Patent GB-522881-A (GB)
Title: Motor vehicle power transmission mechanisms
IPC: B60K17/08
Filed: 1939-03-17
Abstract: 522,881. Controlling change-speed gearing; fluid-pressure servomotors. GENERAL MOTORS CORPORATION. March 17, 1939, No. 8545. Convention date, April 1, 1938. [Class 80 (ii)] [Also in Group XXIX] Power transmission mechanism for vehicles comprises valve-controlled, servo-actuated change-speed gearing providing stepped driving ratios and controlled by a valve operable by fluid pressures determined respectively by engine speed and throttle opening to give four forward speeds, and such that for increasing speeds the tendency is to select a high gear ratio and for increasing actuation of the throttle, a low gear ratio. In Fig. 1 the driving shaft 5 from the engine drives a shaft 8 either in a forward or reverse direction by direct clutching of a sliding gear 19 thereon with a gear 7 or the meshing of the gear with an idler 18 driven by counter shaft gearing 7, 16, 17 respectively. Secured to the shaft 8 is a driving annulus 12 of the first or front epicyclic unit the planet wheels 24 of which are carried by a carrier 22 secured to an intermediate shaft 21 whilst a sun gear 25 is freely rotatable on this shaft but adapted to be braked by a brake band 80 or clutched to the carrier 22 by a clutch 33, 36 to give a reduced or direct drive respectively of the shaft 21. The second or rear epicyclic unit is a compound train and is driven by shaft 21 which has fixed sun wheels 37, 38 thereon engaging with planet pinions 43, 44 on carriers 46, 54 respectively. The carrier 46 is secured to the final driven shaft 50 while the carrier 54 is integral with an annulus 52 meshing with the planets 43: The annulus 42 meshing with the planets 44 is freely rotatable but is adapted to be braked by a brake 90 or clutched to the shaft 21 by a clutch 55, 60 to give a reduced or direct drive through this rear unit. The clutches 33, 36 and 55, 60 are adapted to be actuated by pressure fluid supplied to pistons 72, 76 respectively and the brakes 80 and 90 are also adapted to be actuated by pressure fluid supplied to pistons such as 291, 294, Fig. 3. The arrangement is such that the application of pressure to each unit engages the direct drive clutch whilst releasing the brake, and the release of the pressure releases the clutch and allows springs 97 to engage the brake for reduced drive. The pressure fluid for actuating the brakes and clutches is obtained from an oil pump comprising two parts in parallel, one part being driven by a gear 174 on the shaft 8, Fig. 1, whilst the other part is driven off a gear 173 on the countershaft gear. The latter part of the pump is always directly driven by the engine but in reverse, the first part driven by the gear 174 acting in opposition so that a considerably reduced output is obtained. For normal forward running the outputs are added. The pump drive gear 175 driven by gear 174 also drives a shaft 110 of a pressure governing device whose function is to regulate the pressure of a part of the pump output in a desired manner in accordance with the engine speed. This governing device is indicated at 112, Fig. 4, and the pressure supply pump at 199, also in Fig. 4 which shows the control mechanism for controlling the executive pressure to clutching and braking cylinders 71, 282 and 75, 292 of the front and rear units respectively. The pump 199 supplies pressure fluid through a relief valve 350 to a pressure line 200 which has branches 234 and 238 leading to control valves 150 and 168 for the front and rear gear units respectively to which the pressure passes through lines 278 and 272. A further branch 123 from the pressure line supplies pressure fluid to the governing device 112 which passes it at a governed pressure to a line 220, 220a which leads to control devices 400 and 300 of the control valves 150 and 168 of the front and rear units respectively. The control valves 150 and 168 are both two-position valves either applying or releasing the pressure to the respective executive pistons 71, 282 and 75, 292 of the gear units and are actuated through spring-toggle mechanisms by the governing pressure acting on pistons 403 and 301 of the control devices 400 and 300. The valve 168 of the rear unit is subject to a further manual control by means of a cam plate 212, Fig. 4a, with a slot 213 and movable into the four positions R, N, L, H co-operating with a pin 211 on a bell-crank lever 320, Fig. 4. The cam plate is actuated from a driver's hand-lever into " reverse, " R, " neutral, " N or " high " H and " low " L positions, these positions corresponding to the positions of the reversing mechanism of Fig. 1. In the " reverse " and " low " positions valve 168 is in the lower position shown, in which the pistons of the rear unit are exhausted and the unit therefore maintained in reduced speed. In the " neutral " position the valve is moved to its upper position so that pressure fluid is supplied to the rear unit to put it in direct drive. When the plate is moved to "high" the pin 211 is in a widened portion of the slot and this allows the piston 301 to snap the toggle 303, 304 over so that link 317 moves lever 320 and valve 168 to the upper position to put the rear unit in direct drive. Thus while the " low " position of the cam plate forces the rear unit to reduction drive, the high position is permissive and the gear goes into direct according to the fluid pressure on piston 301. A further device is adapted to prevent the valve 168 being moved down to change the rear unit from direct to reduction drive when the engine speed is above a predetermined valve which comprises a piston 215, subjected to the governor pressure in lines 220a, 222, which abuts against the lever 320. Thus the operator cannot make a manual downshift from direct to indirect drive in the rear unit when the engine speed is too high. Associated with the valve 168 is a modifying valve 240 which is actuated from the engine accelerator pedal 141 and modifies pressure from the pump line 238 to a compensator line 277. The arrangement is such that as the accelerator is depressed and the throttle opened the compensating pressure is reduced and vice versa. This compensating pressure is passed by lines 276 and 277 to compensating cylinders 295 and 285 of the brake actuating mechanisms (see also Fig. 3) and by lines 228 and 229 to pistons 404 and 313 of the servo-control cylinders 400 and 300 respectively. The compensating pressure acting on the brake mechanisms reduces the value of the line pressure to a greater or less extent according to whether the throttle is closed or open so that a greater line pressure is required to release the brakes when the throttle is open and a high torque is being transmitted and consequently a higher pressure is available for the engagement of the clutches. Thus the clutch engaging pressure as it takes up the drive is proportional to the degree of actuation of the engine control device or accelerator pedal. The action of the compensating pressure on the pistons 404 and 313 of the control mechanisms 400 and 300 assists the governing pressure to move the gear control valves 168 and 150 to their direct drive positions so that with small throttle openings and a high compensating pressure the tendency is to maintain, or change to, a higher gear ratio whilst for large throttle openings and a low compensating pressure the tendency is to maintain, or change to, a lower gear ratio. Thus for each gear a range of speed depending upon the engine speed and the throttle opening is obtained for the change to a lower or higher gear ratio. The position of the front unit control valve 150 is also dependent on the gear ratio of the rear unit by reason of the pressure in the rear unit line 272 through a branch 432 acting on a piston 406 which pressure acts in opposition to the governor and compensating pressures so that under certain conditions of engine speed and throttle the pressure acting for direct drive in the rear unit will move the control valve 150 of the front unit into reduced drive position, which corresponds to the third speed ratio. The rear unit pressure also acts on the piston 313 of the rear unit control device in opposition to the governor and compensating pressures. Fluid pressure is admitted to the front of piston 313 whenever it is in lines 431 and 272, and is admitted behind the piston from lines 229 and 277 and this pressure is admitted into those lines by the modifying valve 240 controlled by the accelerator pedal 141. At a low engine speed the governor output pressure is low and both front and rear gear units will be in reduced ratio drive. Upon forward acceleration of the vehicle the governor pressure rises and acts against piston 403 of the front control unit to move valve 150 to its direct drive position through the snap-action toggle. At a small opening of the throttle full compensating pressure is applied to piston 404 and the change to second speed will occur at about 1150 r.p.m. If however the accelerator is fully depressed the compensating pressure is reduced to zero and the change is delayed until a speed of 2800 r.p.m. is reached to obtain the necessary governor pressure for effecting the switch over of valve 150. The change may of course occur at any intermediate speed according to the combined effect of engine speed and throttle opening. For the change from second speed to third speed the rear unit is required to shift to direct drive with a simultaneous shift of the front unit to reduced drive. With a small throttle opening the governor delivers sufficient pressure at about 1330 r.p.m. which acts on piston 301 of the rear unit and assisted by full compensating pressure on piston 313 operates the toggle device 303, 304 to move valve 168 to its higher position to put the rear unit in direct drive. If the engine speed falls to below 680 r.p.m. the front unit will be changed to reduction drive by the governor pressure being overcome by the various springs of the control unit to shift the