Is Safety Wire Safe for Flywheels?

Several teams in the past have wrapped safety wire around their flywheels to limit the expansion. I first became aware of this method a couple years ago in 254’s 2016 technical binder thread and have seen more recent discussion in the thread Flywheel Do’s and Don’ts.

While experimenting with Fairlane wheels for our shooter this season, this solution was proposed. In response, a few people expressed concerns regarding the safety of this method; thinking that the wire may come loose and become a dangerous projectile. I am looking for input from the community regarding this concern.

  1. Why is or isn’t it safe to rotate a piece of stainless steel at 6000rpm?
  2. What is the likelihood of the wire coming loose and being launched if attached using the method in the second link above? Why?
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The danger of the flywheels by themselves is, to my understanding, that at high speeds, the urethane/neoprene expands significantly to the point where it may delaminate from the hub. The risk in the flywheels is because of the material they are made of.

Why is this not an issue with stainless steel? Assuming you do it the way 1678 has shown they do theirs, you can see in an example them spinning such a wheel at very high speeds and not having a problem with the stainless steel coming off. This is, of course, anecdotal, but it also makes sense logically: stainless steel will not expand in the same way these plastics/rubbers on the flywheel will. There is no load on the stainless steel that could cause it to slide off of the wheel, especially if you twist the wire around a few times and embed in the wheel like 1678 showed.

I think you should think a little about what your specific concerns with this method are. For example, I could very easily make the claim that “driving a robot is very dangerous, because there is a risk that the battery could catch fire.” This claim is based on my knowledge of batteries catching on fire in the past – but it turns out that in normal robot operations, the risk of this is negligible for many reasons (in this case, I should consider what specific circumstances beyond just “driving the robot” led to those batteries catching fire).

It’s important, in general, to consider the actual risks (and potential ways they may occur) before you determine that something “feels” risky. Of course, you can’t always predict every risk, and it’s possible that something weird may happen that you had no way of foreseeing. But if you do have a specific concern such as “the safety wire may get flung off of the wheel,” you should think about why you have that particular concern and whether the concern is evidence based.

EDIT: for more examples of things that “feel” risky even though you have evidence that they are not, look at bungee jumping, ziplining, roller coasters, or any other thrill ride. The concept is similar: try to understand the physics of why jumping off of a cliff is different than bungee jumping, and once your logical brain is satisfied, that should probably be enough to go for it.

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One other possible action would be to anticipate the wheel coming apart, and building as much of a shield as practical to contain the debris. Even if your spinning assembly does NOT disintegrate, this kind of safety is inexpensive and, in my own opinion, absolutely mandatory.

Calculate tension in the wire as a function of 4” Fairlane wheel RPM, assuming it is installed per the linked procedure. Compare calculated tension with the wire’s yield strength.

Edit:

an initial stab at the analysis.

Using RCF (g-Force) = RPM^2 x radius x 1.118e-5 [radius in inches], a 4 inch wheel spinning at 6000 RPM experiences about 805g at its surface.

Urethane’s density is about 0.04 lb/in^3 so the 4 inch wheel will weight about half a pound per inch of width.

Pressure on the surface of the urethane wheel will be about 805 x 0.5 / 4pi() = 32 lb/in^2, at 6000 RPM. 4x that at 12000 RPM.

If the wheel o.d. is retained by a 0.032" thick strap, hoop stress in the strap will be surface pressure x (radius/thickness) = 32 X 2 / 0.032 = 2000 PSI. If the strap is stainless steel, with about 40 ksi yield strength, that stress can be held with a safety factor of 20.

However, the safety wire installation we are considering here is not an ideal band. That means the urethane’s own elastic strength (as it expands where the wire isn’t touching it) will take some of the surface strain, and the rest will be concentrated in the safety wire.

So I cannot say from this simplified analysis that the safety wire is safe. A more detailed analysis that accounts for wheel expansion “around” the safety wire will be needed. It seems reasonable to me that the safety wire would hold at 6000 RPM, and break at 12,000 RPM or lower, as is supported by some test results.

Of course improperly installed safety wire will have weak points where it is deformed by kinking or over-twisting, and not all wire is as strong as the 40 ksi yield strength assumed for SS would indicate, so actual results will vary. This is my biggest concern regarding use of safety wire as described by the earlier link.

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Hi Richard,

Should the radius be in cms for the constant you are using? Based on other RCF formulas I have looked up, it looks like radius should be in cm (https://www.promegaconnections.com/converting-rpm-to-g-force-rcf-and-vice-versa-2/#more-20017). Based on cm, your RCF = 2000g for a 4" wheel spinning at 6k rpms. That results in a surface pressure of 81 psi and your safety factor for a .032 wire with 40ksi yield drops down to approx. 8 (using your simplified hoop stress method).

Thanks,

Ren

Good catch! I had almost given up on finding someone to check my over-simplified calculation.

Can someone suggest an analytic method that properly accounts for the wire geometry, and wheel elastic deformation?

One way to get a reasonable estimate is to re-run this analysis but with new geometry parameters. The inner layer in the analysis would become the entirety of the rubber wheel and the outer layer would be a stainless steel layer with meridional cross-sectional area equal to that of whatever wire wrapping you proposed. This would represent the case where the 60A rubber debonded from the core and was supported by itself and the wire.

Beware that the strength of a wire with a twist joint is not likely to equal the yield strength of the wire alone.

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You MUST use “safety wire” and not any stainless steel wire, the two are NOT the same.

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One wrap of .032-in-diameter wire had a cross-sectional area of (pi/4)*0.032^2 = 8.04e-4 in^2. That would be the same area as a 0.000415-in-thick band over the entire width of a 1-15/16-in-wide Fairlane, because 1.9375 * 0.000415 = 8.04 in^2. If I drop that geometry into the two-layer centrifugal stress analysis with 60A rubber, the stress in the steel is over 700 ksi. In real life of course it isn’t. This implies that the reason the assembly hangs together in real life is that, as the original stress analysis shows, the rubber can support itself without the safety wire, it isn’t delaminating from the core, and the wire is able to untwist a little to accommodate the strain without breaking. The wire (“safety” or otherwise) definitely will not prevent the rubber from delaminating from the core if that bond is weak.

It could also imply that preloaded installation is required. In other words, it will fail if you install incorrectly.

Further, there is plenty of empirical evidence that safety wire has prevented delamination. Unless I’m misunderstanding that 254, 1678, etc. have experienced delamination after wrapping in safety wire.

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See my reply at Flywheel Do’s and Don’ts.

“Further, there is plenty of empirical evidence that safety wire has prevented delamination.” How do we know delamination would have occurred if the wires were absent? We have not seen any at up to at least 6k RPM with the 4" 60A Fairlanes. It is true in theory that, if the wire had unrealistic strength, it would limit the hoop stress at the inner bore of the rubber if it was already delaminated.

Because when using 35A wheels we had them delaminate all the time in testing before. We added the banding and had WAY fewer delaminations.

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Somebody needs to do the proper 2D (at least) finite element analysis.

I don’t need math to tell me I’m correct, I have a large body of testing that tells me that.

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Without the math, we have no idea why the wire seems to work. Trying to do engineering with testing and no math is a really really bad idea.

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If the math can’t explain the testing then the math is poor engineering.

Engineering is an empirical science. Empirical evidence is what validates the math, not the other way around.

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What’s the stress in the wire? No clue, right? How much stress margin do you have? No clue, right? What is the delamination stress at the core vs speed with and without the wire? No clue, right? We either need to strain-gauge the wire and core, or do math (FEA nowadays). Which approach is easier to generalize to other speeds, wheel sizes, and materials? The analytical approach, of course. That’s why FEA is so commonly used in industry.

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Math =! Blue Banners.

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As is testing…

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