Effect of thread pitch on the fatigue strength of steel bolts

The paper explores the effect of the thread pitch on the fatigue strength of ISO steel bolts fitted with standard nuts. By means of boundary element analyses and cross-comparison with theory and photoelasticity, the load capacity of the bolt is ultimately related to a comprehensive notch factor which embodies both stress concentration and notch sensitivity dependencies. The notch factor is conveniently plotted as a function of nominal diameter and thread pitch for a selection of steel grades. The charts show that the endurance load slightly increases as the pitch is decreased for small bolts of low-grade steel. Conversely, the endurance load markedly increases with the pitch (improvements above 40 per cent can be achieved) for large bolts of high-grade steel.