Abstract:
[Object] To provide a self-tapping screw which is the most suitable for tightening a workpiece made of a soft material such as a resin or an aluminum alloy. 
     [Solution] In the self-tapping screw  1  including a normal thread  10  and a female screw molding thread  12  having a larger diameter than that of the normal thread  10,  out of pitches of the normal thread  10  positioned in a head portion side from the female screw molding thread  12,  any one pitch is set to be larger than other pitches. The self-tapping screw  1  not only reduces driving torques but also improves fastening force, and thus can be applied to various workpieces made of the soft material as well as a hard material.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a screw for mounting a component on a workpiece made of a soft material such as an aluminum alloy, a magnesium alloy or a resin, and particularly to a self-tapping screw which is screwed to a prepared screwless hole formed on a workpiece made of such a soft material while molding a female screw. 
       BACKGROUND ART 
       [0002]    In recently popular electronic products such as mobile phones, personal computers and portable music players, an aluminum alloy is widely used in view of weight saving, downsizing and good workability and a plurality of screws are used to mount a component thereon. 
         [0003]    Therefore, as an example of such a screw, Japanese Patent No. 4490358 discloses a self-tapping screw. The self-tapping screw includes a normal thread molded in a leg portion and a female screw molding thread having a larger diameter than that of the normal thread, and is configured such that the female screw molding thread is screwed to a workpiece while molding a female screw. In addition, any pitch of the normal thread is set to be the same as each other. According to the configuration, in fastening and rotating, both flank surfaces of the normal thread do not come into contact with a female thread. Thus, it is possible to reduce tightening torques and to prevent the workpiece from being cracked. 
       CITATION LIST 
     Patent Literature 
       [0004]    PTL 1: Japanese Patent No. 4490358 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0005]    However, in compensation for the configuration, in the self-tapping screw, any flank surface of the normal thread does not come into contact with the female thread in fastening and fixing. Therefore, fastening force cannot be sufficiently obtained and the self-tapping screw is likely to be loosened. 
       Solution to Problem 
       [0006]    In a self-tapping screw in which a normal thread and a female screw molding thread having a larger diameter than that of the normal thread are molded in a leg portion, out of pitches of the normal thread positioned in a head portion side from the female screw molding thread, any one pitch is set to be larger than other pitches. 
         [0007]    According to such a self-tapping screw, a distance between a pressure flank surface of the normal thread and a pressure flank surface of the female thread is closer than a distance between a clearance flank surface of the normal thread and a clearance flank surface of the female thread. However, in fastening and rotating, since the normal thread is screwed while being pressed in a fastening direction, contact resistance is insignificant between the pressure flank surface of the normal thread and the pressure flank surface of the female thread. On the other hand, in fastening and fixing, action of axial force causes the pressure flank surface of the normal thread to approach and come into contact with the pressure flank surface of the female thread, thereby increasing fastening force. Therefore, the self-tapping screw of the present invention not only reduces driving torques but also improves the fastening force. Accordingly, it is possible to sufficiently achieve the fastening force without causing cracks even when the self-tapping screw is used in a workpiece made of a soft material such as an aluminum alloy or a resin. 
         [0008]    In addition, it is preferable that a pressure flank angle of the normal thread be set to be smaller than a pressure flank angle of the female screw molding thread. 
         [0009]    According to the self-tapping screw, in fastening and fixing, a crest of the normal thread is wedged into the pressure flank surface of the female thread. Therefore, it is possible to expect more increased fastening force. 
       Advantageous Effects of Invention 
       [0010]    A self-tapping screw of the present invention can be applied to various workpieces made of a soft material as well as a hard material by using reduced driving torques and increased fastening force. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is an overall view illustrating an embodiment of the present invention. 
           [0012]      FIG. 2  is a partially enlarged cross-sectional view illustrating a state of fastening and rotating according to the present invention. 
           [0013]      FIG. 3  is a partially enlarged cross-sectional view illustrating a state of fastening and fixing according to the present invention. 
           [0014]      FIG. 4  is a partially enlarged cross-sectional view illustrating a state of fastening and rotating in a second embodiment of the present invention. 
           [0015]      FIG. 5  is a partially enlarged cross-sectional view illustrating a state of fastening and fixing in a second embodiment of the present invention. 
           [0016]      FIG. 6  is a partially enlarged cross-sectional view illustrating a third embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0017]    Hereinafter, a first embodiment of the present invention will be described with reference to  FIGS. 1 to 3 . Referring to  FIG. 1 , the reference numeral  1  represents a self-tapping screw formed from a head portion  2  and a leg portion  3  formed integrally therewith, and is made of a hard material such as carbon steel or stainless steel. The head portion  2  has a drive hole  4  in which screw driving force is transmitted to a self-tapping screw  1  from a driver bit (not illustrated). In the leg portion  3  formed integrally with the head portion  2 , a normal thread  10  is extended in a spiral shape from the vicinity of a bearing surface  5  of the head portion  2  in a direction where a tip of the leg portion  3  is positioned. 
         [0018]    On the other hand, in the tip side of the leg portion  3 , a guide thread  11  having a smaller diameter than that of the normal thread  10  is molded. The guide thread  11  is set to have a diameter which is the same as or slightly larger than the diameter of a prepared hole  21  formed on a workpiece  20 . The workpiece  20  is made of a soft material such as a resin or an aluminum alloy. 
         [0019]    In addition, a female screw molding thread  12  having a larger diameter than that of the normal thread  10  is molded between the normal thread  10  and the guide thread  11 . The normal thread  10 , the female screw molding thread  12  and the guide thread  11  are continuously connected to molding threads in the leg portion  3 . 
         [0020]    As illustrated in  FIG. 2 , a pitch (P) in a bottom of a female screw  14  molded by the female screw molding thread  12  is determined by a pitch (P) in a crest between the female screw molding thread  12  and the guide thread  11 . In addition, the pitch (P) in the crest of the normal thread  10  which is positioned in the head portion side from the female screw molding thread  12  is set to be similar to the pitch (P) in the bottom of the female screw  14 . However, a pitch (P′) between the crest of the female screw molding thread  12  and the crest of the adjacent normal thread  10  is set to be larger than the pitch (P) in the crest of the other normal thread  10 . This causes the normal thread  10  to be wedged into the female screw  14 . Accordingly, the crest of the normal thread  10  is disposed at a position shifted in a direction where a pressure flank surface  13   a  of a female thread  13  is positioned. According to this configuration, a distance between a pressure flank surface  10   a  of the normal thread  10  and the pressure flank surface  13   a  of the female thread  13  is closer than a distance between a clearance flank surface  10   b  of the normal thread  10  and a clearance flank surface  13   b  of the female thread  13 . 
         [0021]    In addition, the normal thread  10  and the female screw molding thread  12  have an asymmetric shape in which a clearance flank angle (α) is large and a pressure flank angle (β) is small. A flank angle of the female screw molding thread  12  is formed to be larger than each flank angle of the normal thread  10  by (θ). 
         [0022]    As illustrated in  FIG. 2 , in a stage of fastening and rotating, the self-tapping screw  1  is screwed while applying thrust force in a direction of an arrow Y 1 . Therefore, the crest of the normal thread  10  of the pressure flank surface  10   a  is slightly in contact with the pressure flank surface  13   a  of the female thread  13 . However, in this configuration, contact resistance thereof is insignificant and driving torques are set not to be high. The configuration may be made such that in the stage of the fastening and rotating, the crest of the normal thread  10  of the pressure flank surface  10   a  is close to an extent not to come into contact with the pressure flank surface  13   a  of the female thread  13 . 
         [0023]    On the other hand, as illustrated in  FIG. 3 , in a stage of fastening and fixing, axial force acting on the self-tapping screw  1  applies a pressure to the self-tapping screw  1  in a direction of an arrow Y 2 , and applies a pressure to the female screw  14  in a direction of an arrow Y 3  which is the opposite direction to the arrow Y 2 . Therefore, in the stage of the fastening and fixing, the crest of the normal thread  10  of the pressure flank surface  10   a  and the pressure flank surface  13   a  of the female thread  13  are closer to each other and come into contact with each other as compared to the stage of the fastening and rotating. Consequently, the contact resistance is increased to obtain strong fastening force. In particular, in the stage of the fastening and fixing, the crest of the normal thread  10  is wedged into the pressure flank surface  13   a  of the female thread  13  by setting the flank angle of the normal thread  10  and the female screw molding thread  12  as described above. Accordingly, the fastening force is increased as compared to the contact between both surfaces. 
         [0024]    Hereinafter, a second embodiment of the present invention will be described with reference to  FIGS. 4 and 5 . In a self-tapping screw  51  illustrated in the second embodiment, a pitch is differently set compared to the self-tapping screw  1  illustrated in the first embodiment. As illustrated in  FIG. 4 , a pitch (P) between the crest of a female screw molding thread  62  and a crest of an adjacent normal thread  60  is set to be similar to a pitch (P) in a bottom of a female screw  64 . On the other hand, a pitch (P′) in a crest of a normal thread  60 A positioned in the head portion side is set to be larger than a pitch (P) in a bottom of the female screw  64 . 
         [0025]    In this manner, in the stage of the fastening and rotating illustrated in  FIG. 4 , the self-tapping screw  51  has the normal thread  60 A which comes into contact with a pressure flank surface  63   a  of a female thread  63  and a normal thread  60  which does not come into contact therewith. Thus, the driving torques are much lower than those of the self-tapping screw  1  in the first embodiment. On the other hand, in the stage of the fastening and fixing illustrated in  FIG. 5 , if the axial force is applied, the normal thread  60 A further approaches and comes into contact with the pressure flank surface  63   a  of the female thread  63 . However, in the normal thread  60  located in a section between the normal thread  60 A and the female screw molding thread  62 , even if the axial force is applied, the crest of a pressure flank surface  60   a  does not come into contact with the pressure flank surface  63   a  of the female thread  63 . Therefore, as compared to the self-tapping screw  1  in the first embodiment, the fastening force is weak. 
         [0026]    As described above, it is possible to obtain the most suitable driving torques or fastening force by selectively using the self-tapping screws  1  and  51  illustrated in the first and second embodiments depending on the materials of the workpieces. 
         [0027]    Hereinafter, a third embodiment of the present invention will be described with reference to  FIG. 6 . In a self-tapping screw  100 , a normal thread  110  is formed to have a trapezoidal shape. In this case, a reference line of a pitch of a normal thread  110  is set based on a crest which is positioned on a pressure flank surface  113   a  side of a female thread  113  in an upper base of the normal thread  110 . 
         [0028]    In addition, a bottom of a female screw  114  molded by a female screw molding thread  112  having a trapezoidal shape is formed to have a plane. In this case, a pitch line in the bottom of the female screw  114  is set based on an end point where the pressure flank surface  113   a  of the female thread  113  is positioned. 
       REFERENCE SIGNS LIST 
       [0029]      1  self-tapping screw 
         [0030]      2  head portion 
         [0031]      3  leg portion 
         [0032]      4  drive hole 
         [0033]      5  bearing surface 
         [0034]      10  normal thread 
         [0035]      10   a  pressure flank surface 
         [0036]      10   b  clearance flank surface 
         [0037]      11  guide thread 
         [0038]      12  female screw molding thread 
         [0039]      13  female screw thread 
         [0040]      13   a  pressure flank surface 
         [0041]      13   b  clearance flank surface 
         [0042]      14  female screw 
         [0043]      20  workpiece 
         [0044]      21  prepared hole