Upset bolt

Abstract

An upset bolt provided with a pressure formed recess on the bolt head. The recess is formed by the combination of three separate surfaces, that is, concave, convex and flat standing surfaces which respectively form lower, middle and upper portions of the recess. By virture of provision of the concave and convex surfaces respectively in lower and middle portions of the recess, stress concentration occurs in the region along the concave and concave surfaces when the recess is pressure formed. Since the region of stress concentration is positioned away from the neck portion of the bolt body, the breaking strength is remarkably increased as compared with conventional upset bolt in which the stress concentration occurs in a region near the neck portion of the bolt body. Provision of the six flat standing surfaces along the periphery of the hexagonal bolt head makes it possible to sharply form the apexes of the bolt head without application of such an extremely great pressure force as to shorten the life of the dies used in deformation of the bolt head.

Dependencies are inferred from the claim text.