Belts in a Drivetrain

[Chris is me]

If you have the manufacturing capability, there is no reason not to use exact centers for belt drivetrains. Absolutely none. It's effective, idiot proof, and reliable. It's not that hard to under tension or over tension a belt, which leads to reduced belt life. I just don't understand how people can prefer tensioners for a system whose tension never needs to be adjusted (in the context of an FRC robot life span).

15mm is far safer than 9mm for a drivetrain. We had poor experiences with 9mm belts and 18T pulleys our first year using the system. Since we've gone to 15mm the problems have gone away for our particular wheel size and pulley tooth counts. This year we used 15mm belts and 24T pulley stock with zero problems.

The bigger the pulley tooth count, the more you can get away with using a 9mm belt in the drive. I'm not entirely convinced yet that 9mm 24T belt drives are a reliable way to do a "West Coast" style drive. Bigger tooth counts should be fine. Any discussion of the strength and validity of belts *must* include the pulley tooth count or else the anecdote isn't useful.

[Bill_B]

One thing belts can do in FRC is pick up tape. We found this wrapped around the drive pulley. Field admin didn't seem to want it back.

[Mason987]

987 has ran 9mm-wide 5mm HTD belts for the last 5 or so years. Our outside wheels are lifted up slightly so we have a center drop, and the center wheel is driven by the gearbox. There is a 20mm wide pulley on the center wheel axle that has two 9mm belts going to 9mm wide pulleys on the outside wheel axles. I'm rebuilding this years assembly right now so I don't have pulleys in this photo, but you can see how we keep our belts tensioned properly.

http://i.imgur.com/k3bs57U.png (attached the file for anyone who cant access imgur at work/school)

There was a thread a few days ago that was very similar, and a lot of teams seemed to avoid tensioning their belts like this. I believe one of the specific arguments against it was that you were now relying on the static friction of the bolts on the rails to hold your bearing blocks in place and keep the belts tensioned, but we've never seen them come loose (although it is one of the things on our prematch checklist). A little bit of locktite and a washer on each bolt also helps immensely.

[MrForbes]

Originally Posted by Bill_B

One thing belts can do in FRC is pick up tape.

This seemed to be a common problem Friday at Arizona. The field crew was repairing the field with short pieces of tape, which the Kit drivetrain likes to suck off the floor, into the belt. The main cause seems to be that the pulleys and belts are about half an inch off the floor so any loose corner will be caught and pulled up. A mentor on another team brought this to the attention of the head referee, and it appears that on Saturday they repaired the field with long pieces of tape, instead of trying to fix it with short pieces.

[craigboez]

Originally Posted by Chris is me

Any discussion of the strength and validity of belts *must* include the pulley tooth count or else the anecdote isn't useful.

Agreed, thought I'd go one step further and say it also needs to include wheel size. What we're really discussing is how much torque can a given belt/pulley setup transmit. If we assume robot weight and wheel CoF are constant, then a pulley on a 6" wheel needs to transmit 50% more torque than a 4" wheel to put an equal force vector to the ground.

[Joey Milia]

Originally Posted by Chris is me

If you have the manufacturing capability, there is no reason not to use exact centers for belt drivetrains. Absolutely none. It's effective, idiot proof, and reliable. It's not that hard to under tension or over tension a belt, which leads to reduced belt life. I just don't understand how people can prefer tensioners for a system whose tension never needs to be adjusted (in the context of an FRC robot life span).

15mm is far safer than 9mm for a drivetrain. We had poor experiences with 9mm belts and 18T pulleys our first year using the system. Since we've gone to 15mm the problems have gone away for our particular wheel size and pulley tooth counts. This year we used 15mm belts and 24T pulley stock with zero problems.

The bigger the pulley tooth count, the more you can get away with using a 9mm belt in the drive. I'm not entirely convinced yet that 9mm 24T belt drives are a reliable way to do a "West Coast" style drive. Bigger tooth counts should be fine. Any discussion of the strength and validity of belts *must* include the pulley tooth count or else the anecdote isn't useful.

I believe one benefit of tensioners is that, because of manufacturing tolerances in the belts, exact centers won't always provide the same tension. For this reason, if you're using 9mm belt I think you should have tensioners to dial in the exact tension you need without relying on perfectly accurate belt lengths

That said, we use fixed centers but add a bit to the distance depending on how the belts fit. We've been very successful the past two years using 15mm wide belt on 22t pulleys inside our DT tubes driving 3.5in wheels. We have 1/2in hex in the pulleys in the tube, 1/2 in round bearings, and 7/16 hex in the wheels. It's a set up I hope stays around for a while.