Figure shows a high low unloading circuit. It consists of two pumps.

1. High pressure, low flow pump

2. Low pressure, high flow pump.

In punching machine hydraulic ram are required to function in following manner:

1) It should initially extend rapidly over a large distance

2) Then during punching operation, it should extend through a very short distance with very high pressure.

The circuit shown in figure uloads both the pumps to drain when the 4/3 way D.C. valve is in central position. When 4/3 way DCV is shifted to position (1), during the rapid extension stroke, portion, both the pumps supply fluid to the system. When the punching operation begins, the system pressure rises. The increased pressure opens the unloading valve for the low pressure pumps. The check valve protects the low pressure pump 2 from high pressure which occurs during the punching operation stage of the extension stroke. The purpose of the relief valve B in to protect the high pressure at the end of the cylinder stroke.

Application: Punching Press, Bench vice,Shaper Machine