· By Buffy
Anatomy 3: The Mainspring and Barrel
The engine: a coiled steel ribbon whose torque falls as it unwinds — and the tricks (barrel, fusee, remontoir) that keep a watch honest about it.
Part 3 of 8 in the Anatomy series.
The mainspring is a flat ribbon of special steel, a meter or so long, coiled inside a drum called the barrel. Wind the crown and you coil it tighter; the stored torque then drives the gear train as it uncoils. Simple. The problem is in the torque curve.
The dishonest battery
A coiled spring’s torque is highest fully wound and lowest nearly spent — easily a 30–40% difference across the power reserve. If that difference reached the escapement directly, the watch’s amplitude would collapse overnight and its rate would wander with the hour of day.
Constant-force tricks
Watchmakers fight back with dilution and cleverness:
- The going barrel. In most watches the barrel is the first wheel, and the spring is sized so the watch runs on the middle of the torque curve. The mainspring’s hooked inner end (the bridle) slips along the barrel wall when fully wound — an automatic clutch, so over-winding is impossible. This is why an automatic watch is always “slipping” near full wind.
- The fusee. The historical answer: the spring drives a cone-shaped pulley via a chain or gut, so that as torque falls, leverage rises. A fusee chronometer keeps nearly constant torque across its entire reserve — it is also tall, fragile, and expensive, which is why it vanished from wristwatches but remains the textbook constant-force device.
- The remontoir. A tiny secondary spring (or weight) re-wound at fixed intervals delivers constant force to the escapement regardless of the main spring’s mood. Beautiful, rare, and covered where it belongs — with the detent escapements that love it.
Power reserve
The useful wind is a countable resource, so of course we display it: a power-reserve indicator is a differential mechanism measuring the difference between how far the barrel has unwound and how far you have wound it. Calibrating one by hand during assembly is one of the least envious jobs in the workshop.
Materials science in one paragraph
Mainsprings are alloys (Nivaflex and kin) designed for high yield strength, minimal set (they must spring back to shape for decades), and corrosion resistance. Before the 1950s, springs broke — literally snapped — as a routine service item; the “unbreakable” mainspring was a genuine marketing revolution. Modern springs in modern barrels deliver 48–70+ hours of power reserve, and the arms race for longer reserves is mostly a barrel-and-efficiency contest, not some new energy source.
Next: the machine’s conscience — The Escapement.