Mecanum wear and dirt resistance

[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.

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.

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.

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.

How valid are each of these concerns (particularly 1 & 2)? Do you have a recommendation for a particular style of mecanum (with respect to these concerns)?

Please don't discuss mecanum vs. tank-style drivetrains -- let's keep this thread centered on the practicality and implementation of mecanum on a robot with the usage scenarios outlined above.

Thank you.

[Tom Ore]

1) I think you will see significant wear on the rollers
2) Dirt could get under the rollers, making them sticky and maybe increasing wear
3) We always put down carpet whenever we run on a surface that needs to be protected. We also put down carpet when we demo outside with our competition robots. (We keep an old roll we inherited at our shop.)

If you do you run mecanums, buy a set rather than trying to make them.

[flameout]

Originally Posted by Tom Ore

1) I think you will see significant wear on the rollers

Do you have any experience to back this up?

2) Dirt could get under the rollers, making them sticky and maybe increasing wear

Would this be an issue in terms of driving performance? Is this a problem that applies more to some mecanum styles than others?

To clarify, by "styles," I'm mainly referring to how the rollers are mounted -- I've seen rollers mounted on the ends (like AndyMark does with their wheels), and rollers that are mounted in the middle (like AirTrax uses).

3) We always put down carpet whenever we run on a surface that needs to be protected. We also put down carpet when we demo outside with our competition robots. (We keep an old roll we inherited at our shop.)

Unfortunately, this is not feasible for our team. However, now that I've looked at it, it appears as if AndyMark's rollers can be bought individually -- we'll do our own testing before purchasing whatever roller material we choose to use if we choose to do mecanum (so I'll remove this from the focus of this thread).

If you do you run mecanums, buy a set rather than trying to make them.

We need to be able to replace the rollers cheaply. Are you suggesting buying the initial set off-the-shelf and manufacturing our own replacement rollers, or do you know of a source for replacement rollers that we don't know about?

Thanks for the help.

[Tom Ore]

Consider what happens when you're straffing. The rollers will be scrubbing and thus wearing.

[flameout]

Originally Posted by Tom Ore

Consider what happens when you're straffing. The rollers will be scrubbing and thus wearing.

What do you mean by "scrubbing"? My understand was that, in mecanum, slip would only occur when applying a large force -- such as accelerating, decelerating, or pushing against something external.

Am I wrong about this?

[Tom Ore]

Originally Posted by flameout

What do you mean by "scrubbing"? My understand was that, in mecanum, slip would only occur when applying a large force -- such as accelerating, decelerating, or pushing against something external.

Am I wrong about this?

Scrubbing is sideways motion on a wheel. Take any wheel, hold it on the floor and push it sideways - you'll get wear. When a mecaunum robot is straffing sideways the rollers are scrubbing. You don't even need pure straffing - any time you turn you'll get some scrubbing.

[flameout]

Originally Posted by Tom Ore

Scrubbing is sideways motion on a wheel. Take any wheel, hold it on the floor and push it sideways - you'll get wear. When a mecaunum robot is straffing sideways the rollers are scrubbing. You don't even need pure straffing - any time you turn you'll get some scrubbing.

So by this definition, scrubbing isn't a problem (as a design feature of mecanum wheels).

To those teams who have used mecanums before, how much wear did you see and how much did you use the wheels? Have any of you run your wheels extensively in dirt and not had issues (or had issues)?

[Tom Ore]

Originally Posted by flameout

So by this definition, scrubbing isn't a problem (as a design feature of mecanum wheels).

To those teams who have used mecanums before, how much wear did you see and how much did you use the wheels? Have any of you run your wheels extensively in dirt and not had issues (or had issues)?

On carpet, the scrubbing doesn't cause any wear. If you use mecanum wheels on pavement and dirt please let me know how it goes - I would expect you're going to see wear. (By the way, we've used mecanum wheels - we have 4 regional wins on mecanum wheels.)

[flameout]

Originally Posted by Tom Ore

On carpet, the scrubbing doesn't cause any wear. If you use mecanum wheels on pavement and dirt please let me know how it goes - I would expect you're going to see wear. (By the way, we've used mecanum wheels - we have 4 regional wins on mecanum wheels.)

So you saw 0 wear on carpet (through the lifetime of your robot), but haven't run it on anything else?

We have no intention of trying mecanums on grass -- we don't drive our current robot on grass either. Dirt, asphalt, and gym floors are unavoidable for us, however.

[Ether]

Originally Posted by Tom Ore

When a mecaunum robot is straffing sideways the rollers are scrubbing. You don't even need pure straffing - any time you turn you'll get some scrubbing.

This is not true Tom. If the rollers are free to spin, there is no scrubbing, even when strafing.

Originally Posted by Tom Ore

Take any wheel, hold it on the floor and push it sideways - you'll get wear.

If you rotate the mecanum as you push it sideways, there will be no scrubbing. That is how a mecanum works. Try it.

[flameout]

Originally Posted by Ether

If you rotate the mecanum as you push it sideways, there will be no scrubbing.

Thanks -- I was 99% sure about this, but wanted verification since Tom seemed so sure that scrub would occur even during low-acceleration conditions.

[Ankit S.]

Over the past 11 months our robot has probably ran for about 5 hours on asphalt, 4 of which during the same day. We also had about 6 hours of drive time on carpet. Our wheels are doing fine, and we have not had switch anything out.

The one thing that I would suggest, however, is to clean the vicinity of the wheel every now and then, as that can get pretty dirty.

Edit: We used the 8" wheels from AndyMark, and all the parts used for the wheel were COTS.

[sithmonkey13]

For use on carpet and gym flooring, meccanums should be fine, with normal wear and tear. However, depending on the surfacing of asphalt/concrete, your rollers could get torn up pretty badly.

[Ankit S.]

Originally Posted by sithmonkey13

your rollers could get torn up pretty badly.

The AM rollers are pretty strong, the asphalt outside our workspace is not the best, but they still held up to the 4 hours of driving that we did.

If you do have any worries though, you could purchase extra rollers, they are not too expensive, and you should not need to switch them out more than once every 24 hours of drive time on bad ground.

One thing I need to ask though, is there a reason why you want to use mechanum wheels? Sure it can turn a lot easier, but it really is not the best for a really uneven surface, as the vectors may not cancel out properly...

[Tom Ore]

Originally Posted by Ether

If the rollers are free to spin

The rollers aren't necessarily free to spin. Some spin fairly well, others are sticky, some are stuck. The newer AM mecanums seem to be more consistent than the older ones. Breakaway was rather rough on the mecanums - bouncing down the back side of the bump bent the screws holding the rollers and the rollers would stick.