[Cothron Theiss]

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.)

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?

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.

Originally Posted by 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.

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.

Originally Posted by Oblarg

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.

Originally Posted by Oblarg

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.

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.

Originally Posted by asid61

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.

Preach.