Accumulators are frequently used in circuits to provide a supplementary flow in applications where the demanded flow varies in a cyclic manner. In this situation the accumulator can provide flow at a higher level than that available from the pump thus allowing a reduced capacity pump to be used. This operation requires a circuit to:

1. Increase the pump flow when the accumulator requires recharging
2. Reduce the pump flow when there is sufficient fluid volume in the accumulator.

Figure 1 shows a circuit that is used to control the pump displacement to satisfy these requirements.

Figure 1. Accumulator Charging  

After a period when the accumulator has discharged its volume and the unloading pilot spring pre-load, the poppet will open. The associated flow through the restrictor will cause the unloading valve to move against its spring and pressurise the pump stroke piston to reduce the pump displacement.

The accumulator pressure will keep the unloading pilot poppet open because of the force on the pilot piston. The closed circuit check valve will maintain pressure in the accumulator circuit whilst the pump flow has reduced to zero. When the accumulator is required to discharge flow to the system, its outlet pressure will reduce until it has reached the level that causes the unloading pilot to close. The area of the pilot piston is greater than that of the poppet sensing area and, consequently, the level of pressure required to close the poppet can be typically 15% lower than that required by the pump pressure to open it.