Steel Nut/Coupler with Oversize Threading Structural Capacity? 4

[Jefftina]

I need to determine the Tension capacity of a Steel Coupler (Long Nut) that has oversize threads. It probably shouldn't matter but this Coupler/Nut is hot dip galvanized. Does anyone know where I can find some equations to calculate this? I have the Material Grade Specification and hence the ultimate tensile also the yield capacity. I also have the dimensional information for the coupler/nut and also for the threaded rod. If anyone knows where to point me I would really appreciate it.

 

[Ron]

This is usually an empirical determination by manufacturer. Too many variables to calculate accurately.

 

[CANPRO]

Manufacturer should be able to give you some values...if you really want to do the calculation you might get better guidance from the mechanicals in another forum.

 

[Jefftina]

I have been hired by the manufacturer to figure this out. Thanks for the replies.

 

[Hokie93]

Roark's Formulas for Stress & Strain gives some treatment to this topic in Chapters 7 and 13 in the sixth edition. The coverage isn't extensive but does provide several references for more in-depth coverage.

 

[dhengr]

“Oversized threads” better mean longer length of engagement than on a normal matching nut, and not non-mating, different class, threads. “Machinery’s Handbook” and several other Mech. Engineering, Strength of Materials, and Machine Design texts cover this subject. In effect you are looking for the shear failure stress (yielding +) at the thread root on the weaker material. Alternatively, you are looking for the length of thread engagement +, so that the coupling fully utilized the tensile strength of the strongest rod, and the coupling must have enough body tensile area to meet this demand. You pick the margin of safety.

 

[CANPRO]

I would test them...if its just a threaded rod and a coupler it would be a fairly easy test set up. I live in a small City and I know of a couple places that could do this no problem...there should be someone close by that would be able to do this for you. Plus your testing something thats fairly cheap, so you could test a couple dozen to failure and have lots of confidence in the results.

 

[Jefftina]

Thank you very much everyone! dhengr- this is a coupler that is threaded 1 1/2"-6 for a 1 1/2" diameter threaded rod with the same class threads but oversized 0.027" in diameter to safely receive the hot dip galvanized threaded rod (and with the coupler being hot dip galvanized as well). The coupler is 4 1/2" long but we don't currently know the minimum length of engagement being used at each end. I will probably have to specify a minimum length of engagement for it to meet and/or exceed the threaded rod overall limiting capacity. A DH nut was specified but the coupler is desired to be used in place of the DH nut (ASTM 1094 heat treated to A194) as long as it is at least as strong as the B7 threaded rod in tension, compression and shear.

 

[dhengr]

The galvanizing people should be able to guide you on the .027" dia. over sizing for their dipping process, so that you still end up with the proper class of fit and tolerances. And, this might vary a little btwn. processors. You might still want to, need to, clean up (wire brush, or some such) maybe ‘chase’ both threads and/or lubricate them. Also, check with the galvanizer to be sure that their process doesn’t have any adversely affect on virgin material strength or any prior heat treatments (Q&T) on the rods, nuts or couplings. Also, understand that there is some amount of yielding in shear and bending of the mating thread profiles just to really fully engage the nut and the bolt. So, you are looking for the material capacity using Fy or Fu, but it is primarily a shearing or stripping failure; as relates to some failure surface dia., not really the root dia. (but a cylindrical shear failure surface parallel to the coupling axis). It’s a compatibility problem btwn. the two mating parts, and a function of their relative strengths and stiffness. This might change the dia. you finally settle on a little. It’s not an exact science/calc., only Mother Nature knows for sure, but you end up with a circumferential shear area as a function of your dia., thread thickness at that dia., and the thread pitch, etc.; and this leads to a shear capacity in kips/inch of engagement length.