Patent GB-1072651-A (GB)
Title: Hydraulic drive for industrial truck
IPC: B60K17/10
Filed: 1964-06-11
Abstract: 1,072,651. Hydrostatic transmissions. EATON YALE & TOWNE Inc. June 11, 1964 [June 25, 1963], No. 24235/64. Headings F2D and F2L. An hydrostatic transmission for an industrial truck comprises two or three engine-driven pumps 10, 11, 12, selectively feeding two constant displacement motors 13, 14, or one variable-displacement motor (92), Figs. 2 and 3, not. shown, through a speed-sensitive combined starting and shift valve 36, which, at engine idle, permits all the pumps to exhaust to a reservoir 23 through motor by-pass passages 46, 48, 50, 54, a restrictor 56 and cooler 55, the restrictor 56 constituting an engine speed sensing device, whereby as engine-speed increases above idle, pressure built up downstream of the restrictor 56 moves the shift-valve 36 to the right, in opposition to a spring 57, to close the by-pass for drive take-up. For ratio-shift control, the shift-valve 36 is moved to the right by speed increase in three steps, the first blocking the by-pass communication of the pump 10 which drives alone for first ratio; the second step blocking the by-pass' of the pump 11, providing second ratio using pumps 10 and 11; the third acting similarly on the pump 12 giving third ratio using all the pumps. A manual forward-neutral-reverse valve 37 is also provided. The pumpa feed to a main delivery passage 60 in a valve body 40 and thence through the reversing valve 37 to the motors. Service brake operation moves the shift valve 36 left to exhaust all the pumps and throttle fluid pumped by the drive motors for assisting the truck brakes. The shift-valve 36 is downshifted on excess line pressure caused by overload and the variable displacement motor when used. Excess wheel load in second ratio causes the motor to be placed at maximum displacement. A truck lift-ram 31 is fed from the pump 12. Operation.-In Fig. 1, the reversing valve 37 is set at forward, the engine 19 is idling and the shift valve 36 allows all three engine-driven pumps 10, 11, 12 to feed past a reduced portion 45 of the spool 36 to an exhaust passage 46 leading fluid around the reversing valve 37, past the left-hand end of the shift valve 36 to a return line 22 which feeds the pumps and connects to a fluid supply reservoir 23. The restrictor 56 builds up pressure on the left-hand end of shift valve 36 in a groove 51 which pressure at engine idle is insufficient to overcome the spring 57. Excess pressures in groove 51 and delivery line 24 of pump 10 are vented through check valves 56a and 66 to return line 22. Increase of engine speed above idle builds up pressure to the left of the shift-valve 36, which is shifted right to prevent fluid from pump 10 reaching the exhaust passage 46 so that it feeds to the motors 13, 14 for truck drive in 1st ratio. Motor exhaust fluid joins the fluid from pumps 11, 12 and passes to the groove 51 at the left-hand end of the shift-valve 36 and on to the return line 22. A chamfer 36c on the shift-valve 36 smooths the cut-off of pump 10 from exhaust to give a smooth truck start. Further engine speed rise moves the shift-valvo 36 right to also cut off pump 11 from exhaust so that it feeds via a check-valve 68 to passage 60 for second ratio drive. Further engine speed rise moves the shift-valve 36 far right to also cut off pump 12 from exhaust so that it feeds via check valve 70 to passage 60 for third ratio drive. The valve 56a relieves any further pressure build-up on the left end of the shiftvalve 36 on engine speed rise. With the shiftvalve 36 in third ratio position excess line pressure due to overload is fed through a check valve 77 to move a piston 74 leftwards pushing the shift-valve 36 into second ratio position. Further excess line pressure in second ratio is fed through a check valve 75 to move a piston 73 leftwards pushing the shift valve 36 into first ratio position. The pistons 73, 74 have axial bores to allow return rightwards motion when pushed by the shift-valve 36. In first ratio, excess line pressure due to overload slows down the engine 19 and the resulting low pressure on the left end of the shift-valve 36 allows it to be sprung left to exhaust all pumps 10, 11, 12, stopping drive. If the truck service brake is applied a coupled rod 78 forces the shiftvalve 36 far left against a spring 82 to exhaust all pumps via an uncovered line 80 to line 22 and the chamfered end 36e of the shift-valve 36 throttles the return flow of motor-pumped fluid in line 50 to reservoir 23. A check valve 86 allows motor pumped fluid upstream of the throttled groove 51 to feed back to prime the motors. Leakage of fluid at right ends of the valves 36, 37 is fed back to reservoir 23 by a restricted line 90 which slows upshift movements of the shift-valve 36. When two pumps only are used to feed a single variable displacement motor (92), Figs. (2, 3) (not shown), the motor is set at maximum displacement, Fig. 2, by a double-acting plunger (97) held in position by a ball valve (98) preventing fluid communication between the ends of the plunger through a restricted line (99). In second ratio fluid tapped from a pump (11) moves a plunger (100) down to unseat valve (98) and allow fluid flow from the left hand to the right hand end of plunger (97) so that fluid tapped from the left hand end of the shift-valve (36b) moves plunger (97) left to decrease the motor displacement for higher truck speeds. Truck load increases in second ratio cause the reaction of motor (92) to force the plunger (97) right to increase the motor displacement to accommodate the load. Any further load increase " downshifts " the shift-valve (36b), as in the Fig. 1 embodiment, and the ball (98) seats and holds the plunger (97) in a maximum displacement position, at the end of its rightward travel, Fig. 2. The restricted line (99) slows the movement of plunger (97).