PQ according to Kurt Klaus

He finally succeeded in convincing the management of IWC, then provided by the duo Gunter Blümlein and Hannes Pantli, to give the go-ahead for the development of a perpetual calendar for wristwatches.

At the time, calendars were generally built within the precise movement, but Kurt Klaus wanted to design a separate module that he could then integrate into different basic movements. With his calendar, he also intends to set new standards in terms of simplicity and operation.

And, true to the spirit of F. A. Jones, founder of IWC, the perfectionist that is Kurt Klaus is already considering the possibility of industrial production. He therefore decided to work with relatively simple shapes and as few parts as possible.

Its initial idea consists in using as a source of energy the date mechanism integrated into the base movement. A simple switching impulse, triggered during the night, would drive a whole chain of gears and advance the displays of the date, the day of the week and the moon phase.

After a month, the month indicator would similarly advance, followed by the decade indicator after ten years and the century indicator after one hundred years. All with perfect regularity and synchronization.

That was the theory, but putting it into practice proved more complicated. Kurt Klaus imagines during his long walks in the Swiss pastures, the basic functions. On his drawing board, he relentlessly rethinks the shape and layout of the rooms.

The head watchmaker constructs the entire mechanism from triangles, assigns coordinates to each position and performs countless calculations. The design phase is intense and sometimes frustrating. Despite many setbacks, he managed to complete three operational prototypes just in time for the launch of the Da Vinci Chronograph Perpetual Calendar at the 1985 Basel Watch Fair.

The operation of the mechanism – made up of only 81 parts – is extremely efficient. Each night, the base movement activates the date control lever. In return, a pawl advances the 31-tooth date wheel by one day.

At the same time, another lever advances the star wheel for the day of the week as well as the moon phase display. The date wheel has a longer tooth than all the others: at the end of each month, this tooth automatically advances the month cam by one position.

This cam is the centerpiece of the mechanical calendar program. Its circumference alternates protruding sections and notches that represent the different durations of the months.

This piece works on the same principle as those punch cards that were used in the early days of computing. To also incorporate leap years into the equation, the cam represents a full cycle of four years, or 48 months. And one notch is deeper than all the others – that of February 29.

Another mechanism occurs during the shorter months. An additional pawl on the date control lever rests on an eccentric which is directly connected to the date wheel. At the end of months less than 31 days, this pawl falls off the eccentric and lands on a stopper.

During the switching sequence which occurs in the middle of the night, it advances all the days preceding the 31st which does not exist for this month, then the usual pawl comes into play and advances the date wheel by a single tooth.

This additional mechanism is indirectly controlled by the month cam. For months with less than 31 days, a feeler arm connected to the date control lever falls into a notch. The deeper this notch, the greater the radius of progression of the date control lever.

A large radius causes the additional pawl which drops from the eccentric to withdraw slightly at the end of the month. The protruding sections and notches of the month cam determine the different radii and whether or not – and, if so, when – the additional pawl comes into action.

Several calendar mechanisms had already been invented, but Kurt Klaus goes further. Starting from the month wheel which controls the display of the month on the dial, it incorporates a transmission chain which successively drives a year wheel, a decade wheel and a secular slider.

The latter advances by only 1.2 millimeters every 100 years. Let’s put this into perspective: during the same period, a point placed on the rim of the pendulum would in theory cover a distance equal to 40 orbits of the Earth.

The solution developed by Kurt Klaus is revolutionary in several respects. The unique and most important feature is the perfect synchronization of all the displays, from the date to the moon phase, including the day of the week and the month. If the watch is not worn for a few days and it stops, all displays can simply be advanced and adjusted, in one-day intervals.

This ingenious mechanism also has a four-digit year display – a rare feature on wristwatches. Another novelty is the extremely precise moon phase display.

The Da Vinci Perpetual Calendar Chronograph met with resounding success and marked a turning point in the history of IWC.

The basic operating principles of the perpetual calendar have hardly changed since 1985. The mechanism, which is made up of less than 100 parts, is distinguished by its incomparable ease of use. It will have to be manually advanced just one day to 2100, when the leap year will have to be omitted – another quirk of Pope Gregory’s calendar.

Since its launch, IWC has continued to develop this complication and modified it slightly. The design engineers from Schaffhausen have, for example, created a version with a digital date and month display.

Another starting point was also imagined: a model with a display of the phases of the Moon as it can be observed in the southern hemisphere. In the current version of the calendar, the display
of the moon phases is so precise that it only requires a correction of one day every 577.5 years.

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