Belt Drive Design Problem

[yarden.saa]

Thanks, I found a way that I like.
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[philso]

In your latest renderings, it looks like the pulleys on the two middle wheels on each side will have two belts on them. Will there be any problems with the edges of the two belts rubbing against each other and causing wear of some sort?

[Chris is me]

Originally Posted by philso

In your latest renderings, it looks like the pulleys on the two middle wheels on each side will have two belts on them. Will there be any problems with the edges of the two belts rubbing against each other and causing wear of some sort?

Provided proper tension or exact spacing is used, nope, teams have done this for years without issue. Remember, the belts are not in motion relative to each other when they're on the pulley.

Originally Posted by Oblarg

Re: 9mm versus 15mm belts, it's worth noting when calculating the loading that in a 6-wheel drive train, the center wheels are taking far more load than the front and back ones. I'm much more comfortable running 9mm from a center wheel to the outer wheels in a 6WD than I would be going between center wheels on an 8WD.

Many teams with similar setups are doing the exact same thing, actually. I wouldn't use six wheel drop alone as a reason to go to 9mm. 42T pulleys are more than big enough.

4464's current preseason design uses 9mm belts, simply because it's extremely convenient to be able to only have one pulley on the center wheel. We're also using 42-tooth pulleys.

IMO this is a really bad reason to pick a profile - you can just buy two 7mm slices with your pulleys to make them 32mm wide. That said, 9mm belts on 42T pulleys doesn't sound too bad.

[Oblarg]

Originally Posted by Chris is me

IMO this is a really bad reason to pick a profile - you can just buy two 7mm slices with your pulleys to make them 32mm wide.

Sure, we could do that, but it would a) cost more money and b) take up about an additional inch of robot width.

Looking at the rated torques, I think it extremely unlikely that we'll ratchet or break a belt with our setup (especially given that the torque ratings Gates gives are for extended use, and are somewhat lower than the effective maximum torque for the short service times they see in an FRC robot). 449 used 9mm belts on 36t pulleys without incident last year.

[Travis Schuh]

Originally Posted by Oblarg

Re: 9mm versus 15mm belts, it's worth noting when calculating the loading that in a 6-wheel drive train, the center wheels are taking far more load than the front and back ones. I'm much more comfortable running 9mm from a center wheel to the outer wheels in a 6WD than I would be going between center wheels on an 8WD.

While I would agree that on average this is true, I would argue that there can be cases where the outer wheels will be loaded with all of the robot torque, and thus it is appropriate to run the FOS calcs assuming this as worst case. Depending on the height of your CG, you can get significant load transfer to your outer wheels when accelerating. If you accelerate and the robot goes up on its back wheels, then the outer wheels are taking all of the torque (our robot this past year would do this).

[AdamHeard]

A few more data points on pulley sizes, etc...

https://docs.google.com/spreadsheets...7UU/edit#gid=0

Originally Posted by Travis Schuh

While I would agree that on average this is true, I would argue that there can be cases where the outer wheels will be loaded with all of the robot torque, and thus it is appropriate to run the FOS calcs assuming this as worst case. Depending on the height of your CG, you can get significant load transfer to your outer wheels when accelerating. If you accelerate and the robot goes up on its back wheels, then the outer wheels are taking all of the torque (our robot this past year would do this).

Very much agreed, giant mistake to neglect this case.

[philso]

Originally Posted by Chris is me

Provided proper tension or exact spacing is used, nope, teams have done this for years without issue. Remember, the belts are not in motion relative to each other when they're on the pulley.

I am more concerned with the radial motion of the belt towards the pulley when the belt is coming onto the pulley and the radial motion of the belt away from the pulley as it is leaving the pulley. I am interested to know if the total run time in an FRC robot application will cause significant wear.

[Chris is me]

Originally Posted by philso

I am more concerned with the radial motion of the belt towards the pulley when the belt is coming onto the pulley and the radial motion of the belt away from the pulley as it is leaving the pulley. I am interested to know if the total run time in an FRC robot application will cause significant wear.

I was also speaking from experience here. We cram belts right next to each other on pulleys without flanges and we've never had significant wear on the belts, even on the belts with over 100 hours of run time. This is totally fine in FRC. The window for belts not moving in the same direction to contact each other is very very small.

Originally Posted by Oblarg

Looking at the rated torques, I think it extremely unlikely that we'll ratchet or break a belt with our setup (especially given that the torque ratings Gates gives are for extended use, and are somewhat lower than the effective maximum torque for the short service times they see in an FRC robot). 449 used 9mm belts on 36t pulleys without incident last year.

I wasn't very clear in my post - I was trying to say 9mm / 42T is likely just fine. I just meant that picking 9mm over 15mm just because of the double pulley feature, independent of any other features, is not great.

[Joey Milia]

Originally Posted by philso

In your latest renderings, it looks like the pulleys on the two middle wheels on each side will have two belts on them. Will there be any problems with the edges of the two belts rubbing against each other and causing wear of some sort?

192 hasn't had a problem with this on the 5mm pitch drive belts that share a pulley. However on the prototypes of the 2013 gearbox the higher tension 3mm pitch belts would drift more and rub. This was made worse by their arrangement where they crossed over each other. We saw the sides of the belts wearing down so we added a flange on the final version to prevent the rubbing. Should be noted no damage was done to the tensile members and only to the rubber backing.

[Oblarg]

Originally Posted by Chris is me

I wasn't very clear in my post - I was trying to say 9mm / 42T is likely just fine. I just meant that picking 9mm over 15mm just because of the double pulley feature, independent of any other features, is not great.

Well, obviously we didn't make the decision in a vacuum, but it was the primary reason we considered 9mm in the first place.

[snoman]

Originally Posted by yarden.saa

H
adding a hex hub over the pulleys will make the thickness of the wheel+pulley+hexhub+bolts+nuts .

why can't you make your off sets longer or move frame rail in

[DampRobot]

Originally Posted by Travis Schuh

Last year I saw a few WCD that chose to run belts with small pulleys, and were running competitions without wheels powered because the belts broke and it is very difficult to replace the belts. My opinion is that if you don't have a good plan for how to change a belt mid competition if it breaks (and preferably a way to tension the belts properly to help keep them from breaking), then you are probably better off with chain. This doesn't mean belts aren't working for teams, I just caution the mass movement to put belt drive trains.

Travis, I assume you're referring to 100. For those that don't know, 100 ran 9mm vex belts last year on 4" wheels (on 24t pullies IIRC, it's been a while). They ran fine all through SAC, but at SVR we broke 4 separate belts. At first, it seemed like an overtension failure, but we replaced the belts and I personally made sure they were undertensioned, they failed in just the same way. As it took a while to replace the belts, we did run without driving some wheels in some matches just to make it out onto the field.

At this point, I blame those failures on a bad run of VP belts rather than the inherent disadvantages of belts or non center to center designs. However, the whole experience tempered my enthusiasm for belts. One of the things that most people fail to realize with belts is that you have to do your whole assembly with them in place, unlike chain, which you can slip around and masterlink after everything's assembled. For WCDs, this means you have to take apart your gearboxes if a belt breaks.

Having done both belt and chain WCDs in the past, I would probably advise on using #25 chain. The weight savings of belts are tiny, noise shouldn't be a major issue and for most teams the efficiency gains of belts could be more easily gotten in other places. On the other hand, teams can definitely be successful with belts even their first time around as long as they spec their components conservatively and have spares.

[Chris is me]

Originally Posted by DampRobot

At this point, I blame those failures on a bad run of VP belts rather than the inherent disadvantages of belts or non center to center designs.

I really don't think it's either of these - I think 9mm belts on 24T pulleys are just very undersized for your application. You will likely continue to have failures even with other brands of belts. If you can't go above 24T pulleys, you have to go to 15mm wide belts if you want to be safe.

(I know you are probably aware of this by now, but I just wanted to say this publicly for the reference of others reading this thread.)

[AustinSchuh]

Originally Posted by Chris is me

I really don't think it's either of these - I think 9mm belts on 24T pulleys are just very undersized for your application. You will likely continue to have failures even with other brands of belts. If you can't go above 24T pulleys, you have to go to 15mm wide belts if you want to be safe.

(I know you are probably aware of this by now, but I just wanted to say this publicly for the reference of others reading this thread.)

971's experience has been that 24 T pulleys are just on the edge, though we have been using 3.5" wheels. The old GT2 belts weren't quite strong enough (failed after a hard season), but the newer GT3 belts are just barely strong enough to last a long season. We haven't had a GT3 belt fail, once we failed a set in the first hour from over-tensioning. If you believe the Gates manuals, GT2 and GT3 belts are quite a bit stronger than HTD, and everything doesn't seem rated for long life with the loads they'll see in FRC.

We have found that proper tension is crucial. If the belts are under-tensioned, they will ratchet. If they are over-tensioned, they will break. The line seems to be finer than we'd like. We run lots of un-tensioned belts, but will never run an un-tensioned drive belt. The reward just isn't there.

That being said, we are going to run 15mm wide next year for that extra peace of mind. We can see the pulleys starting to wear.

[Joe G.]

Curious -- has anyone in this thread experimented with belt pitches other than 5mm? A quick look at the gates datasheet suggests that a 22t (minimum size) 8mm pitch sprocket with a 12mm wide belt will have slightly more carrying capacity than a 15mm wide 5mm pitch belt with a similar sprocket diameter, and significantly more than the equivalent 9mm wide belt. I'm always trying to squeeze some extra width out of drive designs, but have heard just enough 9mm belt horror stories to discourage me from pursuing them.