternative for work surfaces with limited space. More recently, touchpads, a form of digitizing tablets integrated into the keyboard, have become popular especially for notebook PCs.
A number of studies have compared these pointing devices, showing clear speed-accuracy trade-offs, i.e., the fastest devices (touch screens and light pens) being quite inaccurate. Keyboard cursor keys were very slow and probably not acceptable. Touchpads were a bit faster than joysticks, but were less preferred by users. Mice and trackballs were generally similarly good in both speed and accuracy and probably indicate why mice are so ubiquitous. However, there is a tendency for users to overgrip a mouse by a factor of 2 to 3 (i.e., use two to three times more than the minimum force required), with potential risk of incurring an injury. Use of a drag-lock, similar to what is found on trackballs, would reduce this risk. For a more detailed review of cursor-positioning devices, please refer to
Greenstein and Arnaut (1988) or Sanders and McCormick (1993).

MONITOR SCREENS
The center of the monitor screen should be placed at the normal line of sight, which is roughly about 15 degrees below the horizontal. For a 15-in screen, at a typical
16-in reading distance, the edges are only slightly beyond the recommended Ϯ15 degree cone of primary visual field. The implication is that within this optimal cone, no head movements are needed and eye fatigue is minimized. Thus, the top of the screen should not be above the horizontal plane through the eyes.
A 16-in reading distance, however, may not be optimal. A comfortable reading distance is a function not only of the size of the displayed characters, but also of the person’s ability to maintain focus and alignment of the eyes. The mean resting focus (measured in the dark or in the absence of a stimulus with a laser optometer) is roughly 24 in. The implication is that the eye may be under greater stress when viewing characters at distances larger or smaller than 24 in, because

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then there will be compromise between the “pull” of the stimulus and the tendency of the eye to regress toward the individual’s resting position. However, there is a large variation in individual resting focus distances. Therefore, office workers who view computer screens for extended periods of time may wish to have their eyes measured for their resting focus. Then if they are not able to set the monitor screen at the appropriate distance (e.g., excessive short or long distances), they can have their eyes fitted with special “computer-viewing” lenses
(Harpster et al., 1989).
The monitor, preferably tiltable, should be placed relatively vertical such that the angle formed by the line of sight and the line normal to surface of the display is relatively small, but definitely less than 40 degrees. Tilting the screen upward should be avoided due to an increased likelihood of developing specular reflections from overhead lighting, leading to glare and decreased visibility. The screen should have minimum flicker, uniform luminance, and glare control if needed
(polarizing or micromesh filters). Further details on hardware and furniture requirements for computer workstations can be found in the ANSI/HFS 100 standard
(Human Factors Society, 1988).

NOTEBOOKS AND HANDHELD PCS
Portable PCs, or laptop or notebook computers, are becoming very popular, accounting for 34 percent of the U.S. PC market in 2000. Their main advantage over a desktop is reduced size (and weight) and portability. However, with the smaller size, there are distinct disadvantages: smaller keys and keyboard, keyboard attached to screen, and lack of a peripheral cursor-positioning device. The lack of adjustability in placing the screen has been found to give rise to excessive neck flexion
(much beyond the recommended 15 degrees), increased shoulder flexion, and elbow angles greater than 90 degrees, which have accelerated feelings of discomfort as compared to using a desktop PC. Adding an external keyboard and raising the notebook computer or adding an external monitor helps alleviate the situation.
Even smaller handheld computers, termed personal digital assistants, have been developed but are too new to have had detailed scientific evaluations performed. Being pocket-sized, they offer much greater portability and flexibility, but at an even greater disadvantage for data entry. Decrements in accuracy and speed, when entering text via the touchscreen, have been found. Alternate input methods such as handwriting or voice input may be better.

7.7 HUMAN-COMPUTER INTERACTION:
SOFTWARE CONSIDERATIONS
The typical industrial or methods engineer will not be developing programs but will, most likely, be using a variety of existing software. Therefore that person should be aware of current software features or standards that allow best human interaction with the computer and minimize the number of errors that could occur through poor design.

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Most current interactive computing software utilizes the graphical user interface (GUI), identified by four main elements: windows, icons, menus, and pointers (sometimes collectively termed WIMP). Windows are the areas of the screen that behave as if they were independent screens in their own right. They typically contain text or graphics and can be moved around or resized. More than one window can be on a screen at once, allowing users to switch back and forth between various tasks or information sources. This leads to a potential problem of windows overlapping each other and obscuring vital information. Consequently, there needs to be a layout policy with windows being tiled, cascaded, or picture-ina-picture (PIP). Usually windows have features that increase their usefulness such as scrollbars, allowing the user to move the contents of the window up and down or from left to right. This makes the window behave as if it were a real window onto a much larger world, where new information is brought into view by manipulating the scrollbars. There is usually a title bar attached to the top of the window, identifying it to the user, and there may be special boxes in the corners of the window to aid in resizing and closing.
Icons are small or reduced representations of windows or other entities within the interface. By allowing icons, many windows can be available on the screen at the same time, ready to be expanded to a useful size by clicking on the icon with a pointer (typically a mouse). The icon saves space on the screen and serves as a reminder containing the dialog. Other useful entities represented by icons include a wastebasket for deleting unwanted files, programs, applications, or files accessible to the user. Icons can take many forms: they can be realistic representations of the objects they stand for or they can be highly stylized, but with appropriate reference to the entity (known as compatibility) so that users can easily interpret them.
The pointer is an important component of the WIMP interface, since the selection of an appropriate icon requires a quick and efficient means of directly manipulating it. Currently the mouse is the most common pointing device, although joysticks and trackballs can serve as useful alternatives. A touchscreen, with the finger serving as a pointer, can serve as a very quick alternative and even redundant backup/safety measure in emergency situations. Different shapes of the cursor are often used to distinguish different modes of the pointer, such as an arrow for simple pointing, cross-hairs for drawing lines, and a paintbrush for filling in outlines. Pointing cursors are essentially icons or images and thus should have a hot spot that indicates the active pointing location. For an arrow, the tip is the obvious hot spot. However, cutesy images (e.g., dogs and cats) should be avoided because they have no obvious hot spot.
Menus present an ordered list of operations, services, or information that is available to the user. This implies that the names used for the commands in the menu should be meaningful and informative. The pointing device is used to indicate the desired option, with possible or reasonable options highlighted and impossible or unreasonable actions dimmed. Selection often requires an additional action by the user, usually clicking a button on a mouse or touching the screen with the finger or a pointer. When the number of possible menu items increases

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beyond a reasonable limit (typically 7 to 10), the items need to be grouped in separate windows with only the title or a label appearing on a menu bar. When the title is clicked, the underlying items pop up in a separate window known as a pull-down menu. To facilitate finding the desired item, it is important to group menu items by functionality or similarity. Within a given window or menu, the items should be ordered by importance and frequency of use. Opposite functions, such as SAVE and DELETE, should be clearly kept apart to prevent accidental misselection. Other menulike features include buttons, isolated picture-in-picture windows within a display that can be selected by the user to invoke specific actions, toolbars, a collection of buttons or icons, and dialog boxes that pop up to bring important information to the user’s attention such as possible errors, problems, or emergencies. Other principles in screen design include simple usability considerations: orderly, clean, clutter-free appearance, expected information located where it should be consistently from screen to screen for similar functions or information.
Eye-tracking studies indicate that the user’s eyes typically first move to the upper left center of the display and then move quickly in a clockwise direction.
Therefore, an obvious starting point should be located in the left upper corner of the screen, permitted the standard left-to-right and top-to-bottom reading pattern found in Western cultures. The composition of the display should be visually pleasing with balance, symmetry, regularity, predictability, proportion, and sequentiality. Density and grouping are also important features.
The appropriate use of uppercase and mixed-case fonts is important, with special symbols thrown in as necessary. Any text should be brief and concise with familiar words with minimal jargon. Simple action terms expressed in a positive mode are much more effective than some negative statements or standard military jargon. Color is appropriate to draw attention but should be used sparingly and limited to no more than eight colors. Note that a relatively high proportion of the population suffers from deficiencies in color vision.
The user should always feel under control and have the ability to easily exit screens or modules and undo previous actions. Feedback should appear for any actions, and the progress should be indicated for any long transactions.
Above all, the main consideration in any display should be simplicity. The simpler the design, the quicker the response. Further information on software interface design can be found in Mayhew (1992), Galitz (1993), and Dix et al.
(1998). As a convenience to the purchaser or user of software, the above desired features have been summarized in the Graphical User Interface Features
Checklist (see Figure 7.17).

SUMMARY
This chapter presented a conceptual model of the human as an information processor along with the capacities and limitations of such a system. Specific details were given for properly designing cognitive work so as not to overload the

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human with regard to information presented through auditory and visual displays, to information being stored in various memories, and to information being processed as part of the final decision-making and response-selection step. Also, since the computer is the common tool associated with information processing, issues and design features with regard to the computer workstation were addressed.
With manual work activities, the physical aspects of the workplace and tools, and the working environment having been addressed in Chapters 4, 5, and 6, the cognitive element is the final aspect of the human operator at work, and the analyst is now ready to implement the new method.

QUESTIONS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.

How can the informational content of a task be quantified?
What is redundancy? Give a good everyday example of redundancy.
Explain the five stages of the human information processing model.
How do information processing stages act to prevent an information overload of the human operator?
What are the four possible outcomes explained by signal detection theory?
Give an example of a task to which signal detection theory can be applied. What effect would a shift in the criterion have on task performance?
What is the meaning of sensitivity in signal detection theory? What techniques can be used to increase sensitivity in an inspection task?
What techniques can be used to improve memory?
What are some of the biases that may negatively affect a person’s decision making? What is compatibility? Give two everyday examples of compatibility.
Compare and contrast the different types of attention.
What is the inverted-U curve in attention?
Under what conditions are auditory displays best used?
What is the difference between absolute and relative judgment? What is the limitation in absolute judgment?
What is the just noticeable difference and how does it relate to the level of the stimulus? Why is redundancy utilized for critical stimuli?
Why is a fixed-scale, moving-pointer display preferred?
What is the purpose of using patterns for a set of dials in a control room?
What key features are used to increase attention in a visual display?
What key features are used to increase attention in an auditory display?
What are the trade-offs between the different types of pointing devices?
What are the main components of a good graphical user interface?

PROBLEMS
1.

a. What is the amount of information in a set of eight signal lights if each light has an equal probability of occurrence?

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Windows Features

Yes

No

1. Does the software use movable areas of the screen termed windows?

2. Is there a layout policy for the windows (i.e., are they tiled, cascaded, or picture-in-picture)? ❑
3. Are there scrollbars to allow the contents of windows to be moved up or down?

4. Are there meaningful titles identifying the windows?

5. Are there special boxes in the corners of the windows to resize or close them?




Icon Features






Yes

No

1. Are reduced versions of frequently used windows, termed icons, utilized? ❑
2. Are the icons easily interpretable or realistic representations of the given feature?




Pointer Features



Yes

No

1. Is a pointing device (mouse, joystick, touchscreen) utilized to move icons?

2. Is the pointer or cursor easily identifiable with an obvious active area or hot spot?

Menu Features

1.
2.
3.
4.
5.
6.




Yes

Other Usability Considerations

1.
2.
3.
4.
5.

Is the screen design simple, orderly, and clutter-free?
Are similar functions located consistently from screen to screen?
Is the starting point for the screen action the upper left-hand corner?
Does the screen action proceed left to right and top to bottom?
Is any text brief and concise and does it use both uppercase and lowercase fonts?
6. Is color used sparingly for attention (i.e., limited to eight colors)?
7. Does the user have control over exiting screens and undoing actions?
Is feedback provided for any action?

No















Yes

Are meaningful menus (list of operations) with descriptive titles provided?
Are menu items functionally grouped?
Are menu items limited to a reasonable number (7 to 10)?
Are buttons available for specific common actions?
Are toolbars with a collection of buttons or icons used?
Are dialog boxes used to notify the user of potential problems?

No





















Figure 7.17 Graphical User Interface Features Checklist.

b. The probabilities of the lights are changed as shown below. Calculate the amount of information and the redundancy in this configuration.
Stimulus
Probability
2.

1
0.08

2
0.25

3
0.12

4
0.08

5
0.08

6
0.05

7
0.12

8
0.22

A large state university uses three-digit mail stops to code mail on campus. The initial step in sorting this mail is to sort according to the first digit (there are 10 possible), which signifies a general campus zone. This step is accomplished by

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pushing a key with corresponding number, which dumps the letter in an appropriate bin. A typical mail sorter can sort 60 envelopes per minute, and it takes a minimum of 0.3 s to just push the key with cognitive processing involved.
a. Assuming that the mail is distributed evenly over the campus zones, what is the mail sorter’s bandwidth?
b. After a while the sorter notices that campus mail is distributed as follows. If the mail sorter used this information, how many pieces of mail could the sorter possibly handle in 1 min?

Zone
0
1
2
3–9
3.

% Distribution
25
15
25
5

Bob and Bill are two weather forecasters for AccurateWeather. Bob is a veteran forecaster, while Bill is fresh out of school. The following are the records (in number of predictions) on both forecasters’ ability to predict whether it will rain in the next 24 h.
a. Which forecaster would you hire? Why?
b. Who is a more conservative forecaster? Why?
c. How would a conservative versus a risky forecaster be beneficial for different geographic regions?
True result

True result

Bob said:

No rain

Rain
No rain
4.

Rain
268
320

56
5,318

Bill said:

Rain

No rain

Rain
No rain

100
21

138
349

The Dorben Electronics Co., manufacturer of resistors, screens potential quality control inspectors before they are hired. Dorben has developed the following preemployment test. Each potential employee is presented with the same set of
1,000 resistors of which 500 are defective. The results for two applicants are as follows: (1) of the 500 good resistors, applicant 1 labeled 100 as defective, and of the 500 bad resistors, applicant 1 labeled 200 as defective; (2) of the 500 good resistors, applicant 2 labeled 50 as defective, and of the 500 bad resistors, applicant
2 labeled 300 as defective.
a. Treat picking a defective resistor as defective as a hit. Fill in the following table.
Applicant 1
Hit rate
False-alarm rate
Miss rate
Correct rejection rate dЈ Applicant 2

___________
___________
___________
___________
___________

___________
___________
___________
___________
___________

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5.

Design of Cognitive Work

b. Given that Dorben places a very large emphasis on quality control (i.e., they don’t wish to sell a defective product at any cost), which applicant would better fulfill its goal? Why?
c. Given that Dorben wishes to hire the most efficient inspector (i.e., most correct), which applicant would the company hire?
The following performance data were collected under similar conditions on two inspectors removing defective products from the assembly line. Comment on the relative performance of the two inspectors. Which one is better at finding defects?
Which would you hire if the cost of releasing a defective product were be high?
Which does an overall better job? (Hint, consider dЈ.)

Case 1
JRS
ABD

6.

0.81
0.21
0.84
0.44

Hit rate
False alarm rate
Hit rate
False alarm rate

Case 2
0.41
0.15
0.55
0.31

The following response-time data (in msec) were obtained on Farmer Brown and his son Big John while operating a tractor using the right foot to control the clutch, brake, and accelerator. The foot is normally kept on the rest position. The location and sizes of the pedals are shown below as well as some sample response times (in msec) for activating a given control from the rest position.
a. What is the index of difficulty value for each pedal?
b. Plot the response times. What law can be used to explain the relationship between response times and the difficulty in activating a given control?
c. What is the simple reaction time for Farmer Brown?
d. What is the bandwidth for Big John?
e. Which farmer is the better tractor operator? Why?

Clutch

Brake

Accelerator

Rest position

2"

2"

2"

2"

3"

3"

2"

Accelerator

Brake

Clutch

Farmer Brown

300

432

510

Big John

270

428

480

321

313

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7. Disregarding the digit 0, which keyboard is fastest for entering digits using one finger?
Assume that the home position is digit 5. (Hint: Calculate the index of difficulty.)

#B
Scale .5 in

7

1

2

3

4

5

6

7

8

9

5

6

1

9

8

4

#A

2

3

8. Pianists must often hit keys in very quick succession. The figure below displays a typical piano keyboard.
a. Compare the index of difficulty for striking C, C#, F, and F#. Assume one is starting from the A key and distances are measured center to center.
b. If starting from the A key, it takes 200 ms to strike the C key and 500 ms to strike the F key, what is the bandwidth of a typical pianist?
1/2"

G#

Note:

A#

A

C#

B

C

D#

D

F#

E

F

G#

G

1 Octave

A#

A

B
1"

9. The knob with numbers and directional arrows on it shown below is used on refrigerators to control temperature. In which direction, clockwise or counterclockwise, would you turn the knob to make the refrigerator cooler? Why?
How would you improve the control to avoid confusion?

3
4
4

2

5
5

C

O

OL
ER

R

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10. The dial shown below represents a pressure gage. The operational range is 50 psi.
The operator must read the scale to the nearest 1 psi. Critically evaluate the dial, indicating the poor design practices. Then redesign the dial, following recommended design practices.

PSI

8

40

16

32

24

11. The scale shown below represents a scale used to measure weight. The maximum weight possible is 2 lb, and the scale must be read to the nearest 0.1 lb. Critically evaluate the dial, indicating the poor design practices. Then redesign the dial, following recommended design practices.

0

0.25

0.5

0.75

1

1.25

1.5

1.75

2

12. Design an EXIT sign which is to be displayed in a public auditorium. Explain the ergonomics principles that need to be considered for this sign.

REFERENCES
ANSI (American National Standards Institute). ANSI Standard for Human Factors
Engineering of Visual Display Terminal Workstations. ANSI/HFS 100-1988. Santa
Monica, CA: Human Factors Society, 1988.

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Bishu, R. R., and C. G. Drury. “Information Processing in Assembly Tasks—A Case
Study.” Applied Ergonomics, 19 (1988), pp. 90–98.
Chapanis, A., and L. Lindenbaum. “A Reaction Time Study of Four Control-Display
Linkages.” Human Factors, 1 (1959), pp. 1–7.
Deatherage, B. H. “Auditory and Other Sensory Forms of Information Presentation.” In
H. P. Van Cott and R. Kinkade (Eds.), Human Engineering Guide to Equipment
Design. Washington, DC: Government Printing Office, 1972.
Dix, A., J. Finlay, G. Abowd, and R. Beale. Human-Computer Interaction. 2d ed.
London: Prentice-Hall, 1998.
Drury, C. “Improving Inspection Performance.” In Handbook of Industrial Engineering.
Ed. G. Salvendy. New York: John Wiley & Sons, 1982.
Drury, C. G., and J. L. Addison. “An Industrial Study on the Effects of Feedback and
Fault Density on Inspection Performance.” Ergonomics, 16 (1973), pp. 159–169.
Eggemeier, F. T. “Properties of Workload Assessment Techniques.” In Human Mental
Workload. Eds. P. Hancock and N. Meshkati. Amsterdam: North-Holland, 1988.
Fechner, G. Elements of Psychophysics. New York: Holt, Rinehart and Winston, 1860.
Fitts, P. “The Information Capacity of the Human Motor System in Controlling the
Amplitude of Movement.” Journal of Experimental Psychology, 47 (1954), pp. 381–391.
Galitz, W. O. User-Interface Screen Design. New York: John Wiley & Sons, 1993.
Giambra, L., and R. Quilter. “A Two-Term Exponential Description of the Time Course of Sustained Attention.” Human Factors, 29 (1987), pp. 635–644.
Green, D., and J. Swets. Signal Detection Theory and Psychophysics. Los Altos, CA:
Peninsula Publishing, 1988.
Greenstein, J. S., and L. Y. Arnaut. “Input Devices.” In Handbook of Human-Computer
Interaction. Ed. M. Helander. Amsterdam: Elsevier/North-Holland, 1988.
Harpster, J .L., A. Freivalds, G. Shulman, and H. Leibowitz. “Visual Performance on
CRT Screens and Hard-Copy Displays.” Human Factors, 31 (1989), pp. 247–257.
Helander, J. G., T. K. Landauer, and P. V. Prabhu, (Eds.). Handbook of HumanComputer Interaction, 2d ed. Amsterdam: Elsevier, 1997.
Hick, W. E. “On the Rate of Gain of Information.” Quarterly Journal of Experimental
Psychology, 4 (1952), pp. 11–26.
Human Factors Society. American National Standard for Human Factors Engineering of Visual Display Terminal Workstations, ANSI/HFS 100-1988. Santa Monica, CA:
Human Factors Society, 1988.
Hyman, R. “Stimulus Information as a Determinant of Reaction Time.” Journal of
Experimental Psychology, 45 (1953), pp. 423–432.
Langolf, G., D. Chaffin, and J. Foulke. “An Investigation of Fitts’ Law Using a Wide
Range of Movement Amplitudes.” Journal of Motor Behavior, 8, no. 2 (June 1976), pp. 113–128.
Mayhew, D. J. Principles and Guidelines in Software User Interface Design.
Englewood Cliffs, NJ: Prentice-Hall, 1992.
Miller, G. “The Magical Number Seven, Plus or Minus Two: Some Limits on
Our Capacity for Processing Information.” Psychological Review, 63 (1956), pp. 81–97.

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Sanders, M. S., and E. J. McCormick. Human Factors in Engineering and Design.
7th ed. New York: McGraw-Hill, 1993.
Stroop, J. R. “Studies of Interference in Serial Verbal Reactions.” Journal of
Experimental Psychology, 18 (1935), pp. 643–662.
Wickens, C. Engineering Psychology and Human Performance. Columbus, OH:
Merrill, 1984.
Wickens, C. D. “Processing Resources in Attention.” In Varieties of Attention. Eds.
R. Parasuraman and R. Davies. New York: Academic Press, 1984.
Wickens, C. D., S. E. Gordon, and Y. Liu. An Introduction to Human Factors
Engineering. New York: Longman, 1997.
Yerkes, R. M., and J. D. Dodson. “The Relation of Strength of Stimulus to Rapidity of
Habit Formation.” Journal of Comparative Neurological Psychology, 18 (1908), pp. 459–482.

SELECTED SOFTWARE
DesignTools (available from the McGraw-Hill text website at www.mhhe.com/ niebels-freivalds), New York: McGraw-Hill, 2002.

WEBSITES
Examples of bad ergonomic design—http://www.baddesigns.com/

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CHAPTER

8

KEY POINTS





Accidents result from a sequence of events with multiple causes.



Increase system reliability by adding backups and increasing component reliability. •

Consider trade-offs of various corrective actions by using cost-benefit analysis. •


Be familiar with OSHA safety requirements.

Examine accidents by using job safety analysis.
Detail the accident sequence or system failure by using the fault tree analysis. Control hazards by






Eliminating them completely, if possible
Limiting the energy levels involved
Using isolation, barriers, and interlocks
Designing fail-safe equipment and systems
Minimizing failures through increased reliability, safety factors, and monitoring W

orkplace safety is an extension of the concept of providing a good, safe, comfortable working environment for the operator, as discussed in
Chapter 7. The primary goal here is not to increase production through more efficient working conditions or improved worker morale, but specifically to decrease the number of accidents, which potentially lead to injuries and loss of property. Traditionally, of foremost concern to the employer have been compliance with existing state and federal safety regulations and avoidance of a safety inspection by federal compliance officers (such as OSHA) with commensurate citations, fines, and penalties. However, more recently, the bigger driving force for implementing safety has been the escalating medical costs. Therefore, it only makes sense to implement a thorough safety program to reduce overall costs. Key issues
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of OSHA safety legislation and workers’ compensation are introduced in this chapter, along with a general theories for accident prevention and hazard control.
However, details for correcting specific hazards are not addressed here, as there are numerous traditional textbooks on safety that cover these details (Asfahl,
2004; Banerjee, 2003; Goetsch, 2005; Hammer and Price, 2001; National Safety
Council, 2000). These books will also address the setup and maintenance of safety management organizations and programs.

8.1 BASIC PHILOSOPHIES OF ACCIDENT
CAUSATION AND PREVENTION
Accident prevention is the tactical, sometimes relatively short-term, approach to controlling workers, materials, tools and equipment, and the workplace for the purpose of reducing or preventing the occurrence of accidents. This is in contrast to safety management, which is a relatively long-term strategic approach for the overall planning, education, and training of such activities. A good accident prevention process (see Figure 8.1) is an orderly approach very similar to the methods engineering program introduced in Chapter 2.
The first step in the accident prevention process is the identification of the problem in a clear and logical form. Once the problem is identified, the safety engineer the needs to collect data and analyze them so as to understand the causation of the accident and identify possible remedies to prevent it or, if not completely prevent it, at least to reduce the effects or severity of the accident. In many cases, there may be several solutions, and the safety engineer will need to select one of these solutions. Then the remedy will have to be implemented and monitored to ensure that it is truly effective. If it is not effective, the engineer may need to repeat this process and attempt another, perhaps better remedy. This monitoring effectively closes the feedback cycle and ensures a continuous improvement process for accident prevention.

DOMINO THEORY
In identifying the problem, it is important to understand some of the theories of accident causation and the sequence of steps in an accident itself. One such
Identify Problem
Monitor Results

Collect Data

Implement Remedy

Analyze Data
Select Remedy

Figure 8.1 Accident prevention process.

(Adapted from: Heinrich, Petersen, and Roos, 1980)

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Lack of
Control

Basic
Causes

Immediate
Causes

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Injury

Figure 8.2 The domino theory of an accident sequence.

(Adapted from: Heinrich, Petersen, and Roos, 1980)

theory is the domino theory, developed by Heinrich, Petersen, and Roos (1980) based on a series of theorems developed in the 1920s which formed the individual dominos (see Figure 8.2):
1. Industrial injuries (or loss of damages) result from accidents, which involve contact with an energy source and the consequent release of that energy.
2. Accidents are the result of immediate causes such as
a. Unsafe acts by people
b. Unsafe conditions in the workplace
3. The immediate causes result from more basic causes:
a. The unsafe acts from personal factors such as lack of knowledge, skill, or simply the lack of motivation or care
b. The unsafe conditions due to job factors, such as inadequate work standards, wear and tear, poor working conditions, due to either the environment or lack of maintenance
4. The basic causes result from an overall lack of control or proper management. This domino (the first in the sequence) is essentially the lack of a properly implemented or maintained safety program, which should include elements to properly identify and measure job activities, establish standards proper standards for those jobs, measure worker performance on those jobs, and correct worker performance as needed.
Heinrich, Petersen, and Roos (1980) further postulated that the injury is simply the natural consequence of the previous events having taken place, similar to dominos falling in a chain reaction. As a proactive preventive measure, one could simply remove one of the previous dominos, thereby preventing the rest from falling and stopping the sequence prior to injury. They also emphasized that it was important to try to remove a domino as far upstream as possible, that is, to implement the corrective procedure as early as possible, at the root causes. The implication is that if effort is put into only preventing the injury, similar accidents will still occur in the future with potential for property damage and other types of injuries.
As an adaptation of the domino theory, Heinrich, Petersen, and Roos (1980) also emphasized the concept of multiple causation; that is, behind each accident

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or injury there may be numerous contributing factors, causes, and conditions.
These combine in a rather random fashion, such that it might be difficult to identify which, if any, of the factors was the major cause. Therefore, rather than try to find just one major cause, it would be best to try to identify and control as many causes as possible, so as to get the biggest overall effect on controlling or preventing the accident sequence. As an example, among unsafe acts caused by the human, which Heinrich, Petersen, and Roos (1980) claim amount to 88% of all accidents, there could be (1) horseplay, (2) operating equipment improperly,
(3) intoxication or drugs, (4) purposefully negating safety devices, or (5) not stopping a machine before cleaning or removing a stuck piece. Among unsafe conditions, which amount to 10% of all accidents (the remaining 2% are unpreventable “acts of God”), there could be (1) inadequate guards, (2) defective tools or equipment, (3) poorly designed machines or workplaces, (4) inadequate lighting, or (5) inadequate ventilation.
Figure 8.3 demonstrates the effects of various corrective actions taken along the domino sequence as well as multiple causation for a scenario in which sparks created by a grinder could ignite solvent fumes and cause an explosion and fire, with resulting burns to the operator. The injury is defined by burns to the operator. The accident leading to the injury is an explosion and fire. The sequence could be stopped by having the operator wear a fire-protective suit. The accident still happens, but a severe injury is avoided. Obviously, this is not the best control method as fire still could occur with other consequences to property. Moving one domino backward, the fire was caused by sparks from the grinder igniting

1: Separate gas, grinder
2: Better inspection

Lack of
Control
(1:Solvent stored at grinder, 2:Poor identification of work activities) 1: Increase ventilation
2: Grinder material

Basic
Causes

Immediate
Causes

(1: Less volatile solvent,
2:Grinder
creates sparks) (Sparks ignite fumes) Accident

Injury

(Explosion/ fire) (Burns)

Fire protective suit
1: Use spark arrester
2: Less concentrated fumes

Figure 8.3 A domino sequence for a grinder spark igniting a fire.

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volatile fumes in the grinding area. The sequence could be stopped at this stage by using a spark arrester or decreasing the concentration of the fumes through better ventilation. This still is a risky control measure as the spark arrester may not stop all sparks, and the ventilator may fail or slow during power brownouts. Moving backward another domino, more basic causes could include a couple of different factors (note the multiple causation) such as having such a volatile solvent and the fact that the grinder wheel acting on the casting creates sparks. The sequence could be stopped here by having a less volatile solvent or by installing a softer grinding wheel made of a different material that would not create sparks. Again these might not be most effective control measures with extremely hot weather increasing the vaporization of even a rather stable solvent and the grinding wheel perhaps creating sparks with harder castings. Furthermore they can have other, less positive consequences, such as a softer grinding wheel being less effective in smoothing the rough edges on the castings. The final domino of lack of control has probably a multitude of factors: poor identification of work activities that allowed the use of solvent in the grinding area, storage of a solvent in a work area, poor safety inspections, lack of awareness of the grinding operator, etc. At this stage, simply separating the dangerous elements, that is, removing the solvent from the grinding area, is the simplest, cheapest, and most effective solution.
Although, strictly speaking, the Heinrich, Petersen, and Roos (1980) accidentratio triangle, which establishes the foundation for a major injury (see Figure 8.4a), is not an accident causation model, it emphasizes the necessity of moving backward in the accident progression sequence. For each major injury, most likely there were at least 29 minor injuries and 300 no-injury accidents, with untold hundreds or thousands of unsafe acts leading up to the base of the triangle.
Therefore, rather than just reactively focus on the major injury or even the minor injuries, it makes sense for the safety engineer to look proactively, further back at the no-injury accidents and unsafe acts leading up to those accidents, as a field of opportunities to reduce potential injury and property damage costs and have a much more significant and effective total loss control program. This accidentratio triangle was later modified by Bird and Germain (1985) to include property damage and revised numbers (see Figure 8.4b). However, the basic philosophy remained the same.

BEHAVIOR-BASED SAFETY MODELS
More recent accident causation models have focused on behavioral aspects of the human operator. The basis for this approach lies in early crisis research of Hill
(1949) followed by the quantification of these crises or more modest situational factors into life change units (LCUs) by Holmes and Rahe (1967) (see Table 8.1).
The basic premise of the theory is that situational factors tax a person’s capacity to cope with stress in the workplace (or life in general), leaving the person more likely to suffer an accident as the amount of stress increases. It was found that
37 percent of individuals who accumulated between 150 and 199 LCUs in 2 years had illnesses. As the LCUs increased from 200 to 299, 51 percent had illnesses,

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Major injury
Minor injuries
Accidents

(a)
Major injury
Minor injuries
Property damage
Near misses

(b)
Figure 8.4 (a) Heinrich accident ratio triangle, (b) Bird and Germain (1985) accident ratio triangle.

(Adapted from: Heinrich, Petersen, and Roos, 1980)

and for those exceeding 300 LCUs, 79 percent had illnesses. This theory may help explain apparently accident-prone individuals and the need for having stressed workers avoid difficult or dangerous tasks.
Another behavioral accident causation model is the motivation-rewardsatisfaction model presented by Heinrich, Petersen, and Roos (1980). It expands on Skinnerian concepts (Skinner, 1947) of positive reinforcement to achieve certain goals. In terms of safety, worker performance is dependent on the worker’s motivation as well as the worker’s ability to perform. In the main positive feedback cycle (see Figure 8.5), the better the worker performs; the better the worker is rewarded, the more the worker is satisfied, the greater the worker’s motivation to perform better. This positive feedback could be applied both to safety performance and to worker productivity (which is the basis for wage incentive systems discussed in Chapter 17).
The most current and popular variation of behavior-based safety training is the ABC model. At the center of the model is behavior (the B part) of the worker, or what the worker does as part of the accident sequence. The C part is the consequence of the worker’s behavior, or the events taking place after the behavior, leading to a potential accident and injury. The first A parts are antecedents (sometimes termed activators) or events that take place before the behavior occurs.
Typically, this will start out as a negative process, in which the safety engineer tries to correct unpleasant consequences and determine what behaviors and antecedents lead to these consequences. For example, an operator takes a shortcut across a moving conveyor—a behavior. The antecedent may be break time as the operator tries to beat the lunchtime rush to get into the cafeteria line first. The

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Table 8.1 Table of Life Change Units

Rank
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43

Life Event
Death of spouse
Divorce
Marital separation
Jail term
Death of close family member
Personal injury or illness
Marriage
Fired at work
Marital reconciliation
Retirement
Changes in family member’s health
Pregnancy
Sex difficulties
Gain of new family member
Business readjustment
Change in financial state
Death of close friend
Change to different line of work
Change in number of arguments with spouse
Mortgage over critical amount
Foreclosure of mortgage or loan
Change in work responsibilities
Son or daughter leaving home
Trouble with in-laws
Outstanding personal achievement
Wife beginning or stopping work
Begin or end school
Change in living conditions
Revision of personal habits
Trouble with boss
Change in work-hours, conditions
Change in residence
Change in schools
Change in recreation
Change in church activities
Change in social activities
Mortgage or loan under critical amount
Change in sleeping habits
Change in number of family get-togethers
Change in eating habits
Vacation
Christmas
Minor violations of the law

Mean Value
100
73
65
63
63
53
50
47
45
45
44
40
39
39
39
38
37
36
35
31
30
29
29
29
28
26
26
25
24
23
20
20
20
19
19
18
17
16
15
15
13
12
11

Source: Heinrich, Petersen, and Roos, 1980.

consequences are typically positive for the operator with more time to eat lunch, but in this particular instance are negative with an injury as the operator slipped on the conveyor. One approach in changing the behavior would be to post warnings on the dangers of jumping across the conveyor and to issue fines for violations.

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Boss

Staff

Management

Style - Climate - Relations
SELF
Personality
Achievement

JOB CLIMATE

THE JOB ITSELF
Any fun?

JOB MOTIVATIONAL FACTORS
Can he or she achieve?
Promotion?
A sense of responsibility

MOTIVATION

PEER GROUP(S)
Norms
Pressures

PERFORMANCE
UNION
Norms
Pressures

SELECTION
Can he or she do it?

REWARD

SATISFACTION

ABILITY
Boss

Peer

Union

Self

TRAINING
Does he or she know how?

Figure 8.5 The motivation reward satisfaction model.

Source: Heinrich, Petersen, and Roos, 1980

However, this a negative approach that would require major enforcement action.
That is, changing antecedents can get behavior started, but in many cases are not sufficient to maintain that behavior, especially if the approach focuses on the negative. A better approach would be to use the motivation-reward-satisfaction model and focus on positive consequences. This could be achieved by staggering lunch breaks for employees so that all would enjoy a relaxing, unrushed lunch break. It is also important to realize that the most effective activators are correlated with the most effective consequences—those that are positive, immediate, and certain.
Generally, behavior-based approaches are quite popular and effective as an accident prevention method, especially considering that the large majority (up to
88 percent) of accidents are due to unsafe acts and behaviors on the part of workers. Unfortunately this approach focuses solely on people and not on physical hazards. So there should also be mechanisms and procedures in place for ensuring safe workplace conditions. Finally, one should be careful that such programs do not become convoluted from the original purpose of promoting safety. From personal experience, a manufacturing company had implemented a positive reinforcement program of providing safety incentives for production workers: all workers in a department achieving a particular safety goal, for example, a month without a recordable injury, were provided a free lunch in the cafeteria. If this record was extended to six months, they received a steak dinner at a popular

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restaurant; and if they reached one year, then they received a $200 gift certificate.
Of course, if there was a recordable injury, they had to start over from scratch. As result, once the stakes got high, injured workers were strongly encouraged by fellow workers not to report the injury to the plant nurse, circumventing the original intent!

8.2

ACCIDENT PREVENTION PROCESS

IDENTIFYING THE PROBLEM
In identifying the problem, the same quantitative exploratory tools (such as
Pareto analysis, fish diagram, Gantt chart, job-worksite analysis guide) discussed in Chapter 2 for methods engineering can also be used in the first step in the accident prevention process. Another tool that is effective in identifying whether one department is significantly more hazardous than another is the chi-square analysis. This analysis is based on the chi-square goodness of fit test between a sample and a population distribution in the form of categorical data in a contingency table. Practically, this is expressed as a difference between m observed and expected cell counts of injuries (or accidents or dollars): x2 ϭ a 1Ei Ϫ Oi 2 2>Ei i m

Ei ϭ expected value ϭ Hi ϫ OT /HT
Oi ϭ observed value
OT ϭ total of observed values
Hi ϭ hours worked
HT ϭ total of hours worked m ϭ number of areas compared
2
2
2
If the resulting ␹ is greater than xa,mϪ1, the critical ␹ at an error level of ␣ and

where

with m Ϫ 1 degrees of freedom, then there is a significant difference between the expected and observed values in injuries. Example 8.1 demonstrates an application for safety while more details on the statistical procedure can be found in
Devore (2003).

Chi-Square Analysis of Injury Data
Dorben Co. has three main production departments: processing, assembly, and packing/ shipping. It is concerned with the apparent high number of injuries in processing and would like to know if this is a significant deviation from the rest of the plant. Chisquare analysis comparing the number of injuries in 2006 (shown in Table 8.2) with an expected number based on the number of exposure hours is the appropriate way to study the problem.
The expected number of injuries in processing is found from
Ei ϭ Hi ϫ OT>HT ϭ 900,000 ϫ 36>2,900,000 ϭ 11 .2

EXAMPLE 8.1

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Table 8.2 Observed and Expected Injuries

Injuries Oi

Department
Processing
Assembly
Packing/Shipping
Total

Exposure (h)

Expected Injuries Ei

22
10
4
36

900,000
1,400,000
600,000
2,900,000

11.2
7.4
17.4
36

The expected numbers of injuries for the other departments are found similarly. Note that the total number of expected injuries should add to the total number of observed injuries, 36. x2 ϭ a i 1Ei Ϫ Oi 2 2>Ei ϭ 111 .2 Ϫ 22 2 2>11 .2 ϩ 17 .4 Ϫ 4 2 2>7 .4 ϩ 117 .4 Ϫ 10 2 2>17 .4 ϭ 15 .1 m The resultant value of 15.1 is greater than x2
0.05,3Ϫ1 ϭ 5 .9, found in Table A3–4
(Appendix 3). Therefore, the number of injuries in at least one department deviates significantly from the expected value based purely on exposure hours. This department, processing with 22 injuries instead of the expected 11.2, should then be studied in further detail to find the cause of this increase in injuries.

COLLECT AND ANALYZE DATA—JOB SAFETY ANALYSIS
The second and third steps of the accident prevention process are the collecting and analyzing of data. The most common and basic tool for this is job safety analysis (JSA), sometimes also termed job hazard analysis or methods safety analysis. In a JSA, the safety engineer (1) breaks down a job into its component elements in a sequential order, (2) examines each element critically for a potential hazard or the possibility of an accident occurrence, and (3) identifies ways of improving the safety of this element. While the safety engineer is performing a
JSA, she or he should focus on four major factors:
1. Worker: the operator, the supervisor, or any other individual that may be associated with this element
2. Method: the work procedures being utilized in this particular process
3. Machine: the equipment and tools being utilized
4. Material: the raw material, parts, components, fasteners, etc., that are being used or assembled in the process.
Thus, any improvements could involve better training of or personal protective equipment for the operator, a new method, safer equipment and tools, and different and/or better materials and components.
As an example, consider the process of machining a relatively large (40-lb) coupling, shown in Figure 8.6, with its associated JSA in Figure 8.7. The process involves (1) picking up the unfinished part from a crate, (2) setting it in the

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Figure 8.6 Steps in the machining of a coupling

Courtesy of Andris Freivalds.

machine fixture, (3) tightening the fixture with a wrench, (4) blowing out machining chips (loosening the fixture and removing the coupling—not shown, but equivalent to elements 3 and 2, respectively), (5) smoothing any rough edges with a hand grinder, and (6) placing the finished part in a packing carton. Potential hazards and appropriate controls corresponding to each element are shown in
Figure 8.7. Common problems include high compressive forces while one is retrieving and placing the coupling in crates or packing cartons. These forces could be reduced by tilting the boxes for easier entry. Another problem is shoulder flexion with large torques while one is placing and/or removing the coupling into the machining fixture and tightening or loosening the fixture. These could be alleviated by lowering the fixture and having the stand closer to the fixture so that the elbows are bent closer to the optimum 90° angle. Personal protective equipment such as a dust mask would help with dust and gel gloves with hand vibration.
JSAs provide several useful features that cross over into methods engineering. They are a simple, quick, and objective means of mapping all the relevant details. They can compare existing and proposed methods with potential effects not just on safety but also on production, which is a very useful application in terms of selling increased safety to management. Although quite qualitative, the
JSA approach can be made more quantitative by adding probabilities, which leads into the very quantitative fault tree analysis, discussed later in Section 8.5.

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Figure 8.7 Job safety analysis.

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SELECT A REMEDY—RISK ANALYSIS
AND DECISION MAKING
Once JSAs have been completed and a variety of solutions have been suggested, the safety engineer will need to choose one for implementation. This can be done by using a variety of decision-making tools in the fourth step of the accident prevention process, select a remedy. Most of these tools are just as appropriate for selecting a new method for improved productivity and are presented in Chapter 9.
However, one of these tools, risk analysis, is more suitable for safety because it calculates the potential risk for an accident or injury and the reduction of risk due to modifications. According to Heinrich, Petersen, and Roos (1980), the analysis is based on the premise that the risk for injury or loss cannot be completely eliminated; that only a reduction in risk or potential loss can be achieved.
Furthermore, any modifications should consider maximum cost effectiveness.
According to the method (Heinrich, Petersen, and Roos, 1980), the potential loss increases with (1) increased likelihood or probability that the hazardous event will occur, (2) increased exposure to the hazardous conditions, and (3) increased possible consequences of the hazardous event. Numerical values are assigned to each of the above three factors (see Table 8.2), and then an overall risk score is computed as a product of these factors (see Table 8.3). Note that these numerical values are rather arbitrary, and consequently the final risk score is also rather arbitrary. This, however, doesn’t negate the method; it still serves as method to provide good relative comparison between different safety features or controls.
As an example of risk analysis, consider an event that is conceivable but rather unlikely with a value of 0.5, with a weekly exposure and value of 3, and
Table 8.3 Risk Analysis Factor Values

Likelihood
Expected
Possible
Unusual
Remote
~ Conceivable
~ Impossible

Values
10
6
3
1
0.5
0.1

Exposure

Values

Continuous
Daily
Weekly
Monthly
Few/Year
Yearly

Possible Consequences
Catastrophe (many fatalities, $108 damage)
Disaster (few fatalities, $107 damage)
Very serious (fatality?, $106 damage)
Serious (serious injuries, $105 damage)
Important (injuries, $104 damage)
Noticeable (first aid, $103 damage)
Adapted from: Heinrich, Petersen, and Roos (1980).

10
6
3
2
1
0.5
Value
100
40
15
7
3
1

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Table 8.4

Risk Analysis and Cost-Effectiveness

Risk Situation
Very high risk, discontinue operations
High risk, immediate correction
Substantial risk, correction needed
Possible risk, attention needed
Risk?, perhaps acceptable

Value
400
200–400
70–200
20–70
Ͻ 20

Source: Heinrich, Petersen, and Roos (1980).

,

,

,

,

,

Figure 8.8 Risk analysis calculation.

Source: Heinrich, Petersen, and Roos, 1980.

very serious consequences with a value of 15. The resulting product yields a risk score of 22.5 (ϭ 0.5 ϫ 3 ϫ 15) which corresponds to a rather low risk, with possible attention needed, but not urgent attention. See Table 8.4. This same result can be achieved by using Figure 8.8 and a tie line to connect the two halves of the chart. The cost-effectiveness of two different remedies for the above risky event can be compared by using Figure 8.9. Remedy A reduces the risk by 75 percent but costs $50,000 while remedy B reduces the risk by only 50 percent, but also costs only $500. In terms of cost-effectiveness, remedy A is of doubtful merit and may have difficulty receiving financial support, while remedy B may well be justified because of its lower cost.

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B

A

Figure 8.9 Risk analysis and cost-effectiveness.

Source: Heinrich, Petersen, and Roos, 1980.

After an appropriate cost-effective remedy has been selected, the remedy needs to be implemented in the fifth step of the accident prevention method. This should occur at several levels. The safety engineer with appropriate technicians will install the appropriate safety devices or equipment. However, for a completely successful implementation, the individual operators and supervisors must also buy into the new approach. If they don’t follow the correct procedures with the new equipment, any potential safety benefits may be lost. As an aside, this also presents an opportunity to discuss the 3 E’s approach: engineering, education, and enforcement. The best remedy almost always is an engineering redesign. This will ensure strict safety and doesn’t rely strictly on operator compliance. The next-best remedy is education; however, this does rely on operator compliance and may not always succeed, especially if workers do not follow the correct procedures. Lastly, there is enforcement of strict rules and use of personal protective equipment. This presumes worker noncompliance, requires strict checkups, instills resentment with negative reinforcement, and should be used as a last resort.

MONITORING AND ACCIDENT STATISTICS
The sixth and final step in the accident prevention process is the monitoring of the situation to evaluate the effectiveness of the new method. This provides feedback on the process and closes the loop by restarting the cycle in case the

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situation is not improving. Typically, numerical data provide a solid benchmark for monitoring any changes. These could be insurance costs, medical costs, or simply numbers of injuries and/or accidents. However, any of these numbers should be normalized to the worker exposure hours so that the numbers can be compared across locations and industries. Furthermore, OSHA recommends expressing injury statistics as incidence rate (IR) per 100 full-time employees per year:
IR ϭ 200,000 ϫ I>H where I ϭ number of injuries in given time period
H ϭ employee hours worked in same time period
For OSHA record-keeping purposes, the injuries should be OSHA-recordable, or more than simple first-aid injuries. However, research has shown that there are considerable similarities between minor and major injuries (Laughery and Vaubel,
1993). Similarly, the severity rate (SR) monitors the number of lost-time (LT) days:
SR ϭ 200,000 ϫ LT>H
In addition to simply recording and monitoring the incidence rates as they change from month to month, the safety engineer should apply statistical control charting principles and look for long-term trends. The control chart (see Figure 8.10) is based on a normal distribution of the data and establishing a lower control limit
(LCL) and a upper control limit (UCL) as defined by
LCL ϭ x Ϫ ns
UCL ϭ x ϩ ns
– ϭ sample mean where x s ϭ sample standard deviation n ϭ level of control limits
For example, for the case in which we would expect 100(1Ϫ␣) percent of the data to fall between the upper and lower control limits, n would simply be the standard normal variable z␣/2. For ␣ ϭ 0.05, n becomes 1.96. However, for many

Figure 8.10 Statistical control limits.

Source: Heinrich, Petersen, and Roos, 1980.

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UCL

Figure 8.11 Red flagging with control chart.

Adapted from: Heinrich, Petersen, and Roos, 1980.

situations a higher lever of control is needed with n ϭ 3 or even n ϭ 6 (the Motorola six-sigma control level). For tracking accidents or injuries, the control chart is rotated sideways, and monthly data are plotted on the chart (see Figure 8.11).
Obviously the lower control limit is of less concern (other than a nice pat on the back) than the upper control limit. Should several consecutive months fall above the upper control limit, this should be a red flag or signal to the safety engineer that there is a problem and a serious effort should be put into finding the cause.
In addition to the red flagging, an alert safety engineer should have noticed the upward trend beginning several months previously and started a control action earlier. This trend analysis could be easily performed using a moving linear regression over varying multiple-month periods.

8.3 PROBABILITY METHODS
The accident causation models discussed previously, especially the domino theory, implied a very deterministic response. That is far from the case. Grinding without safety glasses or walking under an unsupported roof in a coal mine at a given moment does not ensure an automatic accident and injury. However, there is a chance that an accident will occur, and the likelihood of that happening can be defined with a numeric probability.
Probability is based on Boolean logic and algebra. Any event is defined by a binary approach; that is, at any given moment, there are only two states—the event either exists and is true (T), or it does not exist and is false (F). There are three operators that define interactions between events:
1. AND, the intersection between two events, with symbol പ or • (the dot sometimes is omitted)
2. OR, the union between events, with symbol ഫ or ϩ
3. NOT, the negation of an event, with the symbol Ϫ.
The interaction of two events X and Y, using these operators, follows a specific pattern termed the truth tables (see Table 8.5). Interactions between more than

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Table 8.5

Boolean Truth Tables

X

Y

X ·Y

X ϩY

T
T
F
F

T
F
T
F

T
F
F
F

T
T
T
F

Table 8.6



(Not)

X
T
F

X
F
T

Boolean Algebra Simplifications

• Basic laws:
X·X ϭ X
XϩXϭX
XT ϭ X

XX ϭ 0
XϩXϭ0
XF ϭ 0

• Distributive laws:
XY ϩ XZ ϭ X(Y ϩ Z)
XY ϩ XY ϭ X
X ϩ XY ϭ X
X(X ϩ Y) ϭ XY

(XϩY)(XϩZ) ϭ X ϩ YZ
X ϩ YX ϭ X ϩ Y
X(X ϩ Y) ϭ X
(XϩY) (XϩY ) ϭ X

two events result in more complicated expressions, which necessitate an ordered processing to evaluate the resultant overall probability of the final accident or injury. The specific order or precedence that must be followed is as follows: ( ),Ϫ, •, ϩ.
Also, certain groupings of events tend to appear repeatedly, so that if one recognizes these patterns, simplification rules can be applied to quicken the evaluation procedure. The most common rules are given in Table 8.6.
The probability of an event P(X) is defined as the number of times event X occurs out of the total number occurrences:
P 1X2 ϭ #X>#Total

and P(X) must necessarily lie between 0 and 1. The probability of ORed events
X ഫ Y, is defined as
P 1X ϩ Y2 ϭ P 1X2 ϩ P 1Y2 Ϫ P 1XY2 if the events are not mutually exclusive, and as
P 1X ϩ Y2 ϭ P 1X 2 ϩ P 1Y2 if the events are mutually exclusive. Two events are defined as mutually exclusive if the two events do not intersect, that is, X ʝ Y ϭ 0. Thus necessarily X and X are mutually exclusive. For the union of more than two events, an alternate expression, based on reverse logic, that is much easier to evaluate is more typically used:
P 1X ϩ Y ϩ Z2 ϭ 1 Ϫ [1 Ϫ P 1X2 ] [1 Ϫ P 1Y2 ] [1 Ϫ P 1Z2 ]
The probability of ANDed events is defined as
P 1XY2 ϭ P 1X2P 1Y2

(2)

if the two events are independent, and as
P 1XY2 ϭ P 1X2P 1Y>X2 ϭ P 1Y2P 1X>Y2

(3)

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if the two events are not independent. Two events are defined as independent if the occurrence of one event doesn’t affect the occurrence of another event. Mathematically this is determined by equating Equations (2) and (3) and removing
P(X) from both sides, yielding
P 1Y2 ϭ P 1Y>X2

(4)

if the two events are independent. Rearrangement of Equation (3) also yields a commonly used expression that is termed Bayes’ rule:
P 1Y>X2 ϭ P 1Y 2P 1X>Y2 >P 1X2

(5)

Note that two events cannot be both mutually exclusive and independent, because being mutually exclusive necessarily defines as the one event defining the other one, that is, being dependent. Example 8.2 demonstrates these various calculations as well as independent and nonindependent events. More details on basic probability can be found in Brown (1976).
EXAMPLE 8.2

Independent and Not Independent Events
Consider the number of occurrences of A being true (or 1) out of the total number of occurrences in Table 8.7a. This determines the probability of A:
P 1X2 ϭ #X>#Total ϭ 7>10 ϭ 0 .7
Note that the probability of A is the number of occurrences of being false (or 0) out of the total number of occurrences:
P 1X2 ϭ #X>#Total ϭ 3>10 ϭ 0 .3

Also P 1X 2 can be found from

P 1X 2 ϭ 1 Ϫ P 1X2 ϭ 1 Ϫ 0 .7 ϭ 0 .3
Similarly the probability of Y is
P 1Y2 ϭ #Y>#Total ϭ 4>10 ϭ 0 .4
The probability of A പ B is the number of occurrences of both A and B being true out of the total number of occurrences:
P 1XY2 ϭ #XY>#Total ϭ 3>10 ϭ 0 .3
Table 8.7 Independent or Not Independent Events

(a) X and Y are not independent

(b) X and Y are independent

X

X

Y

0

1

Total

0
1
Total

2
1
3

4
3
7

6
4
10

Y

0

1

Total

0
1
Total

2
4
6

3
6
9

5
10
15

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The conditional probability of X given that Y has occurred (or is true) is defined as the number of occurrences of X from the Y ϭ 1 row:
P 1X>Y2 ϭ #X>#Total Y ϭ 3>4 ϭ 0 .75

Similarly,
P 1Y>X2 ϭ #Y>#Total X ϭ 3>7 ϭ 0 .43
Note also Bayes’ rule:
P 1Y>X2 ϭ P 1Y2 P 1X>Y2 >P 1X2 ϭ 0 .4 ϫ 0 .75>0 .7 ϭ 0 .43 ϭ P 1Y>X2
Finally, for two events to be independent, Equation (4) must be true. But for Table 8.7a, we found that P(Y) ϭ 0.4 while P(Y/X) ϭ 0.43. Therefore events X and Y are not independent. However in Table 8.7b, we find
P 1X2 ϭ #X>#Total ϭ 9>15 ϭ 0 .6
P 1X>Y2 ϭ #X>#Total Y ϭ 6>10 ϭ 0 .6

Since both expressions are equal, events X and Y are independent. The same equivalence is found for P(Y) and P(Y/X):
P 1Y2 ϭ #Y>#Total ϭ 10>15 ϭ 0 .67

P 1Y>X2 ϭ #Y>#Total X ϭ 6>9 ϭ 0 .67

8.4

RELIABILITY

The term reliability defines the probability of the success of a system, which necessarily must depend on the reliability or the success of its components. A system could be either a physical product with physical components or an operational procedure with a sequence of steps or suboperations that need to be completed correctly for the procedure to succeed. These components or steps can be arranged in combinations using two different basic relationships: series and parallel arrangements. In a series arrangement (see Figure 8.12a), every component must succeed for the total system T to succeed. This can be expressed as the intersection of all components
T ϭ A ʝ B ʝ C ϭ ABC which if independent (in most cases), yields a probability of
P 1T2 ϭ P 1A2 P 1B2P 1C2

or if not independent, yields

P 1T2 ϭ P 1A2P 1B>A2P 1C>AB2

In a parallel arrangement, the total system succeeds if any one component succeeds. This can be expressed as an union of the component
T ϭ A ´ B ´ C ϭ A ϩ B ϩ C

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(a) Series system

A

B

C

T=ABC

(b) Parallel system
A

B

T=A+B+C

C

Figure 8.12

Series and parallel components.

which if mutually exclusive (typically), yields a probability of

P 1T2 ϭ 1 Ϫ [1 Ϫ P 1A 2 ] [1 Ϫ P 1B2 ] [1 Ϫ P 1C 2 ]

Two examples (Examples 8.3 and 8.4) demonstrate the calculation of system probabilities. Reliability of a Two-Stage Amplifier
Consider two prototypes of a two-stage amplifier with backup components. Prototype
1 (see Figure 8.13) has a backup for the full amplifier, while prototype 2 has a backup for each stage. Which has the better system reliability, given that all the components are independent but identical with the same reliability of 0.9?
The best approach is to write all possible paths for system success. For prototype
1 there are two possible paths, either AB or CD. Written as an expression, system success is
T ϭ AB ϩ CD
Expressed as a probability, this expression becomes
P 1T2 ϭ P 1AB2 ϩ P 1CD2 Ϫ P 1AB2P 1CD2 where P 1AB2 ϭ P 1A2P 1B2 ϭ 0 .9 ϫ 0 .9 ϭ 0 .81 ϭ P 1CD2

EXAMPLE 8.3

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#1
B

A

T = AB + CD

C

A

#2

D

B
T = AB + AD + CD + CB

D

C

Figure 8.13 Two prototypes of a two-stage amplifier.

The overall system reliability becomes
P 1T2 ϭ 0 .81 ϩ 0 .81 Ϫ 0 .81 ϫ 0 .81 ϭ 0 .964
For prototype 2 there are four possible paths: AB or AD or CB or CD. Written as an expression, system success is
T ϭ AB ϩ AD ϩ CB ϩ CD which simplifies to

T ϭ 1A ϩ C 2 1B ϩ D2

[Note that complicated probability expressions will need to be simplified; otherwise there may be an incorrect calculation. There are two basic distributive laws that form the basis for all simplification rules. These are:
1X ϩ Y 2 1X ϩ Z2 ϭ X ϩ YZ
XY ϩ XZ ϭ X 1Y ϩ Z 2

The others in Table 8.6 can be derived by substituting X for X, Y, or Z above.]
Now substituting values for the variables yields

P 1A ϩ C 2 ϭ P 1A2 ϩ P 1C2 Ϫ P 1A2P 1C2 ϭ 0 .9 ϩ 0 .9 Ϫ 0 .9 ϫ 0 .9 ϭ 0 .99

The overall system reliability becomes

P 1T2 ϭ 0 .99 ϫ 0 .99 ϭ 0 .9801

Therefore prototype 2 is the better amplifier with higher system reliability.

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EXAMPLE 8.4

Reliability of a Four-Engine Airplane
Consider an airplane with four independent but identical engines (see Figure 8.14).
The airplane can obviously fly with all four engines working, with any three engines working, and also with two engines working, as long as there is one engine working on each side of the plane; that is, two engines working on one side would cause the plane to be so unbalanced as to crash. What is the overall reliability of the airplane given that the reliability of each engine is 0.9?
Writing out all the possible engine scenarios results in these expressions:
4 engines 1 ABCD
3 engines 1 ABC ϩ ABD ϩ BCD ϩ ACD
2 engines 1 AC ϩ AD ϩ BC ϩ BD
T ϭ ABCD ϩ ABC ϩ ABD ϩ BCD ϩ ACD ϩ AC ϩ AD ϩ BC ϩ BD
The expression must be simplified. Note that the three-engine combinations are roughly redundant to the two-engine combinations:
AC ϩ ABC ϭ AC 11 ϩ B2 ϭ AC
Similarly, the four-engine combination is redundant to any of the two-engine combinations, resulting in a final expression for system reliability of
T ϭ AC ϩ AD ϩ BC ϩ BD
This expression further simplifies to
T ϭ 1A ϩ B2 1C ϩ D 2
The probability for each expression in parentheses is
P 1A ϩ B2 ϭ P 1A2 ϩ P 1B2 Ϫ P 1A2 P 1B2 ϭ 0 .9 ϩ 0 .9 Ϫ 0 .9 ϫ 0 .9 ϭ 0 .99 and the total system probability becomes
P 1T 2 ϭ 0 .99 ϫ 0 .99 ϭ 0 .9801

A

B

C

D

X

X

X

X

T = ABCD + ABC + ABD + BCD + ACD + AC + AD + BC + BD

Figure 8.14

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Reliability of a four-engine airplane.

Examples 8.2 and 8.3 indicate one of the basic principles for increasing system reliability—increasing redundancy by adding components in parallel to the original component. Thus two or more components or operations are providing the same function. Note that if two or more operations or components are required

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(a) Basic event mapping
_
Y X
0(X)

1(X)

_
0(Y)

Neither X nor Y occurs

1(Y)

Y occurs, but Both X and
X does not
Y occur

X occurs, but
Y does not

X

0

1

X

0

1
X

0
Y

1

XY

Y

X

0

1

0

0
1

Y

_
(d) T = X

(c) T = XY
Y

(b) T = X + Y

XY

1

Figure 8.15 Basics of Karnaugh mapping.

to prevent an accident, then those elements are in series and do not provide redundancy. System reliability can also increased by increasing the reliability of the individual components. If the reliability of each engine in Example 8.3 is increased to 0.99, the total system reliability increases to 0.9998 as opposed to the original 0.9801. Note, however, that there is a trade-off—increasing the reliabilities of individual components or increasing the number of parallel components will necessarily increase the cost of the system. At some point, the increasing costs may not merit the marginal increase in overall system reliability. This decision point may vary considerably, depending on whether the system of interest is a simple consumer product as opposed to commercial airlines.
As the Boolean expressions for system reliability become more complex, the simplification process can become correspondingly more complex and tedious.
At that point, the use of Karnaugh maps will provide a definitive solution to the problem. A Boolean algebra map is developed so as to represent the spatial representation of all possible events. Each event represents either a row or a column in a gridlike matrix. At its simplest, with two events, the matrix is 2 ϫ 2, with both conditions (either true or false) of one event Y represented as rows, while both conditions of another event X are represented as columns (see Figure 8.15a). Then the expression X ϩ Y is represented as three cells of the matrix (see Figure 8.15b), the expression XY is represented as one cell of the matrix (see Figure 8.15c), and X is a vertical column under 0 (see Figure 8.15d). For more events, the matrix will increase in size; for example, with 4 variables, the matrix will have 16 cells, and for
6 variables the matrix will have 36 cells. Beyond six variables, it will become difficult to handle the matrix, and a computerized approach will become more practical. Also, with more complicated expressions, the expression must be written in the form of a simple sum of products.
Each term or product is then marked appropriately on the matrix. Undoubtedly there will be overlapping areas, or cells that are marked several times.

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A simplified expression is then written of nonoverlapping groups of cells, each having a unique characterization of those cells. If done correctly, each group is mutually exclusive, and the addition of the probabilities for each group is quite straightforward. Note that it is better to identify the largest possible areas so that fewer calculations are needed later. These areas will consist of a certain number of cells as determined by powers of 2, that is, 1, 2, 4, 8, etc. This is best demonstrated by Example 8.4, the previous airplane example but using Karnaugh maps.
More details on Karnaugh maps can be found in Brown (1976).

Reliability of a Four-Engine Airplane Using Karnaugh Maps
Consider the same airplane of Example 8.4 with four independent but identical engines and the event expression
T ϭ ABCD1 ϩ ABC2 ϩ ABD3 ϩ CDB4 ϩ CDA5 ϩ AD6 ϩ BC7 ϩ AC8 ϩ BD9
Each combination of events can be delineated on a Karnaugh map (see Figure 8.16a) as indicated by the appropriate numbers. Many of the events or areas are overlapping, and only the nonoverlapping areas should be evaluated for the probability. Four such nonoverlapping areas are shown in Figure 8.16b, although there could be many other possible combinations of nonoverlapping areas. In this case the largest possible combination of areas was selected: one of four cells, two two-cell combinations, and one remaining cell. The resulting expression is
T ϭ AD1 ϩ ACD2 ϩ ABC3 ϩ ABCD4 with a value of
P 1T2 ϭ 10 .92 10 .09 2 ϩ 10 .9 2 10 .9 2 10 .1 2 ϩ 10 .1 2 10 .9 2 10 .9 2 ϩ 10 .12 10 .9 2 10 .1 2 10 .9 2 ϭ 0 .9801

Obviously the nine cells could be individually identified and calculated, but that would entail a much greater effort. As noted previously, larger areas will be characterized by powers of 2, that is, 1, 2, 4, 8, etc. To simplify the calculations even further, a reverse logic can also be used to define the nonmarked areas. This value is then subtracted from 1 to obtain the true probability of the event of interest (see
Figure 8.16c).
T ϭ AB1 ϩ BCD2 ϩ ABCD3
This results in a probability of
P 1T 2 ϭ 1 Ϫ P 1T 2 ϭ 1 Ϫ 10 .1 2 10 .1 2 Ϫ 10 .9 2 10 .1 2 10 .1 2
Ϫ 10 .92 10 .1 2 10 .1 2 10 .9 2 ϭ 0 .9801

and the same value is calculated in the direct method.

EXAMPLE 8.5

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(a)
CD AB

00

01

11

10

00
9

369

6

479

2345

568

01
11

6789
1278

8

11

7

10

10

(b)
CD AB

00

01

00
01

4

11

1

3
2

10

(c)
CD AB

00

01
2

00
01

11

10
3

1

11
10

Figure 8.16

8.5

Reliability of a four-engine airplane using Karnaugh maps.

FAULT TREE ANALYSIS

Another approach to examining accident sequences or system failures uses fault tree analysis. This is a probabilistic deductive process using a graphical model of parallel and sequential combinations of events, or faults, leading to the overall undesired event, for example, an accident. It was developed by Bell

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Fault event that needs to be evaluated further

Basic event at base level with no further development

Normal event that is expected to occur

Undeveloped event that is inconsequential or does not have sufficient information for further development

AND gate that needs all inputs to occur before output can occur

OR gate that needs any input to occur for output to occur

Figure 8.17

Fault tree symbols.

Laboratories in the early 1960s to assist the U.S. Air Force in examining missile failures and later used by NASA to ensure overall system safety for the manned space program. These events can be of various types and identified by different symbols (see Figure 8.17). In general, there are two main categories: fault events, identified by rectangles, that are to be expanded further, starting from the top head event; and basic events, identified by circles, at the bottom of the fault tree that cannot be developed any further. Formally, there can also be house-shaped symbols indicating that “normal” events can be expected to occur and diamond-shaped symbols indicating inconsequential events or events with insufficient data for further analysis. The events are linked with gates that involve the same Boolean logic described previously (see Figure 8.17 for the symbols). An AND gate requires that all the inputs occur for the output to occur. An OR gate needs at least one of the inputs to occur for the output to occur. Obviously this implies that several of or all the inputs could occur as well, but only one is needed. It also helps to define the input events to the OR gate to cover all possible ways that the output event could occur. There could also be some situations in which the gates may need to be modified by labels indicating certain situations such as a conditional AND: event A must occur before event B occurs or an exclusive OR: either event A or event B must occur for the output to occur but not both events. The first case can be alleviated by defining the AND gate such that the second event is conditioned on the first happening, and a third event is conditioned on the first two happening. The second case is a special case of mutually exclusive events and must be handled as such.

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The development of a fault tree starts with identifying all the events that are deemed undesirable for normal operation. These events need to be separated into mutually exclusive groups according to similar causes with one head event for each group. For example, in a grinding operation, there could be several mutually exclusive fault events leading to different head events or accidents: getting a chip in the eye, having sparks from the grinder start a fire, the operator losing control of the casting while pushing on it and having his or her fingers scraped by grinder, etc. Next, the relationships between the various causal events and head event are established through a combination of the
AND and OR gates. This is continued until basic fault events are reached, which cannot be developed any further. Then probabilities are assigned to each of the basic events, and an overall probability of the head event is calculated using AND and OR expressions. In the final step, appropriate controls are attempted along with estimated reductions in probabilities, leading to a decrease in the probability of the final head event. A simple fault tree analysis is shown in Example 8.6. Obviously, the cost of these controls or modifications has to be considered, and this will be examined through cost-benefit analysis discussed in the next section.

EXAMPLE 8.6

Fault Tree Analysis of a Fire
The grinding shop of Dorben Co. has had several relatively small fires that were quickly put out. However, the company is concerned that a fire could get out of control and burn down the whole plant. One way to analyze the problem is to use the fault tree approach with a major fire as the head event. There are three requirements
(well actually four, but neglect oxygen which is ubiquitous): (1) combustible material with a probability of 0.8, (2) an ignition source, and (3) the probability of a small fire getting out of control with a value of 0.1. There could be several ignition sources: (1) an operator smoking in spite of No Smoking signs, (2) sparks from the grinding wheel, and (3) an electrical short in the grinder. The company estimates the probabilities of these events as 0.01, 0.05, and 0.02, respectively. All the events in the first set are required and thus are connected with an AND gate. Note that the second event is conditional on the first, and the third event is conditional on the first two events. For the ignition set of events, any one input is sufficient for ignition to occur, and thus the events are connected with an OR gate. The complete fault tree is shown in Figure 8.18a.
An alternate approach would be to draw a sequence of events, similar to components for a product or operations for a system, as shown in Figure 8.18b. The three main events—combustible, ignition, getting out of control—are drawn in series, since the path must go through all three to have the plant burn down. The ignition sources can be considered in parallel, since the path has only to go through any one of them to have ignition.
The expression for the final head event or system success (in this case, the plant burning down shouldn’t really be considered a success, but that is the general term used for a system) is
T ϭ ABC

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Plant burns down

Ignition
B

Combustibles

Fire out of control A

C

Smoking

Sparks

Short

B1

B2

B3

(a)
B
B1
Smoking
A

C
B2

Combustibles

Sparks

Fire out of control B3
Short

(b)
Figure 8.18

355

(a) Fault tree for a fire in the grinding shop. (b) Component approach for a fire in the grinding shop.

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where B ϭ B1 ϩ B2 ϩ B3. The probabilities are calculated as
P 1B2 ϭ 1 Ϫ 31 Ϫ P 1B1 2 4 31 Ϫ P 1B2 2 4 31 Ϫ P 1B3 2 4

ϭ 1 Ϫ 11 Ϫ 0.01 2 11 Ϫ 0.05 2 11 Ϫ 0.02 2 ϭ 0.0783

P 1T 2 ϭ P 1A2P 1B2P 1C 2 ϭ 10 .8 2 10 .0783 2 10 .1 2 ϭ 0 .0063
Thus, there is approximately a 0.6 percent chance of the plant burning down.
In spite of the relatively low probability, the company would still like to reduce this probability. Sparks are a natural part of grinding, and shorts are unpredictable occurrences. So neither is a likely avenue for control measures. Two more reasonable approaches would be to enforce the No Smoking ban with the severe punishment of immediate firing. However, even if the probability of smoking went down to zero, the resulting overall probability is still 0.0055 with only a 12 percent reduction. That may not be worth the antagonization of workers by the severe penalties. On the other hand, removing the unnecessary combustibles, perhaps oily rags, from the grinding area may significantly reduce the probability from 0.8 to 0.1. It would not be completely reduced to zero, because there still may be wooden shipping cartons for the castings.
In this case, the resulting overall probability goes to 0.00078, which almost a factorof-10 reduction and is probably more cost-effective.

COST-BENEFIT ANALYSIS
As mentioned in the previous section, a fault tree is very useful in studying a safety problem and understanding the relative contributions of various causes to the head event. However, for this approach to be ultimately effective, the cost of any controls or modifications to the system or workplace also need to be considered, and this provides the basis for cost-benefit analysis. The cost part is easy to understand; it is simply the money being spent to retrofit an old machine, to purchase a new machine or a safety device, or to train workers in a safer method, whether it is in a lump sum or prorated over some useful life.
The benefit part is a bit more difficult to understand because it is typically a reduction in accident costs or lost production costs or money saved in reduced injuries and medical costs over a period of time. Consider, for example, the medical costs associated with injuries from a 200-ton press collected over a
5-year period (see Table 8.8). The severity levels are based on workers’ compensation categories (see Section 8.6), and the costs and probabilities are derived from the company’s medical records. The severities range from relative minor skin laceration treatable with first aid to permanent partial disabilities, such as the amputation of a hand, to permanent total disabilities from a major crushing injury or even death. The total expected costs for an injury using the press can be calculated by the summation of the relative expected costs for each level of injury severity based on the product of the respective costs and the proportion for each type of injury. This resultant total expected cost of approximately $10,000 multiplied by the probability of the head event yields a measure of criticality associated with using a press. Note that this criticality is usually expressed for a given exposure time (for example, 200,000 worker-hours) or

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Example Expected Costs for an Injury from a Press

Severity

Cost ($)

Probability

Expected Cost ($)

First aid
Temporary total disability
Permanent partial disability
Permanent total disability

100
1,000
50,000
500,000

0.515
0.450
0.040
0.015

51.50
450.00
2,000.00
7,500.00
10,001.50

certain amount of production. The benefit part of the cost-benefit analysis will then be a reduction in this criticality, obtained from either reducing the probability of the head event or decreasing the severity and resulting costs of any injuries. Example 8.7 demonstrates the use of cost-benefit analysis to find an appropriate redesign of a coffee mill to prevent finger injuries. More details on fault tree and cost-benefit analyses can be found in Bahr (1997), Brown (1976),
Cox (1998), and Ericson (2005).

Fault Tree and Cost-Benefit Analyses of Coffee Mill Finger Lacerations
With the rise in popularity of speciality coffees, many consumers have purchased coffee mills in order to grind their own coffee beans for fresher coffee. As a result, there has been an increase of finger lacerations from the rotor blade due to the inadvertent activation of the coffee mill with the fingers still in the mill. Possible contributing factors to such an accident and injury are shown in the fault tree of Figure 8.19 with estimated probabilities for each event. This assumes a simple coffee mill with a switch to activate the rotor on the side of the mill. Working downward from the head event, the rotor must be in motion and the finger must be in the path of the rotor, indicating an
AND gate. The reasons for the finger being in the path of the rotor, whether to remove the ground coffee or to clean the container, could be varied and for this example are not developed further. For the rotor to be in motion, the power must be connected and the circuit closed, again indicating an AND gate. The circuit could be closed either normally or abnormally, indicating an OR gate. Note that for either scenario, there is the presumption that the finger is in the container and in the path of the rotor. The normal closure could also include possibilities that the switch failed in a closed position or that the switch was assembled in a closed position, again indicating an OR gate. The abnormal closure could be due to a variety of conditions—broken wire, incorrect wiring, conductive debris, or water in the container—only one of which needs to occur for the circuit to short and close, thus indicating an OR gate. The calculated probabilities are as follows:
P 1C1 2 ϭ 1 Ϫ 11 Ϫ 0 .001 2 11 Ϫ 0 .01 2 11 Ϫ 0 .001 2 11 Ϫ 0 .012 ϭ 0 .0199
P 1C2 2 ϭ 1 Ϫ 11 Ϫ 0 .001 2 11 Ϫ 0 .01 2 11 Ϫ 0 .001 2 ϭ 0 .1
B ϭ C1 ϩ C2
P 1B2 ϭ 1 Ϫ 11 Ϫ 0 .0199 2 11 Ϫ 0 .1 2 ϭ 0 .12
P 1A2 ϭ P 1B2 112 ϭ 0 .12

P 1H 2 ϭ P 1A2 10 .22 ϭ 0 .12 10 .22 ϭ 0 .024

EXAMPLE 8.7

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Rotor cuts finger (H)

Rotor in motion (A)
Finger in path of rotor (0.2)
Power
connected (1)

Circuit closed (B)

Switch closed (C2)

Abnormal closure (C1)

Wired incorrectly (0.001)

Broken wire
(0.001)
Conductive debris (0.01)

Figure 8.19

Moisture across path
(0.01)

Fails in closed position
(0.001)

Switch closed normally
(0.1)

Switch assembled closed
(0.001)

Fault tree for a finger injury using a coffee mill.

Assuming an expected $200 cost for finger injuries ranging from simple laceration to complete amputation (developed similarly as in Table 8.8), the resulting criticality C of a coffee mill finger injury is then
C ϭ P 1H2 1$200 2 ϭ 10 .024 2 1200 2 ϭ $4 .80
Now comes the interesting part in examining alternative redesigns and safety measures in order to reduce the likelihood of incurring a finger injury. An obvious redesign found on most coffee mills is an interlock switch in the cover of the coffee mill (interlocks are discussed in greater detail in Section 8.8). The basic premise is that the finger cannot be in the bowl and the switch activated simultaneously.
This would reduce the probability of “switch closed normally” from 0.1 to 0.0.
However, the other failure modes could still occur, and the probability of the head event would not go completely to 0.0 but instead reduces to 0.0048 with a new criticality of $0.96.
The resulting decrease in criticalities from $4.80 to $0.96 is a benefit of $3.84.
However, there is an increased associated cost of approximately $1.00 per coffee mill to insert a switch in the cover of the coffee mill versus the simpler switch in the side of the mill. Therefore, the cost-benefit (C/B) ratio is

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C>B ϭ $1 .00>$3 .84 ϭ 0 .26
An alternative, cheaper approach could be to apply a warning sticker to the side of the coffee mill stating, “Always Disconnect Power Before Removing Coffee or
Cleaning Bowl” at a minimal cost of $0.10 per mill. The probability of a “power connected” event would be reduced, perhaps to 0.3, but not to 0.0, because consumers will be likely to forget or ignore the warning. The resulting probability of the head event is reduced to 0.0072, and the criticality to $1.44. The new benefit is $3.36, yielding a cost-benefit ratio of
C>B ϭ $0 .10>$3 .36 ϭ 0 .03
This approach, on the surface, appears to be much more cost-effective. However, the probability of “power connected” is probably greatly underpredicted, as most consumers will forget to unplug the power before entering the bowl. Therefore, this would not be the preferred solution. Note that if an additional $1.00 were applied to each mill for additional quality control to catch all the wiring and switch errors before shipping, reducing each of those probabilities to 0.0, the resulting cost-benefit ratio at 1.25 is much larger than the installation of the interlock switch in the cover.

8.6 SAFETY LEGISLATION AND WORKERS’
COMPENSATION
BASICS AND TERMINOLOGY
In the United States, safety legislation, as well as the rest of the legal system, is based on a combination of common law, statute law, and administrative law.
Common law was derived from unwritten customs and typical usage in England, but adjusted and interpreted by the courts through judicial decisions. Statute law is written law enacted by legislators and enforced by the executive branch.
Administrative law is established by the executive branch or government agencies. However, since common law came first, many of our legal terms and principles are derived from that. Thus, the ancient terms of master, servant, and stranger eventually came to represent an employer, an employee, and a guest or visitor, respectively. Liability is the obligation to provide compensation for damages or injury, while strict liability is a higher level of liability, in which the plaintiff need not prove negligence or fault. The plaintiff is the person, typically injured, originating a suit in court. The defendant is the entity defending the suit, typically an employer or the manufacturer of a product. Negligence is the failure to exercise a reasonable amount of care in preventing injury. Higher forms of negligence include gross negligence, with failure to show the slightest care, and negligence per se, with no proof needed. Any resulting awards to the plaintiff fall into two categories: compensatory damages for medical costs, lost wages, and other direct losses on the part of the plaintiff, and punitive damages in the form of additional monetary amounts specifically to punish the defendant.

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Under the English common law system, and later under statute law, the employer did have some legal obligations to provide a safe workplace, protect employees against injury, and pay for injuries and damages that could result if the employer failed to fulfill those obligations. These obligations also extended to customers and the general public, for example, visitors to the workplace. However, in practice, these legal obligations didn’t amount to much as the burden of proof fell on the worker to prove in court that the employer’s negligence had been the sole cause of his or her injury. Several factors made it especially difficult for the employee to prove his or her case. First, the doctrine of privity required a direct relationship, as in the form of a contract, between the two contesting parties. Therefore, any workers not having a direct contract with the employer would likely have little success in court. Second, the assumption of risk concept implied that a worker who was aware of the hazards of the job, but continued working there, assumed the risks and could not recover damages in case of injury even though it occurred through no fault of her or his own. Third, fellow employee negligence or contributory negligence by the worker himself or herself severely limited the worker’s case. Finally, there was always the fear of loss of jobs for the worker or fellow employees, which generally restricted legal actions against employers. In addition, any legal action took many years, delaying the compensation needed for medical expenses, and resulted in inconsistent and relatively insufficient compensation, with much of the money going to the lawyers involved. As a result there arose demands for workers’ compensation legislation that would correct these inequities and force employers to take corrective action to safeguard their employees.

WORKERS’ COMPENSATION
In the United States, the first workers’ compensation laws were enacted in 1908 for federal employees and eventually were enacted in all 50 states and the U.S. territories. These all operate on the general principle of recompensing workers for medical expenses and lost wages without establishing fault. Typically they will have set amounts to be paid for given conditions and even occupations, which could vary among the states. Approximately 80 percent of the U.S. workforce is covered, with some notable exceptions: independent agricultural workers, domestic workers, some charity organizations, railroad and maritime workers, and smaller independent contractors. To ensure that worker benefits do not end if the employer were to go bankrupt, companies are required to obtain insurance. This could be on an exclusive basis through state funds set up for this purpose or on a competitive basis through private insurance companies. In some cases, very large, financially secure companies or entities can be self-insured.
There are three main requirements for a worker to claim compensation:
1. The injury must have resulted from an accident.
2. The injury must have arisen out of employment.
3. The injury must have occurred during the course of employment.

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Injuries not considered accidents include those caused by intoxication, those that are self-inflicted, or those that arose out of a heated argument. Also anything that could have occurred normally, such as a heart attack, would not be covered, unless the work were considered so stressful as to be contributing to the heart attack. Injuries arisen out of employment apply to work assigned by a supervisor or work normally expected of that employee. A typical exception that would negate compensation is doing “government work” or using company equipment for personal use. Injuries during the course of employment apply to normal work-hours and not to commuting time to and from work, unless the company provides transportation.
Workers’ compensation is typically broken down into four categories of disability: (1) temporary partial, (2) temporary total, (3) permanent partial, and (4) permanent total. Temporary partial disabilities are ones in which the worker receives a minor injury and full recovery is expected. The worker can still perform most duties but may suffer some lost time and/or wages. Temporary total disabilities are ones in which the worker is incapable of performing any work for a limited time, but full recovery is expected. This category accounts for the majority of workers’ compensation cases. Permanent partial disabilities are ones in which the worker will not fully recover from injuries but can still perform some work. This category accounts for the majority of workers’ compensation costs and is further subdivided into schedule and nonschedule injuries. A schedule injury receives a specific payment for a specified time according to a schedule, as shown in Table 8.9. Note that there may be considerable differences in payments among states, as shown between federal and Pennsylvania workers. A nonschedule injury is of less specific nature, such as disfigurement, with payments prorated to a schedule injury. Permanent total disabilities are sufficiently serious that they will prevent the employee from ever working in regular employment. Again, there may be considerable differences in what constitutes total disability, but in many states this is accorded by the loss of sight in both eyes or the loss of both arms or both legs. In approximately one-half of the states, the compensation is for the duration of the disability or the injured worker’s lifetime. In the other half, the duration is limited to 500 weeks. The compensation is some percentage of wages, the majority being two-thirds. In case of the worker’s death, benefits are paid to the widow

Table 8.9

Lost-Limb Award Schedule (in weeks) for a Permanent Partial Disability

Amputation or 100% Loss of Use
Arm
Leg
Hand
Foot
Eye
Thumb
First finger (little)
Great toe (other)
Hearing—one ear (both)

Federal
312
288
244
205
160
75
46 (15)
38 (16)
52 (200)

Pennsylvania
410
410
325
250
275
100
50 (28)
40 (16)
60 (260)

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for life or until remarriage and to children until the age of 18 or until the maximum period of payments (for example, 500 weeks).
There may be some other important conditions depending on the states. In some cases, the companies may be able to require the injured worker to see a company physician and to perform suitable lighter-duty employment. If the worker refuses, workers’ compensation benefits can be terminated. In most cases, though, the workers’ compensation cases are settled quickly and amicably, and the worker receives a direct settlement. In some situations, the case can be contested with the employee and employer settling either directly or through the workers’ compensation legal system. From the worker’s standpoint, it is generally a positive trade-off—she or he accepts a guaranteed lesser amount of compensation in place of the uncertain option to sue the employer for negligence. However, the worker does not give up the rights to sue a third party, such as the manufacturer of faulty equipment or defective tools, the architect or contractor of faulty construction, or even an inspection agency that certified the safety of a building or machines.
From the company’s perspective, it is important to try to decrease workers’ compensation costs as much as possible. This can be done through a variety of means. First and foremost, implement a safety program so as to reduce workplace hazards and to train operators in the proper procedures. Second, and almost equally important, implement a proper medical management program. This means hiring a good occupational nurse and selecting a knowledgeable local physician to visit the plant and understand the various jobs. This will aid in proper diagnoses and the assignment of workers to light-duty jobs. It is also very important to get the injured employees back to work as quickly as possible, even if only on a lightduty job. Third, review the employment classification of each employee based on the job she or he performs. It makes no sense to have a misclassification result in increased premiums, for example, an office worker misclassified as a grinder operator. Fourth, conduct a thorough payroll audit. Overtime is charged as straight time on workers’ compensation. Therefore, double overtime wages of $20 an hour would greatly inflate the company’s costs compared to straight $10 an hour charges. Fifth, compare self-insurance and various group insurance programs for lowest cost, and use a deductible. Sixth, check your mod ratio frequently. This is the ratio of actual losses to losses expected of similar employers, with 1.00 being average. By implementing a good safety program and reducing accidents, injuries, and consequently workers’ compensation claims, the mod ratio will drop significantly. A mod ratio of 0.85 means a 15 percent savings on premiums. Through proper management, workers’ compensation costs can be controlled.

8.7 OCCUPATIONAL SAFETY AND HEALTH
ADMINISTRATION (OSHA)
OSHA ACT
The Occupational Safety and Health Act of 1970 was passed by Congress “to assure so far as possible every working man and woman in the Nation safe and

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healthful working conditions and to preserve our human resources.” Under the act, the Occupational Safety and Health Administration was created to
1. Encourage employers and employees to reduce workplace hazards and to implement new, or improve existing, safety and health programs.
2. Establish “separate but dependent responsibilities and rights” for employers and employees for the achievement of better safety and health conditions.
3. Maintain a reporting and record-keeping system to monitor job-related injuries and illnesses.
4. Develop mandatory job safety and health standards, and enforce them effectively. 5. Provide for the development, analysis, evaluation, and approval of state occupational safety and health programs.
Since the act can intimately affect the design of the workplace, methods analysts should be knowledgeable regarding the details of this act. The general-duty clause of the act states that each employer “must furnish a place of employment which is free from recognized hazards that cause or are likely to cause death or serious physical harm to employees.” Furthermore, the act brings out that it is the employers’ responsibility to become familiar with standards applicable to their establishments and to ensure that employees have and use personal protective gear and equipment for safety.
OSHA standards fall into four categories: general industry, maritime, construction, and agriculture. All OSHA standards are published in the Federal
Register, which is available in most public libraries, in a separate book of regulations (OSHA, 1997), and on the Web (http://www.osha.gov/). OSHA can begin standards-setting procedures on its own initiative or on the basis of petitions from the Secretary of Health and Human Services (HHS), the National Institute for Occupational Safety and Health (NIOSH), state and local governments, nationally recognized standards-producing organizations such as the ASME, and employer or labor representatives. Of these groups, NIOSH, an agency of HHS, is quite active in making recommendations for standards. It conducts research on various safety and health problems and provides considerable technical assistance to
OSHA. Especially important are the investigation of toxic substances by NIOSH and its development of criteria for the use of such substances in the workplace.
OSHA also provides free on-site consultation services for employers in all
50 states. This service is available on request, and priority is given to smaller businesses, which are generally less able to afford private-sector consultations.
These consultants help employers identify hazardous conditions and determine corrective measures. A listing of such consultants is found on OSHA’s Web page
(http://www.osha.gov/dcsp/smallbusiness/consult_directory.html).
The act also requires employers of 11 or more employees to maintain records of occupational injuries and illnesses on the OSHA 300 log. An occupational injury is defined as “any injury such as a cut, fracture, sprain or amputation which results from a work-related accident or from exposure involving a single incident in the work environment.” An occupational illness is “any abnormal condition or

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disorder, other than one resulting from an occupational injury, caused by exposure to environmental factors associated with employment.” Occupational illnesses include acute and chronic illnesses that may be caused by inhalation, absorption, ingestion, or direct contact with toxic substances or harmful agents. Specifically, they must be recorded if the result is death, loss of one or more workdays, restriction in motion or ability to do the work that had been done, loss of consciousness, transfer to another job, or medical treatment other than first aid.

WORKPLACE INSPECTIONS
To enforce its standards, OSHA is authorized to conduct workplace inspections.
Consequently, every establishment covered by the act is subject to inspection by
OSHA compliance safety and health officers. The act states that “upon presenting appropriate credentials to the owner, operator, or agent in charge,” an OSHA compliance officer is authorized to enter without delay any factory or workplace to inspect all pertinent conditions, equipment, and materials therein and to question the employer, operator, or employees.
OSHA inspections, with few exceptions, are concluded without advance notice. In fact, alerting an employer in advance of an OSHA inspection can bring a fine of up to $1,000 and/or a 6-month jail term. Special circumstances under which OSHA may give notice of inspection to an employer include those where
1. Imminently dangerous situations exist that require correction as soon as possible. 2. Inspections necessitate special preparation or must take place after regular business hours.
3. Prior notice ensures that the employer and employee representatives or other personnel will be present.
4. The OSHA area director determines that advance notice would produce a more thorough or more effective inspection.
Upon inspection, if an imminently dangerous situation is found, the compliance officer asks the employer to abate the hazard voluntarily and to remove endangered employees from exposure. Notice of the imminent danger must also be posted. Before the OSHA inspector leaves the workplace, he or she will advise all affected employees of the hazard.
At the time of the inspection, the employer is asked to select an employer representative to accompany the compliance officer during the inspection. An authorized employee representative is also given the opportunity to attend the opening conference and to accompany the compliance officer during the inspection. In those plants with a union, the union ordinarily designates the employee representative to accompany the compliance officer. Under no circumstances may the employer select the employee representative for the inspection. The act does not require an employee representative for each inspection; however, where there is no authorized employee representative, the compliance officer must consult with a reasonable number of employees concerning safety and health matters in the workplace.

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After the inspection tour, a closing conference is held between the compliance officer and the employer or the employer representative. Subsequently, the compliance officer reports the findings to the OSHA office, and the area director determines what citations, if any, will be issued and what penalties, if any, will be proposed. CITATIONS
Citations inform the employer and employees of the regulations and standards alleged to have been violated, and the proposed time set for their abatement. The employer will receive citations and notices of proposed penalties by certified mail. The employer must post a copy of each citation at or near the place where a violation has occurred, for three days or until the violation is abated, whichever is longer.
The compliance officer has the authority to issue citations at the worksite, following the closing conference. To do so, he or she must first discuss each apparent violation with the area director and must receive approval to issue the citations. The six types of violations that may be cited, and the penalties that may be imposed, are as follows:
1. De minimis (no penalty). This type of violation has no immediate relationship to safety or health, for example, number of toilets.
2. Nonserious violation. This type of violation has a direct relationship to job safety and health, but probably would not cause death or serious physical harm. A proposed penalty of up to $7,000 for each violation is discretionary. A penalty for a nonserious violation may be decreased considerably depending on the employer’s good faith (demonstrated efforts to comply with the act), history of previous violations, and size of business.
3. Serious violation. This is a violation in which there is substantial probability that death or serious harm could result, stemming from a hazard about which the employer knew or should have known. A mandatory penalty of up to $7,000 is assessed for each violation.
4. Willful violation. This is a violation that the employer intentionally and knowingly commits. The employer either knows that his or her actions constitute a violation, or is aware that a hazardous condition exists and has made no reasonable effort to eliminate it. Penalties of up to $70,000 may be proposed for each willful violation. If an employer is convicted of a willful violation that has resulted in the death of an employee, there may also be imprisonment for up to 6 months. A second conviction doubles these maximum penalties.
5. Repeated violation. A repeated violation occurs when a violation of any standard, regulation, rule, or order is reinspected and another violation of the previously cited section is found. If, on reinspection, a violation of the previously cited standard, regulation, rule, or order is found, but it involves

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another piece of equipment and/or a different location in the establishment or worksite, it may be considered a repeated violation. Each repeated violation can bring a fine of up to $70,000. If there is a finding of guilt in a criminal proceeding, then up to 6 months’ imprisonment and a $250,000 fine for an individual or a $500,000 fine for a corporation may be imposed.
6. Imminent danger. This is a situation in which there is reasonable certainty that a danger exists that can be expected to cause death or serious physical harm either immediately or before the danger can be eliminated through normal enforcement procedures. An imminent danger violation may result in a cessation of the operation or even complete plant shutdown.
Other violations for which citations and proposed penalties may be issued are as follows:
1. Falsifying records, reports, or applications, on conviction, can bring a fine of $10,000 and 6 months in jail.
2. Violating the posting requirements can bring a civil penalty of up to
$7,000.
3. Failing to abate or correct a violation can bring a civil penalty of up to
$7,000 for each day the violation continues beyond the prescribed abatement date.
4. Assaulting, interfering with, or resisting an inspector in his or her duties can result in a fine of up to $5,000 and imprisonment for up to 3 years.

OSHA ERGONOMICS PROGRAM
In 1990, the high incidences and severity of work-related musculoskeletal disorders found in the meatpacking industry led OSHA to develop ergonomics guidelines to be used in protecting meatpackers from these hazards (OSHA, 1990).
The publication and dissemination of these guidelines were meant to be a first step in assisting the meatpacking industry in implementing a comprehensive safety and health program that would include ergonomics. Although the guidelines were initially meant to be advisory in nature, they were eventually developed into new industrywide ergonomics standards. The guidelines were meant to provide information so that the employers could determine if they have ergonomics-type problems, identify the nature and location of those problems, and implement measures to reduce or eliminate them.
The ergonomics program for meatpacking plants is divided into five sections: (1) management commitment and employee involvement, (2) worksite analysis, (3) recommended hazard prevention and controls, (4) medical management, and (5) training and education. Detailed examples tailored for the meatpacking industry are also provided.
Commitment and involvement are essential elements in any sound safety and health program. Commitment by management is especially important in providing both the motivating force and the necessary resources to solve the problems.
Similarly, employee involvement is necessary to maintain and continue the

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program. An effective program should have a team approach, with top management as the team leader, using the following principles:
1. A written program for job safety, health, and ergonomics, with clear goals and objectives to meet these goals, endorsed and advocated by the highest levels of management.
2. A personal concern for employee health and safety, emphasizing the elimination of ergonomics hazards
3. A policy that places the same emphasis on health and safety as on production.
4. Assignment and communication of the responsibility of the ergonomics program to the appropriate managers, supervisors, and employees.
5. A program ensuring accountability from these managers, supervisors, and employees for carrying out these responsibilities.
6. Implementation of a regular review and evaluation of the ergonomics program. This might include trend analyses of injury data, employee surveys, “before and after” evaluations of workplace changes, logs of job improvements, etc.
Employees can be involved via the following:
1. A complaint or suggestion procedure for voicing their concerns to management without fear of reprisal
2. A procedure for prompt and accurate recording of the first signs of workrelated musculoskeletal disorders, so that prompt controls and treatment can be implemented
3. Ergonomics committees that receive reports of, analyze, and correct ergonomics problems
4. Ergonomics teams with the required skills to identify and analyze jobs for ergonomics stress
An effective ergonomics program includes four major program elements: worksite analysis, hazard control, medical management, and training and education.
Worksite analysis identifies existing hazards and conditions, as well as operations and workplaces where such hazards may develop. The analysis includes a detailed tracking and statistical analysis of injury and illness records, to identify patterns of work-related musculoskeletal disorder development. The first step in implementing the analysis program should be a review and analysis of medical records, insurance records, and OSHA 300 logs using chi-square analysis and tracking incidence rates.
Next, baseline screening surveys can be conducted to identify jobs that put employees at risk of developing work-related musculoskeletal disorders. The survey is typically performed with a questionnaire to identify potential ergonomics risk factors in the job process, workplace, or work method, as well as the location and severity of the potential musculoskeletal problems for the individual worker, using the body discomfort charts of Chapter 5. Then a physical worksite analysis should be conducted with a walk-through of the plant and videotaping and analysis of critical jobs, using the work design checklists and analyses tools presented in earlier

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chapters. Finally, as in any methods program, periodic reviews should be conducted.
These may uncover previously missed risk factors or design deficiencies. Trends of injuries and illnesses should be calculated and examined at regular intervals, as a quantitative check on the effectiveness of the ergonomics program.
Hazard control involves the same engineering controls, work practice controls, personal protective equipment, and administrative controls as discussed throughout this book. Engineering controls, where feasible, are the OSHApreferred method of control.
Proper medical management, including the early identification of signs and the effective treatment of symptoms, is necessary to reduce the risk of developing work-related musculoskeletal disorders. A physician or occupational nurse with experience in musculoskeletal disorders should supervise the program. The person should conduct periodic, systematic workplace walk-throughs to remain knowledgeable about the jobs, identify potential light-duty jobs, and maintain close contact with employees. This information will allow the health providers to recommend assignments of recovering workers to restricted-duty jobs with minimal ergonomic stress on the injured muscle and tendon groups.
Health care providers should participate in the training and education of all employees, including supervisors, on different types of work-related musculoskeletal disorders, means of prevention, causes, early symptoms, and treatments. This demonstration will assist in the early detection of work-related musculoskeletal disorders prior to the development of more severe conditions.
Employees should be encouraged to report early signs and symptoms of workrelated musculoskeletal disorders, for timely treatment without fear of retribution by management. Written protocols for health surveillance, evaluation, and treatment will assist in maintaining properly controlled procedures.
Training and education are critical components of an ergonomics program for employees potentially exposed to ergonomics hazards. Training allows managers, supervisors, and employees to understand the ergonomics problems associated with their jobs, as well as the prevention, control, and medical consequences of those problems.
1. General training on work-related musculoskeletal disorder risk factors, symptoms, and hazards associated with the job should be given annually to those employees who are potentially exposed.
2. Job-specific training on tools, knives, guards, safety, and proper lifting should be given to new employees prior to their being placed on a full-time job.
3. Supervisors should be trained to recognize the early signs of work-related musculoskeletal disorders and hazardous work practices.
4. Managers should be trained to be aware of their health and safety responsibilities. 5. Engineers should be trained in the prevention and correction of ergonomics hazards through workplace redesign.
A rough-draft version of the guidelines for general industry, as a precursor to an ergonomics standard, was released in 1990, and the final version was signed

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early in 1992. It contained primarily the same information as found in the guidelines for the meatpacking industry. However, there was considerable negative reaction from industry, and with the Republicans gaining control of Congress in
1992, the ergonomics standard was effectively shelved for the time being.

8.8

HAZARD CONTROL

This section presents the basic principles in controlling hazards. A hazard is a condition with the potential of causing injury or damage while danger is the relative exposure to or potential consequences of that hazard. Thus, an unprotected worker on scaffold is exposed to a hazard and has the danger of serious injury. If the worker wears a safety harness, there is still a hazard, but the danger of the hazard has been reduced considerably.
Hazards can occur in several general categories: (1) due to inherent properties such as high voltage, radiation, or caustic chemicals; (2) due to potential failure, either of the operator (or some other person) or of the machine (or some other equipment); or (3) due to environmental forces or stresses, for example, wind, corrosion, etc. The general approach is to first completely eliminate the hazard and prevent the accident, and then, if not successful, to reduce the hazard level to the point that, should the accident still happen, the potential injury or damage is minimized. Elimination of a hazard can be achieved through good design and proper procedures, for example, use of noncombustible materials and solvents, rounding edges on equipment, automating corrosive dips (that is, removing the operator from the hazardous environment, building an overpass at railroad and highway intersections, etc.).
If the hazard cannot be completely eliminated, then a second-level approach is to limit the hazard level. For example, an electric power drill in a wet environment has the potential for electrocution. Using a cordless drill would reduce the power level for serious injury, although some shock may still occur. Of course, the trade-off is a reduced torque level and drilling effectiveness. Using a pneumatic drill would completely eliminate the electrocution hazard, but may have limited use, especially for homeowners who don’t have compressed air available, may increase the cost of the drill, and, may introduce a new hazard with the release of high-pressure air. The safest solution would be to use a mechanical hand drill, with no minimal hazards due to energy. However, the effectiveness of the tool would be significantly reduced and could introduce musculoskeletal fatigue (a completely separate hazard). Another example of limiting the hazard level is the use of governors on school buses to limit the maximum speed of the vehicles.
If the hazard level cannot be limited due to the inherent nature of electromechanical equipment or power tools, the next approach is to use isolation, barriers, and interlocks to minimize the contact between the energy source and the human operator. Isolation and barriers impose either a distance or a physical impediment between the two. Placing a generator or a compressor outside the plant will limit normal daily contact between operators and the energy source.
Only maintenance workers at irregular intervals will have some exposure to the energy source. Fixed machine guards or pulley enclosures are good examples of

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barriers. The third item, an interlock, is a more complex approach or device that prevents incompatible events from occurring at the wrong time. At the most basic level it could simply be a lockout (as in OSHA 1910.147, Lockout/Tagout) of a dangerous area to prevent unauthorized individuals from entering this area or a lock in of a switch in an energized status so that it can’t be accidentally turned off. The more typical active interlock is a mechanism that ensures that a given event doesn’t occur at the same time as another event. The previously mentioned switch in the cover of a coffee mill (Example 8.7) is a good example of an interlock that prevents the user’s finger from being in the bowl at the same time that the switch is activated.
Another approach is the use of fail-safe designs. Systems are designed such that, in the case of failure, they go to the lowest energy level. This can be accomplished through simple, passive devices such as fuses and circuit breakers, which upon experiencing high current levels physically open the circuit and drop the current immediately to zero. This can also be accomplished operationally through the design of valves that either fail in an open position so as to maintain fluid flow (that is, the valve disk is forced away from the seat by the flow) or fail in a closed position to stop current flow (that is, the valve disk is forced into the seat by the flow). Another good example of an operational design is the deadman control on a lawnmower or a waverunner. In the first case, the operator holds down a lever to keep the blade turning; if the operator trips and loses control of the lever, the blade stops through either release of the clutch or cutting of the engine.
In the second case, the engine key is attached to the operator’s wrist by a leash; if the operator is thrown from the craft, the key is pulled out, stopping the engine.
Another approach in hazard control is failure minimization. Rather than allow the system to fail completely even in a fail-safe mode, this approach decreases the probability of system failure. This can be accomplished by increasing safety factors, monitoring system parameters more closely, and replacing key components regularly or providing redundancy for these components. Safety factor is defined as the ratio of strength to stress and should obviously be well above 1. Given that there can be considerable variance in the strength of material, for example, two-by-fours used in construction and also some variance in environmental stresses, for example, snow in northern locales, it would make sense to increase the safety factor appropriately to account for these variances and minimize the collapse of the building. Monitoring of key temperatures and pressures, with appropriate adjustments or compensations, can help forestall the system reaching critical levels. Automobile tire wear markers are one commonly used parameter monitoring system. The OSHA requirement of a buddy system for work in hazardous areas is another such example. Regular replacement of the above tires even before the wear markers are exposed is an example of regular replacement of components, or using more two-by-fours (set at 12-in distances, rather than 16 in) would be an example of redundancy in the system.
Finally, in case the system does finally fail, the organization must provide for personal protective equipment, escape and survival equipment, and rescue equipment so as to minimize the resulting injuries and costs. Fire protective clothing, helmets, safety shoes, earplugs, etc., are common examples of personal protective

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equipment to minimize injury. The storage of self-rescuers at regular locations in mines provides miners extra oxygen in case of methane leaks or fires using up the ambient oxygen and extra time until rescuers can reach them. Similarly, large companies will have their own firefighting equipment to save time compared to relying on local fire departments.

8.9

GENERAL HOUSEKEEPING

General safety considerations related to the building include adequate floorloading capacity. This is especially important in storage areas, where overloading may cause serious accidents. The danger signs of overloading include cracks in walls or ceilings, excessive vibration, and displacement of structural members.
Aisles, stairs, and other walkways should be investigated periodically to ensure that they are free of obstacles, are not uneven, and are not covered with oil or other material that could lead to slips and falls. In many old buildings, stairs should be inspected, since they are the cause of numerous lost-time accidents.
Stairs should have a slope of 28 to 35 degrees, with tread widths of 11 to 12 in
(28 to 30.5 cm) and riser heights of 6.5 to 7.5 in (16.5 to 19 cm). All stairways should be equipped with handrails, should have at least 10 fc (100 lx) of illumination, and be painted in light colors.
Aisles should be plainly marked and straight, with well-rounded corners or diagonals at turn points. If aisles are to accommodate vehicle travel, they should be at least 3 ft wider than twice the width of the broadest vehicle. When traffic is only one-way, then 2 ft wider than the broadest vehicle is adequate. In general, aisles should have at least 10 fc (100 lx) of illumination. Color should be used throughout, to identify hazardous conditions (see Table 8.10). More details on aisles, stairs, and walkways can be found in OSHA 1910.21-1910.24, Walking and Working Surfaces.
Most machine tools can be satisfactorily guarded to minimize the probability of a worker being injured while operating the machine. The problem is that many older machines are not properly guarded. In these instances, immediate action should be taken to see that a guard is provided and that it is workable and routinely used. An alternate approach is to provide a two-button operation, such as that shown for the press operations in Figure 8.20. Note that the two hand buttons are spread well apart, so the operator’s hands are in a safe position when the press starts. These buttons should not require high levels of force; otherwise repetitive-motion injuries are likely to occur. In fact newer buttons can be activated through skin capacitance rather than relying on mechanical pressure. A better alternative may be to automate the process, completely freeing the operator from the nip point or using a robotic manipulator in place of the operator. Further details on machine guarding can be found in OSHA 1910.211-1901.222, Machinery and Machine Guarding.
A quality control and maintenance system should be incorporated in the tool room and the tool cribs, so that only reliable tools in good working condition are released to workers. Examples of unsafe tools that should not be released to operators include power tools with broken insulation, electrically driven power tools lacking grounding plugs or wires, poorly sharpened tools, hammers with

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Table 8.10

Color

Color Recommendations

Used for:

Examples

Red

Fire protection equipment, Fire alarm boxes, location of fire danger, and as a stop signal extinguishers and fire hose, sprinkler piping, safety cans for flammables, danger signs, emergency stop buttons
Orange
Dangerous parts of
Inside of movable guards, safety machines, other hazards starting buttons, edges of exposed parts of moving equipment
Yellow
Designating caution,
Construction and material handling physical hazards equipment, corner markings, edges of platforms, pits, stair treads, projections. Black stripes or checks may be used in conjunction with yellow
Green
Safety
Location of first-aid equipment, gas masks, safety deluge showers
Blue
Designating caution
Warning flags at starting point of against starting or using machines, electrical controls, valves equipment about tanks and boilers
Purple
Radiation hazards
Container for radioactive materials or sources
Black and white Traffic and housekeeping
Location of aisles, direction signs, clear markings floor areas around emergency equipment

mushroomed heads, cracked grinding wheels, grinding wheels without guards, and tools with split handles or sprung jaws.
There are also potentially dangerous materials and hazardous chemicals to be considered.These materials can cause a variety of health and/or safety problems and typically fall into one of three categories: corrosive materials, toxic materials, and flammable materials. Corrosive materials include a variety of acids and caustics that can burn and destroy human tissue upon contact. The chemical action of corrosive materials can take place by direct contact with the skin or through the inhalation of fumes or vapors. To avoid the potential danger resulting from the use of corrosive materials, consider the following measures:
1. Be sure that the material handling methods are completely foolproof.
2. Avoid any spilling or spattering, especially during initial delivery processes.
3. Be sure that operators exposed to corrosive materials have used and are using correctly designed personal protective equipment and waste disposal procedures. 4. Ensure that the dispensary or the first-aid area is equipped with the necessary emergency provisions, including deluge showers and eye baths.
Toxic or irritating materials include gases, liquids, or solids that poison the body or disrupt normal processes by ingestion, absorption through the skin, or inhalation. To control toxic materials, use the following methods:
1. Completely isolate the process from workers.
2. Provide adequate exhaust ventilation.

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Figure 8.20

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Two-button press operation. © Morton Beebe/CORBIS.

3. Provide workers with reliable personal protective equipment.
4. Substitute a nontoxic or nonirritating material, wherever possible.
More details on toxic materials can be found in OSHA 1910.1000-1910.1200,
Toxic and Hazardous Substances.
Furthermore, per OSHA regulations, the composition of every chemical compound must be ascertained, its hazards determined, and appropriate control measures established to protect employees. This information must be clearly presented to workers with clear labels and material safety data sheets (MSDSs).
Further information on this process (termed HAZCOM) can be found in OSHA
1910.1200, Hazard Communications.
Flammable materials and strong oxidizing agents present fire and explosion hazards. The spontaneous ignition of combustible materials can take place when there is insufficient ventilation to remove the heat from a process of slow oxidation. To prevent such fires, combustible materials need to be stored in a well-ventilated, cool, dry area. Small quantities should be stored in

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Work Design, 12th Edition

8. Workplace and Systems
Safety

Text

© The McGraw−Hill
Companies, 2009

CHAPTER 8

covered metal containers. Some combustible dusts, such as sawdust, are not ordinarily known to be explosive. However, explosions can occur when such dusts are in a fine enough state to ignite. To avoid explosions, prevent ignition by providing adequate ventilation exhaust systems and by controlling the manufacturing processes to minimize the generation of dust and the liberation of gases and vapors. Gases and vapors may be removed from gas streams by absorption in liquids or solids, adsorption on solids, condensation, and catalytic combustion and incineration. In absorption, the gas or vapor becomes distributed in the collecting liquid found in absorption towers, such as bubble-cap plate columns, packed towers, spray towers, and wet-cell washers.
The adsorption of gases and vapors uses a variety of solid adsorbents such as charcoal with an affinity for certain substances such as benzene, carbon tetrachloride, chloroform, nitrous oxide, and acetaldehyde. Further information on flammable materials can be found in OSHA 1910.106, Flammable and
Combustible Liquids.
In case the flammable materials ignite, suppression of the resulting fire is based on the relatively simple principles of the fire triangle (although the actual implementation may not always be so simple). There are three required components, or legs of a triangle, to a fire: oxygen (or oxidizer in chemical reactions), fuel (or reducing agent in chemical reactions), and heat or ignition. Removal of any one component will suppress the fire (or collapse the triangle). Spraying of water on a house fire cools the fire (removes heat) and also dilutes the oxygen.
Using foam on fire (or covering with a blanket) removes oxygen from the fire.
Spreading out the logs in a campfire removes fuel. More practically, in a plant, there will be both fixed extinguishing systems such as water sprinklers and portable fire extinguishers. These are categorized by types and sizes. The four basic types are class A, for ordinary combustibles and could use water or foam; class B, for flammable liquids typically using foam; class C, for electrical equipment using nonconductive foams; and class D for oxidizable metals. Further information on fire suppression can be found in OSHA 1910.155-1910-165, Fire
Protection.

SUMMARY
This chapter covered the basics on safety, including the accident prevention process, starting with various theories on accident causation; the use of probability in understanding system reliability, risk management, and fault tree analysis; the use of cost-benefit analysis and other tools for decision making; various statistical tools for monitoring the success of the safety program; basic hazard control; and federal safety regulations relating to industry. Only the basics of hazard control were presented here. Specific details on specific workplace hazards can be found in numerous traditional safety textbooks such as by Asfahl (2004),
Banerjee (2003), Goetsch (2005), Hammer and Price (2001), National Safety
Council (2000), and Spellman (2005). However, there should be sufficient information for the industrial engineer to start a safety program with the goal of providing a safe working environment for the employees.

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8. Workplace and Systems
Safety

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Companies, 2009

Niebel's Methods, Standards, and Work Design, 12th Edition

CHAPTER 8

Workplace and Systems Safety

QUESTIONS
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What is the difference between accident prevention and safety management?
What are the steps in the accident prevention process?
Describe the “dominoes” of the domino theory? What is the key aspect to this approach?
How does multiple causation affect accident prevention?
Compare and contrast life-change-unit, motivation-reward-satisfaction, and ABC models. What is the common link for all these models?
Explain the significance of using chi-square analysis in accident prevention.
What is the purpose of risk analysis in accident prevention?
What is red flagging?
Discuss the difference between independent and mutually exclusive events.
In what ways can the reliability of a system be improved?
Compare and contrast AND and OR gates.
What is criticality and what role does it play in cost-benefit analysis?
Compare and contrast common law and statute law.
What is the difference between liability and strict liability?
What is the difference between negligence, gross negligence, and negligence per se?
What is the difference between compensatory and punitive damages?
Prior to workers’ compensation, what three common law conditions were used by employers to disqualify an injured worker from receiving benefits?
What are the three main requirements to obtain workers’ compensation?
Compare and contrast the four categories of disabilities recognized under workers’ compensation. What is the difference between schedule and nonschedule injuries?
What is a third-party suit?
What are some ways that a company could try to keep its workers’ comp costs down?
Why is the OSHA general duty clause so important?
What types of citations can be issued by OSHA?
What are the key elements of OSHA’s proposed ergonomics program?
What is the difference between a hazard and a danger?
What is the general approach used in hazard control?
Explain why a deadman switch is a good example of a fail-safe design. Give an example where it might be used.
What is a safety factor?
What is the fire triangle? Explain how its principles are used in fire extinguishers.

PROBLEMS
1.

For the injury data given in the table below:
a. What are the incidence and severity rates for each department?
b. Which department has significantly more injuries than the others?
c. Which department has a significantly higher severity rate than the others?
d. As a safety specialist, which department would you tackle first? Why?

375

367

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Methods, Standards, and
Work Design, 12th Edition

8. Workplace and Systems
Safety

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Companies, 2009

Text

CHAPTER 8

Department
Casting
Spruing
Shearing
Grinding
Packing
2.

Injuries

Lost Days

Hours Worked

13
2
5
6
1

3
0
1
3
3

450,000
100,000
200,000
600,000
500,000

An engineer was adjusting the gearbox of a large steam engine while it was in operation and dropped a wrench into the path of the gears. As a result, the engine was thrown out of alignment and badly wrecked. Luckily the engineer only suffered minor lacerations from ejected pieces of metal. It was hypothesized by the engineer that the metal-handle wrench simply slipped out of his oily hands.
a. Use the domino and multiple-causation theories to examine this accident scenario. b. Use job safety analysis to suggest control measures (and indicate relative effectiveness for each) that could have prevented the injuries and damage.
What is your ultimate recommendation?
3. Given P(A) ϭ 0.6, P(B) ϭ 0.7, P(C) ϭ 0.8, P(D) ϭ 0.9, P(E) ϭ 0.1 and independent events, determine P(T) for
a. T ϭ AB ϩ AC ϩ DE
b. T ϭ A ϩ ABC ϩ DE
c. T ϭ ABD ϩ BC ϩ E
d. T ϭ A ϩ B ϩ CDE
e. T ϭ ABC ϩ BCD ϩ CDE
4. Perform a cost-benefit analysis using fault tree analysis on a stairway. Assume that over the last year, three accidents were caused by slippery surfaces, five were caused by inadequate railings, and three were caused by someone negligently leaving tools or other obstacles on the steps. The average cost for each accident was $200 (total for first aid, lost time, etc.). Assume that you have been allocated
$1,000 to improve the safety of the stairway (although you don’t need to spend all the money). Three alternatives could be employed:
1. New surfaces, which will reduce accidents caused by slippery surfaces by
70 percent and cost $800
2. New railings, which will reduce accidents caused by inadequate railings by only 50 percent (since not all pedestrians use railings!) and cost $1,000
3. Signs and educational programs, which are estimated to reduce both railing and obstacle-related accidents, each by 20 percent (people forget easily) but cost only $100
(To calculate basic event probabilities, assume the stairway is used 5 times per hour, 8 h/day, 5 days/week, 50 weeks/year.)
a. Draw a fault tree of the situation.
b. Evaluate all alternatives (or combinations) to determine the best allocation of the $1,000.
5. Widgets are painted and cured with heat from a drier in the paint shop. Three components are required to start a fire in the paint shop and cause major damage: fuel,

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Methods, Standards, and
Work Design, 12th Edition

8. Workplace and Systems
Safety

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Companies, 2009

Text

Niebel's Methods, Standards, and Work Design, 12th Edition

CHAPTER 8

Workplace and Systems Safety

ignition, and oxygen. Oxygen is always present in the atmosphere and thus has a probability of 1.0. Ignition can occur either from a spark due to static electricity (with a probability of 0.01) or from overheating of the drier mechanism (0.05). Fuel is provided by the volatile vapors that can arise from three sources: paint vapors during the drying process (0.9), paint thinner used to thin the paint (0.9), and solvent used for the cleanup of equipment (0.3). Property damage from a fire could amount to as much as $100,000.
You have three choices of solutions in minimizing the likelihood of a major fire:
1. Spend $50 to move cleanup operations to a different room, which would reduce the probability of having solvent vapors in the paint area to 0.0.
2. Spend $3,000 on a new ventilation system which reduces the probability of having vapor from each of the three fuels to 0.2.
3. Spend $10,000 for a new type of paint and spraying system that doesn’t release volatile vapors, reducing the probability of paint and paint thinner vapors to 0.0.
a. Draw a fault tree and recommend the most cost-effective solution.
b. Consider the domino theory as applied to this scenario. Name each of the dominoes in the proper order, and provide two other possible solutions that would apply to this scenario.
6. a. Draw the fault tree given by the Boolean expression T ϭ AB ϩ CDE ϩ F.
b. The severity of T is 100 lost workdays per accident. The probabilities of the basic events are A ϭ 0.02, B ϭ 0.03, C ϭ 0.01, D ϭ 0.05, E ϭ 0.04, and F ϭ 0.05. What is the expected loss associated with the head event T as given?
c. Compare two alternatives from a cost-benefit standpoint. Which would you recommend? (i) 100 to reduce C and D to 0.005
(ii) $200 to reduce F to 0.01
7. Calculate the system reliability. Assume events are independent.
A

B

0.9

0.7
C
0.8
D

0.9

E
0.9

8. One of the early propeller planes (a Ford Tri-Motor) had three engines, one directly in the middle and one on each wing. In this configuration, the plane can fly with any two engines or just the middle one alone. Assuming the reliability of each engine is 0.9, what is the reliability of the overall plane?
9. NASA uses four identical onboard computers (that is, thre