Abstract:
A multi-directional coupling ( 10 ) consists of a tubular slipper ( 14 ) between a tubular member ( 12 ) and a race ( 16 ), there being engageable tubular friction surfaces ( 20, 36 ) on the slipper ( 14 ) and the race ( 16 ), and facing bearing surfaces ( 18, 22 ) on the tubular member ( 12 ) and the slipper ( 14 ) defining therebetween a channel ( 28 ) with pockets ( 28   a ) receiving roller members ( 30 ), there being an actuator (e.g. a tooth ( 30 )) on the tubular member ( 12 ) engaging a recess ( 40 ) in the slipper ( 14 ), or a spigot or a pin(s) or a cam 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 Provisional appln. 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 full complement of bearings and 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 conventional one-way or overturning clutches include sprag clutches and roller-ramp clutches, such as those disclosed in GB 309 372 and WO 92/14072. 
     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. Since both 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 are 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 clutch 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 due 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. 
     Furthermore, spiral-type one-way clutches characteristically have larger than desirable wind-up angles. For example, it is not uncommon for a race of a conventional spiral-type one-way clutch to rotate over 10° at 6000 ft-lb of torque before full lock-up. Such large wind-up angles render the development of high-performance automotive equipment difficult. Accordingly, it would be desirable to provide a coupling having multiple modes of operation and small wind-up angles 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 coupling having multiple modes of operation. 
     The coupling, according to the invention, consists of a race including a first tubular friction surface; a tubular member including a first circular bearing surface; a tubular slipper including a second tubular friction surface for coupling to the first tubular friction surface, and a second circular bearing surface opposite the second friction surface, the second bearing surface being coaxial to the first bearing surface and, together with the first bearing surface, defining a channel disposed therebetween. A plurality of roller elements is disposed in the channel in abutment against the bearing surfaces, the channel including a pocket retaining at least one of the roller elements therein for coupling the race to the tubular member as the tubular member and the slipper rotate relative to one another. An actuator is provided for selectively restricting rotational movement of the slipper relative to the tubular member so as to control the coupling mode of the coupling. 
     Preferably, the pocket consists of a first arched recess disposed in the first bearing surface, and a complementary arched recess disposed in the second bearing surface substantially in close proximity to the first arched recess. The pockets are disposed along the channel for retaining the roller elements in abutment along the channel and are shaped according to at least one of an involute curve, a logarithmic series, a geometric series and an Archimedes spiral. 
    
    
     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 full-complement multi-directional coupling according to a first aspect of the invention; 
     FIG. 1 b  is a transverse cross-sectional view of the full-complement multi-directional coupling shown in FIG. 1 a;    
     FIG. 1 c  is a magnified schematic view of the actuator of the full-complement multi-directional coupling shown in FIG. 1 a;    
     FIG. 2 a  is an axial cross-sectional view of a full-complement multi-directional coupling according to a second aspect of the invention; 
     FIG. 2 b  is a transverse cross-sectional view of the full-complement multi-directional coupling shown in FIG. 2 a;    
     FIG. 2 c  is a magnified schematic view of the actuator of the full-complement multi-directional coupling shown in FIG. 2 a;    
     FIG. 3 are magnified schematic views of a variation of the actuator shown in FIG. 2 c;    
     FIG. 4 a  is a front plan view of an actuator of a full-complement multi-directional coupling according to a third aspect of the invention; 
     FIG. 4 b  is an axial cross-sectional view of the full-complement multi-directional coupling for the actuator shown in FIG. 4 a;    
     FIG. 4 c  are transverse cross-sectional views of the full-complement multi-directional coupling shown in FIG. 4 b;    
     FIG. 5 is a magnified schematic view of a variation of the actuator shown in FIG. 4 a;    
     FIG. 6 a  is a front plan view of an actuator of a full-complement multi-directional coupling according to a fourth aspect of the invention; 
     FIG. 6 b  is an axial cross-sectional view of the full-complement multi-directional coupling for the actuator shown in FIG. 6 a;    
     FIG. 6 c  are transverse cross-sectional views of the full-complement multi-directional coupling shown in FIG. 6 b;    
     FIG. 7 are magnified schematic views of the actuator according to fifth, sixth and seventh aspects of the invention; 
     FIG. 8 a  is an axial cross-sectional view of the full-complement multi-directional coupling according to an eighth aspect of the invention; 
     FIG. 8 b  is a transverse cross-sectional views of the full-complement multi-directional coupling shown in FIG. 8 a;    
     FIG. 9 a  is an axial cross-sectional view of the full-complement multi-directional coupling according to an ninth aspect of the invention; 
     FIG. 9 b  is a transverse cross-sectional views of the full-complement multi-directional coupling shown in FIG. 9 a;    
     FIG. 10 are various views of a gear-less differential incorporating the full-complement multi-directional couplings; and 
     FIG. 11 are various views of a four-wheel drive transfer case incorporating the full-complement multi-directional couplings. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning to FIGS. 1 a ,  1   b  and  1   c , a full-complement multi-directional coupling, denoted generally as  10 , is shown comprising a tubular clutch housing  12 , a resilient tubular C-shaped slipper  14  coaxial to and disposed externally to the slipper  14 . The clutch housing  12  has an outer cylindrical bearing surface  18  which includes a plurality of arched recesses  18   a.    
     The slipper  14  includes an outer cylindrical friction surface  20 , and an inner cylindrical bearing surface  22 . The slipper  14  is provided with a slit  24  extending between the inner cylindrical bearing surface  22  and the outer cylindrical friction surface  20  axially along the length of the slipper  14  for allowing the slipper  14  to expand and contract. The inner cylindrical bearing surface  22  is coaxial to the outer cylindrical bearing surface  18  and includes a plurality of arched recesses  22   a . The arched recesses  22   a  are positioned substantially opposite to the first arched recesses  18   a , and are complementary to the arched recesses  18   a  in that the arched recesses  22   a  are arched outwardly and the arched recesses  18   a  are arched inwardly. 
     The outer cylindrical bearing surface  18  and the inner cylindrical bearing surface  22  together define a channel  28  disposed between the clutch housing  12  and the slipper  14  extending the length of the clutch housing  12  and the slipper  14 , with the arches  18   a ,  22   a  defining a plurality of pockets  28   a  extending the length of the channel  28 . A plurality of roller bearings  30  are disposed within the channel  28  and abut against the bearing surfaces  18 ,  22  to provide restricted rotational movement between the slipper  14  and the clutch housing  12  in a manner to be described below. Preferably, each arched recess  18   a ,  22   a  is shaped to retain a portion of a single roller bearing  30  therein, such that each pocket  28   a  retains a single roller bearing  30  therein. 
     The race  16  includes an inner cylindrical friction surface  36  which mates with the outer cylindrical friction surface  20 . An actuator is provided for coupling the race  16  to the slipper  14  and the clutch housing  12  by selectively restricting rotational movement of the slipper  14  relative to the clutch housing  12 . The actuator comprises a flange  38  extending radially outwards from the outer cylindrical bearing surface  18 , and a mating channel  40  provided in the inner cylindrical bearing surface  22 . 
     The two modes of operation of the coupling  10  will now be described. If a torque is applied to the clutch housing  12  in the clockwise direction, the roller bearings  30  will move clockwise along the channel  28  in each pocket  28   a . However, as shown in the rightmost breakout figure of FIG. 1 c , shortly after the roller bearings  30  begin to move, they are forced to roll up the side walls  42  of the recesses  18   a ,  22   a . Due to the slit  24  and the resilient nature of the slipper  14 , the roller bearings  30  force the channel  28  and the slipper  14  to expand radially outwards, thereby increasing the radial force exerted by the slipper  14  against the race  16 . As the clutch housing  12  continues to rotate clockwise, the channel  28  will continue to expand until the slipper  14  and the race  16  become locked to the clutch housing  12 . At this point, the input torque from the clutch housing  12 . Alternately, if input torque is applied to the race  16  rather than to the clutch housing  12 , and in the counterclockwise direction, the input torque from the race  16  will be coupled to the clutch housing  12  causing the clutch housing  12  to rotate counterclockwise with the race  16 . 
     If the direction of rotation of the clutch housing  12  is reversed, the width of the channel  28  will decrease as the roller bearings return to the valleys of the recessed portions  18   a ,  22   a . If the clutch housing  12  was not provided with the actuator flange  38  and the mating channel  40 , the roller bearings  30  would again force the channel  28  and the slipper  14  to expand radially outwards, thereby coupling the clutch housing  12  to the race  16 . However, as shown in the leftmost breakout figure of FIG. 1 c , the actuator flange  38  is not disposed at the midpoint. As a result, the actuator flange  38  will engage the left side wall of the mating flange  40  well before the roller bearings  30  force the channel  28  and the slipper  14  to expand radially outwards. Consequently, the radial force exerted by the slipper  14  against the race  16  does not increase as the clutch housing  12  rotates counterclockwise, and the clutch housing  12  remains uncoupled from the race  16 . 
     It will be appreciated that the embodiment shown in FIG. 1 is a form of one-way clutch. However, the embodiment shown in FIG. 1 is a vast improvement over the prior art one-way clutches in that the strut angle is greater, allowing the coupling  10  to lock-up with less deformation of the roller bearings  30 . Furthermore, the full complement structure ensures that the input torque is transferred to the race  16  over the entire set of roller bearings  30  and that lock-up occurs with a smaller wind-up angle and with little risk that roller bearings  30  may jam up when the clutch is subjected to transient rotational forces. This configuration is to be contrasted with sprag-type one-way clutches and roller ramp-type one way clutches in which the wedge elements act individually, rather than as a group. Accordingly, it is preferred that a full complement of abutting roller bearing surfaces  18 ,  22 , with only a single bearing  30  disposed in each pocket  28  to prevent the roller bearings  30  from jamming. However, depending upon the application, the arched recesses may be applied to only one of the bearing surfaces  18 ,  22 , or only over a portion thereof. Additionally, multiple bearings  30  could be retaining in a single pocket  28  if the clutch was not to be subjected to transient rotational forces. 
     The arched recesses  18   a ,  22   a  may adopt any shape suitable for retaining a roller bearing  30  therein and for allowing the roller bearing  30  to roll up the side walls of the recess and thereby lock the race  16  to the clutch housing  12 . Examples of suitable shapes include those based on an involute curve, a logarithmic series, a geometric series, and Archimedes spiral or combinations thereof. However, a recess shape based on a logarithmic series is preferred as it allows the strut angle to increase as the clutch housing  12  is rotated relative to the slipper  14 . As a result, the torque applied to the clutch housing  12  will initially be translated into a relatively small compression force on the bearings  30 , allowing the bearing  30  to bite into the bearing surfaces  18 ,  22  with little deformation. Once the bearing  30  has engaged the bearing surfaces  18 ,  22  and the clutch housing  12  continues to rotate relative to the slipper  14 , the increased strut angle will allow the race  16  to lock to the clutch housing  12  with minimal Hertizian stress and spalling. 
     One variation of the full complement multi-directional coupling  10  is shown in FIGS. 2 a - 2   c . As shown therein, the coupling  110  includes a channel  44  extending through the slipper  114  and the clutch housing  112 . The actuator comprises a spigot  46 , tapered on one side thereof, and sized to pass into the channel  44 . When the spigot  46  is partially inserted into the channel  44  to the depth shown in the breakout figure of FIG. 2 c , the coupling  110  responds in much the same way as the coupling  10 . Specifically, if the clutch housing  112  is rotated clockwise, clutch housing  112  will engage the left side wall of the spigot  46  well before the roller bearings  30  force the channel  28  and the slipper  114  to expand radially outwards. Consequently, the radial force exerted by the slipper  114  against the race  16  does not increase as the clutch housing  112  rotates clockwise, and the clutch housing  112  is reversed, the clutch housing  112  will be able to move relative to the slipper  114  sufficiently far so as to cause the slipper  114  to expand radially outwards, thereby coupling the clutch housing  112  to the race  16 . Thus, when the spigot  46  is partially inserted into the channel  44  to the depth shown in the breakout figure of FIG. 2 c , the coupling  110  acts as a one-way coupling. 
     If the spigot  46  is fully inserted into the channel  44 , the clutch housing  112  will be prevented from rotating relative to the slipper  114  in both directions, allowing the race  16  to freewheel in both directions. 
     A particularly advantageous variation of the spigot  46  is shown in FIG.  3 . As shown therein, the spigot  146  includes a less tapered portion  146   a  adjacent the leading edge of the spigot  146 , and a more tapered portion  146   b  positioned rearwardly of the less tapered portion  146   a  for releasing the coupling  110  under load. As will be appreciated, the less tapered portion  146   a  is inserted first into the channel  44 , and due to the degree of taper, readily rotates the slipper  114  in the direction opposite to that of the applied torque. Once the slipper  114  is so rotated, the degree of coupling between the clutch housing  112  and the slipper  114  is reduced sufficiently to allow the more tapered portion  146   b  to be inserted into the channel  44 . Once the spigot  46  is fully inserted into the channel  44 , the race  16  becomes uncoupled from the clutch housing  112 . 
     Other variations of the full complement multi-directional coupling  10  are shown in FIGS. 4,  5  and  6 . The multi-directional coupling  210  shown in FIGS. 4 a - 4   c  comprises a tubular clutch housing  212 , a resilient tubular C-shaped slipper  214  coaxial to and disposed externally to the clutch housing  212 , and a cylindrical race  16  coaxial to and disposed externally to the slipper  214 . Axially-disposed channels  244  extend through the slipper  214  and the clutch housing  212 . The coupling  210  includes an actuator comprising an actuator disc  248  having a plurality of spigots  246  disposed thereon and extending axially through the channels  244 . As shown in FIG. 5, the spigots  246  are tapered on two opposite sides thereof, and are sized to pass into the channels  244 . AS a result, when the actuator disc  248  is disposed such that the spigots  246  are only partially entered into the channels  244 , the coupling  210  will lock the clutch housing  212  to the race  16  in both directions of rotation. However, when the actuator disc  248  is disposed such that the spigots  246  are fully entered into the channels  244 , the coupling  210  will unlock allowing the clutch housing  212  to freewheel with respect to the race  16  in both directions of rotation. 
     The multi-directional coupling  310  shown in FIGS. 6 a - 6   c  is substantially identical to the coupling  210 , but comprises a tubular clutch housing  312 , a resilient tubular C-shaped slipper  314  disposed internally to the clutch housing  312 , and a cylindrical race  316  disposed internally to the slipper  314 . 
     Three additional actuator structures will now be described with reference to FIG.  7 . In the bottom row of figures, the actuator comprises a pin  346  which extends radially inwards through the clutch housing and engages a flange  348  provided on the slipper for providing two separate modes of operation. IN the first mode, the pin  346  engages the flange provided on the slipper for preventing relative rotation of the slipper in the clockwise direction while permitting limited relative rotation of the slipper in the counterclockwise direction. In the second mode, the pin  346  is displaced from the flange  348  and the coupling is locked in both directions. Accordingly, this structure provides a programmable one-way clutch which freewheels either in the clockwise direction or locks up in both directions 
     In the middle row of figures of FIG. 7, the actuator comprises a rotatable cam  350  having a stem  352  and pair of opposing cam surfaces  354 ,  355  respectively engageable with flanges  356 ,  357  provided on the slipper. When the cam  350  has its stem in the neutral or upright position, as shown in the leftmost figure, the coupling is locked in both directions. When the cam  350  has its stem rotated counterclockwise, the cam surface  354  engages the flange  356  when the slipper is rotated in the clockwise direction. Therefore, in this mode, the coupling acts as a one-way coupling which couples the race to the clutch body when the race is rotated in the counterclockwise direction. When the cam  350  has its stem rotated clockwise, the cam surface  355  engages the flanges  357  when the slipper is rotated in the counterclockwise direction. Therefore, in this mode, the coupling acts as a one-way coupling which couples the race to the clutch body when the race is rotated in the clockwise direction. 
     In the top row of figures of FIG. 7, the actuator comprises a pair of pins  358   a ,  358   b  which extend radially inwards through the clutch housing and respectively engage flange  360   a ,  360   b  provided on the slipper for providing four separate modes of operation. In the first mode, the pins  358   a ,  358   b  engage the flanges  360   a ,  360   b  for allowing freewheeeling in both directions. In the second mode, the pin  358   a  is retracted from the flange  360   a , thereby coupling the race to the clutch body when the race is rotated in the clockwise direction but allowing freewheeling in the counterclockwise direction. In the third mode, the pin  358   b  is retracted from the flange  360   b , thereby coupling the race to the clutch body when the race is rotated in the counterclockwise direction but allowing freewheeling in the clockwise direction. In the fourth mode, the pins  358   a ,  358   b  are retracted from the flanges  360   a ,  360   b  for allowing full coupling in both directions. 
     Turning now to FIGS. 8 a  and  8   b , a multi-directional coupling  410  is shown comprising a tubular clutch housing  412 , a conical slipper  414  coaxial to and disposed externally to the clutch housing  412 , and a conical race  416  coaxial to and disposed externally to the conical slipper  414 . The conical slipper  414  is a full conical slipper in that it lacks the slit found in the previous variations. A pocketed channel  428  formed between the inner cylindrical surface of the slipper  414  and the outer cylindrical surface of the clutch housing  412  retains a plurality of roller bearings  30  disposed therein. The slipper  414  includes an outer conical friction surface  420 , and the race  416  includes an inner conical friction surface  436  which mates with the outer conical friction surface  420 . An actuator ring (not shown) selectively presses the conical race  416  into engagement with the conical slipper  414  so that the inner conical friction surface  436  engages the outer conical friction surface  420 . When so engaged, the race  416  becomes coupled to the clutch housing  412  in both directions. When the actuator ring withdraws the conical race  416  from the conical slipper  414 , the clutch housing  412  freewheels with respect to the race  416  in both directions. In one variation of the coupling  410 , the actuator ring acts upon an axially-slidable conical slipper  414  slidably engages the clutch housing  412  and the conical race  416 . 
     FIGS. 9 a  and  9   b  show a multi-directional coupling  510 , substantially identical to the coupling  410 , but comprising a tubular clutch housing  512 , a full conical slipper  514  disposed internally to the clutch housing  512 , and a conical race  516  disposed internally to the slipper  514 . An actuator ring (not shown) selectively presses the conical race  516  into engagement with the conical slipper  514 . Alternately, the actuator ring presses the conical slipper into engagement with the conical race  516  and the clutch housing  512 . 
     FIG. 10 shows a gear-less differential cage assembly for use in a differential, and which employs the inventive coupling and actuator assembly described with reference to the middle row of figures of FIG.  7 . The differential cage assembly, denoted generally as  600 , comprises a differential cage  602 , a left wheel output shaft  604 , and a right wheel output shaft  606 . A first multi-directional coupling  610   a  is coupled between the cage  602  and the left wheel output shaft  604 . A second multi-directional coupling  610   b  is coupled between the cage  620  and the right wheel output shaft  606 . 
     The multi-directional coupling  610   a  comprises a tubular clutch housing  612   a  coupled to the cage  602 , a resilient tubular C-shaped slipper  614   a  coaxial to and disposed internally to the clutch housing  612   a , and a cylindrical race  616   a  coaxial to and disposed internally to the slipper  614   a . The cylindrical race  616   a  is coupled to the left wheel output shaft  604 . The coupling  610   a  includes an actuator comprising a rotatable toggle  650   a , similar to the cam  350  of FIG. 7, and having a stem  652   a  and pair of opposing cam surfaces  654   a ,  655   a  which respectively engage flanges  656   a ,  657   a  provided on the slipper  612   a.    
     Similarly, the multi-directional coupling  610   b  comprises a tubular clutch housing  612   b  coupled to the cage  602 , a resilient tubular C-shaped slipper  614   b  coaxial to and disposed internally to the clutch housing  612   b , and a cylindrical race  616   b  coaxial to and disposed internally to the slipper  614   b . The cylindrical race  616   b  is coupled to the right wheel output shaft  606 . The coupling  610   b  includes an actuator comprising a rotatable toggle  650   b , similar to the cam  350  of FIG. 7, and having a stem  652   b  and pair of opposing cam surfaces  654   b ,  655   b  which engage respective flanges provided on the slipper  612   b.    
     A cross-actuator is coupled between the first and second couplings  610   a ,  610   b  for rotating the toggle  650   a  of the first multi-directional coupling  610   a  to the neutral or upright position in response to an increase in rotational speed of the right wheel output shaft  606  over the left wheel output shaft  604 , and for rotating the toggle  650   b  of the second multi-directional coupling  610   b  to the neutral or upright position in response to an increase in rotational speed of the left wheel output shaft  604  over the right wheel output shaft  606 . The cross-actuator comprises a first friction disc  66   a  coupled to the right wheel output shaft  606  and having a series of teeth  662   a  provided on the outer edge thereof, and a rotatable cam trigger  664   a  coupled to the friction disc  660   a  and having guides  666   a  for receiving the stem  652   b  and teeth  668   a  for engaging the teeth  662   a . The cross-actuator also comprises a second friction disc  660   b  coupled to the left wheel output shaft  604  and having a series of teeth  662   b  provided on the outer edge thereof, and a rotatable cam trigger  664   b  coupled to the friction disc  660   b  and having guides  666   b  for receiving the stem  652   a  and teeth  668   b  for engaging the teeth  662   b . The cross-actuator is enclosed on both sides by plates  670   a ,  670   b  which couples the cross-actuator to the cage  602 , causing the cross-actuator to rotate with the cage  602 . 
     In operation, when the vehicle is moving forward in a straight line, torque applied to the cage  602  causes the clutch housings  612  to rotate in the forward direction, thereby dragging the plates  670  in the forward direction. As a result, the stems  652  experience drag from the hydraulic fluid in the differential, causing the toggles  650  to rotate in the opposite direction and the coupling  610  to lock in the forward direction. Accordingly, torque is applied to both the left and right wheel output shafts  604 ,  606 . 
     If the input torque is reduce to the cage  602 , the left and right wheel output shafts  604 ,  606  will overrun, causing the friction discs  660  to rotate ahead of the cage  602 . As a result, the cam trigger  664   a  on the right wheel output shaft  606  flips the toggle  652   b  on the left wheel output shaft  604  to the neutral position, causing the coupling  610   a  to lock in both directions. Similarly, the cam trigger  664   b  on the left wheel output shaft  604  flips the toggle  652   a  on the right wheel output shaft  606  to the neutral position, causing the coupling  610   b  to lock in both directions. AS a result, the rear wheels are forced to drive the engine. 
     During vehicle maneuvers, such as when the right wheel output shaft  606  is forced to rotate faster than the left wheel output shaft  604 , the right wheel output shaft  606  will overrun, causing the friction disc  660   a  to rotate ahead of the cage  602 , in effect acting as a relative rotational velocity sensor for sensing the difference in rotational speeds between the left and right wheel output shafts  604 ,  606 . As a result, the cam trigger  664   a  on the right wheel output shaft  606  flips the toggle  652   b  on the left wheel output shaft  604  to the neutral position, causing the coupling  610   a  to lock in both directions. Since the position of the toggle  652   a  on the right wheel output shaft  606  will not change, the right wheel is allowed to overrun while torque is delivered to the left wheel. Since the left wheel is coupled to the cage in both directions, torque will be applied to the left wheel if torque is continually applied to the differential, and torque will be delivered to the engine from the left wheel if input torque is reduced. 
     FIG. 11 shows the front wheel drive portion of a four-wheel drive transfer case which also employs the inventive coupling and actuator assembly described with reference to the middle row of figures of FIG. 7 The four-wheel drive transfer case, denoted generally as  700 , comprises a torque input shaft (not shown), a rear wheel torque output shaft (not shown) coupled to the torque input shaft; a front wheel torque output shaft  702 , a first front wheel drive sprocket and the torque transfer assembly  710 . As will becomes apparent, the torque transfer assembly  704  transfers torque from the first front wheel drive sprocket to the front wheel torque output shaft  702  while allowing overrunning of the front wheel torque output shaft  702  independently of the direction of rotation of the input shaft. 
     The torque transfer assembly  710  comprises and externally toothed annular sprocket  712  rotatably disposed around the front wheel torque output shaft  702 , a resilient tubular C-shaped slipper  714  coaxial to and disposed internally to the annular sprocket  712 , and a cylindrical race  716  coaxial to and disposed internally to the slipper  714 . The cylindrical race  716  is coupled to the front wheel output shaft  702 . The torque transfer assembly  710  includes an actuator comprising a rotatable toggle  750 , similar to the cam  350  of FIG. 7, and having a stem  752  and pair of opposing cam surfaces  754 ,  755  which respectively engage flanges  756 ,  757  provided on the slipper  712 . 
     In operation, when the input shaft is rotating in the forward direction, the rear wheel torque output shaft will be forced to rotate in the forward direction, thereby causing the front wheel annular sprocket  712  and the front wheel torque output shaft  602  to rotate in the forward direction. As a result, the stem  752  will experience drag from the hydraulic fluid in the transfer case, causing the toggle  750  to rotate in the opposite direction and the torque transfer assembly  710  to lock in the forward direction. Accordingly, torque will be applied to the front wheel output shaft  702 . 
     If the vehicle enters a turn, causing the front wheels to rotate faster than the rear wheels, the front wheel torque output shaft  702  will overrun the annular sprocket  712 , allowing the turn to be smoothly completed. If at any time the rear wheels begin to slip, the vehicle will maintain its speed due to the power delivered to the front wheels through the torque transfer assembly  710 . 
     When the direction of the input shaft is reversed, the rear wheel torque output shaft and the annular sprocket  712  will be forced to rotate in the counterclockwise (reverse) direction, thereby urging the automobile to move backwards. Therefore, the front wheel torque output shaft  702  will be forced to rotate in the counterclockwise direction, causing the toggle  750  to rotate in the opposite direction and the torque transfer assembly  710  to lock in the reverse direction. 
     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 be described embodiments which, although not explicitly disclosed herein, are encompassed by the scope of the invention, as defined by the appended claims.