Hydraulic apparatus

956,895. Hydraulic transmission of power. DOWTY HYDRAULIC UNITS Ltd. March 8, 1963 [Jan. 16, 1962], No. 1578/62. Heading F2C. The displacement regulator of a variable reversible positive displacement pump is operated on by a servomotor the control of which is responsive to hydraulic flow through the pump and operates only through a small range of movement from the zero displacement position. A hydrostatic transmission for a vehicle comprises a positive displacement pump 1 and a fixed displacement motor 4, the displacement of the reversible pump 1 being controlled manually by a lever 11. The lever 11 is connected to a lever 15 bearing two pegs 18, 19, acting as stops for a lever 17 frictionally engaged by the motor shaft 5 to sense rotation of that shaft. The pegs 18, 19 are spaced just sufficiently apart such that at zero displacement position of the lever 11, any rotation of the shaft 5 will cause the lever 17 to open a relative vent valve 21 or 22 to vent a servo unit 28 or 29 causing the non- vented servo, under the pressure from a makeup pump, to move the lever 11 through a cam 31 to reduce the pump displacement until the flow through the pump is zero. The pegs are so spaced that movement of the lever 11 to produce a desired rotation of the shaft 5 cannot be reduced by the opening of a vent valve, since the lever 17 abuts a peg before reaching a valve opening position. The return of pump displacement to zero may also be performed by electrically controlled servo means controlled by flow metering devices in the pump/motor pipelines.