Balance Spring

A balance spring, or hairspring, is a part used in mechanical timepieces. Attached to the balance wheel, it controls the speed at which the wheels of the timepiece turn, and thus the rate of movement of the hands. A regulator lever on the spring is used to adjust the speed so the timepiece keeps accurate time.

The balance spring is a fine spiral or helical torsion spring used in mechanical watches, alarm clocks, kitchen timers, marine chronometers, and other timekeeping mechanisms to control the rate of oscillation of the balance wheel. The balance spring is an integral part of the balance wheel, because it reverses the direction of the balance wheel causing it to oscillate back and forth. The balance spring and balance wheel together form a harmonic oscillator, which uses resonance to oscillate with a precise period or "beat" resistant to external disturbances, which is responsible for their timekeeping accuracy.

The addition of the balance spring to the balance wheel around 1657 by Robert Hook and Christian Huygens greatly increased the accuracy of portable timepieces, transforming early pocketwatches from expensive novelties to useful timekeepers. Improvements to the balance spring are responsible for further large increases in accuracy since that time. Modern balance springs are made of special low temperature coefficient alloys like nivarox to reduce the effects of temperature changes on the rate, and carefully shaped to minimize the effect of changes in drive force as the mainspring in the clockwork runs down. Before the 1980s, balance wheels and balance springs were used in virtually every portable timekeeping device, but in recent decades electronic quartz timekeeping technology has replaced mechanical clockwork, and the major remaining use of balance springs is in mechanical watches.