Counterbalance valve

Claim 1

A hydraulic system for raising and lowering a load including a hydraulic motor operable in one direction to raise the load and operable in the opposite direction to lower the load, a pump which supplies pressure fluid to operate the motor, a four-way valve alternatively movable between a first position which connects the pump output to one side of the motor to raise the load, a second position which connects the pump output to the other side of the motor to lower the load and a neutral position in which the pump output is routed to tank and the hydraulic motor is stationary, characterized by a remote, pilot-operated counterbalance valve connected between the four-way valve and the hydraulic motor, which valve comprises a metering spool, a variable metering orifice which passes fluid from the motor when the load is lowered, means biasing the metering spool to close the metering orifice, a pilot fluid conduit connected between one end of the metering spool and the unloaded side of the motor, wherein pressure fluid in said pilot fluid conduit moves the metering spool to open the metering orifice when the load is lowered, a pressure compensator valve which comprises a pressure compensator spool, means for connecting the pressure compensator spool to the upstream and downstream sides of the metering orifice, a compensator spool seat and the compensator spool cooperates with the compensator spool seat to form a compensator orifice which receives fluid from the metering orifice and the pressure compensator valve maintains a fixed pressure differential across the metering orifice to thereby maintain a fixed fluid flow through the metering orifice when the load is lowered, including means for hydraulically closing the compensator orifice when the hydraulic motor is stationary.