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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.)
The main issue is fatigue, not ultimate strength. These particular bolts must be strong enough to have the reserve strength to withstand severe fatigue loads. When you consider that the stock high-strength bolts, with rolled threads, fail regularly due to fatigue, you really want to replace these bolts with something that is at least equal in strength.
It would not be unusual for the bolt to be 8 or 9 times stronger than the peak load it will see. This "extra" strength is needed to take in for the stress concentration at the threads (and at the head) and to allow the bolt last under the cycling (or vibration) load.
The stress concentration factor for a bolt is something a bit over three. The typical "fatigue limit" (or endurance limit) for steel is about 2/5 of the tensile strength. When you put all these factors together, you end up needing a bolt that is 8 or 9 times stronger than the peak load.
>>>>> Fatigue soap box <<<<<<<
Most folks don't understand fatigue failure very well. Fatigue failure is very often the reason that auto engines fail in aircraft applications. Most folks just don't take fatigue into account. Each load cycle (ie. engine revolution) is a grain of sand trickling through the hourglass. When the sand runs out, the part suddenly breaks in two.
This sort of failure happens not only in bolts, crankshafts, and connecting rods, it also happens in radiator supports, exhaust pipes, oil line fittings, motor mounts, etc. Anything that has a varying load (like vibration) is subject to fatigue failure.
It is difficult, if not impossible, to know what the cycle rate and the peak load are going to be in every single part. That is why prototypes are particularly prone to fatigue failures. It's hard to predict that you have exceeded the endurance limit on some portion of a design until you actually build it and test it.
The main reason I selected a rotary for my RV-7 is that Tracy has 1400 hours on one. Others have significant hours on rotaries as well. This tells me that there are no internal fatigue problems inherent to the Mazda rotary engine. It also tells me that the rest of the design is sound as well.
>>>>> Soap box mode off <<<<
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