hairspring

A hairspring, also known as a balance spring, is a fine, circular or spiral spring attached to the balance wheel of a mechanical watch or clock. Its function is to provide a restoring torque that, together with the balance wheel’s inertia, creates a regular oscillation. This oscillation serves as the timekeeping element, regulating the rate at which the gear train advances.

Construction and operation are relatively simple in principle but exacting in practice. The hairspring is typically a small coil made from a resilient alloy. One end is fixed to the balance staff (the pivot around which the balance wheel turns) and the outer end attaches to the balance wheel. As the balance wheel swings, the hairspring winds and unwinds, producing a restoring force that returns the wheel toward its center position. The rate of oscillation depends on the stiffness of the spring, the geometry and inertia of the balance wheel, and any external influences such as temperature or magnetic fields.

Isochronism, or the consistency of rate regardless of amplitude, is a key design concern. To improve it, engineers have developed specialized shapes and end curves, such as the overcoil (or Breguet overcoil), which helps the outer turns distribute more evenly during expansion and contraction. Terminal curves on the outer end also aid concentric expansion and reduce timing errors.

Materials and variants have evolved over time. Early hairsprings were simple steel, but modern spiral springs frequently use advanced alloys designed for low temperature sensitivity and reduced magnetism. In some cases, silicon-based springs are employed for their favorable properties.

Historically, the hairspring was a major advance in watchmaking, introduced in the 17th century and refined by later designers. Today it remains a central component of mechanical timepieces, with ongoing development aimed at improving precision, resistance to environmental effects, and longevity.