Page 374 - Intro Predictive Maintenance

A Total-Plant Predictive Maintenance Program 365

coupled shafts, as well as coupled shafts of machines that are separated by distance—
even those using flexible couplings. It is important because misalignment can intro-
duce a high level of vibration, cause bearings to run hot, and result in the need for
frequent repairs. Proper alignment reduces power consumption and noise level, and
helps achieve the design life of bearings, seals, and couplings.

Alignment procedures are based on the assumption that one machine-train component
is stationary, level, and properly supported by its baseplate and foundation. Both
angular and offset alignment must be performed in the vertical and horizontal planes,
which is accomplished by raising or lowering the other machine components and/or
moving them horizontally to align with the rotational centerline of the stationary shaft.
The movable components are designated as “machines to be moved” (MTBM) or
“machines to be shimmed” (MTBS). MTBM generally refers to corrections in the hor-
izontal plane, whereas MTBS generally refers to corrections in the vertical plane.

Too often, alignment operations are performed randomly and adjustments are made
by trial and error, resulting in a time-consuming procedure.

Alignment Fundamentals. This section discusses the fundamentals of machine align-
ment and presents an alternative to the commonly used trial-and-error method. This
section addresses exactly what alignment is and the tools needed to perform it, why
it is needed, how often it should be performed, what is considered to be “good
enough,” and what steps should be taken before performing the alignment procedure.
It also discusses types of alignment (or misalignment), alignment planes, and why
alignment is performed on shafts as opposed to couplings.

Shafts are considered to be in alignment when they are colinear at the coupling point.
The term colinear refers to the condition when the rotational centerlines of two mating
shafts are parallel and intersect (i.e., join to form one line). When this is the case, the
coupled shafts operate just like a solid shaft. Any deviation from the aligned or co-
linear condition, however, results in abnormal wear of machine-train components such
as bearings and shaft seals.

Variations in machine-component configuration and thermal growth can cause mount-
ing-feet elevations and the horizontal orientations of individual drive-train compo-
nents to be in different planes. Nevertheless, they are properly aligned as long as their
shafts are colinear at the coupling point.

Note that it is important for final drive-train alignment to compensate for actual oper-
ating conditions because machines often move after startup. Such movement is gener-
ally the result of wear, thermal growth, dynamic loads, and support or structural shifts.
These factors must be considered and compensated for during the alignment process.

The tools most commonly used for alignment procedures are dial indicators, adjustable
parallels, taper gauges, feeler gauges, small-hole gauges, and outside micrometer
calipers.