Page 141 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Advanced Machine Alignment

the nature of a rim reading is such that it measures the offset at the
top and the bottom (double the actual misalignment). This value
can now be plotted on the line representing the rim indicator.
Continuing, if a reading of minus eight (–8) mils is recorded
on the rim indicator, first the algebraic sign is changed and the
value is divided by two. This yields plus four (+) 4 mils. Since the
vertical scale was set at one square equals one mil, the point is
located four squares above the horizontal final alignment line, and
on the rim indicator line. This point is marked and circled.
A horizontal line is now drawn from this point to the same
point on the line representing the face indicator. The point on the
face indicator line has a box drawn around it. The recorded value
of the face indicator is now counted off from this point.
Remember, positive numbers are measured up and negative
numbers are measured down. Assuming that in this case the face
indicator reading was a plus two (+2), changing the sign yields a
minus two (–2). A point two squares below the horizontal line just
drawn represents the face indicator reading. This point is marked
and a circle is drawn around it.
Using a long straightedge, align it so a line can be drawn
through the two-circled points. This line should extend to the
right far enough to cross the adjustable outboard foot line. Label
this line the center of the adjustable machine shaft.
Assume in this case that the A distance is 10 inches and the
B distance is 18 inches. Where this line crosses the feet of the
adjustable machine indicates the amount of shims to be added or
removed. If the adjustable machine centerline crosses the foot line
above the final alignment line, shims must be removed; if it is
below, shims must be added.
The amount of shims to be removed or added is determined
by counting the squares (up or down) from the final alignment line
to the adjustable machine centerline at each of the foot locations. In
our example, the inboard foot would require .5 mil to be removed
and the outboard would require 5.5 mils to be added. Since we are
only to deal with whole mils, the inboard foot would be left as is,
and the outboard foot would have 5 mils of shims added.