Propeller bolts

Rowdyflyer1903!

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There is some discussion and even disagreements on how many threads of a bolt must extend past the nut to be safe. However if you look behind many of the propeller flanges with or without inserts, this rule of thumb of safety for AN bolts seems to not apply. In my case, a metal tractor propeller has been replaced with a pusher wooden propeller. I have no way of knowing if my wooden prop with the crush ring is the same thickness as the original pro But that is moot.The propeller bolts, as four as I can tell, are AN bolts with standard thread length. Even with the propeller off and the bolts threaded into the flange with inserts, the threads will not extend past the inserts the expected one to two threads. The unthreaded grip section of the bolt will bottom out long before any threads will extend past the inserts.

I have asked on on-line forum for my engine and no one seems to know the answer nor able to point me to a reference or guide.

In my installation it is impossible for the threads to extend past the threaded flange inserts as the bolt simply does not have enough threads. If propeller bolts do not follow the standard AN bolt guide, then besides measurement, how from a visual inspection does the installer know how many threads are actually protruding to interface with the flange inserts? It seem all AN bolts of a given diameter ( AN 5,6, or whatever) have the same thread count. I have been told, it is common practice to use AN bolts as propeller bolts.

If you happen to see me at Bensen Days peeking my head around and looking at the back side of your propeller and scratching my head, this is why.
 

Abid

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If you are using threaded insert in engine prop flange then this will be a problem using AN bolts. As you say AN bolts are fixed in their short thread length as they are made to reduce weight and have shear or other loads mainly on the grip length.
My advise would be to switch to SAE grade 5 bolts that have longer thread lengths and won’t bottom out. Do note that AN bolts are Cadmium plated which means their required clamping force is achieved at a lower torque value versus zinc plated SAE or metric bolts that will achieve the same clamping force at a different and usually higher torque.

There is nothing wrong with using good quality SAE bolts in this application. You may get too many threads sticking out and in that case you can mark and cut the bolt down with a nice saw while cooling the surface with liquid. Then dip the exposed non plated end into a rust preventing paint like Rustoleum primer. Sometime it’s necessary to chamfer the cut end a little on a grind wheel so they thread in cleanly into a nut.
If you were using AN5 bolts then 5/16” SAE zinc plated bolt is appropriate and will need about 16 foot pounds of torque generally to reach proper clamping force it’s rated for. With wood props torque should be checked every 25 hours. You can use 5/16” Nordloc washers under the head to lock the torque down. Or drill the heads to safety wire the bolts.
McMaster Carr sells SAE bolts that have drilled heads by the way.
 

Doug Riley

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I like to wire the bolt heads regardless of what other locking methods are also employed. Safety wire is "inspectable," while other methods are not so much.

The rule-of-thumb about leaving two or three threads protruding at the far end of the bolt is mainly applicable to setups where a Nyloc (elastic stop) nut is the locking device. The nylon insert must cling to a full-diameter thread to lock properly. The threads' O.D. tapers a bit right at the end, so a "zero thread" setup may not lock. Having 2-3 threads showing also may give you a visual warning of the nut's beginning to back off. You can paint the nuts and protruding threads to help detect any backing-off.

I believe that the 2-3 thread rule is less important where wire or other methods are used to lock the bolts. It is MUCH worse to thread the bolts in so deeply that they bottom out; this sets up the bolt to break at the threads.

BTW, classic prop-mounting "theory" has it that, if the bolts are properly torqued, the engine torque is transferred to the prop by its friction with the flange, not by shear-loading the bolts. It would be nice if that were true, since otherwise we're putting shear loads on the threads. I'm skeptical that this is entirely true, though, because we can't torque the bolts on a wood prop very tight without crushing the wood. Less torque, less drive friction.

If you use Grade 5's, make sure they are genuine US-made hardware. There is a ton of imported junk hardware out there that is actually Grade Zero. Some "AN" bolts are counterfeit as well. Some hardware outlets charge extra if you want American-made hardware.
 

JETLAG03

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A French airbus engineer told me that 1-3 threads beyond the nut is their norm.
 

Vance

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In my opinion if your propeller bolts are not the correct length you should purchase AN bolts that are the correct length.

Unless you have elastic stop nuts there is no reason to have threads showing assuming there is an appropriate thread engagement in the inserts.

In my opinion using an elastic stop nut on a propeller is not best practice.

Most of the props used on modern gyroplanes use elastic stop nuts and I feel it is important to have a couple of threads showing to have the elastic stop nut work and the nuts should be replaced regularly.

A typical AN bolt intended for a propeller will have a drilled head and should be safety wired properly with the thickness of safety wire that is appropriate.

Torque is critical and with a wood prop should be checked often (25 flight hours or seasonally twice a year because moisture makes a wood propeller expand).

Torque the bolts cross wise (going from one side of the propeller to the other rather than in sequence) and in steps.

Bolts holding a propeller on operate in a very hostile environment and care should be exercised and the bolts should be replaced from time to time.

Each installation is unique and the best information will come from the propeller manufacture.

Below is an FAA advisory circular about propellers that you may find interesting.

https://www.faa.gov/documentlibrary/media/advisory_circular/ac_20-37e.pdf
 

Abid

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Just a note.
AN bolts are good but they are not magical. In applications where bottoming out in a critical area will cause terrible consequences, AN bolt should be skipped. Prop bolt is just such an area.
AN bolts would be better used if threaded insert was not there and you were going through and threads were on the back side with a nut. We do that along with Nordlock washers. Even Sensenich does the same thing. When you use threaded inserts, Sensenich advises to use metric (class 8.8) or SAE grade 5 bolts because they also are concerned about AN bolts bottoming out.
Bottoming out does not create the friction and that creates shear loads. No bolt stands those shear loads more than a few hours. The key is to have the right clamping force to make engine and prop turn as one unit. That's the key.
Second Nordlock washers hold clamping force even better than locking safety wire. A torque seal inspection marker across the head to adjacent surface allows inspection at a glance.

 
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Rowdyflyer1903!

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In my opinion if your propeller bolts are not the correct length you should purchase AN bolts that are the correct length.

Unless you have elastic stop nuts there is no reason to have threads showing assuming there is an appropriate thread engagement in the inserts.

In my opinion using an elastic stop nut on a propeller is not best practice.

Most of the props used on modern gyroplanes use elastic stop nuts and I feel it is important to have a couple of threads showing to have the elastic stop nut work and the nuts should be replaced regularly.

A typical AN bolt intended for a propeller will have a drilled head and should be safety wired properly with the thickness of safety wire that is appropriate.

Torque is critical and with a wood prop should be checked often (25 flight hours or seasonally twice a year because moisture makes a wood propeller expand).

Torque the bolts cross wise (going from one side of the propeller to the other rather than in sequence) and in steps.

Bolts holding a propeller on operate in a very hostile environment and care should be exercised and the bolts should be replaced from time to time.

Each installation is unique and the best information will come from the propeller manufacture.

Below is an FAA advisory circular about propellers that you may find interesting.

https://www.faa.gov/documentlibrary/media/advisory_circular/ac_20-37e.pdf
The odd thing is AN bolt thread length will not fill and exceed the insert even if the propeller is not installed. With the propeller off and one of the bolts threaded into the flange insert till the non threaded part of the bolt can go no further, the thread count is still not adeqhate enough. According to aircraft spruce, all AN bolts have the same length or count of threads. AN bolts may be ordered for different lengths but the thread Count will not vary. One could, I suppose, order longer grip bolts and cut additional threads. If your prop flange inserts exceeds the standard AN thread count, you are out of luck.
 

Rowdyflyer1903!

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If you are using threaded insert in engine prop flange then this will be a problem using AN bolts. As you say AN bolts are fixed in their short thread length as they are made to reduce weight and have shear or other loads mainly on the grip length.
My advise would be to switch to SAE grade 5 bolts that have longer thread lengths and won’t bottom out. Do note that AN bolts are Cadmium plated which means their required clamping force is achieved at a lower torque value versus zinc plated SAE or metric bolts that will achieve the same clamping force at a different and usually higher torque.

There is nothing wrong with using good quality SAE bolts in this application. You may get too many threads sticking out and in that case you can mark and cut the bolt down with a nice saw while cooling the surface with liquid. Then dip the exposed non plated end into a rust preventing paint like Rustoleum primer. Sometime it’s necessary to chamfer the cut end a little on a grind wheel so they thread in cleanly into a nut.
If you were using AN5 bolts then 5/16” SAE zinc plated bolt is appropriate and will need about 16 foot pounds of torque generally to reach proper clamping force it’s rated for. With wood props torque should be checked every 25 hours. You can use 5/16” Nordloc washers under the head to lock the torque down. Or drill the heads to safety wire the bolts.
McMaster Carr sells SAE bolts that have drilled heads by the way.
I appreciate the information and the attention to detail even to the reminder to cool the bolt as you are cutting the bolt to length. This is something which is often overlooked and is damaging to the temper of the steel. I can see how using nylock nuts how the one to three threat protruding became the rule of thumb and tends to applied to all.
 

Vance

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In my opinion an AN bolt has plenty of thread engagement for maximum strength.

I would hesitate to cut a bolt to length.

You have not mentioned what engine is the challenge.

My Lycoming IO-320 uses AN bolts and comes up on the torque nicely.

Propeller blots are special and you may need to go somewhere like Genuine Aircraft Hardware which is where I ordered my bolts.

I have a wood core composite covered Catto propeller that he made special for me because flying air shows and making radical maneuvers in a gyroplane is hard on the blade roots.

Good luck with your project.
 

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Rowdyflyer1903!

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In my opinion an AN bolt has plenty of thread engagement for maximum strength.

I would hesitate to cut a bolt to length.

You have not mentioned what engine is the challenge.

My Lycoming IO-320 uses AN bolts and comes up on the torque nicely.

Propeller blots are special and you may need to go somewhere like Genuine Aircraft Hardware which is where I ordered my bolts.

I have a wood core composite covered Catto propeller that he made special for me because flying air shows and making radical maneuvers in a gyroplane is hard on the blade roots.

Good luck with your project.
Lycoming 0-320 D2A 160 hp similar to yours but normal carburetor. I have no doubt the AN bolts which I have will, as you say, meet torque values. What worries me is when the DAR or FAA peeks and could rule it un-airworthy and not issue the sign off during final inspection. I find there is so much debate on what is proper and good practice. No one really seems to know much less agree. Truly I believe the bolts I have are safe if torqued properly and safety wired. I’m good with it. I just did not want to take the chance or be in a position to debate the inspecton. I am swinging a Tennessee Propeller 68x46. I really appreciate your input.
 

Vance

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My camera with the flash is at home so I took this with my GoPro Hero 7 and hanger lighting.

This is the back side of my ring gear and those threads you see sticking through are the propeller bolts.

It is probably .375 protruding and about six threads.

The propeller has been on the Predator for about 1,700 hours. I have probably had it off five times to replace the alternator belt or refurbish the propeller.

If you call Genuine Aircraft Hardware they will either find or have the specifications you call out in stock.

Toll free (888)247-2738.
 

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Rowdyflyer1903!

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My camera with the flash is at home so I took this with my GoPro Hero 7 and hanger lighting.

This is the back side of my ring gear and those threads you see sticking through are the propeller bolts.

It is probably .375 protruding and about six threads.

The propeller has been on the Predator for about 1,700 hours. I have probably had it off five times to replace the alternator belt or refurbish the propeller.

If you call Genuine Aircraft Hardware they will either find or have the specifications you call out in stock.

Toll free (888)247-2738.
Thanks, I will do some measurements to have the information should they needed it. BTW are you going to be at Bensen Day
s tHis year?
 

Vance

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Thanks, I will do some measurements to have the information should they needed it. BTW are you going to be at Bensen Day
s tHis year?
Most propeller manufactures have specifications for the propeller bolts, you might give them a call.
My intention is to go to Bensen days 2022. If my house sells and the buyer wants me out that could occupy my Bensen days time.
 
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