Controls for variable pumps or motors

Claim 1

In a control system for variable displacement pumps, motors and the like, including a servo-motor having a pilot element operably associated therewith for control of said servo-motor in response to positioning of said pilot element, which positioning effects changes in the displacement of the pump, motor, and the like with which it may be associated, the combination of a main control means operative to position said pilot element, a primary servo-motor includng a cylinger having a piston mounted therein, said piston defining a chamber at one end of said cylinder, said piston being operably movable within said cylinder by pressures developed in said chamber, and being operably interconnected to said pilot element whereby movement of said piston varies the position of said pilot element in precedence to the operative effect thereon of said main control means, and vent means for reducing the pressure level within said chamber, said vent means including at least one vent valve having a movable valve element and a coacting valve seat, spring means resiliently urging said valve element to a closed position against said valve seat, a variable fluid pressure to be monitored and sealed plunger means subject to said pressure to be monitored and acting on said valve element in opposition to said spring means, whereby, when said pressure to be monitored reaches a predetermined value, said valve element is unseated to vent said chamber.