Mainspring

The mainspring is the sole source of energy in a mechanical movement: everything else in the train, from the escapement to the hands, depends on the torque it releases as it unwinds. It's a laminated steel ribbon, twenty to forty centimetres long depending on the calibre, wound in a spiral inside the barrel. When you wind it—by hand or through an automatic rotor—it compresses; the energy then releases progressively, coil after coil.

The fundamental problem with the mainspring is that the torque it delivers is not constant: at full wind it pushes hard, nearly depleted it pushes little. This force gradient transfers directly to the movement's precision. Manufacturers address it in different ways. The fusée-and-chain transmission is the historical solution: a chain runs from a conical barrel which, as the spring unwinds, works on an increasing diameter, compensating for the loss of torque. It works well, but takes up space and requires a chain of absolute precision. Modern stop-work systems, like the declutch patented by Armin Strom, instead choose to exclude the first and last coils of the cycle—those where force is at extreme values—limiting the barrel to work only within the optimal tension range.

The bridle is a less discussed but important element: it's a metal lamella fixed to the outer end of the spring and to the inner wall of the barrel. In movements with sliding barrels, the bridle allows the spring to slip when it has reached maximum load, preventing breakage from over-tension. Removing it, as in certain custom projects, requires much more careful charge management.

The material changes its properties. Nivaflex—a nickel-cobalt-based alloy—is paramagnetic: it doesn't magnetise and for this reason has replaced almost everywhere carbon steel, which in the presence of magnetic fields alters rigidity and throws off timekeeping. Springs in amorphous materials, still rare, instead aim for a more stable elastic coefficient over time.

In ultra-thin movements the barrel architecture comes to involve the case itself: in some calibres the caseback serves as the main plate, and the barrel is mounted directly on it. You gain thickness by eliminating a structural layer, but you lose the separability between movement and case—any service requires completely reopening the system.

Power reserve depends on how many coils the spring contains and how much the train consumes: more coils, more stored energy, but also a wider or taller barrel. It's always a compromise between autonomy, size, and regularity of timekeeping.