Shaft Alignment

When it comes to the topic of alignment,
let’s accept the fact that “coupling alignment”
is a misnomer. We are not concerned
about bringing coupling halves into alignment—
we’re only interested in ensuring that the
shafts of the pump and its driver will rotate on a
common axis. If the shafts are not coaxial, the
resulting moments will increase the forces on the
pump shaft and bearings, causing accelerated wear
and premature failure.

In most installations, it’s accepted that perfect
shaft alignment is unlikely throughout the operating
cycle. In such conditions, the coupling selection
should be able to accommodate the maximum
amount of the misalignment anticipated. This
should be confirmed with the coupling supplier, as
even flexible couplings have limitations that are
often ignored, resulting in premature bearing failure
and unreliable operation.

Shaft Offset and Angularity

Alignment occurs when two lines that are
superimposed on each other form a single line.
Misalignment is a measure of how far apart the two
lines are away from forming that single line. The
two lines we’re concerned with here are the centerlines
of the pump shaft and the driver shaft. In one
condition, the two lines can be parallel with each
other, but at a constant distance apart. This is
referred to as offset or parallel misalignment. In the
other, one line will be at an angle to the other. This
is referred to as angular misalignment.

Parallel misalignment can be considered as the
distance between the driver shaft centerline and the
pump shaft centerline at any given point along the
length—and it can happen in any plane.
Consequently, it is worthwhile to take the necessary
measurements on the top and on the bottom for
vertical offset, and also on each side for the horizontal
offset.

Angular misalignment refers to the difference
in slope of the two shafts. If the pump, base and
foundation have been properly installed, the shaft
centerline of the pump can be considered as level,
and therefore, as the reference or datum line. The
slope of the driver shaft can be calculated by determining
the offset measurement at two different
points, subtracting one from the other, and dividing
the result by the axial distance between the two
points. (See Figure 2.) This misalignment should be
measured and calculated in both the vertical and
horizontal planes

Figure 1. Diagram of shaft offset and angular misalignment

Figure 2. Angular misalignment calculation

High Temperature Corrections

When a foot-mounted process pump must
operate at elevated temperatures, some adjustment
will be necessary to allow for the thermal growth
that takes place between the cold condition and the
high operating temperatures. As the pump heats up,
the shaft centerline will be moved up, creating an
offset with the motor shaft.

One method of handling this situation is to
misalign the motor by the amount of growth anticipated
from the pump prior to starting it up. Most
pump manufacturers can provide the cold setting
figures corresponding to the higher operating temperatures.
This will require the pump and motor
shafts to run in a misaligned setting until the pump
is fully up to temperature, by which time, the
expansion of the pump will raise it into position to
align with the motor.

A second method is to start the pump and
motor following a cold alignment, without any
adjustment. As the pump heats up and expands, it
will gradually move up, out of alignment with the
motor. When the pump is fully up to temperature,
the unit is stopped and hot alignment takes place.
For both of these methods, a flexible coupling,
capable of accommodating the total amount of
anticipated misalignment, will be required.

Typical Acceptance Values

Bringing the motor shaft into alignment with
the pump shaft usually involves moving the front
and rear feet of the motor, vertically and horizontally,
until the shafts are aligned within acceptable
tolerances.

In addition to their dependency on data such as
speed of rotation, horsepower, spacer length, shaft
size, etc., acceptable alignment tolerances also
depend, to a large extent, on the level of reliability
the pump user expects. Accordingly, every end user
should develop acceptance levels that provide their
particular desired outcomes.

The tolerances in Table 1 are not intended as
definitive values, but can be used as a starting point
for developing tolerances that will be specific to an
individual company or equipment. They represent
the maximum allowable deviation from the desired
value, whether that value is zero or a targeted misalignment
to allow for thermal growth of the equipment.

Table 1: Alignment tolerances