In the circuit in Fig. 3.15, the pump builds pressure in the accumulator until the setting of the unloading valve is reached. At this point, the unloading valve opens, and flow bypasses to the reservoir. The pressurized fluid is trapped in the accumulator by the check valve and the closed-center directional control valve.
A functional diagram of an unloading valve is shown in Fig. 3.16. Two features are added to a pilot-operated relief valve to create the unloading valve. A check valve is built in, and a small piston is included in the top section in line with the dart and pilot spring. When the unloading valve is closed, fluid flows through the check valve to charge the accumulator.
As with the pilot-operated relief valve, it is helpful to assign values to the springs. For our discussion, we assume the accumulator has a 1000 psi rating. The pilot spring is assigned a value of 975 psi, and the spool spring is assigned a value of 25 psi. When pressure reaches 975 psi, the dart is unseated, allowing fluid to flow through the internal drain to the reservoir. Pressure in the upper chamber cannot increase above 975 psi. The spool is held in place by the 25-psi spring. The small piston has balanced hydraulic forces, because the same pressure acts on both sides. The projected areas of both sides are equal, thus the hydraulic forces are equal.
As pressure continues to build and reaches 975 + 25 = 1000 psi, the unloading valve opens and flow bypasses to the reservoir. The pressure drops, and the check valve closes. Pressure on the accumulator side of the piston pushes it to the right (Fig. 3.17), where the rod pushes the dart off its seat. As long as the dart is held off its seat, the unloading valve is vented, and the pump is unloaded at 25 psi, the pressure required to compress the main spool spring.
When the directional control valve is shifted, fluid drains from the accumulator and the pressure drops. The hydraulic force on the piston drops and, when the pilot spring force becomes greater, the dart reseats. At this point, pressure equalizes on both sides of the spool skirt. The spool spring reseats the spool and the pump begins to build pressure.
Pressure drops at the directional control valve as the accumulator empties. Simultaneously, the pump is building pressure. The resulting pressure vs. time curve has a shape as shown in Fig. 3.18. The minimum pressure is a function of the load, the characteristics of the accumulator, and the characteristics of the pump.