Screw tightening structure, screw member, and screw tightening tool

A threaded-fastener wrenching structure comprising:

• a fitting hole which is provided in one of a threaded fastener and a wrenching tool for wrenching said threaded fastener;
• and
• a fitting protrusion which is provided in the other of said threaded fastener and said wrenching tool and which is to be brought into fitting engagement with said fitting hole;
• wherein each of said fitting hole and protrusion has a contour having a plurality of torque transmitting portions which are equi-angularly spaced apart from each other about a center line of said each of said fitting hole and protrusion and which project outwardly in a radial direction of said each of said fitting hole and protrusion, so that a wrenching torque is transmitted through said torque transmitting portions to said threaded fastener, as a result of rotation of said wrenching tool about said center line when said fitting hole and protrusion are held in fitting engagement with each other,
• said threaded-fastener wrenching structure being characterized in that each of said torque transmitting portions has an engaging portion through which the wrenching torque is transmitted to said threaded fastener, wherein a tangent line tangent to said engaging portion substantially coincides with a straight line passing through said center line in a cross section perpendicular to said center line,
• that said threaded fastener has an external thread,
• that said fitting protrusion is provided in said threaded fastener,
• and that said fitting protrusion has a maximum diameter Dmax, a minimum diameter Dmin and a height k as measured in a direction parallel with said center line such that said maximum diameter Dmax, said minimum diameter Dmin and said height k satisfy the following expressions (4), (5), (6) which represent relationships with a major diameter d of said external thread:
• Dmax≦1.5d (4) 1.1d<Dmin (5) 0.3d≦k<0.45d (6).