It is well know that the belt friction is expressed in an exponential function of a product μ and θ, the coefficient of friction and the angle of contact between the flexible belt and the cylindrical surface respectively. So the frictional force increases greatly with an increment of contact angle θ. Using this property, many kinds of buckles were developed to fasten belt. But the locking condition of belt is not obtained from the equation unless θ is of infinity. Their locking conditions were not clarified theoretically. In practice, the product of μθ is usually less than θ, so that the exponent of the product μθ is not so large. Then some slippage may occur in case of severe loading condition. This study is focusing on a self-locking mechanism of a simple buckle developed for flat belt. The belt in the buckle is partially wound again over the belt. According to the equation derived, the fraction of the tight side belt tension to the loose side belt tension is significantly affected by the angle of double-layered segment. With an increment of angle of doublelayered segment, the fraction increases to infinity, which means the occurrence of belt locking. The locking condition is determined by the geometry of the buckle and the coefficient of frictions. The frictional force is automatically generated by the tension of belt so that the self-locking mechanism is realized in the buckle. The equation derived was confirmed by the experiments.

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