Improvement of anti-loosening resistance of locking bolts based on single-coarse-thread/multiple-coarse-thread mechanism by using spring washer between nuts

An innovative anti-loosening bolt with a double-thread mechanism (denoted as DTB-IIC) consisting of coaxial single and multiple coarse threads was previously devised and its structure and performance were optimized. The results of a previous study showed that increasing the bottom rise ratio $\beta$, which is the ratio of the bottom rise for the multi-thread groove to the thread height, to 70 % significantly improved the formability during the thread-rolling process, but clearly reduced the loosening resistance. In the present study, an attempt was made to address this problem in a simple manner by inserting a left-handed spring washer (SW) between the inner multi-thread nut and the outer single-thread nut. The value of $\beta$ was set to 50 %, 60 %, or 70 %. Comparative Junker vibration loosening tests based on the ISO 16,130 standard were conducted and the change in the residual ratio for the axial load, $\kappa$, was evaluated. Without the SW, the final $\kappa$ (${\kappa }_f$) for $\beta =50\%$ was above 90 %, while ${\kappa }_f$ for $\beta =60\%$ and $\beta =70\%$, it was approximately 73 % and 64 %, respectively. Attachment of the SW caused an increase in ${\kappa }_f$ for all $\beta$ values, with a greater increase for larger $\beta$, reaching 82 % for $\beta$ = 60 % and 75 % for $\beta$ = 70 %, respectively. It was found that the contact force between the nuts is an indicator for determining the degree of locking between the DTB-IIC and the nut. The initial loosening process was simulated using a three-dimensional finite element method model, and the $\kappa$ curves obtained in the analysis agreed well with the experimental results by setting the gap $\delta$ between the inner multi-thread nut and the DTB-IIC bolt in the range 0.125–0.15 mm. The simulation results indicated that there were clear differences in mating state between the outer nut and the DTB-IIC depending on the $\beta$ value, and the use of a SW achieved a more robust locking state when $\beta$ was 50 %.