Balance wheel (Unruh)
The oscillating weighted wheel in a mechanical watch movement that, together with the hairspring, regulates the rate of timekeeping. Oscillates at typically 21,600 to 36,000 vibrations per hour. Visible through the case back on movements with display backs.
The balance wheel (German Unruh) is the oscillating weighted wheel in a mechanical watch movement that, together with the hairspring, regulates the rate of timekeeping. The balance wheel is the heart of the movement — its consistent back-and-forth oscillation creates the time-keeping reference against which everything else in the movement is synchronised.
How the balance wheel works
The balance wheel is a circular weighted disc, typically 7 to 14 millimetres in diameter, suspended on a pivot. A spiral hairspring attached to its rim provides the restoring force that returns the balance to its rest position whenever it is displaced. Energy delivered through the escapement gives the balance a small impulse each oscillation, sustaining the rotation indefinitely while the mainspring has energy.
The balance oscillates at a defined frequency:
- 18,000 vph (2.5 Hz) — slow, used in some vintage pocket watches and Lange's classic calibers.
- 21,600 vph (3 Hz) — common in 1950s-1970s movements and modern Patek Philippe.
- 28,800 vph (4 Hz) — the modern standard (ETA, Rolex, AP, most contemporary movements).
- 36,000 vph (5 Hz) — high-beat, used in Zenith El Primero, Grand Seiko 9SA5.
Higher beat rate gives more positional stability but increases wear and power consumption.
Balance design types
- Inertia-screw / regulator-arm balance. Classical design; rate adjustment via a movable regulator arm on the balance bridge. Used in older movements and many ETA-based calibers.
- Free-sprung balance. No regulator arm. Rate adjustment via timing screws or weights set directly on the balance rim. Less susceptible to shock, more precise but harder to regulate. Used in modern Rolex (Microstella nuts on the balance arms), AP, Patek, and Glashütte calibers.
- Variable-inertia balance. Modern proprietary designs (Rolex Microstella) allow fine inertia adjustment via tiny screws on the balance wheel itself.
Materials
- Brass (traditional) — basic, adequate for most uses.
- Beryllium-copper alloy (Glucydur is the proprietary trade name) — temperature-stable, used in mid-to-high-end Swiss movements.
- Modern paramagnetic alloys — Rolex uses proprietary alloys for the balance wheel that reduce magnetisation susceptibility.
Why the balance wheel matters
For dealers, balance wheel issues manifest as:
- Erratic timekeeping when the balance has been damaged by impact (chipped jewel, bent pivot, deformed wheel).
- Magnetisation effects when the balance has been magnetised — fixed by demagnetisation.
- Authentication. A movement's balance design and material is a key feature in identifying the caliber and detecting non-OEM replacement parts.
Related: hairspring, escapement, caliber, regulation.