NR prop bolt concern

[Delta Whisky]

Brian - it would take a very experienced structural engineer with plenty of metallurgical experience to give a full and complete answer to your question on this subject. John touched on two of the high level answers so I'm just saying what he did in an different way.
1. you can more often trust the properties of AN, MS, NAS, etc. spec'd bolts sourced from respected aircraft hardware businesses than more common bolts sourced from places that serve other industries. (And, yes, we have experienced forgeries/counterfeit in the aviation world but those events are much more likely to be detected earlier than "grade" bolt variation.) and,
2. their ductility is felt to be beneficial in that it might offer a chance for detection of an overloaded condition in a properly designed application. (The designer will often want the bolt to yield - not break - rather than one of the fastened parts breaking.) It is really a case of "strong enough" with other performance benefits being included by the designer.

I too run a NR prop and really like its performance. I was surprised to see the bolts that came with it though. Not knowing much about 8.8 bolts, I used them and haven't (yet?) had a problem but that is what the designer spec'd so I went with them.

[avidflyer]

Delta Whisky, Have you ever taken any bolts off to get a look on the hub of the prop to see if you are having the same issues as I did? JImChuk

[jiott]

Yes, grade 8 bolts are stronger but are less ductile, meaning they won't stretch as far before breaking. Therefore, when you torque them to spec they don't stretch as far, causing less "spring effect" in the joint. This is very critical in high vibration applications which can cause a threaded joint to back off slightly. Under this condition the grade 8 bolt will lose its clamp load faster than a grade 5. So the engineer designs the required clamp loads to be compatible with grade 5 hardware and gains a much more forgiving joint before failure. This is why its so important to properly torque your critical bolts to specified torque. You want to take full advantage of that stretch and spring effect.

[Delta Whisky]

Delta Whisky, Have you ever taken any bolts off to get a look on the hub of the prop to see if you are having the same issues as I did? JImChuk

Jim - good question. This forum subject got me to thinking that I should. Because the plane is down for an equipment swap and because I've wanted to change the pitch for cold weather, your question is the final motivation I needed. If I can get it all in before the rain comes in tomorrow; tomorrow will be the day. Darrel

[Av8r3400]

So why do we use the equivalent to grade 5 bolts on something that is obviously very critical, high stress, high-vibration (possibly)?

Yield strength and ductility are more desirable than hardness in this application.

[Delta Whisky]

Delta Whisky, Have you ever taken any bolts off to get a look on the hub of the prop to see if you are having the same issues as I did? JImChuk

Jim - a short vacation and a bout with Covid kept me from getting to this until today. To me, all look normal. There's about 280 hours on these bolts since last loosened for a prop adjustment.
Propeller bolts at 280 hours.jpg

Darrel

[skyguy04]

Great information Delta Whisky and jiott. I learned something today thanks to you guys! ;-).

[avidflyer]

I see you have different bolts than I have. Mine are allen head bolts. Not sure if there was any other difference though.