Crab Drive
 

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 :D

Re: Crab Drive
 

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?

Re: Crab Drive
 

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.

Re: Crab Drive
 

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.

Re: Crab Drive
 

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.

Re: Crab Drive
 

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?

Re: Crab Drive
 

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.

Re: Crab Drive
 

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.

Re: Crab Drive
 

We didnt think we needed a tensioner. We can tension by hand and there should be about 180 degrees of contact i think...

Re: Crab Drive
 

We were going to mount a POT on the verticle shaft. Still need to work on a mount though...

Re: Crab Drive
 

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.

Re: Crab Drive
 

Thanks for the support. :D

Re: Crab Drive
 

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.

Re: Crab Drive
 

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.

Re: Crab Drive
 

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

http://www.youtube.com/watch?v=6fLf71xlVhE

If it is just one piece, then you will probably need a idler to get enough wrap around the globe anyways.