Patent GB-1461163-A (GB)
Title: Pivot steering vehicle
IPC: B62D5/06
Filed: 1974-01-24
Abstract: 1461163 Hydrostatic transmission control CLARK EQUIPMENT CO 24 Jan 1974 [24 Jan 1973] 03292/74 Heading F2W A motor vehicle, such as a fork-lift truck, has two driving wheels, each driven by an hydraulic motor, and two steerable wheels (Fig. 1, not shown) the hydraulic motors being supplied by respective pumps of variable displacement whose displacement is varied together by an accelerator valve 38 and differentially in response to the steering means, the differential increasing towards the extreme steering positions. As shown, the accelerator valve 38 is operated by an accelerator pedal 36 (also controlling the prime mover) to move a valve spool 42 to supply pressure fluid from a pump 74 through valves 80L and 80R, a direction control valve 98, and reversing valves 102L, 102R to actuators 118L, 118R to thereby move pump 17L, 17R adjustment members 22L, 22R. A cam member 34 is moved as the vehicle is steered to differentially affect valves 80L, 80R and thereby cause differential movement of the pump adjustment members 22L, 22R. Operation.-On depression of accelerator pedal 36 a valve spool 64 of accelerator valve 38 is moved through a spring 52 to supply pressure fluid from a pump 74 to the valves 80L, 80R from which it passes by lines 96L, 96R to direction control valve 98 in relative amounts depending on the positions of valve spools 84L, 84R which in turn depends on the position of cam 34 which moves the spools 84 through springs 86 and cam followers 82R, 82L. The direction control valve 98 (in neutral as shown) if moved, for example, to the forward setting F supplies fluid through line 104L-104F from line 96L to reversing valve 102L and through line 104R-104F from line 96R to valve 102R which valves supply fluid to the forward sides of actuators 118L and 118R. The valves 102 are held in position to allow this operation by pressure from a pump 115 supplied through a relief valve 114 to chambers 110 of valves 102 to hold spools 106 against springs 108. The pressure to each of the chambers 110 can be exhausted through a connection 112 between each supply line 117 to the chambers 110 and the respective cam follower 82 so that, at the extreme positions of steering a cam surface 94 can lift a cam follower and so vent a chamber 110 to cause reversal of the supply to the respective actuator 118, the motors thus running in opposite directions to permit pivoting of the vehicle about a point between its driving wheels, the other cam follower here riding up the opposite side of a cam surface 92 to reopen the supply to it (cut-off when the follower 82 reached the bottom of the surface 92 and the pump 17 associated therewith was at neutral). An inching and braking pedal 72 can, through a spring 67, move the spool 64 against spring 52 to reduce the fluid supply from pump 74 irrespective of the position of accelerator pedal 36. The reaction pressure from the actuators 118 also acts on the spool 64 (through a bore 66 in the spool allowing pressure inside the spool to bias it against a spring 58 tending to shut off the supply to line 78, so that increased pressure is supplied to compensate for increased resistance to actuator movement. For steering movements less than that which brings cam followers 82 in contact with surfaces 92 or 94 the pumps 17 allow differential movements of the driven wheels to occur, through reaction in the supply lines to the motors.