Balance Wheel

Specification

Technical parameters

Overview

The balance wheel is the timekeeper. Together with its hairspring it forms a torsional oscillator with a natural period: the hairspring stores energy and returns it, the balance provides inertia, and the period of the resulting oscillation is what divides time. Change the spring's effective length or the wheel's inertia and you change that period — which is exactly how regulation works.

Amplitude is the wheel's rotation from rest, measured in degrees from the point of reversal. A healthy modern movement in the flat runs roughly 270° to 310°. Low amplitude points to a power problem — friction, dried oil, a weak mainspring, or excessive endshake. High amplitude risks knocking, where the impulse pin strikes the back of the fork horn and the watch runs wildly fast.

Poising matters as much as timing. A balance heavy at one point runs at different rates in different positions, which no amount of regulation will fix. Screwed balances are poised by moving screws; smooth balances are poised by removing metal from the heavy side.

What matters about it

• Period set by hairspring length and wheel inertia
• Amplitude is the primary health indicator
• Poise errors show up as positional variation
• Shock-protected bearings in modern movements
• Free-sprung balances regulate by inertia, not by a regulator index

Practical notes

• Let the power down completely and remove the pallet fork before touching the balance.
• A folded hairspring can sometimes be coaxed flat with pegwood and patience, but assume it will need replacing.
• Check amplitude before adjusting rate; regulating a starved balance just hides the real fault.
• Compare dial-up and dial-down amplitude — a large gap points to a worn upper or lower pivot.

Compatible with