Vibration resistant screw

Claim 1

A threaded fastener comprising an elongated shank having a torque receiving means at one end and thread forming means at the other end for forming internal threads of a predetermined shape in the wall of a previously formed hole, said shank having a fastening section with external threads adjacent said torque receiving means, said external threads being continuous and having substantially the same pitch as the pitch of the thread forming means and being of a thread form different from but adapted to mate with the internal threads formed by the thread forming means, said shank having a longitudinal axis and said thread forming means and external threads being coaxial therewith, said shank having a reduced diameter portion defining an empty annular space around the shank in said hole and also defining a thread forming torque transmission connection section between said external threads and said thread forming means which is the minimum cross section and weakest part of said shank with respect to axial tension therein whereby elongation under tension occurs primarily in the connection section and said section is more susceptible to fracture under tensile load than the other sections, said thread forming means being shaped to form internal threads with wedge ramps at the roots thereof and said external threads having crests that engage said ramps and move down the ramps as axial load on the shank increases.