The Jaeger-LeCoultre Gyrotourbillon Part 3:
Perpetual Calendar
By Ron DeCorte
April, 2005
It’s been a real pleasure putting together these three technical articles about the JLC Gyrotourbillon. I want to thank everyone at Jaeger-LeCoultre in LeSentier for their excellent support.
For those who haven’t been following this series, I suggest visiting the two previous articles, Part 1 about the multi-axis tourbillon and Part 2 about the Equation of Time, prior to reading this final installment.
The fact that the multi-axis tourbillon takes center stage is to be expected as this is the one complication that is in constant motion and visible to the owner. But the equation of time complication, and the perpetual calendar with double retrograde date indication, are nothing short of innovative. Over the years I’ve written about many complicated watches and never encountered one that made me think harder to understand (yet alone try to explain) than the JLC Gyrotourbillon (Gyro). From the mechanical design to the fabrication processes, there isn’t much Old School thinking here, other than excellent quality.
So here we are at the 3rd and final installment about the Gyro. Previously I explained the multi-axis tourbillon and the equation of time complications. In this installment I will explain the quantieme perpetual calendar.
As an appetizer I thought I would share some drawings that show the complete movement. (Be sure to enlarge these drawings by clicking them - they are quite amazing! Very large versions are available at the end of the article.)
Quantieme Perpetual Calendar
Mechanical calendar devises have been around for centuries and most of us still have one strapped to our wrist. Over time these calendar mechanisms have evolved to show the month, date, and phase of the moon. And along came the Quantieme Perpetual.
Simply translated Quantieme means “days of the month”, and of course Perpetual means “never ending”. So I suppose in English we would say “never ending days of the month”. Too bad the Quantieme Perpetual Calendar (QPC) mechanism isn’t so easily translated. Several companies have developed inexpensive QPC quartz watches, but in mechanical watches this complication is anything but inexpensive, and for good reason. I don’t know about you but I’m constantly forgetting to adjust the date at the end of the month, not to mention remembering how many days there are in each month (what sadistic person developed this calendar system anyway…). The sheer vision of designing and fabricating a mechanical complication that remembers the exact date, day, month, and year is quite amazing. And did I mention leap years!
The Gyro QPC is unique in several ways. The most obvious is the use of two retrograde hands to indicate the date. One hand on the left indicates the dates 1 through 16, and a second hand on the right indicates the dates from 16 through 31. In the above picture we see the right hand indicating a date of 18 while the left hand is resting to the far left. At the end of the month, be it 28, 29, 30, or 31 days, the right hand will rest and the left hand will start its job.
For one day each month, the 16th, both hands will indicate the same date, as shown above.
Photo #1
The retrograde date hands are controlled primarily by the components shown above. In the upper right are the hands, upper left are two cam-follower arms with small racks of teeth, lower right are two control-racks with spiral tension springs, and lower center is the date snail assembly with two snails.
How it works:
Snail “A” and “B” are directly connected and are indexed each day at midnight making one revolution each month of 28 29, 30 or 31 days.
Cam “A” is followed (traced) by cam-follower arm “C” that controls date hand “F” (16 through 31 days). While cam-follower arm “D” traces cam “B” controlling date hand “E” (1 through 16 days).
If you look closely at Photo #! You will notice that cam “A” and “B” are not totally in step. In other words cam-follower arm “C” will drop off cam “A” before cam-follower “D” drops off cam “B”.
Therefore, when cam-follower “C” falls off the outer diameter of cam “A” date hand "F" will indicate day 16, the same as date hand “E”. But one day later (on the 17th) cam-follower “D” will fall off cam “B” and rest until the beginning of the next month.
Now I suppose you are wondering how this cam assembly is adjusted for the variable length of each month, read on…
The brain of any QPC mechanism is a cam. Early pocket watches with QPC had a month cam with 48 steps making one revolution every 4 years. This was a fairly simple way of adjusting the length of each month over a 4-year cycle, including leap years. With time the 4-year cams were replaced with 1-year cams, primarily because they required a lot less space inside the watch allowing for other complications and features. And of course in wristwatches space is a valuable commodity.
In the picture above the 1-year cam is mounted directly under the equation of time cam (see Part 2 of this series). There are twelve steps on the 1-year cam each corresponding to a month of the year. The deeper the step, the fewer days in that month, hence it’s easy to see that February has the fewest days.
With the advent of 1-year cams it became necessary to find a way of adjusting for leap years. This was originally accomplished by integrating a small 4-step cam into the 1-year cam at the February position. This smaller cam had three steps of the same size for those years with 28 days, and one step that was slightly larger for those years with 29 days. This “February cam” would rotate ¼ turn each year adjusting the length of February as required.
In the drawing above from the Gyro, we have 1-year cam “A”, lever “B”, February cam “C”, and wheel “D”. You will notice that the February cam is separate from the 1-year cam. This separation was a clever way of keeping the 1-year cam thinner than it would be if the February cam were integrated.
And so, in very simple terms:
Cam “A” is making one revolution each year.
Lever “B” drops onto cam “A” at the end of each month - the deeper it drops, the larger the end of month correction. In the drawing above lever “B” will make its deepest drop and a correction from February 28th to March 1st in one instantaneous movement.
February cam “C” makes 3¼ turns each year and hence the larger radius portion will be in position to block the tail of lever “B”, reducing its drop slightly, and adjusting for leap year once every four years.
And why does the February cam “C” make 3¼ turns each year when ¼ turn would be sufficient? Since cam “C” is mounted to a pinion driven by wheel “D” the pinion would need to be 3¼ times larger, requiring a lot more space in the watch, in order to make only ¼ turn each year.
In these drawings above we can see the complete Gyro QPC mechanism.
Notice that there is yet another retrograde hand on the back of the watch indicating the four year cycle of leap years. This makes adjusting the calendar much easier if the watch is not in operation for an extended period of time.
Click here and here to view monster size versions of the top and side-view drawings of the Gyrotourbillon mechanism.
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