Abstract:
To make a bellows section ( 1 ) of a boot to be flexibly and repeatedly deformable without deterioration in durability, to thereby reduce rotational resistance when a joint angle is large, the boot has the bellows portion ( 1 ) where three or more circumferentially extending crest portions ( 11   1 - 11   3 ) and root portions ( 12   1 - 12   2 ) therebetween are alternately formed; a large-diameter attaching portion ( 2 ) formed at one end of the bellows portion; and a small-diameter attaching portion ( 3 ) formed at the other end, a rising wall ( 1   a ) closest to the large-diameter attaching portion ( 2 ) and a rising wall ( 1   f ) closest to the small-diameter attaching portion ( 3 ) have smaller wall thickness than other portions of the bellows section ( 1 ), and the ratio (t 1 /t 2 ) of the thickness (t 1 ) of the rising walls ( 1   a   , 1   f ) to the thickness (t 2 ) of the other portions of the bellows portion ( 1 ) is set to 0.6≦t 1 /t 2 &lt;1.0.

Description:
[0001]    This is a national stage of the International Application No. PCT/JP2006/319658 filed Oct. 2, 2006 and published in Japanese. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a boot installed to a universal joint, for example, a constant velocity joint (CVJ) or the like used in a steering wheel of a motor vehicle or the like, for sealing the universal joint. 
         [0004]    2. Description of the Conventional Art 
         [0005]    The constant velocity joint in the steering wheel of the motor vehicle is sealed by a boot for preventing leakage of grease in an inner portion and intrusion of muddy water or dust from an outer portion.  FIG. 8  is a side view showing this kind of a boot  100  for universal joint by partly cutting it together with a constant velocity joint  200 , and  FIG. 9  is a cross sectional view showing a state in which the boot  100  for universal joint in  FIG. 8  is deformed, together with a part of the constant velocity joint  200 . 
         [0006]    The boot  100  shown in  FIG. 8  is formed by a rubber-like elastic material or the like in accordance with a blow molding, an injection molding or the like, and is constituted by a flexible bellows portion  101  in which a plurality of crest portions  101   a ,  101   c  and  101   e  extending in a circumferential direction and root portions  101   b  and  101   d  between them are alternately formed, a large-diameter attaching portion  102  formed at one end of the bellows portion  101 , and a small-diameter attaching portion  103  formed at the other end of the bellows portion  101  (refer, for example, to Japanese Unexamined Patent Publication No. 2000-266072, and Japanese Unexamined Patent Publication No. 2003-049944). 
         [0007]    The boot  100  is structured such that the large-diameter attaching portion  102  is fastened and fixed to an outer peripheral surface of an end portion of an outer ring  201  of the constant velocity joint  200  formed at one rotating shaft  202  side of the steering wheel by a metal fastening band  104 , and the small-diameter attaching portion  103  is fastened and fixed to an outer peripheral surface of the other rotating shaft  203  of the steering wheel rotatably coupled so as to be freely displaced angularly with respect to an axis of the outer ring  201  (the rotating shaft  202 ) by an internal bearing mechanism of the constant velocity joint  200 , by means of another metal fastening band  105 . 
         [0008]    Further, since the boot  100  is rotated with the outer ring  201  (the rotating shaft  202 ) and the rotating shaft  203 , there is repeated such a deformation that the bellows portion  101  is extended in a half cycle of the rotation and is compressed in the other half cycle, in a state in which the rotating shaft  203  is angularly displaced with respect to the rotating shaft  202 . Further, in the case that a joint angle (an angle of the rotating shaft  203  with respect to the rotating shaft  202 ) is high, the crest portions  101   a ,  101   c  and  101   e  and the root portions  101   b  and  101   d  respectively come into contact with each other in the half cycle in which the bellows portion  101  is compressed as shown in the lower half part of  FIG. 9 . Thus, there is a risk that a contact surface pressure is high, whereby steering torque is enlarged due to increase of rotational resistance and early abrasion is generated. 
       SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
       [0009]    The present invention is made by taking the above-mentioned problem into consideration and the object of the present invention is to make it possible that the bellows portion is repeatedly deformed flexibly without deterioration in durability to thereby reduce rotational resistance in the case of the joint angle being high. 
       Means for Solving the Problem 
       [0010]    As a means for effectively solving the technical problem mentioned above, in accordance with a first aspect of the present invention, there is provided a boot for a universal joint comprising: 
         [0011]    a bellows portion in which three or more crest portions extending in a circumferential direction and root portions therebetween are alternately formed; 
         [0012]    a large-diameter attaching portion formed at one end of the bellows portion; and 
         [0013]    a small-diameter attaching portion formed at the other end of the bellows portion, 
         [0014]    wherein at least one of a rising wall closest to the large-diameter attaching portion in the bellows portion and a rising wall closest to the small-diameter attaching portion is formed thinner than the other portions in the bellows portion. 
         [0015]    In accordance with a second aspect of the present invention, there is provided a boot for a universal joint as recited in the first aspect, 
         [0016]    wherein a thickness ratio t 1 /t 2  is set to 0.6≦t 1 /t 2 ≦1.0, in which t 1  is thickness of the rising wall closest to the large-diameter attaching portion, which is relatively thin, or thickness of the rising wall closest to the small-diameter attaching portion, which is relatively thin, and t 2  is thickness of the other portions in the bellows portion. 
         [0017]    In accordance with a third aspect of the present invention, there is provided a boot for a universal joint as recited in the first aspect, wherein an angle θ 1  of inclination of the rising wall at the small-diameter attaching portion side of the crest portion closest to the large-diameter attaching portion is formed larger than an angle θ 2  of inclination of the adjacent rising wall at the small-diameter attaching portion side, and an angle θ 4  of inclination of the rising wall at the large-diameter attaching portion side of the crest portion closest to the small-diameter attaching portion is formed larger than an angle θ 3  of inclination of the adjacent rising wall at the large-diameter attaching portion side. 
         [0018]    Further, in accordance with a fourth aspect of the present invention, there is provided a boot for a universal joint as recited in the first aspect, wherein a distance P 1  in an axial direction between the crest portion closest to the large-diameter attaching portion and a bottom portion thereof at the large-diameter attaching portion side is formed smaller than a distance P 2  in an axial direction between the crest portion closest to the large-diameter attaching portion and the adjacent root portion at the small-diameter attaching portion side, and a distance P 4  in an axial direction between the crest portion closest to the small-diameter attaching portion and a bottom portion thereof at the small-diameter attaching portion side is formed smaller than a distance P 3  in an axial direction between the crest portion closest to the small-diameter attaching portion and the adjacent root portion at the large-diameter attaching portion side. 
       EFFECT OF THE INVENTION 
       [0019]    In accordance with the boot for a universal joint on the basis of the first aspect of the present invention, even if an angle of the joint is high, the bellows portion can be deformed largely to an outer diameter side at the side of being compressed in accordance with the rotation. Further, since a rigidity of at least one of the rising wall closest to the large-diameter attaching portion in the bellows portion and the rising wall closest to the small-diameter attaching portion is small, an increase of contact force between the crest portions and between the root portions is restricted and an increase of rotational resistance in a steering operation is restricted, even if the crest portions and the root portions respectively come into contact with each other in accordance with the rotation. 
         [0020]    In accordance with the boot for a universal joint on the basis of the second aspect of the present invention, it is possible to reduce the rotational resistance without lowering the durability, and it is possible to restrict the increase of the contact force between the crest portions and between the root portions. 
         [0021]    In accordance with the boot for a universal joint on the basis of the third and fourth aspects of the present invention, since a start of the contact between the crest portions and between the root portions at the compression side of the bellows portion in accordance with the rotation is delayed, that is, a contact start angle is enlarged, it is possible to restrict the increase of the contact force, and it is restrict to suppress the increase of the rotational resistance in a steering. 
     
    
     
       BRIEF EXPLANATION OF DRAWINGS 
         [0022]      FIG. 1  is a side view showing a boot for a universal joint in accordance with a preferable first embodiment of the present invention together with a constant velocity joint  2  in a partly cutting manner; 
           [0023]      FIG. 2  is a side view showing a boot for a universal joint in accordance with a preferable second embodiment of the present invention together with the constant velocity joint  2  in a partly cutting manner; 
           [0024]      FIG. 3  is a side view showing a boot for a universal joint in accordance with a preferable third embodiment of the present invention together with the constant velocity joint  2  in a partly cutting manner; 
           [0025]      FIG. 4  is an explanatory view showing a relation between a joint angle and a contact force between both sides of a root portion  12   1 , which is obtained in accordance with an FEM, to compare an embodiment with a comparative example; 
           [0026]      FIG. 5  is an explanatory view showing a result of conducting a relation between a joint angle and a contact force between both sides of a root portion  12   2  in accordance with the FEM, to compare the embodiment with the comparative example; 
           [0027]      FIG. 6  is an explanatory view showing a relation between the joint angle and a sum of the contact forces applied to the bellows portion  1 , to compare the embodiment with a boot in accordance with the comparative example (t 1 =t 2 ) in which only the thickness of the bellows portion  1  is different from the embodiment; 
           [0028]      FIG. 7  is an explanatory view showing a measurement result of a crack generating time and a rotational torque at a time of variously changing the thickness ratio t 1 /t 2  and rotating at the joint angle of 40 deg in a boot rotation durability test; 
           [0029]      FIG. 8  is a side view showing a conventional boot  100  for a universal joint together with a constant velocity joint  200  in a partly cutting manner; and 
           [0030]      FIG. 9  is a cross sectional view showing a state in which the boot  100  for the universal joint is deformed in  FIG. 8 , together with a part of the constant velocity joint  200 . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0031]    A description will be given below of preferable embodiments of a boot for a universal joint in accordance with the present invention with reference to the accompanying drawings.  FIG. 1  is a cross sectional view showing a boot for a universal joint in accordance with a first embodiment. 
         [0032]    A boot shown in  FIG. 1  is formed by a rubber-like elastic material or a synthetic resin having a flexibility in accordance with a blow molding, an injection molding or the like, and is basically constituted by a flexible bellows portion  1  in which three crest portions  11   1 ,  11   2  and  11   3  extending in a circumferential direction and two root portions  12   1  and  12   2  therebetween are alternately formed, a large-diameter attaching portion  2  formed at one end of the bellows portion  1 , and a small-diameter attaching portion formed at the other end of the bellows portion  1 , in the same manner as the conventional structure described above. 
         [0033]    A band attaching groove  2   a  which is continuous in a circumferential direction for attaching a metal fastening band is formed on an outer peripheral surface of the large-diameter attaching portion  2 , and a fitting protruding stripe  2   b  which is continuous in the circumferential direction is formed in an inner peripheral surface. Further, in the same manner, a band attaching groove  3   a  which is continuous in the circumferential direction for attaching another metal fastening band is formed on an outer peripheral surface of the small-diameter attaching portion  3 , and a fitting protruding stripe  3   b  which is continuous in the circumferential direction is formed on the inner peripheral surface. 
         [0034]    Thickness t 1  of a rising wall  1   a  closest to the large-diameter attaching portion  2  in the bellows portion  1  and a rising wall  1   f  closest to the small-diameter attaching portion  3  is, for example, 1.2 mm, and thickness t 2  of the other portion (root portions  12   1  and  12   2  and rising walls  1   b  to  1   e  constructing inclined surfaces at both sides of the root portions  12   1  and  12   2 ) in the bellows portion  1  is, for example, 1.5 mm. Accordingly, a relation t 1 &lt;t 2  is established. Further, in accordance with test results mentioned below, it is preferable to set a ratio (a thickness ratio) t 1 /t 2  between t 1  and t 2  is set to 0.6≦t 1 /t 2 &lt;1.0. 
         [0035]    Further, an angle θ 1  of inclination of the rising wall  1   b  at the small-diameter attaching portion  3  side of a first crest portion  11   1  closest to the large-diameter attaching portion  2  is formed larger than an angle θ 2  of inclination of the rising wall  1   c  which is adjacent at the small-diameter attaching portion  3  side beyond the root portion  12   1  (θ 1 &gt;θ 2 ), and an angle θ 4  of inclination of the rising wall  1   e  at the large-diameter attaching portion  2  side of a third crest portion  11   3  closest to the small-diameter attaching portion  3  is formed larger than an angle θ 3  of inclination of the rising wall  1   d  which is adjacent at the large-diameter attaching portion  2  side beyond a root portion  12   2  (θ 3 &lt;θ 4 ). In this case, the angles θ 1  to θ 4  of inclination correspond to angles of inclination with respect to planes which orthogonally intersect an axis. 
         [0036]    Further, a pitch P 1  in an axial direction between the first crest portion  11   1  and a bottom portion  13   1  at the large-diameter attaching portion  2  side is formed smaller than a pitch P 2  in an axial direction between the first crest portion  11   1  and the adjacent root portion  12   1  at the small-diameter attaching portion  3  side, and a pitch P 4  in an axial direction between the third crest portion  11   3  and a bottom portion  13   2  at the small-diameter attaching portion  3  side is formed smaller than a pitch P 3  in an axial direction between the third crest portion  11   3  and the adjacent root portion  12   2  at the large-diameter attaching portion  2  side. 
         [0037]    In the boot provided with the structure mentioned above, in the same manner as the prior art in  FIG. 8  described above, the large-diameter attaching portion  2  is fastened and fixed to the forming portion of the fitting groove on the outer peripheral surface of the outer ring of the constant velocity joint by a fastening band, and the small-diameter attaching portion  3  is fastened and fixed to the forming portion of the fitting groove on an outer peripheral surface of a rotating shaft of a steering wheel by another fastening band, whereby it is possible to prevent foreign matters from making an intrusion into an internal bearing mechanism of the constant velocity joint from an external portion, and it is possible to prevent lubricating grease or the like filled in the bearing mechanism from leaking to an outer portion. 
         [0038]    Further, since the boot is rotated together with the outer ring or the constant velocity joint and the rotating shaft in accordance with the operation of the steering wheel, there is repeated such a deformation that the bellows portion  1  is extended in a half cycle of the rotation and is compressed in the other half cycle in accordance with a joint angle. 
         [0039]    Further, in accordance with this aspect, since the thickness t 1  of the rising wall  1   a  closest to the large-diameter attaching portion  2  in the bellows portion  1  and the rising wall  1   f  closest to the small-diameter attaching portion  3  is thinner than the thickness t 2  of the other portions (the root portions  12   1  and  12   2  and the rising walls  1   b  to  1   e  constructing the inclined surfaces at both sides of the root portions  12   1  and  12   2 ) in the bellows portion  1 , the bellows portion  1  can be deformed largely to the outer diameter side, at the side where the bellows portion  1  is compressed in accordance with the rotation, even if the joint angle is high. Further, since the thickness t 1  of the rising walls  1   a  and  1   f  is small, whereby the rigidity thereof is small, the increase of the contact force between the crest portions  11   1  to  11   3  and between the root portions  12   1  and  12   2  can be restricted even if the crest portions  11   1  to  11   3  and the root portions  12   1  and  12   2  come into contact with each other at the side where the bellows portion  1  is compressed in accordance with the rotation. As a result, the increase of the rotational resistance in the steering operation can be restricted. 
         [0040]    Further, since the relation θ 1 &gt;θ 2  and θ 3 &lt;θ 4  is established, the rising walls  1   b  and  1   e  rising while sandwiching the root portions  12   1  and  12   2  at both sides of the intermediate second crest portion  11   2  are formed in such a shape as to incline more largely to the axial direction than the rising walls  1   c  and  1   d  of the second crest portion  11   2 . Accordingly, the second crest portion  11   2  tends to be readily folded in the half cycle in which the bellows portion  1  is compressed in accordance with the rotation. Further, since the crest of the first crest portion  1  is unevenly positioned at the large-diameter attaching portion  2  side due to the relation P 1 &lt;P 2 , and the crest of the third crest portion  11   3  is unevenly positioned at the small-diameter attaching portion  3  side due to the relation P 3 &gt;P 4 , it is possible to set a pitch P 5  between the first crest portion  11   1  and the second crest portion  11   2  and a pitch P 6  between the second crest portion  11   2  and the third crest portion  11   3  comparatively large in the limited size of the boot. 
         [0041]    Accordingly, at the side where the bellows portion  1  is compressed at a time of enlarging the joint angle, such a deformation that the second crest portion  11   2  is folded is first started, the pitches P 5  and P 6  between the second crest portion  11   2 , and the first crest portion  11   1  and the third crest portion  11   3  are then narrowed, and the crest portions  11   1  to  11   3  and the root portions  12   1  and  12   2  are respectively brought into contact with each other. Therefore, it is possible to delay the contact start at the compression side of the bellows portion  1  (enlarge the contact start angle). In other words, the increase of the contact force can be thereby restricted, and the increase of the rotational resistance in the steering operation can be restricted. 
         [0042]    In this case, in the first embodiment mentioned above, both of the rising wall  1   a  closest to the large-diameter attaching portion  2  in the bellows portion  1  and the rising wall  1   f  closest to the small-diameter attaching portion  3  are formed thinner than the other portions in the bellows portion  1 , however, only one of the rising walls may be formed thinner than the other portions. 
         [0043]      FIG. 2  is a side view showing a boot for a universal joint in accordance with a preferable second embodiment of the present invention together with the constant velocity joint  2  in a partly cutting manner. The boot is structured such that a thickness t 1  of the rising wall  1   a  closest to the large-diameter attaching portion  2  in the bellows portion  1  is formed thinner than a thickness t 2  of the other portions (the root portions  12   1  and  12   2 , the rising walls  1   b  to  1   e  constructing the inclined surfaces of the root portions  12   1  and  12   2 , and the rising wall  1   f  closest to the small-diameter attaching portion  3 ) in the bellows portion  1 . The other portions are structured in the same manner as  FIG. 1 . 
         [0044]    Further,  FIG. 3  is a side view showing a boot for a universal joint in accordance with a preferable third embodiment of the present invention together with the constant velocity joint  2  in a partly cutting manner. The boot is structured such that a thickness t 1  of the rising wall  1   f  closest to the small-diameter attaching portion  3  in the bellows portion  1  is formed thinner than a thickness t 2  of the other portions (the root portions  12   1  and  12   2 , the rising walls  1   b  to  1   e  constructing the inclined surfaces of the root portions  12   1  and  12   2 , and the rising wall  1   a  closest to the large-diameter attaching portion  2 ) in the bellows portion  1 . The other portions are structured in the same manner as  FIG. 1 . 
         [0045]    In the case of the boot for a universal joint in accordance with the second or third embodiment, there can be expected the effect that the increase of the contact force is restricted at a time when the crest portions  11   1  to  11   3  and the root portions  12   1  and  12   2  come into contact with each other at the side where the bellows  1  is compressed, and the increase of the rotational resistance in the steering operation is restricted. 
       EMBODIMENT 
       [0046]    Table 1 shows a result obtained by comparing a boot in accordance with an embodiment of the present invention structured such that the contact start angles between both sides of the root portion  12   1  and both sides of the root portions  12   2  at the compression side of the bellows portion  1  are the same as those shown in  FIG. 1 , at a time of enlarging the joint angle, with a boot in accordance with a comparative example in which the attaching shape, the attaching dimension and the set length are the same as those of the embodiment, but the thickness of the bellows portion  1  is made uniform (all set to t 2 ), θ 1  is equal to θ 2  and θ 3  is equal to θ 4 . From this table 1, it is known that the contact start at the compression side is widely delayed in accordance with the boot of the present invention. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
             
             
               
                   
                   
               
               
                   
                 Contact start angle (deg) 
                   
               
             
          
           
               
                   
                 Comparative example 
                 Embodiment 
               
               
                   
                   
               
             
          
           
               
                   
                 Root portion 12 1   
                 17.5 
                 45 
               
               
                   
                 Root portion 12 2   
                 22.5 
                 45 
               
               
                   
                   
               
             
          
         
       
     
         [0047]    Further,  FIG. 4  is an explanatory view showing a result of conducting a relation between the joint angle and the contact force between both sides of the root portion  12   1  in accordance with the FEM, to compare the boot in accordance with the embodiment with the boot in accordance with the comparative example,  FIG. 5  is an explanatory view showing a relation between the joint angle and the contact force between both sides of the root portion  12   2  to compare the boot in accordance with the embodiment with the boot in accordance with the comparative example, and  FIG. 6  is an explanatory view showing a relation between the joint angle and the sum of the contact forces applied to the bellows portion  1  to compare the boot in accordance with the embodiment with the boot in accordance with the comparative example (t 1 =t 2 ) in which only the thickness of the bellows portion  1  is different from the embodiment. As is apparent from these results, it is known that the increase of the contact force is small in the boot in accordance with the embodiment even if the joint angle becomes high. 
         [0048]    Further,  FIG. 7  is an explanatory view showing a measurement result of a crack generating time (a lasting time) and rotational torque at a time of variously changing a thickness ratio t 1 /t 2  between the thickness t 1  of the rising wall  1   a  closest to the large-diameter attaching portion  2  in the bellows portion  1  and the rising wall  1   f  closest to the small-diameter attaching portion  3 , and the thickness t 2  of the other portions in the bellows portion  1 , and rotating at the joint angle of 40 deg, in a boot rotation durability test. 
         [0049]    As is apparent from  FIG. 7 , it is known that no large change is observed in the lasting time in the case that the thickness ratio is equal to or more than 0.6, but the durability is suddenly lowered in the case that the thickness ratio is less than 0.6. Further, it is known that the rotational torque is increased at an approximately fixed rate in accordance with the increase of the thickness ratio in the case that the thickness ratio is equal to or less than 1.5, but the torque is suddenly increased in the case that the thickness ratio is more than 1.5. Further, since it is effective to set the relation t 1 &lt;t 2  in order to restrict the increase of the contact force due to the compression of the bellows portion  1 , as is described above, it is known that the thickness ratio t 1 /t 2  should be preferably set to 0.6≦t 1 /t 2 &lt;1.0. 
       INDUSTRIAL APPLICABILITY 
       [0050]    In the boot for a universal joint in accordance with the present invention, since the increase of the contact force is restricted and the increase of the rotational resistance is restricted, even if the crest portions and the root portions of the bellows portion respectively come into contact with each other in accordance with the rotation, the boot can be utilized as the means for sealing a universal joint by being installed to the universal joint, for example, a constant velocity joint or the like used in a steering wheel of a motor vehicle.