Polybelt, polyurethane flat belt, polycord flat belt, etc.
“Flat urethane rubber belting” on various sites such as McMaster
Question about those: I’ve seen teams use them with pvc pipe (sometimes adding makeshift crowning to sustain centering) to make a conveyer mechanism. How small can you go on pvc pipe diameter without increasing risk a well-welded belt will fail?
Depends on the belt thickness; McMaster will specify a minimum pulley diameter.
I’ve run them on a hex shaft with a not crazy larger OD printed crown pulley successfully.
ETA: well below manufacturer minimum diameter.
If your weld is done properly it is very nearly as strong as the parent material and this stuff is DURABLE. You can really abuse this stuff a lot without risking failure. Generally you should plan to use 10% less belt length than a string would use for the same path and stretch it to fit. The biggest issue with this belt material is if some part of your system stalls often you can wear out a section more that the rest and have that part fail.
I highly recommend this tool for urethane belt welding. https://www.harborfreight.com/130-watt-hot-knife-60313.html It also helps to keep a jig to maintain alignment, but a straightedge will work in a pinch.
You can make an alignment tool from scraps, but I really appreciate this tool. Urethane Belt Welding Clamp – Practical Parts
You can often go well below this rating, depending on the thickness of the belt. I’ve seen these used on tubing as small as 1" before. The VersaRoller tubing that is popular these days will work well with most common sizes and thicknesses (at least in aluminum).
When using flat belts we cut the ends at a 45 degree angle. That spreads the load over 40% more well area and makes it run smoother over the pulleys.
It looked like the iupui ri3d team welded like an inch and a half of the main faces of the belt together using heat gun and clamp (creating an extra thick little section of the belt). If that thick segment doesn’t matter, it seems like maybe an easier (if cruder) way to weld the belt closed. Any belt gurus care to comment?
If you want to avoid failure that is how much overlap I would do to be safe. I’ve done less and it’s been fine but depends on your system and how much compression the belts will see.
I don’t really see how else to go about welding a belt.
Urethane belts can, and really should, be butt welded, no overlap necessary. When properly welded, the weld retains something like over 95% of the base material strength. I have never been able to break a proper urethane belt butt weld, even pulling with hundreds of pounds of force in attempts to do so. Doing a proper butt weld with no overlap, the welded section will be almost indistinguishable from the rest of the belt.
Looking at the responses, I can conclude butt welding and overlapping face welding both can work. I assume if you want to go for really small diameter pulleys like the small 3D-printed crowned pulleys on hex shaft someone mentioned, it’s worthwhile to learn to do good butt welds. Thanks all.
I would like to add that before you start using these belts you should carefully consider all of your options. I highly recommend just using 15mm wide HTD belts as conveyor belts instead of these (see 2910’s 2020 and 2021 robots for examples of heavy use of these). They’re about the same as the urethane in terms of grip and you don’t have to deal with the welding process.
I moved away from the urethane several years ago and have never looked back.
We do a ~1/8" overlap on the bias cut weld. If the belt is soft enough and enough pressure is applied while it cools the bump is very small.
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