Sunday, November 20, 2016

Clock drive updates: Chain sprocket failure and ratcheting friction drive

The friction drive I devised earlier for my clock has generally proved to be somewhat unreliable.  The drive barrel ended up getting slippery, so I glued sandpaper around it to grip the drive cord.  This worked well enough to drive, but winding was then a problem.  Although you could release the friction enough to slip the cord upwards to wind, this slowly eroded the cord... and eventually the cord would break! 

Since the timer ratchet mechanism seemed to work so well, I figured that might be a good idea. But holding the friction barrel with a ratchet wasn't the first thing that jumped into mind for whatever reason, although in the end, it's what I've chosen to go with.  First, I embarked on an experiment to see whether I could use a chain drive with a ratchet mechanism.  I found some "sash chain" at the hardware store that was apparently rated for 35 lb, which is more than enough for the clock's weight of 10 lb. 

Oddly, no one seems to carry sprockets for sash chain, because it's really for window sashes, which don't actually need to be driven.  (There are sash chain pulleys, though.)  So I attempted to make my own sprocket, shown above, by cutting a barrel, drilling holes for nails and then pressing nails into the holes for teeth.  The nails were originally finishing nails that I cut to size using a cutoff wheel and a jig to ensure they were all the same size. 

Although the above picture is just an indication (don't spin with the nails installed!), I cut the barrel to a nice round circle on the lathe.  I find I'm using the lathe more and more to ensure accuracy of circles.  I find it helpful to grip a large bolt in the three-jaw chuck to use as an arbor for the work. 

Here's a picture of the mechanism in place, with a click spring and the chain installed.

Here's another picture with the clock assembled.  Notice the large gap to the left... if the winding barrel slips too far to the left, the click disengages and the weight falls! 

So, I installed a brass pipe...

... that keeps the barrel pushed to the right against the click.

Unfortunately, the mechanism was nothing but trouble.  The placement of teeth is way more critical than I imagined, and the chain kept jumping off the sprocket.  Initially, the chain would lift and jam on the gear above it, which was annoying, but not hazardous.  But as I got the teeth a little closer, the chain would jump off entirely, sending the weight falling.

So, back to the original friction drive...  Here are the parts: a ratchet, barrel, end cap, and brass pipe to keep the barrel in the right spot.

Again, I used the lathe to make perfect circles, and indeed to drill and bore all holes.

Here is the mechanism installed.  Time will tell if it proves reliable!