Abstract:
A differential assembly having first, second and third structures, a differential gear set and a biasing mechanism. The first structure is configured to rotate along a differential axis in response to receipt of a rotational input. The second structure is supported for rotation on the differential axis. The third structure is supported for rotation on the differential axis and disposed between the first and second structures. The third structure can be operated in an engaged condition for transmitting torque from the first structure to the second structure and a disengaged condition for inhibiting the transmission of torque from the first structure to the second structure. The differential gear set is coupled to and rotatably supported within the second structure. The biasing mechanism biases the third structure in the disengaged condition. The third structure is placed in the engaged condition if a torsional magnitude of the rotational input exceeds by a predetermined amount a torsional magnitude of a rotational force exerted through the differential gear set. A vehicle drive train is also provided.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Technical Field  
           [0002]    The present invention generally relates to vehicle drivelines and more particularly to a differential assembly for a vehicle driveline that selectively transmits power to a set of vehicle wheels.  
           [0003]    2. Discussion  
           [0004]    Modernly, vehicle manufacturers are employing vehicle drivetrains having more than one drive axle to improve vehicle traction. Common arrangements include part-time four-wheel drive systems that employ a front axle disconnect to selectively disconnect the front wheels from the front of the vehicle drivetrain. These arrangements are commonly known as rear drive/front assist drivetrains. Disconnection of the front wheels from the front of the vehicle drivetrain prevents the front drive wheels from rotating the front of the vehicle drive train at road speed, thereby saving wear and tear on the vehicle driveline. The front axle disconnect also controls the coupling of the front wheels to the front of the vehicle driveline such that the front driveshaft will spin at the same speed as the rear driveshaft.  
           [0005]    Despite the relatively widespread use of such drivetrain arrangements, several drawbacks are known to exist, such as their cost and the amount of time that is sometimes necessary for the front axle disconnect to engage and disengage the front of the vehicle driveline to the front wheels. In isolating the front wheels from the rest of the front driveline, front axle disconnects typically use a sliding sleeve to connect or disconnect an axle shaft from the front differential side gear. Vehicle manufacturers typically use either vacuum or heat to move the engagement sleeve and as such, the time that is required to shift the sliding sleeve to a desired position can be relatively long, particularly when heat is employed to heat a fluid to generate sufficient pressure to cause the engagement sleeve to move.  
           [0006]    Accordingly, there remains a need in the art for a vehicle driveline that is less costly and which provides improved response in the time for the engagement and disengagement of the vehicle drivetrain to the vehicle wheels.  
         SUMMARY OF THE INVENTION  
         [0007]    In one preferred form, the present invention provides a differential assembly having first, second and third structures, a differential gear set and a biasing mechanism. The first structure is configured to rotate along a differential axis in response to receipt of a rotational input. The second structure is supported for rotation on the differential axis. The third structure is supported for rotation on the differential axis and disposed between the first and second structures. The third structure can be operated in an engaged condition for transmitting torque from the first structure to the second structure and a disengaged condition for inhibiting the transmission of torque from the first structure to the second structure. The differential gear set is coupled to and rotatably supported within the second structure. The biasing mechanism biases the third structure in the disengaged condition. The third structure is placed in the engaged condition if a torsional magnitude of the rotational input exceeds by a predetermined amount a torsional magnitude of a rotational force exerted through the differential gear set.  
           [0008]    In another preferred form, the present invention provides a vehicle drive train having a transfer case assembly and first and second axle assemblies. The transfer case assembly receives a rotational input from a vehicle power source and produces first and second intermediate rotational outputs therefrom. The first axle assembly is coupled to the transfer case assembly, receives the first intermediate rotational output therefrom and produces a first drive wheel output for rotating a first set of drive wheels. The second axle assembly has a differential assembly with a differential housing member configured to rotate about differential axis in response to receipt of the second intermediate rotational output, a differential case member supported for rotation on the differential axis, a cam member supported for rotation on the differential axis and disposed between the differential housing member and the differential case member and a differential gear set. The cam member can be operated in an engaged condition for transmitting torque from the differential housing member to the differential case member and a disengaged condition for inhibiting the transmission of torque from the differential housing member to the differential case member. The differential gear set is coupled to and rotatably supported within the differential case member. Operation of the cam member in the engaged condition permits the differential gear set to produce a second drive wheel output to rotate a second set of drive wheels. Operation of the cam member in the disengaged condition inhibits the differential from producing the second drive wheel output and permitting the second set of drive wheels to rotate freely. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein:  
         [0010]    [0010]FIG. 1 is a schematic view of the drivetrain of an exemplary motor vehicle constructed in accordance with the teachings of the present invention;  
         [0011]    [0011]FIG. 2 is an exploded perspective view of a portion of the drivetrain of FIG. 1 illustrating the rear axle assembly in greater detail;  
         [0012]    [0012]FIG. 3 is an exploded perspective view of a portion of the drivetrain of FIG. 1 illustrating the front axle assembly in greater detail; and  
         [0013]    [0013]FIG. 4 is an exploded perspective view of a portion of the front axle assembly of FIG. 3 illustrating the differential assembly in greater detail.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]    Referring now to FIG. 1 of the drawings, a drivetrain  10  for a part-time four-wheel drive vehicle  12  is schematically shown interactively associated with a differential assembly  14  constructed in accordance with the teachings of the present invention. The drivetrain  10  includes a rear driveline  20  and a front driveline  22  which are both drivable from a source of power, such as an engine  24 , through a transmission  26  which may be of either the manual or automatic type. In the particular embodiment shown, the drivetrain  10  is a rear drive/front assist system which incorporates a transfer case  28  for transmitting drive torque from the engine  24  and the transmission  26  to the rear and front drivelines  20  and  22 . The transfer case  28  is preferably a non-differentiating transfer case that causes the rear and front transfer case output shafts  30  and  32 , respectively to rotate at the same rotational speed.  
         [0015]    With additional reference to FIG. 2, the rear driveline  20  is conventional in its construction and operation and includes a pair of rear wheels  36  connected at the opposite ends of a rear axle assembly  38  having a rear differential assembly  40  coupled to one end of a rear prop shaft  42 , the opposite end of which is interconnected to a rear transfer case output shaft  30  of the transfer case  28 . The rear axle assembly  38  includes a rear axle housing  44 , a rear pinion shaft  46  and a pair of rear axle shafts  48  that are interconnected to a respective one of the left and right rear wheels  36 . The rear axle housing  44  has a wall member  50  that defines a differential cavity  52  into which the rear differential assembly  40  is rotatably supported. The rear pinion shaft  46  has a pinion gear  54  that is fixed thereto which drives a ring gear  56  that is fixed to a differential case  58  of the rear differential assembly  40 . A gearset  60  supported within the differential case  58  transfers rotary power from the differential case  58  to the rear axle shafts  48  to facilitate relative rotation (i.e., differentiation) therebetween. Thus, rotary power from the engine  24  is transmitted to the rear axle shafts  48  for driving the left and right rear wheels  36  via the transmission  26 , the transfer case  28 , the rear prop shaft  42 , the rear pinion shaft  46 , the differential case  58  and the gearset  60 .  
         [0016]    With reference to FIGS. 1 and 3, the front driveline  22  includes a pair of front wheels  66  connected at the opposite ends of a front axle assembly  68  having the differential assembly  14  coupled to one end of a front prop shaft  72 , the opposite end of which is interconnected to the front transfer case output shaft  32  of the transfer case  28 . The front axle assembly  68  includes a front axle housing  74 , a front pinion shaft  76 , the front differential assembly  14 , a pair of front axle shafts  78  that are interconnected to left and right front wheels  66 . The front axle housing  74  has a wall member  80  that defines a differential cavity  82  into which the front differential assembly  14  is supported for rotation about a differential axis  83 . The front pinion shaft  76  has a pinion gear  84  that is fixed thereto which drives a ring gear  86  that is fixed to a differential housing assembly  88  of the front differential assembly  14 .  
         [0017]    With reference to FIG. 4, the front differential assembly  14  is shown in greater detail to also include a cam member  90 , a differential case member  92 , a biasing mechanism  94 , a gearset  96  and a thrust washer  98 . The differential housing assembly  88  includes a first housing member  100  and a second housing member  102  that collectively define a differential cavity  104 . The first housing member  100  is generally hollow and includes a retaining flange  106 , an extending portion  108  and a first housing aperture  110 . The retaining flange  106  is operable for receiving a plurality of fasteners  114  to permit the first and second housing members  100  and  102  and the ring gear  86  to be fixedly but removably coupled together. The extending portion  108  is configured to at least partially extend into a second housing aperture  118  formed into the second housing member  102 . The extending portion  108  terminates at an abutting face  120  that is configured to abut an abutting face  122  formed in the cam member  90 . Each of the abutting faces  120  and  122  are illustrated to be formed by a plurality of peaks  124  and valleys  126 , the purpose of which will be discussed in greater detail, below.  
         [0018]    The cam member  90  is illustrated to have a generally hollow cylindrical configuration and is rotatably supported within the differential cavity  104  between the first housing member  100  and the differential case member  92 . The cam member  90  includes a cam portion  130  into which the abutting face  122  is formed, a collar portion  132 , a plurality of teeth  134  and an aperture  136  extending through the cam member  90  and formed along the longitudinal axis of the cam member  90 . Bushings or bearings (not specifically shown) may be mounted within the second housing member  102  in the second housing aperture  118  to support the cam member  90  for rotation within the differential cavity  104  about the differential axis  83 . Each of the plurality of teeth  134  formed into the cam member  90  are illustrated to have a generally square configuration that is configured to meshingly engage a plurality of teeth  140  formed in the differential case member  92  to permit rotary power to be transferred between the cam member  90  and the differential case member  92 . Those skilled in the art will understand, however, that the particular configuration of the teeth  134  and  140  which is illustrated is merely exemplary and not intended to be limiting in any manner. Accordingly, those skilled in the art will understand that the teeth  134  and  140  may have another configuration or that they may be omitted altogether if another means for transferring power between the cam member  90  and the differential case member  92 , such as one that utilizes friction between the mating surfaces of the cam member  90  and the differential case member  92 , is employed.  
         [0019]    The cam member  90  is operable in a disengaged condition and an engaged condition. When positioned in the disengaged condition, the peaks  124  and valleys  126  of the abutting face  120  of the first housing member  100  are positioned against the valleys  126  and peaks  124 , respectively, of the abutting face  122  of the cam member  90  and the teeth  134  formed in the cam member  90  are spaced apart from the teeth  140  formed into the differential case member  92 . As such, rotary power cannot be transmitted between the cam member  90  and the differential case member  92 . When positioned in the engaged condition, the peaks  124  and valleys  126  of the abutting face  120  of the first housing member  100  are positioned against the peaks  124  and valleys  126 , respectively, of the abutting face  122  of the cam member  90  and the teeth  134  formed in the cam member  90  are meshingly engaged with the teeth  140  formed into the differential case member  92 , thereby facilitating the transmission of rotary power therebetween.  
         [0020]    The differential case member  92  is also illustrated to have a generally hollow cylindrical configuration. In addition to the teeth  140  that are formed into an extending portion  144 , the differential case member  92  includes a flange member  146  and a pinion shaft aperture  148  which is positioned generally perpendicularly to the longitudinal axis of the differential case member  92 . As with the cam member  90 , bushings or bearings (not specifically shown) may be mounted within the second housing member  102  in the second housing aperture  118  to support the differential case member  92  for rotation within the differential cavity  104  about the differential axis  83 . The end of the differential case member  92  opposite the end having the teeth  140  terminates at a thrust flange  150  that is configured to contact the thrust washer  98 . The thrust washer  98  is disposed between the thrust flange  150  and an end portion  154  of the second housing member  102  being configured to reduce the friction between the thrust flange  150  and the end portion  154 .  
         [0021]    The gearset  96  is illustrated to include a pinion shaft  170 , a pair of pinions  172  and a pair of side gears  174 . The pinion shaft  170  extends through the pinion shaft aperture  148  and is fixedly coupled to the differential case member  92 . The pinion shaft  170  rotatably supports the pair of pinions  172 , each of which is meshingly engaged to the pair of side gears  174 . The front axle shafts  78  are coupled at a first end to an associated one of the side gears  174  and at an opposite end to an associated one of the left and right front wheels  66 .  
         [0022]    The biasing mechanism  94  is operable for maintaining the cam member  90  in the disengaged condition until a predetermined condition has occurred. In the particular embodiment illustrated, the biasing mechanism  94  is a compression spring  180  that encircles the teeth  134  and  140  of the cam member  90  and the differential case member  92 . The spring  180  is operable for generating a biasing force that is transmitted to the collar portion  132  and the flange member  146  to thereby axially space the cam member  90  and the differential case member  92  apart along the differential axis  83 .  
         [0023]    Rotary power from the engine  24  is transmitted to the differential assembly  14  via the transmission  26 , the transfer case  28 , the front prop shaft  72  and the pinion shaft  76 , causing the differential housing assembly  88  to rotate about the differential axis  83 . When the cam member  90  is in the disengaged condition, rotary power is not transmitted through the cam member  90  to the differential case member  92 , and as such, the rotary power is not transmitted to the front wheels  66  via the front axle shafts  78 . The front wheels  66 , however, are free to rotate at the road speed of the vehicle and as such, cause the front axle shafts  78 , the gearset  96  and the differential case member  92  to rotate about the differential axis  83 . When the cam member  90  is in the engaged condition, rotary power is transmitted through the cam member  90  to the differential case member  92 , and as such, the rotary power is transmitted to the front wheels  66  via the differential case member  92 , the gearset  96  and the front axle shafts  78 . In the particular example provided, the configuration of the gear set  96  provides the differential assembly  14  with a bias ratio of one (1) when the cam member  90  is in the engaged condition.  
         [0024]    In operation, drive torque produced by the engine  24  is transmitted via the transmission  26  and the transfer case  28  to the rear and front transfer case output shafts  30  and  32 . In normal operating conditions where the rear and front wheels  36  and  66  have good traction, the engine drive torque is substantially transmitted through the rear prop shaft  42  to the rear axle assembly  38  for driving the left and right rear wheels  36 . This distribution of the engine drive torque results from the biasing of the cam member  90  in the disengaged condition. As traction in the rear wheels  36  is sufficiently good, the driveline  10  is not able to transmit enough of the drive torque to the front prop shaft  72  to cause the first housing member  100  to rotate relative to the cam member  90 , and as such, the cam member  90  will remain in the disengaged condition and the front wheels  66  are permitted to spin freely.  
         [0025]    When the rear wheels  36  begin to slip in excess of a predetermined amount, however, the drive torque transmitted through the front prop shaft  72  will exceed the magnitude of the torque that is exerted through the gearset  96  by the rotation of the front wheels  66 , permitting the first housing member  100  to overcome the biasing force generated by the biasing mechanism  94  and rotate relative to the cam member  90  causing the cam member  90  to be positioned in the engaged condition.  
         [0026]    As such, engine drive torque is distributed to the front wheels  66  through the gearset  96 .  
         [0027]    Construction of the drivetrain  10  in this manner is highly advantageous in that the differential assembly  14  produces a relatively simple and inexpensive part-time four-wheel drive system that may be instantaneously actuated in response to wheel slip without the use of sensors or electronic control mechanisms.  
         [0028]    While the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the description of the appended claims.