Mainspring

Winding the timepiece, by turning a knob or key, stores energy in the mainspring by twisting the spiral tighter.

8-Day movements, used in clocks meant to be wound weekly, provide power for at least 192 hours but use longer mainsprings and bigger barrels.

Known to watchmakers as "white metal" springs (as opposed to blued carbon steel), these are stainless and have a higher elastic limit.

This type is often used on alarm clocks, music boxes and kitchen timers where it doesn't matter if the mechanism stops while winding.

The winding mechanism always has a ratchet attached, with a pawl (called by clockmakers the click) to prevent the spring from unwinding.

Before servicing, mainsprings are “let down” gently by pulling the click back while holding the winding key, allowing the spring to slowly unwind.

It replaced the weight hanging from a cord wrapped around a pulley, which was the power source used in all previous mechanical clocks.

Many sources erroneously credit the invention of the mainspring to the Nuremberg clockmaker Peter Henlein (also spelled Henle, or Hele) around 1511.

Timekeeping mechanisms are never perfectly isochronous, meaning their rate is affected by changes in the drive force.

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.

Its curving shape continuously changed the mechanical advantage of the linkage to even out the force of the mainspring as it ran down.

This was a small secondary spring or weight which powered the timepiece's escapement, and was itself rewound periodically by the mainspring.

Because they are subjected to constant stress cycles, up until the 1960s mainsprings generally broke from metal fatigue long before other parts of the timepiece.

[12] 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.

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.

Even if the mainsprings were not prone to breakage, too much force during winding caused another problem in early watches, called 'knocking' or 'banking'.

So the watch ran with excessive drive force for several hours, until the extra tension in the end of the spring was relieved.

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.

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.

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.

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.

[citation needed] After decades of use, mainsprings in older timepieces are found to deform slightly and lose some of their force, becoming 'tired' or 'set'.