Belts 9mm or 15mm?

[IndySam]

So after four years of gear drives we are looking at maybe switching it up and moving to belts (assuming the game calls for standard 6 wheel drive.)

So in your experience what is the better choice 9 or 15mm belts?

Or maybe a better question are 9mm adequate for typical FRC loads?

Unlike 25 vs 35 chain the trade off isn't weight but size.

There are a few threads that mention this choice but none that really answered the question.

Thoughts?

[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.

[s_forbes]

We have been using 15mm belts in our drivetrains recently, and everybody likes the belts over the chains. I'd really prefer to go to 9mm belts on whatever we build next though to save space. 987 has been using them for years and they seem to do just dandy, even with big wheels and small pulleys. Seems like if you get the belt tension correct they will survive easily on a 6wd setup with the center wheel direct driven by the transmission. Other configurations might be different.

[Nuttyman54]

Not all belts are equal, FWIW. If you're looking at HTD profile (what VexPro and AndyMark sell) I would recommend 15mm for drivetrain. If you're running GT2/GT3 belts and pulleys, 9mm could be sufficient.

I believe 971 ran 9mm GT2 in 2013 and 2014, but switched to 15mm in 2015.

9mm can definitely work, but 15mm gives you a lot more headroom.

[Deke]

Originally Posted by s_forbes

Seems like if you get the belt tension correct they will survive easily on a 6wd setup with the center wheel direct driven by the transmission. Other configurations might be different.

This.

If you aren't direct driving a 6wd center wheel with a transmission, I would not recommended using 9mm wide belts. That is the only drive configuration that would make them manageable. If there are small pulleys driving large high traction wheels, the 9mm wide belts will still give you problems in the 6wd direct drive center wheel.

[Chris is me]

Originally Posted by Deke

This.

If you aren't direct driving a 6wd center wheel with a transmission, I would not recommended using 9mm wide belts. That is the only drive configuration that would make them manageable. If there are small pulleys driving large high traction wheels, the 9mm wide belts will still give you problems in the 6wd direct drive center wheel.

This is also a function of pulley diameter. I am not familiar with 987's setup, but from what I remember they use pulleys at least 30 tooth, not tiny like 24 tooth or smaller.

I (and many other teams) have had some problems with 24T pulleys, 9mm belts, direct driving 6wd from the center etc etc. It's right on the margin of can or can't work, and it depends on how careful the user is. I don't like my drives to ride the safety margin that closely.

[MoistRobot]

We've used 9mm vexpro belts with 30 or 36 tooth pulleys for 2013-2015 (Rhino last year) with no issues. Never broken a belt.

[AustinSchuh]

Originally Posted by Deke

This.

If you aren't direct driving a 6wd center wheel with a transmission, I would not recommended using 9mm wide belts. That is the only drive configuration that would make them manageable. If there are small pulleys driving large high traction wheels, the 9mm wide belts will still give you problems in the 6wd direct drive center wheel.

We ran 9mm wide, 5mm pitch belts in 2014, GT3 with a 3.5" diameter wheel and a 24 tooth pulley. The center wheel was direct driven, with belts to the front and back wheels. We carefully measured the tension, and worked hard to keep them properly tensioned to avoid ratcheting or over-tensioning. They all broke right after as the season ended.

We are going to go to 15 mm wide belts next year unless the game is something weird again. It's just not worth pushing the limits. If you read the spec sheets, we are well outside the rated range for the belts.

[ARampantBrian]

Originally Posted by IndySam

So after four years of gear drives we are looking at maybe switching it up and moving to belts (assuming the game calls for standard 6 wheel drive.)

What would 829 even be anymore?

[frcguy]

We ran 15mm belts from AndyMark in our KOP drive train last year. We've had no issues after 40+ official and practice matches and at least 50 hours of driver practice and demos.

I am a little curious - why are you going away from chain?

[IndySam]

Originally Posted by frcguy

I am a little curious - why are you going away from chain?

We haven't had a drive chain on a robot since 2012.