Patent GB-982133-A (GB)
Title: Variable speed transmission control
Inventor: KEMPSON BERTRAM CARL
IPC: B60W30/18
Filed: 1961-04-20
Abstract: 982,133. Hydrostatic transmissions. DOWTY HYDRAULIC UNITS Ltd. July 20, 1962 [April 20, 1961], No. 14334/61. Headings F2C and F2D. A variable speed prime mover driving a variable ratio hydrostatic transmission is controlled by means tending to maintain the prime mover at any selected speed, and the speed ratio of the transmission is automatically varied to maintain the prime mover at the selected speed. irrespective of variations in the load. As shown a diesel engine 1 drives a shaft 2 driving a variable displacement hydraulic pump 3 connected by pipe lines 5, 6 to a fixed displacement hydraulic motor 7 driving the ground engaging wheels of a tractor. The displacement of pump 3 is controlled by a servo piston 25 supplied with liquid by an engine driven gear pump 4 under the control of valve 45 actuated by a manually operated lever 22 which also determines the speed of the diesel engine 1. The engine has a variable delivery fuel injector pump 9 adjusted by a pivoted lever 11 which is adjusted by means of a bellows 12 the opposite end of which reacts against a cam 16 formed on the lever 22. The bellows 12 is extended by a spring 13 and is subject to the pressure existing at a Venturi 14 in the inlet to the engine manifold 15. For any selected speed position of the cam 16 the bellows 12 will adjust its overall length to maintain the engine approximately at the selected speed. The pump displacement servo piston 25 is biased to a central (zero displacement) position by a spring 34 reacting between loose stops 39, 41 co-operating with stops 37, 38 fixed on a piston rod 28, the loose stops and spring 34 being located within a fixed cylinder 33. The gear pump 4 is permanently connected through fixed restrictors 43, 44 to each end of the servo cylinder, and is also connected to a pair of tapered ports 52, 53 of the control valve 45. Ports 56, 57 of the control valve are connected by a line 58 and non-return valves 59, 61 to the lines 6, 5. On movement of the lever 22 to any speed setting, valve member 47 of the valve 45 is moved to uncover port 52 or 53 a predetermined amount and to uncover port 56 or 57 so that the pump 4 supplies through the appropriate pair of ports to whichever line 5 or 6 is the low pressure line. Assuming that the lever 22 is moved in the forward direction, valve member 47 is moved to the left to uncover port 53 a corresponding amount, and there is a corresponding pressure drop in the right-hand chamber of the valve cylinder which pressure communicates through a line 55 with the right-hand chamber of the servo piston 25 which is urged to the right by the pressure in the left-hand chamber which is the full pressure of the pump 4. The opening of the port 53 is so related to the speed setting of the lever 22 that the pressure drop in the valve chamber is sufficient for normal operation under light load to move the piston 25 the maximum amount against the loading of the spring 32 to give full displacement to the pump 3. If an increase in load causes the engine speed to drop, the pressure drop occurring at the port 53 will be reduced thus causing the piston 25 to move to the left to reduce the pump displacement and thus reduce the speed ratio of the transmission to reduce the load on the engine and thus prevent excessive reduction of engine speed.