Belts in a Drivetrain
 

So I have been working on a WCD style drive train, and I was wondering what peoples experiences with belts have been. I have been looking at HTD timing belts, and from the reading I have done, some people have had issues with the 9mm wide belts. I was wondering if this was a common issue or the result of improper tensioning. Are 9mm belts sufficient in an FRC drivetrain or do I need to look at 15mm wide belts?

Our team has never built a drivetrain with belts before so I want to do as much research as I can before we do this in the offseason. If you have any other tips you learned from your experiences, please share them as they will be much appreciated.

Thanks

Re: Belts in a Drivetrain
 

We've used belts a few times and we've had no issues (apart from actually sourcing the parts one time). Some of the key things to remember are, of course, to tension properly, but also make sure that there is nothing caught in the sprockets. This past competition (district event) we had some tape and cables get caught in the sprockets for our drive train and it was not driving properly.

Kitbot has a really nice sprocket system built into it (well, the holes at least), and we've not had any issues with tensioning. If that's an option for you you may want to consider it.

Re: Belts in a Drivetrain
 

We run 9 mm belts on our 6 CIM 6WD Versa-chassis. We often play a strong defensive position and so far they have held up reasonably well. We've had a few break after hard pushing matches but we believe it was because of them going out of tension. If our center wheel wasn't direct driven we'd probably be more comfortable running 25 chain or a 15 mm belt that can better handle being out of tension.

Re: Belts in a Drivetrain
 

We used 9mm belts this season with 30 tooth pulleys. Exact Center-Center distance, we added in some delrin blocks because of some rubbing issues on our bellypan but they weren't needed at all for tension. We're going to try 24 tooth pulleys next, as it will fix the rubbing.

Here's a Google Doc you may find interesting.

Re: Belts in a Drivetrain
 

That is a pretty awesome spreadsheet! It looks to me like proper tensioning and/or proper C-C design is more important than belt width. On a side note, where do people buy colson wheels? I have looked online and found a few combat robotics websites that sell them, as well as regional distributors. Belts and colsons seem to go hand in hand for some teams and I'm curious.

Re: Belts in a Drivetrain
 

9 or 15mm belts should do it - 15mm is the 'safer' bet.

You can get colsons from various robotics sites or McMaster. McMaster is, of course, ridiculously overpriced though. When we bought ours for this season, they were out of stock and I found them on some sketchy website. They came with bearings, but we just popped them out.

Re: Belts in a Drivetrain
 

We went with 9mm belts and 24t pulleys tensioned by cams on VersaBlocks. It's amazing, we've never had problems with ratcheting, and the drivetrain is noticeably more efficient. If you tension properly, 9mm shouldn't be a problem (given reasonably sized pulleys).

Re: Belts in a Drivetrain
 

Yep, according to the google doc I listed above (I believe I got that through Picone, so credit there), I wouldn't go under 24t.

Re: Belts in a Drivetrain
 

Could you put your information in the spreadsheet? With the new versachassis + belts + pulleys from VEXpro your specific setup will become more common, and your data point would be useful.

Re: Belts in a Drivetrain
 

So how do you know what the correct tension is for a belt? I have always just gone by feel but wouldnt an over tensioned belt be bad?

I know there are official force measurements on Gates' website but is there an easier way to measure this like over a certain length it should deflect this much with this much force?

Re: Belts in a Drivetrain
 

Done. Thanks for reminding me, I've found looking at that spreadsheet very useful in the past.

Re: Belts in a Drivetrain
 

This year we went with a 6 CIM 2 speed 6 wheel drive using colsons and geared with a high speed of ~15.5fps and low of ~5.5fps. We used 15mm belts and pulleys with nearly everything purchased from VexPro.

We were the single strongest drivetrain in terms of pushing power of any other robot in both Dallas and Oklahoma. We faced a self-proclaimed "unpushable robot" and bulldozed them from one side of the field to the other.

While all of the above is true, and the belts did well, I personally would rather recommend using chain for the following reasons:

-Breaking:
At Oklahoma (2nd regional), we broke 3 different belts. I think (I am not 100%) that they were a set used either partway in Dallas, or on our practice bot, so they were not brand new. The cause of the breaking is known. In Dallas and on our practice bot, we used encoders as servos (Im not a coder dont get mad at me). The class in the code also contained a system that would quickly ramp up the power from 0 to 100%, even if full yoke was suddenly applied. In early Oklahoma, one of our encoders broke (the mechanical linkage broke), so the servo class was temporarily removed, and thus the power ramp function. With 3 CIMS geared to 5.5fps, there is an utterly massive amount of torque. Without the power ramp, going from 100 to -100 instantaneously is little problem for the CIMs, but a major problem for the momentum, so somewhere something had to give. The belts did. I would expect the larger gauge of chain would be capable of handling that torque, even if it meant we were spinning the tires.

-Replacing:
In many cases, replacing a chain is much easier than a belt. In our drive you had to remove both pulleys to slip a new belt on, which took a fair amount of time. With chain you can replace one with everything else still fixed. If you break a belt, you throw it away. You break a chain, it is usually repairable.

-Buying:
On our robot, each pair of belts (there were 3 on each side) was a different length. This was mostly due to the fact that with our compact robot, the gearboxes had to go where there was room for everything else. We bought 4 pair of belts (8 of each size). The problem was, when we broke belts, we kept breaking the same one (even with our redundant system: if any one breaks, at least 1 wheel would still turn). When we ran out of that one kind of belt, there were no more, even though we had 2 extra of a different belt. With chain, you just chop it to length. Bring 20' of chain and you could replace each loop once, or one single loop 10 times, depending on your needs.

-Space:
We needed 15mm belts for our drive, as already stated we still snapped them. 9mm would have been worse. ANSI 35 chain is 0.1" thinner than 15mm, and both require slight clearances (guides on pulleys!). While you don't save much, it does add up. Using ANSI 25 chain like we did last year (with no breakages on our 4 CIM 2 Speed 6wd robot) you save 1/8" vs 15mm

-Weight:
While belts are lighter than chain, I think it is not light enough to be worth it.



Overall I was satisfied with the belts this year, and they definitely do work, but as much as I hate to say it, I prefer chain much, much more.

Re: Belts in a Drivetrain
 

Added our experience with Versa blocks. We like them. Easy to tension. Sprocket centers just where the calculator said they should be. Two speed versa gearbox with 36/30 belt drive to the center wheel which drives the outboard wheels. Outboard wheels are driven by 30 tooth pulleys Started with 9mm belts all around. We changed the primary drive belt to 15mm because the 9mm belts were breaking. We have left the outboard drive 9mm with slightly less tension that allows them to ratchet rather than break.



We are using Cheesy Poof style control which seems to help with ramping the violent direction changes.

Re: Belts in a Drivetrain
 

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.

Re: Belts in a Drivetrain
 

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. :rolleyes:

Re: Belts in a Drivetrain
 

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. :D

Re: Belts in a Drivetrain
 

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.

Re: Belts in a Drivetrain
 

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 :p ), 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!

Re: Belts in a Drivetrain
 

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.

Re: Belts in a Drivetrain
 

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.

Re: Belts in a Drivetrain
 

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.

Re: Belts in a Drivetrain
 

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

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.

Re: Belts in a Drivetrain
 

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.

Re: Belts in a Drivetrain
 

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.

Re: Belts in a Drivetrain
 

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.

Re: Belts in a Drivetrain
 

B&B manufacturing is another decent option.

Re: Belts in a Drivetrain
 

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

Re: Belts in a Drivetrain
 

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.

Re: Belts in a Drivetrain
 

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.