Abstract:
An axle case assembly is provided with a speed sensitive torque coupling mechanism used to transmit torque from the ring gear to a planetary differential assembly. The differential assembly provides torque transfer proportional to the speed difference between the ring gear sub-assembly and a planetary gear set sub-assembly, wherein the invention splits a differential case assembly into two primary pieces and a speed sensitive mechanism is installed between each piece. The mechanism is entirely contained inside an axle differential case assembly. An optional limited slip device may be provided for the differential gears. The torque transmission coupling assembly eliminates the need for a center differential in the transfer case, i.e. an interaxle differential, thereby reducing the driveline complexity and cost without requiring a separate torque coupling in the transfer case or in-line with the driveline.

Description:
BACKGROUND OF THE INVENTION 
     a) Field of the Invention 
     The present invention relates to a full time-four-wheel-drive vehicle in which front and rear wheels are always connected to each other through a torque transmission coupling. 
     b) Description of Related Art 
     Torque applied to a tire through a drive shaft propels a vehicle by the friction between the tire and the surface of the road for the vehicle. Occasionally, slip takes place between the road surface and the tire. The ratio of the slip depends on the coefficient of friction between the tire and the road surface. The coefficient of friction fluctuates due to the states of the road surface and the tire, the grounding load upon the tire, the magnitude of the torque transmitted to the tire, the driving speed of the vehicle, and so forth. 
     As for an ordinary two-wheel-drive vehicle, high torque is transmitted to each driving wheel through a transmission at the start of the vehicle so that a large slip could take place between the road surface and the tire of the wheel. The torque transmitted through the transmission decreases as the driving speed of the vehicle rises, so that the ratio of the slip falls. 
     When the torque transmitted to the tire is so high that the tire slips, the torque does not fully act to propel the vehicle, resulting in wasted motive power, lowered fuel efficiency, and adverse vehicle handling. 
     When the fluctuation in the coefficient of friction is large or the coefficient of friction is very small, as on a muddy road, a partially icy road, a snowy road, a graveled road, or the like, the stability of movement of the vehicle is reduced and the stopping distance increases in the case of locking of the wheel in braking. Moreover, it is sometimes difficult to maintain the direction of movement of the vehicle in the case of locking of the rear wheel (in particular, in braking). 
     For the above-mentioned reasons, four-wheel-drive vehicles have become popular vehicles for driving on a wide range of road conditions. In four-wheel-drive vehicles, the driving power of an engine is dividedly transmitted to four wheels to eliminate the above-mentioned drawbacks and problems. 
     Since a rotation speed difference arises between the front and rear wheels of the four-wheel-drive vehicle due to the turning radius difference between the front and the rear wheels at the time of turning of the vehicle, torsional torque is caused (a tight corner braking phenomenon) between the drive shafts for the front and the rear wheels if the turning is performed on a high-friction-coefficient road (such as a paved road), on which the driving wheel and the surface of the road are less likely to slip relative to each other. For that reason, different types of four-wheel-drive vehicles have been developed in order to prevent the deterioration of the moving property of each vehicle due to the torsional torque, the increase in the wear of the tire, the shortening of the life of the vehicle, and so forth. 
     One of the different types of four-wheel-drive vehicles is a part time four-wheel-drive vehicle in which the driver shifts from the four-wheel drive mode to the two-wheel drive mode when running on a high-friction-coefficient road such as a paved road. Another type of four-wheel-drive vehicle is a full time-four-wheel-drive or all-wheel-drive vehicle equipped with a center differential unit for dividedly transmitting motive power to a front and a rear wheel drive shafts. Another type of four-wheel-drive vehicle is a full time-four-wheel-drive vehicle in which the front or rear wheels are always driven and in which the rear or front wheels are driven through a viscous clutch which transmits torque by the viscosity of silicone oil or the like. 
     Although the part time-four-wheel-drive vehicle can be manufactured at a relatively low cost, it is troublesome to shift between the two-wheel drive and the four-wheel drive and it is likely that the vehicle is slowly turned when the driver mistakenly fails to properly choose between four-wheel drive and two-wheel drive. It is less likely that every driver can precisely predict the occurrence of slip of the driving wheel and take appropriate action. 
     Full time-four-wheel-drive vehicle, that are equipped with the center differential unit, have a front wheel drive differential unit, which dividedly transmits motive power to the right and left front wheels, and a rear wheel drive differential unit, which dividedly transmits motive power to the right and left rear wheels. These full-time four-wheel-drive vehicles suffer from a problem that no motive power is transmitted to any of the remaining three of four driving wheels when one wheel is caused to spin or loses the tire grip due to overhanging on the road side or ditch, a slip on an icy road, or the like. For that reason, the center differential unit is provided with a differential locking mechanism. The differential locking mechanism is of the mechanical type or the electronic control type. In the mechanical type, a differential rotation which takes place in the center differential unit is stopped through manual shifting when no motive power is transmitted to the three of the four driving wheels in order to put the vehicle into the state of direct-connection four-wheel drive. In the electronic control type, the speed of the vehicle, the angle of turning of the vehicle, the racing of the drive shaft, and so forth are detected by sensors in order to put the differential locking mechanism into a locking or unlocking state through an electronic controller. As for the mechanical type, it is difficult to set a differential locking start time point, the time point cannot be changed depending on the moving condition of the vehicle, and it is more difficult to automate the differential locking mechanism. As for the electronic control type, a device for controlling the differential locking mechanism is more complex and the cost of production of the mechanism is very high. 
     Since the center differential unit comprises an input shaft which receives motive power transmitted from an engine through a transmission, a differential case connected to the input shaft, a pinion shaft which is driven by the differential case, pinions rotatably attached to the peripheral surface of the pinion shaft, a first side gear which is engaged with the pinion and connected to a first differential means for driving the front or rear wheels, a second side gear which is engaged with the pinion and connected to a second differential means for driving the rear or front wheels, and the differential locking mechanism which engages the differential case and the side gear with each other through mechanical operation or electronic control, the cost of production of the center differential unit is very high and the weight of the vehicle is increased. 
     It is also known to replace the aforementioned center differential with a torque transmission coupling that includes an input shaft drivingly connected to the transmission and a first differential, an output shaft drivingly connected to a second differential, an oil pump driven by the relative rotation between the input and the output shafts to generate oil pressure corresponding to the speed of the relative rotation, and a friction clutch mechanism engaging the input shaft and the output shaft with each other by the oil pressure generated by the oil pump. The torque transmitted by the torque coupling is proportional to the speed of the relative rotation. When the rotation speed of the wheels driven by the first differential is higher than that of the wheels driven by the second differential, a rotation speed difference takes place between the input and the output shafts. The oil pump generates the oil pressure corresponding to that rotation speed difference. The oil pressure is applied to the friction clutch mechanism so that torque is transmitted from the input shaft to the output shaft depending on the magnitude of the oil pressure. When torque is transmitted to the second differential, the rotation speed of the wheels drivingly connected to the second differential is raised to approach that of the wheels driven by the first differential, thereby reducing the rotation speed difference between the input and the output shafts. In short, the torque transmission coupling operates in response to the rotation speed difference that takes place depending on the environmental situation of the vehicle and the moving conditions thereof. In other words, a prescribed slip is always allowed. 
     The conventional torque coupling assemblies, however, suffer from drawbacks inherent in their assembly and location within the vehicle drivetrain. Conventional torque coupling assemblies are installed in the transfer case or in-line with the driveline or driveshaft. 
     The need therefore exists for a torque coupling assembly that eliminates the need for a center differential in the transfer case, i.e. an interaxle differential, thereby reducing the driveline complexity and cost without requiring a separate torque coupling in the transfer case or in-line with the driveline. 
     SUMMARY OF THE INVENTION 
     The present invention was made in consideration of the above-described circumstances. Accordingly, it is an object of the present invention to provide a four-wheel-drive vehicle which does not have the above-described drawbacks and problems and fulfills the functions of four-wheel drive under all conditions and whose constitution and cost are simple and low, respectively. 
     The present invention provides an axle case assembly with a speed sensitive mechanism used to transmit torque from the ring gear to a planetary differential housing. The inventive differential assembly provides torque transfer proportional to the speed difference between the ring gear sub-assembly and a planetary gear set sub-assembly, wherein the invention splits a differential case assembly into two primary pieces and a speed sensitive mechanism is installed between each piece. In the preferred embodiment, the mechanism is entirely contained inside an axle differential assembly. An optional limited slip device may be provided for the differential gears. 
     The primary features, structures and objectives of the invention will be more fully understood with reference to the following description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of a four-wheel-drive vehicle incorporating the torque coupling assembly of this invention. 
     FIG. 2 is a partially exploded view showing the ring gear assembly, the bearing assembly and the differential assembly of the invention with the speed sensitive torque coupling device omitted. 
     FIG. 3 is a view of the torque coupling and differential assembly of FIG. 2 shown in the assembled state with a pump system and clutch pack interposed between the ring gear sub-assembly and the differential sub-assembly. 
     FIG. 4 is a perspective view of the axle differential case housing the torque coupling assembly of this invention; 
     FIG. 5 is a side view of the axle differential and torque coupling assembly shown in FIG. 4; 
     FIG. 6 is a left-end view of the differential and torque coupling assembly shown in FIG. 5; 
     FIG. 7 is a sectional view of the differential and torque coupling assembly as viewed along section line VII—VII of FIG. 6; 
     FIG. 8 is a sectional view of the differential and torque coupling assembly as viewed along section line VIII—VIII of FIG. 6; 
     FIG. 9 is left end view of the differential and torque coupling assembly as shown in FIG. 5 with components of the pump actuator shown in dotted lines; 
     FIGS. 10-13 are exploded views of the primary components of one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1, the four-wheel-drive vehicle provided in accordance with the present invention comprises an engine  110 , a transmission  130  which is driven through a clutch  120  by the engine  110  to change the speed of the output rotation of the engine  110 . A transfer case  150  divides torque transmission between a first differential means  140  that drives one of a front and a rear wheel systems and a second differential means  170  that drives the other of the front and the rear wheel systems. 
     A torque transmission coupling is provided between a ring gear and a planetary differential housing. The torque transmission coupling comprises an oil pump that is driven by the relative rotation between the ring gear sub-assembly and a planetary gear set sub-assembly to generate oil pressure corresponding to the speed of the relative rotation. A friction clutch mechanism engages the ring gear sub-assembly and the differential gear set sub-assembly with each other by the oil pressure generated by the oil pump. The torque transmission coupling has such a property that the torque transmitted by the coupling is proportional to the speed of the relative rotation. 
     With reference to FIGS. 2 and 3, the torque coupling assembly comprises a ring gear sub-assembly  210 , a differential sub-assembly  220 , and a bearing sub-assembly  230  (see FIG.  2 ). The ring gear subassembly  210  includes a ring gear  212  fastened to a side case member  214  via fasteners  216 . While FIG. 3 shows fasteners  216  in the form of bolts fixing the ring gear  212  to the side case member  214 , it will be understood that various fastening assemblies may be employed without departing from the objectives of this invention. The differential sub-assembly  220  comprises a differential case  222 , a shaft  223  driven by the rear differential case  222 , pinions  224   a,    224   b  rotatably attached to the peripheral surface of the shaft  223 , and side gears  226   a,    226   b  engaged with the pinions  224 . The side gears  226   a,    226   b  drive the right and left axles (not shown in FIG.  3 ). 
     Interposed between the ring gear sub-assembly  210  and the differential sub-assembly  220  is a bearing assembly  230  which permits relative rotation between the ring gear sub-assembly  210  and the differential sub-assembly  220 . 
     Further provided between the ring gear sub-assembly  210  and the differential sub-assembly  220  is a speed-sensitive torque coupling assembly, shown generally as assembly  240 . The speed-sensitive torque coupling assembly  240  included in the preferred embodiment of the present invention comprises a fluid pump  250  and a clutch pack  260 . The fluid pump shown and described herein is a Gerotor type pump of the automatically reversible unidirectional flow type. The specific structure of the fluid pump  250  and clutch pack  260  will be described below. 
     With the assembly of FIGS. 2 and 3, a torque coupling assembly is provided within the axle differential case assembly. Therefore, when the rotation speed of the wheels driven by the first differential  140  is higher than that of the wheels driven by the second differential  170 , a rotation speed difference takes place. In that case, the pump  250  generates the oil pressure corresponding to that rotation speed difference. The oil pressure is applied to the friction clutch mechanism  260  so that torque is properly distributed between the first differential  140  and the second differential  170  depending on the magnitude of the oil pressure. When the torque is transmitted to the second differential  170 , the rotation speed of the wheels drivingly connected to the second differential  170  is raised to approach that of the wheels driven by the first differential  140 , thereby reducing the rotation speed difference between the front and rear wheels. 
     In the low speed running of the vehicle, the absolute value of the speed of rotation transmitted to the first differential  140  is small, and the rotation speed of the ring gear sub-assembly  210  is therefore small as well. Even if the speed of the rotation of the differential sub-assembly  220  output shaft is zero or very low, the absolute value of the rotation speed difference between the sub-assemblies  210 ,  220  is small. In addition, the rising of the oil pressure generated by the oil pump at the low rotation speed is generally slow due to the internal leak of the pump. For these reasons, the torque transmitted through the friction clutch mechanism  260  is very low, so that the ring gear sub-assembly and the differential sub-assembly are allowed to slip relative to each other. As a result, torsional torque caused between a front and a rear wheel drive shafts at the time of slow turning of the vehicle is absorbed by the friction clutch mechanism  260  to prevent a tight corner braking phenomenon. 
     In the high speed running of the vehicle, if the rotation speed of the wheels driven by the second differential  170  is even slightly lower than that of the wheels driven by the first differential  140 , the absolute value of the rotation speed difference between the ring gear sub-assembly  210  and the differential sub-assembly is certain to increase, because the absolute value of the speed of rotation transmitted to the first differential  140  is large in proportion to the driving speed of the vehicle. Therefore, the torque transmitted through the friction clutch mechanism  260  is also high, corresponding to the absolute value of the rotation speed difference between the ring gear sub-assembly  210  and the differential sub-assembly  220  shafts so that these shafts are maintained in a torque transmission state approximate to a directly connected state. For that reason, in the rapid running of the vehicle, the torque of the engine is transmitted to the front and the rear wheels, while the torque is divided nearly at a ratio of 50:50 between them, so that the stability of the running of the vehicle and the fuel efficiency thereof are enhanced. 
     Since the second differential  170  is always connected to the first differential  140  through the torque transmission coupling  240 , it is not necessary to perform troublesome shifting between two-wheel drive and four-wheel drive as is done in the conventional part time-four-wheel-drive vehicle. 
     When some driving wheel slips during the running of the vehicle provided in accordance with the present invention, the rotation speed difference between the ring gear sub-assembly  210  and differential sub-assembly  220  of the torque transmission coupling increases immediately so that the oil pressure corresponding to the rotation speed difference increases. Consequently, the friction clutch mechanism  260  immediately acts to prevent the increase in the rotation speed difference between the ring gear sub-assembly  210  and the differential sub-assembly  220  to keep the slipping driving wheel from skidding sideways. Excess torque is transmitted to the other non-slipping driving wheels instead of the slipping driving wheel, so that the torque of the engine transmitted through the transmission is dividedly transmitted to the first and the second differentials  140 ,  170 . Appropriate driving forces are thus automatically and constantly applied to the front and the rear driving wheels with good response. 
     With reference to FIGS. 4-13, a specific embodiment of the invention will now be described. FIG. 4 is a perspective view of the preferred embodiment of the invention in an assembled state, and FIG. 4 shows the differential case  222  disposed within an outer housing  215  which is affixed to the side housing  214 . The bolt holes  214   a  pass through both the side case member  214  and a flange  215   a  formed on the outer housing  215 . Apertures  214   b  are provided to receive bolts  216  to mount a ring ger (not shown) to the side housing  214  and the outer housing  215 . FIG. 5 is a side view of the torque coupling differential of FIG.  4 . 
     FIG. 6 is a left side view of the torque coupling differential of FIG. 5 illustrating the side housing  214  and bolts  216  mounted within the bolt holes  214   b.    
     FIG. 7 is a cross sectional view of the torque coupling differential as viewed along section line VII—VII of FIG. 6, and FIG. 8 is a cross sectional view of the torque coupling differential as viewed along section line VIII—VIII of FIG.  6 . FIG. 9 is left end view of the differential and torque coupling assembly as shown in FIG. 5 with components of the pump actuator shown in dotted lines. 
     FIGS. 6-8 show the components of the torque coupling mechanism disposed between the ring gear sub-assembly  210  and the differential sub-assembly  220 . The ring gear  212  has been omitted from FIGS. 4-8. The differential assembly shown in cross section includes the differential case  222 , pinion gears  224   a,    224   b  and side gears  226   a,    226   b,  wherein the pinion gears  224   a,    224   b  are mounted on the shaft  223 . 
     Disposed adjacent the side gear  226   a  is an inner clutch sleeve  242  having external splines  242   a.  A clutch pack is disposed between the ring gear sub-assembly  210  and the differential sub-assembly  220 . Forming the clutch pack are clutch plates  244  and  245  alternately mounted between the inner clutch sleeve  242  and the outer housing  215 . The clutch plates  244  mate with the splines  215   b  formed on the clutch sleeve  242 , and the clutch plates  245  mates with splines  215   b  formed on the inner surface of the outer housing  215 . The clutch plates  244  frictionally engage the clutch plates  245  to form a torque coupling arrangement between the ring gear sub-assembly  210  and the differential sub-assembly  220 . Torque is transferred from the ring gear to the outer housing  215 , then to the clutch plates  245 . The clutch plates  245  transmit torque to the clutch plates  244  which, in turn, transmit torque to the clutch sleeve  242 . The clutch sleeve  242  then transmits torque to the differential case  222 . 
     A speed sensitive fluid pump arrangement  250  actuates the clutch pack to increase the frictional engagement between the clutch plates  244 ,  245 . The speed sensitive fluid pump arrangement  250  comprises an outer ring member  252 , an outer rotor  254  and an inner rotor  256 . The inner rotor  256  mates with the clutch sleeve  242 , and the outer ring member  252  mates with the ring gear sub-assembly  210  via pin  253 . 
     As illustrated in FIG. 11, the inner rotor  256  has one less tooth than the outer rotor  254  and when the inner rotor  256  is driven it will drive the outer rotor  254 , which can freely rotate within the outer ring member  252  thus providing a series of decreasing and increasing volume fluid pockets by means of which fluid pressure is created. 
     External to the pump itself, the inner rotor  256  is matingly connected to the clutch sleeve  242 , and the sleeve  242  meshes with clutch plates  244 . When relative motion takes place between ring gear sub-assembly  210  and the differential sub-assembly  220 , the clutch sleeve  242  will rotate the inner rotor  256  of pump  250  to create fluid pressure. 
     The torque transmission coupling provided in accordance with the present invention comprises the ring gear sub-assembly  210 , the differential sub-assembly  220 , the friction clutch mechanism  260 , and the oil pump  250  for engaging the clutch mechanism depending on the rotation speed difference between the ring gear sub-assembly  210  and the differential sub-assembly  220 . Therefore, the torque coupling is an improvement over the conventional center differential unit of the full time-four-wheel-drive vehicle, the cost of production of the coupling is very low, and the weight of the four-wheel-drive vehicle provided in accordance with the present invention is reduced. 
     When the front wheel system of the four-wheel-drive vehicle provided in accordance with the present invention is driven by the first differential means, torque is transmitted to the rear wheels at the side of the second differential means as long as the front wheels are not locked at the sharp braking of the vehicle. For that reason, an anti-locking effect is produced. In other words, the torque is transmitted to the rear wheels from the front wheels through the torque transmission coupling. This serves to prevent the early locking of the rear wheels, which would be likely to occur at the time of braking on a low-friction-coefficient road such as an icy road. 
     As described above, the four-wheel-drive vehicle provided in accordance with the present invention fulfills the functions of four-wheel drive well through the action of the compact, lighter weight torque transmission coupling which does not need an electronic controller of high production cost and whose constitution is relatively simple, so that the cost of production of the coupling is lower. Of course, an electronic controller may be employed in conjunction with the torque coupling differential assembly described herein, whereby the fluid pump is eliminated in favor of an electronic controller or other suitable type of controller for the clutch mechanism or other torque transfer mechanism. 
     While the foregoing invention has been shown and described with reference to a specific arrangement and design, it will be understood by those of skill in the art that various changes in form and detail may be made therein without departing from the spirit and scope of this invention.