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Unread 08-11-2018, 10:15 AM
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Rubber Belting

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?
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Unread 08-11-2018, 10:27 AM
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Re: Rubber Belting

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
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Last edited by AriMB : 08-11-2018 at 10:32 AM.
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Unread 08-11-2018, 10:29 AM
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Re: Rubber Belting

Youíll also want to be familiar with crowned pulleys. Itís easy enough to make them using some string and electrical tape.
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Unread 08-11-2018, 10:31 AM
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Re: Rubber Belting

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Originally Posted by AriMB View Post
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.
I see so then you only need some sorta pulley? Doesn't necessarily need to have teeth or something?
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Unread 08-11-2018, 10:35 AM
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Re: Rubber Belting

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Originally Posted by risho900 View Post
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.
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Unread 08-11-2018, 10:36 AM
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Re: Rubber Belting

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Originally Posted by risho900 View Post
I see so then you only need some sorta pulley? Doesn't necessarily need to have teeth or something?
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.
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Unread 08-11-2018, 10:37 AM
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Re: Rubber Belting

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Originally Posted by marshall View Post
Youíll also want to be familiar with crowned pulleys. Itís easy enough to make them using some string and electrical tape.
Do you mind explaining that?
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Unread 08-11-2018, 10:43 AM
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Re: Rubber Belting

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Originally Posted by Billfred View Post
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.
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.
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Unread 08-11-2018, 11:06 AM
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Re: Rubber Belting

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Originally Posted by AriMB View Post
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 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.
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Unread 08-11-2018, 12:20 PM
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Re: Rubber Belting

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.
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Unread 08-11-2018, 01:47 PM
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Re: Rubber Belting

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.
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Unread 08-12-2018, 09:31 AM
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Re: Rubber Belting

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Originally Posted by nuclearnerd View Post
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?
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Unread 08-12-2018, 09:48 AM
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Re: Rubber Belting

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Originally Posted by risho900 View Post
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).
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Unread 08-12-2018, 06:14 PM
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Re: Rubber Belting

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Originally Posted by risho900 View Post
Do you mind explaining that?
Place some string under some electrical tape to act as the crowned pulley.
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Unread 08-12-2018, 06:26 PM
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Re: Rubber Belting

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Originally Posted by risho900 View Post
Do you mind explaining that?
https://www.youtube.com/watch?v=6sM0Qjumyro
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