Barrel
The cylindrical container for the mainspring
What it is
The barrel is a cylindrical drum, typically brass, that contains the mainspring and serves as the first wheel of the gear train. Its toothed outer edge meshes with the center wheel. The barrel's diameter and depth determine how much mainspring can be stored; and therefore how long the movement can run on a full wind. Larger barrel, longer power reserve, all else being equal.
History
The fixed barrel; in which both ends of the mainspring were anchored; was the early solution, but it had a catastrophic flaw: if the spring was overwound, it broke, often damaging other parts of the movement. The going barrel, developed in the 18th century, solved this by attaching only the mainspring's inner end to the arbor and allowing the outer coil to slip against the barrel wall when maximum tension is reached. This slipping-clutch behaviour makes overwinding essentially impossible. The fusée-and-chain system placed a separately-rotating cone between the barrel and the gear train to compensate for declining torque across the spring's full range; elegant engineering that persists in a handful of wristwatch movements, including the IWC Portuguese 7-Day and Jaeger-LeCoultre Duomètre. Multiple barrels in series; two or three linked sequentially; multiply power reserve; A. Lange & Söhne's Lange 31 uses two barrels to achieve a 31-day power reserve.
How it works
The mainspring's outer end hooks to the barrel wall; its inner end hooks to the arbor. The arbor is fixed to the movement plate by the ratchet wheel assembly; the barrel rotates around the arbor as the spring unwinds. A click spring prevents the ratchet wheel from backwinding when the crown is released. The barrel's teeth engage the center wheel directly, making the barrel both a storage container and the driving wheel of the entire gear train. Barrel size is the primary lever a movement designer pulls to achieve longer power reserve; which is why movements with extended reserves tend to be thicker or require a larger movement diameter.
In the catalog
Related
- Mainspring: The coiled steel strip that stores the watch's energy
- Gear train: The series of wheels reducing the mainspring's speed to readable time
