Efficient Plant April 2018 - 14
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Any one specific coefficient of friction bolt test can differ from the next one.
may be able to back off/loosen if the applied vibrations occurring in service are sufficient.
Remember the original problem with the loose bolts
in Acme Power's diesel-engine-generator set? Personnel had installed them without lubrication, as prescribed by the manufacturer. They also ensured there
was no residual lubrication on the threads and collars
by wiping them with soft, white, cotton cloths-every
time. And, as noted previously, they were careful to
torque the bolts as specified.
Unfortunately, each time the equipment was
disassembled, the bolts were stored in a bucket in a
hot, humid area. Over time, their exterior surfaces
developed a visible patina, something that could have
been easily detected by comparing the old bolts to
those fresh from the box.
Further, the thread and collar mating surfaces had
become roughened as the bolts were repeatedly tightened and loosened. Close inspection of these surfaces
with a 20X hand lens found that the mating surfaces
of the old bolts were pitted and abraded. They no
longer looked like the smooth surfaces of new bolts.
In other words, the COF had increased such that
the resulting tensile stress preload in the bolt was
below design requirements. When the engine operated, the alternating vibratory forces were sufficient to
cause some of the bolts to loosen.
As for Apex Industries' problem of overly tight
bolts, the OEM had specified that the bolts be installed dry, i.e., without lubrication. That's why Apex
personnel had been consistent in cleaning the bolts
with alcohol and wiping the threads and collars with
clean cotton cloths prior to reassembly.
Observations of maintenance work had found
that during disassembly, the crew used various
penetrating oils and lubricants to remove the bolts
and prevent galling. Unfortunately, while the bolts
themselves were thoroughly cleaned of residual lubricant before they were re-used, no one considered
that penetrating lubricant might still remain in the
Despite the fact that the bolts had been cleaned,
the mating female threads within the bolt holes were
still lubricated by the penetrating oil that had been silently collecting in the bottom of the holes. As a bolt
was tightened, that residual lubricant spread onto its
previously cleaned threads.
In the end, when a bolt was tightened to the specified torque, the resulting clamping force was higher
than the design value-and some of the bolts were
Randall Noon is a registered professional engineer
and author of several books and articles about failure
analysis. He has conducted root-cause investigations
for four decades, in nuclear and non-nuclear power
facilities. Contact him at email@example.com.