pic: 2009 Mecanum Hub - Team 1595

[Qbranch]

Oooh... pretty!

I feel like this thing should be in some kind of fancy impeller assembly... wait a minute.

I know! You're fooling us! It's actually a propeller, and you're on the inside of the conspiracy trying to convince us all there won't be a water game this year! Ahah! Caught mechanum-wheel-hub in hand!

-q

[willson.thomas]

To answer more questions...

We are using a key rather than hex because that is what the Banebots planetary transmissions use, which are what we have used in the past.

We need a 5-axis because it has to be able to drill the hole from one side of the hub, then flip around, and drill the hole exactly on the other side. I do not believe that it could be done manually, regardless of the center design. I do not see how the center would be milled out or how we would get those holes drilled at an angle like that.

[dtengineering]

I think it may be possible for you to create a functionally, if not aesthetically, identical hub without using a 5 axis mill. In fact, it may be possible to do without using anything more than a good lathe, turret head mill, and indexing head.

I don't say this to discourage you from trying to get time on a 5 axis machine... go for it! It may be that given current economic conditions that some machine time is an easier donation for a shop to make than financial support. Rather I suggest this because one of the things I really like about FRC is that it encourages designers to think like builders... in other words, it doesn't matter how good it looks in CAD if you can't build it (quickly and efficiently) in real life!

Just as you've found it to be an interesting and useful exercise to design this hub, working in various machining constraints ("How can we make this without a 5 axis machine?") can make it an even more beneficial exercise. Talk to some machinists about what they can do with non-CNC equipment and you may be amazed.

Ooooorrrr.... why not go all the way down the rapid prototyping track and use a 3D printer and lost wax casting to go almost directly from CAD to metal?

Oh, right... no metal allowed this year.

Jason

[dlavery]

Wilson-
Other than the obvious problems with the "no metal" rule I do have one serious question. How will the roller axles be attached to the hub, and have you looked at the stresses across that joint? From your hub design, it appears that the rollers will be split in the middle, with half-rollers cantilevered out on either side of the tang through which the axle passes. As the wheel rotates, (assuming a proper profile on the rollers - which many of the mechanum wheels used by FRC teams do not have) the contact patch between the wheel and the ground should start at the end of the roller and move across the entire length and off the other end. When each roller first makes contact with the ground and the contact patch is at the end of the roller, the torque applied to the axle/hub joint can be significant (for a typical ~150 pound FRC robot, 600-1000 in-oz would not be unusual). That obviously is a dynamic load as the contact patch moves from one end of the roller to the other. So expect those values to at least double as shock loads if the roller is not shaped properly and you get intermittent contact between the roller and the ground as the wheel rotates.

Many designs avoid the center joint/cantilevered roller approach and go with an axle that is supported on both ends. This can add in a different set of issues (e.g. the axle supports must be designed to avoid striking the ground when the wheel rotates), but it does avoid one of the most common failure modes. The alternative is to really beef up the axle/hub joint to make sure it can handle the loads you are going to see.

I am not familiar with the mechanum wheel design you have used previously, so you may have already addressed this. In which case - "never mind." It is just not obvious from this image how the design handles this.

-dave

[MrForbes]

Originally Posted by dtengineering

Talk to some machinists about what they can do with non-CNC equipment and you may be amazed.

Or just look at some old "stuff", you'll be amazed at what we were capable of making before computers came along.

It actually looks like it should not be too hard to support the wheel at an angle for the milling operation. An indexing head would be handy, of course.

Also the design of the center could be slightly modified to make it much easier to machine...just change those three strange shaped openings to simple round holes. What you lose in artistic appeal, you might more than make up for in ease of fabrication.

[Brandon Holley]

Originally Posted by dlavery

Wilson-
Other than the obvious problems with the "no metal" rule I do have one serious question. How will the roller axles be attached to the hub, and have you looked at the stresses across that joint?

I have to agree with dave here.

I've designed some mecanums on my own time as a little side project and figuring out how you are going to hold those shafts with 2 fairly large cantilevers on the end is an issue.

If you've addressed this already I'd love to hear your solution. Thanks!

Brando

[Triple B]

very nice , but i will agree that a hex drive is a better choice than the key way.
mike d

[techtiger1]

Very nice design work. Gotta to agree with Studman Dan, Mike and DL in this thread though all very good points made. Also how many materials do you have available to use for roller testing. Durometer and polyurethane seems to work the best ask 357. I do love the web in between the petals of the wheel very good addition to your generic design. Good luck and I hope you get them made the way you want them to come out. Ohh and if you do make them take some pics please becuase these are sure to come out purty.

[willson.thomas]

Above is the complete mecanum assembly. We first used mecanums in 2006 and discovered that the axle needed to be supported. Our current drive base design includes this. I will put this mecanum hub and put it on our drive base along with the old hub and upload it when I get a chance. The reason we won't build this without a 5-axis is that the time required would not really be justifiable during the six week build season when we can get the old one made really easily.