WatchTech

DonCorson
[AHCI]
3358

• Show profile
• Show other posts

Sign of the Zodiac Complication

A Sign of the Zodiac indication on a watch or clock is a complication that is not often seen.  Let's assume without discussion that it has some kind of value, so we want it.
Before making a Sign of the Zodiac complication in a wrist watch I wanted to be able to test it, so I made a Zodiac clock using the system I had envisioned.

What should the complication do?  Well the zodiac signs repeat once a year, so what is needed is a way to turn a disk displaying the signs once a year.  Turning once a year is once every 365 days or once every 8'760 hours or once every 525'600 minutes or once every 31'536'000 seconds or once every 126'144'000 oscillations of the balance wheel of my watch.

To keep the gear train managable it is obvious that I will not be using even the once a minute of the second hand as my time base, but the slowest turning wheel of the watch or clock.  On my pendulum clock the great wheel is the slowest turning, at one turn every 15 hours.  So using that as my time base the zodiac wheel needs to turn 8760h / 15h=584 times slower.  Simply dividing by 584 can't be that hard, can it?  

Unfortunately division ratios of much over 15 are not really practicle because of the size of the wheels.  On top of that, because of the big wheels the reduction needs a lot of real estate in the movement.  Of course perpetual calandars do this, but I want something simple.

What other possibilities do we have? Differential gearing?  Intermittant motion using a pawl?  Somewhere in the back of my head I remember reading about extreme gear reduction using wheels of slightly different tooth size and a planet gear.  This has two advantages, very high reductions are possible and the gears are concentric with the driving gear which would reduce the movement real estate necessary (while adding to the movement height).  

The idea of this reduction gearing is to have two gears of the same diameter with the tooth count differing by one tooth.  A planet gear runs around the outside of these gears engaging both of them.  If the gear with the larger tooth count is held still and the planet gear goes around it one full turn, the gear with the smaller tooth count will advance by one tooth.  Let's imagine that the reducing gears have 146 and 147 teeth.  The tooth size is slightly different so that the gears have the same diameter, the planet gear that engages both of them is given a little more play than usual so it has no problems with binding.  When the planet gear has turned around the reducing gears 146 times the rotating gear has turned one complete turn.  As such we have a reduction of (division by) 146.  146 just happens to be one quarter of the 584 we are looking to divide by.  I will now add a reduction by 4 to the drive of the Zodiac disk and all is well.

Let's take a closer look.  In the image below we see the two reduction gears, with 146 and 147 teeth, and the planet gear which is mounted on a small bearing so it can turn in the rim of the great wheel.  As the great wheel turns the planet gear engages in the reduction gears and turns so the teeth of the reduction gears are always aligned together at the position of the planet gear.



In the image below we see better how this looks.  The teeth are aligned on the right where the planet gear is positioned.  180° away at the left side we can see that there is a ½ tooth difference between the two gears.  With each complete turn of the great wheel the rotating reduction gear advances by one tooth.  We thus get our needed division by 146 in the turning speed of the wheels.





In the image below we see the rest of the zodiac complication, a 42 tooth gear on the rotating reduction gear, a transfer gear which is just to bring the zodiac indication to the front of the movement and the zodiac disk with it's 168 tooth external gearing.  The final reduction of 4 needed is in the 168/42=4.




In the image below we see how this added complication fits into the existing clock movement.
There was some added work to fit this into the movement.  For example the reduction gear and the zodiac disk need to be put on very large diameter bearings that fit around existing parts of the movement.  To this end I designed large diameter ball bearings.  As the loads are almost nonexistant 5 balls for each are enough to securely position the wheels.




And finally the finished clock on the wall.





Of course a complete test will take at least a year.  For a test each wheel must turn at least one full turn.  But for the time being everything is working very well.  

There are certainly those who will say that claiming a year is equal to 365 days is far from reality, (according to which definition?) the length being more or less 365.256363 days.  I don't want to think about it…. today.  That is work for another day, but actually  am thinking more about equation of time these days.

Site Info.