Page 375 - Intro Predictive Maintenance

366 An Introduction to Predictive Maintenance

Why Perform Alignment and How Often? Periodic alignment checks on all coupled
machinery are considered one of the best tools in a preventive maintenance program.
Such checks are important because the vibration effects of misalignment can seriously
damage a piece of equipment. Misalignment of more than a few thousandths of an
inch can cause vibration that significantly reduces equipment life.

Although the machinery may have been properly aligned during installation or during
a previous check, misalignment may develop over a very short period. Potential causes
include foundation movement or settling, accidentally bumping the machine with
another piece of equipment, thermal expansion, distortion caused by connected piping,
loosened hold-down nuts, expanded grout, rusting of shims, and others. Indications
of misalignment in rotating machinery are shaft wobbling, excessive vibration (in both
radial and axial directions), excessive bearing temperature (even if adequate lubrica-
tion is present), noise, bearing wear pattern, and coupling wear.

Many alignments are done by the trial-and-error method. Although this method may
eventually produce the correct answers, it is extremely time consuming and, as a result,
it is usually considered “good enough” before it really is. Rather than relying on “feel”
as with trial-and-error, some simple trigonometric principles allow alignment to
be done properly with the exact amount of correction needed either measured or cal-
culated, taking the guesswork out of the process. Such accurate measurements and
calculations make it possible to align a piece of machinery on the first attempt.

What Is Good Enough? This question is difficult to answer because there are vast
differences in machinery strength, speed of rotation, type of coupling, and so on. It
also is important to understand that flexible couplings do not cure misalignment
problems—a common myth in industry. Although they may somewhat dampen the
effects, flexible couplings are not a total solution.

An easy (perhaps too easy) answer to the question of what is good enough is to align
all machinery to comply exactly with the manufacturers’ specifications; however, the
question of which manufacturers’ specifications to follow must be answered because
few manufacturers build entire assemblies. Therefore, an alignment is not considered
good enough until it is well within all manufacturers’ tolerances and a vibration analy-
sis of the machinery in operation shows the vibration effects caused by misalignment
to be within the manufacturers’ specifications or accepted industry standards. Note
that manufacturers’ alignment specifications may include intentional misalignment
during “cold” alignment to compensate for thermal growth, gear lash, and the like
during operation.

Coupling Alignment versus Shaft Alignment. If all couplings were perfectly bored
through their exact center and perfectly machined about their rim and face, it might
be possible to align a piece of machinery simply by aligning the two coupling halves;
however, coupling eccentricity often results in coupling misalignment. This does not
mean that dial indicators should not be placed on the coupling halves to obtain align-
ment measurements. It does mean that the two shafts should be rotated simultaneously