Arm swings when turning, how to brace it?
 

Or arm is made of a single four foot aluminum c-channel with a fabricated 8020 bracket at the top to hold our window motors for the shoulder joint. the next stage is another three foot c-channel with the gripper on the end. we noticed in the little practice time we had that the channel tends to twist when we turn. Does anyone know of a way to brace the channel so that it won't twist? there are some pictures on the portal of our robot from last monday (team 3329) if that helps anyone. Thanks!

Re: Arm swings when turning, how to brace it?
 

Twist? In what way? Without knowing more about the problem, I can't help you. Sorry.

Re: Arm swings when turning, how to brace it?
 

the c-channel literally twists. like your ringing out a towel, just not as dramatic. like someone is holding the ends and trying to make a metallic twizzler out of it. hasn't had any damage yet, my concern is just that we don't want to enter a match like that.

Re: Arm swings when turning, how to brace it?
 

a c channel or angle will always be easy to twist, if you could replace it with some kind of tube, round or square etc, it would be much more rigid.

Re: Arm swings when turning, how to brace it?
 

You may try bolting another C-channel over the open side of your existing C-channel. I assume your using the AndyMark C-channel from the kit of parts. There is another C-channel that fits over the AndyMark - maybe from IFI - someone can probably confirm this. This isn't an ideal fix but it may be easier than replacing the entire arm section.

Re: Arm swings when turning, how to brace it?
 

Actually we are using donated c-channel from the Trident Refit Facility here on Kings Bay Naval Base. They fabricated the channel from Aluminum sheet, as that is all they could do. other channel could be hard to fit into the grove as we have various screw and bolts going through the arm...

Re: Arm swings when turning, how to brace it?
 

If its not too complicated to do, switching to a tube round or square is the best option. C-channels are just by design quite flexy like you have seen.

Re: Arm swings when turning, how to brace it?
 

if you post some pictures of the robot, and the parts that are twisting, we might be able to help you figure something out. Otherwise we're just guessing..and that doesn't help you much

Re: Arm swings when turning, how to brace it?
 

Best way to strengthen a channel is to box it in. One way is to bolt on another C as sugested or you could weld a plate to the open end of the C.

Re: Arm swings when turning, how to brace it?
 

Ill say it again, square tubing 1/16in thick, its light, durable, and some-what cheap. If you don't have a local hardware store that has any you can get some from McMasters for under 10$ and it will ship in 18-24hrs.

Here's the URL for it http://www.mcmaster.com/#tubing/=b829jk

Re: Arm swings when turning, how to brace it?
 

I live in a small city, there are at least 4 local stores that sell 1x1x1/16" wall aluminum square tubing. But I don't know if it would fit onto your robot easily. Also, round tubes such as ABS sewer pipe will not twist, and are readily available in several sizes (ID from 1.5" to 4" is very common), and might be more easily adapted.

Re: Arm swings when turning, how to brace it?
 

Squirrel, what local stores sell 1x1x1/16 sq al tube? We use a lot of it and always have to special order it from a local metals supplier.
Bruce

Re: Arm swings when turning, how to brace it?
 

We managed to strengthen a bit of our cbase from twisting the way you mention by fitting in some 1010 8020 (1" x 1" extruded aluminum) into the channels and bolting it down. Maybe you can try that with some square tubing. We also used the 8020 in the cbase for mounting purposes but for just strengthening i'm sure just square tubing will work alright.

Re: Arm swings when turning, how to brace it?
 

Ace, Sutherland's, Lowe's, and Home Depot. Although I have to double check Lowe's, as I didn't buy it there this year.

Re: Arm swings when turning, how to brace it?
 

I can attest that Lowe's carries it.
So does Menard's.

Re: Arm swings when turning, how to brace it?
 

Our arm twists also, it is pretty light.

What we found out in many hours of practice driving is that it didn't matter; it twisted but didn't break. i was sure it would break, but it didn't.

What we are doing is making sure that the arm and gamepiece is in "stow" position whenever we travel. That will also somewhat protect it from entanglement with other arms.

we are not sure whether our arm can take the rough and tumble of the competition. we made plenty of spare parts!

jim wick

Re: Arm swings when turning, how to brace it?
 

Our arm twisted a bunch the first time we built it, we found the weak points and redesigned them, and now it twists a bit, but it's acceptable.

But we used tubular materials for the long parts.

If you can use the robot, and it doesn't get damaged by things moving around, then some deflection is acceptable.

Did you ever sit in an airliner and watch the wings bounce up and down?

Re: Arm swings when turning, how to brace it?
 

Have you ever flown in a 707 and sat in a window seat? The wings move so much it looks like they're flapping like a bird's wings.

Re: Arm swings when turning, how to brace it?
 

heh....i'm not THAT old. My first ride was in a 737 in the late 70s

Re: Arm swings when turning, how to brace it?
 

You can also find 1x1 thinner than 1/16th at places that sell sign materials.

We buy alumalite & light aluminum tubing from http://www.harborsales.net/Default.aspx
tubing here: http://www.harborsales.net/tabid/130...oductTypeId=27

Re: Arm swings when turning, how to brace it?
 

Thanks a lot :-(

Re: Arm swings when turning, how to brace it?
 

From my experience, the 1x1 I get at Home Depot here is 1/20 in. wall. Light for an arm by itself, but when used to stiffen it could be sufficient.

Re: Arm swings when turning, how to brace it?
 

Yes, thanks for the correction. I bought a stick of it there and noticed it said 1/20, but forgot until you just reminded me.

Re: Arm swings when turning, how to brace it?
 

It likely is. When I took Aero Structures last semester, one of our homework assignments was meant to drive home the fact that a closed section (like a tube with a square or circular cross section) is significantly more resistant to twist than an open section (imagine a tube after you've run a sawzall down one side).

You can show that for the same stress a circular thin walled closed structure (like a length of tubing) can carry 60x the torque of the equivalent structure with a slit down one side.

In layman's terms, you get about 60x less twist once you close the structure. Tubing is your friend. :)