Prop Nuts

[krush]

LOCK NUT
There are two common usage's of this term:
1. A nut which provides extra resistance to vibration loosening by either providing some form of prevailing torque, or, in free spinning nuts, by deforming and/or biting into mating parts when fully tightened.
2. The term is sometimes used for thin (or jam) nuts used to lock a thicker nut. When used in this way the thin nut should be adjacent to the joint surface and tightened against the thick nut. If placed on top of the thick nut the thin nut would sustain loads it was not designed to sustain.

http://www.boltscience.com/pages/glossary.htm#l

................The idea of a jam nut is to cause the two nuts to press on the threads in opposite directions, so that the thread is stretched between them, and this stretch is not relieved if the nuts should turn slightly on the bolt, so that unscrewing is discouraged. If this condition does not exist, and it usually will not be if the thinner nut is on top and tightened second, then the top nut and after it the lower nut can loosen without restraint, and there will be no locking action. Two nuts in this case are not much better than one. The proper connection is made by tightening the jam nut snugly first, then tightening the upper nut so tightly that the stress on the jam nut is reversed as the bolt strains. The two nuts could be the same thickness, but it saves space if the lower one is thinner. The thinner nut must counter only part of the compressive force of the regular nut, which does double duty. It is surprisingly difficult to find illustrations of a jam nut used properly, or torque specifications. This is also a matter that is of no interest to the academic engineer, who only talks about locknuts and does not actually use them..............

http://www.samsmarine.com/forums/new...uote=1&p=76930


Bill, the book I has is just a design book that covers the forces and basic methods of figuring out "how big" of material to use when one makes a machine. It's premise is that the nut is of equal grade to the bolt (and does nto yield), which isn't true in this case. I'd have to go look through more specific material to find a definite answer.

And THIS is where I can see it may not even matter which nut goes first. If the nuts are so "soft" that they deform and do not "stretch" the bolt, then all the reason can probably be tossed out the window. The only preload placed on the joint is by the stretching of the threds in the soft nut...but this most likely isn't an elastic stretch (what causes preload in bolts); instead it may be permanent deformation.

Like I said before, bolted joints are NOT intuitive. I checked the wiki writeup, and it is fairly correct:

When a cap screw is tightened it is stretched, and the parts that are captured are compressed. The result is a spring-like assembly. External forces are designed to act on the parts that have been compressed, and not on the cap screw.

The result is a non-intuitive distribution of strain; in this engineering model, as long as the forces acting on the compressed parts do not exceed the clamp load, the cap screw doesn't see any increased load.

http://en.wikipedia.org/wiki/Bolted_joint

I can honestly say, this is one of the couple things I learned in school that I NEVER would've learned in the field.

[Trojan]

Most or all of the jam nut situations are in reference to steel on steel. Not brass on steel. What would you do with brass on brass, aluminium? Direct me to a torque chart that references something other than steel. It just boils down to the fact that you cannot torque the thin nut to the same torque as the thick nut. And most people don't even torque the nuts at all. They just give them all they got and call it tight. That's how they get away with it. Some get lucky. The old shade tree mechanic. As said before, if the props are properly seated you almost don't need the nuts. Most people don't even have a torque wrench and the ones that do, most don't know the correct way to use one. And that can be another issue. LOL.

BILL

[mike]

Here is the data your requested.....

The attached link lists torque specs for SS, Al & Bronze.

Source:http://www.engineershandbook.com/Tables/torque3.htm



"In God we trust; all others bring data!!!"

Cheers, mike

[Trojan]

Neat charts. Nice to have, But there is still no chart for brass to steel or torque for half nuts. Oh well.

BILL

[GaryNW]

I thought I would be able to stay out of this....
My copy of SAE J755- "Marine propeller-shaft ends" shows this standard dates to 1929. I like to be up to date so I have the 1950 revision. The standard has a picture of the small nut first. There is no explaination as to why. There are also no torques in the standard. This does not surprise me, as the standard has no idea what alloys one might use.
My theory is that in 1929, boatyards did not have Snap-On trucks stopping by with the latest cool wrenches. Wouldn't surprise me if most yards cut their own big wrenches from steel plate. So- it is simply more difficult to get on the half nut to apply a really big force (with any wrench, let alone a crude one). To benefit from a really big force "jamming" the two nuts together, you would want to be able to grab that big nut; hence put it on last. I think Trojan's posts capture the realities best.
In my aerospace engineering experience, I have never seen double nuts as a locking scheme. We would have some designs that used two nuts, but it was for FAA required redundancy applications (such as engine or strut pin retention). The problem with locking nuts is that they are generally considered single use. I like the traditional boat prop double nut approach for this very reason. The nuts will last the life of the boat.
Gary

[Boss Lady]

I found this document that in fact shows the small nut first.

https://www.americanboatandyachtcoun...ttees/P-06.pdf

[Pete]

Boss Lady, thanks for the proof to vindicate us small nut first guys. It is tough to be in the minority vote count, when the truth is on your side. What a weight has been lifted from my shoulders. Time will tell if any large nut first voters convert.

Pete

[Boss Lady]

I was convinced that Hatteras had this right with the large nut first. Most boats have the large nut first. The logic is large nut first, but the Society of Automotive Engineers would not have let a mistake perpetuate itself for decades. So the published SAE standard is the correct method for re-useable bronze nuts on a steel shaft.

[Trojan]

Boy! That's a relief off my alleged mind. I rest my case and anything else. Thanks Boss Lady for documented proof. Trojan boats have it correct in there yacht manuals. Maybe It should be placed in the Frequently Ask Service Questions. Just so the wrong NUT doesn't get up tight. LOL.

BILL

[Tony Athens]

Gentlemen,

May I add something here for thought................

For the time being, forget about the little nut all together but do everything else per my politics on the subject. This is how I would look at it..

1) Everything is fitted just right.

2)The BIG nut is installed with "lube" on all--taper, key, threads and nut faces where is pushed hard up againt the prop hub.

3) the BIG nut is brought up to to an applicable torque. Be resonable as you do have things to think about besides just nut tightness.

4) Go have a sandwitch. When that's done, come back and re-torque to spec. Guaranteed you will get some nut rotation...

5) Now, just put some thought into what you have just done.. The prop is installed as right as it's ever going to be, and all you now need to accomplish is to keep that nut in place under any possible scenario..

You can do that is many ways.. Drill and safety wire ( ala air craft or the "Jesus Nut" on the main rotor on a helicopter), use a machined and PROPERLY drilled shaft and Castlated Nut, or you can do what we prefer, and that is, use EPOXY on the thin nut (after cleaning the shaft), and then never have a worry..

Anyway, since somebody did ask for an explanation, this is it with one more caveat. During the prop installing and tightening process, I often see the first nut rotate slighty when tightening the 2nd nut and will then re-check the first nut ( big in this case). The typical wrench that FITS the nut is would be TOO wide to fit if the "little nut" if it was installed first.. So, there it is....Does that meet any written and accepted ay engineering principle? Of course not, but what does in most of the work that I do..

Cheers,

Tony