9 mm Wide Belts

Hi everyone, my team is working on our final drive train and we were wondering how strong the 9mm belts are compared to the 15 mm belts? How would we even go about calculating if we need the thicker belts or is it just knowlege that you should use the wider 15mm belts for drive train applications. I didn’t see any calculation tools on the vex site and just wanted to try and figure it out. Thanks you guys!

We used 9mm wide on our drive in the offseason and it worked perfectly.

motors? Gearbox reduction? Belt reduction? Pully teeth? Wheel size? How many matches?

6 motors, 6.11 ratio, belts were at 1:1, 24 tooth, 4" wheels, 12 competition matches, about 5-6 practice matches, and 4-5 hours of standalone practice at our shop.

Thank you! That really makes my cad a lot prettier so I’m glad that you had a good experience with them!

Go to the belt manufacturer site to get strength ratings.

And when the belt manufacturers site says the belts are way undersized, realize that those ratings are based on working forever (rather then 100 hours) under continual torque loads and at much higher RPMs. Personally, I think the collective experience of the FRC community would be better then the manufacturers site.

My experience says to never use 9mm wide belts in the drivetrain. Sometimes they will work but sometimes they won’t We have run belts the past 4 seasons and have gone back to chain for this season.

Uh - this worries me. We’re trying belts ourselves for the first time - 9mm. (2 CIM driving 6" AM high-grip wheels via a SuperShifters, WCD.) What were your failure points? Why did you go back to chain?

  • Mr. Van
    Coach, Robodox

What pullies are you running, specifically tooth count? 6" wheels complicates the matter even more, as the load in the belt is higher than with a smaller diameter wheel. Also will you have a means for tensioning or are you setting it up for exact center to center distance?

The failures we experienced were mostly ratcheting, teeth skipping as we changed direction rapidly. This of course accelerates wear and leads to failure quicker than normal. That said, we never had a belt totally fail, although we replaced belts every regional.

From a straight-forward strength comparison, the different widths are the same material - just different width…so 9mm would theoretically have 9/15 the tensile strength of 15mm. Anecdotally, and just from my observation - it seems to me the biggest difference would be the amount of surface engagement between belt and pulley. The only problem we have had with belts is the situation where the required torque causes the belt to ratchet over the pulley. Less surface contact would lower the threshold. We have not observed this problem with a drive-train application, rather with other systems, trying to pivot exceptionally heavy mechanisms.

We used 15mm belts on the drive train for the first time last year and loved it. It seems so much smoother, quieter, and lighter than a chain-driven system. Unless presented with some special circumstance, I can’t see going back to chain.

Sounds like your belts weren’t correctly tensioned. Too much tension on a belt will strip the teeth.

I didn’t say the teeth stripped. I said the teeth ratcheted or skipped teeth during rapid direction changes.

Then that could be not enough tension, or too small a pulley size. How many teeth did you use?

Or the belts are outside of their safe working limit. They were 26 teeth, and slightly on the overtight side. In my experience, belts are not very well suited for rapid direction changes with relatively high torque and minimal tooth engagement which is what happens in a west coast drive since pullies are generally kept smaller than 2" diameter.

That’s very interesting. I’ve never heard of problems on correctly tensioned belts in a drive. Were you using GT2? HTD is better rated for rapid direction change, though it really shouldn’t matter.

GT2. I know of plenty of teams experiencing ratcheting with 9mm wide belt. Once you go up to 15mm ratcheting only happens in extreme cases, but then you’re much wider than running #25 chain

We ran 9mm wide GT2 belts last year, and didn’t have any ratcheting. We had some wear, but it was what you would expect for a device running outside the rated specifications. 1 set of belts lasted the entire season (2 competitions, 1 offseason, and lots of practice.) Ratcheting occurs when your belts are under-tensioned, which means that your belts were under tensioned. You can look at the failed belt to figure out what was wrong. For comparison, we had a 24 tooth pulley with a 3.5" diameter wheel.

We’re using the small 18 tooth pulleys (9 teeth in contact). We will have the means to tension the belts.

I suppose that in the worst case scenario, we’ll switch back to 25 chain. Now were are we going to get the weight for that…?

  • Mr. Van
    Coach, Robodox

We experienced ratcheting problems with 9mm belts. Our experience mirrors Sean’s. We now use 15mm belts and 24T pulleys for our 6 CIM WCD.

I would be very hesitant to take risks with belt sizing, unless you have a good reason. Looking pretty is not.