# Flywheel Do's and Don'ts

they are much tougher and harder to delaminate, however they are more prone to material pickup and can, depending on the game piece start building up a hard coating of “game piece”

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I didn’t have the safety wire last night and am quite annoyed about that. These are some pics from a wheel I pulled out the wire to show the last steps. I’ll try and find the safety wire today and show how you do the twist step.
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Thanks! I see the model index, but not the ID for the material and durometer. I would assume those are the D60 Neoprene?

These are 35A, you probably won’t need to band the 60A wheels

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Here is my walk through video.

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Nice video, but where are the safety glasses.

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We’ve had a pair of those pliers in the shop at work for years and I’ve never known what they’re supposed to be used for. Thanks for the video! This is exactly what I’ve been looking for.

He’s using safety wire so there’s no need for more safety

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Thanks for posting this. Couple of questions:

1. is that 304 Stainless 0.041" diameter “safety wire”?

2. rated tensile for the above is about 275 ksi; that would imply about 350 lbf maximum tension on the band. Will there be sufficient safety factor when the wire is wrapped on a 4" Fairlane as shown and it spins at 5000 RPM? Is there some analysis to show the robot inspector (Rule S1)?

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The first year they did it, 1678 tested it above the target RPM with their lathe and provided video. I think that’s a more effective approach than a calculation package that I don’t think the average RI would be prepared to evaluate the accuracy of.

Because hand calcs are fun...

The hand calculation is tricky, the simplest approaches all seem too conservative.

Consider a 2lb (1kg) 4" (0.1m) dia, 2" (0.05m) width Fairlane with 3ksi (20Mpa) neoprene rubber as the governing hoop stress.
Solve for rotational velocity as controlled by hoop stress
https://en.wikipedia.org/wiki/Flywheel

Rearrange for angular velocity from stress, density, radius

w^2=stress/(density*r^2)

Plug in density = mass/volume (solid cylinder assumed) & simplify

w^2=stress/(mass/(pir^2l)r^2)=stress/(mass/(pil))=stresspil/mass

Add numbers & convert to RPMs
w=sqrt(20MPa3.140.05m/1kg)=1,800 radians/sec=17,000RPM

Doing the same calculation with 80ksi (550Mpa) safety wire as the governing stress results in
w=sqrt(550MPa3.140.05m/1kg)=9,300 radians/sec=89,000RPM
But this is effectively saying I have a complete cylinder of welded stainless foil around the wheel, rather than just the twin wires. There will be additional stress on the rubber that’s not captured in this calculation.

Taking experimental data showing wheel deformation at speed before and after application of safety wire and calculating the load implied by the difference in deformation that the wire is responsible for could be more accurate - but still neglects some of the load sharing with the structure of the rubber.

At that point you’re already taking experimental data, so might as well just run to 1.5x-2x the target RPM and find out if it breaks.

Do you know anyone who does tires professionally that could help with the hand-calc analysis approach?

(Also - did you mean 275Mpa? 40ksi is a common stainless yield strength… A580 doesn’t list any Stainless alloy over 220 ksi tensile…)

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I’m sure you’re better qualified than I am to do this calc. I was looking here when I cited 275 ksi.

What I was looking for was stress on the stainless steel safety wire caused when the rubber wheel expands at high speed. My only design experience-based reference for comparison is stainless steel sleeves on the rotors of permanent-magnet motors. I have typically made those about 1/2 mm (0.020") thick on rotor diameters up to about 50 mm (2") running at speeds up to 8000 RPM. That case is somewhat like the one we’re considering here, because the in both cases the stainless steel retainer must handle tension loads due to centripetal acceleration of the retained mass; however, the magnets don’t deform elastically, and as you pointed out, a wire retainer is not much like a sleeve.

In any case, I agree that testing is the best approach. Will look for 1678’s data.

I am absolutely not, just an ME with a hand calc (one of the most dangerous types of ME… )
That’s why I’m hoping a tire (or even better, engine flywheel) engineer could chime in

That is some strong wire! Over double the requirements of ASTM A580 table 2, though A580 doesn’t say you can’t get those results, just that the minimum requirement to sell it as stainless wire is much lower…

I think the key difference is it sounds like you are not planning for the structure of the magnets to support their own weight - whereas the rubber in the flywheel does the work of holding it together, and the stainless wire simply suggests a shape for the rubber to fit.

Ah, so that’s how to dynamically change your compression.

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Does anyone have a good idea of how much the 35A ones expand at 9000 ish rpm? Looking to mount a motor adjacent to one of these, and looking for a good clearance distance.

We ran a banded wheel up to 7500 rpm and the banding disappeared into the rubber, I was trying to destroy the wheel but couldn’t push it hard enough. The wire is 0.032"

@sstew at 9000 rpm I feel you need a different wheel or solution, I feel like the wheel will experience sudden unplanned disassembly at that point.

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We’ve taken a 4" black fairlane wheel up to an estimated 16,000 RPM with no wire and no ill effects so far (other than the thing expanding), but I’m not sure it’s a good idea in the long run, and there are definitely some safety considerations.

You are very lucky… We’ve exploded fairlanes @ 40A @ 8k-10rpm. 16,000 is just ridic.

With safety wire?

David mentioned he did it without safety wire. So i’m comparing the same experience we had without safety wire.

With safety wire no issue.

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Are the WCP shooter wheels actually fairlane rollers? Have you tested them to see what RPM they handle, and would they need the same safety wire treatment?