Designing arms against buckling

[Gasperini]

Originally Posted by Gasperini

Here's what we were working on:

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We already figured it out. We've got a FOS of about 8 against buckling, and 11 for crushing. I'm posted the solved problem so the FIRST community can benefit.

Gdeaver, our arm will not be seeing significant torsional stress. We looked at the worst case scenario involving a robot trying to stack, and would be relatively motionless. The force we used would result from a piston device jamming, and we would not deploy this device if the robot was turning or driving. This formula addresses a 3-dimensional beam.

You might have been talking about a bending moment. We do have one of those, espicially since out column has an eccentric load. All that is accounted for in the problem. Given our L/r ratio is about 150, buckling needed to be addressed.

(Students - You can calculate this ratio based on your beam geometry. I didn't actually find the L/r ratio in the problem because I correctly assumed it was bad. I did find the moment of inertia, I, of the beam cross section, and the area. rg=(I/A)^(1/2), and our L/rg comes out to 158.50.)

[Gdeaver]

If a robot is not interfered with, I agree the torsional stresses are relatively low. However you can bet the other alliance is going to be hitting your robot and arm. This is going to cause major torques on the arm. IF a team has not designed for this, they could be looking at some ugly twisted metal. Last year most teams where fighting for the platform and we could cap with out interference . This year interference is part of the game and we are going to have a good amount of weight hanging out there. We're adding reinforcements to deal with it, but we're worried.