Control system for a hydraulically driven vehicle

Claim 1

A control system for a hydraulically driven vehicle including a variable displacement hydraulic pump driven by a prime mover, and a hydraulic motor connected with said variable displacement hydraulic pump in a closed loop and driven thereby, said control system comprising in combination: a fixed displacement first charge pump driven by said prime mover; a pilot-operated servo valve means for controlling the displacement of said variable displacement hydraulic pump, said pilot-operated servo valve means having a pilot port formed therein and being connected with said first charge pump; a fixed displacement second charge pump driven by said prime mover; a manually operated change-over valve means connected at the input side thereof to said second charge pump, said change-over valve means having a communication position and an offset position said change-over valve means being normally held in the communication position, said change-over valve means being changed over to the offset position for connecting the output side of said second charge pump to a tank; and servo control valve means connected at the input side thereof with said change-over valve means and at the output side with the pilot port of said pilot-operated servo valve means, wherein when said change-over valve means is in the communication position, said servo control valve means is connected to said second charge pump through said change-over valve means.