Watch Fact of the Day: The Watch with 1.85-Meter Mainsprings That Requires a Key to Wind

📅 Monday, August 10, 2026

When A. Lange & Söhne introduced the Lange 31, it became the world’s first mechanical wristwatch with a massive 31-day power reserve. To store that much energy, its movement houses two colossal mainsprings that each measure 1.85 meters long—roughly ten times the length of a standard spring. Because winding such immense tension by hand via a traditional crown would be agonizingly slow, the watch must be wound using a miniature mechanical key inserted directly into the caseback!

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Imagine wearing a mechanical watch that can run for an entire month after a single winding. When A. Lange & Söhne created the extraordinary LANGE 31, it became the world’s first mechanical wristwatch with a 31-day power reserve—an astonishing 744 hours of stored mechanical energy.

Achieving that endurance required something enormous by wristwatch standards. Hidden inside the L034.1 movement are two mainsprings, each measuring 1.85 metres (1,850 mm) long—roughly ten times the length found in a conventional mechanical wristwatch. Together, that’s an incredible 3.7 metres of mainspring packed inside a wristwatch.

But all that stored energy created another problem: how do you wind it? Turning a conventional crown enough to tension two enormous mainsprings would be cumbersome and time-consuming. Lange’s wonderfully mechanical solution was to give the owner a separate winding key. Inserted into a square socket recessed into the sapphire caseback, the key provides substantially greater leverage than an ordinary crown-based winding system.

And storing the energy was only half the challenge. Fully wound mainsprings this powerful deliver enormous torque, which gradually declines as they unwind. Lange therefore developed a patented constant-force escapement that sits between the twin mainspring barrel and going train, releasing a controlled amount of energy to the escapement. In the LANGE 31, its remontoire spring is recharged every ten seconds, helping maintain consistent balance amplitude throughout the enormous power reserve.

Category: Fun Quirk

Disclaimer: Every effort has been made to keep these watch facts accurate, but the world of horology is full of myths, disputed stories, and fascinating trivia. Consider these facts a starting point for discovery rather than the final word. If you have corrections or additional insights, let us know—we’re always happy to keep the record ticking in the right direction.

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