Patent ID: 8186082

Claim:
A fluid pressure drive unit, which gives a fluid pressure driving force to a driven body independently, comprising: a fluid pressure pump for sending a working fluid used in the unit under pressure in both of the positive and reverse directions; a single-rod double acting cylinder operated by the working fluid; a tank for storing the working fluid; a pair of operate check valves which are interposed between the fluid pressure pump and the single-rod double acting cylinder to control the flow in both of the positive and reverse directions of working fluid therebetween; and a change valve, which is interposed between the fluid pressure pump and the tank to control the flow in both of the positive and reverse directions of working fluid therebetween, characterized in that there is provided a first excessive load relief valve for letting the working fluid escape from a bottom-side pipe line, which connects the bottom-side port of the single-rod double acting cylinder to the bottom-side operate check valve corresponding to the bottom-side port of the paired operate check valves, to a rod-side pipe line, which connects the rod-side port of the single-rod double acting cylinder to the rod-side operate check valve corresponding to the rod-side port of the paired operate check valves, and there is provided a second excessive load relief valve for letting the working fluid escape from the rod-side pipe line to the tank, and there is provided a negative pressure preventing check valve for allowing the working fluid to flow into the bottom-side pipeline from the tank.