Achieving uniform thread load distribution in bolted joints using different pitch values

Bolted joints are a critical component of machines and structures under cyclic loading. Bolt fatigue failure usually takes place in the first engaged thread for being the most loaded one. In this sense, a uniform thread load distribution improves its mechanical response and consequently the reliability of the joint. For this purpose, different thread pitch values can be used in the bolt and the nut, and there is an optimum value that leads to the most uniform load distribution for each particular joint configuration (geometry, preload level, materials and boundary conditions). Sopwith developed an analytical model to calculate this value, but no validation was carried out. This work presents a semianalytical model to estimate the load distribution for any differential thread spacing, which can also be used to calculate the optimum value. The results of this model are nearly coincident with Finite Element results and more accurate than those obtained from Sopwith model.