Mainspring

The mainspring is the energy reservoir of any mechanical watch: a tempered steel blade coiled inside a barrel that stores torque during winding and releases it progressively to the gear train. From my bench, I've seen every type of failure, and most precision problems start here, not at the escapement.

The barrel is not a simple container. Its dimensions directly determine the length and thickness of the spring, and therefore the power reserve and torque curve. A 35 mm barrel with three meters of spring and thirty turns, like the one in the Honoris machined directly from the plate, delivers relatively stable torque for nearly the entire reserve. A double barrel, instead, sums the energies of two springs in parallel or in series: Grand Seiko's 9SA5 caliber reaches eighty hours while maintaining a thickness of just over five millimeters. More barrels also mean that torque peaks are dampened, because each spring works over a narrower unwinding interval.

The central problem of the mainspring is non-linearity: it delivers high torque when fully wound, declining torque when nearly depleted. This variation transmits to the balance wheel and shows up as amplitude variation, then as rate variation. Historic manufacturers responded with the fusée-and-chain transmission: a chain links the barrel to a conical pulley of variable diameter, mechanically compensating for the torque decay. Berthoud marine chronometers and British naval instruments made this a standard. Today you find it on haute horlogerie pieces like the A. Lange & Söhne Tourbograph Perpetual. The remontoire d'égalité and constant-force escapement work on the same principle but downstream, decoupling the escapement from direct transmission and metering uniform impulses to the anchor at fixed intervals.

The stop-works mechanism, often a Maltese cross, instead cuts the extremes of the curve: it blocks the last quarter and first quarter of unwinding, where torque is respectively too high or too low. Nominal power reserve is lost, but actual linearity is gained. The power reserve indicator shows where you are on this curve: a tool useful not only for convenience, but for understanding in which torque zone the movement is operating at any given moment. The 2002 Royal Oak Concept Dymograph took this concept further, displaying the spring's instantaneous torque directly. Seiko's Spring Drive technology solves the problem another way: it uses a traditional mainspring but regulates release with a battery-free electromagnetic brake, achieving one second per day on a constant basis. I'm not sure whether this solution will find widespread heirs outside Seiko.