If a bolted joint is subjected to axial vibration, the internal force the bolt receives is determined based on the load factor that is a function expressed by bolt stiffness and clamped part stiffness. The stiffness of the bolt and clamped part are generally calculated based on VDI2230, but if the structure of the clamped parts is complicated or made of multiple materials, we have to calculate the clamped part stiffness using FE analysis. FE analysis is very useful for calculations of clamped part stiffness. However it is not easy to conduct FE analysis for all parts and structures. A new method to detect clamped part stiffness of bolted joints was developed in this study. The proposed method is based on a clamp force detection method that we had already proposed. In the method, a protruding bolt thread portion is first pulled while holding down the nut's upper surface and the displacement at the pulling point is measured. The gradient of pulling force in relation to the displacement changes when the pulling force reaches the clamp force of bolted joint. In the clamp force detection method, we detected the pulling force and when the pulling force reached the clamp force, we took that as the clamp force. In the proposed method, we detected the clamped part stiffness using the gradient of pulling force before and after the pulling force reaches the clamp force. To verify the usefulness of the proposed method we conducted an experiment to detect the clamped part stiffness. In the experiments, M10 hexagon head bolts and hexagon nuts were used, and the stiffness of clamped parts made of stainless steel were detected using the proposed method. The results showed that the proposed method can detect clamped part stiffness with about ±20 % accuracy.
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