Finite element analyses of female-thread forming process and loosening proof of self-tapping screw

Abstract

Tapping screw fabricates the female thread due to plastic deformation during screw rotation. Since the tapping screw skips the preliminary fabrication of female thread, it is widely used for the tightening in the fields of such as electric parts and car parts. However, there is a few theoretical studies for its self-tapping process and loosening proof. In this paper, we have investigated the mechanism of tapping torque and shape of female thread using three-dimensional finite element analysis. In addition to the plastic flow caused by the friction force due to the rotation of male thread, the plastic flow at the top of male thread separated by the up and down directions occurs. That contact at thread top mostly contributes the torque of self-tapping. Also, plastic flow along the lower surface of frank thread separated by the outside and inside radial directions creates the characteristic shape of female thread. The analysis results well reproduce the tapping torque and the shape of female thread. We also investigate the loosening proofs due to shear loading both for tapping screw and normal screw using there-dimensional finite element analysis. The shape of female screw obtained by self-tapping analysis is incorporated into the analysis model. It is found that the loosening proof of the tapping screw is superior to that of normal screw since contact at the top of thread in the lower part of tapping bolt inhibits the thread slip.

Keywords

• finite element analysis
• self-tapping screw
• screw
• fixing
• plastic working
• contact problem