GB945425A - Improvements in or relating to hydrostatic transmissions - Google Patents

Abstract

945,425. Motor vehicle hydraulic driving gear. R. C. PEARSON. April 6, 1962 [Oct. 18, 1961; Nov. 6, 1961], Nos. 37379/61 and 39695/61. Heading B7H. [Also in Division F2] An hydraulic transmission for a motor vehicle comprises a variable displacement pump driving two variable displacement motors in parallel, the displacement of each of which is separately adjustable by a spring loaded hydraulic jack and a master control valve having an oppositely spring-loaded pressure responsive spool controls the fluid supply to such jacks in an arrangement such that normally with the spool in midposition, each jack receives fluid at the same pressure but on its being displaced, one or other jack supply is reduced or cut off and that jack exhausted or partially exhausted, and steering control is effected by varying the pressure acting on one or both ends of the spool of the master valve to cause it to move and vary the supply to one of the jacks to bring about desired relative operation of the motors to impart steering movement to the vehicle in which the transmission is fitted. As shown, a variable displacement pump 2 controlled by a servo and follow up valve 5 operated by links 9, 10 is driven by a motor 1 and is connected by output and return lines 15 and 16 with motors 21 and 22 which drive the track-driving sprockets 27 of an endless track vehicle through bevels 25 and 26. The engine also drives auxiliary pumps 3 and 4. The pump 4 supplies make-up fluid to the lines 15 and 16 through check valves 17 or 317, at a constant pressure controlled by relief valve 91. Pressure relief valves 319 and 19 are provided on the lines 15 and 16. The pump 3 supplies the various servo auxiliaries. The motors 21 and 22 are controlled by jacks 30 and 31, each of which comprises a piston 34 acted upon by a spring 35 and having an aperture 36, the position of the piston being determined by the dynamic balance between the pressures on opposite sides and the aperture 36 tending to damp sudden movements. Fluid is supplied to the jacks 30 and 31 from lines 63 or 64 through a valve 302, the spool of which is positioned by the pressures in lines 15 and 16. Each motor 21 and 22 is coupled to a pump 201 and 202 respectively, drawing fluid from the reservoir 11 or being supplied by the pump 4 the outputs of which, determined by an arrangement of check valves is directed through the lines 73 and 74 and act upon the ends of a change over valve 45 which receives fluid from the higher pressure of lines 15 and 16 through a shuttle valve 397, or from the auxiliary pump 3, whichever is at the higher pressure, through a shuttle valve 393. According to the position of its spool 44, the output from valve 45 is either returned to drain or is directed through a reversing valve 399 to the valve 302 and thence to the jacks. In the normal position, both jacks receive fluid at equal pressures. The outputs from pumps 201 and 202 lead through flow restrictors 102 and 103 and variable pressure flow restrictors 303 and 304, respectively, and thence to drain. The flow restrictors also receive fluid from the pump 3 through flow restrictor 106 and 107 so that there is always pressure on the ends of the spool 44. The variable flow restrictors 303 and 304 comprise spring-loaded cores 377 and 378 and are operable by levers 90 and 370, adjustable stops 381, 382 and 383, 384 being provided to determine the maximum and minimum pressures at which they operate. By pressing one or other of the levers 90 or 370, a pressure build-up in the line 73 or 74 is caused and consequent adjustment of the position of the valve spool 44, so that only one jack 30 or 31 received pressure fluid, the other being opened to drain, with consequent operation of its associated motor to slow down, stop or reverse, whereby the vehicle is steered. As the motor slows down, the output from the connected pumps 201 or 202 is diminished and consequently the pressure in the line 73 or 74 is restored with consequent operation of the valve spool 44. The steering operation is the same when the motors are being overdriven, e.g. when acting as brakes when the vehicle is running downhill. The steering is correspondingly effected when the vehicle is running in reverse, the valve 399 being operated to reverse the steering effect. Alternatively, the valves 302 and 399 may be omitted, whereby the steering effect in forward and reverse directions of drive is consistent but is reversed with reverse directions of drive. In a modified circuit, shown in the drawing of the second Provisional Specification the valves 302 and 399 are omitted and fluid pressure is supplied to either end of the steering valve 45 from the pumps 201 or 202, or from the pump 4 under the control of flow restrictors 102, 106 and 103, 107 and, optionally, non-return valves 104, 105. Specification 902,185 and 941,735 are referred to.