Pressure control system for a variable displacement hydraulic pump

Abstract

A displacement control valve includes a valve element having a first operative position communicating a displacement controller of a variable displacement hydraulic pump with a tank for increasing the pump displacement and a second operative position communicating a discharge passage of the pump with the control port for decreasing pump displacement. The discharge passage communicates with a first end of the valve element to generate a force biasing the valve element toward the second position. An opposing biasing force is exerted on the valve element in opposition to the pressure generated force acting on the first end of the valve element so that a predetermined pressure level is established in the discharge passage when the forces acting on the valve element are equalized. The opposing biasing force is generated by a spring disposed to exert a substantially constant biasing force on the valve element and an orifice device for generating a fluid pressure at the second end of the valve element commensurate with the pressure in the discharge passage. A variable mechanical biasing force can be exerted on the valve element in concert with the opposing biasing force to change the pressure level in the discharge passage proportional to the level of the mechanical biasing force.