Patent ID: 8545156

Claim:
A screw tightening structure comprising an indented concave recess formed on a screw, and mating convex sections provided on a screw tightening tool for tightening the screw: the recess and the mating convex sections including three or more torque transfer portions, respectively, which protrude outward and are disposed at equiangular intervals for concentrically mating engagements; rotating the screw tightening tool causing the torque transfer portions of the mating convex sections to transfer a tightening torque to the screw via the torque transfer portions of the recess; under which outer circumferential edge portions of the torque transfer portions of the mating convex sections to be brought into abutting contact with sidewall of the torque transfer portions of the recess with the tightening torque being transferred to the screw via force application points Q representing resulting abutment portions; whereby in a cross-sectional shape or an end face configuration of the recess on a plane perpendicular to a center line O 1 of the recess and involving the force application points Q, a surface perpendicular force F, perpendicular to a sidewall of the recess at the force application point Q, is oriented in a direction at a driving angle q, representing an angle that is inclined with respect to a tangential direction of the force application point Q with a center on the center line O 1 , which is expressed as θ≦0° with the driving angle θ being oriented outward from the tangential direction and assigned to be positive; in the cross-sectional shape or the end face configuration of the recess on the plane perpendicular to a center line O 1 of the recess and involving the force application points Q, the three or more torque transfer portions of the recess being connected to each other via concave circular arc contours having circular arc each with a radius R which form a part of or a whole of sidewalls of the torque transfer portions; the radius R falling in a range expressed as 0.15 G≦R≦0.30 G when G represents an outer diameter of the torque transfer portion of the recess, and the center point P 1 of the concave circular arc contours each with the radius R being located at a circumference of a circle of a center-point diameter GR falling in a range expressed as 0.85 G≦GR≦0.95 G with the center line O 1 as a center point such that the force application points Q are present on the concave circular arc contours having a diameter in a range from 0.87 G to 0.93 G with the center line O 1 as a center point, respectively; outer circumferential walls of the torque transfer portions of the recess having circular arc configurations on the outer diameter G and the outer circumferential walls and the concave circular arc contours in areas closer to the force application points Q being smoothly connected to each other via circular arcs each with a radius Rc and having the relationship Rc<(G−GR)/2.