Hairspring
An ultra-thin coiled spring that controls the balance wheel's oscillation and is central to the watch's accuracy.
What is a Hairspring?
The hairspring — also called the balance spring — is one of the most delicate components in a mechanical watch. It is a flat, spiral spring coiled from a strip of metal roughly 0.02 mm thick and typically spanning 12–15 coils. It is attached at its inner end to the balance staff and at its outer end to a fixed stud on the movement.
Its job is to act as the restoring force for the balance wheel: after each swing, the hairspring's tension pulls the wheel back, setting up the next oscillation. The precision of these oscillations is what makes the watch tick accurately.
Why Hairsprings Are So Critical
Even a tiny distortion in the hairspring — a bent coil, a coil touching its neighbor, rust, or magnetization — can cause the watch to gain or lose minutes per day. Watchmakers inspect hairsprings under high magnification. If a hairspring is magnetized (a common problem), it can sometimes be demagnetized; if it is physically damaged, replacement is the only remedy.
Materials
Traditional hairsprings were made from Elinvar, a nickel-iron alloy chosen for its low sensitivity to temperature changes. Modern movements often use silicon hairsprings, which are immune to magnetism and require no lubrication. Vintage watches will almost always have metal hairsprings, which require careful handling during service.