Component record · Escapement

Balance Spring (Hairspring)

Verified

A very fine, coiled spring, typically of specialist alloy, attached to the balance staff and to a fixed stud, that provides the restoring force for balance oscillation.

What it does

Governs the timekeeping rate of the watch by determining, together with the balance's inertia, the natural period of oscillation.

How it works

The balance spring behaves like a torsional spring: as the balance rotates one way, the spring winds up and stores energy, then unwinds and reverses the balance's rotation, repeating in a near-isochronous cycle whose period depends mainly on the balance's inertia and the spring's stiffness rather than on the amplitude of swing, provided the spring's geometry is well designed.

The addition of a balance spring to the verge watch in the 1670s was transformative: earlier balances without a spring had no strong restoring force and were highly sensitive to positional and power variations, whereas a sprung balance behaves far more consistently, improving accuracy by an order of magnitude.

An overcoil (Breguet) terminal curve, bending the outer coil of the spring up and inward, was developed to make the spring 'breathe' more concentrically, reducing positional error; flat springs without an overcoil are simpler but generally less accurate in multiple positions.

Evolution

From plain flat blued-steel springs, through the Breguet overcoil for improved isochronism, to modern temperature-stable alloys and, in some contemporary high-end movements, silicon springs immune to magnetism.