Mecanum wear and dirt resistance

[Ether]

Originally Posted by Tom Ore

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.

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.

[Tom Ore]

Originally Posted by Ether

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.

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.

[flameout]

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?

[PAR_WIG1350]

Originally Posted by flameout

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?

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.

[flameout]

Originally Posted by PAR_WIG1350

Either lay out something to cover the ground

Unfortunately, this is not possible.

cover the individual rollers with a sleeve of some sort ... likely be time consuming and trying to keep the roller covers in place would be challenging

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 might be affected.

Performance is not very important, although reliability is.

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.

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.

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

That's something I haven't really thought about... good idea.

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.

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.

[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.

[flameout]

Originally Posted by eagle33199

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.

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.

[Ether]

Originally Posted by eagle33199

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.

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.

[Hawiian Cadder]

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.

[Ether]

Originally Posted by Hawiian Cadder

When we drove our mechanum (sic) robot on our wood gym floor it left massive skid marks everywhere it drove.

what kind of mecanum rollers were you using

[Hawiian Cadder]

Originally Posted by Ether

what kind of mecanum rollers were you using

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.

[Ether]

Originally Posted by Hawiian Cadder

the skid marks were in pairs and about ever 2.5 inches.

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?

[kws4000]

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.

[flameout]

Originally Posted by kws4000

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

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.

[Hawiian Cadder]

Originally Posted by Ether

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?

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).