| 摘要 |
PROBLEM TO BE SOLVED: To prevent too tight fastening of a screw, prevent breakage of thread groove, and prevent a state in which a screw is hard to return, by abutting thread tightener to a head, including a groove constituted in a longitudinal direction and a lateral direction for thread fastening, and comprising steps in a radial direction of the head at an approximately central part of the groove. SOLUTION: This screw comprises a head 1, a thread part 2, a thread groove 3 to which a thread tightener is abutted for rotation, and step parts 4 at an approximately central part of the thread groove 3. Each of the step parts 4 descends from a left side to a right side. When fastening of the screw terminates, the thread tightener rides on the steps 4, and is disengaged from the thread groove 3, thereby preventing too tight fastening of the screw. The thread groove 3 is formed so as to be shallower at both ends than at a central part. By this constitution, a breakage of the thread groove 3 can be prevented, and too tight fastening of the screw can be avoided, whereby the screw is easy to return when loosing the screw. When a resin such as a plastic which is likely to be broken is used as the head 1 of the screw, breakage can be prevented. When not only the thread groove 3 but also the head 1 and the thread part 2 are not integrally formed, cracking of a joint part can be prevented.
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