Re: Andymark Churros
 

Excuse me. Please read the rest of my post:
"This might have been due to to other drivetriain problems that stopped us from moving too much though.
6063 is considerably weaker than 6061, so I would be wary of using it. For prototyping, it might be fine, but it would be much safer to just purchase some 1/2" 7075 hex stock from Online Metals instead."
Emphasis on my own post mine. If I could edit my post, I would, but I can't. I posted later in the thread to clarify this.
As a side note, please keep in mind that I am indeed a real person, and I have my own experiences, which might be different from yours. Different does not mean incorrect.

Now, to clarify my statement:
My team used some 6061 axles due to 7075 being unavailable late season. We experienced a multitude of drivetrain problems, leading to us only being able to move during half the matches. However, post-season while we were debugging the drivetrain, we discovered that axle bending was not one of the issues in our drivetrain. More likely it was due to incorrect tensioning of chain.
So, I can say this: 6061 is fine for axles, provided is is not under undue stress. It is cheap, which would be the main reason why you would use it. For a prototype drivetrain, it should work fine. However, once the season rolls around, switching 6061 for 7075 should not be a huge issue.

There have been threads in the past detailing shaft strength as compared to shaft diameter (for rounds). There is one here: http://www.chiefdelphi.com/forums/sh...t=38737&page=2
It would be wise to run some calculations, regardless of material being used. It's just plugging in a few numbers.

Good point. I was thinking of just a proof-of-concept drivebase, but for a proven design like WCD you're right, it would be better to design realistically instead. Might be a good idea to have some 7075 on hand anyway then...

Re: Andymark Churros
 

We have made a mistake on the AndyMark website, incorrectly stating the wrong alloy of aluminum for these extruded churros. They are actually made of 6005A aluminum.

As for using them on an FRC-sized drivetrain as a cantilevered axle, I would not recommend this, for many reasons stated previously in this thread.

Sorry for this error in stating the wrong alloy! The website is now corrected.

Sincerely,
Andy Baker

Re: Andymark Churros
 

Wow this thread got a lot of responses, thank you all for your input!

In regards to the material: After seeing that the churro extrusion was probably not going to hold up in a high stress environment like a drive train (thanks to all of the input from you guys), we researched the different alloys of solid hex stock we have available to purchase. Now Vexpro has issues with being out of stock right when we need something, so we opted for McMaster. We chose the high strength 2024 alloy hex shaft, and will be using this for our drivetrain project. This shaft had the highest yield strength of all the shaft available: 47,000 psi.

Re: Andymark Churros
 

C1018 steel yield is 54K and C12L14 steel yield is 70K if you need stronger. We've gotten some recommendations from local teams to look directly at tool steel for direct driven cantilevered wheels after this past seasons abuse to avoid bending.

Re: Andymark Churros
 

You also have the option of using AndyMark 1/2 Hex that made from 7075, although not sure if it is -O, -T6, or something else.

Re: Andymark Churros
 

I really can't think of why you would possibly need to use tool steel axles for anything in FRC. Online Metals has 7075 hex aluminum, which even 6 cim drivers like 254 use.

Re: Andymark Churros
 

All cantilevered wheel setups aren't the same. If you have the wheel far out from the bearings, and you're using a large diameter wheel (more torque needed to drive it), I might consider using something stronger than 7075.

Re: Andymark Churros
 

It would have to be an absolutely massive wheel (like 8") and cantalievered out 2" from the chassis bearings to have a factor of safety of 2, according to my calculator. And tool steel axles is still overkill. 4140 steel would be my next choie.
Interestingly, the cantaliever only slightly affects the factor of safety.
Of course, if they want to run something lie 24" wheels, then I could see tool steel being a valid choice. But before doing that I would just increase the axle diameter.

EDIT: Actually, is tool steel even capable of taking axle type loads? It might be too brittle.

Re: Andymark Churros
 

I know teams that used steel axles this year that had bending issues with aluminum and mild steel. Progression of materials landed them at tool steel. Calculators have issues with being based in theory. The problem with theory is that it doesn't always jive with reality.

Re: Andymark Churros
 

What setup were they running?
What type of tool steel, out of curiosity? Some steels are labeled as such, even if they aren't technically tool steels.

Re: Andymark Churros
 

There are many varieties. Tool steels become brittle due to their heat treatment. Of course the composition of various tool steels can cause the hardness to be different from each other pre-heat treat. I have only worked with D2 and it bends before heat treating.

Re: Andymark Churros
 

As a senior in EET, I have to at least partially disagree.

More times than not, in EET at least, that IS how designs are implemented. Why? Because more times than not the math is, in practice, an ideal figure that is not as precise as one would like it to be. For example, this is why they still teach Smith charts in Electromagnetics classes (for those that don't know it's a graphical way of analyzing RF circuits), because while the modern math is easy given a good calculator (we used TI-86/89's), in reality, there are so many factors at play that the realistic accuracy is equivlent to using graphical methods.

Another example is with microcontrollers and DACs. While often times what the DAC is controlling (in my case, it was an VCO tuning an FM radio) can be modeled with an equation, in practice in a fair number of cases it works better to build the system and then through trial and error find the values needed. In my case, it was the 100 values corresponding to the 100 "channels" in the FM band. While, even with an optimized "scrolling" set of code to expidite the process, this took a couple of hours to find all the values (in the 12 bits the DAC would see), the result was a radio that despite being in an RF lab with poor reception, would pick up stations with reasonably clear sound. Using an equation to find the values probably would have worked, yes, but the results wouldn't have been as good.

Yes, math has a place, like initial designs (you have to stat somewhere), ballpark values (for situations that are hard to model), and feasibility analysis, but any good design is backed up with a tested and tuned prototype, which in some cases require some educated "guess and check" to get dialed in.

Relying on math alone may get a good grade on an exam but for actual design work, relying on math alone is a poor practice. To me, engineering is ultimately about good design work, not good test scores.

(yes, it's a long post, but I had something to say)

Re: Andymark Churros
 

I am going to disagree, and agree. (I know, can't lose, right?)

Disagree: Sometimes, you have only one chance to get something right. Or for whatever reason, you don't have the resources to build a prototype and tune it prior to the final design. In such cases, it's actually a lot better to run the best numbers you have available--factoring in edge cases, outside factors. Relying on math in this case is actually good design work, primarily because there is minimal chance of actually being able to test your work. (There are a number of cases like this--think of large Civil Engineering projects, where you don't get X number of retakes. You just get a case of "OK, we found a problem, how do we fix it before we continue?")

Agree: For most cases, particularly high-volume production cases, it's usually a good idea to build a prototype or other test just to make sure that X will actually work. Even on those one-shot cases, your best math is actually going to come from scale testing, where you build a nice, cheap model and run it through various simulations to figure out exactly what's going on. For more complicated systems, a multitude of tests will be conducted.

tl;dr: Sometimes, the math just needs to be done, and done RIGHT, because no prototype will serve the purpose.

Re: Andymark Churros
 

Does your calculator include torsion? Or the combined stresses of bending and torsion? Do you make some allowance for the fatigue strength of aluminum?

Re: Andymark Churros
 

Like when you press the "fire" button on your lunar lander & hope it will take off and mate up with your command capsule somewhere in orbit?