Double nut

[bluedog225]

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

 

[HPRifleman]

Tightening a second nut against one that is already there transfers the load to the second nut.

 

[mcbane]

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.

 

[Firebrick43]

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

 

[nadogail]

When weight is a consideration, Double Nuts will have about twice the weight of a single nut and Lock Tight.

This assumes that the weight of the Lock Tight is negligible.

 

[Zeus36]

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.

 

[strutaeng]

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.

 

[bluedog225]

Thanks all. It sounds like the rod fails, not the nut. That being the case, a single seems sufficient in this application.

 

[finn]

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.

 

[WNYflyer]

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.

 

[rancherbill]

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_2066.html

 

[firebirdparts]

double nut is for security, vibration resistance, not strength. Designers know that you can’t predict how you have the load distributed, so if they need more strength, hey always just use a harder fastener or a bigger fastener.

 

[bluedog225]

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]

​

 

[WNYflyer]

If truly ASTM A193-B7 rod material then per Portland Bolt which has a lot of good information but should always verify though.

https://www.a193bolts.com/hardware/

Grade of Bolt	A194 Heavy Hex Nut Grade
A193 Grade B7	2H or 4 or 7

 

[bluedog225]

Great info. Thanks.

It’s got to be a bit frustrating for the engineers how much information is lost between the engineering specifications and what the counter guy gives the client.

 

[mcbane]

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 1980s. 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.

 

[strutaeng]

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.

 

[bluedog225]

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

 

[larry_g]

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

 

[Zeus36]

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?

 

[CraigStu]

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.

 

[bluedog225]

As far as I can tell, they add no strength to the overall system. I’ll locktite and or spray the exposed threads with rustoleum to keep them from creeping back.

 

[bluedog225]

[duplicate deleted]

 

[Drill Sergeant Arc]

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.

 

[DAustin]

Maybe back when I was younger.

 

[TomC750]

Maybe back when I was younger.

If I remember correctly, when double nutting is specified and two are furnished, the thicker one is installed on top.

 

[bluedog225]

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

 

[larry_g]

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

Your beyond my area of expertise. I would tighten them till tight on the wood without crushing any of the fibers in the wood that gives it strength. This is why you have engineered drawings and unless you have experience and information beyond what the designer has then build it as designed.

lg
no neat sig line

 

[bluedog225]

Agreed. This may be close.

 

[HenryAZ]

When I do have to double nut, I always put a flat washer between the two nuts. Then each nut has something to tighten against, and they cannot spin together in or out.

 

[WNYflyer]

Agreed. This may be close.

To clarify, the delta T is typically relative to the temperature of the object when fabricated so you could have lengthening or shortening of the rod.

 

[bluedog225]

Hmmm. I was thinking of tightening them very gently on a cold day. Expansion will create slack and though not ideal, should not cause an issue.