Mecanum wear and dirt resistance
 

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.

Re: Mecanum wear and dirt resistance
 

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.

Re: Mecanum wear and dirt resistance
 

Do you have any experience to back this up?
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).
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).
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.

Re: Mecanum wear and dirt resistance
 

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

Re: Mecanum wear and dirt resistance
 

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?

Re: Mecanum wear and dirt resistance
 

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.

Re: Mecanum wear and dirt resistance
 

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)?

Re: Mecanum wear and dirt resistance
 

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.)

Re: Mecanum wear and dirt resistance
 

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.

Re: Mecanum wear and dirt resistance
 

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


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

Re: Mecanum wear and dirt resistance
 

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

Re: Mecanum wear and dirt resistance
 

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.

Re: Mecanum wear and dirt resistance
 

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.

Re: Mecanum wear and dirt resistance
 

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...

Re: Mecanum wear and dirt resistance
 

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.

Re: Mecanum wear and dirt resistance
 

If the rollers are not free to spin, then the wheel is no longer operating as a mecanum wheel.

... and if a swerve steering motor burns out or jams, the wheel is no longer swerving and the affected wheel will scrub.

BTW, don't use mecanum on dusty dirt, you are asking for trouble. The roller "bearings" will get contaminated and it won't operate properly any more.

Re: Mecanum wear and dirt resistance
 

The wheel can still function as a mecanum but at a reduced performance - only some of the rollers are sticky or stuck. With our Breakaway bot it still ran fairly well even though some of the roller shafts were bent.

Re: Mecanum wear and dirt resistance
 

Okay, so it sounds like they won't wear too fast (we won't run on asphalt bad enough to cause horrible control issues).

However, since it sounds like we'll have to replace them about once per year, do people have recommendations for how to replace the rollers? AndyMark sells them for $3-4 apiece, which is too much for us to spend each year (we have no problem buying replacements -- I would like to get the yearly maintenance cost down to $20 or less, which necessitates cheap roller replacements).

We've talked about fashioning rollers out of PVC ourselves, but I fear that it is too slick, negating the mecanum effect (by sliding just as easily along the axis of rotation as it rotates around that axis). Is there a less expensive way for us to make our own rollers (I know other teams have used urethane, but I recall that being above our budget)?

It sounds like the best option would be to buy a set of AM mecanums (or other COTS mecanums) and replace the rollers with something cheaper than AM's rollers each year -- does anyone disagree?

Re: Mecanum wear and dirt resistance
 

if you buy a full set of 12 rollers (for 8 inch wheels) it only costs $40 times 4 wheels means it would cost $160 per year IF you used AM rollers. To build new replacements for 12.5% of the cost of COTS rollers might be difficult, but there are other options that might be cheaper:

1) protect the rollers when driving on rough ground
Either lay out something to cover the ground or cover the individual rollers with a sleeve of some sort, the former is easy and quick, but a ground cover sturdy enough to drive on might need a lot of storage space. The latter would likely be time consuming and trying to keep the roller covers in place would be challenging, and performance might be affected.
2) have a different set of more durable rollers for driving on rough ground
Skateboard wheels come to mind, put 2 or three on an axle and shape them to the approximate contour of the mecanum rollers (the sponsors with machining capabilities might be able to help). Just switch the rollers when you switch surfaces.
3) instead of replacing the rollers, maybe new rollers could be cast over the old ones
This saves money on urethane, but the layered structure might fail, which wouldn't be good
4) this one is like number three, but the rollers are cast over PVC cores, this allows for more control over the structure of the wheel so that the rollers are more uniform, every time the rollers wear out, remove the remaining tread and cast a new one over the core.

I think the even numbered ones sound like they might work better, but if you could find rollers within your budget, that might be even better.

Re: Mecanum wear and dirt resistance
 

Unfortunately, this is not possible.
This is something I have thought of... however, the rollers are closely spaced IIRC, so we'd need to grind them down first, but I think it'd be doable. However, coming up with a suitable cover seems like the hard part... any ideas?
Performance is not very important, although reliability is.
Switching the rollers would be quite time-consuming, but I don't see us doing it more than a few times per year, so it's plausible. This would make dealing with gym floors easy -- we'd only switch for the duration of our presentations (the rest of the time we'd use our low-wear wheels).

I'm not familiar with skateboard wheels, but I'm worried that they might not have enough traction. Although we don't need a lot of traction, I think the kinematics of a mecanum drivetrain break down when the wheel isn't significantly more difficult to skid than it is to roll.
That's something I haven't really thought about... good idea.
That does sound like it would work.

Also, I don't see why we need to use urethane. Is there anything less expensive that would still do the job? I think I recall something a while back about casting holiday ornaments out of hot glue. What do you think about this idea?

EDIT:
Earlier I mentioned a concern about dirt -- it is possible to avoid traveling through the dirt (just a minor inconvenience). Therefore, in my opinion, it comes down to our ability to cheaply repair/replace the rollers.

Re: Mecanum wear and dirt resistance
 

I don't think you're going to achieve savings trying to develop a process for making replacement rollers. Assuming your team members' labour is worth anything (either in terms of money, or in terms of an alternative activity), you're going to be hard pressed to beat a cost of $160 per year. Plus you'll have to pay for raw materials as well.

If you really want to spend time developing better Mecanum wheels, that's another story. Then the R&D necessary to investigate new methods of roller production will be a good project.

How much mileage do you intend to put on these wheels in a year, anyway? Depending on the team, it could range from hundreds of metres to tens of kilometres.

Re: Mecanum wear and dirt resistance
 

Let's assume that team labor is free ;)

I'm really looking into casting hot glue right now. We could make a mold using a pristine roller (we'd order an extra set of 12 to replace rollers that we mess up beyond repair) -- I believe there are commercial products for building reusable molds for stuff like this.

Further, by using hot glue, it might be possible to simply re-heat the roller (I'm not sure what the maximum temperature of AM roller before damage is done is) to remove the hot glue before re-casting the glue around the roller (we would use this as a roller-repair method).

An added benefit would be the knowledge of how to perform a casting.

Looking online, it appears as if enough hot glue to last us for many years can be obtained for $10, and most of the materials for making molds is around the same price, putting this process well within our team's budget (and freeing up funds for other projects).

We will be putting in perhaps a kilometer or two of mileage on these wheels on asphalt each year. The more I think about it, the less it seems like wear will even be an issue.

I'll propose that we simply buy AM mecanum wheels (probably 8-inch) and, if necessary, attempt to repair the rollers at the end of the year by casting hot glue around them. I'm not sure if casting hot glue like this would even work, but if it doesn't, then we can simply buy new rollers to replace the old ones.

Thanks for all the advice.

Re: Mecanum wear and dirt resistance
 

That hot glue technique sounds innovative, but definitely consider the heat buildup in the tread when a wheel spins in place (i.e. the wheel breaks loose before stalling the motor—generally a good thing for preserving motors). You don't want the glue to become soft and tacky as a result. Also, if you're running outdoors on asphalt, consider the ambient temperature of the surface, and the added heat due to insolation, especially in the summer. Again, you don't want a sticky mess.

Why not get in touch with AndyMark, and ask them about their durability tests on their rollers? (Andy might respond right here, if you're lucky.) See if they can estimate the wear for you, based on their tests and your usage patterns (e.g. mostly driving forward, with occasional sideways maneouvres, in moderate temperatures on asphalt, with a little bit of mileage indoors).

Another problem with soft rollers in general, and hot glue in particular, will be wheels picking up abrasive material from outdoors, and dragging it inside of buildings. You don't want tiny stones embedded in the treads, if you plan to transition from outdoors to a shiny, polished floor.

Re: Mecanum wear and dirt resistance
 

I have used hot glue to cast a lot of things, but usually things that do not need much strength or abrasion resistance. If you have built many robots, or similar devices, you probably realize the importance of selecting rugged materials for wheel surfaces and the amount of wear even tough materials experience. And that is just with normal wheels. Mecanum wheels place even more demands on these surfaces. I expect you will find any rollers cast from hot glue to be a poor substitute for more conventional plastics.

If you do plan to try to fabricate your own rollers, try to think about all of the properties that will be important in the finished roller. How much weight will each roller need to support? Can you calculate the PSI at critical load bearing locations? Do you plan to lubricate each roller regularly or do you require some material that is mostly self lubricating such as Nylon or UHMW?

While I admire the desire to save money and try unique fabrication techniques, hot glue does have some significant drawbacks in a number of material properties. It may have good applicability if developing a rapid prototype, model, or even as a finished part if it is being used in a low stress application. But I doubt you will have much luck in this very demanding application.

Re: Mecanum wear and dirt resistance
 

Okay, so it sounds like hot glue doesn't have the durability to work as tread on the rollers.

In that case, I'll pitch the idea of mecanum wheels to our team with the caveat that it would be a yearly expense.

Thanks for all the help.

Re: Mecanum wear and dirt resistance
 

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.

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.

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.

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.

Re: Mecanum wear and dirt resistance
 

On our minibot, we stretched surgical tubing around otherwise slick tubing for increased grip. However, for a mecanum bot, we are worried that the tubing would slip if wrapped around a mecanum roller.

Did you have issues with the tubing slipping on your robot?

Unfortunately, I don't think we have the capability to manufacture our own rollers in this way (since we'd need to decrease the diameter while keeping a very specific shape), but I can ask about it if wear does end up being a problem.

Thanks for the help.

Re: Mecanum wear and dirt resistance
 

For a roller design such as you have described: When there is torque on a mecanum wheel, the roller slides axially through any free play until it butts up against the plate, possibly with great force. Here it encounters friction. If there is fine gritty dust or dirt in there, it can affect performance. Stay away from dirt.

Re: Mecanum wear and dirt resistance
 

When we drove our mechanum robot on our wood gym floor it left massive skid marks everywhere it drove. this was not however a problem when driving on the rubbery wood substitute used in some gyms.

Re: Mecanum wear and dirt resistance
 

what kind of mecanum rollers were you using

Re: Mecanum wear and dirt resistance
 

I have used hot glue to cast a lot of things, but usually things that do not need much strength or abrasion resistance. If you have built many robots, or similar devices, you probably realize the importance of selecting rugged materials for wheel surfaces and the amount of wear even tough materials experience. And that is just with normal wheels. Mecanum wheels place even more demands on these surfaces. I expect you will find any rollers cast from hot glue to be a poor substitute for more conventional plastics.

If you do plan to try to fabricate your own rollers, try to think about all of the properties that will be important in the finished roller. How much weight will each roller need to support? Can you calculate the PSI at critical load bearing locations? Do you plan to lubricate each roller regularly or do you require some material that is mostly self lubricating such as Nylon or UHMW?

While I admire the desire to save money and try unique fabrication techniques, hot glue does have some significant drawbacks in a number of material properties. It may have good applicability if developing a rapid prototype, model, or even as a finished part if it is being used in a low stress application. But I doubt you will have much luck in this very demanding application.

Re: Mecanum wear and dirt resistance
 

They were the newer 8 inch mechanisms with the Grey rollers. A theory that was presented was that because the hardwood floor had so little give in terms of rollers being able to compress the surface, that during the brief point when two rollers touch a skid mark would be created. This idea seemed to fit the pattern, the skid marks were in pairs and about ever 2.5 inches.

Re: Mecanum wear and dirt resistance
 

Even when going straight forward?

Or only when strafing?

Or only when spinning?

... more detail please


Oh, and this: were the rollers free-spinning? Or were some (or all) binding?

Re: Mecanum wear and dirt resistance
 

Big point from my team...

This past year we used mecanum. I have no say on wear, as all we used it was at 2 competitions.

Following AM directions, using certified material, the rollers still leave marks when straffing.

We had a mentor custom-torch and press the sideplates instead of buying them, the only part we bought were the rollers from AM.

Here's the rub... After being left in a grarage from April to September, the provided bolts in the AM kit have rusted a significant amount. They should still be reusable for one more year. If you have the money, I think I would reccommend getting stainless steel bolts simply so that when you do have to replace the rollers, it is not neccessary to cut them apart.

My $.02, not adjusted for inflation.

Re: Mecanum wear and dirt resistance
 

Ouch... thanks for telling me this.

One of the schools we do demos at has a gym floor that is extremely susceptible to marks -- we had 4 Lunacy wheels on our robot, which we wrapped in masking tape after seeing it leave some marks (the floor was waxed; fortunately there was no permanent damage). The masking tape still left marks (I suspect it was dragging around dirt particles, scraping the surface of the wax). We finally found success by wrapping the wheels in electrical tape.

I guess mecanum wheels are out, then. Thanks for all the help.

Re: Mecanum wear and dirt resistance
 

The wheels were all free spinning. The marks appeared primarily when that wheel was powered, so going forward left 4 trails, while going diagonal only left two from the wheels being powered. the distance between marks remained consistent regardless of speed, while their intensity increased the faster the robot went. When changing rapidly from one direction to another the marks got substantially worse (darker).

Re: Mecanum wear and dirt resistance
 

I did a shadow-graph of that design several months ago, and there is a slightly bumpy transition (circumferencial gap) from one roller to the next. I wonder if a mec with a continuous transition would make any difference.

Long shot I know, but did you happen to measure the axial free-play of the rollers?

Re: Mecanum wear and dirt resistance
 

Our Mecanum Robot leaves massive skid marks when it drives sometimes. My main guess is that it is attempting to accelerate too fast and the wheels are slipping. I think part of this is due to the dusty shop floor (robot slips more easily) we drive on and the fact that our team tends to drive the mecanum robot pretty hard (accelerates too fast).

Re: Mecanum wear and dirt resistance
 

The pro's would obviously be manuvarability. However, mecanum wheels are not great durability-wise, especiallily off of the field. Doing demos in the off-season with our mecanum-driven robot from breakaway tore them apart. they are more expensive, so keep in mind that they will not last very long on aspault.

Re: Mecanum wear and dirt resistance
 

You are leaving out a MAJOR factor which IMO is VERY important: Mecanum wheels are SWEET!! In particular, if you are doing this at a place where many spectators have no idea what is happening, this will look like straight up MAGIC!!! Isn't the point of a T-shirt shooter to make more people aware of your robotics team? If so, why not make them AMAZED at how you can drive STRAIGHT SIDEWAYS!!! I'm serious. non-robotics people are ALWAYS amazed by this... As far as the mecanum vs. 6WD debate, I wonder how the debate would go if the only question was "which looks more awesome??"