Abstract:
When there travels a vehicle equipped with a steering apparatus with a heavy constant velocity universal joint, e.g. double Cardan joint or Rzeppa joint, vibrations of the wheels and the vehicle body cause the joint to vibrate. A vibratory moment applied to the pinion shaft caused by the vibration provokes undesirable engagement of the pinion and the steering rack. A steering apparatus of the present invention is provided with an elastic coupling between the joint and the pinion so as to prevent the vibration from being transferred and absorb the vibration.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a steering apparatus. More particularly, it relates to a steering apparatus in which vibrations and bending moments applied to a pinion are reduced, the pinion being attached to an extension shaft to convey rotation of a steering wheel to a rack of a steering gear.  
       BACKGROUND OF THE INVENTION  
       [0002]     A steering apparatus having such an extension shaft is disclosed in U.S. Pat. No. 6,742,621. In the steering apparatus disclosed in the patent document, to convey rotation of a steering wheel to an extension shaft which is not linearly aligned with the steering wheel, an intermediate shaft is connected to a lower end of a steering shaft which is connected to the steering wheel, and the extension shaft is connected to a lower end of the intermediate shaft via a universal joint.  
         [0003]     When vibrations of wheels and a body of a traveling vehicle are applied to the steering apparatus, the universal joint at an upper end of the extension shaft vibrates. The vibrations of the universal joint then cause a pinion shaft at a lower end of the extension shaft to be subjected to large bending moments and vibrations. Hence, there have been cases in which not only steering stability of the steering wheel is degraded but the pinion shaft is subjected to a heavy load. Particularly, incases where the steering apparatus uses a large, heavy double Cardan constant velocity universal joint, the pinion shaft is subjected to large bending moments often creating a big problem.  
       SUMMARY OF THE INVENTION  
       [0004]     An object of the present invention is to provide a steering apparatus in which vibrations and bending moments attributable to vibrations of wheels and a body of a traveling vehicle and applied to a pinion shaft and a pinion provided at a lower end of an extension shaft are reduced so as to improve steering stability and prevent the pinion shaft from being subjected to a heavy load.  
         [0005]     The above object is achieved as follows. A first invention provides a steering apparatus which includes: a steering apparatus comprising; a steering column for rotatably supporting a steering shaft with which a steering wheel is to be fitted at the rear end; an intermediate shaft connected directly or indirectly to the front end of said steering shaft; a constant velocity universal joint connected to said intermediate shaft; an extension shaft consisting of an upper part connected to said constant velocity universal joint and a lower part having a pinion engaging with a steering rack; and an elastic coupling connecting said upper part to said lower part.  
         [0006]     A second invention provides the steering apparatus according to the first invention, characterized in that said elastic coupling is provided at the substantial midpoint of said extension shaft.  
         [0007]     A third invention provides the steering apparatus according to claim  1 , characterized in that said elastic coupling includes an elastic body for transmitting torque applied to upper part to said lower part when said torque is smaller than a predetermined value and a stopper pin for transmitting said torque when said torque is larger than said value.  
         [0008]     A fourth invention provides the steering apparatus according to claim  1 , characterized in that said intermediate shaft comprises an outer shaft and an inner shaft engaging with said outer shaft for transmitting torque but being allowed to slide axially in said outer shaft.  
         [0009]     A fifth invention provides the steering apparatus according to claim  1 , characterized in that said constant velocity universal joint is a double Cardan joint or a Rzeppa joint.  
         [0010]     sixth invention provides the steering apparatus according to claims  1  to  5 , further comprises a variable gear ratio steering device for changing the transmission ratio provided between said upper and lower parts.  
         [0011]     In the steering apparatus of the present invention, an extension shaft includes an upper extension shaft and a lower extension shaft with a lower end of the upper extension shaft and an upper end of the lower extension shaft being connected by an elastic coupling, so that vibrations of a constant velocity universal joint are absorbed by the elastic coupling. Therefore, vibrations and bending moments applied to a pinion shaft and a pinion are reduced, and steering stability is improved. As a result, the pinion shaft and the pinion are prevented from being subjected to a heavy load. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is an overall perspective view showing a steering apparatus according to the present invention in a state of being mounted on a vehicle.  
         [0013]      FIG. 2  is a partly sectional side elevation view showing principal parts of a steering apparatus according to a first embodiment of the present invention.  
         [0014]      FIG. 3  is an enlarged sectional view of a double Cardan constant velocity universal joint shown in  FIG. 2 , taken along line A-A thereof.  
         [0015]      FIG. 4  is a sectional view of the double Cardan constant velocity universal joint in a state reached when a steering shaft in the state shown in  FIG. 3  is rotated by 90 degrees.  
         [0016]      FIG. 5  is an enlarged sectional view of an elastic coupling shown in  FIG. 2 .  
         [0017]      FIG. 6  is a partly sectional side elevation view showing principal parts of a steering apparatus according to a second embodiment of the present invention.  
         [0018]      FIG. 7  is an exploded perspective view of an elastic coupling shown in  FIG. 6 .  
         [0019]      FIG. 8  is a partly sectional side elevation view showing principal parts of a steering apparatus according to a third embodiment of the present invention.  
         [0020]      FIG. 9  is a partly sectional side elevation view showing principal parts of a steering apparatus according to a fourth embodiment of the present invention.  
         [0021]      FIG. 10  is a cross-sectional view of a constant velocity universal joint shown in  FIG. 9 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
     First Embodiment  
       [0022]     A first embodiment of the present invention will be described in the following with reference to the accompanying drawings.  FIG. 1  is an overall perspective view showing a steering apparatus according to the present invention in a state of being mounted on a vehicle.  FIG. 2  is a partly sectional side elevation view showing principal parts of a steering apparatus according to the first embodiment of the present invention.  FIG. 3  is an enlarged sectional view of a double Cardan constant velocity universal joint shown in  FIG. 2 , taken along line A—A thereof.  FIG. 4  is a sectional view of the double Cardan constant velocity universal joint in a state reached when a steering shaft in the state shown in  FIG. 3  is rotated by 90 degrees.  FIG. 5  is an enlarged sectional view of an elastic coupling shown in  FIG. 2 .  
         [0023]     As shown in  FIGS. 1 and 2 , a cylindrical column  1  is fixed to a vehicle body and a steering shaft  12  is rotatably and axially supported by the column  1 . A steering wheel  13  is connected to a right end (toward a rear of the vehicle body) of the steering shaft  12  and an intermediate shaft  3  is connected, via a Cardan joint  2 , to a left end (toward a front of the vehicle body) of the steering shaft  12 .  
         [0024]     A double Cardan constant velocity universal joint  4  is connected to a left end of the intermediate shaft  3 , and an extension shaft  5  is connected to the double Cardan constant velocity universal joint  4 . Constant velocity universal joints are available in such types as Birfield, tripot, double offset, cross groove, double Cardan, and Rzeppa. The following description of the first embodiment is based on a case in which a double Cardan constant velocity universal joint is used.  
         [0025]     A pinion  54  attached to a pinion shaft  53  which is connected to a lower end of the extension shaft  5  is engaged with a rack  61  of a steering gear  6 . The extension shaft  5  includes an upper extension shaft  51 , a lower extension shaft  52 , and an elastic coupling  7 , a lower end of the upper extension shaft  51  and an upper end of the lower extension shaft  52  being coupled by the elastic coupling  7 . The elastic coupling  7  is provided at about an axial midpoint of the extension shaft  5 . The pinion shaft  53  and the pinion  54  are not integral with the extension shaft  5 . They are fixedly clamped to a coupling  523  formed at a lower end of the lower extension shaft  52 .  
         [0026]     When a driver rotates the steering wheel  13 , a torque generated is transmitted to the steering gear  6  via the steering shaft  12 , the Cardan joint  2 , the intermediate shaft  3 , the double Cardan constant velocity universal joint  4 , the extension shaft  5 , and the pinion  54  attached to the pinion shaft  53 . As a result, a tie rod  62  is moved via a rack and pinion mechanism to change the steering angle of wheels.  
         [0027]     The intermediate shaft  3  includes an intermediate inner shaft  31  on which a male spline is formed and which is positioned toward the rear of the vehicle (upper side) and a hollow cylindrical intermediate outer shaft  32  on which a female spline is formed and which is positioned toward the front of the vehicle (lower side). The male spline of the intermediate inner shaft  31  is fit into the female spline of the intermediate outer shaft  32  such that the male spline is slidable along the female spline and such that a torque is transmissible from the male spline to the female spline. The end toward the rear of the vehicle of the intermediate inner shaft  31  is connected to the Cardan joint  2 , and the end toward the front of the vehicle of the intermediate outer shaft  32  is connected to the double Cardan constant velocity universal joint  4 .  
         [0028]      FIGS. 3 and 4  show details of the double Cardan constant velocity universal joint  4 . The double Cardan constant velocity universal joint  4  includes an intermediate housing  41 , first and second yokes  42  and  43 , a first spider  44  connecting the first yoke  42  to the intermediate housing  41 , and a second spider  45  connecting the second yoke  43  to the intermediate housing  41 . It is a double Cardan constant velocity universal joint which absorbs, by means of a sliding centering disc  46 , changes in positional relationship among the first and second yokes  42  and  43  and the intermediate housing  41 .  
         [0029]     The intermediate housing  41  is provided with a pair of first support arms  411  of a same phase at an axial end (right end as viewed in  FIGS. 3 and 4 ) and a pair of second support arms  412  of a same phase at the other axial end (left end as viewed in  FIGS. 3 and 4 ). The first support arms  411  are provided, in their edge portions, with a corresponding pair of mutually concentric first support holes  413 . The second support arms  412  are provided, in their edge portions, with a corresponding pair of mutually concentric second support holes  414 .  
         [0030]     The first yoke  42  has a coupling cylinder portion  421  to which an end portion toward the front of the vehicle of the intermediate outer shaft  32  can be fixed. A pair of third support arms  422  is provided at an axial end (left end as viewed in  FIGS. 3 and 4 ) of the coupling cylinder portion  421 . A pair of mutually concentric third support holes  423  is formed in end portions of the third support arms  422 , respectively. Furthermore, a spherical first engaging protrusion  425  protruding in a direction away from the coupling cylinder portion  421  is formed at an intermediate portion of a first connecting part  424  which connects ends of the third support arms  422 .  
         [0031]     The second yoke  43  has a coupling cylinder portion  431  to which an upper end portion of the upper extension shaft  51  is fixable. A pair of fourth support arms  432  is provided at an axial end (right end as viewed in  FIGS. 3 and 4 ) of the coupling cylinder portion  431 . A pair of mutually concentric fourth support holes  433  is formed in end portions of the fourth support arms  432 , respectively. Furthermore, a spherical second engaging protrusion  435  protruding in a direction away from the coupling cylinder portion  431  is formed at an intermediate portion of a second connecting part  434  which connects ends of the fourth support arms  432 .  
         [0032]     First and second axial portions  441  and  442  make up, in a state of being perpendicular to each other, the first spider  44 . Ends of the first axial portion  441  are rotatably supported at inside the first support holes  413 , respectively, by radial needle bearings  415 . Ends of the second axial portion  442  are rotatably supported at inside the third support holes  423 , respectively, by radial needle bearings  415 .  
         [0033]     Third and fourth axial portions  451  and  452  make up, in a state of being perpendicular to each other, the second spider  45 . Ends of the third axial portion  451  are rotatably supported at inside the second support holes  414 , respectively, by radial needle bearings  415 . Ends of the fourth axial portion  452  are rotatably supported at inside the fourth support holes  433 , respectively, by radial needle bearings  415 .  
         [0034]     The intermediate housing  41  includes a first intermediate housing element  416  which includes the first support arms  411  and a second intermediate housing element  417  which includes the second support arms  412 , the first and second intermediate housing elements  416  and  417  being coupled by plural bolts  418 . A space for sliding  419  is provided between the first and second intermediate housing elements  416  and  417 . The centering disc  46  is disposed in the space for sliding  419  such that it is displaceable in a plane perpendicular to a central axis of the intermediate housing  41 .  
         [0035]     First and second spherical engaging holes  461  and  462  are formed, in the same phase (concentrically), in axial end portions of the centering disc  46 . The first and second engaging protrusions  425  and  435  are oscillatingly displaceably engaged with the first and second engaging holes  461  and  462 , respectively. The engagement between the first and second engaging protrusions  425  and  435  and the first and second engaging holes  461  and  462  works to equalize the inclinations with respect to the intermediate housing  41  of the first and second yokes  42  and  43 .  
         [0036]     Therefore, when the first yoke  42  is rotated, a torque generated is transmitted to the second yoke  43  via the first spider  44 , the intermediate housing  41 , and the second spider  45 . As the torque is transmitted, the positional relationship among the first and second yokes  42  and  43 , and the intermediate housing  41  changes, but the change is absorbed as the centering disc  46  disposed in the space for sliding  419  provided in the housing  41  slides in a plane perpendicular to the central axis of the intermediate housing  41 .  
         [0037]     Next, the elastic coupling  7  will be described. The elastic coupling  7  includes, as shown in  FIG. 5 , an inner sleeve  71 , an elastic body  72 , an outer sleeve  73 , and a stopper pin  74 . The stopper pin  74  is a metallic pin which is fixed in an upper end portion of the lower extension shaft  52  in a diameter direction thereof. To allow the stopper pin  74  to be fixed as described above, a pair of through holes  521  facing each other in the diameter direction are formed in the upper end portion of the lower extension shaft  52 .  
         [0038]     The stopper pin  74  inserted through the through holes  521  is supported at its two intermediate portions. Both end portions of the stopper pin  74  have an outer diameter slightly larger than the inner diameter of the through holes  521 . In a state where the stopper pin  74  is fixed as described above, each end portion of the stopper pin  74  protrudes outward from an outer periphery of the lower extension shaft  52  in the diameter direction thereof.  
         [0039]     The elastic body  72  is made of an elastomeric material such as rubber and has a cylindrical shape. The metallic inner and outer sleeves  71  and  73  both having a cylindrical shape are concentrically disposed. An outer peripheral surface of the inner sleeve  71  and an inner peripheral surface of the elastic body  72  are bonded by heating or by using an adhesive. An inner peripheral surface of the outer sleeve  73  and an outer peripheral surface of the elastic body  72  are bonded in a similar way.  
         [0040]     The inner sleeve  71  is fixedly fitted onto an outer periphery  522  of an upper end portion of the lower extension shaft  52 . The outer sleeve  73  is fixedly fitted in an inner periphery  512  of the upper extension shaft  51 .  
         [0041]     There are one pair each of through holes  711 ,  721 ,  731  and  511  formed at one pair each of locations in the inner sleeve  71 , the elastic body  72 , the outer sleeve  73  and a part of cylindrical portion  513  of the upper extension shaft  51 . The one pair each of through holes are aligned straight with the through holes  521  formed in the lower extension shaft  52 , each pair of the through holes facing each other in a diameter direction of the lower extension shaft  52 . Two end portions of the stopper pin  74  supported by the lower extension shaft  52  are inserted in the through holes  711 ,  721 ,  731 , and  511  formed in the inner sleeve  71 , the elastic body  72 , the outer sleeve  73 , and the upper extension shaft  51 , respectively.  
         [0042]     The through holes  711  formed in the inner sleeve  71  have an inner diameter approximately equal to the inner diameter of the through holes  521  formed in the lower extension shaft  52 . The through holes  721  and  731  formed in the elastic body  72  and the outer sleeve  73 , respectively, and the through holes  511  formed in the cylindrical portion  513  of the upper extension shaft  51  have an inner diameter larger than the inner diameter of the through holes  521  formed in the lower extension shaft  52 .  
         [0043]     Therefore, a gap is formed between an outer peripheral surface at each end portion of the stopper  74  and the inner peripheral surface of each of the corresponding one of the through holes  721  formed in the elastic body  72 , the corresponding one of the through holes  731  formed in the outer sleeve  73 , and the corresponding one of the through holes  511  formed in the upper extension shaft  51 .  
         [0044]     The elastic coupling  7  configured as described above operates as follows. When the torque applied from the steering wheel  13  to the steering shaft  12  is small, each end of the stopper pin  74  is positioned in a neutral position or in a position slightly shifted from the neutral position inside each of the through holes  511  formed in the upper extension shaft  51 . In this state, there is a gap between the outer peripheral surface of each end portion of the stopper pin  74  and the inner peripheral surface of the corresponding one of the through holes  511 , the gap extending completely around the outer peripheral surface of each end portion of the stopper pin  74  without allowing the outer peripheral surface of each end portion of the stopper pin  74  to come in contact with the inner peripheral surface of the corresponding one of the through holes  511 .  
         [0045]     In the above state, the small torque is transmitted from the lower extension shaft  52  to the pinion shaft  53 , the pinion  54 , the rack  61 , and the steering gear  6  via the outer sleeve  73 , the elastic body  72 , and the inner sleeve  71 .  
         [0046]     When the vehicle travels, vibrations of the wheels and the vehicle body cause the heavy double Cardan constant velocity universal joint  4  to vibrate. However, with the elastic coupling  7  provided at about the axial midpoint of the extension shaft  5 , vibrations of the double Cardan constant velocity universal joint  4  are absorbed by the elastic body  72  included in the elastic coupling  7 .  
         [0047]     Thus, the vibrations and bending moments applied to the pinion shaft  53  and the pinion  54  are reduced. As a result, steering stability is improved and the pinion shaft  53  is prevented from being subjected to a heavy load. As in the present embodiment, disposing the elastic coupling  7  at about the axial midpoint of the extension shaft  5  makes it possible to appropriately restrain the amplitude of vibrations and the magnitude of bending moments applied to the pinion shaft  53 .  
         [0048]     When the torque applied from the steering wheel  13  to the steering shaft  12  is large as in the case of giving a large steering angle to front wheels, the outer peripheral surface in each end portion of the stopper pin  74  and the inner peripheral surfaces of the corresponding through holes  511 ,  731 , and  721  abut against each other. As a result, a large portion of the torque applied from the steering wheel  13  to the steering shaft  12  is transmitted to the lower extension shaft  52  via the stopper pin  74 . In such a state, the torque transmitted via the elastic body  72  is limited, so that the elastic body  72  is not subjected to an excessive force.  
         [0049]     Even though, in the first embodiment, the pinion shaft  53  and the pinion  54  are not integral with the extension shaft  5 , and the pinion shaft  53  is connected to the lower end of the lower extension shaft  52 , the pinion shaft  53  and the pinion  54  may be configured to be integral with the lower extension shaft  52 . Also, in the first embodiment, the elastic coupling  7  is provided at about the axial midpoint of the extension shaft  5 , but it may be disposed anywhere along the extension shaft  5 , for example, at a point close to the pinion shaft  53  or at a point close to the double Cardan constant velocity universal joint  4 .  
       Second Embodiment  
       [0050]     Next, a second embodiment of the present invention will be described.  FIG. 6 , which is equivalent to  FIG. 2  for the first embodiment, is a partly sectional side elevation view showing principal parts of a steering apparatus according to the second embodiment of the present invention.  FIG. 7  is an exploded perspective view of an elastic coupling shown in  FIG. 6 . In the following, only parts of the steering apparatus of the present embodiment configured differently from those used in the first embodiment will be described. Description which duplicates the foregoing description will be omitted. Parts identical with those used in the first embodiment will be denoted by the same reference numerals as used in describing the first embodiment. The elastic coupling used in the first embodiment is of a stopper pin type. The elastic coupling used in the second embodiment is of a coupling type.  
         [0051]     As shown in  FIGS. 6 and 7 , in the steering apparatus according to the second embodiment, a cylindrical column  1  is fixed to a vehicle body, and a steering shaft  12  is rotatably and axially supported by the column  1 . A steering wheel  13  as shown in the first embodiment is connected to a right end (toward a rear of the vehicle) of the steering shaft  12 . An intermediate shaft  3  is connected to a left end (toward a front of the vehicle) of the steering shaft  12  via a Cardan joint  2 .  
         [0052]     A double Cardan constant velocity universal joint  4  which is identical in configuration with the one used in the first embodiment is connected to a left end of the intermediate shaft  3 . An extension shaft  5  is connected to the double Cardan constant velocity universal joint  4 . A pinion  54  attached to a pinion shaft  53  connected to a lower end of the extension shaft  5  is engaged with a rack  61  of a steering gear  6 .  
         [0053]     The extension shaft  5  includes an upper extension shaft  51 , a lower extension shaft  52 , and an elastic coupling  7  of a coupling type, a lower end of the upper extension shaft  51  and an upper end of the lower extension shaft  52  being connected by the elastic coupling  7 . The elastic coupling  7  of a coupling type is provided at a position closer, than an axial midpoint of the extension shaft  5 , to an upper end thereof. The pinion shaft  53  and the pinion  54  are not integral with the extension shaft  5 . They are fixedly clamped to a coupling  523  formed at a lower end of the lower extension shaft  52 .  
         [0054]     As shown in  FIGS. 6 and 7 , the elastic coupling  7  of a coupling type used in the second embodiment includes a vibration-proof disc-shaped rubber  75  made of an elastomeric material such as rubber. Four bolt through-holes are formed through, along a left-to-right direction as viewed in  FIG. 7 , the vibration-proof rubber  75  to be 90 degrees apart. Metallic cylindrical members  751   a  to  751   d  are fit in the bolt through-holes, respectively.  
         [0055]     A pair of the cylindrical members  751   a  and  751   b  opposed to each other are fit such that they protrude from a left face of the vibration-proof rubber  75 . A pair of the cylindrical members  751   c  and  751   d  opposed to each other is fit such that they protrude from a right face of the vibration-proof rubber  75 . Plastic collars  77   a  to  77   d  are fit onto outer peripheries of protruding portions of the cylindrical members  751   a  to  751   d , respectively, the plastic collars  77   a  to  77   d  being about as high as the protruding portions.  
         [0056]     The lower extension shaft  52  and the upper extension shaft  51  are connected to each other using the elastic coupling  7 . At this time, a metallic stopper plate  76   a  having notched portions  761   a  and  761   b  corresponding to the plastic collars  77   a  and  77   b  is fit between the lower extension shaft  52  and the vibration-proof rubber  75 , and a metallic stopper plate  76   b  having notched portions  761   c  and  761   d  corresponding to the plastic collars  77   c  and  77   d  is fit between the upper extension shaft  51  and the vibration-proof rubber  75 .  
         [0057]     A pair of bolt holes  762   c  and  762   d  are formed on both sides of a pair of the notched portions  761   a  and  761   b  of the stopper plate  76   a , the bolt holes  762   c  and  762   d  corresponding to a pair of the cylindrical members  751   c  and  751   d . A pair of bolt holes  762   a  and  762   b  are formed on both sides of a pair of the notched portions  761   c  and  761   d  of the stopper plate  76   b , the bolt holes  762   a  and  762   b  corresponding to a pair of the cylindrical members  751   a  and  751   b.    
         [0058]     The stopper plates  76   a  and  76   b  are to prevent the vibration-proof rubber  75  from being twisted when the extension shaft  5  rotates. The notched portions  761   a  to  761   d  abut against the plastic collars  77   a  to  77   d , respectively, to restrict turning of the vibration-proof rubber  75 .  
         [0059]     At the upper end (right end as viewed in  FIG. 7 ) of the lower extension shaft  52 , a flange  524  to be fixed to the vibration-proof rubber  75  is formed integrally with the lower extension shaft  52 . A pair of bolt through-holes  525   a  and  525   b  is formed in two end portions of the flange  524 , respectively. At the lower end (left end as viewed in  FIG. 7 ) of the upper extension shaft  51 , a flange  514  to be fixed to the vibration-proof rubber  75  is formed integrally with the upper extension shaft  51 . A pair of bolt through-holes  515   c  and  515   d  is formed in two end portions of the flange  514 , respectively.  
         [0060]     To fix the lower extension shaft  52  and the upper extension shaft  51  to the vibration-proof rubber  75  using bolts, first the flange  524  of the lower extension shaft  52  is positioned on the cylindrical members  751   a  and  751   b  protruding from the left face of the vibration-proof rubber  75 , then bolts  79   a  and  79   b  are inserted, respectively, through the bolt through-holes  525   a  and  525   b  formed in the flange  524 . The bolts  79   a  and  79   b  are then further inserted through the cylindrical members  751   a  and  751   b  fit in the vibration-proof rubber  75  and the bolt holes  762   a  and  762   b  formed in the stopper plate  76   b  disposed on the right face of the vibration-proof rubber  75 . Subsequently, nuts  78   a  and  78   b  are fixedly screwed to threaded end portions of the bolts  79   a  and  79   b , respectively.  
         [0061]     Next, the bolt through-holes  515   c  and  515   d  formed in the flange  514  of the upper extension shaft  51  are aligned with the cylindrical members  751   c  and  751   d  protruding from the right face of the vibration-proof rubber  75 , then bolts  79   c  and  79   d  are inserted, respectively, through the bolt through-holes  515   c  and  515   d  formed in the flange  524 . The bolts  79   c  and  79   d  are then further inserted through the cylindrical members  751   c  and  751   d  fit in the vibration-proof rubber  75  and the bolt holes  762   c  and  762   d  formed in the stopper plate  76   a  disposed on the left face of the vibration-proof rubber  75 . Subsequently, nuts  78   c  and  78   d  are fixedly screwed to threaded end portions of the bolts  79   c  and  79   d , respectively.  
         [0062]     In cases where the elastic coupling  7  of a coupling type, configured as described above, of the second embodiment is used, wheel and vehicle body vibrations generated when the vehicle travels cause the heavy double Cardan constant velocity universal joint  4  to vibrate, but the vibrations of the double Cardan constant velocity universal joint  4  are absorbed by twisting movement of the vibration-proof rubber  75 . Thus, the vibrations and bending moments applied to the pinion shaft  53  and the pinion  54  are reduced. As a result, steering stability is improved and the pinion shaft  53  is prevented from being subjected to a heavy load.  
         [0063]     When the torque applied from the steering wheel  13  to the steering shaft  12  is large as in the case of giving a large steering angle to front wheels, the notched portions  761   a  to  761   d  formed in the stopper plates  76   a  and  76   b  abut against the plastic collars  77   a  to  77   d , respectively.  
         [0064]     As a result, a large portion of the torque applied from the steering wheel  13  to the steering shaft  12  is transmitted to the lower extension shaft  52  via the stopper plates  76   a  and  76   b  and the plastic collars  77   a  to  77   d . In such a state, the torque transmitted via the vibration-proof rubber  75  is limited, so that the vibration-proof rubber  75  is not subjected to an excessive force.  
       Third Embodiment  
       [0065]     Next, a third embodiment of the present invention will be described.  FIG. 8 , which is equivalent to  FIG. 2  for the first embodiment, is a partly sectional side elevation view of a steering apparatus according to the third embodiment of the present invention. In the following, only parts of the steering apparatus of the present embodiment configured differently from those used in the foregoing embodiments will be described. Description which duplicates the foregoing description will be omitted. Parts identical with those used in the foregoing embodiments will be denoted by the same reference numerals as used in describing the foregoing embodiments.  
         [0066]     The third embodiment represents a case in which a variable gear ratio steering (VGRS) device is provided between a double Cardan constant velocity universal joint  4  and an extension shaft  5 , the variable gear ratio steering (VGRS) device being for changing a ratio between a steering angle of a steering wheel and a turning angle of front wheels, that is, a steering angle ratio depending on the running speed of a vehicle.  
         [0067]     As shown in  FIG. 8 , in the steering apparatus according to the third embodiment, a cylindrical column  1  is fixed to a vehicle body, and a steering shaft  12  is rotatably and axially supported by the column  1 . A steering wheel  13  as shown in the first embodiment is connected to a right end (toward a rear of the vehicle) of the steering shaft  12 . An intermediate shaft  3  is connected to a left end (toward a front of the vehicle) of the steering shaft  12  via a Cardan joint  2 .  
         [0068]     A double Cardan constant velocity universal joint  4  which is identical in configuration with the one used in the first embodiment is connected to a left end of the intermediate shaft  3 . An extension shaft  5  is connected to the double Cardan constant velocity universal joint  4 . A pinion  54  formed at a lower end of the extension shaft  5  integrally therewith is engaged with a rack  61  of a steering gear  6 .  
         [0069]     The extension shaft  5  includes an upper extension shaft  51 , a lower extension shaft  52 , and an elastic coupling  7  which is identical in configuration with the elastic coupling  7  used in the second embodiment, a lower end of the upper extension shaft  51  and an upper end of the lower extension shaft  52  being connected by the elastic coupling  7 . The elastic coupling  7  of a coupling type is provided at a position closer, than an axial midpoint of the extension shaft  5 , to a lower end thereof.  
         [0070]     A variable gear ratio steering (VGRS) device  8  is mounted at about an axial midpoint of the upper extension shaft  51 . The variable gear ratio steering (VGRS) device  8  internally includes a differential transmission mechanism such as a harmonic drive gearing mechanism for speed reduction, for example, like the one disclosed in JP-A No. 211541/2000 or a planetary gear mechanism. It is a device to continuously change a steering angle ratio according to the running speed of a vehicle, decreasing the turning angle of the steering wheel when the vehicle is running at low speed and increasing the turning angle of the steering wheel when the vehicle is running at high speed.  
         [0071]     When the steering apparatus, configured as described above, of the third embodiment is used, wheel and vehicle body vibrations generated when the vehicle travels cause the heavy double Cardan constant velocity universal joint  4  and the variable gear ratio steering (VGRS) device  8  to vibrate, but the vibrations of the double Cardan constant velocity universal joint  4  and the variable gear ratio steering (VGRS) device  8  are absorbed by twisting movement of the vibration-proof rubber  75 . Thus, the vibrations and bending moments applied to the pinion  54  are reduced. As a result, steering stability is improved and the pinion  54  is prevented from being subjected to a heavy load.  
       Fourth Embodiment  
       [0072]     Next, a fourth embodiment of the present invention will be described.  FIG. 9 , which is equivalent to  FIG. 2  for the first embodiment, is a partly sectional side elevation view of a steering apparatus according to the fourth embodiment of the present invention.  FIG. 10  is a cross-sectional view of a constant velocity universal joint shown in  FIG. 9 . In the following, only parts of the steering apparatus of the present embodiment configured differently from those used in the foregoing embodiments will be described. Description which duplicates the foregoing description will be omitted. Parts identical with those used in the foregoing embodiments will be denoted by the same reference numerals as used in describing the foregoing embodiments. In the fourth embodiment, a Rzeppa constant velocity universal joint is used instead of the double Cardan constant velocity universal joint  4  used in the foregoing embodiments.  
         [0073]     As shown in  FIG. 9 , in the steering apparatus according to the fourth embodiment, a cylindrical column  1  is fixed to a vehicle body, and a steering shaft  12  is rotatably and axially supported by the column  1 . A steering wheel  13  as shown in the first embodiment is connected to a right end (toward a rear of the vehicle) of the steering shaft  12 . An intermediate shaft  3  is connected to a left end (toward a front of the vehicle) of the steering shaft  12  via a Cardan joint  2 .  
         [0074]     A Rzeppa constant velocity universal joint  47  is connected to a left end of the intermediate shaft  3 . An extension shaft  5  is connected to the Rzeppa constant velocity universal joint  47 . A pinion  54  formed at a lower end of the extension shaft  5  integrally therewith is engaged with a rack  61  of a steering gear  6 .  
         [0075]     The extension shaft  5  includes an upper extension shaft  51 , a lower extension shaft  52 , and an elastic coupling  7  which is identical in configuration with the elastic coupling  7  used in the second embodiment, a lower end of the upper extension shaft  51  and an upper end of the lower extension shaft  52  being connected by the elastic coupling  7 . The elastic coupling  7  of a coupling type is provided at about an axial midpoint of the extension shaft  5 . In the fourth embodiment, an Oldham&#39;s coupling may be disposed on the intermediate shaft  3  or on the extension shaft  5  instead of using a Cardan joint  2 .  
         [0076]      FIG. 10  is a detailed sectional view of the Rzeppa constant velocity universal joint  47 . A spherical inner coupling member  471  is formed at an upper end of the upper extension shaft  51  integrally therewith. A ball guide groove  472  is formed on the spherical surface of the inner coupling member  471 . An outer coupling member  473  having a spherical concave portion is formed at a lower end of an intermediate outer shaft  32  integrally therewith, the intermediate outer shaft  32  being included in the intermediate shaft  3 . A ball guide groove  474  is formed in the concave portion.  
         [0077]     Plural torque transmitting balls  475  are rollably fit in the ball guide grooves  472  and  474 , enabling torque transmission between the inner coupling member  471  and the outer coupling member  473 . A ball retainer  476  having an inner surface complementary to the spherical surface of the inner coupling member  471  can rotate about a center of sphere of the inner coupling member  471  while retaining the torque transmitting balls  475 .  
         [0078]     Even though, in the above embodiment, the steering shaft  12  and the intermediate inner shaft  31  are connected by the Cardan joint  2 , the Cardan joint  2  may be omitted in cases where the steering shaft  12  and the intermediate inner shaft  31  can be arranged straightway with no assembly error. Even though the above embodiment has been described based on a case where a steering apparatus not allowing a position of the steering wheel  13  to be adjusted depending on a physical size of a driver is used, the embodiment may also be applied to a steering apparatus which allows at least either one of a tilted position or a telescopic position of the steering wheel  13  to be adjusted.