Efficient Plant April 2018 - 13
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friction, i.e., µ = µc . Be aware, however, that while
this may often be the case, it isn't always so.
The inaccuracy of Equations 1 and 2 with
respect to torque and its resulting clamping force
occurs because the two coefficients of friction can
vary significantly due to, among other things:
less than full contact between engaged surfaces
degree of surface roughness between the
presence of a lubricant
non-presence of a required lubricant
IN THE FIELD
Coefficients of friction (COFs) aren't usually
measured in the field during maintenance work-
and torque specifications aren't then re-computed
based upon the newly measured COFs prior
to tightening a bolt. A bolt's COF is generally
When the initial torque-versus-clamping-force
computation is done, the designer typically
assumes the mating surfaces are smooth and
clean, and that the machined surfaces are within the usual manufacturing tolerances. In other
words, the designer assumes that the coefficients
of friction match those listed in a COF bolt reference table.
If any lubricant is specified, the designer also
assumes that the correct lubricant is evenly spread
over all the contacting surfaces in such a thickness
as to match his "COF with lubricant" values in a
Bear in mind, though, that reference-table
numbers are typically an average of many tests.
That means the values are statistical in nature. Any
one specific COF bolt test can differ from the next
one-and differ from the average of the lot that
Further, field conditions don't always match
conditions under which testing was done to
prepare the COF reference table. This is especially
true after bolts have been in service for a time,
frequently tightened and loosened, and exposed,
when not in use, to various conditions.
Consider, for example, the differences between
non-lubricated and lubricated installations of the
same bolt. If a steel-on-steel dry COF of 0.20 is
assumed for the threads and collar, and the bolt
has 12 threads/in., then the torque coefficient K in
Equation 2 computes to 0.285.
If, on the other hand, a lubricant is applied to
the threads and bolt collar so that the COF between surfaces is 0.15, then the torque coefficient
computes to 0.224.
To achieve the same clamping force in the
same bolted joint, the non-lubricated bolt
requires 27% more tightening torque
than its lubricated counterpart.
Conversely, if the same tightening torque is used in both cases,
the non-lubricated bolt will
have only 79% of the clamping
force of that of the lubricated
bolt. This reduced amount of
clamping force may then be
sufficiently low enough to allow
the bolt to loosen when vibrations are present.
Now consider what occurs when
old bolts are reused. Compare an
old bolt, that has surface rust, adhesive chemical remnants, or otherwise has
become discolored due to surface corrosion,
to a new bolt. In most cases, the COF of the old
bolt will be higher than that of a "fresh from the
box" new one. If a new, non-lubricated bolt, for
example, has a COF of 0.2, the corresponding K
torque coefficient is 0.285. If a rusted or discolored
version of the same bolt has a COF of 0.3, the
corresponding K torque coefficient is 0.41 (refer
to Fig. 1).
If a rusty or discolored bolt is tightened to the
same torque specification intended for a new
bolt, Equations 1 and 2 indicate that the resulting
clamping force in the rusty or corroded bolt will
be about 30% less than the clamping force produced with a new bolt.
Thus, the rusty bolt will result in a looser clamping force than the designer specified. As a result, it
Watch for bolt problems after
a machine is disassembled and
reassembled a few times.