POWER TRANSMISSION APPARATUS

Abstract: To realize three functions of twin viscous coupling in four-wheel drive, free coupling in two-wheel drive, and further drive (lock) coupling in four-wheel drive, the power transmission apparatus for transmitting power from a propeller shaft (9A) to two right and left rear wheel drive shafts (21A, 23A), comprises first and second hubs (4, 5) coupled to the drive shafts; cylindrical inner and outer casings (3a, 3b) for forming two working chambers (11, 12) in cooperation with the two hubs; first and second resistance plates (8, 9), and a driven member (17) for selectively engaging the inner casing with the outer casing (TWIN VISCO IN 4WD) and disengaging the inner casing from the outer casing (FREE IN 2WD). Further, it is also preferable to use hollow clutch pipes (99, 101) disposed between the two drive shafts (21A, 23A) and the two hubs (49, 51), for selectively engaging the two drive shafts with the two hubs (TWIN VISCO IN 4WD) or with the cylindrical casing (29) (LOCK IN 2WD), and disengaging the two drive shafts from both the two hubs and the cylindrical casing (FREE IN 2WD).

Description: Background of the Invention Field of the Invention The present invention relates to a power transmission apparatus, and more specifically to a power transmission apparatus provided with viscous coupling suitable for use in four-wheel drive vehicles by which an engine power transmission system can be switched from two-wheel drive to four-wheel drive or vice versa. Description of the Prior Art Fig. 1 is a diagram for assistance in explaining an engine power transmission system for a four-wheel drive (4WD) vehicle. In the drawing, power generated by an engine 1A is speed-changed by a tansmission 3A, and then transmitted from a transfer box 5A directly to a front wheel differential gear 7A and to a rear wheel differential gear 11A via a propeller shaft 9A. The front wheel differential gear 7A distributes the transmitted power differentially to two (right and left) front wheels 19A and 17A. The rear wheel differential gear 11A distributes the transmitted power differentially to two (right and left) rear wheels 27A and 25A. The power transmission apparatus of the present invention provided with a viscous coupling is related to the rear wheel differential gear 11A shown in Fig. 1. In conventional four-wheel drive vehicles for transmitting torque from an internal combustion engine 1A to front and rear wheels, there exists a problem in that braking occurs when the vehicle is driven around a sharp corner as when the vehicle is being put into a garage. To overcome this problem, the four-wheel drive vehicle is usually provided with a differential gear 11A for absorbing a difference in rotational speed between the front and rear wheel drive shafts. In this four-wheel drive vehicle, when the vehicle travels on a muddy road surface (whose friction coefficient is small) and therefore a front wheel slips, only a small torque (required for a slipping front wheel whose friction coefficient is small) is transmitted from the internal combustion engine 1A to the front and rear wheels in dependence upon the differential gear construction, so that it is impossible to extricate the vehicle from the muddy road. To overcome this problem, the four-wheel drive vehicle is usually provided with a viscous coupling including a differential limiting device by which differential operation of the differential gear is limited so that the vehicle can e extricated from a muddy road, as disclosed in "Automobile Engineering" page 62, June 1987. On the other hand, a part-time four-wheel drive vehicle is usually provided with a transfer box 5A by which two-wheel drive can selectively be switched to four-wheel drive or vice versa. However, when two-wheel drive is selected through the transfer box 5A, the driven power transmission system from the transfer box to the rear wheels is rotated by the running vehicle. When the driven power transmission system is rotated, there exist problems in that vehicle travelling resistance increases and therefore vehicle vibration and noise increase. To overcome the above-mentioned problems, there has been proposed a power interruption apparatus for an four-wheel drive vehicle, by which a part of the driven power transmission system is disconnected, when the vehicle is driven in two-wheel drive mode, to allow the driven system to be free in order that the above-mentioned travelling resistance, vibration and noise can be reduced as much as possible, as disclosed in Japanese Published Unexamined (Kokai) Pat. Application No. 60-215428. In the former prior-art four-wheel drive vehicle provided with a viscous coupling, although difference in the number of revolutions between the front and rear wheel drive shafts can be absorbed to allow the vehicle to be driven away from a muddy road, since the four wheels are always driven by the engine, there exists a problem in that the fuel consumption is high during vehicle travelling. On the other hand, in the latter prior-art four-wheel drive vehicle provided with a power interruption apparatus, since the vehicle is driven in two-wheel drive maode, it is possible to improve the fuel consumption rate and also reduce the travel resistance, vibration and noise due to the rotation of the driven power transmission system. However, there exists a problem in that braking occurs when the vehicle is driven around a sharp corner and therefore the vehicle cannot travel smoothly because the difference in the number of revolutions between the front and rear drive shafts cannot be absorbed during four-wheel drive travelling. Further, Japanese Published Unexamined (Kokai) Patent Application No. 60-172764 discloses a power transmission apparatus usable in four-wheel drive (4WD) vehicles. This prior-art apparatus is disposed between a suboutput shaft of a transaxle and a rear wheel driving apparatus and is constructed in such a way that an output shaft of the viscous coupling is connected to the rear wheel driving apparatus and the suboutput shaft of the transaxle is connected to or disconnedted from input and output shafts of the viscous coupling. In this prior-art apparatus, three drive modes can be obtained as follows: (1) when the suboutput shaft of the transaxle is disconnected from the input shaft of the viscous coupling, a two-wheel drive (2WD) can be attained; (2) when the suboutput shaft of the transaxle is connected to the input shaft of the viscous coupling, an automatic four-wheel drive (4WD) can be attained (a torque corresponding to a difference in relative revolution speed between the input and output shafts of the viscous coupling can be transmitted); and (3) when the suboutput shaft of the transaxle is connected to the output shaft of the viscous coupling, a direct coupled four-wheel drive (4WD) can be attained. In this prior-art apparatus as described above, however, there sill exist problems in that since the viscous coupling and the rear wheel drive apparatus are rotated by the rear wheels even during the two-wheel drive (2WD) operation, the fuel consumption is high and vibration and noise are produced due to the agitation resistance of the differential gear oil. Summary of the Invention With these problems in mind, therefore, it is an object of the present invention to provide a power transmission apparatus provided with three functions of free coupling (power interruption) in two-wheel drive, twin viscous coupling in four-wheel drive and direct coupling (power lock) in four-wheel drive. According to the invention these problems are solved by a power transmission apparatus for transmitting power from a propeller shaft to first and second coaxially aligned rear wheel drive shafts in a four-wheel drive vehicle, which comprises: (a) a first hub surrounding the first rear wheel drive shaft; (b) a second hub surrounding the second rear wheel drive shaft and aligned coaxially with said first hub; (c) a cylindrical casing formed with a ring gear, said casing being disposed coaxially with and being rotatable relative to said first and second hubs so as to surround said first and second hubs, an annular working chamber being formed between said first and second hubs and said cylindrical casing; (d) a plurality of first plates engaged with an inner circumferential surface of said casing; (e) a plurality of second plates engaged with an outer circumferential surface of said first and second hubs, each of said second plates intervening between an adjacent two of said first plates; (f) a drive pinion gear coupled to the propeller shaft and in mesh with the ring gear of said cylindrical casing; and (g) clutch means, disposed between the first and second rear wheel drive shafts and said first and second hubs, for selectively engaging the two rear wheel drive shafts with said two hubs in a first operating operating mode to realize four wheel drive through the twin viscous coupling for directly engaging the two rear wheel drive shafts with said cylindrical casing in a second operating mode to realize four-wheel drive with the viscous-couplings bypassed, and for disengaging the two rear wheel drive shafts from both said two hubs and said cylindrical casing in a third operating mode to realize two-wheel drive. When the hub members are engaged with the rear wheel drive shafts by the clutch means, torque can be transmitted from the cylindrical casing to the shafts via the viscous coupling. Under these conditions, the more the difference in revolution speed between the casing and the shafts, the more will the revolution difference be restricted and therefore the higher will be the transmittable torque. The less the difference in revolution speed is, the more will the revolution difference be allowed and therefore the lower will be the transmittable torque. Further, when the shafts are directly connected to the casing by the clutch means, torque can be transmitted to the shafts without passing through the viscous coupling. Furthermore, when the shafts are disengaged from both the casing and hub members torque transmission is interrupted. Brief description of the drawings The features and advantages of the power transmission apparatus according to the present invention will be more clearly appreciated from the following description taken in conjunction with the accompanying drawings in which; Fig. 1 is a diagram for assistance in explaining an engine power transmission system for a four-wheel drive vehicle; Fig. 2 is a cross-sectional view showing an embodiment of the power transmission apparatus according to the present invention, in which the casing is disengaged from the viscous coupling to realize the free mode in two-wheel drive operation to eliminate vibration and noise; Fig. 3 is a perspective view showing a sleeve connecting member; Fig. 4 is a cross-sectional view showing the embodiment of Fig. 2 in which the casing is engaged with the viscous coupling to realize a twin viscous coupling mode in four-wheel drive operation to extricate the vehicle from a muddy road; and Fig. 5 is a cross-sectional view showing the embodiment of Fig. 2 in which the casing is directly connected to the two rear wheel drive shafts to realize the lock mode in four-wheel drive operation to drive the vehicle on a muddy road at high speed. Detailed description of the preferred embodiment The present invention will be described in detail hereinbelow with reference to Figs. 3 to 5. A cylindrial differential casing 29 is composed of a casing body 31 and a cover 33, and rotatably supported by a differential gear carrier 39 via two bearings 35 and 37. A ring gear 41 is fixed to the differential casing 29 by bolts 43. A drive pinion gear is formed integral with a rear end of a drive pinion shaft 47 connected to a propeller shaft 9A and geared with the ring gear 41. As described above, the differential casing 29 is driven via the propeller shaft 9A from the engine 1A. Within the differential casing 29, a pair of first (left) and second (right) hub members 49 and 51 are arranged coaxially with each other. The hub members 49 and 51 are rotably supported by axle support portions 53 and 55 formed in the differential casing 29, and rotatably and coaxially fit together at 57 so as to provide mutual support for their free ends. An annular working chamber 59 is formed between the differential casing 29 and the hub members 49 and 51 and filled with a high viscosity silicone oil. Further, two X-shaped (in cross section) sealing rings 61 and 63 and two backup rings 65 and 67 are arranged between the differential casing 29 and the hub members 49 and 51 at the two shaft support portions 53 and 55. Further, an X-shaped sealing ring 69 and a backup ring 71 are arranged between the two hub members 49 and 51 at 57. The working chamber 59 is therefore sealed. Within the working chamber 59, the differential casing 29 is formed with a spline 73 and the hub members 49 and 51 are also formed with splines 75 and 77. A ring-shaped partition wall member (spacer) 79 is engaged with the spline 73 so as to surround the interfitting end portions of the two hub members 49 and 51. That is, the working chamber 59 is partitioned by this wall member 79. This partition wall member 79 is formed with a through hole 81 providing communication between the right and left sides of the working chamber 59. A plurality of outer plates 83 are engaged with the spline 73 on either side of the partition wall member 79 so as to be movable axially of the casing 29. A plurality of inner plates 85 are engaged with the splines 75 and 77 being arranged alternately with the outer plates 83 and are also movable axially of the casing 29. A spacer 87 is arranged between each two outer plates 83, and a spacer 89 is arranged between each two inner plates 85. These spacers serve to keep the plates spaced apart at an appropriate distance. The arrangment described provides a pair of viscous couplings 91 and 93. Within the inner circumferential surfaces of the hub members 49 and 51, two slidable clutch sleeves 99 and 101 (clutch means) are supported by two slide bearings 95 and 97 so as to be rotatable circumferentially and slidable axially. The clutch sleeves 99 and 101 are connected to each other via a coupling member 103. As shown in Fig. 3, this coupling member 103 is a cylindrial member formed with three flexible arms 105 on each end thereof. Further, each arm 105 is formed with a claw 107 extending from an axially outward end of the arm 105 in the radially inward direction of the member 103. An outer circumferential groove 109 is formed at the adjacent ends of each sleeve 99 and 101. When this coupling member 103 is fitted to the adjacent ends of the sleeves 99 and 101 by engaging the claws 107 of the arms 105 with the two circumferential grooves 109, the sleeves 99 and 101 are coupled so as to be rotatable relative to each other in the circumferential direction but slidable together in both axial directions. The first hub member 49 is formed with an inner spline 111 on the inner circumferential surface thereof; the second hub member 51 is formed with two inner splines 113 and 115 on the inner circumferential surface thereof; and the differential casing 29 is formed with an inner spline 117 on the inner circumferential surface thereof. The sleeve 99 is formed with an outer spline 119 engageable with the inner splines 111 and 113; and the sleeve 101 is formed with an outer spline 121 engageable with the inner splines 115 and 117. As shown in Fig. 2 when the clutch spline 119 is disengaged, the clutch spline 121 is also disengaged. However, when the clutch spline 119 is engaged with the hub spline 111, the clutch spline 121 is also engaged with the hub spline 115 as shown in Fig. 4. Further, when the clutch spline 119 is engaged with the hub splines 111 and 113, the clutch spline 121 is also engaged with the hub spline 115 and the casing spline 117, as shown in Fig. 5. Within the sleeves 99 and 101 respectively first and second rear wheel drive shafts 21A and 23A are arranged coaxially and rotatably relative to each other. The lefthand drive shaft 21A is connected to the left rear wheel via a coupling device (not shown), and the righthand drive shaft 23A is connected to the right rear wheel via a coupling device (not shown). The drive shaft 21A is formed with an outer spline 127 on the outer circumferential surface thereof and the drive shaft 23A is formed with an outer spline 129 on the outer circumferential surface thereof. The sleeve 99 is formed with an inner spline 131 engageable with the outer spline 127 of the drive shaft 21A, the sleeve 101 is formed with an inner spline 133 engageable with the outer spline 129 of the drive shaft 23A. The sleeves 99 and 101 are fitted to the drive shafts 21A and 23A so as to be slidable axially thereof. A ring 137 formed with an outer circumferential groove 135 is fitted to the sleeve 101 and located in position by two stopper rings 139 and 141. An arm of a shift fork (not shown) is engaged with the groove 135 in the ring 137, so that the shift fork may slide the sleeve 99 and, via the ring 137, the sleeve 101 axially. This shift fork is actuated manually by a driver or automatically according to vehicle steering conditions or road surface conditions. The operation of this embodiment will now be described. As shown in Fig. 2 when the sleeves 99 and 101 are disengaged from the hub members 49 and 51 the viscous couplings 91 and 93 are disconnected from the drive shafts 21A and 23A and engine power transmission to the drive shafts is interrupted so that the rear wheels 25A and 27A can rotate freely (FREE IN 2WD). As shown in Fig. 4, when the sleeves 99 and 101 are slid in the rightward direction to engage the clutch splines 119 and 121 with the hub splines 111 and 115, engine power is transmitted from the engine 1a to the left rear wheel 25A via the viscous coupling 91 and to the right rear wheel 27A via the viscous coupling 93 as shown by thick arrows in Fig. 4 (TWIN VISCO IN 4WD). Under these conditions, since the two hub members 49 and 51 are freely rotatable relative to each other, the two viscous couplings 91 and 93 can be independently differentially rotated to transmit engine power. Therefore, the differential revolution of the viscous coupling 91 or 93 increases with increasing drive resistance of the right and left rear wheels 27A and 25A respectively, so that the rotation difference is increasingly limited and therefore the transmission torque also increases. Further, the differential revolution of the viscous coupling 91 or 93 decreases with decreasing drive resistance of the right and left rear wheels 27A and 25A, so that the rotation difference is decreasingly limited and therefore, the transmission torque also decreases. As described above, the power from the engine 1A can be differentially distributed to the right and left rear wheels 27A and 25A. Simultaneously, the rotational difference between the right and left rear wheels 27A and 25A and the rotational difference between the front wheels 17A and 19A and the rear wheels 25A and 27A can be controlled (limited or allowed). As shown in Fig. 5, when the sleeves 99 and 101 are slid further to the right to engage the clutch spline 119 with the hub splines 111 and 113 and the clutch spline 121 with the hub spline 115 and the casing spline 117, since the differential casing 29 is engaged with the hub member 51 and further the hub member 51 is engaged with the hub member 49, the two viscous couplings 91 and 93 are both locked (LOCK IN 4WD). Therefore the differential operation of the viscous coupling 91 and 93 are disabled. Under these conditions, engine power is directly transmitted from the engine 1A to the right rear wheel 27A via the differential casing 29 and to the left rear wheel 25A via the hub members 51 and 49, respectively, as indicated by thick arrows in Fig. 5. Further, since the right and left hub members 49 and 51 are both locked, the differential distribution of the engine power and the differential action between the two front wheels 17A, 19A and the rear wheels 25A, 27A are disabled. The function of the apparatus according to the embodiment described will be described in relation to the performance of the vehicle shown in Fig. 1. When the rear differential gear 11A is allowed to be free as shown in Fig. 2 (FREE IN 2WD), since the two rear wheels 25A and 27A are disconnected from the engine 1A, the vehicle is driven in 2WD mode. In this case, the rotational power of the rear wheels 25A and 27A rotated by the vehicle is not transmitted to the viscous couplings 91 and 93. Therefore, since the power transmission of the rear wheel (25A,27A) drive system is interrupted by the 2-to-4 switching transfer mechanism, the viscous couplings 91 and 93 and their related drive system are not rotated wastefully, so that it is possible to improve the fuel consumption and reduce mechanical abrasion, vibration and noise. As described above, it is possible to obtain the same effect as when the vehicle is provided with a free-wheel clutch, without the need to provide a free-wheel clutch. Further, the operation is more stable than that of the free-wheel clutch. When the rear differential gear 11A is changed to the state as shown in Fig. 4, the vehicle is driven in 4WD mode (TWIN VISCO IN 4WD). Therefore, when the vehicle is turned by an external force and therefore a rotational difference is produced between the inner and outer wheels, the rotational difference is limited by the differential restriction function of the rear differential gear 11A and therefore a yawing moment is generated to return the vehicle position to its original state, so that the vehicle forward stability can be improved. Further, since the drive power can be distributed to the four wheels, each wheel can not easily slip, so that vehicle driving manipulatability and stability can be improved. In addition, when the front wheels 17A and 19A slip on a muddy road, since the rotational difference between the two viscous couplings 91 and 93 increases, a large torque can be transmitted to the rear wheels 25A and 27A, so that the extricability from the muddy road and the running capability over a long distance can be improved. Further, when the vehicle travels around a corner, since the rotational difference between the inner and outer wheels (the two rear wheels 25A and 27A) and between the front and rear wheels can be absorbed by the viscous couplings 91 and 93, it is possible to obtain smooth cornering operation. Further, when the rear differential gear 11A is locked as shown in Fig. 5 (LOCK IN 4WD), since the rear wheels 25A and 27A are directly coupled to the engine 1A, a large drive power can be transmitted, so that the running capability over a long distance on muddy roads and high speed straight drive capability can be improved. As described above, the power transmission apparatus of the present invention can transmit torque under control (restriction and permission) of the rotational difference and further can interrupt the torque transmission. When the torque is interrupted, it is possible to eliminate the loss due to the operation of the viscous couplings.

Claims: 1. A power transmission apparatus for transmitting power from a propeller shaft (9A) to first and second coaxially aligned rear wheel drive shafts (21A,23A) in a four-wheel drive vehicle, which comprises: (a) a first hub (49) surrounding the first rear wheel drive shaft (21A); (b) a second hub (51) surrounding the second rear wheel drive shaft (23A) and aligned coaxially with said first hub; (c) a cylindrical casing (29) formed with a ring gear (41), said casing being disposed coaxially with and being rotatable relative to said first and second hubs so as to surround said first and second hubs, an annular working chamber (59) being formed between said first and second hubs and said cylindrical casing; (d) a plurality of first plates (83) engaged with an inner circumferential surface of said casing; (e) a plurality of second plates (85) engaged with an outer circumferential surface of said first and second hubs, each of said second plates intervening between an adjacent two of said first plates; (f) a drive pinion gear (45) coupled to the propeller shaft and in mesh with the ring gear of said cylindrical casing; and (g) clutch means (99,101), disposed between the first and second rear wheel drive shafts and said first and second hubs, for selectively engaging the two rear wheel drive shafts (21A,23A) with said two hubs (49,51) in a first operating mode to realize four wheel drive through the twin viscous coupling (91,93), for directly engaging the two rear wheel drive shafts (21A,23A) with said cylindrical casing (29) in a second operating mode to realize four-wheel drive with the viscous couplings (91,93) bypassed, and for disengaging the two rear wheel drive shafts from both said two hubs and said cylindrical casing in a third operating mode to realize two-wheel drive. 2. The power transmission apparatus of claim 2, wherein said clutch means (99,101) is a slidable sleeve spline-coupled to circumferential surfaces of said first and second rear wheel drive shafts and formed on an outer circumferential surface thereof with two outer clutch splines (119,121) selectively engageable with a first hub spline (111) formed in an inner circumferential surface of said first hub (45), with two second hub splines (113,115) formed in an inner circumferential surface of said second hub (51), and with a casing spline (117) formed in an inner circumferential surface of said cylindrical casing (29).

Inventors: KURIHARA, SAKUO, TERAOKA, MASAO
Applicants: VISCODRIVE JAPAN LTD
Application year: 1989
Main classification: B60K17
IPC codes: F16D35, B60K17, F16D47