A. LANGE & SÖHNE
The Lange 31

No other watch runs longer: The new manually wound A. Lange & Söhne timepiece has a power reserve of one month. That is why it has been named the Lange 31. Another innovation, a constant-force escapement, assures that power delivery is uniform throughout the entire 31-day period. Additionally, Lange reintroduces the venerable key winding mechanism.

The construction of a mechanical wristwatch with a power reserve of a whole calendar month, not to mention constant rate accuracy across the entire period, was one of the few as yet unmastered horological feats. Now, it is reality: the Lange 31. With it, the horological year is now subdivided into 12 “winding sessions”. To store this amount of energy, the Lange 31 features two stacked mainspring barrels with an inside diameter of 25 millimetres. The prominent twin barrels occupy three quarters of the movement’s footprint. Each measuring 1,850 millimetres in length, their combined energy is enough to lift a 100-gram bar of Swiss chocolate 320 centimetres – more than 10 feet. The leverage delivered by a key makes it possible to choose a transmission ratio that would be inconceivable with a crown. A square key, inserted through an aperture in the sapphire-crystal caseback, delivers the energy to the spring barrels. The key features a built-in backstop ratchet to allow smooth winding as with a regular crown, and a torque limiter prevents accidental over tightening of the springs.

It stands to reason that a movement with such an extraordinary power reserve cannot constantly run at a stable rate without special technical precautions. So Lange’s engineers sought – and found – a different solution: the constant-force escapement. It is based on an auxiliary spring that is periodically rewound by the mainspring and always builds up the same torque, regardless of the state of wind of the mainspring. As it relaxes, this pretensioned so-called remontoir spring on the fourth-wheel arbour always delivers the same amount of energy to the escape wheel. Every ten seconds, this spring, attached to a stud, is retensioned by 60 degrees at its outer end.

The motion sequence of the constant-force escapement can be observed through the sapphire-crystal caseback. A transparent sapphire jewel reveals the fascinating interaction of the three-point cam with the pivoting lever. On the bottom line, the constant-force escapement prevents the waning torque provided by the mainspring barrel from diminishing the rate accuracy of the watch. The result: uniform energy delivery, constant amplitude, same rate accuracy from the first to the thirty-first day on which a shutoff mechanism halts the movement.

The platinum case of the Lange 31 has an impressive diameter of 46 millimetres and an overall height of 15.9 millimetres. After all, it takes space to store so much time. And the circular 31-day power-reserve indicator that occupies nearly the entire right-hand half of the solid-silver dial requires space as well. It is the hallmark of a true breakthrough in horology. The last segment, in red, reminds the owner that after a full month has elapsed, it is finally time to rewind the watch. The Lange outsize date on the left side harmoniously balances the face. Needless to say, the Lange 31 also showcases all of the quality features that make timepieces by A. Lange & Söhne so coveted around the world. A glance through the sapphire-crystal back reveals the screw balance, the whiplash precision index adjuster on the hand-engraved balance cock, screwed gold chatons, and lavishly decorated elements in the classic Lange style.