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Date: Wed, 18 May 2005 14:24:54 -0500
From: Charlie England <ceengland@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Fatigue limit
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wrjjrs@aol.com 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. With high strength bolt materials a stress 
> crack is far more likely to form at the thread. Since it should be 
> easy to get bolts long enough why take that risk? The fact that "he" 
> has flown with cut thread bolts on his prop proves nothing. The prop 
> material is the weakest link in that chain. Ask him about thermal 
> cycling in a steel/aluminum sandwhich next time you see him. I would 
> also say reguardless of his luck, using cut threads on a prop bolt 
> smacks of being aggressively idiotic. I have seen all six prop bolts 
> SHEARED on a BD-5 with vibration problems. Not using the best you can 
> get there is just silly. BTW if your using a 10mm bolt with a good 
> steel, (35,000 psi tensile or better) the bolt is good for 3,650 
> pounds in tension. That never was the problem! I have suggested before 
> that people look up a paper on prop drive systems and tortional 
> vibration by Donald Hessenaur. He worked on the BD-5 prop drive 
> system. WWW.prime-mover.org/Engines/Torsional/contact1/contact1.html 
> <http://www.prime-mover.org/Engines/Torsional/contact1/contact1.html> Simple 
> stress is rarely the reason for fastener failure! DO NOT TAKE 
> SHORTCUTS HERE!
> Bill Jepson
>  

Can't let Bill take the blame for that; it was I that posted the
original message.

My choice of words was pretty poor. In my mind I was expecting the
resonant stress to be included in the stress calculation. My attempted
point/question is that rolled threads don't help if the bolt is excited
at its resonant frequency (accelerating the fatigue rate). Therefore, if
you avoid the resonant frequency are cut threads really that big a deal
if the bolts are 15 or 20 times the needed strength?

I'm not advocating doing anything that's clearly stupid & agree that if
rolled bolts are readily available they make more sense.

Charlie