Prop Nuts

Keelboater

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Yeah, that is always a good topic. I follow "the good engineering" method with the thin nut on first followed by the larger nut. When the large nut is torqued up, it tends to push the small nut forward, off of the threads just enough, for it to function more like a spacer rather than a nut. At that point the large nut is taking all of the load. But if the large nut becomes loose and backs off, the thin nut springs back to the point where the preloaded threads re-engage with the threads on the shaft, and it becomes preloaded once again. Then it will not back off the shaft and serves it's purpose as a safety device. At least that's my take on it.
 

starrfish

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so basically that guy uses everything on his garage shelf, wd40. rector seal, epoxy, 5200, safety wire...to put a prop on, wow.
 

FPTMarineDiesel

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Yeah, that is always a good topic. I follow "the good engineering" method with the thin nut on first followed by the larger nut. When the large nut is torqued up, it tends to push the small nut forward, off of the threads just enough, for it to function more like a spacer rather than a nut. At that point the large nut is taking all of the load. But if the large nut becomes loose and backs off, the thin nut springs back to the point where the preloaded threads re-engage with the threads on the shaft, and it becomes preloaded once again. Then it will not back off the shaft and serves it's purpose as a safety device. At least that's my take on it.
Hello Keel, can you explain why the smaller nut should go on first as a "good engineering method"? The smaller nut usually is a back up nut in most engineering cases, in fact we use a double nut method for our turbo charger mounting and the smaller nut always goes on last as it is not the nut that is meant to be torqued because it does not have as many threads to distribute torque/clamping load across as the larger nut does. Therefore the smaller nut acts only as a backup lock type nut. At least in our engineering analysis it is this way. Just curious. Thanks.
 

starrfish

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peen over any showing threads and weld the rest. ( just being a smart ass)
I personally have done the big nut 1st then the smaller, then told I was doing it wrong. so ive tride the small nut then the big nut....
still have not lost a prop.. 20+ years
 

BillD

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I do believe the guy that wrote the article has never "lost a prop" either :D

80% of the nuts I look @ are big 1st, little 2nd.

I install small 1st big 2nd.

Installed both ways over 22 years, never lost a prop.
 

FPTMarineDiesel

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Now you did it. Gotta get out the popcorn!
Gotta love it! :)! I agree with both Bill D and Starr, I have seen it done both ways and with cotter pins and not seen the propeller back off or fall off because the nuts were lost at sea. However, I can tell you if you give it some rational thought, it is no different than a bolt, if you have only 2-4 threads carrying a load vs. 8 threads, the load is better retained and distributed differently/evenly for clamping force than it is with just 2 or 4 threads. You wouldn't want your head bolts or head studs to be held in place with 4 threads for clamping the head and gasket down, granted there is a lot more thermal expansion and contraction and sudden peak pressure differences on a gasket and cylinder head as well as the bolts, but it is the same principal.
 

Badlatitude

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The little nut sets the proper place and holds it there. The big nut pressures the small one ( big nuts threads fully engaged small nuts pressed.

Why would you want to press the big nut and then hold it all in place with less threads of the small nut. If anything two large nuts is better than using the small nut 2nd but its over kill.

ETA
In before Travis hits this thread with the wrath of the boat building gods. Most likely supported by CAD and a stress strain lecture. Hopefully with some cool shit like this
bolt-10-32-with-failure.gif
 
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FPTMarineDiesel

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That is a very interesting article, if that truly be the case with today's hardened fastners and rolled thread system, than I completely stand corrected and I myself have learned something new today! Thank you for sharing, that is a great article that I have not seen discussed that way before. In our case we don't use the larger not for applying more load, we only use the smaller nut for a lock nut but this describes a method of using the larger nut for more clamping force, but it does also state the smaller nut can be used as a lock nut at the bottom of the article too. I guess we need to find the person that started the two nut method for prop mounting and ask that person the method behind it, if it was used to create more force in which case than the torque method is critical of both nuts, or if it was just a safety, like with our mounting of our turbo chargers. With most high load systems where there is a lot of stress, the bolts are made with a lot less carbon so that they can have greater elasticity for torque turn procedures with much higher clamping loads than probably was available when they used to use a double nut method like being discussed in this article, but I have always believed that studing a block for main cap retention and head retention created a better clamping force and I also believe that after reading this article, you could improve on that process even more for high performance engines. Cool.
 
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Keelboater

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Hello Keel, can you explain why the smaller nut should go on first as a "good engineering method"? The smaller nut usually is a back up nut in most engineering cases, in fact we use a double nut method for our turbo charger mounting and the smaller nut always goes on last as it is not the nut that is meant to be torqued because it does not have as many threads to distribute torque/clamping load across as the larger nut does. Therefore the smaller nut acts only as a backup lock type nut. At least in our engineering analysis it is this way. Just curious. Thanks.

FPT - I just took the term "good engineering method" from the story in the link that was provided, but it really is the good method. Yes, you are correct in my opinion. Most of the time the "accepted use" of the thin lock nut is after the heavier nut has already been torqued in place. I will try my best to explain why that is and hopefully it will not be too confusing. The reason for this is referred to as the required joint stiffness, where pre-load of the bolt is critical in establishing a tension loaded joint that overpowers the working loads applied to it. The added lock nut is there just to prevent the larger nut from loosening, just as you described. Now, factor in unusual conditions where fatigue life of the fastener may come into play. A bolt acts like a spring when it is tightened, and it stretches (within it's elastic limit). The length to diameter ratio of the bolt plays a big part in just how much "stretch" is reliably obtained. The longer the threaded fastener is, the more it can stretch before it fails. So a 1/2-13 x 3" bolt would fail before a 1/2"-13 x 6" bolt when subjected to pure tension. In other words, the greater the stretch within the fastener, the more energy it can absorb from the working load before it fails. One method of increasing fatigue life of a bolted joint is to actually use shaft collars under the nut and the bolt head, thereby requiring a longer bolt than normally required for the given joint. The increased length to diameter ratio enables more stretch in the bolt......and more energy to be absorbed before failure. See where this is leading? Finally, look at the prop nut arrangement. If the larger nut were to be torqued up first, it would be closest to the end of the threaded section of the shaft, and the tension in the shaft threads would be examined "in a similar manner to a short bolt". Now use the lock nut first which kind of acts as a shaft collar, and then torque the large nut in place. The large nut displaces the small nut, and as I mentioned in my previous post, the threads on the small nut become disengaged (in theory). The large nut now carries all of the load, but is now further away from the tapered end of the shaft. This new arrangement would be examined as if the length to diameter ratio had increased, or a longer fastener was used. More energy could be absorbed in the threaded section of the shaft prior to shaft failure. What holds the prop in place besides the friction in the taper seat? The threaded portion of the shaft. So it's just a safety precaution on fatigue failure and reduces the possibility of shaft failure. It really has nothing to do with torque or nuts coming loose. That's it in a nut shell. Make sense? :confused: ;)
 

FPTMarineDiesel

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FPT - I just took the term "good engineering method" from the story in the link that was provided, but it really is the good method. Yes, you are correct in my opinion. Most of the time the "accepted use" of the thin lock nut is after the heavier nut has already been torqued in place. I will try my best to explain why that is and hopefully it will not be too confusing. The reason for this is referred to as the required joint stiffness, where pre-load of the bolt is critical in establishing a tension loaded joint that overpowers the working loads applied to it. The added lock nut is there just to prevent the larger nut from loosening, just as you described. Now, factor in unusual conditions where fatigue life of the fastener may come into play. A bolt acts like a spring when it is tightened, and it stretches (within it's elastic limit). The length to diameter ratio of the bolt plays a big part in just how much "stretch" is reliably obtained. The longer the threaded fastener is, the more it can stretch before it fails. So a 1/2-13 x 3" bolt would fail before a 1/2"-13 x 6" bolt when subjected to pure tension. In other words, the greater the stretch within the fastener, the more energy it can absorb from the working load before it fails. One method of increasing fatigue life of a bolted joint is to actually use shaft collars under the nut and the bolt head, thereby requiring a longer bolt than normally required for the given joint. The increased length to diameter ratio enables more stretch in the bolt......and more energy to be absorbed before failure. See where this is leading? Finally, look at the prop nut arrangement. If the larger nut were to be torqued up first, it would be closest to the end of the threaded section of the shaft, and the tension in the shaft threads would be examined "in a similar manner to a short bolt". Now use the lock nut first which kind of acts as a shaft collar, and then torque the large nut in place. The large nut displaces the small nut, and as I mentioned in my previous post, the threads on the small nut become disengaged (in theory). The large nut now carries all of the load, but is now further away from the tapered end of the shaft. This new arrangement would be examined as if the length to diameter ratio had increased, or a longer fastener was used. More energy could be absorbed in the threaded section of the shaft prior to shaft failure. What holds the prop in place besides the friction in the taper seat? The threaded portion of the shaft. So it's just a safety precaution on fatigue failure and reduces the possibility of shaft failure. It really has nothing to do with torque or nuts coming loose. That's it in a nut shell. Make sense? :confused: ;)
Good explanation! I am buying that, from a load and fatigue stress factor that is perfectly realistic. No doubt that there is a lot of cyclic loading and vibration in some cases, so that makes sense too.
 

Keelboater

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I just read the post by Chortle. That's good info, but it doesn't get into the bolt strength, or in this case shaft strength at the threads. Makes sense from a locking perspective and explains why nobody follows that procedure in industry. But who has actually suffered a prop loss because the fasteners came loose? It always seems to be from shaft failure of one form or another. I love the FEA demo by Badlad. Now picture that with a longer fastener and you would see it stretch further before failing. Good stuff.
 

FPTMarineDiesel

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I just read the post by Chortle. That's good info, but it doesn't get into the bolt strength, or in this case shaft strength at the threads. Makes sense from a locking perspective and explains why nobody follows that procedure in industry. But who has actually suffered a prop loss because the fasteners came loose? It always seems to be from shaft failure of one form or another. I love the FEA demo by Badlad. Now picture that with a longer fastener and you would see it stretch further before failing. Good stuff.
A lot of the shaft strength at the threads can also be affected by method of the threads being made, like cutting or rolling the threads, this would also affect how torque can be applied, based off of stress raiser already created by method of manufacturing. Most fasteners that are under extremely high load and for torque to yield are rolled threads so that there is a radius and no sharp edges in the shaft area where a fatigue crack can be created before the bolt is even torqued. If cut threads are used there is a high probability of high cycle fatigue failure with a torque to yield method. I wonder how the threads are machined into the propeller shafts being that most of them are made from very durable materials. This would also add important information that would help conclude why the two nut method might be used as well, would it not?

I have to step away from this conversation now, Patriots are in the second half of the game, seems to be the only half they like to play these days, so it could be a good game from here on out. However, I will check back as this has always been a question I have had myself for prop installation and I too have not seen a propeller fall off with two nuts tightened in either sequence. I have only seen high cycle torsional fatigue fractures/failures and lost props in the abyss.

Great article started by Tunamojo!
 
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captainlarry84

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I go little nut 1st then the large one. The big thing is when tightening always block your wheel at the hub and never the tip. Especially with DQX wheels. You will bend a blade. Just lay the wood between the rudder & keel as close to the hub as possible.

kristen 006.jpg

kristen 009.jpg

zzz.jpg
 

Keelboater

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A lot of the shaft strength at the treads can also be affected by method of the threads being made, like cutting or rolling the threads, this would also affect how torque can be applied, based off of stress raiser already created by method of manufacturing. Most fasteners that are under extremely high load and for torque to yield are rolled threads so that there is a radius and no sharp edges in the shaft area where a fatigue crack can be created before the bolt is even torqued. If cut threads are used there is a high probability of high cycle fatigue failure with a torque to yield method. I wonder how the threads are machined into the propeller shafts being that most of them are made from very durable materials. This would also add important information that would help conclude why the two nut method might be used as well, would it not?

I have to step away from this conversation now, Patriots are in the second half of the game, seems to be the only half they like to play these days, so it could be a good game from here on out. However, I will check back as this has always been a question I have had myself for prop installation and I too have not seen a propeller fall off with two nuts tightened in either sequence. I have only seen high cycle torsional fatigue fractures/failures and lost props in the abyss.

Great article started by Tunamojo!

Absolutely. Rolled threads are much stronger than cut threads. In designing our steel mill equipment, we will normally spec rolled threads on all of the bolts for mounting hydraulic screwdown cylinders. They can be subjected to well over 1 million lbs. of separating force. Nothing on a downeaster even comes close. :D
 

Toolate

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All this talk make me want to mention sharp edges. Lots of talk about prop installation and what to do with the key and keyway and I dont know what the industry standard/accepted method is but it seems intuitive to me to use a fine file to smooth and round all corners of both items before installation to avoid the buildup of stress at sharp points. Threads of a shaft would also seem to fall into this category (any nicks should be filed down and smoothed).

You guys are nuts :eek:
 

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