Patent ID: 7214020

Claim:
A looseness resisting screw formed with an external thread comprising: a first thread area having a substantially uniform outer diameter and a second thread area extended from the first thread area toward a leading end of the screw, wherein the second thread area has a plurality of fractional lead parts each of which has an outer thread diameter and an axial thickness at the root of the thread increasing along a helical direction of the fractional lead part from a start point to an end point, and said outer thread diameter is substantially equal to a thread root diameter at the start point, while said outer thread diameter is equal to a thread top diameter at the end point, the shoulder having a surface substantially perpendicular to a central axis of the screw is provided between the two of the fractional lead parts wherein the starting point is at the shoulder; wherein the fractional lead part has a thread sectional area increasing along the helical direction from the start point to the end point, and the thread top diameter of the fractional lead part is slightly larger at the end point than the substantially uniform outer diameter of the first thread area so that the second thread area provides a tapping thread section, and wherein the second thread area has an introductory screw section extended from the tapping section toward the leading end of the screw.