Hydraulic Closed-Center Circuit

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The closed-center circuit shown in Fig. 4.18 has a variable displacement pump like that shown in Fig. 4.15. The compensator valve spring is a 2500 psi spring. The pump builds 2500 psi before the compensator valve moves to open a pathway for fluid to flow to the yoke actuating piston. The piston extends and forces the swashplate to the zero displacement position. The pump then maintains the 2500 psi and supplies only enough flow to replace leakage.

The operator cracks open the DCV to start the load. Pressure drops slightly at the pump, and the compensator valve moves slightly to partly close the pathway to the yoke actuating piston. This piston retracts slightly, and the swashplate tilts slightly so that the pump is now delivering fluid to the DCV and thus the load. This sequence of events continues until the operator has opened the valve to the position where 9 GPM is flowing to the load. The pressure is something less than 2500 psi, depending on the characteristics of the compensator valve spring. To produce the power diagram, we again assume that the pressure is 2500 psi.

 Hydraulic Closed Center Circuit

Load pressure is 1300 psi, thus the pressure drop across the DCV is 2500 ?1300 = 1200 psi. The pump is delivering 9 GPM, thus the power loss is

 Hydraulic Closed Center Circuit

This loss is considerably less than the 16 hp for the open-center circuit but is still quite large. We are now ready to see what load sensing can accomplish.

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