Back Up Next

Calendar remontoire and demonstration overdrive safety clutch - March 2015

In this segment Buchanan begins the fabrication of a remontoire that will power the entire calendar mechanism. Why do we need a remontoire to do this? Well one reason should be obvious, I truly like these mechanisms as they are applied to clockwork. So far this project employs two of the three remontoire commonly used  in clocks. First is the gravity driven train remontoire, of which there are many types. We use Bernard-Henri Wagner’s design c.1840 in the time train. Next is Christiaan Huygens endless chain remontoire, 1662 in the celestial train, (also commonly known as the Robert Robin remontoire who popularized this device as a way to compensate for error in spring driven clocks in 1772). Used here is the third and possibly earliest, the spring remontoire and was invented by Jost Bürgi, c. 1600 in his second experimental clock designed for astronomical observations. It is the spring remontoire that will power the calendar.

But aside from my interest in these devices we need a remontoire to provide a periodic power supply to the calendar mechanism. The calendar is tripped once daily and needs a stored energy source to provide more power than can be delivered in real time by the slow moving celestial train. The remontoire allows the celestial train to wind up its spring over a twenty four hour period and then releases this energy to turn the calendar train of wheels at once. In conventional calendar clockwork the dials are tripped via a lever that engages a set of pins on the rim of a star wheel and upon which is mounted the dial hand. The star wheel is held steady by a spring, usually with a roller attached that engages the star wheel teeth to keep it in position until it is turned and then is held steady again by the spring into the next space on the star wheel until pushed again by the wheel’s pin. In this way the hands on the calendar ‘click over’ or jump in discrete increments. A typical calendar is driven first by the indicator that turns most frequently, normally the date of which there are between 28 to 31 positions per month, and then this is carried through to those indicators that turn more slowly, the day, the month and then maybe the season or even year.

Of course things get quite a bit more complicated when one wants to correctly calculate the proper trip of the date given the differing days of the month, the leap years and the 400 year correction cycles. All of which is dealt with in the months prior this one.


Here Buchanan has made an innovative design for the clicks on the remontoire flies for the calendar function. He wants to create a ratchet effect that does not employ the use of a spring for drag. This will increase efficiency of the design. At this small scale energy becomes an issue and efficiencies are needed to make sure that there is enough power throughout the very complicated calendar assembly as supplied by the finite energy provided by the spring remontoire.


The pawls have evolved from an organic shape to a more symmetrical design. The wheels also have fewer teeth, seven verses twelve as shown above. There is a pair of ratchets for each of the two reversing fly drive wheels in the remontoire. The pair is needed since the fly fans as well as remontoire must be able to work in both forward and reverse since the calendar must work properly in both directions.



The ratchets are now beginning to be set in place on the remontoire reversing wheels.

Buchanan had to add another plate to hold the center bearing for the fly fan wheel pair, giving the calendar remontoire a triple plate. This will be later fretted out to just a small bridge.


Now Buchanan experiments with various springs to give the proper torque to the remontoire. A balanced pair must be chosen for the forward and reverse functions.

The barrels are made for the pair of remontoire springs. Note the ‘C’ shaped slot on one of the barrel covers. This allows for the half turn needed for the safety overdrive clutch first designed and illustrated in the December 2014 installment.

Now begins the fabrication of the overdrive safety clutch.

To understand how this safety clutch works look at a video of the working mockup of the safety overdrive clutch which was was made in December of 2014. The first three photos above show the fabrication of the inner cage that turns inside the outer case which is shown in the next two photos. The last photo is the two-start spiral mounted to the outer case that will direct the movement of the safety mechanism. For those who have experience in machining, they will recognize the difficulty of making this type of two-start spiral.

These photos show the armature upon which a roller will be mounted on each end. Notice the decorative machining in the third photo as compared to the second on a part which will largely be buried within the final mechanism and would have worked perfectly as well unrefined.


The next two photos show the armature and the rollers which will travel along the length of the inner rotating cage.


Here we have one of the two spring barrels, each controlling forward and reverse, next the pair are shown back to back. Note the detail of the barrel covers; each is rounded with a decorative reveal.

The safety over speed clutch is complete. As the inner rectangular cage turns in relation to the cylindrical outer envelope, the silver armature within the inner cage is driven leftward away from the wheels in the background through the action of rollers running within the spiral ring mounted to the external cylinder. The armature is driven in the same rearward direction whether the inner cage is rotating clockwise or anticlockwise relative to the outer cylindrical envelope. This withdraws the remontoire detent cam allowing the detent to spin past the cam, avoiding damage in the event the operator tries to crank the demonstration drive too quickly. This and the photo below was taken prior to the decorative machining of the spring barrels described above.

This photo shows the overrun safety clutch in place within the machine. It has filled in nicely one of the very few remaining vacant pieces of space between the main frames.


These photos show the beautiful serpentine remontoire detent. This is milled from one piece. The thin curved ends were not soldered or otherwise attached to the central hub. First Buchanan milled the part from a solid block to obtain the full depth for the hub. Then he flipped the part over and secured it with auto body repair material like Bondo. Once the adhesive set, he milled the final thin width for the arms. 


Here is the calendar remontoire detent along with its four armed latching cam. The detent is released once daily, so the cam turns once every four days.


A component for the calendar output dial work to be fabricated next month.

Begin work on decorative veneering for clock stand.

Here the first sets of flame mahogany veneer are assembled. Buchanan will be doing this work as he proceeds with the rest of the clock. I think it will be better to consolidate this work into a separate set of presentations rather than intersperse it within the rest of the body of work on the clock. In this way the case and stand can be presented as a separate and unified project. I show this photo to let the viewer know this will be is proceeding simultaneously with what is otherwise being presented and that it will be detailed at a future date.

Back Up Next