Sanity check regarding urethane flat belting

Technical Technical Discussion
1

Doing urethane flat belts for the first time. One run is 24” center to center, one is 32”. Pulleys are 1.3” or so before the crowns, belt is 1.75” wide.

Is a 1/2” undersizing about right? I’ve heard 1/4” in the past, but the demos I’ve seen had far shorter runs.

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I can’t be 100% sure but I believe we took the exact length and then took off like 5 or 10% of that and then that would be a proper tight belt. Idk how well that will work for a 24" or 32" one but its worth the test.

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we are new to it this year, but I’ve heard 7-10%

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We are also new, but this video helped alot. We basically cut to exact length then overlap by like .625 inch. Works great if you get it hot enough

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You really just need to experiment to find the right tension for your application. C2C distance, stiffness of rollers, and belt thickness and width are all factors which play into how much you want to overlap your joints. Our normal starting point is around 1/4 to 1/2" overlap with a 1" wide belt with about 8-12" C2C distance. Then we install and run the mechanism to be sure it behaves how we need it to. It often takes a couple tries to get it right. We use a mini torch to melt the ends, because… fire.

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This ^

We also found we couldn’t get the desired support on one of our long belt runs because if we made it shorter it would start to bend the hex shaft. We supported the belt with polycarbonate and low friction tape which solved the issue. If you aren’t comfortable making belts super tight over long runs, back then with some sort of support and it will give the same effect.

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To add some more data points, we are running ~22" and 24" C2C runs and found 1" of stretch (2%) is working well. That is with a pair of 1" wide orange urethane belts from McMaster (aka Eagle brand) on each side.

Less is more with these flat belts, be very careful about how much you trim as we wasted a whole weekend build day to redoing belts from miscalculations and trying the wrong recommendations. A few notes from our mishaps:

  • The 5 - 10% stretch recommendation is typically for round belts, not flat belts (unless maybe you had a really short belt run).
  • The same 2% stretch was still too short for 2" wide black urethane belts from VEX. I don’t know if this is more related to the belt width or material (VEX is a different durometer than the orange belt) but it really didn’t like to stretch much at all.
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PLEASE Ignore all advice about 5-10% - that is WAY too much for flat belting. That is good advice for round belting.

I would take about 3% off the length for flat belting, particularly if you are using more than one belt per shaft.

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9

3% (and then somewhere between 1/2" and 3/4" of overlap) was effective and juuuuuuuust able to be pulled even so we could slot the shafts into the bearings. Thanks for the sanity check!

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