Hairspring
The coiled spring that gives the balance wheel its restoring force
What it is
The hairspring; also called the balance spring; is a spiral spring attached at its inner end to the balance wheel staff and at its outer end to a fixed stud. It provides the restoring force that makes the balance wheel oscillate at a consistent frequency. The hairspring is typically less than 0.1mm thick, and minor variations in its geometry, mass distribution, or material properties directly affect the watch's rate.
History
Robert Hooke and Christiaan Huygens both claimed priority for the hairspring around 1675; the dispute was never fully resolved. The immediate practical result was undeniable: adding a hairspring to the balance wheel improved portable watch accuracy from minutes per day to seconds per day. Abraham-Louis Breguet developed the overcoil (or Breguet overcoil) around 1795; a terminal curve that lifts the outer coil above the plane of the spring, improving isochronism (the property of oscillating at the same rate regardless of arc amplitude). The alloy challenge occupied the 19th and 20th centuries: plain steel hairsprings are magnetic and temperature-sensitive. Invar (nickel-iron) improved temperature stability; Nivarox (nickel-iron-chromium-beryllium, developed by Paillard in 1933 and now produced by Nivarox-FAR, a Swatch Group subsidiary) became the modern standard. Silicon hairsprings, introduced by Patek Philippe in 2000 and subsequently by Rolex (Syloxi), Omega, and others, are anti-magnetic, require no lubrication, and are produced by deep reactive-ion etching with atomic-scale dimensional consistency.
How it works
The hairspring attaches at its inner end to the collet; a tiny collar press-fitted to the balance staff; and at its outer end to the stud, which is fixed to the balance bridge. As the balance wheel swings, the spring alternately winds and unwinds, storing and releasing energy. The effective length of the hairspring; the length between the collet and the regulating pins (or index); determines the oscillation frequency. Moving the regulating pins lengthens or shortens this effective length, slowing or speeding the rate. Free-sprung designs (Gyromax, Kif Flector) fix the outer terminal and adjust rate via the mass of the balance wheel itself, which is considered more stable and consistent than index regulation.
In the catalog
Related
- Balance wheel: The oscillating wheel that divides time
- Escapement: The mechanism that divides time into equal steps
