Page 36 - PNEUMATIC AND HYDRAULIC

2.5.1 Pressure Relief Valve

One of the most important pressure controls is the relief valve. Its primary function is

to limit the system pressure. Relief valve is found in practically all the Hydraulic
system. It is normally a closed valve whose function is to limit the pressure to a

specified maximum value by diverting pump flow back to the tank.

2.5.2 Unloading Valve

An unloading valve is used to permit a pump to operate at minimum load. It operates

on the principle that pump delivery is diverted to the tank, when sufficient pilot
pressure is applied to move the spool against the spring force. The valve is held open

by pilot pressure until the pump delivery is again needed by the circuit.

2.5.3 Sequence Valve

A sequence valve’s primary function is to divert flow in a predetermined sequence.

The sequence valve operates on the principle that when main system pressure
overcomes the spring setting, the valve spool moves up allowing flow from the

secondary port.

2.5.4 Counterbalance Valve

A Counterbalance valve is used to maintain back pressure to prevent a load from
failing. One can find application in vertical presses, lift trucks, loaders and other

machine tool that must position or hold suspended loads.

2.5.5 Pressure Reducing Valve

Pressure reducing valve is used to limit its outlet pressure. Reducing valves are used

for the operation of branch circuits, where pressure may vary from the main system
pressures

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