Poppet or check shut off valves
Pressure relief valve simulation
Function
Leak free sealing of flow in one direction.
Allows flow in only one direction.
Provides positive back pressure on a system when fitted with a spring.
Also used as simple low pressure relief valves.
Setting
Fixed spring to control the opening or cracking pressure
Fit valves without springs vertically
Variations
Pilot operated – can be opened by pilot pressure to allow flow back through valves.
Can be used as leak free directional valves. Typically for low flow, high pressure situations or for very high flows where directional valves are expensive or not suitable.
Directional control valves
Function
Stops/starts flow or directs flow into A or B service lines.
Variations
Manual or electrical operation.
Spool centre positions vary depending on free flow path or pressure venting.
Pilot or direct operated valves. Low flows generally direct acting but large flows require pilot operation.
Pressure relief valves
Function
Limits the maximum pressure in a circuit.
Setting
Generally set 15 bar above the working pressure of system.
Back pressure - Changes in pilot drain or backpressure will generally be added to the set pressure; making a sequence valve.
Variations
Direct acting - quick response, low flow, leak free.
Pilot operated - high flow, internal or external leakage.
Unloading valve - e.g. relief valve that switches to minimum pressure for start up.
Flow control or throttle valves
Function
Controls the flow passing through the valve.
Generally a sharp edged orifice or other restrictor.
Variations
Fixed or variable - fixed orifii provide constant flow or adjustable provide variable flow.
Pressure compensation - Without pressure compensation the flow will vary with changes in pressure either side of the orifice.
Often come with in-build check valves to control flow in one direction only.
Flow control accuracy can vary widely between valves.
Pressure reducing valves
Function
Maintains the downstream pressure in a circuit.
Setting
Can only reduce system pressure not increase it.
Pilot pressure - Changes in pilot drain or backpressure can effect the pressure setting.
Variations
Pilot or direct operated
Backflow capability - Some systems require backflow capability in case downstream pressures go up as well as down.
Pilot operated - controls setting remotely.
Proportional control valves
Function
Proportionally control the flow into the A or B lines depending on the angle of a hand leaver or value of electrical control signal.
Also available as proportional relief and reducing valves.
Setting
Proportional valves require a significant pressure drop across the valve to accurately control the flow, say 30 bar.
Require good fluid cleanliness
Require suitable electrical control cards
Variations
Manual or electrical operation.
Spool centre leakages vary depending on free flow path or pressure venting.
Pilot or direct operated valves. Low flows generally direct acting, large flows piloted.
Valve response times may vary.
Control pressure drop across valve to give repeatable flow accuracy independent of system.
Servo directional control valves
Function
Very accurately control the flow into the A or B lines depending on the electrical control signal. Generally used in high performance closed loop control systems.
Setting
Servo valves require a significant pressure drop across the valve to accurately control the flow, say 30 bar, and stiff, high natural frequency, systems to operate correctly.
Require very good fluid cleanliness
Variations
Spool centre positions bias may vary.
Valve response times may vary.
Hydraulic Cylinder Parts

Hydraulic cylinders derive power from hydraulic fluid. The various parts of the hydraulic cylinder are cylinder barrel (which is actually a piston connected to a piston rod), a separate barrel which is closed on each end with a cylinder bottom, and with a cylinder head and rod gland. Let's have a look at various hydraulic cylinder parts in detail. * Cylinder Barrel: The cylinder barrel is the area where the piston makes back and forth moments. The barrel is a unlined thick walled pipe that is situated inside the cylinder. The cylinder barrel is perfected internally. * Cylinder Head: Cylinder head is connected to the barrel with a lock or screw. Most engineers say that a flanged connection should be used to connect the head and barrel. Flanged connections are ideal, but they cost a lot. The advantage of having a flanged connection is they are always simple to remove for maintenance and other purposes. * Cylinder Bottom: The barrel and bottom portion are welded together in most hydraulic cylinders. In most cases this damages the inside of the barrel if done poorly. However many upcoming new cylinders have a screwed connection from the cylinder end cap to the barrel, so that the question of how to rebuild hydraulic cylinder can be answered in the future. * Piston: A piston is defined as a short cylinder shaped metal component which is used to separate the two sides of the cylinder barrel. The piston is usually forged with grooves or metal seals which prevent the hydraulic oil to pass by the piston through the chamber on the opposite side. Seals are of different designs and are made to suit the temperature and pressure requirements of the cylinder. * Piston Rod: A piston rod is chrome plated piece of cold steel which is attached to the piston. The piston rod joins the hydraulic actuator to the machine component which helps the cylinder to accomplish a given task. A machine thread or a mounting attachment can be used for this connection. * Rod Gland: The last names on the list of hydraulic cylinder parts is rod gland. To make sure the pressurized oil doesn't leak the cylinder head is fitted with seals. This area is termed as rod gland. It also has another seal called rod viper which works as a block to stop contaminants from entering in the cylinder.
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What is a Hydraulic Valve and Why Do You Need It? A hydraulic valve properly directs the flow of a liquid medium, usually oil, through your hydraulic system. The direction of the oil flow is determined by the position of a spool. A hydraulic system can only function - as per requirements - by using valves. Thus, you should always look for the correct type of hydraulic valve to serve your intended purpose.

Hydraulic valves are available in a variety of sizes. The size required is determined by the maximum flow of the hydraulic system through the valve and the maximum pressure in the hydraulic system. Hydraulic valves are available with different mountings: e.g. mounting in pipe lines, threaded connection as cartridges, subplate mounting, etc.
How DTA Can Help You:
DTA has extensive expertise with hydraulic valves and carries a substantial inventory of valves in different sizes and spool configurations. Depending on your requirements, DTA can supply high-quality hydraulic valves, taking into account a wide variety of functional and hydraulic system requirements.

Hydraulic valves are available from several major vendors; our preferred hydraulic valve suppliers are Parker Hannifin and Denison Hydraulics. Additionally, we supply hydraulic valve parts and accessories such as sub plates, seal kits, O-rings, solenoids and hydraulic valve control electronics.
Different Types of Hydraulic Valves
Hydraulic valves are sometimes referred to as hydraulic components. These are subdivided into three main categories: directional control valves, pressure control valves and flow control valves. We have added proportional and servo control hydraulic valves as a separate category below:

Directional control valves * Check valves * Directional spool valves * Directional poppet valves
Pressure control valves * Control task: variable throttle valves * Switching task: fixed throttle valves
Flow control valves * Throttle valves: flow Δp dependent * Control valves: flow Δp independent
Electro-hydraulic valves * Servo hydraulic valves * Proportional hydraulic valves
Different valves function in different ways. Check valves permit free flow in one direction and block flow in the opposite direction. The directional control valve is used to pass on the pressure medium (i.e. flow) in an orderly fashion to a particular direction. Pressure control valves switch (or control) at a certain pressure; the switching pressure may be adjusted on the valve. Flow control valves regulate the flowthis is done by adjusting the size of the bores (orifices).

Note that proportional hydraulic valves and servo control valves may be used to perform a directional control, pressure control or flow control function.

Back to hydraulics overview - Hydraulic Parts

Directional Control Valves Directional control valves can control the start, stop and change in direction of flow of a pressure medium (i.e. hydraulic oil). For this reason, they are also referred to as switching valves. Directional control valves are available as spool valves, poppet valves or rotary slide valves. Rotary slide valves, however, are no longer commonly used due to low operating pressure (up to 70 bar).

The performance of a directional control valve depends on the following: * Dynamic power limit (as a function of maximum flow and pressure) * Static power limit (adhesive force may be produced between the spool and housing due to standstill) * Resistance to flow (internal resistance, i.e. pressure drop) * Leakage (spool valves only) * Switching time (time between the actuating force and the completion of the stroke of the control element)
The function of a directional control valve is determined by the number of working ports (excluding control ports) and the number of spool positions. A directional control valve comprises at least two spool (switching) positions and two working ports. The most common directional control valve is a 4/3-way valve, which means the valve has four working ports and three spool positions, in accordance with DIN ISO 1219.

Back to top - Hydraulic Valves Check Valves
A check valve is the simplest type of directional control valve used in hydraulic systems. Check valves stop the flow of fluid in one direction and allow free flow in the opposite direction. They are also known as non-return valves. Check valves may be used as: * Prefill valves (anti-cavitation) * Bypass valves (e.g. throttling points or return-line filters) * Stops for flow in one direction * Pre-tensioning by creating a certain backpressure * Protection of hydraulic components against pressure surges
Most check valves are spring-loaded and use a ball or plate to seal the flow in one direction. Check valves are designed with seats and thus are able to isolate circuits with no leakage. Balls, plates, poppets or poppets with soft seals are used as isolating elements.

There is a special type of check valve that prevents pistons or cylinder plungers from coming down and causing accidents. This is called a line rupture valve. When the line ruptures, the flow through the line rupture valve increases substantially, causing an increased pressure drop. This in turn creates a stronger force on the ball, which will close immediately.

Other special types of check valves are pilot-operated check valves and shuttle valves. A pilot-operated check valve allows flow in either direction by application of an external pilot pressure signal. A shuttle valve permits free flow at the highest operating pressure.

Back to top - Directional Control Valves

Directional Spool Valves
Directional spool valves comprise a moving spool situated in the valve housing. When an actuating force moves the control spool, the annular channels in the housing are connected or separated. Directional spool valves have several unique features, such as: * Low cost due to simple design * Low actuating force (due to good pressure compensation) * High switching power * Low losses (even though oil leakage flows continuously from the high pressure to the low pressure side) * Wide variety of control functions
Directional spool valves may be direct-operated or pilot-operated. A direct-operated spool valve is either electrically controlled with solenoids, mechanically (e.g. manually) controlled with levers or rollers, or controlled with hydraulics or pneumatics. Whether a directional spool valve is direct- or pilot-operated depends on the actuating force needed to move the spool. Thus, this is dependent on the flow, i.e. nominal size of the directional spool valve.

When valves are operated at higher hydraulic system operating pressures, leakage losses around the spool and the housing should be taken into account, especially at system pressures over 350 bar. The leakage loss is determined by the size of the gap between the spool and the housing, the viscosity of the hydraulic fluid and the hydraulic system pressure.

Typical parameters of spool valves are: * Nominal sizes: NG6 and NG10 (up to NG120 for pilot-operated valves) * Maximum flow: up to 150 l/min (up to 7,000 l/min for pilot-operated valves) * Maximum pressure: up to 350 bar
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Directional Poppet Valves
Directional poppet valves can operate at a maximum operating pressure of up to 1,000 bar. Directional poppet valves are directional control valves that are fitted into housing bores with a threaded connection. This is why they are often referred to as hydraulic cartridge valves. The moveable seating elements may consist of balls, poppets or plates.

Key features of hydraulic cartridge valves: * No leakage * Long product life * High maximum and continuous operating pressures due to zero leakage losses * Very good sealing characteristics without additional isolating elements
The design permits the valve to become more tightly sealed when operating pressure increases. The maximum flow of cartridge valves is limited compared to spool valves. Larger flows would require larger cartridges to fit into housing bores; the threaded connection will not hold.

Typical parameters of poppet valves are: * Nominal size: NG6 and NG10, SAE, and ¼" - 2" BSP * Maximum flow: up to 60 l/min (up to 400 l/min for pilot-operated valves) * Maximum pressure: up to 1,000 bar
Back to top - Directional Control Valves

Pressure Control Valves Every pressure control valve switches (or controls) at a predetermined pressure setting. The switching pressure is generally a variable setting on the valve itself. A change in position of the pressure control valve (i.e. open or closed) occurs either gradually (by control) or suddenly (by switch). Pressure control valves can be subdivided into control and switching task:

Control task (variable throttle) * Pressure relief valves * Pressure reducing valves * Pressure drop valves * Pressure difference valves * Pressure valves with additional electric switch-off
Switching task (fixed throttle) * Pressure shut-off valves * Pressure sequence valves
Sequence valves are used to direct the flow to a secondary circuit. When the primary hydraulic circuit reaches a preset pressure, flow is permitted to the secondary circuit. Unloading valves are remotely-piloted pressure shut-off valves that direct flow to the tank when the preset pressure in a particular location of the hydraulic circuit has been reached.

The most common types of pressure control valves are the pressure relief valve and the pressure reducing valve. Pressure relief valves control the system pressure by relieving part, or all, of the flow to tank. Pressure reducing valves reduce the pressure supplied to a sub-system of a hydraulic system.

Pressure control valves may be directly or pilot operated. Larger flows require larger spools or poppets, which increases the area of the spring diameter: the spring force increases proportionally. Pilot-operated pressure control valves require only a small spring, are suitable for compact space requirements and have maximum flows of up to 650 l/min.

Back to top - Hydraulic Valves

Pressure Relief Valves
There are two pressure control valves: one is a pressure relief valve, and the other is a pressure reducing valve. Pressure relief valves have higher flow capacities than pressure reducing valves because pressure relief valves have to be capable of directing all flow to the tank.

Pressure relief valves are used in hydraulic systems to limit the system pressure to a specific set level. If this set level is reached, the pressure relief valve responds and feeds the excess flow from the system back to the tank.

Note: the pressure relief valve is a normally closed pressure control valve.

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Pressure Reducing Valves
In contrast to pressure relief valves, which affect the level of input pressure (i.e. pump pressure) fed into the hydraulic system, pressure reducing valves are used to influence the output pressure of the hydraulic system to a subsystem (e.g. actuator pressure).

Reducing the input pressure (primary pressure) or maintaining a particular output pressure (secondary pressure) is achieved at a set value which is below the varying pressure occurring in the main hydraulic circuit. It is thus possible to reduce the pressure in one part of the circuit to a lower level than the system pressure.

Note: the pressure control valve is normally open.

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Flow Control Valves Flow control valves manage the flow by decreasing or increasing the opening at the throttling point. This helps to determine speed of movement for the actuators. The simplest design for a flow control valve is a needle or longitudinal slot mounted in the pipeline and connected to a screw that adjusts the opening at the throttling point.

These are called throttle valves and they are regularly used in combination with a check valve, i.e. the throttle check valve for speed control in one direction of flow. A disadvantage of throttle valves is that at varying loads a change in pressure drop will change the flow; thus, the speed of the moving actuator will also be affected. Flow control valves are divided into two types:

Throttle valves (flow depends on Δp) * Viscosity dependent * Viscosity independent
Flow control valves (independent of Δp) * 2-way flow control valves * 3-way flow control valves
The difference in pressure before and after the throttling point, i.e. the pressure drop (Δp), determines the rate of flow through the throttle valve at a particular setting. If the pressure drop over the throttling point remains the same, the flow of oil that passes the throttling point remains the same as well. This allows operation at constant speeds, regardless of the load.

Flow control valves are used to influence the speed of movement of actuators by changing the opening for the flow (decreasing or increasing it) at the throttling point. In fact, these are two flow control valves placed in series and built together. The throttling point operates at a fixed setting, but the throttle opening before the throttling point varies with pilot pressure of the load.

Note that flow dividers have a certain special standing: they divide an oncoming flow into two or more flows. Usually it is used to distribute the flow of a single pump to two or more sub-systems that have different operating pressures.

Back to top - Hydraulic Valves Throttle Valves
Throttle valves come in piston and needle-valve structures, but we also know them as turn plugs with axial or radial grooves. The smothering of grooves is more accurate but provides more variability than those with piston structures. The needle throttle has one disadvantage: a great deal of power is required to move the needle only a slight amount.

The pressure difference (Δp) determines the amount of flow that passes the throttle. In systems with varying loads (and therefore varying pressures), the flow passing the throttle will always be different.

You can find non-adjustable (fixed) smothering in situations where it is needed to gauge against excessive pressure surges. Also, one can find non-adjustable throttles in control systems.

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Flow Control Valves
A pressure-compensated flow control valve has a fixed throttling flow at all pressures. The two-way pressure-compensated flow control valve is also called a series valve. The pressure reducing valve and the smothering of this valve are placed in series with each other. At a three-way pressure control valve, the pressure relief valve and the throttle in this valve are parallel to each other.

The two-way and three-way pressure compensated flow control valves are quite similar. The only difference is that in the latter type, the excess oil flow is not drained through the pumps safety valve but through the flow control valve itself. The pump pressure for a three-way pressure compensated flow control valve is always equal to the pressure exerted by the actuator plus the spring pressure.

Note that using three-way flow control valves increases efficiency because the system pressure will never be higher than as required for moving the load. With two-way flow control valves, this is not the case because the pump keeps feeding the system with oil until the preset pressure of the pump's safety - pressure relief - valve has been reached.

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Electro-hydraulic Valves When your hydraulic application requires a very accurate control of the flow, hydraulic valves may be equipped with advanced control electronics. This allows the use of inductive path measuring devices that monitor the position of the spool continuously to ensure optimum position of the spool.

Proportional hydraulic valves are able to control the opening to flow proportionally instead of gradually, as is the case for most standard hydraulic valves. The simplest type of proportional hydraulic valve is a variable throttle controlled by a manual lever, as illustrated below:

Proportional and servo hydraulic valves are usually classified as high-performance valves. This distinction gives an expected indication of performance, which tends to generalise the true differences between various types of servo and proportional hydraulic valves. Each type has its own unique performance characteristics either in controlling pressure or controlling flow.

Servo hydraulic valves use closed-loop control to monitor and feed back the main-stage spool position to a pilot stage (hydraulic/pneumatic) or driver (electronic). Proportional hydraulic valves, on the other hand, move the main-stage spool in direct proportion to an external piloted control signal and do not have any means of feedback within the valve.

Back to top - Hydraulic Valves Servo Valves
The main stage spool of a servo hydraulic valve is equipped with a path measuring system for accurate adjustment. This system measures the exact position of the main-stage spool and feeds its position directly to the electronic control unit. If sworn position is not consistent, the control signal corrects the position of the main-stage spool.

Servo hydraulic valves are widely used in aerospace and industrial applications.

Back to top - Electro-hydraulic Valves

Proportional Valves
The most common proportional hydraulic valves are directional control valves, pressure relief valves, flow controllers and adjustable throttling. Proportional hydraulic valves convert an incoming mechanical or electrical signal directly proportional to a shear mode. The movement follows a continuously incoming signal. Different types of available proportional hydraulic valves are: * Various directional control valves * Flow control valves * Pressure relief valves * Pressure reducing valves * Counter balance valves
Typical applications of proportional hydraulic valves include cranes and industrial applications such as injection moulding