Belts 9mm or 15mm?

[Chris is me]

15mm is what I would feel safest with in a drivetrain, particularly with the smaller pulleys typically used in west coast drives. 9mm is very marginal for most standard drivetrains using 24T or 27T pulleys, and can definitely fail. Once you go above 30T (for example, with a dead axle setup bolted to the wheels directly), you can probably get away with 9mm. It's a little extra space, but the result is a zero maintenance, highly reliable drivetrain. Go for it!

[Oblarg]

Originally Posted by Chris is me

15mm is what I would feel safest with in a drivetrain, particularly with the smaller pulleys typically used in west coast drives. 9mm is very marginal for most standard drivetrains using 24T or 27T pulleys, and can definitely fail. Once you go above 30T (for example, with a dead axle setup bolted to the wheels directly), you can probably get away with 9mm. It's a little extra space, but the result is a zero maintenance, highly reliable drivetrain. Go for it!

"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.

[Chief Hedgehog]

4607 typically uses 9mm when we use belts. In 2016 we used them with no problems. However, there was no reduction; just strictly transfer of power (1:1 ratio). We purchased extra belts and even though we sheered a few drive shafts, our belts were never a problem. And we played in some pretty tough matches throughout the regular FRC season and in the offseason (looking at you 2052 and 5172).

Maybe this year if we have the room (we won't) we will try out the 15mm belts and pulleys we bought so long ago...

Good luck!

[sanddrag]

We managed to break a 15mm belt by the end of the season, and I know we're not the only ones. I would be very skeptical of 9mm belts in a FIRST robot drivetrain.

[Chris is me]

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.

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.