NR prop bolt concern

[avidflyer]

Hi all, I have about 175 hrs on my NR two blade prop on my Kitfox 4 with 80 HP 912. I've adjusted the prop a number of times between winter and summer, but never actually took the bolts out. The other day I put a balance master on it, and got new AN5 bolts for the install. Here is what I found. I've always torqued the bolts to correct torque, but a few times it did seem like a couple of them acted weird. Like not turning as much as I expected at the next torque setting. I would say that apparently, they weren't tightening up to correct torque, at least some of them. I'm back to working on my new built Kitfox 4 now, and I took the NR two blade prop off of it, so I could work on the engine and cowl install better. I had ran this engine/prop about 15 or 20 minutes last summer, before I got side tracked. Both times I used the bolts that NR supplied with the props. Last prop I know of that came from them didn't come with bolts. Maybe I'm not the only one who had an issue. I like the prop, gives good performance, but if you are running their bolts, you may want to have a look. First two pictures and the fourth one are the 80 HP, and the third and fifth ones are of the 100 HP 912 with 15 or 20 minutes run time. JImChuk

[Shadowrider]

When we bought our NR props and they showed up, I did notice the bolts seem to be low grade hardware so we changed them out from day 1. I would also recommend to change out the hardward to aircraft grade bolts. Another kitfox friend gave me the number to a company in Texas that can get you all the replacement hardware.

PH: (817) 326-6293 E-MAIL: SABERMFG@SABERMFG.COM

[avidflyer]

I did change out the bolts to AN5-36 bolts before I reinstalled the prop. JImChuk

[Shadowrider]

Good call.

[avidflyer]

The NR bolts had an 8.8 marking on the heads. JImChuk

[jrevens]

The NR bolts had an 8.8 marking on the heads. JImChuk

If that marking denotes a grade 8 bolt (I don’t know if it does - are they metric?) then those bolts are likely supposed to have a greater tensile strength than a normal AN bolt. AN bolts are roughly equivalent to a grade 5. NAS bolts are available in higher strength than AN bolts also.

Just as an aside, I know that a few American wood prop makers used to specify stronger industrial bolts for their props also. Coming from trusted US manufacturers, they were usually equivalent to grade 8.

[Av8r3400]

If that marking denotes a grade 8 bolt (I don’t know if it does - are they metric?) then those bolts are likely supposed to have a greater tensile strength than a normal AN bolt. AN bolts are roughly equivalent to a grade 5. NAS bolts are available in higher strength than AN bolts also.

Just as an aside, I know that a few American wood prop makers used to specify stronger industrial bolts for their props also. Coming from trusted US manufacturers, they were usually equivalent to grade 8.


8.8 is NOT a grade 8 bolt. It is a metric designation closer to an SAE grade 5.

I ordered special size metric 8.8 hardware for mine that attached into the metric threaded hub inserts. I needed longer hardware for the prop spacer, balance ring and the prop hub. I put the bolts into my milling machine and cross drilled the heads for safety wire.

33348

https://www.engineeringtoolbox.com/metric-bolts-minimum-ultimate-tensile-proof-loads-d_2026.html#gsc.tab=0

[jrevens]

“8.8 is NOT a grade 8 bolt. …”

Thanks Larry, I should have known that. As I asked in my first sentence - (…”are they metric?”).

[jrevens]

Just to clarify my previous comments, specifically about some propeller manufacturers in previous times specifying non-AN bolts, I am not suggesting that it’s wise or safe to substitute any type of bolt other than what is specified by the particular propeller designer/manufacturer. Although a grade 8 bolt has greater tensile strength than a grade 5 for instance, that doesn’t necessarily make it better or safer for a particular application, even if you can be certain that it meets the specifications for it’s type. It may be stronger in tension but more brittle for instance. And (unless it’s counterfeit) you can trust AN hardware to meet design specs. There are usually important factors involved in choosing parts & materials that may not be readily apparent. Sorry for the long diatribe about things that are likely very obvious to many people reading this.

[skyguy04]

Thanks for this information. It's made me want to learn more about fasteners. In my readings, I thought this was interesting:

https://troweltrades.net/2023/06/05/bolts-are-graded-based-on-their-material-properties-including-tensile-strength-yield-strength-and-hardness/#:~:text=Grade%205%20bolts%20are%20made,yield%20st rength%20of%20130%2C000%20PSI.


Bolts are graded based on their material properties, including tensile strength, yield strength, and hardness. Grade 5 bolts are made of medium carbon steel with a tensile strength of 120,000 PSI and a yield strength of 92,000 PSI. On the other hand, grade 8 bolts are made of alloy steel with a tensile strength of 150,000 PSI and a yield strength of 130,000 PSI. The higher strength of grade 8 bolts makes them more suitable for demanding applications that require more durability and reliability.
One of the primary applications for grade 8 bolts is in the automotive industry, where high-stress and high-vibration conditions are common. In this industry, grade 8 bolts are preferred for critical components such as engine mounts, suspension systems, and steering components. These bolts are also used in heavy equipment, construction, and aerospace industries, where safety and reliability are paramount.
In contrast, grade 5 bolts are typically used in low to medium-stress applications, such as general construction, automotive, and machinery. They are also commonly used in non-critical applications such as holding non-structural components in place.

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

[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.
33457

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.