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Spinner bolts break

Old 03-18-2009, 08:51 PM
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aghost
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Default Spinner bolts break

I have had spinner bolts break on two different planes at about the 100th flight. The recent break (3/17/09) was definitely fatigue and the one last fall probably was also.

The recent break was an aluminum spinner (3.3") and bolt from Skyshark with a Tru-Turn adapter. The engine is an OS 91FX on a TF FW-190 60 size. This spinner and bolt have 92 flights over three years.

The break last fall was an aluminum spinner (4.5"), adapter, and bolt from Vortech. The engine is an OS BGX on a giant scale P-40. The spinner and bolt had about 106 flights over three years.

Both bolts( 10-32) went about 3/8" into the adapter nut, but they were not bottomed out. Both breaks were right at the outside face of the adapter nut.

No unusual vibrations in either plane. Spinner bolts are checked periodically for tightness.
The breakages are not obviously related to any recent equipment changes.

I am considering changing out spinner bolts somewhere around 70 flights. Anyone have any other suggestions?

Brian

PS The propellers (APC and MAS) sustained nicks but did not break. Got both planes down OK.

Old 03-18-2009, 09:03 PM
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Default RE: Spinner bolts break

The 70 flight changeout might be a good idea. If you stop and think about it, the speinner bolt is fairly long, and is normally cantilevered out quite a bit. Do you balance your spinners carefully? Harmonics will do serious damage to pieces of metal. I have not computed at what order of harmonics the spinner bolt would vibrate, but it's probably pretty far up in the order of things, and the juncture of the adaptor nut and the unsupported part of the bolt would be just about at that place. I don't think that using a hardened bolt is the answer, so I'd say change it out before it fails.

Bill, Waco Brother #1
Old 03-19-2009, 01:23 AM
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Default RE: Spinner bolts break

Now, don’t get me wrong, I’m not having a crack at you or anything like that, but I haven't heard of this happening before, and you've had two cases at roughly the same period of operation, so if we analyse what information we have it MIGHT lead us to look at what’s going on in your setups in particular.

How tight are you doing them up? If it's a fatigue failure then the original load it’s been done up to will play a major roll. I often see guys wrench on stuff at the field with way more force than is really required. That could definitely lead to premature failure of the bolt.

Also, have look at the adaptor bolt. What does the begining of the threads look like where the spinner bolt enter's into in? and roughness, sharp edges etc...

Hope you can sort out the problem, those aulminium spinners are not cheap!

Thanks

Dave
Old 03-19-2009, 07:32 AM
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Default RE: Spinner bolts break

Over time they tight too much.

Spinner bolts should be just tight just a tad - which means not to change them out, just check them. If they are to tight they will brake. The spinner plate and propeller has the tendency to spin faster then the spinner.

Just loose the bolt sometimes and re tight - very little as possible.

This fix that problem before happening.
Old 03-19-2009, 12:39 PM
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Default RE: Spinner bolts break

Spinner bolts should be mild steel and not hardened. If you are using hardened steel bolts they will eventually fail because they do not have the flexability to stand the vibrations. We learned this the hard way flying Formula One.

Larry
Old 03-19-2009, 01:19 PM
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Default RE: Spinner bolts break

Hi all, thanks for the comments so far.

As for tightning the bolts. I just hand tightnen them using the small end of the hex wrench as leverage. I doubt they are too tight, but I will pay attention to that.

The spinner bolts are those that came with the spinner. I don't remember seeing any specs on them. I will make sure the new bolts are mild steel. Seems like I saw some specs on Tru-Turns site. I may order some of theirs.

The threads on the adapter nut appear to be OK. A bolt screws into it without any sign of a problem anyway. I replaced the adapter nut involved in the break last fall.

thanks again,

Brian
Old 03-19-2009, 01:47 PM
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Default RE: Spinner bolts break

ORIGINAL: [email protected]
Spinner bolts should be mild steel and not hardened. If you are using hardened steel bolts they will eventually fail because they do not have the flexability to stand the vibrations. We learned this the hard way flying Formula One.
Larry
The answer is not that easy about mild and hardened steel.

Are the bolts sinter hardened, surface or core - what is the base steel A1, A2...? Or is it a prepped 1010-1020? Even was it a pure steel or a stainless steel (btw, mechanicly formed SS is magnetic- so no hint there).

What I'm saying is, different bolts different grades - different applications. General to say a hardened bold is not a good fit for a spinner is not good enough. You looking for a bold with a higher torsion rating not necessary more tensile strength. BUT what you really looking for is a bold with a high shear resistance - like many stainless steel bolts.

Vibration has nothing to do with a failure in that application - just think about a turbine shaft....

Now you are in my playground (work)....
Old 03-19-2009, 01:49 PM
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Default RE: Spinner bolts break

the vibration makes it possible for the bold to turn....that's all....
Old 03-19-2009, 01:59 PM
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Default RE: Spinner bolts break

I just went in the shop and show you some of our broken bolts - this one takes 585000 pounds - and we broke it anyway
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Old 03-19-2009, 03:08 PM
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Default RE: Spinner bolts break

I’ve got planes it 30years and thousands of flights on spinner bolts with no failures. First I’m assuming the break is in the middle of the threads of the screw. I would look for something that would through the spinner hole out of alignment with the engines crank. Weather dirt in the back plate alignment grove or a hole that is drilled slightly off center. Also check for the screw head to be not on center with the shaft. You do this by mounting the screw in a drill press/ drill held in a vice then with it rotating hit it with a felt tipped pen stop the drill and look at the marks, they should line up.

If the screw is centered in the drill you will get a line of constant width all the way around it. If the screw is bent or not lined up correctly the line thickness will vary or disappear completely on one side.

Joe
Old 03-19-2009, 03:45 PM
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Default RE: Spinner bolts break

Hi!
Do you balance your props?
Old 03-19-2009, 05:49 PM
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Default RE: Spinner bolts break

jaka: Yes. I balance a prop prior to installation, then about once a season (or more often if something indicates it might be out of balance).

Brian

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