Unloading valve for hi-lo-hydraulic system

Abstract

An unloading valve for satisfying the requirements of a hydraulic system while minimizing transient pressure fluctuations. When the hydraulic system requires full fluid flow, fluid under pressure passes through the valve for discharge to the hydraulic system. If the system does not require full fluid flow, the excess fluid is automatically discharged from the unloading valve to a reservoir. Incoming fluid under pressure is communicated against one end of a locking spool for causing it to operate against an unloading pin. An orifice plate is mounted within the valve body adjacent to the unloading pin with the opening through the orifice plate being closed by a spring biased unloading poppet. A relief poppet is provided in the valve body and incoming fluid under pressure into the interior of the relief poppet is communicated to a fluid chamber between the seated end of the unloading poppet and a mounting plug holding the unloading pin. The hydraulic system pressure increases until the unloading poppet opens slightly permitting fluid under pressure to escape as pilot flow to the reservoir. A further increase in fluid pressure in the system causes the relief poppet to unseat thereby permitting drainage flow to the reservoir. A point is reached where the system pressure shifts the locking spool against the unloading pin and unloading poppet until the unloading poppet is fully opened and the relief poppet is moved away from its seat to the limit of its travel. If the hydraulic system pressure drops, the locking spool is shifted back thereby reloading the valve until the pressure in the system again rises to the unload pressure. Thus, the unloading pin provides for substantially reduced oscillations in the operation of the unloading valve as compared to conventional ball-seat arrangements.