Mecanum Wheels

[Ether]

nice photo.

Did you use ball, roller, or journal bearings for the rollers? Can't tell from the photo.

[Jeff 801]

Originally Posted by Ether

nice photo.

Did you use ball, roller, or journal bearings for the rollers? Can't tell from the photo.

The bearing surface is nylon on a 1/4-20 bolt (the nylon rolls on the shank of the bolt and not the threads) And between the neoprene roller and the fin there is a Hard Fiber washer from mcmaster

[Ether]

Originally Posted by Jeff 801

between the neoprene roller and the fin there is a Hard Fiber washer from mcmaster

When the roller is axially loaded toward the fin, is the neoprene rubbing against the Hard Fiber washer ?

[Jeff 801]

Originally Posted by Ether

When the roller is axially loaded toward the fin, is the neoprene rubbing against the Hard Fiber washer ?

Yes but with that said the coefficient of friction between the neoprene and the hard fiber washer is greater than that of the hard fiber washer and the aluminium hub so there is just about no rotation of the washer relative to the roller. I hope that make sense. If I was to remake them i would have done it more like how 357 did it this past year with a nylon core and a molded urethane rubber outside.

[Ether]

Originally Posted by Jeff 801

the coefficient of friction between the neoprene and the hard fiber washer is greater than that of the hard fiber washer and the aluminium hub so there is just about no rotation of the washer relative to the roller. I hope that make sense.

Yeah, I realized that after I hit the "send" button.

If I was to remake them i would have done it more like how 357 did it this past year with a nylon core and a molded urethane rubber outside.

The axial loading forces on the rollers can be quite high, so anything you can do to reduce end-loading friction would be beneficial. Perhaps three or four very thin Teflon washers, for example. Is the bolt nickel plated?

Minimizing axial free-play also minimizes losses.

How did you determine the roller profile?

[Marc S.]

Originally Posted by dudefise

It's for a shifting drivetrain project. It will have mecanum for speed and agility and tank for brute strength (with traction wheels) and the ability to switch between the two.

I would argue that mecanum wheels are actualy slower and less agile than ,for instance, a 6wd. Mecanum wheels are usualy heavier than standard traction wheels making it slower to accerlate. And being that mecanum wheels have rollers and no real hard pushing point, the robots momentum would dicide how quick it can stop/change direction(basic agility).

When was the last time you saw a mecanum robot going 18ft/s(?), and then stop in less than 5ft?

[Hawiian Cadder]

http://www.fingertechrobotics.com/pr...mecanum-set-hg

what if instead of using 4 6 inch mechs, you used 8 or even 12 of those. that would greatly decrease the amount of rotational inertia in the wheels, those little wheels are also higher quality.

[Ether]

Originally Posted by Hawiian Cadder

what if instead of using 4 6 inch mechs, you used 8 or even 12 of those

The software would get interesting, unless you plan to mount 3 coaxially at each of 4 locations and drive all three 3 wheels within each coaxial set at the same speed (which would be approximately correct but not quite, resulting in friction).

[Hawiian Cadder]

i was actualy thinking of mounting all 3 right next to each other on a standard frame setup. i would probably use #25 chain directly driven from a cim ( no gearbox)

[Ether]

Originally Posted by Hawiian Cadder

i was actualy thinking of mounting all 3 right next to each other on a standard frame setup. i would probably use #25 chain directly driven from a cim ( no gearbox)

By "right next to each other" I assume you mean "in the same plane" and not "coaxially".

Such being the case, a correct inverse kinematic translation would require that each of the 3 wheels in a set be driven at slightly different speeds.

If you just chain them together and drive all 3 at the same speed, you'll get some scrubbing of the contact patch which means additional loss of power and traction.

[Hawiian Cadder]

i think that the same thing would happen if the were mounted co-axially. while it is true that while using all 3 degrees of freedom there would be some scrub, during normal translation there would be none, conceptually it is 3 entirely separate mechanum bases, driving in the same space, at the same speed.

[Ether]

Originally Posted by Hawiian Cadder

i think that the same thing would happen if the were mounted co-axially. while it is true that while using all 3 degrees of freedom there would be some scrub, during normal translation there would be none, conceptually it is 3 entirely separate mechanum bases, driving in the same space, at the same speed.

I believe that is correct.

[Ether]

Originally Posted by Marc S.

When was the last time you saw a mecanum robot going 18ft/s(?)...

I suppose it would get pretty heavy and costly to have four shifting gearboxes, but it would be possible I guess.

...and then stop in less than 5ft?

Doing a little back-of-the envelope math...

assuming constant deceleration

a = (V*V)/(2*x) = (18*18)/(2*5) ~ 32 f/s/s ~ 1g

... so you'd need an effective CoF of 1.

To get that with mec, you'd need about 1.4 CoF between the tread material and the carpet. So you'd have to make your own custom rollers with high-traction tread.

[EricH]

There aren't exactly a lot of sources of mecanum wheels in the FRC size range. AndyMark is the biggest (and to some extent, the only) one.

You may want to get in touch with 357 (or, the Behind the Design books from the 2006 and 2007 FRC seasons); they've got casting their own down to a fine art.

Or, if you can find someone who can build them directly with a direct manufacturing machine, that might do it. Just watch out for FRC cost rules--the material can get pretty expensive.

Matt, offseason is the best time to do mecanum. Not during the season. 330 spent 2 offseasons prototyping a set before giving them a chance to make a competition robot in the build season.