Re: Has any team or anybody made a 80% - 100% lexan bot?
 

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.

Re: Has any team or anybody made a 80% - 100% lexan bot?
 

I have great respect for all the people to commit their time to FIRST and I have respect for the work they accomplish. That is in no way mutually exclusive to these same people not doing perfect work and having "significant design errors" (would we even be talking about any other kind of design error?).

When I was talking about design errors, I was referring to different requirements then you were. Not the requirements that are set by teams for themselves (the best teams probably nail all of these requirements consistently), but the requirements of the job. The requirement is simple: make a robot that wins. If it was possible to make a more robust robot, or save enough weight to add an aditional useful mechanism, etc, then I don't think the design was "correct". (I'm not speaking about the more general design as it is very complex to analyze wether a ball herder is better than a ball carrier or wether a hanging mechanism is worth the weight, etc, etc.)

The only thing I have been saying is that a very common design mistake is a bad choice in materials. As Mr. Baker pointed out very nicely, and as I didn't and don't disagree with, certain materials do have unique properties that aren't encompassed by the single number I was using for analysis.

If I have offended anyone, I apologize, but nothing was meant to be offensive, and I hope I have explained my intentions to you. This thread has gone way off topic, so if you really wish to continue this discussion, perhaps do it in private.

Re: Has any team or anybody made a 80% - 100% lexan bot?
 

I probably posted this before but Lexan/Polycarbonate has a pretty cool effect to spot wear and tear. Take two polarizing lens and rotate them so that you can't see any light coming out of them. Now look at a clear light source (glass, lexan, polycarbonate) and at points where there is stress you should be able to see rainbows/something.

Re: Has any team or anybody made a 80% - 100% lexan bot?
 

Team 1038 did use the Lexan pictured in this thread. It wasn't acrylic. We only had minimal damage - a chunk taken out of a side plate during the 2003 IRI when a very strong and fast team tried to remove us from the top of the ramp. We also used 80% Lexan in 2002 when we were team #144. There may be some pics in the image galleries from that season. That machine was great. We like polycarb because it is workable for our students and it has many close characteristics to 6061 Aluminum. We also get it donated by a couple of our sponsors, therefore very affordable! One of our students (CD username crispyc) did a Strength of Materials project while studying at Miami University. I'll see if he has any of his research around. Good luck with your project Alex. I'll PM you if I find Crispyc, or his project.

Re: Has any team or anybody made a 80% - 100% lexan bot?
 

A few points about Lexan that we've learned over the years:

1) All polycarbonate is not created equal. We have tried cheaper forms of polycarbonate and found the the GE LEXAN brand name is definitely stronger / more rugged than others. There are different varieties of GE LEXAN as well, but in general they are all better than other manufacturers.

- Also, please don't confuse Lexan or polycarbonate with acrylic. You might as well make your robot out of glass if you go that route.

2) We started using Lexan when it was an unlimited allowable material because it made very nice assemblies. It is easy to machine and it makes nice square corner joint assemblies - we drill pilot holes through the edges using 3/8 inch thick Lexan and use self tapping drywall screws. You need to use Lithium grease or the screws will shear while torquing (use the low torque setting on your cordless drill when installing them), and don't use locktite because it heats up, expands and cracks/delaminates the lexan.

3) An important point to understand when talking material choice is that for impact loading (like running into other robots or them running into you or your components) the imparted load is NOT INDEPENDANT of the material or construction of your robot. The kinetic energy input (1/2mv^2) is absorbed by the structure based on its spring rate; a very stiff structure will take a very high effective g-loading, where a soft structure will see a much lower load. So you could say for instance that aluminum is twice as strong as Lexan, but it the aluminum structure will see 10 g's and the lexan structure will only see 2 g's when they run into something, the lexan may be the better choice if you can tolerate the deflection. This is frequently the case with some mechanisms that don't require precision, and why PVC is a very good choice for them in many cases.

One other point about aluminum if you're making the comparison for a structure: 6061-T6 loses 2/3 of it's strength when welded. The Aluminum Association Handbook takes it from 35 ksi yield to 12 ksi yield in the heat affected zone (within 1 inch of the weld) so be careful.

Re: Has any team or anybody made a 80% - 100% lexan bot?
 

Funny, there haven't really been any references to plywood in this thread.
For my team's first 2 years we used 3/8" plywood. Though in 2002 we had this large steel "knee breaker" apparatus that was used to grab ahold of goals. In 2003, however, the only metal you could see on the robot was the angled alluminum, 1/4-20 bolts and lock nuts used to hold the chassis together. It was sweet.

Re: Has any team or anybody made a 80% - 100% lexan bot?
 

While that would probably work quite well, I would highly recommend that anyone using lexan first glue the pieces together using an unthickened methylene chloride solvent glue. You can get this from any plastics supplier. It is applied using an applicator bottle that has a needle tip and uses capillary action to flow between the pieces of lexan. The solvent breaks down the plastic's surface, allowing the pieces to chemically weld themselves together. Edge preparation prior to glue application is of utmost importance. Your final seam will never be better than your beginning edge, so you may need to use a mill to make a flat edge if you are gluing a cut side.

Once the edges are glued, you can drill and tap them (or use self tapping screws if you are feeling lucky), and they will never come apart.