04-11-2008 09:38

JesseK

Re: pic: 2009 Mecanum Hub - Team 1595

Hope you're ready to re-CAD it from scratch on kickoff day

I'm having trouble visualizing the full wheel, any chance a photo of that could be posted? It seems like the rollers would interfere with the roller-mount supports (the angled struts coming off of the roller mount flanges).

04-11-2008 09:49

NickE

Re: pic: 2009 Mecanum Hub - Team 1595

According to Rule R27 from 2008, you can CAD all you want before build season, as long as nothing gets built. Unless this rule changes, having this CADed now shouldnt be an issue.

04-11-2008 09:57

JesseK

Re: pic: 2009 Mecanum Hub - Team 1595

The paragraph above <R27> states the spirit of the rule. Though it's not 'technically' a rule, it's still in the rulebook and still explicitly governs as such: (emphasis is mine)

I do not mean for this to take away from the discussion of the design itself, and apologize for that

04-11-2008 10:10

KINGOFCLUBS454

Re: pic: 2009 Mecanum Hub - Team 1595

Rules aside, that is a beautiful looking hub, I can't wait to see it with rollers. I noticed you had some extra support between the spokes, what's your reasoning behind that?

04-11-2008 10:57

Lowfategg

Re: pic: 2009 Mecanum Hub - Team 1595

Those are very cool looking. Hope you can find someone to machine them.

Is not andymark going to release an updated version of the mecanum wheels this year? I know a lot of teams did not like the thin sheet the old ones are made of.

04-11-2008 11:11

EricH

Re: pic: 2009 Mecanum Hub - Team 1595

Rules aside (as you can argue either way), there is one other issue. Mecanum is cool, but don't make design decisions before Kickoff. You don't know what the game is, or if mecanums are going to be a good option. (Or even if you can get that 5-axis CNC...) If you lock in now, you may easily be at a competitive disadvantage come competition.

04-11-2008 13:13

Dan Richardson

Re: pic: 2009 Mecanum Hub - Team 1595

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 )

04-11-2008 16:35

AustinSchuh

Re: pic: 2009 Mecanum Hub - Team 1595

Any reason you are using a key instead of hex?

04-11-2008 17:01

dlavery

Re: pic: 2009 Mecanum Hub - Team 1595

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


-dave



.

04-11-2008 17:41

EricH

Re: pic: 2009 Mecanum Hub - Team 1595

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.

04-11-2008 18:50

IKE

Re: pic: 2009 Mecanum Hub - Team 1595

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.

04-11-2008 19:08

willson.thomas

Re: pic: 2009 Mecanum Hub - Team 1595

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.

04-11-2008 19:14

Andrew Schreiber

Re: pic: 2009 Mecanum Hub - Team 1595

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?

04-11-2008 19:18

RyanCahoon

Re: pic: 2009 Mecanum Hub - Team 1595

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...

04-11-2008 19:39

Qbranch

Re: pic: 2009 Mecanum Hub - Team 1595

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

05-11-2008 00:45

willson.thomas

Re: pic: 2009 Mecanum Hub - Team 1595

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.

05-11-2008 02:58

dtengineering

Re: pic: 2009 Mecanum Hub - Team 1595

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

05-11-2008 10:45

dlavery

Re: pic: 2009 Mecanum Hub - Team 1595

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

05-11-2008 11:24

MrForbes

Re: pic: 2009 Mecanum Hub - Team 1595

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.

05-11-2008 12:52

Brandon Holley

Re: pic: 2009 Mecanum Hub - Team 1595

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

06-11-2008 12:58

Triple B

Re: pic: 2009 Mecanum Hub - Team 1595

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

06-11-2008 15:58

techtiger1

Re: pic: 2009 Mecanum Hub - Team 1595

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.

06-11-2008 17:02

willson.thomas

Re: pic: 2009 Mecanum Hub - Team 1595

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.