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Wheel train (horology)
Encyclopedia
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 gear
s.
Watch movement
s 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
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:
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 ratio
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
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
s. The going train in a modern clock or watch consists of:
The motion work is the small 12-to-1 reduction gear train that turns the timepiece's hour hand from the minute hand
. It is attached to the going train by the friction coupling of the cannon pinion, so the minute and hour hands can be turned independently to set the timepiece. It is often located on the outside of the movement's front plate, just under the dial. It consists of:
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.
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.
Horology
Horology is the art or science of measuring time. Clocks, watches, clockwork, sundials, clepsydras, timers, time recorders and marine chronometers are all examples of instruments used to measure time.People interested in horology are called horologists...
, a wheel train (or just train) is the gear train
Gear train
A gear train is 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, this provides a smooth transmission of rotation from one gear to the next.The transmission of...
of a mechanical watch
Mechanical watch
A mechanical watch is a watch that uses a mechanical mechanism to measure the passage of time, as opposed to modern quartz watches which function electronically. It is driven by a spring which must be wound periodically...
or clock
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...
. 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
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....
s.
Watch movement
Movement (clockwork)
In horology, a movement is the internal mechanism of a clock or watch, as opposed to the case, which encloses and protects the movement, and the face which displays the time. The term originated with mechanical timepieces, whose movements are made of many moving parts...
s 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
Pinion
A pinion is a round gear used in several applications:*usually the smallest gear in a gear drive train, although in the case of John Blenkinsop's Salamanca, the pinion was rather large...
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
Movement (clockwork)
In horology, a movement is the internal mechanism of a clock or watch, as opposed to the case, which encloses and protects the movement, and the face which displays the time. The term originated with mechanical timepieces, whose movements are made of many moving parts...
, 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 mainspringMainspring
A mainspring is a spiral torsion spring of metal ribbon that is the power source in mechanical watches and some clocks. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it...
or weight, to the escapement
Escapement
In mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...
to drive the pendulum
Pendulum
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced from its resting equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position...
or balance wheel
Balance wheel
The balance wheel is the timekeeping device used in mechanical watches and some 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 spiral spring, the balance spring or hairspring...
. 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
Gear ratio
The gear ratio of a gear train is the ratio of the angular velocity of the input gear to the angular velocity of the output gear, also known as the speed ratio of the gear train. The gear ratio can be computed directly from the numbers of teeth of the various gears that engage to form the gear...
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
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
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
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 jewel material is usually synthetic sapphire...
s. The going train in a modern clock or watch consists of:
- Center wheel - which turns once per hour. Its pinion is turned by the teeth on the mainspringMainspringA mainspring is a spiral torsion spring of metal ribbon that is the power source in mechanical watches and some clocks. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it...
barrelBarrel (horology)Used in mechanical watches and clocks, a barrel is a cylindrical metal box closed by a cover, with a ring of gear teeth around it, containing a spiral spring called the mainspring, which provides power to run the timepiece. The barrel turns on an arbor . The spring is hooked to the barrel at its...
in watches and spring driven clocks, and by the weight pulleyPulleyA pulley, also called a sheave or a drum, is a mechanism composed of a wheel on an axle or shaft that may have a groove between two flanges around its circumference. A rope, cable, belt, or chain usually runs over the wheel and inside the groove, if present...
in weight driven clocks. Its arbor projects through a hole in the faceClock faceA clock face is the part of an analog clock that displays the time through the use of a fixed numbered dial or dials and moving hands. In its most basic form, recognized universally throughout the world, the dial is numbered 1–12 indicating the hours in a 12-hour cycle, and a short hour hand...
and is attached to a frictionFrictionFriction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...
coupling called the cannon pinion, which holds the minute handClock faceA clock face is the part of an analog clock that displays the time through the use of a fixed numbered dial or dials and moving hands. In its most basic form, recognized universally throughout the world, the dial is numbered 1–12 indicating the hours in a 12-hour cycle, and a short hour hand...
. It also drives the pinion of the third wheel. - 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 handClock faceA clock face is the part of an analog clock that displays the time through the use of a fixed numbered dial or dials and moving hands. In its most basic form, recognized universally throughout the world, the dial is numbered 1–12 indicating the hours in a 12-hour cycle, and a short hour 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 escapementEscapementIn mechanical watches and clocks, an escapement is a device that transfers energy to the timekeeping element and enables counting the number of oscillations of the timekeeping element...
, 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.
Motion work
Clock face
A clock face is the part of an analog clock that displays the time through the use of a fixed numbered dial or dials and moving hands. In its most basic form, recognized universally throughout the world, the dial is numbered 1–12 indicating the hours in a 12-hour cycle, and a short hour hand...
. It is attached to the going train by the friction coupling of the cannon pinion, so the minute and hour hands can be turned independently to set the timepiece. It is often located on the outside of the movement's front plate, just under the dial. It consists of:
- Cannon pinion - a 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 this 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.
- Minute wheel - Its pinion drives the hour wheel. During setting, this is driven by the intermediate wheel in the keyless work and it turns both the cannon pinion and the hour wheel, moving the hands.
- 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 Adrien PhilippeAdrien Philippe
Jean Adrien Philippe was French horologist and cofounder of watchmaker Patek Philippe & Co. of Geneva, Switzerland.In 1842 Adrien Philippe invented a mechanism for watches which allowed them to be wound and set by means of crown rather than a key. His patented invention earned him a Gold Medal at...
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
Mainspring
A mainspring is a spiral torsion spring of metal ribbon that is the power source in mechanical watches and some clocks. Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it...
. 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
In striking clockStriking 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 AM, twice at 2 AM, continuing in this way up to twelve times at 12 noon, then starts over, striking once at 1 PM, twice at 2...
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.