Output control device for a hydraulic pump of the variable displacement type

Claim 1

In an output control device for a hydraulic pump of the variable displacement type including a cylinder having a control piston and a piston rod reciprocally movable therein, a yoke arranged to vary the delivery of said pump by change in the inclination of said yoke in response to movement of said piston, and means for feeding a delivery pressure of said pump into said cylinder to move said piston thereby to effect variation in the inclination of said yoke, the improvement comprising a cam formed on the piston rod of said control piston, a hollow cylindrical sleeve mounted for slidable movement in directions perpendicular to said piston rod and arranged to remain in sliding contact with said cam to be displaced by said cam away from said piston rod with displacement of said control piston due to a rise in fluid pressure, a fluid passage leading to said cylinder, a pressure pickup hole defined in said sleeve in flow communication with said fluid passage, a compensation piston slidably fitted in said sleeve and adapted to close said pressure pickup hole to shut off flow of pressure fluid into said cylinder when the fluid pressure is zero and to be moved upon a rise of fluid pressure away from said pickup hole to open said pickup hole, a compensation piston fluid passage extending from one end of said compensation piston facing said piston rod to the cylinder hole in order to effect balancing of the forces applied to both ends of said sleeve by the fluid pressure exerted in said compensation piston fluid passage, and means for urging said compensation piston with a preset force into engagement with said piston rod, the position of said pressure pickup hole being shifted according to the delivery of said pump in order to attain a desired pressure and to control the pump output.