Design Feedback?

[Chris is me]

Originally Posted by Jared

This is slightly off topic, but I'm curious; has anybody had an actual problem with over constraining a hex shaft in an FRC application?

Absolutely. In 2012, we had an intake / ball conveyor powered by a gearbox with two bearings capturing the extended output shaft. We then tried to slide this extended output shaft through another bearing, the conveyor roller, and finally a fourth bearing. As you might expect, the system was heavily binding.

In general, we have found that once you hit the third bearing, in a third part, you get reduced efficiency, and four is a death sentence. The farther apart the bearings are, the less of an issue this is (I.e. if you have a 24" long shaft, you can probably do two bearings per side).

If you need to use more than two bearings per shaft, it can be done, but you need to make an extremely deliberate effort to ensure they are all aligned. In our drivetrain, our gearbox has two bearings, and then the output shaft goes through a third bearing in our tube. We use counterbored standoffs to hold the two gearbox plates captive relative to each other, so those two holes are as aligned as possible. We also counterbore the output bearing so it sticks out of the gearbox. This lets us tuck the bearing into the hole in our drive tube - this hole is cut through both sides of the tube and thus the third bearing is mounted through it as well. When we go to this much effort, we don't run into problems.

The versaplanetary gearboxes have two bearings on the output shaft, so anybody who supports the vp output shaft with an additional bearing is locating the shaft in three places. We ran setups with four bearings on the same shaft both this year and last year with no problems, including a bearing that was pressed into a welded sleeve that couldn't have been extremely accurate.

The reason this is okay is because two of the three bearings are held concentric with the same feature in the same part. It's very hard for two things in the exact same hole to be misaligned. It's much, much easier for two parts to be misaligned, so it is much more of a problem.

I'd guess we can get away with this because of the fit between hex shafts and bearings and the clearance holes for standoff bolts. A #10 is a slip fit in a .1875" hole, so it has almost .01" of slop in the recommended .196" clearance.

This is actually why I would say you can't get away with it. The slop leads to the misalignment we're talking about. Because both bearings are tucked into the same part in the VP, I think those bearings are as close to perfectly aligned as possible and for alignment purposes count as one of your two bearings.

I'd also assume that the hex bearings which tend to come in a little oversize are also pretty forgiving.

Honestly, every hex bearing I have ever used has had a far, far tighter fit than any of the gears I put on those shafts. It's annoying.