Some hydraulic circuits, such as in laminating presses, need to hold at pressure for long periods. A pressure-compensated pump could maintain pressure, but energy loss from pump leakage generates heat. Another way to hold pressure for long periods is with a fixed-volume pump and an accumulator. Figure 1-24 shows a press cylinder that must stay extended under pressure for several minutes.

Tee small accumulator D into the cylinder cap-end line through flow control C. Flow control C allows the accumulator to fill quickly but discharge slowly when directional valve A centers or shifts to retract the cylinder. Flow control C should pass enough flow to let the accumulator discharge quickly without system shock when directional valve A shifts to retract the cylinder. Any oil left in the accumulator when the directional valve centers will make the cylinder extend a small amount. Tee dump valve B into the cylinder cap-end line to automatically discharge the accumulator when the pump stops. Tee pressure switch E into the cap-end cylinder line to set pump load and unload pressure. Pressure switch E sets high and low pressures to control maximum and minimum tonnage.

When the pump starts, Figure 1-25, backpressure check valve F gives 75 psi pressure, closing accumulator dump valve B and supplying pilot oil for solenoid pilot-operated directional valve A. When directional valve A shifts, the cylinder starts to extend, Figure 1-26, at whatever pressure it takes to overcome the counterbalance valve. The signal to the extend coil of directional valveA goes through the normally closed contacts on pressure switch E. Because gas pre-charge pressure in the accumulator is approximately 85% of working pressure, no fluid will enter it yet.

When the cylinder contacts the work, Figure 1-27, pressure increases and oil fills the accumulator. Upon reaching the maximum working pressure set by pressure switch E, the normally closed contacts open, de-energizing the solenoid on directional valve A. Directional valve A spring centers, the pump unloads, and oil stored in the accumulator maintains pressure while making up for cylinder and valve leakage.

Bypass at the cylinder seals and/or valve causes pressure to drop slowly to the low-pressure setting of pressure switchE. This low-pressure setting is normally adjustable but must be high enough to keep the parts firmly together. Upon reaching the low-pressure setting, pressure switch E shifts, allowing the normally closed contacts to shift directional valve A to refill the accumulator. Upon reaching maximum working pressure, directional valve A again spring centers to unload the pump, while the accumulator holds its pressing force and makes up for leaks.

A pilot-operated check valve in the cap-end cylinder line between the directional valve and the pressure switch would have less leakage than the blocked port of the spool valve. With a pilot-operated check valve and resilient seals in the cylinder, it is possible to maintain pressure for 2 to 5 min or more. Use an all-ports-open directional valve with the pilot-operated check valve. This accumulator circuit maintains pressure in the cylinder while unloading the pump. It also conserves energy while using an inexpensive fixed-volume pump.