Patent Publication Number: US-2015059505-A1

Title: Gearbox

Description:
TECHNICAL FIELD 
     The present invention relates to a gearbox including a first shaft having a first gear, a second shaft arranged to intersect with the first shaft and having a second gear configured to mesh with the first gear and a first case configured to rotatably support the first shaft. 
     BACKGROUND ART 
     A description is made with reference to  FIG. 15 .  FIG. 15  is an exploded perspective view of a gearbox embedded in a seat track according to the background art. In  FIG. 15 , a drive shaft (a first shaft)  1  configured to rotate by a motor provided for an upper rail (not shown) has a worm (a first gear: drive gear)  3 . 
     A fixed shaft  5  is arranged to intersect with the drive shaft  1 . Both end portions of the fixed shaft  5  are supported at a state where rotations thereof are locked by shaft holders  13  provided for a lower rail. A circumferential surface of the fixed shaft  5  is formed with a male screw. 
     The fixed shaft  5  is provided with a cylindrical nut member  7 . The nut member  7  is formed on its inner peripheral part with a female screw configured to screw with the male screw of the fixed shaft  5 . The nut member  7  functions as a driven shaft (a second shaft). The nut member  7  is formed on its outer peripheral part with a worm wheel (a second gear: driven gear) configured to mesh with the worm  3 . 
     A pair of cases  9 ,  11  is arranged to interpose the worm  3  therebetween from an axial direction. Surfaces  9   d,    11   d  of the cases  9 ,  11  intersecting with the drive shaft  1  are formed with holes  9   a,    11   a  into which the drive shaft  1  is inserted. Also, one surface  9   e  and one surface  11  e orthogonal to the surface  9   d  of the case  9  and the surface  11   d  of the case  11  are formed with semi-circular recess portions  9   b ,  11   b  configured to cooperatively hold one side of the outer peripheral part of the nut member  7 . Further, the other surface  9   f  and the other surface  11   f  orthogonal to the surface  9   d  of the case  9  and the surface  11   d  of the case  11  are formed with semi-circular recess portions  9   c,    11   c  (the recess portion  9   c  is not shown) configured to cooperatively hold the other side of the outer peripheral part of the nut member  7 . 
     The drive shaft  1 , the nut member (driven shaft)  7 , the case  9  and the case  11  are integrated using screws  14 ,  15 , thereby configuring a gearbox  2 . The gearbox  2  is attached to the upper rail by using a bracket  17 . 
     CITATION LIST 
     Patent Literature 
     [Patent Literature 1] Japanese Patent Application Publication No. 2007-513005A 
     SUMMARY OF THE INVENTION 
     Technical Problem 
     However, the gearbox having the above configuration is integrated with using the screws  14 ,  15 . Therefore, there is a problem of requiring an effort for assembling. 
     It is therefore an object of the present invention to provide a gearbox not requiring an effort to assemble the gearbox. 
     Solution to Problem 
     In order realize at least one of the problems, a gearbox reflecting one aspect of the present invention includes A gearbox includes: a first shaft having a first gear; a second shaft arranged to intersect with the first shaft and having a second gear configured to mesh with the first gear; a pair of first cases arranged to interpose the first gear therebetween from an axial direction and formed with holes into which the first shaft is inserted; a pair of second cases arranged to interpose the second gear therebetween from an axial direction and formed with holes into which the second shaft is inserted, and a pressing mechanism in which the second cases, in response to pressing in a direction toward the first cases, press the pair of the first cases to each other. 
     Effects of the Present Invention 
     According to the present invention, the gearbox includes the pair of second cases arranged to interpose the second gear therebetween from the axial direction and formed with the holes into which the second shaft is inserted, and the pressing mechanism configured to push the pair of the first cases to each other when the second cases are pressed in the direction toward the first cases. Thereby, the man-hour for assembling a component for which a mechanical coupling such as a screw is performed is not necessary. Therefore, a gearbox not requiring an effort for assembling is provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded perspective view of a gearbox according to a first illustrative embodiment. 
         FIG. 2  is a perspective view of one first case of a pair of first cases shown in  FIG. 1 . 
         FIG. 3  is a perspective view of one second case of a pair of second cases shown in  FIG. 1 . 
         FIGS. 4A-4D  illustrate a process of assembling the gearbox shown in  FIG. 1 . 
         FIG. 5  illustrates a process of assembling the gearbox shown in  FIG. 1 . 
         FIG. 6  is a schematic view illustrating a pressing mechanism. 
         FIG. 7  is an enlarged view of an A part of  FIG. 6 , illustrating an inclined angle of an inclined surface. 
         FIG. 8  illustrates a modified embodiment of the first illustrative embodiment. 
         FIG. 9  is a schematic view illustrating a pressing mechanism of a gearbox according to a second illustrative embodiment. 
         FIG. 10  is a schematic view illustrating a pressing mechanism of a gearbox according to a third illustrative embodiment. 
         FIG. 11  is a schematic view illustrating a pressing mechanism of a gearbox according to a fourth illustrative embodiment. 
         FIG. 12  is a schematic view illustrating a pressing mechanism of a gearbox according to a fifth illustrative embodiment. 
         FIG. 13  is a schematic view illustrating a pressing mechanism of a gearbox according to a sixth illustrative embodiment. 
         FIG. 14  is a schematic view illustrating a pressing mechanism of a gearbox according to a seventh illustrative embodiment. 
         FIG. 15  is an exploded perspective view of a gearbox embedded in a seat track according to the background art. 
     
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     First Illustrative Embodiment 
     A gearbox according to a first illustrative embodiment will be described with reference to  FIGS. 1 to 3 .  FIG. 1  is an exploded perspective view of a gearbox according to a first illustrative embodiment,  FIG. 2  is a perspective view of one first case of a pair of first cases shown in  FIG. 1  and  FIG. 3  is a perspective view of one second case of a pair of second cases shown in  FIG. 1 . 
     In  FIG. 1 , a drive shaft (a first shaft)  51  is provided with a worm (a first gear)  53 . 
     A driven shaft (a second shaft)  55  orthogonal to (intersecting with) the drive shaft  51  is provided with a worm wheel  57  configured to mesh with the worm  53  of the drive shaft  51 . 
     A pair of first cases  59 ,  61  is arranged to interpose the worm  53  therebetween from an axial direction. The first case  59  and the first case  61  are formed with holes  59   a,    61   a  in which the drive shaft  51  is inserted and is rotatably supported. 
     A pair of second cases  63 ,  65  is arranged to interpose the worm wheel  57  therebetween from an axial direction. The second case  63  and the second case  65  are formed with a hole  63   a  and a hole  65   a  in which the driven shaft  55  is inserted and is supported. 
     Meanwhile, in this illustrative embodiment, a degree of elasticity of a material of the second cases  63 ,  65  is set to be smaller than that of a material of the first cases  59 ,  61 . That is, the second cases  63 ,  65  are likely to be more easily deformable than the first cases  59 ,  61 . 
     As shown in  FIGS. 1 and 2 , a surface of the first case  59  facing the first case  61  is formed with a protrusion  59   b  protruding towards the axial direction of the drive shaft  51  and a hole  59   c.  The protrusion  59   b  and the hole  59   c  are formed so that the hole  59   a  is positioned therebetween. In the meantime, a surface of the first case  61  facing the first case  59  is formed with a hole  61   b  into which the protrusion  59   b  of the first case  59  is fitted and a protrusion  61   c  protruding towards the axial direction of the drive shaft  51  and fitted into the hole  59   c  of the first case  59 . The hole  61   b  and the protrusion  61   c  are formed so that the hole  61   a  is positioned therebetween. In the meantime, the fitting of the protrusion  59   b  and the hole  61   b  and the fitting of the protrusion  61   c  and the hole  59   c  are loose fittings. For this reason, the fittings are not to fix the first case  59  and the first case  61  each other but to temporarily position the same. 
     The surface of the first case  59  facing the first case  61  is formed at its lower part with semi-cylindrical recess portions  59   d,    59   e  configured to interpose and support the driven shaft  55  from one side thereof. In the meantime, the surface of the first case  61  facing the first case  59  is formed at its lower part with semi-cylindrical recess portions  61   d,    61   e  configured to interpose and support the driven shaft  55  from the other side thereof. 
     The first case  59  is formed with a chamfered surface (an inclined surface)  59   f  and a chamfered surface  59   g  at vertical corner parts of sides of a back surface thereof. Likewise, the first case  61  is formed with a chamfered surface (an inclined surface)  61   f  and a chamfered surface  61   g  (the chamfered surface  61   g  is not shown) at vertical corner parts of sides of a back surface thereof. 
     The first case  59  is formed at upper parts of side surfaces thereof with protrusions  59   h,    59   i  protruding towards the axial direction of the driven shaft  55 . Likewise, the first case  61  is formed at upper parts of side surfaces thereof with protrusions  61   h,    61   i  (the protrusion  61   i  is not shown) protruding towards the axial direction of the driven shaft  55 . 
     Further, a surface (an outer surface) of the second case  63  opposite to the surface thereof facing the second case  65  is formed with a bead (a rib)  63   f  and a bead (a rib)  63   g  so that an opening of the hole  63   a  is interposed therebetween. Likewise, a surface (an outer surface) of the second case  65  opposite to the surface thereof facing the second case  63  is also formed with a bead (a rib)  65   f  (not shown) and a bead (a rib)  63   g  (not shown) so that an opening of the hole  65   a  is interposed therebetween. 
     As shown in  FIGS. 1 and 3 , the surface of the second case  63  facing the second case  65  is formed at its sides with an inclined surface  63   b  configured to abut on the inclined surface  59   f  of the first case  59  and an inclined surface  63   c  configured to abut on the inclined surface  61   f  of the first case  61 . Likewise, the surface of the second case  65  facing the second case  63  is formed at its sides with an inclined surface  65   b  configured to abut on the inclined surface  59   g  of the first case  59  and an inclined surface  65   c  configured to abut on the inclined surface  61   g  of the first case  61 . 
     When the second case  63  and the second case  65  are pressed in the direction of the first case  59  and the first case  61 , the inclined surface  63   b  and inclined surface  63   c  of the second case  63  press the inclined surface  59   f  of the first case  59  and the inclined surface  61   f  of the first case  61 , respectively. Also, the inclined surface  65   b  and inclined surface  65   c  of the second case  65  press the inclined surface  59   g  of the first case  59  and the inclined surface  61   g  of the first case  61 , respectively. Thereby, the pair of first cases  59 ,  61  is pushed to each other by force components of forces generated on the inclined surface  63   b  and inclined surface  63   c  of the second case  63  and the inclined surface  65   b  and inclined surface  65   c  of the second case  65 . That is, the inclined surface  63   b  and inclined surface  63   c  of the second case  63  and the inclined surface  65   b  and inclined surface  65   c  of the second case  65  function as pressing mechanisms that are pressing force generation surfaces from which the force components of pushing the pair of first cases  59 ,  61  to each other are generated when the second case  63  and the second case  65  are pressed in the direction of the first case  59  and the first case  61 . 
     The surface of the second case  63  facing the second case  65  is formed at its upper part with a hole  63   d  in which the protrusion  59   h  of the first case  59  is fitted and a hole  63   e  in which the protrusion  61   h  of the first case  61  is fitted. Likewise, the surface of the second case  65  facing the second case  63  is formed at its upper part with a hole  65   d  in which the protrusion  59   i  of the first case  59  is fitted and a hole  65   e  in which the protrusion  61   i  of the first case  61  is fitted. In the meantime, the fitting of the protrusion  59   h  and the hole  63   d,  the fitting of the protrusion  61   h  and the hole  63   e,  the fitting of the protrusion  59   i  and the hole  65   d  and the fitting of the protrusion  61   i  and the hole  65   e  are to enable the first case  59  and the first case  61  to move in the pushing direction when the second case  63  and the second case  65  are pressed in the direction of the first case  59  and first case  61  and the pair of first cased  59 ,  61  is thus pushed to each other. 
     Subsequently, a method of assembling the gearbox having the above configuration is described with reference to  FIGS. 4 and 5 .  FIGS. 4 and 5  illustrate a method of assembling the gearbox shown in  FIG. 1 . 
     First of all, as shown in  FIGS. 4A and 4B , the drive shaft  51  having the worm  53  is inserted into the hole  61   a  of the first case  61 . Also, one side of the driven shaft  55  having the worm wheel  57  is supported to the semi-cylindrical recess portions  61   d,    61   e  of the first case  61 . 
     Then, as shown in  FIG. 4C , the first case  59  is assembled to the first case  61 . At this time, the protrusion  61   c  of the first case  61  is loosely fitted into the hole  59   c  of the first case  59  and the protrusion  59   b  of the first case  59  is loosely fitted into the hole  61   b  of the first case  61 . Thereby, the first case  61  and the first case  59  are temporarily positioned. Also, the drive shaft  51  is inserted into the hole  59   a  of the first case  59 . Further, the other side of the driven shaft  55  having the worm wheel  57  is supported to the semi-cylindrical recess portions  59   d,    59   e  of the first case  59 . 
     Then, as shown in  FIG. 4D , the second case  63  and the second case  65  are assembled. At this time, as shown in  FIG. 6 , the inclined surface  63   b  of the second case  63  abuts on the inclined surface  59   f  of the first case  59 . The inclined surface  63   c  of the second case  63  abuts on the inclined surface  61   f  of the first case  61 . Also, the inclined surface  65   b  of the second case  65  abuts on the inclined surface  59   g  of the first case  59 . The inclined surface  65   c  of the second case  65  abuts on the inclined surface  61   g  of the first case  61 . 
     Also, the protrusion  59   h  of the first case  59  is loosely fitted into the hole  63   d  of the second case  63 , the protrusion  61   h  of the first case  61  is loosely fitted into the hole  63   e  of the second case  63 , the protrusion  59   i  of the first case  59  is loosely fitted into the hole  65   d  of the second case  65  and the protrusion  61   i  of the first case  61  is loosely fitted into the hole  65   e  of the second case  65 . 
     At this time, at least one of the first case  59  and first case  61  and the second case  63  and second case  65  is elastically deformed by the pressing force of the pressing mechanisms. Static friction between the first cases  59 ,  61  and the second cases  63 ,  65 , which is generated by an elastically repulsive force thereof, keeps the assembled state of the first cases  59 ,  61  and the second cases  63 ,  65 . 
     Finally, as shown in  FIG. 5 , the assembling-completed gearbox is assembled to a bracket  71 . The bracket  71  has a base part  71   c,  a base part  71   d,  an upstanding wall part  71   f,  an upstanding wall part  71   g  and a bottom part  71   h.  The base part  71   c  and the base part  71   d  are formed with holes  71   a,    71   b  for upper rail assembling. 
     The upstanding wall part  71   f  is bent from an end portion of the base part  71   c  at the base part  71   d -side and is configured to face the second case  63  of the gearbox. Also, the upstanding wall part  71   f  is formed with a hole  71   i  facing the hole  63   a  of the second case  63 . 
     The upstanding wall part  71   g  is bent from an end portion of the base part  71   d  at the base part  71   c -side and is configured to face the second case  65  of the gearbox. Also, the upstanding wall part  71   g  is formed with a hole  71   j  facing the hole  65   a  of the second case  65 . 
     The bottom part  71   h  is configured to bridge lower end portions of the upstanding wall part  71   f  and the upstanding wall part  71   g  and to face a bottom part of the gearbox. 
     When the gearbox is assembled to a space surrounded by the upstanding wall part  71   f,  upstanding wall part  71   g  and bottom part  71   h  of the bracket  71 , the bead  63   f  and bead  63   g  of the second case  63  and the bead  65   f  and bead  65   g  of the second case  65  are elastically deformed and the second case  63  and the second case  65  are pressed in the direction of the first case  59  and the first case  61 . 
     Here, the pressing mechanism is described with reference to  FIGS. 6 and 7 .  FIG. 6  is a schematic view illustrating the pressing mechanism, and  FIG. 7  is an enlarged view of an A part of  FIG. 6 , illustrating an inclined angle of the inclined surface. 
     As shown in  FIG. 6 , the inclined surface  63   b  of the second case  63  presses the inclined surface  59   f  of the first case  59 , and the inclined surface  63   c  of the second case  63  presses the inclined surface  61   f  of the first case  61 . Also, the inclined surface  65   b  of the second case  65  presses the inclined surface  59   g  of the first case  59 , and the inclined surface  65   c  of the second case  65  presses the inclined surface  61   g  of the first case  61 . 
     Here, in this illustrative embodiment, the inclined surface  59   f  and inclined surface  59   g  of the first case  59  and the inclined surface  61   f  and inclined surface  61   g  of the first case  61  have the same inclined angle. Also, the inclined surface  63   b  and inclined surface  63   c  of the second case  63  and the inclined surface  65   b  and inclined surface  65   c  of the second case  65  have the same inclined angle. As shown in  FIG. 7 , when the inclined angle of the inclined surfaces of the first cases  59 ,  61  is denoted with θ1 and the inclined angle of the inclined surfaces of the second cases  63 ,  65  is denoted with θ2, θ1&lt;θ2. 
     By the force components of the force generated on the inclined surface  63   b  and inclined surface  63   c  of the second case  63  and the inclined surface  65   b  and inclined surface  65   c  of the second case  65 , which are the pressing force generation surfaces, the pair of first cases  59 ,  61  is pushed to each other. 
     As shown in the A part of  FIG. 6 , a force component Fy of force components Fx, Fy of a force F generated on the inclined surface  65   b,  which is the pressing force generation surface, is the pressing force pushing the first case  59  and the first case  61  to each other. 
     According to the above configuration, since the first case  59  and the first case  61  can be integrated without using a screw, an effort is not required for the assembling. 
     In the meantime, the present invention is not limited to the above illustrative embodiment. In the above illustrative embodiment, when the inclined angle of the inclined surfaces of the first cases  59 ,  61  is denoted with θ1 and the inclined angle of the inclined surfaces of the second cases  63 ,  65  is denoted with θ2, θ1&lt;θ2. However, θ1 may be equal to θ2. 
     Also, as shown in  FIG. 8 , when the inclined angle of the inclined surfaces of the first cases  59 ,  61  is denoted with θ1 and the inclined angle of the inclined surfaces of the second cases  63 ,  65  is denoted with θ2, θ1 may be larger than θ2. 
     In the case of θ1&lt;θ2 shown in  FIG. 7 , the pressing force is greater, as compared to the case of θ1&gt;θ2 shown in  FIG. 8 . However, since the greater force is applied to the inclined surfaces of the second case  63  and second case  65 , the second case  63  and the second case  65  having higher strength are required. 
     Second Illustrative Embodiment 
     Subsequently, a gearbox according to a second illustrative embodiment is described with reference to  FIG. 9 .  FIG. 9  is a schematic view illustrating a pressing mechanism of a gearbox according to a second illustrative embodiment. 
     This illustrative embodiment is different from the first illustrative embodiment as regards the first case, and the other configurations are the same. Therefore, the same parts as the first illustrative embodiment are denoted with the same reference numerals and the overlapping descriptions thereof are omitted. 
     When a pair of first cases  159 ,  161  is assembled, the first cases configure a substantial cuboid. A corner part  159   a  of the first case  159  is pressed to the inclined surface  63   b  of the second case  63  and a corner part  161   a  of the first case  161  is pressed to the inclined surface  63   c  of the second case  63 . Also, a corner part  159   b  of the first case  159  is pressed to the inclined surface  65   b  of the second case  65  and a corner part  161   b  of the first case  161  is pressed to the inclined surface  65   c  of the second case  65 . 
     By the inclined surface  63   b  and inclined surface  63   c  of the second case  63  and the inclined surface  65   b  and inclined surface  65   c  of the second case  65 , which are the pressing force generation surfaces, the pair of first case  159  and first case  161  is pushed to each other. 
     As shown in  FIG. 9 , the operation is described with reference to the inclined surface  65   b  and the corner part  159   b.  A force component Fy of force components Fx, Fy of a force F generated on the inclined surface  65   b,  which is the pressing force generation surface, is the pressing force pushing the first case  159  and the first case  161  to each other. 
     According to the above configuration, since the first case  159  and the first case  161  can be integrated without using a screw, an effort is not required for the assembling. 
     Third Illustrative Embodiment 
     A gearbox according to a third illustrative embodiment is described with reference to  FIG. 10 .  FIG. 10  is a schematic view illustrating a pressing mechanism of a gearbox according to a third illustrative embodiment. This illustrative embodiment is different from the second illustrative embodiment as regards the second case, and the other configurations are the same. Therefore, the same parts as the second illustrative embodiment are denoted with the same reference numerals and the overlapping descriptions thereof are omitted. 
     In this illustrative embodiment, the pressing force generation surfaces of the pressing mechanisms of a second case  163  and a second case  165  are a circular arc surface  163   a  and a circular arc surface  165   a.    
     The corner part  159   a  of the first case  159  and the corner part  161   a  of the first case  161  are pressed by the circular arc surface  163   a  of the second case  163 . Also, the corner part  159   b  of the first case  159  and the corner part  161   b  of the first case  161  are pressed by the circular arc surface  165   a  of the second case  165 . 
     By the circular arc surface  163   a  of the second case  163  and the circular arc surface  165   a  of the second case  165 , which are the pressing force generation surfaces, the pair of first cases  159 ,  161  is pushed to each other. 
     As shown in  FIG. 10 , the operation is described with reference to the circular arc surface  165   a  and the corner part  159   b.  A force component Fy of force components Fx, Fy of a force F generated on the circular arc surface  165   a,  which is the pressing force generation surface, is the pressing force pushing the first case  159  and the first case  161  to each other. 
     According to the above configuration, since the first case  159  and the first case  161  can be integrated without using a screw, an effort is not required for the assembling. 
     In the meantime, the present invention is not limited to the above illustrative embodiment. In the above illustrative embodiment, the first case  159  and the first case  161  are formed with the circular arc surfaces. However, any curved surface from which the force pushing the first case  159  and the first case  161  to each other is generated is also possible. 
     Fourth Illustrative Embodiment 
     A gearbox according to a fourth illustrative embodiment is described with reference to  FIG. 11 .  FIG. 11  is a schematic view illustrating a pressing mechanism of a gearbox according to a fourth illustrative embodiment. This illustrative embodiment is different from the third illustrative embodiment as regards the first case, and the other configurations are the same. Therefore, the same parts as the third illustrative embodiment are denoted with the same reference numerals and the overlapping descriptions thereof are omitted. 
     When a pair of first cases  259 ,  261  is assembled, the first cases configure a box shape having a circular arc surface of which a facing surface  259   a  and a facing surface  261   a  towards the second case  163  are continuous and a circular arc surface of which a facing surface  259   b  and a facing surface  261   b  towards the second case  165  are continuous. 
     A radius of the circular arc surface having the facing surface  259   a  and facing surface  259   b  of the first case  259  and a radius of the circular arc surface having the facing surface  261   a  and facing surface  261   b  of the first case  261  are the same, i.e., R 1 . In the meantime, a radius of the circular arc surface  163   a  of the second case  163  and a radius of the circular arc surface  165   a  of the second case  165  are the same, i.e., R 2 . Here, R 1 &gt;R 2 . 
     An outer end portion of the facing surface  259   a  of the first case  259  and an outer end portion of the facing surface  261   a  of the first case  261  are pressed to the circular arc surface  163   a  of the second case  163 , and an outer end portion of the facing surface  259   b  of the first case  259  and an outer end portion of the facing surface  261   b  of the first case  261  are pressed to the circular arc surface  165   a  of the second case  165 . 
     By the circular arc surface  163   a  of the second case  163  and the circular arc surface  165   a  of the second case  165 , which are the pressing force generation surfaces, the pair of first cases  259 ,  261  is pushed to each other. 
     As shown in  FIG. 11 , the operation is described with reference to the circular arc surface  165   a  and the outer end portion of the facing surface  259   b.  A force component Fy of force components Fx, Fy of a force F generated on the circular arc surface  165   a,  which is the pressing force generation surface, is the pressing force pushing the first case  259  and the first case  261  to each other. 
     According to the above configuration, since the first case  259  and the first case  261  can be integrated without using a screw, an effort is not required for the assembling. 
     Fifth Illustrative Embodiment 
     A gearbox according to a fifth illustrative embodiment is described with reference to  FIG. 12 .  FIG. 12  is a schematic view illustrating a pressing mechanism of a gearbox according to a fifth illustrative embodiment. 
     In this illustrative embodiment, a second case  263  is formed with a circular arc surface  263   a  and a second surface  265  is formed with a circular arc surface  265   a.    
     When a pair of first cases  359 ,  361  is assembled, the first cases configure a box shape having a circular arc surface of which a facing surface  359   a  and a facing surface  361   a  towards the second case  263  are continuous and a circular arc surface of which a facing surface  359   b  and a facing surface  361   b  towards the second case  265  are continuous. 
     The circular arc surface  263   a  of the second case  263  and the circular arc surface having the facing surface  359   a  of the first case  359  and the facing surface  361   a  of the first case  361  have the same radius, i.e., R 1 . Also, the circular arc surface  265   a  of the second case  265  and the circular arc surface having the facing surface  359   b  of the first case  359  and the facing surface  361   b  of the first case  361  have the same radius, i.e., R 2 . 
     Here, R 1 ≠R 2 . 
     According to the above configuration, the pressing force pushing the pair of the first case  359  and the first case  361  to each other is generated over entire areas of the circular arc surface  263   a  of the second case  263  and the circular arc surface  265   a  of the second case  265 . 
     According to the above configuration, since the first case  359  and the first case  361  can be integrated without using a screw, an effort is not required for the assembling. Further, when being assembled, the first case  359  and the first case  361  are not rotated. 
     Sixth Illustrative Embodiment 
     A gearbox according to a sixth illustrative embodiment is described with reference to  FIG. 13 .  FIG. 13  is a schematic view illustrating a pressing mechanism of a gearbox according to a sixth illustrative embodiment. This illustrative embodiment is different from the second illustrative embodiment as regards the second case, and the other configurations are the same. Therefore, the same parts as the second illustrative embodiment are denoted with the same reference numerals and the overlapping descriptions thereof are omitted. 
     In this illustrative embodiment, a second case  363  is formed with a pressing part  363   a  capable of abutting on an outer end surface of the first case  159  and a pressing part  363   b  capable of abutting on an outer end surface of the first case  161 . Likewise, a second case  365  is formed with a pressing part  365   a  capable of abutting on the outer end surface of the first case  159  and a pressing part  365   b  capable of abutting on the outer end surface of the first case  161 . Also, an interval a between the pressing part  363   a  and pressing part  363   b  of the second case  363  is set to be the same as an interval a between the pressing part  365   a  and pressing part  365   b  of the second case  365 . 
     When the interval between the pressing part  363   a  and pressing part  363   b  of the second case  363  and the interval between the pressing part  365   a  and pressing part  365   b  of the second case  365  are denoted with ‘a’ and a height of the integrated first case  159  and first case  161  is denoted with ‘b’, it is set to be a&lt;b. 
     When the second case  363  and the second case  365  are pressed in the direction of the integrated first case  159  and first case  161 , the pressing part  363   a  and pressing part  363   b  of the second case  363  and the pressing part  365   a  and pressing part  365   b  of the second case  365  are elastically deformed and are pressed to hold the respective outer end surfaces (two surfaces of the pair of the first case  159  and the first case  161  intersecting with the pushing direction) of the integrated first case  159  and first case  161 . 
     Therefore, the pressing part  363   a  and pressing part  363   b  of the second case  363  and the pressing part  365   a  and pressing part  365   b  of the second case  365  function as a pressing mechanism configured to push the pair of the first case  159  and the first case  161  to each other when the second case  363  and the second case  365  are pressed in the direction of the pair of the first case  159  and the first case  161 . 
     According to the above configuration, since the first case  159  and the first case  161  can be integrated without using a screw, an effort is not required for the assembling. 
     Seventh Illustrative Embodiment 
     A gearbox according to a seventh illustrative embodiment is described with reference to  FIG. 14 .  FIG. 14  is an exploded perspective view of a gearbox according to a seventh illustrative embodiment. In  FIG. 14 , the same parts as  FIG. 1  showing the first illustrative embodiment are denoted with the same reference numerals and the overlapping descriptions thereof are omitted. 
     In  FIG. 14 , a pair of first cases  459 ,  461  is arranged to interpose the worm  53  therebetween from the axial direction. The first case  459  and the first case  461  are formed with a hole  459   a  and a hole  461   a  in which the drive shaft  51  is inserted and is rotatably supported. 
     A pair of second cases  463 ,  465  is arranged to interpose the worm wheel  57  therebetween. The second case  463  and the second case  465  are formed with a hole  463   a  and a hole  465   a  in which the driven shaft  55  is inserted and is supported. Meanwhile, in this illustrative embodiment, the first case  459  and the first case  461  are made of a resin. The second case  463  and the second case  465  are made of metal. 
     A surface of the first case  459  facing the first case  461  is formed with a protrusion  459   b  protruding towards the axial direction of the drive shaft  51  and a hole (not shown) with the hole  459   a  being positioned therebetween. In the meantime, a surface of the second case  461  facing the first case  459  is formed with a hole  461   b , into which the protrusion  459   b  of the first case  459  is fitted, and a protrusion  461   c  protruding towards the axial direction of the drive shaft  51  and fitted into the hole of the first case  459  with the hole  461   a  being positioned therebetween. Meanwhile, the fitting of the protrusion  459   b  and the hole  461   b  and the fitting of the protrusion  461   c  and the hole are loose fittings. Therefore, the fittings are not to fix the first case  459  and the first case  461  each other but to temporarily position the same. 
     The surface of the first case  459  facing the first case  461  is formed at its lower part with semi-cylindrical recess portions  459   d,    459   e  configured to interpose and support the driven shaft  55  from one side thereof. In the meantime, the surface of the first case  461  facing the first case  459  is formed at its lower part with semi-cylindrical recess portions  461   d,    461   e  configured to interpose and support the driven shaft  55  from the other side thereof. 
     Both sides of the second case  463  are bent to form a bent part  463   b  and a bent part  463   d  configured to hold the first case  459  and the first case  461  therebetween. Likewise, both sides of the second case  465  are bent to form a bent part  465   b  and a bent part  465   c  configured to hold the first case  459  and the first case  461  therebetween. 
     The first case  459  is formed with a bead (a rib)  459   f  at a place on which the bent part  463   b  of the second case  463  abuts. The first case  461  is formed with a bead (a rib: not shown) at a place on which the bent part  463   c  of the second case  463  abuts. 
     The first case  459  is formed with a bead (a rib)  459   g  at a place on which the bent part  465   b  of the first case  461  abuts. The first case  461  is formed with a bead (a rib: not shown) at a place on which the bent part  465   c  of the second case  463  abuts. 
     When assembling the first case  459  and the first case  461 , a length from a top surface of the bead  459   f  of the first case  459  to a top surface of the bead (not shown) of the first case  461  abutting on the bent part  463   c  of the second case  463  is set to be slightly longer than a length from an inner surface of the bent part  463   b  of the second case to an inner surface of the bent part  463   c.  Likewise, when assembling the first case  459  and the first case  461 , a length from a top surface of the bead  459   g  of the first case  459  to a top surface of the bead (not shown) of the first case  461  abutting on the bent part  465   c  of the second case  465  is set to be slightly longer than a length from an inner surface of the bent part  465   b  of the second case to an inner surface of the bent part  465   c.    
     In the meantime, the top surface of the bead  459   f  is a surface of surfaces of the bead  459   f  facing towards the axial direction of the drive shaft  51 . Likewise, the top surface of the bead  459   g  is a surface of surfaces of the bead  459   g  facing towards the axial direction of the drive shaft  51 . 
     When assembling the second case  463  and the second case  465  to the first case  459  and the first case  461 , the bent part  463   b  and the bent part  463   c  of the second case  463  and the bent part  465   b  and the bent part  465   c  of the second case  465  can be assembled while elastically deforming the first case  459  and the first case  461 , respectively. 
     Therefore, the bent part  463   b  and the bent part  463   c  of the second case  463  and the bent part  465   b  and the bent part  465   c  of the second case  465  function as a pressing mechanism configured to push the pair of the first case  459  and the first case  461  to each other when the pair of the second case  463  and the second case  465  are pressed in the direction of the pair of the first case  459  and the first case  461 . 
     According to the above configuration, since the first case  459  and the first case  461  can be integrated without using a screw, an effort is not required for the assembling. 
     Although the present invention has been specifically described with reference to the specific illustrative embodiments, it is apparent to one skilled in the art that a variety of changes and modifications can be made without departing from the spirit and scope of the present invention. 
     The present application is based on Japanese Patent Application No. 2012-73130 filed on Mar. 28, 2012, the contents of which being here incorporated for reference. 
     INDUSTRIAL APPLICABILITY 
     According to the present invention, the man-hour for assembling a component for which a mechanical coupling such as a screw is performed is not necessary. Therefore, a gearbox not requiring an effort for assembling is provided. 
     DESCRIPTION OF THE REFERENCE NUMERALS 
       51 : drive shaft (first shaft) 
       53 : worm (first gear) 
       55 : driven shaft (second shaft) 
       57 : worm wheel (second gear) 
       59 ,  61 : first case 
       59   a,    61   a : hole 
       63 ,  65 : second case 
       63   a,    65   a : hole 
       63   b,    63   c,    65   b,    65   c : inclined surface (pressing mechanism)