pic: 604's Arm

[AdamHeard]

Originally Posted by Daniel_LaFleur

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.

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.

[JYang]

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

[Daniel_LaFleur]

Originally Posted by JYang

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.

[burkechrs1]

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

[spinmunky]

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.

[=Martin=Taylor=]

Quick Fix:

Buy a piece of channel with a 1" slot and 1/8" wall (they sell it at Allen Steel and other suppliers in the Bay Area).

Unbend your arm and press-fit the channel onto it. You won't even need any bolts. It will be so tight it won't ever come off.

That should get you into the first practice matches

[burkechrs1]

Originally Posted by spinmunky

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.

Yea same here. That 290-2 match may not have been such a great idea to start the day off ;-) haha, but hey it was the highest score at the regional. it was great playing with you guys...

[funstuff]

Yet again, you guys were a great alliance, both 604 and 668. It is too bad about your arm. At least you only need to replace the tubing. And now you can take off the steel and add a beefier arm. Good luck.

[DonRotolo]

Originally Posted by Daniel_LaFleur

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.

Also note exactly where it failed - right where the hole on the top & bottom starts. As you get closer to the fixed point (the end held in position) the force on the metal gets higher very quickly given a small force at the free end.

The hole is a "stress concentrator" - the 'lines' of stress (or force) cannot 'flow' through the hole, they must go through a solid, so all the stresses focused at the end of the hole, and that being a weaker point, it failed.

As stated, it can be calculated with some reasonable assumptions. This is called "engineering"

Of course, even with double the arm, there can always be a hit that'll bend it. I, too, hope that your repair efforst go smoothly and you whomp them all in competition...

Don