Andymark Churros

[philso]

I believe our drive train team used the churros for all the axles in our octanum modules (2 per module). I do not recall seeing any fail. The holes in the ends were handy for holding the modules together. Please keep in mind that these axles were not cantilevered. They were always supported on both sides of the wheels with hex bearings.

[Gray Adams]

Originally Posted by Chowmaster4695

http://www.chiefdelphi.com/forums/sh...0&postcount=16

You stated they would bend here. Please give us accurate information. Our students use chief delphi to help them make decisions where others have more experience. If they see contradictory posts like yours then they and many others will be confused. Please edit your post.

Chow Out

I can't say that I recommend blindly following random posts on CD and expecting success. You could always either go through to math with the max torque seen on the shaft during a direction reversal at top speed, calculating the stress for a hex shaft, a churro, or a keyed shaft, and then selecting an appropriately strong material.

Or you could just test it and see what happens. Both are totally valid.

[EricH]

Originally Posted by Gray Adams

I can't say that I recommend blindly following random posts on CD and expecting success. You could always either go through to math with the max torque seen on the shaft during a direction reversal at top speed, calculating the stress for a hex shaft, a churro, or a keyed shaft, and then selecting an appropriately strong material.

Or you could just test it and see what happens. Both are totally valid.

Why not do both? Run the math, then run the tests, and see how they stack up. Then if (when?) the test doesn't match the math, run more math and more tests to see which one forgot to take reality into account. There've been cases where tests showed a bit of a reality that doesn't show up at all in high school physics (or college physics, for that matter).

[Alan Anderson]

Originally Posted by philso

I believe our drive train team used the churros for all the axles in our octanum modules (2 per module). I do not recall seeing any fail.

Were they live axles, actually transmitting torque? That's the twizzler-inducing situation that people are worried about.

[philso]

Originally Posted by Alan Anderson

Were they live axles, actually transmitting torque? That's the twizzler-inducing situation that people are worried about.

Yes, they were live axles. The drive train team may have used the churros so they would not have to wait to get the proper hex shaft material. I don't recall them having the "twizzler" problem but I can understand that it could happen. We may just be lucky.

[AndreaV]

Originally Posted by philso

Yes, they were live axles. The drive train team may have used the churros so they would not have to wait to get the proper hex shaft material. I don't recall them having the "twizzler" problem but I can understand that it could happen. We may just be lucky.

They may have been live, but i am sure these live axles were supported by bearings on both sides, with the shaft section probably being around 3". In a butterfly drive whatever is driving the shaft is bolted directly to the wheel or driven by the hex in close proximity to the wheel. On a live axle WCD the sprocket/belt is on the opposite side so the whole axle is susceptible to torsional loading, which is the twisting action mentioned earlier.

With a WCD you have traction wheels being cantilevered, the shaft has to deal with axial and radial loading. Sure all shafts have to deal with this in some regard, but in a WCD the wheel(which is acting like a lever) transfers it directly to one side of the shaft, instead of evenly distributing it between both. This is when these forces become a problem.

I busted out my paint skills to illustrate what this looks like on a wcd with live axles.

[JamesCH95]

Originally Posted by AndreaV

They may have been live, but i am sure these live axles were supported by bearings on both sides, with the shaft section probably being around 3". In a butterfly drive whatever is driving the shaft is bolted directly to the wheel or driven by the hex in close proximity to the wheel. On a live axle WCD the sprocket/belt is on the opposite side so the whole axle is susceptible to torsional loading, which is the twisting action mentioned earlier.

With a WCD you have traction wheels being cantilevered, the shaft has to deal with axial and radial loading. Sure all shafts have to deal with this in some regard, but in a WCD the wheel(which is acting like a lever) transfers it directly to one side of the shaft, instead of evenly distributing it between both. This is when these forces become a problem.

I busted out my paint skills to illustrate what this looks like on a wcd with live axles.

This is called a cantilever axle.

Simplistically, a cantilevered axle sees six times the bending stress compared to a axle (beam) fixed on both sides. I would surprised if AM churro survived as a cantilevered axle for very long because of its low XC area and relatively poor material properties compared to 7075T6.

I would also be surprised if the shaft failed in torsion instead of bending.

[Nate Laverdure]

Polar moments of inertia:

AndyMark 1/2" churro tube       .00535 in^4  [Solidworks]
VexPro 1/2" round tube          .00570 in^4  [Shigley 8th ed. Table A-18]
1/2" round shaft w/ 1/8" keyway .00570 in^4  [Solidworks]
1/2" round bar                  .00614 in^4  [Shigley 8th ed. Table A-18]
1/2" hex bar                    .00752 in^4  [Wikipedia]

[AdamHeard]

Originally Posted by Nate Laverdure

Polar moments of inertia:

AndyMark 1/2" churro tube .00535 in^4  [Solidworks]
1/2" hex bar              .00752 in^4  [Wikipedia]
1/2" round bar            .00614 in^4  [Shigley 8th ed. Table A-18]
VexPro 1/2" round tube    .00570 in^4  [Shigley 8th ed. Table A-18]

It's worth noting that the 6063 that the Churros are made of has a substantially lower yield.

[Greg Woelki]

Originally Posted by Nate Laverdure

Polar moments of inertia:

AndyMark 1/2" churro tube .00535 in^4  [Solidworks]
1/2" hex bar              .00752 in^4  [Wikipedia]
1/2" round bar            .00614 in^4  [Shigley 8th ed. Table A-18]
VexPro 1/2" round tube    .00570 in^4  [Shigley 8th ed. Table A-18]

While the polar moment of inertia of the churros is decent, not only is it 6063, but also the dimples will cause it to perform poorly in torsion.

[MechEng83]

Originally Posted by Nate Laverdure

Polar moments of inertia:

AndyMark 1/2" churro tube .00535 in^4  [Solidworks]
1/2" hex bar              .00752 in^4  [Wikipedia]
1/2" round bar            .00614 in^4  [Shigley 8th ed. Table A-18]
VexPro 1/2" round tube    .00570 in^4  [Shigley 8th ed. Table A-18]

*WARNING* Summary of Graduate-Level Mechanics of Materials concept ahead

The polar moment of area is only useful in terms of torsional rigidity. The torsion constant requires a much more complex formulation (the Prandtl membrane analogy). It is only identical to polar moment of area for circles.

Then, you have to use the modulus of rigidity (G) of the material and the distance from the central axis to the outer-most point to determine the shear stress. and compare this to the maximum allowable shear stress of the material.

The dimpled sides of the churro profile actually make it incredibly weak in torsion compared to a solid section or even a full hexagon with a hole in the middle. The membrane or "soap bubble" analogy lets you have a bit of understanding as to how rigid something is in torsion. If you imagine a membrane or soap bubble is attached to the outside edges and the membrane is inflated the volume is analogous to the torsion constant of the section. If there are open sections that are completely contained, the membrane is "flat" in that area. This is why a thin walled tube is very strong in torsion compared to a 359 degree non-closed section.

tl;dr - science says churros are weak in torsion.

[Briansmithtown]

Nothing beats Andy Bakers Churros... Nothing!

[hrench]

it's great that someone has actually posted the equations for torsion, but I want to weigh-in and agree with the churros being the 'worst shape' for transmitting torsion.

In an engineering class somewhere along the line, I learned the 'membrane analogy' http://en.wikipedia.org/wiki/Membrane_analogy for torsion in a shaft.

Essentially, the calculus is the same for an imagined 'membrane' (think square of latex of a balloon) that's stretched over the end of the shaft. So if you could stretch a balloon over the end of the shaft and then air it up, if it holds alot of air then the shaft will withstand alot of torque. But if the shaft has cuts in it--like a churro--then it won't hold much air and hence not much torque. This is also true for shafts that have been through-drilled with a pin--if you do that, you cut the torsional limit to about half to a third of what it would've been. Also shows how keyways and/or splines are good because they transmit torque without hurting the torsional limit of the shaft very much.

Also, in the membrane analogy, if you air up the balloon-stretched over the end, the slope of the transition indicates stress. If it produces gentle-slow changes in slope, these indicate the stress will be even. If you stretched the balloon over a churro, the slope in the vee-grooves would be very steep, indicating high stress. This also shows how if you use a square-section for torsion, the failure will be in the middle of the sides, not at the corners.

Will it fail? I don't know--it would be difficult to estimate the loading. Is it a bad shape? yes.

[Chris is me]

Originally Posted by llamadon

Hi, our team is prototyping a wcd this summer, and we have an excess of andymark Churro extrusion (http://www.andymark.com/product-p/am-2595.htm) and plan to use it as a live axle. Does anyone have experience using this material? If so, is it strong enough?

Originally Posted by Akash Rastogi

I wouldn't even use regular 6061 hex shaft for this application, let alone the 6063 churros. Bad experiences with bending and snapped axles. (Just ask 2791, among others).

Akash is right. Do not use churro tube as West Coast Drive cantilevered axles!!! I would go so far as to say that churro tube shouldn't ever be used as a live axle in any drivetrain.

There are a multitude of reasons churro tube is bad for a cantilevered axle, most of which have been touched on already. The profile shape is atrocious in torsion, which means your shaft will twist and possibly snap under heavy drivetrain loads. The material is 6063 aluminum, which generally has a lower yield strength than even 6061 aluminum. I suspect you would also have problems with bending or transverse shear loads, especially since the shaft has a giant clearance hole going all the way through it.

Originally Posted by asid61

6061 1/2" hex will not bend in drivetrain applications, as we had to substitute a few 7075 axles for 6061 during build and they held up through 2 competitions.

This is absolutely, positively not true. It's okay to post your experiences on CD, but don't use that information to make blanket statements about all drivetrains. At least not without solid evidence or reasoning.

This might have been due to to other drivetriain problems that stopped us from moving too much though.

This is exactly why you didn't have any failures. Failure is not always instantaneous.

To share a story, 2791 used 6061 hex stock to make drive axles in 2011, and we had numerous failures. Both center drive axles failed halfway into our first event. One center axle, if I recall correctly, actually stripped itself from hex to a rounded profile. Another split in half in an apparent torsion + shear failure. These were solid 6061 axles without intermediate snap ring grooves or other stress risers.

Originally Posted by asid61

I'm sorry, it did not bend for us. Our drivetrain didn't see very heavy use this year, and that might be why it worked fine. Teams like 254 probably would need the extra durability afforded by 7075, but for a prototype low-load drivetrain 6061 should be fine.

What's the point of building a prototype drive if you're not going to test durability? If you're building a drivetrain before the season starts, you should build as close to how you would in build season as you can. That way, if and when you do have failures, you can directly apply them to your future robot. If you use parts you know will fail to cut corners, you won't know if the part you intended to use in build will work properly.

Originally Posted by cbale2000

We used some Churro for our collector on this years bot with no issues...

You can't really see it in that picture because the PVC spacers cover it, but it extends across the entire length of the collector, drives the wheels, and is only supported on the ends. Overall we've been very happy with it, especially because it's light and rigid (and because we already had some around when VEXPro ran out of their long hex stock during build season).

Now granted this application probably undergoes far less force than something like a drive system would, so keep that in mind when planning its use.

We also used churro tube for our collector this year. This was partially because we were out of hex stock and partially because a collector is probably the least loaded part of our entire robot. The loads a collector shaft sees, unless exposed to a high speed collision, are extremely low.

[hrench]

I also want to weigh-in on the 'build it and see if it breaks' philosphy that's being encouraged here.


That is NOT SCIENCE. What Science IS is using previous observation to determine what will happen deductively. The reason we engineers make books of statistics about materials and books of equations about stresses is because science works. If we design something that will work because we've used science, we've taught the kids the value of STEM.

if we design something with the guess that it might break or might not, then we didn't teach math, we didn't teach use of historical empirical statistics and instead we've taught 'trial and error.'

Not a good way to be an engineer.

Don't get me wrong, I was a 'farmboy' and stuff on a farm is most always pushed till it's about to break--or does. Seeing how stuff breaks is a great learning experience and I recommend it.

But engineering --doing the math--works better in the real world and I think that's what we're supposed to be teaching the kids.