Quote:
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.