It is well known that bolted joints are loosened when lateral loads are applied after they are tightened. This phenomenon is proved by experiments or by the real operations, but the mechanism is not yet elucidated why the bolts are loosened. In this paper several finite element analyses were performed and reproduced the phenomenon that the bolts are loosened, or the nut rotates, when the lateral loads are applied. Based on this result, the mechanism of the bolt loosening (the nut rotation) is investigated. Additionally, it is also examined that bolts are more difficult to be loosened when double nut procedure is used or that they are almost not loosened when the eccentric bolt/nut systems are used. As the result, the bolt loosening is caused by the shape of the thread that is the wedge inclined to the bolt axis. When lateral loads are applied, the bolt/nut threads slide relatively, which causes the relative nut rotation. When the loads are released, the bolt returns to the normal position keeping the relative slide to the nut. This nut rotation causes a reduction of the bolt tension, too. Although the magnitude of this rotation may be quite small, many times of repetition makes the bolt loosened.
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