Friday, March 07, 2008


Beaded Belt Drive Success

I've bought some "ball chain" or "beaded belt" from the local Mitre 10 hardware store for NZ$6.95 per metre. The balls on it are 4.60mm in diameter and have 1.80mm exposed length bars between them. I have plastic beaded belt of the same dimensions, so this is likely to be some kind of standard. Investigations continue, do clue us in.

I've designed a pulley (shown left) that can be fabricated on a RepRap - and made one. After a mechanical test, Adrian ran 4 off on the Strat. Note: Make one with a rim that lifts off in the future so I can fit the damn belt without resorting to violence.

Here is the video below showing them in operation. Note that the orange pulley under the bed in the lower left corner of the frame is rotating and that the old drive belt is not fitted. The red temporary join is a cable tie and is obviously not going to go smoothly around the corners. The chain is made from chromed brass, by the way.

To join the ends of the loop, I filed down the two terminating balls on the chain into approximate halves (measured with a badly-calibrated Mk I eyechrometer) while holding them in medical clamps. Then I held the clamps in a couple of angled vices so that the halves fitted together properly, finally soldering them together with a 30W soldering iron and highly toxic lead solder (yum). Better ideas welcomed! The joined chain is robust enough to turn the whole Z axis assembly with some enthusiasm. When I build the next RepRap the gears will go in the usual place.

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Re the joins, there's a "standard" component for making joins on these things -- the pens at your bank probably have a short segment coming out of the pen, connected to a long segment coming out of the table/counter. However, it's not shaped like a normal ball, so won't fit well along the gear.
That could be worked around, OTOH, by having a double space somewhere on the gear, but that limits the sizes of gear and chain we can use so that the joiner always hits the right place on the gear.
On the plus side, I think these might be available in the same size in hardware stores across much of the world -- they're used for lightswitch pulls in the US, and I *think* at the same size.
(In the UK, ceiling switches are used almost entirely where the electrical code would disallow a metal pull, but that's not the case in 120V-land, where we assume the people putting switches in bathrooms are competent enough not to connect the switch to live.)
You might try filing the tops and bottoms of the end balls down. If the balls are hollow that will leave you with a washer-like shape at each end. You can overlap the two and slide a pin through to secure them.

If the balls aren't hollow you'd need to drill them.

You won't get perfect alignment between the two sides, but I doubt that it will be off enough to cause problems.
Brilliant Vik! Those timing belts are ridiculously expensive.

Do you have any feel for how "lumpy" the drive is? That may be a problem for X and Y, especially as their drive wheels need to be quite small in diameter (which could be alleviated by using smaller balls, as you say).
The pull feels amazingly smooth, but you do need to maintain quite a lot of pressure on it as the friction coefficient of chrome on ABS is much lower than neoprene on CAPA.

I've got some chain with smaller balls much more like the stuff that holds pens and I can have a go at making an X or Y axis if you like?

Vik :v)
Good idea. Could the increased pull be reduced by a more conformal shape to the gear teeth so the balls can't slip out?
I think the way blind drive chains work is that the chain loop is more than twice the length of the blind, so the link can be placed such that it never reaches the gear.
I'll have a look at reshaping the teeth, but I really need to get the Darwin working again. I took a day off yesterday to celebrate by 44th.

Vik :v)
If I understand correctly, the beads are stamped from sheet brass.

You may be able to pry one open, cut off the one on the other end, and clamp the opened one around the nailhead-shaped ends of the remaining rods.

The solder is a brilliant solution. Are the balls plated on the inside as well? If so, there may be some chemical means of stripping the plating off wihtout harming the brass to get a stronger bond; I'm fairly certain unplated chain is also available.
I got some of these today and have tried the other way of connecting them (what joel said) by prying open the seams on the end balls, taking one off and reclosing the other on the dumbell shaped rod that links the hollow balls.
I had the same idea and am working on making a gear with better fitting shape like Adrian suggested. I don't even have all the parts to my cartesian system yet, but I'm working on it. I have an idea on how to get the tension to work effectivly, but I can't go into detail without trying it out first.
A thought comes to mind: Put an idler on each side of the driven pulley. This will allow you to wrap the ball chain more than 90° on the driven pulley AND removing one idler will provide a way to get the chain on the flanged pulleys. Since the pulleys and the supports are fabbed this should not add much to the parts list.
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