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Author Topic: Using a timing belt and pulley
airzone
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Posts: 33
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Post Using a timing belt and pulley
on: January 7, 2013, 12:50
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Hi,

I'm looking to set up one of these bad boys here, however the dilema I have is that I cannot easily get my hands on the beaded cord hardware (i.e. shipping costs from the 3d printers, and kids tangling themselves in curtain chains make it hard to come by here)... So I was thinking a 1:1 alternative would be some T5 timing belt with some pulleys.

There is additional cost per meter of belt but it will probably work out even in the end, factored in shipping, etc, but with the added bonus of better precision.

Does anyone have experience with this type of belt? Would a 15 tooth pulley be necessary over a 10 tooth?

Also, roughly how much chain / belt / cord do people use for a given paper size?

sandy
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sandy
Post Re: Using a timing belt and pulley
on: January 7, 2013, 13:50
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Hi there, yep the hardware for the beaded chain doesn't exist off-the-shelf, so it can end up an expensive way of doing it, even though in principle it's a twentieth of the price of timing belt and pulleys. The reason I used is was because it was more able to be hacked together - I could carve sprockets from balsa or make them from laminating sheet materials together.

Precision is an issue but only really in principle. The issue is that while the beaded cord is nominally 12mm pitch, it actually varies a couple of percent from batch to batch, whereas timing belts are manufactured to a much higher quality. The number of teeth (beads) is not related to precision.

Lots (probably most) other hanging plotter devices use timing belts, the best example probably being Der Kritzler (http://tinkerlog.com/2011/09/02/der-kritzler/), but of course Hektor, the granddaddy of them all uses toothed belt too.

I would probably go for a 15 tooth, that means there will always be at the very least 3 or 4 teeth engaged at any one time (a quarter of the pulley), and that seems better than 2.5 teeth.

good luck!
sn
airzone
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Posts: 33
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Post Re: Using a timing belt and pulley
on: January 8, 2013, 00:00
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Thanks for your feedback Sandy.. The precision is more about preventing the cord from slipping cogs in the very likely event that any pulley I make being not nearly accurate.

To answer my own question on how long the piece of string should be, I came up with this (remembering old high school maths!), which might be helpful to anyone with the same questions:

An A1 sheet is approx 600mm wide and 850 high.
Given a 120mm height from the pulleys to the home point, that gives a depth of 970mm.
For the width, there should be 150mm buffer between edge of the machine and the page, therefore giving a max stretch of 750mm across... My mate Pythagoras reckons that the distance from the pulley to the bottom far corner of the page will be ~1227mm for an A1 page, everything lined up perfectly: Sqr( 750*750 + 970*970 ).

The 15 tooth pulleys have a pulley width of 21mm, which gives a circumference of ~65mm (21*3.141 - granted the belt will never fully wrap the circumference, but more is merrier).

This gives a total of ~1290mm, so by the time we attach the gondola and counterweights, and allow for slightly more room (220mm overheads), approx 1500mm of belt per side of the machine will net an A1 sized page.

Calculated for other page sizes (mm per side of the machine, rounded)
A0 = 1930
A1 = 1500
A2 = 1200
A3 = 900
A4 = 840
A5 = 730
RedStar
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Posts: 11
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Post Re: Using a timing belt and pulley
on: January 8, 2013, 03:29
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Be carful with the type of timing pully you end up with..
I Had the same idea when I was waiting for my SD kit to arive and bought some HTD 3M timing belt and pulleys from eBay... When it arrived the shaft hole on the pulleys where bigger than what I asked for, so when mounted to my steppers the pulleys was off center(not good)

The belt I was thinking would be nice and flexible was far to stiff to bend around a pulley as a replacment for the beaded cord

Just my 2cents,
airzone
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Posts: 33
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Post Re: Using a timing belt and pulley
on: January 8, 2013, 05:56
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RedStar - what type of core was your belt? Steel, kevlar, nylon? How many teeth did the pulley have? I take your cautions and if I do go ahead with it, it'll be from a local CNC shop (where I can blast them if the pulleys are off). The flexibility of the belt is my biggest question mark though
RedStar
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Post Re: Using a timing belt and pulley
on: January 8, 2013, 08:30
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Hi airzone, Good question!
I've just got home from work and had another look at the belt and pulleys..

Turns out the belt has a steel core.. that would be a reason why its stiff! kevlar/nylon would be much better. In my deference the ebay listing didn't mention the steel core and I didn't stop to think about it because it was such a good price.
The belt is 3mm pitch and mates in the pulleys really well.

I just measured the pulleys, they are 6mm ID. I thought I had ordered 5mm ones.. Maybe they just got them mixed up? I can confirm they fit a 6mm shaft really well!

Again lucky it all didn't cost to much, the belt/pulleys wont work for a polargraph but I have a new project for them where the steel core will be a bonus and a motor with a 6mm shaft will be what I use (now) 😉

maybe if I paid more attention when buying things online or didn't go for the cheapest option I wouldn't be writing this, but then I would not have learnt anything either...
I firmly believe that using flexible timing belt is the way to go for accuracy on a drawing machine,
not my only thought now is: Because the timing belt has more weight to it that the beaded cord, should there be more weight added to the gondola (and counter weights) to help keep the timing belt held tight and not allow it sag in the middle (like power lines on a hot day)... just a thought..

Good luck!
sandy
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sandy
Post Re: Using a timing belt and pulley
on: January 8, 2013, 09:03
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Redstar did you ever use your off-centre pulleys for drawing? I think they would show a very subtle wave in the arc of the line, but each row would be synchronised at least - you wouldn't have got gaps. That's one of the beautiful things about this machine / setup - not only is it very very subject to the law of diminishing returns (when it comes to accuracy), sometimes it also actively rewards flaws by rendering something unexpectedly pretty or smart. The tip dragging behind the gondola and the size of the tip introduces far more margin for error than is caused by the drive chain.

sn
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