How to Choose Double Head Screw Nut?

Many types of old machinery have two nuts on the bolts. A thin nut is frequently used in these applications. Sometimes the thin nut can be observed below the standard thickness nut and on other installations, it’s on top. Although it may seem counter-intuitive, the thin nut should go next to the joint and not be put on last. In other applications, for example on column attachments, two standard thickness nuts are frequently used.

In this article the effectiveness of this locking method is investigated and the tightening procedure that should be used if effective locking is to be achieved.

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The use of two plain nuts goes back at least 150 years based upon observation of historic machinery. Tightening one nut down and then simply tightening another nut on top of it achieves little locking effect. A specific procedure needs to be followed if locking is to be achieved. When a thin and thick nut are used, it may be thought that the thick nut should go next to the joint since this would take the entire load. However, by placing the thin nut on first, when the thick nut is tightened on top of it, the load on the threads of the thin nut are relieved of their load.

The thin nut should be placed on the bolt first. This nut is typically tightened to between 25% to 50% of the overall tightening torque. The second (thick) nut is then placed on the bolt and the thin nut held to prevent rotation by a spanner whilst the thick nut is tightened to the full torque value. The series of diagrams show the effect that the procedure has on forces present between the nuts and in the bolt.

When the thick nut is tightened onto the thin nut, as the load increases, the load is lifted from the pressure flanks of the thin nut. As tightening continues a point is reached when the bolt thread touches the top flanks of the thin nut. At this point F3 = F2. Continuing to tighten the top nut results in the jamming of the threads leading to F3 > F2. If tightening is continued, the force between the two nuts will continue to increase. If the thick nut is overtightened, there is the risk of thread stripping or the tensile fracture of the bolt between the two nuts.

The reason why the two nut system is effective in resisting self loosening is due to the way the threads are jammed together (hence the term jam nut being frequently used for the thin nut). Since the bolt thread is in contact with the top flank of the small nut and the bottom flank of the top nut, relative thread movement is not possible. For self-loosening to occur, relative movement between the bolt and nut threads must occur. It is this jamming action that is the secret of the two-nut method.

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In order to achieve the appropriate bolt preload prior to the threads jamming it is necessary to tighten the smaller nut. The greater the grip length of the joint, the greater is the extension needed to achieve a given preload and hence the higher the initial load that must be sustained by the small nut. Although the axial backlash can be calculated for given tolerance conditions of the nut and bolt threads, there can be a factor of 10 difference between the minimum and maximum values. Such variation makes it difficult to establish the correct preloading of the small nut. As a result, the bottom nut is tightened to a simple percentage (i.e. 25% to 50% of the overall torque value). Two full height nuts can be used if the principles that have been outlined above are followed. Small (jam nuts) are frequently used since there is no need to have a full height nut on the bottom since the threads do not carry the load. An advantage of a thin nut in this application is that a greater amount of axial backlash will be provided for a given tolerance class.

On occasions, two thin, or jam nuts used together can be observed, one placed on top of the other. If the nuts are tightened to any extent, this is a poor and undesireable practice. It may be believed that the strength of two thin nuts used in this way is greater than that of a single normal nut. This is true if the two thin nuts are on top of each other and a proof load test is completed on both simultaneously, but not true if the nuts are tightened, which is the norm. What can happen is that when the first nut is tightened, some partial thread stripping can occur, often of the bolt due to the small length of engagement that the thin nut provides. This may not be readily detectable at the time of assembly. The second nut is then tightened on top of the first. Because of the partial stripping that's occurred, plastic deformation in the threads occurs after the tightening is completed. This results in a loss of preload and the joint potentially coming loose. If two nuts are desired to be used, for whatever reason, use of two normal height nuts would avoid the thread stripping issue.

The two videos shown below presents the results of a Junker fastener vibration test performed on the two arrangements that a thick and thin nut on can be arranged. The tests were conducted to investigate the effectiveness of the two-nut method in terms of resistance to self-loosening. A Junker transverse vibration test machine was used with M10 nuts and bolts. The results are illustrated in the graph below. With the small nut on top, both nuts can be observed to rotate together and can subsequently come completely loose. The results are slightly better than is normally observed with a single plain nut. With the small nut next to the joint, some relaxation occurs but not a significant amount of self-loosening . The performance of the two-nut method, when properly applied, provides a superior locking capability when compared to many so-called lock nuts. The proper application of the two-nut method is time intensive and requires a degree of skill and is hence unlikely to make a major comeback on new machinery any time soon.

The first video shows a test in which the normal height nut is tightened onto the joint first followed by the thin nut being tightened on top.

In this second test shown below, the thin nut was placed next to the joint and the normal height nut tightened on top.

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Is there any advantage to two nuts?

Seriously, I know that two nuts is a bad way to try to lock something in place. I’ll use locktite.

I’m specifically wondering where threaded rod fails. Does the rod fail randomly in its length or does the nut fail?

The application is 5/8 threaded rod with heavy hex nuts used to tie rafters to the foundation. With a straight pull, where does rod fail? Im sure the engineers have worked this out. And thought you guys may have run across it.

Thanks

Is there any advantage to two nuts?

Seriously, I know that two nuts is a bad way to try to lock something in place. I’ll use locktite.

I’m specifically wondering where threaded rod fails. Does the rod fail randomly in its length or does the nut fail?

The application is 5/8 threaded rod with heavy hex nuts used to tie rafters to the foundation. With a straight pull, where does rod fail? Im sure the engineers have worked this out. And thought you guys may have run across it.

Thanks

Adjustability. Many double nut applications on industrial equipment need adjusted from time to time especially, loctite would be a really poor choice in those situations.

Aircraft commonly use double nuts on push pull tubes for flight controls for the same reason

There is nothing inherently wrong with using double nuts anywhere, it’s just cost more and takes up more space. They are better than lock washer, especially the spring split ones

Is there any advantage to two nuts?

Seriously, I know that two nuts is a bad way to try to lock something in place. I’ll use locktite.

I’m specifically wondering where threaded rod fails. Does the rod fail randomly in its length or does the nut fail?

The application is 5/8 threaded rod with heavy hex nuts used to tie rafters to the foundation. With a straight pull, where does rod fail? Im sure the engineers have worked this out. And thought you guys may have run across it.

Thanks

Two nuts create a mechanical lock vs. a polymer chemical lock with Loctite for fasteners. You also can use the locking nut to drive the threaded rod in or the locked nut to back the threaded rod out. If using Loctite, you would have to destroy the bond then reapply. If vibration was a concern, then the use of two nuts plus Loctite would be a better solution than using either alone. If you really want to be certain, then you can employ aviation safety wire in the mix. No advantage in that application that I can think of.

The only time I specify double nuts is in structural steel construction. Sometimes you want to attach a post to a structure on the ground level but don't want the post to carry unintended load from above (if second floor beams deflect, they will induce load into the post.) So to avoid that, the connection is with a vertical slotted hole on the top of the post to the steel structure.

That detail says something like "hand tighten, back 1/4 turn and..." Options are: tack weld nut, damage threads or double nut. These are usually 5/8 or 3/4 structural bolts.

I specify double nut personally.

Like anchor bolts, quality threaded rod is designed to stretch plastically before it snaps. This is why threaded rods and bolts used in structures sometimes appear loose after an earthquake. With enough cyclical loading it can fail in a brittle manner and you would expect that failure point to be where it is restrained, at an anchor point, where both tension and incidental bending forces might be present.

Ungraded threaded rod from China will suffer a brittle fracture even when subjected to design loads.

I don’t think China, or any other country for that matter, enters into the calculations.

At least that wasn’t a constant or variable in any of the calculations in courses I took in engineering school. In my world of engineering structures I just make sure that the ASTM grade of nut is one of those recommended in the ASTM specification for the actual bolt/rod material. If I am using the correctly matched nut and bolt/rod per ATSM I doubt the nut is going to fail before the body of the bolt/rod. Double nutting for purely strength purposes only may do something for you but if the nut and bolt/rod are correctly matched double nutting is not required for strength purposes. For anchor bolts for vibration, cyclic loading, etc , using a second nut as a lock nut, staking/damaging the bolt/rod threads and such to keep the first nut from backing off is done. Though I haver done it I have seen anchor bolt nuts wired together in an "S" pattern to the nut of an adjacent anchor bolt such that if the nut tries to back off it can't because the wire is trying to tighten the other anchor bolt's nut. Now structural bolts subjected to vibrations, fatigue cycling we just have them get torqued as required to pre-load them and thus induce a clamping force which resists vibrations, etc.

I have papers on the nut versus rod/bolt thing but seldom have used it since someone has already done the work for the materials I'm typically specifying, i.e matching nut and bolt/rod material.

The application is 5/8 threaded rod with heavy hex nuts used to tie rafters to the foundation. With a straight pull, where does rod fail? Im sure the engineers have worked this out. And thought you guys may have run across it.

Thanks

Assuming you are following a plan or code speced by a technologist or engineer neither will fail in the intended application.

But for the GJ discussion the potential weak spot is the rod. IIRC hardware store rods are grade 2 and their performance characteristics are low. It is used widely to tie down buildings. I gets down to the spec for your application.

https://www.engineeringtoolbox.com/us-bolts-tensile-proof-load-d_.html It’s an engineered structure but this detail is not specified. I’ve got 36”-7/8” Simpson SSTB anchor bolts embedded in the foundation with hdu’s to anchor posts at various points.

I’m using coupling nuts and where needed shackle and cable/chain to tie the rafters down to the anchor bolts with additional hdu’s. . I got “good” 5/8” threaded from American bolt company. From memory, I think it was a193-b7.

All in an effort to secure the roof against extraordinary winds. I just don’t have faith in the specified (tiny) Simpson rafter ties in the spf top plate. [edit-the h2.5]

​

I don’t think China, or any other country for that matter, enters into the calculations.

At least that wasn’t a constant or variable in any of the calculations in courses I took in engineering school.

Lots of things changed since I went to school in the s. Counterfeit bolts are commonplace these days. I have seen 1" supposedly high strength bolts snap off when being snugged up with a hand wrench. The counterfeits are house blend alloy with the specified bolt grade stamped on the head to look like the real thing. Why inspection and testing have become so important.

It’s an engineered structure but this detail is not specified. I’ve got 36”-7/8” Simpson SSTB anchor bolts embedded in the foundation with hdu’s to anchor posts at various points.

I’m using coupling nuts and where needed shackle and cable/chain to tie the rafters down to the anchor bolts with additional hdu’s. . I got “good” 5/8” threaded from American bolt company. From memory, I think it was a193-b7.

All in an effort to secure the roof against extraordinary winds. I just don’t have faith in the specified (tiny) Simpson rafter ties in the spf top plate. [edit-the h2.5]

​

Have you seen those full-scale model lateral tests they have on their website (Simpson Strong-Tie)?

I want to say the controlling failure mechanism appears to be the wood fibers crushing, hardware deforming and/or hardware fasteners deforming. I don't recall ever seeing the actual threaded rod fails. I'm thinking of the holdowns at the shearwall ends.

Can you post of photos of what you are doing? What was the structure designed for as far as wind? You can PM if you'd like. I’d have to go check the plan notes regarding wind. The PE and I did not specifically discuss it. He was pretty well seasoned and I’m sure it is appropriate for the area.

A couple of samples below. Simply connecting the rafters to lower down on the structure. This particular one runs down to a point above a door. Otherwise to the foundation. This spans a joint in the exterior 3/4” ply.

I will remove the double nuts.

Edit-and I’ll look for the lateral tests. Thanks

Is there any advantage to two nuts?

Seriously, I know that two nuts is a bad way to try to lock something in place. I’ll use locktite.

I’m specifically wondering where threaded rod fails. Does the rod fail randomly in its length or does the nut fail?

The application is 5/8 threaded rod with heavy hex nuts used to tie rafters to the foundation. With a straight pull, where does rod fail? Im sure the engineers have worked this out. And thought you guys may have run across it.

Thanks

If your dealing with wood then the fastener can loosen as the wood shrinks. In this case double nuts have the advantage of holding position on the rod as the wood shrinks and loosens the tension that keeps the nut in place. I'm not sure if Loctite will hold on an untensioned fastener as well as a double nut.

lg
no neat sig line

I’d have to go check the plan notes regarding wind. The PE and I did not specifically discuss it. He was pretty well seasoned and I’m sure it is appropriate for the area.

A couple of samples below. Simply connecting the rafters to lower down on the structure. This particular one runs down to a point above a door. Otherwise to the foundation. This spans a joint in the exterior 3/4” ply.

I will remove the double nuts.

Edit-and I’ll look for the lateral tests. Thanks

Why remove them? In custom automotive applications I use double nuts. I much prefer nylocks but, when access to the nut is difficult, I often spin one nut on by hand so I only need 1-3 more turns w/ a wrench to tighten it. Then I spin on a second nut and tighten it. The advantage is it can be much faster than having to manually wrench a nylock. Sometimes I just can't find the nut I'd really like to use so a double nut to the rescue. Locking two nuts together was one of the first things I learned about as a kid in my dad's shop. And there were usually two situations where I would use it. The most common was not for compression. For example, any situation where movement was wanted, but kept to a specific tolerance, like say the handle on my wagon. I could have it tight enough to stay vertical and not fall down or loose enough that it would. We had many types of lock washers of all sizes in our shop but they never were as capable of maintaining a specific tolerance as a double nut assembly. Trial and error taught that certain flat washers would extend the life of the connection and a little drop of oil could provide added performance satisfaction. We did have a few of those lock nuts that were dimpled or slightly misshaped to produce friction immediately as it was started on the thread but I preferred what seemed the quicker two nut solution. The other application of the two nut solution would be any place you could have a wide variety of dynamic loading like the U-bolts on a axil connection to the leaf springs or the spring's shackle's themselves, they were a place we could have added an extra nut to a questionable friction nut or just did the double up with two regular ones. But it was always in consideration of a dynamic load and not specific to just a static, compression or tension force/load.

If your dealing with wood then the fastener can loosen as the wood shrinks. In this case double nuts have the advantage of holding position on the rod as the wood shrinks and loosens the tension that keeps the nut in place. I'm not sure if Loctite will hold on an untensioned fastener as well as a double nut.

lg
no neat sig line

You bring up a good point. I wonder how tight I should make these? How much will 20’ of 5/8” rod or chain or wire rope contract from say 115F to 15F?

It would be a little tragic to ovettighten them on a warm day and have significant shrinkage/contraction. I guess same deal with humidity.

Thanks