Patent GB-1117666-A (GB)
Title: Hydrostatic transmission
IPC: B62D11/18
Filed: 1965-06-29
Abstract: 1,117,666. Selective drive. for steering. SUNSTRAND CORP. 29 June, 1965 [2 July, 1964], No. 2.7594/65. Heading B7H: [Also in Division F2] A variable-ratio and steering gear for wheeled or tracked amphibious vehicles comprises a hydrostatic transmission driving the vehicle output shafts, and a differential gear driven by an auxiliary hydrostatic transmission and rotating the output shafts in opposite directions to provide positive steering. Gearing.-As shown in Fig. 1, the main transmission comprises a variable stroke pump 18 supplying variable stroke motors 16, 17 respectively driving output shafts 10, 11. The pump andmotors are all of the "crosshead" type, having a rotatable cylinder block e.g. 16a with axial pistons connected to rods 16b which are pivotally connected to axially reciprocatable sliders in a rotatable crosshead 16c. The sliders are pivotally connected to rods 16d co-operating with a variable inclination swash plate 16e. An engine shaft 24 is geared to the pump crosshead 23, and the motor crossheads are geared to the output shafts. A steering pump 40 is also driven from the shaft 24 and supplies a steering motor 38 which drives the sun 51 of an epicyclic gearing having output carriers 52, 58 fast with the shafts 10, 11, interconnected ring gears 55, 56, and a permanently station sun 59. The steering pump and motor are both of the variable stroke crosshead type. Steering is effected by varying the pump displacement; the motor displacement can be set to one of two values according as the vehicle is being driven on land or on water. Control system components.-As shown in Fig: 2 servo pressure is supplied by an input driven pump 28 and an output driven pump 150 which is connected through a differential 155 to the output shafts 10, 11 so as to rotate at the sum of the speeds of the shafts. The differential 155 also drives a centrifugal governor 84 controlling a valve which controls the displacement of the main hydraulic motors 16, 17. The main pump 18 is controlled by a direction control valve 76 connected in series with an accelerator valve 77; the steering pump 40 is controlled by a steering servo 79 and the steering motor 38 is controlled by a steering mode valve 80. The centrifugal governor 84 controls a valve 305 controlling supply of liquid to either side of a servo piston 312 which is connected to a quadrant 314. Quadrant 314 is pivoted at 315 and meshes a gear meshing a quadrant 318 which is pivoted at 319. The quadrants are respectively connected to valves 335, 350, which co-operate with servo pistons of servos 82, 83 controlling the swash plates of motors 16, 17. The swash plates are thus connected for simultaneous movement, and each increment of movement of the piston 312 produces a corresponding adjustment of the swash plates. The quadrant 317 is connected to a spring loading the centrifugal governor to increase governor loading with increase of the swash plate angle. The pump 18 is controlled by a servo 81 supplied with liquid under the control of the accelerator valve 77 which is actuated by a manual control 232 acting. through a spring 230, the valve being subject to the pressure in the high pressure transmission line acting through a line 234, so that the pump swash plate is set to produce a pressure corresponding to the movement of the manual control 232. The swash plate is set to give forward or reverse according to the setting of the direction control valve 76 The steering pump 40 is controlled by a follow-up servomotor 79 having a piston connected to the swash plate 40e and a valve 470 co-operating; with the piston and connected to the manual steering control. The steering motor swash plate is adjusted by a servo piston 451 shifted from one end to the other of its cylinder under the control of the steering mode valve 80 so that the motor displacement is reduced when steering on water. Make up liquid is supplied by an input driven pump 26 to the main transmission through a shuttle valve 100 and to the. steering transmission through non-return valves 172, 174. To provide for the withdrawal of heated fluid from the main transmission a shuttle valve 120 is provided adjacent the motors and liquid from the low pressure line 71 or 70 passes via a cooler and a line 125 to a reservoir 127. Operation.-Initially the motors 16, 17 are set to maximum displacement and the pump 18 to zero stroke. The valve 76 is set for forward or reverse, and on actuating the accelerator valve 77 fluid is supplied to the pump servo 81 to shift the pump swash plate. The high pressure in line 71 or 70 is fed back to the accelerator valve through line 234 to close the valve and hold the pump at a displacement providing the pressure called for by the accelerator valve. The system is sized such that when the vehicle reaches a predetermined speed, of about 5 miles an hour, the demands of the system are met by the pump 18 at approximately three quarters of its maximum displacement, and the pump swash plate remains at this three quarter position except for minor temporary variations during speed changing. When the vehicle attains 5 miles an hour, and the pump attains its three quarter full stroke position, if the demand of the accelerator valve has not been satisfied the increasing vehicle speed moves the governor valve upwardly to start reducing the strokes of the motors 16, 17. To reduce speed, the manual pressure on the accelerator valve is relieved, and transmission pressure 234 moves the valve to the left to reduce the angle of the pump swash plate 18e and consequently reduce the vehicle speed, whereupon the governor valve moves downwardly to increase the motor displacements and thereby decrease the speed. During normal operation a valve 78 in a by-pass between the high and low pressure transmission lines functions as a high pressure relief valve, being subject to the high pressure line 70 or 71 acting on a small diameter portion 374 and subject to servo pressure 142, which is supplied in the forward and reverse positions of the direction control valve 76, to a line 297 to act on a large diameter land 372. Depression of the brake pedal acts to shift the direction control valve 76 to its central neutral position in which the pump displacement is zero and in which line 297 is open to exhaust so that servo pressure no longer acts on the valve 78, and at the same time a plunger 387 is shifted rightwards by the brake pedal and engages the end of a spring, from which it is normally slightly separated, to load the spring and hence load the valve 78 to the right so as to adjust the transmission pressure 70 or 71 at which the valve opens to thereby determine the braking effect in accordance with the degree of depression of the brake pedal. In a modification of the control system, Fig. 7, a valve 550 controls the speed and direction of travel of the vehicle, being displaced to the right by a link 580 acting through its spring 577 for forward travel and being displaced to the left by a link 585 acting through a spring 578 for reverse. The ends of the valve are subjected to the transmission pressures through lines 565, 566; shifting the valve to the right or left opens the left or right hand end respectively to exhaust port 591 or 607 so that in either case the high pressure of the transmission acts to restore the valve 550 to its central position. The linkage of the pump control servo 600 has cams 624, 625 acting on a valve 610 controlling the supply of control fluid for actuating valves 611, 612 of servos controlling the motor swash plates. When the pump displacement reaches three quarters of the maximum displacement, the cam 264 or 265 actuates the valve 610 to thereby displace the valves 611, 612 and actuate the servos to reduce the motor displacement. The control of the steering pump is unaltered, but the steering motor in this case is of fixed displacement. The charge and make-up pump 510 additionally supplies jets 710 located adjacent the cylinder blocks of the various units for cooling them. In a modification of the gearing (Fig. 8, not shown) the main transmission comprises a variable displacement pump driving a single variable displacement motor which is geared to the interconnected ring gears of a pair of epicyclic trains the planet carriers of which are fast with the output shafts. The steering hydrostatic transmission drives the sun gears of the two trains in opposite directions.