Pump supplementing circuit with full pressure when work is contacted

In some cases, a pump-supplementing accumulator circuit can speed up cylinder extension and/or retraction without having to go above working pressure. Normally in a pump-supplementing circuit, the relief valve is set as high as possible above the working pressure to store ample fluid. As the cycle progresses, oil from the accumulator and pump move the actuator quickly, but circuit pressure drops steadily. If pressure drops below the actuator’s need, the pump must refill the accumulator before the cycle finishes. To overcome this problem, a larger pump and/or more accumulators are necessary.

The next circuit shows an accumulator arrangement that provides high volume to move the cylinder rapidly with the relief valve set at working pressure. The accumulator and pump supply volume to fill the large bore cylinder as it extends. The cylinder then moves to working pressure while a check valve isolates the accumulator.

Like all accumulator circuits, there must be time for refilling between cycles, as shown in Figure 1-31. Pre-charge the accumulator to a pressure slightly higher than it takes to retract the cylinder. The cylinder will then retract when directional valve A and normally open, solenoid-operated relief valve H shift. (Also see Figure 1-34.) The large piston rod reduces the return volume, although retract pressure will be higher. When the cylinder fully retracts, pressure climbs and the accumulator starts to fill through check valve E and the bypass check valve around flow control C. Piston-type accumulators are best for this circuit because they can have a low pre-charge pressure and a high final pressure without internal damage. The accumulator can discharge a large volume of oil because the pressure in it is not important when the cylinder needs full tonnage.

When pressure in the circuit reaches 2000 psi, pressure switch G de-energizes the solenoid on normally open, solenoid-operated relief valve H, unloading the pump to tank.

When directional valve A and normally open, solenoid-operated relief valve H shift, Figure 1-32, pump flow and accumulator flow provide a large volume of oil to quickly stroke the cylinder to the work. Because accumulators can discharge at a very high rate, use flow control C to set the desired advance speed. Pressure in the circuit will fall as the cylinder extends and will be well below working pressure before the cylinder meets the work.

When the cylinder contacts the work, Figure 1-33, check valve F keeps pump flow from going to the accumulator. The pump will continue filling the cylinder and pressure will build to whatever it takes to do the work. Check valve F blocks flow to the accumulator to isolate it during the high-pressure work stroke.

When directional valve A shifts to the retract position, Figure 1-34, pump flow goes to the cylinder rod end. The accumulator pre-charge is high enough to force all pump flow to the cylinder, causing it to quickly retract.

Figure 1-31 shows the cylinder reaching the top of the stroke. The accumulator now accepts all pump flow through check valve E until pressure switch G unloads the pump.