Efficient Plant April 2018 - 11

feature | mro solutions

PREVENTING BOLTS FROM
becoming too loose and discouraging
personnel from over-tightening them
can prove to be challenging. These
two true stories and details about the
physics that determine whether a bolt
is properly installed will help you
solve your own bolting mysteries.
Names, of course, have been changed
to protect the innocent.
Let's start with Acme Power Co.
After several years of good service,
its diesel-engine-generator set began
to exhibit bolting issues. During a
maintenance teardown and reassembly, several bolts were found
to have loosened after only about a
week of running time. A check of
the quality-assurance (QA) records
from the previous engine reassembly and follow-up interviews with
maintenance personnel revealed that,
in every case, the problem bolts had
been tightened to the requisite torque
specification. There was no reason to
doubt this.
Apex Industries Co. was also dealing with bolting issues. Its problem,
though, wasn't loose bolts, but rather
those that were too tight.
Bolt threads in some of the company's equipment were found to be
galled, stripped, and, in some cases,
showing cracks between the bolt
heads and shanks. A check of QA
records and personnel interviews
confirmed that the bolts had been
tightened as specified-and no more
than that. There was no mistake
about this.
In each of these situations, the
same bolts were being reused on the
respective machinery. Furthermore,
both companies verified that all
problem bolts were original with the

APRIL 2018

equipment. No incorrect substitutions had been made. So, what was
going on?
Most machine designers go to
a lot of trouble to ensure that the
torque-tightening specifications for
a particular bolted joint are sufficient to, among other things, keep
a bolt from loosening, backing off,
breaking, galling, and having threads
stripped. Occasionally, however, after
a machine has been disassembled
and reassembled a few times, bolt
problems begin to occur-even when
torque-tightening specifications are
meticulously followed.

BOLT FUNDAMENTALS
In the process of tightening to create
a clamping force, a bolt is stretched.
This causes a tensile stress to be
developed across the bolt shank. For
the bolt to function as desired, the
designer requires it to be stretched
until it has reached a certain amount
of tensile stress.
Tensile stress is best determined by
actually measuring the elongation in
the loaded portion of the bolt as it is
tightened. Elongation is a direct measure of material strain that can readily be converted into a stress number
by Hooke's Law. This method,
though, requires access to both ends
of the bolt, accurate instrumentation,
a skilled person, and a bit of time. As
such, this method isn't amenable to
normal maintenance work.
The more common way of determining and setting the clamping
force in a bolted joint is to measure
the torque applied to tighten the bolt.
Tightening torque is directly related,
at least algebraically, to the amount of
tensile stress created in the bolt and
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11


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Table of Contents for the Digital Edition of Efficient Plant April 2018

Editorial
Implementations
What's Wrong With My Bolts?
Understand Vertical Motor Bearings
Bourbon Boom Drives Distillery Expansion
Lubricant Myths That Can Cost You
IIoT
SAP: When To Update SAP Master Data
Give Your Chain A 'Hand'
Why Do Hackers Hack?
Why Compressor Size Matters
Equipment-Rental Considerations
On The Floor
Microgrids Are On The Rise
Solution Focus
Products
Index
Showcase
Seeking Reliability
Efficient Plant April 2018 - Cover1
Efficient Plant April 2018 - Cover2
Efficient Plant April 2018 - 1
Efficient Plant April 2018 - 2
Efficient Plant April 2018 - 3
Efficient Plant April 2018 - 4
Efficient Plant April 2018 - 5
Efficient Plant April 2018 - Editorial
Efficient Plant April 2018 - 7
Efficient Plant April 2018 - Implementations
Efficient Plant April 2018 - 9
Efficient Plant April 2018 - What's Wrong With My Bolts?
Efficient Plant April 2018 - 11
Efficient Plant April 2018 - 12
Efficient Plant April 2018 - 13
Efficient Plant April 2018 - 14
Efficient Plant April 2018 - 15
Efficient Plant April 2018 - Understand Vertical Motor Bearings
Efficient Plant April 2018 - 19
Efficient Plant April 2018 - 20
Efficient Plant April 2018 - Bourbon Boom Drives Distillery Expansion
Efficient Plant April 2018 - 22
Efficient Plant April 2018 - 17
Efficient Plant April 2018 - 18
Efficient Plant April 2018 - 19
Efficient Plant April 2018 - 20
Efficient Plant April 2018 - Bourbon Boom Drives Distillery Expansion
Efficient Plant April 2018 - 22
Efficient Plant April 2018 - 23
Efficient Plant April 2018 - 24
Efficient Plant April 2018 - Lubricant Myths That Can Cost You
Efficient Plant April 2018 - 26
Efficient Plant April 2018 - 27
Efficient Plant April 2018 - 28
Efficient Plant April 2018 - IIoT
Efficient Plant April 2018 - SAP: When To Update SAP Master Data
Efficient Plant April 2018 - Give Your Chain A 'Hand'
Efficient Plant April 2018 - Why Do Hackers Hack?
Efficient Plant April 2018 - Why Compressor Size Matters
Efficient Plant April 2018 - Equipment-Rental Considerations
Efficient Plant April 2018 - On The Floor
Efficient Plant April 2018 - 36
Efficient Plant April 2018 - Solution Focus
Efficient Plant April 2018 - 38
Efficient Plant April 2018 - 39
Efficient Plant April 2018 - 40
Efficient Plant April 2018 - Products
Efficient Plant April 2018 - 42
Efficient Plant April 2018 - 43
Efficient Plant April 2018 - 44
Efficient Plant April 2018 - 45
Efficient Plant April 2018 - Index
Efficient Plant April 2018 - Showcase
Efficient Plant April 2018 - Seeking Reliability
Efficient Plant April 2018 - Cover3
Efficient Plant April 2018 - Cover4
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