Next clock?

I have several ideas for clocks I'd like to build next...  Here they are in no particular order.

• A torsion pendulum clock.  Maybe with a pivoted detent escapement (see below)
• A gravity escapement.  Not sure which kind; there are several good options.
• A clock with a conical pendulum.  (This doesn't seem like it ought to even work... though of course it apparently does.)

Aesthetics and adjustments

Since I started constructing the clock, I imagined cutting out the front plate to reveal the gears.  Here's a sketch of where to cut, drawn by hand on a tracing of the front plate.

I cut out the paper plan and traced the edges on to the plate itself.

This was easily cut out on the scroll saw and finished with fine sandpaper.

The clock with the cutout front plate shows the gears as desired...

But the weight pulleys were still held on with wire.   Aside from looking sloppy, the weight has a tendency to be very close to the pendulum, sometimes colliding with the pendulum.  I added two small, but sturdy wings to hold the pulleys farther out.  This was easy, and keeps the weights clear of the pendulum.

Regulating

Now that I've been running the clock for some time, it looks like it was running consistently fast.  Disassembling the clock and examining, I found that

• The pendulum rod weighed 200g
• The pendulum rod (from arbor center to end) was 43"
• The pendulum bob weighed 100g

Putting this together, the farthest the center of mass could be from the arbor is about 29".  That's far shorter than the nearly 39" it should be.   Tweaking these values a bit, it looked like I needed to reduce the weight of the rod to around 50g and increase the bob weight to over 300g in order to be able to make a band of adjustments around the correct length.

Given that direction, I used a router to grind down the pendulum rod to quite bit thinner.  This took off about 100g, but also made somewhat of a mess.  Fortunately, I could make this the back so it's not so obvious.  I then tapered the rest of the rod using a plane.  In the end, the pendulum rod weighed 60g and is quite delicate.

I also cut two more circles of plywood to add additional weight to the back of the pendulum bob -- now it is 325g.

Given those two changes, I found that the clock had difficulty running.  Earlier calculations suggested that this would happen, because more lifting force needs to be applied at the anchor.  Therefore, I very carefully filed back the anchor teeth on the escapement to reduce the lift.  I may have overdone the exit tooth, but after applying more Slip-It to the gear teeth and pivots, the clock runs. 

After these changes, with the pendulum bob roughly in the middle of its range, the clock actually runs slow by about a minute every hour.  That's a success: I can now adjust both faster and slower as needed.  So now it remains to slowly bring it into regulation over the next few days.

Edwin's relay latch circuit

My son Edwin loves Elenco Snap Circuits.  He has many, many components, and usually builds circuits that are not in the books that come with the kits.  Recently, he's been playing with relays, I think because he likes how they control things.  For instance, about a week ago, he built an oscillator by putting the relay coil in series with the normally closed contacts.

Yesterday, he discovered relay latches.  He did so in an odd way, by building the circuit into his Snap Rover.  He used the rover's motor as a generator to drive the relay coil.  Once the coil activated, the normally open contacts supplied an alternate path for the battery to run the motor.  The effect is that you can push the rover to start it, and then it continues along its way.

Here's a different version of that circuit that he built today, where instead of the motor he used a lamp.

His layout is a little confusing, so here's a scan of the circuit diagram I drew.

As the diagram indicates, there are two power sources: one (on the left) that drives the light when the relay is deactivated (S1 is open), and the other (on the right) that both controls the relay and supplies an alternate source of power for the light.

There are a few nonstandard things about this circuit even though the latch circuit is completely standard (he's never been told about it before).  The two power supplies could be combined by just using the one on the right.  More surprising is that the lamp is in parallel with the relay coil; usually a designer would place them in series.  I think this is because of Edwin's design history.  If you replace the lamp with a motor (removing S2 and the 4.5 V source), then the circuit sustains the motor once it's been pushed.  Although I didn't draw the diagram for his previous circuit, I suspect that is what he had done.

Wooden clock #1 lessons

1. Make the pinions have more than 6 leaves.  Pinions with a small number of leaves are more difficult to cut, have more friction, and just cause more trouble.  Cutting extra teeth on a larger gear is actually easier
2. Be a little more careful with computing gear ratios, especially involving the action of the escapement.  Although I got the minute hand wrong, I managed to make a second error in the motion work so the hour hand runs properly
3. Make the pendulum longer than you'd expect to need -- you can always trim it back
4. Although it seems unbalanced, consider putting the winding barrel at the top of the mechanism, to give more room for weight drop.
5. Connecting the anchor without a crutch assembly works, although tolerances are probably improved by using one.
6. Along those lines, mounting the pendulum in a pivot also seems to work but probably contributes excess friction
7. Slipit!

Frictional winding

Due to the fact that the clock seems to need about 10 lb of weight, it seems like the endless chain mechanism I had intended to use probably won't work.  In particular, it basically requires doubling the drive weight, which doesn't seem like a good idea.  So instead, I've opted to try a simpler mechanism for winding.  Merely winding a few turns of cord around the drive pulley seems to give quite a bit of friction -- enough to hold the drive weight with a small amount of force on the other end. 

Specifically, it looks like if the drive weight is the 9 lb 8 oz from before, a 7.5 oz counterweight seems to hold it in place.  In this case, since there are two weights that need to be held clear of the pendulum and each other, I've used a pair of pulleys to run the cord.

This seems to work well enough, when the drive weight runs down, you can lift the drive weight and gently take up the slack with the counterweight.

Pins and slip-it

Now that the clock "runs," I've been tuning it so that it runs well.  The largest apparent problem was that that the anchor slipped on its arbor.  This meant that sometimes the clock would run with an even beat, and then lose this...  So to fix the problem, I made a small pin, like the one shown below:

In the end, the pin shown here didn't grip well, so I turned a second one that was tapered.  Then, with the anchor in place, I drilled through the anchor into its arbor and gently hammered the pin in place.

This helped with keeping the beat even, but the clock still was stopping abruptly.  After talking with my father, we settled on lubricating the teeth.  Nearly all wooden clock enthusiasts argue strongly against such a move, most wood lubricants or finishes turn hopelessly gummy and become hard to remove.  However, one caught our attention: slipit.  They sell two kinds, a wood-working kind (without silicone) and a "mechanics" kind (with silicone).  The reviews indicate that the kind without silicone doesn't work well, but the kind WITH silicone seems to work better.

So we settled on the stuff with silicone, and I applied it to all gear teeth with a small paintbrush.  After putting on a frightfully large weight (9 lb 8 oz), the clock has worked absolutely smoothly since!  Amazing!

Now, I did notice something odd.  Although the minute hand seems to run at about half speed, the hour hand runs at the right speed.  Weird.  I guess I made TWO gear ratio calculation errors...