The LANGE 31 is the world's first mechanical wristwatch with a power reserve of 31 days and a constant-force escapement that powers the going train with constant torque delivery. The tremendous amount of energy needed to keep the watch running so long is transferred to the movement with a winding key and stored in a twin mainspring barrel. The Lange 31 is yet another example of A. Lange & Söhne's ability to redefine benchmarks in precision watchmaking. For an entire month, the mechanism assures the highest level of rate stability and rate accuracy. Thanks to its unmatched autonomy, the LANGE 31 offers a useful benefit – you may forget to wind it once in a while. Because even if you take the watch off, it keeps telling the right time for as long as 31 days. To emphasize this special functionality, the solid-silver dial bears the inscription Monats-Werk. Additionally, the power-reserve indicator was accorded ample space on the right-hand side of the solid-silver dial. The patented outsize date on the left-hand side harmoniously balances the face.
A glance at the case back reveals another construction that is very rare in precision watchmaking the key-winding mechanism. It was explicitly designed for and matched to this watch. It allows a considerably larger winding transmission ratio than would be possible with a crown. So fewer revolutions are needed, and the energy can be built up faster and more conveniently. The key is inserted through an aperture in the sapphire-crystal caseback. Three seals between the winding mechanism and the case protect the watch against ingress of water up to a pressure of 3 bar. To prevent inadvertent over-tightening of the mainsprings, the key features an integrated torque limiter. An attempt to turn the key further - once the mainspring is fully wound - disengages the triangular toothing of two wheels inside the key and interrupts the flow of power.
The L034.1 manufacture movement in the LANGE 31 is a true power pack. In a space with a diameter of 37.3 millimetres and a height of 9.6 millimetres, it stores the energy needed to keep running non-stop for 31 days. Two crucial questions had to be answered by Lange's calibre engineers: how can so much energy be stored in a movement? And, just as important: how can this energy be delivered in a controlled manner? The energy storage function is handled by the large twin mainspring barrel - two stacked spring barrels, in fact, with an inside diameter of 25 mm. They contain the two mainsprings, each of which is 1,85 meters long – five to ten times as long as the springs of conventional mechanical wristwatches. When fully wound, their combined energy is equivalent to more than 3 joules. The torque delivered by the mainspring barrel has its maximum when the spring is fully wound and then declines perceptibly as the spring relaxes. This changes the rate parameters of the movement, especially in watches with large power reserve values. To eliminate this unwanted effect in the LANGE 31, the manufactory integrated a constant-force escapement. The main element of this mechanism is a pretensioned spiral spring that is quickly re-tightened on the fly at intervals of ten seconds. During the ten seconds that follow, the energy stored in the spring is delivered to the going train in a controlled manner. The torque output is virtually constant and powers the watch uniformly throughout 31 days.
Apart from its innovative technology, this movement stands out by virtue of its aesthetic appearance. It showcases everything that makes Lange watchmaking artistry so desirable: attractive traditional elements such as the screw balance, the whiplash precision index adjuster on the hand-engraved balance cock, and three screwed gold chatons. Not to mention the superb finish of all parts that is so typical for Lange.