Credits: Article and images by Joshua Munchow @ Quill & Pad. See the original article here - https://quillandpad.com/2024/04/06/hairsprings-origins-progress-and-dare-i-say-exciting-future-2/
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What is a hairspring?
Let’s kick this off with the basics: what is the hairspring?
The hairspring is conventionally a flat spiral of steel that is finer than a human hair. It attaches to the balance wheel and, after the balance wheel receives an impulse from the escape lever, it expands (or contracts), thereby limiting the balance wheel rotation’s before returning it to the beginning. The process then repeats in the opposite direction, often occurring between five and ten times a second.
Its precise size, shape, and mounting position leads directly to the balance wheel’s frequency, which, once combined with the appropriate gearing, determines a watch’s rate and accuracy.
When the hairspring was first invented, it consisted of a somewhat crude semi-spiral of steel wire that was attached to a balance wheel. The early hairspring didn’t have many spirals, making it inconsistent as it expanded and contracted during rotation due to uneven “breathing.”
These early hairsprings weren’t hardened or tempered, either, so the spring force varied greatly and tended to weaken over time, not to mention that corrosion was also an issue with the unstable steel. This is where the first foray into exotic materials began.
Some watchmakers, including John Arnold (1736-1799), experimented with gold-alloy hairsprings as it was a much better understood metal during that time and could be more easily shaped; on top of that it was completely corrosion-free.
But like most metals, it also suffered from fatigue over time, gradually weakening and introducing inconsistent rates. I also have seen a few examples of movements featuring palladium hairsprings, likely attempted for reasons similar to gold.
Eventually the art of hardening and tempering steel was introduced by John Harrison (1693-1776), greatly reducing the effects of metal fatigue as well as corrosion over time and making much more pliable and long-lasting hairsprings. This led to the development of much more consistent hairsprings as experimentation with the form factor was added to just trying to get a material that even functioned as a spring.
This is when technicians began adding spirals and calculating the optimal terminal curves to allow for increased isochronism and consistent rates. But as these became better, other issues took center stage like the effects magnetism and temperature fluctuations would have on the now more accurate hairsprings. These effects had already been discovered and some attempts at remedying them had been undertaken, though none were successful.
This led to more research and development. Many watchmakers, most notably Abraham-Louis Breguet (1747-1823) and the firm Arnold & Dent, experimented with glass hairsprings as the material provided a lot of positives over the aforementioned tempered steel.
In the plus column, it was non-magnetic, corrosion-free, much less susceptible to temperature changes, had increased purity (as the production process of glass was much cleaner than that of metal alloys), and it even had lower susceptibility to inertial issues (bumps and shocks) due to a lower density. The glass hairsprings were actually rather robust and seemed to be a viable alternative.
Alas, it was not meant to be as other negative issues also became apparent. First, glass is never truly a solid, which means it can move and change over time, basically leading to disintegration as a hairspring. It also was tricky to attach the ends to metal components, and it was practically impossible to adjust or modify the spring once formed.
But the nail in the coffin was the rate change over time. Like metals, glass can fatigue and weaken, leading to a change in rate. While steel hairsprings will have a rate that accelerates over time from these effects, that of glass is even worse.
This meant that especially for use during nautical navigation the calculations would change over time and sailors could end up way, way off course. Steel hairsprings proved less susceptible to this rate change and when combined with forms such as a helix spiral resulted in much more accurate timepieces over much longer periods of time.
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Credits: Article and images by Joshua Munchow @ Quill & Pad. See the original article here - https://quillandpad.com/2024/04/06/hairsprings-origins-progress-and-dare-i-say-exciting-future-2/