Knex Clock
https://www.youtube.com/watch?v=zvmOLhV_NYE
Brilliant design for the hand. Over comes the lack of correct gearing.
Differential: https://www.youtube.com/watch?v=rBfQeRWtpxQ
Sizing
The whole thing will be 7' tall.
I'm not sure I want to use corner connectors - I don't think I have enough. Instead, alternate yellows with corner bracing to hold the shape.
Hands
Rather than gears, we'll use rods for transmission. The minute hand will have essentially 2 teeth (two rods at 180°). That goes to a 3:1 reduction, which goes to an 8:1 reduction; for a total of 24. The 3:1 will be a little janky, but feasible.
Winding and Weights
I'd like to do all this with Knex chains rather than string - more authentic; but I'm afraid of how much friction that incurs.
For string, I think I'll use the mason line I have. Fishing line would hold it's shape, causing weird fluctuations in power.
The weights will drop down the center, I need to add a guide track.
The winding mechanism will of course go through a differential gear. Also need a ratchet for that to work.
Pendulum
As tall as the clock. The pendulum will actually rock in a cradle rather than spinning on a rod - lower friction.
A 6' swinging pendulum has a period of about 2.7s.
Torsion Tests
| Arm Length | String length | String Type | String Spacing | Weights | period |
|---|---|---|---|---|---|
| 1 red | 18" | mason | 1 connector | 1 wheel each | 16s |
| 1 red | 18" | mason | 2 connectors | 1 wheel each | 8s |
| 2 red | 18" | mason | 1 connectors | 1 wheel each | 32s |
| 2 red | 18" | mason | 1 connectors | 2 wheel each | 39 |
I had to do a little testing and adjustment to get this to work. I didn't even try single stranded, double through one connector was to finnicky to be practical. Double spaced with one connector seemed pretty solid. At 2 connector spacing, the loss seemed pretty high, I'm not sure if that could be maintained.
The 2-red, single spaced worked really well. Adding a shaft down the center with the spacer seemed to help even more.
Math
I think I can get 1 tick every 15 seconds to work. Also, we only have 2 large gears.
Gears teeth
- S 14 = 7*2
- M 34 = 17*2
- L 84 = 7*3*2*2
Ratios
- S to
- M: .412
- L: .167
- M to
- S: 2.429
- L: 0.405
- L to
- S: 6
- M: 2.471
1 tick is going to be the equivalent of an 8 tooth gear, so the escapement shaft will make a full rotation about every 120 seconds. So the minute hand needs to make 1 full rotation about every 22.5 escapement rotations.
That leaves us with two reasonable gearing configurations S : L-S : L, which gives us 1:36. Or S : L-M : L, which gives us 1:14.826. The other option would be to have the last stage be spokes instead of gears and do an 8:2, giving us 1:24. Yeah, let's do that.
Working backwards; the minute will make a full rotation for every 24 escapement rotations, so the escapement needs to make 150 rotations per hour. 8 ticks is one rotation, so 1200 ticks per hour.
Escapement
I've seen some good Arnfield escapements.
I'd really like to do an Anniversary (Torsion pendulum) escapement. It's Lego, but this is a good demo.
I may be able to combine an Arnfield with a torsion pendulum.