Which Goes On First: Jam Nut or Full Nut?

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Which Goes On First: Jam Nut or Full Nut?

OK.

The description is correct.

Some of the shop guys have tried this for years. I don't know why they keep trying. I don't know why I keep laughing. They just joke about it.

Most of them are convinced it is impossible to put the second nut on a bolt before the first.

Yes there are two-part nuts but that's cheating.

If I do it with modern components, The load carrying nut gets tightened properly, then a selflocking nut tightened enough to prevent loosening of the main lug. In high reliability situation (airplanes controls) the castellated crowns of the locking nut + holes drilled thru the non-tensioned shaft allows safety wiring, in a true belt and suspender fashion.

For a repair of old fashioned locknuts I would use Loctite, as a bare minimum.

There is a really, simple fix that always works and any idiot will always get it right:-

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ALWAYS USE TWO IDENTICAL NORMAL NUTS!!!

Its always to hand and costs usually in small quantities less than small nuts.

Remember big nuts are always best!!!!

Put a lockwasher between the nuts to keep them jammed.

I was on the fence until I read your response. A very well reasoned answer. Thank you.

I didn't come back here after posting so this is catching up.

Your answer is interesting and logical.

The difference in our answers is context. My answer is framed by my background in structural engineering, static connections. The only time we really use these is when we need a slip connection, as in a dummy bracing required by the Architect for appearance, and then only when QC will not let us "upset" the threads. In this case the only force developed is when nuts are jammed together.

You are correct. If the jam nut goes on first it will increase the friction when torquing the main nut and thus reduce the pressure on the prop. You would have no idea how much pressure is on the prop. And, you are correct, you always use two wrenches to lock the nuts together. This is the main mistake that the jam nut first people make.

Hi

Bandership gave a very good description of along debated topic, here is a good site for many related thread issues but describes exactly this problem in depth (good little video as well). Pull this one out for apprentices and newbes

www.boltscience.com/pages/twonuts.htm

I hadn't considered the application of jam nuts on prop shafts where changing centrifugal forces play a large role. Without giving it a great deal of thought, I suppose that this is one application where jam nuts can actually be effective. However, in typical mechanical joints where a fastener is tightened to apply a clamp load on members, these things are vestiges from the early days of the industrial revolution where brute force beating'n'bashing was employed in attempt to keep things from falling down or blowing apart. Alas, this wasn't always successful, was it?!

Yes, through the use of a second nut we can make sure that the first doesn't back off. However, there is something even more fundamental that many seem to be forgetting: fatigue failure.

Let's consider what happens when a jamb nut is applied:

• the threads of the thinner nut deform thus increasing the friction on the mating thread flanks of the nut and the bolt
• faying surface friction (only between the two nuts, not between the bottom nut and the flange!) increases

Certainly both of the above conditions may help to prevent the two nuts from separating yet this may have no positive effect at all on the joint. This is particularly true if the first nut (either full or thin) is only tightened to a certain percentage of the necessary load.

The second nut doesn't apply any additional load to the joint. Its effective grip length is only the thread pitch divided by two; the grip length of the joint doesn't even come in to play. As a result, the joint remains loose. If the system is dynamic with cyclic loads in excess of this insufficient fastener preload, the fastener will eventually fail in fatigue. For those who've been around nuts'n'bolts, think of the broken fasteners that you've come across where the jamb-nut combination is still tightly mated even though the fastener is no longer attached to anything!

The videos are flawed, as is the conclusion. How are they flawed, you ask? Well, in the first video, the full nut is tightened with a socket wrench, then the jam nut is tightened on top of it, without the full nut being held by a wrench. This does not create the same kind of jamming effect as is found in the second video, where both nuts are torqued against each other with wrenches. It's quite obvious that much more force is being used in the second video to secure the nuts together. If the author wanted to do an honest study, then the bottom nut should be tightened to a specific torque, then held with a wrench while the top nut is also tightened to a specific torque.

I work in the envelope manufacturing business. Every envelope machine I've ever worked on has always used jam nuts to keep the full nut from moving and hence keep the adjustment setting in place. This assembly is used to set pressure on rollers mounted on shafts. There's quite a bit of vibration, but not a great deal of force. The manufacturer has always put the full nut on first, and the jam nut second. We make the adjustment, then lock the jam nut to the full nut. It's easier to use regular thickness wrenches to lock the nuts together this way. Otherwise you would need a special thin wrench to hold the thinner jam nut if it was on the bottom.

This link backs up what Bandership said:

http://sharpesurveying.com/Propeller_Nut_Installation.html

Perhaps there are different answers for different applications.

I would argue that in the case of the propeller, if the jam nut is put on last, the jam nut should not be tightened to a higher force as the main nut, and thus take all the load from it. Example... main nut torqued to 55ft-lbs, jam nut torqued against the main nut while it is being held with another wrench, to 20ft-lbs. This should not, in my humble opinion, take all the load off the main nut, and the propeller is still being held on with the original amount of force. The jam nut, by virtue of being jammed against the main nut, will be taking some kind of load anyway, regardless of it's position on the shaft. At least in this example all the force is being directed towards the propeller. Now if you torque the main nut on first, to 55ft-lbs, then take it off and put the jam nut on and torque it to 20ft-lbs, then replace the main nut and torque it back to 55ft-lbs, won't the jam nut absorb some of that force from the main nut? Wouldn't you have to torque the main nut to say 75ft-lbs to create the 20ft-lbs of jamming torque and end up with the original amount of force on the propeller? Please follow my line of thinking, and not nit-pick my terminology or specifics. I'm sure I'm missing something, but jam nut first just seems wrong.

Another thing that may be coming into play here is for holding boat propellers on, the full nut is sometimes castellated, therefor making it neccessary to put the jam nut on first. Due to habit, non-castellated nuts may be put on last, but there is really no reason for that. ABYC standard P-6 does NOT specify that jam nut first is the ONLY way to mount the prop. I can't get a hold of SAE j755, but I'll bet it doesn't specify that the jam nut MUST go on first.

There is no need in fact is counterproductive to use a jam nut in a castelated fastener. The pin or wire properly installed provides the security required. A boat prop only has to stay on, to much tightness on the nut may cause bearing and seal failure. As was explained to me by an old hardware engineer, the larger nut, with or without a flat washer depending on application, is torqued to specs. This nut is held with a wrench while the jam nut is torqued to its specs. The holding power was due to deformation of the mating threads of the nuts and shaft. Mostly in the nuts, which is why hardware should not be reused. Anyhow, that is what I was tought, seemed to work. The important part is torquing to a specific value. All she 'll take is not a value.

You are absolutely right, and there is no reason to put the jam nut on first. However, someone is doing it, and I tried to come up with a reason for it. Can anyone else come up with a logical reason for it beyond the full nut being castellated?

The videos about the jam nuts are flawed. Why give me a "off topic" for being right about them? Only a child would be fooled by them.

Read some of the earlier posts, and you will see the reason to put the jam nut first. I don't know if the video explains it well. The castellated nut is irrelevant.

None of the earlier posts explained it at all and the videos are a complete joke. How any intelligent person can fall for those I'll never understand.

Look at this catalog page from Marine Hardware, Inc.:

https://docs.google.com/viewer?url=http://www.marinehardware.com/downloads/catalog.pdf&chrome=true

Page 5 shows what appears to be the jam nut going on first, but page 7 shows the shaft assembled with the jam nut going on after the full nut. If you want to say I'm wrong, then they are wrong, too. But I doubt it. They do this for a living.

The OP and post 9 both explained it quite straightforwardly. What did you not consider explanatory about them? Did you not understand the comments about loading?

Quote from post #9, also relates to OP's post:

"If the jamb is tightened up enough it will push against the full nut so hard that the full nut is no longer in contact with the threads at all, in which case the jamb nut will be carrying all the load and doing all the pushing against the threads."

Who in their right mind is going to do this? As I said before, you would torque the full nut to specs, which some manufacturers have put at 55ft-lbs. Then you add the jam nut, and while holding the full nut with a wrench, tighten the jam nut against it to around 20ft-lbs. The load on the full nut has not transfered to the jam nut, and any thought that it did is crazy. The full nut is still applying it's initial force against the propeller, and the jam nut is locking it in place. Removing the full nut to install the jam nut will remove that initial force the full nut put on the propeller, and the jam nut will not re-apply it. Now if you tighten the full nut on after the jam nut, and don't hold the jam nut with a wrench, you stand a chance of them both turning and applying the initial force back on the propeller. I personally would think the jam nut would interfere with the full nut re-applying the load, to some extent, but that could work. But if you don't lock the two nuts together with two wrenches, the full nut will come loose. The flawed videos in the earlier post shows what happens when you use one wrench vs two wrenches to tighten two nuts together, not what happens when you swap the positions of the jam nut with the full nut.

Might as well add another comment lest somebody who isn't quite as bolting-astute as some of the posters to this thread appear to be, misinterprets.

Attaching a nut onto a propshaft is quite a different animal when compared to a typical flanged mechanical joint.

In the former, the integrity of the prop-to-shaft mating has the benefit of the fact that the propellor is continually being driven up onto the shaft during forward thrust. Conceivably, one wouldn't even need a single nut aft of the prop if the vessel never engages in reverse!

In the latter application, jamb nuts are often as useless as wheels on a boat (other than the one on the bridge). Furthermore, their use often contributes to joint failure. Particularly if the first nut, either thin or thick, doesn't apply the proper preload required by the joint (with so much talk here of "torquing" the nut to the "right torque", it's quite likely that such misinformation would certainly lead to such a situation). The second nut would only bind itself to the first through face friction and thus would not contribute anything to the bolt/stud preload. As mentioned earlier, if the process is dynamic, the ensuing cyclic loads would eventually result in the fatigue failure of the fastener.

Fortunately, most of the applications using double nuts, are simple ones where the process is fairly static (for example, light standards). Hence, the extra nut is even superfluous. When one sees such an arrangement on process equipment, it's a fair bet that the designer didn't really understand bolted joint design and decided to cover his buttocks with what misguided ancient 'wisdom' suggested might work. Alas, the reliability of bolted joints doesn't depend on adding extra nuts, wires, locks or compounds to the interface but rather on ensuring that sufficient preload is applied in the first place.

You mentioned it and others have mentioned it before, bur why would turning a prop in reverse undo the nut??????

All the props that I have seen are dogged/keyed to the prop shaft in some manner (I forget the technical term, its a long time ago!), either a single one or the shaft has been pre-machined with several slots as on some inboard/outboard drives.....the prop also of course!

All the nut (usually held in position with a pin or wire or similar) has to do is to hold the prop in position......

In reverse, (or forward) there is little or no turning motion applied to either the shaft (forward) or the nut (in reverse), the pressure is linear along the length of the shaft...

In fact, in all the propellers I have looked at, from the rear, they turn anticlockwise for forward, the direction that might theoretically anyway, turn the nut, unless its a LH thread.....going in reverse will only tighten the nut!!!!! If anything.

See here:-

You can also clearly see the slot which allows the shaft key to engage and prevent turning.....

I expect some props are made for the reverse direction as well, but I could not find any pictures quickly.....

The propeller shaft, where the propeller fits, is tapered and keyed. If the propeller is always driving the boat forward it is pushed forward against the taper and does not need anything to hold it on.

If the propeller is stopped and the boat is slowing down, or being driven astern (backing up), the forces involved will tend to pull the propeller off the shaft, thus requiring some arrangement to keep the propeller on the shaft.

Boats' propellers most commonly turn clockwise as viewed from astern on single screw boats, but not always. Twin screw boats' propellers almost always turn in opposite directions.

All the above assumes inboard engines. Outboards and outdrives are another story.

That still does not answer as to why a prop with a keyway and taper can turn a nut......

The pressure is linear along the length of the shaft, its not a turning motion....as long as the nut/shaft threads are strong enough not to shear......nothing nasty should happen.....

Now if the keyways are loosely made..........

I was taught to make such keyways on equipment a light interference fit. A puller would be needed to pull off such a pulley/prop.....

BoltIntegrity states: "Alas, the reliability of bolted joints doesn't depend on adding extra nuts, wires, locks or compounds to the interface but rather on ensuring that sufficient preload is applied in the first place."

Folks who fly aircraft (and spacecraft) would disagree strongly as they routinely safety-wire all threaded parts on their engines and make liberal use of shake-proof nuts (distorted or nylon insert types) as well as safety-wire elsewhere. Indeed, it is difficult to find fasteners anywhere on an aircraft that rely only on "sufficient preload" to stay secure.

Arrangements that must withstand widely varying forces require more complex attention.

Personally, my biggest problem of fasteners loosening up was on a pair of propeller shaft couplings on my boat. Between the output of my main engine transmission and the input of my pitch control mechanism is a 18" steel shaft. At each end of the shaft is a flexible coupling connecting the shaft to the transmission and the pitch control. Each end of the shaft slides into the inner part of each coupling with a square key and two set screws in each. The set screws were invariably loosening up with use. I tried locking compounds, using two set screws - one short one in first with another on top of it (much like full nut/jamb nut situation) - and drilled-for-safety-wire square head set screws safety wired properly. The only thing that worked is to make a loop of safety wire through a set screw head, and another loop of safety wire around the 2nd set screw. The loop on the head of the set screw is wrapped around the screw clockwise and then connected to the other loop with a strong spring so it is pulling hard at keeping the set screw tightened. This is repeated for all four set screws. This is the only arrangement I have tried that has kept the set screws tight.

LocTite, if used on a properly cleaned thread (both the casting and the grub screw/bolt), then priming with LocTite primer before glueing, will hold really well.

I'm just trying to grasp the argument for putting the jam nut on first. So you put the jam nut on first, and torque it to specs. Then you put the full nut on. Now you have one of two choices. You either torque the full nut higher or lower than the jam nut. Everyone who supports this procedure would torque it higher so it jams against the jam nut and takes the load from it. But what has happened to the jam nut? Doesn't it unload it's threads and become nothing more than a threaded washer? If you torque the full nut enough to load the threads of the jam nut the opposite direction, then the threads of the full nut is taking on the main load and the load from the jam nut in the same direction at the same time. Plus you've increased the stress you've put on the propeller by the amount you are locking the full nut to the jam nut. If you torque the full nut lower than the jam nut, then the jam nut is still carrying the load, which is what this procedure is supposed to be preventing, according to it's supporters.

Now my argument for putting the full nut first allows you to get your torque reading with the full nut, then tighten the jam nut against the full nut at a lower torque level. This does not unload the full nut's threads completely, and it does not put any more force on the propeller than you originally had. Furthermore, you have spread the load of the force against the propeller to the threads of the full nut and the jam nut, making for a assembly that could handle more force without the threads severely distorting or failing. Unless the jam nut is made of a softer metal than the full nut, the argument for the full nut needing to take 100% of the load isn't valid.

GA

I like your style!!!!

"Bandership" responding to "Envelope Guy"

EG - I'm just trying to grasp the argument for putting the jam nut on first. So you put the jam nut on first, and torque it to specs. Then you put the full nut on. Now you have one of two choices. You either torque the full nut higher or lower than the jam nut. Everyone who supports this procedure would torque it higher so it jams against the jam nut and takes the load from it. But what has happened to the jam nut? Doesn't it unload it's threads and become nothing more than a threaded washer?

BS - A washer would be free to move laterally along the threaded part of the shaft but the jamb nut cannot. The concept of the jamb nut is to prevent the full nut from becoming loose if the main load (the propeller) drops to less than zero by keeping the both nuts pressing against each other and the threads (in the opposite directions) insuring interference fits to keep them from moving.

The reason the full nut is on last is because a shock load will most likely be pushing in the aft direction so the aft nut should be the biggest to be more likely to absorb the shock without distorting its threads and loosening up.

EG - If you torque the full nut enough to load the threads of the jam nut the opposite direction, then the threads of the full nut is taking on the main load and the load from the jam nut in the same direction at the same time. Plus you've increased the stress you've put on the propeller by the amount you are locking the full nut to the jam nut.

BS - Yes to all the above.

EG - If you torque the full nut lower than the jam nut, then the jam nut is still carrying the load, which is what this procedure is supposed to be preventing, according to it's supporters.

BS - True, but in actuality the jamb nut is snugged and the full nut is tightened hard.

EG - Now my argument for putting the full nut first allows you to get your torque reading with the full nut, then tighten the jam nut against the full nut at a lower torque level. This does not unload the full nut's threads completely, and it does not put any more force on the propeller than you originally had. Furthermore, you have spread the load of the force against the propeller to the threads of the full nut and the jam nut, making for a assembly that could handle more force without the threads severely distorting or failing. Unless the jam nut is made of a softer metal than the full nut, the argument for the full nut needing to take 100% of the load isn't valid.

BS - You have an interesting idea here about spreading a possible shock load across both nuts, but unless the first-nut-on is pushed by the second-nut-on away from its original thread contact and up against the opposite threads the two nuts will not maintain the interference fit needed to keep them in place if the main load goes to zero - the whole reason for using two nuts. The nuts must continuously push against each other to be stable, much like two sumo wrestlers.

There are much more reliable ways of securing the propeller to the shaft other than the jamb nut, such as a castle nut/cotter pin and tack-welded keeper. Even using two full nuts will make the discussion moot. In fact, in my own situation, the main use I have for jamb nuts is to reduce how far a nutted fastener protrudes from the workpiece. As for my own propeller, it is not held on by nuts at all. See: hundestedpropeller.dk

I am about out of fuel for this discussion so I'll start another one.

Which weighs more, a pound of gold or a pound of feathers?

Hint: it's the feathers, but why?

Hi all

Now we are getting into why I throw this question to apprentices the arguments can sort of add up in both directions, but one query I have is that most jam nuts wont take full tension so putting them on first is very hard to achieve correct tensioning, which is often critical not just do they come loose. Two full nuts are not the same as you are relying on the extra deformation in the jam nut to lock them. All in all a good debate is occuring and more people will be quering the "standard practices"

Displacement.

Yep, you are right. If the full nut loses it's load against the propeller, the jam nut releases, while having the jam nut first creates problems with applying the correct amount of pressure against the propeller and keeping it at what it should be. So the best answer is not to use a jam nut, but to lock the full nut with some other means, as other posters have said.

But what if you use a lockwasher between the two nuts...

To check out this theory, and to prove you were correct in what you said earlier, I did an experiment. I threaded a single nut onto a piece of 3/8 threaded rod. I then inserted the rod through a piece of metal and tightened a full nut against it with a lot of force. I then tightened a jam nut against it, but not with as much force, and held the full nut with a wrench. When I released the single nut I had first put on the threaded rod, the full nut/jam nut combination came loose. I then reassembled it, but used a lockwasher between the jam nut and the full nut. When I released the single nut, the full nut/jam nut stayed together, and actually required two wrenches to separate them. Hmmm...