Centrifugal Pumps Design and Application 2nd ed - Val S. Lobanoff, Robert R. Ross (Butterworth-Heinemann, 1992)

516 Centrifugal Pumps: Design and Application
Methods of Calculating Alignment Movements
Several methods are available for calculating the required alignment
movements using data recorded from the alignment measurements. Table
19-4 describes these, and Figure 19-5 shows the alignment plotting
board, a popular graphical calculating device for determining alignment
movements.
Jig Posts
For dial indicator measurements, it is sometimes useful to use auxiliary
surfaces, sometimes called jig posts. These are most often used for rim
measurements, but if necessary, can be used for face measurements.
They accomplish the task of moving the measurement surface to a more
convenient location rather than measuring directly on the shaft or coupling
surface. Also, if the jig post has a flat surface, it eliminates curvature
error that can sometimes reduce measurement accuracy on small diameters.
Table 19-4
Methods of Calculating Alignment Movements
Method Advantages Disadvantages
Nelson's
Method
Two-Step
Mathematical
Methods
Graph Paper
Plot
Simple pencil and paper
math, using face-and-rim
data. Easy to visualize and
figure. Accurate.
Pencil and paper
mathematics, using any type
of measurement data. More
efficient than Nelson's
method. Accurate.
Pencil, straightedge, and
graph paper, using any type
of measurement data. Easier
to visualize than straight
mathematical methods. Can
use on multi-element trains
for optimum move
calculations.
Applicable only to
face-and-rim measurement.
A four-move method, less
efficient than two-move
methods. The second and
fourth moves may reverse
some of the corrections
applied in the first and third
moves.
Harder to visualize than
Nelson's method. More
likelihood of calculation
error due to incorrect choice
of sign.
Slightly less accurate than
straight mathematical
methods, but not usually
enough so to matter.
Awkward to use in field.