Crab drive question

If i put this in the wrong spot, sorry, im new to this forum, my team and i are trying to put together a crab drive in the off season and a just had a few questions:

How do we keep shaft bores and bevil gear holes are inline?
how do we make sure our sprockets are inline?

any help is appreciated

I think it would help me in understand your question if you could more specifically reference the parts your talking about. I have seen several implementations so maybe relate to a similar design picture.

I’m not an expert on crab drive by any means but I do have some experience with the 118 design. Similar to:

http://www.chiefdelphi.com/media/photos/31078
http://www.chiefdelphi.com/media/photos/28442
http://www.chiefdelphi.com/media/photos/28443

oh sorry. this is for putting together the wheel housing

i just wanted to know when fabricating the housing how to keep the wheel shaft bores even so we dont have crooked wheels and such,
how we drill the holes fro the bevil gears to they line up properly
and any tips for getting even drive sprockets

Precision tools, and a skilled machinist. Crab drive should be done on at least a mill, and preferably a CNC one. Use CAD to create a detailed model of your design, and work from that.

From a machining point of view you’re best served with access lathe and mill (CNC probably needed for the nicer, lighter designs) while access to waterjet/laser cutter will make some of the machining much faster and simpler.

To get the all the gears and sprockets lined up and keep the keys in you just need to cut some bushing that are the right lengths to keep everything lined up.

From McMaster we use:
6391K215 1/2" ID Sleeve Bearings (custom cut these to length for vertical and upper horizontal shafts)
1570K43 1/2" Keyed Aluminum Shaft (for vertical and upper horizontal shafts)
5911K42 3/8" Aluminum Shaft (for lower, wheel, shaft)
Small PVC or sleeve bearing for the 3/8" shaft

The vertical shaft comes down through the middle of a turned piece guided by bearings at the top and the bottom which keeps the shaft lined up with the upper horizontal one. If you are transmitting power form the upper horizontal shaft to the wheel via #35 chain then that alignment isn’t too critical, but the bevel gear spacing needs to be more carefully adjusted with the custom cut bushings.

Don’t get discouraged, but crab drive is very resource and time intensive, lots of CAD work before you start cutting with reduce mistakes and simplify your design… There are 21 machined pieces, 8 bearings, plus shafts and spacers in each of our wheel boxes.

A really simple way to do this is to make the entire module out of boxed aluminum (like this). That way you will only have to drill one hole, and it will match up on both sides.

As for the miter gear; be careful. Make sure the two drive axles are directly over each other at right angles, and leave some room above the miter gear so you can position it up and down.

Have you used those shafts in the past? The first is 2024, which is probably OK. The second is 6061, and I would almost guarantee that it will fail if used in such a high load application. Especially at 3/8" diameter. I’d recommend 2024, 7075, or 7068 for shaft applications. 2024 is the most commonly available (and cheapest).

Agreed, on any applications that a shaft does not need to be welded to the frame or to something else, use aircraft grade.

I think you will be unhappy with the performance of the bevel gears transmitting power. The design linked above with the motor in the crab module has worked for us when we use crab. It is easier to get wires into a turning assembly.
We always find ourselves reminding teams that they shouldn’t dedicate themselves to a particular drive system this far in advance of the kickoff. We use crab drive if and only if the game needs it. There are tradeoffs for weight and programming that could be used for other devices.

This was our first year to try crab and that is the shaft material we used, we didn’t have any problems, but then again I wouldn’t say the robot experienced any real competition level stresses… it’s funny when all the mentors are EEs but the wiring turns out to be the problem. :rolleyes:

It’s really nice to be able to have different drives depending on the game, but newer teams or ones with limited resources should consider picking a drive and building it preseason as practice and staying with that frame and drive for a few years until they are more established. Switching drives to match the game can be very resource intensive, and it also seems that for most any game any drive system will allow teams to still score and not having the reinvent the drive means they can spend more time on other aspects like manipulator. (I wish someone had told me this when starting out…)

For that exact reason is why it is a bad idea to lock into a very resource intensive drivetrain preseason. They’d be better suited developing a 6 wheel or other simpler drive if that was their goal.

I agree with your overall sentiment, just not in the case of a crab drive.

Don’t be discouraged though - if your team wants to make a crab drive for an off-season project, DO IT! What better time to learn than now and not have the build season time constraints placed upon your team? Better to try it now and mess up than in January. After you’ve made a successful one you’ll probably be able to make the next one in half the time and you’ll have that experience under your belt already.

Our team plans to work on some practical yet fun off-season projects. It’s a great way to give the new people some experience prior to build season.

Sorry I didn’t mean that to specifically apply to crab, but more generally to the concept of always considering a change of your drive for the game. Crab is intensive and unless you can get one working preseason most teams shouldn’t try it for competition. This is not to say don’t try it, everyone will learn from this kind of project. “Stretch goals” right?

Although I don’t know 2338’s situation, sometimes challenging yourself can be a catalyst for your team. I’d say by most metrics that for us(1817) to do crab this past year would have been in the “bad use of resources” category, but the level of complexity and dedication required plus the “cool” factor has gotten us more mentors, money, and attention.

I’d say that designing/building/programming/utilizing a swerve drive in the off-season is a great way for students on any level team to work on their all around robotics skills. Get everyone involved, you can teach wayward freshmen the finer parts of manufacturing precision parts, your programmers get some new and very interesting “opportunities” (problems in disguise), and it is just a great way to build team work in general. Be sure to set deadlines though, our team (1625) tends to find as many reasons as possible to procrastinate when there are no actual deadlines set or impending on them. Once you have built said crab drive, be sure to never stop thinking of ways to improve it, make it lighter, more durable, more maintainable, anything to better it. Even if you don’t wind up building one for next years game or it doesn’t feasibly apply to the gameplay, your team will have walked away with invaluable machining experience and just experience in general in the ways of teamwork and comradery. Also be sure to have a few LAN parties to offset the idea that it’s actually work :wink: (that’s what we do).

I was quite happy with the performance of our bevel gears in our drivetrain, part of it is deciding whether to sacrifice the efficiency for the pro’s of coaxial (no limits). Which i chose coaxial for the simple fact that no matter what the programmers shouldn’t be able to break the drivetrain:D .

And the better you align them the more efficient they’ll be, we had a 13fps high gear and if you watch matches with us in them we get moving pretty good, so the efficiency loss is not very noticeable.

FIRST is very much a pick your poison sort of deal.

If you are going to let your robot get anywhere NEAR bevel gears (especially the bits associated with the drive system) then I recommend that you follow the example of The Dread Pirate Roberts when dealing with Iocane powder*: spend a “few years building up an immunity.”

Bevel Gears are serious poison, at least as deadly as Iocane powder :wink:

Consider yourself warned.

Joe J.

from the Princess Bride:Dread Pirate Roberts*: **Good. Then pour the wine. <Westley pulls out a small
cylinder> Inhale this, but do not touch.

** Vincini:** I smell nothing.

Dread Pirate Roberts**:** What you do not smell is called Iocane powder. It is
odorless, tasteless, dissolves instantly in liquid, and is among the
more deadly poisons known to man.

<snip>

Dread Pirate Roberts: You guessed wrong.

Vizzini: You only think I guessed wrong! That’s what’s so funny! I switched glasses when your back was turned! Ha ha! You fool! You fell victim to one of the classic blunders! The most famous is never get involved in a land war in Asia, but only slightly less well-known is this: never go in against a Sicilian when death is on the line!! Ha ha ha ha ha ha ha!! Ha ha ha ha ha ha ha!! Ha ha ha-- <Vizzini stops suddenly, and falls dead to the right>

<snip>

Buttercup: And to think, all that time it was your cup that was poisoned.

Dread Pirate Roberts: They were both poisoned. I spent the last few years building up an immunity to iocane powder.