Andymark Churros

[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.

[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.

[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.