Shaft slop help?

[Arpan]

Hi all!

We're making a west coast drive this year. Ours is "unique" in that instead of putting sprockets and chain like normal people do, we're using 15mm belts on the outside of the aluminum tubes, making our cantilever fairly long. The FEA indicated that even under worst case loads the bearings and tube would be fine, but that the shafts might take a little bit of heat.

Because of this, we decided to go with hot-rolled steel hex shaft from mcmaster carr. Everything was great until we put the shafts in. They appear to be able to wobble in their bearings by a few mm, meaning that the wheels wobble by up to an eighth of an inch.

Anybody know a good, simple solution to this problem other than replacing the shafts or where I could get tighter toleranced steel .5" hex shafts? (we can harden them ourselves if need be)

[Dr.Gusta]

Would you post a picture I am not sure I fully understand what you are describing but believe I could help if I understood.

[Teched3]

Cold-finished steel probably would have a better size tolerance. Your "wobble" is compounded not only by bearings, but the fit in the wheel hub. Vex aluminum shafting has a really good fit, and we used them on a cantilevered drive shaft with no bending all season, and are still running that robot for public demos.

We also ran the shafts through our frame and used a bearing there to relieve some of the load on the outboard tranny bearing, reducing the cantilevered shaft distance. Hope that helps.

[Arpan]

Originally Posted by Teched3

Cold-finished steel probably would have a better size tolerance. Your "wobble" is compounded not only by bearings, but the fit in the wheel hub. Vex aluminum shafting has a really good fit, and we used them on a cantilevered drive shaft with no bending all season, and are still running that robot for public demos.

We also ran the shafts through our frame and used a bearing there to relieve some of the load on the outboard tranny bearing, reducing the cantilevered shaft distance. Hope that helps.

Thanks a lot! I didn't notice the the wheels , but you are correct.
Well, guess it's time to buy aluminum. Worst case, I think we have some long steel toughbox shafts we can cut down.

[jee7s]

Even if you have the tight tolerances, there will always be some movement with a slip fit. It sounds like you have a slip fit because you didn't mention any pressing. Even if you get a precision ground shaft, you'll still have a few thousandths of space in there that will cause some wobble.

This can be compensated by taking up all the space on the shaft and putting it into slight tension with a spring. If you do this, you'll use the face of your tubing to hold the shaft axis in line with the bearing axis (or more appropriately keep the shaft axis normal to the bearing face). That does a lot to prevent wear on the bearing and sprockets.

[R.C.]

Originally Posted by Arpan

Thanks a lot! I didn't notice the the wheels , but you are correct.
Well, guess it's time to buy aluminum. Worst case, I think we have some long steel toughbox shafts we can cut down.

Arpan

You won't have a problem with the undersize steel. In the grand scheme of things if the hex is .495 and the hexed mating piece is .500-.502 you'll be more than fine. Sure it won't be a perfect fit but at least your not spending all day grinding the shaft to go into the hex bearing/wheels.

I do recommend while buying steel hex that you buy 4140 or 4340 from McMaster.

[magnets]

Hex shafts and hex bearings are never going to be perfectly sized. There's a reason nobody uses hex bearings outside of FRC, and why McMaster sells the unpolished "mill" finish slightly oversize hex shaft.

To get a perfect fit, make sure that the bearing hole in the frame is perfect, and turn down the end of the hex shaft and use normal round bearings. Then, for the wheel, use a vex hex shaft and vex hub. They work real well together.

[GeeTwo]

This may not be the best thread to ask this, but at least it's about West Coast Drive. How do WC drives comply with R26(c), "the BUMPER must be backed by the FRAME PERIMETER at least every 8 in"?

[nathannfm]

Originally Posted by GeeTwo

This may not be the best thread to ask this, but at least it's about West Coast Drive. How do WC drives comply with R26(c), "the BUMPER must be backed by the FRAME PERIMETER at least every 8 in"?

Usually people put wide stand offs between the wheels less than every 8 inches to act as the frame perimeter.

Now let me ask a question of the OP. I think I understand your set up, but just to clarify, from the outside of the bot in it's wheels, pullies, aluminum tubing with hex bearings. If this is the case why are you using hex? You could just use dead axles and bolt the pullies right to the wheels.