“If my watch winds while I wear it, how does it know when it’s fully wound?” It was a question posed to me by a first-time watch owner. While I had a reasonable response at the time - “it just does, don’t worry about it” - my curiosity led me down a path of research to understand the parts that power both modern and vintage watch.
When the crown is turned, the center of the spring turns and compresses, thereby drawing in the coils to store energy. The reverse process happens as the movement runs; the spring slowly unwinds as it doles out its stored power reserve. The red handle below illustrates that this spring would be wound counter-clockwise in order to store power,
As the outer end is affixed, the wearer can feel the spring as it starts to tighten, letting them know that the watch is fully wound. Relying on this feedback becomes a problem when we are no longer looking at a manual watch. Creating an automatic movement means that we must be able to cope with constant winding and movement.
In an automatic movement, such as the one found in a Rolex Submariner, the spring can be wound either through twisting the crown or the movement of the wrist. However, given the uncontrollable nature of wrist movement, a solution had to be found to address the potential that the spring would be overwound.
To solve this problem, a mechanism was created to allow an overwound spring to dispose of some tension without breaking the part. This led to the creation of a bridle, or small piece of metal pointing in the direction of the spring tension.
The model below shows where the spring bridle would press into the notch in the barrel wall under normal tension. Once the tension is too much, it will simply slip out of the notch and settle into another notch inside of the barrel wall.
This elegant solution has proven to be extremely reliable and durable as well. Through the abrupt movement of the wrist during daily wear as well as residing inside of a watch winder, this mechanism provides an efficient way to slip and prevent breakage.
In most cases, no, you cannot overwind a modern automatic watch. The smart design on these winding mechanisms allow for any excess tension to be released as the spring slips inside of the barrel. Despite that, some wearers choose to store their watches in a watch roll to prevent excess wear on the movement. Whatever solution you choose to protect your Swiss watch, take a moment to enjoy the years of design that created the final product you can enjoy on your wrist!