Direct-Acting Unloading Valve
A direct-acting unloading valve consists of a spool held in the closed position by a spring. The spool blocks flow from the inlet to the tank port under normal conditions. When a high-pressure fluid from the pump enters at the external-pilot port, it exerts force against the pilot piston. (The small-diameter pilot piston allows the use of a long, low-force spring.) When the system pressure increases to the spring setting, the fluid bypasses to the tank (as a relief valve would function). When the pressure goes above the spring setting, the spool opens fully to dump the excess fluid to the tank at little or no pressure.
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