Gears for kinetic scuplture?

[Dork]

I'm planning on building a simple kinetic sculpture over the next couple of months that will be powered by a bike and hopefully by a windmill. It will be sort of like a clock mechanism, only with 2:1 gearing between hands.

My first thought is to use bike gears but I want 8 hands so that means a lot of bikes and chains to collect and assemble.

Has anyone else come up with some alternate playa-worthy gearing? What about wooden gears?

[MikeVDS]

I don't completely understand your design but you could use peg gears. They are much easier to make than what people usually think of when they imagine gears.



From what it sounds like you may have to fabricate your own gearing. If chains and sprockets sounds like a pain the peg gears might work (can be made of wood dowels and can look cool too if exposed). You could use pullies and belts which end up the same as chains basically.

[axolotl]

I love those wooden peg gear mechanisms, and I have a kinetic sculpture mechanism floating around in my head that uses them.
If they're good enough for Leonardo, they're good enough for me...

The problem I'm stuck on is a wooden escapement to go with them. Perhaps "escapement" is not the exact term I'm looking for, but it gives the general idea.
Something that turns a constant downward pull (as from a heavy weight- think of a grandfather clock) into a slow back-and forth rhythmic rotation.
Without dropping the weight too far or hammering itself to bits when it changes direction.

[geekster]

http://www.bhi.co.uk/hints/clock.htm

[Dork]

Yeah Mike - there's a similar inline gear you can make using one peg gear and another one with matching slots. I was worried about the wood splitting but I guess there's no reason I couldn't just use steel. Has anyone seem them actually being used on the playa?

[MikeVDS]

Why are you worried about wood? That playa is not *that* harsh. My wood chairs seemed to do fine all week. You can make gears that should be strong enough for what you're doing (unless you want to make REALLY heavy clock hands). Humans on bikes can produce from around 1 to 4 HP for an extended period, so using that as a starting point you can figure out how much stress you'll put on your gears. If you plan on sitting it out all year long and use it for years then you could use a protective sealant or just use metal. Wood is much cheaper and easier to work with. Also if it's a large structure wood may be significantly lighter and cheaper.

[Bob]

Yeah, my wrist hourglass is only about five pounds.

[MikeVDS]

^ If you created that structure out of a metal it could be closer to 20.

[Bob]

Don't understand your wood v steel comparison. Nobody builds wooden bikes unless they want them to float, or burn.

In any case, rub a few coats of varnish on those wooden gears so you can dust them off easily.

[MikeVDS]

Actually lots of people around the world build bikes out of wood. There are probably a number of reasons we don't see commercially made wooden bikes in industrialized countries. Steel is easy to mass manufacture reliably, and makes nearly indestructible bike frames for normal use.

Obviously different materials have their places, I'm just defending wood here because he originally wanted to go with wood and seemed to change his mind because he's worried about it out on the playa. Really knowing what we know from this thread there is no reason I see to not use wood if it will work better. Depending on the exact application he'll have to choose the best material. If he wants it small, metal will probably work best, if he wants it large and artsy then wood might make for easier transport.

[Dork]

Why are you worried about wood? That playa is not *that* harsh.

I'm not concerned about playa dust damaging the wood, I'm worried about it splitting under the stress or something shifting and kinking the works. I'm not finding many people using wood gears for anything other than clocks, which are very low stress.

As for the size, I'm imagining the smallest arm (moving the fastest, driven directly by the rider) about 4 feet long, the longest and slowest being about 8 feet long. The gears need to be sized to reliably support them. Sorry it's hard for me to describe.

[Zhust]

Has anyone else come up with some alternate playa-worthy gearing?

Wow ... 8 hands ... so will each subsequent hand be 1/2 speed of the last? That might get complicated.

Regardless -- you may wish to consider bicycle wheels -- say, 12" kids wheels to 24" teen-bike wheels. Setting up a soft rubber coupling might be easier because it could be as imprecise as chains (compared to metal gears) but with fewer moving parts. For that matter, bike wheels can rub against wood, pipes, or other (round) materials for different ratios.

Humans on bikes can produce from around 1 to 4 HP for an extended period

Whoa there Trigger -- a human on a bike can typically maintain about 150 watts or 1/4 horsepower or so for extended periods. With training you can get into the 1/2 horsepower range, and the world-record peak output is something like 4,000 watts (about 5 horsepower) but that was just for 30 seconds.

Remember, though, that the torque can be pretty high. A 200-pound person standing on the pedal with a 9-inch crank yields 150 foot-pounds of torque. If they are in a position to jump on it or push from a sitting position, peak might be something like 600 pounds for 450 foot-pounds of torque. Of course, at 80 rpm (a typical pace for a cyclist) with a 200-pound person and a 9-inch crank arm is about 0.36 horsepower or 270 watts.

[Bob]

...I'm worried about it splitting under the stress or something shifting and kinking the works...

Plywood. Might want to laminate multiple layers of plywood if the parts are very large. Spend money on bearings and bring plenty of shims. But I think we have very little idea of what you're imagining here, function-wise and size-wise.

[MikeVDS]

I tried to draw out a rough sketch of a clock with 8 hands working independently. Seems like each shaft that drives the hand will have to be inside the next hand. So in the end there will be 8 shafts each slightly larger to fit around the previous one.

Given this set-up I would use a single driven rod out from your power source with 8 pulleys on it.roughly a foot away would be the centerline for the driving rods for the clock hands. Each separate rod would have a pulley on it. The size of the pulley depends on the speed ratio you want. The pulleys and drive train is easy until you have to manufacturer the 8 rods inside each other. I would do that out of metal and spend some time designing it. really it's simple but hard to make. Once you find a good way to do it you should have a good time with this project. Also with this set-up you could fairly easily make some of the rods run backwards with an additional supported rod with pulleys.

[Zhust]

As for the size, I'm imagining the smallest arm (moving the fastest, driven directly by the rider) about 4 feet long, the longest and slowest being about 8 feet long. The gears need to be sized to reliably support them. Sorry it's hard for me to describe.

If you've seen clock hands, you'll note that there's an arrow end and a short end with a decoration on it. The decoration is a counterweight so the clock mechanism only needs to overcome the torque of moving the hand, not the weight of the arrow when it is coming up to the top.

So if you apply counterweights, the fastest hand will need to handle the most force when a "rider" quickly accelerates.

[MikeVDS]

^ That's a great tip. I never realized that.

[phil]

> Actually lots of people around the world build bikes out of wood.
> There are probably a number of reasons we don't see commercially
> made wooden bikes in industrialized countries

I see lots of designs for renewable bikes, including wood and bamboo. My suspicion of the reasons we don't see commecial ones is that wood frames to support one or two people would be too big and heavy and that lighter frames are too flexible to transfer torque effeciently from rider to ground.

[Bob]

Okay, enough with the wooden bikes. I don't see any of you Gen-X fuckwits riding around in wooden bikes.

Dork -- FWIW, I built a wood & steel windmill at the event on Hualapai Flat in 1997 that, last time I looked, was still standing, and I built it all onsite, during the event, out of scrounged parts. Granted, it wasn't a clock, and I'm no Genius.

Small note: I'd avoid Foucault pendulums made from bowling balls. Utterly feeble.

PS I'm not Belgian, so I don't waffle, but I'm known to get in Dutch.