A hairspring is the fine spiral spring attached to the balance wheel in a mechanical watch. It is also called a balance spring. Although it is tiny, it plays a major role in setting the movement’s rhythm.
How the hairspring works
The escapement gives the balance wheel an impulse. As the wheel turns, the hairspring coils tighter or opens wider. Its elasticity pulls the balance back in the opposite direction. This repeated expansion and contraction creates the oscillation used to divide time.
The balance wheel supplies inertia. The hairspring supplies the restoring force. Changing the effective behavior of either component changes the rate of the watch.
Why precision is difficult
Grand Seiko calls the hairspring a critical component governing mechanical precision. Its shape, attachment, material, and adjustment all matter. A hairspring must breathe as evenly as possible while avoiding contact with nearby parts.
Accuracy can change when:
- Magnetism makes coils attract or stick.
- Shock bends or displaces the spring.
- Temperature changes its physical behavior.
- Oil, dust, or damage interferes with free movement.
- A previous adjustment leaves the spring off-center.
Modern makers use special alloys or silicon to reduce some of these effects, but no mechanical system is completely isolated from its environment.
Do not try to straighten one
A hairspring is much finer than it looks in photographs. Touching it can permanently change its geometry. If a watch suddenly gains a large amount of time, magnetism is one possible cause, but a watchmaker should confirm the diagnosis. Demagnetizing will not fix a bent or contaminated spring.
The hairspring is one reason mechanical watches are both impressive and imperfect. Their rate comes from a physical oscillator that responds to position, power, temperature, and condition.
Next, see what the balance wheel does.
Sources: Grand Seiko on mechanical movements and Rolex movement components.