Balance wheel
The oscillating wheel that divides time
What it is
The balance wheel is the timekeeping oscillator of a mechanical watch: a weighted wheel that swings back and forth at a fixed frequency determined by its moment of inertia and the stiffness of the hairspring attached to it. Each complete oscillation (one swing in each direction) is one "beat." The rate of a watch is expressed in vibrations per hour (vph): a 28,800 vph movement beats 8 times per second; a 36,000 vph hi-beat movement beats 10 times per second.
History
The balance wheel replaced the foliot; a crude oscillating bar; in the 16th century. The crucial advance came around 1675, when Robert Hooke and Christiaan Huygens independently attached a spiral spring to the balance, giving it a precise restoring force and transforming portable timekeeping accuracy by an order of magnitude. The 19th century tackled the temperature problem: steel hairsprings change stiffness with temperature, causing timekeeping errors. The bimetallic compensation balance; with arms made of two bonded metals that curve to change the effective rim mass as temperature varies; partially corrected this. The modern solutions are metallurgical: Nivarox hairspring alloys greatly reduce temperature sensitivity, and silicon balance wheels (Patek Philippe's Gyromax variants, Rolex's Chronergy escapement) eliminate it nearly entirely. Adjustable-inertia balance wheels; Patek's Gyromax, Audemars Piguet's Microstella; use eccentric mass screws to fine-tune frequency without removing the wheel from the movement.
How it works
The balance wheel is mounted on a staff (axle) that pivots in jeweled bearings. The hairspring is attached at its inner end to the collet on the staff and at its outer end to the stud holder on the balance bridge. As the balance wheel swings one way, the hairspring winds; its stored energy drives the balance back in the other direction. The escapement delivers a small impulse to the balance wheel on each half-swing to replace the energy lost to friction. The frequency of oscillation; and therefore the rate of the watch; depends on the balance wheel's moment of inertia and the hairspring's effective length; the regulator or free-sprung adjustment system changes the effective length to correct rate.
In the catalog
Related
- Hairspring: The coiled spring that gives the balance wheel its restoring force
- Escapement: The mechanism that divides time into equal steps
- Tourbillon: A rotating cage that carries the entire escapement
