Steel belt stretches 2.45mm over 150mm at 38 lbf — 2.45/150/38lbf *1000mm/m = 0.430mm/m/lbf
Kevlar belt stretches 2.65mm over 150mm at 38 lbf — 2.65/150/38lbf *1000mm/m = 0.465mm/m/lbf
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@The_real_janderson Can you switch your axis on the plot so we can get Youngs from your regression line? Since you used a standard gauge length it would be easy to directly compare stiffness!
Never mind… I need to learn to read, @gmack did it. My bad 
Still though it would be cool to see it, if time permits !
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Is 0.035mm/m/lbf a BIG difference or does that mean there isn’t significant difference?
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It’s only about 8% of the overall stretch - not very significant!
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It doesn’t seem very significant to me. I read that as steel and kevlar being interchangeable in terms of stretch.
There’s other differences worth considering: cost, availability, and durability (is one more prone to snapping, fraying over time, or permanently deforming?). I don’t know the answers to those.
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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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