Belt Drive Design Problem

[Mike Marandola]

Originally Posted by Travis Schuh

Why are you looking to switch from chain to belts? I caution against using belts in situations where you do not have active tensioning. Chain (even #25) is more forgiving for a design like this.

I agree that chain may be better in this design, but what about the AM14U? It has no active tensioning.

[Chris is me]

Originally Posted by Monochron

Can you explain why this is the case? As I understand it, belts are great if you have perfect C-C distance with no need for tensioning. Chain however, due to their slight stretching, does require tensioning.

We're now entering our 5th season with belts, and our 4th season using exact centers with no tensioners. As long as you pick a strong enough belt / pulley combination, this setup will have no problems at all. There is no noticeable stretch or wear over an FRC robot's life span. We put the drive together Week 4 and never touch it again. Perhaps for other applications you may want a tensioner, but in a drive we have done this year after year and it's just "set it and forget it".

Originally Posted by sdcantrell56

Due to belt tolerances, the only way to get perfect c-c distances would be to lay out and measure the actual belt and pullies. This solves it for that particular belt but what happens when you have to replace it? Last time I checked the belt tolerance is +-.010 on the circumference so the chances of it being perfect with another belt are slim. With a means of tensioning the belt this problem disappears.

I would actually argue (it sounds totally nuts, but hear me out) that for the majority of teams, adding tensioners to a belt system will make it more likely to fail. For the precise, detail oriented teams (e.g. 971, 254, etc) tensioners are probably a performance boost to account for that tolerance.

However, for the average team, adding tensioners makes it very easy to over or under tension a belt. It is surprisingly easy to over-tension a belt as a "perfect" center distance belt has more slack than you would expect. Overtensioning a belt significantly weakens the system. In some specific cases with under-sized belts and pulleys, this can and absolutely has led to drive failure. Another common problem is include differing tension in two belts on the same driveline. Again, teams that pay a lot of attention to detail and design great tensioners can find success, but it's easier to fail a tensioned system than an untensioned system in my experience.

Despite the wider tolerance in belt length, we've just never had a problem doing it this way. It just works. We've done this to at least 16 individual belts in different drivetrains now.

[pfreivald]

Originally Posted by Chris is me

We're now entering our 5th season with belts, and our 4th season using exact centers with no tensioners. As long as you pick a strong enough belt / pulley combination, this setup will have no problems at all. There is no noticeable stretch or wear over an FRC robot's life span.

Agreed. 15mm belts are plenty durable for the abuse of one FRC season--though you should design your drive for easy swap-outs just in case.

[Travis Schuh]

Originally Posted by Mike Marandola

I agree that chain may be better in this design, but what about the AM14U? It has no active tensioning.

My statement is colored by our experience, which was with small pulleys (22T I think) with 9mm wide 5mm GT3 belts. We know that we are running these belts out of spec, and it shows because we do have belts break. Previous years we used the same set of belts until practice for off-seasons, but last year we went through some belts during season practice due we think to the higher CG design. I would not feel comfortable running our current setup without a tension. For this year, we are looking at moving to 15mm belts so we are not as under the rated specs (we are currently running ~2X the rated load for the belts based on my memory of the calculations we did).

I see that the AM14U runs 42T pulleys and 15mm belts. This is should be 3X better on the loading than what we are doing (not including the rating difference of HTD vs GT3), but without running the life numbers, I still bet the belts are still close to the rating for this application. If you want to run belts without tensioners, I would follow with this pattern (looking back at your pictures, it appears like this is what you are doing).

Based on quick calculations from AM's listed weights, it looks like a AM14U has under 0.4lb of belts, and would require about 1lb of chain. I don't see that as a huge weight difference, particularly to pay for drivetrain reliability. 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.

[Deke]

Originally Posted by Travis Schuh

Why are you looking to switch from chain to belts? I caution against using belts in situations where you do not have active tensioning. Chain (even #25) is more forgiving for a design like this.

Originally Posted by Travis Schuh

My statement is colored by our experience, which was with small pulleys (22T I think) with 9mm wide 5mm GT3 belts. We know that we are running these belts out of spec, and it shows because we do have belts break. Previous years we used the same set of belts until practice for off-seasons, but last year we went through some belts during season practice due we think to the higher CG design. I would not feel comfortable running our current setup without a tension. For this year, we are looking at moving to 15mm belts so we are not as under the rated specs (we are currently running ~2X the rated load for the belts based on my memory of the calculations we did).

...(snip)...

Based on quick calculations from AM's listed weights, it looks like a AM14U has under 0.4lb of belts, and would require about 1lb of chain. I don't see that as a huge weight difference, particularly to pay for drivetrain reliability. 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.

Just curious if you can explain your team's design decisions on going with belts over chain for drive train. I don't have much experience with belts in a drive application, it would be helpful to understand the thought process.

It seems like there is a lot of caution with your advice on belted drives, and chain seems like the superior approach with this guidance.

[Travis Schuh]

Originally Posted by Infinity2718

Just curious if you can explain your team's design decisions on going with belts over chain for drive train. I don't have much experience with belts in a drive application, it would be helpful to understand the thought process.

It seems like there is a lot of caution with your advice on belted drives, and chain seems like the superior approach with this guidance.

It isn't as dreary as I paint it. We have just been recently thinking about how to get that last bit of reliability and performance out of our drive train, so this has been on my mind.

971 uses belts primarily because the pulleys integrate into our design better than a sprocket (we can bore out and glue modified COTS pulleys into our integrated wheel module, where there isn't a COTS sprocket that I know of that we could make do this). Beyond that, there is a nice benefit that belts are lighter than chain and run pretty quiet. If we ran a WCD, I would run #25 chain like 254 does. It turns out that #25 is also out of spec for a drive application, but it appears to handle it more gracefully.

[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.

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.

[yarden.saa]

Thanks, I found a way that I like.
See attachments

[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.

[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.

[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.

[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.