Chief Delphi - Mecanum wear and dirt resistance

Chief Delphi (http://www.chiefdelphi.com/forums/index.php)

- Technical Discussion (http://www.chiefdelphi.com/forums/forumdisplay.php?f=22)

- - Mecanum wear and dirt resistance (http://www.chiefdelphi.com/forums/showthread.php?t=98247)

Re: Mecanum wear and dirt resistance

Quote:

Originally Posted by flameout (Post 1084989)

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.

Thanks for all the advice.

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

Quote:

Originally Posted by Ether (Post 1085084)

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.

Re: Mecanum wear and dirt resistance

Quote:

Originally Posted by Hawiian Cadder (Post 1085276)

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?

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

Quote:

Originally Posted by kws4000 (Post 1085351)

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.

Re: Mecanum wear and dirt resistance

Quote:

Originally Posted by Ether (Post 1085284)

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).

Re: Mecanum wear and dirt resistance

Quote:

Originally Posted by Hawiian Cadder (Post 1085276)

They were the newer 8 inch mechanisms with the Grey rollers.

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.

Quote:

Originally Posted by Hawiian Cadder (Post 1085454)

The wheels were all free spinning

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

Quote:

Originally Posted by flameout (Post 1084915)

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.

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

Quote:

Originally Posted by flameout (Post 1084915)

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??"