Pilot-Operated Unloading Valve

A pilot-operated unloading valve has less pressure override than its direct-acting counterpart.So it does not dump part of the flow prematurely.

A pilot-operated unloading relief valve is the same as a pilot-operated relief valve with the addition of an unloading spool. Without the unloading spool, this valve would function just like any pilot-operated relief valve. Pressure buildup in the pilot section would open some flow to the tank and unbalance the poppet, allowing it to open and relieve excess pump flow.

Schematic diagram of unloading valve is shown in Fig. 1.10.In a pilot-operated unloading valve; the unloading spool receives a signal through the remote-pilot port when pressure in the working circuit goes above its setting. At the same time, pressure on the spring-loaded ball in the pilot section starts to open it. Pressure drop on the front side of the unloading spool lowers back force and pilot pressure from the high-pressure circuit forces the spring-loaded ball completely off its seat. Now there is more flow going to the tank than what the control orifice can keep up with. The main poppet opens at approximately 20 psi. Now, all high-volume pump flow can go to the tank at little or no pressure drop and all horsepower can go to the low-volume pump to do the work. When pressure falls approximately 15% below the pressure set in the pilot section, the spring-loaded ball closes and pushes the unloading spool back for the next cycle.

An unloading valve requires no electric signals. This eliminates the need for extra persons when troubleshooting. These valves are very reliable and seldom require maintenance, adjustment or replacement.An unloading valve unloads the pump when the desired pressure is reached.It allows rapid discharge of pressurized oil near atmospheric pressure.As soon as the system pressure reaches the setting pressure that is available at the pilot port, it lifts the spool against the spring force.When the spool is held by the pilot pressure, the delivery from the pump goes to the tank.An unloading valve is used to perform operations such as stamping, coining, punching, piercing, etc.

Figure1.11shows the application of unloading valve in a punching press. It is a circuit that uses a high-pressure, low-flow pump in conjunction with a low-pressure, high-flow pump. In a punching press, the hydraulic cylinder must extend rapidly over a great distance with low-pressure, high-flow requirements. This rapid extension of cylinder occurs under no external load (when the punching tool approaches the sheet metal).But during punching operation for short motion, the pressure requirements are high due to punching load. During this cylinder travel, high-pressure, low-flow requirements are needed.

When punching operation begins, the increased pressure opens the unloading valve to unload the low-pressure pump. The purpose of relief is to protect the high-pressure pump from over pressure at the end of the cylinder stroke and when direction control valve (DCV) is in its spring centered mode. The check valve protects the low-pressure pump from high pressure, which occurs during punching operation that occurs at the end of cylinder extension and when the DCV is in its spring centered mode.

The above circuit given in Fig. 1.11 eliminates the necessity of having a very expensive high-pressure, high-flow pump.

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