Studs sheared and magnetic particles found

[cbk14]

Hello all,

A friend of mine had two (out of twenty) OEM wheel studs shear off. One sheared while installing wheels and the other sheared while taking the wheels off (about a week later). Both stud failures occurred while tightening or loosening the lug nuts (also OEM).

Being curious, I investigated a little bit. I noticed the lug nuts have a chrome-like plating which was readily flaking off in areas. I also noticed on a few of the nuts that came off normally, there were chunks taken out of some of the threads.

We replaced all studs recently but while removing the wheels, I noticed a few nuts were more difficult to unscrew. The corresponding studs had metal particles which appeared to be standing up on the thread tips magnetically (picture attached).

According to my friend, all lug nuts were installed using a torque wrench properly set to the to OEM provided torque spec of 88 ft•lb. I checked his torque wrench against a couple of mine and they all seem very close.

One interesting fact is that my car uses the same stud pattern and thread (M12 x 1.25) yet the torque spec for my car is 66 ft•lb vs 88 ft•lb for my friend's car.

My questions are:

1) why did the studs fail?
2) what do the magnetic particles indicate?
3) is over-torquing a possible explanation?

Personally, I believe the nuts galled and seized on the studs, hence why they sheared while rotating them. Galling is evidenced by the observed metal particles and chrome flaking. The magnetic effect could be explained by very high friction / heat and movement in a single direction.

I have one of the nuts with the broken off stud tip still in it, which I plan to cut in half to inspect. Any other recommendations to get to the bottom of this?

 

[Tmoose]

"We replaced all studs recently "
"the lug nuts have a chrome-like plating which was readily flaking off in areas."

Are the new studs the ones that are giving you problems?

Were these chrome lug nuts installed just prior to the problems beginning?
Were/are the lug nuts' threads chromed ?

 

[CapriRacer]

Sorry, but your post is a bit unclear.

Did you replace the lug nuts at the same time as you replaced the studs?

If not, maybe the lug nuts are the source of the problem.

Also, why did you replace the studs? Is it because of the breakage? - and the metal particles came after? - meaning you are trying to diagnose the breakage by looking at what you have after the repair?

 

[cibachrome]

This is actually a (not so) pretty but common problem when installers are too lazy to start the nuts by hand for a few turns and then use a high torque nut runner to start and then finish the job. Instead, they load the nut into the gun and drive it home until seated. It's seated all right. Permanently. The nut actually cuts a new thread on the stud and shears off segments of the old thread. It's hardened steel so there is only powder, pieces, and shrapnel.

Oh, I know, your buddy would never do this, but in a past universe, somebody ran a secondary thread onto the stud and bingo, we have a snapper. The gun can run so fast as to weld the nut onto the stud and they snap, crackle and pop when the same sized power wrench attempts to remove the nut. The wheel comes off ok, but there will be fewer fasteners !

Ax me how I know. It cost a tire shop employee his job and almost his life via a self feeding commercial wood chipper, feet first, mouth duct taped shut.

 

[NormPeterson]

It's a common enough problem that dealerships and parts stores generally have replacement studs in stock in whatever size you need. I'm sure that one of the mechanics at my local Nissan dealership tech is guilty of this exact lugnut installation crime.

cbaker should consider himself fortunate that it wasn't a locking lug nut that stripped out its keying groove instead of breaking off the stud. Sometimes even the remover sockets don't/can't work and the only way to recover at that point is by drilling (in stages, with frequent bit re-sharpening). Been there with a lug bolt on my daughter's car.


Norm

 

[cbk14]

To clarify, we replaced the studs and lug nuts after the stud failures. My friend decided it would be wise to replace all the studs and lug nuts at the same time rather than just the two broken studs.

My findings posted above are all post-breakage and were all during the replacement of the studs (I inspected the parts as they were disassembled, hoping to find a smoking gun).

Cibachrome: my first thought, too, was those damn air-gun wielding "mechanics" at a tire shop "torquing" the lug nuts to the specified three ugga-duggas past tight. But apparently my friend is the only person who has touched the wheels, studs, or lug nuts in the past couple of years. So that's pretty much ruled out. At least it rules out air-gunning a second thread on (he doesn't own an air gun). But it does not rule out improper nut installation (my friend is mechanically inclined but not necessarily mechanically adept).

Couple new bits of potentially relevant information. My friend firstly mentioned how difficult it was to get the old lug nuts started on the studs (as opposed to the new ones which are much longer and thus easier to get started straight). Secondly, he told me he's probably had the lug nuts on and off maybe 15-20 times over the past three years (swapping between sets of wheels/tires).

I interpret this new information like so:

1) difficulty starting a nut means it is very likely to start a nut incorrectly, especially if you are unfamiliar with or not paying close attention to proper threading.
2) increasing frequency of installation/removal of nuts increases the wear on the threads, and probability of failure of the threaded joint is thereby increased. This is especially true for torques approaching the limit of material strength (which these lugs and studs may have been).

Thoughts?