and the lower one just above the coupling flange. On the turbine shaft, the lower band should be
as low as possible, with the top band just below the coupling flange. Ladders or scaffolding may
be required to provide access to the upper band. If a ladder is used, it must not rest against the
Taking two readings per shaft makes the assumption that the individual shafts are straight and
any bends will be at the coupling. If there is any reason to believe that a bend exists in a shaft,
more reading elevations should be used. If there is only a short section of the generator shaft
accessible below the rotor, readings above the rotor may be required.
Shaft plumb readings allow a plot of shaft centerline to be drawn as in figure 15. This plot uses
the data from figure 14. The plot of the shaft will provide information on straightness of the
shaft. Once the shaft is plotted, the relative position of other components can be plotted as well.
From the plot, plumb and concentricity of the stationary components can be determined.
5.3 Static Runout
Due to non-perpendicularity between the thrust runner and the shaft, as the shaft rotates, the shaft
centerline will scribe a cone shape, as shown in figure 12, when the guide bearings are removed.
This is referred to as static runout. A bent shaft or dogleg and offset at the coupling can also
contribute to excessive static runout. The larger the static runout, the higher the loading on the
guide bearings and, in most cases, the higher the vibration levels.
Static runout cannot be measured on units with self equalizing thrust bearings. The self
equalizing bearings correct for non-perpendicularity of the thrust runner, making static runout
data impossible to obtain, as well as unnecessary.
Static runout is measured in either of two ways, both requiring rotating the shaft. To rotate the
shaft, the high pressure lubrication system must be operational. This may require providing a
temporary source of oil because, in some cases, it is necessary to remove the oil tub during the
alignment. If this is the case, some temporary method of routing the oil from the bearings to the
drain is required as well. If a high pressure lubrication system is not installed, it will be
necessary to jack the unit to get oil under the shoes prior to each rotation. In this case, the rotor
is jacked, and then, immediately after the jacks are released, the rotor is rotated.
The first method of taking static runout readings requires taking plumb readings with the shaft
rotated to the 0, 90, 180, and 270 degree positions. Readings are usually taken only at two
elevations to speed up the process because the straightness of the shaft should already be verified.
From the plumb readings, it is possible to determine the diameter of runout at the turbine bearing
and the location of the center of runout with respect to plumb. Figure 16 is an example of the
form used to record the data and perform the calculations.