Prop bolts....what's the consensus?

[wesley.]

Just a quick question.....  Is there really a concern between 8.8 and 12.9 on hardness when the hub is aluminium?  I cant see where that would make a difference.  

 

[Vern.Smith]

This will undoubtedly bring slings and arrows but the engineering and product specs on fasteners has been around since the nineteen thirties. Problem is they are all based on the proposition that the bolt will be holding two things together in tension and that the two objects are non compressible. This is not the case with our propellers, they do compress. Not to get overly technical but bolts are designed and rated in relation to their elongation properties (how much the metal they are made of will stretch before it reaches the point that it will not return to it's original length). For a bolt to provide it's maximum clamping force it should be stretches to around 80% of it elongation factor.

You may do this when you install your prop but within a couple of hours the prop will have compressed under the bolt. If you don't believe me tighten your prop bolts with a torque wrench (or make one) and come back later and see how much you can turn the same bolts to achieve the same torque reading.

In our case the IMPORTANT factor is to keep the bolt at the same position you put it during the first tightening. All mechanical locking devices depend on the amount of the torque on the bolt remaining stationary which is not the case in our situation. The simple answer for us is loctite 242 (the blue stuff). It does not matter what the bolts elongation may be, it glues the internal and external threads together.

All the engines I am aware of come with socket head cap screws which are quite hard, rockwell 55 or so. Our loads are not tension loads (stretching the bolt) but shear loads ( the bolt being flexed in a bending motion with each cylinder ignition). This is not the best environment for a hard (brittle) bolt. We would be better off with either stainless socket head cap screws or grade 5 AN hex bolts with AN washer under them.

I will now put on my flack jacket :^ )

 

[Alky6]

Good question, but yes it does. The yield strength of the bolt (think force/area) in psi (pounds per square inch) or more commonly dentoted as ksi, or 1,000 times psi). The big concerning factor is shear strength - and this is also why shooting for 2x the diameter as noted above - coupled with combined tensile stresses and again with oscillating stresses, or vibration. Shear strength, rule of thumb, is about half that of the tensile strength. Further, in cases where the oscillation occurs, as long as the maximum stresses seen remain below 50% of the allowable stress, then the member in question will survive infinite cycles of vibration (for steel). As those stresses increase there is a curve at which point they will ultimately fail after a certain number of cycles. By using higher yield strength bolts (and preloading them - a topic I'll leave out) it helps reduce the the number of cycles obtained above that magic 50% of yield stress mark, therefore increasing the longevity of the fastener. Aluminum, BTW, has a finite life cycle regardless of the amount of stress observed. In other words, aluminum, at some point in its life cycle will crack and fail. A penalty with the higher yield stresses is brittleness. You need a certain amount of malleability also - it is all a balance.

At the hub - the aluminum hub has a larger area (supposedly) that the resulting shear stresses are much lower than those seen by the bolts. Hope that make sense.

-Paul

 

[Alky6]

Vern:

Well put as to why the propeller bolts would fail in shear.

-Paul

 

[stangflyer]

Awesome information to have... With everything I have learned in the last few hours, I can feel much better for being a little bit better informed. 

 

[Ohio AV8TOR]

Terryscustom said
Ohio.AV8TOR said
I buy a box of 20 from Fastenal or McMaster and always weight them to exact weight batches for use.  I do not change out on a frequency but I do from time to time.  Thread depth should always be at least 2x thread diameter so 1/4" is not enough. 5mm screw x2 = 10mm (0.393").  I always use a torque wrench as well.  Torque is a whole other subject.   

The actual general rule in engineering is the engagement should be equal to or greater than one times the dia. of the fastener.      But the common sense rule of thumb is two times the dia. for fasteners under load.      In our case we have load and vibration so I would never fly with 1/4" of threads in the hub.
Exactly Terry, I mentioned the 2x because of our loads

 

[stangflyer]

Ohio.AV8TOR said
Terryscustom said
Ohio.AV8TOR said
I buy a box of 20 from Fastenal or McMaster and always weight them to exact weight batches for use.  I do not change out on a frequency but I do from time to time.  Thread depth should always be at least 2x thread diameter so 1/4" is not enough. 5mm screw x2 = 10mm (0.393").  I always use a torque wrench as well.  Torque is a whole other subject.   

The actual general rule in engineering is the engagement should be equal to or greater than one times the dia. of the fastener.      But the common sense rule of thumb is two times the dia. for fasteners under load.      In our case we have load and vibration so I would never fly with 1/4" of threads in the hub.

Exactly Terry, I mentioned the 2x because of our loads
Thank you both Ohio and Terry....for getting me lined out on the prop bolts. I feel a lot more at ease now. Damn this site is great huh?

 

[Capt.Roll]

Yes change out, I do every 5 times I have the prop off (with carbon props). Reason is they stretch and get weaker. I get boxes of 20-25 from McMasters because Fastenal wants an arm and part of a leg for them. I also use nord-lock fasteners on the prop bolts instead of lock washers for a little added security. Washers are expensive but especially the stainless ones can be used countless times. Rule is 2-2.5 times the dia. I normally shoot for 1/2-5/8" minimum with 3/4 being my goal. Class 12.9 seems stronger than most bolts included with engines, I even replace the ones included with my DA's after a couple times of use: http://www.mcmaster.com/#catalog/120/3094/=ucjdg6

Terry is the only one that stated a number, 5, to change the prop bolts. Any one else have a number or process they follow regarding how often prop bolts should be changed?

Here’s why I ask. Went out Saturday morning for some stick time with my 104” EF Extra 300, powered with a GP123 and spinning a Falcon 28X10 CF. First two flights had zero issues; didn’t notice anything unusual before shutting down the engine after the 2nd flight, all seemed good.

Went back to the pit area and refueled and chatted with a couple other pilots for about 30 minutes. Took the plane back out to the flight line for my third flight of the morning and when flipping the prop to start the engine there was a loud slapping sound as the prop was spinning, but not metallic in nature. It didn’t sound normal so I had one of the other pilots come out and listen while I flipped. He agreed that something didn’t sound right. On the flight line we did a quick visual; the spinner and prop looked fine and they felt tight. Mufflers were tight and the engine mount to the firewall was solid. Turning the engine over slowly there was no drag or unusual noise coming from anything, no play beyond what I would consider customary, and the compression felt normal.

I took it back to the pit area for a better inspection. Started with the spinner to check for a crack in the structure or aluminum backplate. As soon as I had the spinner removed, the problem was pretty clear. There were three broken prop bolts, one loose and the other two were still tight. Went ahead and checked the spinner, backplate and prop; all were in great shape.

Once I removed the prop from the hub, the three broken bolts were a couple of threads deep into the hub so I was unable to get a grip and back them out. I don’t carry the tools to deal with something like this to the field so my day was done. The pic below shows the broken pieces, probably about 12-15mm length.

These were the stock bolts that came with the engine. I don’t know how many on/off cycles were on the bolts (I’d say at least a half dozen) but the plane had 47 flights, right around 9 hours run time with a little over 6 gallons of fuel through the engine. The prop bolts are installed with around 70 in. lbs. torque. It’s unlikely that I overtighten the bolts so I’m thinking that they broke due to the repeated fatigue of the slight stretching during tightening.

Happy ending……..was able to remove the broken bolts, installed six new class 12.9 bolts and was able to fly Sunday morning. No issues, all good.

So, what are your opinions regarding changing prop bolts? Every time the prop is removed, every other time, after every ten flights, every 40 flights, annually, only when they fail, etc.

 

[Terryscustom]

Number of flights is not an issue ever as the forces are not as great as when installing the bolts in which you have pulling and twisting forces, the number of torque cycles from tightening that heats and stretch the bolt each time is what kills them.

I think safest bet if you are testing props is to test till you find your favorite, then install that favorite prop with new bolts and forget about it.

(oops, just noticed you copied me so at least I'm consistent in my answer)

 

[-Rick-]

Very interesting read. So much can be learned from this site and you gents.