Wheel train

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. 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.

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:

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