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Watch Works: Fusée And Chain

Watch close up

We’ve covered the basics of how a mechanical watch works in a previous write-up which you can find here. Today we highlight another marvel you will find in a few watches out there: The Fusée-and-Chain transmission. This is of course perfectly timed in anticipation to our Master Class in Haute Horlogerie with Maison De Greef in Brussels.

watch movement gears
Close-up by A. Langhe & Söhne

What’s in a name?

Fusée and Chain is not French for Freaking Awesome, but it could be, keep reading.

Why does a mechanical watch need it?

If you’ve done a bit of reading on mechanical watch movements, you understand that the mainspring is what harnesses the watch’s core power (check out our excellent starter guide here to understand mechanical watch movements). When fully wound, the power it generates will (by physical fact) be higher than when the spring is nearly unwound. As such, your watch gradually loses power, in a downward sloping curve. This  as such then affects the overall accuracy of your wristwatch. Think of when you wind an old music box and near the end of the song as the spring is nearly unwound you hear the music slowing down until it finally stops.

The Fusée and chain mechanism, or ‘transmission’, exists for the sole purpose of negating that effect to the maximum extent. Its purpose in life is to avoid an uneven energy release as the mainspring unwinds and ensure that the movement receives a constant amount of energy. No downward curve here, but a straight line that comes to a hard stop when the power runs out.

detailed watch movement
Dionysius Lardner, The Museum of Science and Art, Vol.6, 1855

How it works

Ok so we know what it does for the accuracy of your watch, but how does it do it?

When the mainspring is fully wound the chain will be covering the entire fusée from top to bottom. The chain will be wrapped up tightly around the top of the fusée. As the mainspring unwinds, the chain will wrap around the barrel and unwind downward. Keep this in mind for what comes next.

Langhe Watch

In a nutshell this is how it works and improves the accuracy of your watch:

  • Mainspring is fully wound, full power. The Fusée sits in ‘high gear’ if you think of gears on a bicycle;
  • As the mainspring unwinds, the chain moves down the ‘gears’ on the fusée, moving in lower gear as the watch gradually ticks along;
  • Through the above ‘automatic’ transmission, the chain moves the pulling force from high gear to low gear. Meaning at the very end the least amount of force is pulling on the lowest gear possible. Same as on a bicycle, where in lowest gear, a small kick will get you a lot of meters, compared to a small kick in the highest gear which will barely move you forward.

Thanks to this you end up with a highly balanced power discharge which ensures your watch keeps time in the most glorious of ways: not a downward sloping graph, but a near straight line which at the end simply drops way down a cliff. The mainspring & fusée essentially keep each-other in check and they are linked to eachother by the chain. Making this at such a tiny scale that it fits in a wristwatch is another feat all in itself. A. Langhe & Söhne for example uses this complication only in very few of its (top range) watches for this reason.

For  a deep dive in how this is mastered at A. Langhe & Söhne, I highly recommend this (mandatory) reading over at their website, right here

Note: Splash macro image courtesy A. Langhe & Söhne


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