The last few days we’ve been experimenting with the VEX 2 inch flat belting to drive our elevator assembly. We are having a hard time keeping the belts from sliding across the conveyer rollers. We don’t wanna add any edges on the ends of the conveyer rollers as this might add added compression to the ball. How are teams fighting this problem. This is our first time using flat belting.
The most common way to keep flat belting stable is running them on crowned pulleys - even though it seems like it’ll be really easy for the belt to slip off of them, the geometry of them keeps the belts on-center. Video on them linked below.
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Crowned pullys are great, but for a simple fix with your current shaft just wrap about 8 to10 wraps of electrical tape where you want them to ride. This creates a crown or a bump on the shaft and the belt will center itself. If that is not enough you can also tape down a o-ring or even a hair tie around the tube to create the bump.
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BTW, you only need one crowned pulley on each belt. Its a great job for a 3D printer. The crown can be pretty informal and still work fine. The key is that the belt needs to be “not flat”. So, if you fake a crown with a straight section and two tapers, the straight section needs to be narrower than the belt for it to work right.
+1 on the tape wrap. I would use something with a lot less goo potential than electrical tape, thought. Or heat shrink over it. But then I hate vinyl electrical tape 
YMMV.
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Like everyone else said - it’s all in the crown.
To give you some practical stats, we ran 2" wide poly belting on a 2 1/2" wide pulley with about a 1/8" high crown in the middle on a 1.6" base diameter pulley. With only 1/4" clearance on each side, we never were in danger of rolling off the pulley because of the self-centering of the crown.
This is one of the mechanically amazing things that are totally non-intuitive at first because people tend to sort of think of gravity (“the belt will slide down!”) instead of differential belt tension.
Once you do it, you’ll be amazed and impressed. You’ll be one of the people next time saying “Look I’ve been there; you need a crown on your pulley”.
The dimension we used was a lot - more than I suspect was needed - but you need something.
This is also why you want your shafts to be parallel because non-parallel shafts will cause the same tension differential across the belt and subject the belt to lateral forces as well.
Do it and you’ll have a sweet belts tracking right machine!
Buy some 1/8" thick O-rings with the ID matching your tubes OD and electrical tape them in the locations on the tube where you want the belts centered.
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Only use the good, 3M branded electrical tape. It doesn’t save a worthwhile amount of money to buy anything else and will save a lot of trouble. I have only had the other brands leave goo.
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This method works great, and looks the cleanest when done without having to make new parts or machine grooves or anything.
Resist any temptation to make flanges go around the sides of flat belts - that doesn’t really work, and as you said it will interfere with the balls. A crown in the center should do just fine. I like crowns on both rollers but you should only need them on one roller.
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In our experience you don’t even need an O-ring under the electrical tape. Just make a ~1/8" thick coil of electrical tape over the roller where you want each belt to be centered and bam - there’s your crown.
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