A self-tapping 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 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.

TECHNICAL FIELD

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

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.

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

SUMMARY OF INVENTION

Technical Problem

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

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.

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.

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.

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

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.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a first embodiment of the present invention will be described with reference toFIGS. 1 to 3. Referring toFIG. 1, the reference numeral1represents a self-tapping screw formed from a head portion2and a leg portion3formed integrally therewith, and is made of a hard material such as carbon steel or stainless steel. The head portion2has a drive hole4in which screw driving force is transmitted to a self-tapping screw1from a driver bit (not illustrated). In the leg portion3formed integrally with the head portion2, a normal thread10is extended in a spiral shape from the vicinity of a bearing surface5of the head portion2in a direction where a tip of the leg portion3is positioned.

On the other hand, in the tip side of the leg portion3, a guide thread11having a smaller diameter than that of the normal thread10is molded. The guide thread11is set to have a diameter which is the same as or slightly larger than the diameter of a prepared hole21formed on a workpiece20. The workpiece20is made of a soft material such as a resin or an aluminum alloy.

In addition, a female screw molding thread12having a larger diameter than that of the normal thread10is molded between the normal thread10and the guide thread11. The normal thread10, the female screw molding thread12and the guide thread11are molded in the spiral shape and continuously connected to molding threads in the leg portion3.

As illustrated inFIG. 2, a pitch (P) in a bottom of a female screw14molded by the female screw molding thread12is determined by a pitch (P) in a crest between the female screw molding thread12and the guide thread11(denoted as a third pitch). In addition, the pitch (P) in the crest of the normal thread10which is positioned in the head portion side from the female screw molding thread12(denoted as a second pitch) is set to be similar to the third pitch (P) in the bottom of the female screw14. However, a pitch (P′) between the crest of the female screw molding thread12and the crest of the adjacent normal thread10(denoted as a first pitch) is set to be larger than the pitch (P) in the crest of the other normal thread10. This causes the normal thread10to be wedged into the female screw14. Accordingly, the crest of the normal thread10is disposed at a position shifted in a direction where a pressure flank surface13aof a female thread13is positioned. According to this configuration, a distance between a pressure flank surface10aof the normal thread10and the pressure flank surface13aof the female thread13is closer than a distance between a clearance flank surface10bof the normal thread10and a clearance flank surface13bof the female thread13. In addition, the portions of the first and third pitches are constantly formed along at least 180 degrees of the periphery as seen inFIGS. 1 and 2.

In addition, the normal thread10and the female screw molding thread12have 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 thread12is formed to be larger than each flank angle of the normal thread10by (θ).

As illustrated inFIG. 2, in a stage of fastening and rotating, the self-tapping screw1is screwed while applying thrust force in a direction of an arrow Y1. Therefore, the crest of the normal thread10of the pressure flank surface10ais slightly in contact with the pressure flank surface13aof the female thread13. 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 thread10of the pressure flank surface10ais close to an extent not to come into contact with the pressure flank surface13aof the female thread13.

On the other hand, as illustrated inFIG. 3, in a stage of fastening and fixing, axial force acting on the self-tapping screw1applies a pressure to the self-tapping screw1in a direction of an arrow Y2, and applies a pressure to the female screw14in a direction of an arrow Y3which is the opposite direction to the arrow Y2. Therefore, in the stage of the fastening and fixing, the crest of the normal thread10of the pressure flank surface10aand the pressure flank surface13aof the female thread13are 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 thread10is wedged into the pressure flank surface13aof the female thread13by setting the flank angle of the normal thread10and the female screw molding thread12as described above. Accordingly, the fastening force is increased as compared to the contact between both surfaces.

Hereinafter, a second embodiment of the present invention will be described with reference toFIGS. 4 and 5. In a self-tapping screw51illustrated in the second embodiment, a pitch is differently set compared to the self-tapping screw1illustrated in the first embodiment. As illustrated inFIG. 4, a pitch (P) between the crest of a female screw molding thread62and a crest of an adjacent normal thread60is set to be similar to a pitch (P) in a bottom of a female screw64. On the other hand, a pitch (P′) in a crest of a normal thread60A positioned in the head portion side is set to be larger than a pitch (P) in a bottom of the female screw64.

In this manner, in the stage of the fastening and rotating illustrated inFIG. 4, the self-tapping screw51has the normal thread60A which comes into contact with a pressure flank surface63aof a female thread63and a normal thread60which does not come into contact therewith. Thus, the driving torques are much lower than those of the self-tapping screw1in the first embodiment. On the other hand, in the stage of the fastening and fixing illustrated inFIG. 5, if the axial force is applied, the normal thread60A further approaches and comes into contact with the pressure flank surface63aof the female thread63. However, in the normal thread60located in a section between the normal thread60A and the female screw molding thread62, even if the axial force is applied, the crest of a pressure flank surface60adoes not come into contact with the pressure flank surface63aof the female thread63. Therefore, as compared to the self-tapping screw1in the first embodiment, the fastening force is weak.

As described above, it is possible to obtain the most suitable driving torques or fastening force by selectively using the self-tapping screws1and51illustrated in the first and second embodiments depending on the materials of the workpieces.

Hereinafter, a third embodiment of the present invention will be described with reference toFIG. 6. In a self-tapping screw100, a normal thread110is formed to have a trapezoidal shape. In this case, a reference line of a pitch of a normal thread110is set based on a crest which is positioned on a pressure flank surface113aside of a female thread113in an upper base of the normal thread110.

In addition, a bottom of a female screw114molded by a female screw molding thread112having a trapezoidal shape is formed to have a plane. In this case, a pitch line in the bottom of the female screw114is set based on an end point where the pressure flank surface113aof the female thread113is positioned.

REFERENCE SIGNS LIST

12female screw molding thread

13female screw thread