A conventional hydraulic system will not operate unless the pump is running. Some machines must be able to cycle to a safe condition after a power or pump failure. Use an accumulator to store enough energy to move the actuators to a safe condition after the pump quits. The operator or setup person can manually cycle the machine into a safe condition by using the stored energy.

The hopper gate cylinder shown in Figure 1-20 must close in case of a power failure. If the gate stays open, the entire hopper could overflow the truck under it, then dump on the ground. This circuit uses a pressure-compensated pump that maintains pressure with minimal heating during normal operation. An accumulator F stores the first pump flow, check valve D stops accumulator back flow, and normally open directional valves C isolate the accumulator from the cylinder and tank during normal operation.

The gate cylinder needs at least 1500 psi, so the pump compensator is set for 2000 psi. This ensures that the accumulator has enough fluid to extend the cylinder when necessary. Because the solenoids on valves C are energized by the pump start command, the accumulator is completely isolated from the cylinder and tank as long as the pump runs. When solenoid B of the 4-way directional valve shifts (as seen in Figure 1-21), the gate opens as fast as the pump moves it.

When solenoid A shifts the 4-way directional valve, as seen in Figure 1-22, the gate closes as fast as the pump moves it. When the power is on, the cylinder extends or retracts partially or all the way at the operator’s command.

If the gate cylinder is partially or completely open and power fails, the circuit automatically goes to the condition shown in Figure 1-23. In this condition the pump stops, the 4-way directional valve centers, and the normally open 2-way shutoff valves C open.

When power fails, the accumulator has a direct path to the cap end of the cylinder while rod-end oil flows to tank. The cylinder will extend and close the gate using the stored energy in the accumulator. Place warning signs at the gate indicating this equipment can operate at any time without operator intervention.

When using an accumulator for emergency power supply it is difficult to automatically drain it during normal operation. Automatically draining the accumulator would defeat its purpose as an emergency power supply. Add a manual drain valve E, with warning signs to tell maintenance persons to manually drain the accumulator before working on the gate circuit.


Size emergency-power accumulators to hold enough oil to move all actuators to the home position before pressure drops to dangerous levels. Most manufacturers provide formulas in their catalogs and offer several offer excellent computer programs to size accumulators for emergency-power supplies.