Coupling Nut Thread Engagement

Coupling Nut Engagement Unfortunately there appears to be no documented information on how much rod engagement into a coupling nut is required to achieve the strength of the rod or bolt being used with it. Coupling nuts are manufactured to the same ASTM specifications as regular hex nuts, and ASME B18.2.2 provides dimensional specifications for coupling nuts. Standard coupling nuts are 3 diameters in length. However, there is no reference to coupling nuts in the actual ASTM A563 or A194 nut specifications.

Your first step is making sure that the ASTM grade of coupling nut being used with the threaded rod or bolt is the same grade as the recommended grade of nut that would be used with the rod or bolt. For example, ASTM A193 B7 rods or bolts require an ASTM A194 2H heavy hex nut, so if you were to use a coupling nut with an ASTM A193 B7 rod, the grade of coupling nut should also be ASTM A194 2H. Although the amount of thread engagement is not specified, our coupling nut supplier recommends the bolt or rod should be threaded a minimum of one diameter in length into one end of the coupling nut.

coupling-nut-hole

There may be an application which requires that the threaded parts going in each end of the coupling nut are engaged approximately one half of the coupling nut thickness. In this case, a hole may be drilled in the middle of one side of the coupling nut. This aids in the visual inspection to ensure that both rod ends are in fact meeting in the middle, not off center. According to IFI-128, note 7, the hole should be located at mid nut thickness and have a diameter of 0.2 to 0.4 times nominal nut size for sizes 2-1/2″ and smaller. For sizes 2-3/4″ and larger, the hole is 1″. Coupling nuts are manufactured without this hole unless otherwise specified.

As long as the correct grade of coupling nut is used and the bolts or rods are engaged a minimum of one diameter, the bolt should theoretically break before the threads strip. Again, this is only a common rule of thumb and we have no documented specifications that cover this issue. In any situation involving coupling nuts, we would recommend consulting with the engineer of record or a structural engineer to determine the appropriate length of coupling nut and amount of thread engagement required.

32 comments

Charlie N. Nunez, PE

October 31, 2018 at 12:15 pm

Dane:
I need to extend 4 – 1″ dia. Galv. anchor bolts with standard threads and would like to use fabricated stand-off, in order to keep the length of the coupler to a minimum. The total length of the stand-off, including the coupler and threaded rod to be 6 1/2″ long. Can you weld a 1″ dia. galvanized threaded rod (ASTM A307) to a 2″ long galvanized coupler, leaving sufficient thread in the un-welded end of the coupler to meet the 1.5 x rod diameter requirement?

Dane McKinnon

November 1, 2018 at 7:41 am

@Charlie- If I understand this correctly, that would leave the extension threaded only 1/2″ in and then welded? If your project engineer has evaluated this scenario and is confident that it will still support the load that it needs to support, I guess it would be acceptable. The rule of thumb is that you want the threads engaged at least one diameter, but that is simply a guideline and not an absolute law. If you feel that your solution will be the best fit for your project, and the project engineer signs off on it, then you should be ok.

Chris Hanna, PE

December 17, 2017 at 8:25 pm

I have an application where I would like to tack weld one-inch diameter threaded rods to coupler nuts. I understand that I can obtain weldable ASTM F1554 grade 55 threaded rod, but I would like also to obtain weldable couplers. Are weldable couplers available? if not, is there another option to welding that I’m not thinking of? Are sufficient chemical properties available for the coupler nuts so that I can calculate the Carbon Equivalent of your 1″ coupler nuts?
Thank you,
Chris
530-604-8090

Dane McKinnon

December 21, 2017 at 8:38 am

@Chris- In the sampling of MTRs for some commonly available imported couplers, I found they were all made of low carbon steel like AISI 1010 and appeared to be weldable. We can’t say that ALL couplers are available, as other suppliers may have other sources, but we are comfortable saying that we could supply weldable couplers. Whomever they are sourced from, ask if they can supply MTRs showing the chemistry. If they can, it should be easy to determine if they are in fact weldable. Plan B would be to have couplers manufactured. That would also guarantee that they are weldable, but would be at a higher cost.

clark allen

May 22, 2017 at 12:04 am

How difficult (more expensive) would it be to manufacture “positive engagement” coupler nuts in the 1/2′ to 3/4″ size so that it eliminates the potential for faulty installation that is more likely to happen at that location than anywhere else.

This webpage has an example of a positive engagement coupler.
https://www.hurribolt.com/compare.htm

Dane McKinnon

May 23, 2017 at 7:23 am

@Clark- We would be able to quote your couplers as a special, so the cost would depend on the specifics (size, grade, quantity, finish) of what you are needing.

Carson Hemphill

May 2, 2016 at 3:05 pm

I’m looking for installation/inspection standards for the these coupling nuts. We plan to use these nuts to hang racks from inserts in a concrete deck. Is their a product spec or manufacturer installation recommendation that indicates whether or not the two rods should be visible touching or if it is acceptable can be an air gap between the two rods. If it is acceptable to have an air gap please specify the maximum that the air gap can be?

Dane McKinnon

May 3, 2016 at 1:29 pm

@Carson- We do not know of any published document regarding this. However, rule of thumb is that you want at least one diameter’s worth of thread engagement each end. Coupling nuts are usually three diameters long, for example a 1″ coupler is 3″ long. So if each end is engaged one diameter, that leaves one diameter of ‘gap’ in the middle. As long as the gap is not more than one diameter, you should be safe.

Perry Nelson

October 28, 2015 at 2:45 pm

Can a coupling nut be used in SAG ROD applications? They want to tie into the tail-end of a sag rod above using a coupling nut, the extreme strength grade. Will the nut tend to wiggle loose over a period of time? Thank for the article!

Dane McKinnon

November 5, 2015 at 11:17 am

@Perry- Yes, it may be acceptable to use a coupler in a sag rod application. However, the project engineer should be consulted to make sure there are not any special circumstances that may come into play.

Coupling Nut Thread Engagement