Abstract:
Disclosed is a structure for fastening a ring gear to a differential case. The ring gear has a gear-side press-fit face annularly formed thereon, a projected portion located at an inner position relative to the gear-side press-fit face, and a notch portion located opposite to the gear-side press-fit face across the projected portion. The differential case has a case-side press-fit face which is annularly formed thereon and over which the gear-side press-fit face is press fitted, a caulk portion which is smaller than the case-side press-fit face in outer diameter and caulked to the notch portion, and a case-side smooth face which contacts the projected portion to position the ring gear with respect to the differential case. This arrangement serves to reduce the differential case in size.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a 371 national phase application of PCT/JP2010/059536 filed on 4 Jun. 2010, the entire contents of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a structure for fastening a ring gear to a differential case, and a differential device employing the same. 
       BACKGROUND OF THE INVENTION 
       [0003]    A diff gear (differential gear) used for a drive mechanism of an automobile is specifically used for a shaft for coupling drive wheels of the automobile as one example of a differential device to absorb a speed difference of an inner wheel and an outer wheel when the automobile goes round a curve. 
         [0004]    To be concise, the differential gear consists of a ring gear held outside the differential case, a pinion gear placed in and attached to the differential case; and a gear attached to an axle to be engaged with the pinion gear. 
         [0005]    A drive force generated by an engine and others of the automobile is transmitted to the ring gear fastened to the differential case, and the force is further transmitted to the axle by rotating the gear attached to the axle through the pinion gear attached to the differential case. 
         [0006]    As another example of the differential device used in an automobile, there is a LSD designed to make up a defect that an axle rotates idle when one of wheels is under no-load condition. This LSD is similar to the above in a structure including a ring gear provided outside a differential case. 
         [0007]    Heretofore, as a method of fastening a ring gear to a differential case of a differential device, a fastening method using a bolt is adopted. 
         [0008]    However, this fastening method using a bolt leads to a problem of increase in weight due to weight of the bolt, a thickness required for fastening, and the like needed to be taken into consideration. 
         [0009]    Instead of using a bolt, it has been studied as another method that the differential case is fastened with the ring gear by swaging or caulking (for example, refer to Patent Document 1).  FIG. 10  is a schematic configuration view of a differential gear  110  of a conventional art.  FIG. 11  is a diagram showing a mounting operation of a differential gear  112 .  FIG. 12  is a view showing a press-fitting step of a ring gear  103  to be press-fitted in a differential case  102 , showing a state before completion of the press-fitting.  FIG. 13  is a view showing the press-fitting step of the ring gear  103  press-fitted to the differential case  102  after completion of the press-fitting.  FIG. 14  is a view showing a swaging step of the ring gear to be swaged and fixed to the differential case  102 . 
         [0010]    The differential gear  110  in  FIG. 10  applies a fastening structure  101  to fasten the ring gear  103  in a manner that the annular ring gear  103  is press-fitted to an outer peripheral surface of the differential case  102  at its one end in  FIG. 11 , and then the ring gear  103  is fastened by swaging. As shown in  FIGS. 12 to 14 , an outer peripheral surface of the ring gear  103  is provided with a gear part  104  to receive a driving force. An inner peripheral surface of the ring gear  103  is formed with a plurality of notches  105  sequentially arranged in a circumferential direction. 
         [0011]    As shown in  FIG. 12 , the differential case  102  is provided with a case-side press-fitting surface  106  coaxially formed with the case  102  to be press-fitted with the ring gear  103 . On an outer side (right-end side in the figure) of the press-fitting surface  106 , a heel part  107  extends vertically to the press-fitting surface  106  to restrict a press-fitting amount of the ring gear  103 . On an inner side (left-end side in the figure) of the surface  106 , a flange  108  extends in an axial direction of the case  102 . A length of the flange  108  extending from the surface  106  is determined such that the flange  108  protrudes out beyond the ring gear  103  when the ring gear  103  is press-fitted to the press-fitting surface  106  until the ring gear  103  comes into contact with the heel part  107 . 
         [0012]    In the differential gear  110  having the above configuration, as shown in  FIG. 12 , the ring gear  103  is press-fitted from the flange  108  side to the case-side press-fitting surface  106  of the differential case  102 . As shown in  FIG. 13 , the ring gear  103  is press-fitted to the press-fitting surface  106  until an end surface  103   a  comes into contact with the heel part  107 . At this time, the ring gear  103  is press-fitted to the surface  106  so that the notches  105  are positioned opposite to the heel part  107  side. Then, as shown in  FIG. 14 , a part of the flange  108  protruding from the ring gear  103  is bent toward the notches  105  to press against the notches  105 , so that the material of the flange  108  plastically flows into the notches  105 . Thereby, the ring gear  103  is fastened to the notches  105  by swaging the flange  108  and held between a swaged part and the heel part  107 . 
       RELATED ART DOCUMENTS 
     Patent Documents 
       [0000]    
       
         Patent Document 1: EP0647789A1 
       
     
       SUMMARY OF INVENTION 
     Problems to be Solved by the Invention 
       [0014]    As shown in  FIG. 14 , the conventional differential gear  110  includes the heel part  107  outside the case-side press-fitting surface  106  (right-end side in the figure), and thereby the length of the differential case  102  in the axial direction is made long by a thickness C of the heel part  107  in the axial direction as shown in  FIGS. 10 and 11 . Especially, in the press-fitting step that the ring gear  103  is press-fitted to the case-side press-fitting surface  106  of the differential case  102 , a load of as much as 800 kg may be exerted on the heel part  107 , for example. Further, when the differential gear  110  is operated for power transmission, the swaged portions of the flange  108  and the notches  105  are subjected to a load of as much as 2 tons, for example, and an engagement reaction force between the flange  108  and the notches  105  acts on the heel part  107 . In order to counteract these press-fitting load and engagement reaction force, the heel part  107  requires a certain thickness C in the axial direction as shown in  FIGS. 10 ,  11 , and  14 , leading to a long length of the differential case  102  in the axial direction. Furthermore, in order to position the ring gear  103  by the heel part  107 , an outer diameter D 2  of the heel part  107  has to be larger than an outer diameter D 1  of the case-side press-fitting surface  106  as shown in  FIG. 14 . For this reason, in the conventional fastening structure  101  and the differential gear  110 , the heel part  107  largely protrudes outside the ring gear  103 . 
         [0015]    The larger the axial thickness C and the outer diameter D 2  of the heel part  107  are, the heavier the material weight becomes, resulting in cost increase. 
         [0016]    Also, when the heel part  107  protrudes outside the case-side press-fitting surface  106 , components collide against each other during conveyance, causing scratches or dents on the heel part  107 . If the heel part  107  has the scratches or the dents on its surface to face with the end surface  103   a  of the ring gear  103 , the ring gear  103  cannot be precisely positioned in place with respect to the differential case  102 . In this case, the case  102  is regarded as a defective piece, and thereby, a yield could be worsened. 
         [0017]    Further, in the differential gear  110 , the differential gear  112  is attached to a mounting space  111  provided in the case  102  as shown in  FIG. 11 . For automatically mounting the differential gear  112  in the mounting space  111 , the mounting space  111  has to be designed such that the differential gear  112  is entirely accommodated in the case  102 . However, when the axial length of the differential gear  102  is long, it is difficult to have an axial length E of the mounting space  111  long enough to entirely store the differential gear  112 . 
         [0018]    The present invention is made to solve the above problem and to provide a structure for fastening a ring gear to a differential case and a differential device employing the same that enables reduction in size of the differential case. 
       Means of Solving the Problems 
       [0019]    To solve the above problem, one aspect of the present invention is a structure for fastening a ring gear to a differential case, wherein the ring gear includes: an annular gear-side press-fitting surface; a protrusion formed more inside than the gear-side press-fitting surface; and a notch formed on an opposite side from the gear-side press-fitting surface with respect to the protrusion, and the differential case includes: an annular case-side press-fitting surface press-fitted with the gear-side press-fitting surface; a flange swaged into the notch, the flange having an outer diameter smaller than that of the case-side press-fitting surface; and a case-side smooth surface placed in contact with the protrusion so that the ring gear is positioned with respect to the differential case, and a length of the case-side press-fitting surface in an axial direction is formed longer than a length of the protrusion. 
         [0020]    In the above fastening structure of the ring gear and the differential case, preferably, the protrusion is provided orthogonal to an axis of the ring gear, and the case-side smooth surface is provided orthogonal to an axis of the differential case. 
         [0021]    In the above fastening structure of the ring gear and the differential case, preferably, the protrusion is positioned inside the gear-side press-fitting surface such that a length of the gear-side press-fitting surface in a press-fitting direction is equal to a length of the case-side press-fitting surface in the press-fitting direction. 
         [0022]    To solve the above problem, another aspect of the present invention is a differential device using the fastening structure of the ring gear and the differential case according to any one of the above structure for fastening a ring gear to a differential case. 
       Effects of the Invention 
       [0023]    According to the above mentioned structure for fastening the ring gear to the differential case and the differential device employing the same, the case-side press-fitting surface and the gear-side press-fitting surface are press-fitted together until the protrusion of the ring gear comes into contact with the case-side smooth surface. Then, the flange is pressed against the notches to be swaged or caulked. The ring gear is placed in position with respect to the differential case by the close contact of the protrusion with the case-side smooth surface. Since the protrusion is formed inside the gear-side press-fitting surface and placed between the notches and the gear-side press-fitting surface, the contact portion with the case-side smooth surface does not extend outside the differential case. Therefore, the above mentioned fastening structure of the ring gear and the differential case and the differential gear employing the same do not need to provide a protrusion outside the case-side press-fitting surface for positioning the ring gear to the differential case, achieving size reduction of the differential case. 
         [0024]    In the above fastening structure of the ring gear and the differential case, the protrusion is formed orthogonal to the axis of the ring gear, and the case-side smooth surface is formed orthogonal to the axis of the differential case. Thereby, when the ring gear is press-fitted to the differential case by bringing the protrusion into contact with the case-side smooth surface, the protrusion is in surface contact with the case-side smooth surface and positioned in place. Accordingly, in the above fastening structure of the ring gear and the differential case, the ring gear can be precisely positioned in place with respect to the differential case. 
         [0025]    In the above fastening structure of the ring gear and the differential case, the protrusion is placed inside the gear-side press-fitting surface such that the length of the gear-side press-fitting surface in the press-fitting direction is equal to the length of the case-side press-fitting surface in the press-fitting direction. Thereby, the ring gear does not extend outside the differential case when the protrusion comes into contact with the case-side smooth surface to be positioned in place. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a schematic view of a configuration of a differential device employing a fastening structure of a ring gear and a differential case according to a first embodiment of the present invention; 
           [0027]      FIG. 2  is a diagram of the fastening structure of the ring gear and the differential case according to the first embodiment of the present invention; 
           [0028]      FIG. 3  is a partial sectional view of the ring gear in a direction orthogonal to an axial direction of the ring gear; 
           [0029]      FIG. 4  is a partial enlarged view of an inner peripheral surface of the ring gear seen from a direction K in  FIG. 3 ; 
           [0030]      FIG. 5  is a partial sectional view of the differential case; 
           [0031]      FIG. 6  is an explanatory view showing a press-fitting step before completion of press-fitting; 
           [0032]      FIG. 7  is an explanatory view showing the press-fitting step after the completion of the press-fitting; 
           [0033]      FIG. 8  is an explanatory view showing a swaging step; 
           [0034]      FIG. 9  is a partial enlarged view of a swaged part; 
           [0035]      FIG. 10  is a schematic view showing a configuration of a conventional differential gear; 
           [0036]      FIG. 11  is a diagram showing a mounting operation of a differential gear; 
           [0037]      FIG. 12  is a view showing a press-fitting step of a ring gear to be press-fitted to a differential case, showing a state before completion of press-fitting; 
           [0038]      FIG. 13  is a view showing the press-fitting step of the ring gear press-fitted to the differential case after the completion of the press-fitting; and 
           [0039]      FIG. 14  is a view showing a swaging step of the ring gear to be swaged and fixed to the differential case. 
       
    
    
     REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1  Structure for fastening a ring gear to a differential case 
               2  Differential case 
               3  Ring gear 
               6  Case-side press-fitting surface 
               8  Flange 
               9  Case-side smooth surface 
               10  Differential gear (one example of a differential device) 
               21  Gear-side press-fitting surface 
               23  Protrusion 
           
         
       
     
       DETAILED DESCRIPTION 
       [0049]    One embodiment illustrating a structure for fastening a ring gear to a differential case and a differential gear employing the same of the present invention is herein described in detail with reference to the accompanying drawings. 
         [0050]      FIG. 1  is a schematic view showing a configuration of a differential device employing a structure  1  for fastening a ring gear  3  to a differential case  2  (hereinafter, referred to as a fastening structure  1 ) in a first embodiment of the present invention.  FIG. 2  is a diagram of the fastening structure  1  of the differential case  2  and the ring gear  3  in the first embodiment of the present invention. 
         [0051]    The fastening structure  1  in  FIGS. 1 and 2  is applied to a differential gear  10  (one example of a differential device) used for a drive mechanism of an automobile as similar to prior arts. In the fastening structure  1 , the ring gear  3  is fastened to the differential case  2  in a manner that the ring gear  3  is press-fitted to the differential case  2  and swaged or caulked. 
         [0052]    In the differential gear  10 , rotation torque is transmitted to the ring gear  3  and further transmitted to the differential case  2  through a swaged part and a press-fitted part with respect to the ring gear  3 , so that the differential case  2  integrally rotates with the ring gear  3 . 
         [0053]    As shown in  FIG. 2 , the differential case  2  is provided with a mounting space  11  for mounting a differential gear  12 . In the mounting space  11 , a not-shown pinion gear is placed in a non-rotatable manner via a not-shown pinion shaft. The differential gear  12  is placed in the mounting space  11  so that the gear  12  is entirely accommodated in the mounting space  11  and engaged with the not-shown pinion gear. A not-shown axle is connected to the gear  12 . In the differential gear  10  having this configuration, when the differential case  2  rotates integrally with the ring gear  3 , the not-shown pinion gear rotates integrally with the differential case  2  via the not-shown pinion shaft and the rotation torque transmitted from the ring gear  3  to the differential case  2  is changed its direction to be transmitted to the gear  12 , and thus the axle rotates. The fastening structure  1  applied to the above explained differential gear  10  has the configuration that a part provided for positioning the ring gear  3  with respect to the differential case  2  in an axial direction is formed so as not to extend outside the ring gear  3 . 
         [0054]    The ring gear  3  shown in  FIGS. 1 and 2  is made from low-carbon steel and formed as a short cylinder extending in the axial direction. A surface of the ring gear  3  is subjected to carburizing. As shown in  FIG. 1 , a gear part  4  is formed in an outer peripheral surface of the ring gear  3  to receive the rotation torque from an external device. 
         [0055]      FIG. 3  is a partial sectional view of the ring gear  3  in a direction orthogonal to the axial direction.  FIG. 4  is a partial enlarged view of an inner peripheral surface  3   c  of the ring gear  3 , seen from a direction K in  FIG. 3 . 
         [0056]    As shown in  FIG. 3 , the ring gear  3  is formed with an annular gear-side press-fitting surface  21  extending from a first end surface  3   a  in a right side of the figure. The gear-side press-fitting surface  21  has an inner diameter A 11  determined larger than an inner diameter A 12  of the inner peripheral surface  3   c  of the ring gear  3  so that the surface  21  is formed coaxial with an axis of the ring gear  3 . The press-fitting surface  21  has a predetermined length W 2  extending from the first end surface  3   a  in the axial direction. The ring gear  3  is further formed with a protrusion  23  having a predetermined length W 1  extending from a second end surface  3   b  in a left side of the figure in the axial direction, the protrusion  23  being annularly formed inside (left side in the figure) the press-fitting surface  21 . A gear-side smooth surface  22  is configured as a surface (a side surface of the protrusion  23  in the press-fitting surface  21  side) to form a stepped portion between the inner peripheral surface  3   c  of the ring gear  3  and the press-fitting surface  21 . The smooth surface  22  is formed flat to be orthogonal to the axis of the ring gear  3 . 
         [0057]    The predetermined length W 1  of the protrusion  23  in the axial direction is determined long enough to have rigidity to prevent deformation of the protrusion  23  due to the press-fitting load caused when the ring gear  3  is press-fitted to the differential case  2  and to prevent deformation of the protrusion  23  due to the engagement reaction force of the gear part  4  caused when the rotation torque acting on the gear part  4  is transmitted from the ring gear  3  to the differential case  2 . 
         [0058]    The ring gear  3  is formed with a plurality of notches  5  positioned on an opposite side from the press-fitting surface  21  with respect to the protrusion  23 . As shown in  FIG. 4 , the notches  5  are mountain-shaped when seen from the second end surface  3   b  side of the ring gear  3  (the direction K in  FIG. 3 ). The notches  5  are sequentially formed along an edge of the inner peripheral surface  3   c  of an opening formed in the second end surface  3   b  of the ring gear  3 . 
         [0059]      FIG. 5  is a partial sectional view of the differential case  2 . 
         [0060]    The differential case  2  is made from cast iron which is softer than the material of the ring gear  3  so that the case  2  is easy to cause plastic flow during swaging. The differential case  2  includes a case-side press-fitting surface  6 , a flange  8 , a case-side smooth surface  9 , the mounting space  11 , and others, which are formed by cutting. 
         [0061]    The case-side press-fitting surface  6  is annularly formed on an outer peripheral surface of the differential case  2  at its one end so that the surface  6  is press-fitted with the gear-side press-fitting surface  21  of the ring gear  3 . The flange  8  has an outer diameter A 2  smaller than an outer diameter A 1  of the press-fitting surface  6  and is to be swaged with the notches  5  of the ring gear  3 . The flange  8  is annularly configured. The press-fitting surface  6  and the flange  8  are formed coaxial with an axis of the differential case  2 . The case-side smooth surface  9  is configured as a stepped portion formed between the press-fitting surface  6  and the flange  8 . The smooth surface  9  is formed flat to be orthogonal to the axis of the differential case  2 . 
         [0062]    The outer diameter A 1  of the case-side press-fitting surface  6  is determined larger than the inner diameter A 11  of the gear-side press-fitting surface  21  shown in  FIG. 3  so that the press-fitting surface  6  includes a press-fitting allowance. The press-fitting surface  6  has a length W 21  in the axial direction determined equal to the predetermined axial length W 2  of the gear-side press-fitting surface  21  so that a first end surface  2   a  of the differential case  2  and the first end surface  3   a  of the ring gear  3  are positioned to be flush with each other when the protrusion  23  comes into contact with the smooth surface  9  and is positioned in place in the axial direction. The axial length W 21  is determined long enough to prevent deformation of the smooth surface  9  due to the engagement reaction force generated on the gear part  4  when the drive force is transmitted from the ring gear  3  to the gear part  4  and due to the press-fitting load generated when the ring gear  3  is press-fitted to the differential case  2 . 
         [0063]    The flange  8  is formed to protrude from the smooth surface  9  by a predetermined length W 11  in the axial direction of the differential case  2 . The flange  8  is annularly formed to be coaxial with the press-fitting surface  6 . The predetermined axial length W 11  is determined to be longer than the predetermined length W 1  of the protrusion  23  as shown in  FIG. 3  such that an end portion of the flange  8  protrudes out beyond the second end surface  3   b  of the ring gear  3  when the gear-side press-fitting surface  21  is press-fitted to the press-fitting surface  6  until the protrusion  23  comes into contact with the case-side smooth surface  9 . A thickness B of the flange  8  in a radial direction is determined to allow deformation of the flange  8 . 
         [0064]    &lt;Fastening Method of a Differential Case and a Ring Gear&gt; 
         [0065]      FIG. 6  is an explanatory view showing a press-fitting step, showing a state before completion of press-fitting.  FIG. 7  is an explanatory view showing the press-fitting step after completion of the press-fitting.  FIG. 8  is an explanatory view showing a swaging step.  FIG. 9  is a partial enlarged view of a swaged part  30 . 
         [0066]    As shown in  FIG. 6 , the press-fitting surface  21  of the ring gear  3  is brought into contact with the case-side press-fitting surface  6  from the flange  8  side of the differential case  2 , and the ring gear  3  is pressed in the axial direction to press-fit the gear-side press-fitting surface  21  to the press-fitting surface  6 . As shown in  FIG. 7 , the press-fitting surface  21  of the ring gear  3  is press-fitted to the press-fitting surface  6  until the gear-side smooth surface  22  comes into contact with the case-side smooth surface  9  of the differential case  2 . 
         [0067]    When the protrusion  23  comes into contact with the case-side smooth surface  9  and the gear-side press-fitting surface  21  is press-fitted to the case-side press-fitting surface  6 , for example, a load of as much as 800 kg is exerted on the smooth surface  9 . However, the smooth surface  9  and others are not deformed by the press-fitting load since the axial length W 21  of the press-fitting surface  6  is determined long enough to counteract the press-fitting load. Further, the protrusion  23  is not deformed since the axial length W 1  is determined long enough to counteract the press-fitting load. 
         [0068]    The gear-side smooth surface  22  and the case-side smooth surface  9  are formed flat with no roughness. Further, the gear-side smooth surface  22  is formed orthogonal to the axis of the ring gear  3 , and the case-side smooth surface  9  is formed orthogonal to the axis of the differential case  2 . Namely, the ring gear  3  is precisely positioned in place with respect to the differential case  2  in the axial direction by the surface contact of the gear-side smooth surface  22  and the case-side smooth surface  9 . 
         [0069]    Furthermore, the gear-side press-fitting surface  21  is annularly formed to be coaxial with the axis of the ring gear  3 , and the case-side press-fitting surface  6  is annularly formed to be coaxial with the axis of the differential case  2 . Thereby, the ring gear  3  is radially positioned in place with respect to the differential case  2  by the press-fitted part of the gear-side press-fitting surface  21  and the case-side press-fitting surface  6 . 
         [0070]    Subsequently, the flange  8  of the differential case  2  extending laterally beyond the second end surface  3   b  of the ring gear  3  is pushed and bent toward the ring gear  3  to be firmly pressed against the notches  5  as shown in  FIG. 8 . Since the flange  8  has hardness lower than the notches  5 , the material of the flange  8  plastically flows to be filled in each notch  5  by pressing the flange  8  to the notches  5 . Thereby, as shown in  FIG. 9 , the flange  8  is plastically deformed to get into the mountain-shaped portion in section of each notch  5  and swaged, thus the swaged part  30  being formed. 
         [0071]    According to the above explained press-fitting step and swaging step, the protrusion  23  is held between the swaged part  30  of the flange  8  with the notches  5  and the contact portion of the case-side smooth surface  9  with the gear-side smooth surface  22 , so that the ring gear  3  is prevented from being misaligned relative to the differential case  2  in the axial direction. The ring gear  3  is also prevented from being misaligned relative to the case  2  in the radial direction by the press-fitted part of the case-side press-fitting surface  6  with the gear-side press-fitting surface  21 . In this state, the ring gear  3  is held in the case  2 . 
         [0072]    &lt;Explanation of Drive Transmission Operation&gt; 
         [0073]    In the differential gear  10  in  FIG. 2 , the differential case  2  integrally rotates with the ring gear  3  when the rotation torque acts on the gear part  4  of the ring gear  3 , and the drive power is transmitted to the differential gear  12 . The power transmission from the ring gear  3  to the differential case  2  is done through the press-fitted part of the gear-side press-fitting surface  21  with the case-side press-fitting surface  6  and the swaged part  30  of each notch  5  and the flange  8 . 
         [0074]    For example, the engagement reaction force is generated on the gear part  4  when the rotation torque is transmitted from the not-shown drive gear. In this case, an engagement reaction force of as much as 2 tons may act on the case-side smooth surface  9  and the protrusion  23 , for example. However, the case-side smooth surface  9  and others are not deformed by the engagement reaction force since the axial length W 21  of the case-side press-fitting surface  6  is determined long enough to counteract the engagement reaction force. Further, the protrusion  23  is not deformed by the engagement reaction force since the axial length W 1  is determined long enough to counteract the engagement reaction force. In addition, the differential case  2  is formed with the case-side smooth surface  9  provided inside the case-side press-fitting surface  6 , so that width (heights) of the case-side smooth surface  9  in the radial direction can be kept equal to or longer than width (heights) of the heel part  107  in the radial direction of the conventional fastening structure  101  shown in  FIGS. 10 to 14 . Accordingly, the gear-side press-fitting surface  21  and the case-side press-fitting surface  6  do not slide each other to cause friction on the press-fitted part during the torque transmission, so that the rotation torque can be reliably transmitted from the ring gear  3  to the differential case  2 . 
         [0075]    &lt;Operational Effects&gt; 
         [0076]    According to the above mentioned fastening structure  1  and the differential gear  10 , the case-side press-fitting surface  6  and the gear-side press-fitting surface  21  are press-fitted together until the protrusion  23  of the ring gear  3  comes into contact with the case-side smooth surface  9 . Then, the flange  8  is pressed against the notches  5  to be swaged. The ring gear  3  is positioned in place with respect to the differential case  2  by bringing the protrusion  23  into contact with the case-side smooth surface  9 . Since the protrusion  23  is formed inside the press-fitting surface  21  and positioned between the notches  5  and the press-fitting surface  21 , the contact portion with the case-side smooth surface  9  does not extend outside the case  2 . Thereby, the fastening structure  1  and the differential gear  10  employing the same in the present embodiment do not need to provide the heel part  107  outside the case-side press-fitting surface  106  (on the first end surface  3   a  opposite to the second end surface  3   b  formed with the notches  5 ) as the conventional differential case  102  in  FIG. 11 . Therefore, the axial length of the differential case  2  can be made short, and size reduction of the case  2  can be achieved. 
         [0077]    The size reduction in the differential case  2  is accompanied with the effect of cost reduction by reducing weight of the material used for the case  2 . 
         [0078]    Further, since the case  2  has the overall axial length shorter than the conventional case  102  by the thickness C of the heel part  107  in the axial direction, the axial length W 3  of the mounting space  11  where the gear  12  is to be mounted (see  FIG. 2 ) can be designed with high flexibility. 
         [0079]    Furthermore, since the protrusion  23  is formed more inside than the gear-side press-fitting surface  21  and the case-side smooth surface  9  is formed more inside than the case-side press-fitting surface  6 , the smooth surface  22  of the protrusion  23  and the smooth surface  9  of the case  2  hardly suffer from scratches or dents due to a bump or collision of components during conveyance of the components. Less scratches and less dents on facing surfaces of the case-side smooth surface  9  and the gear-side smooth surface  22  lead to accurate positioning of the ring gear  3  and the case  2  in the axial direction, so that yield of the ring gear  3  and the case  2  can be improved. 
         [0080]    In the above fastening structure  1 , the protrusion  23  is formed orthogonal to the axis of the ring gear  3 , and the case-side smooth surface  9  is formed orthogonal to the axis of the differential case  2 . Specifically, the protrusion  23  is in surface contact with the case-side smooth surface  9  to be positioned in place when the ring gear  3  is press-fitted to the case  2  and the protrusion  23  comes into contact with the case-side smooth surface  9 . Therefore, according to the fastening structure  1  of the present embodiment, the ring gear  3  can be accurately positioned in place with respect to the case  2 . 
         [0081]    In the above fastening structure  1 , the protrusion  23  is positioned inside the gear-side press-fitting surface  21  such that the length W 2  in the press-fitting direction of the press-fitting surface  21  is equal to the length W 21  of the case-side press-fitting surface  6 , so that the ring gear  3  does not extend outside the differential case  2  when the protrusion  23  comes into contact with the case-side smooth surface  9  to be positioned in place. 
         [0082]    The present invention may be embodied with various modification without limited to the above mentioned embodiment. 
         [0083]    For example, in the above embodiment, the protrusion  23  is annularly formed in the ring gear  3 . Alternatively, the protrusion  23  may be divided into three or more in a circumferential direction of the ring gear  3 .