Belts in a Drivetrain

[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.

[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.

[Teamcodeorange]

This is 3476's first year using belts on a drivetrain. We are a 6wd "wide" configuration using 4" AM performance wheels (w/ nitrile tread) and 3/32 center drop. We are using VexPro HTD 15mm belts, Ball Shifters with 4 CIMs (plus some runtime with 2 550s) and 24tooth VexPro pulleys on all axles. Geared for 4/22 fps. I do admit our shafts are a bit overbuilt using 5/8 7075 aluminum stock through round bearings (hex bearings...don't get me started on those). Machined to exact C-C + 0.003.

We have driven this thing through some very tough defense at SDR and many nights of drive practice, and have run it at very high speeds with very quick stops. Not a single ratchet or failure (with everything together, that is ), and it would be far from the truth to say that we have perfect manufacturing capabilities.

We have still quite a few official and unofficial events until the season is over so we won't draw any conclusions yet...but with the way we drive and the nature of this year's game I am pretty happy with our decisions so far.

Good luck!

[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.

[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.

[kevincrispie]

971 has run belts in their drivetrains for 3 years now so we have seen both the failures and successes of belt drivetrains. Below are some thoughts from what the team has learned. Sorry for the long post, but we've learned a lot, so I think it's best to get the information out there.

Tensioning belts "correctly" is not necessarily obvious. I see people saying that you should always tension belts coreectly, but to many teams the correct tension is difficult to attain. Some teams might take "correct" tensioning to be a tight belt, but this is not the case. An overtensioned belt can be just as bad as an undertensioned one. Tensioning belts correctly often invovles finding some sort of sweet spot that is not too loose or too tight. 971 has started to tension based on the tone created when strumming the belt. We have found that depending on the tension, the frequency of the tone is different. We are using 45 Hz right now. A smartphone app such as this one can be used to analyze the tone. The frequency can be calculated given certain equations; I think you can find one on the Gates website.

A rule of thumb is also if teeth are coming off, the belt is undertensioned. The reason this happens is that the teeth ride up out of the pulleys on the slack side. This puts a bending force on them, and the root of the belt tooth is a good stress riser, so the tensile elements break there as opposed to somewhere else.

If tensile elements are tearing out of both sides, the belt is wearing out, possibly prematurely because of overtensioning. If you have wear or some sort of discoloration in the recesses of the belt and noticeable wear on the outer parts of the pulleys, then the belt is probably overtensioned.

For many FRC applications, running belts a little overtensioned is at times necessary, but it's still best to know the amount you're overtensioning your belts. Overall, knowing the warning signs and being consistent with how you tension your belts is going to help your team in the long run.

Another thing to consider when analyzing belt failure is where your robot's center of gravity is. When your cg is higher, you'll get more load on your outer wheels. If you are rocking on the outer wheels of a 6wd while applying full power, you can assume that all torque is being put through your drive belts. We think this is one reason why we we have had belts break under the same conditions last year. As many people can attest, 971's 2014 robot has a much higher cg than 2013's. This could be the case for your team. Be wary of how high your cg is and remember that it can affect your drivetrain too.

As for junk getting caught in belt drivetrains, I have seen this too. Junk seems to get caught in many FRC drivetrains, so its not an isolated problem. With belts, its important to inspect the drivetrain and clean in before a match. Cleaning debris from a drivetrain early is going to be a lot lower maintenance than removing gobs of carpet off a drive pulley.

Another thing: In terms of belt quality, we prefer the Gates GT2 over the VEX HTD belts. We like the manufacturing quality of the GT2 belts so far.

We've found documents like this and this are extremeley useful. It's important that before a team jumps from chain to belt that they understand how to use them properly. Belts aren't a magic elixir that will make your drivetrain better immediately. A major change in the drivetrain requires thorough research and testing. Before running belts in our drivetrain, we ran a timing belt elevator, and before we did that we did stress tests to make sure we could do it.

That being said, we still really like belts, and will continue to use belts in many applications, including but not limited to our drivetrains.

[Chris Fultz]

We did an evaluation a few years ago, comparing Belt to Chain for a drive system. There are good and bad with both options. The report is linked below.

In 2013 we used 15mm belts on an 8 wheel drive with no problem. We had two belts from the gearbox to the center-most wheels, and another belt from that wheel to the outer wheels (total of 4 belts per side). These were Gates.

For 2014, we are direct driving the center wheel on a 6 WD and have one 9 mm belt going to front and one to the back (total of 2 per side). We have seen some wear on our prototype robot and it has probably 100 or more hours. So far no wear on the competition bot but it has limited practice time and about 45-50 matches on it. We have VEXPRO belts on the prototype robot and Gates on the Competition robot.



http://www.chiefdelphi.com/media/papers/2216

[m1506m]

This belt set up had been solid for us all season. And it's amazing just how quiet belts are in comparison to chains. No sign of wear on the belts, these are the ones from vex pro.

[Oblarg]

I have been an advocate of 15mm timing belt over chain for drives for as long as I've had experience with both systems; there's simply less servicing, fewer failures, fewer losses, and less headache with belts.

The kitbot belt setup this year has worked wonderfully for us.

[JeremyLansing]

Originally Posted by kevincrispie

snip

Another thing: In terms of belt quality, we prefer the Gates GT2 over the VEX HTD belts. We like the manufacturing quality of the GT2 belts so far.

snip

Since you did not buy your belts and pulleys from Vex, where did you get them from? Should I be looking for local distributors, or are there some good websites to look at.

[kevincrispie]

Originally Posted by JeremyLansing

Since you did not buy your belts and pulleys from Vex, where did you get them from? Should I be looking for local distributors, or are there some good websites to look at.

We got our drivetrain belts from Royal Supply this year. There is an option for Gates belts on this page. We got belts for other applications at SDP-SI. Both vendors sell pulleys as well.

[AdamHeard]

Originally Posted by kevincrispie

We got our drivetrain belts from Royal Supply this year. There is an option for Gates belts on this page. We got belts for other applications at SDP-SI. Both vendors sell pulleys as well.

B&B manufacturing is another decent option.

[Rauhul Varma]

Originally Posted by JeremyLansing

Since you did not buy your belts and pulleys from Vex, where did you get them from? Should I be looking for local distributors, or are there some good websites to look at.

We have been using belts exclusively from SDP-SI for the last 2 years without any problems.

[kevincrispie]

Some more resources people might find useful:

Gates has a failure analysis guide that can be used to diagnose belt failures. Gates also has a FIRST Robotics webpage with videos and technical notes.

[Aren Siekmeier]

Originally Posted by kevincrispie

971 has started to tension based on the tone created when strumming the belt. We have found that depending on the tension, the frequency of the tone is different. We are using 45 Hz right now.

Won't the fundamental frequency depend considerably on belt length, width, and composition? Or is this test always applied to similar size belts?

Edit: Haha, cool the Gates guide you linked to has a page about using a sonic meter and computing tension force from belt width, weight, etc. Neat I imagine you took this into account. The formula's really not so bad.