if the shaft is considerably out-of-plumb, as is the case many times on the first reading after
reassembly, a scale of 0.002 inch or more per division may be required.
Once an acceptable scale is laid out, draw two vertical lines on the graph. These lines represent
zero, or perfect plumb, for the north-south and the east-west plots. Label north, south, east, and
west on their respective sides of the lines. The point for the first reading elevation will be
directly on the vertical line for both the north-south and east-west plots. The second, third, and
fourth reading elevation points are all plotted the amount indicated in column 5 away from the
vertical line in the direction indicated in column 6.
With all the points plotted, draw a line from the first elevation point to the second elevation point
and extend the line to the shaft coupling elevation on both the north-south and the east-west
plots. This line represents the generator shaft. Draw a line from the fourth to the third elevation
points and extend it up to the coupling elevation. This line represents the turbine shaft. The
horizontal distance between the lines at the coupling is the amount of offset. Any angle between
the two lines indicates dogleg.
To determine the total effect of the dogleg and offset on the static runout, extend the generator
shaft line down to the fourth elevation. The horizontal distance at the fourth reading elevation
from the extended generator shaft line to the turbine shaft line, multiplied by two, is the total
effect of dogleg and offset on the static runout at the fourth elevation. If this value is near or
exceeds the maximum allowable runout as calculated in the next section, some correction will
probably be required. If the dogleg and offset are acceptable, only the first and fourth elevation
readings are required for subsequent readings.
If the generator and turbine shaft are straight, the total out-of-plumb can be determined by
drawing a line from the first to the fourth elevation points and extending it upward to the thrust
bearing elevation. From the point where this line intersects the thrust bearing elevation, draw a
vertical line downward to the fourth reading elevation. The horizontal distance from where the
projected line crosses the fourth reading elevation is the total out-of-plumb at that elevation.
If the dogleg is significant enough to require readings at all four elevations, the total out of plumb
is determined by extending the generator shaft line upward to the thrust bearing elevation. Again
a vertical line is drawn downward from the point where this line crosses the thrust bearing
elevation down to the fourth reading elevation. The horizontal distance from where the projected
line crosses the fourth reading elevation is the total out-of-plumb at that elevation.
Bearing and seal ring centerlines can be plotted by taking half of the difference between the
north-south and east-west clearances and plotting that value against their respective shaft
centerline plot. The bearing centerline will lie on the side of the shaft centerline in the direction
of largest clearance reading. In the example in figure 15, the difference between the north-south
readings is 0.040 inch. The centerline is half of that value, 0.020 inch to the north of the shaft
centerline. In the east-west direction, the difference is 0.008 inch, so the bearing centerline is
0.004 inch to the west of the shaft centerline.