Hydraulic circuit for a hydraulically driven vehicle

Claim 1

In a hydraulic circuit for a hydraulically driven vehicle including a pair of variable displacement hydraulic pumps, each driven by a common engine, a pair of variable displacement hydraulic motors, each connected with said respective variable displacement pumps in a closed loop circuit and driven thereby, means for controlling the displacement of volume of said variable displacement pumps and means for controlling the displacement of said variable displacement motors, the improvement comprising: a fixed displacement charge pump driven by said engine; a passage means leading to tank via a relief valve means; and a pair of pilot-operated shuttle valve means, each connected in parallel with said respective closed loop circuits and each operated by fluid pressure therein, each said shuttle valve means having a neutral position and two offset positions formed therein wherein, in each of said offset positions of said shuttle valve means a lower pressure side of a respective closed loop circuit is in open communication with said passage means and in the neutral positions of said shuttle valve means, both sides of each closed loop are connected together and both said closed loop circuits are connected with each other through said passage means and said shuttle valve means and with said charge pump, thereby equalizing the hydraulic fluid pressure in both said closed loop circuits.