[Chris is me]

Originally Posted by dellagd

I'm currently designing a prototype frame for possible adaptation in the coming FRC season. To save space, I'm looking to run one of the 15mm belts on each side of the drive train inside of the frame element itself. I know this is commonly done with 9mm belts, but to stay on the safer side I figured we should design for 15mm. The problem I'm having is I'd like to stay with 2 inch x 1.5 inch frame without having to move up to 3 x 1.5, and this mean the only HTD pulley I can fit (accounting for a center wheel drop of 1/8 inch) in the frame is a 18T one. The 24T is just a bit too big from my calculations.

You're right, the biggest HTD pulley you can fit in a 2" frame is 18t. You are also right to not even consider 9mm wide an option if you try this.

Also if it matters I'd like to do this all exact C-C. I've heard it works well and we do have access to machining that is accurate down to the thousandth, so I think its the best solution.

If you can hold tolerance in the <10 thou range, exact CC works just fine in our experience.

My question is whether or not 18T pulleys are big enough to handle the loads that an average drive train subjects them to. Has anyone used 18T pulleys on their drive train? Is 9 teeth of engagement enough?

(note that it's not just number of teeth engaged but the size of the moment arm that makes bigger pullies stronger)

We used 18t, 9mm belts in 2011 and had huge issues with ratcheting. If we didn't fail a belt, we were definitely about to. Going to 15mm would help, but we have never done that with 18T, so I can't speak from experience. Honestly I'd say it's a bit too close to the margin for my comfort.

I would bite the bullet and go with bigger tubing and 24T pulleys. It is a weight penalty which is annoying but you have plenty of room to rivet to both sides of the tube and the combination of 24T pulleys and 15mm belt has served us very well in the past few years.

Note that if using 1.5" wide tube, you'll either have to do some kind of clearance counterbore on the pulley or use some kind of spacer to space your bearings out of the tube. We take oversize washers and bore them to 1.125, then place them between the bearing flange and the tube. I'd like to go with the counterbore method for next year.

As a side note, I'm also wondering if 6063 aluminum is good to use for frame elements over 6061. Online its about half the price, so I'm hoping we could use that since we would probably be able to buy more stock.

I can tell you that we've used 6063 in our 3x1 and 3x1.5 frames in 1/8" wall without anything resembling a failure. It's not best practice but it works.