pic: 2009 Mecanum Hub - Team 1595

[JesseK]

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).

[NickE]

Originally Posted by JesseK

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

Originally Posted by R27

Prior to the Kick-off: Before the formal start of the Robot Build Season, teams are encouraged to think as much as they please about their ROBOTS. They may develop prototypes, create proof-of-concept models, and conduct design exercises. Teams may gather all the raw stock materials and COTS COMPONENTS they want. But absolutely no fabrication or assembly of any elements intended for the final ROBOT is permitted prior to the Kick-off presentation

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.

[JesseK]

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)

Teams can not be prevented from thinking about their hardware and software designs, and it is not our intention to do so. However, the timeline permitted for the development of the actual competition version of the ROBOT is severely, and intentionally, restricted. Pondering software issues to be resolved, researching general case solutions, discussing solutions with teammates, collecting raw materials, sketching mechanisms, preparing tools, and outlining high-level descriptions of software algorithms are all reasonable activities outside of the scheduled build periods. But completing detailed dimensioned drawings of parts, and any actual fabrication of any hardware items intended to go on the actual competition ROBOT is prohibited outside of the approved fabrication periods.

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

[KINGOFCLUBS454]

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?

[Lowfategg]

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.

[EricH]

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.

[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?