After our rookie season, we decided to take on building a crab drive. It has taken us a while since none of us knew inventor, it is also fairly primitive. The sprockets used to connect the wheel modules are missing, (they refused to go parallel with each other…) and there is no chain or screws yet. We would like some feedback on our design. Pictures are posted on http://s296.photobucket.com/albums/mm173/rshanor/
Thanks 
Regarding the sprockets…
Constrain them using the “flush” constraint. That should get the connecting sprockets coplanar/parallel to each other. Then mate them to the wheel assemblies.
So far, it looks good; I’m a little puzzled why there is a ring around the wheel. Is it to provide a rotational platform?
Cool, looks a bit like wildstang’s crab design.
How much does it weigh as shown?
Will it be welded, or will it use the angle brackets as shown? If you use the brackets, its in general a good idea to have at least two bolts mounting it to each surface, so that they cannot rotate. It looks like there are several places with only one bolt hole.
Crab Drive is a little iffy to me. Other then high maneuverability there aren’t many advantages IMO. I mean if you lose one motor or chain it’s game over for the match. Also it’s a lot of work to build. I find that keeping a chassis simple is better and often lighter.
That ring will be used to mount the module to the lazy susan. As of welding, we hope to if we can find a location to weld at. We have not yet calculated weight yet, we are going to cut weight anter we build our 1st design.
That is very impressive. Especially from a rookie. You guys are definitely one of the up and coming young FIRST teams.
What sensors are you going to use to get the wheels to re-center? Or are you going to rely on driver skill?
It looks like a really solid start for a crab drive. You have the overall concepts down, but there is a decent amount of detail work that can be improved.
Even with the bottom support for the module, I wouldn’t use such a small shaft for the module rotation: It could potentially be taking a lot of load. Also, what is your method for attaching the shaft to the module?
I would definitely get it welded. It will be stronger than those L brackets by far, and won’t loosen like the bolts will (You’d have to be rechecking the bolts constantly to be safe).
There doesn’t seem to be a tensioner for the steering chain loop, what do you plan on doing?
I would try to make the bottom row of outer extrusion extend all the way into the corners for maximum strength. I see how it may interfere with that ring some, but you’ll have to work that out. Since you are taking a lot of the module’s load at the bottom with that turntable, the structure really needs to be strong there. I can see how the plate would strengthen it, but I would have the extrusion go the whole way just to be sure.
Don’t let my comments get you down, it is definitely a promising start.
Good luck, and keep it up until you get a truly great design.
You can do this fairly easily in inventor, by setting the material through iProperties, and then viewing the physical properties of the completed assembly
I suggest looking at wildstang’s 2007 design. It is of the same basic design, but you can probably pick up a few ideas from it.
We didnt think we needed a tensioner. We can tension by hand and there should be about 180 degrees of contact i think…
We were going to mount a POT on the verticle shaft. Still need to work on a mount though…
Depends on exactly how it’s set up. It is quite possible to have a motor go out and still be able to drive (e.g. 118 or 111). It is a lot of work to build, yes, but if they build one post-season, then it’ll be a little easier.
There are easier ways to get maneuverability, but you can lose lots of power. Any drive system (or any system for that matter) that is designed during the offseason has an element of risk; the rules or game design may force a different setup.
And, in this case, there are definite advantages, if not to applying it for the 2009 game, then simply to doing it. For one thing, they’re learning CAD. For another, they’re thinking about new tricks to try in their second year. If they do build it, then it’ll be a good experience and a pretty good freshman introduction.
Thanks for the support. 
True, I’m so use to doing things the same way every year when it comes to drive trains. It is good for learning how to build and for using CAD. I was just talking about the drive it self.
I’m not sure what you mean by tension by hand. Do you mean try to stretch the train onto the sprockets? that won’t work.
you could do exact center to center distances, but that would be difficult with all the different parts, and once the chain stretches it wouldn’t work.
Really, you should be a tensioner in there, and position it so you get more wrap on the globe motor. Also, the globe motor output shaft should be supported.
As for the replies to the thread, he’s designing crab drive, he already decided that. Help him with that, or don’t reply.
its a good idea on any chain run of this distance, regardless of wrap. Chains stretch considerably, and do crazy things under power. If you are planning on having seperate chains running to each module, and mounting space is a concern, then you can use this design, with a sprocket floating within the chain
If it is just one piece, then you will probably need a idler to get enough wrap around the globe anyways.
Awesome job! Even if you never build this, you’ll have learned a lot!
My Opinion on Crab:
Crab will only offer an advantage when you must maintain a specific angle of orientation with the game piece or goal. Crab will NOT offer any major advantage when the game pieces can be picked up from any angle, and/or the goal can be accessed from any angle. A good example of this is 2008. The balls were round… It didn’t matter what angle you approached them from (same for the overpass). There was no need to go sideways, you could just turn and approach them from a different angle. As a result robots that used crab were almost exactly as successful as robots that didn’t. Of course this is all just my own opinion, take it or leave it.
On another note…
I was recently researching a battlebot when I found something very cool which yall might be interested in seeing. If I didn’t know better I’d say they built this for FIRST. (click ‘robots,’ and then ‘Mechanical Maniac’). Its certainly a very innovative Crab design - uses no gears and looks like it might be easy to build.
Check it out! They are using some neat bearings you might want to incorporate into your swerve design.
We were able to tension by hand two years ago–we had 25 chain running on two sides of our robot in two different loops to turn the wheels, and we used turnbuckles as our “master-links” in order to tension it by hand. Easy as pie.
It seemed to work okay, although after a competition or two the turnbuckle would break because it was some cheap thing from a local hardware store. If you got a bigger/nicer turnbuckle, that could work. But you should be sure that you have enough chain that the turnbuckles don’t go anywhere near the sprockets!
Just an option.
Nikhil
Maybe next design, but that is an awsome drive system. I think you have to use gears just to gear down the CIM enough. We were shooting for 12 fps, a big reduction for just 1 sprocket.
Yeah, that guy makes amazing battlebots. Greg actually got his big one destroyed by one of his robots. 
The main feedback I wanted to give you is NOT to do a crab drive until you’ve successfully prototyped one during the off-season. There is a lot of development involved and you don’t want to be learning how to make a crab drive behave when you’re in the middle of robot design for a new game. This is a lot for a team to handle, especially a team as young as yours.
Don’t bite off more than you can chew, especially for minimal gain. The best way to be successful is to simplify!
Good Luck,
John
PS - I lied a little bit; my real advice is to never to a swerve drive at all, they don’t provide any huge benefit to your robot design beyond a basic skid steer.