A micromechanical masterpiece composed of 636 parts
It is 15 centimetres long as well as half a millimetre wide and weighs a scant 0.12 grams. Nonetheless, it can lift a load of two kilograms. The object in question is the filigreed chain in the Tourbograph Perpetual “Pour le Mérite” that delivers torque from the mainspring to the fusée, thus assuring constant power across the entire power reserve duration. Six macrophotographs document the craftsmanship invested in the complex component.
The highly complicated manufacture calibre L133.1 of the Tourbograph Perpetual “Pour le Mérite” consists of 684 parts if the chain is counted as one component. But the chain alone consists of nearly as many parts as the entire movement. Its 636 microscopically small pins, chain links and hooks are produced by computer-controlled electrical discharge machines and automatic lathes to a tolerance of thousandths of a millimetre before the watchmakers painstakingly assemble them by hand in an arduous process.
Like a miniature bicycle chain
Like all chains used to transfer torque, the chain in the Tourbograph Perpetual “Pour le Mérite” is articulated, in that the overlapping links rotate on riveted pins. Its surface is straight-grained, which, apart from the aesthetic aspect, also plays an important technical role because this finish stabilises the connection between the pin and the chain link.
There’s a catch
To establish the connection between the mainspring barrel and the fusée, the beginning and the end of the chain are fitted with special hooks that are attached to the two components after having passed a stress test. This closes the torque flow path. In the front third of the chain, a stopwork rivet assures that the ratchet wheel of the movement is blocked just before the mainspring is fully wound. The rivet prevents the chain from being accidentally overstretched. A tiny planetary gearing mechanism inside the fusée assures that the flow of power is not interrupted while the watch is being wound.
Assembly of the fusée-and-chain mechanism
Once the watchmaker has wrapped the delicate chain around the previously assembled mainspring barrel, he can integrate both elements into the basic movement. As the torque of the mainspring weakens, the chain wraps around the barrel and pulls at the gradually increasing circumference of the fusée. As a result, the torque at the arbor of the fusée – and thus the amplitude of the balance – remains constant during the entire power reserve period.