Abstract:
A vehicle drive system includes a compact drive mode-shifting mechanism that is mounted in the final drive gearset of a vehicle. The drive mode-shifting mechanism includes a first shaft, a second shaft, and a clutch assembly. The first shaft forms an input shaft for the differential for a pair of wheels, the second shaft transmits engine power and the clutch assembly connects and disconnects the first and second shafts to one another to effect either the two-wheel-drive mode or the four-wheel-drive mode. The first shaft has an outer surface provided with a first spline, the second shaft has an outer surface provided with a second spline, and said clutch assembly includes an axially movable shifting member provided with an internal spline. The shifting member is movable between a first position in which the internal spline on the shifting member is in engagement with the first spline on the first shaft and the second spline on the second shaft to achieve the four-wheel-drive more and a second position in which the internal spline on the shifting member is in engagement with the first spline on the first shaft and is out of engagement with the second spline on the second shaft to achieve the two-wheel-drive mode.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a drive train shifting mechanism. More particularly, the present invention pertains to a vehicle drive system provided with a mechanism for shifting the drive train through which engine power is transmitted between two-wheel drive and four-wheel drive. 
     BACKGROUND OF THE INVENTION 
     One known mechanism for shifting the drive wheels to which engine power is transmitted from front or rear wheel drive to all four wheel drive is a transfer case. Generally speaking, the transfer case is fabricated integrally with the transmission. The transfer case is specifically adapted for use a connection with a particular transmission and the use of a different transfer case requires the use of or replacement of many differed parts. Thus, the transfer case is not well suited for use with different vehicles mod different transmissions. 
     Another known arrangement for effecting a shift between two-wheel drive and four-wheel drive is disclosed in Japanese Patent Publication No. 23330/1989. Here, a clutch is mounted to one output shaft of the differential on the axle side for coupling and uncoupling the engine power from a part-time four-wheel drive vehicle. When the engine power is coupled, the clutch is engaged to obtain the four-wheel-drive mode. When the engine power is uncoupled the clutch is disengaged to achieve the two-wheel-drive mode. In this way, the drive mode can be switched between the two-wheel-drive mode and the four-wheel-drive mode. Also, in the two-wheel-drive mode, the parts of the drive mechanism necessary for the four-wheel-drive mode are not needlessly driven. 
     In the arrangement disclosed in the above-cited Japanese Patent Publication No. 23330/1989, the clutch assembly in mounted an the output shaft side of the differential which undesirably increases the width of the differential. This clutch assembly also transmits torque after the final gear reduction. In the four-wheel-drive mode, large torque is imposed on the clutch assembly. Therefore, there arises the problem that the clutch assembly must be made of a material having great strength at must be made rather large in size to obtain sufficient strength. 
     In light of the foregoing, a need exists for a drive-mode shifting mechanism that does not suffer from the foregoing disadvantages and drawbacks. 
     It would thus be desirable to provide a drive-mode shifting mechanism for switching between two-wheel drive and four-wheel drive that need not be made of excessively strong material and that is not excessively large in size. 
     It would also be desirable to provide a drive-mode shifting mechanism for switching between two-wheel drive and four-wheel drive that has a wide range of use and is not limited to use in connection with a particular vehicle or transmission. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a vehicle drive system switchable between the two-wheel-drive mode and the four-wheel-drive mode includes a transmission, and a housing in which is disposed a differential gear connected to a pair at wheels and a drive-mode-shifting mechanism. The drive-mode-shifting mechanism includes a first shaft forming an input shaft for the differential gear, a second shaft for transmitting power from the transmission, and a clutch assembly for connecting the first and second shafts during the four-wheel-drive mode and for disconnecting and second shaft during the two-wheel-drive mode. 
     In the present invention the clutch assembly is mounted in the final drive gearset and so the mechanism for shifting between the two-wheel-drive mode and four-wheel-drive mode can be constructed rather simply by replacing one unit (final drive gearset) including the differential. Because the clutch assembly is mounted on the input side of the differential, the clutch assembly beings the first shaft into or out of engagement with the second shaft before speed is reduced by the gears of the differential. That is, the clutch assembly according to the present invention does not require the same degree of high strength as assemblies that are mounted an the output side of the differential. Consequently, where the same material in used a smaller clutch assembly can be built. 
     The first and second shafts are preferably mounted coaxially with one another, with the first shaft having a first spline on its outer surface and the second shaft having a second spline on its outer surface. The clutch assembly includes a shifting member having an internal spline that corresponds to the splines on the first and second shafts. The shifting member is able to move axially to couple the first and second shafts together by aligning the internal spline of the shifting member to the splines on both the first and second shafts. 
     The clutch assembly is advantageously mounted between the bearing that supports the first shaft and the bearing that supports the second shaft. In this way advantageous use is made of space which would otherwise be dead space. This permits the size of the assembly to be reduced. Furthermore, this clutch assembly can be used with different transmissions and different vehicles with greater ease. 
     The differential referenced herein refers to a differential assembly for absorbing the difference in rotational speed between the right and left drive wheels or between front and rear drive wheels to permit smooth driving. The differential can be a limited slip differential. The final drive gearset referenced herein refers to the land drive/differential assembly. 
     According to another aspect of the present invention, a vehicle drive system for a vehicle having four wheels is switchable between a two-wheel-drive mode in which only two of the wheels are driven and a four-wheel-drive mode in which all four of the wheels are driven, The vehicle drive system includes a transmission, a differential gear, and a drive mode-shifting mechanism positioned between the transmission and the differential gear for shifting between the two-wheel-drive mode and the four-wheel-drive mode. The drive mode-shifting mechanism is housed in a housing that is separate from and spaced from the transmission. The drive mode-shifting mechanism includes a first shaft, a second shaft and a clutch assembly for connecting the first shaft and the second shaft during the four-wheel-drive mode and for disconnecting the first shaft and the second shaft during the two-wheel-drive mode. 
     Another aspect of the present invention involves a vehicle drive system switchable between a two-wheel-drive mode and a four-wheel-drive mode, wherein the vehicle drive system includes a differential gear connectable to a pair of wheels, and a drive mode-shifting mechanism for shifting between the two-wheel-drive mode and the four-wheel-drive mode. The drive mode-shifting mechanism includes a first shaft forming an input shaft of the differential gear, a second shaft for transmitting power and a clutch assembly for connecting the first shaft and the second shaft during the four-wheel-drive mode and for disconnecting the first shaft and the second shaft during the two-wheel-drive mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The foregoing and additional features and characteristics of the present invention will become more apparent from the following detailed description considered with reference to the accompanying drawing figures in which like reference numerals designate like elements and wherein: 
         FIG. 1  in a schematic diagram of a portion of a vehicle having a vehicle drive system provided with a drive mode-shifting mechanism in accordance with the present invention; 
         FIG. 2  is a cross-sectional view of main portions of the drive mode-shifting mechanism shown in  FIG. 1 ; and 
         FIG. 3  is an enlarged cross-sectional view of a portion of the drive mode-shifting mechanism taken along the section line III—III in FIG.  2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A vehicle having a vehicle drive system equipped with a drive mode-shifting mechanism in accordance with the present invention is schematically shown in FIG.  1 . In the embodiment discussed below, the vehicle is a rear wheel drive vehicle in which engine power is transmitted to the rear wheels at all times. In the two-wheel-drive mode, power transmission to the front wheels is interrupted. 
     As illustrated in  FIG. 1 , the vehicle includes an engine  1  acting as a power source, a transmission  2  connected to the output of the engine  1 , a drive axle  4  having one end connected to the output side of the transmission  2  for transmitting power from the engine  1 , and a shaft  17  (see  FIG. 2 ) having one end connecting to one end of the drive axle  4  by way of a yoke  16  and the other end connected to a final drive gearset  5 . A pair of front axles  6 ,  8  in connected to the final drive gearset  5 , and a front wheel  7 ,  9  is attached to each of the front axles  6 ,  8  respectively. The vehicle also includes a pair of rear wheels  13 ,  15 , each of which is connected to a respective rear axle  12 ,  14 . The rear axles  12 ,  14  are connected to a final drive gearset  11  which is also connected to the transmission  2  by way of a shaft  30  and a yoke  32 . 
     As seen in  FIG. 2 , a first shaft  18  in located in the final drive gearset  5 . This first shaft  18  is mounted coaxially with respect to the shaft  17  that is connected to the drive axle  4  by way of the yoke  16  and to the first drive gearset  5 . The shaft  17  constitutes a second shaft. The first shaft  18  is an input shaft to the front differential  20 . 
     The drive mode-shifting mechanism of the present invention is mounted in the final drive gearset  5  and comprises the first shaft  18 , the second shaft  17  mounted coaxially with the first shaft  18 , and a clutch assembly capable of connecting and disconnecting the first shaft  18  and the second shaft  17 . The first shaft is rotatable relative to a housing  21 . A pinion gear  18 b meshing with a ring gear  23  is mounted on the side of the differential  20 , which in turn is mounted inside the housing  21 . This differential  20  is housed in a differential case  22  that s mounted so as to be rotatable perpendicular to the axles  6 ,  8 . The ring gear  23  meshing with the pinion gear  18 b is mounted in the differential case  22 . The second shaft  17  is adjacent to the outer end of the first shaft  18  and is rotatable relative to the housing  21 . The yoke  16  is fixedly mounted to the output shaft of the transmission and to an end of the second shaft  17 , and transmits engine power to the second shaft  17 . The yoke  16  permits the second shaft  17  to move or pivot relative to the drive axle  4 . 
     The construction of the clutch assembly is as follows. The clutch assembly includes a shifting member  19  located between the first shaft  18  and the second shaft  17 , and shifting member  19  is capable of connecting and disconnecting the first shaft  18  and the second shaft  17 . An internal spline  19 a is formed on the inner surface of the shifting member  19 . This internal spline  19 a corresponds to and is capable of engaging a first spline  18 a formed on the outer surface of the first shaft  18  and a second spline  17 a formed on the outer surface of the second shaft  17 . This shifting member  19  is adapted to slide axially along the splines  18 a,  17 a on the first and second shafts  18 ,  17 . 
     A shifting device for producing axial sliding movement of the shifting member  19  includes an output gear  25  and a rack  26 . As the output gear  25  turns, the rack  26  slides relative to a shaft  27  that in mounted to the housing  21 . The output gear  25  in rotated through operation of an electric motor and the operation of the motor in controlled by an electronic control unit (ECU). The ECU is connected to, for example, a switch which affects operation of the motor. The output gear  25  is mounted on the motor via a spiral spring. When the motor is driven, power is transmitted to the output gear  25  via the spring. As the output gear  25  rotates, the rack  26  moves axially. An element in the form of a fork  28  is firmly fixed to the rack  26  and so axial movement of the rack  26  also causes axial movement of the fork  28 . Accordingly, the shifting member  19 , which is rigidly affixed to one end of the fork  28 , can be axially slid along the splines on the first and second shafts  18 , 17  through movement of the fork  28 . 
     In accordance with the present invention, a switch-depressing portion  28 a in mounted integrally with the fork  28 . During four-wheel drive, the fork  28  is moved to push the switch-depressing portion  28 a against a switch  31 . In this way, the four-wheel-drive mode is electrically detected. Because the switch  31  in turned on and off, using the fork  28  independent of the rotation of the first shaft  18  and the second shaft  17 , wear of the switch  31  is prevented. As a result, the durability of the switch  31  is improved. 
     The operation of the drive-mode switching mechanism of the present invention is as follows. When the vehicle is in the four-wheel-drive mode, the shifting member  19  is splined to both the first shaft  18  and the second shaft  17 . Torque from the output shaft of the transmission  2  is transmitted to the yoke  16 , the second shaft  17 , the first shaft  18 , the ring gear  23 , and the differential case  22  in this order. Power is then transmitted via the front axles  6 ,  8  to the front wheels  7 ,  9  that are the drive wheels. When the vehicle makes a turn, the front wheels  7 ,  9  produce a difference in travel with respect to the rear wheels  13 ,  15 , resulting the a difference in rotational speed between the front wheels  7 ,  9  and the rear wheels  13 ,  15 . Therefore, if the four-wheel-drive mode is maintained, the steering effort is undesirably increased. 
     Accordingly, if the front and rear wheels produce any difference in rotational speed, the output gear  25  is rotated in the opposite direction from that described above, causing the shifting member  19  to slide axially. This rotational speed differential can be determined by speed sensors. This disconnects the shifting member  19  from the second shaft  17 . Under this condition, the two-wheel-drive mode is achieved, i.e., engine power is no longer transmitted to the first shaft  18 . In this way, the difference in rotational speed between the front and rear wheels can be absorbed and converted into rotation between the first shaft  18  and the second shaft  17 . This reduces the steering effort. 
     However, where the torque between the shifting member  19  and the second shaft  11  is large and thus the shifting member  19  cannot slide axially, even if the motor operates, the shifting member  19  cannot axially slide. The aforementioned spiral spring mounted between the output shaft of the motor and the output gear  25  then deflects. When the torque in the second shaft  17  is reduced to a level that allows the shifting member  19  to slide, the output shaft  25  rotates, thus producing sliding movement of the shifting member  19 . The shifting member  19  splined to both the first shaft  18  and the second shaft  17  is then splined to only the first shaft  18 . As a result only the rear wheels  13 ,  15  we allowed to be driven. That is, the two-wheel-drive mode is achieved. The shifting member  19  and the fork  28  thus assume the state indicated by the solid line in FIG.  2 . 
     The drive mode is shifted from the two-wheel-drive mode to the four-wheel-drive mode in the manner described below. To return to the four-wheel-drive mode, the shifting device actuates the output gear  25  so that the shifting member  19  slides axially. The shifting member  19  can then engage both the first shaft  18  and the second shaft  17 . Thus, torque from the output shaft of the transmission  2  is transmitted to The front wheels  7 ,  9 , as well as to the rear wheels  13 ,  15 . If the internal spline  19 a on the shifting member  19  and the second spline  17 c on the second shaft  17  are out of phase and the sliding member  19  cannot slide, the shifting member  19  in unable to slide even if the motor is operated. The aforementioned spiral spring located between the output shaft of the motor and the output gear  25  thus deflects. When the internal spline  19 a on the shifting member  19  and the second spline  17 a on the second shaft  17  are located in phase due to slippage of the rear wheels, for example, the output gear  25  actuates to slide the shifting member  19 , thus splining the shifting member  19  to both the first shaft  18  and the second shaft  17 . This in the instant when a shift is made from the two-wheel-drive mode to the four-wheel-drive mode. The shifting member  19  and the fork  28  are then in the state indicated by the dot-and-dash him in FIG.  2 . 
     In, accordance with the present invention, a shift is made at will between the two-wheel-drive mode and the four-wheel-drive mode by operating the clutch assembly. That is the driver operates a shifting switch (not shown) to electrically turn on or off the motor or to switch the direction of electrical energization. Alternatively, rotational speed sensors may be mounted on the front and rear wheels. Any difference in rotational speed between the front and rear wheels is then detected by the electronic control unit (ECU). The motor is then electronically energized to effect a shift from the four-wheel-drive mode to the two-wheel-drive mode. If the ECU senses that the rotational speed difference bas decreased to zero, the direction of energization of the motor is reversed as compared to the situation where a shift is made to the two-wheel-drive mode. In this way, a shift is made from the two-wheel-drive mode to the four-wheel-drive mode. 
     According to the present invention, when the splined shafts are out of phase or the torque is large, the sliding movement of the shifting member  19  is limited through use of the spiral spring. It is to be understood, however, that the use of the spiral spring is not essential to the present invention. Rather, any type of mechanism may be used as long as the shifting member  19  is inhibited from sliding before the splines are located in phase with one another. 
     Also, in the present invention, the switching member is mounted between the bearing  29  supporting the first shaft  18  and the bearing  30  supporting the second shaft  17 . Consequently, use is advantageously made of space which would have been dead space in other known constructions. This permits the assembly to be made smaller. 
     Furthermore, this clutch assembly can be used without having to make any modifications to the transmission or without having to use a different transmission. In addition, the steering force can be reduced by making a shift from the four-wheel-drive mode to the two-wheel-drive mode. 
     According to the present invention, engine power is transmitted directly to the output axle without being transmitted via a transfer ease. The invention can also be applied to a structure in which a transfer case is mounted between the transmission and the output axles of the front and rear wheels. In the embodiment described above, the differential  20  is a limited-slip type differential. However, as an alternative, an open type differential can be used. 
     In accordance with the present invention, the clutch assembly is mounted in the final drive gearset. Therefore, a mechanism permitting the driver to effect a shift between the two-wheel-drive mode and the four-wheel-drive mode can be constructed simply by replacing only the final drive gearset including the differential. Also, the clutch numbly is mounted on the input side of the differential. Consequently, the clutch amounts and disconnects the first And second shafts before the speed is reduced by the gears of the differential. That is, the clutch assembly according to the present invention does not require high strength, unlike other known clutch assemblies mounted an the output side of the differential. Consequently, where the same material is used, a smaller clutch assembly can be built. 
     As noted above, by mounting the clutch assembly between the two bearings that support the first and second shafts, advantageous use is made of space which would otherwise have been dead space. This permits the size of the assembly to be reduced. Further, the clutch assembly of the present invention can advantageously be used without the need for significant modifications to the transmission and without the need for different transmissions. 
     The principles, a preferred embodiment and mode of operation of the present invention have been described in the foregoing specification. However, the invention which is intended to be protected is not to be construed as limited to the particular embodiment described. Further, the embodiment described herein is to be regarded as illustrative rather than restrictive. Variations and changes may be made by others, and equivalents employed, without departing from the spirit of the present invention. Accordingly, it is expressly intended that all such variations, changes and equivalents which fall within the spirit and scope of the invention be embraced thereby.