Improving Fatigue Resistance of Threaded Fasteners Using a Novel Mathematical Model for Selecting Optimal Nut Pitch Value

ABSTRACT Threaded fasteners are prone to fatigue fracture under axial alternative load conditions. Recent studies demonstrated that the pitch difference between bolt and nut can significantly impact the fatigue life. Currently most standards specify equal nominal pitch for bolt and nut that can be one of the reasons of their low fatigue resistance. Choosing optimal pitch difference value can enhance fasteners durability. However, the exact relationship between the pitch difference and the fatigue life remains unclear. This study analytically demonstrates that the relationship between stress concentration level under static load and the pitch difference follows inverse normal distribution. Then, based on the experiments under alternative load, we divided fatigue life into two stages: the initial fatigue life and the residual fatigue life. The results show that the stress concentration level and initial fatigue life exhibit a geometrically inverse relationship, i.e., when the stress concentration is minimized, initial fatigue life is maximized. Finally, we propose a novel mathematical model based on the pitch difference–stress concentration level–fatigue life relationship. The model offers a practical solution for improving fatigue resistance in engineering applications without incurring any additional cost.