Hydraulic Component Descriptions

Learn about the main hydraulic control valve functions

 

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Last Updated
2010-06-17

 


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Summary of common hydraulic components. Please note that this is a basic guide and components may also be used in situations that are not described below.
Pressure relief valve simulation
Counterbalance valve simulation
Proportional valve simulation

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.

 


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