Abstract:
The outer diameter of a joint outer ring is reduced using simple means, so that a constant velocity universal joint that can easily be reduced in weight and size is provided. It is an object to improve the unnerving vibrations, muffled noises, and the like. The constant velocity universal joint includes a joint outer ring having a plurality of track grooves formed in the inner circumference, a joint inner ring provided with track grooves corresponding to the track grooves of the joint outer ring, a plurality of balls provided in the ball tracks interposed between the joint outer ring and the joint inner ring and formed by cooperation of the track grooves to transmit torque, and a cage having pockets for retaining the balls. In the constant velocity universal joint having a plurality of vehicle body attachment flanges provided apart from each other in the outer circumferential direction of the joint outer ring and partly protruding in the radial direction. The joint outer ring has a flower outer circumferential shape corresponding to the inner circumferential shape, and the vehicle body attachment flanges are provided at outer recesses positioned between the track grooves of the joint outer ring. In this joint, the number of the balls is six, and the pitches of the ball tracks are random, unequal and not less than 55°.

Description:
BACKGROUND ART  
       [0001]     The present invention relates to sliding type constant velocity universal joints for use in power transmission mechanisms, for example, in automobiles and various kinds of industrial machines that allow axial displacement and angular displacement between two axes on the driving side and the driven side.  
         [0002]     A power transmission mechanism that transmits power from the engine of an automobile to a drive wheel must respond to angular displacement and axial displacement based on changes in the relative positional relation between the engine and the wheel. Therefore, for example as shown in  FIG. 21 , an intermediate shaft  1  is interposed between the engine side and the drive wheel side, one end of the intermediate shaft  1  is coupled to a differential  3  through a sliding type constant velocity universal joint  2 , and the other end thereof is coupled to the drive wheel  6  through a fixed type constant velocity universal joint  4  and a wheel bearing  5 .  
         [0003]     In the sliding type constant velocity universal joint  2  described above, not only angular displacement but also axial displacement is absorbed by so-called plunging, while in the fixed type constant velocity universal joint  4 , only the angular displacement can be absorbed. The sliding type constant velocity universal joint  2 , the fixed type constant velocity universal joint  4 , and the intermediate shaft  1  constitute a drive shaft  7  as a unit, and as the drive shaft  7  is mounted in the vehicle body, the constant velocity universal joints  2  and  4  are set at prescribed operation angles. The operation angles of the constant velocity universal joints  2  and  4  sequentially change, and therefore, in general, among these joints  2  and  4 , the fixed type constant velocity universal joint  4  is used on the outboard side and the sliding type constant velocity universal joint  2  is used on the inboard side to respond to the changing operation angles.  
         [0004]     A double offset type constant velocity universal joint (DOJ) is well known as the sliding type constant velocity universal joint  2 . As shown in  FIGS. 22   a  and  22   b , the constant velocity universal joint includes, as essential elements, a joint outer ring  8  attached to a differential  3  on the vehicle body side, a joint inner ring  9  attached to one end of the intermediate shaft  1 , a plurality of balls  10  incorporated between the joint outer ring  8  and the joint inner ring  9 , and a cage  11  interposed between the joint outer ring  8  and the joint inner ring  9  to support the balls  10 . Note that a lid  16  to cover the opening is provided at the end of the joint outer ring  8  on the differential side.  
         [0005]     The joint outer ring  8  is in the shape of a cup having a plurality of linear track grooves  12  parallel to its axial line and in its inner circumference at equal intervals in its circumferential direction. A plurality of linear track grooves  13  parallel to its axial line and corresponding to the track grooves  12  are provided in the outer circumference of the joint inner ring  9 . The track grooves  12  and  13  in the joint outer ring  8  and the joint inner ring  9  cooperate with each other to define ball tracks in which the balls  10  transmitting torque are provided. The balls  10  are supported in the cage  11  interposed between the joint outer ring  8  and the joint inner ring  9 . In the constant velocity universal joint, when an operation angle is set between the joint outer ring  8  and the joint inner ring  9 , the cage  11  controls the balls  10  to be on the bisector plane of the operation angle so that the constant velocity is maintained.  
         [0006]     Various types of rings may be used for the joint outer ring  8  in the constant velocity universal joint  2  depending on how the joint is attached to the vehicle body, and the one shown in  FIGS. 22   a  and  22   b  is of flange type. The flange type joint outer ring  8  has protruding vehicle body attachment flanges  14  integrally formed at equal intervals in the circumferential direction at the outer circumferential end, and is attached to the differential  3  (see  FIG. 21 ) by fastening bolts using the bolt holes  15  formed through the flanges  14 . In the field of constant velocity universal joints, products having a joint outer ring  8  with a flower outer circumferential shape formed corresponding to the inner circumferential shape have been used in order to meet recent demands for lightweight and compact products (see for example, Japanese Patent Laid-Open Application No. Hei 5-231436).  
         [0007]     The constant velocity universal joint having the flange type joint outer ring  8  has the plurality of vehicle body attachment flanges  14  protruding radially outwardly at the outer circumference of the joint outer ring  8  as described above, and the bolts are inserted through the bolt holes  15  in the vehicle body attachment flanges  14  for attachment to the differential on the vehicle body side.  
         [0008]     As shown in  FIGS. 23   a  and  23   b , when the bolts are fastened to attach the joint outer ring  8 , a fastening tool (socket  18  as shown) is used, and therefore there should be a space a from the outer circumference of the joint outer ring  8  for inserting the tool. Therefore, in consideration of the attaching process using the fastening tool, the necessity of providing the space a from the outer side of the joint outer ring  8  and the bolt holes  15  in the flanges  14  causes the outer diameter size of the vehicle body attachment flanges  14  to increase, which increases the weight of the constant velocity universal joint.  
         [0009]     In the constant velocity universal joint, the number of balls  10  is typically six or eight, and the balls  10  are normally arranged in the circumferential direction at six equal pitch intervals (60°) or eight equal pitch intervals (45°). In this constant velocity universal joint, as shown in  FIG. 23   b , the balls  10  are provided at equal pitch intervals of 60°. If the number of the balls is not six or eight, the balls are arranged at equal pitch intervals in the circumferential direction.  
         [0010]     In the constant velocity universal joint of this kind, when the torque is loaded and rotation is carried out, in other words, when power is transmitted, thrust force is induced in the axial direction of the constant velocity universal joint (induced thrust force), and the induced thrust force changes as many times as the number of the track grooves in one rotation. In the conventional constant velocity universal joint, the track grooves are arranged at equal intervals of 60°, and therefore the number of vibration frequency is six, which sometimes causes unnerving vibrations or muffled noises in resonance with the natural vibration frequency of the underbody of the vehicle.  
       DISCLOSURE OF THE INVENTION  
       [0011]     An object of the present invention is to provide a constant velocity universal joint that can readily achieve reduction of the weight and size thereof by reducing the outer diameter of the joint outer ring using simple means.  
         [0012]     The invention is directed to a constant velocity universal joint including an outer member provided with a plurality of track grooves formed in an inner circumference thereof, an inner member provided with track grooves corresponding to the track grooves of the outer member in an outer circumference, a plurality of balls provided in ball tracks defined by cooperation of the track grooves between the outer member and the inner member to transmit torque, and a cage having pockets for retaining the balls. The constant velocity universal joint has a plurality of vehicle body attachment flanges provided apart in a circumferential direction at an outer end of said outer member so as to outwardly protrude partially. In the universal joint, the outer circumferential shape of the outer member is in a flower shape corresponding to the inner circumferential shape, and the vehicle body attachment flanges are formed at outer circumferential recesses positioned between the track grooves of the outer member.  
         [0013]     According to the present invention, since the outer circumferential shape of the outer member is formed in a flower shape corresponding to the inner circumferential shape, the weight thereof can be reduced while the load capacity of the constant velocity universal joint is maintained in the present level. In addition, the vehicle body attachment flanges provided at the outer circumferential recesses between the track grooves in the outer member in the flower shape allows the outer diameter size of the vehicle body attachment flanges to be reduced, and therefore the constant velocity universal joint can be more compact. Therefore the weight reduction and compactness of the constant velocity universal joint can improve the performance of the constant velocity universal joint and expand the applicable field thereof.  
         [0014]     Regarding the outer circumferential shape of the outer member according to the invention, the ratio DN/DT of the outermost diameter size DT where the track grooves are positioned and the innermost diameter size DN where the vehicle body attachment flanges are located is desirably set in the range of from 0.85 to 0.95. The ratio of the outermost diameter size and the innermost diameter size is defined in the above-described range, so that the weight and size can be reduced as described above and the strength of the outer member can be secured.  
         [0015]     Relative to the number of the track grooves of the outer member described above, an arbitrary number of the vehicle body attachment flanges can be provided. In other words, instead of providing vehicle body attachment flanges in all the outer circumferential recesses positioned between the track grooves of the outer member, vehicle body attachment flanges may be provided only in part of the outer circumferential recesses.  
         [0016]     The present invention is applicable to a constant velocity universal joint having eight balls incorporated. With the eight balls, the ball PCD can be reduced as compared to a constant velocity universal joint with six balls and the size can effectively be reduced.  
         [0017]     Another object of the invention is to attempt to improve countermeasure against the unnerving vibrations, muffled noises and the like.  
         [0018]     According to the invention, a constant velocity universal joint includes an outer member having a plurality of axially extending track grooves formed in a cylindrical inner circumferential surface thereof, an inner member having a plurality of axially extending track grooves in a spherical outer circumferential surface thereof, balls each incorporated in a ball track formed by a pair of the track groove of the outer member and the track groove of the inner member, and a cage having pockets for holding the balls. The center of the outer spherical surface of the cage and the center of the inner spherical surface are offset from each other by an equal distance axially in the opposite directions from the cage center. The number of the balls is six, and the pitches of the ball tracks are random unequal pitches that are at least 55°. In the DOJ type, sliding type constant velocity universal joint, the track grooves of the outer member and the track grooves of the inner member are arranged with unequal pitches in the circumferential direction, so that in the DOJ with six balls, for example, the 6th order induced thrust force can be reduced, and vibrations and muffled noises in the vehicle can be prevented.  
         [0019]     In the DOJ with six torque transmission balls, in order to reduce the 6th order induced thrust force described above, the track grooves of the outer member and the track grooves of the inner member may be arranged with unequal pitches in the circumferential direction (see Japanese Patent Laid-Open Publication No. Hei 1-50767), but simply providing the tracks with unequal pitches might prevent other important requirements (such as strength and durability) for the constant velocity universal joint from being satisfied. The pitch between ball tracks that can satisfy the strength, durability, and NVH characteristics of a constant velocity universal joint should be at least 55°. In this case, the positions of the pockets of the cage should be in phase with the pitches of the track grooves of the outer member and the track grooves of the inner member. Note that this applies to products with the maximum operation angle in the range of from 20 to 25°, and the upper limit for the ball track pitch is 55° in order to secure the inter-pocket column width W 1  of the cage and the inter-track spherical surface width W 2  of the inner member. If the ball track pitch is less than 55°, the inter-pocket column width W 1  of the cage ( FIG. 9   a ) and the spherical surface width W 2  of the inner member ( FIG. 8   a ) are too small, and sufficient strength for a constant velocity universal joint cannot be provided.  
         [0020]     The invention is characterized in that, in the constant velocity universal joint, the ball track pitch is a random unequal pitch within the range of 60°±3°. Since the pitches of the track grooves of the outer member and the track grooves of the inner member are set to 60°±3°, the pockets of the cage can have an equal window length and an equal pitch (60°). Note that this applies to constant velocity universal joints with the maximum operation angle in the range of from 20 to 25°. The ball track pitch is limited to the range of 60°±3° in order to secure the inter-pocket column width W 3  ( FIG. 12   a ) necessary for securing the strength of the cage.  
         [0021]     The invention is characterized in that, in the constant velocity universal joint, the pockets are provided with equal pitch in the circumferential direction and the window lengths are equal to each other. In this case, the window length L 2  of the pocket is set in consideration of deviations between track pitches (60°±3°) and the circumferential movement of the ball based on the maximum operation angle of the constant velocity universal joint. When the pockets of the cage have an equal window length, and can be set at equal pitch intervals, the constant velocity universal joint can be assembled significantly easily. More specifically, the outer member and the inner member need only be in phase.  
         [0022]     The invention is characterized in that, in the constant velocity universal joint, in a section including the axial line of the joint, the inner spherical surface of the cage has the center of curvature in a location radially shifted from the center of curvature of the spherical outer circumferential surface of the inner member, and is formed with a greater radius of curvature than that of the spherical outer circumferential surface of the inner member. Here, axial clearances δ 2 +δ 2 ′ in the range of from 5 to 50 μm are provided between the ball and the pocket of the cage. In this way, axial clearances δ 1  and δ 1 ′ are provided between the inner member and the cage, and the slide resistance in the joint is significantly reduced. Therefore, even when the constant velocity universal joint is used for a drive wheel in an automobile, and a relatively small torque is loaded for example during idling in an AT automobile, vibrations from the engine side can be absorbed and prevented from being transmitted to the vehicle body, and therefore the vibration of the vehicle body can be prevented.  
         [0023]     The invention is characterized in that, in the constant velocity universal joint, the inner circumferential surface of the cage is formed by connecting the cylindrical surface extending for an arbitrary axial size in the center, and the spherical outer circumferential surface of the inner member and a partial spherical surface having the same radius of curvature located on the sides of the cylindrical surface, and axial clearances δ 2 +δ 2 ′ in the range of from 5 to 50 μm are provided between the ball and the pocket of the cage. In this way, axial clearances δ 3  and δ 3 ′ are provided between the inner member and the cage, so that the slide resistance in the joint is significantly reduced. Even when the constant velocity universal joint is used for a drive wheel in an automobile, and a relatively small torque is loaded for example during idling in an AT automobile, vibrations from the engine side can be absorbed and prevented from being transmitted to the vehicle body, and therefore the vibration of the vehicle can be prevented.  
         [0024]     According to the invention, in the DOJ type, sliding type constant velocity universal joint having a plurality of balls, the pitch of the ball track formed by a pair of the track groove of the outer member and the track groove of the inner member is randomly set in such a range that various characteristics (such as strength, durability, and NVH) necessary for a constant velocity universal joint are provided as described above. In this way, the vibration cycle by induced thrust force is not constant, so that the vibrations, muffled noises, and the like in the vehicle can be reduced.  
         [0025]     FIGS.  15  to  20  show measurement results of induced thrust force for a conventional DOJ with six balls and the inventive product with six balls. In these figures, the abscissa represents the operation angle (0° to 15°), and the ordinate represents induced thrust (N). The broken line represents the measurements for the conventional product, and the solid line represents the measurements for the inventive product. In the inventive product, not only the 6th order induced thrust force can sufficiently be reduced, but also the induced thrust force in all the other orders are not more than that of the conventional product. The ball track pitch in the inventive product is as shown in Example 1 in Table 1. Note that measurement was carried out for combinations in Examples 2 to 4 in Table 1, and substantially the same effect as that in Example 1 was observed.  
                               TABLE 1                       Pitch   Example 1   Example 2   Example 3   Example 4                   α 1     55   55   55   58       α 2     70   65   59   61       α 3     55   55   65   61       α 4     55   65   61   63       α 5     70   55   57   60       α 6     55   65   63   57                  
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0026]      FIG. 1   a  is a front view showing an embodiment of a joint outer ring of a constant velocity universal joint according to the present invention.  
         [0027]      FIG. 1   b  is a sectional view taken along the line A-O-B in  FIG. 1   a.    
         [0028]      FIG. 2   a  is a diagram for comparison in the size of a joint outer ring between a conventional product and the inventive product, wherein is a front view showing the conventional product in the left part from the line X-X as the boundary and the inventive product in the right part.  
         [0029]      FIG. 2   b  is a diagram for comparison in the size of a joint outer ring between a conventional product and the inventive product, wherein is a sectional view showing the conventional product in the upper part above the line Y-Y as the boundary and the inventive product in the lower part.  
         [0030]      FIG. 3  is a front view of a six-ball constant velocity universal joint having three vehicle body attachment flanges showing another embodiment of the present invention.  
         [0031]      FIG. 4  is a front view of an eight-ball constant velocity universal joint having eight vehicle body attachment flanges showing another embodiment of the present invention.  
         [0032]      FIG. 5  is a front view of an eight-ball constant velocity universal joint having four vehicle body attachment flanges showing another embodiment of the present invention.  
         [0033]      FIG. 6  is a front view of a six-ball DOJ according to an embodiment of the invention.  
         [0034]      FIG. 7  is a longitudinal sectional view of the DOJ shown in  FIG. 6 .  
         [0035]      FIG. 8   a  is a cross sectional view of the inner member in the DOJ of  FIG. 6 .  
         [0036]      FIG. 8   b  is a longitudinal sectional view thereof.  
         [0037]      FIG. 9   a  is a front view of the cage in the DOJ of FIG.  6 .  
         [0038]      FIG. 9   b  is a longitudinal sectional view thereof.  
         [0039]      FIG. 10  is a front view of a six-ball DOJ according to another embodiment.  
         [0040]      FIG. 11   a  is a cross sectional view of the inner member in the DOJ of  FIG. 10 .  
         [0041]      FIG. 11   b  is a longitudinal sectional view thereof.  
         [0042]      FIG. 12   a  is a front view of the cage in the DOJ of  FIG. 10 .  
         [0043]      FIG. 12   b  is a cross sectional view thereof.  
         [0044]      FIG. 13  is a longitudinal sectional view of an inner ring and a cage according to another embodiment.  
         [0045]      FIG. 14  is a longitudinal sectional view of an inner ring and a cage according to yet another embodiment.  
         [0046]      FIG. 15  is a graph representing measurement results for a 1st order component of induced thrust.  
         [0047]      FIG. 16  is a graph representing measurement results for a 2nd order component of induced thrust.  
         [0048]      FIG. 17  is a graph representing measurement results for a 3rd order component of induced thrust.  
         [0049]      FIG. 18  is a graph representing measurement results for a 4th order component of induced thrust.  
         [0050]      FIG. 19  is a graph representing measurement results for a 5th order component of induced thrust.  
         [0051]      FIG. 20  is a graph representing measurement results for a 6th order component of induced thrust.  
         [0052]      FIG. 21  is a cross sectional view of a sliding type constant velocity universal joint that constitutes a drive shaft of an automobile.  
         [0053]      FIG. 22   a  is a cross sectional view taken along the line C-O-D in  FIG. 22   b  showing a conventional sliding type constant velocity universal joint.  
         [0054]      FIG. 22   b  is a partly omitted front view showing a joint outer ring of a conventional sliding type constant velocity universal joint.  
         [0055]      FIG. 23   a  is a cross sectional view showing the state in which a bolt and a socket are mounted to the joint outer ring of  FIG. 22   a.    
         [0056]      FIG. 23   b  a front view thereof. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0057]     Embodiments of the constant velocity universal joint according to the present invention shown in FIGS.  1  to  5  will be described in detail. The same or corresponding parts as or to those in the conventional constant velocity universal joints shown in  FIGS. 22   a ,  22   b ,  23   a , and  23   b  will be denoted by the same reference numerals.  
         [0058]     A sliding type constant velocity universal joint according to the embodiment shown in  FIGS. 1   a  and  1   b  is a double offset type constant velocity universal joint (DOJ) that constitutes a drive shaft  7  (see  FIG. 21 ) serving as a power transmission mechanism in an automobile and is coupled to a differential  3  on the vehicle body side. The constant velocity universal joint includes, as essential elements, a joint outer ring  21  as an outer member attached to the differential  3  on the vehicle body side, a joint inner ring  9  as an inner member attached to one end of an intermediate shaft  1 , a plurality of balls  10  incorporated between the joint outer ring  21  and the joint inner ring  9 , and a cage  11  interposed between the joint outer ring  21  and the joint inner ring  9  to support the balls. (refer to  FIGS. 22   a ,  22   b ,  23   a , and  23   b , because the structure is the same as the conventional structure except for the joint outer ring  21 .)  
         [0059]     The joint outer ring  21  is in the shape of a cup having a plurality of linear track grooves  22  parallel to its axial line and in its inner circumference at equal intervals in the circumferential direction. The joint inner ring  9  has a plurality of linear track grooves  13  parallel to its axial line and corresponding to the track grooves  22  in its outer circumference. The track grooves  22  and  13  in the joint outer ring  21  and the joint inner ring  9  cooperate with each other to define the ball tracks in which the torque transmitting balls  10  are provided. The balls  10  are supported in the pockets of the cage  11  interposed between the joint outer ring  21  and the joint inner ring  9 . In the constant velocity universal joint, when an operation angle is provided between the joint outer ring  21  and the joint inner ring  9 , the cage  11  controls the balls  10  to be on the bisector plane of the operation angle, so that the constant velocity is maintained.  
         [0060]     The joint outer ring  21  in the constant velocity universal joint is classified as a flange type ring based on how it is attached to the vehicle body. The flange type joint outer ring  21  uses a plurality of vehicle body attachment flanges  23  integrally provided at equal intervals in its circumferential direction at the outer end portion, and is attached to the differential  3  (see  FIG. 21 ) by bolts fastened through bolt holes  24  formed through the vehicle body attachment flanges  23 .  
         [0061]     The joint outer ring  21  has a flower outer circumferential shape formed corresponding to the shape of the inner circumference (track grooves) for reducing the weight and size. Herein, the “flower shape” refers to a shape that has recesses  25  that are formed, between the positions of the track grooves  22  formed in the inner circumference, at the outer circumferential surface of the joint outer ring  21  so as to extend along the track grooves  22 . The vehicle body attachment flanges  23  are provided at the outer circumferential recesses  25  positioned between the track grooves  22  of the joint outer ring  21 .  
         [0062]     In this way, the joint outer ring  21  has the flower outer circumferential shape corresponding to the inner circumferential shape, so that the constant velocity universal joint can be reduced in weight with its load capacity maintained in the present level. In addition, the vehicle body attachment flanges  23  are provided at the outer circumferential recesses  25  positioned between the track grooves  22  in the flower joint outer ring  21 , so that the outer diameter size of the vehicle body attachment flanges  23  can be reduced and the constant velocity universal joint can be made compact.  
         [0063]      FIG. 2  shows the conventional joint outer ring  8  and the inventive joint outer ring  21  as they are compared in size. In  FIG. 2   a , the left part from the line X-X as the boundary shows the conventional product and the right part shows the inventive product. In  FIG. 2   b , the upper part above the line Y-Y as the boundary shows the conventional product, and the lower part shows the inventive product.  
         [0064]     In the comparison between the conventional product and the inventive product, the load capacity (size) of the constant velocity universal joint and the space a for inserting the tool are the same. In the comparison in the outer diameter size between the vehicle body attachment flanges  14  and  23 , the inventive product can be reduced by about 10% with respect to the conventional product in size, and by about 20% in weight.  
         [0065]     Herein, the joint outer ring  21  of the inventive product has a flower outer shape that is advantageous in terms of weight reduction, but the shape has a limitation in thickness in order to keep certain strength. More specifically, in order to reduce the weight by employing the flower shape and still keep satisfactory strength for the constant velocity universal joint, not only the thickness of the track groove portions but also the thickness of the portion between the track grooves is crucial.  
         [0066]     Therefore, as shown in  FIG. 1   a , the ratio DN/DT of the outermost diameter size DT where the track grooves  22  are located and the innermost diameter size DN where the vehicle body attachment flanges  23  are located between the track grooves  22  should be set in the range of from 0.85 to 0.95. When the ratio of the outermost diameter size DT and the innermost diameter size DN is set in the above-described range, the weight and size can be reduced and the strength of the joint outer ring  21  can be secured simultaneously.  
         [0067]     If the ratio DN/DT is smaller than 0.85, the portion of the joint outer ring  21  where the flanges  23  are located is too thin to provide strength required by the constant velocity universal joint. If the ratio DN/DT is greater than 0.95, the outer diameter size of the vehicle body attachment flanges  23  is too large, and the weight and size cannot be reduced.  
         [0068]     Note that the number of the vehicle body attachment flanges  23  can arbitrarily be set based on the number of the track grooves  22  (balls  10 ) of the joint outer ring  21  described above. More specifically, instead of providing the vehicle body attachment flanges  23  in all the outer circumferential recesses  25  positioned between the track grooves  22  for all the track grooves  22  in the joint outer ring  21  as shown in  FIGS. 1   a ,  1   b ,  2   a , and  2   b , vehicle body attachment flanges  23  may be provided only in part of the outer circumferential recesses  25 . For example as shown in  FIG. 3 , the vehicle body attachment flanges  23  may be provided in three outer circumferential recesses  25  arranged at equal intervals in the circumferential direction of the joint outer ring  21 .  
         [0069]     In the above example, although six balls  10  are incorporated in the constant velocity universal joint, the embodiment may be applied to a constant velocity universal joint in which eight balls  10  are incorporated. With eight balls  10 , the ball PCD may be reduced and the joint may be more compact than the constant velocity universal joint with six balls. In this case, vehicle body attachment flanges  23  may be provided in all the eight outer circumferential recesses  25  as shown in  FIG. 4 , or the flanges  23  may be formed in four outer circumferential recesses  25  at equal intervals in the circumferential direction of the joint outer ring  21  as shown in  FIG. 5 .  
         [0070]     A constant velocity universal joint of an embodiment of the invention shown in FIGS.  6  to  9  includes an outer ring  110 , an inner ring  120 , balls  130 , and a cage  140  as essential elements. The outer ring  110  is in the shape of a cup having one end opened and has a shaft portion  116  coupled to a rotating shaft on the opposite side to the open end. The inner circumferential surface  112  of the outer ring  110  is cylindrical, and six axially extending track grooves  114  are formed in the inner circumferential surface of the cylinder. The inner ring  120  has a spherical outer circumferential surface  122 , and six axially extending track grooves  124  are formed in the spherical outer circumferential surface  122 . The inner ring  120  has a serration hole  126  to couple with the rotating shaft. The track grooves  114  of the outer ring  110  and the track grooves  124  of the inner ring  120  are paired to define ball tracks, and one ball  130  is incorporated in each ball track. The balls  130  are interposed between the outer ring  110  and the inner ring  120  to transmit torque. The balls  130  are held in pockets  146  in the cage  140 . The cage  140  is in contact with the cylindrical inner circumferential surface portion  112  of the outer ring  110  at the outer spherical surface portion  142 , and in contact with the spherical outer circumferential surface  122  of the outer ring  120  at the inner spherical surface portion  144 . Therefore, angular displacement can be made between the outer ring  110  and the cage  140  and between the cage  140  and the inner ring  120 . A sub unit consisting of the inner ring  120 , the balls  130 , and the cage  140  can slide relative to the outer ring  110  in the axial direction of the outer ring  110 . As shown in  FIG. 9   b , the center Oo of the outer spherical surface portion  142  of the cage  140  and the center Oi of the inner spherical surface portion  144  are offset from each other by an equal distance axially in the opposite directions from the center O of the pocket. Therefore, when the joint transmits torque at a certain operation angle, the balls are always located in the bisector plane of the angle formed by the rotating axis of the outer ring  110  and the rotating axis of the inner ring  120 , so that the constant velocity of the joint can be secured.  
         [0071]     According to the embodiment, the pitches α 1  to α 6  of the ball tracks are random and not less than 55°. More specifically, as shown in  FIGS. 6 and 8 , the pitches of the track grooves  114  of the outer ring  110  and the track grooves  124  of the inner ring  120  are random and not less than 55° (see Examples 1 to 3 in Table 1). The lower limit for the pitch is set as 55°, so that prescribed sizes for the spherical surface width W 2  of the inner ring  120  and the inter-pocket column width W 1  of the cage  140  necessary in consideration of the strength of the inner ring  120  and the cage  140  can be secured. According to the embodiment, as shown in  FIG. 9 , the pitch of the pockets  146  of the cage  140  is also random and not less than 55° as with the pitches of the track grooves  114  of the outer ring  110  and the track grooves  124  of the inner ring  120 . Consequently, at the time of assembling the joint, the outer ring  110 , the inner ring  120 , and the cage  140  should be adjusted to be in phase. The window length L 1  of the pockets  146  of the cage  140  is equal. The window length L 1  of the pocket  146  is set in consideration of the circumferential movement of the ball  130  based on the maximum operation angle of the joint.  
         [0072]     Now, an embodiment of the invention shown in FIGS.  10  to  12  will be described. Note that the basic structure of the DOJ is the same as that of the embodiment in FIGS.  6  to  9 , and therefore substantially the same elements or parts will be denoted by the same reference characters. As shown in  FIGS. 10 and 11 , according to the embodiment, the pitches α 1  to α 6  of the track grooves  114  of the outer ring  110  and the track grooves  124  of the inner ring  120  are unequal pitches in the range of 60°±3° (see Example 4 in Table 1). When the pitch is limited to the range of 60°±3°, the necessary size for the inter-pocket column width W 3  in consideration of the strength of the cage  140  is secured. In this example, as shown in  FIG. 12 , the pockets  146  of the cage  140  are provided at equal pitch intervals (60°), and the window length L 2  of the pockets  146  is equal. The window length L 2  of the pocket  146  is set in consideration of the deviation of the ball track pitch (60°±3°) and the circumferential movement of the balls  130  based on the maximum operation angle of the joint. The pockets  146  of the cage  140  are equal in length and provided with equal pitch, phase adjustment is necessary only for the outer ring  110  and the inner ring  120  at the time of assembling the joint, which can be carried out significantly easily.  
         [0073]     According to an embodiment shown in  FIGS. 13 and 14 , the inner ring  120  and the cage  140  can move axially relative to each other, and the balls are released from restriction, so that they can more easily turn. In the embodiment shown in  FIG. 13 , the radius curvature (r) of the spherical outer circumferential surface  122  of the inner ring  120  is set to be smaller than the radius curvature (R) of the inner spherical surface portion  144  of the cage  140 , and the center of curvature of the spherical outer circumferential surface  122  of the inner ring  120  and the center of curvature of the inner spherical surface portion  144  of the cage  140  are radially shifted. In this way, axial clearances δ 1  and δ 1 ′ are formed between the outer spherical surface  122  of the inner ring  120  and the inner spherical surface portion  144  of the cage  140 , and the clearances δ 1  and δ 1 ′ allow the inner ring  120  to be axially displaced relative to the cage  140 .  
         [0074]     In the embodiment shown in  FIG. 14 , the inner circumferential surface of the cage  140  is formed by connecting a cylindrical surface  144   a  for a size (L) in the axial direction in the center and partial spherical surfaces  144   b  on its both sides. The radius of curvature (R) of the partial spherical surface  144   b  is equal to the radius of curvature (r) of the spherical outer circumferential surface  122  of the inner ring  120 , and there is a clearance  63  and  631  between the spherical outer circumferential surface  122  of the inner ring  120  and the inner circumferential surfaces ( 144   a  and  144   b ) of the cage  140 .  
         [0075]     In the embodiment shown in  FIGS. 13 and 14 , there are clearances δ 2  and δ 2 ′ between the wall of the cage  140  opposing the axial direction of the pocket  146  and the ball  130 . The clearances δ 2  and δ 2 ′ are set in the range of from 5 to 50 μm in order to release the ball  130  from restriction, and in consideration of the effect of collision between the ball  130  and the cage  140 . The upper limit for the clearances δ 2 δ 2 ′ is 50 μm because for a clearance larger than 50 μm, not only the striking noise caused by the collision between the ball  130  and the cage  140  is large, but also the stability of the cage  140  is impaired by the impact upon the collision, which gives rise to increased vibrations. The lower limit is 5 μm though it would be possible to set the lower limit to zero in theory since the ball  130  is to be released from restriction. This is for surely eliminating fastening allowance and securing δ 2  and δ 2 ′ for convenience of manufacture and maintenance.  
         [0076]     In the embodiment shown in  FIGS. 13 and 14 , the clearances δ 1  and δ 1 ′ or δ 3  and δ 3 ′ allow the inner ring  120  and the cage  140  to be relatively moved in the axial direction, and the ball  130  can turn without resistance as it is not restricted by the pocket  146  of the cage  140 , so that the slide resistance for the axial relative movement of the outer ring  110  and the inner ring  120  is very small. Therefore, vibrations from the engine side as the torque is loaded are absorbed by smooth, slight relative movement between the outer ring  110  and the inner ring  120  through the cage  140  and are not transmitted to the vehicle body. Since the slide resistance inside the joint is small, angular displacement and axial displacement are extremely smoothly carried out.  
         [0077]     In the described embodiment, the six balls  130  are used, and the induced force can similarly be reduced by employing unequal pitches in cases other than where the number of the balls  130  is six. Note however that the range of setting the pitches is determined based on the relation between the number of balls  130  and the operation angle. The relation between the operation angle and the ball track pitch for a six-ball DOJ and an eight-ball DOJ is given in following Tables 2 and 3.  
                                     TABLE 2                           Maximum               operation   Ball track pitch            angle   Pockets with   Pockets with       (°)   unequal pitches   equal pitch               15 to 20   at least 53°   60° ± 4°       20 to 25   at least 55°   60° ± 3°       25 to 30   at least 57°   60° ± 2°                  
 
         [0078]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                   
               
               
                 Maximum 
                   
                   
               
               
                 operation 
                 Ball track pitch 
               
             
          
           
               
                 angle 
                 Pockets with 
                 Pockets with 
               
               
                 (°) 
                 unequal pitches 
                 equal pitch 
               
               
                   
               
               
                 15 to 20 
                 at least 39° 
                 45° ± 3° 
               
               
                 20 to 25 
                 at least 41° 
                 45° ± 2° 
               
               
                 25 to 30 
                 at least 43° 
                 45° ± 1°