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Date: Wed, 18 May 2005 13:26:55 -0600
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
From: Bill Dube <bdube@al.noaa.gov>
Subject: Re: [FlyRotary] Re: Fatigue limit 
In-Reply-To: <list-948873@logan.com>
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At 11:44 AM 5/18/2005, you wrote:
>  >Anyone care to do the math on the stress on each bolt used to hold a 
> 6.5B >together? I'm no engineer but I do know one & he has pointed out 
> that if >stresses are well below the danger zone, rolled threads vs. cut 
> threads >just ain't that big a deal. (To back that up, he's flown with 
> re-cut prop >bolts on wood props for probably more hours than I have 
> total time.)
>
>Bill I totally disagree. While the stress level is well below the danger 
>zone on either the 6.5 or 13B the problem with the bolts has come from 
>harmonic vibration. Since the 6.5 bolts are so much shorter it is less 
>likely to be a problem, but using cut thread bolts is simply looking for 
>trouble.

         Doesn't sound like you "totally" disagree.

         The harmonic vibration issue for these specific bolts is not all 
that well understood. Even though these bolts are shorter, they are still 
long and slender and subject to vibration, just at a different frequency. 
With a one-rotor set-up, the vibration will likely be much greater in 
amplitude, but with a different spectrum. Willing to bet your life that the 
shorter bolt will not fatigue in a similar manner as the longer bolt? I 
agree that vibration would likely be less of a problem, but who knows how 
much less?

>With high strength bolt materials a stress crack is far more likely to 
>form at the thread.

         Fatigue cracks will always form at the stress concentration. If a 
bolt fails in fatigue, it will typically fail at the threads or where the 
head meets the shank. The strength of the bolt material doesn't alter the 
location of the stress concentration so it doesn't alter the location of 
the fatigue failure.