Patent GB-1211695-A (GB)
Title: Hydraulic power transmission
Inventor: KEMPSON BERTRAM CARL
IPC: B60W10/10
Filed: 1967-02-10
Abstract: 1,211,695. Hydrostatic transmissions; fluid pressure brakes. DOWTY HYDRAULIC UNITS Ltd. 9 Feb.. 1968 [10 Feb., 1967], No. 6426/67. Headings F2D and F2F. An hydraulic power transmission comprises a power source driving a reversible positive displacement pump 1 connected to a motor 5 by a pair of passages, a reverse control 21 for reversing the flow through the passages, hydraulic pressure means 7 to apply a friction brake to the motor output or on the load driven by the motor and an hydraulic change-over valve 41 mechanically connected to the reversing control to connect the passage directly connected to the pump inlet and connect it to the hydraulic pressure means whereby excessive pressure at the pump inlet causes application of the friction brake. Pump 1 is driven by a diesel engine, to give infinitely variable displacement between forward and reverse maxima by adjustment of the control rod 2 of a servomotor 3. For forward delivery the flow, to the fixed displacement motor 5 is via passage 4 and return is via passage 6. For reverse delivery the flow paths are reversed. The vehicle wheels (not shown), which are driven by motor 5, have friction brakes operable jointly by a piston/cylinder 7. A pump 8 delivers liquid from reservoir 9 to a variable orifice unit 11 in which a tapered needle 13, moved by the driver's speed control and a speed governor, controls the flow through an orifice 12. The maximum limit of pressure drop is controlled by a spring-loaded valve 14 and the liquid leaving the unit 11 passes via a valve unit (not shown) which connects pipe 15 to prime the passage 4 or 6 which is carrying the lower pressure. The pressure drop at orifice 12 is fed to a direction and power override control 19 which includes the valve 21 to connect pipes 17 and 18 in one direction or the other to pipes 22 and 23 which are connected to the two ends of a cylinder 28 in a servo-actuator 24. The cylinder 28 is centralized by springs 29 on piston 25 and piston-rods 26, 27. The deflection of cylinder 28 is proportional to the pressure difference supplied through pipes 22, 23. In the direction and power override control 19 the flow from passage 17 enters cylinder 31 and is prevented from flowing through rod 34 and ports 36 into chamber 35 by a sleeve 37 axially positioned by a brake control lever 38. The position of sleeve 37 determines the required movement of piston 32 against its spring load until ports 36 are opened to allow flow via chamber 35 into passage 18. This arrangement determines the maximum pressure drop which can occur at orifice 11 and thereby the proportioned displacement of pump 1. Brake circuit.-A rod 43 ensures that the valve 41 operates in the same way as change-over valve 21 to connect either passage 4 or 6 to valve 42. Pressure acting on a piston 46 moves it against spring 47 to vary the pressure of a spring 49 which acts on a plunger 51 to control the pressure fluid supply from pump 8 via port 56, groove 54, passage 58 and cylinder 57 to the cylinder/piston 7. The pressure in cylinder 57 reacts on piston 51 against the load of spring 49. Operation.-With valves 21 and 41 in the " forward " position the pressure difference acting on cylinder 28 moves it downwards causing liquid displacement from pump 1 through passage 6 to motor 5. Vehicle movement is caused by moving brake lever 38 to the " off " position and moving the driver's speed control to raise the engine speed. This causes needle 13 to move into orifice 12 to develop a substantial pressure drop which is supplied to the reversing valve 19 and the servo actuator 24. For a slow speed reduction the driver reduces the speed setting causing needle 13 to reduce to orifice size to increase the pressure drop. This is prevented by valve 14 and pump 1 continues at maximum displacement whilst the engine governor reduces the fuel supply and the engine ceases to drive the pump. Vehicle inertia causes the motor to displace pressure liquid through pipe 4 to pump 1 to cause the pump to drive the engine to dissipate energy as heat. The pressure in passage 4 is insufficient to operate the friction brake circuit. For increased braking lever 38 is moved so that sleeve 37 uncovers ports 36 thereby reducing the pressure difference to a value determined by the position of sleeve 37. Cylinder 28 takes up a position corresponding to the pressure drop and the pump 1 operates at a proportioned displacement. This alters the transmission gear ratio to allow engine acceleration and increase dissipation of heat energy. The pressure in passage 4 is sufficient to move piston 46 thereby compressing spring 49 and moving plunger 51. This connects the delivery pressure of pump 8 to the piston/cylinder device 7 to apply the friction brake with a force proportional to that applied to lever 38. In a modification the braking circuit may be replaced by a simple piston and cylinder including a non-linear spring. In further modifications the transmission pump may be of fixed displacement, a throttle valve may be included in the passage 4 and the system may have a non-vehicular application.