Belt Stretch and Stepper Holding Measured

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.


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;

  1. Original Shapeoko belts - 3.5mm stretch over 1000mm (0.35% for 22lbs)
  2. 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…


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.


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.


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


Nope, I just left it hanging for 24 hours with the 12.5kg total weight applied to see if the stretch was greater in a new belt and whether it would change after being left under tension, figured that was fair to the belt. It didn’t, within the rather limited precision of my measurements at least.

I was comparing with the_real_janderson’s graph above “Strain Induced in 9mm GT2 Timing Belts” as that gave me a reasonable force to test at and comparison points, not sure if that’s the best choice.

The strain recorded on the chart for Polyester looks to be around 0.45% at around 20lbs-f and around 0.25% for Steel and Kevlar.

In my tests I saw, for an incremental 22lbs-f I saw 0.35% stretch for what I believe to be an old Polyester cored belt and 0.2% for the Gates GT3.

Given the improvised nature of my measurement rig I think I can say;
a) The measured values seem reasonable compared to the measurements performed on a proper machine, the values are in the range expected
b) The measured values seem to be lower in my case, but this is easily attributable to poor precision and resolution in my measurement, perhaps I have a slightly less stretchy Polyester belt?
c) The Gates GT3 was around 50% to 60% of the stretch of the old Polyester belt, thus putting it in a very similar range to the Steel and Kevlar cored belts

Does that make more sense or am I still missing something?


No, you just confused me with this statement.

IMO you’ve shown that the GT3 belts are potentially the best option for Shapeokos.
Your measurements are also probably pretty accurate since you tested such long lengths. (I use 12 inch calipers to set bandsaw blade tensions).


OK good

They may be, what we don’t know yet is how well they hold up and whether they show better lifetime and failure behaviour than the steel or kevlar cored belts which they may.

In the meantime, I’m not in any rush to take the cheaper Kevlar cored belts off mine, or wait another 2 weeks for delivery from the only UK supplier I could find for the other two belts I’d need for the Y axes.


Per @Microwave_Monkey’s request, I tested 5 more belts!

Same as mentioned prior: Belt Stretch and Stepper Holding Measured

Similar to what @LiamN saw, the Gates GT3 belts perform better than the GT2 fiberglass belts. Steel belts are still the best in terms of stiffness, as to be expected, with the fiberglass belts most likely winning in longevity. 10mm fiberglass belts are about 11% better than 9mm belts (as 10mm is 11% wider than 9mm), however, 10mm steel-cored belts are about 19% better than 9mm steel-cored belts. Perhaps the extra 1mm allows for a disproportionate increase in steel.