This document summarizes different types of pressure control valves used in hydraulic systems. It discusses pressure relief valves, which limit pressure in the system by diverting excess flow to the reservoir. Unloading valves perform similarly but are actuated externally rather than with an internal spring. Sequence valves maintain primary system pressure while allowing a secondary operation. Counterbalance valves control cylinders to prevent free falling due to gravity. Brake valves prevent overspeeding of hydraulic motors under load. Pressure reducing valves are used to maintain a reduced pressure in parts of the system and close to prevent pressure increases above the valve setting.

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Basics of Hydraulics
PRESSURE CONTROLS
Pressure control valve performs two main functions as,
1. Limiting maximum pressure.
2. Maintaining the reduced pressure in the certain portions of the circuit.
Functioning of the valves mainly characterized by the control spring movement.
There are many types of pressure control valves used for industrial purpose, such as
relief valve, sequence valve, counterbalance valve, brake valve etc.
In this study we are going to learn about the pressure reducing valve and pressure
relief valve, and brief importance about the other pressure controls.
A. Pressure Relief valve:
Its very important that, this valve found in every hydraulic system in one or other
form. It is normally closed valve position between the pressure line (Pump input) and
the reservoir. Its purpose is to limit pressure in the system. The system pressure is
maintain to relief valve setting pressure by delivering the extra flow to the reservoir
through relief valve. It protect the hydraulic system from over pressure.
A simple or direct acting relief valve consist of a ball or poppet held seated in the
valve body by a heavy spring. When pressure at the inlet is sufficient to press the
spring flow of fluid goes to reservoir maintaining the system pressure to the set range.
In most if the cases adjustable screw is provided to change the pressure on the spring
i.e. Pressure relief setting.
B. Unloading valve:
This is same as relief valve but instead of internal poppet/ spring actuation, in
unloading valve it is done through external source. This allows system to operate
remotely and change to unload the pressure. It also has a check valve to allow the
return flow from the reservoir to the primary input.
C. Sequence valve:
Valve is used in a system when sequencing of two operation is required. This valve
maintains the pre-pressurized primary system pressure while doing secondary
operation.
D. Counterbalance valve:
Its application is to maintain control over vertical cylinder so that it will not fall
freely because of gravity.

2. RS
Basics of Hydraulics
E. Brake valve:
Brake valve is used in the exhaust line of a hydraulic motor to prevent over speeding
when an overrunning load is applied to the motor shaft and to prevent excessive
pressure build up while stopping the load.
F. Pressure Reducing Valve:
Pressure reducing valves are normally open pressure controls used to maintain
reduced pressure in the certain portion of the system. It is not necessary to use the
valve in every hydraulic system. They are actuated by pressure sensed in the
branched circuit and tends to close as it reaches the valve setting. This prevent further
increase in pressure in the branch circuit. These valves are available in both direct
and pilot operated versions.
This is summarized report of pressure control valve written by reading online
resources and experience.