Patent GB-985894-A (GB)
Title: Hydrostatic transmission
IPC: F16H61/46
Filed: 1963-06-06
Abstract: 985,894. Hydrostatic transmission control. NEW YORK AIR BRAKE CO. June 6, 1963 [July 5, 1962], No. 22530/63. Heading F2D. In a forward and reverse hydrostatic transmission driving the road wheels of a front end loader, wherein an engine-driven, variable displacement pump 13 feeds a fixed displacement reversible motor 11 through conduits 19, 21, and the pump displacement is manually controlled through a position-responsive fluidpressure servo control comprising a follow-up valve 33 and a double-acting servomotor 26, to prevent engine stalling the servomotor 26 is vented by an override valve 68 when pump output pressure acting thereon reaches a predetermined value, and an override valve control sleeve 76 responsive to pump displacement increases and decreases said value respectively with decrease and increase of pump displacement. In addition to pump output pressure, the override valve 68 is also acted on by pressure in a separate circuit 102 actuating the materialhandling bucket. The pump 13 and the motor 11 are of the rotary cylinder barrel, longitudinally reciprocating piston type. The pump swash-plate 16 is tiltable, by the servomotor 26, to maximum stroke positions on opposite sides of a neutral position. The axis of swash-plate trunnions 17 is so positioned that the pump reaction forces urge the swash-plate 16 towards its neutral position. The hydrostatic circuit is replenished by an engine-driven charge pump 22 connected through check valves 24, 25 to the conduits 19, 21 which respectively transmit pump output pressure during forward and reverse drive. The pump 22 also feeds a control system main line 46. The follow-up valve 33 comprises a fixed housing connected to the motor 26 by lines 31, 32; a plunger 53 respectively leftwardly and rightwardly moved for forward and reverse by pedals 63, 63a acting through springs 67, 67a and a lever 59; and a sleeve 39 rightwardly urged by a spring 52 to engage a pin 48 carried by a lever 49 pivoted at 51 and connected to the swash-plate 16 by pin and slot means. The over-ride valve 68 comprises a fixed housing connected to the servomotor 26 by lines 69, 71, the valve 33 housing and the lines 31, 32, and a plunger 88 rightwardly movable against a spring 93 by motors 98, 98a respectively energized by output pressure of the pump 13 and pressure in the bucket actuating circuit 102. A valve 101 connects the motor 98 to whichever of the conduits 19, 21 is transmitting pump output pressure. The control sleeve 76 is rightwardly urged by a spring 84 to engage pins 81 carried by a disc 82 rotatable with the lever 49. A motor 86 energized from the main line 46 biases the swash-plate 16, the servomotor 26 and the sleeves 39, 76 towards the neutral positions shown. The bucket actuating circuit 102 comprises a further engine-driven pump 103 feeding at least one double-acting servomotor 109 through a four-way valve 105. In operation, with, say, the " forward " pedal 63 acted on to move the plunger 53 leftwardly and hence the piston of the servomotor 26 rightwardly, the swash-plate 16 clockwise, and the follow-up valve sleeve 39 leftwardly, the override valve control sleeve 76 is leftwardly moved, by the anti-clockwise rotated disc 82 and the upper pin 81, to determine the position to which the plunger 88 must be rightwardly moved, by pressure in the motors 98, 98a, to vent the servomotor 26. On venting of the servomotor 26 the swash-plate is returned towards its neutral position and the control sleeve 76 is spring returned towards the right to close the override valve 68. The return movement of the swash-plate returns the sleeve 39 towards the right and re-opens the valve 33; however, a restriction 47 limits the supply rate to the valve 33 to a value less than the vent rate through the override valve 68. On closure of the override valve 68 after venting the motor 26 tends to return the swash-plate to the position dictated by the pedal 63; however, the then leftwardly moved control sleeve 76 once again vents the servomotor 26. On decrease of load pressure, the plunger 88 is returned towards the left and the servomotor 26 returns the swash-plate to the position dictated by the pedal 63. Specification 883,129 is referred to.