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A gear train is a mechanical system formed by mounting gears on a frame so that the teeth of the gears engage. Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, providing a smooth transmission of rotation from one gear to the next. The transmission of rotation between contacting toothed wheels can be traced back to the Antikythera mechanism of Greece and the south-pointing chariot of China. Illustrations by the Renaissance scientist Georgius Agricola show gear trains with cylindrical teeth. The implementation of the involute tooth yielded a standard gear design that provides a constant speed ratio. Features of gears and gear trains include: * The ratio of the pitch circles of mating gears defines the speed ratio and the mechanical advantage of the gear set. * A planetary gear train provides high gear reduction in a compact package. * It is possible to design gear teeth for gears that are non-circular, yet still transmit torque smoothly. * The speed ratios of chain and belt drives are computed in the same way as gear ratios. See bicycle gearing. == Mechanical advantage == Gear teeth are designed so that the number of teeth on a gear is proportional to the radius of its pitch circle, and so that the pitch circles of meshing gears roll on each other without slipping. The speed ratio for a pair of meshing gears can be computed from ratio of the radii of the pitch circles and the ratio of the number of teeth on each gear. The velocity ''v'' of the point of contact on the pitch circles is the same on both gears, and is given by : where input gear ''A'' has radius ''rA'' and meshes with output gear ''B'' of radius ''rB, therefore, : where ''NA'' is the number of teeth on the input gear and ''NB'' is the number of teeth on the output gear. The mechanical advantage of a pair of meshing gears for which the input gear has ''NA'' teeth and the output gear has ''NB'' teeth is given by : This shows that if the output gear ''G''B has more teeth than the input gear ''G''A, then the gear train ''amplifies'' the input torque. And, if the output gear has fewer teeth than the input gear, then the gear train ''reduces'' the input torque. If the output gear of a gear train rotates more slowly than the input gear, then the gear train is called a ''speed reducer''. In this case, because the output gear must have more teeth than the input gear, the speed reducer amplifies the input torque. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Gear train」の詳細全文を読む スポンサード リンク
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