Fine screw threads are widely used for the bolted joints under severe running conditions. It is well known that they are effective to prevent thread loosening due to fine pitch. As for other mechanical characteristics, it has been reported in the previous paper using complex stress functions that the stress concentration at bolt thread root is higher than coarse screw threads and the fatigue strength of threaded fasteners shows a minimum value for varying pitch. However, the latter is questionable since the calculations were conducted under lots of hypotheses. In this study, stress concentration and stress amplitude along thread root are evaluated by three-dimensional finite element analysis, in which numerical models of the bolted joints are constructed so as to accurately represent the effect of thread helical geometry. It is shown that the stress concentration at thread root of fine screw threads is higher than that of coarse screw threads, and the maximum stress amplitude is likely to be lower on the contrary. Meanwhile, it is sometimes recognized that clamping forces of fine screw threads are smaller comparing to those of coarse screw threads when tightened with same torque. To clarify this contradictory phenomenon, tightening experiments are conducted, and it is found that the difference of the energy needed for tightening screw threads is found to be the major cause.

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