Of winding and overwinding
Winding a mechanical watch, for the uninitiated, might be felt as a daunting task. These tiny mechanical wonders look like they are sort of frail, when we listen to their soft clicking as we wind them. And we worry about breaking them. So, how much should we wind them? And how often?
To dispel our doubts, we have to get back to their basic elements and functions. And discover that, in older times, our little ticking friends had a much lesser autonomy than they have right here, right now.
I describe how watches work in much more detail in my book, The Watch Manual), but here follows the short version of the story.
As you all know, mechanical watches are powered by a special steel spring called mainspring. This spring has been around since the 1500s, and is still very similar today to what it was back then: a coiled spring that slowly unfolds and gives energy to the mechanism of the watch.
It was soon understood that the mainspring had to be protected and properly greased to ensure it worked well. So watchmakers put it inside a hollow cylinder called barrel, and devised a way so that the mainspring released its energy into letting the barrel rotate.
The inventor of the so-called “going barrel” was Jean-Antoine Lépine, around 1750. The going barrel also allowed the watch to continue working when it was wound.
As a note, Lépine was one of the most influential horologists of all time, and the first developer of modern (flat) watches.
In very simple terms, the concept of the going barrel is straightforward. The mainspring is wound around itself, and it releases its accumulated energy by making the barrel rotate around a central axis called arbor (not shown in the photo). The arbor has a nail that slides into a hole of the mainspring, so when it rotates, it coils the spring around itself.
As you can see here, the barrel has a set of teeth on its outer rim, so it gives motion to the time train of the watch.
The issues of mainsprings
Mainsprings, though, had (and have) several natural “enemies”. Springs are made in steel, which is a material that can extend and then return to its previous state.
However, steel is an alloy, and as all alloys, it is not homogeneous. Even the best modern steels have inclusions and weaknesses which depend on the processes used in their making, which make them faulty and brittle. Imagine the ones made in older times: mainspring breaking was understandably common.
More, any spring is subject to “metal fatigue”. In short, the more a spring is used, the less it returns to its original state (with its potential energy output decreasing).
Modern steel alloys have done much to overcome these issues, and ensure that the springs are less prone to breaking (which was a common issue with older springs) and metal fatigue. Also, the advances in metallurgy now allow for the precise inclusion of specific elements to the base alloys. This have made it possible to increase the performance of modern mainsprings, resulting in more power and more autonomy for the watch.
To make some examples, old pocket watches in the 1800s had to be wound up once a day. The first wristwatches that were developed in the 1920s had a power reserve of 24–36 hours, increasing to reach almost 48 hours of autonomy after WWII. The “bibles” of watch servicing, written by DeCarle and Freid in the Fifties, inform us of these stats.
Two days of autonomy was about perfect for everyone. More, the diffusion of automatic winding movements, which gained traction in the 1950s, extended the operative life of watches almost indefinitely, as watches were recharged by the movement of the arm.
The development of the power reserve
The introduction of new materials into watchmaking has helped power reserves rise tremendously from the early days of watchmaking, and among the most used solutions we find silicon.
Today, almost every mechanical watch has an autonomy of a couple days, with some modern movements like the Baumatic by Baume et Mercier and the Powermatic 80 by the Swatch Group reaching 80 hours of autonomy — a feat that in the late 1800s would have sounded almost like wizardry.
The only way to achieve such a power reserve back then was to use a huge mainspring (which had many other disadvantages). The Hebdomas was such a timepiece: it touted itself as having a power reserve of 8 days, even if in practice, it averaged around 4. Still, it was exceptional, if confronted with the power reserves that were available at the time.
The myth of overwinding
Sometimes people hear about “overwinding”. That is, turning a mainspring so much that it becomes stuck, without the watch working. Well, I want to inform you that overwinding does not actually exist anymore, and was effectively resolved with the development of automatic watches.
Automatic watches posed the issue of overcharging a mainspring by applying too much force to it. Up to the point that the mainspring could snap, possibly ruining the delicate mechanisms of the timepiece in the process.
More, every spring has a more constant output of its force when it is in an intermediate state between fully wound and completely unwound, because of an effect known as Hooke’s Law.
To avoid this occurrence, watchmakers developed a sort of “safety valve” inside the barrel. Instead of applying directly the mainspring to the barrel, they attached it to another small and strong spring called bridle that had a protruding “tooth”.
As you know, when you charge a watch the mainspring gets coiled around the arbor. The more it is coiled there, the less pressure it makes on the sides of the barrel. If too much tension is applied to the mainspring, it would make the bridle slip along the inner surface of the barrel to shrug off the excessive tension, and after that, stick again in place because of its elastic force.
This simple but effective trick permits to reduce the strain on the mainspring, and avoid it to break inside the watch.
Some mainsprings still snap from metal fatigue, especially when they are very old, but a snapping mainspring is a very rare exception to the rule in modern horology.
To come back to you and your winding up your mechanical watch, you are free to recharge your watch as often as you like without having to worry too much about the mainspring breaking. However, depending on the characteristics of the watch, a recharge every couple days should be enough to keep it going.
When you do recharge manually a watch, do it gently, and stop when you find too much resistance on the crown: remember that watches do not really need to be fully charged to operate well.