AskDefine | Define mainspring

Dictionary Definition

mainspring n : the most important spring in a mechanical device (especially a clock or watch); as it uncoils it drives the mechanism

User Contributed Dictionary



  1. The principal spring of a clockwork mechanism, that drives it by uncoiling.
  2. The most important reason for something.
    Editing Wiktionary is the mainspring of my life.

Extensive Definition

A mainspring is a spiral 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 unwinds, until the next winding is needed. The adjectives wind-up and spring-wound refer to mechanisms powered by mainsprings, which also include kitchen timers, music boxes, wind-up toys and clockwork radios.

Modern mainsprings

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 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 the center (hour) wheel pinion and drives the gear train. The barrel usually rotates once every 8 hours, so the common 40 hour spring requires 5 turns to unwind completely.
Disassembling a mainspring-powered watch or clock is dangerous; even if not wound up, the spring contains energy and can release suddenly, causing injury. The mainspring should be 'let down' gently first, by holding the winding key and pulling the click back, allowing the spring to slowly unwind.


Mainsprings appeared in the first spring powered clocks, in 15th century Europe. Springs were applied to clocks to make them smaller and more portable than previous weight driven clocks, evolving into the first pocketwatches by 1600. Around 1400 coiled springs appeared in locks, Many sources erroneously credit the invention of the mainspring to the Nürnberg locksmith Peter Henlein (or Henle, or Hele) around 1511. However, many descriptions from the 1400s 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 earliest existing spring driven clock is the chamber clock given to Peter the Good, Duke of Burgundy, around 1430, now in the Germanisches Nationalmuseum. 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. 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 caused the balance wheel to rotate too far and 'knock', and the watch to gain time. 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.

Motor or safety barrel

Around 1900, when broken watchsprings were more of a problem, some watches 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 spun the barrel in the reverse direction. This applied great force to the delicate gear 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 delicate 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, 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 it's running period. If the owner continues to wind and use the watch, 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 watches, 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'.

'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 2 1/2 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.



mainspring in French: Ressort moteur

Synonyms, Antonyms and Related Words

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