So I’ve seen some teams, especially during Steamworks that use rubber belting to intake balls. This is an example: https://youtu.be/ewTCvLp5EUo?t=51s
How do you make sure that the belt actually spins as the rod spins and prevent the belt from just slipping and only the rod spinning?
Those belts are colloquially known as “flat polycord”, technically “Flat Polyurethane Belting” from McMaster. This is the 3/4" wide version.
The product page specifies a minimum pulley diameter. The belt is pretty grippy (hence why it’s used for intakes). If you use a pulley larger than the one specified that’s not made from a very low friction material, and tighten the belt sufficiently, the belt grips on to the pulley like it grips on to the ball being intook and rolls with it. Using a crowned pulley helps keep the belt aligned without needing flanges. I’ve used the round variant of this, and seen a number of teams using the flat variant, and it is a pretty easy-to-implement solution.
edit: Here’s video I found very helpful in explaining how crowned pulleys work
You’ll also want to be familiar with crowned pulleys. It’s easy enough to make them using some string and electrical tape.
I see so then you only need some sorta pulley? Doesn’t necessarily need to have teeth or something?
Nope, no teeth necessary. In fact, teeth will probably make the pulley less effective because there’s less surface area for the belt to grip on to.
If you have the ability, I would recommend 3D printing the pulleys. It should be easy to make a simple model of a cylinder with a slight bulge in the center and a hex broach for driving. 100% infill and you shouldn’t have any problem with strength unless you plan on banging them against the ground or something.
If you can’t do that, you can probably make them using the Vex Delrin hex spacers and some tape for the crown without too much effort.
Correct–torque is transferred by the friction between the belt and the pulley.
Round belts are also available, which is what I’ve used in the past. Mostly because, for the longest time since they started appearing in robots, urethane flat belts were only available in orange and we were building robots on the campus of a college that feels a certain way about orange. With access to a lathe, pulleys weren’t particularly tough to groove.
Do you mind explaining that?
The only thing I didn’t like about my experience with round polycord was that the pulleys need substantial flanges to not travel. That meant that we had to space the two sets of opposite belts farther apart to provide clearance for the non-compliant balls, which made the belts grip the balls looser than we would have liked. If we had used flat polycord with crowned pulleys, we wouldn’t have needed flanged pulleys and the belts could have been closer together. Certainly there are cases where the round ones would work (maybe even better sometimes), but I think I would use the flat ones if presented with the choice again.
Agreed. The last time I needed to use them was 2017, when 5402 was not under the same constraints about orange being in Indiana…but our (my) knowledge base was in round belts and we had easy lathe access, so we stuck with round belts.
Now that VEX and WCP have black flat belts, I’m inclined to give them a shot next time we need them.
One thing we discovered this offseason is that AndyMark wheels make good pulleys. If you use a belt that’s > than the width of a wheel it works just like a crowned pulley.
Something else to be aware of: The polyurethane belts have to have a decent amount of pre-tension in order to transfer any torque. Multiply that by several belts in a wide conveyor, and you can easily end up with enough tension to deflect a 1/2" hex shaft. Plan to use a stronger tube axle in that case.
Can this remedied by putting an additional bearing support in like the middle?
If you were that hard up, probably. But you’ll increase the chances of binding in the system, which may or may not be a problem depending on your power inputs and desired speed outputs.
Planning to use a VersaRoller or some other tube axle setup from the start is probably going to be simpler and more effective for 99% of applications that use flat belts. For round belts, you’d have to get a little more inventive (but then, you’re already busting out a lathe so there).
Place some string under some electrical tape to act as the crowned pulley.
We used the small colson hex wheels for pulies and the belts never fell off.
I’ll second this. The pre-tension can also make assembly a bit of a pain.
For adding crowns to rollers, our standard has become adhesive-backed foam tape (https://www.mcmaster.com/#5109k53/=1e4z9bf) with a rivet on each end. Credit to 111 for first bringing this method to our attention a few years ago. https://www.chiefdelphi.com/forums/showthread.php?t=145805
Essentially any thin, localized bulge will serve as an effective crown. I’ve seen everything from wrapping electrical tape around a roller over and over, to the string method above. I personally like to get undersize O-rings onto the roller and then tape those down - it’s easier for it to come out straighter. If you want to get fancy, have the rings fall into a small groove you turn into the roller. But there’s really no wrong way to do this, as long as the crown isn’t wider than the belt and that you aren’t trying to capture the belt between flanges (they’ll just ride up on them).
Belt tension is everything here - you need enough tension to transmit torque without slipping, not so much that you overly preload your system. Lots of guess and check here, and get some practice welding the belts effectively.
In addition to the flat belting people have already posted, I’ve had pretty good luck with Round “Poly-Cord” belts. IMO, they’re easier to work with, especially if you use the hollow-tube quick-connect versions (both flat and solid-core round belts require you to melt the ends together which can be tricky). We also 3D printed our rollers, which worked fine for us.
In some sense they are easier - you don’t have to weld the hollow ones, in particular - but be careful. On some game pieces they have noticeably less traction than the flat belts, and they essentially require flanged pulleys or at least some sort of comb-like guide along the path to keep them from walking too much.
The hollow ones are also very prone to stretching over time, so get lots of extra barbs and be prepared to shorten the belts between matches to accomodate the extra tension added.