Patent ID: 8075233

Claim:
A bolt, the axial force of which is to be determined by a supersonic wave, consisting of a bolt head having a polygonal shaped outline, and a bolt shaft around which a spiral thread is formed, wherein a recess having a polygonal shaped outline corresponding to the polygonal shaped outline of said bolt head is formed on said bolt head, with each corner of the polygonal shaped outline of said recess and each corresponding corner of the polygonal shaped outline of said bolt head being on a radial line from the center of said recess, with a projection having a plane upper face being formed on the inner side of said recess, said plane upper face being set to be on the same level as, or a little higher level than, the upper face of said bolt head, said bolt shaft having a plane end face, said plane end face being surrounded with a wall, the outer diameter of which is set to reduce gradually toward the end of said bolt shaft, said wall being formed during the thread rolling process by stretching the outside of the bolt shaft in which the spiral thread is formed.