double-pump hydraulic system

DOUBLE-PUMP HYDRAULIC SYSTEM

(DOUBLE-PUMP HYDRAULIC PRESS CIRCUIT)

1. Functional and Operational Requirements

It may be noted that in a punch press, the hydraulic cylinder must extend rapidly over a great distance with low-pressure but high-flow requirements and this occurs under no load. But during the punching operation for short motion, the pressure requirements are high and the flow requirements are low due to shortened cylinder travel. This requires very expensive high-pressure, high-flow pump. By employing an unloading valve and two pumps viz. low- pressure high-flow pump and high-pressure low-flow pump in the hydraulic press circuit, the very expensive high-pressure high-flow pump can be eliminated.

2. Circuit,

The application of unloading valve in a hydraulic press circuit is illustrated in Fig.8.8.

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 and the pressing/punching operation commences. When the pressing/punching operation starts, the increased pressure of oil opens the unloading valve to unload the low-pressure pump. Thus - only high-pressure low-flow pump does the pressing/punching operation.

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 retraction (idle return stoke without any pressing/punching) starts, 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 high-pressure pump to the tank. Thus only low-pressure high- flow pump does the idle return stroke operation.

Cylinder locking: When the 4/3 DC valve is shifted to the spring-centred position in tandem design, the both pumps unload i.e., the pumps flow completely go to tank.

The check valve protects the low-pressure pump from high pressure during the pressing/punching operation, at the end of cylinder stroke and when DC valve is in spring- centred position. When the DC valve is in its spring-centered mode, the relief valve protects the high-pressure pump from over-pressure at the end of the cylinder stroke.