pump-unloading circuit

PUMP-UNLOADING CIRCUIT

1. Functional and Operational Requirements

• Pump unloading is necessary to avoid the damage to the components when the pressure line is blocked in the circuit.

• Pump unloading is required at the end of extending and retraction strokes as well as in the spring-centred position of the DC valve.

• It may be noted that the unloading valve is used to unload the pump to tank when pressure in the separate part of the circuit reaches a preset level. When the pressure drops to approximately 85% of the unload pressure, the valves will close which will cause the circuit to reload.

2. Circuit

Fig. 8.7 depicts a circuit used for unloading a pump using an unloading valve.

3. Operation

Extension: When the 4/3 DC valve is manually shifted to the left mode, oil flows from the pump to the blind end of the cylinder. This pump flow extends the cylinder. When the cylinder reaches the end of its extension stroke, the pressure of oil rises because the check valve keeps the high-pressure oil. Due to high-pressure oil in the pilot line of the unloading valve, it opens and unloads the pump pressure to the tank.

Retraction: When the 4/3 DC valve is manually shifted to the right mode, the oil from the pump flows to the rod side of the cylinder and the cylinder retracts. When the cylinder retracts, the motion of the piston reduces the pressure in the pilot line of the unloading valve.

This resets the unloading valve until the cylinder is fully retracted. When this happens, the unloading valve unloads the pump due to high-pressure oil.

Cylinder locking: When the 4/3 DC valve is shifted to the spring centred position in tandem design, the pump unloads i.e., the pump flow completely goes to tank.

Thus, the unloading valve unloads the pump at the ends of the extending and retraction - strokes as well as in the spring-centered position of the DC valve.