Abstract:
A spiral type coupling ( 10 ) consists of a tubular slipper ( 14 ) between a tubular member ( 12 ) and a race ( 6 ), there being engageable tubular friction surfaces ( 20, 36 ) on the slipper ( 14 ) and the race ( 16 ), and facing pairs of clockwise and counterclockwise spiral surfaces ( 18   a   , 18   b ,  22   a,    22   b ) on the tubular member ( 12 ) and the slipper ( 14 ) defining therebetween a channel ( 28 ) receiving roller members ( 30 ), there being an actuator engageable with the slipper ( 14 ) to provide two, three or four different modes of operation (e.g., lock up, freewheel in both directions, freewheel in one direction and/or the other).

Description:
This application claims benefit of Ser. No. 60/066,666 filed Nov. 26, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a coupling for selectively transferring torque from a torque input member to a torque output member. In particular, the present invention relates to a programmable coupling having a plurality of operating modes for selectively transferring torque between a clutch housing and a race. 
     BACKGROUND OF THE INVENTION 
     One-way clutches and couplings are widely used in the automotive industry for transferring torque between an input shaft and an output shaft when the input shaft is rotating in one direction relative to the output shaft and for allowing the input shaft to freewheel in the opposite direction. As a result, one-way clutches have been used in torque converters and automatic transmissions to allow an input member to drive an driven member while allowing freewheeling to occur between the input member and the driven member when necessary. Examples of such one-way or overrunning clutches presently in use include sprag clutches and roller-ramp clutches. 
     The sprag clutch generally comprises an outer race, an inner race, and a plurality of wedge-like elements disposed between the inner and outer race. The geometry of the sprag element is such that it allows the clutch to freewheel in one direction, but becomes wedged between the inner and outer race to lock up the clutch in the opposite direction. The roller-ramp clutch is similar to the sprag clutch but includes a plurality of roller elements in replacement of the sprag elements. 
     A variant of the conventional roller ramp clutch is taught by Rockwell (U.S. Pat. No. 2,085,606) and includes a plurality of graduated-sized roller elements. Since these devices rely on a wedging action to lock up, the sprag elements, roller elements and races are subjected to extremely high radial stresses during lock up. Further, the sprag and roller elements subject the clutch to vibrations while freewheeling. As a result, such one-way clutches an prone to frequent failure. 
     Spiral-type one-way clutches have been developed as an improvement over sprag and roller-ramp clutches. State of the art spiral-type one-way clutches, such as that taught by Kerr (EP 0 015 674) comprise an outer member having an inner spiral race, an inner member having an outer spiral race congruent with the inner spiral race, and a plurality of elongate roller bearings disposed between the inner and outer race. The elongate roller bearings reduce the frictional resistance to the differential rotation of the spiral surfaces while providing an even distribution of compression forces on the roller bearings and races. However, as conventional spiral-type one way clutches, and one-way clutches in general, only have a single mode of operation, namely lock up in one direction and freewheeling in the opposite direction, the design of automotive equipment using such clutches is unnecessarily over complicated. 
     Although Rockwell teaches a two-way roller ramp-type clutch, the graduated-sized roller elements can cause spalling of the roller elements and limit the indexing rate of the clutch. Therefore, it would be desirable to provide a reliable coupling having multiple modes of operation and high indexing rates, but without drastically increasing the cost of the coupling. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to overcome or reduce the problems associated with the prior art one-way clutches by providing a spiral-type coupling having multiple modes of operation. 
     The spiral-type coupling, according to the invention, comprises a tubular member including a first tubular surface; a resilient tubular slipper coaxial to the tubular member and including a first tubular friction surface, a second tubular surface opposite the first tubular friction surface which, together with the first tubular surface, defines a channel disposed between the tubular member and the slipper; a plurality of roller elements disposed in the channel for allowing limited rotational movement between the tubular member and the slipper; a race including a second tubular friction surface disposed adjacent the first tubular friction surface; and an actuator for selectively engaging the second tubular friction surface with the first tubular friction surface. 
     The tubular slipper includes a pair of adjacent end walls extending between the first tubular friction surface and the second tubular surface along the length of the slipper for allowing the diameter of the slipper to vary in accordance with the position of the actuator. 
     The first tubular surface comprises at least one clockwise-oriented spiral surface and at least one counterclockwise-oriented spiral surface. The second tubular surface is substantially congruent with the first tubular surface. Preferably, the spiral surfaces are involute spiral surfaces so that the first tubular surface and the second tubular surface remain parallel to each other as the slipper is rotated relative to the tubular member. 
     In one embodiment of the invention, the slipper comprises a plurality of slipper segments, and the actuator comprises a single actuator ring which engages the slipper segments for selecting between a first mode in which the race freewheels in both directions relative to the tubular member, and a second mode in which the race is locked in both directions to the tubular member. 
     In another embodiment of the invention, the slipper comprises a plurality of slipper segments, and the actuator comprises a pair of actuator rings which engage alternate slipper segments for selecting between a first mode in which the race freewheels in both directions relative to the tubular member, a second mode in which the race is locked in both directions to the tubular member, a third mode in which the race freewheels in the clockwise direction but is locked to the tubular member in the counterclockwise direction, and a fourth mode in which the race freewheels in the counterclockwise direction but is locked to the tubular member in the clockwise direction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described, by way of example only, with reference to the drawings, in which like reference numerals indicate like elements, and in which: 
     FIG. 1 a  is an axial cross-sectional view of a spiral-type dual-mode coupling according to a first embodiment of the invention, utilizing a full internal slipper and a cone race activator; 
     FIG. 1 b  is a transverse cross-sectional view of the spiral-type dual-mode coupling shown in FIG. 1 a;    
     FIG. 2 is a transverse cross-sectional view of one variation of the spiral-type dual-mode coupling shown in FIGS. 1 a  and  1   b , utilizing a segmented internal slipper and cone race activator; 
     FIG. 3 a  is an axial cross-sectional view of a second variation of the spiral-type dual-mode coupling shown in FIGS. 1 a  and  1   b , utilizing a full external slipper and ring activator; 
     FIGS. 3 b - 3   c  is a transverse cross-sectional view of the spiral-type dual-mode coupling shown in FIG. 3 a;    
     FIG. 4 a  is an axial cross-sectional view of a spiral-type quad-mode coupling according to a second embodiment of the invention, utilizing a segmented internal slipper and dual opposed ring activators; 
     FIG. 4 b  is a transverse cross-sectional view of the spiral-type quad-mode coupling shown in FIG. 4 a;    
     FIG. 4 c  is a perspective view of the spiral-type quad-mode coupling shown in FIG. 4 a;    
     FIG. 5 a  is an axial cross-sectional view of a variation of the spiral-type quad-mode coupling shown in FIG. 4, utilizing a segmented internal slipper and dual coplanar ring activators; 
     FIG. 5 b  is a transverse cross-sectional view of the spiral-type quad-mode coupling shown in FIG. 5 a;    
     FIG. 5 c  is an exploded view of the spiral-type quad-mode coupling shown in FIG. 5 a;    
     FIG. 6 a  is an axial cross-sectional view of a spiral-type tri-mode coupling according to a third embodiment of the invention, utilizing a cam plate actuator for switching modes; 
     FIG. 6 b  is a plan view of the cam plate shown in FIG. 6 a ; 
     FIG. 7 is an axial cross-sectional view of a four wheel drive transfer case, utilizing a variation of the cam plate actuator shown in FIG. 6; and 
     FIG. 8 is an axial cross-sectional view of an all wheel drive transfer case, utilizing a variation of the cam plate actuator shown in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning to FIGS. 1 a  and  1   b , a dual-mode spiral-type coupling, denoted generally as  10 , is shown comprising a tubular clutch housing  12 , a resilient tubular C-shaped slipper  14  coaxial to and disposed within the clutch housing  12 , and a conical race  16  coaxial to and disposed within the slipper  14 . The clutch housing  12  has an inner tubular surface  18  which includes a plurality of clockwise outwardly spiraling surface portions  18   a , and a plurality of counterclockwise outwardly spiraling surface portions  18   b . As shonw in FIG. 1 b , the spiraling surface portions  18   a ,  18   b  are disposed around the inner tubular surface  18  in a sequence of alternating clockwise spiraling surface portions  18   a  and counterclockwise spiraling surface portions  18   b . As will be explained, the clockwise spiraling surface portions  18   a  and the counterclockwise spiraling surface portions  18   b  serve to selectively restrict rotational movement between the race  16  and the clutch housing  12  in both the clockwise and the counterclockwise directions. Accordingly, it will be appreciated that a sequence of spiraling surface portions  18   a ,  18   b , other than that shown in FIG. 1 b , may be adopted without departing from the scope of the invention. 
     The slipper  14  includes an inner conical friction surface  20 , and an outer tubular surface  22 . The slipper  14  is provided with a slit defined by a pair of adjacent end walls  24 ,  26  extending between the inner conical friction surface  20  and the outer tubular surface  22  axially along the length of the slipper  14  for allowing the slipper  14  to expand and contact in response to axial movement of the race  16 . The outer tubular surface  22  is substantially congruent with the inner tubular surface  18  and includes a plurality of clockwise outwardly spiraling surface portions  22   a , and a plurality of counterclockwise outwardly spiraling surface portions  22   b . Preferably, the spiraling surface portions  18   a ,  18   b ,  22   a ,  22   b  are involute spirals so that as the slipper  14  rotates relative to the clutch housing  12 , the inner tubular surface  18  remains parallel to the outer tubular surface  22 . However, other spiral shapes may be adopted as the application demands. 
     The inner tubular surface  18  and the outer tubular surface  22  together define a counterclockwise-oriented and clockwise oriented channels  28  disposed between he clutch housing  12  sad the slipper  14  extending the length of the clutch housing  12  and the slipper  14 . A plurality of elongated roller bearings  30  are disposed within the channel  28  to provide restricted rotational movement between the slipper  14  and the clutch housing  12  in a manner to be described below. The clutch housing  12  is provided with a plurality of flanges  32  extending radially inwards from the inner tubular surface  18 , and the slipper  14  is provided with a plurality of flanges  34  extending radially outwards from the outer tubular surface  22  for restricting radial movement of the roller bearings  30  along the channels  28  by separating the clockwise-oriented channels from the counterclockwise-oriented channels  28  and for reducing the locking and unlocking times of the coupling  10 . However, it will be appreciated that in applications where rapid locking and unlocking times are not critical, the flanges  32 ,  34  may be eliminated. 
     The conical race  16  includes an outer conical friction surface  36  congruent with the inner conical friction surface  20 . An actuator, not shown, is provided for moving the conical race  16  towards and away from the slipper  14  along a line coaxial to the centre of rotation of the slipper  14 . 
     The two modes of operation of the coupling  10  will now be described. When the actuator is inactive and the conical race  16  is in the position shown in FIG. 1 a , the functional forces between the slipper  14  and the race  16  are sufficiently small such that the race  16  can be driven in either the clockwise direction or the counterclockwise direction. Therefore, in this position the coupling  10  is in the bi-directional freewheeling mode with the race  16  free to rotate in both directions. 
     When the actuator is active, the conical race  16  is driven towards the slipper  14  such that the outer conical fictional surface  36  engages the inner conical friction surface  20 . Due to the resilient nature of the slipper  14  and the slit defined by the end walls  24 ,  26 , the slipper  14  expands radially in response to the axial movement of the race  16 , thereby increasing the radial force exerted by the slipper  14  against the race  16 . If the race  16  is driven sufficiently deeply into the slipper  14  and then subsequently rotated in the clockwise direction, the clockwise outwardly spiraling surface portions  22   a  will rotate towards the clockwise outwardly spiraling surface portions  18   a  causing the width of the channel  28  between the spiral surface portions  18   a ,  22   a  to narrow and the inner conical friction surface  20  to be held with greater force against the outer conical friction surface  36 . As the race  16  continues to rotate clockwise, the channel  28  will continue to narrow until the slipper  14  and the race  16  become locked to the clutch housing  12 . At this point, the input torque from the race  16  is coupled to the clutch housing  12  causing the clutch housing  12  to rotate clockwise with the race  16 . Alternately, if input torque is applied to the clutch housing  12  rather than to the race  16 , and in the counterclockwise direction, the input torque from the clutch housing  12  would be coupled to the race  16  causing the race  16  to rotate counterclockwise with the clutch housing  12 . 
     If the direction of rotation of the race  16  is reversed, the width of the channel  28  will initially increase, and then subsequently decrease as the counterclockwise outwardly spiraling surface portions  22   b  are driven towards the counterclockwise outwardly spiraling surface portions  18   b . As above, as the race  16  continues to rotate counterclockwise, the channel  28  will continue to narrow until the slipper  14  and the race  16  become locked to the clutch housing  12 . At this point, the input torque from the race  16  is coupled to the clutch housing  12  causing the clutch housing  12  to rotate counterclockwise with the race  16 . Accordingly in this mode, the coupling  10  is in the bi-directional full coupling mode with the race  16  coupled to the clutch housing  12  in both directions. 
     Turning now to FIG. 2, a dual-mode spiral-type coupling  110  is shown substantially identical to the spiral-type coupling  10  but with the C-shaped slipper  14  replaced with a segmented slipper  114 . As shown therein, the slipper  114  comprises a plurality of slipper segments  114   a ,  114   b  interlocked through tongue and groove means. The slipper segments  114   a  include clockwise outwardly spiraling surface portions  122   a , while the slipper segments  114   b  include counterclockwise outwardly spiraling surface portions  122   b . As above, the inner tubular surface  18  of the clutch housing  12  includes a plurality of clockwise outwardly spiraling surface portions  18   a , and a plurality of counterclockwise outwardly spiraling surface portions  18   b , with the spiraling surface portions  122   a ,  122   b  being substantially congruent to the spiraling surface portions  18   a ,  18   b . As shown in FIG. 2, the spiraling surface portions  122   a ,  122   b  are disposed around the inner tubular surface  18  in a sequence of alternating clockwise spiraling surface portions  122   a  and counterclockwise surface portions  22   b . However, as discussed above, some other sequence of spiraling surface portions  122   a ,  112   b  may be adopted. 
     FIGS. 3 a ,  3   b  and  3   c  show a variation of the dual-mode spiral-type coupling  10 . The spiral-type coupling  210  shown therein comprises a tubular clutch housing  212 , a resilient tubular C-shaped slipper  214  coaxial to and disposed externally to the clutch housing  212 , a cylindrical race  216  coaxial to and disposed externally to the slipper  214 , and an actuator  235  coupled to the slipper  214 . The clutch housing  212  has an outer tubular surface  218  which includes a plurality of clockwise outwardly spiraling surface portions, and a plurality of counterclockwise outwardly spiraling surface portions. 
     The slipper  214  includes an outer cylindrical friction surface  220 , and an inner tubular surface  222 . The slipper  214  is provided with a chamfered edge  240 , and a slit  242  extending between the outer cylindrical friction surface  220  and the inner tubular surface  222  axially along the length of the slipper  214 . The inner tubular surface  222  is substantially congruent with the outer tubular surface  218  and includes a plurality of clockwise outwardly spiral surface portions, and a plurality of counterclockwise outwardly spiraling surface portions. The cylindrical race  216  includes an inner cyclical fiction surface  236  congruent with the outer cylindrical friction surface  220 . 
     The outer tubular surface  218  and the inner tubular surface  222  together define a channel disposed between the clutch housing  212  and the slipper  214  for receiving a plurality of roller bearings  230  and resilient elements  244  within the channel. As will be appreciated, the resilient element  244  serve to soften the locking and unlocking action of the coupling  218  by maintaining parallel alignment of the roller bearings. 
     The actuator  238  comprises an actuator ring  246  for engaging the chamfered edge  240 , and a piston  248  coupled to the actuator ring  246  for axially moving the actuator ring  246  towards and away from the slipper  214 . 
     The two modes of operation of the coupling  210  will now be described. When the piston  248  is inactive, the inner friction surface  236  of the race  216  engages the outer frictional surface  220  of the slipper  214 . As a result, clockwise and counterclockwise rotation of the race  216  causes the channel to narrow, as described above, until the race  216  becomes locked to the clutch housing  212 . When the piston  248  is active, the actuator ring  246  engages the chamfered edge  240 , causing the slipper  214  to move radially inwards away from the race  216 . As a result, the race  216  becomes free to rotate in either direction about the clutch housing  212 . 
     Turning now to FIGS. 4 a ,  4   b  and  4   c , a preferred quad-mode spiral-type coupling  310  is shown comprising a tubular outer clutch housing  312 , a segmented inner slipper  314  coaxial to and disposed within the clutch housing  312 , a cylindrical race  316  coaxial to and disposed within the slipper  314 , and a pair of first and second actuators  338   a ,  338   b  coupled to opposite side edges of the slipper  314 . The slipper  314  comprises a plurality of slipper segments  314   a ,  314   b . Each slipper segment  314   a  includes a chamfered edge  340   a , an inner frictional surface  320   a  and a clockwise spiraling surface portion, while each slipper segment  314   b  includes a chamfered edge  340   b , an inner frictional surface  320   b  and counterclockwise spiraling surface portion. As shown in FIG. 4 b , the slipper segments  314   a ,  314   b  are disposed around the inner tubular surface  318  of the clutch housing  312  in a sequence of alternating clockwise slipper segments  314   a  and counterclockwise slipper segments  314   b . In addition, the chamfered edges  340   a  are all disposed on one side of the coupling  310 , while the chamfered edges  340   b  are all disposed on the opposite side of the coupling  310 . 
     The first actuator  338   a  comprises a first actuator ring  346   a  for engaging the chamfered edges  340   a , and a first piston  348   a  coupled to the first actuator ring  346   a  for axially moving the first actuator ring  346   a  towards and away from the slipper  314 . Similarly, the second actuator  338   b  comprises a second actuator ring  346   b  for engaging the chamfered edges  340   b , and a second piston  348   b  coupled to the second actuator ring  346   b  for axially moving the second actuator ring  346   b  towards and away from the slipper  314 . 
     The four modes of operation of the coupling  310  will now be described. When the first and second pistons  348   a ,  348   b  are both inactive, the outer friction surface  336  of the race  316  engages the inner frictional surface  320   a  of the clockwise slipper segments  314   a  and the inner frictional surface  320   b  of the counterclockwise slipper segments  314   b . As a result, the race  316  becomes locked to the clutch housing  312 . When the first piston  348   a  is active but the second piston  348   b  inactive, the race  316  becomes freed from the clockwise slipper segments  314   a  but remains coupled to the counterclockwise slipper segments  314   b . As a result, the race  316  is free to rotate in the clockwise direction only. When the first piston  348   a  is inactive but the second piston  348   b  active, the race  316  becomes freed from the counterclockwise slipper segments  314   b  but remains coupled to the clockwise slipper segments  314   a . As a result, the race  316  is free to rotate in the counterclockwise direction only. When the first and second pistons  348   a ,  348   b  are both active, the race  316  becomes freed from the clockwise slipper segments  314   a  and the counterclockwise slipper segments  314   b . As a result, the race  316  is free to rotate about the clutch housing  312  in both the clockwise direction and in the counterclockwise direction. 
     FIGS. 5 a ,  5   b  and  5   c  show a quad-mode spiral-type coupling  410  substantially similar to the quad-mode spiral-type coupling  310  except that the slipper segments  414   a ,  414   b  are all chamfered on a common edge, and the first and second ring actuators  438   a ,  438   b  are both disposed on the same side edge of the slipper  414 . The first actuator  438   a  is coupled to the clutch housing  412  and rotates therewith to maintain alignment with the slipper segments  414   a , and the second actuator  438   b  is coupled to the clutch housing  412  and rotates therewith to maintain alignment with the slipper segments  414   b.    
     Turning to FIGS. 6 a ,  6   b ,  6   c , a tri-mode spiral-type coupling  510  is shown similar to the quad-mode spiral-type coupling  410 , but replacing the segmented inner slipper  414  with a chamfered segmented outer slipper  514  and including a single novel cam actuator  538  assembly in replacement of the pistons of the first and second ring actuators  438   a ,  438   b . The cam actuator  538  comprises a cam plate  550  rotatably coupled to the clutch housing  512 , a first cam follower  552   a  coupled between the cam plate  550  and the first actuator ring  546   a , and a second cam follower  552   b  coupled between the cam plate  550  and the second actuator ring  546   b . The cam actuator is provided with a plurality of first cam slots  554   a  for engaging the first cam follower  552   a , and a plurality of second cam slots  554   b  for engaging the second cam follower  552   b.    
     The three modes of operation of the coupling  510  will now be described. When the cam plate  550  is oriented in the position denoted by reference numeral  1  in FIG. 6 b , the first cam follower  552   a  engages the first actuator ring  546   a , causing the race  516  to be freed from the clockwise slipper segments  514   a  but to remain coupled to the counterclockwise slipper segments  514   b . As a result, the race  516  is allowed to rotate in the clockwise direction only. When the cam plate  550  is oriented in the position denoted by reference numeral  2  in FIG. 6 b , the second cam follower  552   b  engages the second actuator ring  546   b , causing the race  516  to be freed from the counterclockwise slipper segments  514   b  but to remain coupled to the clockwise slipper segments  514   a . As a result, the race  516  is allowed to rotate in the counterclockwise direction only. When the cam plate  550  is oriented in the position denoted by reference numeral  0  in FIG. 6 b , neither of the first or second cam followers  552   a ,  552   b  engage the actuator rings  546 , thereby causing the race  516  to remain coupled to the clockwise slipper segments  514   a  and the counterclockwise slipper segments  514   b . As a result, the race  516  remains coupled to the clutch housing  512  in both directions. Other modes of operation can be made available by varying the relative shapes of the first and second cam slots  554   a ,  554   b.    
     Turning now to FIG. 7, a four-wheel drive transfer case  600  is shown which embodies the inventive programmable couplings described above. The transfer case  600  comprises an input shaft  602 , a rear wheel output shaft  604 , a front wheel output shaft  606 , and a torque transfer assembly  608  for transferring torque from the input shaft  602  to the front wheel output shaft  606  while allowing overrunning of the front wheel output shaft  606  independently of the direction of rotation of the input shaft  602 . The torque transfer assembly  608  comprises an input disc  616  splined to the input shaft  602 , a spiral-type two-way coupling  610  coupled to the input shaft  602 , a first front wheel drive sprocket  666   a  coupled to the two-way coupling  610 , a second front wheel drive sprocket  666   b  splined to the front wheel output shaft  606 , and a chain  668  trained around the front wheel drive sprockets  666   a ,  666   b.    
     The coupling  610  is similar to the tri-mode spiral-type coupling  510 , and comprises a clutch body  612 , and a slipper  614 , with the input disc  616  acting as the race. The coupling  610  also includes a rotatable cam plate  650  coupled to the front wheel drive sprocket  666   a  through a friction plate  674 , a first cam follower  652   a  coupled between the cam plate  650  and the clockwise slipper segments, and a second cam follower  652   b  coupled between the cam plate  650  and the counterclockwise slipper segments. The cam plate  650  is provided with a plurality of ball cams  654   a ,  654   b  for engaging the first and second cam followers  652   a ,  652   b . However, unlike the coupling  510 , the cam plate  650  only provides two modes of operation: clockwise freewheeling and counterclockwise freewheeling. 
     The transfer case  600  also includes an actuator  676  coupled to the cam plate  650  through an axially-movable yoke  678  for enabling or disabling four-wheel drive mode. 
     In operation, with the vehicle moving in a straight line, torque from the input shaft  602  is applied to the rear wheel output shaft  604 , and to the front wheel output shaft  606  through the coupling  610 . When the vehicle turns and the front wheels rotate faster than the rear wheels, the coupling  610  allows the slipper  612  to overrun the input disc  616  to allow the vehicle turn to be completed without rear wheel slippage. 
     Turning now to FIG. 8, an all-wheel drive transfer case  700  is shown which embodies the inventive programmable couplings described above. The transfer case  700  comprises an input shaft  702 , a rear wheel output shaft  704 , a front wheel output shaft (not shown), and a coplanar reverted gear train loop  706  coupled between the input shaft  702 , the rear wheel output shaft  704  and the front wheel output shaft for splitting input torque between the rear wheel output shaft  704  and the front wheel output shaft. A spiral-type two-way coupling  710  is coupled between the input shaft  702  and the rear wheel output shaft  704  for allowing overrunning of the front wheel output shaft independently of the direction of rotation of the input shaft  702 . 
     The coplanar reverted gear train loop  706  comprises an externally-toothed pinion  756 , an eccentric cage  758  disposed around the pinion  756 , and an internally-toothed annular gear  760  disposed around the cage  758  and being coplanar to the pinion  756  and the cage  758 . The eccentric cage  758  comprises a ring gear  762  and an eccentric guide  764  for providing the ring gear  762  with an axis of rotation eccentric to that of the pinion  756  and the annular gear  760 . The ring gear  762  has an external set of teeth which mesh with the internally-toothed annular gear  760 , and an internal set of teeth which mesh with the externally-toothed pinion  756 . The cage  758  is coupled to the rear wheel output shaft  704 , and the annular gear  760  is coupled to the input shaft  702 . 
     A first front wheel drive sprocket  766  is splined to the pinion  756 , and a second front wheel drive sprocket (not shown) is splined to the front wheel output shaft. A chain is trained around the first front wheel drive sprocket  766  and the second front wheel drive sprocket for coupling the pinion  756  to the front wheel output shaft. 
     The spiral-type two-way coupling  710  is coupled to the extension  770  of the annular gear  760  and the extension  772  of the cage  758 . The coupling  710  is substantially identical to the dual-mode spiral-type coupling  610 , and comprises a rotatable cam plate  750  coupled to the front wheel drive sprocket  766  through a friction plate  774 , a first cam follower  752   a  coupled between the cam plate  750  and the clockwise slipper segments, and a second cam follower  752   b  coupled between the cam plate  750  and the counterclockwise slipper segments. The transfer case  700  also includes an actuator  776  coupled to the cam plate  750  through an axially-movable yoke  778  for enabling or disabling all-wheel drive mode. 
     In operation, the coplanar reverted gear train loop  706  unequally splits the input torque from the input shaft  702  between the pinion  756  and the cage  758  in accordance with the ratio of the number of teeth on the pinion  756 , the ring gear  762  and the annular gear  760 . As a result, one of the output shafts is driven more actively than the other. However, the relative sizes of the first front wheel drive sprocket  766  and the second front wheel drive sprocket are such that the less actively driven output shaft is rotated more slowly than the other output shaft to allow the less actively driven output shaft to overrun up to a predetermined threshold. 
     In one implementation of the invention, the coplanar reverted gear train loop  706  directs 60% of the input torque to the rear wheel output shaft  704  and the remainder to the front wheel output shaft. However, the relative sizes of the first front wheel drive sprocket  766  and the second front wheel drive sprocket are such that the front wheel output shaft is rotated by the coplanar reverted gear train loop  706  about 15-20% slower than the rear wheel output shaft  704 . Accordingly, when the vehicle enters a turn, or the front freewheels lose traction, the front wheels will be allowed to rotate up to 15-20% faster than the rear wheels with the relative proportions of torque remaining the same. If the front wheels attempt to rotate at a greater relative rate, the coupling  710  will couple the annular gear  760  to the cage  758 , thereby causing the coplanar reverted gear train loop  706  to rotate as a solid coupling and the differential rate of rotation to be limited to the 15-20% factor described above. 
     If vehicle direction is reversed, the rotatable cam plate  750  coupled to the front wheel drive sprocket  766  will select the opposite freewheeling mode of the coupling  710  to again allow the front wheels to rotate at up to 15-20% faster than the rear wheels before the coplanar reverted gear train loop  706  locks up. 
     The foregoing description of the preferred embodiment is intended to be illustrative of the present invention. Those of ordinary skill will be able to envisage certain additions, deletions and/or modifications to the described embodiments which, although not explicitly disclosed herein, are encompassed by the scope of the invention, as defined by the appended claims.