Hairspring

The hairspring is the fine spiral spring, typically only a few hundredths of a millimeter thick, fitted to the balance staff and anchored at its outer end to a stud on the balance cock. It supplies the force that pulls the balance back toward rest after every swing. Its stiffness and the inertia of the balance wheel together set the frequency of the oscillation, which means the hairspring, more than any other single part, decides how fast the watch runs.

What it is doing

A wristwatch hairspring is roughly a dozen coils, three to four millimeters across, of ribbon thinner than a human hair. At 28,800 vibrations per hour the balance completes four oscillations a second, so the spring opens and closes about 345,000 times a day and 126 million times a year.

Rate follows from stiffness and length. Shorten the part free to flex and the spring stiffens and the watch gains: a one percent change in effective length moves the rate by roughly half a percent, about seven minutes a day. That number explains both how a watch is regulated and why a magnetized one goes so far wrong.

Why the shape matters

A flat spiral does not breathe evenly. As it winds and unwinds its center of gravity shifts, which introduces positional errors. Two classical corrections address it. A Breguet overcoil, credited to Abraham-Louis Breguet in the late 18th century, lifts the last part of the spring above the plane of the coil and curves it inward so the spring expands concentrically. A Phillips terminal curve, from Edouard Phillips' work of the 1860s, applies related geometry. Both are costly and appear on higher grades.

The property at stake is isochronism, the ideal of a period that stays constant regardless of amplitude. No real spring achieves it, and getting close is much of what separates an adjusted movement from an unadjusted one.

Materials

Steel gave way in the 20th century to iron-nickel alloys developed to hold their elasticity as temperature changes, the family Charles Edouard Guillaume named Elinvar and for which he won the 1920 Nobel Prize in Physics. Nivarox is the best-known trade name in it. The chemistry that makes them stable also makes them ferrous, and so magnetizable, which is the trade-off makers have spent decades working around.

Three routes out are in production. Rolex's Parachrom is a niobium and zirconium alloy that cannot be magnetized as a ferrous spring can, used in the caliber 3235. Swatch Group's Nivachron is titanium-based, fitted to the Powermatic 80. Silicon springs, etched from wafers since the mid-2000s and used in the Omega 8800, are non-magnetic and very consistent in shape, but cannot be bent to adjust and come only from the maker.

Regulation at the bench

On a conventional balance a regulator arm carries curb pins that grip the outer coil. Move the arm and you change the spring's free length, and with it the rate. A nudge is worth a few seconds a day, which is why regulating is done on a timing machine and not by watching the watch for a week. A free-sprung balance has no regulator: rate is set by weights or screws on the balance rim, slower to adjust and far less likely to shift after a knock.

One more hairspring fault shows up on a timing printout. Beat error means the collet is not set so the balance rests symmetrically, and it is corrected by rotating the collet. Our guide to timing a watch explains that trace and the rest.

In practice

Magnetism is the classic hairspring failure and the easiest to fix. A magnetized ferrous hairspring has coils that cling to one another, shortening the effective spring and making the watch run minutes a day fast. Demagnetizing takes a watchmaker seconds. ISO 764, the antimagnetic standard, asks only that a watch survive about 60 gauss and then hold its rate within 30 seconds a day; the METAS Master Chronometer specification is 15,000 gauss. Our guide to magnetism and watches lists the everyday sources.

Physical damage is a different matter: a bent, kinked or tangled spring usually means specialist repair or a new balance assembly. Regulation on a timing machine is an indicative $60 to $150 in the US before sales tax, far less than a service, and worth trying first.

Terms it gets confused with

The hairspring is not the mainspring. The mainspring stores power; the hairspring regulates its release and is never wound. Nor does "antimagnetic" on a dial mean immune: it usually means the movement met the modest ISO 764 threshold, not that it will shrug off a magnetic phone case. And a watch running minutes fast is far more likely to be magnetized than worn out, as our answer on a watch running fast sets out.