Hi, I’ve been wondering about how to strengthen the rigidity and raise the resistance of wear of certain materials.
So far, what I’ve found is that you can strengthen the rigidity of aluminum and certain plastics by using carbon fiber. However, carbon fiber is a very time consuming process and can also be expensive. The carbon fiber itself isn’t expensive, however, the machinery needed to spool and thread each part can be expensive.
Has anyone come up with any other solutions? I feel like I’ve heard of people talk about some sort of vinyl/adhesive where you apply an epoxy after you stick on the vinyl to aluminum/plastic and it hardens, giving the aluminum/plastic extra rigidity.
I’ve also researched that the carbon fiber vinyl wraps don’t actually strengthen the material, but they look cool and reduce some of the wear.
You can also raise the resistance against wear of certain metals such as aluminum and titanium(Really really expensive), by anodizing it. However, anodizing can be hard when you have to put large parts in an electric bath. Are there any other better methods? Maybe a film adhesive? I know that cars sometimes have a film adhesive they apply on the front exterior of the car in order to prevent scratches in the paint.
I also understand that some parts if designed better with structural supports will obviously have more rigidity, but sometimes the weight limit or just the lack of manufacturing tools prevents teams from doing so.
Are there any better alternatives to the conventional materials most teams use?
The debate between aluminum and steel have been somewhat controversial, but what about acrylic? Acrylic’s known to shatter. I’ve seen teams use Lexan or some other sort of plastic, but what do you guys recommend?
Sorry for the wall of questions, but I would be grateful for any advice.
With regards to anodizing, try looking online for local companies that do anodizing professionally. Go in and explain what it is you do. You may find that they’re willing to do a few parts as a donation to you. We managed to get a company this year to do our gearbox plates (purely aesthetics) and have now since formed what is hopefully, a lasting relationship.
Acrylic is definitely a no-go because it shatters very easily. Polycarbonate is much stronger than acrylic, but if you’re looking for stiffness then you’re looking in the wrong place. PC is good for light-weight mechanisms and out-of-frame extensions that need to be flexible to withstand hits.
I wouldn’t really be worried about wear in FRC robots as long as your robot is designed well. FRC robots will very rarely see 100 hours of use never for more than 5 minutes at a time, which is nothing compared to machines in industry that run constantly for years. Aside from wear on rubber parts and tread (and battle scars), the most wear your robot should get is maybe a bit of rust on steel parts over the years. Anodizing makes parts look nice, but for FRC it’s not really required.
Overall, I would say the best way to increase the rigidity of your mechanisms is to design the parts better. I would venture to say that there are very few (if any) designs for FRC that would need to be made from non-standard materials for rigidity or wear purposes (whether you want to for other reasons is a different story). If you aren’t a powerhouse team looking to optimize every little detail, there are probably better uses of your team’s time and resources than experimenting with non-standard materials.
In general, new designers in FRC focus way, way too much on material and not enough on changing geometry for strength. Making a piece thicker, changing the profile to something more rigid (e.g. replacing flat plates with c-channel or tube), increasing the size of the cross section (replacing small-diameter solid axles with large-diameter hollow ones), etc. are all easy ways to get strength without really changing the material or how much weight is used.
It’s hard to talk about this stuff in the “generic case”, so it would help to have some examples to work from.
In general, if you’re running into the limits of the strength of your material in FRC, either the material is completely inadequate for FRC (acrylic) or you’ve done something very wrong in the design.
This summer, one of the the things my team has experimented with is Carbon Fiber! Despite my initial hesitation, it wasn’t as time consuming and grueling as I had imagined. Alyssa, who was once the carbon master on 2485 ran a workshop with us, and it took us a few hours to cut our sheets/let the eposy set, and another few hours the next sunday (you could do the next day) after it cures to cut out the final shape and stuff like that. We didn’t actually need a machine to spool the carbon as we just bought a long length pre-woven and some other smaller stuff per Alyssa’s reccomendations. While The Milkenknights will not be making every part in Carbon Fiber next year, its pretty clear that using the Carbon is totally feasible. That said, Carbon is a nice alternative when you want to trade some extra work for weight. In many cases, using thicker material will do the trick for much less time and other resources and the Milkenknights will likely go that route in most cases for a hypothetical 2018 game.
Unless your bot is really hurtin for weight, I wouldn’t risk using those weaker materials or devote resources to things like carbon fiber *
A few exceptions to that would be 2015 can grabbers,
2011 massive arms, and anywhere else something needs to be super strong and light.
All depends on your teams resources like money, manpower, machines, etc.
You are touching upon one of the fundamental issues of engineering: Stronger, lighter, and not expensive.
The short answer is: It’s not always easy, but anything can be done.
The longer naswer is that it really depends on the exact usage a part will get.
Two trivial examples: If two parts rub against each other, therefore are subject to wear, to reduce wear you can add a low-friction material between them, and that will reduce wear. “Low-fricthion” can be oil, grease, teflon, or even paper.
If Part “X” needs to to resist bending from a load “Y” placed upon it, there is an optimal shape and size for a given material that will do that. This can be determined experimentally (make smaller and smaller X until it fails, then go back tot he last one that didn’t fail) or theoretically (do the engineering math).
There is no easy answer - “Just wrap all parts in zirconium foil” - because each and every part is unique in its requirements. You don’t need an engineering degree, but some level of understanding of Statics & Dynamics, and Materials, will answer most of your questions.
The classic engineering conundrum since ‘stiffer’ means more brittle. Why glass is very stiff but when it does get pushed hard enough instead of flexing it shatters into a million pieces. Same concept on why acrylic is horrible for robots, an impact and you are gonna have acrylic everywhere.
Polycarb is a bit more so the other way it’s fairly flexible so when it does take an impact it bends instead of breaking but it’s sometimes more flexible then you want for a lot of robot parts. Plastics like ABS/Delrin/UHMW are in the middle of the stiffness vs strength area.
Aluminum (6061 and 7075 mainly) is a great balance between stiffness and overall strength. Generally pretty sturdy and will flex before snapping. As it seems you already understand geometry and proper design is usually more important then whatever fancy material you can find or dream about. This is something most FRC folks learn the hard way as the hivemind generally is “add more material”. It’s hard but getting comfortable understand loading is one of the key aspects of design.
For example this year we used a combination of bolted together aluminum to frame basic structure of the robot but then welded on supports and gussets to increase stiffness in the directions we wanted to. Made it easy to mill all the parts and have the kids bolt them together square. Which then meant the welded on pieces can be simply clamped on and the welder lays the bead. And we can run thin 1/16 wall instead of 1/8.
Carbon fiber has the best stiffness to weight ratio, period. Why it’s used in aircraft and drones a lot. Since well they can’t fall apart and but still be light enough to fly efficiently. But work with carbon fiber has a lot of issues in the FRC world. You can by per-weaved sheets or even sheets with epoxy already embedded in them (call prepreg needs to be stored in a fridge). But simply taking an aluminum structure and warping it in carbon fiber + coating it in a layer of epoxy isn’t gonna give you the strength you want in the ways you want. The team this year moved away from it and it saved a good deal of time and effort. But using carbon fiber plates as covers, or using carbon fiber tubes for rollers and such did work out very well on some mechanisms.
You can post some designs on CD and get some quick pointers from the various more experienced folks to if you are practicing design over the fall/summer.
There’s always the option of changing your alloy and/or temper. (Cue metallurgical debate.) That being said, that’s a “procurement” solution, and probably not what you’re looking for.
Material selection 101:
Define what exactly you are looking for this material–we’ll call it “unknownium” for now–to do.
Are you looking for:
Stiffness?
Toughness?
Strength (and in which direction)?
Wear resistance (and why would it be wearing)?
Fatigue resistance?
Ductility?
Ease of working?
Light weight?
Different materials have different answers to each of these questions, and even the same material may have slightly different answers for different applications.
If you want better wear resistance, though… May I suggest a couple of extra washers at the pivot/fastening joints? Sounds like you want a little bit of air in between two pieces. The other decent alternative is a sacrificial coating that will wear out, tell you it’s worn out, and be replaced.
Just to back some of this up, my team had an arm that a Bosch Seat motor would rotate using a milled down hex shaft. The arm wasn’t that big or heavy, but at the beginning of season we used 7075 aluminum for it. We noticed a slight twist so we replaced it with stainless (Alloy unknown). That was too strong of a material and it ended up destroying our gearbox from not having the ability to flex. We made a new set of shafts using 6061 (us not watching the alloys when purchasing) and those twisted to a breaking point about every match.
Long story short, alloys can make a huge difference in the strength, while weighing nearly the same.
Dimpled eyes!! Our robot last year had a ton of them… for that reason!! (Also, it makes the robot look super cool) But basically, you cut a circle into a flat plate, and bend the rim of the hole. It adds a third dimension to the plate, making it more durable… also you are cutting material, making it lighter. We used this extensively last year… it saved us from the 120 pound limit.
The issue was due to using a worm drive gearbox on this arm. When our drivers weren’t careful, they would hit things, causing a sudden stress on the arm, breaking a couple teeth off the gear. Once those couple teeth were gone, it never functioned correctly under load (arm attached to the shaft).
There’s always the option of changing your alloy and/or temper.
This does fit under the design of parts, but do not underestimate the effect that properly choosing the alloy of metal that you use for a part can have. You can buy alloys of aluminum that are comparable to the strength of steel and much lighter, but they are expensive. Our team decided to use 7075 aluminum for our climber axle, and that let us run it nearly the length of the robot with only minimal support. Running finite element analysis in Solidworks or a similar program is a great way to get an idea of how some materials will react to different stresses, and can aid with your decision.
Just FYI, all of the hex axles sold by AndyMark and Vex are 7075 except for churros. We also had our climber axle all the way across the chassis with minimal deflection.
FEA is good to a point, but it can sometimes give you a false sense of confidence. It’s hard to estimate the forces in play in an FRC robot, especially before you’ve seen the game play out. FEA can be useful for ruling out ideas, but I’d be careful using it to “rule in” ideas without making sure you’ve got a good factor of safety.
I have seen teams weld beads into a very thin piece of sheet metal, and although it added little material to the metal, it greatly increased its strength. I think the metal was either 1/16in aluminum or 1/32. Either way, that’s very thin!
Did adding weld beads increase the strength of the material? Or did it increase the stiffness? Also, what material was this? Because I would be very careful before laying down beads on thin aluminum sheet depending on what alloy it is.
I don’t see putting weld beads on thin AL sheet as productive for stiffness. You mess with the metallurgy, kill whatever heat treatment or cold working present, add heat distortion etc. Rolling ridges or making dimples is the way to go. Look at the pictures of 4910’s robot as the way it is done.
The punch in the second picture most likely. Drill a hole for the screw, and then tighten the screw which drives the punch through (and also acts as a die for the dimple). Same idea as a greenlee punch if you’ve seen or used those before.
Increase stiffness and FOS for static loading by increasing cross section area or section moment of inertia, as appropriate. Or pick a stiffer or stronger material. A material change will generally result in strength or stiffness changes on the order of 1-3x, whereas geometry changes can quickly reach 10x improvements.
Increase impact resistance is generally increased as stiffness is decreased. Pick a material with high impact resistance when needed. More generally, target materials with a high strength:elastic modulus ratio.
Wear resistance is quite complicated. Generally a high surface hardness is good, and two mating materials should differ significantly in hardness to enhance wear properties. Like-materials tend to cold-weld to each other, called galling, and should be avoided. There are many surface treatments that can do nothing for wear, or dramatically improve it. Proceed with caution when picking a surface treatment.
Addressing your comments in order…
Depending on how the carbon is wrapped and bonded it can be done relatively inexpensively. I got a setup to do basic vacuum bonding for <$1k.
Vinyl wraps are just that, vinyl, and add negligible strength.
Titanium isn’t ‘really really expensive’ when mentioned in the same breath as carbon fiber. The raw materials are of comparable cost per pound and titanium can be cut, welded, machined, and formed with traditional metal-working tools. Titanium is also isotropic, potentially making it stronger than CF in combined loading situations.
Anodizing titanium offers little or no wear protection. Hard anodizing aluminum offers decent wear protection, but cosmetic anodizing (colored/dyed) offers little wear protection. There are many, MANY surface coatings for wear purposes. My day job use Nibore, nickelon, hard chrome, enbio teflon blasting, TiN, PCD, and a few others.
Often the /best/ way to get iimproved strength or rigidity is not with adding material(s) but rather with better structural design. I do not buy your general statement that ‘lack of manufacturing tools’ would prevent this approach while simultaneously allowing CF or other composite manufacturing techniques.
Acrylic has a very low impact resistance, making it a generally bad material for robots. Acrylic has a high impact resistance, so you will see it. Delrin, HDPE, Nylon, PTFE, and other plastics all have good applications in robotics.
The other decent alternative is a sacrificial coating that will wear out, tell you it’s worn out, and be replaced.
