Mainspring
Encyclopedia
A mainspring is a spiral torsion spring
of metal ribbon that is the power source in mechanical watch
es and some clock
s. 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 unwinds, until the next winding is needed. The adjectives wind-up and spring-wound refer to mechanisms powered by mainsprings, which also include kitchen timer
s, music boxes, wind-up toy
s and clockwork radio
s.
es. 8-Day movements provide power for at least 192 hours but use longer mainsprings and bigger barrel
s. Clock mainsprings are similar, only larger.
Since 1945, carbon
steel
alloys have been increasingly superseded by newer special alloys (iron
, nickel
and chromium
with the addition of cobalt
, molybdenum
, or beryllium
), 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. 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.
In their relaxed form, mainsprings can have three distinct shapes:
The reverse coils provide extra force at the end of the running period, in order to keep the timepiece running at a constant rate to the end.
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 arrangement is that while the mainspring is being wound, its drive force is removed from the clock movement, so the clock may stop. The winding mechanism must always have a ratchet
attached, with a pawl
(called by clockmakers the click) to prevent the spring from unwinding.
In the form used in modern watches, called the going barrel
, the mainspring is coiled around an arbor and enclosed inside a cylindrical box called the barrel
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
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 clocks gears, usually the center wheel pinion
and drives the wheel train
. The barrel usually rotates once every 8 hours, so the common 40 hour spring requires 5 turns to unwind completely.
s and clockmaker
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.
(also spelled Henle, or Hele) around 1511. However, many descriptions from the 15th century of portable clocks 'without weights', and at least two surviving examples, show that spring driven clocks existed by the early years of that century. The oldest surviving clock powered by a mainspring is the Burgunderuhr (Burgundy Clock), an ornate, gilt spring driven chamber clock, currently at the Germanisches Nationalmuseum
in Nurnberg, whose iconography suggests that it was made around 1430 for Philippe the Good, Duke of Burgundy.
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.
) provided by a spring is not constant, but diminishes as the spring unwinds (see graph). Timepieces, however, have to run at a constant rate to keep accurate time. Timekeeping mechanisms are never isochronous
; meaning their rate is affected by changes in the drive force. This was especially true of the primitive verge and foliot type used before the advent of the balance spring
in 1657. So early clocks slowed down as the mainspring ran down.
Two solutions to this problem appeared in the early spring powered clocks in the 15th century: the stackfreed and the fusee. The stackfreed was an eccentric cam
mounted on the mainspring arbor, with a spring-loaded roller that pressed against it. The cam was shaped so that early in the running period when the mainspring was pushing strongly, the stackfreed would provide an opposing force, while later when the mainspring was almost run down and pushing weakly, it would provide a helping force. The stackfreed added a lot of friction and probably reduced a clock's running time substantially; it was rarely used and was abandoned after about a century.
The fusee was a much longer lasting innovation. This was a cone-shaped pulley
that was turned by a chain wrapped around the mainspring barrel. Its curving shape continuously changed the mechanical advantage
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
s until the 1970s.
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 or 'Maltese cross'. Stopwork isn't needed in modern watches.
A fourth device used in a few precision timepieces was the remontoire
. This was a small secondary spring or weight which powered the timepiece's escapement
, and was itself rewound periodically by the mainspring. This isolated the timekeeping element from the varying mainspring force.
The modern going barrel
, invented in 1760 by Jean-Antoine Lépine
, produces a constant force by simply using a longer mainspring than needed, and coiling it under tension in the barrel. In operation, only the inner turns of the spring are used. Mathematically, the tension creates a 'flat' section in the spring's 'torque curve' (see graph at right) 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
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.
, up until the 1960s mainsprings generally broke from metal fatigue
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. Broken mainsprings were the largest cause of watch repairs until the 1960s. Since then, the improvements in spring metallurgy
mentioned above have made broken mainsprings rare.
Even if the spring didn't break, too much force caused another problem, called 'knocking' or 'banking'. Since no more slack was left in the spring, the pressure of the last turn of the winding knob put 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
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
), to allow the arbor to rotate backward after winding by about two ratchet teeth, enough to remove excess tension.
are greatest. When the mainspring broke, the outer part recoiled and the momentum
spun the barrel in the reverse direction. This applied great force to the delicate wheel train
and escapement
, 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.
, 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.
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
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' .
Torsion spring
A torsion spring is a spring that works by torsion or twisting; that is, a flexible elastic object that stores mechanical energy when it is twisted. The amount of force it exerts is proportional to the amount it is twisted. There are two types...
of metal ribbon that is the power source in 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...
es and some 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...
s. Winding the timepiece, by turning a knob or key, stores energy
Energy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
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-wound refer to mechanisms powered by mainsprings, which also include kitchen timer
Timer
A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Whereas a stopwatch counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass.Timers can be mechanical, electromechanical,...
s, music boxes, wind-up toy
Toy
A toy is any object that can be used for play. Toys are associated commonly with children and pets. Playing with toys is often thought to be an enjoyable means of training the young for life in human society. Different materials are used to make toys enjoyable and cuddly to both young and old...
s and clockwork radio
Clockwork radio
A windup radio or clockwork radio is a radio that is powered by human muscle power rather than batteries or the electrical grid. In the most common arrangement, an internal electrical generator is run by a mainspring, which is wound by a hand crank on the case. Turning the crank winds the spring...
s.
Modern mainsprings
A modern watch mainspring is a long strip of hardened and blued steel, or specialised steel alloy, 20-30 centimeters long and 0.05-0.2 millimeters thick. The mainspring in the common 1-day movement is calculated to enable the watch to run for 36 to 40 hours, i.e. with a power-reserve for 12 to 16 hours, which is the normal standard for hand-wound as well as self-winding watchAutomatic watch
An automatic or self-winding watch is a mechanical watch, whose mainspring is wound automatically by the natural motion of the wearer's arm, providing energy to run the watch, to make it unnecessary to manually wind the watch. A watch which is not self-winding is called a manual watch...
es. 8-Day movements provide power for at least 192 hours but use longer mainsprings and bigger barrel
Barrel (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...
s. Clock mainsprings are similar, only larger.
Since 1945, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
alloys have been increasingly superseded by newer special alloys (iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
and chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
with the addition of cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
, molybdenum
Molybdenum
Molybdenum , is a Group 6 chemical element with the symbol Mo and atomic number 42. The name is from Neo-Latin Molybdaenum, from Ancient Greek , meaning lead, itself proposed as a loanword from Anatolian Luvian and Lydian languages, since its ores were confused with lead ores...
, or beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
), and also by cold-rolled alloys ('structural hardening'). Known to watchmakers as 'white metal' springs (as opposed to blued carbon steel), these are stainless
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
and have a higher elastic limit. 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.
In their relaxed form, mainsprings can have three distinct shapes:
- Spiral coiled: i.e. coiled in the same direction throughout, viz. that of a spring inside the barrel
- Semi-reverse: The outer end of the spring is coiled in the reverse direction to form an angle less than 360 degrees.
- Reverse (resilient): the outer end of the spring is coiled in the reverse direction to form an angle exceeding 360 degrees.
The reverse coils provide extra force at the end of the running period, in order to keep the timepiece running at a constant rate to the end.
How they work
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 arrangement is that while the mainspring is being wound, its drive force is removed from the clock movement, so the clock may stop. The winding mechanism must always have a ratchet
Ratchet (device)
A ratchet is a device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. Because most socket wrenches today use ratcheting handles, the term "ratchet" alone is often used to refer to a ratcheting wrench, and the terms "ratchet"...
attached, with a pawl
Pawl
Pawl may refer to:* A common component of a ratchet* A part of the adjustable height locking mechanism of an extension ladder* Pawl , a former racing car constructor...
(called by clockmakers the click) to prevent the spring from unwinding.
In the form used in modern watches, called the going barrel
Barrel (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...
, the mainspring is coiled around an arbor and enclosed inside a cylindrical box called the barrel
Barrel (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...
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 (device)
A ratchet is a device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. Because most socket wrenches today use ratcheting handles, the term "ratchet" alone is often used to refer to a ratcheting wrench, and the terms "ratchet"...
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 clocks gears, usually the center wheel 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...
and drives the wheel train
Wheel train (horology)
In horology, a wheel 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...
. 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 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 in barrels contain dangerous residual tension. WatchmakerWatchmaker
A watchmaker is an artisan who makes and repairs watches. Since virtually all watches are now factory made, most modern watchmakers solely repair watches. However, originally they were master craftsmen who built watches, including all their parts, by hand...
s and clockmaker
Clockmaker
A clockmaker is an artisan who makes and 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 to repairing clocks and watches...
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. Around 1400 coiled springs began to be used in locks, 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 pocketwatches by 1600. Many sources erroneously credit the invention of the mainspring to the Nürnberg clockmaker Peter HenleinPeter Henlein
Peter Henlein , a locksmith and watchmaker from Nuremberg, is often considered the inventor of the portable timekeeper, making him the inventor of the watch, but this claim is disputed...
(also spelled Henle, or Hele) around 1511. However, many descriptions from the 15th century of portable clocks 'without weights', and at least two surviving examples, show that spring driven clocks existed by the early years of that century. The oldest surviving clock powered by a mainspring is the Burgunderuhr (Burgundy Clock), an ornate, gilt spring driven chamber clock, currently at the Germanisches Nationalmuseum
Germanisches Nationalmuseum
The Germanisches Nationalmuseum is a museum in Nuremberg, Germany. Founded in 1852, houses a large collection of items relating to German culture and art extending from prehistoric times through to the present day...
in Nurnberg, whose iconography suggests that it was made around 1430 for Philippe the Good, Duke of Burgundy.
The first mainsprings were made of steel without tempering
Tempering
Tempering is a heat treatment technique for metals, alloys and glass. In steels, tempering is done to "toughen" the metal by transforming brittle martensite or bainite into a combination of ferrite and cementite or sometimes Tempered martensite...
or hardening
Hardening (metallurgy)
Hardening is a metallurgical and metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain...
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.
Constant force from a spring
A problem throughout the history of spring driven clocks and watches is that the force (torqueTorque
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....
) provided by a spring is not constant, but diminishes as the spring unwinds (see graph). Timepieces, however, have to run at a constant rate to keep accurate time. Timekeeping mechanisms are never isochronous
Isochronous
Isochronous : From Greek iso, equal + chronos, time. It literally means regularly, or at equal time intervals. In general English language, it refers to something that occurs at a regular interval, of the same duration; as opposed to synchronous which refers to more than one thing happening at the...
; meaning their rate is affected by changes in the drive force. This was especially true of the primitive verge and foliot type used before the advent of the balance spring
Balance spring
A balance spring, or hairspring, is a part used in mechanical timepieces. The balance spring, attached to the balance wheel, controls the speed at which the wheels of the timepiece turn, and thus the rate of movement of the hands...
in 1657. So early clocks slowed down as the mainspring ran down.
Two solutions to this problem appeared in the early spring powered clocks in the 15th century: the stackfreed and the fusee. The stackfreed was an eccentric cam
Cam
A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path...
mounted on the mainspring arbor, with a spring-loaded roller that pressed against it. The cam was shaped so that early in the running period when the mainspring was pushing strongly, the stackfreed would provide an opposing force, while later when the mainspring was almost run down and pushing weakly, it would provide a helping force. The stackfreed added a lot of friction and probably reduced a clock's running time substantially; it was rarely used and was abandoned after about a century.
The fusee was a much longer lasting innovation. This was a cone-shaped pulley
Pulley
A 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...
that was turned by a chain wrapped around the mainspring barrel. Its curving shape continuously changed the mechanical advantage
Mechanical advantage
Mechanical advantage is a measure of the force amplification achieved by using a tool, mechanical device or machine system. Ideally, the device preserves the input power and simply trades off forces against movement to obtain a desired amplification in the output force...
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
Marine chronometer
A marine chronometer is a clock that is precise and accurate enough to be used as a portable time standard; it can therefore be used to determine longitude by means of celestial navigation...
s until the 1970s.
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 or 'Maltese cross'. Stopwork isn't needed in modern watches.
A fourth device used in a few precision timepieces was the remontoire
Remontoire
In mechanical horology, a remontoire, 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 source, such as a mainspring...
. This was a small secondary spring or weight which powered the timepiece's 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...
, and was itself rewound periodically by the mainspring. This isolated the timekeeping element from the varying mainspring force.
The modern going barrel
Barrel (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...
, invented in 1760 by Jean-Antoine Lépine
Jean-Antoine Lépine
Jean-Antoine Lépine , was a revolutionary and inventive French watch and clockmaker, who contributed with crucial inventions for watchmaking still used nowadays...
, produces a constant force by simply using a longer mainspring than needed, and coiling it under tension in the barrel. In operation, only the inner turns of the spring are used. Mathematically, the tension creates a 'flat' section in the spring's 'torque curve' (see graph at right) 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
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....
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 cyclesCyclic stress
Cyclic stress in engineering refers to an internal distribution of forces that changes over time in a repetitive fashion. As an example, consider one of the large wheels used to drive an aerial lift such as a ski lift. The wire cable wrapped around the wheel exerts a downward force on the wheel...
, up until the 1960s mainsprings generally broke from metal fatigue
Metal Fatigue
Metal Fatigue , is a futuristic science fiction, real-time strategy computer game developed by Zono Incorporated and published by Psygnosis and TalonSoft .-Plot:...
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. Broken mainsprings were the largest cause of watch repairs until the 1960s. Since then, the improvements in spring metallurgy
Metallurgy
Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...
mentioned above have made broken mainsprings rare.
Even if the spring didn't break, too much force caused another problem, called 'knocking' or 'banking'. Since no more slack was left in the spring, the pressure of the last turn of the winding knob put 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
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...
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 (engineering)
In mechanical engineering, backlash, sometimes called lash or play, is clearance between mating components, sometimes described as the amount of lost motion due to clearance or slackness when movement is reversed and contact is re-established...
), 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 stressesStress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...
are greatest. When the mainspring broke, the outer part recoiled and the momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
spun the barrel in the reverse direction. This applied great force to the delicate wheel train
Wheel train (horology)
In horology, a wheel 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...
and 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...
, 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 pinionPinion
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...
, 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 are often found stopped with the mainspring fully wound, which led to a myth that winding a watch all the way up damages it. What actually happens is that as time passes and 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 until 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'.Self-winding watches and 'unbreakable' mainsprings
Self-winding or automatic watchAutomatic watch
An automatic or self-winding watch is a mechanical watch, whose mainspring is wound automatically by the natural motion of the wearer's arm, providing energy to run the watch, to make it unnecessary to manually wind the watch. A watch which is not self-winding is called a manual watch...
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
Clutch
A clutch is a mechanical device which provides for the transmission of power from one component to another...
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
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 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 USA and inventive clockmakers built clocks powered by a stack of leafsprings, similar what has traditionally served as a suspension spring for wagons.
- Other spring types are conceivable and have been used occasionally on experimental timepieces, such as e.g. torsion springs.
- Occasionally one finds an odd clock with a spring made of material other than metal, such as e.g. synthetic elastic materials.