In this case study, we are examining a high-quality perpetual calendar mechanism. This is a unique mechanism - because once it is set, you don’t have to alter the date for 100 years! The mechanism we’re looking at here was in a watch that needed a good cleaning and overhauling to ensure that it ran properly once more – so this presented an excellent opportunity to share with you what goes on inside a watch with this kind of complex and sophisticated mechanism.
This is the date wheel with 31 teeth - and with a steel cam screwed to the wheel. The cam allows the operating lever to drop early and advance the wheel 1,2,3 or 4 teeth, depending on whether it is a shorter month than 31 days , i.e. 30, 29 or 28 days.
This wheel has seven teeth and represents day of the week.
This intermediate wheel also has 31 teeth and connects the date wheel and perpetual wheel. One tooth that is longer than the rest and only comes into contact on the 31st day of the month. This will advance the perpetual wheel one month.
The moon phase has 59 teeth and will revolve once every two months (this is why there are two moons). The dial this is made of gold and is enamelled. The cycle of the moon is 29 and a half days – though it is not possible to make half a tooth. After three months the phase would be out by one-and a-half teeth and would need to be continuously adjusted.
This is an operating lever, and has four functions.
The top claw shape changes the day wheel, whilst the middle claw drops into the perpetual wheel, which rotates close to this claw. The bottom two claws advance the month, whilst the longer of the levers will sit on the month wheel cam and drop early – and if the month is short, this will advance the wheel. The bottom claw will advance the month wheel on the 31st day only.
This is the perpetual wheel with 48 teeth and 48 divisions screwed on top. This rotates once every four years – and the area circled is for the month of February. There are also three other cut outs at the same depth; the division at the 1 o’clock position is slightly shorter, since it is the 29 days and leap year indicator. Usually, on the dial, there is a leap year indication - otherwise the calendar would need to be set to every February until the leap year is found. Then, to set the calendar correctly, it would be set from the previous leap year.
In this time-lapse video, you can see described levers in position and on the lift. It took six hours to complete a leap year cycle, so to save time we used the 30th day of the month. The operating lever with longer finger piece (or claw) has dropped early into the date wheel cam (see first image).
This has already advanced a tooth, and you can now watch as it slowly changes all the wheels - apart from moon phase, which usually takes place at midday. With all these watches, you will find correctors, which allow the user to advance the calendar; however breakages can happen if this is adjusted on change-over.
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