1380729 Controlling vehicles DOWTY HYDRAULICS UNITS Ltd 19 May 1972 [20 May 1971] 16051/71 Heading B7H [Also in Division F2] In a hydrostatic transmission for a fork lift truck wherein an engine driven pump 2 supplies pressure liquid through a flow-dividing valve 7 and a forward-reverse throttle valve 25 to a motor 3, the by-pass liquid from the flowdividing valve 7 returns to a reservoir 5 through a restriction 199 and an engine speed control servo 158 is connected to the upstream side of the restriction 199 so that a reduction in the by-pass flow will cause an increase in engine speed and vice versa. The pump 2 may, as shown, be a fixed displacement gear type pump or may be a variable displacement pump and the motor 3 may, as shown, be a variable displacement swash-plate motor or may be a fixed displacement motor. The flow-dividing valve 7 adjusts the flow to the motor 3 in response to the pressure drop across the throttle valve 25. The motor 3 has its displacement controlled by a follow-up servomotor 124. The forward-reverse throttle valve 25 and the follow-up servomotor 124 are adjusted in a predetermined sequence by slave cylinders 41, 42, 136 supplied from a manually controlled master cylinder 105. A braking valve 51 throttles motor exhaust during overrun for vehicle braking. A make-up valve 73 supplies liquid to the motor supply line 47 (or 48) during vehicle braking. An override valve 141 overrides the slave cylinder control of the follow-up servomotor 124, to increase motor displacement, when the motor supply pressure exceeds a predetermined value. The forwardreverse throttle valve slave cylinders 41, 42 are selectively supplied from the master cylinder 105 through a reversing valve 104. A detent 99 prevents movement of the valve 104 between forward and reverse positions whilst the vehicle is moving. The master cylinder 105 is actuated by a follow-up servomotor 109 in response to movement of a pedal 117. The by-pass liquid from the flow-dividing valve 7 may be selectively supplied through control valves 161, 162 to lift and tilt servomotors 152, 153. Operation. Drive.-Movement of a lever 87 into a forward (or reverse) position moves a spool 92 of the reversing valve 104 left (or right) to connect a master cylinder supply line 137 to the forwardreverse throttle valve slave cylinder 41 (or 42). The movement of the valve spool 92 additionally closes a vent duct 123 which, in the neutral position of the spool 92, vents a supply line 111 to the servomotor 109. Depression of the pedal 117 moves a servomotor control sleeve 116 left to close an exhaust duct 115 and permit pressure liquid supplied from the pump supply line 6 through a pressure reducing valve 119, the reversing valve 104 and the line 111 to move the servo piston 108 and master cylinder piston 106 left. On initial depression of the pedal 117 the master cylinder pressure acting in the slave cylinder 41 (or 42) moves a valve spool 34 of the forward-reverse throttle valve 25 right (or left) to provide a throttled connection between the pump supply line 24 and the motor supply line 47 (or 48) for forward (or reverse) drive. The pressure supplied to the motor supply line 24 is also fed through a port 53 (or 52) and an orifice 23 to a chamber 17 of the flow-dividing valve 7. The pressure differential on the spool 8 of the flow-dividing valve 7 (i.e. the pressure drop across the throttle valve 25) adjusts the flow from the pump 2 to the line 24 in accordance with demand. Increased depression of the pedal 117 within the range A-B reduces the throttling effect of the valve 25 and thus increases motor speed. Within the range A-B the by-pass liquid from the flow dividing valve 7 flowing through the restriction 199 is sufficient to generate a pressure in the servo 158 to maintain the engine speed governor 159 at a half-speed setting. When the pedal 117 is in the position B the master cylinder pressure is sufficient to move the throttle valve spool 34 against the bias of a spring 43 to a position in which an additional loading spring 44 acts on the valve spool 34. The spring 44 prevents further movement of the valve spool 34 during further depression of the pedal 117 in the range B-C. The increased master cylinder pressure provided during depression of the pedal 117 in the range B-C overcomes the spring loading of a pilot valve 131 of the follow-up servomotor 124. Upward movement of the pilot valve spool 131 interconnects differential area servo chambers 127, 128 through a restriction 132 whereupon the servopiston 126 moves upwardly to reduce motor displacement. Within the range B-C the engine speed governor 159 is maintained at its half speed setting by the servo 158. At the pedal position C motor displacement is a minimum and further depression of the pedal 117 beyond the position C moves the throttle valve spool 34 further right against the combined action of springs 43, 44 to further reduce the throttling effect of the valve 25 and thus further increase motor speed. The increased flow to the motor 3 is accompanied by a reduced by-pass flow through the restriction 199 and thus a reduced pressure generated in the servo 158 which permits a spring 203 to increase the engine speed setting of the governor 159. The resulting increased engine speed produces an increased output from the pump 2. During drive the pressure in the line 22 opens a valve 67 to vent the right end of the braking valve 51 which thus moves right to connect a motor return line 49 with the make-up valve 73 which in turn is moved right by the pressure in the line 22 to connect the motor return line to the reservoir 5. Braking.-On overrun reduced motor supply pressure acting in the line 22 permits the valve 67 to close whereupon pressure build-up in the righthand chamber 62 of the braking valve 51 moves the valve spool 57 left to throttle motor exhaust for vehicle braking. The motor return pressure in the line 49 acts on a valve 66 to meter pressure from the chamber 62 and thus limit braking. During overrun the make-up valve 73 is moved to the position shown in which the pump supply line 6 is connected through the braking valve 51 and one-way valve 84 or 85 to the motor supply line to prevent cavitation. Override and safety controls.-Excessive pressure in the motor supply line 47 (or 48) acts through a line 142 on the override valve 141 which vents the servo chamber 128 and thus permits the servo piston 126 to move down and increase motor displacement. The liquid displaced from the slave cylinder 136 acts on the throttle valve 25 to increase flow to the motor 3. The increased flow to the motor 3 may reduce by-pass flow to the restriction 199 sufficiently for the servo 158 to increase engine speed and thus pump delivery. During vehicle motion, return line pressure acts through a restricted duct 64 in the braking valve spool 58 and a line 102 on the detent 99 to move the latter into the path of the reversing valve spool 92 to thereby prevent a shift from forward to reverse or vice versa whilst the vehicle is in motion. Lift and tilt control.-Manually controlled lift and tilt valves 161, 162, selectively supply bypass flow to lift and tilt servomotors 152, 153. The valves 161, 162 additionally throttle the flow of by-pass fluid to the restriction 199 to increase the pressure supplied to the servo cylinders 152, 153 and also to increase engine speed and thus pump delivery.