Abstract:
A light-weight transfer case is provided for implementation with a four-wheel drive vehicle. The light-weight transfer case includes a single-piece housing formed through either a lost-foam magnesium or die cast process. First and second output shafts are included which are formed from single-piece tubing through either a hydro-forming or swaging process. The first and second output shafts are lighter weight and maintain increased strength over traditional transfer case output shafts. A gear reduction unit is also included for establishing high, low and neutral speeds of the first and second output shafts. Furthermore, a mode selection device is included for selectively providing drive to either a single output shaft, in a two-wheel drive mode, or both the first and second output shafts, in a four-wheel drive mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. patent application Ser. No. 10/091,793 filed on Mar. 6, 2002, which claims the benefit of U.S. Provisional Application No. 60/278,140, filed Mar. 23, 2001. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to transfer cases for use in four wheel drive vehicles. More particularly, the present invention is directed to a light-weight transfer case improving overall vehicle cost and efficiency.  
       BACKGROUND OF THE INVENTION  
       [0003]     As is known, the majority of four-wheel drive vehicles are equipped with a transfer case mounted to a multi-speed transmission for directing power from the engine to all four wheels. To accommodate different road surfaces and conditions, many transfer cases are equipped with a mode shift mechanism which permits the vehicle operator to selectively couple the non-driven wheels to the driven wheels for establishing a part-time four wheel drive mode in addition to the two-wheel drive mode. As an alternative, some transfer cases are equipped with a transfer clutch that is passively or actively controlled in response to driveline slip for automatically delivering drive torque to the non-driven wheels for establishing an on-demand four-wheel drive mode. In addition, some transfer cases are also equipped with a two-speed range shift mechanism for permitting the vehicle operator to select between high-range and low-range four-wheel drive modes.  
         [0004]     Automobile manufacturers continuously strive to reduce vehicle weight and improve vehicle noise, vibration and harshness (NVH) characteristics. In particular, sport utility vehicles (SUV) enjoy a significant portion of the overall vehicle market. The majority of these SUV&#39;s provide a four-wheel drive mode and, therefore, are typically equipped with a transfer case. As part of the vehicle&#39;s driveline, a transfer case has significant influence on the NVH characteristics of the vehicle. For example, vibrations and excitations generated by the transmission are transferred through the transfer case to front and rear propshafts. Additionally, the transfer case itself can be a source of NVH excitation.  
         [0005]     On critical characteristic of four-wheel drive vehicles is the weight of the transfer case. Specifically, the shafts used in transfer cases are generally manufactured from solid forgings which are machined to form various gear segments, bearing and stop surfaces, as well as other features along the length of the shaft. Furthermore, traditional transfer cases include a multi-piece cast housing which includes at least two housing sections that are bolted together for enclosing and supporting the internal components. Because the housing sections are bolted together, each section requires a peripheral flange through which the bolts extend. In view of the recognized needs to reduce vehicle weight for improved fuel economy and to improve vehicle NVH characteristics, it is desirable to develop a light-weight transfer case providing improved NVH characteristics.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention is directed to a transfer case for use in a four-wheel drive vehicle having improved weight and NVH characteristics. These improvements are provided by transfer case having tubular shafts and a one-piece housing enclosed with end plates.  
         [0007]     To this end the transfer case of the present invention includes a one-piece housing defining first and second apertures and an opening, a first cover plate enclosing the first aperture of the housing and defining an opening, and a second cover plate enclosing the second aperture of said housing and defining an opening. The transfer case also includes an input shaft extending through and rotatably supported in the opening in the first cover plate, a first output shaft driven by the input shaft and extending through and rotatably supported in the opening in the housing, a second output shaft extending through and rotatably supported in the opening in said second cover plate, and a mode clutch for transferring drive torque from the first output shaft to the second output shaft. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]     Further objects, features and advantages of the present invention will become apparent to those skilled in the art from studying the following description and the accompanying drawings in which:  
         [0009]      FIG. 1  is a schematic representation of a drivetrain for a four-wheel drive vehicle equipped with a light-weight transfer case according to the present invention;  
         [0010]      FIG. 2  is a sectional view of the light-weight transfer case of the present invention;  
         [0011]      FIGS. 3A and 3B  are partial sectional views of two shafts comparing grain structure according to the principles of the present invention; and  
         [0012]      FIG. 4  is a sectional view of an alternative construction for the light-weight transfer case of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0013]     The present invention relates generally to light-weight transfer cases for use in four-wheel drive vehicles for providing drive torque and rotational motion to front and rear drivelines. In particular, the light-weight transfer case comprises components having reduced weight but which retain required strength and stiffness properties. These light-weight components include a one-piece housing and tubular front and rear output shafts. Additionally, as a result of the forming process used to manufacture the tubular output shafts, each can be tuned to reduce the noise, vibration, and harshness (NVH) characteristics of the transfer case.  
         [0014]     With reference to  FIG. 1 , an exemplary motor vehicle drivetrain  10  of a type suitable for use with the present invention is schematically shown. Drivetrain  10  has a pair of front wheels  12  and a pair of rear wheels  14  drivable from a source of power, such as an engine  16  through a transmission  18 . It is foreseen that transmission  18  may be either of the automatic or manual types commonly known in the art. In the particular embodiment shown, drivetrain  10  is a rear wheel drive system which incorporates a light-weight transfer case  20  that is operable to receive drive torque from transmission  18  for normally driving rear wheels  14  in a two-wheel drive mode of operation. Additionally, light-weight transfer case  20  is adapted to permit a vehicle operator to selectively transfer drive torque to front wheels  12  for defining a four-wheel drive mode of operation.  
         [0015]     Typically, front and rear wheels  12 ,  14  have a common rolling radius and are part of front and rear wheel assemblies  24 ,  26  which, in turn, are connected at opposite ends of front and rear wheel axle assemblies  28 ,  30 , respectively. A front differential  32  is mechanically coupled between front axle assembly  28  and a front prop shaft  36  such that front wheel assemblies  24  are driven by front prop shaft  36  when light-weight transfer case  20  is operating in the four-wheel drive mode. Similarly, rear axle assembly  30  includes a rear differential  34  coupled in driven relationship to a rear prop shaft  38  for driving rear wheel assemblies  26 . It is to be understood that the orientation of drivetrain  10  is merely exemplary in nature and that the drivetrain could be reversed for normally driving the front wheels  12  in the two wheel drive mode.  
         [0016]     With reference now to  FIGS. 1 and 2  rear prop shaft  38  is adapted to be connected to a rear output shaft  40  of light-weight transfer case  20  via a suitable rear coupling  42 . Similarly, front prop shaft  36  is adapted to be connected to a front output shaft  44  via a suitable front coupling  46 . A transmission output shaft (not shown) couples transmission  18  to an input shaft  48  of light-weight transfer case  20  for supplying power thereto. Transfer case  20  is shown to include a one-piece housing  50 . Housing  50  is preferably cast from aluminum or magnesium utilizing a lost foam casting process. Housing  50  includes a first aperture  52  and a second aperture  54 , each sized to permit assembly of various components into an internal chamber  56 . As described hereinbelow, input shaft  48  and rear output shaft  40  rotatably support various components within chamber  56  and are themselves rotatably supported at one end by housing  50  and at an opposite end by a first cover plate  58  which encloses first aperture  52  of housing  50 .  
         [0017]     First cover plate  58  includes a plate segment  58   a  interconnecting an inner annular hub  58   b  and an outer annular hub  58   c . Outer hub  58   c  of cover plate  58  is seated in first aperture  52  and includes a ring seal  60 . As seen, a stop face  62  of outer hub  58   c  abuts a radial shoulder  64  formed in first aperture  52 . First cover plate  58  is held in position with stop face  62  against shoulder  64  by a circlip  66 . A bearing assembly  68  is retained between inner hub  58   b  of first cover plate  58  and input shaft  48  to facilitate rotation of input shaft  48  relative to housing  50 . A seal assembly  70  provides a fluid-tight rotary seal between input shaft  48  and first cover plate  58 .  
         [0018]     Rear output shaft  40  is a tubular component aligned on the longitudinal axis of input shaft  48  and has a small diameter pilot segment  72  and a large diameter shaft segment  74 . Rear output shaft  40  is preferably made using a swaging process with a tubular member having the diameter of shaft segment  74  drawn or elongated at one end to form pilot segment  72  and a tapered transition segment  75  therebetween. Pilot segment  72  is rotatably supported by a bearing assembly  76  in an axial bore  78  of input shaft  48 . An end plate  79  encloses the terminal end of pilot segment  72 . A seal cap  80  provides a seal between bore  78  of input shaft  48  and an internal chamber  82  of rear output shaft  40 . Throughbores  83  in end plate  79  and pilot segment  72  permit hydraulic fluid in chamber  82  to lubricate various rotary components through which lubricant flows. Hydraulic fluid is supplied to chamber  82  from a shaft-driven pump  84  which draws fluid from a sump provided with chamber  56  of housing  50 .  
         [0019]     The axial position of pilot segment  72  of rear output shaft  40  is maintained relative to input shaft  48  via a thrust washer  86  which accommodates relative rotation therebetween. The opposite end of rear output shaft  40  is shown with end portion of shaft segment  74  extending through a first cylindrical opening  88  formed in housing  50  and rotatably supported therein by a bearing assembly  90 . A rotary seal assembly  92  is also shown to extend between shaft segment  74  of rear output shaft  40  and first opening  88 . Internal splines  93  are formed (i.e., rolled) in the open end of rear output shaft  40  and are adapted to receive an externally splined component of rear coupling  42 .  
         [0020]     With continued reference to  FIG. 2 , front output shaft  44  is shown to be a shaped tubular component having a first end segment  94 , a second end segment  96 , and a central sprocket segment  98 . First end segment  94  is cylindrical and is enclosed by an end wall  100 . First end segment  94  is shown to be retained in a boss segment  102  of housing  50  and rotatably supported therein via a bearing assembly  104 . Second end segment  96  is also cylindrical and is mounted by a bearing assembly  106  and a rotary seal assembly  108  to a second cover plate  110 . Second cover plate  110  encloses second aperture  54  of housing  50 . Second cover plate  110  includes a radial plate segment  110   a  and an annular hub segment  110   b . Plate segment  110   a  of second cover plate  110  is seated in second aperture  54  and includes a ring seal  112 . A stop face  114  on plate segment  110   a  abuts a shoulder surface  116  on housing  50  while a circlip  118  secures second cover plate  110  to housing  50 . Second end segment  96  of front output shaft  44  extends through an opening  120  in second cover plate  110  and is adapted for connection to front prop shaft  36  via coupling  46 . Specifically, second end segment  96  has internal splines  121  formed therein adapted to receive externally splined component of front coupling  46 .  
         [0021]     Input shaft  48  has an input sun gear  122  of a planetary gearset  124  formed integral therewith. Planetary gearset  124  is a speed reduction apparatus operable for defining high and low speed ratios relative to input shaft  48 . It will be understood that planetary gearset assembly  124  is merely exemplary of a suitable two speed gear apparatus for use in light-weight transfer case  20 . Sun gear  122  is shown meshed with a plurality of planet gears  126 . Each planet gear  126  is rotatably journalled on a pin  128  supported in a planetary carrier  130 . Planetary carrier  130  includes fore and aft ring members  132  and  134  secured together by bolts (not shown). Planet gears  126  also mesh with an annulus gear  136  that is non-rotatably mounted to housing  50 . Specifically, annulus gear  136  is retained against rotational movement by a plurality of radially extending tabs  138  which are received in corresponding longitudinal grooves formed in housing  50 . Annulus gear  136  is additionally retained against axial movement away from a stop shoulder  140  formed in housing  50  by retention lugs  142  formed on first cover plate  58   
         [0022]     Transfer case  20  also includes a range clutch  150  and a shift mechanism  152 . Range clutch  150  includes a range sleeve  154  supported via a spline connection  156  for rotation with rear output shaft  40  and axial movement thereon between three distinct positions. In the first position, denoted by a “H” position line, external clutch teeth  158  on range sleeve  154  are meshed with internal clutch teeth  160  formed on input shaft  48 , thereby establishing a direct or high-range drive connection between input shaft  48  and rear output shaft  40 . In a second position, denoted by a “L” position line, external clutch teeth  158  on range sleeve  154  are meshed with internal clutch teeth  162  formed on aft ring  134  of planetary carrier  130 , thereby establishing a reduced or low-range drive connection between input shaft  48  and rear output shaft  40 . Finally, in its third position, denoted by a “N” position line, a non-driven neutral mode is established with range sleeve  154  disconnected from both input shaft  48  and carrier  130  such that no drive torque is transferred from input shaft  48  to rear output shaft  40 . Spline connection  156  includes external splines  164  that are roll formed on an external surface  166  of shaft segment  74 .  
         [0023]     Shift mechanism  152  is operable for selectively moving range sleeve  154  between its three distinct positions. Shift mechanism  152  includes a range fork  170  journalled for axial movement on a shift rail  172  and having a C-shaped fork setment  174  retained in a peripheral groove  176  formed in range sleeve  154 . One end of shift rail  172  is retained in a closed cylindrical boss  178  formed in housing  50  while its opposite end is retained in a cylindrical bore  180  formed in housing  50 . An end cap  182  is shown to enclose bore  180 . A cam follower  184  secured to a tubular section  175  of range fork  170  is retained in the helical groove  186  of a cam  188  that is shown secured to drive shaft  190 . One end of drive shaft  190  is retained in a closed boss  192  formed in housing  50  and its opposite end extends out of a bore  194  also formed in housing  50 . The second end of drive shaft  190  is coupled to a geartrain of an electric motor assembly  196 .  
         [0024]     With continued reference to  FIG. 2 , a mode clutch  200  is provided to selectively shift light-weight transfer case  20  between a two-wheel drive mode and a four-wheel drive mode. Mode clutch  200  includes a hub member  202  that is splined to rear output shaft  40  and an axially moveable mode sleeve  204  shown in a central disengaged or two-wheel drive mode (2WD) position. Mode sleeve  204  is formed with internal spline teeth  206  which are in constant axial sliding engagement with external spline teeth  208  on hub member  202 . A mode fork  210  is coupled to mode sleeve  204  for permitting axial movement of mode sleeve  204  via selective actuation of shift mechanism  152 . A tubular section  211  of mode fork  210  is secured via pin  212  to rail  172  and is biased by a spring  214  such that a cam follower  216 , mounted to mode fork  210 , is biased against an outer surface  218  of cam  188 . Thus, mode sleeve  204  may be selectively shifted from the two-wheel drive mode (2WD) position shown to a four-wheel drive mode (4WD) position whereat internal spline teeth  206  drivingly engage external spline teeth  220  formed on a chain carrier  222 . Chain carrier  222  is journalled on shaft segment  74  of rear output shaft  40  and also includes a drive sprocket  224 . Drive sprocket  224  engages a chain  226 , shown in dashed lines, which is coupled to a driven sprocket  228 . Driven sprocket  220  is secured to or an integral portion of sprocket segment  98  of front output shaft  44 . It should also be noted that front output shaft  42  is formed from tubular material similarly to rear output shaft  40 , as discussed above. For example, an expandable mandrel tool may be inserted into a tubular work piece and expanded to form the shaped configuration of front output shaft  44 . As such, front output shaft  44  incorporates the weight and NVH advantages resulting from the tubular forming process.  
         [0025]     A mode selector  230  permits the vehicle operator to select any one of the available two-wheel and four-wheel high-range and low-range drive modes. A mode signal from mode selector  230  is sent to a controller  232  which sends the appropriate electric control signal to motor assembly  196  to control rotation of cam  188 . As will be understood, the contour of helical cam track  186  associated with range fork  170  and the contour of cam surface  218  associated with mode fork  210  acts to coordinate movement of range sleeve  154  and mode sleeve  204  to establish the various drive modes in response to the rotated position of cam  188 . As is well know, the mode clutch  200  can be replaced with a passive coupling (i.e., viscous coupling, geared traction unit, i-activated clutch, etc.) or an electronically controlled active coupling (i.e., power-operated transfer clutch) as known in the art.  
         [0026]     As a result of the various components which rear output shaft  40  must support and the rotatable interface between rear output shaft  40  and housing  44  and first cover plate  52 , rear output shaft  40  requires various diameter changes along its length. As such, a stepped segment  75  is formed between larger diameter segment  74  and smaller diameter segment  72 . Output shaft  40  is a formed tube which offers significant weight and strength advantages over traditional forged shafts. The tube can be formed through any of several forming processes known in the art. For example, hydro-forming or swaging could be used. Because the tube is formed, as opposed to turned, the metal&#39;s grain structure continuously flows along the entire length. As a result, the strength of output shaft  40  is maximized, while minimizing the amount of material required (i.e. in a cross-section a formed tube has thinner walls than an analogous turned bar shaft). With particular reference to  FIGS. 3A and 3B  an exemplary cross-section of each of a traditionally machined shaft  40 A and a formed shaft  40  are shown, respectively. Machined shaft  40 A and formed shaft  40  each include a step  75 A and  75 , respectively. In forming step  75 A of machined shaft  40 A, excess material is cut away. This material is represented by the shadowed section labeled “A”. As such, the grain structure of machined shaft  40  is discontinuous at step  75 . In contradistinction, the grain structure of formed shaft  40  is continuous through step  75 , resulting in improved strength. Additionally, the wall thickness ‘X 1 ’ of machined shaft  40 A is much thicker than the wall thickness ‘X 2 ’ of formed shaft  40 .  
         [0027]     Another significant advantage of formed shaft  40  is the unique ability to ‘tune’ it for particular excitation frequencies. Resonant frequencies through driveline components, including transfer case shafts, can result in significant NVH problems. To minimize these problems, shaft  40  can be formed to include additional steps or other features which effectively tune shaft  40  out of the excitation range. In comparison, traditional shafts require increased mass or additional dampers for curing these types of NVH problems. However, increased mass results in increased weight and dampers increase both cost and weight, as well as increasing packaging complexity within the transfer case. Obviously, the teachings relative to shaping a tubular rear output shaft  40  are also applicable to front shaft  44 .  
         [0028]     Referring now to  FIG. 4 , transfer case  20 ′ is shown to now be equipped with a modified rear output shaft, identified by reference numeral  40 ′. Rear output shaft  40 ′ is a two-piece assembly having a shaft segment  74 ′ and a pilot segment  76 ′. Pilot segment  76 ′ is secured (i.e., welded) to a forward end of shaft segment  74 ′. This arrangement of a two-piece shaft  40 ′ eliminates the need to perform a shaft forming operation. In addition, a modified front output shaft  44 ′ is shown installed in transfer case  20 ′. As shown, front output shaft  44 ′ has a tubular shaft segment  250  to which drive sprocket  228 ′ is secured (i.e., welded, splined, etc.) for common rotation. Tubular shaft segment  250  has a uniform wall thickness across its length such that bearing assemblies  104  and  106  are supported thereon. Radial plate segment  110 A′ of second cover plate  110 ′ has be slightly modified to accommodate retention of seal assembly  108  on shaft segment  250 .  
         [0029]     Finally, a cylindrical insert  254  is secured (i.e., welded) in the forward open end of shaft segment  250  and includes internal splines  256  adapted for meshed engagement with an externally-splined component of coupling  46 . Obviously, similar splined inserts can be used in conjunction with rear output shafts  40 ,  40 ′ as well. An end cap  256  is shown to enclose the rear end of shaft segment.  
         [0030]     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and the following claims.