balancewheel

The balance wheel is a key component of the escapement in most mechanical clocks and watches. It is a circular, weighted wheel mounted on a fixed pivot that oscillates back and forth. Paired with a hairspring, or balance spring, it forms a harmonic oscillator whose regular swings regulate the release of energy from the mainspring to the gear train.

Functionally, each swing of the balance wheel advances the escape wheel by a small amount, allowing the hands to move in discrete steps. The hairspring provides a restoring torque that brings the wheel back toward its central position, while the wheel’s inertia determines the period of oscillation. The balance wheel’s rate is approximately proportional to the square root of the ratio between its moment of inertia and the spring constant, and is adjusted by altering the wheel’s inertia (via screws on the rim) or the hairspring tension.

Construction and regulation vary, but modern balance wheels are typically made from metal alloys chosen for strength and stability. Many high-quality movements use temperature-compensating designs and overcoil hairsprings to improve isochronism. Rates are commonly specified in vibrations per hour (vph), with typical ranges from about 18,000 to 36,000 vph in contemporary watches. Precision is kept through careful poising, fine regulation, lubrication, and sometimes shock protection.

History and context: the balance wheel with a hairspring emerged in the late 17th century and became the basis for accurate portable timekeeping, replacing earlier verge and foliot mechanisms. Today it remains central to the function of most mechanical timepieces, even as electronic systems have become common in other applications.