Patent GB-1212673-A (GB)
Title: Hydraulic apparatus
Inventor: KEMPSON BERTRAM CARL
IPC: F15B11/10
Filed: 1966-09-09
Abstract: 1,212,673. Fluid-pressure servomotor systems. DOWTY HYDRAULIC UNITS Ltd. Dec. 4, 1967 [Sept. 9, 1966], No.40419/66. Heading G3P. [Also in Division F2] A hydrostatic transmission comprises a variable displacement pump 1 and a fixed or variable displacement motor 2 and having the pump displacement controlled by a servomotor 37 which is itself controlled by an hydraulic actuator 16 which is slidably mounted on a fixed piston 18 and defines working spaces 21, 22 therewith. A pump 8 supplies control liquid to a line 11 at a high-pressure determined by a spring-loaded relief valve 12 which maintains a lower pressure in a line 7, which lower pressure is maintained in the return line of the transmission by way of a valve 6. Parallel paths comprising fixed restrictions 46, 47, lines 48, 49, and variable throttle valves 51, are provided between the high and low pressure control lines 11, 7, the working spaces 21, 22 being respectively connected to the lines 48, 49. The throttle valves 51 are respectively opened by pivotal movement of a handle in opposite directions from the position shown; the extent to which the selected valve is opened depending on the degree of movement of the handle. The opened valve maintains a pressure corresponding to its degree of opening in the associated line 48 or 49 and hence in the working space 21 or 22. The actuator 16 is thereby displaced from its central position, compressing centring springs 35, and displacing a servovalve 36 to give follow-up movement to the pump adjusting servopiston 38. In order to limit the transmission pressure a valve 78 having ports 76, 77 communicating with the working spaces 21, 22 may be provided. Valve 78 has a control port connected to the transmission high-pressure line, or by way of a shuttle valve to the transmission line at the highest pressure. The valve may be as described with reference to Fig. 2 of Specification 1, 212, 675 or as described in Specification 1, 212, 676 and serve to inter-connect the working chambers 21, 22 on excess transmission pressure during normal drive only (that is, not during overrun) so that the pump displacement is reduced, alternatively the valve 78 may be as described with reference to Fig.1 of Specification 1, 212, 675 and serve to inter-connect the working chambers on the occurrence of excessive pressure in the line 3 or 4 irrespective of the direction of the pressure difference between the ports 76 and 77. The transmission pressure may also be limited by a valve which is intended to replace a cover plate 99 and has ports 103, 101, 102 communicating respectively with the interior of the casing (and hence with a reservoir), with the working space 22, and with the line 49. The valve is subject to the pressure in the line 3 and on the occurrence of excessive pressure disconnects the ports 101, 102 and connects the port 101 to the exhaust port 103. Such a valve is described with reference to Fig.1 or Specification 1, 212, 674 and is particularly intended to control the rotation of the motor 2 when the motor drives a cable winding drum and it is desired to maintain the predetermined tension in the cable irrespective of whether the cable is being wound in or paid out.