Component record · Escapement

Balance Wheel

Verified

The oscillating wheel-and-staff assembly that, together with the balance spring, forms a watch's timekeeping oscillator, analogous to a clock's pendulum.

What it does

Swings back and forth at a regular beat rate, its period regulated by the balance spring, and receives impulse from the pallet fork to sustain oscillation.

How it works

The balance oscillates under the restoring force of the balance spring, its period governed largely by its moment of inertia and the spring's stiffness, in close analogy to a pendulum's period being governed by length and gravity.

Each swing, the balance receives a small push (impulse) from the escapement via the impulse pin and pallet fork, replacing energy lost to friction and air resistance, so oscillation is sustained rather than decaying.

Balance wheel design historically addressed temperature compensation (since metals expand and change a plain balance's rate with heat) through devices like the bimetallic cut compensation balance, later largely superseded by temperature-stable alloys that made compensation balances unnecessary.

Evolution

From plain uncompensated balances, through screwed and bimetallic compensation balances in precision work, to modern monometallic alloys (e.g. Glucydur) that need no mechanical compensation.

Known variations

Plain balance
Bimetallic compensation balance with timing screws
Monometallic temperature-stable alloy balance
Free-sprung balance with adjustable mass (no regulator index)

Identification for collectors

Screws around the rim indicate a compensation or timing balance; a plain uninterrupted rim suggests a later monometallic balance or a simple economical movement