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1. Valves Used in Hydraulic Circuits

Valves are used in hydraulic circuit in order to control hydraulic pressure, direction of flow and amount of flow.

1) Relief Valves

If there is no escape route for the hydraulic oil that is continuously pumped out of the hydraulic circuit can burst.
A relief valve is a valve that allows the oil in the circuit to escape back into the tank when the pressure exceeds a certain limit in order to prevent the kind of malfunction described above.

2) Pressure Compensated Flow Control Valve

These are flow control valves that maintain a constant pressure difference between the intake and the outlet, so that even if the pressure changes, the flow is constant.
These are used in winches, and make more precise inching operation possible.

3) Directional Control Valve

In order to expand the cylinder in the figure, oil is pumped into A; to contract it, oil is pumped into B. The valve that changes the direction of this oil is called a directional control valve.

A directional control valve is a valve that changes the direction in which the oil flows by moving a spool. By changing the direction of oil flow, the direction of a hydraulic cylinder's motion can be changed, or the direction of rotation of a hydraulic motor can be changed.

In a crane, the operator uses a control lever to manipulate the valve. There is also a type of directional control valve called a solenoid valve that is controlled with a magnet(solenoid).

Operation of the crane is performed by using the directional control valve to control the flow of oil. The speed of operation of the actuators changes with the flow of oil.

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