As the tension in the mainspring of a mechanical watch decreases, so too does the balance amplitude. This has a negative impact on the watch's accuracy. IWC's constant-force mechanism ensures that the escapement delivers an absolutely even supply of power and delivers unprecendented precision.
The only constant – so runs the adage – is change. In the world of haute horlogerie, however, its aptness is limited, because in watchmaking all our efforts are channelled into creating constancy: in other words, into ensuring that the oscillations in the balance are always exactly equal. For centuries now, one specific challenge has faced inventors and watchmakers. “When a watch is fully wound, the mainspring generates its maximum torque. And that results in
maximum amplitude. But as the tension in the barrel decreases, so too do the oscillations,” says Thomas Gäumann, Head of In-House Movements at IWC in Schaffhausen. This phenomenon has a negative effect on a mechanical watch’s accuracy.
For the balance oscillations to remain even at all times, the power transmitted through the wheel train and the escapement must likewise always be constant. However, as long as the flow of power to the balance is continuous, decreasing tension in the mainspring inevitably influences the amplitude. As Gäumann explains: "Various solutions have been designed with the aim of converting diminished energy from the spring into constant momentum with the help of an additional mechanism."
The constant-force mechanism permits unprecendeted precision
IN SEARCH OF CONSTANT FORCE
One possibility is to insert an infinitely variable transmission with a fusee and chain between the barrel and wheel train. Leonardo da Vinci had sketched such a design, which resembles the gearing on a bicycle, as early as the 15th century. In this system, the barrel revolves around itself and in the process winds up a chain mounted on a cone-shaped fusee. When fully wound, it pulls on the pointed end of the cone. The leverage there is at its lowest, with the result that less torque is transmitted to the wheel train. The lower the tension in the spring, the larger the leverage at the base of the fusee and thus the amount of torque delivered. Over the entire length of the process, the force transmitted to the balance remains constant.
The fusee-and-chain drive was particularly successful in large clocks. It was used on ships in marine chronometers, for example, where the need for precision was extremely high. Similar mechanisms were also used in pocket watches. However, an infinitely variable transmission requires a great deal of space, which meant that its use in wristwatches was limited.
INTEGRATING AN ADDITIONAL ESCAPEMENT
It was a challenge that also occupied the engineers at IWC for many years. But eventually they found an efficient and technically elegant alternative: "Our patented constant-force mechanism integrates an additional escapement between the escape wheel and the fourth wheel. Every second, this winds a balance spring that serves as a temporary storage space and keeps the escape wheel supplied with sufficient energy to keep the balance moving." says Gäumann, summing up the principle. And the trick is simply this: the angle by which the balance spring is wound every second remains the same, which in turn means that the energy supplied to the escapement also remains constant. Even when the tension in the spring decreases, the balance continues to oscillate with virtually the same amplitude.
The constant-force mechanism ensures that the watch’s rate is extremely precise. It is found in the Portugieser Sidérale Scafusia and the Ingenieur Constant Force Tourbillon and has even been integrated in a tourbillon. The frequency of the constant-force tourbillon has purposely been set at 2.5 Hz to enable the system to wind the balance spring every second.
—94800 calibre
A BALANCE SPRING PROVIDES TEMPORARY ENERGY STORAGE
At the heart of the mechanism is a form of Swiss club-tooth lever escapement. A triangular cam is mounted on the escape wheel pinion. The cam engages with the fork-shaped constant-force lever, which grips the so-called stop wheel with its two pallets at the other end. When the escape wheel has advanced five steps, it releases the stop wheel. It revolves through 30 degrees before being locked again. The process is repeated after every five beats of the balance. At 18,000 beats per hour, this sequence also determines the progress of the seconds hand mounted on the tourbillon cage. Every rotation of the cage also turns a pinion on the escape wheel staff, which meshes with the fixed fourth wheel. It winds the balance spring (situated below the escape wheel), which supplies a constant impulse of force to the balance.
“To drive the tourbillon and the constant-force mechanism, we equipped the 94800 and 94900 calibres with twin barrels. Together, they supply enough power to keep the mechanism running for about 48 hours” explains Gäumann. After two days, the available torque is no longer sufficient. At this point, the tourbillon automatically reverts to normal mode and advances at the rate of five steps a second, or at the same speed as the beats in the balance.
A CHALLENGE FOR THE DESIGN ENGINEERS
For the engineers involved, the design and manufacture of the constant-force mechanism was a Herculean task. The filigree construction comprises around 20 additional parts that had to be integrated in a tourbillon with a diameter of 15.8 millimetres. “Defining the various sequential processes, such as releasing and stopping the constant-force lever, was especially demanding. To achieve that, we had to strike a balance between safety and functionality. We also had to build in a certain reserve in order to ensure that the complex sequences of movements and the lever actions were always reliably executed” stresses Gäumann.
—With the patented constant-force tourbillon, IWC elegantly combines two complications that serve to increase timekeeping precision. It ensures an extremely regular and precise rate over a period of at least 48 hours.
EXTREME PRECISION CALLS FOR NEW PRODUCTION TECHNOLOGY
It is all made more difficult by the fact that the constant-force mechanism is situated between the escape wheel and the fourth wheel, right in front of the balance, and has a direct influence on the latter’s oscillations. As a result, the permitted tolerances are extremely small – in some cases as little as one thousandth of a millimetre. The constant-force lever and cam are manufactured using the LIGA process combined with X-ray exposure. “This version of the LIGA process uses X-rays and permits us to produce extremely homogeneous microstructures with a degree of precision that conventional manufacturing technologies would not be even remotely capable of” says Gäumann. Another important factor is the choice of materials: the cam is made of solid gold and the constant-force lever of nickel phosphorus. Gold is self-lubricating, a fact that has made this the optimum combination for friction partners that run dry.
Only three highly experienced and qualified specialists at IWC are equal to the task of assembling a constant-force tourbillon alone
A CHALLENGE FOR THE DESIGN ENGINEERS
Assembling a constant-force tourbillon is a severe test of patience for even the most experienced watchmaker. It takes them a full two weeks to assemble a mechanism that weighs in at just 0.7 grams and consists of 104 individual parts. Only three highly qualified specialists at IWC are equal to the task. For Gäumann, the constant-force tourbillon is not only a unique selling point but also a perfect example of the Schaffhausen-based company’s aspiration to engineering and innovation at the highest level. “The fact we’ve been able to surmount a centuries-old watchmaking challenge with a solution that’s both functional and technically elegant makes me extremely proud,” he says.
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