Optimization of anti-loosening bolt based on double thread mechanism: Development of ground rolling die and effect of thread accuracy on loosening resistance.

We have devised innovative anti-loosening bolts with a double thread-mechanism (denoted DTB-IIC) composed of coaxial single and multiple coarse threads. In this study, we first developed a high-speed and high-precision grinding system for dedicated thread rolling dies of the DTB-IIC. Compared to conventional electro-discharge machining dies, ground dies significantly reduced processing time and costs, and achieved more than 10 times higher durability in thread rolling tests. Comparative Junker vibration loosening tests based on an ISO standard were conducted on several types of DTB-IICs. The amount of backlash δ between the inner multiple-thread nut and DTB-IIC bolt has a great effect on the initial drop of the bolt axial preload, and the rolled DTB-IIC with a relatively small δ secured a residual axial load rate of ≥85%, which satisfies the rating-1 of good self-locking behavior. A 3D FEM model was employed to simulate the initial loosening process in the Junker test, and the analytical results agreed well with the experimental ones by adjusting δ appropriately. Both experimental and analytical results indicate that the high loosening resistance of DTB-IIC is due to the synergistic effect of the interference mechanism of two types of nuts and the jammed locking state.