Anxious to see @Microwave_Monkey’s results with Gates GT2 and GT3 belts!
Unfortunately, it might be a hot minute. Shipping dates keep getting pushed back on all my recent online orders. If all goes well, I’ll be able to run some tests and post a couple pictures or a video of the test itself here in a month or so, at best.
I should note they are knock off gt2 belts from some random factory in china.
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The issue with trying to turn what I measured into Young’s modulus is that the timing belts are non-standard geometries. In standard tensile and elongation measurements, you have to measure the thickness of your sample because the stress is dependent on the cross sectional area of the sample. Since these samples don’t have a constant thickness, the only thing I could do is estimate one and then the samples would only be accurate compared to one another - you can’t compare it to actual, properly measured thickness samples.
tl;dr changing the graph to “Young’s modulus” wouldn’t tell you anything other than what you already see.
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My fault, I’m used to pulling stress strain off of an Instron. I need to learn to read the axis on a graph😳. I thought you used a constant minimum cross section and calculated stress. Didn’t notice till after the post.
Granted you aren’t dealing with a material that has a typical stress strain curve, it has elastic properties, a non constant cross section, technically it’s a composite… so I agree the comparison to a standard material is a stretch(pun intended,) and likely the very reason you had to force a linear fit. So I suppose it would be more out of pure curiosity than anything else. You’re right though, stress/strain vs. force/strain would look identical, as you would have approximately the same minimum cross section for all samples. Only thing you would get by switching axis and expressing it as mm/mm instead of % is the same thing @gmac has, so no point. 
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Have anyone considered the 15mm wide GT2 belts? And what could be the downside?
6 and 9mm Gates GT3 belts would likely perform better, possibly as well or better than Kevlar and steel belts. They’re also compatible with existing hardware.
15mm wouldn’t fit on the existing pulleys, so you’d have to make some extra changes.
Yes you have to do more then belt change. But it would be a more simple task than ballscrew. It would even out the flex in x and y axis?
I expect going to 15mm from 9mm would give you about 9/15 the amount of stretch. I did try 10mm belts and saw about a 10% improvement, in line with that.
I don’t know if the drawbacks from 15mm will be noticeable. Increased belt mass, backlash, maybe other things? Maybe it’ll be fine.
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I would be willing to send a length of gt3 to you, @The_real_janderson , if you want to test it on your equipment. I am a little swamped right now with things outside the shop to devote any real time to it right now, and I much rather have the community get some “legit” numbers on them. Just let me know and I will send it.
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Yeah, I could totally do that, although just letting you know that it will be at least a month or maybe two before I don’t have access to the machine currently.
I figured that would be the case. No worries, just pm me your address at some point and I will send it over along with a length of some extra fiberglass core gt2 I have laying around (sourced from amazon almost a year ago). It might be interesting to see how it stacks up to the other gt2 that has been tested.
And have no worries about the COVID monster. Everything will be sanitized, including the box, before pick up by USPS. I’m not trying to disrespect anyone’s fear/feelings about this crap or put anyone at jeopardy.
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Thanks guys! These real-world measurements are very useful!!!
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Gates GT3 vs. Shapeoko belts;
I got my sample Gates GT3 belt finally from the UK supplier and improvised a stretch test against a vertical surface to compare the Gates to the original stretchy belts from my Shapeoko (pre steel belt changeover).
I’m measuring, on my less accurate test jig, for 10kg test weight;
- Original Shapeoko belts - 3.5mm stretch over 1000mm (0.35% for 22lbs)
- Gates GT3 belt - 2mm stretch over 1000mm (0.2% for 22lbs)
My ‘base’ measurement is reasonably close to the 0.4% that the_real_janderson measured for that type of belt in a proper rig so I’m reasonably confident in the comparison that the Gates GT3 is no better than the 0.2% recorded for the Kevlar or Steel cored belts, in fact it looks a little stretchier.
Test approach:
- 1.5 metres of ply clamped vertically to a solid wall
- Clamp both belts to the top of the ply, pinched between two pieces of ply with a large C clamp, hang test weights, check no slippage at clamp
- Affix a clamp to bottom of belt, from which to hang test weights
- Establish mark at 1 metre point from top clamp to observe belt stretch
- Hang 2.5kg initial weight to straighten belt and apply basic tension
- Add 10kg test weight and observe additional belt movement at the 1m mark
This one says “made in UK” so maybe it’s Brexit quality…
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Don’t you know only the Chinese can make good belts, at least that is what they told me. I am sure seeing “Made in the UK” made you proud and it should. I hope the US has woken up to the fact that we need to make things here. Let China make all the plastic food containers they want but high tech needs to be made in the country that will use it.
I got word back from @The_real_janderson that the belts I sent him for testing showed up. Can’t wait to see his results.
I sent him some lengths of 9mm gt2 fiber and steel core, 10mm gt2 fiber and steel core, and a length of 9mm gt3.
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Me too, but I’m not hopeful for the GT3
I’ve re-measured the Gates GT3 after leaving it for 24 hours hanging with 12.5kg weight to see if it “stretched in”, nope, still 2mm displacement for 10kg weight at 1 meter.
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I’m confused! Did it stretch another 2mm?
Steel stretches 0.49% / 38lbf = 0.0129%/lbf
Kevlar stretches 0.53% / 38lbf = 0.014%/lbf
Gates GT3 stretches (2mm/1000mm)*100% / 22lbf = 0.009%/lbf
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