![Mainspring wind-up keys](https://upload.wikimedia.org/wikipedia/commons/7/7c/Mainspring_wind-up_keys.jpg)
A mainspring is a spiral
torsion spring
A torsion spring is a spring that works by twisting its end along its axis; that is, a flexible elastic object that stores mechanical energy when it is twisted. When it is twisted, it exerts a torque in the opposite direction, proportional ...
of metal ribbon—commonly
spring steel
Spring steel is a name given to a wide range of steels used in the manufacture of different products, including swords, saw blades, springs and many more. These steels are generally low-alloy manganese, medium-carbon steel or high-carbon steel ...
—used as a power source in
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 watches ...
es, some
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 the ...
s, and other
clockwork
Clockwork refers to the inner workings of either mechanical devices called clocks and watches (where it is also called the movement) or other mechanisms that work similarly, using a series of gears driven by a spring or weight.
A clockwork mec ...
mechanisms. ''Winding'' the timepiece, by turning a knob or key, stores
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
in the mainspring by twisting the spiral tighter. The force of the mainspring then turns the clock's wheels as it unwinds, until the next winding is needed. The adjectives wind-up and spring-powered refer to mechanisms powered by mainsprings, which also include kitchen
timer
A timer is a specialized type of clock used for measuring specific time intervals.
Timers can be categorized into two main types.
The word "timer" is usually reserved for devices that counts down from a specified time interval, while devices th ...
s,
metronome
A metronome, from ancient Greek μέτρον (''métron'', "measure") and νομός (nomós, "custom", "melody") is a device that produces an audible click or other sound at a regular interval that can be set by the user, typically in beats pe ...
s,
music box
A music box (American English) or musical box (British English) is an automatic musical instrument in a box that produces musical notes by using a set of pins placed on a revolving cylinder or disc to pluck the tuned teeth (or ''lamellae'') ...
es, wind-up
toy
A toy or plaything is an object that is used primarily to provide entertainment. Simple examples include toy blocks, board games, and dolls. Toys are often designed for use by children, although many are designed specifically for adults and pet ...
s and
clockwork radio
Human power is work or energy that is produced from the human body. It can also refer to the power (rate of work per time) of a human. Power comes primarily from muscles, but body heat is also used to do work like warming shelters, food, or other ...
s.
Modern mainsprings
A modern watch mainspring is a long strip of hardened and blued steel, or specialised steel alloy, 20–30 cm long and 0.05-0.2 mm thick. The mainspring in the common 1-day movement is calculated to enable the watch to run for 36 to 40 hours, i.e. 24 hours between daily windings with a power-reserve of 12 to 16 hours, in case the owner is late winding the watch. This is the normal standard for hand-wound as well as
self-winding watch
An automatic watch, also known as a self-winding watch or simply an automatic, is a mechanical watch where the natural motion of the wearer provides energy to wind the mainspring, making manual winding unnecessary if worn enough. It is distinguis ...
es. 8-Day movements, used in clocks meant to be wound weekly, provide power for at least 192 hours but use longer mainsprings and bigger
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, ...
s. Clock mainsprings are similar to watch springs, only larger.
Since 1945,
carbon steel
Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:
* no minimum content is specified or required for chromium, cobalt ...
alloys
An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
have been increasingly superseded by newer special alloys (
iron
Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...
,
nickel
Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...
and
chromium
Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal.
Chromium metal is valued for its high corrosion resistance and hardne ...
with the addition of
cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, pr ...
,
molybdenum
Molybdenum is a chemical element with the symbol Mo and atomic number 42 which is located in period 5 and group 6. The name is from Neo-Latin ''molybdaenum'', which is based on Ancient Greek ', meaning lead, since its ores were confused with lea ...
, or
beryllium
Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to form mi ...
), and also by
cold-rolled alloys ('structural hardening'). Known to watchmakers as 'white metal' springs (as opposed to blued carbon steel), these are
stainless and have a higher
elastic limit
In materials science and engineering, the yield point is the point on a stress-strain curve that indicates the limit of elastic behavior and the beginning of plastic behavior. Below the yield point, a material will deform elastically and wi ...
. They are less subject to permanent bending (becoming
'tired') and there is scarcely any risk of their breaking. Some of them are also practically non-
magnetic
Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particle ...
.
In their relaxed form, mainsprings are made in three distinct shapes:
*''Spiral coiled'': These are coiled in the same direction throughout, in a simple spiral.
*''Semi-reverse'': The outer end of the spring is coiled in the reverse direction for less than one turn (less than 360°).
*''Reverse'' (resilient): the outer end of the spring is coiled in the reverse direction for one or more turns (exceeding 360°).
The semi-reverse and reverse types provide extra force at the end of the running period, when the spring is almost out of energy, in order to keep the timepiece running at a constant rate to the end.
Operation
The mainspring is coiled around an axle called the arbor, with the inner end hooked to it. In many clocks, the outer end is attached to a stationary post. The spring is wound up by turning the arbor, and after winding its force turns the arbor the other way to run the clock. The disadvantage of this ''open spring'' arrangement is that while the mainspring is being wound, its drive force is removed from the clock movement, so the clock may stop. This type is often used on
alarm clock
An alarm clock (or sometimes just an alarm) is a clock that is designed to alert an individual or group of individuals at a specified time. The primary function of these clocks is to awaken people from their night's sleep or short naps; they ar ...
s,
music box
A music box (American English) or musical box (British English) is an automatic musical instrument in a box that produces musical notes by using a set of pins placed on a revolving cylinder or disc to pluck the tuned teeth (or ''lamellae'') ...
es and kitchen
timer
A timer is a specialized type of clock used for measuring specific time intervals.
Timers can be categorized into two main types.
The word "timer" is usually reserved for devices that counts down from a specified time interval, while devices th ...
s where it doesn't matter if the mechanism stops while winding. The winding mechanism always has a
ratchet
Ratchet may refer to:
Devices
* Ratchet (device), a mechanical device that allows movement in only one direction
* Ratchet, metonomic name for a socket wrench incorporating a ratcheting device
* Ratchet (instrument), a music instrument and a ...
attached, with a
pawl
A pawl is a movable lever that engages a fixed component to either prevent movement in one direction or restrain it altogether. As such, it is a type of latch and can also be considered a type of dog. It typically consists of a spring-loaded le ...
(called by clockmakers the ''click'') to prevent the spring from unwinding.
In the form used in modern watches, called the ''
going barrel {{unreferenced, date=September 2009
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 th ...
'', the mainspring is coiled around an arbor and enclosed inside a cylindrical box called the
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, ...
which is free to turn. The spring is attached to the arbor at its inner end, and to the barrel at its outer end. The attachments are small hooks or tabs, which the spring is hooked to by square holes in its ends, so it can be easily replaced.
The mainspring is wound by turning the arbor, but drives the watch movement by the barrel; this arrangement allows the spring to continue powering the watch while it is being wound. Winding the watch turns the arbor, which tightens the mainspring, wrapping it closer around the arbor. The arbor has a
ratchet
Ratchet may refer to:
Devices
* Ratchet (device), a mechanical device that allows movement in only one direction
* Ratchet, metonomic name for a socket wrench incorporating a ratcheting device
* Ratchet (instrument), a music instrument and a ...
attached to it, with a click to prevent the spring from turning the arbor backward and unwinding. After winding, the arbor is stationary and the pull of the mainspring turns the barrel, which has a ring of gear teeth around it. This meshes with one of the clock's gears, usually the ''center wheel''
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 ...
and drives the
wheel train In horology, a wheel train (or just train) is the gear train of a mechanical watch or Mechanical clock, clock. Although the term is used for other types of gear trains, the long history of mechanical timepieces has created a traditional terminology ...
. The barrel usually rotates once every 8 hours, so the common 40-hour spring requires 5 turns to unwind completely.
Hazards
The mainspring contains a lot of energy. Clocks and watches have to be disassembled periodically for maintenance and repair, and if precautions are not taken the spring can release suddenly, causing potentially serious injury. Mainsprings are 'let down' gently before servicing, by pulling the click back while holding the winding key, allowing the spring to slowly unwind. However, even in their 'let down' state, mainsprings contain dangerous residual tension.
Watchmaker
A watchmaker is an artisan who makes and repairs watches. Since a majority of watches are now factory-made, most modern watchmakers only repair watches. However, originally they were master craftsmen who built watches, including all their par ...
s and
clockmaker
A clockmaker is an artisan who makes and/or repairs clocks. Since almost all clocks are now factory-made, most modern clockmakers only repair clocks. Modern clockmakers may be employed by jewellers, antique shops, and places devoted strictly t ...
s use a tool called a "mainspring winder" to safely install and remove them. Large mainsprings in clocks are immobilized by "mainspring clamps" before removal.
History
Mainsprings appeared in the first spring-powered clocks, in 15th-century Europe. It replaced the weight hanging from a cord wrapped around a pulley, which was the power source used in all previous mechanical clocks. Around 1400 coiled springs began to be used in locks,
[, p.126-127] and many early clockmakers were also locksmiths.
Springs were applied to clocks to make them smaller and more portable than previous weight-driven clocks, evolving into the first
pocketwatch
A pocket watch (or pocketwatch) is a watch that is made to be carried in a pocket, as opposed to a wristwatch, which is strapped to the wrist.
They were the most common type of watch from their development in the 16th century until wristwatc ...
es by 1600. Many sources erroneously credit the invention of the mainspring to the
Nuremberg
Nuremberg ( ; german: link=no, Nürnberg ; in the local East Franconian dialect: ''Nämberch'' ) is the second-largest city of the German state of Bavaria after its capital Munich, and its 518,370 (2019) inhabitants make it the 14th-largest ...
clockmaker
Peter Henlein
Peter Henlein (also spelled Henle or Hele) (1485 - August 1542), a locksmith and clockmaker of Nuremberg, Germany, is often considered the inventor of the watch., p.31 He was one of the first craftsmen to make small ornamental portable clocks wh ...
(also spelled Henle, or Hele) around 1511. However, many references in 15th-century sources to portable clocks 'without weights', and at least two surviving examples, show that spring-driven clocks existed by the early years of that century.
[, p.121] The oldest surviving clock powered by a mainspring is the ''Burgunderuhr'' (Burgundy Clock), an ornate, gilt chamber clock, currently at the
Germanisches Nationalmuseum
The Germanisches National Museum is a museum in Nuremberg, Germany. Founded in 1852, it houses a large collection of items relating to German culture and art extending from prehistoric times through to the present day. The Germanisches National ...
in Nuremberg, whose iconography suggests that it was made around 1430 for
Philip the Good, Duke of Burgundy
Philip III (french: Philippe le Bon; nl, Filips de Goede; 31 July 1396 – 15 June 1467) was Duke of Burgundy from 1419 until his death. He was a member of a cadet line of the Valois dynasty, to which all 15th-century kings of France belonged ...
.
The first mainsprings were made of steel without
tempering or
hardening processes. They didn't run very long, and had to be wound twice a day. Henlein was noted for making watches that would run 40 hours between windings. The 18th century methods of making mainsprings are described by Berthoud
and Blakey
Constant force from a spring
A problem throughout the history of spring-driven clocks and watches is that the force (
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 ...
) provided by a spring is not constant, but diminishes as the spring unwinds (see graph). However, timepieces have to run at a constant rate in order to keep accurate time. Timekeeping mechanisms are never perfectly
isochronous
A sequence of events is isochronous if the events occur regularly, or at equal time intervals. The term ''isochronous'' is used in several technical contexts, but usually refers to the primary subject maintaining a constant period or interval ( ...
, meaning their rate is affected by changes in the drive force. This was especially true of the primitive
verge and foliot
The verge (or crown wheel) escapement is the earliest known type of mechanical escapement, the mechanism in a mechanical clock that controls its rate by allowing the gear train to advance at regular intervals or 'ticks'. Its origin is unknown. V ...
type used before the advent of the
balance spring
A balance spring, or hairspring, is a spring attached to the balance wheel in mechanical timepieces. It causes the balance wheel to oscillate with a resonant frequency when the timepiece is running, which controls the speed at which the wheels of ...
in 1657. So early clocks slowed down during their running period as the mainspring ran down, causing inaccurate timekeeping.
Two solutions to this problem appeared in the early spring-powered clocks in the 15th century; the ''
stackfreed'' and the ''
fusee
Fusee or fusée may refer to:
* Fusee (horology), a component of a clock
* Flare, a pyrotechnic device sometimes called a Fusee
* Fusee, an old word for "flintlock
Flintlock is a general term for any firearm that uses a flint-striking lock (fi ...
'':
Stackfreed
The stackfreed was an eccentric
cam
Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an intracellular target of the secondary messenger Ca2+, and the bind ...
mounted on the mainspring arbor, with a spring-loaded roller that pressed against it. The cam had a 'snail' shape so that early in the running period when the mainspring was pushing strongly, the spring would bear against the wide part of the cam, providing a strong opposing force, while later in the running period as the force of the mainspring decreased, the spring would bear against the narrower part of the cam and the opposing force would also decrease. The stackfreed added a lot of friction and probably reduced a clock's running time substantially; it was only used in some German timepieces and was abandoned after about a century.
Fusee
The
fusee
Fusee or fusée may refer to:
* Fusee (horology), a component of a clock
* Flare, a pyrotechnic device sometimes called a Fusee
* Fusee, an old word for "flintlock
Flintlock is a general term for any firearm that uses a flint-striking lock (fi ...
was a much longer-lasting innovation. This was a cone-shaped
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 ...
that was turned by a chain wrapped around the mainspring barrel. Its curving shape continuously changed the
mechanical advantage
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. The device trades off input forces against movement to obtain a desired amplification in the output force. The model for t ...
of the linkage to even out the force of the mainspring as it ran down. Fusees became the standard method of getting constant torque from a mainspring. They were used in most spring-driven clocks and watches from their first appearance until the 19th century when the going barrel took over, and in
marine chronometer
A marine chronometer is a precision timepiece that is carried on a ship and employed in the determination of the ship's position by celestial navigation. It is used to determine longitude by comparing Greenwich Mean Time (GMT), or in the modern ...
s until the 1970s.
Stopwork
Another early device which helped even out the spring's force was ''stopwork'' or ''winding stops'', which prevented the mainspring from being wound up all the way, and prevented it from unwinding all the way. The idea was to use only the central part of the spring's 'torque curve', where its force was more constant. The most common form was the
Geneva stop
The Geneva drive or Maltese cross is a gear mechanism that translates a continuous rotation movement into intermittent rotary motion.
The ''rotating drive'' wheel is usually equipped with a pin that reaches into a slot located in the other whe ...
or 'Maltese cross'. Stopwork isn't needed in modern watches.
Remontoire
A fourth device used in a few precision timepieces was the
remontoire In mechanical horology, a remontoire (from the French ''remonter'', meaning 'to wind') is a small secondary source of power, a weight or spring, which runs the timekeeping mechanism and is itself periodically rewound by the timepiece's main power so ...
. This was a small secondary spring or weight which powered the timepiece's
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 ...
, and was itself rewound periodically by the mainspring. This isolated the timekeeping element from the varying mainspring force.
Going barrel
The modern ''
going barrel {{unreferenced, date=September 2009
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 th ...
'', invented in 1760 by
Jean-Antoine Lépine
Jean-Antoine Lépine (L’Pine, LePine, Lepine, L’Epine), born as Jean-Antoine Depigny, was an influential watchmaker. He contributed inventions which are still used in watchmaking today and was amongst the finest French watchmakers, who were c ...
, produces a constant force by simply using a longer mainspring than needed, and coiling it under tension in the barrel. In operation, only a few turns of the spring at a time are used, with the remainder pressed against the outer wall of the barrel. Mathematically, the tension creates a 'flat' section in the spring's 'torque curve' (see graph) and only this flat section is used. In addition, the outer end of the spring is often given a 'reverse' curve, so it has an 'S' shape. This stores more tension in the spring's outer turns where it is available toward the end of the running period. The result is that the barrel provides approximately 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 ...
over the watch's designed running period; the torque doesn't decline until the mainspring has almost run down.
The built-in tension of the spring in the going barrel makes it hazardous to disassemble even when not wound up.
Broken mainsprings
Because they are subjected to constant
stress cycles, up until the 1960s mainsprings generally broke from
metal fatigue
In materials science, fatigue is the initiation and propagation of cracks in a material due to cyclic loading. Once a fatigue crack has initiated, it grows a small amount with each loading cycle, typically producing striations on some parts o ...
long before other parts of the timepiece. They were considered expendable items. This often happened at the end of the winding process, when the spring is wound as tightly as possible around the arbor, with no space between the coils. When manually winding, it is easy to reach this point unexpectedly and put excessive pressure on the spring. Another cause was temperature changes. If a watch was fully wound in the evening and the temperature dropped at night, without any slack between the coils the thermal contraction of the long spring could break it loose from its attachments at one end. In earlier times, watch repairers noted that changes in the weather brought in a rash of watches with broken mainsprings. Broken mainsprings were the largest cause of watch repairs until the 1960s. Since then, the improvements in spring
metallurgy
Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys.
Metallurgy encompasses both the sc ...
mentioned above have made broken mainsprings rare.
'Knocking' or 'banking'
Even if the mainsprings were not prone to breakage, too much force during winding caused another problem in early watches, called 'knocking' or 'banking'.
[, p.91] If very little slack was left in the spring after winding ('overwinding"), the pressure of the last turn of the winding knob put the end of the spring under excessive tension, which was locked in by the last click of the ratchet. So the watch ran with excessive drive force for several hours, until the extra tension in the end of the spring was relieved. This made the
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 ...
rotate too far in each direction, causing the impulse pin on the wheel to knock against the back of the fork horns. This caused the watch to gain time, and could break the impulse pin. In older watches this was prevented with 'stopwork'. In modern watches this is prevented by designing the 'click' with some 'recoil' (
backlash
Backlash may refer to:
Literature
* '' Backlash: The Undeclared War Against American Women'', a 1991 book by Susan Faludi
* ''Backlash'' (Star Wars novel), a 2010 novel by Aaron Allston
* Backlash (Marc Slayton), comic book character
* ''Backla ...
), to allow the arbor to rotate backward after winding by about two ratchet teeth, enough to remove excess tension.
Motor or safety barrel
Around 1900, when broken watchsprings were more of a problem, some pocketwatches used a variation of the going barrel called the ''motor barrel'' or ''safety barrel''. Mainsprings usually broke at their attachment to the arbor, where bending
stresses are greatest. When the mainspring broke, the outer part recoiled and the
momentum
In Newtonian mechanics, momentum (more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object. It is a vector quantity, possessing a magnitude and a direction. If is an object's mass an ...
spun the barrel in the reverse direction. This applied great force to the delicate
wheel train In horology, a wheel train (or just train) is the gear train of a mechanical watch or Mechanical clock, clock. Although the term is used for other types of gear trains, the long history of mechanical timepieces has created a traditional terminology ...
and
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 ...
, often breaking pivots and jewels.
In the motor barrel, the functions of the arbor and barrel were reversed from the going barrel. The mainspring was wound by the barrel, and turned the arbor to drive the wheel train. Thus if the mainspring broke, the destructive recoil of the barrel would be applied not to the wheel train but to the winding mechanism, which was robust enough to take it.
Safety pinion
A ''safety pinion'' was an alternate means of protection, used with the going barrel. In this, the center wheel
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 ...
, which the barrel gear engages, was attached to its shaft with a reverse screw thread. If the spring broke, the reverse recoil of the barrel, instead of being passed on to the gear train, would simply unscrew the pinion.
The myth of 'overwinding'
Watches and clocks are often found stopped with the mainspring fully wound, which led to a myth that winding a spring-driven timepiece all the way up damages it.
Several problems can cause this type of breakdown, but it is never due to "overwinding", as timepieces are designed to handle being wound up all the way.
One cause of “overwinding” is dirt. Watch movements require regular cleaning and lubrication, and the normal result of neglecting to get a watch cleaned is a watch stopped at full wind. As the watch movement collects dirt and the oil dries up, friction increases, so that the mainspring doesn't have the force to turn the watch at the end of its normal running period, and it stops prematurely. If the owner continues to wind and use the watch without servicing, eventually the friction force reaches the 'flat' part of the torque curve, and quickly a point is reached where the mainspring doesn't have the force to run the watch even at full wind, so the watch stops with the mainspring fully wound. The watch needs service, but the problem is caused by a dirty movement or other defect, not "overwinding".
Another common cause of a watch stopped at full wind is that if a watch is dropped then the balance staff can break and the watch can no longer run even when the mainspring is fully wound.
Self-winding watches and 'unbreakable' mainsprings
Self-winding or
automatic watch
An automatic watch, also known as a self-winding watch or simply an automatic, is a mechanical watch where the natural motion of the wearer provides energy to wind the mainspring, making manual winding unnecessary if worn enough. It is distinguis ...
es, introduced widely in the 1950s, use the natural motions of the wrist to keep the mainspring wound. A semicircular weight, pivoted at the center of the watch, rotates with each wrist motion. A winder mechanism uses rotations in both directions to wind the mainspring.
In automatic watches, motion of the wrist could continue winding the mainspring until it broke. This is prevented with a slipping
clutch
A clutch is a mechanical device that engages and disengages power transmission, especially from a drive shaft to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or line shafts). ...
device. The outer end of the mainspring, instead of attaching to the barrel, is attached to a circular expansion spring called the ''bridle'' that presses against the inner wall of the barrel, which has serrations or notches to hold it. During normal winding the bridle holds by friction to the barrel, allowing the mainspring to wind. When the mainspring reaches its full tension, its pull is stronger than the bridle. Further rotation of the arbor causes the bridle to slip along the barrel, preventing further winding. In watch company terminology, this is often misleadingly referred to as an 'unbreakable mainspring'.
'Tired' or 'set' mainsprings
After decades of use, mainsprings in older timepieces are found to deform slightly and lose some of their force, becoming 'tired' or 'set'. This condition is mostly found in springs in barrels. It causes the running time between windings to decrease. During servicing the mainspring should be checked for 'tiredness' and replaced if necessary. The British Horological Institute suggests these tests:
* In a mainspring barrel, when unwound and relaxed, most of a healthy spring's turns should be pressed flat against the wall of the barrel, with only 1 or 2 turns spiralling across the central space to attach to the arbor. If more than 2 turns are loose in the center, the spring may be 'tired'; with 4 or 5 turns it definitely is 'tired'.
* When removed from the barrel, if the diameter of the relaxed spring lying on a flat surface is less than 2½ times the barrel diameter, it is 'tired'.
Power reserve indicator
![Orient SFD0H001B](https://upload.wikimedia.org/wikipedia/commons/e/e4/Orient_SFD0H001B.jpg)
Some high-grade watches have an extra dial on the face indicating how much power is left in the mainspring, often graduated in hours the watch has left to run. Since both the arbor and the barrel turn, this mechanism requires a
differential gear
A differential is a gear train with three drive shafts that has the property that the rotational speed of one shaft is the average of the speeds of the others, or a fixed multiple of that average.
Functional description
The following descr ...
that measures how far the arbor has been turned, compared to the barrel.
Unusual forms of mainspring
A mainspring is usually a coiled metal spring, however there are exceptions:
* The wagon spring clock: During a brief time in American clockmaking history, coilable spring steel was not available in the United States, and inventive clockmakers built clocks powered by a stack of
leaf spring
A leaf spring is a simple form of spring commonly used for the suspension in wheeled vehicles. Originally called a ''laminated'' or ''carriage spring'', and sometimes referred to as a semi-elliptical spring, elliptical spring, or cart spring, it ...
s, similar to what has traditionally served as a suspension spring for wagons.
* Other spring types are conceivable and have been used occasionally on experimental timepieces.
* Occasionally one finds an odd clock with a spring made of material other than metal, such as synthetic elastic materials.
Notes
References
* ''G.A. Berner 4-language Glossary'', 1988 re-edition, courtesy FH, Federation of the Swiss Watch Industry, Bienne, Switzerland.
* {{cite web , last=Murray , first=Michael P. , title=Everything you always wanted to know about clock mainsprings , date=2005-02-01 , url=http://www.atmos-man.com/spring.html , publisher=Mike's Clock Clinic , access-date=2009-07-06
External links
Federation of the Swiss Watch Industry Dictionary
Timekeeping components
Springs (mechanical)
Horology