Abstract:
A four-wheel drive transmission is provided which combines a continuously variable transmission unit and a transfer case into a common assembly. The four-wheel drive transmission includes a continuously variable transmission unit, a drive selector unit and a torque transfer unit. Variable speed control of a worm/worm gear transmission drives a component of a differential associated with the continuously variable transmission unit to provide continuous ratio control between an input shaft and an intermediate shaft. The drive selector units establishes forward and reverse drive connections between the intermediate shaft and a first output shaft. Finally, the torque transfer unit controls the selective/automatic transfer of drive torque from the first output shaft to a second output shaft to provide four-wheel drive operation.

Description:
This application claims the benefit of provisional application Serial No. 60/159,892 filed Oct. 15, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to continuously variable transmissions for motor vehicles, and more particularly, to a four-wheel drive continuously variable transmission. 
     BACKGROUND OF THE INVENTION 
     In general, power transfer mechanisms, such as transfer cases, are operative for selectively directing power to the non-driven wheels of a motor vehicle for establishing four-wheel drive modes. Many automotive transfer cases are equipped with a speed reduction gear assembly for providing “high” and “low” speed ranges in conjunction with two and four-wheel drive modes. As such, a range shift mechanism is provided for operatively shifting components of the speed reduction gear assembly between a high range position providing a direct ratio drive connection and a low range position providing a reduction ratio drive connection. 
     Conventionally, transfer cases are provided in combination with standard automatic and manual transmissions. However, conventional automatic and manual transmissions are heavy and contribute greatly to the weight of the vehicle and are only capable of establishing discrete gear ratios. Furthermore, automatic transmissions are inefficient due to the requirement that the hydraulic system utilized for controlling the sequential shifting of the automatic transmission requires a large amount of power in order to maintain the hydraulic fluid pressure even when shifting is not necessary. Thus, it would be desirable in the art of automotive transmissions to provide a four-wheel drive transmission which has improved efficiency, a continuously variable operating range, and reduced size and weight. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a four-wheel drive transmission which combines a continuously variable transmission unit and a transfer case unit into a common assembly. 
     It is another object of the present invention to provide a four-wheel drive transmission which is lighter in weight and more compact than conventional transmission and transfer case combinations. 
     It is still another object of the present invention to provide a four-wheel drive transmission which does not require an engine clutch. 
     A further object is to provide a rear-wheel drive transmission integrating a continuously variable transmission unit and a geared drive selector unit into a common assembly. A related to object is to arrange the continuously variable transmission unit and the geared drive selector unit to provide a front-wheel drive transaxle. 
     These and other objects of the present invention are obtained by providing a four-wheel drive transmission having a continuously variable transmission unit, a planetary drive selector unit, and a torque transfer unit. The four-wheel drive transmission is further equipped with a worm/worm gear transmission which is controlled by a transmission controller to provide continuous ratio control of the continuously variable transmission unit. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a sectional view of a worm/worm gear transmission with the worm gear having three teeth according to the principles of the present invention; 
     FIGS. 2 and 3 are views of a worm/worm gear transmission with the worm gear having six teeth according to the principles of the present invention; 
     FIG. 4 is a side view of an enveloping worm having two threads; 
     FIG. 5 illustrates an enveloping thread of a worm used for generating the gear teeth profile of the worm gear; 
     FIG. 6 is a view of a shortened thread of a worm used for generating the gear teeth profile of the worm gear; 
     FIG. 7 shows an enveloping worm gear according to the principles of the present invention; 
     FIG. 8 shows a modified enveloping worm gear; 
     FIGS. 9 and 10 show a worm/worm gear transmission utilizing a modified worm; 
     FIG. 11 shows a worm/worm gear transmission with a modified worm in an off-center position; 
     FIG. 12 shows a worm/worm gear transmission with two modified worms in off-center positions; 
     FIG. 13 shows a worm/worm gear transmission with two modified worms aligned on the same axis of rotation and connected to a common shaft; 
     FIG. 14 shows a worm/worm gear transmission with two modified worms located on different axes of rotation; 
     FIG. 15 shows an enveloping worm gear with a different tooth profile; 
     FIG. 16 shows a worm/worm gear transmission with two enveloping worms placed on different axes of rotation; 
     FIG. 17 is a perspective view of the worm/worm gear transmission shown in FIG. 1 with three worm gear teeth; 
     FIG. 18 is a perspective view of the worm/worm gear transmission shown in FIG. 2 with six worm gear teeth and two threads on the worm; 
     FIG. 19 is a perspective view of a worm/worm gear transmission with ten worm gear teeth and with a single thread worm; 
     FIG. 20 is a perspective view of a worm/worm gear transmission with nine worm gear teeth and a modified worm having three threads on the worm; 
     FIG. 21 is a perspective view of a worm gear with six teeth with darkened spots illustrated on the surface of the teeth to indicate the contact surface with the worm in mesh; 
     FIG. 22 is a perspective view of a worm with two threads with darkened spots illustrated on the surface of the thread to indicate the contact surface with the worm gear in mesh; 
     FIG. 23 illustrates the size difference of the worm/worm gear transmission of FIG. 20 in comparison to the size of a typical hypoid gear; 
     FIG. 24 is a sectional view of the continuously variable four-wheel drive transmission according to the principles of the present invention; 
     FIG. 25 is a schematic view of a rear-wheel drive transmission according to the present invention; and 
     FIG. 26 is a schematic view of a front-wheel drive transmission according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Continuously variable transmissions according to the principles of the present invention will be described below with reference to FIGS. 24 through 26. However, the following discussion relating to FIGS. 1-23 provides a detailed description of the unique enveloping-type worm/worm gear transmission which can be utilized with the present continuously variable transmission of the invention. 
     The reason for using an enveloping-type worm gear is that this type of worm gear has a natural profile of tooth surface which is distinct from other types of thread followers. The configuration of the worm gear teeth is generated by the profile of the thread or threads of the worm. A computer model simulation is utilized to generated the configuration of the worm gear teeth of the worm gear. The worm gear is then formed using known techniques such as hobbing or casting. When the worm gear teeth are generated by the profile of the threads of the worm having different lengths for the same enveloping angle (shortened), the profiles of the worm teeth is different. The main advantage for using the enveloping-type worm gears is more torque capacity. 
     The worm thread has a rolling action contract relationship with the teeth of the worm gear which provides an increased efficiency. Furthermore, it is beneficial to have the pitch diameter in the center of the worm gear. With standard worm designs, with more than one thread and a large enveloping angle, the inability to assemble the worm and worm gear was considered a major obstacle. With the worm and worm gear of the present invention, the worm and worm gear are easily assembled by properly orienting the worm thread and worm teeth. 
     According to the present invention, the greater enveloping angle for one revolution of the worm thread permits the use of worm gear teeth without undercut portions. Enveloping worm/worm gear transmissions with a worm gear having less than twenty-four teeth have not been commercially used because it was believed impossible to build such a transmission due to the need to undercut the root of the worm gear tooth. Thus, those skilled in the art did not consider enveloping type worm gears with less than twenty-four teeth to be feasible for commercial applications. In contrast, the enveloping worm/worm gear transmission of the present invention utilize a worm gear without undercut gear teeth because of a greater enveloping angle for one revolution of the worm thread. 
     With less than twenty-four gear teeth and a greater enveloping angle for one revolution of the thread, as compared to prior enveloping worm/worm gear transmissions, the minimum ratio for one thread could be reduced to two, with an increase in achieved efficiency, in contrast, prior enveloping worm/worm gear transmissions had a minimum ratio of twenty-four for one thread of the worm and a ratio of five for five threads of the worm. The efficiency of the new worm/worm gear transmission is even greater than that of well-known hypoid gearsets which are used in low ratio right-angle drives. Thus, the present invention can replace hypoid or bevel gearing in many applications by reason of the low ratio. In addition, this new worm/worm gear transmission is able to back drive by transmitting torque from the worm gear to the worm. For the same size, this invention has more than twice the capacity of traditional hypoid gearing. 
     In this application, it is possible to have “surface to surface” contact between the worm gear teeth and the worm thread, thereby increasing the torque capacity of the enveloping worm/worm gear transmission. This became feasible when the enveloping angle for one revolution of worm thread is equal or greater than 15 degrees. In all standard enveloping worm/worm gear transmissions, only “line” contact is obtained between the thread and worm gear teeth or thread followers. This physical distinction has realized new and unexpected results with regard to the torque capacity of the worm/worm gear transmission of the present invention. The efficiency of the new worm/worm gear transmission is equal or even greater than in well-known hypoid gearing, which are used in right angle drives with low ratio. For back drive, when the worm gear is a driven member and the worm is a driving member, this worm/worm gear transmission also has high efficiency compared to a hypoid gear set. 
     In accordance with one feature of the present invention, a worm/worm gear transmission is utilized to transmit rotation with the smallest ratio between the worm gear teeth and one worm thread. In the past, it has been believed that at least 24 teeth were required for a worm gear to be used with a double enveloping worm/worm gear combination. However, in the present invention, the big difference from the traditional worm/worm gear is not only in the number of teeth, but also in the enveloping angle of the worm thread, which is used for generation of the profile for the worm gear teeth. This enveloping angle can be as large as 180 degrees for one revolution of the thread when the number of worm gear teeth is only two but is preferably larger than 15 degrees. Further detail of the use of the unique worm/worm gear transmission discussed herein can be found in U.S. Pat. No. 5,992,259 entitled “Worm/Worm Gear Transmission and Apparatus for Transmitting Rotation Utilizing An Oscillating Input” and U.S. application Ser. No. 09/290,911 filed Apr. 12, 1999 titled “Worm/Worm Gear Transmission,” both of which are owned by the inventor hereof and which are also expressly incorporated by reference hereto. 
     In the past, the worm and worm gears have been formed of materials having low coefficients of friction; worm gears typically were made only from bronze. With the present invention however, the worm and worm gear can be made from a high strength material, such as steel. The preferable shape of the teeth and threads for the worm gear and the worm are shown in the drawings, but could be different. Even so, a worker of ordinary skill in the art would recognize that other shapes would come within the scope of this invention. 
     Referring now to the drawings, one embodiment of a worm/worm gear transmission  8  of the present invention is illustrated in FIG.  1 . Transmission  8  has an enveloping type worm  10  with at least one screw thread  12 . Enveloping type worm  10  is supported on a shaft  13 . Thread  12  is engaged by at least one tooth  14  of an enveloping type worm gear  16 , which is shown to have three teeth  14 . As shown in FIG. 1, enveloping worm  10  has a single thread  12  in a preferred embodiment and worm gear  16  has three teeth  14  spaced about its circumference. As shown, a gap “G” exists between any tooth on worm gear  16  and thread  12  on enveloping worm  10 . Enveloping worm  10  wraps around enveloping worm gear  16 , and enveloping worm gear  16  also wraps around enveloping worm  10 . 
     Worm gear  16  and worm  10  are preferably enclosed in a housing (not shown) in FIG.  1 . Typically, the housing is made from metal and forms a reservoir for a lubricant to both lubricate and cool the gears, bearings, and seals for the unit. The housing forms a rigid support to mount the gears, bearings, seals and their associated parts (not shown). 
     FIG. 17 is a perspective view corresponding with worm/worm gear transmission  8  shown in FIG.  1  and which includes an enveloping worm  10  having a single thread  12  and a worm gear  16  having three gear teeth  14 . As can be understood, as worm  10  rotates in the direction of Arrow A, thread  12  which is engaged with tooth  14   a  presses downward on tooth  14   a  to cause rotation of worm gear  16  in the direction of Arrow “B”. As worm gear  16  rotates, gear tooth  14   b  then comes into engagement with thread  12  and is acted on to cause further rotation of worm gear  16  as gear tooth  14   a  disengages from thread  12 . 
     Another embodiment of a worm/worm gear transmission  20  of the present invention is illustrated in FIG.  2 . This transmission has an enveloping-type worm  22  with two identical screw threads  24 . Threads  24  are engaged by at least one tooth of an enveloping-type worm gear  26  shown to have six teeth  28 . Worm gear  26  is connected to a shaft  30  while worm  22  is connected to a shaft  32 . In FIG. 3, worm gear  26  is shown in cross-section. FIG. 4 is a side view of enveloping worm  22  with two identical threads  24  and supporting shaft  32 . FIG. 18 is a perspective view corresponding with worm/worm gear transmission  20  shown in FIG.  2  and which includes enveloping worm gear  26  having six teeth  28  in mesh with enveloping worm  22  having two threads  24 . 
     FIG. 5 shows an enveloping angle of 120° for enveloping worm thread  24  that is used to generate the six teeth  28  on worm gear  26 . This enveloping worm thread  24  has one revolution of thread or 360° of revolution around its axis of rotation. For illustration of one revolution for the enveloping worm thread, we could use this example: the ends of worm threads have the same cross-sections but could be placed from one position to another position, which is a distinct 120°. This is possible by movement of the cross-section of the worm from one end along the worm thread  24  to another end. In this case, the cross-section will rotate 360° around the axis of rotation for shaft  32 . 
     The enveloping worm/worm gear transmissions of the present invention provide for a worm gear having fewer than twenty-four teeth and also provides surface contact between the thread of the worm and the teeth of the worm gear as illustrated in FIGS. 21 and 22. FIG. 21 illustrates two surface contact spots  100   a ,  100   b  for a worm gear  26  having six teeth  28 . FIG. 22 illustrates two corresponding surface contact spots  102   a ,  102   b  for a worm  22  with two threads  24 . 
     FIG. 6 shows a worm thread  38  used for generating worm gear teeth and which is a shortened portion of a thread having an enveloping angle of 120°. 
     FIG. 7 shows a side view of enveloping worm gear  26  with six teeth  28 . FIG. 8 shows an enveloping worm gear  44  having six teeth  34  which is modified from worm gear  26  shown in FIG. 7 by shortening the gear along its axis of rotation around a shaft  46 . Practically, worm gear  44  could be longitudinally split into two halves and using only one shortened part or generated worm gear from blank, which is already shortened. Modified worm gear  44  is easy to assemble in a single reduction unit. This is very important for gears with a small pressure angle when it is difficult to assemble an enveloping worm with an enveloping type of worm gear. For many applications, only the modified worm gear  44  may be adequate. Enveloping worm gear  44  could connect to drive shaft  46  for supporting worm gear  44  from only one side or could be supported on both sides. 
     The bodies of enveloping worm gears  26  and  44  have axially extending end flanges that hook underneath flanges of adjacent collars to hold the worms in place. One or both of the worm and worm gear bodies are keyed or otherwise fastened to the shaft for driving or being driven. Relatively slight longitudinal movement of one or both the worm or worm gear allows for disassembling the entire worm gear—collars—shaft assembly. 
     In the present invention, it is preferred that the ratio of the number of teeth  14  on worm gear  16  relative to the number of threads  12  on worm  10  is 11 to 1 and less. Most preferably, the ratio is three or even less, as shown. It is possible that only two teeth  14  need to be utilized on worm gear  16 . The worm/worm gear transmission used in the present application could also self-lock. The term “self-locking” as it is utilized in this application to describe the inventive worm and worm gear combinations, means that the teeth of the worm gear, when in contact with the thread of the worm, are not capable of rotating the worm about the axis of the worm. For example, teeth  14  do not slip on thread  12 , thereby causing thread  12  to rotate about its own axis. By carefully selecting the material of teeth  14  and threads  12 , and the respective angles, a worker of ordinary skill in the art would be able to achieve this goal. The worm/worm gear transmission of the present invention particularly lends itself to a geometric as opposed to a purely frictional type self-locking device. 
     FIG. 9 shows a shortened enveloping worm  50  with an enveloping type of worm gear  52 , which has a different profile of teeth  53 , compared to teeth  28  of worm gear  26  (shown in FIGS. 2 and 7) even for the same number of worm gear teeth. This difference is due to the fact that the profile of teeth  53  was generated by a shortened enveloping thread  54  for shortened enveloping worm  50 . 
     In FIG. 10, enveloping worm  50  is connected to a drive shaft  56  which supports worm  50  from one side. FIG. 11 shows a view of a worm/worm gear transmission with a modified enveloping split worm  60  having two threads  61  in an off-center position relative to an enveloping-type worm gear  62  having six teeth  63 . In contrast, FIG. 12 shows a side view of a worm/worm gear transmission with two modified worms  60  having two threads  61  in off-center positions and respectively connected to different drive shafts  62  and  64  and each meshingly engaged with worm gear  62 . 
     FIG. 13 shows a view of a worm/worm gear transmission with two modified worms  60  in off-center positions placed on the same axis of rotation and both connected to drive shaft  32 . When the modified worms are connected to a common shaft with a different angular phase of the threads, it means that in motion, the threads of one worm are entering mesh with the worm gear teeth while the thread of the other worm are released from mesh at different times. The purpose of the phase difference is to increase the contact ratio and to provide smooth mesh. 
     FIG. 14 shows a view of a worm/worm gear transmission with two modified worms  60  having worm threads  68  each placed on different axes of rotation and connected to different drive shafts  70  and  72 . Each of worms  60  meshingly engages the worm gear  62  having teeth  64 . 
     FIG. 15 shows a side view of an enveloping worm gear  62  with teeth  64  have a different profile which is generated by enveloping thread  68  of worm  60  as shown in FIG.  14 . 
     FIG. 16 shows a view of a worm/worm gear transmission with two enveloping worms  22  having corresponding worm threads  24  placed on different axes of rotation and which are connected to drive shafts  32  and  23 . Each of worms  22  meshingly engages enveloping worm gear  26 . 
     FIG. 19 is a perspective view of a worm/worm gear transmission including worm gear  80  having ten teeth  82  in mesh with an enveloping split worm  84  including a thread  86 . 
     FIG. 20 is a perspective view of a worm gear  90  having nine teeth  92  in mesh with a modified enveloping split worm  94  having three threads  96 . 
     For the inventions described in the present patent application, there could be two different types of operations. When the worm/worm gear transmission does not incorporate the self-lock feature, the motion could be provided from the drive shaft through enveloping worm  12  and enveloping-type worm gear  16  to an output shaft or back from the output shaft to the drive shaft  32 . The same operation is applicable for motion from the drive shaft to the driven shafts or from the driven shafts to drive shaft of the various other embodiments shown. Alternatively, when the worm/worm gear transmission does include the self-lock feature, rotary motion can be is provided only from the drive shaft to the enveloping worm and through the enveloping type worm gear to the output shaft. Thus, the worm/worm gear transmissions shown in FIGS. 12,  14  and  16 , with independent drive shafts connected to the worms, could be used in a split-power transmission to transmit energy from a high-speed engine to a rotor drive shaft. 
     In the present invention, a self-locking worm/worm gear combination can have a worm gear to worm thread ratio that is preferably ten and less. Such a system is desirable so that each one of the worm and worm gear combinations described above can transmit very high torque loads when compared to prior systems. The lower noise of the worm/worm gear transmission, compared with hypoid and bevel gear transmission, make using the worm/worm gear transmission of the present invention more beneficial, particularly in motor vehicle powertrain applications. For the same size, this invention can provide more than twice the capacity of hypoid gearing, where the hypoid gear also has more than 24 teeth. The smaller number of teeth of the present invention than in a hypoid gear of the same circumference makes each tooth thicker and therefore stronger. In the illustration shown in FIG. 23, a modified worm  94  with three threads  96  is shown which has a shape and size similar to a pinion of a hypoid transmission. Assuming the modified worm  94  is the same size as the pinion of a hypoid gear set, the diameter of hypoid gear  106  is twice the diameter of worm gear  90 . Up to now, those skilled in the art were of the opinion that an enveloping type worm gear with less than twenty-four teeth would not work and/or that it presented an insurmountable barrier to commercial applications. 
     Referring now to FIG. 24, a four-wheel drive transmission  150  is shown which includes an input shaft  152 , and intermediate shaft  154 , a continuously variable transmission (CVT) unit  156  operably coupling input shaft  152  to intermediate shaft  154 , a rear output shaft  160 , a drive selector unit  158  operably coupling intermediate shaft  154  to rear output shaft  160 , a front output shaft  166 , and a torque transfer unit  162  operably disposed between rear output shaft  160  and front output shaft  166 . Torque transfer unit  162  includes a coupling  163 , a first sprocket  164  rotatably supported on rear output shaft  154  and adapted to receive drive torque therefrom via actuation of coupling  163 , a second sprocket  170  fixed to front output shaft  166 , and a chain  168  connecting sprockets  164  and  170 . Four-wheel drive transmission  150  also includes a housing assembly  172  which supports all the afore-mentioned components and maintains a sealed sump of lubricant. Housing assembly  172  includes a first section  172   a  for enclosing CVT unit  156 , a second section  172   b  for enclosing drive selector unit  158 , and a third section  172   c  for enclosing torque transfer unit  162 . These sections can be bolted together to define a common assembly or, in the alternative, housing assembly  172  could be configured as a unitary housing. 
     CVT unit  156  includes a differential  174  and a drive mechanism  176 . Differential  174  is shown to be a bevel gearset including a carrier assembly  178  with pinion gears  180  rotatably supported from pinion shaft  182 . Pinion gears  180  both mesh with an input side gear  184  and an output side gear  186 . Input side gear  184  is fixed to input shaft  152  and output side gear  186  is fixed to intermediate shaft  154 . Drive mechanism  176  is comprised of a worm/worm gear transmission and a power-operated drive unit. Specifically, a tubular extension  187  of carrier assembly  178  is rotatably supported on input shaft  152  and has an enveloping worm gear  188  fixed thereto. The thread(s) of an enveloping worm  190  are in meshing engagement with the teeth of enveloping worm gear  188 . Worm  190  is selectively driven by an electric motor  192 . Enveloping worm  190  and worm gear  188  define a worm/worm gear transmission preferably of the type described above wherein the gear ratio is low. Specifically, it is desirable that enveloping worm gear  186  have fewer than twenty-four teeth. 
     As can be understood by one having ordinary skill in the art, the rotational speed of intermediate shaft  154  which provides input to drive selector unit  158  can be varied relative to the rotational speed of input shaft  152  by controlling the rotational speed of carrier assembly  178  of differential  174 . When electric motor  192  is not operated, worm  190  and worm gear  188  provide a self-lock function which prevents carrier assembly  178  from rotating. Thus, when carrier assembly  178  is prevented from rotating, the rotation of input shaft  152  is directly transmitted via input side gear  184  through pinion gears  180  output side gear  186  for directly driving intermediate shaft  154  at a one-to-one drive ratio. However, if electric motor  192  is operated to rotate worm  190  at a variable speed relative to worm gear  188 , carrier assembly  178  provides a differential speed to differential  174  which allows for a continuously variable transmission of speed to intermediate shaft  154  based upon the speed at which electric motor  192  drives worm  190 . As such, variable control of the rotary speed of worm  190  via motor  192  provides a continuously variable transmission of power to intermediate shaft  154 . 
     Drive selector unit  158  is operable to control the rotary direction and ratio between intermediate shaft  154  and rear output shaft  160 . Drive selector unit  158  includes a planetary gearset  196  and a mode shaft mechanism  198 . Planetary gearset  196  includes a sun gear  200  fixed to intermediate shaft  154 . A planetary carrier  202  is mounted for concentric rotation about sun gear  200 . A plurality of pinion gears  204  are rotatably supported on planetary carrier  202  in meshing engagement with sun gear  200  and a drive annulus  206 . Drive annulus  206  is provided with a splined connection to rear output shaft  160 . Mode shift mechanism  198  includes a lock collar  208  and a gearshift lever  210  which permits a vehicle operator to move lock collar  208  between a DRIVE (D) position, a NEUTRAL (N) position and a REVERSE (R) position. Lock collar  208  is splined for rotation with and axial movement on an axial extension  212  of carrier  202 . 
     When lock collar  208  is in its DRIVE position (as shown in FIG.  24 ), its external clutch teeth are engaged with internal clutch teeth on drive annulus  206 . As such, carrier  202  and drive annulus  206  are locked together to prevent relative rotation therebetween such that planetary gearset  196  is disabled. Thus, the rotation of intermediate shaft  154  is transmitted directly through sun gear  200  to drive carrier  202  and drive annulus  206  at a common speed and in a common direction, thereby causing rear output shaft  186  to be driven at the same common speed and direction for establishing a forward motive condition. 
     When lock collar  208  is in the NEUTRAL position, lock collar  208  is totally disengaged with respect to drive annulus  206  and a ground ring  214  that is non-rotatably secured to housing  172   b . Thus, rotation of intermediate shaft  172  causes sun gear  200  to rotate, thereby driving pinion gears  204 . If there is resistance applied to the rear output shaft  160  and thus, drive annulus  206 , carrier  202  then rotates relative to sun gear  200  so that pinion gears  204  can planetate around sun gear  200  without transferring drive torque to rear output shaft  160 . 
     When lock collar  208  is moved to its REVERSE position, the external clutch teeth on lock collar  208  become engaged with internal clutch teeth on ground ring  214  in order to prevent rotation of carrier  202  relative to housing  172   b . Thus, rotation of intermediate shaft  154  causes rotation of sun gear  200  which drives pinion gears  204 . Because planetary carrier  202  is prevented from rotating, the rotation of pinion gears  204  acts to rotate drive annulus  206  in a direction opposite to the rotation of sun gear  200  in order to obtain a reverse rotation of rear output shaft  160 . 
     Coupling  163  can be of any known type in order to provide two-wheel drive to four-wheel drive control. Known torque transfer couplings  163  include a viscous coupling or a pump-type speed sensitive coupling which are well known from commonly assigned U.S. Pat. Nos. 5,597,369 and 5,704,863 which are herein incorporated by reference. In addition, known active torque transfer couplings include an electromagnetic clutch coupling which is automatically controlled based upon sensor based inputs which detect a rotational different in the front and rear wheel drives as is well known from commonly assigned U.S. Pat. Nos. 5,215,160 and 5,275,253 which are herein incorporated by reference. In addition, adaptively activated clutch pack clutches can also be utilized as active torque transfer couplings as is well known from commonly assigned U.S. Pat. No. 5,704,867 which is herein incorporated by reference. 
     Torque transfer coupling  163  supplies torque from rear output shaft  160  to drive sprocket  164  which, in turn, delivers torque to driven sprocket  170  via chain  168 . It should be understood that other known torque delivery systems such as a pulley and belt or a geartrain can be utilized for providing torque between rear output shaft  160  and front output shaft  166 . For purposes of example, coupling  163  is a mutli-plate clutch having an actuator  220  that can be variably actuated to modulate the clutch engagement force across the clutch, thereby modulating the torque transferred to front output shaft  166 . 
     Transmission  150  further includes a transmission controller  230  which is operable to control actuation of motor  192  and actuator  220 . Input signals are delivered to controller from various sensors, identified by block  232 , and which are used by controller  230  to adaptively control torque transfer between input shaft  152  and output shafts  160  and  166 . Contemplated input signals include, among others, is the rotary speed of all the shafts, brake actuation status, throttle position and steering angle. 
     Referring now to FIG. 25, a rear-wheel drive version of a continuously variable transmission  250  is shown. Transmission  250  is similar to transmission  150  with the exception that torque transfer unit  162  has been eliminated such that all drive torque is transferred to rear output shaft. FIG. 26 shows a front-wheel drive version of a continuously variable transmission  350  which is similar to transmission  250  with the exception that a final drive unit  352  is driven by output shaft  160  to transfer drive torque to a pair of axle-shafts  354  and  356 . Final drive unit  352  includes an output gear  358  fixed to output shaft  160 , a drive gear  360  meshed with output gear  358  and secured to a carrier  362  of a differential  364 . Differential  364  has a geartrain  366  interconnecting carrier  362  to axle-shafts  354  and  356 . 
     Various embodiments of the present invention have been disclosed. A worker of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.