Transmission system for a two and four-wheel drive motor vehicle

Abstract: In a vehicle having a transmission (3) capable of being coupled to either; the front-wheels (F) or the front-wheels and the rear-wheels, a front differential (4) is provided between the front-wheels (F) and a rear differential (8) between the rear wheels (R). The transmission (3) is permanently coupled to one differential (4) and can be coupled and uncoupled from the rear differential (8) by a wet clutch (7). For compactness the wet clutch casing (24) is mounted on the differential casing. So that the differential gear lubricating oil does not mix with the clutch/transmission lubricant an oil seal (28) is located between the interior of the differential casing and the clutch casing.

Description: The present invention relates to a power transmission system for a two and four-wheel drive motor vehicle, and more particularly to a system having a fluid-operated multiple-disk friction clutch for distributing the power of and engine to front and rear wheels. Japanese Patent Publication 61-28531 discloses a system in which a fluid-operated multiple-disk friction clutch is provided as a transfer device for distributing the power of the engine to the front wheels and rear wheels. The clutch is disposed in an extension case of an automatic transmission. The clutch comprises a clutch drum and a clutch hub, each having a plurality of disks, and a piston oil chamber formed between the clutch drum and a piston. The piston is slidably mounted on the hub with splines for pushing the disks of the clutch drum to the disks of the hub. When the clutch hub rotates, the piston rotates together with the hub. The multiple-disk friction clutch uses the oil in the automatic transmission as actuating oil for actuating and lubricating the clutch, since the oil has a good friction characteristic and a low oil viscosity. On the other hand, the differential, arranged between the right and left driving wheels employs hypoid gears. In such a differential, it is necessary to use a high-pressure lubricant which contains additives to improve their ability to adhere to the surfaces of metals under high bearing pressures. Such a lubricant is liable to cause the multiple disks of the clutch to stick and slip, which causes unpleasant vibration of the vehicle body. Further, the friction characteristic of the lubricant varies considerably with temperature, which affects the durability and reliability of the clutch. Japanese patent Application Laid-Open 61-155027 discloses a system in which a pair of transfer clutches for front wheels and for rear wheels are provided in a transfer device provided for distributing the torque to the front wheels and the rear wheels. The reader will be further enlightened as to the state of the art by reference to DE-A-37 03 165, with respect to which claim 1 of this application has been characterised. The present invention seeks to provide a system in which an exclusive oil supply system is provided for the transfer clutch. According to the present invention, there is provided a power transmission system in a two or four-wheel drive motor vehicle having a transmission coupled to an engine, a front wheel differential between the front wheels of the vehicle and a rear wheel differential between the rear wheels of the vehicle, the transmission being coupled to one of the differentials via a transmission output, and a corresponding front or rear power train and a fluid operated transfer clutch mounted in a clutch casing and coupled to one of the differentials, said transfer clutch being hydraulically actuated via a piston reciprocally mounted in a cylinder, to couple the transmission output to the other of the differentials, a hydraulic circuit including an oil pump and control valves responsive to driving conditions of the vehicle, said hydraulic circuit being located inside the clutch casing to actuate the transfer clutch, characterised in that the clutch casing is mounted directly on the differential casing and there is provided an oil seal arranged to isolate the interior of the clutch casing from the interior of the differential casing so that the differential lubricating oil is separate from the clutch oil. An embodiment of a transmission system constructed in accordance with the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:- Fig. 1 is a schematic diagram shown in a four-wheel drive power transmission system according to the present invention; Fig. 2 is a sectional view showing a differential and transfer clutch; Fig. 3 is a sectional view taken along a line III-III of Fig. 2; Fig. 4 is a graph showing characteristics of duty ratio for control pressure and clutch pressure; Fig. 5 shows a hydraulic control circuit for controlling a transfer clutch of the system; Fig. 6a is a sectional view partly showing a modification of the transfer clutch; Fig. 6b is a perspective view showing a race of the transfer clutch; Fig. 1 shows a power transmission system of a four-wheel drive motor vehicle having a trans-axle power transmission system to which the present invention is applied. In a front portion of the motor vehicle, an engine 1, a clutch 2 and a manual transmission 3 are laterally mounted. The power transmission system has a front power train and a rear power train. The front power train has a front differential 4. The rear power train comprises a transfer device 5, a propeller shaft 6, a transfer clutch 7 in the form of a fluid operated multiple-disk friction clutch and a rear differential 8. The power of the engine 1 is transmitted to the transmission 3 through the clutch 2. The output torque of the transmission is transmitted to front wheels F through an output gear 9, a final reduction gear 10, the front differential 4, and axles 4a. The final reduction gear 10 engages with an input gear 11 of the transfer device 5, which is securely mounted on a transfer shaft 5a. The transfer shaft 5a is operatively connected to the propeller shaft 6 through a pair of bevel gears 12, thereby transmitting the power to the transfer clutch 7. The power is transmitted to rear wheels R through a final reduction gear 13, the rear differential 8 and axles 8a at a predetermined distribution ratio determined by the clutch7. Referring to Fig. 2 showing the transfer clutch 7, a carrier 20 of the rear differential 8 which is suspended from a body of the vehicle has a cylindrical portion 24 as a casing for the clutch 7. The cylindrical portion 24 is integrally formed with the carrier 20 to form an oil chamber 24d therein. An extension case 23 is secured to a front end of the cylindrical portion 24. An input shaft 22 is rotatably mounted in the extension case 23 and connected to the propeller shaft 6. A drive pinion shaft 21a having a drive pinion is rotatably mounted in the carrier 20 of the rear differential 8. An end of the driven pinion shaft 21a is rotatably mounted in a hollow end 22c of the input shaft 22. The drive pinion 21 engages with the final reduction gear 13. The transfer clutch 7 is mounted on the drive pinion shaft 21a in the cylindrical portion 24 and an oil pump 14 is secured to a side of the cylindrical portion 24 for supplying oil to the transfer clutch 7. The transfer clutch 7 comprises a drive drum 7a secured to a flange 22a of the input shaft 22 and a driven drum 7b splined on the drive pinion shaft 21a. A front end of the driven drum 7b engages with a thrust washer 26 which abuts on the end of the input shaft 22. A plurality of drive disks 7c are splined on the drum 7a together with a pair of retainers 7d provided adjacent both end disks 7c, and a plurality of driven disks 7e are splined on the drum 7b, disposed alternatively with the disks 7c. A piston 7f is slidably mounted on an inner wall of the cylindrical portion 24 and on a guide cylinder 24c thereof with splines. The piston 7f engages with the outermost retainer 7d through an angular contact bearing 27. A piston chamber 7g is defined by the piston 7f, a partition 24b and the guide cylinder 24c of the cylindrical portion 24. The bearing 27 has an outer race 27a engaged with an operating end of the piston 7f, and splined with the guide cylinder 24c at a projecting end 27b, and an inner race 27c engaged with the retainer 7d and splined on the drum 7a at the outer periphery thereof. Thus, the outer race 27a is engaged with the guide cylinder 24c for preventing the rotation of outer race 27a and the inner race 27c is engaged with the drum 7a rotating in the same direction. Oil seals 28 and 29 are provided in inner peripheries of the partition 24b and the extension case 23, respectively. Since characteristics of the oil for the transfer clutch 7 and for hypoid gears of the differential 8 are different from each other, the oil seal 28 is provided for preventing the oils from mixing. Oil passages 22b and 21b are formed in the hollow end 22c of the input shaft 22 and in the drive pinion shaft 21a respectively, so as to communicate an outer space of the drive drum 7a with an inner space of the driven drum 7b. When the clutch drum 7a rotates, the oil in the oil chamber 24d in the cylindrical portion 24 is drawn up by the outer wall of the drive drum 7a, and supplied to the opening of oil passage 22b through an oil guide (not shown). The oil passes through the passages 22b and 21b and enters the chamber in the driven drum 7b. Further, the oil supplied to disks 7c and 7e through passages (not shown) radially formed in the spline portion of the driven drum 7b. The oil pump 14 is secured to a housing portion 24a formed on the side of the cylindrical portion 24. As shown in Fig. 3, an oil reservoir 14g is provided on the underside of the cylindrical portion 24. A valve unit 25, which will be described hereinafter, is provided in the reservoir 14g. The oil pump 14 has an electric motor 14a, an inner rotor 14b mounted on an output shaft of the motor 14a and an outer rotor 14c engaged with the inner rotor 14b. These rotors are mounted in the housing portion 24a and covered by a cover 14d which is secured to the housing portion 24a by bolts together with the motor 14a. An inlet port 14e is communicated with an oil strainer of the oil reservoir 14g and a discharge port 14f is communicated with the valve unit 25. A hydraulic control system for the transfer clutch 7 is described hereinafter with reference to Fig. 5. The hydraulic control system comprises the valve unit 25 and a control unit 19. The valve unit 25 has a pressure regulator valve 15, a transfer control valve 16, a solenoid operated control valve 17 and a pilot valve 18. Oil from the oil pump 14 is supplied to the pressure regulator valve 15 by which the pressure of oil is regulated to a predetermined value. The oil is fed to the piston chamber 7g of the transfer clutch 7 through the transfer control valve 16. The pilot valve 18 operated to provide a constant pilot pressure . The solenoid operated control valve 17 is operated by pulses from the control unit 19 at a duty ratio dependent on driving conditions of the vehicle, so that the drain of the oil from the control valve 17 provides a control pressure. The control pressure is applied to an end of the transfer control valve 16 to control the oil supplied to the clutch 7 so as to control a clutch pressure. The control unit 19 has a CPU 150 having a memory 151, input interfaces 152, 153 and an A/D converter 154, an output interface 155 and a constant voltage source 156. Output signals from a throttle position sensor 157, a battery voltage sensor 158, a rear wheel speed sensor 159, a front wheel speed sensor 160, a shift range switch 161 and an idle switch 162 are applied to the CPU 150 through the interfaces 152, 153 and the A/D converter 154 for determining the driving conditions of the motor vehicle. An optimum duty ratio is derived from a map stored in the memory 151 in accordance with the vehicle speed and a throttle opening degree detected by the front and rear wheel speed sensors 160, 159 and the throttle position sensor 157, and applied to the solenoid operated control valve 17 through the output interface 155 to provide a control pressure for the transfer control valve 16. Fig. 4 shows a relationship between the control pressure, the clutch pressure and the duty ratio. Describing the operation of the system, the power of the engine 1 is transmitted to the transmission 3 through the clutch 2. The torque is directly transmitted to the front differential 4 and the torque is further transmitted to the rear differential 8 through the transfer clutch 7 for providing the four-wheel drive. The disks 7c and 7e of the transfer clutch 7 are soaked in the optimum actuating oil so that a desired friction characteristic is ensured. The controlled pressure is applied to the chamber 7g so that the piston 7f pushes the disc 7c and 7e. Accordingly, the power is distributed to the front wheels and to the rear wheels in accordance with the transmission torque of the transfer clutch 7. The torque distributing ratio is changed in accordance with the clutch pressure controlled by the duty ratio signal from the control unit 19. When the drive drum 7a rotates, the disks 7c and the inner race 27c of the bearing 27 are rotated. However, since the outer race 27a of the bearing 27 and the piston 7f are splined to the guide cylinder 24c of the cylindrical portion 24 through the projecting end 27b, the outer race 27a and the piston 7f do not rotate. Further, the piston chamber 7g is formed by the piston 7f and the cylindrical portion 24 and hence it does not rotate. Accordingly, centrifugal force of the oil does not generate in the chamber 7g. Thus, the pressure of oil in the chamber 7g is accurately controlled, and unnecessary pressure is not exerted on the disks of the clutch 7, so that the torque distribution ratio can be properly controlled. Figs. 6a and 6b show a modification of the transfer clutch 7. In place of the bearing 27 for the piston of the transfer clutch 7, a thrust bearing 30 is employed. A race 31 of the thrust bearing 30 is engaged with grooves of the guide cylinder 24c at an inner projection 31a. Accordingly, the same effect as described hereinbefore in the previous embodiment is achieved. In accordance with the present invention, the torque distributing ratio is changed to a front-wheel drive mode in which the torque of 100% is transmitted to the front wheels, and to a four-wheel drive mode in which the distribution of torque to the rear-wheels is increased. It is possible to control the torque distributing ratio dependent on the driving conditions of the motor vehicle and surface conditions of the road, thereby improving driving stability and driveability. Since the transfer clutch 7 is provided with the oil having a desired characteristic, the stick/slip of disks does not occur. Thus, uncomfortable oscillation and noise are prevented, and the reliability and durability of the power transmitting system are improved. Since the piston of the clutch is disposed outside the clutch, the diameter of the piston can be increased and the diameter of the clutch can be reduced. Accordingly, resistance of the oil in the clutch is reduced to increase the power transmission efficiency of the clutch. The oil pump and the valve unit for the transfer clutch are disposed adjacent the transfer clutch. Thus, external pipes for the hydraulic system of the clutch are unnecessary and the system is compact in size. Further, the leakage of the oil is prevented and the response of the clutch is quickened.

Claims: A power transmission system in a two or four-wheel drive motor vehicle having a transmission (3) coupled to an engine (1), a front wheel differential (4) between the front wheels (F) of the vehicle and a rear wheel differential (8) between the rear wheels (R) of the vehicle, the transmission (3) being coupled to one of the differentials (4, 8) via a transmission output, and a corresponding front or rear power train and a fluid operated transfer clutch (7) mounted in a clutch casing (24) and coupled to one of the differentials (8), said transfer clutch (7) being hydraulically actuated via a piston (7f) reciprocally mounted in a cylinder (7g), to couple the transmission output to the other of the differentials (8), a hydraulic circuit (25) including an oil pump (14) and control valves responsive to driving conditions of the vehicle, said hydraulic circuit (25) being located inside the clutch casing (24) to actuate the transfer clutch (7), characterised in that the clutch casing (24) is mounted directly on the differential casing (20) and there is provided an oil seal (28) arranged to isolate the interior of the clutch casing (24) from the interior of the differential casing (20) so that the differential lubricating oil is separate from the clutch oil.

Inventors: KOBAYASHI TOSHIO, MARUYAMA KEIICHI, OHNUKI YUKIO, KOBAYASHI,TOSHIO, MARUYAMA, KEIICHI, OHNUKI, YUKIO
Applicants: FUJI HEAVY IND LTD, FUJI JUKOGYO KABUSHIKI KAISHA
Application year: 1989
Main classification: B60K23
IPC codes: B60K17, B60K23