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Loosening failure of bolted joints often occurs in a vibration environment. This may induce the separation of clamped components and even result in catastrophic consequences in certain situations. Various anti-loosening bolted connections have been developed and widely applied to prevent loosening. Tightening and fatigue resistance performances are also two important characteristics of anti-loosening bolted connections. However, a comprehensive comparison study of the tightening, anti-loosening, and fatigue resistance performances of typical anti-loosening bolted connections is lacking, meaning that there is a lack of material to assist in the selection of anti-loosening bolted connections in engineering. In this study, accurate three-dimensional finite element models of typical anti-loosening bolted connections with the forms of washers or nuts are established. The processes of tightening, vibration-induced loosening, and fatigue are simulated via finite element analysis (FEA). Based on the results of FEA, the tightening, anti-loosening, and fatigue resistance performances of regular bolted joints and several anti-loosening bolted connections are comprehensively compared. The results reveal that bolted joints installed with both a variable-diameter nut and an eccentric double nut are difficult to tighten. We confirm that the researched structures all have superior anti-loosening capacities compared to regular bolted joints. Additionally, the results show that the regular bolted joint achieved the worst fatigue resistance performance, i.e., the application of anti-loosening bolted connections improves fatigue resistance. Finally, the effects of preload distributions on the two nuts in bolted joints with double nuts are discussed in terms of tightening, anti-loosening, and fatigue resistance performances.

Abstract Self-loosening of bolted joints can occur in a vibration environment, and it may induce bolt fatigue fracture with catastrophic consequences. It is essential to clarify the self-loosening mechanism, based on which novel anti-loosening thread structures can be developed. In this paper, we propose the concept of radial slippage propagation and provide new insights into the self-loosening process. The new theory states that the slippage along the radial direction of the thread surface induces more slippage areas (slippage propagation), and self-loosening occurs due to the dynamic evolution and propagation of contact states on the thread and bearing surfaces with an increase in the number of vibration cycles. Finite element analysis (FEA) was used to validate the propagation process of slippage areas on the thread surface. A novel bolted joint with step thread engagement was developed, which could prevent the occurrence of relative motion of the external and internal threads in the radial direction and thus block slippage propagation. A three-dimensional (3D) finite element model (FEM) of the novel thread structure was established, and a test specimen was manufactured using two special tools. FEA and experiments validated its superior anti-loosening and anti-fatigue performances, and the convenience of installation and removal. Experimental validation of the radial slippage propagation theory and the performance optimisation of the step-thread structure should be performed in the future.