THE PATEK ANNUAL CALENDAR MECHANISM
by Walt Odets
At the 1997 Basel Fair, Patek Philippe introduced its "annual calendar" watch, the Reference 5035. Based on Patek’s familiar caliber 315-SC automatic, the 5035 carries a calendar plate of new concept and design. Perhaps most significantly, this semi-complex calendar provides a guichet (displayed-in-window) date, which has proven very popular for its superior legibility over traditional date-on-dial displays. Displaying the day of the week and the month (both by dial), the semi-complexity of the 5035 provides the additional convenience of a program wheel distinguishing months with 30 days (from those with 31) and automatically switching to the first of the following month at the end of "short" months. Manual date setting is required only once a year, at the end of February. At this time it is necessary to manually advance the day to March 1. Date, day-of-the-week and month corrections are accomplished with band-mounted case pushers. Unlike some designs, each pusher is intuitively located adjacent to its dial display.
DETAILS OF THE MOVEMENT
The basic caliber 315-SC is a central rotor, unidirectionally-winding automatic running at 21,600 beats per hour. The movement displays center seconds ("SC" for secondes centre), and simple guichet date at 3 o'clock. A 12 ligne (27 mm) movement, it is 3.22 mm in height, uses an eight-weight Gyromax (adjustable mass) balance with overcoil spring, and 30 jewels. (For comparison, the Jaeger LeCoultre 889 is 26.5 mm, 3.25 mm and 36 jewels. The uncomplicated 315-SC was very much a response to JLC’s "men’s, thin automatic" movement, beginning with the JLC caliber 888.)
The 315-SC with annual calendar plate—known as the caliber 315 S-QA (secondes-calendre complication-annuaire)—is, by comparison, 13.3 lignes (30mm), has a height of 5.22 mm and runs in 35 jewels. The S-QA moves the guichet date to 6 o'clock and adds day-of-the-week and month displays at 9 and 3 o'clock respectively. A 24 hour indicator displays at 6 o'clock for convenience in setting the watch. The two millimeter (62 percent) increase in thickness for the S QA is, of course, due to the calendar plate attached to the bottom plate (behind the dial). Patek cites a "38 to 48" power reserve for both the 315-SC and 315 S-QA. I suspect that if midnight were to be approached with a low state of wind, the drag of the calendar mechanism in both watches might lead to irregular running or stop the watch. Thus, the lower power reserve figure is probably cited for this possibility.
The full calendar plate of the S-QA is shown in Figure 1. The calendar plate is driven from the base movement through the hole in the center of the calendar plate. Wheel 1a is driven clockwise, and via 1b, drives the 24 hour driving wheel (9) counter-clockwise. In turn, the 24 hour driving wheel—which makes a complete rotation every 24 hours—drives the 31-tooth date wheel (10C). The month star wheel, not visible under the date wheel, is driven by the date wheel. The star wheel pinion carries the month display hand on the dial and assures that the month hand jumps from month to month (and does not fall between months). By means of intermediate wheels 12 and 13, the 31 tooth date wheel drives the calendar display daisy-wheel, 14. This daisy-wheel drives the date ring, 15. Wheel 1a also drives the 24 hour display via 2C, the pinion of which carries the 24 hour hand on the dial.
DETAILS OF THE ANNUAL MECHANISM
Figure 2 illustrates the annual mechanism in detail. This detail can be located in Figure 1, just to the right of the central opening in the calendar plate.
The regular calendar functioning (on 31 day months) has already been described: the 24 hour driving wheel (Figure 2, #1) drives the 31-tooth date wheel (3C). Via a gear train, the 31-tooth date wheel drives the actual display of date. In the detail of Figure 2, it can be seen that as the 24-hour wheel (1) rotates counter-clockwise, cam 2/2a trips one tooth on the 31-tooth date wheel every 24 hours. The 12-pointed month star wheel can be seen at 5 in Figure 2. The month star wheel, of course, makes one complete rotation each year.
As complex calendar mechanisms go, the annual mechanism has the relatively simple task of performing "irregular" switching only four times a year, at the end of 30-day months. At the end of "short" months (April, June, September, and November), the date display is advanced twice to bring the date display to "1." (At the end of February, leap year or not, the calendar must be manually advanced to March 1.) The extra switching on 30-day months is accomplished with the lobed calendar program wheel, shown in detail in Figure 3, #4. Mounted on a common staff (which runs through the 31-tooth date wheel) with the month star wheel (5), the calendar program wheel also makes one complete rotation per year. As shown in Figure 3, the calendar program wheel (rotating clockwise) carries five cams for the short months: April (white arrow), June (turquoise), September (green), November (red), and February (magenta). July and August are represented at the blue arrow, May at the orange. The lobe for February allows the calendar to switch to February 29 every year and thus be correct for leap year. This switching also reduces the amount of manual adjustment required to change to March 1 in non-leap years.
The mechanics of the extra advance are illustrated in Figure 4. At the end of a 30-day month, one of the lobes on the calendar program wheel (4) contacts the inner lobe on the short month rocker cam (6) as indicated at the red arrow. The rocker cam rotates slightly counter-clockwise and its outer lobe (6a) is brought into the path of the lobe (7a) on the extra advance cam (7). This contact is shown at the white and red arrow. As the 24-hour wheel (1) continues to rotate, cam 7a rotates the 31-tooth date wheel an extra tooth via the rocker cam, which is attached to the date wheel. This process takes about four hours. Note that, because of the offset between cams 7a and 2a, the "extra" advance (by lobe 7a) actually takes place before the "regular" advance (by lobe 2a). Thus, at the end of short months, the date change begins well before midnight and extends for about four hours.
Interestingly, the relatively simple switching function of the annual calendar is accomplished with a higher parts count than Patek's own perpetual: 316 compared to 275 for the perpetual. For the user, the annual's advantages over the perpetual lie almost completely in the superior legibility of the annual. While Patek touts the "entirely rotary" construction of the mechanism (and the lower power consumption during switch over), the penalty of such a design is a four hour switch over on short months.
One clear advantage of the annual mechanism is ease of service. Because the calendar program wheel and month star wheel share a common shaft, the only critical aspect of set up after disassembly is positioning the month hand on the common shaft to indicate the actual position of the program wheel. That done, all indications will be correct. Day-of-the-week indication must, of course, be corrected with the case pusher after assembly.
The pricing of the the annual (at about 65% of a Patek perpetual, Ref. 3940) is an interesting issue. The annual is probably no less expensive to produce than the perpetual. But its legibility and ease of use suggest that it will have a much larger market than the perpetual and that it will draw customers from the less expensive simple calendars like the Ref. 3998 (which retail for approximately 60% of the annual) rather than from the more expensive watches. Thus, if it is successful, the annual may well increase the dollar volume of Patek's total calendar watch production. While many would eschew the need for a true perpetual, the apparently simple and clearly sensible annual begins to make a true simple calendar seem a bit primitive.
If there is an anomaly in the "simple and sensible" theme of the annual calendar, it is the use of three case pushers for corrections. For example, the fully perpetual IWC caliber 375-7 allows all corrections though the rotation of the crown. Such a mechanism in the annual, however, would have required significantly more complication and pushed the cost beyond that of Patek's own perpetual.
As it stands, the Ref. 5035 is an extremely practical, legible, and elegant calendar watch at a fair price (as such things go). It is also, needless to say, impeccably built and finished, and it is supported by the best manufacturer-run service and parts organization in the luxury watch world.