Underspeed actuator for a hydrostatic transmission having a shunt valve

Claim 1

In a control system for controlling the displacement of a first pump driven by a prime mover, comprising: control means for adjusting the displacement of said first pump; an underspeed actuator having a piston, first and second actuating chambers at opposite ends of said piston, and a biasing means, said piston being connected to the control means and movable between a first position at which said first pump is at minimum displacement and a second position at which said first pump is at maximum displacement, said biasing means urging the piston toward the first position and said piston being movable between said first and second positions in response to pressure differential in said first and second chambers; signal means for controllably developing first and second control signals in response to the operating speed of the prime mover, said signal means having a first conduit for delivering said first control signal to said first chamber and a second conduit for delivering said second control signal to said second chamber; and shunt valve means connected to the first and second conduits for passing fluid expelled from the first chamber to the second chamber in response to said flow of fluid expelled from the first chamber exceeding a preselected flow rate.