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Unread 14-12-2004, 07:49
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Re: Has any team or anybody made a 80% - 100% lexan bot?

Quote:
Originally Posted by JVN
Well if I remember correctly--
The tkats used that mean arm as a "stiffarm" to prevent teams from getting onto the puck. Perhaps they wanted a "flexy" arm that would give a little bit when it was under "stress". This would protect their gearboxes from damage.

Best guess...

John
Correct. Not only did this design protect gearboxes from damage, but the arm could withstand a significant deflection and then spring back into its original straightness. I remember a few times when the arm's polycarbonate box structure would buckle and bend at a 90 degree angle, and then it would spring back after the load was removed. Sure, there was some damage to the arm, but it did not fracture and we could easily repair it.



Many plastics have great elasticity, while most metals do not. Shown above and linked here is a stress strain curve for a high grade polycarbonate. As stress begins (lower left corner of the graph), the material goes through its elastic stage. Until the stress hits the material's Yield Point, the material can bounce back to its original structure. As the strain increases, the yield point is passes and the material is in the "plastic region". This is the area where you see the material being bent, and then maintaining that bent shape (not springing back to original form). Then, at the end, to the right is the Ultimate Strength point, where the material fractures.

All materials have these stress/strain curves, just like all motors have torque/speed curves. The main difference between plastics and metals is the distance between the yield point and the ultimate breaking point. With metals, the distance is short. With plastics, it is much longer. Therefore, choosing plastic (Lexan, PVC, etc.) as a construction member for a FIRST robotics design is a good choice, since these members see much deflection at times.

Andy B.
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