The original wristwatch Starwheel was released by AP c. 1991, drawing its inspiration from pocket watch designs dating from as far back as 1800 (a Vaucher Freres signed piece with a cylinder escapement and quarter repeater) The original AP Starwheel was reputed to suffer from a few reliability issues in the field, including excessive sensitivity to abrupt movements, difficulty of the starwheel plates holding their correct positions, and the correct adjustment of the tension of the plates.
A large part of the difficulty of the starwheel design is the tight tolerances required for the innovative display system to function correctly - too tight or too much friction, and the whole display system seizes up; too loose, and the whole system runs the risk of slipping out of adjustment and position.
The basic idea behind the starwheel display system is the placement of the starwheel plate planetary mounting ring on the center staff, where the hour and minute hands usually go. (click on the picture to view a full frame jpg)
This planetary ring is geared to complete a full 360 degree rotation once every 3 hours, or, put another way, traverse an arc of 120 degrees per hour. Thus, as the planetary ring rotates, so too do the sapphire plates imprinted with the hour numerals.
On the watch dial, instead of a 360 degree, 12 hour calibration typical on standard watch displays, there is a printed arc of 120 degrees calibrated with 60 minute indices. As the sapphire plate travels over the printed minute arc, the hour indicator points to the current minute, and thus the time is read - the hour numeral, and the corresponding minute index indicated by the pointer.
Thus, the time indicated in the picture at left would be 3:44
At first, such a time display system would seem to be counter-intuitive and hard to read, but one grows accustomed surprisingly quickly, and soon after strapping the watch onto the wrist, one can read the precise time as quickly as the standard center minute and hour hand display system.
(click on the picture to view a full frame jpg)
As the mounting ring turns, the three sapphire disks travel with it, mounted on star gears, buffered by what look like teflon washers, to the mounting ring. The three sapphire disks are marked with four hour numberals each, in increments of 3 (disk 1=1,4,7,10; disk 2=2,5,8,11; disk 3=3,6,9,12).
Through a system of locking springs, advancing posts, and the star gear on each disk, the sapphire disks are calibrated and positioned so that the proper hour numeral is displayed against the minute indices on the printed 120 degree arc.
Because the planetary ring is in constant rotation, versus instantaneous or semi-instantaneous "jumps," there are a number of significant implications, both visual and mechanical.
First, the movement of the hour numeral across the minute arc is accomplished by the rotation of the planetary ring, rather than by some independent motion of the sapphire disk. Thus, at the end of the hour, the previous hour marker on its sapphire disk will leave at 59 minutes, at the bottom of the minute arc, as the next hour hour marker, on its own sapphire disk mounted opposite, moves into position at the top of the minute arc, or 0 minutes. This reminds me of the graceful synchronous dance of a well coordinated chorus line. With this system, there is no need for a spring or other torque loading device to engage or disengage at any time during the hour.
Second, because the planetary ring and the sapphire disks are in constant motion, without any "jump," there is the micro-mechanical problem of torque loading twice an hour, when first one disk at the normal 6 o'clock position, then the other disk, at the normal 12:00 position, need to be rotated to properly align the hour numeral on the sapphire disk for display against the minute arc. The amount of rotation is controlled by the fingers of the star gear under the sapphire plate. Here is where the micro-mechanics and physics get interesting -
Much of the physics and difficulty of designing complications are the additional loads placed on the power train whenever a complication is added and activated. This applies to something as simple as the basic turning of the hour, minute, and second hands, which because they are constant and designed in, are "part of the equation." As each additional "complication" is added, torque and stress loads are added, which must be calculated into the capacities of the power train and adjusted for in the regulation system. Thus, a date wheel will add additional loads to the power train when it is engaged; so would a moonphase plate, or the perpetual calendar plates, or a jump hour, and so on. In the case of a multi-complication like a perpetual calendar with moonphase, each of the functions - day, date, month, year, moonphase - is designed so that the engagement of each of the functions is staggered throughout the day. This avoids the overwhelming loads that would be presented if all functions were to engage at the same time, say at 12:00am, that would likely bring the power train and the regulation sub-system to its knees.
Now consider that with date and other classic complications, the marginal loads are presented to the power train at most once or twice per day (for the day/date, and moonphase), and less frequently for the month and year. The
available system torque and marginal load stresses are thus affected for short, relatively infrequent periods.
AP stock photos
Now consider the Starwheel display system. Besides the planetary ring, which itself is more massive than a standard minute, hour, and second hand, you have the three sapphire disks and assorted mechanical paraphernalia. These all present load conditions in the way of inertia that need to be overcome. Add to this the mechanical friction presented by the design, and the micro-mechanical dynamics
become really complicated.
Rather than engaging at most twice per day, like a moonphase complication, the disks need to be rotated and repositioned TWICE PER HOUR, first the lower one at 6:00 (at the lower fixed engaging post) then the upper one at 12:00 (at the upper fixed engaging post - see photo of star gear engaged with the post.) So 48 times per day, the power and regulation trains must
accommodate for the inertial loads presented by the rotating of the sapphire disks, and the frictional loads of that rotation on their own mounting staffs, as well as the resistive force of the
locking spring and the engaged star gear.
Suffice it to say, I marvel at the accuracy and stability of the Jubilee Starwheels I have, given all of the previous. They all three run from +4 to -2 on my AP winders (selectable uni-directional,) and +6 to -4 on my Underwood winders (non-selectable bi-directional.)
The Jubilee Limited Edition Starwheel in the Millenary case represents well the motto of Audemars Piguet as they celebrate their 125th Anniversary - 125 ans d'audace...
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