Abstract: A non-feedback proportional system is provided to achieve desired operating characteristics which will allow the control of a hydrostatic transmission to closely approximate the control performance of a displacement feedback control system by establishing certain parameters relating to (1) the servo spring rate; (2) the valve plate index; (3) valve plate crossport/trap; (4) valve plate porting geometry; (5) swashplate offset; and (6) piston bore volume.

Abstract: A dual pressure source, or selectively alternative high pressure source, control system for a variable displacement hydraulic unit servo mechanism wherein normal control of the unit is provided by the low pressure source, such as a transmission charge pump, and wherein a high pressure source of control fluid with a restrictive orifice is provided to initiate servo movement, especially under conditions generating high internal friction. The flow restrictive orifice generates a pressure drop in a high pressure control line once the servo mechanism starts to move, inducing flow through such line. Thus, the control system provides a momentary source of high pressure fluid but permits control rate changes which are substantially independent of the hydraulic unit working pressure. Preferably, the high pressure source is a shuttle selected highest pressure of a transmission main loop incorporating the hydraulic unit.