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In industrial fields, the bolt-nut connection is widely used and unitized as an important machine component. In the previous studies, a bolt-nut connection with slight pitch difference was considered towards realizing anti-loosening performance and high fatigue strength through axisymmetric finite element method (FEM) as well as experiment. Also by applying three-dimensional FEM, the nut screwing process was analyzed to obtain the prevailing torque Tp confirming anti-loosening performance. In this study, the tightening and untightening processes after the nut touched the clamped body are analyzed through 3D FEM as well as experiment. The FEM results are in good agreement with the experimental results. The clamping force and tightening torque relationship under the pitch difference is compared to the normal nut. Then, the behaviors of the pitch difference nut in tightening and untightening process is discussed by varying the pitch difference. For the normal nut, the equation of clamping force and tightening torque relation in the untightening process is clarified by FEM. Since the nut untightening property after the nut tightening is more important to prevent the nut self-loosening, the loosening resistance torque TRu is newly defined in this process. It is found that with increasing the pitch difference the loosening resistance torque TRu increases. The large loosening resistance torque TRu suggests that the pitch difference nuts have good anti-loosening performance.

In the previous studies, a bolt-nut connection with slight pitch difference was considered towards realizing anti-loosening performance and high fatigue strength through axisymmetric finite element method (FEM) as well as experiment. In this study, screwing process before the nut touching the clamped body is analyzed through three dimensional FEM. Then, it is found that the prevailing torque during this screwing process is in good agreement with the experimental result. The prevailing torque varies depending on the pitch difference from 5 to 25 Nm and anti-loosening behavior is confirmed. After the nut toughing a clamped body, the tightening process is also analyzed. For normal nut, the tightening force coincides with the empirical formula. The tightening torque and tightening force relations are discussed with varying the pitch difference.