to the journal. On turbine bearings that use a tapered fit, a jig or some other means must be used
to insure that the readings are taken at the same point of the taper at all four measurement points.
When measurements are taken from the shaft to the bearing housing with an inside micrometer, it
is not necessary to calibrate the micrometer because only the differences between readings and
not absolute dimensions are of interest. The bearing clearances should always be verified after
installation in case the bearing surface is not concentric to its fit.
6. PLOTTING THE DATA
6.1 Plumb Data
Plumb readings from either plumb wires or the Hamar laser system are used with the worksheet
in figure 14. The actual readings are entered in column 1. As mentioned above, the electric
micrometer readings are not calibrated, so these readings mean nothing by themselves. The
difference between readings is what is used to determine the plumb of the unit. Since the wires
will not be the same distance from the shaft, an amount is added to each reading in column 2 to
mathematically make all four wires the same distance from the shaft at the first reading elevation.
This will simplify subsequent calculations. The first elevation is considered the origin for the
plot of the shaft. The values in column 2 are calculated by taking the largest value of column 1
in the first reading elevation and subtracting each of the other three measurements. As three
wires have been mathematically moved these distances at the first elevation, these values must be
carried through the rest of the reading elevations. Column 3 is the sum of columns 1 and 2. If
the values in column 3 at the first elevation are all equal to the largest value in column 1, the
values in column 2 are correct. Column 4 is the difference between north and south and east and
west. Column 5 is one half of column 4, which is the amount the shaft is out of plumb from the
first elevation, the origin of the plot. Column 6 indicates the direction the shaft is out of plumb
from the first reading. Columns 7 and 8 are used to calculate the accuracy of the readings.
Column 7 is the sum of the north and south and east and west readings. As most shafts are
machined to a high degree of accuracy regarding roundness, any value in column 8 of more than
0.002 inches is considered excessive and is probably due to an error in a measurement or in
reading the micrometer.
To plot the plumb of the shaft centerline, the values in column 5 and the directions in column 6
are used. Two separate plots will be required, one for the north-south profile and one for the
east-west profile. Usually, both plots are drawn on a single sheet of graph paper. To determine
the vertical scale for the plot, the vertical distances shown on the sketch on the bottom of figure
14 are used. The distances between the thrust bearing and coupling and the distances from the
coupling to the seal rings are obtained from the manufacturer's drawings. After choosing a
suitable scale on graph paper, mark on the vertical scale the elevation marks for the thrust
bearing, the reading elevations, and the shaft coupling. To plot the centerline of the guide
bearings, seal rings, and generator stator, their elevations will have to be added to the graph as
well. Figure 15 is an example of a shaft plumb plot.
The horizontal axis will be the plumb of the shaft. The horizontal scale should be chosen based
on the total out-of-plumb of the shaft. Usually, a scale of 0.001 inch per division will work, but