What tension is put on a weld stud when torquing is applied?

[DanKane]

I have a CD weld stud that requires a 3.1NM torque test. I torque the stud with out anything assembled and it passes the 3.1NM torque (weld holds)

When My team assembles the part and they attempt to torque the same stud with hardware the downward force causes the weld to fail and stud to fall off. Is there a conversion to measure the upward pull on the weld when I am assembling? I want to send these back to the welder but every test that they can perform passes the simple torque test. The failure I am sure is in the upward tension.

Thanks

 

[Tmoose]

"I torque the stud with out anything assembled"

So you are winding a nut down until it jams on the threads ?

Tables for Approximate bolt tension/clamping force vs torque are all over the place.
Here is one, for inch threads.

https://spaenaur.com/pdf/sectionD/D48.pdf

Here is one for metric fasteners.

https://www.fastenal.com/content/feds/pdf/Torque-Tension Chart for Metric Fasteners.pdf

Many posts will follow that remind us how variable the torque/tension relationship is due to friction.

 

[desertfox]

Hi

Maybe you should do test and tighten the same equipment down.
One thing that might be happening, are the studs when being assembled being lubricated on the threads? if so that might be the problem, you might be testing the stud dry and they pass but your colleges are tightening the threads with lubricant on either by accident or intentional.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[DanKane]

Thank you both for the replies. I am sure that we are violating the tensile load capacity therefore the stud is being torn away from the welded area.

 

[desertfox]

Hi

Well if the threads get lubricated at assembly then the tensile load for the same torque can double and hence yes failure will ensue.
Find out of any lubricant is getting on the threads at assembly stage.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[DanKane]

There is no lubricant present at either stage. I believe the issue is that the torque check is not the same application as the assembly. The torque is nothing more than twisting of the stud. The application applies upward pull (away from the weld). Overall bad design. Could have went with a PEM fastener and been done by now. Customer's design makes it even harder to change though.

 

[12345abc6ttyui67]

I assume you have have a rod which is welded at one end, all round the rod, onto a plate which is perpendicular to the axis of the rod.

If you simply twist / apply a torque to the rod after welding (e.g. hold the bottom plate still and twist the rod) the weld will only be seeing shear load, distributed over the weld length (e.g. rod circumference).

When you start to turn a nut down the rod, and try to tighten it against something, the torque you are applying to the nut presses the nut and rod threads together. As you continue to try and turn the nut down the rod (e.g. apply more torque), an upward (tension) force is generated in the rod. The weld at the bottom of the rod is therefore now experiencing a tensile load, which is determined by the amount of torque applied to the nut. As the torque on the nut increases (e.g. as you tighten it more against whatever is below the nut) the tension in the rod also increases.

There is a standard equation for this relationship. From my old metric structural standards (check whether this is applicable for your case!) the equation is:

Torque (Nm) = 0.2 x bolt tension (kN) x bolt dia (mm)

The 0.2 factor is the 'friction factor' or 'k-value' and is 0.2 for threads in an 'as delivered / slightly oily' condition.

You will be able to rearrange this to determine what tension is being applied for a given nut tightening torque.

 

[3DDave]

I'm curious about where the test torque load comes from. What I've seen in the past is that the weld process is tested on a sample by hammering the stud to 90 degrees to demonstrate weld quality.

It's entirely possible that the test load is inappropriate as an installation load. Without knowing the fastener size it's not possible to tell more.

Weld studs are typically in tension.

 

[chicopee]

Obviously your shop test should duplicate field practices.

 

[desertfox]

Hi

Follow the link it gives procedures for testing stud welds.

https://www.pfsno.com/blog/2015/12/15/procedures-for-mechanical-testing-of-stud-welds/

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[gtaw]

You might find AWS D1.1 provides a reasonable approach to testing studs. The testing of the stud involves applying a torque to a nut that is seated on a washer that is seated on the end of a short length of "pipe" such that tightening the nut applies a tensile force on the stud. If the stud is intact after applying the load, all is good. If the tensile force from the applied torque causes the weld to fail, no so good.

From what I read, it sounds like the approach used is to run a nut down to the end of the threads and trying to "twist" the stud. I believe that is not the typical means of testing a stud correctly.

The attached file is extracted from AWS D1.1. It may not be applicable to your situation, but then again, maybe it is.

Best regards - Al

 

[gtaw]

OK, so the file didn't load. I'll give it another go.

Best regards - Al