Hydraulic motor control

Claim 1

In a hydraulic circuit, a hydraulic motor having working pistons which are reciprocal therein, fluid supply means for supplying said pistons with a working fluid under pressure, a cam plate which is angularly positioned relative to the axis of reciprocation of said pistons, and cam plate angle varying means including motor control means for selectively varying the angle of said cam plate to vary the length of reciprocation of said pistons and the torque of said motor when the load upon the motor is varied, wherein the improvement in said motor control means comprises in combination with said motor: piston means which directly contacts said cam plate, fluid conduit means communicating said fluid supply means with said piston means such that said piston means continuously exerts a variable force on said cam plate during operation of said motor which varies in response to changes in pressure in said fluid supply means and in a direction which tends to vary the angular position of said cam plate toward its maximum angular position, and preload means which also directly contacts said cam plate and continuously exerts a force of predetermined magnitude on said cam plate in a direction which tends to vary the angular position of said cam plate to its minimum angular position, said predetermined magnitude being such that the angular position of said cam plate remains substaNtially unchanged for load increases on said motor below a given load during the operation of said motor, but wherein the force exerted by said preload means is substantially overcome and the angular position of said cam plate is increased for load increases on said motor above said given load, whereby the increase in the torque of said motor is substantially greater than the change in the pressure of the fluid in the fluid supply means when the load on the motor is increased above said given load than the increase in torque below said given load during the operation of said motor.