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From the Bench: Mainspring Mechanics

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From the Bench: Mainspring Mechanics
I previously talked about overwinding, but I skipped over some of the finer details for the sake of brevity. Let’s dig into the mechanics about your watch's mainspring and winding mechanism.

What is a Watch Mainspring?

Mainsprings are what make your watch tick consistently. They consist of a flat spring steel band that's coiled when wound, storing energy to keep your watch ticking. 

Mainsprings come in two main types, traditional and automatic. 
Manually-wound mainspring with tongue end

Traditional Mainspring

Traditional mainsprings have a tongue at the end, which fits into a hook in the barrel wall to push the barrel around.
Manually-wound mainspring barrel
Winding a traditional manual-wind mainspring will start easily, become progressively tighter, and then will be fully wound at a distinct point. The spring just won’t wind anymore, and the tongue/hook keeps it fully tensioned.
Automatic mainspring with sliding bridle

Automatic Mainspring

Automatic mainsprings have a very different end. Instead of a tongue, they have a “sliding bridle” that pushes the mainspring tail against the barrel wall. Likewise, instead of a hook in the barrel wall, automatic barrels have multiple soft-sided cutouts and a sticky grease around the perimeter.

Automatic barrels are designed differently because of the way that they’re wound. Automatic watches can theoretically be wound infinitely, since the automatic module never stops working, even when the watch is fully wound. If automatic mainsprings had a hard stop, the automatic module would bind up at full wind, causing all sorts of problems.

Imagine an athlete out on a run, wearing a fine automatic mechanical watch (yes, this hypothetical runner is a WIS and leaves their Garmin at home). Runners move their arms vigorously and regularly, perfect for winding an automatic. But imagine that this athlete is running a marathon—the watch will be fully wound early in the race, and if the mainspring stops, the heavy oscillating weight will bounce against delicate reversing wheel teeth over and over and over and over. The weight will win this fight, the teeth will shear off, and the watch will need a service.
Automatic mainspring barrel
Instead, automatic barrels use a special high-friction braking grease that provides a strong “base” for the automatic mainspring to push against, while also allowing the mainspring to wind infinitely. Those cutouts in the barrel walls are reservoirs for the grease, rather than a metal hook for the spring as in the traditional barrel.

A Quick History of Mainsprings

A quick historical diversion, if I may. Automatic watches haven’t always had this design, but they have always had this problem.
A wildly complex solution from the past, which I personally love, came in the Jaeger LeCoultre Futurematic. Instead of a slipping mainspring, a small hook would extend at full wind, catching a matching hook on the oscillating weight and holding it steady until the mainspring ran enough for the automatic to run freely again. A wonderfully complex Rube Goldberg-esque solution to the problem.
Geneva stopwork in a vintage Vacheron Constantin pocket watch movement
While we’re talking about history, another fascinating mechanism to prevent overwinding of manual-wind watches is a component called the “Geneva stopwork.” A finger is attached to the barrel arbor, which turns a “Maltese Cross” wheel once per rotation. Since one of the cross points is solid, it prevents the barrel arbor from being wound or unwound past a certain point in both directions. This was critical in the past when mainsprings were made of steel and tended to be more shatter-prone.

Too Long; Didn't Read (TL;DR)

Manually-wound mainsprings come to a hard stop, and automatic mainsprings slip forever. Oh, and don’t try to force anything on your watch—you’re stronger than the watch, and you can break it if you really put your fingers into it.