Pressure compensated spool valve

Abstract

A control valve for actuating a double acting hydraulic jack or lift cylinder features a pressure compensator assembly for limiting the rate of retraction of the cylinder and piston relative to each other under heavy loading conditions to prevent dropping a load or letting the load get out of control. Pressure compensation is provided by a floating piston which is movably coupled in releasable engagement with a spool, wherein engagement of the piston with the spool defines a restricted flow path for limiting hydraulic flow out of the blind end of the cylinder to tank during retraction. According to a preferred embodiment, the spool is provided with a plurality of notches defining restricted flow passages which are covered and uncovered by the piston as it moves into and out of engagement with the spool. A compression spring is coupled between the piston and the spool for biasing the piston for movement away from the spool. The piston is provided with axially spaced pressure faces whose effective areas are balanced with respect to each other for driving the piston against the spring to a hydraulic detent position in which the notches are exposed. As the valve assembly opens to increase the orifice area of the notches, a pressure drop occurs across the notches, and the piston is held in equilibrium in the detent position as the forces developed by the piston pressure faces and by the spring balance each other. Variations in the cylinder port hydraulic pressure are compensated by the piston to maintain a constant pressure drop across the notches as it moves to choke off or throttle flow through the notches in response to the forces developed by the spring and pressure faces.