In a horological deep dive, quattro, a respected WatchProSite contributor, meticulously details the fusee-and-chain transmission, a sophisticated mechanism ensuring constant force delivery in mechanical watches. His comprehensive post not only elucidates the technical principles but also showcases nine exquisite examples from seven distinct brands, making this complex complication accessible and visually stunning for enthusiasts and newcomers alike. This article serves as an invaluable reference for understanding one of watchmaking's most revered constant force systems.
The
fusee-and-chain transmission is a way of providing a
constant even force to the
escapement as the
mainspring unwinds itself throughout its range of power reserve.
The fusee (from the French fusée, wire wound around a spindle) is a cone-shaped pulley with a helical groove around it, wound with chain attached to the mainspring barrel.
How it works is fairly easy to understand if you have in mind a bicycle deraileur, which allows to change the gears:
- When the mainspring is wound up (Fig. 1), all the chain is wrapped around the fusee from bottom to top, and the end going to the barrel comes off the narrow top end of the fusee: so, the strong pull of the wound up mainspring is applied to the small end of the fusee, and the torque on the fusee is reduced by the small lever arm of the fusee radius.
- As the mainspring runs down (Fig. 2), more of the chain is wrapped on the barrel, and the chain going to the barrel comes off the wide bottom grooves of the fusee: so, the weakened pull of the mainspring is applied to the larger radius of the bottom of the fusee and the greater turning moment provided by the larger radius at the fusee compensates for the weaker force of the spring, keeping the drive torque constant.
credit: Wikipedia
Here are nine examples from seven different brands of this transmission system integrated into a wristwatch.
The brands are presented in alphabetical order.
1/ FERDINAND BERTHOUD: THREE EXAMPLES
a/ NAISSANCE D’UNE MONTRE 3 
credit: Hodinkee
b/ FB 1.2 
credit: Monochrome, A Blog to Watch & The Naked Watchmaker
c/ FB 2RE.1
credit: Deployant
2/ BREGUET TRADITION TOURBILLON 7047 ref. 7047PT/11/9ZU
credit: Monochrome & @winewhiskywatches
3/ CABESTAN TERRA LUNA
credit: The Horophile
4/ ROMAIN GAUTHIER LOGICAL ONE
credit: Quill & Pad
5/ A. LANGE & SÖHNE RICHARD LANGE TOURBILLON POUR LE MÉRITE ref. 760.026F
credit: Quill & Pad & Hodinkee
6/ LEROY OSMIOR CHRONOMÈTRE À TOURBILLON ref. L102/1
credit: GPHG & Quill & Pad
7/ ZENITH ACADEMY GEORGES FAVRE-JACOT ref. 18.2210.4810
credit: Fratello & A Blog to WatchThanks for reading.
Best, Emmanuel
