POWER TRANSMISSION DEVICE FOR A FOUR WHEEL DRIVE VEHICLE

Abstract: A centre differential gear (33) for use in a power transmission device for a full time four wheel drive vehicle, comprising a diff-carrier (4l) coupled to a power transmitting front casing (25) covering a front diff-carrier (26) of a front wheel differential gear (5) and arranged co-axially with the front diff­ carrier (26), one side gear (42) coupled to the front wheels and another side gear (43) coupled to a gear mount casing (32) which transmits power to the rear wheels. The diff-carrier (4l) of the centre differential gear (33) is supported between the side gear (42) on the side driving the front wheel differential gear (5) and the gear mount casing (32) in cantilever fashion such that the other side gear (43) is spline coupled directly to the gear mount casing (32) without passing through the diff-carrier (4l).

Description: The present invention relates to a power transmission device for a four wheel drive vehicle, and particularly to an improvement in the structure of a centre differential gear in a power transmission unit for a full time four wheel drive vehicle, which transmits rotation to both front and rear wheels at all times. Heretofore, there have been proposed various kinds of power transmission devices for four wheel drive vehicles, which are obtained by slightly modifying the power transmission devices for transverse front engine front wheel drive vehicles. In the above-mentioned conventional devices the manner of operation is as shown in Figure 3, which has been labelled as Prior Art to indicate that it does not embody the present invention, rather than necessarily indicating that its features as illustrated are known. Drive power from the engine is transmitted through a hollow shaft 45' to a differential gear carrier (diff-carrier) 41', which is rotatably supported on shaft sections 42'a,43'a of both side gears 42' ,43' of centre differential gear 33', and is distributed among both side gears 42' ,43' from pinion 50' on the diff-carrier 41'. The drive power transmitted to the left hand side gear 42' is transmitted through another hollow shaft 46' to the front wheel differential gear (not shown) and distributed among side gears to be transmitted to the front axles on both sides (only the right front axle 31' is shown in Figure 3). The drive power transmitted to the right hand side gear 43' is transmitted through a ring gear mount casing 32' and a rear-wheel drive ring gear 35' to a gear 40', and then transmitted from a drive pinion 39' through a propeller shaft and a final speed reduction gear (not shown) to be distributed among the rear axles on both sides. Further, in the above-mentioned transmission of drive power, axial forces generated at side gears 42', 43' are transmitted to the diff-carrier 41' and the ring gear mount casing 32' through thrust washers 51', 52' 53', 55' to be borne by the carrier 41' and the casing 32', which are both housed within a transfer casing 12'. The above-mentioned conventional power transmission device has a disadvantage in that, if the dimensions of the side gears 42', 43' are increased in order to withstand larger input torques, not only the outer diameter of the diff-carrier 41' but also those of the ring gear mount casing 32' and the transfer casing 12' would become larger, so much so that the transfer casing 12' could no longer be contained within the confinement of the restricted space available behind the engine. It is also known from EP-A-131892 for a power transfer device for a four wheel drive vehicle to be provided in combination with a power transmission 22. It comprises a transfer casing 76 detachably secured at one side thereof to the transmission casing, first and second inner casings 56,78 respectively assembled within the transmission and transfer casings 38,76 in a coaxial relationship, a first ring gear 60 integral with the first inner casing 56 and in mesh with an output gear 34 of the transmission, a second ring gear 82 integral with the second inner casing 76 and drivingly connected to rear-wheel axles, a first differential gear unit 58 for front-wheel drive assembled within the first inner casing 56, a second differential gear unit 80 assembled within the second inner casing 78 and drivingly connected to the first inner casing 56 and to the first differential gear unit, and a locking mechanism 104 for locking the second differential gear unit 80. Although the side gear 96 is directly coupled to the inner casing 78, the differential gear unit 80 has no extremity with an open end not supported by the side gear 96, and thus in the arrangement of EP-A-131892 its side gears cannot be designed to have large dimensions. An object of the present invention is to provide an improved structure for a centre differential gear in a power transmission device for a four wheel drive vehicle, and preferably the diff-carrier is capable of being supported on the side gear on the front wheel drive side only and not on the other side gear for transmitting power to the rear wheels, thereby eliminating the above-mentioned problem and allowing the side gears to be of optimum size even within a confined space. According to the present invention, a power transmission device for a four wheel drive vehicle comprises a front wheel differential gear and a transfer unit having a centre differential gear, the centre differential gear comprising a diff-carrier coupled to a power transmitting front casing covering a front diff-carrier in said front wheel differential gear and arranged co-axially with said front diff-carrier, one side gear coupled to said front diff-carrier and another side gear coupled to a gear mount casing for transmitting power to the rear wheels, the diff-carrier in said centre differential gear being supported between one of said side gears and said gear mount casing and including an extremity having an open end not supported by the other of said side gears, and said other side gear being connected directly to said gear mount casing. With the above-mentioned arrangement, drive power transmitted to the diff-carrier from the engine is distributed from a pinion on the diff-carrier to said one side gear coupled to the front wheel differential gear and to said other side gear coupled to the rear wheel side. The drive power transmitted to said one side gear is transmitted to the front wheel differential gear where the drive power is distributed to rotate the front axles, while the drive power transmitted to said other side gear is transmitted through the gear mount casing directly coupled to said other side gear and through a rear wheel drive ring gear to drive a pinion shaft, and is then transmitted to the rear axles to rotate the rear wheels. During the drive power transmission, the axial force generated in said one side gear is borne by the diff-carrier and ring gear mount casing, while the axial force generated in said other side gear is directly borne by the gear mount casing. An example of a power transmission device for a four wheel drive vehicle in accordance with the present invention is described below with reference to the accompanying drawings wherein:- Figure 1 is a cross sectional view illustrating part of a power transmission device according to the present invention; Fig. 2 is a cross-sectional view illustrating a power transmission device for a four wheel drive vehicle to which the present invention is applied; and Fig. 3 is a cross-sectional view illustrating a transfer unit provided with a conventional center differential gear. The present invention shall now be explained in one embodiment form with reference to the drawings. The power transmission device for a full-time transverse front engine four wheel drive vehicle, is provided, as shown in Fig. 2, with a torque converter unit 2, an automatic transmission 3, a front-wheel differential gear 5, a differential motion limiting mechanism 6 and a transfer unit 7, these being disposed in a trans-axle housing 9, a trans-axle casing 10 and a transfer casing 12 which are joined to each other. The torque converter unit 2 is composed of a torque converter 13 and a lock-up clutch 15, the rotation of the engine output shaft 16 being transmitted through them to an input shaft 17. The automatic transmission 3 is composed of a three stage planetary gear unit which is controlled by clutch or brake, the rotation of the input shaft 17 being subjected to speed reduction in any of the first, second, third overdrive and reverse gear stages and being transmitted to an output gear 20 rotatably supported on shaft 19. The above-mentioned front wheel differential gear 5 is coaxially disposed on the first ring gear 21 meshed with the gear 20 and mount casing 25 to which the above-mentioned ring gear 21 is secured and which is supported by casings 10, 9 through tapered roller bearings 22, 23. Its front diff-carrier 26 is rotatably incorporated in the mount casing 25, and side gears 27, 29 which are meshed with pinion 28 supported on the diff-carrier 26 are coupled to front axles 30, 31 so that power can be transmitted. A hydraulic multiple-disc clutch 47 constituting the differential motion limiting mechanism 6 is coaxially disposed between the ring gear mount casing 25 and the front diff-carrier 26. The two piece transfer casing 12 is incorporated to the right side of the first ring gear mount casing 25 and front wheel differential gear 5 in the rear of the engine, and the transfer unit 7 is constituted within the transfer casing 12, coupled coaxially with the first ring gear mount casing 25 and the front-wheel differential gear 5. The transfer unit 7 includes a two piece second ring gear mount casing 32 which supports a rear wheel drive second ring gear 35 composed of a hypoid gear, and which is rotatably supported by the transfer casing 12 through a pair of tapered roller bearings 36, 37. The second ring gear 35 is always meshed with a gear 40 on the drive pinion shaft 39 which is coupled to the rear axles through a known propeller shaft and a known rear wheel drive differential gear (not shown), so that power can be transmitted. The center differential gear 33 of the present power transmission device 1 comprises, as shown in Fig. 1, a diff-carrier 41 which rotatably supports pinion 50 through pinion shaft 49, and left and right hand side gears 42, 43 meshed with the pinion 50, the diff-carrier 41 having such a cantilever structure that it is supported between the inside of the second ring gear mount casing 32 and the outside of the left hand side gear 42. The rotation of the first ring gear mount casing 25 is transmitted to the diff-carrier 41 through the first hollow shaft 45, and the second ring gear mount casing 32 is spline-coupled directly to the right hand side gear 43. The diff-carrier 41 is rotatably incorporated in the second ring gear mount casing 32, and the front axle 31 passes through both side gears 42, 43 and projects rightward from the right side end of the transfer casing 12. The left hand side gear 42 in the center differential gear 33 is coupled to the front diff-carrier 26 in the front wheel differential gear 5 by means of a second hollow shaft 46 which is spline-coupled with the left side end of the side gear 42 and rotatably fitted over the front axle 31, so that power can be transmitted. The first and second hollow shafts 45, 46 are attached integrally to the first gear mount casing 25 and the front diff-carrier 26, respectively, and the diff-carrier 41 is spline-coupled with the first hollow shaft 45 by press-fitting. In the above-mentioned power transmission device 1, the first gear mount casing 25 is arranged to be engaged to and disengaged from the front diff-carrier 26 in the front differential gear 26 by means of the differential motion limiting mechanism 6. That is, when the hydraulic multiple-disc clutch 47 is actuated and the first ring gear casing 25 and the front diff-carrier 26 are integrally coupled with each other, the center differential gear 33 is inhibited from operating or is locked, and when the clutch is not actuated, relative rotation between side gears 42, 43 are allowed so that the center differential gear 33 is operational. With the above-mentioned arrangement, the rotation of engine is transmitted to the automatic transmission 3 through the torque converter 13 or the lock-up clutch 15, and is then transmitted from its output gear 20 to the mount casing 25 through the first ring gear 21. The rotation is further transmitted to the diff-carrier 41 of the center differential gear 33 through the first hollow shaft 45, and is delivered from the pinion 50 on the pinion shaft 49 to side gears 42, 43. The drive power transmitted to the left hand side gear 42 is transmitted to the front-wheel differential gear 5 through the second hollow shaft 46 so that the front-wheel differential gear 5 distributes the drive power to both side gears 27, 29, thereby transmitting the drive power to both front axles 30, 31. Meanwhile, the drive power transmitted to the right hand side gear 43 is transmitted to the gear 40 through the second gear mount casing 32 and the rear wheel drive ring gear 35, and is distributed from the drive pinion shaft 39 to both rear axles through the propeller shaft and the rear-wheel differential gear which are not shown. As described above, according to the present invention, since the diff-carrier 41 in the center differential gear 33 is supported between the side gear 42 on the front axle drive side and the rear-wheel gear mount casing 32 in cantilever fashion the side gear 43 transmitting power to the mount casing 32 being coupled directly to the latter without passing through the diff-carrier 41, the diff-carrier 41 can have a compact and lightweight structure with smooth surface, and therefore, even if both side gears 42, 43 are designed to have sufficiently large dimensions in order to withstand the input torque, it allows the transfer casing 12 to be stored in a confined space in the rear section of the engine. Also, even with the above-mentioned modification to the power transmission device, a substantial part of the remaining component parts can be used in common with conventional power transmission devices, so that it is greatly advantageous in terms of cost thereof. Further, as the axial force generated in the side gear 43 is borne directly by the mount casing 32 it is possible to eliminate the thrust washers necessary for smooth relative rotation between the side gears, the diff-carrier and the mount casing, in the conventional power transmission devices, thereby. promoting the reduction in the number of necessary component parts.

Claims: A power transmission device (1) for a four wheel drive vehicle, comprising a front wheel differential gear (5) and a transfer unit (7) having a centre differential gear (33), the centre differential gear (33) comprising a diff-carrier (41) coupled to a power transmitting front casing (25) covering a front diff-carrier (26) in said front wheel differential gear (5) and arranged co-axially with said front diff-carrier (26), one side gear (42) coupled to said front diff-carrier (26) and another side gear (43) coupled to a gear mount casing (32) for transmitting power to the rear wheels, the diff-carrier (41) in said centre differential gear (33) being supported between one of said side gears and said gear mount casing and including an extremity having an open end not supported by the other of said side gears, and said other side gear being connected directly to said gear mount casing (32). A power transmission device according to claim 1, in which the diff-carrier (41) in said centre differential gear (33) is coupled to said front casing (25) through a first hollow shaft (45), and said one side gear (42) is coupled to said front diff-carrier (26) through a second hollow shaft (46) located inside said first hollow shaft (45) and fitted over a front axle (31) extending from said front wheel differential gear (5) , said front wheel differential gear (5) being arranged co-axially with said centre differential gear (33) A power transmission device according to claim 1 or claim 2, in which a differential motion limiting device (6) comprises a hydraulic friction clutch (47), said friction clutch (47) being disposed between the front diff-carrier (26) in said front wheel differential gear (5) and said front casing (25) covering said front diff-carrier (26), with said front wheel differential gear (5), said centre differential gear (33) and said differential motion limiting device (6) being arranged co-axially with each other. A power transmission device according to any preceding claim, in which said front casing (25) is a ring gear mount casing having a ring gear (21) meshed with an output gear (20) of an automatic transmission (3). A power transmission device according to any preceding claim, in which said gear mount casing (32) of said centre differential gear (33) is a ring gear mount casing having a hypoid gear (35) meshed with a rear wheel drive gear (40).

Inventors: IWATSUKI, TATSUYA@AISIN-WARNERK.K., KANO, TAKENORI@AISIN-WARNER K.K., KAWAMOTO, MUTSUMI@AISIN-WARNERK.K., Iwatsuki, TatsuyaµAisin-WarnerK.K., Kano, TakenoriµAisin-Warner K.K., Kawamoto, MutsumiµAisin-WarnerK.K.
Applicants: AISIN-WARNER KABUSHIKI KAISHA
Application year: 1987
Main classification: B60K17
IPC codes: B60K17