| Mike Ciance |
02-02-2005 18:15 |
Re: multiple tetra stacking
Quote:
Originally Posted by Matt Adams
Alrighty...
I'll go ahead and just do some math. (And engineering, it's much better than math.)
We're gonna take 3 tetra at 10 lbs each, 30 pounds. We'll be crazy people and make an arm that's 8 feet around a pivot (it telescopes out)
We'll just use a piece of 3 x 3 by 1/8" box aluminum, 6061 with a yield around 36000 PSI.
That arm joint will weigh (3in²-2.75in²) * 96 in * .0975 lbs / in³ = 13.455 lbs
That's 30 lbs * 96 inches = 2880 in-lbs for tetras
and
13.455 lbs * 48 inches = 645.9 in-lbs for the arm weight itself (not really negligible)
bending stress = M * y / I
M = moment
y = distance from center to extreme edge
I = moment of inertia (for a beam = h³*b/12 where b = base width and h = height in the direction of applied force)
(645.9+2880) * 1.5 / ((3^4-2.75^4)/12) = 2665.68 PSI
Factor of safety... about 13.5.
This proves that you can create something that will endure the stress using just simple materials.
The torque is a lot, but only when you start trying to twist around that axle of your pivot. Start thinking pulleys, counter weights, large moment arms on the back end...
I didn't mean to go all formulae crazy... but... you dared me!
Matt
|
Matt, as a veteran of FIRST I have seen this happen in 2002, 2003, and 2004, and I see it happening again. you are making a grave mistake.
mathematically, yes, you can build a robot capable of lifting multiple tetras at the same time. what you fail to take into account, however, is the competition environment. teams will play defense, and if they see a robot trying to cap multiple tetras they are going to push it. it doesnt take much of a push to send a robot holding that much weight that high toppling over. it won't even be counted for a flag by the judges, because they were preventing a score with a perfectly legal maneuver. even if you don't fall over, that much weight will make the arm sway when you get nudged. that will make it very hard to cap, and tetras may even come out of your grip with all that movement. in all three seasons that i have seen, one thing that remains constant is the failure of top-heavy robots. the more you carry up high at one time, the more top-heavy you are. if you do choose to make a robot capable of such a task, please be advised to use this strategy sparingly. any aggressive opponant will take immediate advantage if they see you lugging around a stack of tetras. |
