Hydrostatic drive system

Claim 1

A hydrostatic drive system for a vehicle providing precise slow speed control of vehicle movement comprising; a variable displacement pump; a hydraulic motor; a closed hydraulic system interconnecting said pump and said motor for communicating fluid therebetween; fluid bypass means disposed within said hydraulic system in parallel with said motor including a pressure responsive automatically variable restrictor valve having a passage extending therethrough, a valve member operatively associated with said passage and a spring resiliently urging said valve member to an open position permitting fluid flow through the bypass means for dampening pressure surges and smoothing fluid flow in the hydraulic system at relatively slow vehicle speeds when the fluid flow in the hydraulic system is below a predetermined flow rate and a predetermined pressure value, said restrictor valve being effective to increase the fluid pressure in the closed hydraulic system as the fluid flow through the bypass means increases and responsive to such increased pressure progressively to reduce fluid flow through the bypass means for increasing the speed of the vehicle, and eventually to block such fluid flow through the bypass means when the fluid pressure exceeds said predetermined value to assure delivery of all fluid flow from the pump to the motor at high speeds and at high vehicle load conditions where the fluid pressure remains above said predetermined pressure even though the flow rate decreases below the predetermined rate at which said restrictor valve is normally opened; and said restrictor valve including means providing an auxiliary flow path for transmitting fluid undEr pressure from said closed hydraulic system to said valve member for moving it toward a closed position against the bias of said spring for blocking fluid flow through said passage when the fluid pressure in the hydraulic system exceeds said predetermined value.