Belts 9mm or 15mm?
 

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?

Re: Belts 9mm or 15mm?
 

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!

Re: Belts 9mm or 15mm?
 

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.

Re: Belts 9mm or 15mm?
 

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.

Re: Belts 9mm or 15mm?
 

What would 829 even be anymore? :confused:

Re: Belts 9mm or 15mm?
 

"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.

Re: Belts 9mm or 15mm?
 

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!

Re: Belts 9mm or 15mm?
 

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.

Re: Belts 9mm or 15mm?
 

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.

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?

Re: Belts 9mm or 15mm?
 

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

Re: Belts 9mm or 15mm?
 

461 has used 15 mm HTD belts every year since 2013 except 2014 where we direct drove. All 6 inch wheels with 42t pulleys. We've never had a belt issue in the drive once.

D'oh! Completely missed the part about gear drives.

Re: Belts 9mm or 15mm?
 

WARNING...a programmer / electrical person is attempting to pretend to talk about mechanical stuff...but this is our experience over the past 14 years.

We have done all of this...

25/35 chain
9/15mm belt
Never done gears like you.

So far, we like 9mm belt, but ONLY if its a Bando brand and specifically "Bando Synchro-Link HT belt". DO NOT USE 9mm VexPro belts on the drive system. They break all the time, but the Bando Synchro-Link HT type 9mm, knock-on-wood, we have never broken a drive train belt with this brand and type. The major drawback with 9mm, is you better have a good way to tension the belt. It's not as forgiving as a 15mm. Kinda reminds me of chain. 25 chain seems to want more tension than 35. 9mm and 15mm seemed to behave the same way with us, 9mm will start pulley skipping, like 25 chain does with loose tension.

Example Part Numbers:
900-5M-9 = list price sample $8.68 (weird price example...LOL)
https://www.bandovbelts.com/products...-belt-by-bando

Good resource:
http://www.bandousa.com/html/pdfs/bu-143manual.pdf

We like the 9mm over 15mm because we can narrow the wheel wells up and have alot more space inside the robot generally is why we favor the 9mm pulley setup, because putting a double 15mm pulley on the center transmission output shaft on both side of your robot, eats up alot of real estate in the middle of your bot.

Of course 15mm is recommended by most you will probably find, like talking with mechanical super geeks in Indiana, no names mentioned.. By design, 15mm was included in Rhino tracks, we did not have any breakage problems with this single 15mm Gates belt supplied but it did stretch over time, by the time we got to world champs, we had to change this belt out with a new one because the tension was so loose it was pulley skipping all the time and there was no way to adjust the belt tension in last years Rhino track setup.

I am not sure if the Bando brand 9mm stretched over time, because generally we used tension blocks on the drive system when we have used 9mm on the drive in the past and the pit crew always made sure the tension was correct on the drive belts, so in other words, if it was stretching, I would not have known because they keep the belts tight.

We like belts for sure when possible everywhere.

Re: Belts 9mm or 15mm?
 

I have quite literally never seen one of these break (in fact, I've only ever seen a HTD belt from any source break once, in almost a decade of FRC). Are you sure you're using them within specs?

One thing to watch out for is that belts can break if they are crimped (this was the cause of the one failure I have seen, and is the cause of most of the ones I've heard about).

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.

Re: Belts 9mm or 15mm?
 

On the sourcing belt front, you have to remember that the GT2 and GT3 profiles are still patented by Gates, whereas the patent on HTD ran out so there's many, many more suppliers of HTD.

On load ratings on the design manual, those also might be based on fatigue and not instantaneous loading.

Re: Belts 9mm or 15mm?
 

Though it's really more the name that's been patented at this point, and not the actual profile, right? Because there are several suppliers, WCP included, that sell a "GT2 compatible" profile. I don't know where the lines are drawn on how close other profiles can get before being in violation of the patents, but I'd say the "GT2 compatible" profiles are pretty close.

And yeah, none of Gates' load ratings are off of instantaneous loading or reversing loads.

Re: Belts 9mm or 15mm?
 

I believe patent on gt2 belts are up but gt3 belts are still protected (source: gates rep)

Re: Belts 9mm or 15mm?
 

My biggest question when it comes to belts on drive teains is what am I gaining buy using two 15mm belt vs any chain. Weight savings is all I can find. I worry that the larger space taken up might not be worth it. That said belts are my go to choice for everything else, 3D printed pulleys anyone? :p

Re: Belts 9mm or 15mm?
 

Unfortunately, when you sum up the extra width of 15mm belts and the weight of pulleys, the theoretical weight savings compared to #25 chain are nulled. :\

Re: Belts 9mm or 15mm?
 

Thanks for the compliment.

We might not. Really going on a simplicity kick and gear drives are not simple. As usual everything depends on the game.

Re: Belts 9mm or 15mm?
 

I know in the most recent instance I've looked at replacing a chain with a belt I was looking to remove slop from the system and would be using GT2 profile belts and pulleys to do so.

Re: Belts 9mm or 15mm?
 

Belt offers more advantages over chain than just weight savings. They are slightly more efficient, they are cleaner, they do not stretch, and they do not require maintenance. We have used 24 tooth pulleys with 15mm HDT belt on both 4" and 6" wheels with 3 CIM single speed gearboxes for 4 seasons. We have never broken a belt. If you size your pulley correctly for the wheel and use exact center to center, you should only have to touch your belts one time, I can not say the same for chain.

Re: Belts 9mm or 15mm?
 

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.