Double nutting? 1

[dhlinva]

I've never seen anything that I would consider official (pro or con) on the subject of using double nuts, but I've never been a fan for some reason. It just doesn't seem like a proper engineering approach to me; however, I wouldn't mind being able to make an educated decision. I'm not really sure the exact problem double nuts are intended to solve, though it seems to be suggested when people want a higher degree of confidence that a fastener is not going to back off. Does it really accomplish anything? If anybody can point me in the direction of where I can read up on this in order to speak more intelligently on the subject, I'd sure appreciate it.

 

[dimjim]

I think this was explored on this site before under
jam nuts. You might want to search on jam nuts.
I am rather certain that the conclusion was that they
do very little good. But check it out.

 

[CoryPad]

The exact problem a jam nut is intended to solve is rotation of the first nut, which could lead to joint failure. They do work if done properly (for example, automobile rear suspension lateral links or automobile steering tie rods), but these are "adjustment" joints, not "preload" joints.

You can find more information in Handbook of Bolts and Bolted Joints edited by Bickford and Nassar, and NASA RP1228 Fastener Design Manual.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[unclesyd]

Here is an explanation on how a jam nut works from an article by Dr. J. B. Calvert.

http://www.du.edu/~jcalvert/tech/jamnut.htm

 

[dimjim]

To quote NASA 1228 catalogue page 9.

Jam nuts. These nuts are normally "jammed" together
as shown in figure 7, although the "experts" cannot agree
on which nut should be on the bottom. However, this type
of assembly is too unpredictable to be reliable. If the innernut is torqued tighter than the outer nut, the inner nut will yield before the outer nut can pick up its full load. On the other hand hand, if the outer nut is tightened more than the inner nut, the inner nut unloads. Then the outer nut will yield before the inner nut can pick up its full load. It would be rare to fet the correcto amount of torque on each nut. A lock nut is a much more practical choice than a regular nut and a jam nut. However, a juam nut can be uses on a turnbuckle, where it does not carry any of the tension load.

Figure 7 shows the jam nut next to the mounting surface with the thicker nut on top.

Not certain if this applies fully to double nutting.
I know they use double nuts in many structural applications
where the loads are basically static. I see them all the time on monopole designs.

 

[dhlinva]

That the use of double nuts is intended to prevent rotation of the fastener seems rather obvious. That should be a concern with any threaded fastener. Why jam nuts in particular would be chosen over the more widely used friction or chemical type locking devices is perhaps the better question. The intended application in this case is for use with pipe hangers in tanks containing diesel oil that will be inaccessible for the most part. I would have expected to see the use of nyloc nuts. If there had been a particular concern regarding loosening, then I probqably would have recommended Loctite. These joints are never going to come apart for any type of normal maintenance. I just never see jam nuts used so I'm trying to figure out why someone would choose this method over the others I mentioned. It seems a questionable solution at best.

 

[CoryPad]

Agreed

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[strokersix]

Well, I have one application where double nutting seems to work quite well. The exhaust systems I build for my hobby autos I use 3-hole flanged connections with high temp gasket material between. Nyloc and Loctite locking don't work due to high temps. Deformed thread locking methods gall the threads. The double nut method is the best solution I've found.

 

[mullins]

Jam nuts are cheaper than nylocs.

The only cases where I have had to use double nutted bolts is when they are going to subjected to intense vibration loads for an extended amount of time. For this use, I was able to use special bolts where they had UNC Right hand threads and UNF Left hand threads. This allowed for everything to be cinched tight.

 

[KENAT]

The first place I ever remember seeing 'Double nutting' was on pillar drills in my schools work shop.

They were used to set the 'plunge' height of the drill.

I used to have a list of approved fastening techniques for on UK military A/C, but can’t remember if jam nuts were on there.

I’ve certainly used them at home in situations where there is rotation or vibration(when I didn't have a nyloc or similar to hand), how well they work seems to depend on how well I manage to torque them against each other!

The thing I have used at work and seen used more is where you have a ‘screw’ that adjusts up and down in a threaded hole and then has a locking ring to set whatever displacement you come up with. I’ve used it on items subject to rotation with some success. I’ve seen it used on things like flow valves too. It’s the same basic principle but instead of 2 nuts you have a threaded hole and a nut.

 

[dhlinva]

I've seen them used in situations where there is an adjustment to be made. That's not the situation here.

If I was at home, if I was working on my car, or if I was in the middle of the ocean and needed to lock a fastener and had no other choice available, I'd use two nuts. In a heartbeat. I know will work to a certain extent. I'm trying to determine the engineering behind the use of jam nuts.

I'm not sure what help anecdotal evidence would be if I was trying to present an argument for or against. I was really looking for reasons why a jam nut would be chosen and the mechanism by which it works. If I know those two things, I'll understand when and why it should be used.

I do appreciate the response. It certainly doesn't seem like there's any consensus, but at least I know that generally, I'm not alone in my skepticism.

 

[KENAT]

You say your application is in a diesel tank.

For Nyloc, or for that matter thread lock, is there any compatibility issues between them and the diesel or other chemicals present?

My materials/chemistry is a bit rusty but I'd want to be sure that the diesel or any contaminants or solvents used when cleaning the tanks etc, wouldn't degrade the nyloc/threadlock.

If compatibility is an issue you could still look at all metal 'stiff nuts' or even something like wire locking or castellated nuts & split pin etc instead of jam nuts.

The link unclesyd gives explains how they work, essentially they 'pre-load' each other so they are less likely to become loose from vibration.

Thinking about it variations on the theme were used on aircraft stores as I recall. They were used on fuze pockets and the like I think. They were more like locking rings than actual nuts but used the same principle. However it was several years ago I was looking at such things and I can’t recall for sure.

http://www.machinedesign.com/BDE/FASTENING/bdefj3/bdefj3_1.html

thread311-164764

thread725-167507

http://en.wikipedia.org/wiki/Castellated_nut#_note-4

Several of them reference the same link as unclesyd.

 

[unclesyd]

We have a lot of gyrating and vibrating screeners for our polymer process that run 24/7/365. A good number of them have overhead support and all the rod suspension system uses a jam nut or double nut (tacked). On other one all the support hardware is double nutted.

Our extensive monorail system has all the support rod double nutted.

 

[dhlinva]

I don't think compatibility is an issue. If the non-metallic insert could not withstand immersion in diesel oil, there are metallic versions available. While I haven't checked specifically for this application, Loctite has thread locking compounds for use in hydraulic systems. I don't think incompatibilty with petroleum products is an issue.

If I asked how to determine the proper torque on a bolt, would anybody think an example of how a bolt was torqued would be a good answer? Or that providing an example of a situation in which a bolt required torqueing would be of any help? None of that presents a compelling argument for their use. I understand they have worked or were used in this or that situation. That's not what I would consider engineering rationale for use anywhere else. Isn't the point of engineering to determine what's happening and why so that those lessons can be applied to other situations totally unrelated? That's always been my approach.

I have yet to see two references that are in agreement. There appears to be disagreement on the torqueing sequence and even the order of the nuts. I would like to see the "Handbook of Bolts and Bolted Joints" but I don't have access to it. I was hoping to find what I needed here, but I don't believe I'm getting anywhere.

I really appreciate all the well intentioned input. Thanks guys.

 

[Dinosaur]

My understanding is that in applications where the "structure" will undergo cyclic loading, double nutting especially with a jam nut and a structural nut are used when threaded fasteners other than high-strength bolts are used to insure the nut does not back off after installation. When using high-strength fasteners including ASTM A325 and A490 bolts, the bolts are installed by turning the nut beyond a snug tight condition producing a tension in the bolt material. This is the condition described in the AISC Manual of Steel Construction as a Type F fastener. This tension load in the fastener creates a friction effect at the nut which should hold it in place. However, for a variety of reasons, other threaded fasteners are often installed with two nuts, one of them being structural. The second nut is turned against the first to jam them together. This forces one nut against one side of its threads and the other nut against the opposite side. This locks the nuts together so they resist turning and keeps the nut from backing off under cyclic loads. I see this most often in mounting hardware for machinery but it is often used elsewhere. If you have the time, I commend the original cited article on using jam nuts. In that you will learn that the jam nut should placed against the material and the structural nut placed against the jam nut. I occasionally specify two structural nuts so I avoid the possibility of having the wrong nut placed against the material. Sorry for the long post.

 

[unclesyd]

Here is an article that echoes my first reference.

http://www.epcserver.com/Structural/Analysis/connection/info_jamnut.asp

I talked with my neighbor, ex-NASA employee and he stated that NASA did use quite a lot of jam nuts on ground applications. He mentioned one reason that they might not be considered for launch vehicles was the weight consideration.

 

[dhlinva]

I looked at the link from the last post and I understand now what is supposed to be happening. I certainly appreciate the help. The only thing I am left to wonder now, is how would the torque of the jam nut and then of the full size nut be determined? And lastly, how would you ever be able to tell what the overall torque on your bolt was?

 

[unclesyd]

I've always used the calculated torque required if was a single nut bolted assembly. If the called out torque is 100 ft lbs then tha is the torque used on the full nut. I have seen a torque value for a jam nut only a very few times as the normal call out is snug tight, as used in structural bolting.

 

[dimjim]

If you watch the vibration test of the Junker test rig
for the different types of locking methods, the double
nut bolts did better than the others but soon vibrated
loose. It was in the Nordlock Washer Video sited earlier.
I assume this was not all a Sales pitch by Nordlock.