GB1212674A - Hydrostatic power transmission - Google Patents

Abstract

1,212,674. Fluid-pressure servomotor systems. DOWTY HYDRAULIC UNITS Ltd. Dec.4, 1967 [Sept. 9, 1966], No.40420/66. Heading G3P. [Also in Division F2] A hydrostatic transmission for a cable winding drum comprises a fixed or variable capacity hydraulic motor 2 supplied by a pump 1 the capacity of which is adjusted by a servomotor, and the pressure in the transmission supply line 3 (for forward drive) acts, when it reaches a predetermined value, to reduce the pump displacement or even give the pump a reverse displacement in order to maintain a constant tension in the cable. As shown in Fig.1 a servopiston 7 is spring- biased to a central position giving zero pump displacement and is supplied with servo pressure by an input driven pump 12. Pump 12 supplies liquid at a pressure determined by a valve 15 to one servo chamber 11 through a restriction 18 and to the other chamber 9 through a restriction 19 and a valve 21. A rotary control valve 23 is connected to the chambers and serves to selectively provide a restricted connection between either servo chamber and a line 16 which supplies make-up liquid to the low-pressure transmission line. For forward displacement, valve 23 is manually turned in the direction F to connect chamber 11 to the line 16 and thus reduce the pressure in this chamber. The valve 21 is subject to the pressure in the transmission line 3, which is the flow line for forward drive, and below a predetermined pressure interconnects ports 32, 33 which are respectively connected to the restriction 19 and to the servo chamber 9. Above the predetermined pressure the chamber port 33 is connected to an exhaust port 34 so that the pump displacement control is shifted towards its reverse displacement position to maintain the predetermined pressure in the transmission line 3. In a modification the rotary control valve 23 is replaced by two selectively operable throttle valves shown in Fig.3 of Specification 1, 212, 673. In the embodiment of Fig. 2 a differential servopiston 42 controls the pump displacement and has a chamber 47 connected to the output of an input driven pump 48, and a chamber 46 connected, through a valve 67, to a port 63 of a control valve 59. Rotation of a handle 53 in a direction F or R to give forward or reverse shifts the valve 59 to connect port 63 with a port 62 connected to the servo chamber 47 or to an exhaust port 64 so that the piston is shifted downwardly or upwardly to give forward or reverse drive. The transmission line 3 is connected to a port 69 of the valve 67 and above a predetermined pressure, the servo chamber 46 is cut off from the control port 63 and opened to an exhaust port 78 so that the piston 42 is shifted upwardly towards its reverse displacement position.