Quote:
Originally Posted by Chris is me
You have it backwards - chain *does* stretch over time, and belts do not. Chains generally do need a tensioning system, and belts generally do not. My team has run 15mm belts in tube with exact center distances for now four seasons in a row without ever having a belt failure or need to tension. Belts just don't stretch in the liftetime of an FRC robot.
If you undersize your belt drive, it can break - generally failures I've seen involve 9mm wide belts with 24t or smaller pulleys. This just isn't up to snuff. 15mm wide is a safe bet if you're using small, west coast drive friendly pulleys. If you are using larger tooth count pulleys such as the bolt on VersaPulleys that are 36 or 48 tooth, 9mm is generally fine for those.
The reason "chain in tube" drivetrains with no tensioning work is because the chain and sprockets are sized so that they just barely fit in the tube at all. Chain in tube does stretch, but since the chain on the sprocket is such a close fit with the tube, the chain can't jump the sprocket since there's just nowhere for the chain to go. It works well, but it is very much an edge case with chain drives.
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My concerns with belt were more geared towards snapping, not the stretch of the belt. As Joey and others pointed out, the 15mm GT2 belts are pretty indestructible, as are 9mm when utilized correctly. I had heard that belts stretch second hand, but its pretty clear now that they don't stretch enough to be noticeable.
In contrast, we've run chain (bearing in tube) with no tensioners the last 3 years and never had an issue. It does wear the sprockets a bit, but that's only at the beginning.