Belt Drive Design Problem
 

Hi CD,
For the first time I am trying to design a drivetrain with belts. I would like to keep everything in the minimum size.
**I will attach a photo of the drivetrain I want to convet to belts.
We are using a direct drive with hex shaft. the problem is that Vex doesn't offer pulleys it the same diameter as the hex pulleys. adding a hex hub over the pulleys will make the thickness of the wheel+pulley+hexhub+bolts+nuts too thick. I looked at andymark solution in their AM14U, they made holes in their pulley so the hub will fit in the pulley. I am looking for a solution that will work with vex wheels because andymark pulleys doesn't have the versa key pattern. Andymark variety of pulleys and belts is pretty small so I can allow my self using it.

Re: Belt Drive Design Problem
 

If you are using VersaWheels with Vex pulleys, you may not need to add a hex hub to both the wheel AND the pulley. Just use a hex hub on the wheel and maybe put a hex bearing in the pulley itself just to support the pulley on the shaft. You can rely on the screws and the VersaKey pattern to transmit torque between the pulley and the wheel. Just like the sprocket your old design has doesn't have a hex bore but works just fine, the same is likely true for pulleys.

Re: Belt Drive Design Problem
 

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.

Re: Belt Drive Design Problem
 

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.

Re: Belt Drive Design Problem
 

1. We switched from chain to belts years ago, and are so glad we did. I would recommend that everybody do it!

2. If the wheel and the pulley are on the same hex shaft, the pulley shouldn't need to be attached to the wheel in any manner--the pulley turns the shaft, which turns the wheel.

Re: Belt Drive Design Problem
 

I think you want VersaPulleys

Re: Belt Drive Design Problem
 

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.

Re: Belt Drive Design Problem
 

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

Re: Belt Drive Design Problem
 

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".

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.

Re: Belt Drive Design Problem
 

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.

Re: Belt Drive Design Problem
 

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.

Re: Belt Drive Design Problem
 

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.

Re: Belt Drive Design Problem
 

To add another data point, my team has converged on 24T pulleys and 15mm wide belts for 4" and 6" wheels without incident.

These are my rules of thumb from the last few years of drivetrains, what i've seen other teams do, etc. Use at your own risk, your mileage may vary:
- 15mm can run 24T and larger without tensioners in a "standard" traction tank drivetrain
- 9mm can run ~36T and larger without tensioners in a "standard" traction tank drivetrain. (I'm less sure on the number I'd start being okay using 9mm belts with)
- 9mm belts with 24T and smaller pulleys run a high risk of failure in a "standard" traction tank drivetrain - I would try at least 27T if you must use the 9mm profile.

Really, the big takeaway here is that 15mm belt is so much more forgiving than 9mm belt that I would much rather run 15mm than 9mm in any drive.

Re: Belt Drive Design Problem
 

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.

Re: Belt Drive Design Problem
 

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.