pic: 604's Arm

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After our quarterfinals round, we realized that our arm was hit. This is what it looks like now.

It’s such a shame that people will try to do anything they can on the field in order to win in this competition, even resort to what in hockey the equivalent would be a “Check from Behind.” I know this is a competition, but these people spent the same 6-weeks that the rest of you did designing and building your robot and this is what they got in the end; a broken arm. At the very least I’d hope that the offending team(s) came over to appologize and offer any assistance they could to help repair it. There are plently of good, honest, and fair ways to play defense in FIRST that don’t come near causing this kind of damage. This is terrible.

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Not to mention this was done to the SVR Chairman’s Award winning team :frowning:
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Heh, I knew someone on your team would post this. I was one of the drivers from 624 (we were admiring your wheels at the end of SVR). It’s too bad that that needs to be replaced now. The thing is, that, and some of the damage we sustained in the quarterfinals should have been penalties for the opposing alliance. Nevertheless, what’s done is done and I hope you guys can repair that arm before your next competition. Good luck!

Edit: Congratulations on winning the Chairman’s Award!

So this is how it went…

1-long ago when we first made the arm- it was straight!!!

2-after tensioning the chain a lot- it started bending slightly to the right…

3-after the first 10 seconds of the Quarterfinal match- bent 20degrees to the right

4-at the end of the match- bent 40 degrees to the LEFT

Btw, great alliance partner, 624

http://i7.photobucket.com/albums/y261/pepgrrl/lelandarm.jpg

Ouch, look at that angle…

604, you guys are an amazing group of kids. :slight_smile: Congrats on getting Chairman’s again, your team deserves that and more!

Yeah… It was a nasty angle. Thanks. You girls rock too, in the near future you girls will soar!

This is exactly why we made our arm team spend several weeks designing before they got to build. The one major thing we learned last year was that competitions are very very rough and the robot must be designed to withstand tons of abuse!

OMG!:ahh: Sorry to find out that happened to your bot. I hope your bot gets fixed up enough for Georgia. Don’t know if we’ll make it or not but if we do, I know Jon wil be excited to see you guys. Good Luck at Champs and congrats on Chairmans.

I am sorry to see that damage to your arm, and I hope you are able to build a replacement.

That being said, this is a great example of how Lightening holes can often affect structure. Your arm failed because (in addition to the rough play on the field) you removed (lateral) load bearing material. If the material for that particular lightening hole was not removed the arm probably would have had enough lateral structure to survive. Removing that material so close to a fixed (lateral) structure (your sproket and tower) means that the entire load of the lever arm is applied to the (now reduced) structure. At this point a simple bending calculation would have told you exactly how much force would need to be applied to bend the material beyond it’s modulus of elasticity and deform the material permanantly.

I cannot tell the material, but it appears to be 1" or 1-1/2" box (1/16" or 3/32" wall) aluminium (6061) and by removing 3/4 of that material you reduced it’s load capacity significantly enough to fail.

Please don’t take this the wrong way, as I’m trying to show other teams that just drilling speed holes blindly is not the answer. You need to understand material properties and applied forces. I did not see the hit that you (in 604) took. That said, anything that gets extended outside the bumpers is allowed to be hit and if part of your robot is gripping on to something, contact in the bumper zone can cause stresses in unexpexted directions. This is a rough game, expect forces to be applied in directions other than the directions your robot can drive it in. Calculate the lever forces on long parts, especially if one end is fixed. In essence, build it robust.

If you give me the material (metal type), size (box size, wall thickness), how much material you removed, and the length of the arm from that point to the end, I can tell you how much (lateral) force will cause the structure to fail. I think you’ll be surprised at how low that dorce actually is.

Again, I hope you are able to build a replacement.

that arm is definately 1x1x1/8th wall sqare tube and should be able to take alot of abuse but theres always that point which appears has been reach by another robot.

I would expect that arm to bend there…it is very weak. Even though it started out as 1" square tubing, it isn’t square tubing now, because of the slots cut in it, presumably for weight reduction. And it probably should have been 1.5" or 2" tubing in the first place. A small increase in tubing size gives a big increase in strength.

A quick fix could be made by straightening the arm, then riveting two pieces of 1" angle over it, one on either side.

I do recall watching that happen out on the field. It wasn’t a special hit for that regional, but it was a lot more than any robot should have to take. SVR had defense to the point where a team spun the rack about 60 degrees. It was impressive.

For that application, I personally would have gone with 2x1 rather than 1x1. The extra strength would have been invaluable.

Hope that gets fixed. If you need any help in Atlanta, let us know!

There was definitely more defense in SVR this year than in most past years. Probably due partially to us (190) being there, and partially do to a number of robots lacking working arms (or designed as purely as a ramp robot), and going on defense.

Which is why our replacement arm for Nationals will have almost no lightening holes in it… xD

Yes, that round where the rack spun around was our first Quarterfinal match, and our arm got bent in second Quarterfinal match.

I was thinking the same thing this year. I was wanting the lighten to two main lateral base supports (2x2x1/8 AL) (seen here) but when I did the math on it the weight saved was minimal compared to the strength loss.

Looking at your arm; Assuming 1x1x1/8 AL, and that those patterns are 1/2" radius x 2" long, inventor says that each cutout weighs .01 lbs. So, the strength loss versus weight saved seems not worth it in my opinion.

When we rebuild the arm, I don’t think we are hollowing it out again. The defense robots at SVR were really good.

Good luck and we’ll see you at the championships.

yea i saw the arm after the quarter finals and it looked pretty bad. team 190 played the strongest defense against us i have ever seen so it left you guys to score everything. you guys did great and put up one heck of a fight. it was great playing with you guys. and 624 your elevator ramps were awesome. very clever. 604, hopefully you guys can get that stuff fixed by nationals. but i think SVR will be some of the toughest defense we will see this year. thanks for the great alliace 624, and 604, you guys were great.

-Chris
Driver, 668

SVR was brutal for us, once everyone saw us in action in the first match of the regional they always sent thier defense at us. We got tipped over 2 times and almost 2 more times. I hated that match when 253 tipped us over. My dad got great video and it’s painfull to watch them tip us.