Mecanum Alignment Problems

So we have made an awesome chassis this year with in frame gearbox’s. We took 4 hex Nano-toughboxs from Andymark and disassembled them so that they could be put back together inside our 1.5 x 3 inch aluminum tubing (see picture). The Gearboxes work great, but that’s beside the point, not all of our mecanum wheels aren’t applying pressure to the floor equally. Furthermore the internal gearbox design doesn’t allow for adjustments with moving the shaft up and down to level the robot. In the picture its the front right and back left that aren’t touching as much as the other two (although they are touching). This makes us not move much on that parrallel but move fine on other vectors. Anyone have any suggestions how to level the robot? we are at a complete loss.

-Members of Team 3324

Your frame is probably too rigid. Good mecanum drives generally have frames that can conform to the carpet, or the mecanum modules have suspensions using springs. You are also not testing it fully weighed down. Add weight and see how things look.

We added more than the 120 lbs to it today and it didn’t do anything. Unfortunately there isn’t much adjusting you can do when you want an internal gearbox’s…

Well, you also welded AND used gussets. Any way you can break your welds and see how it is with just the gussets?

We will have to try that tomorrow. We were hoping to avoid that but might have to do it. Unfortunately part of our goal was to build the base chassis like a tank. Which doesn’t allow for much adjustment. Thanks for your input.

Our current idea to avoid taking everything apart and breaking the welds is to lag bolt the chassis into the concrete loading dock behind our work space and use a lever to fine tune the chassis by bending it.

You’re going to have issues if any of your wheels are not in contact with the carpet. This year’s field may appear to be flat, but invariably even the flattest fields will have dips or ridges in them. Your best bet is to allow flex in your chassis or engineer a suspension.

Regardless of what you do, good luck.

It’s worse than that. Each wheel has to be in contact with the carpet with enough force to provide traction for that wheel.

Your totally right, All 4 of our wheels are touching the floor but two are not touching the same amount so we have trouble on certain vectors. Would cutting the welds only 1/4 of the way down the height of the chassis allow enough flexibility to make this tiny adjustment (given we take the corner gusset plates off)?

About the only way to know that answer would be to try it.

i have a pop sci from the 60’s that shows old racing cart that allowed for one wheel to be 6" off the ground and the rest to be in-contact with he ground.
The designer did it all with out any moving parts just CrMo tubing and good welds.

I don’t know if you’ll like my suggestions… but here goes:

I REALLY don’t think you’ll want to lag-bolt it down and try to bend things into alignment. I think you will run a significant risk of distorting the tubes that house your gearboxes, and thus screwing them up with misaligned shafts, bearings, and gears.

A few possible paths forward:

Option A

  1. Cut the front and rear cross-members in half
  2. Use sleeves, blocks, or plates to reconnect the two halves of your chassis
  3. Leave enough slop (or better yet, use slots) in whatever connecting part(s) you use to adjust the chassis for flat ground

It will still be fairly strong, but won’t conform to variations in playing fields.

Option B

  1. Cut the front and rear cross-members in half
  2. Connect them together with sliding plates, rod ends, spherical bearings, some combination of these, or something I didn’t list that allows relative movement in multiple axes between both halves
  3. Mount a flexible ‘deck’ to the whole structure (1/8in polycarb?) to act as a suspension spring

Perhaps the linkages get complicated and run the risk of binding, but it would somewhat conform to playing field surfaces.

Option C

  1. Completely remove the front and rear cross-members
  2. Connect the two ‘drive pods’ with two sheets of a flexible material (again, polycarb comes to mind) on the upper and lower surfaces, acting as springs and allowing the whole chassis to conform to the floor.

My favorite solution of the three, allows a lot of wheel movement (relatively speaking) and could be selectively stiffened to optimize performance. It also provides a lot of surface area for electronics/cRIO, etc.

Best of luck!

Thanks for the ideas Guys. The plastic idea is brilliant! we will definitely try it.

Ok so we tried the plastic and it helped slightly but did not completely resolve the problem. We are currently working on a jig to C-clamp the chassis to. We hope that we can then level it by slowly tightening it to a level surface with the C-clamps

This should work. I am guessing that when you welded the frame together that it expanded and contracted making it not square. So just like they do on automobiles you need to straighten out the frame on a big flat table with a bunch of clamps and straps to yank it back into proper shape. I know our team uses a big welding table that is square and flat and also is topped with a piece of 3/8" steel plate. That way we can weld tabs to clamp on to or other jigs to help facilitate the straightening process.
When flexing it back into shape you will probably need to flex it beyond square because the aluminum will spring back slightly as well.

If you cant bend it back I would suggest cutting some of the welds and then slowly bend it back into square. Then once you have measured your frame to insure squareness, slowly weld each of the cut welds from before.While doing this make sure to equally heat up each area to try and minimize the expansion and contraction caused by welding. I know our welding mentor will go around to each weld on the piece we are fabricating only welding about 1/4 of the area and then moving on to the next one to insure equal heating.

The way they straighten axles in cars is to weld a bead down one side. As it cools it shrinks pulling the axle in that direction. Practice on a piece of scrap.