Abstract:
This invention relates to an apparatus for driving a wheel of an automobile, and more particularly to a bearing unit for a wheel called a 3.5-generation hub unit in which a constant-velocity joint and hub unit are integrated into one unit. This apparatus is used for supporting a driven wheel that is supported by an independent-type suspension such that the driven wheel rotates freely with respect to the suspension, as well as for rotating and driving the driven wheel.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to an apparatus for driving a wheel of an automobile, and more particularly to a bearing unit for a wheel called a 3.5-generation hub unit in which a constant-velocity joint and hub unit are integrated into one unit. This apparatus is used for supporting a driven wheel that is supported by an independent-type suspension {front wheels for a FF car (front engine, front-wheel drive), rear wheels for a FR car (front engine, rear-wheel drive), rear wheels for a RR car (rear engine, rear-wheel drive) and all the wheels for a 4 WD car (4 wheel drive)} such that the driven wheel rotates freely with respect to suspension, as well as for rotating and driving the driven wheel.  
           [0003]    2. Description of the Related Art  
           [0004]    In order to support a wheel such that it can rotate freely with respect to the suspension, a wheel-support bearing unit, having an outer race and inner race that rotate freely by way of rolling members is used. Moreover, in the independent-type suspension, the wheel-support bearing unit, which supports the driven wheel, is combined with a constant-velocity joint, and it must smoothly transmit the rotation of the drive shaft to the driven wheel (maintaining constant velocity) regardless of relative displacement between the differential gear and the driven wheel or the steering angle applied to the driven wheel. FIG. 3 shows a typical apparatus for the driven wheel of an automobile in which the wheel-support bearing unit  1  is combined with the constant-velocity joint  2 .  
           [0005]    Here, the wheel-support bearing unit  1  comprises an outer race  3  on the radially inside of which a hub  4  and inner race  5  rotate freely by way of a plurality of rolling members  6 . Of these, the outer race  3  is fastened to the knuckle  8  (see FIG. 4) of the suspension by a first flange  7  that is formed around the outer surface of the outer race  3  such that it does not rotate during operation. In addition, there are rows of outer-ring raceways  9  formed around the inner peripheral surface of the outer race  3 , and the hub  4  and inner race  5  are supported on the radially inside of this outer race  3  such that they are concentric with the outer race  3 .  
           [0006]    On the outside end (the outside end in the width direction of the automobile when the bearing unit is installed in the automobile, the left end in FIGS.  1  to  4 ) around the outer peripheral surface of the hub  4 , there is a second flange  10  for supporting the wheel. Moreover, there is a first inner-ring raceway  11  formed around the outer peripheral surface in the middle of the hub  4 , and likewise, the inner race  5  fits around a small-diameter section  12  that is formed on the inside end (the inside end in the width direction of the automobile when the bearing unit is installed in the automobile, the right end in FIGS.  1  to  4 ), and a second inner-ring raceway  13  is formed around the outer peripheral surface of the inner race  5 . There is also a first spline hole  14  formed in the center of the hub  4 .  
           [0007]    On the other hand, the constant-velocity joint  2  comprises an outer ring  15  for the constant-velocity joint, an inner ring  16  for the constant-velocity joint, and a spline shaft  17 . Of these, the outer ring  15  for the constant-velocity joint and the spline shaft  17  form a drive shaft member  18 . In other words, this spline shaft  17  is formed on the outside end of this drive-shaft member  18 , and it freely fits in the first spline hole  14  described above, and the outer ring  15  for the constant-velocity joint is formed on the inside end of the drive-shaft member  18 . At a plurality of locations in the circumferential direction around the inner peripheral surface of this outer ring  15  for the constant-velocity joint, there are outside engaging grooves  19  which are formed such that they are each orthogonal with respect to the circumferential direction. Moreover, in the center of the inner ring  16  for the constant-velocity joint there is a second spline hole  20 , and there are inside engaging grooves  21  that are formed such that they are each orthogonal with respect to the circumferential direction and are located around the outer peripheral surface of the inner ring  16  such that they correspond with the location of the aforementioned outside engaging grooves  19 . There are balls  22  located between these inside engaging grooves  21  and outside engaging grooves  19 , and they are supported by a retainer  23  such that they can roll freely along the engaging grooves  21 ,  19 . The shape of each of the components of the aforementioned constant-velocity joint  2  are substantially the same as the well-known Rzeppa type constant-velocity joint, and is not essentially related to this invention, so a detailed description will be omitted here.  
           [0008]    In the case of a constant-velocity joint  2  and the wheel-support roller-bearing unit  1  as described above, the spline shaft  17  is inserted into the first spline hole  14  in the hub  4  toward the outside from the inside (from right to left in the figure). Also, by screwing a nut  25  onto the male screw section  24  formed on the outside end of the spline shaft  17  on the part that protrudes from the outside end of the hub  4 , and then tightening the nut  25 , the hub  4  and the spline shaft  17  are fastened together. In this condition, the surface on the inside end of the inner race  5  comes in contact with the surface on the outside end of the outer ring  15  for the constant-velocity joint, so the inner race  5  does not move in a direction that would cause it to come apart from the small-diameter step section  12 . At the same time, the rolling members  6  are properly pre-loaded.  
           [0009]    Furthermore, when installed in the suspension of an automobile, the male spline  27  that is formed on the outside end of the drive shaft  26  makes a spline fit with the second spline hole  20  that is formed in the center of the inner ring  16  for the constant-velocity joint. A stop ring  29  that fits in the installation groove  28  that is formed all the way around the outer peripheral surface on the outside end of the male spline  27  fits in an anchoring step section  30  that is formed around the edge on the opening on the outside end of the second spline hole  20 , and this prevents the male spline  27  from coming out from the second spline hole  20 . The output shaft section of the differential gear, not shown in the figure, is provided with the trunnion of a tripod-type constant-velocity joint, also not shown in the figure, and the inside end of the drive shaft  26  is connected to the center of the trunnion. As the automobile moves, the drive shaft  26  rotates at constant rpm, however, a thrust load is repeatedly applied in both axial directions due to the resistance of the tripod-type constant-velocity joint that occurs during rotation.  
           [0010]    In the wheel drive apparatus for an automobile described above and shown in FIG. 3, the wheel-support roller-bearing unit  1  and constant-velocity joint  2  are fastened together by screwing and tightening a nut  25  to the screw section  24 , so the weight of the unit is large. In other words, it is necessary to lengthen the splint shaft  17  by the amount of the male screw section that is formed on the spline shaft  17  on the outside of the constant-velocity joint  2 , as well as a nut  25  is necessary. Therefore, the dimension in the axial direction, as well as the weight, of the wheel drive apparatus for an automobile increases by the amount of the screw section  24  and nut  25 .  
           [0011]    In regards to this, as shown in FIG. 4, U.S. Pat. No. 4,881,842 discloses a more simple construction that makes it possible to fasten the wheel-support bearing unit and constant-velocity joint in a way such that the dimension in the axial direction is shortened and the weight is reduced. In this second example of prior construction shown in FIG. 4, the hub  4  is supported on the radially inside of the outer race  3 , which is fastened to the knuckle  8 , such that it can rotate freely by way of the rolling members  6  that are arranged in a plurality of rows. In addition, the spline shaft  17  of the drive-shaft member  18   a  fits in the first spline hole  14  that is formed in the center of the hub  4 . There is an installation section  31  formed on the surface of the outside end of this spline shaft  17  for attaching a fitting tool for taking the spline shaft  17  into the first spline hole  14 . Also, this spline shaft  17  is prevented from coming out of the hub  4  by a stop ring  33  that is fastened in an installation groove  32  formed around the outer peripheral surface on the tip end of the spline shaft  17 . In this state, an elastic ring  34  is elastically compressed between the hub  4  and the outer ring  15  for the constant-velocity joint of the drive-shaft member  18   a , to prevent the spline shaft  17  from rocking and moving inside the hub  4 . In this second example of prior construction, the wheel drive apparatus for an automobile is made more compact and lighter by using the stop ring  33  for connecting the wheel-support bearing unit  1   a  and the constant-velocity joint  2   a.    
           [0012]    In the case of the second example of prior construction described above, although the apparatus is made more compact and lighter, it is difficult to maintain adequate durability as is. The reason for this is explained below referring to FIGS. 3, 4.  
           [0013]    When the wheel-drive apparatus for an automobile is operating and the driving power (torque) is transmitted while the center axes of the outer ring  15  and the inner ring  16  for the constant-velocity joint of the Rzeppa-type constant-velocity joint  2  (FIG. 3),  2   a  (FIG. 4) are not concentric (angle of intersection is not 180 degrees), then the force that acts on the balls  22 , and the inside engaging grooves  21  and outside engaging grooves  19  of the constant-velocity joint  2  is no longer applied to the same horizontal surface orthogonal to the center axis. Therefore, a bending moment acts on the drive-shaft members  18  (FIG. 3),  18   a  (FIG. 4), and the drive shaft  26 . Furthermore, in the case of unevenness in the pitch of the outside and inside engaging grooves  19 ,  21  due to manufacturing error, a component of load also occurs in the radial load direction as a resultant force of contact loads on the outside engaging grooves  19  and inside engaging grooves  21  and the balls  22 . When a load component in the radial load direction occurs due to this kind of resultant force, the cross section of the drive-shaft members  18  (FIG. 3),  18   a  (FIG. 4) and the drive shaft  26  are not only equally loaded by a twisting torque and bending moment, but they are also loaded by a bending moment due to the aforementioned radial load that is multiplied with a distance in the axial direction from the balls  22  of the constant-velocity joint  2  (FIG. 3),  2   a  (FIG. 4).  
           [0014]    In the case of the drive shaft  26 , since the distance in the axial direction from the balls  22  of the constant-velocity joint  2  to the base end of the male spline  27 , which is the weakest part, is short, so that the load due to the bending moment of the aforementioned radial load is small. On the other hand, in the case of the drive-shaft members  18  (FIG. 3),  18   a  (FIG. 4), the distance in the axial direction from the balls  22  of the constant-velocity joint  2  to the base end (inside end) (of the spline shaft  17 , which is the weakest part, is longer than the distance to the weakest point of the drive shaft  26 . Therefore, the bending moment due to the aforementioned radial load becomes larger than the bending moment at the weakest part of the drive shaft  26 . In addition, the effect of the load that acts between the outside and inside engaging grooves  19 ,  21  and the retainer  23  can also be taken into consideration, however, it is considered that the difference between the drive-shaft members  18  (FIG. 3),  18   a  (FIG. 4) and the weakest part of the drive shaft  26  in fatigue strength, is outstandingly affected by the bending moment due to the aforementioned radial load.  
           [0015]    The inventors of this invention performed tests on a driven wheel apparatus for an automobile in which the wheel-support bearing units  1  (FIG. 3),  1   a  (FIG. 4), are combined with the constant-velocity joints  2  (FIG. 3),  2   a  (FIG. 4) to investigate which part received the most damage. In the test, various joint angles (supplementary angle of the intersecting axis angle) were applied to the constant-velocity joints  2  (FIG. 3),  2   a  (FIG. 4), and a ±1470 N·m torque was repeatedly applied to the drive shaft  26 . With the spline fitting area  35  on the hub side, where the first spline hole  14  fits with the spline shaft  17 , and with the spline fitting area  36  on the joint side, where the second spline hole  20  fits with the male spline  27 , the small spline diameter (male bottom diameter, female peak diameter) is 24 mm, and they form a module  1 . The results of the test are shown in Table 1.  
                                                         TABLE 1                                   Hub and constant-       Cycles               velocity   Joint   until failure           joint connection   angle   (x 10 4 )   Failure location                                        Stop ring       19.1   Base of spline                       shaft 17                   29.8   Base of spline                       shaft 17               0°   30.1   Base of spline                       shaft 17                   30.3   Base of spline                       shaft 17                   28.3   Base of spline                       shaft 17               15°   8.0   Base of spline                       shaft 17                   11.9   Base of spline                       shaft 17           Nut Screwing       40.8   Spider of the                       tripod-type cons-                       tant-velocity joint                   38.4   Base of the male                       spline 27               0°   52.3   Base of the male                       spline 27                   47.1   Base of spline                       shaft 17                   51.3   Base of spline                       shaft 17                   45.8   Base of spline                       shaft 17               6°   53.4   Base of the male                       spline 27                      
 
           [0016]    As can be clearly seen from the test results, in the case of using a nut  25 , as shown in FIG. 3, to fasten the wheel-support bearing unit  1  with the constant-velocity joint  2 , the spline fitting section  35  on the hub side and the spline fitting section  36  on the constant-velocity joint side are damaged at the same rate, and the number of cycles until failure was large (long life). Also, there was a case where the tripod-type constant-velocity joint was damaged before either of the spline fitting sections. On the other hand, when a stop ring  33 , as shown in FIG. 4, is used to fasten the wheel-support bearing unit  1   a  with the constant-velocity joint  2   a,  the spline fitting section  35  on the hub side was damaged in all cases, and the number of cycles until failure was small (short life).  
           [0017]    The reason that the life of the spline fitting section  35  on the hub side is short due to the difference in the connection of the wheel-support bearing units  1 ,  1   a  and constant-velocity joints  2  (FIG. 3),  2   a  (FIG. 4) is as follows. First, in the case of construction using a nut  25  as shown in FIG. 3, the surface on the outside end of the hub  4  and the inner surface of the washer  37  attached to the nut  25 , and the surface on the inside end of the inner race  5  and the surface on the outside end of the outer ring  15  for the constant-velocity joint come in strong direct contact (friction engagement) with each other. As a result, the substantial part of the twisting torque or bending moment are supported by the respective contact surfaces, so that the twisting torque and bending moment applied to the base of the spline shaft  17  are reduced by that amount and the durability of this base is improved.  
           [0018]    On the other hand, in the case of construction using a stop ring  33 , as shown in FIG. 4, there is no strong contact (friction engagement) between the inside surface of the stop ring  33  and the surface on the outside end of the hub  4 , and the surface on the inside end of the inner race  5  and the surface on the outside end of the outer ring  15  for the constant-velocity joint. The contact force is limited (small) by the elastic force of the elastic ring  34 , so that the force of the friction engagement on the contact surface is small, and the amount that this portion supports the twisting torque and the bending moment is very limited. Therefore, most of the twisting torque and bending moment act on the base of the spline shaft  17 . The cross-section of each surface is loaded by an uneven load, and when considering the size of the bending moment due to the aforementioned radial load, the base of the spline shaft  17  is loaded by a bending moment that is larger than the bending moment acting on the base of the male spline  27 , that is formed on the outside end of the drive shaft  26 , so when the shaft diameters are the same, the base of the spline shaft  17  is damaged first.  
         SUMMARY OF THE INVENTION  
         [0019]    Taking the above problems into consideration, an object of this invention is to provide a wheel-drive apparatus for automobile that makes it possible to effectively maintain adequate durability even when the wheel-support bearing unit and constant-velocity joint are connected by a stop ring. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a cross sectional view showing a first example of the embodiment of the present invention.  
         [0021]    [0021]FIG. 2 is a cross sectional view showing a second example of the embodiment of the present invention.  
         [0022]    [0022]FIG. 3 is a cross sectional view showing a first example of the prior art structures.  
         [0023]    [0023]FIG. 4 is a cross sectional view showing a second example of the prior art structures. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Accordingly, the wheel-drive apparatus for automobile of this invention comprises: an outer race, hub, a plurality of rolling members, a drive-shaft member, stop ring, outside engaging grooves, inner-ring for constant-velocity joint, a plurality of balls for constant-velocity joint, and a drive shaft.  
         [0025]    Of these, the outer race does not rotate during operation, and comprises: a first flange around its outer peripheral surface for fastening to the suspension apparatus, and a plurality of outer-ring raceways around its inner peripheral surface.  
         [0026]    Moreover, the hub comprises a second flange around its outer peripheral surface near the outside end for supporting the wheel, a first inner-ring raceway that is formed in the middle of the hub directly or on a separate inner ring, a first spline hole that is formed in the center of the hub, and an inner ring, that has a second inner-ring raceway formed around its outer peripheral surface, fits around and is fastened to the outer peripheral surface of the hub on the inside end.  
         [0027]    Furthermore, a plurality of rolling members are located between each of the outer-ring raceways and the first and second inner-ring raceways, such that they rotate freely.  
         [0028]    The drive-shaft member comprises a spline shaft on its outside end that fits in the first spline hole, and its inside end functions as the outer ring for the constant-velocity joint.  
         [0029]    The stop ring is located between and engaged with the inner fitting section, that is formed all the way around the outer peripheral surface on the outside end of the spline shaft, and the outer fitting section, that is formed around the inner peripheral surface of the hub in the section that faces the inner fitting section, and this stop ring prevents the spline shaft from coming out of the spline hole.  
         [0030]    The outside engaging grooves are located at a plurality of locations in the circumferential direction around the inner surface of the outer ring for the constant-velocity joint, and they are each formed such that they arc orthogonal with respect to the circumferential direction.  
         [0031]    The inner-ring for the constant-velocity joint is located on the radially inside of the outer ring for the constant-velocity joint, and there is a second spline hole formed through its center, and on its outer peripheral surface, there are inside engaging grooves that are formed such that they correspond with the outside engaging grooves and are orthogonal with respect to the circumferential direction.  
         [0032]    The balls are located between the inside engaging grooves and outside engaging grooves such that they can roll freely along the grooves.  
         [0033]    The drive shaft comprises a male spline section that is formed on it outside end, and the male spline makes a spline fit inside the second spline hole.  
         [0034]    Furthermore, with the wheel-drive apparatus for an automobile of this invention, the minimum outer diameter of the base end of the spline shaft is larger than the diameter of the groove bottom in the male spline formed on the outside end of the drive shaft.  
         [0035]    With the wheel-drive apparatus for an automobile of this invention, constructed as described above, it is possible to maintain the strength of the spline shaft of the drive-shaft member during operation even when the wheel-support bearing unit and constant-velocity joint are connected with a stop ring, and it is possible to maintain adequate durability regardless of the twisting moment and bending moment that are repeatedly applied during operation.  
         [0036]    [0036]FIG. 1 shows a first embodiment of the invention. This invention is characterized by construction that makes it possible to effectively maintain adequate durability even when the wheel-support bearing unit  1   b  and the constant-velocity joint  2   a  are fastened together by a stop ring  33   a.  The construction and function of other parts are nearly the same as those of the prior construction shown in FIGS. 3 and 4, so the like code numbers are given for identical parts and any redundant explanation is omitted or simplified. The following explanation centers on the features of this invention and parts that are different from the prior construction.  
         [0037]    With this invention, in order to prevent the inner race  5  that fits over the small-diameter step section  12  formed on the inside end of the hub  4   a  from coming apart from this small-diameter step section  12 , there is a crimped section  38  on the inside end of the hub  4   a.  In other words, there is a cylindrical section formed on the inside end of the hub  4   a,  and after the inner race  5  is fitted over the small-diameter section  12 , the cylindrical section is plastically deformed outward in the radial direction to form the crimped section  38 , and the surface on the inside end of the inner race  5  is held by this crimped section.  
         [0038]    An engaging groove  32 , that is the inside engaging groove mentioned above, is formed all the way around the end section (outside end) of the spline shaft  17   a  of the drive-shaft member  18   b.  When the spline shaft  17   a  is inserted in the first spline hole that is formed through the center of the hub  4   a,  the outer-diameter half of the stop ring  33   a,  whose inner-diameter half is fastened to the engaging groove  32 , fits with the fitting step section  39  that is formed around the inner peripheral surface on the outside end of the hub  4   a,  so that the spline shaft  17   a  is prevented from coming out of the first spline hole  14 . The fitting step section  39  corresponds to the outer-diameter fitting section previously mentioned. There is no spline formed in the center hole of the hub  4   a  at the section from the fitting step section  39  to the opening on the outside end, and it is a simple cylindrical surface.  
         [0039]    The stop ring  33   a  is attached to the engaging groove  32  before the spline shaft  17   a  is inserted into the first spline hole  14 . When inserting the spline shaft  17   a  into the first spline hole  14 , the diameter of the stop ring  33   a  is elastically compressed as it passes through the first spline hole  14 . After the stop ring  33   a  has passed through the spline hole  14 , it is elastically restored to its original shape, and as described above, fits between the engaging groove  32  and the fitting step section  39 . In order to do this, the diameter of the groove bottom in the engaging groove  32  is made sufficiently small.  
         [0040]    Moreover, the opening on the outside end of the center hole in the hub  4   a  is covered by a cap  40 . On the other hand, the seal ring  41 , that is located between the inside surface of the crimped section  38  and the surface of the outside end of the outer ring  15  for the constant-velocity joint, is elastically compressed and then installed between these surfaces, and it covers the space between the crimped section and the outer ring  15  for the constant-velocity joint. The cap  40  and seal ring  41  prevent foreign matter, such as rain water, from getting inside the spline fitting section  35  on the hub side between the spline  17   a  and the first spline hole  14 , and prevent this spline fitting section  35  on the hub side from rusting.  
         [0041]    In this way, when the spline shaft  17   a  is prevented from coming out of the first spline hole  14  by the stop ring  33   a,  and with the seal ring  41  attached between the inside surface of the crimped section  38  and the surface on the outside end of the outer ring  15  for the constant-velocity joint, there exists a space between the inside surface of the crimped section  38  and a step section  42  that is formed on the outer surface in the middle of the drive-shaft member  18   b.  In this invention, the dimension of this space in the thrust direction is kept to 0.8 mm or less. The reason for this is, to prevent wear of the spline fitting section  35  on the hub side regardless of displacement due to the thrust load applied during operation, and to prevent the seal ring  41  from being over compressed even when the hub  4   a  and the drive shaft member  18   b  come close together to the point where the inside surface of the crimped section  38  comes in contact with the step section  42  due to the thrust load. The reason the step section  42  on the drive-shaft member is tapered is to make this step section  42  inclined to correspond with the radially inner section of the crimped section  38 , in order to make it possible in a small cross-sectional height to install the seal ring  41 , and to set the step section  42  for controlling the movement in the axial direction.  
         [0042]    Furthermore, with the wheel-drive apparatus for an automobile of this invention, the minimum outer diameter of the base (inside end) of the spline shaft  17   a  is larger than the groove diameter of the male spline  27  that is formed on the outside end of the drive shaft  26 . In other words, at the base of the spline shaft  17   a,  there are two cylindrical portions, one of which is a neck or constricted section  43  with a minimum outer diameter slightly smaller than the groove diameter of the male spline in order to provide relief when forming the male spline on the spline shaft  17   a  by a broaching process. The outer diameter D 43  of this neck section  43  is larger than the groove diameter D 27  of the male spline  27  by a range of 1 to 3 mm, or preferably, 2 mm (D 43 =D 27 +2 mm). This dimension is for a typical wheel-drive apparatus for an automobile,  
         [0043]    With the wheel-drive apparatus for an automobile of this invention. constructed as described above, it is possible to maintain the strength of the splint shaft  17   a  of the drive-shaft member  18   b  during operation, even in the structure where the wheel-support bearing unit  1   b  and the constant-velocity joint  2   a  are fastened together by a stop ring  33   a  so that the construction of the apparatus is made more compact and lightweight. In other words, by the amount that the outer diameter of the weakest section or neck section  43 , that is formed on the base end of the spline shaft  17   a,  is increased, it is made possible to prevent the strength of the neck section  43  from becoming severely less than the strength of the outside end of the drive shaft  26 . Therefore, the durability of part of the components of the wheel-drive apparatus for an automobile of this invention is prevented from becoming severely less than the durability of other parts, and thus it is possible to maintain the durability of the entire wheel-drive apparatus for an automobile regardless of the bending moment that is repeatedly applied during operation. The spline shaft  17   a  can be processed by rolling. In that case, there is no need for relief, so it is possible to make the dimension of the diameter at the base end such that it is the average value of the spline groove diameter and outer diameter, however, this kind of shape has a high concentrated stress coefficient regardless of the outer diameter being increased, and it has been experimentally confirmed that the strength decreases. So, even if the spline shaft  17   a  is processed by rolling, it is preferable to form a neck section. In that case, the outer diameter D 43  of the neck section  43  should be larger than the groove diameter D 27  of the male spline  27  to equalize the each strength also.  
         [0044]    Next, FIG. 2 shows a second embodiment to the invention. In this embodiment, an engaging groove  44  is formed all the way around the base end of the spline shaft  17   b.  The space between the outer peripheral surface on the base end of the spline shaft  17   b  and the inner peripheral surface on the inside end of the hub  4   a  is sealed by an O-ring  45  that fits in the engaging groove  44 . The O-ring  45  corresponds to the seal ring previously mentioned. Moreover, the seal ring  41  (see FIG. 1), as, described in the first embodiment, is omitted. In this embodiment, the groove diameter D 44  of the engaging groove  44  is larger than the groove diameter D 27  of the male spline  27  that is formed on the outside end of the drive shaft  26  by 1 to 3 mm, and preferably 2 mm (D 44 ≈D 27 +2 mm). In addition, in this embodiment, the spline diameter D 17b  of the spline shaft  17   b  is larger than the diameter D 27  of the male spline  27  by 4 mm (D 17b ≈D 27 +4 mm). In order that it is possible to sufficiently maintain the diameter D 44  of the groove bottom in this engaging groove  44  when this engaging groove  44  is formed, a relatively thin ring with cross-sectional diameter of about 3 mm in uncompressed state is used as the O-ring  45 .  
         [0045]    Incidentally, in this example, there are four cylindrical portions at the base (from portion  43  to portion  44 ) of the spline shaft  17   b,  and the minimum diameter in the cylindrical portions is the minimum outer diameter of the base.  
         [0046]    With the construction of this embodiment, in addition to the function and effects that are the same as the first embodiment, the following function and effects can be obtained. In this embodiment, by the fact that the seal construction, for preventing foreign matter from getting into the spline fitting section  35  on the hub side, is located in the radial direction, it is possible to reduce the distance L between the center of displacement of constant-velocity joint  2   a  and the installation surface of the second flange  10  for supporting the wheel, more than when the construction is in the thrust direction as described for the first embodiment. Therefore, it is possible to reduce the force (steering moment) required for moving the hub  4   a , that applies the steering angle, back and forth around the center of displacement. This contributes to reducing the overall weight of the automobile by making it possible to make the power-steering apparatus more compact. In the case of this embodiment, the inside surface of the crimped section  38  that is formed on the inside end of the hub  4   a  directly faces the surface on the outside end of the outer ring  15  for the constant-velocity joint, however, the dimensions and shapes of all the parts are precisely regulated such that the space existing between these surfaces is within 0.5 mm, in order to control the movement in the axial direction of the spline fitting section  35  on the hub side.  
         [0047]    The wheel-drive apparatus for an automobile of this invention functions as described above, so it is possible to maintain sufficient durability even when the wheel-support bearing unit is fastened to the constant-velocity joint by a stop ring, and the apparatus is made more compact and lightweight.