Abstract:
A “torque-on-demand” (“TOD”) four wheel drive system comprising a slipper clutch or roller clutch positioned between a first rotatable component and a second rotatable component. The clutch comprises a first tubular component having a first axial slot and a second tubular component having a second axial slot. A control pin extends through and is axially moveable within the first and second slots. One of the first or second slots has a constant circumferntial width W and the other of the first and second slots has at least first and second portions along its axial length with the first and second portions having different circumferential widths. Axial movement of the control pin along the slots changes the clutch between 2WD and 4WD/TOD modes.

Description:
BACKGROUND  
       [0001]     A ‘torque-on-demand’ (TOD) four wheel drive system automatically applies torque to the front wheels when the rear wheels slip. An overrunning clutch can provide a low cost method for TOD. Such a system is explained in U.S. Pat. No. 6,602,159 and provides either TOD or full lock four wheel drive (4WD). In such a system, the front axle is always turning which adds parasitic drag to the vehicle, increasing fuel consumption. It is desirable to provide a mode that allows the front axle to be stopped, i.e. two wheel drive mode (2WD). Another undesirable feature of the current system using an overrunning clutch to provide TOD is that a drag brake must be used for clutch control which increases fuel consumption.  
         [0002]     Referring to  FIG. 1 , a prior art four wheel drive control device  8  with TOD mode is shown. The control device  8  comprises a through shaft  1  delivering torque to the rear wheels, a sprocket  3  capable of driving the front wheels through a chain  2 , an inner race  4  fitted over the shaft  1 , a slipper  5  between the sprocket  3  and the inner race  4 , and a brake  6  capable of causing drag torque on the slipper  5  by way of an actuator ring  6 A which is keyed into the slipper  5 . The inner race  4  has multiple axially oriented recesses  9  disposed around it&#39;s outer periphery and the slipper  5  has multiple recesses  10  disposed around it&#39;s inner periphery aligned with the recesses  9  in the inner race  4  to form pockets into which rollers  7  are placed. The slipper  5  is circumferentially discontinuous by virtue of an axial cut  11 . The slipper  5  is generally loose in the bore of the sprocket  3 . In conditions without wheel slip, the drive ratio to the front wheels is different to the rear such that the sprocket  3  rotates faster than the shaft  1 . The friction of the slipper  5  in the sprocket  3  would tend to rotate the slipper  5  relative to the sprocket  3 , but the friction of the drag brake  6  is greater than the slipper drag preventing such relative rotation. If the rear wheels slip, the sprocket  3  will tend to rotate slower than the shaft  1  because the vehicle speed reduces. The slipper drag is now in the same direction as the drag from the brake  6  causing the slipper  5  to rotate relative to the sprocket  3 . Such relative rotation causes the rollers  7  to climb the sides of the recesses  9 ,  10  in the inner race  3  and the slipper  5 . The slipper  5  expands in diameter as the rollers  7  climb in the recesses  9 ,  10  causing the slipper  5  to lock in the sprocket  3  and thereby transfer torque to the front wheels. Since the drag brake torque reverses in reverse rotation, the identical functions occur in reverse rotation. Removing the drag brake torque causes the slipper  5  to lock unconditionally. A substantial drag is required from the drag brake, which increases fuel consumption.  
       SUMMARY  
       [0003]     The present invention provides a “torque-on-demand” four wheel drive system comprising a slipper clutch or roller clutch positioned between a first rotatable component and a second rotatable component. The clutch comprises a first tubular component having a first axial slot and a second tubular component having a second axial slot. A control pin extends through and is axially moveable within the first and second slots. One of the first or second slots has a constant circumferential width W and the other of the first and second slots has at least first and second portions along its axial length with the first and second portions having different circumferential widths. Axial movement of the control pin along the slots changes the clutch between 2WD and 4WD/TOD modes. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0004]      FIG. 1  is a cross section through a plane perpendicular to the axis of the main shaft illustrating the principle of the bidirectional slipper clutch with drag brake providing TOD operation.  
         [0005]      FIG. 2  is a cross section through a plane containing the axis of the main shaft of a slipper clutch assembly that is a first embodiment of the present invention.  
         [0006]      FIG. 3  is a radially inward view along the line  3 - 3  in  FIG. 2 .  
         [0007]      FIG. 4  is a schematic diagram of the mode select device for the slipper clutch assembly of  FIG. 2 .  
         [0008]      FIG. 5  is a cross section through a plane containing the axis of the main shaft of a roller clutch assembly that is a second embodiment of the present invention.  
         [0009]      FIG. 6  is a cross section through a plane containing the axis of the main shaft of a self contained slipper clutch that is a third embodiment of the present invention.  
         [0010]      FIG. 7  is a cross section through a plane containing the axis of the main shaft of a self contained slipper clutch that is a fourth embodiment of the present invention.  
         [0011]      FIG. 8  is a cross section through a plane containing the axis of the main shaft of a self contained slipper clutch that is a fifth embodiment of the present invention.  
         [0012]      FIG. 9  is a radial view along the line to the main shaft showing the slot profiles of the device in  FIG. 8 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     The present invention will be described with reference to the accompanying drawing figures wherein like numbers represent like elements throughout. Certain terminology, for example, “lop”, “bottom”, “right”, “left”, “front”, “frontward”, “forward”, “back”, “rear” and “rearward”, is used in the following description for relative descriptive clarity only and is not intended to be limiting.  
         [0014]     Referring to  FIGS. 2 and 3 , a slipper clutch assembly  20  that is a first embodiment of the present invention is shown. The slipper clutch assembly  20  generally includes an inner tube  22  rotationally fixed to the inner race  4  and outer tube  24  rotationally fixed to slipper  5 . The inner tube  22  has an axial slot  26  and the outer tube  24  has an axial slot  30  overlying the inner tube axial slot  26 . A pin  28  extends through both axial slots  26  and  30  and is moveable along the axis of the shaft  1  by an actuator plate  29 . The inner tube axial slot  26  has a uniform width W. As illustrated in  FIG. 3 , the outer tube slot  30  has variable widths. Namely, the outer tube slot  30  has a first portion  32  that is substantially equal to the width W of the inner tube slot  26 ; a second portion  34  that is wider than the inner tube slot  26  in both circumferential directions; a third portion  36  that is wider than the inner tube slot  26  in one circumferential direction and a forth portion  38  that is wider than the inner tube slot  26  in the opposite circumferential direction.  
         [0015]     When the pin  28  is located in the first portion  32  of the outer tube slot  30 , the recesses  9 ,  10  of the inner race  4  and slipper  5  are aligned so that the rollers  7  cannot climb up the sides of the recesses  9 ,  10 , preventing the slipper  5  from locking. This prevents torque from being transmitted to the front wheels, thereby providing a 2 wheel drive mode. The second portion  34  of the slot  30  allows relative rotation between the inner race  4  and the slipper  5  in both directions to provide fill freedom to unconditionally lock the slipper  5 . When the pin  28  is in the third portion  36  of the slot  30 , the system provides a forward TOD mode wherein locking is prevented when the sprocket  3  overruns the shaft  1  in forward and locks when the sprocket  3  is slower than the shaft  1 . The fourth slot portion  38  provides a reverse TOD position in which the free and locking directions are reversed from that of the forward TOD mode. Various mechanisms may be utilized to provide the axial motion of the actuator plate  29  to select the desired operating mode.  
         [0016]     Referring to  FIG. 4 , an illustrative mechanism  40  for axially moving the actuator plate  29  is shown. The mechanism  40  includes a gearmotor that turns a sector plate  42  to select the mode of the transfer case. A hi-low shift fork  44  is moved by the sector plate  42  as well as the slipper clutch control fork  46 . The slipper clutch control fork  46  moves the actuator plate  29 . A spring loaded solenoid  48  moves the pivot point  49  of a sector plate follower  47 . When the transfer case is in TOD mode, it is in forward mode unless the solenoid  48  is actuated to move it to the TOD reverse mode.  
         [0017]     Referring to  FIG. 5 , a roller clutch assembly  50  that is a second embodiment of the present invention is shown. The roller clutch assembly  50  includes an outer race  52  that is press fitted into the sprocket  3 . The outer race  52  is formed with a plurality of axial recesses on its inner periphery similar to the recesses  10  described in the previous embodiment. The outer race  52  also includes an axial slot  54  having a variable configuration similar to the configuration of the outer tube slot  30  of  FIG. 3 , with portions  32 ,  34 ,  36 ,  38 . Rollers  7  are placed between the outer race recesses and the shaft  1 . The rollers  7  are held in location by a cage  56 . The cage  56  is rotationally fixed to an inner tube  58  having an axial slot  57  with a configuration similar to inner tube slot  26 . The rotational position of the cage  56 , and thereby the rollers  7 , relative to the outer race  52  is determined by the axial location of pin  28  which is controlled by the actual actuator  29 . For 2WD operation, the pin  28  is retained in the slot portion  32  such that the cage  56  maintains its relative position to the outer race  52  and the rollers  7  are held centered in the outer race recesses. When locking is desired in a mode, the pin  28  is moved axially along the slots  52 ,  56 , such that the outer race slot portions  34 ,  36 ,  38  provide freedom for relative rotation between the outer race  52  and cage  56  such that the rollers  7  climb the sides of the recesses and lock against the shaft  1  functionally similar to the  FIG. 2  arrangement.  
         [0018]     Referring to  FIG. 6 , a self contained slipper clutch assembly  60  that is a third embodiment of the present invention is shown. The slipper clutch assembly  60  includes an inner race  4  and a slipper  5 . The inner race  4  includes splines  62  axially alignable and engageable with splines  64  on the slipper  5 . While splines are described, other interlocking features may also be utilized. When the splines  62  and  64  are axially aligned, the splines  62  and  64  engage one another such that there is no relative rotation between the inner race  4  and slipper  5 . As such, the recesses  9 ,  10  are maintained in alignment and the assembly  60  is prevented from locking. This provides 2WD mode. A stack of wave springs,  65 ,  66 , holds the splines  62 ,  64  engaged. An axial actuator plate  67  is aligned with and contacts the slipper  5  and the wave springs  65 ,  66 . A shift fork  68  or the lice is utilized to move the actuator plate  67  against the slipper S and wave springs  65 ,  66  to achieve the 4WD and TOD modes. Initial movement of the actuator plate  67  to the right causes the splines  62  and  64  to disengage as the weak wave spring  65  in the stack collapses. Once the splines  62  and  64  are disengaged, the inner race  4  and slipper  5  are free to rotate relative to one another in both directions. This provides an unconditional, full lock operation. Further movement of the shift fork  68 , and thereby the actuator plate  67 , causes a higher force to develop as the stiffer wave springs  66  collapse. The higher force causes a drag torque higher than the slipper friction to put the system into TOD mode similar to the function of the device described in  FIG. 1 . A cup  69  holds the sprocket  3  in a fixed axial location.  
         [0019]     Referring to  FIG. 7 , a self contained slipper clutch assembly  70  that is a fourth embodiment of the present invention is shown. The clutch assembly  70  is similar to that shown in  FIG. 6  and includes an inner race  4  and slipper  5 , with an interengaging feature  72 , for example, splines, therebetween. An axial actuator plate  73  is moved against the slipper  5  in a manner similar to the previous embodiment to achieve the various modes of operation. To reduce the frictional wear between the slipper  5  and the sprocket  3  when the slipper  5  rotates relative to the sprocket  3 , a pair of conical plain bearings  75  is positioned between the slipper  5  and tapered surfaces  78  of the sprocket  3  and support the sprocket  3  away from the rotating slipper  5 . The plain bearings  75  are loaded by springs  76  and  77 , which allow the plain bearings  75  to back away as the slipper S expands to lock the clutch. The plain bearings  75  do not interfere with the locking action of the clutch. Ball bearings can be used instead of plain bearings. A spacer  79  may also be provided to position the rollers  7 .  FIG. 7  also illustrates an alternative construction of the device of  FIG. 6  where the wave springs  74  are located in the cup  69 .  
         [0020]     Referring to  FIGS. 8 and 9 , a self contained slipper clutch assembly  80  that is a fifth embodiment of the present invention is shown. The clutch assembly  80  is similar to that shown in  FIG. 2  and includes an inner race  4  and slipper  5 . Rather than providing independent tubes, the inner race  4  and slipper  5  are each provided with an axially extending flange  82  and  84 , respectively. Each flange  82 , 84  includes a respective slot  83 ,  85  with a pin  28  extending therethrough. An actuator plate  29  is axially moveable to move the pin  28  within the slots. Referring to  FIG. 9 , slipper slot  85  has a constant width W similar to slot  26  while the inner race slot  83  includes a first portion  86  with a width substantially equal to the slipper slot width W and a second portion  87  with an expanded width in both circumferential directions.  
         [0021]     The actuator plate  29  is moveable between two actuator positions. In the right position in which the pin  28  is in inner race slot portion  86 , the inner race  4  and slipper  5  are closely aligned to prevent the clutch from locking to provide 2WD operation. When the pin  28  is moved to the left position aligned with the inner race slot portion  87  (as shown), there is freedom for the clutch to lock in either direction. This position allows for either full lock or TOD. To change between full lock and TOD, a drag band  88  is provided about a disc member  89  adjacent the second position of the actuator plate  29 . When the drag band  88  is engaged, the actuator plate  29 , through the pin  28  applies the drag band torque to the slipper  5  by way of the pin  28  and slot  85  to operate the clutch in TOD mode. The clutch assembly  80  may also include conical bearings  75  similar to those described in the previous embodiment. A roller clutch similar to that shown in  FIG. 5  can also incorporate the feature of the present embodiment be providing the cage with a slot similar to  83  and the race has a slot similar to  85 .