Quote:
Originally Posted by Oblarg
"Marginal" is the crucial term here. 9mm belts can be fine, or not, depending on your drive. 15mm are basically always safe, however, unless you're doing something really weird (exceptionally big wheels w/ exceptionally small pulleys). That said, there's certainly nothing wrong with using 9mm belts - you just have to be careful.
Also keep in mind that the stated belt specs from Gates are not necessarily accurate - those are calculated for lifetimes waaaaaaaay beyond what we expect in FRC, and we actually have a fair bit more headroom than they indicate.
If I were to give a rule of thumb for 9mm HTD belts based on my personal experience, for standard FRC drive parameters (traction-limited with a wheel COF around 1), I'd feel comfortable with 9mm belts up to, say, 42t pulleys with 5'' wheels (that yields a tooth-per-wheel-diameter ratio of ~8 teeth/in, meaning for a 3'' wheel you'd want no smaller than a 24t pulley, etc). Keep in mind that the pulley diameter/wheel diameter ratio is indeed the operative quantity here, since that determines the force on the belt.
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I wasn't going off of the load ratings, but mostly personal experience. 9mm pulleys and belts have failed me at 18T and 24T before. 9mm / 24T can work, with active tensioning and very careful management, but this eliminates the big advantage of belts for me - lack of maintenance. Just go 15, and set it and forget it, is what I'd suggest.
I think your rule of thumb is pretty fair.
Anyone saying "X size belt is fine" without naming the pulley size they were using (and ideally the wheel size too, but I guess four inch is implicit for this) isn't giving the full story.