POWER TRANSMITTING SYSTEM FOR A FOUR-WHEEL DRIVE VEHICLE

Abstract: A power transmitting system for a four-wheel drive vehicle has a transmission (8), a central differential (36), and a viscous coupling (40). The viscous coupling (40) is provided coaxially with the central differential (36), and a drive shaft (21) is provided passing through axes of the central differential (30) and the viscous coupling (40). An output shaft (9) of the transmission (8) is operatively connected to the casing of the differential (36). One of the differential gears (35) is operatively connected to rear wheels through the drive shaft (21), and the other differential gear (34) is secured to a housing (38) of the viscous coupling (40). The housing (38) is operatively connected to front wheels, and the hub (39) is secured to the drive shaft (21).

Description: The present invention relates to a system for transmitting the power of an engine to four driving wheels of a four-wheel drive vehicle. In a conventional four-wheel drive vehicle having a central differential, which is called a full time four-wheel drive vehicle, the central differential operates to uniformly distribute the torque of an engine to front wheels and to rear wheels. However, if one of four wheels skids, the torque is not transmitted to four wheels, so that the vehicle stops. In order to prevent such a problem, a lock mechanism for locking the central differential is provided. As a lock mechanism for the system, a system employing a solenoid operated dog clutch is known. Further, Japanese Patent Application Laid-Open 59-179425 discloses a system in which front wheels are connected to rear wheels by a fluid clutch or a viscous clutch. When rotational speeds of rear wheels are lower than a predetermined speed range compared with those of front wheels, the rear wheels are driven by the power for front wheel drive through a one-way clutch. However in the former case, since the clutch is operated by a driver in accordance with conditions of the surface of a road, it causes troublesome operations. In the latter system, it is difficult to determine the capacity of the viscous clutch for setting a viscous mode and a hump mode. Further, in order to obtain an appropriate operational characteristic, the viscous clutch must have a comparatively large capacity. The present invention seeks to provide a power system which is compact in size and automatically operates to restrict the differential operation of the central differential in accordance with driving conditions. According to the present invention, there is provided a power transmitting system for a four-wheel drive vehicle comprising a transmission, a central differential having a casing, a pinion shaft secured to the casing, a pair of pinion gears mounted on the pinion shaft, and a pair of differential gears engaged with the pinion gears, and a viscous coupling having a housing and hub provided in the housing. The system of the present invention is characterized in that the viscous coupling is provided coaxially with the central differential, a first drive shaft is provided passing through axes of the central differential and the viscous coupling, the pinion shaft comprises a pair of shafts, an inner end of each of which is rotatably mounted on the first drive shaft, an output shaft of the transmission is operatively connected to the casing of the differential, one of the differential gears is secured to the first drive shaft, and the first drive shaft is operatively connected to either front or rear wheels, the other differential gear is secured to the housing of the viscous coupling, the housing is operatively connected to the other wheels, and the hub is secured to the first drive shaft. Preferably, the output shaft is a cylindrical shaft in which a second drive shaft for the other wheels is rotatably mounted, and the housing of the viscous coupling connected to the second drive shaft. The housing is preferably rotatably mounted on the first drive shaft through a bearing. A preferred embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a schematic diagram of a four-wheel drive power transmission system of the present invention; Figure 2 is an enlarged sectional view showing part of the system. Figure 1 shows a power transmission system for a four-wheel drive motor vehicle to which the present embodiment is applied. An engine 1 is longitudinally mounted on the motor vehicle at a front portion thereof, thus forming a transaxle type. The power transmission system housed in a transmission case comprises a clutch 2, sub transmission 4, main transmission 8 and transfer device 20. The sub transmission 4 connected to the clutch 2 through a drive shaft 3 has a reduction gears 6 disposed between the drive shaft 3 and an input shaft 5, and a selector mechanism 7. The sub transmission 4 is adapted to selectively operate to directly engage the drive shaft 3 with the input shaft 5 for providing a high speed range and to engage the drive shaft 3 with the input shaft 5 through the reduction gears 6 for providing a low speed range. The main transmission 8 connected to the input shaft 5 has a tubular output shaft 9 parallel with the input shaft 5, five pairs of change-speed gears 10 to 14 corresponding to first to fifth speed gears, synchronizers 15 to 17, and a reverse drive gear (not shown). A front drive shaft 18 is rotatably supported in the output shaft 9. A drive pinion 9a at the front end of the shaft 18 meshes with a crown gear of a front differential 19 so as to transmit the power to front wheels. The transfer device 20 is disposed behind the main transmission 8 at the rear end of the output shaft 9. The transfer device 20 has a drive shaft 21 disposed in alignment with the front drive shaft 18, and is connected to a rear drive shaft 23 through transfer gears 22. The rear drive shaft 23 is disposed in parallel with the front drive shaft 18. The rear drive shaft 23 is connected to rear wheels through a propeller shaft 24 and a rear differential 25. Accordingly, torque of the engine is distributed to front wheels and rear wheels, and the difference between rotational speeds of these wheels is absorbed. Referring to Fig. 2, the transfer device 20 has a front cylindrical casing 30 mounted on the output shaft 9 and a differential casing 31 connected to the front casing 30. In the casing 31, a central differential 36 and a viscous coupling 40 are provided coaxially with the drive shaft 21. The central differential 36 comprises a pair of pinion shafts 32, each secured to the casing 31 and to a centre ring 31a which is rotatably mounted on the shaft 21, and a pair of pinion gears 33, each rotatably mounted on the pinion shaft 32. A differential gear 35 meshes with the pinion gears 33 and is securely mounted on the drive shaft 21, and a differential gear 34 meshes with the pinion gears 33. The viscous coupling 40 comprises a housing 38 having a plurality of outer disks 38a, a tubular output shaft 37b integral with the housing 38 and secured to the front drive shaft 18, a hub 39 secured to the drive shaft 21 and having a plurality of inner disks 39a, an input shaft 37a integral with the differential gear 34 and secured to the housing 38, and a high viscous fluid contained in a space defined by the hub 39, the shaft 37a and the housing 38. The disks are alternately arranged in the axial direction of the coupling 40. When there is a speed difference between the housing 38 and the hub 39, the viscous coupling 40 operates to generate torque in dependence on the difference. Thus, the torque is transmitted from a higher speed disk side to a lower speed disk side. A bearing 41 is disposed between the housing 38 and the drive shaft 21 for providing appropriate clearances 41a, 41b between the hub 39 and the housing 38 and between the hub 39 and the shaft 37a. The power of the engine 1 is transmitted to the sub transmission 4 through the clutch 2 and the drive shaft 3, and further to the main transmission 8. The torque of the main transmission 8 is transmitted to pinion gears 33 of the differential 36 through the output shaft 9, front casing 30, differential casing 31, and pinion shafts 32. At normal driving, the torque is transmitted to the front wheels through differential gear 34, shaft 37a, housing 38, output shaft 37b, and front drive shaft 18. The torque is further transmitted to the rear wheels through differential gear 35, drive shaft 21, transfer gears 22, rear drive shaft 23, propeller shaft 24 and rear differential 25. When the rear wheels skid, a speed difference between the front and rear wheels occurs. The torque of the rear drive shaft 23 is transmitted to the drive shaft 21. The hub 39 secured to the drive shaft 21 rotates faster than the housing 38. The difference of rotational speeds between outer disks 38a and inner disks 39a generates coupling torque which is transmitted to the lower speed side, namely to the outer disks 38a. Accordingly, the torque is transmitted to the differential gear 34, thereby preventing the pinion gears 33 from rotating. Thus, the differential operation is restricted. This means that the central differential 36 is automatically locked. In case of skidding of the front wheels, the central differential is locked in a similar manner to the above described operation. From the foregoing, it will be understood that the present embodiment provides a system in which a viscous coupling is combined with a central differential for automatically locking the central differential. The viscous coupling is combined with the central differential to achieve a compact structure, so that increase of size of the transmission is prevented. While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.

Claims: 1. A power transmitting system for a four-wheel drive vehicle, comprising: a transmission (8); a central differential (36) having a casing (30, 31); a pinion shaft (32) secured to the casing (30, 31); a pair of pinion gears (33) mounted on the pinion shaft (32); and a pair of differential gears (34, 35) engaged with the pinion gears (33); and a viscous coupling (40) having a housing (38) and a hub (39) provided in the housing (38), characterised in that the viscous coupling (40) is provided coaxially with the central differential (36); a first drive shaft (21) is provided passing through axes of the central differential (36) and the viscous coupling (40); the pinion shaft comprises a pair of shafts (32), an inner end of each of which is rotatably mounted on the first drive shaft (21); an output shaft (9) of the transmission (8) is operatively connected to the casing (30, 31) of the differential; one of the differential gears (35) is secured to the first drive shaft (21), and the first drive shaft (21) is operatively connected to either the front or rear wheels; the other differential gear (34) is secured to the housing (38) of the viscous coupling (40); the housing (38) is operatively connected to the other wheels; and the hub (39) is secured to the first drive shaft (21). 2. A system as claimed in claim 1, wherein the output shaft (9) is a cylindrical shaft in which a second drive shaft (18) for the other wheels is rotatably mounted; and the housing (38) of the viscous coupling (40) is connected to the second drive shaft (18). 3. A system as claimed in claim 1 or 2, wherein the housing (38) is rotatably mounted on the first drive shaft (21) through a bearing (41). 4. A system as claimed in claim 1, wherein each pinion shaft (32) is secured to a ring (31a) which is rotatably mounted on the first drive shaft (21). 5. A four-wheel drive vehicle comprising an engine, and a transmitting system as claimed in any preceding claim for transmitting power from the engine to wheels of the vehicle.

Inventors: MURA, SHINICHIRO
Applicants: FUJI JUKOGYO KABUSHIKI KAISHA
Application year: 1988
Main classification: B60K17
IPC codes: F16H48, B60K17