Brass threaded stud failure 1

[mcgyvr]

Can anyone shed any light on why this threaded stud is failing?
The manufacturer of this stud is looking into it but I'm looking for some additional insight.
The part is brand new and installed in an indoor office type environment. This is happening during installation at the end users sites. They "claim" they are NOT over torquing these connections. Its a straight cut not the typical 45 deg angle I always see with a typical over torque situation.
The material is 360 brass (nickel plated). It is a 1/4"-20 threaded stud that is used with an electrical connection. (larger terminal block). There is a compression lug that is slipped over the stud (approximately the thickness of where the break happened to where the mating/landing surface it) and then a zinc plated steel flat washer/keps nut is torqued down (40 in-lbs. max)

This terminal block is used in one of our products and are customers are having failures like this and they claim its happening way before the stated max torque is being applied.

I can get more pictures if necessary.
I do not have any method of testing hardness or anything other than visual examination.

 

[mcgyvr]

another image

 

[dhengr]

I’ll bet it’s a combined stress failure caused by a stress raiser on the thread, or the thread itself as the stress raiser. The initiation point is certainly at, or near, the thread root and for an instant the failure was yielding or plastic, then total fracture, predominantly due to tension. I’ll bet you are applying sufficient tension at the root of the thread to cause failure; it will be max. at the thread root and at that elevation on the stud, even though you haven’t reached your spec’d. torque yet.

On that design, shouldn’t the compression (bearing stress) on the connector which is directly proportional to the tension in the stud be the design control? Can you get the desired bearing stress on the connector at a lower torque? The compression lug area, (Pi/4)(d^2 - di^2) or the washer may have too much bearing area to get the compression stress (bearing stress) you want on the connector; or the stud should be larger to provide that compression at a lower tensile stress in the stud. Then you add the torsional stress in the stud, again a max. at that location, and you see what you get, some percentage of the time. This is probably happening where the thread die is leaving a ding of some sort in the cut thread. Would a rolled thread be better, for its shape and cold working of the thread shape?

 

[hydtools]

Typical tensile failure through the thread root.
Questions:
Correct material?
Correct torque control/calibration?
Kep washer/nut behaving correctly as it is tightened?
Have you run any testing independantly of the customer/end user?

Ted

 

[Tmoose]

I'd torque a dozen or two to spec, and then to failure myself to see what happens

 

[hydtools]

One more question.
Was someone being 'helpful' by adding a lubricant to the assembly process?

Ted

 

[mcgyvr]

Thanks for all your suggestions,etc...

The stud continues into the inside of our product and we have torqued thousands in our assembly environment without a single breakage..

We did find out what seems to be the issue though...
The installers were using a nut driver to start the nut before using a torque wrench and if the handle of the nut driver is pulled off axis (@ about 30 degrees off the stud axis) the studs were simply snapping off. It took very little shearing force to snap them off.

I guess we need to include a warning.. "Nut drivers are not handles or pull up bars"

We are having the manufacturer of the block investigate different alloys or heat treating to prevent this in the future..

 

[btrueblood]

Was gonna ask if they were driving the nut crooked.

A similar problem happened awhile back, and we finally realized that a batch of cheap flanged nuts had thread axis mis-aligned to the flange/washer, resulting in point contact and bending of the stud when the nut bottomed.

 

[hydtools]

Suggest a flex extension or wobble extension for the nut drivers.

http://www.nkbaxter.com/Stanley-Proto-Sockets.html#drivetools

Ted

 

[mcgyvr]

It was actually a hand nut driver (like a regular screw driver).

We had the sales rep from the company come in today and torque down a lug and remove it from EVERY stud on the customers side for all the blocks we had in house.. Not a single failure.

Hi Michael if you come across this post. Good job buddy

 

[hydtools]

Looks like it is the end users problem.

Ted

 

[mcgyvr]

Yes...But the end users problem quickly becomes mine...

 

[hydtools]

Yes, I have found that to be true, too.
It is good when you can solve it for them. Old fashioned customer service.

Ted

 

[mcgyvr]

Yep.. Our customer service has been what is allowing us to take quite a bit of business away from our competition these past few years..

 

[Cockroach]

That is a tensile load failure at the thread relief or perhaps the root of the thread. You may find that 40 in lbf is too large a torque specification for that. Easy to figure out, to the stress mechanics on it and find out what the load is over the shank of threading compared to material strength.

You also may need to look at the combined load of bending and axial stresses in the bolt. Looking at your attachement, brittle failure with raised elements is indicative of excessive tensile loading.

Has to be coming from somewhere. Look at the nature of the application.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada