Last Updated on May 22, 2024 by Jason
The balance wheel, combined with the hairspring, is the critical timekeeping component of the movement. It forms an oscillator that controls the release of power from the mainspring barrel via the wheel train and the escapement and regulates the timekeeping of the watch. The balance wheel has a central arm which is mounted on the balance staff and carries a circular rim that is the oscillating mass.
The balance wheel appeared with the first mechanical clocks in 14th century Europe. Early balance wheels lacked a hairspring and were pushed in one direction by the escapement until the verge flag that was in contact with a tooth on the escape wheel slipped past the tip of the tooth. The action of the escapement was reversed, pushing the balance wheel back in the other direction.
Hairspring
The Dutch mathematician Christiaan Huygens developed the balance wheel, in its current form, in the 1670s and was granted a French patent for it in 1675. Huygens’s contribution was to introduce a spiralled balance spring or hairspring to the balance wheel, which causes it to oscillate with a constant frequency.
The hairspring is mounted above the balance. It is secured to the balance cock at its outer end by the stud. Its inner end is secured to the balance staff by the collet. The hairspring is constantly trying to hold the balance wheel in one central position. If the balance is moved from that central position in either direction, the spring pulls it back. A regulator lever is often fitted to the balance cock. This can be used to alter the free length of the spring and thereby adjust the rate of the timepiece. A fast or slow running watch can be adjusted by the regulator to improve its timing by several seconds a day.
The balance cycle
As the balance swings back and forth, a small impulse jewel, attached to the balance staff, knocks the pallet lever from one side to the other. Each time this happens, the pallet jewel that is currently locking the escape wheel is released. This frees one tooth of the escape wheel and the wheel rotates. The other pallet jewel then locks with the escape wheel. As such, each time that the balance wheel swings, the escape wheel advances one tooth. Immediately after the locking pallet jewel has released a tooth of the escape wheel, the escape wheel gives the pallet jewel a push, which in turn makes the lever give a push to the balance wheel through the impulse pin. This push or impulse, as it is called, is what keeps the balance swinging backwards and forwards.
Once the balance has nudged the lever it carries on turning as far as its momentum will take it against the resistance of the hairspring. When the balance has run out of energy it pauses and then the hairspring accelerates it in the reverse direction. In passing it knocks the lever back in the other direction, and the cycle repeats. This process occurs thousands of times per hour.
The period of oscillation
Each swing in a single direction of the balance wheel is referred to as a beat. A complete cycle, or period of oscillation, consists of two beats, one in each direction. Each beat represents a tick and the forward movement of the hands. Because these beats are happening so quickly it is barely noticeable to the naked eye. As a result, the sweeping second hand of a mechanical watch is actually an optical illusion. The balance wheel is ticking or beating around 5 beats per second (18,000 BPH). The second hand ticks forward for each of these beats, that is 5 times a second, this creates the appearance that the hand is actually sweeping forward. If you listen closely, you can hear the multiple “ticks” that occur with each beat.
The effects of temperature
The balance wheels and hairsprings are both highly susceptible to changes in temperature. Early watches had hairsprings made of plain steel and balances of brass or steel. The effects of temperature on these noticeably affected the rate. For example, an increase in temperature causes the metal of the hairspring and the balance to expand. The effect on a hairspring made of plain steel is that the elasticity of the spring decreases significantly as the temperature increases. This results in a weaker spring with increasing temperature.
An increase in temperature also increases the diameter of the balance wheel, increasing its rotational inertia. This makes it harder for the hairspring to accelerate the balance wheel. A weaker hairspring and the increased inertia of the balance wheel combine to slow down the number of beats and the movement loses time. The opposite occurs with colder temperatures. However, these issues were gradually compensated for by using different alloys and materials that were more resistant to changes in temperature. These temperature-adjusted movements are accurate to within just a few seconds a day. All good quality watches, modern and vintage, will include a compensated balance that offsets the effects of temperature.
Related content
Christiaan Huygens at Wikipedia.