Beads Instead of Teeth?
December 14, 2007 10:18AM
Google Alerts sent this page to me this morning: [angry-economist.russnelson.com]

The author suggests using bead garlands instead of toothed gears. Neat idea. I wonder how you'd splice the ends together. I've got some of these at home. I might have to experiment.


Re: Beads Instead of Teeth?
December 14, 2007 10:38AM
What an interesting idea! I'm worried about tension stretching/breaking the links between the beads though.

I had a friend who made a two-color school-pride sort of woven garland of these (it's hard to describe without seeing it, I realize I'm not being clear). Anyway, the thing staying together relied on the tension between beads continuing to remain pretty constant, and after a couple of days of wearing it (it was a spirit week) the thing started to fall apart because the distance between the links slowly increased, and a few actually snapped. The point is I'm not sure you can count on those beads to reliably transmit power.
Re: Beads Instead of Teeth?
December 14, 2007 10:54AM
Yeah, the strength of the string would be key. I know from personal experience that these garlands snap easily in the hands of a four-year-old.
VDX
Re: Beads Instead of Teeth?
December 14, 2007 02:45PM
... look here, maybe some more ideas and hints for fabbing and connecting: [forums.reprap.org]

Viktor
Anonymous User
Re: Beads Instead of Teeth?
December 18, 2007 01:09PM
You could use beaded metal chains of the type used for keychains, but these they aren't all that durable either. Besides, if you wanted to use metal chain, you might as well use sprocket (bicycle) chain.
Re: Beads Instead of Teeth?
December 18, 2007 06:20PM
Which also stretches.
Actually, when I get around to starting, I'm considering motorcycle chain. I live on a farm. My access to junk is excellent.
Re: Beads Instead of Teeth?
December 18, 2007 09:50PM
Sean,

Poke around on cnczone; I think people tend to avoid roller chain in larger cnc machines in favor of belt drives or gear rack.

Not to say that you can't get it to work:
[www.cnczone.com]

I brought up chain drives with Ed some time ago. He'd considered using them with darwin but they wouldn't fit.
Re: Beads Instead of Teeth?
December 18, 2007 09:58PM
What about a cable drive? A cable is a whole series of threads all twisted right? So if the pulley had the negative of those threads molded into it, than it would work the same way any belt or beaded drive would.


Jay
VDX
Re: Beads Instead of Teeth?
December 19, 2007 01:58AM
Hi Jay,

... a twisted cable isn't so dimensional accurate, that a negative shape would fit over the complete length ...

In past i drived with thin steel-wires (0,3 to 0,7 mm diameter) self-developed pen- and laser-plotters.

For a good friction i wound the wire 3 times around DELRIN/POM-rolls (with 20 respective 40mm diameter) which were driven by stepper-motors and stretched the wires at one end with a strong spring - worked very fine and my accuracy and repeatability for the laser-plotter was in the range of 0,05 mm!

Viktor
Re: Beads Instead of Teeth?
December 19, 2007 12:59PM
when i was disassembling a large laserjet printer, they had used thin steel wires as well for driving. they simply wrapped them around a spool multiple times. it looks like a very good method for driving things. the steel wire is very cheap, and the spools and pulleys would be easy to reprap.
Re: Beads Instead of Teeth?
December 19, 2007 01:38PM
> when i was disassembling a large laserjet printer, they had used thin
> steel wires as well for driving

I have an old XY plotter around somewhere that uses this method. The Y axis is the most interesting in that the cable is strung in a sideways figure 8 so that the left and right sides always move in the same direction. Ed had looked at using a similar arrangement for the Z stage on Darwin at one point: [blog.reprap.org]
Re: Beads Instead of Teeth?
December 19, 2007 03:26PM
Basically it's all about accuracy and backlash i think beads etc are unlikely to be accurate enough, chains etc have a lot of backlash with multi-turn cable it runs along the length of the pulley 1 cable diameter per rev but it can transmit massive power just look at sailing rigging, cranes etc. If the movement of cable could be worked with it would be very cheap and universally available.

If you use a 1/8th step driver and a 50.9mm


Ian
[www.bitsfrombytes.com]
VDX
Re: Beads Instead of Teeth?
December 19, 2007 03:47PM
... the atached images in this post [forums.reprap.org] show, how i move the tool-head of a laserploter with two stationary motors over steel-wires.

In the two last types with this setup i used wounded steel-wires with 0,5mm diameter and had an overall accuracy of nearly 0,025mm in a drawing area of DIN-A3 or 45x30 mm ...

With this method it's no problem to apply high forces and speed - but it's not so rigid, that i'll use it for milling winking smiley

Viktor
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