Patent GB-1228135-A (GB)
IPC: F16H61/42
Filed: 1967-06-06
Abstract: 1,228,135. Controlling hydrostatic transmissions; linkwork. DOWTY HYDRAULIC UNITS Ltd. 5 June, 1968 [6 June, 19671, No. 26001/67. Headings F2D and F2K. In a control for an hydrostatic transmission, a ratio determining control rod 8 is actuated by a variable bell-crank lever comprising a handle 16 fast with a cylinder 21 and movable about a fixed pivot 18 and a piston 22 slidable within said cylinder and connected to said control rod by a long link 29, the piston being rightwardly moved to the position shown by hydraulic pressure generated by a make-up pump 9 driven at a speed proportional to that of the transmission input, and the arrangement being such that on a reduction of transmission input speed below a predetermined value the piston 22 is moved leftwardly by a spring (33) to reduce transmission output speed. The transmission may comprise a variable displacement pump in hydraulic connection with either a rotary motor of fixed or variable displacement or an hydraulic ram. The ratio may be varied by varying the displacement of the pump and/or motor. In the transmission shown, arranged between a vehicle engine and ground engaging wheels, a variable displacement pump 1 is connected in closed circuit with a fixed displacement motor 2 by lines 3, 4. The make-up pump 9 feeds a line 12 feeding the line 3 or 4 at lowest pressure through a non-return valve 14 or 15. The line 12 is also connected to the cylinder 21 through a manually controlled valve 34. The control rod 8 acts to vary pump displacement. The long link 29 is screw adjustable in length such that when the piston 22 is fully leftwardly moved by the spring (33) against a stop 26, the pivot 24 between the piston and the link coincides with the fixed pivot 18 and the transmission output is stationary. A relief valve 13 is set to relieve the line 12 at the lowest power-giving speed of the engine, say 1400 R.P.M. In operation, when the engine is stationary, the pivots 18, 24 coincide and the displacement of the pump 1 is zero. When the driver increases the engine speed to the lowest powergiving speed the pressure in the line 12 moves the piston 22 rightwardly against a stop 25 so that movement of the handle 16 effects displacement of the pump 1. Should the engine speed be reduced, as by the vehicle meeting resistance or by load applied to the engine by an independent device, pressure in line 12 will drop and allow the spring (33) to leftwardly move the piston 22 and reduce displacement of the pump 1 and hence output speed and engine torque requirement. At about 1000 R.P.M. engine speed, the pivots 18, 24 coincide to reduce output speed to zero. Should the driver wish to stop quickly the valve 34 is rotated to dump the pressure in the cylinder 21. In Fig. 3 of the Provisional Specification drawings (not shown), wherein two separate transmissions are driven by one engine to drive, e.g. two ground engaging tracks of a tractor, two of the above piston and cylinder arrangements are provided side by side, the cylinders being mounted on a common fixed pivot 18. Both cylinders are fed by one, only, of two make-up pumps of the transmissions such that reduction of pressure delivered by this one pump effects displacement reduction of both transmission pumps. Further, only one dump valve 34 is provided. In an alternative arrangement, a floating lever connected between the two piston and cylinder arrangements may be moved bodily or rotated for similar or differential adjustment for speed or steering control respectively.