[Jon Stratis]

Originally Posted by flameout

I am considering presenting the idea of switching our T-shirt launcher over to mecanum to Team 957. Before doing so, I would like to have a definite list of pros and cons (as we have never done mecanum before).

While there are many threads discussing mecanums in the context of competition, I have a few concerns specific to this robot that I have not found adequate information on.

Here are my primary concerns:

1) Wear. We will be running this bot a lot, often on asphalt and other abrasive surfaces. We'd rather not have to periodically replace wheels (even once a year would be too much), although, if necessary, we might be willing to switch out the rollers (if they can be made/obtained cheaply) every year or so.

We had a mecanum bot for Breakaway. We ran it through two intense competitions and as practice at our build space ever since. We've been running it as a second robot this past year against our practice bot when we felt the need to have a "real" experience practicing against someone playing defense. Our practice area is mostly cement flooring, although we have some carpet we put down in important places (like a strip to practice line following in autonomous this year). the robots gotten rather significant use, and the rollers still look fine.

2) Resistance to dirt. We do not always run the robot inside, and it is not uncommon to have it running right on dirt (or being pushed over the dirt). I'm afraid that the dirt may gum up the bearings.

We've never run ours on dirt, so I don't know what it would do to the rollers themselves - that's going to be highly material dependent. However, the rollers are not attached with bearings - there is simple a long bolt that goes all the way through the roller and through the plates on either end supporting the roller. The tightness of the bolt determines how free the roller is to rotate. I wouldn't really be worried about dirt getting in the way here.

3) They must not damage gym floors. We often run our robots on gym floors -- the kitbot wheels work fine for this, but other types damage the flooring. This is likely to be a smaller issue than the others (as mecanums don't typically skid very much), but it's still a consideration.
EDIT: Let's remove this from the topic of this conversation, as mentioned in post #3 -- we'll do our own testing to verify that a specific roller material does not damage the floors prior to ordering a set of wheels.

The biggest worry here are the side plates - you need to make sure the rollers are big enough that the plates won't hit the ground. This is mostly an issue if you're making your own replacement rollers.

In terms of machining (for creating replacement rollers if it's necessary), our team isn't very capable. We have neither a lathe nor a mill, and have no experience with casting parts. However, we have a sponsor with a high level of machining capability who would be able to machine roller casts for us, as long as it is a one-time deal.

We made our own mecanum wheels for Breakaway (although we ended up only using those on the practice bot - we used AndyMark wheels on the real one). The rollers were done on a 3D printer, and probably would only need to be done once. Since those obviously wouldn't have much grip, we got some tubing of some sort (I really don't remember what, something grippy) and stretched it over the rollers. That makes it easily replaceable if need be - you can take the rollers off, cut off the old tubing, and put new tubing on, all without needing to make new rollers. If you want to make it real easy, make 2 complete sets of rollers at the start - you can have one on the robot, the second set off and ready to go. Then if you need to switch them, it's a rather quick process, and then you can take your time putting tubing on the worn out set until you need to switch them again.