MECHANISM OF SCREW THREAD LOOSENING IN BOLTED JOINTS WITH DISSIMILAR CLAMPED PARTS UNDER REPEATED TEMPERATURE CHANGES

摘要

It is well known that bolted joints are sometimes loosened under transverse repeated loadings. Recently, some studies have been carried out on the mechanism of the bolt loosening under transverse repeated loadings. In the present research, the mechanism of bolt loosening was examined for a bolted joint using Junker's type loosening test machine under transverse repeated loadings. Furthermore, a rotational deformation of bolt axis and nut was analyzed using FEM. As a result, it is found that the rotational load is applied and the bolt loosening occurs due to the deformation between the bolt axis and the nut. The measured change in axial bolt force was fairly good agreement with the numerical results. In addition, the bolt loosening in bolted joints with dissimilar clamped parts was examined under temperature changes. Due to the temperature changes, it is assumed that a relative displacement among the bolt bearing surface and the bearing surfaces of the clamped parts and then it is found that a bolt loosening occurs. In FEM calculations, the behavior of the bolt loosening was examined and found to occur a slight loosening. In the experiments, the bolt loosening was measured under temperature changes. The material of dissimilar clamped parts is assumed steel and aluminum. The thickness of the clamped parts is 10mm. The tendency of the bolt loosening is fairly coincided between the numerical and the measured results. It was observed that the bolt loosening occurred easily as the clamped length (gap length) decreased. In the FEM calculations, the slope θ of the bearing surfaces between the bolt head and the clamped part is assumed to be 0.5 degree to 2 degree. The effects of the slope of the bearing surfaces and on the bolt loosening are examined in the FEM calculations. The calculated results are in a fairly good agreement with the measured results. As the conclusions, 1) the bolt loosening of bolted joints under transverse repeated loadings was elucidated using 3-D FEM calculations, 2) Under repeated temperature changes, it was found that the bolt loosening occurred due to the slips between the bearing surfaces and the interfaces of clamped parts. Using FEM calculations, the behavior of the bolt loosening was elucidated.