Rolling Formability Optimization of Locking Bolt Based on a Double-Thread Structure Composed of Coaxial Single and Multiple Threads

Abstract In previous studies, we developed innovative anti-loosening bolts and nuts with a double-thread structure (denoted DTB-IIC) composed of coaxial single and multiple coarse threads. It was also experimentally proven that the DTB-IIC has high anti-loosening performance. In this study, we analytically and experimentally investigated the effects of multiple thread groove depths and rolling methods on the thread rolling formability of DTB-IIC fasteners. The bottom rise rate, which is the ratio of the bottom rise amount of the multi-thread groove to the reference thread height, was set in three ways of 50%, 60%, and 70%. As the bottom rise rate increased, peeling on the thread surface was suppressed and the high temperatures produced by thread rolling decreased significantly, but the loosening resistance against vibration clearly decreased. We compared three typical mass production processes for bolts: the round die method, the flat die method, and the planetary method, with the bottom rise rate set at 50%. It was found that the flat die method had the best rolling formability in terms of screw-thread shape and surface quality. A finite element simulation consistently reproduced the deformation process of the complex DTB-IIC thread and revealed that the material shear flow due to over-rolling led to the occurrence of surface peeling.