Perpetual Calendar No.3

 Yet another perpetual calendar. This one is a design by Dr. Ludwig Oechslin of the ochs und junior watchmaking company in Switzerland.(Yes, that really is the name).  It is quite different from other designs of perpetual calendars in that it works on a geared principal rather than having many levers and ratchets. It is also simple in that it  only adds 9 extra parts to a base watch while others have  many tens of parts. It is also easy to set using the crown of the watch with no extra pushers involved, and can be rotated backward or forward through the dates.

A demonstration model of the calendar

The Program Wheel

The heart of the calendar is the Program wheel  (PW) consisting of a set of stacked  gears, all of which have one or more teeth that are longer than the others. The PW rotates once per calendar month and particular teeth (here coloured) pop in and out to aid its action.

The wheel at the bottom with the four coloured teeth is the 24hr wheel and is rotated once per day clocwise by the watch mechanism. Every day at midnight it interacts with the PW using its black tooth and the PW is moved on one day anticlockwise. The PW has 31 teeth and is always turned at least one tooth per day and sometimes more.

Taking the cover off shows the single white tooth in the centre of the PW. This  is fixed, and the PW rotates around it. The single tooth interacts with the 8 tooth gear next to it and turns it two teeth every time it passes. When it is not turning it locks the 8 tooth gearl

The month wheel has now been added. It has 24 teeth with 5 longer ones, here painted red, for the months with less than 31 days. This wheel rotates 2 teeth per month, turned by the 8 tooth wheel and thus goes round once per year. On 30 day months the red teeth are protruded as they pass the 24hour wheel. Interacting with the red tooth on the 24 hour wheel at 11pm and then the black tooth at midnight the PW is turned on two days to complete the month.

On the underside of the month wheel is a single white tooth which once a year turns the first small white gear by two teeth. This in turn  drives the February 28th wheel that has 3 long teeth that protrude for three years running on a normal leap year. These wheels go round once every four years. 

Now is added the Feb. 29th wheel which rotates slightly off centre of the month wheel but is driven by the same 8 tooth gear as the month wheel. It has a single long tooth, here coloured blue, that protrudes as it passes the 24hr wheel in February but is retracted otherwise. As the 24hour wheel rotate first the blue teeth touch and move the PW wheel on one at 10pm, then the red at 11pm and the black at midnight thus moving the PW on 3 days in a leap year with 29 days.

 On a February with only 28 days a long  white tooth on the small wheel is extended together with the blue 29th and the red 30th and of course the black on the base. The sequence starts at 9pm on the 28th with the white teeth and moves four teeth on in one night to complete the month.

With the cover on it can display the date directly or the PW can be linked to some other display of date . The tooth on the cover can trip over the month wheel and the 24hr wheel can drive a day display.

The video below  shows the operation of the device. I think we have to  agree it is very innovative design yet simple in form and perfect in function. I salute the genius of Dr. Ludwig Oechslin.