THE IWC PERPETUAL CALENDAR MECHANISM

IWC greatly surprised the watch-making world in 1985 with its introduction of the Da Vinci (Ref. 3750) at that year's Basel Fair. Here was an automatic chronograph perpetual calendar at about half the price of its nearest competitor. Additionally, it displayed the century and year in numeric display, and the calendar mechanism was of an entirely new design that allowed all its indications to be adjusted simply by rotating the crown. This perpetual mechanism was to become the most widely sold in history. Shortly after the release of the DaVinci, IWC was to produce a perpetual without chronograph, the Ref. 3541 Portofino Perpetual. This has, in turn, been followed the the introduction of the JLC Odysseus perpetual with the same calendar mechanism; the IWC Novocento (Ref. 3546); the JLC Master Perpetual (Ref. 140.240.802B); the IWC split-second version of the DaVinci (Ref. 3751); and, most recently, the IWC hand-wound Portofino Perpetual (Ref. 2050), with a movement thickness of 3.15mm, the slimmest perpetual calendar every produced.

The versatility of the IWC perpetual mechanism is attested to by the variety of base movements it has been used with. The movement used in the original DaVinci is IWC's iteration of the 7750, IWC calibre 79061. In the Ref. 3541 Portofino, the calendar mechanism is used on an ETA 2892 as the IWC calibre 375-7. The early rectangular Novocentos used the Piguet 953 as a base, while the current Novocento uses the JLC calibre 960. In the more recent Portofino hand-wound, the calendar is attached to the very thin JLC calibre 849 (itself only 1.85mm thick). In its two perpetuals, JLC has used the calendar plate with the 889/1 and 889/2 automatic movements as the calibre 889/440 and 889/440/2 respectively.

The IWC calendar plate is not only an entirely IWC-developed mechanism, it is unique in the history of watchmaking. Unlike all other perpetuals to date, the IWC calendar mechanism is a truly integrated design. Most perpetuals provide the four functions (date, day of the week, month, and moonphase) through essentially separate mechanisms, each of which may be driven separately from the base movement. As a result, resetting of the watch requires either three or four pushers ("correctors") to reset the calendar if the watch has stopped. The IWC calendar mechanism is attached to the underlying movement at only one point. A "calendar driving wheel" in the movement tilts the date ring (yellow arrow, Figure 1A) of the calendar plate by means of a lever and eccentric screw (neither visible in the figures.) The pivot point of the date ring in indicated by the red arrow in Fig. 1A. As will shortly be seen, this design allows the calendar mechanism to be advanced (but not retarded) by simply turning the crown, or, in the case of the JLC watches, a single pusher. As shown in Figure 1, the entire calendar mechanism is sandwiched between an upper and lower plate held by four screws. The entire assembly is called a "calendar block."

The calendar mechanism is show in Figure 2 set to February 28, one year after leap year in the four year cycle (this is considered the "beginning" of the four year cycle). The date ring (1) is moved from under the lower plate and pivots at the point indicated by the red arrow. The magenta arrows indicate the space allowed in the lower plate for the ring to rotate. The date ring merely pivots several degrees clockwise (down and right) on its pivot each midnight and then counterclockwise (up and left) back to rest position. This pivoting action causes the date click (1B) to advance the 31 tooth date wheel (2) by one day every midnight.

On the dial, the the date is indicated by a hand attached to the orange pivot of the date wheel (2).

The program drive wheel (4A) (and the program wheel (4) attached to it) make one counter-clockwise revolution every four years. They are driven by a single large tooth (Figure 3, purple arrow) on the clockwise-rotating date wheel (2), which catches a tooth on the program drive wheel at midnight on the 28th of each month. The date ring (1) has a tip at it furthest end (1A) that rests on the edge of the program wheel (4). It can be seen resting in the deep notch for February 28 on a non leap year. It is the "information" derived from how far the tip (1A) must rise out of the notch that determines extra advances at the end of short months. The further the tip must rise, the more extra advances are made. Extra advances range from none (31 day months) to three (non-leap year Februaries). This operation is accomplished by the following means.

As the program disk (4) rotates, the tip rises, forcing the date ring (1) to rotate further clockwise that it normally would. With reference to Figure 2, this rotation causes the additional click (1C) to further advance the date wheel (2) via the snail wheel (2B) attached to the top of the date wheel.

The details of the program wheel (4) are shown in Figure 3. The purple arrow identifies the single large tooth on the date wheel that drives the program drive wheel. The three green arrows on the program wheel itself indicate the slots for 28-day (non-leap year) Februarys. The red arrow indicates the slot for February of leap year. The orange arrow indicates the surface against which the tip (blue arrow) of the date ring (1) slides. The magenta arrow points to the pivot for the date ring (1). As the tip of the date ring rises out of a notch, it can be imagined that a deeper slot forces the date ring to rotate further than it normally would. A shallower notch causes less rotation. Thus leap year, which has a 29-day February has a shallower notch than other Februarys and provides only two extra days of advance at the end of the month instead of the 3 days required on non-leap years. The same principle applies to 30 day months (the shortest slots) and 31 day months (the tip of the date ring is resting on the top of a "turret" between two notches and provides no extra advance). Note that December and January both have 31 days and are represented by the turret indicated by the light blue arrow. July and August, also with 31 days each are at the white arrow. The tip of the date ring remains on top of each of these two turrets for two consecutive months.

As the program disk wheel (4A, Figure 3) rotates, it rotates the 12 tooth month wheel (8, Figure 3 and Figure 5) one tooth per month. The pivot (green arrow) of the month wheel (8) carries the hand indicating the month on the dial of the watch at the six o'clock position. Each time the month wheel makes a full revolution, the year display is advanced by one year. The month wheel has a pin (red arrow, Figure 5) which moves intermediate wheel (8D) by one tooth each year. The power flow from 8D to the year display itself is shown by the blue arrows in the diagram.

While the date ring is rotating the date wheel, it is simultaneously performing other functions. An oblong hole in the left extension of the date ring (blue arrow, Figure 6) operates the week-day switch lever (5) which rotates the seven-point week day star wheel (6). This action occurs each midnight with the rotation of the date ring. The week-day hand on the dial is, of course, attached to the pivot of this wheel. The week-day star wheel (like the date wheel and program wheel) is stabilized by a beautiful ruby tipped spring (6A and illustration right) which also prevents the wheel from falling between days.

The toothed wheel at the base of the week day star wheel drives the intermediate moon gear (7B) and the attached second intermediate gear (7A). The latter, in turn, drives the moon display disc itself (7). The moon position is displayed through an aperture in the dial at 12 o'clock.

The relative simplicity and reliability of this calendar plate, as well as the convenience of being able to operate it solely by rotation of the crown (or single pusher) have made it a very popular mechanism. As the description of the operation should make clear, simply rocking the date ring back and forth (with crown or pusher) will advance the entire calendar in synchronization. Should the watch stop, advancing the date (and corresponding month) to the current date will automatically correct the week day and moon phase.

This simplicity of operation, however, places a special responsibility on the watchmaker servicing the calendar block. After cleaning, all the elements of the calendar are laid in the bottom plate. The watchmaker must assure that every tooth is properly aligned, for once the top plate is screwed on, the entire gear train will be locked in that position. Any error of assembly will reduce the accuracy built into the movement, including that of the moonphase which, properly assembled, shows an error of only one day (behind) every 122 years. (The IWC's moon cycle is geared to 29.53125 days compared to the absolutely correct 29.53059 days. More accuracy would have required excessively fine gearing on the moonphase gear train).

There are a few other interesting observations about the IWC calendar mechanism and its use in various watches. In IWC's Novocento, the calendar block is rotated 90 degrees clockwise from its original position in the DaVinci. Dial displays are correspondingly rotated. In both of JLC perpetuals the calendar block is rotated 180 degrees, also rearranging the Da Vinci's original dial displays. Only in the very recent Ref. 2050 hand-wound has IWC returned to the original orientation of the DaVinci. The Novocento, as well as both of JLC's perpetual also omit the century indication. These variations have, in all likelihood, been done simply to vary the appearance of the watches. The JLC's uniquely include a useful 24 hour indicator around the central hands, indicating in red the time during which the calendar should not be manually adjusted. For approximately 1 hour either side of midnight the advancing pawls are engaged with the calendar wheel.

The calendar will function properly until the year 2499, though it will require an internal adjustment on March 1, 2100 (in which year the normally expected leap year will be skipped). Those watches with century display will also require a special service on January 1, 2200 to replace the century slide with one numbered 22, 23, and 24. Interestingly, those watches with the century display create one of the most extreme ratios in the history of watchmaking: for each movement of the century slide, the escape wheel has completed about 6.3 billion rotations and the balance has completed approximately 94.5 billion half-swings.

I offer my appreciation to Jack Freedman (whose company, Superior Watch Service, Inc. of New York is an IWC factory-authorized service center) for his help in clarifying some of the technical details of the calendar block. Jack is among the few DaVinci calendar experts in the U.S.

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