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Jam nuts :(

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vic syracuse

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I am about ready to start a movement like Nancy Reagan's "just say No to drugs", only mine is going to say "Just say NO to loose jamnuts!"

I thought I had started a thread on this some time ago, but I can't find it right now, so you get to hear me again.

I performed a prebuy on an RV-10 today that has been flying for 8 years. So that's an Airworthiness inspection, and at least 7 Condition Inspections, and amongst the 800 hours, hopefully some preflights.

I found 7 loose jamnuts on the tail surfaces! They had never been tightened, as there was no scoring on the mating surfaces. I just reached in with my fingers and turned them.

So, please go check your jamnuts on your next preflights. This is a high stress area to begin with, with a SB even being issued across most of the models for cracking around the nutplate rivets. Perhaps someone on this forum can set up a poll that says "I have checked my aircraft jamnuts." It would be interesting to see if we could achieve 100% in the next year. :)



Vic
 
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The temptation might be to look at this as an issue of the jam nuts preventing the rod ends from unthreading themselves. You'd see that the angle brackets on the stabilizer prevent the rod end from turning very far, decide that the jam nuts have no particular purpose, and carry on.

However, what the jam nuts actually do is apply clamping pressure that helps transfer bending loads from the shank of the rod end into the control surface of the spar. In the absence of this clamping pressure, all that's resisting the bending loads are a couple of wimpy AN427AD3 rivets, and these rivets react the bending loads in tension. And rivets, especially when there's only one or two of them, are not all that good at reacting tension.

Once those tiny rivets break, the control surface has a lot of play to move out of plane. And that's very bad from a flutter perspective.

20180604_1701561.jpg
 
The temptation might be to look at this as an issue of the jam nuts preventing the rod ends from unthreading themselves. You'd see that the angle brackets on the stabilizer prevent the rod end from turning very far, decide that the jam nuts have no particular purpose, and carry on.

However, what the jam nuts actually do is apply clamping pressure that helps transfer bending loads from the shank of the rod end into the control surface of the spar. In the absence of this clamping pressure, all that's resisting the bending loads are a couple of wimpy AN427AD3 rivets, and these rivets react the bending loads in tension. And rivets, especially when there's only one or two of them, are not all that good at reacting tension.

Once those tiny rivets break, the control surface has a lot of play to move out of plane. And that's very bad from a flutter perspective.

20180604_1701561.jpg

Most excellent drawing to go with explanation. I have sketched many such drawings on my whiteboard to visualize what is really happening.
 
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While I’ve taken these warning to heart and check the jam nuts on every preflight, I appreciate the helpful discussion and drawings of what’s really going on. It’s always good to understand the “why.”
 
The temptation might be to look at this as an issue of the jam nuts preventing the rod ends from unthreading themselves. You'd see that the angle brackets on the stabilizer prevent the rod end from turning very far, decide that the jam nuts have no particular purpose, and carry on.

However, what the jam nuts actually do is apply clamping pressure that helps transfer bending loads from the shank of the rod end into the control surface of the spar. In the absence of this clamping pressure, all that's resisting the bending loads are a couple of wimpy AN427AD3 rivets, and these rivets react the bending loads in tension. And rivets, especially when there's only one or two of them, are not all that good at reacting tension.

Once those tiny rivets break, the control surface has a lot of play to move out of plane. And that's very bad from a flutter perspective.
....

Given that explanation, wouldn't a regular washer under the jam nut be a good idea?
 
Vic, I have found the same thing on several occasions. When I preflight I often check the jam nuts. Not every time, but regularly, and every time on a plane I haven?t flown before.
 
I do a visual on the jam nuts before every flight in my Cherokee. I don't always check them with my fingers though. It looks like it would be a good practice.
 
On my inboard gear door - a high vibration area on takeoff - I found a loose jamb nut which allowed the rod end to partially unscrew. I didn't notice it but my tail-cam video showed that the door wasn't closing all the way. I'll be adding that to my preflight checklist.
 
Loose nuts

Isn't everyone using a torque seal after torquing their fasteners? I would think that it's an excellent way to indicate that the fastener has been torqued and whether it has moved since then. Check and retorque at annual and reseal with a new color.
 
Kitplanes May 2018

For those of you who receive KitPlanes, I wrote a column in the May 2018 issue regarding jamnuts. The column was entitled "The 3 common Discrepancies."

Torque seal can be your friend when it comes to jamnuts. :)

Vic
 
I have a reputation of finding lose jam nuts on almost every airworthiness inspection. Once I arrived to inspect an RV-7 and was told, "I promise you will not find a loose jam nut on this airplane!" Sounds like a challenge.
Toward the end of the inspection, I was asked, "You didn't find a loose jam nut, did you?" "Well, not exactly." "What do you mean, not exactly?" "Take a look at the right aileron pushrod and explain the function of the jam nut." He had tightened the jam nut in the wrong direction.
Moral: Do your best to catch everything, but don't get too cocky about it.
 
Given that explanation, wouldn't a regular washer under the jam nut be a good idea?

Actually, given that these jam nuts seem to be famous for loosening up (and often not being caught by the owner), wouldn't a lock washer under the jam nut be an even better idea?

On the other hand, I think the jam nuts on my tail control surfaces tighten down onto painted surfaces, and I guess a lock washer would bite into and screw up my paint, opening a pathway for possible corrosion - which wouldn't be good. A lock washer on top of a plain washer then? I bought my plane already built, so I may be missing something here(?).
 
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Not ashamed to admit that the jam nuts on all the pushrods was the major entry on my punch list from yesterday morning's airworthiness inspection. Major difference between "I thought they were tight and even finger checked them" and /really/ torquing them down to the pushrods.
 
Mine were torqued (hinge rod end jam nuts) but loose after 100 hours or so. I attributed it to torquing on a top coat painted surface and subsequent cold flow of the paint out of the bearing surface. I installed the rod ends after paint. Re-torqued and all good after 400 more hours. I?m putting thin flat washers under them on the 8 to avoid the nuts bearing directly on the aluminum spars.
 
Experts chime in please

Ok so reading this thread I've now heard both to torque the jam nuts and then to just tighten them. Which is it?

I'm under the camp of gettinga tight snug tightening then applying torque paint to visually verify during walk-around that they haven't moved or loosened over time.

??
 
The only way to really tighten torque tube jams nuts is to have a wrench on both jam nuts at each end of the tube otherwise you can't really get it tight enough and tend to twist the assy. Loctite is certainly an option and can't hurt. Like Vic I find loose ones all the time.
 
OK Vic, Can you please educate me. (You know I was a buyer, and not a builder) Could you maybe make a drawing or take a picture, first from a distance, (so I know where on the plane you're talking about), and then up close, so I know exactly where to look??

Don
 
Here's an RV-8 elevator picture from the plans. I've marked where the hinge is. You can pretty easily see where the rod end is, the nut and the nutplate. Although I'm building an RV-3B, this design is common for all the RVs that I'm familiar with. And besides the elevators, the rudder uses this design, too.

Hinge.jpg


Also, thanks, Bob, for your clear explanation of how and why this design works. It's a good example of one of those subtle details that aren't actually all that obvious.

Dave
RV-3B, skinning the fuselage
 
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OK, Thanks Dave. I will try to figure it out next time at the airport. But per the drawings on post #2, and your schematic here, it looks?? like it's inside the elevator, ie, you couldn't see it. I guess I'm wrong, because you guys say you can see it, I'll just have to look. The "eyeball" shaped screw and the washer against it (I guess it's a washer), is something I most definitely do check on all my pre flights, on my elevators, rudder and ailerons, just to make sure they are there and look right. But I've never actually touched them.
 
Tighten with a wrench

Some (most, probably) are kind of hard to get a torque wrench on them. Most of them are also just 10-32 threads, so just tighten them with a small wrench and then use some torque seal or other indicating fluid.

Vic
 
Some (most, probably) are kind of hard to get a torque wrench on them. Most of them are also just 10-32 threads, so just tighten them with a small wrench and then use some torque seal or other indicating fluid.

Vic

Aren't we talking about the rod end bearings with the jam nuts that act as hinges?

They are 3/8 in my -6A and similar in other models I presume.
 
Perfect

Some (most, probably) are kind of hard to get a torque wrench on them. Most of them are also just 10-32 threads, so just tighten them with a small wrench and then use some torque seal or other indicating fluid.

Vic

10-4 Vic thanks. That's what I've been doing
 
Aren't we talking about the rod end bearings with the jam nuts that act as hinges?

They are 3/8 in my -6A and similar in other models I presume.

Yes, Gil, you are right. Those on the tail that started the thread take a 9/16" wrench. Same with all of the rod end bearings on the long push tubes. Some of the push tubes in the RV's have 1/4" rod end bearings and take a 1/2" wrench on the jam nut.

Still very hard to get a torque wrench on them.

I responded too fast about the 10-32 jam nut because I had found one of those as well and had it in my mind. Sorry for the confusion.

Same principle on all of them---tighten and use torque seal. For those who want, you could use the loctite as well once you are all finished adusting everything.
 
Mine were torqued (hinge rod end jam nuts) but loose after 100 hours or so. I attributed it to torquing on a top coat painted surface and subsequent cold flow of the paint out of the bearing surface. I installed the rod ends after paint. Re-torqued and all good after 400 more hours. I?m putting thin flat washers under them on the 8 to avoid the nuts bearing directly on the aluminum spars.

Exact same thing happened to my -6 and I also attributed it to paint cold-flow.
 
Perhaps...

I can understand that might happen. These ones I am talking about on this particular airplane, and a host of others that I have taken pictures of, were never tightened and there is absolutely no scoring of the paint underneath the nut.



Vic
 
Loose Nuts

This thread got me checking-out my 8A for loose jam nuts...

Elevator jam nuts = tight. OK, very good.

I pulled the floor and checked the elevator push rod = loose nut. Didn't need torque seal to see it backed off 2 threads! :eek:

Thanks, Vic.

Ron B.
 
in general, jam nuts are bad

jam nuts are unreliable as fasteners used for secondary retention. this is because they don't allow for bolt stretch between the fasteners to maintain clamp because they are normally adjacent to the nut they are to retain. no bolt stretch = no clamp. that's why they become loose. the only help is loctite. for control rods, never count on a jam nut to be secure, always verify the rod could not become loose even if it unthreaded to maximum. it should bump and stop at the other end, essentially be trapped.
 
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jam nuts are unreliable as fasteners used for secondary retention. this is because they don't allow for bolt stretch between the fasteners to maintain clamp because they are normally adjacent to the nut they are to retain. no bolt stretch = no clamp. that's why they become loose. the only help is loctite. for control rods, never count on a jam nut to be secure, always verify the rod could not become loose even if it unthreaded to maximum. it should bump and stop at the other end, essentially be trapped.

I have been using torque seal on jam nuts for 25 + years when I finish torquing them. I have never found one loose that had been proper torqued at installation (The torque seal being the reminder that it had been).
 
Thanks for checking!!!

For those of you who are checking the jam nuts, and adding them to your condition inspection, I say thank you! That's the whole purpose of me starting this thread----not to say "look what I found" but to help everyone understand that I know I haven't found every loose jam nut. There are many out there still waiting to be found. :)

That being said, with regards to some comments on jam nuts not being appropriate for some of these functions, I am not an engineer and don't pretend to be one. I can only speak from experience. Having built quite a few airplanes over 40+ years, along with about 4500+ flight hours in those airplanes, I have NEVER found a loose one. Now, I am always hesitant to use the word NEVER, so let's just say "so far."

But, "so far" does seem like a pretty good field test that says if they are properly tightened, they should stay tight. The torque seal is just a quick indicator to be used during inspections. For those who want to use loctite, that's probably OK, but kind of like a belt and suspenders approach.

The bottom line is that I don't think we have an engineering problem to be solved. Just tighten them as they are supposed to be tightened and they will serve their purpose. :)

Vic
 
Great thread. One thing not discussed is control tube length. This is not a factor in the rod ends used in the tail surfaces.
At one time exact accurate measurements were not given in the plans for control tubes. The ideal length of a control tube is one that if both jam nuts were loose the control tube could turn all the way one way and the opposite rod end bearing would not fall out of the tube. Ideally, and I am sure there is a code, you should have a full nut worth of threads still buried in the control rod. Yes the jam nuts are supposed to be tight but this thread has shown that they are not always.
Test this sometime on your airplane, especially during the building phase. Back off both nuts and turn the tube all the way both directions to make sure the tube has at least a safe number of threads in the tube and worse case that it does not fall out!

The elevator control tube system is a place where this could happen. Also wing control systems. Especially if there are more then a few tubes in the system. Should you have one of the tubes with the rod ends too far in other tubes in the system may not have the proper number of threads left should the jam nuts come loose.

Check it out!
 
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...Just tighten them as they are supposed to be tightened and they will serve their purpose...

Not to argue or dogpile this issue, but rather to add some context for new builders as to why we find so many control surface hinge jam nuts loose:

When you're fitting the control surfaces, the freedom of motion of the elevator and rudder is extremely sensitive to how the distance between the rod ends on the surface matches the distance between the hinge brackets on the matching stabilizer. If they mismatch by even 0.015" or so, the control surface can show considerable binding. So there is usually a bit of fiddly work tweaking the brackets or washer thicknesses so that they match perfectly.

However, many builders have discovered that they don't have this problem when the jam nuts for the hinge rod ends are left slightly loose. This allows the rod ends to wiggle around so that they match the spacing of the hinge brackets, so the control surface pivots freely. But when the builder tightens up the jam nuts, the control surface binds. The bad thing that happens next is that the builder assumes that the cause of the problem is tightening the jam nuts, not the fact that they haven't gotten the spacing of the hinge brackets just right.

So they leave the jam nuts loose, or perhaps resolve to tighten them and address the spacing issues later, and then forget to do so.

This is a pet peeve of mine. I happily concede that using rod ends as control surface pivots works just fine for airplanes of the general size and speed range of the RVs. However, I and engineers familiar with the RVs believe that it is not a particularly scalable or forgiving solution, and that you don't have to be very careless with either building or flying to get outside the envelope in which they function.

So use some care about those hinge rod ends. They make construction of the RVs a lot simpler than it might otherwise be, but you have to respect their limitations.

--Bob K.
 
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Given that explanation, wouldn't a regular washer under the jam nut be a good idea?

I don?t believe Gil?s question was addressed in this thread. I agree with Gil, given the purpose of the jam nut, it seems like this situation begs for a flat washer. I could see the reason for not using Loctite. You could even make the argument for adding a lock washer too. Is there a reason, at least a flat washer isn?t called out in the plans?

Although as Vic stated, I?ve never had a problem with my jam nuts despite 11 years and 1500 hours of flying including a lot of hard aerobatics, the idea of a steel nut bearing down on soft aluminum has made me cringe since the day I installed them. This is an area I inspect carefully on every preflight considering the type of flying I do.
 
I am not an engineer, however, a washer, locking or otherwise, has little value in my opinion. Lock washers are an anti-rotation device. Jam nuts can get loose after torquing only if the material has expanded or contracted between them and the bolt or rod end they are securing. If this happens, lock washer or not, the nut will no longer be tightened. The lock washer may offer some additional protection but the joint will no longer be tight regardless and loosening of the assembly may continue.
I am not sure where or when we started calling an AN-316 a “Jam Nut”. They are a “Check Nut”. Symantec’s perhaps.
A Jam Nut is a thin nut put between a regular nut and the part. The jam nut is tightened first, but not to the full torque value, then the regular nut is torqued. When the regular nut is torqued, the threads of the bolt under the jam nut are relieved and the regular nut captures the thin nut jamming it against the part at the designed torqued value. Hence “Jam Nut”. We do not have any joints in an RV that use a jam nut that I am aware of.
Although Check Nuts are called out in AC-43-1b, I could not find any reference to their proper use in that document.
Most importantly, a “Check Nut” should never be confused with a “Nut Check”.
 
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washers

Flat washers DO serve a purpose. They spread the load over a larger area (minimal in this case) and, more importantly, prevent the steel nut from galling the aluminum while tightening...
 
Flat washers DO serve a purpose. They spread the load over a larger area (minimal in this case) and, more importantly, prevent the steel nut from galling the aluminum while tightening...

Minimal, but measurable. The bearing area is increased by about 50%.
 
I don?t disagree, but Vans doesn?t call out a washer nor do any of the airframe manuals I have for the same application. Again, I am not an engineer, so I will yield to those who are.
I am sticking with the plans and doing as Vic suggests; always check the nuts....
 
Just tighten and check....

Keep in mind that there are over 10K Van's aircraft flying, and there doesn't seem to be a problem here that needs engineering. It is a practice across the entire aviation industry as well.

Candidly, if they are tightened properly in the first place, and then checked for tightness after the completion of Phase I or during Phase I, and then at the first Condition Inspection, they probably aren't going to come loose. Yes, there may be an exception here or there as a few have mentioned. But with an average of 10-15 check nuts/jam nuts across each of those 10K airplanes, that's an extremely low percentage of failures.


My intention on this post was to get everyone to go check their airplane, whether you built it or not. IF you find a loose one, and based upon my experience and from some of the replies, you will----then tighten it and don't loose any sleep over it. :)

Vic
 
Keep in mind that there are over 10K Van's aircraft flying, and there doesn't seem to be a problem here that needs engineering. It is a practice across the entire aviation industry as well.

.....

Vic

Vic is the configuration of a jam nut into a plate nut really an industry practice?

Jam nuts on push-pull control rod end bearings and similar parts certainly are, but isn't the rod end control surface hinge application fairly unique to our RVs?

The forces involved on the joint are quite different as the sketch in post #2 of this thread shows.
 
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Hmmm...

I don't remember where else I saw them, but will try to find it. However, even with an average of 6 on each of the 10K airplanes flying, that is a really big number. If there was a problem to be solved I honestly think it would have been noticed by now. Van's has done a pretty good job of issuing SB's where there has been cracking or other wear items.

I've got 1500 hours on my current RV-10 and they have never come loose. I know we have quite a few who do A LOT of aerobatics, consistently, so you would think we would see some cracking or other problems arise.

Just my .02. Not that of Van's. :)

Vic
 
Adding a large are washer would distribute the bending load of the rod end over a wider area but suggesting it is needed should require us to make lots of other changes because they would make localized areas stronger or less susceptible to fatigue.
The question that should be asked is "Is there in service evidence that indicates that if the parts are assembled correctly, that there are no problems?"

I think there is.
 
Adding a large are washer would distribute the bending load of the rod end over a wider area but suggesting it is needed should require us to make lots of other changes because they would make localized areas stronger or less susceptible to fatigue.
The question that should be asked is "Is there in service evidence that indicates that if the parts are assembled correctly, that there are no problems?"

I think there is.

But aren't those exact hinge assemblies, and maybe poor construction, the exact reason for this SB?

https://www.vansaircraft.com/pdf/service_bulletins/sb14-02-05.pdf

It certainly looks like this fix is spreading the loads over a larger area.

My suggestion was actually for a standard AN960 washer to get 50% more bearing area over a straight nut.
 
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