With neat sketch explain the working of a pressure compensated flow control valve and pressure unloading valve.

 

Oil from the pump flows through the flow central valve and the flow is restricted. The approach to the adjustable restrictor is also through the pressure compensator spool. There is enough passage and space around the compensator for the oil to reach the restrictor.

Let us say the pump flow is 221/min and the restrictor is adjusted to allow 101/min. There is a balance flow of 121/min which will be building up pressure in the pipe line P. The load pressure from port A of the cylinder is felt on the outlet of the flow control valve.

If the spring rating is 1 bar and the load pressure at port A is 50 bar then the pressure felt on the left-hand side of the pressure compensator is 51-bar.

The pressure on the right side of the compensator slowly builds-up. When this pressure reaches a pressure level just above 51-bar, the pressure compensator moves towards the left compressing the spring. In moving the passage to the restrictor is narrowed by the landing of the pressure compensator. This in turn will allow less oil to flow to the flow control valve.

The movement of the pressure compensator depends on the load pressure and the spring rating.

Unloading circuit is used to unload a pump or accumulator. In many operations, the cylinder has to extend first at no load and it should extend slowly when there is high load. For example consider punching operation. The actual punching starts only when the punch touch the work-piece.