Patent GB-1536580-A (GB)
Title: Controlling hydrostatic transmissions
IPC: B60K17/10
Filed: 1976-07-13
Abstract: 1536580 Hydrostatic transmission control; change-speed control CATERPILLAR TRACTOR CO 13 July 1976 [2 Sept 1975] 29053/76 Headings F2D and F2W A control for a vehicle drive unit including, a hydrostatic transmission 20 (Fig. 1) in series with a two-speed gear 26 comprises a manual control valve 112 (Fig. 2) which meters control pressure liquid to pump and motor displacement adjusting follow-up servomotors 211, 50 and to an accumulator 120 which, when the control pressure exceeds a predetermined value corresponding to maximum output speed from the hydrostatic transmission, supplies a signal to a range selector valve 118 that changes over to effect ratio shift in the two-speed gear and additionally increases motor displacement to thereafter permit further increase in final output speed by reducing motor displacement again in response to further increase in control pressure. Function summary.-The control valve 112 meters pressure liquid from a constant volume flow supply 82 to a line 132 to provide differential control pressure signals 174, 132. A modulating valve 114 restricts the rate of increase of the high pressure control signal 196 supplied to the pump and motor displacement adjusting servos 50, 211 and thus restricts the rate of ratio change in the hydrostatic transmission. A reversing valve 116 selectively supplies the high pressure control signal to opposite ends of the pump displacement adjusting servo 211 for forward or reverse drive and supplies the signal to the motor servo 50 when maximum pump displacement is attained. The valve 116 additionally restricts the rate of decrease of the high pressure signal from the pump and motor servos 50, 211 during a forward-reverse shift The accumulator 120 selectively supplies the low pressure signal 132 to opposite ends of the range selector valve 118 in response to the high pressure signal 222 supplied to the motor servo 50. The range selector valve 118 selectively supplies servo pressure 84 to low ratio and high ratio clutches 272, 292 of the two-ratio gear. The range selector valve 118 additionally supplies the high pressure signal through a line 264 to the motor servo 50 during low ratio in the twospeed gear. An underspeed control valve 124 meters pressure liquid from the high pressure signal line to the low pressure signal line to cause a reduction in output speed when the engine speed is below a predetermined value. An override speed control valve 126 is actuated manually to meter pressure liquid from the high pressure signal control line to the low pressure signal control line and thus reduce output speed. A speed limiting control valve 128 reduces pressure in a brake control line 370 and increases the high pressure control signal when engine speed exceeds a predetermined value. The reduction in pressure in the brake control line 370 acts on a brake pressure control valve 129 to cause engagement of a transmission brake 360. A safety control valve 122 vents a line 156 to permit engagement of a parking brake and interconnects lines 160, 162 connected to opposite ends of the pump servo 211 to neutralize the hydrostatic transmission when the supply of pressure liquid to the system ceases, e.g. by stopping the driving engine. When the engine is restarted the safety control valve maintains the transmission neutralized and the parking brake engaged until the speed control valve is moved to a neutral position. System pressure is supplied by an engine driven pump 54 which delivers through a venturi 60 and flow control valve 76 which provides a constant volume flow 82 to the speed control valve 112 and a servo supply 84. A valve 86 meters pressure liquid from the servo supply 84 to a line 88 connected to a replenishing and relief valve system 52 of the hydrostatic transmission. Liquid from the relief valve system 52 is returned to sump through a relief valve 92, cooler 94 and jet pump 98 which draws liquid from the two-speed gear 26. The venturi 60 is provided with thermal compensating means 62 and provides an engine speed differential signal through lines 64, 66. Pump and motor displacement control.-With the spool 130 of the control valve 112 in its neutral position pressure liquid is supplied through a line 144 to the lower end of the safety control valve 122 to move a spool thereof upwardly against spring bias to a position in which it blocks communication between lines 160, 162 and supplies pressure liquid from a line 150 to the brake disengaging line 156. The pressure in the line 150 is additionally supplied through a restriction to the lower end of the spool to maintain the latter in its shifted position while pressure is supplied to the system. Rightward movement of the spool 130 restricts communication between the lines 82, 132 and thus increases the pressure in the high pressure control line 174. The pressure signal 174 is transmitted through the modulating valve 114 and line 196 to the reversing valve 116. A manually actuated spool 208 of the reversing valve selectively connects the line 196 with the line 210 or 228. The increasing pressure in the line 210 or 228 acts on a spring centred piston of the pump displacement servo 211. The piston is connected to the control valve of a followup servo unit 246 which adjusts pump displacement and bodily moves the pump servo 211 to return the servo unit control valve to its null position. When the pump displacement attains its maximum value the pressure signal in the line 196 which acts on the lower end of a sequencing valve 212 (Fig. 6) is sufficient to move the valve 212 upwardly against the bias of a spring 220. The line 196 is thus connected through a line 222 to a chamber 300 of the motor displacement adjusting servo pilot valve 248 (Fig. 11). The high pressure line 222 is additionally supplied to the accumulator 120 and through the range selector valve 118 and line 264 to a second chamber 310 of the pilot valve 248. Further increase in the high pressure signal 222 thus acts in the chambers 300, 310 to move the pilot valve 248 right. The valve 248 meters pressure liquid from the servo line 84 to a chamber 322 to move a motor displacement adjusting piston 46 left to reduce motor displacement. Movement of the piston 46 additionally acts through a follow-up linkage on the pilot valve 248 to return the latter to its null position. When the motor displacement attains its minimum value the pressure in the line 222 moves the piston 278 of the accumulator 120 (Fig. 6) to a position in which it blocks communication between lines 132, 252 and establishes communication between lines 132, 280. The range selector valve spool 258 (Fig. 7) is thus shifted leftwardly to vent the low range clutch 272 and supply servo pressure 84 to the high ratio clutch 292. In addition the shift of the range selector spool 258 blocks communication between the high pressure signal line 222 and the line 264 and establishes communication between the low pressure signal line 132 and the line 264. The reduced pressure in the line 264 and chamber 310 causes an increase in motor displacement to its maximum value. Thereafter a further increase in the high pressure signal line 222 acts in the chamber 300 to reduce motor displacement and thus effect a further increase in the final output speed. If, during operation of the transmission in one direction, the reversing valve 208 is changed over, a valve 240 restricts the rate of decrease of the high pressure signal and thus the rate of deceleration of the vehicle prior to drive in the opposite direction. Override controls.-The underspeed control valve 124 has a valve spool which is normally held in a position blocking communication between the lines 140, 132 by the engine speed responsive differential pressure 64, 66. When the engine speed is below a predetermined value the drop in pressure in the line 64 permits a spring to move the valve spool to a position in which it meters pressure liquid from the line 140 and thus line 222 to the line 132 to increase motor displacement and thus reduce output speed. The override speed control valve 126 comprises a manual control rod 344 which acts through a lever 352 on a valve spool 354 which meters pressure liquid from the line 140 to the line 132 to increase motor displacement and thus reduce output speed. The speed limiting control valve 128 (Fig. 10) has a valve spool 366 which is normally held in the position shown by a spring 368 to supply servo pressure 84 to the brake control line 370. When the engine speed exceeds a predetermined value the engine speed responsive differential pressure 64, 66 acting on the spool 366 moves the latter up to restrict communication between the line 84 and the brake control line 370. The reduction in pressure in the line 370 permits a spring 410 to move a brake control valve spool 408 upwardly to a position in which it supplies pressure liquid to engage the brake 360. The brake engaging pressure is supplied from the hydrostatic transmission high pressure line through lines 422, 424 and a shuttle valve 420. The speed limiting control valve 128 additionally includes a valve spool 384, which, when engine speed exceeds a predetermined value, is moved upwardly to meter pressure liquid through an orifice 387 to the high pressure signal line 222 to effect a reduction in motor displacement and thus an increase in output speed. The valve spool 384 and an additional valve spool 390 provide a feed-back signal 380 which is proportional to engine speed and the difference between the high and low pressure signals. The feedback signal acts through the override speed control valve on the manual control 344 to provide "feel" proportional to braking.