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Semi-instantaneous month-driving mechanism

Semi-instantaneous month-driving mechanism

1. General description

The indication of the current month constitutes an additional calendar complication. It is systematically associated, at the very least, with the indication of the Date / Calendar, of which it constitutes the natural extension. Together, these two pieces of information — the day number of the month and the name of the month — allow the user to read a complete date without resorting to an external reference. It is this complementarity that explains their joint presence in almost all calendar watches.

The indication of months is found mainly on simple calendars — also known as “triple calendars” — and on annual Date / Calendars, perpetual and secular.

2. The Gregorian calendar and its months: a fundamental irregularity

The Gregorian calendar, gradually adopted in most countries from 1582 onward, is the universal reference for calendar horology. It comprises twelve months of unequal length: seven months of 31 days (January, March, May, July, August, October, December), four months of 30 days (April, June, September, November) and one month of 28 days in a common year and 29 days in a leap year (February). This irregularity is the origin of all the complications of calendar mechanics.

From the strict standpoint of the month Indicator, this twelve-month cycle is perfectly regular: one month always follows the next, without exception, without a jump and without going backward. The month Indicator simply needs to advance one position at each month change, then return to January after December. In this respect, its logic is identical to that of the Indicator for Days of the week, which advances one step per day over a seven-day cycle.

The difficulty therefore does not lie in the month indicator itself, but in the coordination between the date indicator and the month indicator. It is precisely at this junction that the calendar complication takes on its full complexity. On a simple calendar, this coordination is entirely manual: the user must correct the date display — and never the month display, which adjusts when the date is corrected — at the end of every short month. On an annual or perpetual calendar, the movement manages this transition itself thanks to a mechanism with a cam that knows the length of each month.

It is important to note that the driving mechanism of the Month Indicator receives its Impulse not directly from the base Train of the Movement, but from the Date / Calendar Indicator itself (see the animation above). It is the passage of the Date / Calendar to the value 1 — that is, the first Day of the new Month — that triggers the advance of the Month Indicator by one position. This functional dependency is a constant feature of calendar mechanics, regardless of the type of calendar.

3. Mechanical principle of Month drive

In the vast majority of calendar Complications, the drive of the Month Indicator is provided by the Date / Calendar mechanism, and not directly by the base Train of the Movement. This design choice is logical: the change of Month is inseparable from the transition to the first Day of the following Month. It is therefore natural that it is the Date / Calendar mechanism which, upon its return to the value 1, transmits an Impulse to the Month mechanism.

This Impulse is generally transmitted by a Finger or by a Cam integral with the Date/Calendar Disc or the Date/Calendar Star (wheel). When passing from the value 31 (or 30, 29 or 28, depending on the Month and the type of calendar construction) to the value 1, this Finger advances the Month Display by one step — a twelve-month Disc or Star (wheel). The Neck chain (sautoir) of the Month mechanism then holds the Display in a stable position throughout the Month, i.e. from 28 to 31 Days depending on the Month in question.

This architecture places the Month Indicator in functional dependence on the Date/Calendar mechanism. This means that any manual correction of the Date/Calendar — for example when winding after a stoppage — must be coordinated with any correction of the Month. If the user manually advances the Date/Calendar from 31 to 1, the Month Indicator will automatically advance by one step, as if a genuine Month change had occurred. This coordination is generally transparent to the user, but it requires particular attention during time-setting operations after a prolonged stoppage of the Movement (notably on annual calendars and Perpetual calendars).

The force required to drive the Month Indicator is generally low. The Disc or the twelve-position Star (wheel) changes position at most once a month, making it one of the least stressed components in the entire Movement. However, the design of the driving Finger and the associated Neck chain (sautoir) must account for the variable duration between two consecutive changes — from 28 to 31 days — during which the Neck chain (sautoir) must keep the display component firmly in position.

4. Display: Disc and Hand

As with the Day of the week, the Month Display takes one of two main forms: the Month Disc where the Hand, depending on whether the Display is provided by a appearing in a Aperture / Window or by a Hand on an offset Dial.

The month disc is a flat ring bearing the twelve names or abbreviations of the months of the Gregorian calendar, inscribed at regular intervals around its periphery. It is positioned beneath the Dial, in the peripheral area of the Movement, and reveals only a single indication at a time in the Aperture / Window. Its design is identical to that of the Date / Calendar disc or the day-of-the-week disc, with twelve positions instead of thirty-one or seven.

When the Display of the months is entrusted to a Hand, the pivot of the twelve-toothed Star (wheel) carries it directly. Each arm of the Star (wheel) corresponds to a month of the year. The drive Finger, attached to the Date / Calendar mechanism, rests against each arm in succession, causing it to rotate one-twelfth of a turn each time the first of the month occurs. The Axis of the Star (wheel) is attached to the months Hand, which indicates the current month on a graduated or inscribed secondary Dial.

In both cases, the twelve-tooth star wheel is held in position by a neck chain (sautoir). This spring, ending in a rounded tip, engages in a notch of the twelve-tooth star wheel. The tension of the neck chain (sautoir) ensures the stability of the indication throughout the month, which can last up to 31 days. The design of the neck chain (sautoir) must therefore offer sufficient resistance to maintain positioning against shocks and vibrations, while remaining compatible with the force available for driving it.

5. Display by disc with aperture / window

5.1 Arrangement and reading

Displaying the month by disc is the most common solution in the watchmaking industry. The aperture / window concerned is generally larger than that of the date / calendar, since the names of the months — even abbreviated — require a wider display space than simple digits. Its position on the dial varies depending on the layout chosen, but it is often placed at 12 o’clock, symmetrical to or close to the date / calendar aperture / window when both indications coexist.

The inscriptions on the month disc come in different forms. The three-letter abbreviations — JAN, FEB, MAR, APR, MAY, JUN, JUL, AUG, SEP, OCT, NOV, DEC — are the most common. They offer the advantage of good legibility, even in a small-sized aperture. The full names of the months, reserved for pieces with large dials, require a wide aperture and carefully chosen typography.

5.2 The twelve-month cycle and the return to January

The month disc has exactly twelve positions, corresponding to the twelve months of the Gregorian calendar. After December, it automatically returns to January to begin a new annual cycle. This return presents no particular mechanical difficulty: it takes place in the same way as any other month change, through a finger pushing the next tooth of the disc, followed by it being held in position by the neck chain (sautoir).

Unlike the Date / Calendar disc — which must be manually corrected at the end of every short month in a simple calendar — the month disc is never manually corrected under normal use (except after a prolonged stop). The advance of the month disc is always triggered by the Date / Calendar passing to the value 1, whether this passage is automatic (during operation) or manually caused by the user (during quick corrections). In the latter case, the user only acts on the Date / Calendar: it is the mechanism that, when the Date / Calendar disc returns to 1, automatically triggers the advance of the month (except in the case of a prolonged stop beyond a month).

6. Hand display

6.1 The hand on offset dial

Month display by Hand is a classic alternative to the aperture disc. A dedicated hand moves across a circular subsidiary dial, on which abbreviations of the twelve months are inscribed. This subsidiary dial is generally positioned at 12 o’clock, at 6 o’clock, or laterally, depending on the composition of the main dial and the presence of other complications.

The hand makes one complete revolution of the subsidiary dial in twelve months, advancing one twelfth of a revolution with each month change. This display mode is particularly well suited to large-diameter watches, where calendar indications are grouped in separate sub-dials. The legibility of the indication depends directly on the diameter of the subsidiary dial and the care given to the typography as well as to the divisions of the subsidiary dial.

From a mechanical point of view, the month hand is driven by the star (wheel) of twelve, whose axis is integral with it. The finger driving the calendar / calendar mechanism rotates the star (wheel) and, with it, the hand, by one twelfth of a turn each time it passes to the first of the month. The neck chain (sautoir) keeps the assembly in a stable position between two consecutive changes. The precision of the angular positioning of the star (wheel) of twelve is particularly important, as a hand poorly centred on the graduation of a small secondary dial results in a visible reading error.

6.2 The retrograde display

The retrograde display of the month is a particularly spectacular variant of hand indication. Rather than making a full turn on a dial, the hand progresses linearly or in an arc from January to December, then instantly returns to its initial position when passing from December to January.

7. Month indication in the different types of calendars

7.1 The simple calendar (triple calendar)

In the simple calendar, the mechanism has no knowledge of the length of the months: it advances one position each time the Date passes to the value 1, whether this passage is natural (at the end of a 31-day month) or manually triggered (quick correction) by the user (at the end of a 28, 29, or 30-day month). The displayed month therefore faithfully follows the Date, without any automation of the calendar correction.

The user must manually correct the Date at the end of each short month. During this correction, the mechanism automatically advances the month Indicator as soon as the Date returns to 1. If the user advances the Date from, for example, 29 to 1 at the end of February, the month Indicator will automatically advance from February to March when the Date Indicator jumps from 31 to 1. The month correction is never performed directly: it always results from the correction of the Date.

7.2 The annual calendar

In the annual calendar, the Month Indicator plays an active role in the automatic month-end correction mechanism. The mechanism has a programming Cam that encodes the duration of each Month — 28, 30 or 31 days. It is this Cam, driven by the Month Indicator itself, that determines the moment at which the Date / Calendar mechanism must trigger the automatic switch to the first of the following Month.

This interdependence between the Month Indicator and the Date / Calendar correction mechanism constitutes one of the most subtle aspects of designing an annual calendar. The two mechanisms are mutually dependent: the switch to the first of the Month causes the Month Indicator to advance, and the position of the Month Indicator determines the triggering of the automatic switch to the first of the following Month. This circularity requires precise design and careful Regulation / Adjustment to ensure the consistency of the system throughout the annual cycle.

The annual calendar automatically handles eleven of the twelve month-end transitions. Only the end of February — whose length varies from 28 to 29 days depending on the year — requires manual intervention. During this correction, the user advances the date from 28 or 29 to 1, which automatically triggers the advance of the month indicator from February to March.

7.3 The Perpetual calendar

In the Perpetual calendar, the month indicator is one of the central parts of the mechanical program. The perpetual mechanism encodes the length of each month for the four years of the leap-year cycle, i.e. 48 distinct pieces of information (12 months × 4 years). This programming is generally achieved by means of a 48-position cam or by a cam and differential lever system, which changes the length of each month according to the position of the month indicator and that of the year indicator within the four-year cycle.

The month indicator of the perpetual calendar is therefore both a display component — it indicates the current month — and a programming component — its position determines the behaviour of the date mechanism. This dual function makes it one of the most critical elements for the accuracy of the perpetual calendar. A positioning defect of the month indicator, even a slight one, can cause an error in the management of month-ends, with consequences on the date display.

A correctly adjusted perpetual calendar requires no manual calendar correction for 400 years, with the exception of non-leap centennial years. In these years — multiples of 100 but not of 400, such as 1900 or 2100 — 29 February does not exist according to the Gregorian calendar, whereas the ordinary perpetual mechanism provides for one. The month indicator must then be manually corrected to move from February to March on 28 February of these exceptional years.

7.4 The century calendar

The century calendarintegrates, in addition to the mechanisms of the perpetual, the automatic management of non-leap secular years. Its mechanical program covers a 400-year cycle, which allows it to manage, without any human intervention, all the exceptions of the Gregorian calendar. The Month Indicator, in this context, is part of a mechanical program of unparalleled complexity in mechanical horology.

In these exceptional pieces, the programming Cam associated with the Month Indicator encodes not only the duration of each Month of the four-year cycle, but also incorporates the secular correction applied every hundred years. The mechanical realization of this program, on a Cam or on a system of stepped Cams, constitutes one of the most demanding challenges in watchmaking design. The Month Indicator is, even more so than in the perpetual, both a programming component and a Display component.

8. The drive mechanisms for Month change

The drive mechanism for the Month Indicator follows the same general principles as those of the Date / Calendar or the Day of the week. Two families of mechanisms coexist: semi-instantaneous mechanisms and instantaneous mechanisms. The choice between these two families is generally dictated by the overall design of the calendar module, the two types of mechanisms rarely being combined within the same Calibre.

8.1 The semi-instantaneous mechanism

In the semi-instantaneous mechanism, the Month change takes place in two phases. During the first phase, which begins the moment the Date / Calendar Disc reaches the value 1, the Month drive Finger progressively pushes the Display — twelve-toothed Star (wheel) — toward its next position. This transition phase is generally very short, as it is not spread out over Time as with the Date / Calendar (whose drive can take several minutes around midnight), but instead occurs during the jump Time of the Date / Calendar itself.

The second phase is instantaneous. When the beak of the sautoir crosses the tooth or the next branch of the display component for the months, it releases the accumulated energy and abruptly positions the star (wheel) in its new position. This phase is imperceptible to the naked eye in most designs, as the entire transition — from the start of the drive to the final positioning — occurs during the brief instant of the date / calendar jump to value 1.

8.2 The instantaneous mechanism

Some calibres adopt an instantaneous mechanism for the month change, the principle of which is similar to that of the indicator for the days of the week. An accumulator spring is progressively wound during the date / calendar jump time, and its energy is released instantaneously to trigger the month change in a fraction of a second. This solution guarantees a clean and unambiguous month change, but requires that the date / calendar mechanism be able to provide additional energy.

In calibres with simultaneous and instantaneous change of all calendar indications — date, day of the week and month — the design of the mainspring must be particularly careful. The power spike resulting from the simultaneous release of several accumulator springs can be significant and must remain compatible with the barrel’s capabilities at the end of the power reserve.

9. Regulation and correction of the month indicator

The month indicator is equipped with a correction system allowing the user to manually adjust the display. This correction is necessary after each prolonged stoppage of the movement, during the initial regulation of a new watch, or when taking back in hand a piece whose calendar has drifted.

The correction method varies depending on the calibre. In certain calendar watches (notably simple calendars), the month has no correctorindependent. Correcting the month is done indirectly, by advancing the Date / Calendar until it reaches the value 1, which automatically causes the month to advance by one position. To reach the desired month from the displayed month, the user must therefore advance the Date / Calendar as many times as necessary, each pass through 1 causing the month Indicator to progress by one position.

Nowadays, most Movements, particularly Perpetual calendar and secular calendar Complications, have one or more independent correction Pushers for each calendar indication. A Pusher dedicated to correcting the month then allows the month Indicator to be advanced directly by one position, independently of the Date / Calendar.

As with all calendar Complications linked to the Date / Calendar mechanism, manual correction of the month Indicator should never be carried out during the operating window of the drive mechanism, which is approximately between 10 p.m. and 2 a.m. During this Period, the drive Finger is in contact with the Date / Calendar display component, and any manual intervention risks damaging the drive parts or the Neck chains (sautoir).