Patent GB-1131456-A (GB)
Title: Hydrostatic transmission
IPC: B62D11/18
Filed: 1965-11-12
Abstract: 1,131,456. Hydrostatic transmissions. SUNDSTRAND CORP. 12 Nov., 1965 [4 March, 1965], No. 48221/65. Headings F2C and F2D. A hydrostatic transmission, for power assist propulsion of a vehicle, has a variable displacement swash-plate motor 30 or 32, the displacement of the swash-plate 36 of which is variable in opposite directions from zero by a fluid piston-motor 52, 53, the direction of displacement being controlled by selectively-operable forward-reverse solenoid valves 220, 221, whilst the amount of the displacement is controlled automatically by a torque-valve 260, responsive to working pressure in the conduit 74 connecting the motor to a driving pump 83, and disabled below a predetermined working pressure (1000 p.s.i.) by a torque-control by-pass valve 262, responsive to working pressure 74 to disable the torque-valve 260 and control the supply of pressure fluid to the displacement motor 52, 53. Motors are provided for all four wheels of the vehicle and controlled differential action by flow dividers is provided. A 6 x 2 dump truck has four variable displacement hydraulic motors 30, 32 adapted to be selectively clutched through slipping clutches 49 to the truck's non-mechanically driven wheels 10, 14 for power-assist on rough terrain. The motors are supplied with fluid from a variable displacement pump 83, Fig. 1A, selectively switched in to provide "drive" pressure for the motors or switched out to provide low "idle" pressure and driven from the truck engine 16. The pump displacement is varied to provide a constant output pressure in the "drive" or "idle" states by an output pressure sensitive valve 100. The pump supplies fluid to the motors through a side-divider valve 170 metering equal flows to the near and off-side motors and divider valves 191, 192 metering equal flows to the fore and aft motors on each truck side. When the hydrostatic transmission is switched in to deliver "drive" line pressure, movement of the mechanical gear-shift selector selects either two (30) or four motors 30, 32 to assist drive. Motors and pumps.-The motors 30, 32 have rotating blocks 34 with axial cylinders communicating with a fixed inlet and outlet valve block 42 and are fast on output shafts 48 clutchable to the adjacent wheel. The axial motor pistons 35 bear on a stationary reaction swashplate 36 adjusted by fluid-actuated pistons 52, 53 and biased by springs 54 to a zero deliver position. The pump 76 is basically similar a d the swashplate 86 is biased to a zero delivery position by the pumping pistons 84. Line pressure supply.-The pump output pressure in line 74 is governed in either the "drive" or "idle" setting, selected by a valve 130, by a valve spool 101, Fig. 1A, moved by output line pressure 74 against a spring 107 to supply pressure 74 to the pump swashplate control pistons 96, 162. Excess line pressure moves spool 101 further left to vent piston 96 to a reservoir 112 giving a smaller swashplate angle. Low line pressure allows spool 101 to move right to supply line pressure to piston 96 to increase the swashplate angle. When the pump is set on "idle" by valve 130 the resulting low line pressure acting on piston land 166 allows the swashplate 86 to be sprung against a stop 164 to give approximately 3 degrees swashplate angle and a low "idle" delivery pressure. An engine-driven make-up pump 120 feeds fluid to the pump return line 80 whose pressure is set by a ball valve 123 controlling a vent passage 125. The hydrostatic transmission is selected by the valve 130. In "idle", line pressure 74 passes through a bore 136, Fig. 1B, in a piston 131 sprung on to a seat 133 and then through an on-off valve 139 to the return low-pressure line 80 to give a low "idle" pressure in line 74 which automatically sets the pump swashplate angle. When the hydrostatic transmission is required for power assisting the truck's normal mechanical drive the valve 139 is closed by energizing a coil 143 and the resulting pressure build up behind piston 131 prevents drain of line fluid 74 to the pump return line 80, thus maintaining a high "drive" line pressure 74. Flow dividers.-Line pressure 74 is fed via two restrictors 187, 188, inlets 171, 172 and axial valve spool passages 185, 186 to opposed ends of a spool 176, which is sprung to the centre of the side-divider valve 170. A larger flow to the near-side motors causes a high pressure drop across the restrictor 187 which causes a pressure drop at the bottom of spool 176, which thus moves down enlarging the flow to the off-side motors, and cutting down the near-side motor supply by land 178 throttling inlet 172 and land 177 throttling inlet 171. Similar divider valves 191 and 192 even out the fluid delivery from the valve 170 to the fore and aft motors 30, 32 on each truck side. This arrangement acts as a limited slip arrangement for pairs of side or fore and aft motors. The left-hand ends of valves 191 and 192 are controlled by spools 195 normally open to allow drive of the rear motors 32 only but selectively closed by pressure from a source 200 under control of a solenoid valve 199 to allow operation of all four motors 30, 32. When the normal mechanical gear-shift lever is placed in low gear the solenoid valve 199 is energized to be opened to bring in all four motors 30, 32 and to energize all clutches 49. In second gear, valve 199 is de-energized and closed, thus allowing valve 195 to open and thus bring the divider-valve spools left so that only the rear motors 32 are driven. In this position loss of motor feed fluid is prevented by the axial bores 185R and 185L in the divider valve spools seating on cones 210. Forward and reverse selection of motors.-The swashplate control pistons 52, 53 of the motors 30, 32 are pressurized from line 74 through distributer valves 220, 221 having spools 224, 225 biased upwards by line pressure 74, metered through spools 248, 249 moved by electromagnets 242, 243, against springs 226, 227. Springs 226, 227 normally bias the spools 225, 224 down allowing pressure from line 74 to pressurize pistons 53 and vent pistons 52 to pump return line 80, thus urging the swashplates 36 to the position shown for full-stroke in a reverse truck direction. In mechanical first gear the solenoids 242, 243 are energized moving the spools 248, 249 up to move spools 224, 225 up by line pressure where pressure from line 74 acting on pistons 52 reverse the swashplate angles from that shown to give full-stroke forward drive of all four motors, pistons 53 being drained to return line 80. In forward second gear only solenoid 243 is energized so that motors 32 have their swashplates moved to a full-stroke forward position and motors 30 have their swashplates moved to a full-stroke reverse position. The clutches 49 between motors 30 and the associated wheels are also automatically disengaged in this gear. At the same time valve 199 is de-energized to make the flow-divider valves 191, 192 feed the rear motors 32 only. Due to the blocking by valves 191, 192 of the pressure feed lines 43, 44 to motors 30, any tendency of the motors 30 to be driven by the wheels is restricted, since the clutches 49 are of the constant slip-type. When the hydrostatic drive is not utilized both solenoids 242, 243 are de-energized and all the clutches 49 de-energized to slip and any tendency of the motors to rotate is restricted by the pumped fluid being restricted by the by-pass valve 131. Motor-stroke adjustment.-A torque valve 260 assisted by a low-pressure by-pass valve 262 controls the motor strokes. When the hydrostatic system is on "idle" line pressure valve spools 264 and 280 of the valves 260, 262 are spring-urged to the left blocking pressure line 74 from line 229 feeding the distributer valves 220, 221. When on "drive" pressure at about 1000 p.s.i. pressure in line 74 moves spool 280 to the position shown so that the valve 260 alone can throttle feed of pressure line 74 to pistons 53 of the motors through valves 220, 221. At about 3000 p.s.i. the spool 264 moves right to fully open lines 273 to the motor swashplate control pistons. A cut-back in line pressure 74 reduces the motor swashplate angles to maintain the system at constant pressure, and thus give constant horsepower output at wheels.