Hairspring (Spiralfeder)
The spiral spring attached to the balance wheel that provides the restoring force for the balance's oscillation. Material and geometry directly determine timekeeping accuracy and resistance to temperature, magnetism, and shock.
The hairspring (German Spiralfeder or Unruhspirale) is the spiral spring attached to the balance wheel that provides the restoring force returning the balance to its rest position after each impulse. It is, alongside the balance wheel, the most critical timekeeping component in a mechanical movement.
How the hairspring works
When the balance wheel rotates in one direction, it stretches the hairspring; the spring's stored tension then pulls the balance back. As the balance swings past rest in the other direction, the spring compresses; tension again pulls back. The cycle repeats, regulated by the spring's specific stiffness and the wheel's mass.
The "natural frequency" of the balance-hairspring system is what determines beat rate. Adjusting either the spring's effective length or the balance's mass distribution shifts the rate.
Hairspring materials
The material is where most modern hairspring innovation has happened:
- Steel (traditional, pre-1930s). Susceptible to magnetisation; temperature-sensitive.
- Nivarox / Glucydur alloys (mid-20th century onwards). Improved temperature stability, less magnetic susceptibility. Standard for mid-tier modern movements.
- Parachrom blue (Rolex proprietary, introduced 2000). Niobium-zirconium alloy. Paramagnetic, significantly more shock-resistant. Visible blue tint.
- Silicon (silicium) (Patek Philippe Spiromax 2005, Omega Si14 2010, many independents). Non-magnetic, lighter weight, geometrically stable. Some collectors prefer traditional metal hairsprings as a heritage feature.
- Silinvar (Patek Philippe variant). Silicon-based with specific shaping for thermal compensation.
Silicon and Parachrom-style alloys are what enabled the modern anti-magnetic certification work — Master Chronometer ratings to 15,000 gauss rely on these materials.
Hairspring geometry
Beyond material, the spring's geometry matters:
- Flat (Breguet-overcoil) hairspring. Traditional flat coil, simple to manufacture, slightly position-sensitive.
- Breguet overcoil. A raised section of the outermost coil that improves positional regulation. Common in high-end mechanical movements.
- Phillips terminal curve. A specific mathematical terminal-coil geometry that minimises positional error.
Why the hairspring matters
- Accuracy. The hairspring's quality and stability are the primary determinants of how accurately a mechanical watch keeps time.
- Magnetism resistance. Modern non-ferrous hairsprings (silicon, Parachrom) are the reason modern watches stay regulated in environments full of magnets that would have stopped older watches within hours.
- Service implications. A damaged hairspring is one of the most delicate watchmaker repairs. Replacement requires matching the exact spring length and stiffness to the original; some independent watchmakers will not attempt hairspring replacement on premium calibers.
For dealers, a malfunctioning hairspring shows up as severely off-rate movements that don't respond to standard regulation or demagnetisation. The fix is service-centre work.
Related: balance-wheel, escapement, caliber, master-chronometer.