Patent GB-927849-A (GB)
Title: Improvements in or relating to automatic hydrostatic transmission for a motor vehicle
IPC: B62D11/18
Filed: 1960-04-21
Abstract: 927,849. Fluid-pressure servomotor control systems; valves; hydraulic transmission of power. INTERNATIONAL HARVESTER CO. April 21, 1960 [April 22, 1959], No. 14014/60. Classes 69 (2) and 135. [Also in Group XXXI] General arrangement.-An hydraulic power transmission for propelling a vehicle is formed by an engine-driven variable displacement pump 18 connected by a flow divider 23 to two variable displacement motors 33, 33<SP>1</SP>, each geared to one vehicle drive wheel 38, 38<SP>1</SP> and each having a servo means 42, 42<SP>1</SP> controlling the motor displacement. The divider 23 has two outlet ports 24, 25, each connected to one of the motors 33, 33<SP>1</SP>   whilst a threaded portion on a steering column 27 is integral with a bracket 30 which in the neutral position provides for equal flow through each port 24 and 25; movement of bracket 30 and an associated slide 53 alters the fluid flow to each motor and so provides the required differential speed of the two wheels when turning the vehicle. Similarly, if a pressure differential occurs in compartments 62, 64, e.g. due to one wheel slipping, then the plunger 53 is automatically moved to counteract this differential. An auxiliary constant-displacement pump 66 supplies fluid to a line 68 having a pressure relief-valve 69 and a pedal actuated valve 71 controlling connection of the line to a tank 21; a spring-loaded ram 73 open to the line 68 is connected via a rod 74 and a lever 75 to the displacement varying means for the pump 18. The lever 75 is attached to another lever 77 which both controls the loading of a maximum pressure ball relief-valve 79 in the line 68, and is pivotally secured by pin-and-slot means 78 to a rod 93, the other end of which is similarly connected to a piston-rod 98 and a set point or normally-operable speed-indicator 100. The piston-rod 98 is fixed to a spring-loaded piston slidable in a cylinder, the position of which is determined by a further spring-loaded piston, the whole providing a cumulative servo device 109, whereby the movement of the rod 98 is dependent upon energizing either or both pistons and represents the average pressure in conduits 32c and 47c. Secured to a power takeoff shaft 132 is a fly-ball governor 133 which controls a relief valve 140 in the line 68 similar to the relief valve 79 controlled by the ram 73. In a modification, the adjustable relief valves 79, 140 are dispensed with and the rod 93 is connected to the engine throttle control lines. Operation.-To operate the vehicle in a straight line the indicator 100 is set at the desired maximum speed and the valve 71 is opened, whereby the pump 18 is set at zero displacement and the motors 33<SP>1</SP>, 33 at minimum displacement with the pistons 43, 43<SP>1</SP> and 109 fully retracted. Upon starting the engine the governor 133 compresses the loading spring of the relief valve 140 and after starting the auxiliary pump 66 the operator closes the valve 71. Initially, the rod 93 will be in a rightward position due to the retraction of the servo device 109 and the co-operation between the rod 93 and a part 102 of the speed indicator. Pressure increase in the line 68 actuates the ram 73 to increase the displacement of the engine-driven pump 18 whereby equal flow takes place through the divider 23 to the servopistons 42, 42<SP>1</SP> and the motors 33, 33<SP>1</SP>. As the mean pressure downstream of ports 24, 25 approaches that representing the vehicle speed set on the indicator 100 the rod 93 is moved to the left to unseat the relief valve 79 and decrease the pump displacement until an equilibrium condition is attained. Should the load on the power take-off be such as to cause the engine speed to decrease then the governor 133 operates the relief valve 140 to reduce the pump displacement, if necessary to zero, the demand of the power take-off having priority over that of the transmission. The vehicle is stopped by opening the valve 71, some braking action being provided by the motor 33, 33<SP>1</SP> acting as pump. Specifications 876,140 and 876,440 are referred to.