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I recently found 2 loose bolts on a slew-ring, both were cracked! We were not allowed by the manufacturer to remove the other 157 bolts because of the load on it.
A Krautkamer USN 50L was used to perform test on the bolts in place.
(bolts are 12.9 - 30mm  11" long)

How reliable is this instrument?
Would some other test been more applicable?

See pictures @ www.towercranesupport.com


RE: Krautkamer

The USN 50L is excellent, though not cutting-edge, UT equipment - note that Krautkramer is now part of GE Inspection Technolgy (GEIT) and information on UT equipment can be reviewed on its website.

You say nothing about the procedure you have utilised to detect the cracks. You would need to set up the equipment correctly using identical threaded bolts with natural indications similar to the one's being searched for at a position where the cracks are expected to be found.

Also operator skills and experience need to be considered, plus choice of the correct transducer and instrument settings.

Perhaps the best course if you have doubts is to contact an NDT consultancy and have them look at your inspection problem and advise any improvements.

All the best

Nigel Armstrong
Karachaganak Petroleum

RE: Krautkamer

Agree with ndeguy but would like to make a few points.
The location of the failures suggest fatigue, but the fracture surface looks a little funny, more like an embrittlement. So I would immediately replace a number of other bolts that tested good and do a closer exam, probably wet MT. You should be able to detect the beginnings of fatigue if that is the root cause.
I also would have done a little more preparation to the bolt end prior to using the UT.
I also would have the failed fasteners immediately sent to a Metallurgical/Forensics lab for evaluation.

Were all the bolts tight?

You NDT guy's hat is too pretty.

RE: Krautkamer

The only thing I saw the tech. input was the length of the bolt! I was surprised. We did have the new bolts on hand which he checked also. We are sending out the removed bolts for further examination.

Thanks for the responce.


RE: Krautkamer

I would like to reemphasise the idea of removing a set of bolts for wet magnetic particle examination. Of the 158 I would replace at least 8 for further examination.

I'm basing this on an incident several years ago where we have contractor loose the cab and boom of a 150 ton crawler crane on our site. The failure occurred after about 25% of the bolts in one sector had completely failed. About 90% of the remaining bolts had significant indications on the flank of the threads.

Another reason is that if the primary mode of failure was fatigue it means that there was relative movement between the two clamped faces. This means that there may be the detectable beginnings of another fatigue failure in the making.

Based on the publicity tower cranes are getting NYC I thinks a little CMA is applicable.

The hat remark was made on the observations of many years the NDT guys always had to worst looking hats of anybody. I always took it as a trade mark.  

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