Swerve Question

[Team1710]

Could anyone recommend a specific type of aluminum for swerve? We were thinking of using 7075.

[Akash Rastogi]

Which part specifically?

Module plates? Frame?...etc..

[Aren_Hill]

It all really depends what road your taking, if you take the route we have (supporting the top and bottom) 6061 should be just fine, if you go 118's route with no bottom support then your sideplates may warrant 7075.

It really all depends on what you have

148 went with the fork style and i dont think they used 7075 (john hop in anytime)

[Team1710]

Yeah the aluminum is for the side brackets. We are going more of a 118 route. I guess 7075 is needed to prevent flexing.

[sgreco]

If you have no bottom support at all, you're going to want to use 7075. My persoanl prefernece would be to use bottom support and 6061, but if you want it look like 118, you should use 7075.

[Nikhil Bajaj]

I think this warrants a brief discussion of what the differences are between the two materials, and what properties mean what.

Most importantly, a discussion of strength vs. stiffness and what alloying can do for aluminum (in broad strokes.)

When you alloy a material, what do you do? You add small amounts of other elements in order to try to make it harder and/or stronger. Why does this work? Well, it's actually a complicated field of engineering and science(which you might want to look into) called Materials Science--specifically Metallurgy. But to a large degree we are adding these other elements in order to make it harder for cracks in the material to form, and for deformations to propagate through the material. What this does is increase the amount of stress it takes for a material to fail, often in both yield strength AND ultimate tensile strength.

In an undergraduate mechanics of materials course, for metals typically two regimes are discussed--the elastic regime, and the plastic regime. Elastic is when any deformation you apply to a material (by pulling on it, twisting it, whatever) is largely recoverable. What do I mean by that? Think of a spring--if you compress it or stretch it, it comes back to its original shape when you let go. The plastic regime, then, corresponds to deformation that isn't recoverable--think a spring that has been over-stretched--its new no-load size and shape is changed from what it used to be.

The idea of yielding is when you go from being just an elastic deformation, which is often okay and doesn't affect the life of a part very much to a plastic deformation, which probably means your part will start to fail shortly. So the yield stress (or yield strength) is the property of a material that is just the value of stress at which you'll transition. We usually want to keep our stress values under that value. Ultimate tensile strength is how much stress it takes to break a part outright.

Okay, so now we know what strength is--what about stiffness? Think about the spring again--if it is a stiffer spring, the same given load will compress it or stretch it less. Materials inherently have this property as well. It is measured in a quantity called elastic modulus, which is how much stretching is caused by how much stress in the elastic region. In fact, it is the slope of the stress-strain curve in the elastic region. But that's not too big of an issue for this discussion.

But when you alloy aluminum, or most materials for that matter, you typically only improve the strength based parameter--hardness, yield stress, sometimes ultimate tensile strength--but you don't change the stiffness very much at all!

So, when you're picking between two aluminum alloys, don't think that one will bend less for the same load--pretty much all aluminum has the same stiffness give or take 10%. So, if the concern about the side plates is flexure, going from one aluminum alloy to another will not help at all. Making the plates thicker will do far more for you.

That said, if you do the analysis and FEA and for the given geometries that you want to use you need a little bit more strength in the same package to prevent it from failing, you might want to bump up your alloy. But to do so on the basis of trying to prevent flexure doesn't make much sense.

[JVN]

The strength of your material is often less important than the strength of your cross-section. I would hope the difference between 6061 and 7075 doesn't affect your design too much. How close to optimized do you think you are? Maybe closer then I would want to be.

-John

[Ken Patton]

Originally Posted by JVN

I would hope the difference between 6061 and 7075 doesn't affect your design too much.

And if you follow John's advice here, you will likely be able to use 2024-T4, which should save you some money.

Ken

[Team1710]

Looking at mcmaster and noticed that there wasn't a huge difference in pricing between 7075 and 2024. In some cases, the cost for 2024 in that specific dimension was more than 7075.

[Brandon Holley]

Originally Posted by Team1710

Looking at mcmaster and noticed that there wasn't a huge difference in pricing between 7075 and 2024. In some cases, the cost for 2024 in that specific dimension was more than 7075.

Mcmaster is not a very good gauge of pricing for raw metal (specifically aluminum) stock. Regardless you are going to be paying much more than you will at a direct metal source.