Re: Belts 9mm or 15mm?
 

The first time trying 9mm belts, VexPro broke all the time, replaced with Bando, same application, same tension, never broke again rest of season, "back in 2012".

Never crimped the belts or altered them, straight off the shelf.

Re: Belts 9mm or 15mm?
 

I would not recommend using 9mm belts on a drivetrain. We tried it last year to save space on our robot. It was the biggest flaw of our robot. Maybe it was because of missallignment or tensioning but we went through several sets of belts at one regional. Eventually we switched to chain and that was much better. Just save yourself the headache and use 15mm belts or chain.

Re: Belts 9mm or 15mm?
 

We have been using 15mm belts for the last couple of years on the drivetrain, and they have been working great. As long as the alignment is correct, no belts should be broken. We have had to tension them a couple of times through the season, but nothing too worrying.

We tried using 9mm belts on a test drivetrain that we built. It weighed only around 50lbs, and after only 30 minutes of use, we ripped one of the belts. We replaced it and haven't ripped one yet. I would go with the 15mm to be safe, but you have to take the extra width into account when designing the robot.

Re: Belts 9mm or 15mm?
 

Can people that list belt sizes please list the pulley and wheel size? There is an direct correlation between pulley size and maximum torque, and maximum torque and wheel size, and maximum torque and force on the belt.

Re: Belts 9mm or 15mm?
 

My 24T pulley worked great the entire season. It never failed once and I would recommend everyone use this pulley.

Re: Belts 9mm or 15mm?
 

Only our Rookie year did we use chain, in 2011. Since then we have used belts with a 99% reliability of our drive train. In 2014, we went to 9mm wide belts figuring that a factor of safety of 5 was enough, as we were trying to fit them in 1 x 2 tubing. The small sprockets and the narrow belts just didn't hold up in defensive play in Rebound Rumble. We quickly became experts at changing a complete drive train side in 5 min, going through 32 belts that year.

Since then we went back to the 15mm belts and use 1.5 x 2.5 tubing to fit them and the sprocket in. Both 2015 & 2016 we had NO drive issues nor repairs for the lifetime of the robot.

So in short, if you are going to use them for drive and plan on doing alot, 15mm wide are the only way to go.

Re: Belts 9mm or 15mm?
 

I am curious as to how you got a safety factor of 5 while fitting belts inside 2''x1'' tubing. Even using (what I'd consider to be) sufficiently large pulleys, FRC drives are usually skirting (or outright violating) Gates' stated torque ratings for 9mm belts.

Re: Belts 9mm or 15mm?
 

Would this be on 9mm or 15mm belts?

Re: Belts 9mm or 15mm?
 

Sorry, is the 24T small enough to be used inside .062" 2x1 or 2x2 tubing? I can't remember.

Re: Belts 9mm or 15mm?
 

We used 42T pulleys on 8" wheels both years. The test drivetrain used 42T pulleys with 6" wheels.

Re: Belts 9mm or 15mm?
 

Only if you don't rivet or bolt to the top or bottom. I'd recommend 3x1.5 if you add a counterbore to your pulley for the bearing clearance or 3x2 if you do not. This gives you plenty of room for hardware so you never have to worry about it. You can probably do 2.5" tubing if you can find it?

Re: Belts 9mm or 15mm?
 

Just because I'm curious, why the switch from gear drives? I still remember seeing your 2013 bot at the Smoky Mountains Regional and being in absolute awe. I also remember yours being the best drivetrain at the Regional.

In answer to your question, yes, 9mm wide (5mm pitch) HTD belts from VexPro can be used on an FRC drivetrain. This will save you space in one dimension (width), but cost you space since your pulleys will need to be larger. I would be uncomfortable using anything smaller than 30T 9mm pulleys with 4" wheels and a reasonably geared speed. If you're using bigger wheels or active belt tensioning or voltage/current ramping or anything else that affects how your robot drives and accelerates, you'll need to change your numbers. Even the center distances make a small difference, though that's more a factor of alignment and tensioning than actual torque. But in my opinion and for my team, I would suggest 15mm just for peace of mind and smaller pulleys. YMMV.




While the GT2/GT3 profile is superior to HTD in almost every way, including service life, backlash, vibration, and load-carrying capacity, HTD belts respond slightly better to reversing shock loads. GT2/GT3 are much better suited for most industrial uses, but reversing shock loads are so common in FRC, what's best for industry isn't the same as what's best for us. The people at AndyMark and Vex knew this and made the informed decision to sell HTD pulleys and belts instead of GT2/GT3 after discussing it with people from Gates and other industry vets.

Just to add on to what Eli is saying about Gates' stated torque ratings not always being applicable to FRC purposes, remember that all of those values are rounded averages of many tests. Really, those values show a range, not an exact value. A few months ago, I tried to copy all of Gates' relevant data in to an Excel sheet, convert it to ft-lbs and rpms, and make a calculator that gave you a minimum pulley size for a given gear ratio and wheel size and motor combination. I was curious about just how precise all those values from Gates were, so I emailed them about it. They were pretty helpful, but the end message was that, since I was really trying to read a lot into a very small portion of their tested range, the precision over the range I was looking at wasn't tight enough to give accurate distinctions between pulley sizes. So, basically, if Gates said a certain pulley size could handle .5 horsepower, the fudge factor in that .5 was the difference between failing and not failing on a specific torque ratio.

So yeah, Gates' user and design manuals are awesome, but the stated load ratings aren't super applicable.

Preach.

Re: Belts 9mm or 15mm?
 

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.

Re: Belts 9mm or 15mm?
 

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.

Re: Belts 9mm or 15mm?
 

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.