Composite Fibreglass Helical Spring

Objective
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To make a helical spring from fibreglass composite with the following properties:

D(mean) = 43 mm D(internal)>30mm K = 250 lb/in Compression max = 3 in = 75mm Length = 130mm N = 6 active + 2 inactive

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Materials Used for Final Sample
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Unidirectional Fibreglass Roving Araldite (epoxy)

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Properties of the Composite
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Taken from the book ”mechanics of composite materials” by Robert M. Jones
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E1 = 7.8 X 10^6 psi E2 = 2.6 X 10^6 psi G12 = 1.3 X 10^6 psi Xt = 150 X 10^3 psi = Xc Yt = 4 X 10^3 psi S = 6 X 10^3 psi Yc = 20 X 10^3 psi

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Formulae's used to Design
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Rate (N/mm) = K 2 G b t 3/ (N D 3) Stress (N/mm 2) = K 1 W D /( b t 2 )
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* b = largest section dimension(mm) * t = Smallest Section dimension(mm) * K 1 = Shape Factor (see table) * K 2 = Shape Factor (see table) * C = Spring Index = D/(radial dimension = b or t)

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For b=t, K 1=2.41, K 2=0.18

Theoritical Parameters Obtained
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Square cross section Side = 10mm D(min) = 31mm D(mean) = 41mm Length = 127mm Compression max = 55mm K = 263 lb/in N = 5 active

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Initial Trial
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Used an aluminium core of 30 mm Dia. Used double sided tape to make the design Used fibreglass unidirectional roving Used unknown matrix material.

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Results Obtained
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Removal of spring from the core was very difficult. Matrix had dipped out of the spring. As very less matrix material was left in the spring it didn't perform its function.

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Changes made to the second sample
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Used araldite instead of unknown material As the matrix material(araldite) was very viscous it didnt sweep out. Used heater to cure it faster.

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Results Obtained
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Need to be tested.

THANK YOU