Combination lock and relief valve for hydraulic systems

A combination lock and relief valve comprising a housing having a valve chamber, a lock cylinder and a relief cylinder, a lock piston dividing the lock cylinder into first and second lock chambers, a relief piston dividing the relief cylinder into first and second relief chambers, said first lock chamber communicating with said valve chamber, a valve poppet movable in said valve chamber for opening and closing the valve chamber relative to said first lock chamber, a first actuator port connected to the valve chamber, a second actuator port connected to said first lock chamber, said first actuator port at times serving as an inlet for fluid and at other times as an outlet for fluid, said second actuator port serving as an outlet for fluid when said first actuator port serves as an inlet and serving as an inlet for fluid when the first actuator port serves as an outlet, a high pressure port connected to the second lock chamber, an exhaust port connected to the first relief chamber, a passage connecting the second relief chamber to the valve chamber, said lock piston being movable by high pressure in said second lock chamber into engagement with said valve poppet for moving the latter to open position, and said relief piston being movable by pressure in said second relief chamber into engagement with said lock piston to move the latter into engagement with said poppet for moving the poppet to its open position.