Abstract: A hydraulic control device to vary the displacement of a hydraulic piston unit, such as pump or motor, in function of the operating pressure, is provided with a pilot-operated servo piston. The pilot system comprises a sensing piston responsive to the operating pressure actuating a pilot piston and a force exerting member acting at an angle to the direction of movement of the sensing piston, the sensing piston and force exerting member acting on a movable bearing. The pilot piston is hydraulically and the movable bearing mechanically, e.g. by a coupling lever, connected to the main piston.

Abstract: A relief valve for limiting the pressure of the output of a feed pump being furnished to a main hydraulic pump is provided not only with a bore through the valve disk leading to a chamber at the rear of the guide piston for counteracting, as relief valving continues, the force of the spring tending to keep the valve closed, but also a pressure piston, in the form of an annular flange on the guide piston, subdividing an outer cylindrical space into two chambers respectively connected to a constriction in a hydraulic conduit, so that the amount of flow through the constriction will additionally modify the limit pressure at which relief valving takes place. The device is used to reduce the limit pressure when the feed pump provides excessive oil feed to the main pump.