Optimal Design of Cap Screw Thread Runout for Transversal and Axial Loads

Abstract

The presented work is focused on a design of a new thread runout of screws and bolts subjected by transversal loads. These types of loads can occur in a normal service of bolt connection and they can lead to a failure. Two failure modes of the screw could occur: self-loosening of the screw or fatigue failure of the screw. The latter failure mode is analysed in the presented project focusing on an improvement of the existing design of the screws in terms of fatigue failure. The fatigue fracture of the screws, which are loaded by the transversal load, occurs normally in a groove of a first thread run of an engagement. A shape of the thread runout has some influence on stress in the thread therefore that shape can optimize to minimize the thread stress. Considering this, free shape optimisation was employed to find the best runout shape. As results of the presented work, the optimum design of the thread runout is proposed for the analysed conditions. The elaborated design of the cap screw brings significant improvement of the lifetime when the cycling tangential load occurs. In addition, the new design gives the better structural performance than the existing designs also for the cycling normal loads.