Abstract:
A transfer case for AWD/4WD vehicle that employs a bushing positioned within a transfer case housing. The bushing is formed to the housing when the housing is cast. The bushing includes one or more axial notches formed in an outer surface of the bushing that fill with the housing metal during the casting process that prevents the bushing from rotating in response to a load from a slip yoke. A helical groove is formed in the inner surface of the bushing and a slot is formed through a housing wall along the bushing that allows lubricant from within the housing to flow through the bushing.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates generally to a transfer case for an all-wheel drive (AWD)/four-wheel drive (4WD) vehicle and, more particularly, to a transfer case for an AWD/4WD vehicle that employs a rear output shaft bushing that is cast with the transfer case housing, where the bushing incorporates a spiral groove for allowing lubricant flow therethrough.  
           [0003]    2. Discussion of the Related Art  
           [0004]    Various sport utility vehicles (SUV), off-road vehicles, four-wheel drive vehicles, etc. are equipped with drive modes that allow the vehicle to be driven in one or more of two-wheel drive high, four-wheel drive high, four-wheel drive low and AWD. Typically, these types of vehicles employ transfer cases that distribute the drive power received from an output shaft of the vehicle&#39;s transmission. Particularly, the output shaft of the transmission is coupled to an input shaft of the transfer case that distributes drive power to a front output shaft that is coupled to a front drive shaft that drives the vehicle&#39;s front wheels and a rear output shaft that is coupled to a rear drive shaft that drives the vehicle&#39;s rear wheels. Known transfer cases have employed various types of couplings, such as viscous couplings, electromagnetic clutches, positionable spur gears, etc., that allow the drive power from the transmission to be distributed to the front and rear drive shafts to provide the various drive modes.  
           [0005]    The rear output shaft of the transfer case is coupled to the rear drive shaft by a slip yoke. The rear drive shaft is coupled to a rear axle of the vehicle, which is mounted to a vehicle suspension system. As the vehicle travels, the rear axle moves up and down in response to road conditions. As the rear axle moves up and down, the slip yoke slides on the rear output shaft so that the suspension load is not significantly transferred thereto. A bushing is typically employed in the opening of the transfer case housing through which the rear output shaft extends. The slip yoke is supported and rotates within the bushing, and is able to reciprocate therein along the axis of the output shaft. This allows the rear drive shaft to slide relative to the rear output shaft in response to rough driving conditions.  
           [0006]    The housing of the transfer case is typically cast in two or more pieces and then bolted together. For example, a portion of the transfer case housing is sometimes cast as a separate cover housing and extension housing. An opening through which the rear output shaft extends is then machined into the appropriate housing piece to accept the bushing. The bushing is then pressed into the housing piece in a friction engagement before the housing pieces are bolted together. An inner surface of the bushing is then machined so that it has an internal diameter suitable for the outer diameter of the slip yoke.  
           [0007]    It is desirable to limit the number of housing pieces to reduce costs and assembly time. However, if the housing piece to which the bushing is mounted is too large, then it becomes too difficult to machine the opening in the transfer case that accepts the bushing. Further, the bushing has a tendency to spin out of the opening in response to the load applied thereto from the slip yoke during operation. Also, the load from the slip yoke significantly increases the temperature of the bushing, that may lead to part failure of the bushing and/or rear output shaft.  
         SUMMARY OF THE INVENTION  
         [0008]    In accordance with the teachings of the present invention, a transfer case for an AWD/4WD vehicle is disclosed that employs a bushing positioned within a housing of the transfer case. A slip yoke is inserted into the bushing and is rotatable therein. A rear drive shaft is rigidly coupled to the yoke. An output shaft of the transfer case is rigidly coupled to the yoke so that they rotate together and the yoke can slide in an axial direction relative to the output shaft. The bushing is formed to the housing when the housing is cast. The bushing includes one or more axial notches formed in an outer surface of the bushing that fill with housing metal during the casting process that prevent the bushing from rotating in response to the load from the slip yoke. A helical or spiral groove is formed in an inner surface of the bushing and a slot is formed through a housing wall along the bushing that allows lubricant from within the housing to flow through the groove.  
           [0009]    Additional advantages and features of the present invention will become apparent to those skilled in the art from the following discussion and the accompanying drawings and claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a plan view of the drive components of an AWD/4WD vehicle employing a transfer case, according to an embodiment of the present invention;  
         [0011]    [0011]FIG. 2 is a cross-sectional view of part of the transfer case of the invention shown in FIG. 1;  
         [0012]    [0012]FIG. 3 is another cross-section view of part of the transfer case of the invention shown in FIG. 1; and  
         [0013]    [0013]FIG. 4 is a rear-view of the transfer case showing a bushing employed therein. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0014]    The following discussion of the embodiments of the invention directed to a transfer case for an AWD/4WD vehicle that includes a rear output shaft bushing is merely exemplary in nature, and is in no way intended to limit the invention or it&#39;s applications or uses.  
         [0015]    [0015]FIG. 1 is a plan view of the drive components of an AWD/4WD vehicle  10 . The vehicle  10  includes an internal combustion engine  12  that provides the power to drive the vehicle  10 , as is well understood in the art. A drive shaft (not shown) is rotated by the engine  12 , and is coupled to a transmission  14  that converts the output power from the engine  12  to a selectively geared output. The operation of the engine  12  and the transmission  14  are well understood to those skilled in the art, and need not be discussed in detail here for a proper understanding of the invention.  
         [0016]    The output drive power from the transmission  14  is provided to an input shaft  18  of a transfer case  16 . The transfer case  16  selectively provides output drive power to a pair of rear wheels  20  and  22  and a pair of front wheels  24  and  26 . In the two-wheel drive mode, drive power is only provided to the rear wheels  20  and  22 . The transfer case  16  can also be shifted to neutral, where the transmission  12  is disengaged from the transfer case  16  and the wheels  20 - 26  can rotate freely for towing and the like.  
         [0017]    The transfer case  16  includes a rear output shaft  30  and a front output shaft  32 . The rear output shaft  30  is coupled to a rear drive shaft  34  by a slip yoke  28 , and the rear drive shaft  34  is coupled to a rear differential  36 . A first rear axle  38  is coupled at one end to the differential  36  and at an opposite end to the wheel  20 . Likewise, a second rear axle  40  is coupled at one end to the differential  36  and at an opposite end to the wheel  22 . The transfer case  16  provides output power on the rear output shaft  30 , which provides rotational energy to the rear drive shaft  34 . This rotational energy is transferred through the rear differential  36  and the axles  38  and  40  to the wheels  20  and  22  in a manner that is well understood in the art. The rear axles  38  and  40  are coupled to a vehicle suspension system (not shown) so that the axles  38  and  40  move up and down in response to the road conditions. The slip yoke  28  allows the rear drive shaft  34  to slide independent of the rear output shaft  30  so that this load is not significantly imparted to the transfer case  16 .  
         [0018]    The front output shaft  32  is coupled to a front drive shaft  44  by a slip yoke  42 , and the drive shaft  44  is coupled to a front differential  46 . A first front axle  48  is coupled at one end to the front differential  46  and at an opposite end to the wheel  24 . Likewise, a second front axle  50  is coupled at one end to the front differential  46  and at an opposite end to the wheel  26 . Drive energy on the front output shaft  32  drives the front drive shaft  44 , and the front differential  46  transfers the drive energy to the wheels  24  and  26  through the front axles  48  and  50 .  
         [0019]    A switch  54 , generally mounted on the dashboard of the vehicle  10 , allows the vehicle operator to select the drive mode for two-wheel drive (2WD), AWD or neutral (N). The switch  54  provides a signal to a controller  56  indicating the drive mode selection. The controller  56  provides a control signal to the transfer case  16  to cause the transfer case  16  to make the shift to the desired drive mode, as will be discussed in detail below.  
         [0020]    [0020]FIGS. 2 and 3 are cross-sectional views of a housing portion  60  of the transfer case  16 . The housing portion  60  is a cast metal member that would be bolted to another housing portion (not shown) to form the complete transfer case housing, as would be understood by those skilled in the art. The housing portions would include various seals, recesses, shoulders, flanges, bores, etc. that accept and position the various components and parts of the transfer case  16 . The rear output shaft  30  is rotatably coupled to the input shaft  18  within the transfer case  16  by various gears, bearings, etc. (not shown) in any suitable manner that would be well understood to those skilled in the art. For example, the input shaft  18  may be coupled to the rear output shaft  30  by a planetary gear assembly (not shown) to provide the desired gear ratio between the input and output of the transfer case  16 . Further, the rear output shaft  30  would be selectively coupled to the front output shaft  32  by a sprocket and chain assembly (not shown) to provide the drive power to the front output shaft  32  for the AWD/4WD drive mode. The coupling between the input shaft  18  and the rear output shaft  30  is not shown in any detail because any type of coupling system suitable for a transfer case can be employed.  
         [0021]    As discussed above, the rear drive shaft  34  is mounted to the rear output shaft  30  by the slip yoke  28 . The rear drive shaft  34  is rigidly mounted to the slip yoke  28 , and the slip yoke  28  is rotatably and slidably mounted to the rear output shaft  30 . Particularly, the rear output shaft  30  includes axial splines  62  on its outer surface and the yoke  28  includes cooperating splines  64  on its inner surface that allow the yoke  28  to slide axially relative to the output shaft  30 . A seal  66  is mounted to an annular extension  74 , as shown, to seal the housing portion  60  of the transfer case  16 .  
         [0022]    According to the invention, the transfer case  16  includes a bushing  70  mounted to an annular shoulder portion  68  of the housing portion  60 , as shown. FIG. 3 does not show the yoke  28  and the rear output shaft  30  for clarity purposes to better show the bushing  70 . FIG. 4 is a rear-view of the housing portion  60  showing the bushing  70 . As is well understood in the art, the slip yoke  28  rotates on an inner surface  72  of the bushing  70 , and reciprocates in an axial direction within the bushing  70  in response to a rebound rear axle suspension system to provide a slip engagement with the rear output shaft  30 . This allows the rear drive shaft  34  to move independent of the transfer case  16 .  
         [0023]    According to the invention, the bushing  70  is mounted to the shoulder portion  68  when the housing portion  60  is cast. Particularly, the bushing  70  is placed in the die cast, and the molten metal forming the housing portion  60  flows around it. In one embodiment, the bushing  70  is a sintered bronze bushing. However, this is by way of a non-limiting example, in that the bushing  70  can be made of any material suitable for the purposes described herein. By casting the bushing  70  with the housing portion  60 , the opening in which the bushing  70  would normally be pressed into does not need to be machined, and the bushing  70  does not need to be later press fit into the housing portion  60 , as was previously done in the art.  
         [0024]    If the slip yoke  28  is cocked or angled relative to the axis of the bushing  70  in response to the up and down movement of the rear wheels  20  and  22 , it can exert a significant load thereon. To prevent the bushing  70  from rotating within the shoulder  68  in response to the load, the bushing  70  includes a series of axial notches  76  formed in an outer surface  78  of the bushing  70  that receive molten metal when the housing portion  60  is cast. In this embodiment, there are twelve notches  76  symmetrically disposed around the outer surface  78  of the bushing  70 . However, this is by way of a non-limiting example, in that any suitable number or size of the notches  76  can be provided within the scope of the present invention. The notches  76  prevent the bushing  70  from rotating within the housing portion  60  under the load from the yoke  28 . Because the bushing  70  cannot spin within the housing portion  60 , it will not push on the seal  66 , causing failure of the transfer case  16 .  
         [0025]    Further, the load applied to the bushing  70  from the slip yoke  28  creates a significant heat build-up. According to the invention, a spiral or helical groove  80  is machined into the inner surface  72  of the bushing  70  after it is cast to the housing portion  60 . Further, a slot  82  is machined through the shoulder  68  proximate the outer surface  78  of the bushing  70 . The transfer case  16  is mounted to the vehicle  10  so that the rear portion of the transfer case  16  is slightly lower than the front portion of the transfer  16 . This allows lubricating oil within the transfer case  16  to collect within the housing portion  60  proximate the bushing  70 . The oil will flow through the slot  82  under gravity and into a chamber  84  between the seal  66  and the bushing  70 . The oil will be pumped by the helical groove  80  as the yoke  28  and the output shaft  32  rotate back into the housing portion  60 . Therefore, a constant supply of cooling and lubricating oil is provided to the space between the inner surface  72  of the bushing  70  and the yoke  28  to control the heat build-up. The level of the oil in the housing portion  60  is not high enough to cover the opening of the groove  80  into the housing portion  60 .  
         [0026]    The foregoing discussion describes merely exemplary embodiments of the present invention. One skilled in the art would readily recognize that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.