12
Center of Runout
90°
180°
0°
Shaft Centerline
Thrust Block
and Runner
and Runner
Static Runout Caused by Nonperpendicularity of
Thrust Runner to Shaft
Figure 12.--Static runout.
3.4 Plumb
A line or plane is considered plumb when it is exactly vertical. In the alignment of vertical shaft
units, plumb is essentially the reference for all measurements. A common misconception in unit
alignment is that the primary goal is to make the shaft itself plumb. The actual goal is to make
the thrust bearing surface level. The levelness of the shoes is checked indirectly by plumb and
runout readings. If the thrust runner was perfectly perpendicular to the shaft when the shaft was
plumb, the thrust shoes would be level. Due to non-perpendicularity of the thrust runner to the
shaft we instead must make the center of runout plumb. Referring again to figure 12, we can see
that if the shaft is plumb in the 0-degree position, it will be out of plumb by the runout diameter
once the shaft is rotated 180 degrees. If the center of runout is plumb, the shaft will be out of
plumb by half the runout diameter in any rotational position. As long as the runout diameter is
within tolerance, this will be acceptable. By making the center of runout plumb, the thrust shoes
are made level (figure 11).
units, plumb is essentially the reference for all measurements. A common misconception in unit
alignment is that the primary goal is to make the shaft itself plumb. The actual goal is to make
the thrust bearing surface level. The levelness of the shoes is checked indirectly by plumb and
runout readings. If the thrust runner was perfectly perpendicular to the shaft when the shaft was
plumb, the thrust shoes would be level. Due to non-perpendicularity of the thrust runner to the
shaft we instead must make the center of runout plumb. Referring again to figure 12, we can see
that if the shaft is plumb in the 0-degree position, it will be out of plumb by the runout diameter
once the shaft is rotated 180 degrees. If the center of runout is plumb, the shaft will be out of
plumb by half the runout diameter in any rotational position. As long as the runout diameter is
within tolerance, this will be acceptable. By making the center of runout plumb, the thrust shoes
are made level (figure 11).
3.5 Straightness
Straightness refers to absence of bends or offset in the shaft. Offset is the parallel misalignment
between two shafts and occurs at the coupling between the generator and turbine shafts. Angular
misalignment at the coupling is referred to as dogleg (figure 13). Usually, the individual
between two shafts and occurs at the coupling between the generator and turbine shafts. Angular
misalignment at the coupling is referred to as dogleg (figure 13). Usually, the individual