Patent GB-878511-A (GB)
Title: Improvements in hydraulic power transmission
Inventor: THOMA OSWALD
IPC: F16H61/40
Filed: 1956-12-21
Abstract: 878,511. Fluid-pressure servo motor-control systems. DOWTY HYDRAULIC UNITS Ltd. Dec. 17, 1957 [Dec. 21, 1956], No. 39038/56. Class 135. An hydraulic transmission suitable for use in agricultural tractors and of the kind comprising a variable positive-displacement pump driven by a prime mover and a positivedisplacement hydraulic motor fed by the pump has manual means to control the pump displacement and pressure-sensitive means to override the manual control if the pump output pressure exceeds a certain value, this value increasing with reduction of pump displacement. As shown, the pump 1 feeds the motor 2 through conduits 3, 4 and its displacement is adjustable by a servomotor 9 under the control of a hand lever 23 acting on the servomotor through a spring link 26. The conduits 3, 4 are connected by conduits 44, 43 respectively to plungers 42, 41 which are loaded by a spring 36 of which the compression is variable by a cam 33 which is positioned by the servomotor 9 so that the compression of the spring is at its maximum when the pump is at zero displacement. If, at any given displacement of the pump, the pressure in either of the conduits 3, 4 should exceed that determined by the cam 33 and spring 36, the corresponding plunger 41, 42 is depressed to enable the fluid pressure to act on a piston 51 which is thereby moved against a loading spring 52 to connect the pressure fluid to a piston 61 which, acting through an abutment carried by the cam 33, returns the pump displacement towards zero. This arrangement prevents stalling of the prime mover by the application through the hand control of too great a load. To minimize wheel slip a pair of spring-loaded valves 63, 64 connected to the conduits 44, 43 is provided, the loading of these valves being adjustable so as to limit the torque that can be applied to the driving wheels of the tractor in accordance with the conditions over which the tractor is to be driven. For example, if the tractor is being driven over muddy ground the valves 63, 64 are set to open at a comparatively low pressure in the conduits 3, 4 so that when this pressure is reached it is applied to one of the pistons 51 to cause the pump displacement to be reduced. An auxiliary pump 5 supplies the servomotor with motive fluid and also supplies make-up fluid to the main circuit 3, 4 through non- return valves 7, 8. The high and low limits of pressure in the main circuit 3, 4 are determined by relief valves 19, 16 respectively, the valve 19 being connected to the circuit through non- return valves 17, 18 and the valve 16 being connected to the circuit through a shuttle valve 14. According to the Provisional Specification the hand control may act on the servomotor through a lever which is moved towards the position of zero displacement by either of a pair of plungers supplied through a relief valve connected with the main circuit. In a modification of this arrangement the loading of the relief valve is adjustable by a cam on the lever, the loading being at a maximum when the lever is in the position of zero pump displacement. In another modification, each plunger acting on the lever has its own associated relief valve and the lever carries a double cam arrangement to control the loading on both of the relief valves simultaneously.