sci.physics

I have an old clock (30 day windup) I want to power with a weight
that automatically raises a couple inches every 4 hours so I don't
have to wind it. The clock will run with a 8 ounce weight hung on an
arm extending about 6 inches from the drive wheel. The main drive
wheel moves about 180 degrees per day and there are 13 clicks on the
drive ratchet per revolution, so each click represents about 4 hours
and about 3 inches of distance at a radius of 6 inches. So, the
winding mechinism has to pull down 3 inches on the arm at 180 degrees
from the weight every four hours, or some combination of radius and
distance.

My idea is to construct a solonoid with a strong magnet inside that
pulls down on the arm, moves it about 3 inches and lifts the weight up
on the other side, but I'm not sure how to construct the solonoid. It
needs to be fast acting (1/2 second or less) so the clock doesn't lose
a second while power is removed. Seems like the work involved is about
1/3 watt for 1/2 second. One foot-pound per second is about watts,
so 1/2 pound and 1/2 second should be around 1/4 of that, or maybe 350
milliwatts ignoring losses. The battery will be 2 lithium cells in
series, or about 7 volts regulated down to 5. The pendulum will also
be synchronized to a small air core electromagnet controlled by a
quartz crystal oscillator for good accuracy.

So, the question is, how to construct the solonoid to do the job?How
may turns of wire, etc. to make it work? Or can I buy one ready made?

Thanks,

-Bill