Abstract:
A constant velocity universal joint is provided, which can ensure a smooth torque transmission, extend the useful life thereof, maintain the constant velocity, and damp vibrations and abnormal sounds. The constant velocity universal joint having at least two sets of link mechanisms. The link mechanism has: link hubs installed in an input shaft and an output shaft, respectively; end link members rotatably each coupled with the link hubs installed in the respective input and the output shafts; and a central link member to which the end link members on the respective input and output shaft sides are rotatably coupled. Geometries of the mechanism on the input shaft side and the output shaft side are identical to each other across a transverse plane in a center of the link mechanism. The universal joint further has rotational resistance reducing means installed in at least either coupling part between the link hub and the end link member or coupling part between the central link member and the end link member. As the rotational resistance reducing means, a ball bearing is installed in the coupling part.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    This invention relates to a constant velocity universal joint that is installed in the steering system of an automobile for torque transmission between two shafts.  
           [0003]    2. Description of the Related Art  
           [0004]    One of the examples of constant velocity universal joints that are installed in the steering system of an automobile for torque transmission between two shafts is a link-type constant velocity universal joint disclosed in Japanese Patent Publication No. Sho. 47-51502.  
           [0005]    Referring now to FIGS.  14 ( a ) and  14 ( b ), this link-type constant velocity universal joint has at least two link mechanism  3  sets that link an input shaft  1  and an output shaft  2  (only one link mechanism  3  set is shown in the drawings). The link mechanism  3  has two link hubs  10  that are each installed in the input shaft  1  and the output shaft  2  and shared with the other link mechanisms  3 , two end link members  20  each rotatably coupled with the individual link hubs  10 , and one central link member  30  that is rotatably coupled with the end link members  20  so as to interconnect these end link members  20  to each other. The link hub  10  and the end link member  20  on the input side make a large operation angle with the link hub  10  and the end link member  20  on the output side. Thus they are located to take positions rotationally symmetric across center line A in the central link member  30 .  
           [0006]    Referring now to FIGS.  15 ( a ) and  15 ( b ), the link hub  10  has a plurality of leg shafts  11  (three in the drawings) thereon that project in the radial direction. The angle, α, between a leg shaft  11  and either input shaft  1  or output shaft  2  is set at 90 degrees, considering the positional relationship of the link mechanism  3  that is configured to define a rotationally symmetric structure. The leg shafts  11  do not need to be spaced evenly in the circumferential direction. On the other hand, the hub links  10  on the input and output sides need to occupy conforming positions in the circumferential direction.  
           [0007]    As shown in FIGS.  16 ( a ) and  16 ( b ), the end link member  20  which is formed into an L-shape has a coupling bore  21  that rotatably receives the leg shaft  11  of the link hub  10  on one side and a coupling bore  22  that rotatably receives a leg shaft  32  of the central link member  30  on the other side. The angle, β, between the coupling bore  21  on the hub link side and the coupling bore  22  on the central link member side is set at 90 degrees, considering the positional relationship of the link mechanism  3  that is configured to define a rotationally symmetric structure.  
           [0008]    As shown in FIGS.  17 ( a ) and  17 ( b ), the central link member  30  has an L-shape base member  31  and leg shafts  32  each coupled with the respective coupling holes  22  of the end link members  20  on the input shaft side and output shaft side, on both sides of its L-shaped base  31 . The angle, φ, between the leg shafts  32  on the input and output shaft sides is set at the 40-100 degrees range for practical use. If this angle is smaller than 40 degrees, the outer diameter of the central link member  30  becomes too big, while if larger than 100 degrees the central link member  30  becomes too long in the axial direction and the operation angle becomes small due to mechanical interference.  
           [0009]    In the link mechanism  3 , when the leg shaft angle α and length of the link hub  10  and the geometry of the end link member  20  on the input shaft side are the same as those on the output shaft side and when the geometry of the central link member  30  on the input shaft side is the same as that on the output shaft side, the link hub  10  and the end link member  20  on the input side and those on the output side move in synchronization with each other and then the input shaft  1  and the output shaft  2  make the same rotational angle and rotate at the same angular speed (see FIGS.  18 ( a ) and  18 ( b )), provided that the angular relation on the input shaft side between the central link member  30  and the end link member  20  coupled with the leg shaft  11  of the link hub  10  is controlled to be the same, with respect to the symmetry plane in the central link member  30 , as that on the output shaft side. If the input shaft and the output shaft rotate at the same angular speed, the symmetry plane in the central link member  30  is referred to as the constant-velocity bisecting plane.  
           [0010]    If a plurality of link mechanisms  3  that have the same geometry and share link hubs  10  on the input and output shaft sides are installed in the circumferential direction, the range of movements with no interference between those link mechanisms  3  is limited to the constant-velocity bisecting plane in the central link member  30 , and the input shaft  1  and the output shaft  2  rotate at the same angular speed whatever operation angle γ they make.  
           [0011]    This link-type constant velocity universal joint, however, cannot transmit torque smoothly because of a large rotation resistance and its life of use is short because of a large frictional resistance experienced during rotation. This is because the coupling part between the leg shaft  11  of the link hub  10  and the end link member  20  as well as the coupling part between the leg shaft  32  of the central link member  30  and the end link member  20  experience large frictional resistance. Besides, the large gaps in those coupling parts cause significant backlash between the input shaft  1  and the output shaft  2 , leading to irregular movements during operation. As a result, it becomes difficult to maintain a constant velocity and vibrations and abnormal sounds are produced.  
         SUMMARY OF THE INVENTION  
         [0012]    An object of this invention is to ensure smooth torque transmission, improve the useful life, maintain a constant velocity by preventing backlash in the input and output shafts, and damp vibrations and abnormal sounds of the universal joint.  
           [0013]    For attaining the above object, the present invention provides a constant velocity universal joint having at least two sets of link mechanisms, the link mechanism having: link hubs installed in an input shaft and an output shaft and shared with other link mechanisms, respectively; end link members rotatably each coupled with the link hubs installed in the respective input and the output shafts; and a central link member to which the end link members on the respective input and output shaft sides are rotatably coupled, geometries of the mechanism on the input shaft side and the output shaft side being identical to each other across a transverse plane in a center of the link mechanism, wherein the joint further has rotational resistance reducing means installed in at least either coupling part between the link hub and the end link member or coupling part between the central link member and the end link member. Herein, the description “geometries of the mechanism on the input shaft side and the output shaft side being identical to each other across a transverse plane in a center of the link mechanism” means that if the constant velocity universal joint is divided across the symmetric plane in the central link member into two parts on the input shaft side and the output shaft side, the geometries on the input shaft side and the output shaft side are identical to each other.  
           [0014]    In the present invention, the rotational resistance reducing means is installed in at least either coupling part between the link hub and the end link member or coupling part between the central link member and the end link member. Then since the frictional resistance in coupling parts is reduced and the rotational resistance is thereby lowered, a smooth torque transmission can be provided and the useful life can be extended.  
           [0015]    Such rotational resistance reducing means may be a structure where a roller bearing is installed in the coupling part. This means may also be a structure where a journal bearing such as a roller bearing and a thrust bearing such as a slide bearing are installed in combination in the coupling part. It is preferable to install a preload-providing means that applies a preload to the bearing. A preload helps reduce irregular movements in the coupling parts, prevent backlash between the input and output shaft sides, maintain a constant velocity and damp vibrations and abnormal sounds. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    In the accompanying drawings:  
         [0017]    [0017]FIG. 1( a ) is a front view of a constant velocity universal joint according to the present invention, and FIG. 1( b ) is a plan view of the universal joint shown in FIG. 1( a );  
         [0018]    [0018]FIG. 2 is an enlarged sectional view of the major part showing a first embodiment of the invention;  
         [0019]    [0019]FIG. 3 is an enlarged sectional view of the major part showing a second embodiment of the invention;  
         [0020]    [0020]FIG. 4 is an enlarged sectional view of the major part showing a third embodiment of the invention;  
         [0021]    [0021]FIG. 5 is an enlarged sectional view of the major part showing a fourth embodiment of the invention;  
         [0022]    [0022]FIG. 6 is an enlarged sectional view of the major part showing a fifth embodiment of the invention;  
         [0023]    [0023]FIG. 7 is an enlarged sectional view of the major part showing a sixth embodiment of the invention;  
         [0024]    [0024]FIG. 8 is an enlarged sectional view of the major part showing a seventh embodiment of the invention;  
         [0025]    [0025]FIG. 9 is an enlarged sectional view of the major part showing an eighth embodiment of the invention;  
         [0026]    [0026]FIG. 10 is an enlarged sectional view of the major part showing a ninth embodiment of the invention;  
         [0027]    [0027]FIG. 11 is an enlarged sectional view of the major part showing a tenth embodiment of the invention;  
         [0028]    [0028]FIG. 12 is an enlarged sectional view of the major part showing an eleventh embodiment of the invention;  
         [0029]    [0029]FIG. 13( a ) is a plan view showing a twelfth embodiment of the invention, and FIG. 13( b ) is a front view of universal joint shown in FIG. 13( a );  
         [0030]    [0030]FIG. 14( a ) is a front view of a link-type constant velocity university joint, and FIG. 14( b ) is a plan view of the universal joint shown in FIG. 14( a );  
         [0031]    [0031]FIG. 15( a ) is a front view of a link hub, and FIG. 15( b ) is a side view of the link hub shown in FIG. 15( a );  
         [0032]    [0032]FIG. 16( a ) is a front view of an end link member, and FIG. 16( b ) is a side view of the end link member shown in FIG. 16( a );  
         [0033]    [0033]FIG. 17( a ) is a front view of a central link member, and FIG. 17( b ) is a side view of the central link member shown in FIG. 17( a ); and  
         [0034]    [0034]FIG. 18( a ) is a front view of the link-type constant velocity universal joint of FIG. 14( a ) that is in the state of taking an operation angle, and FIG. 18( b ) is a rear view of the universal joint shown in FIG. 18( a ). 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]    FIGS.  1 ( a ) and  1 ( b ) illustrate an embodiment of a link-type constant velocity universal joint in accordance with the present invention. This constant velocity universal joint has at least two link mechanism  3  sets to couple an input shaft  1  with an output shaft  2  (in the figure, only one link mechanism  3  set is shown). A link mechanism  3  has link hubs  10  each installed in the input shaft  1  and output shaft  2  respectively, two end link members  20  each rotatably coupled with the individual link hubs  10 , and a central link member  30  that is rotatably coupled with the end link members  20  to interconnect these end link members  20  to each other. The link hub  10  has a plurality of leg shafts  11  that radially project (three in the drawings). The end link member  20  which is formed into an L-shape has a coupling bore  21  that rotatably receives the leg shaft  11  of the link hub  10  on one side and a coupling bore  22  that rotatably receives a leg shaft  32  of the central link member  30  on the other side. The central link member  30  has an L-shaped base member  31  and leg shafts  32  on both sides of the L-shaped base member  31 . The leg shafts  32  are each coupled with the respective coupling bores  22  of the end link members  20  on the input shaft and output shaft sides. This link mechanism  3  is configured such that its geometry is symmetric on the input and output shaft sides across the transverse plane in the center. Because of this geometric symmetry, the link hub  10  and the end link member  20  on the input shaft side synchronize in rotation with the link hub  10  and the end link member  20  on the output shaft side. Then the input shaft  1  and the output shaft  2  make the same rotational angle and they rotate at the same angular speed whatever operation angle they take.  
         [0036]    In a first embodiment shown in FIG. 2, the leg shaft  11  of the link hub  10  is inserted into the coupling bore  21  of the end link member  20 , and the leg shaft  11  of the link hub  10  is rotatably coupled with the end link member  20  via two ball bearings  40  installed between the leg shaft  11  and the coupling bore  21 . The ball bearing  40  has an inner ring  41  fitted on the outer peripheral surface of the leg shaft  11  of the link hub  10 , an outer ring  42  fitted in the coupling bore  21  of the end link member  20  and a plurality of rollers (for example, balls)  43  installed between the inner ring  41  and the outer ring  42 .  
         [0037]    A washer  12  mounted on the base portion of the leg shaft  11  and a cap  14  secured on the leg shaft  11  with a bolt  13  retain the inner ring  41  to prevent its slipping off. An inward collar  21   a  formed in the coupling bore  21  of the end link member  20  and a fastener  21   b  formed by plastic deformation in the coupling bore  21  of the end link member  20  prevent the outer ring  42  from slipping off. The plurality of rollers  43  are rotatably retained at controlled intervals by a cage (not shown). The ball bearing  40  has a seal member  44  only on the side exposed to the outside to prevent the leakage of grease filled in the bearing and intrusion of water and foreign matters from the outside.  
         [0038]    Since two bearings  40  are installed in the first embodiment of the invention, the frictional resistance in the coupling part between the end link member  20  and the link hub  10  is lowered and the rotational resistance is reduced. As a result, a smooth torque transmission is ensured and the useful life is extended.  
         [0039]    In a second embodiment shown in FIG. 3, a preload is applied to two ball bearings  40  in the coupling structure for the end link member  20  and the link hub  10  in the first embodiment. Namely, the two ball bearings  40  are installed in parallel on the leg shaft  11  of the link hub  10  at a predetermined interval, and an inward collar  21   c  formed in the center of the coupling bore  21  of the end link member  20  is engaged between the outer rings  42  of the ball bearings  40 . When the bolt  13  is tightened to push the cap  14  in the axial direction of the leg shaft  11  and to press the inner ring  41  of the ball bearing  40  positioned upper in the figure, a preload is applied to the pair of bearings  40  in order to eliminate radial gaps and thrust gaps.  
         [0040]    According to the second embodiment, since the bolt  13  applies a preload to the two ball bearings  40  to eliminate radial gaps and thrust gaps, the backlash in the coupling part between the end link member  20  and the link hub  10  is prevented. Then the input shaft  1  and the output shaft  2  are synchronized in their rotary movements. As a result, a constant velocity is maintained, and vibrations and abnormal sounds are damped.  
         [0041]    As shown in a third embodiment of FIG. 4, a double row angular ball baring  50  may be installed between the leg shaft  11  and the coupling bore  21 . The double row angular ball baring  50  has two inner rings  51  fitted in parallel on the outer peripheral surface of the leg shaft  11 , an outer ring  52  fitted in the coupling bore  21  of the end link member  20  and a plurality of rollers (balls)  53  installed in two rows between the inner ring  51  and the outer ring  52 .  
         [0042]    A snap ring  15  fitted on the leg shaft  11  secures the inner ring  51  to prevent its slipping off. An inward collar  21   a  formed in the coupling bore  21  of the end link member  20  and a fastener  21   b  formed by plastic deformation in the coupling bore  21  of the end link member  20  prevent the outer ring  52  from slipping off. The plurality of rollers  53  installed in two rows are rotatably held at controlled intervals by a cage (not shown). As is the case with the first embodiment, the double row angular ball bearing  50  has seal members  54  at both ends.  
         [0043]    Since the double row angular ball bearing  50  is installed in the third embodiment of the invention, the rotational resistance is reduced as is the case with the first embodiment. As a result, a smooth torque transmission is ensured and the useful life is extended. Furthermore, since the double row angular ball bearing  50  is fastened with the snap ring  15 , the retention structure for the bearings can be simplified, compared with the first embodiment shown in FIG. 2.  
         [0044]    In a fourth embodiment shown in FIG. 5, a preload is applied to the double row angular ball bearing  50  in the coupling structure of the third embodiment shown in FIG. 4. Namely, the inner rings  51  of the double row angular ball bearing  50  are spaced at a predetermined gap H and for the control of this gap a shim  16  is inserted between the snap ring  15  and the inner ring  51  positioned upper in the figure of the double row angular ball bearing  50 . By controlling the gap H between the inner rings  51  with the shim  16 , a preload is given to the double row angular ball bearing  50  via the shim  16  to eliminate radial gaps and axial gaps.  
         [0045]    Since the radial and thrust gaps are eliminated by applying a preload to the double row angular ball bearing  50  via the shim  16  in the fourth embodiment, the backlash in the coupling part is prevented as is the case with the second embodiment. Then a constant velocity is maintained, and vibrations and abnormal sounds are reduced. Moreover, since a preload is applied to the double row angular ball bearing  50  via the shim  16  in the fourth embodiment, the bolt  13  used in the second embodiment shown in FIG. 3 becomes unnecessary and the height of the leg shaft  11  can be reduced.  
         [0046]    In a fifth embodiment shown in FIG. 6, a plate spring  17  gives a preload to the double row angular ball bearing  50  in the coupling structure of the fourth embodiment shown in FIG. 5. Namely, a resilient plate spring  17  is inserted between the base portion of the leg shaft  11  and the inner ring  51  of the double row angular ball bearing  50  positioned lower in the figure. The resilient force of the plate spring  17  pushes the inner ring  51  to provide a preload for the double row angular ball bearing  50  via the inner ring  51 , and thereby the radial and thrust gaps are eliminated.  
         [0047]    In addition to the maintenance of constant velocity and prevention of vibrations and abnormal sounds attained by the fourth embodiment, the fifth embodiment can eliminate the need of the gap control mechanism using the shim  16  employed in the fourth embodiment and reduce backlash even if there is some wear in the coupling structure.  
         [0048]    A sixth embodiment shown in FIG. 7 has a structure in which a four-point contact ball bearing  60  is installed between the leg shaft  11  and the coupling bore  21 . The four-point contact ball bearing  60  has two inner rings  61  fitted on the outer peripheral surface of the leg shaft  11  of the link hub  10 , an outer ring  62  fitted in the coupling bore  21  of the end link member  20  and a plurality of rollers (balls)  63  installed between the inner rings  61  and the outer ring  62 .  
         [0049]    A snap ring  15  prevents the inner rings  61  from slipping off, while a collar  21   a  formed in the end link member  20  and a fastener  21   b  prevent the outer ring  62  from slipping off. The rollers  63  are rotatably held at controlled intervals by a cage (not shown), providing four contact points between each of the inner rings  61  and the outer ring  62 . The four-point contact ball bearing  60  has seal members  64  at its both ends.  
         [0050]    A resilient plate spring  17  is installed between the base portion of the leg shaft  11  and the inner ring  61  positioned lower in the figure illustrating the four-point contact ball bearing  60 . When the resilient force of the plate spring  17  pushes the inner ring  61 , a preload is applied to the four-point contact ball bearing  60  to eliminate radial and thrust gaps.  
         [0051]    Since the four-point contact ball bearing  60  is installed and the preload applied by the plate spring  17  to the four-point contact ball bearing  60  eliminates radial and thrust gaps in the sixth embodiment, the friction in the coupling part is reduced. Then a smooth torque transmission is ensured and the useful life is extended. At the same time, the backlash in the coupling part is prevented, a constant velocity is maintained, and vibrations and abnormal sounds are damped.  
         [0052]    These embodiments that have been described have the ball bearing  40 , double row angular ball bearing  50  or four-point contact ball bearing  60  in the coupling part between the end link member  20  and the link hub  10 . However, the present invention is not limited thereto, and other roller bearings may be used.  
         [0053]    A seventh embodiment shown in FIG. 8 has a plurality of needle bearings  71  between the leg shaft  11  and the coupling bore  21 . Slipping off of the end link member  20  is prevented by a washer  18  retained by a snap ring  15 . The needle bearings  71  are rotatably held as they roll with no cage.  
         [0054]    Since the plurality of needle bearings  71  are installed in the seventh embodiment, a smooth torque transmission is ensured and the useful life is extended. At the same time, the load tolerance can be raised without enlarging the diameter of the coupling bore  21  of the end link member  20 .  
         [0055]    In an eighth embodiment shown in FIG. 9, sliding members (sliding bearings)  72  are inserted between the end link member  20  and the base portion of the leg shaft  11  and between the end link member  20  and the washer  18  in the coupling structure of the seventh embodiment. A plurality of needle bearings  71  receive the load in the radial direction, while the sliding members  72  receive the load in the thrust direction. The sliding member  72  is made of resin materials having low friction coefficients such as, for example, fluororesin, polyimide, polyethylene, polyamideimide.  
         [0056]    In the eighth embodiment, the sliding member  72  further lowers the frictional resistance in the coupling part between the end link member  20  and the link hub  10 . Besides, the backlash in the axial direction is also prevented by the sliding member  72 .  
         [0057]    In a ninth embodiment shown in FIG. 10, a shell-type needle bearing  80  is inserted between the leg shaft  11  and the coupling bore  21 . The shell-type needle bearing  80  has a cup-shape shell outer ring  81  fitted in the coupling bore  21  of the end link member  20  and a plurality of needle bearings  82  inserted between the inner surface of the outer ring  81  and the outer peripheral surface of the leg shaft  11  of the link hub  10 .  
         [0058]    Slipping off of the outer ring  81  is prevented by a fastener  21   b  formed by plastic deformation in the coupling bore  21  of the end link member  20 . The displacement of the end link member  20  in the axial direction is restricted by an inward collar  21   a  formed in the coupling bore  21  and the snap ring  19  secured to the leg shaft  11 . A seal member  83  is inserted between the inner peripheral surface of the collar  21   a  of the end link member  20  and the base portion of the leg shaft  11 . A sliding member (sliding bearing)  84  is inserted between the collar  21   a  of the end link member  20  and the snap ring  19  to receive the load with the sliding member  84  in the thrust direction. The sliding member  84  is made of resin materials having low friction coefficients such as, for example, fluororesin, polyimide, polyethylene, polyamideimide.  
         [0059]    Since the shell-type needle bearing  80  is installed between the end link member  20  and the leg shaft  11  and the displacement of the end link member  20  in the axial direction with respect to the leg shaft  11  are restricted with the snap ring  19  in the ninth embodiment, the frictional resistance in the coupling part is reduced. Then a smooth torque transmission is ensured and the useful life is extended. At the same time, the backlash in the coupling part is prevented, a constant velocity is maintained, and vibrations and abnormal sounds are damped. The frictional resistance in the coupling part can be further reduced by installing the sliding member  84 .  
         [0060]    In a tenth embodiment shown in FIG. 11, a spherical bearing  90  is inserted between the leg shaft  11  and the coupling bore  21 . The spherical bearing  90  has an inner ring  91 , which is fitted on the outer peripheral surface of the leg shaft  11  and has a convex outer peripheral surface, and an outer ring  92  that is fitted in the coupling bore  21  of the end link member  20  and has a concave outer peripheral surface that fits on the outer peripheral surface of the inner ring  91 . Slipping off of the inner ring  91  is prevented by a fastener  11   a  formed in the leg shaft  11 . Slipping off of the outer ring  92  is prevented by the collar  21   a  formed in the coupling bore  21  of the end link member  20  and the fastener  21   b.    
         [0061]    Since the spherical bearing  90  is installed in the tenth embodiment of the invention, a smooth torque transmission is ensured in the coupling part and the useful life is extended. In addition, the coupling structure can be made compact because the spherical bearing  90  has a simple structure consisting of a small number of constituting components.  
         [0062]    In an eleventh embodiment shown in FIG. 12, an inner-ring-split type spherical bearing  100  is installed between the leg shaft  11  and the coupling bore  21 . The inner-ring-split type spherical bearing  100  has two inner rings  101 , which are fitted on the outer peripheral surface of the leg shaft  11  and has a convex outer peripheral surface, and an outer ring  102  that is fitted in the coupling bore  21  of the end link member  20  and has two concave outer peripheral surfaces that are fitted on the outer peripheral surface of the inner ring  101 .  
         [0063]    Slipping off of the inner ring  101  is prevented by the snap ring  15  fitted on the outer peripheral surface of the leg shaft  11 . The collar  21   a  formed in the coupling bore  21  of the end link member  20  and the fastener  21   b  prevent the outer ring  102  from slipping off. A plate spring  17  is installed between the base portion of the leg shaft  11  and the inner ring  101  positioned lower in the figure of the spherical bearing  100 . A preload is applied to the spherical bearing  100  by the inner ring  101  that is pushed by the resilient force of the plate spring  17  so as to eliminate radial and thrust gaps.  
         [0064]    In the eleventh embodiment of the invention, the inner-ring-split type spherical bearing  100  is installed and the preload applied to the inner-ring-split type spherical bearing  100  by the plate spring  17  eliminates radial and thrust gaps. Then a smooth torque transmission is ensured and the useful life is extended. At the same time, the backlash in the coupling part is prevented, a constant velocity is maintained, and vibrations and abnormal sounds are damped.  
         [0065]    In a twelfth embodiment shown in FIGS.  13 ( a ) and  13 ( b ), the coupling bore  21  of the end link member  20  is expanded or shrunk by a clamping structure using a fastener bolt  22  in the tenth embodiment. The bearing gap in the spherical bearing  90  is controlled by expanding or shrinking the coupling bore  21  with the fastener bolt  22 .  
         [0066]    In the twelfth embodiment of the invention, the bearing gap in the spherical bearing  90  is adjusted by expanding or shrinking the coupling bore  21  of the end link member  20  with the fastener bolt  22 . Then the backlash in the coupling part is prevented, a constant velocity is maintained, and vibrations and abnormal sounds are damped.  
         [0067]    In the embodiments that have been described so far, the bearing installed between the end link member  20  and the link hub  10  lowers the rotational resistance in the coupling part. However, it is possible to employ materials having small friction coefficients and treat the surface for lowering frictional resistant, in order to reduce rotational resistance. As such material having small friction coefficients, copper alloys, graphite and fluororesin, for example, may be used in the coupling part between the end link member  20  and the link hub  10 . In order to lower the friction coefficient by surface treatment, molybdenum dioxide, polytetrafluoroethylene (PTFE) and soft metals such as gold and silver may be coated on the surface of the coupling part.  
         [0068]    Although the above embodiments have referred to the structure of the coupling part between the end link member  20  and the leg shaft  11  of the link hub  10 , the embodiments can be applied to the coupling part between the end link member  20  and the leg shaft  32  of the central link member  30 .