Retorque nut on Class 10.9 stud? 1

[dozer]

We've torqued a nut to 560 N-m onto a M20 Class 10.9 stud. We would like to back the nut off, adjust the depth of the stud and torque the nut back down. The stud has not seen any additional load since torqueing the nut.

Is this acceptable or should we throw the nut and/or the stud away and try again?

 

[dimjim]

How long has it been in service.
How are you changing the length
of the stud? Simply driving it
deeper? Why are you doing this?

 

[CoryPad]

What were the environmental conditions. What are the coatings, if any?

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dozer]

dimjim,
Length of service is zero. The studs were turned in too deep (they go in a threaded through hole) so we are going to back them out some. Why? Because there is an allen wrench blind hole in the top of the stud that is . . . oh, about an inch deep. We've threaded the stud so deep that the nut is adjacent to this hole. One of the studs broke right at the base of the hole when we torqued the nut. See what happens when you don't follow the directions exactly?

CoryPad,
Inside a building under construction that's dried in. Temperature ranges from I would say 40's F to 70's F. Relative humidity usually is around 60% I think. Just atmospheric. The studs are dacrometized. I don't know about the nuts, but they do have a relatively bright finish whatever it is.

 

[MintJulep]

Then no, I would not reuse the studs. If you broke one you might have yielded others.

 

[Metalguy]

Go ahead and reuse them *if* your torque did not exceed ~75% of the yield strength of the studs. If you have certs for the studs you can use that YS value instead of the min. required.

Sounds like your broken stud broke in an area where the new preloaded section won't be located (that's why your wanting to do this, right?).

"When the eagles are silent, the parrots begin to jabber."
Winston Churchill

 

[CoryPad]

I agree with MintJulep. You should not use the other parts since you experienced a fracture.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[MintJulep]

Sorry Metalguy, but I don't agree.

Reuse if the actual stress in the stud didn't exceed 75% yield maybe.

Problem is, it is really difficult to know that.

A small difference in the "k" factor from stud to stud - which could be caused by many factors - may have resulted in a wide scatter of achieved preload.

If they did load the reduced cross section portion of the stud there are things like stress concentration factors, and the possibility of micro cracks produced by whatever process was used to form the internal hex.

Cheap insurance to throw them away and use new studs.

 

[dimjim]

Since you are using torque and not
certain what type of load you are
imposing on the studs, no one can
tell you what to do. I do not know
how dicronite would effect the results.
You probably were using .14 factor
for tightening assuming you were going
to 70 percent of yield. The fact that
you broke one of the studs indicates
that the torque might be too high.
If the blind allen hole was at the
bottom of the nut, maybe you are right.
If it happened to be at the top of the
nut, the torque is probably too high.
I think I would torque several of the
studs to breakage to give you some
idea of where you are and if the studs
are not pseudo grade studs.

 

[gfbotha]

Just adding my vote - I agree with Metalguy. Having made the adjustment you are not going to be near the hex hole again. Your torque is 75% yield (theoretical). This allows for variation and is the recommended level for reused bolts/studs. If your torque was near 90% yield (or sometimes even more), right through the stud (not only near the hole as initially) then you had to throw them away.

Gert

 

[dozer]

Thanks for all the input. We only torqued six studs so we're going to play it safe and chunk 'em and try again.

 

[deanc]

Check the last link in this thread.

http://www.eng-tips.com/viewthread.cfm?qid=168718&page=1