pic: Another Belt-In-Tube Drivetrain



Playing with trying to fit 15mm HTD belts into 2"x1.5"x.125" tubing.

Custom 24T pulleys are machined from 5mm HTD pulley stock. Pulleys are 30mm wide to accommodate two 15mm belts.

Belts and pulleys fit inside tube with .125" center drop (techincally .0625" down in the center and .0625" up in the corners).

30mm is a little wider than the 1" gap between the bearings in the tube walls. To compensate, the custom pulleys are pocketed on the ends.

Album here
OnShape model here.

I love this thing! Very clever to counterbore the pulley so that it can use the full width of the tube.
Looks super compact and lightweight, and above all very clean, great work. Interesting that you used OnShape for this, I have a few friends who love it or hate it. :smiley:

Thanks. I’m a little bit concerned about the lack of flanges on the pulleys, but with less than .035" of space on either side I just couldn’t fit them in. That said, the .035" is small enough to let the tube wall be the “flange” if anything goes wrong and the belt tries to walk.

As for OnShape, I’ve expressed my thoughts here.

2 inch square .125 wall 6061-T6 is readily available ( On-line Metals, et al). Seems like this would eliminate machining of cots parts, and potential rubbing of the belts.

It seems like many teams just use 1.5x2, but there’s no need to constrain yourself to it.

I would think the reasoning is to save space rather than just fit the belts in the tube. Losing 1" of width doesn’t seem like a lot, but especially for this year that 1" would have been the deal breaker for me.

Saving space is nice, but my main reason for designing this was an exercise in whether or not I could fit a belt-in-tube drivetrain with 15mm belts into a 2x1.5. When I have the time (aka, offseason) I like to give myself tight constraints in an effort to force myself to think outside the box.

(or inside the tube… eyyyyy)

If you’re doing 15mm belts with a single pulley, you already have to machine your own pulleys from stock. You also already want to leave a nub on the pulley to touch the inner race of the bearing. So adding the counterbore for the bearing isn’t THAT much harder, and it makes the entire chassis more compact to do it this way.

The extra inch of space really does help sometimes.

How do you plan to attach your belly pan to this setup? We use belt in tube and I have wanted to go to a smaller tube but I always worry about the rivets rubbing the belt.

An alternative to pocketing the belts is to add .125" thick washer (spacers) to the bearings before putting them into the tube, that makes them flush with the interior or the tube.

We’re fortunate enough to have the ability to weld our belly pan on. You could get clever with staggering your rivets for your belly pan as the belts only take up one side of the tube between the pulleys, but it would make it almost impossible to disassemble if you needed to.

Everywhere there is a hole it is safe to mount to.


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what if you have to pull the chain/belt out of the tube?

Wouldn’t you just pull the shafts out and pull them out that way? I always thought that how everybody did it. You can pull them out to avoid the rivets pretty easily, I would think, especially given that you only have 2 belt runs in a tube.

Are you saying to pull the belt out through the bearing hole? That’s kind of a tight fit.

Regardless, I think this is mostly an academic exercise - this particular belt in tube configuration has by now been battle tested on dozens of FRC robots without failures. It’s a very reliable drivetrain.

Chris, what’re your thoughts on this belt in tube design? IIRC, 2791 has had success with belt in tube.

I mean pull them out from the front or back of the tube. It looks like the front and back are open, so one should be able to just draw them out from there.

The issue is the pulleys. There’s only .030"-.060" of space around the pulleys, so they’d get caught on any rivets poking into the tube (like rivets do).

Could you pocket the tube right above the pulley such that you could pull it out that direction?

I think you’ll be fine with the 1.5 tube and no flanges, good way to do it. I am more concerned about fitting the 24T pulleys in there, are you sure your belt thickness is correct? When I was choosing 22T for our drives I was pretty sure that was as large as could fit. Maybe i was using a different drop than you or something, but there isn’t much room when we assemble, they’re almost rubbing the tube on some of our drives.

I don’t know when this will be but next time i get access to one of the robots i’ll try and take lots of pictures and show how we do assembly and everything. We have it so assembly and disassembly are really really fast.