ES360704A1 - Hydrostatic front wheel drive for a tractor - Google Patents

Abstract

In a hydrostatic transmission (see Division F2) the front wheels of a tractor are each driven by hydraulic motors supplied by a main engine driven pump which also supplies other hydraulic circuits, the system being arranged so that priority of the pressure supply is ensured for the latter. Forward or reverse motor operation can be selected without reversing the pump, the motors being connectible in series or parallel. The front wheel drive is disconnected when the main transmission is shifted to neutral or park or when the tractor speed, as determined by the main transmission ratio exceeds a predetermined value or when the main transmission clutch is disengaged. Each motor drives the wheel through a planetary gear train having an associated hydraulic actuated clutch which is automatically engaged or disengaged when the motors are respectively pressurized or connected to the reservoir. Tractor arrangement.-An engine-driven main pump (30), Fig. 1, supplies fluid pressure to power brakes (not shown), a rockshaft cylinder (not shown) and a power steering cylinder (32) which actuates the front wheels (20, 22). The rear wheels are driven conventionally from an infinitely variable or step type transmission (26) via a main drive clutch disengageable by a clutch pedal 44, Fig. 2. Control levers (50) control the rockshaft and other hydraulic functions whilst a lever (48) controls the movement of members 220 and 230 to control the transmission ratio or the forward, neutral and reverse conditions. Front wheel drive and steering arrangement.- The steering rod (34) moves housing 94 about the axis of pins 88, 90, Fig. 4, through which feed and return hydraulic pressure pipes are connected respectively to passages 146 and 150. The passage 146 is connected via passages 152, 154 to the inlet port of motor 112 and passage 150 is connected to passages 156, 158 to the motor outlet port. The motor output shaft 116 is splined to a shaft 121 carrying an integral first sun gear 122, a ring gear 124, planetary pinion 126 being journalled on a carrier 128 splined to a second sun gear 130 which drives a second annular ring gear 132 engageable by an hydraulically actuated clutch 133 with an annular member 134 connected to the housing 94 by a member 110. The clutch includes a plate 136 shiftable against spring bias by pistons 138 under pressure admitted through a toroidal passage 142. The latter is fed from the inlet passage 152 through passage (160), Fig. 5 (not shown), an orifice (163) and passages (162, 168). The pressure in passage 142 is relieved on clutch disengagement through passages (168, 162) check valve (166), passages (164, 156) and 150. One of the planetary elements may be shiftable out of the drive train and the clutch 133 may be partly disengageable to assist steering. Control circuit.-The main pump 30, Fig. 2, supplies the other hydraulic circuits 61 and, i.e. priority valve 186 which closes when the pressure in line 182 is below a predetermined value, e.g. when the pressure is being supplied to the other circuits. Pressure is directed to a solenoid actuated direction control valve 188 which is spring-loaded into a neutral condition when the front wheel drive system is inoperative in which line 182 is blocked and lines 70, 72 are connected to the return line 184. Inlet line 72 is connected to line 182 and return line 70 is connected to the return line 184 for forward drive, inlet 182 being connected to line 70 and line 72 connected to line 184 for reverse drive. A solenoid-actuated series-parallel control valve 190 connects the motors in parallel as shown or in series in the position of Fig. 3 (not shown). The electric current which energizes the valve solenoids is supplied from a battery 28 through a switch 206 actuated by the clutch pedal 44 whereby when the main drive is disengaged the current supply to valve 188 is cut off to cause the latter to shift to its neutral condition. The engagement and series-parallel connection of the front wheel units is controlled by a manuallyactuated switch 208 including switches 210, 212 and 214. When switch 208 is moved to the series position, Fig. 3 (not shown) switch element 210 closes to supply current to solenoid of valve 190 which is shifted to its series condition and switch 212 closes to supply current to line 216 which is connectible to a solenoid lead 202 through a switch 218. The latter is opened or closed by a transmission control element 220, connected to the transmission control lever 48, in response to the shifting of the transmission into forward, neutral or reverse condition. The element 220 may form part of the transmission control linkage or part of the transmission itself. Whne the transmission is in either neutral or forward drive switch 218 is open to disconnect lead 202 from the battery. When element 220 is in the reverse drive position switch 218 is closed to connect lead 202 to lead 216 thereby shifting the valve 188 into the reverse drive condition, provided switch 206 is closed. A switch 226 is operated in a similar manner to move valve 188 into its forward drive condition. A further element 230 which is connected to the transmission control lever 48 actuates switches 228, 236. The element 230 can be connected to a different transmission control element from that connected to element 220. In the park condition the switch 228 and 236 are open and the lead 200 is disconnected from the battery. In the five lowest ratios switch 228 is closed so that current is supplied to lead 200 to maintain valve 188 in its forward drive position when switches 206, 212 and 226 are closed. When the motors are connected in parallel switch 208 is shifted so that switch element 212 contacts the upper branch of the lead 216 and the switch element 214 is disconnected from lead 238. Thus, if the transmission is shifted to ratios above fifth, switch 228 is opened and since lead 238 is not connected to the lead 200 the latter is disconnected from the current source so that the valve 188 moves to its neutral condition. In sixth and seventh forward speeds the switch 236 is closed and if the switch 208 is moved to the motors-in-series condition the switch element 214 connects the lead 238 to the lead 200 to maintain the valve in its forward condition. In eighth gear both switches 228 and 236 are open as in the park condition.