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In horology, a wheel train (or just train) is the gear train of a mechanical watch or clock. Although the term is used for other types of gear trains, the long history of mechanical timepieces has created a traditional terminology for their gear trains which is not used in other applications of gears. Watch movements are very standardized, and the wheel trains of most watches have the same parts. The wheel trains of clocks are a little more varied, with different numbers of wheels depending on the type of clock and how many hours the clock runs between windings (the "going"). However, the wheel trains of clocks and watches share the same terminology, and are similar enough that they can be described together. The large gears in timepieces are generally called ''wheels'', the smaller gears they mesh with (large to small, large to small) are called pinions, and the shafts that the wheels and pinions are mounted on are called ''arbors''.〔(Britannica 2008 )〕 The wheels are mounted between the plates of the movement, with the pivots rotating in holes in the plates. The pivot holes have semicircular depressions around them, called oil cups, to hold the oil in contact with the shaft by capillary action. There are several wheel trains in a typical clock or watch. ==Going train== The going train is the main gear train of the timepiece. It consists of the wheels that transmit the force of the timepiece's power source, the mainspring or weight, to the escapement to drive the pendulum or balance wheel. The going train has two functions. First, it scales up the speed of rotation of the mainspring or weight pulley. This allows the use of a very strong, slow turning mainspring or heavy weight that will run the timepiece for days or weeks. Second, its gear ratios divide the rotation of the escape wheel into convenient time units of seconds, minutes, and hours, to turn the timepiece's hands. The going train wheels are the only ones under load in a timepiece, since they bear the constant torque of the mainspring which is applied to the escapement, so these wheels are the only ones that receive significant wear. In watches and some high quality clocks their arbors have jewel bearings. The going train in a modern clock or watch consists of: *First or Great wheel - is attached and ratcheted to the main spring, or cable, barrel. The ratchet allows the main spring or cable barrel to be wound without turning the wheel. In horology jargon the pawl of the ratchet is called "the click". The first wheel turns the pinion of the Center wheel. *Center or second wheel - which turns once per hour. Its pinion is turned by the teeth on the mainspring barrel in watches and spring driven clocks, and by the weight pulley in weight driven clocks. Its arbor projects through a hole in the face and drives, via a friction coupling, the ''cannon pinion'', which carries the minute hand. It also drives the pinion of the third wheel. In wrist watches with centre seconds, that is with the seconds hand pivoted coaxially with the minute and hour hands, this wheel is positioned off centre to allow the fourth wheel to be placed at the centre of the movement. In this arrangement the wheel is called the second wheel, because it is still the second wheel in the train but no longer at the centre of the movement. *Third wheel - which drives the pinion of the fourth wheel. (this is called the third wheel because the mainspring barrel is the first wheel and the center wheel is the second wheel in the gear train) *Fourth wheel - In clocks and watches with the second hand in a subdial on the face, this turns once per minute, and the shaft projects through the face and holds the second hand. The fourth wheel also turns the escape wheel pinion. Many clocks don't need this wheel, because of their slower moving escapements, and in these the third wheel drives the escape wheel directly. *Escape wheel - This wheel is released one tooth at a time by the escapement, with each swing of the pendulum or balance wheel. The escape wheel keeps the pendulum or balance swinging by giving it a small push each time it moves forward. 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「Wheel train」の詳細全文を読む スポンサード リンク
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