Page 222 - Machinery Component Maintenance

204 Machinery Component Maintenance and Repair

-[.0015 + .0185]
r4 + :: 26)

+ .0015 = -.074 + .0015 = -.0725
+
say move OB .072 in. or .073 in. south
Again, the answers come out all right if you get the signs right, but the
visualization is difficult unless you make scale drawings or graphical
plots representing the “as found” conditions.

Graphical Techniques

Now let’s see how we can do the same thing more easily. To do this, we
will turn to graphical techniques. Reference 7 shows one version of this
in an article on alignment of barrel-type centrifugal compressors. Its au-
thor stretches a strip-chart out on a drafting table, and rules in the ma-
chine element shafts, to scale, oriented as shown by reverse-indicator
readings, much as we did in our previous example. Movements to
achieve zero offset, or to compensate for thermal growth, can then be
plotted on the same sheet. This technique has the advantage of giving a
complete and permanent written visual record of what is happening, and
is especially useful on multi-element machinery trains such as the article
discusses.
Another graphical technique, which we prefer for most situations, is
the Alignment Plotting Board. It is fast, easy, and accurate, particularly
on the simple two-element trains comprising the majority of all ma-
chines. It can be used on multi-element trains, but is less efficient than
graph paper beyond two elements. To our knowledge, the plotting board
is the only method which handles both the face-and-rim and reverse-indi-
cator setups with equal ease and can convert from R-I to F&R. An addi-
tional advantage is its portability-it is only 8V2 X 11 in. and is made
from an “indestructible” polycarbonate plastic. Figure 5-22 shows it in
use.
Let’s return to our first example. For convenience, we will refer to the
data sheet in Figure 5-15, then show how to find the corrections on the
plotting board. This is graphically explained in Figure 5-23. By flipping
back and forth between data sheet and plotting board sketches, you can
see where the answers come from. It will help to also read the instruc-
tions that come with the plotting board, if you have one. With practice,
the answers will appear in a minute or two.