pic: 2009 Mecanum Hub - Team 1595

[Dan Richardson]

Pretty! Do you have any back up solutions if you don't gain access to a 5 axis cnc?

And as far as I understand the intent of the rule and clarifications last season, if you were to make the drawings publicly available on Delphi white papers or any other viable source, then you would be able to use the same drawings. ( I.e. nothing but dewalts )

[AustinSchuh]

Any reason you are using a key instead of hex?

[dlavery]

Originally Posted by Stud Man Dan

Pretty! Do you have any back up solutions if you don't gain access to a 5 axis cnc?

Of course, that may be a moot point if the new "no metal parts allowed" rule stays in play...


-dave



.

[EricH]

Originally Posted by dlavery

Of course, that may be a moot point if the new "no metal parts allowed" rule stays in play...


-dave



.

They'll just use wood, if it's strong enough. Of course, at that point, the "freshman on a wood platform" is probably the best robot, as the controllers won't be allowed.

[IKE]

Originally Posted by dlavery

Of course, that may be a moot point if the new "no metal parts allowed" rule stays in play...


-dave



.

Okay Dave,
I think I have the structures, motors and the power source worked out. The controls are going to be very interesting though. May have to check out a "Idiot's Guide to Programming with Rope" from the Library in Alexandria.

[willson.thomas]

I have just uploaded the hub with the rollers finished. If we do not get a 5-axis we will just use our previous mecanum design. We have had mecanums for the past three years. This is just a significantly lighter way of creating them. Will post pics of old ones soon.

We are in no way locking ourselves into these hubs. We will not request them to be manufactured until after we know the challenge and have evaluated the advantages and disadvantages. For overdrive we seriously considered not using them, but decided the maneuverability we would gain we more than make up for the speed loss when trying to pick up the ball.

[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

[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

[Andrew Schreiber]

Originally Posted by IKE

Okay Dave,
I think I have the structures, motors and the power source worked out. The controls are going to be very interesting though. May have to check out a "Idiot's Guide to Programming with Rope" from the Library in Alexandria.

You know, I have a copy of that book in my room, I was going to go with Pigeon over IP protocol personally ^_^

Seriously guys, gorgeous wheels. Would you really need a 5 axis cnc mill to get them done? If you accept that the interior wont be as pretty you could do them with a mill by hand. But hey, if your sponsor wants to make em for you I say what better advertising for them eh?

[RyanCahoon]

Nah... we'll just program dna and rat brains to control the robots this year.

Completely organic game: First alliance to eat all the food items on the field wins. Of course some teams will opt for herbivores, some carnivores. Rule: no eating the other robots.

Better start working on getting those biomedical sponsors...

[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

[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.

[Triple B]

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