Not true as a general proposition. Depends on the type of steel, and type of aluminum being compared.
Can you weld galvanized steel?
Yes.
Would I feel comfortable doing it around or with high school students?
No.
Not true. Aluminum (as a generalization for FRC, you can find exceptions) has a significantly higher strength to weight than steel. In fact, for shafts/gears, your 7075 aluminum is both lighter and stronger that the common stainless steels places like andymark use. In terms of ultimate yield strength, aluminum is roughly 1/3 the weight and almost twice as strong.
I wouldn’t say “significantly stronger” but yeah, it can be stronger per unit of weight.
It’s also a more expensive. Something else to note is that steel generally is more stiff than aluminum per unit weight. When you’re building something like a bridge, those two factors can be important.
The real reason aluminum is thought of as lighter (for FRC purposes) is that we rarely design on weight, we usually design on size. There’s a lot of arbitrary standards in FRC (1/4" thick bearings, 20 DP gears, 2x1 tubing, etc) where stuff just needs to be that big to conform to a standard. Your WCD will be a ton harder to design if you choose .75x.75 steel tubing vs alu 2x1, for example. Also, for stuff like shafts, aluminum (especially 7075) usually has an adequate total yield strength, and since the shaft needs to be 1/2" hex, the alu will be a lot lighter than steel. In other places, we need certain plate thickenesses to get the right number of threads in a tapped hope. Again, steel will be heavier because we just care about overall thickness, not strength. Of course, if we designed all our parts to just be a certain strength, steel and aluminum would be a lot more comparable in terms of weight.
Sure, we could crazily pocket steel or change our FRC wide standards to conform to steel robots. But until then, the lower density of aluminum and comparable strength will make it seen lighter for FRC purposes.
The difference is large enough to be noticed, for purposes of FRC use.
Um, we are talking about galvanized steel, so the copper would be coming into contact with zinc. Go look up what happens when copper and zinc are in contact with each other. Can you say… “battery”?
You can perform electrolysis with many materials, not just water.
Thank you MrBasse for the elaboration.
My concern was not the zinc fumes, but weld spatter. Glavanized spits out molten globs of metal a LOT more than other materials.
I did not say it couldn’t be welded, just that this particular person should not try it.
I stand corrected.
I disagree, although I will point out that stainless steel (302, 303, 304, 316) is a specific and (typically) less durable subset of steels (without getting into hardenable stainless such as 440C). The 3x price differential on stainless makes it less than desirable for teams to work with in the raw, aside from the additional tooling needed to properly work it. However, I will support my original contention with the following links and tables…
from reference #2…
…Aluminium…Steel
Density, ρ kg m−3…2,700…7,800
Young modulus, E N mm−2…70,000…210,000
Shear modulus, G N mm−2…27,000…81,000
Poisson ratio, ν…0.33…0.3
from matweb.com…
1018 steel versus 6061-T6 aluminum versus ASTM A526 galvanized steel
I could never bring myself to use aluminum for a shaft. It is too weak (comparably) to take that shock and torsion loads, as well as being more prone to unannounced catastrophic failure over steel (steel bends and then breaks … aluminum has a much smaller deformation period before it snaps under the same loads). I’d rather use a steel shaft so that I could press-fit it into a bearing. The surface properties of aluminum just aren’t good enough (at least for me) for rotating or direct-contact sliding.
So if I have the tooling and can keep the weight down why not save the money? And if I get 3 times as much free sheet steel wouldn’t it just be better to save the money and be smart about its use?
Basically what I’m getting at is it isn’t as simple as Steel is denser or aluminum is less rigid. For instance at the beginning of last build I found that a welded steel version of the kit-bot frame we used would be several times more rigid and cheaper. It would only have been about 2-5lbs heavier and that robot was 98lbs at inspection. We didn’t posses the tooling to work it quick enough yet so we scraped it.
Witch metal to use has to do with the circumstances surrounding the team and the build.
This statement can be generalized to “material choices are context-dependent,” or “design choices are context-dependent,” or even “essentially all utility calculations are context-dependent.” It is good to keep in mind the difference between a heuristic rule and a fundamental principal - general statements about “optimal materials for FRC” are quite clearly of the former sort, and you should thus interpret them as such.
I would agree that 6061 alloy aluminum is a poor choice for shafts, but 2024 alloy aluminum and particularly 7075-T6 have excellent material properties that are competitive with many steel alloys. These two aluminum alloys are much harder and have significantly higher yield strengths than 6061.
I’ve been using 2024 and 7075 for shafts on FRC robots since I was in college in 2010, and have never had one fail. Plus, I absolutely love machining 7075 aluminum on mills and lathes. With good carbide tools, you can machine it like a hot knife through butter, as it does not have any of the galling issues that plague softer aluminum alloys. It’s my second favorite material to machine after Delrin.
// Note: I didn’t directly link to the Matweb pages for 2024 and 7075 since apparently the links are tied to cookies and 404 after a while.
Well… 7075 is twice as strong as certain steels, so yeah, it is “significantly stronger”
You’re not looking at the right alloys. AndyMark steel parts are 4140 stainless with a yield strength of 60200 psi, but 7075 aluminum (vex pro parts) have a yield strength of 78,000 psi. Also, I don’t really follow the shaft argument. Any 3 CIM gearbox that I’ve ever seen uses an aluminum shaft. Same with our climber.
I will totally agree that aluminum, especially 7075, machines really well. However, we have a tough time getting anything but 6061 locally. One of our former mentors went out and got some cheap aluminum from a different alloy in the 6xxx series that machined like garbage. Regardless of speed, feed, or cutter age, it rolled instead of cutting. It reached a point where I would have preferred to be cutting gummy stainless (AL6XN).
However, I digress. We’ve bent a number of aluminum shafts and torn up the surface of others in bearings, so we’re gunshy about going that route again. I’d be interested to know what installation and mounting techniques you use for the aluminum shafts.
Whoops. :o Didn’t realize that. Sorry. The short form was that the materials listed matched the chart in the post for properties and that the galvanized was not significantly different from the 1018.
That’s possible. However, 4140 is not stainless (under AISI and ASTM coding). It is a steel which, among other things, is excellent for shafting.
We’re all shafted now and then.:o
We’ve digressed significantly. I think the OP has a better idea of how to work with Galvanized Steel, and aluminum versus steel probably deserves a thread of its own. Or?
I don’t really have a problem with a materials science class in a thread now and then. 
Also on the steel vs 7075 shafting thing, personally reducing drive train inertia is a priority and also reduces overall weight. Design wise I like to go a little heavy in the lower frame and keep all the scoring stuff up top light. Coupled with lightweight drive train parts and as few as possible. I can’t tell you how much time I spent trying to get a 2" colsin to work so I’d only need a 4:1 of the CIMs.
PS I really like the torque/speed combo from 8:1 on 4" wheels. PSS really dislike drive train inertia.
This isn’t as bad as the “Driver Station Power” thread. That started out with legalities of marine batteries in the driver station/battery cart, but diverted to microcontrollers, the propeller chip and comparing and contrasting it’s functions.
At least this thread is still on topic.
What do you mean by “drivetrain inertia” in this context? Are you concerned about the moment of inertia of the output shaft of your gearbox? And if so, why?
I mean to say the total mass of all my moving parts and the friction between them to increase the efficiency of the drive train itself.