Patent GB-1074523-A (GB)
Title: Vehicle transmission
IPC: F16H61/46
Filed: 1965-01-11
Abstract: 1,074,523. Hydrostatic transmission. FORD MOTOR CO. Ltd. Jan. 11, 1965 [Feb. 20, 1964], No. 1149/65. Headings F2C and F2D. An hydrostatic transmission for a tractor comprises a variable displacement pump 18 supplying a variable displacement motor 36 and control means 156 which below a predetermined output speed provide maximum motor displacement and above the predetermined speed varies the motor displacement to provide a constant rate of flow to the motor. Summary.-Both the pump 18 and motor 36 are of the axial piston rotary cylinder block type having adjustable swash plates. The pump 18 supplies the motor 36 through a reversing valve 32. The displacement of the pump 18 is controlled in response to engine speed, and throttle pedal position. The motor displacement control valve 156 is responsive to the pressure drop across a restriction 158 and varies the motor displacement to maintain a constant pressure drop. If the pressure drop across the restriction 158 exceeds a predetermined value a pressure-sensitive element 180 causes the reversing valve 32 to move to its neutral position in which it connects the motor supply and discharge lines 70, 72. On overrun of the motor output shaft 40 excessive pressure in the motor discharge line is prevented by a relief valve 186. An engine driven make-up pump 52 supplies make-up fluid through a filter 80 and one-way valve 84 and also supplies the control fluid the pressure of which is regulated by a valve 144. The motor 38 drives the wheels through an hydraulically operated clutch 56 which is automatically disengaged when either the output speed or the clutch temperature exceed predetermined values. A high pressure relief valve 78 is provided. Operation.-The reversing valve 124 and its pilot valve 126 are, as shown, biased towards their central neutral positions by springs 128, 130, 146, 148. Energization of a solenoid 152 or 154 moves the pilot valve 126 right or left to supply pressure fluid from the line 140 through a line 138 or 136 to servomotors 132 or 134 which move the reversing valve 124 left or right for reverse or forward drive. The motor displacement control valve 156 is biased by a spring 172 to a position in which it vents the servomotor 50 and produces maximum motor displacement. The pressure difference across the restriction 158 is applied to hydraulic servomotors 162, 164 which act on the valve 156. When the pressure difference exceeds a predetermined value which corresponds to a predetermined output speed the valve 156 connects the pressure supply line 140 through lines 170, 160 to the servomotor 50 which reduces motor displacement. Thereafter the valve 156 Hunts and continuously changes the displacement of the motor 36 to provide a constant pressure drop across the restriction 158. If the restriction 158 becomes blocked and produces an excessive pressure difference a pressure-responsive element 180 opens a switch in the circuit to the solenoids 152, 154 and neutralizes the reversing valve 124. The motor supply and discharge lines 70, 72 are connected through lines 183, 184 and a valve 185 to a pressure relief valve 186 which on overrun of the motor 36 relieves the discharge pressure to the supply line. The solenoid-operated valve 185 reverses the connections to the relief valve 186 during reverse drive. The pump displacement control 86 normally reduces pump displacement as the pressure in a line 88 increases under the control of relief of valves 90, 92, 94. The pressure at which the relief valve 90 opens is increased with engine speed by a servomotor 102 which increases the compression of a spring 98 in response to the pressure difference across a restriction 104 through which the engine driven make up pump 52 delivers. The pressure at which the relief valve 92 opens is increased as the throttle pedal 114 is depressed. The line 88 is normally disconnected from the relief valve 94 by a valve 121 but is connected thereto when the output speed sensed by a governor 60 exceeds a predetermined speed (e.g. 10 m.p.h.). The hydraulically operated clutch 56 is engaged by supplying pressure fluid from the line 140 through a solenoid-operated valve 194 to a servomotor 190. Electrical control.-Initially switches 206, 208, 210, 220, respectively responsive to the governor 60, the engine clutch being engaged, the temperature-responsive element 200 of the hydraulically operated clutch 56, and the pressureresponsive device 180, are closed. Switches 214, 224 are selectively closed for forward and reverse drive respectively. Closure of a manual switch 212 energizes a relay C3 which closes switches CR3-1, CR3-2, CR3-3, the former of which energizes the solenoid 198 of the clutch control valve 194 to supply engaging pressure fluid to the clutch 56. If the forward drive switch 214 is closed closure of the switch CR3-2 energizes a relay C1 which closes a switch CR1-1 to energize the pilot valve solenoid 154 for forward drive, whilst if the reverse drive switch 224 is closed closure of the switch CR3-3 energizes the relay C2 to close switches CR2-1, CR2-2, the former of which energizes the pilot valve solenoid 152 for reverse drive, whilst the latter energizes the solenoid 188 to change over the valve 185. Opening of any of the switches 206, 208, 210, 220, de-energizes the relay C3 to neutralize the transmission. In addition opening of the switch 206 energizes a relay C4 which opens a switch CR4-1 and closes a switch CR4-2, the latter energizing the solenoid 123 of the changeover valve 121. Opening the switch CR4-1 disables a manual override forward reverse selector switch 215.