horology Horology (; related to Latin '; ; , interfix ''-o-'', and suffix ''-logy''), . is the study of the measurement of time. Clocks, watches, clockwork, sundials, hourglasses, clepsydras, timers, time recorders, marine chronometers, and atomic clo ...

, a wheel train (or just train) is the

gear train A gear train is a mechanical system formed by mounting gears on a frame so 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 a

mechanical watch A mechanical watch is a watch that uses a clockwork mechanism to measure the passage of time, as opposed to quartz watches which function using the vibration modes of a piezoelectric quartz tuning fork, or radio watches, which are quartz watc ...

clock A clock or a timepiece is a device used to measure and indicate time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month and ...

. 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

gear A gear is a rotating circular machine part having cut teeth or, in the case of a cogwheel or gearwheel, inserted teeth (called ''cogs''), which mesh with another (compatible) toothed part to transmit (convert) torque and speed. The basic ...

s. 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

pinion A pinion is a round gear—usually the smaller of two meshed gears—used in several applications, including drivetrain and rack and pinion systems. Applications Drivetrain Drivetrains usually feature a gear known as the pinion, which may ...

s, 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.

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 An escapement is a mechanical linkage in mechanical watches and clocks that gives impulses to the timekeeping element and periodically releases the gear train to move forward, advancing the clock's hands. The impulse action transfers energy ...

to drive the

pendulum A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward th ...

balance wheel A balance wheel, or balance, is the timekeeping device used in mechanical watches and small clocks, analogous to the pendulum in a pendulum clock. It is a weighted wheel that rotates back and forth, being returned toward its center position by a ...

. 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 ratio A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage. Gear teeth are designed to ensure the Pitch circle diameter (gears), pitch circles of engaging gears roll on each other without slipping, pr ...

s 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 In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...

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 bearing A jewel bearing is a plain bearing in which a metal spindle turns in a jewel-lined pivot hole. The hole is typically shaped like a torus and is slightly larger than the shaft diameter. The jewels are typically made from the mineral corund ...

s. The going train in a modern clock or watch consists of: * First or Great wheel 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 A barrel or cask is a hollow cylindrical container with a bulging center, longer than it is wide. They are traditionally made of wooden staves and bound by wooden or metal hoops. The word vat is often used for large containers for liquids, ...

in watches and spring driven clocks, and by the weight

pulley A pulley is a wheel on an axle or shaft that is designed to support movement and change of direction of a taut cable or belt, or transfer of power between the shaft and cable or belt. In the case of a pulley supported by a frame or shell that ...

in weight driven clocks. Its arbor projects through a hole in the

face The face is the front of an animal's head that features the eyes, nose and mouth, and through which animals express many of their emotions. The face is crucial for human identity, and damage such as scarring or developmental deformities may aff ...

and drives, via a

friction Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction: *Dry friction is a force that opposes the relative lateral motion of ...

coupling, the ''cannon pinion'', which carries the

minute hand A clock face is the part of an analog clock (or watch) that displays time through the use of a flat dial with reference marks, and revolving pointers turning on concentric shafts at the center, called hands. In its most basic, globally recogn ...

. It also drives the pinion of the third wheel. In wristwatches with center seconds (i.e. with the seconds hand pivoted coaxially with the minute and hour hands) this wheel is positioned off center to allow the fourth wheel to be placed at the center 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 center of the movement. * Third wheel which drives the pinion of the fourth wheel. It 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 which, in clocks and watches with the second hand in a subdial, turns once per minute and the arbor 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; in these the third wheel drives the escape wheel directly. * Escape wheel which is released one tooth at a time by the

, with each swing of the pendulum or balance. The escape wheel keeps the pendulum or balance swinging by giving it a small push each time it moves forward.

Motion work

The motion work is the small 12-to-1 reduction gear train that turns the timepiece's

hour hand A clock face is the part of an analog clock (or watch) that displays time through the use of a flat dial with reference marks, and revolving pointers turning on concentric shafts at the center, called hands. In its most basic, globally recogn ...

from the

. It is attached to the going train by the friction coupling of the cannon pinion, so the minute and hour hands can be turned to set the time. It is often located on the outside of the movement's front plate, just under the dial. It consists of: * A Cannon pinion with a hollow shaft that fits friction-tight over the center wheel shaft, projects through the face, and holds the minute hand. While the timepiece is not being set this is turned by the center wheel and drives the minute wheel. While being set, it is turned by the setting mechanism – in modern clocks, a setting knob on the back of the clock. In watches during setting it is turned by the minute wheel, which is turned by the keyless work. In older clocks the setting was done by opening the face and manually pushing the minute hand which rotated the cannon pinion directly. * A Minute wheel whose pinion drives the hour wheel. During setting it is driven by the intermediate wheel in the keyless work and it turns both the cannon pinion and the hour wheel, moving the hands. * An Hour wheel which fits over the shaft of the cannon pinion and whose shaft holds the hour hand. The hour wheel rotates once for every 12 rotations of the cannon pinion.

Keyless work

Used in watches, the keyless work are the gears that wind the mainspring when the ''crown'' is turned, and when the crown is pulled out allow the hands to be set. The term originated because, before the modern form of keyless work was invented by the French watchmaker

Adrien Philippe Jean Adrien Philippe (April 16, 1815, La Bazoche-Gouet, Eure-et-Loir – January 5, 1894) was a French horologist and cofounder of watchmaker Patek Philippe & Co. of Geneva, Switzerland. In 1842, Adrien Philippe invented a mechanism for watche ...

in 1843, watches were wound and set by inserting a separate key into holes in the back and turning it. The core of the keyless mechanism is a gear on the watch's ''winding stem'', the ''clutch'' (or ''castle wheel'' in Britain), with two sets of axial gear teeth on it, which slides in and out. When the stem is pushed in, a lever slides the clutch out, and the outer set of teeth engages a small wheel train which turns the mainspring arbor, winding the mainspring. When the stem is pulled out, the clutch slides in, and the inner teeth engage another wheel, which turns the hour wheel in the motion work, turning the watch's hands.

Striking train

striking clock A striking clock is a clock that sounds the hours audibly on a bell or gong. In 12-hour striking, used most commonly in striking clocks today, the clock strikes once at 1:00 am, twice at 2:00 am, continuing in this way up to twelve t ...

s, the striking train is a gear train that moves a hammer to strike the hours on a gong. It is usually driven by a separate but identical power source to the going train. In antique clocks, to save costs, it was often identical to the going train, and mounted parallel to it on the left side when facing the front of the clock.Milham 1945, p.198

Footnotes

{{Gears Timekeeping components Gears de:Analoguhr