Thorough understanding on the mechanism of vibration-induced loosening of threaded fasteners based on modified Iwan model

Under transversal vibration, the local slippage on the thread and bearing surfaces is typically nonlinear and its gradual accumulation with vibration cycles contributes to the loosening of threaded fasteners. The classical Iwan model is used widely to model the nonlinearity of contact interface, but not appropriate for analyzing the local slippage accumulation on the thread and bearing surfaces. In this paper, we proposed modified Iwan model to represent the nonlinear local slippage behavior. The acting forces on the internal thread were decomposed into three component forces along different directions. The effect of each component on the local slippage accumulation was analyzed theoretically based on the modified Iwan model. It was found that the cyclic component force along the radial direction dominates the local slippage accumulation of thread surface and other two cyclic component forces have no effects on it. Then, we also analyzed the local slippage accumulation on the bearing surface using the modified Iwan model and developed a thorough understanding on loosening mechanism. Based on the loosening mechanism, we proposed that the key to preventing local slippage accumulation and sustained loosening was to inhibit the slippage or relative motion occurring on the thread surface along the radial direction. Thus, several novel thread structures were designed for resisting loosening based on above findings. Finally, their superior anti-loosening abilities were validated by FEA method.