pic: Coax Crab, another variation.

Well, I got a PM from someone noticing that I hadn’t put any CAD up in a while… So I figured I’d update you all. Here’s a new iteration of my coax crab, lightened, compacted, and strengthened. Totally square frame this time, ala 118. Single gearbox, so whatever manipulator used would need a turret.

Weight: 38lbs sans chain and steering. Steering will be done via Kevlar belt.

Questions and comments welcome, as always.

looks very nice! what are you making the yokes out of that hold theh wheels? This looks very similar to our setup, and we made our yokes out of .25" steel plate, and had numerous problems with them bending right below the uppermost axle. There are 2 ways around this:
1: Go to more of a wildstang setup and support the system on the bottom as well
2: Run a pair of straps around the wheel right where the lower axle is. This would join the forks together, and give you more support on the bottom side where it would be prone to bending

Also, how do you plan on on tightening the chain to drive the wheels themselves, or do you plan on using spur gears like the beast does?
I would personally swap your setup between the steering and the drive: aka, use the timing belt for the drive of the wheels, and the chain for the steering, but that’s just my opinion. Good Job, that is a real nice looking setup though.

About bending of the wheel forks: There are three standoffs there, not easily visible. The whole thing would be made of 1/4inch hard anodized 7075, so it’d be pretty strong.

As for tensioning, ever seen 118? Floating idler is all I have to say. :stuck_out_tongue:

I used a floating Idler before after seeing one in one of 118’s videos and it worked great, my mentor looked at me and said “how the heck is that gonna work” and I replied just watch. But, yeah nice cad work. Are you only using one motor to turn all of the modules. If so how do you steer or is it that you don’t?

The robot can’t rotate, which is why he said it would need a turreted manipulator


Yeah, the floating idler is pretty awesome. I do, however, have a backup tensioner designed in case the floating one doesn’t work.

I think you misunderstood me on the tensioning. Let me see if I understand how this works correctly to drive the wheels:
1: motor turns chain which powers the drive sprockets on top of modules
2: drive sprockets turn a shaft going through the center of the module which has a bevel gear on the other end
3: bevel gear meshes with another bevel gear, turning the rotation along the vertical axis to the horizontal axis
4: This second bevel gear powers a chain drive which goes down the interior of the module directly to the wheel.

I realize how easy it is to keep tension on the 1st step, what I am worried about is in the 4th step, especially if you are using #25 chain. With #35, you don’t have near as big of problem, but there’s still a problem.

Nice work though.

Why the massive wheels?

Ah. It’s close enough to not need tensioning, and may even be changed to use kevlar belting if I ever end up making the thing.

They’re not massive, they’re only 4 inches in diameter, and 2 inches wide. The whole robot actually isn’t that big…

FINALLY! I’ve been asking you for quite a while for your version of the 118 Crab and I finally got it! Awesome job! :smiley:

Wow! That is small. Looked more like 6" to me.

Yeah, the whole robot is only 26x26, and about 10ish inches tall. It’s really quite small.

And yes, Pavan, this is ONE of my 118 type crabs. There are more. IM me sometime and I’ll send you a render of another one.

Ok, good to know, If you do plan on making it sometime, 648 used a similar setup as far as the individual crab modules go. The whole yoke was extremely tight clearance. The bevel gear cleared the tread on the wheel by about 1/16", and we had a real PITA trying to tighten up the chain. What we ended up doing though worked pretty slick once it was set. In our setup, the bearings which you have on the yokes, were actually on the wheels, and the sprocket was bolted to the wheel. This let us mill a slot where you currently have your bearings, and attach a small threaded block to which had a bolt and jamb nut in it. The axle was milled flat at each end, and the bolt was tightened down equally onto the axle on each side until the chain tightened. It worked real slick. 1625 had a similar crab setup, but used a small cam tightener to tighten the chain. Needless to say, some sort of chain tightening device is a necessity if it is to see the floor.


Is there any reason to use chain in the 4th step instead of two spur gears? Seems like gears might be more reliable option to connect two shafts that are so close together. But, I’ve never built a crab drive system.

I just love crab drive, and I’m thinking of designing my own for our team next year, but I’m not sure I completely understand the concept. Do you think you could maybe give me a short explanation of the basics.

Also, how many CIMs are on there? It looks like four on just the one side, and if so is it possible to use less?

Mmm, Craig you could make it so the robot could turn if you used two motors to control the rotation. You could have a warthog esk drive. Your cads make me want to start cadding again =]

This particular crab uses a “powerplant” style gearbox to drive it. As you can tell, there are no motors in the individual crab modules, but rather one cluster of them in an isolated location. If you look, you’ll notice two sprockets on each of the 5 vertical shafts. Those 5 shafts are all connected with the chain. So each shaft is connected to the one before it and the one after it.

What you may be confused on is how it turns. Right now, it doesn’t. But it looked like he was planning on adding some timing belts. The steering would come from a separate motor and not the powerplant. So the shafts are spinning on the same axis as the modules rotate. They call this a “coaxial” design.

Below are pics of another crab with very similar setup but different frame. It should give you a better idea as far as the gearbox works.


Joe, I think his intentions were to show 118’s method of crab. Which doesn’t really have a correct orientation. But I do like your suggestion a lot. Turning a crab drive tank style is very frusterating and really the best way I’ve seen is to have two sets of steering… or 6 wheel crab…

Exactly. Also, WOW this is old… I re-did this one, making it about 2/3rds the weight, and MUCH shorter. The new one can be checked out here.

Also, 6 wheel crabs are a step in the wrong direction. You don’t gain anything out of the extra two wheels, except for a more complex frame, more parts to machine, more weight, and more hassle. Crab is a LOT of work, and should only be chosen if the game demands it. (Note: in my opinion, 2006 was the most recent game that could be owned by a crab.)

Tell that to Nasa. In all honestly though, I was joking. It’s still really cool…

Craig, I was hoping to get my hands on the cad files for your “Bullet Proof Drive”. I tried shooting you an email but I must’ve sent it to the wrong address. If you get a chance, I’d appreciate it.