Patent GB-1016870-A (GB)
Title: Variable-ratio drive assemblies
IPC: B60T11/10
Filed: 1964-06-25
Abstract: 1,016,870. Hydraulic transmission of power; band- and shoe-drum brakes; hydraulic brakes. GENERAL MOTORS CORPORATION. June 25, 1964 [July 9, 1963; Sept. 27, 1963], No. 26277/64. Headings F2D, F2E and F2F. In a continuously variable speed hydrostatic drive for a vehicle, including an engine-driven variable displacement pump 312 with delivery and return lines 326, 331 to at least one variable displacement motor 316, actuation of a service brake control 410 to apply a friction brake 499 to a motor output member 321 also moves a pump displacement control 391 towards a minimum displacement position to provide increased engine braking. Function summary.-A pump displacement control valve 391 responsive to engine throttle variable (TV) pressure, engine speed, and pump suction pressure, and motor displacement control valves 416-419 responsive to TV pressure and to motor fluid flow rates, act to select the maximum efficiency engine speed for each throttle position and to control the pump 312 and motors 316-319 to maintain this speed throughout the entire range of motor output conditions. Progressive service brake operation causes increase of the motor displacement and progressive reduction of the pump displacement for increased engine braking. Engine overspeed causes a governor valve 467 to actuate each motor displacement control valve to open a restricted by-pass between its motor supply and return lines to increase braking while reducing pump flow, the pump displacement being simultaneously increased. Speed differential between the motors, and individual motor overspeed protection in the case of temporary loss of traction by one wheel, are provided. Hydraulic power circuit.-An engine 309 with a speed governor 310 drives a variable displacement swashplate pump 312 whose outlet and suction lines 326, 331 connect through a manual reversing valve 327 with lines 328, 329 supplying parallel connected variable displacement motors 316-319. In neutral, lines 326, 328, 329, 331 all communicate together. Each motor comprises a piston, cylinder, and valve assembly 501, a swashplate 502, and a displacement control motor 507, which on two motors has a stop 514 preventing zero displacement setting. Fluid pressure supply.-An engine driven charge pump 334 supplies charge and make-up oil from a sump 332, through a line 336, a filter 339, a pressure regulating valve 363, a line 335, a cooler 338, and a non-return valve 340, to a system supply valve 346. In forward drive, the pressure in the motor forward pressure line 328, sensed by a line 353, holds the spool of the valve 346 to the right to direct charge oil through a line 352 to the motor reverse pressure line 329. In reverse, or in forward overrun, the pressure in the reverse pressure line 329, sensed by a line 356, moves the spool of the valve 346 to the left, to direct oil through the line 353 to the forward pressure line 328. To allow push-starting, an electricallydriven pump 476 is provided in parallel with the charge pump 334 to charge the system when the engine 309 is dead. The line 336, which supplies servo-shift pressure to the pump and motor displacement control valves 391, 416- 419, and brake slack take-up pressure to a brake valve 405, is maintained by the valve 363 at or above a minimum pressure controlled by a bias spring 371, motor supply pressure in line 362, and take-up pressure 375. Control system valves.-To provide TV pressure 387 a TV valve 376, supplied by a line 358 at pump suction pressure, limited by a relief valve 360, is actuated by a link 381 of the engine throttle control acting through a bias spring 383. The spool 392 of the pump displacement control valve 391 is loaded leftwards for displacement decrease by TV pressure 387, and rightwards for displacement increase by pump suction pressure 352<SP>1</SP> and engine speed responsive pressure acting on a governor plug 401 and derived from the pressure differential between lines 342, 343 connected to the throat and entry of a Venturi 341 in the delivery line of charge pump 334. The spool 392 directs servo-shift pressure 336 to lines 396 or 397 respectively to decrease or increase the displacement of the pump 312. Each motor displacement control valve comprises a spool 424 having a land e sliding within a piston 424 to enclose a chamber connected through a bore 433 to an engine speed responsive governor pressure 466 regulated by the governor valve 467 in accordance with loading by the Venturi 341, which pressure 466 normally urges the valve spool 421 and the piston 424 apart. The spool 421 is urged rightwards for displacement decrease by TV pressure 387 and by motor oil flow rate responsive pressure acting on a governor plug 436 and derived from the pressure differential between lines 451, 452 connected to the throat and entry of a Venturi 446 in the reverse pressure line 329 of the respective motor 316, and leftwards for displacement increase by the pressures 453, 454 in the supply lines 328, 329 of the respective motor. The spool 421 directs servo-shift pressure 336 to lines 456, 461 respectively to increase or decrease the displacement of the respective motor. Service brake system.-A cam-operated internal brake band or shoe 499 with a return spring 500 is operated by hydraulic pressure in an actuator 483 under the control of a brake valve 405 actuated by a brake control link 410. A manual lever 488<SP>1</SP> allows emergency brake application. Initial movement of the link 410 connects the servo-pressure 336 to a line 375 to act on land b of the brake actuator piston to take up slack in the brake linkage. The pressure 375 also increases the bias on the regulator valve 363 to increase the servo-pressure 336. Further movement of the link 410 connects pump suction pressure 358 to a line 415 to supply oil to the actuator 483 to apply the brake 499, operating pressure being proportional to the displacement of link 410, which also acts on the TV valve 376 to progressively increase engine braking. A line 496 from the sump 332, together with non-return valves 491, 495, keeps the main bore of the actuator 483 full of oil to ensure rapid take-up of slack. Operation.-In forward, with engine idling and throttle closed, the pump 312 will have zero displacement and the motors 416-419 maximum displacement. On opening the throttle, TV pressure 387 is increased, but increasing engine speed raises the governor pressure differential 342, 343 acting on the pump displacement control tube 391 to overcome this to progressively increase pump displacement until a balance position is reached, with the correct engine speed for the chosen throttle setting. As the pump 312 starts to circulate oil to the motors, the resulting high pressure in the line 328 acts through the line 453 to maintain maximum motor displacement for high starting torque. As motor flow and TV pressure rise, the motor displacement will be progressively reduced until a balance position is reached. At low throttle settings in steady running, the two motors without stops 514 will reduce to zero displacement and cease to drive. Reverse operation is similar except that the line 454 from reverse pressure line 329 assumes the function of the line 453. When the brake pedal is actuated the link 410 acts on the TV valve 376 to increase TV pressure which acts on the pump displacement control valve 391 to reduce the pump displacement. Increased pump suction pressure 3521 acts on the valve 391 to oppose the decrease of pump displacement, as does the governor pressure due to increased engine speed, so that engine braking proportional to brake pedal displacement is obtained. The pressure 453, 454 are sufficiently large to overcome TV pressure and governor plug pressure in the motor displacement control valves, to move these valves to their maximum displacement positions. Maximum displacement of the brake pedal holds the spool of the pump displacement control valve at its minimum displacement position, causing the engine to be motored at its governed speed. The power circuit pressure reaches a maximum permitted value, which is sufficient to act through the line 453 or 454 of each motor displacement control valve to overcome the governor pressure 466 and collapse the spool 421 into the piston 424, withdrawing a land a from a bore 422 to interconnect the motor lines 328, 329. This increases the motor flow to provide maximum hydrostatic braking. On engine overspeed other than during maximum braking, the governor pressures 342, 343 move the spool of the governor valve 467 leftwards to vent the pressure 466 to exhaust, allowing each motor displacement control valve to interconnect the motor lines 328, 329 as above. This by-pass flow reduces the flow to the pump 312, while the governor pressures 342, 343 act on the pump displacement control valve 391 to increase pump displacement. Should any motor overspeed, e.g. due to loss of traction, the resulting high flow through the Venturi 446 of that motor will cause governor pressures 451, 452 to act on the corresponding motor control valve to reduce displacement of that motor. Excessive uncontrolled differential action is thus prevented.  Modifications.-In a second system (not shown) having similarly arranged pump, motors, and motor displacement control valves, displacement of a link in the service brake control linkage actuates a valve to vent the TV pressure to the motor displacement control valves to exhaust, allowing these to move to the maximum motor displacement position. Simultaneously, the TV valve is moved to increase TV pressure, which acts on the pump displacement control valve to reduce pump displacement, which action is opposed by pump suction pressure, to give engine braking proportional to brake pedal displacement. The motor displacement control valves have no governor plugs, and the single Venturi in each motor branch circuit is replaced by two Venturis, one in each b