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Polycarbonate vs. Aluminum
My team is currently in a debate over whether we should use polycarbonate or aluminum for an intake which extends past the bumpers of our robot. The concern that some members of the team have is that 1/4" polycarbonate would crack or shatter if impacted by other robots. Teams that have had experience with using polycarbonate (or other plastics) structures that extend past the bumper and would be susceptible to impacts, how has it gone? Did your drivers need to exert caution when driving or could the plastic hold up to sudden impacts?
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Re: Polycarbonate vs. Aluminum
See the original intake from the 111 2012 robot or the 1678 2014 robot for examples of polycarbonate intakes.
The idea here is that you want a material that is tough, not strong. While strength is a measure of the amount of stress a material can take before failure, toughness is a measure of how much energy the material can absorb before failure, and is a mixture of the strength and ductility of the material. You want the polycarbonate intake to deflect when it is hit instead of breaking, but this naturally means that your intake won't be stiff. On the other hand, you can have a very stiff aluminum intake that will always be in the same spot relative to your robot frame, but you will then have to build it to be strong/tough enough to withstand impact. |
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Our intake arms for our 2014 robot were 1/4 poly carb and in more than 1 match ended up getting caught on another bot and dragged them around the field by our intake. Those arms lasted for at least 150 matches, they finally died after being exposed to UV and then the heat of our crate during our last offseason event. Polycarb does not shatter like acrylic. Aluminum will just die this year. 11/10 Highly recommended. ![]() Ask me your questions. |
Re: Polycarbonate vs. Aluminum
In contrast to RoboChair...
![]() Our 2014 robot used 1/4in aluminum plate for its intake and we had zero issues with it all season. We too got our intake caught up in other robots that year and never broke it. The air cylinders used to deploy them were about the smallest air cylinders we could use, so they collapsed neatly when hit. And we got hit a lot when opening up to catch a ball. It doesn't matter what material you pick, it matters how you execute your design. |
Re: Polycarbonate vs. Aluminum
I recommend polycarbonate. Any mechanism extending out of our bumpers (within volume constraints of course) are polycarbonate. Polycarbonate rarely cracks or breaks on impact in standard conditions. The only time we have ever cracked polycarbonate was in 2015 with our polycarbonate tote intake. The only reason we thought it shattered on impact was because loctite was used on the screws that attached our motors to the polycarb plate. We think that the loctite reacted with the polycarbonate during its assembly and made it brittle instead of flexible, and that's just our hypothesis. The only areas where there was evidence of shattering was in our motor mount holes. After avoiding loctite on the rebuilt intake, it never experienced any cracking after several more regional and championship matches.
Our lesson was to be careful of what possible reactive chemicals come in contact with polycarbonate. And DON'T use acrylic. That's guaranteed to shatter. |
Re: Polycarbonate vs. Aluminum
Polycarbonate is tough stuff, and will likely survive robot-to-robot interaction. It usually bends out of the way and springs back to shape. It's more likely to get bent or tear if you overload it, rather than cracking into pieces like other hard plastics.
Don't let loctite get near it though! We are designing an intake that goes out past the bumper this year, but are planning on using aluminum with pneumatics that take the impact load, similar to 95's robot above. Both methods are feasible. |
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As a professional Plastics Engineer - you will likely what to use Poly-carbonate. You can get to any strength by changing the thickness of the material. Polycarb will flex, bend and take an impact without permanent deformation, while aluminum will deform and require you to reform. At similar strain so PC should work out best. hard to confirm WO a design to review.
Senior Mentor team 6443 Mr. Krafty |
Re: Polycarbonate vs. Aluminum
I would definitely recommend polycarb for the reasons people posted above.
Another point is that it is extremely easy to cut and drill. We use our small CNC router for polycarb but a hand drill and a jig saw could be used to make even the most advanced looking polycarb mechanisms. |
Re: Polycarbonate vs. Aluminum
Another point is that polycarb stands up well to cold bending without too much worry on cracking at the bends after lots of abuse, so all sorts of shapes can be made with it.
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I found a couple sources that confirm these numbers: http://www.usplastic.com/knowledgeba...contentkey=446 Wherein the minimum bend radius for .125in pc is 12.5in, 100x material thickness! I have only heard of teams having success with hot-bending PC, but I would be very interested to see good cold-forming results. |
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If you're using a router, aluminum should be chipped at about the same rate as PC because aluminum can be machined at around 2x the SFM at about 1/2x the chipload per tooth. |
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Definitely take a look at their After the Game video on their 2012 intake. Stacking up lexan plates will give you strength and flexibility and are incredibly easy to make replacements of if you break them and you can stack up more plates to add strength and rigidity. |
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I think the important thing to realize here is that both materials will work and the best material for you depends on availability and your ability to work with the material.
Try not to fall into the trap of choosing a material because it had the most supportive comments on CD. |
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It's certainly a material that is for the most part easier to work with. --- I like polycarb intakes. I like them to be a bit stiffer than 1678's 2014 intake (that violent shaking back and forth gives me the heebie jeebies even though it was fine), so I like to use either Vex's 2x1 polycarbonate tubing or McMaster's 1x1 tubing for some structure. It certainly can be strong enough to do the job - a rough rule of thumb is to double the thickness of the equivalent aluminum part. Aluminum can work too, it just likes to stay bent and doesn't absorb impact energy in the same way. |
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My team (3357) switched out our aluminum shooter deck/intake aluminum for polycarb in the middle of the season because we had a small crack in it. We also did it because we wanted to decrease weight. It worked well and was able to bend without cracking.
Here's a video of us with it: https://m.youtube.com/watch?v=o-mdnlaoFbs |
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I don't understand why teams don't take advantage of material properties like that more often. |
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The key was 2 bearings in the VP mount tube (one on each side of the tube), and then a third bearing on the other polycarb arm. This meant that, even as the arms twisted, the C-C for the chain was held constant. But since the polycarb arm is flexy, the intake shaft isn't over-constrained, even with three bearings (as it would be if all three bearings were held constant relative to each other). Hope this makes sense. Best, -Mike |
Re: Polycarbonate vs. Aluminum
To play devil's advocate: consider for this year is the potential violations of R03 with a highly compliant mechanism. An intake that can bend over 45° sideways is great for avoiding damage, but may not meet R03 to an inspector's satisfaction.
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This is a pretty significant tangent, but it's not worth making a new thread over. What actually happens when you overconstrain a shaft? Does it automatically bind? Is it somewhat more prone to binding? Is the resistance (load) substantially increased? I know not to do it, but I don't know that actual effects of doing it.
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Some potential effects: 1) You won't be able to install the shaft. This is an easy problem to spot. You have three bearings, try to slide the shaft in, and no dice. 2) You figure out a way to install the shaft, with a axially mis-aligned set of three bearings (maybe you constrained the bearings to a rigid member AFTER installing the shaft). In this scenario: 2a) The shaft rotates, but with additional load due to the shaft constantly bending/additional radial load on bearing. This inefficiency can sometimes be hard to spot right away, but can suck power from your system 2b) The whole thing seizes up and won't move. I've seen this happen, especially when the torque applied to the shaft is fairly weak (obviously, since the system is more susceptible to inefficiencies). Like Chris said, do it right, and you won't have issues. But people don't always do it right, so it is important to understand where things can go wrong. Best, -Mike |
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As to the OP, we generally treat 1/8" and 1/16" polycarbonate as if it's indestructible in terms of impact damage, and it has yet to let us down. I've been tempted to prototype a "sheet metal robot" made from poly instead of aluminum and take it to an off-season competition, just to see how well it would stand up to full gameplay. ...maybe some day. |
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Just a reminder, if your intake is made of polycarb, it will probably deflect some. And if that deflection makes your robot exceed the maximum size constraints (of which there are two possible combinations), you theoretically could get flagged for violating G04. I'm not advocating for aluminum over polycarb, but do be aware that they each come with their own set of challenges, and this is one of them.
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Bonus points if you can show the residual stress fields in the plastic. |
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Thank you everyone for the input thus far, all of this has been helpful for us and we will take this information into account.
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Re: Polycarbonate vs. Aluminum
Okay, so, guess what snapped in my hand this evening?
A piece of 1/16" polycarbonate that we'd bent on the break. We've been doing it for years with no problems, and have played some awfully aggressive games with pieces made like this, without any problems. It took almost no effort to break. It was, mind, bent much farther than 90 degrees, and I'd warned my students that this might be a problem. So as with most things, you might want to try things for yourself before implementing them based on advice from some internet blowhard (i.e. me). |
Re: Polycarbonate vs. Aluminum
Another poly-card intake from 2014, MORE Robotics.
https://www.thebluealliance.com/team/1714/2014 This one freaked me out a little when I first saw it, not because of the design, but I had never seen a polycarb intake of that scale before, it had significant wobble but worked just fine. |
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Of course, this is a very different use case than an outside-frame intake, but my point is mainly that what you can get away with depends on use context and runtime. FRC teams can and do get away with doing a lot of objectively awful things to our robots. |
Re: Polycarbonate vs. Aluminum
Thanks s_forbes!
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However, I reserve the right to be skeptical. I'd rather see one example in practice than a dozen people insisting something is okay without any tangible proof. |
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Here is the only picture of one of the Cold bent parts I have on my phone. https://drive.google.com/file/d/0B5V...ew?usp=sharing I apologize for the google drive upload. I'm at school ;). If you want any more pictures of parts Ill take more when I can get into the lab after school. |
Re: Polycarbonate vs. Aluminum
We have a few scraps in the shop. I'll bend some up and post a picture today.
We do cold bends on non crucial parts. Things we want to have survive for a long time we heat bend. |
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Don't confuse polycarb with Acrylic. People do it all the time because they look the same but drastically different properties. Acrylic will shatter and crack. Poly is the same stuff bulletproof windows in banks are made from.
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EDIT: apologies, it was only explicitly stated once, now I look like a hypocrite... |
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I think polycarbonate intakes are great, but you need to ensure it is functional under defense with the added flexibility (though this is really applicable to any intake). |
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