turning to fig1 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 fig1 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 fig1 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 fig1 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 fig2 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 fig2 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 . fig3 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 fig4 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 fig4 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 . fig5 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 fig6 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 fig6 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 fig6 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 fig6 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 fig7 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 fig8 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 .