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6.2 Correcting Excessive Dogleg and Offset
Correcting a dogleg between the generator and turbine shafts requires installing a shim pack
between the coupling faces. If readings indicate that a dogleg exists, the first step in correcting it
is to verify that it really exists. A dogleg may show up when a shaft is in a bind or is not totally
free. Check for a free shaft. If the shaft is free, rotate the shaft 90 degrees and take another set of
plumb readings. If the dogleg is real, the second set of readings should verify this. The dogleg
should simply move from the north-south plot to the east-west plot or vice versa. If the dogleg is
still in the same plane, the shaft is not free.
between the coupling faces. If readings indicate that a dogleg exists, the first step in correcting it
is to verify that it really exists. A dogleg may show up when a shaft is in a bind or is not totally
free. Check for a free shaft. If the shaft is free, rotate the shaft 90 degrees and take another set of
plumb readings. If the dogleg is real, the second set of readings should verify this. The dogleg
should simply move from the north-south plot to the east-west plot or vice versa. If the dogleg is
still in the same plane, the shaft is not free.
When calculating the amount of shims to install in the coupling, several consistent readings are
important. Installing shims in the coupling is a very time consuming process and, preferably,
should be done only once. The amount of shims required should be calculated for several sets of
readings, and, if there are any major differences between calculations, more readings should be
taken until an acceptable level of consistency is achieved. It should be remembered that the
shims should be installed so that the shim pack creates a wedge to prevent distortion of the
coupling.
important. Installing shims in the coupling is a very time consuming process and, preferably,
should be done only once. The amount of shims required should be calculated for several sets of
readings, and, if there are any major differences between calculations, more readings should be
taken until an acceptable level of consistency is achieved. It should be remembered that the
shims should be installed so that the shim pack creates a wedge to prevent distortion of the
coupling.
Excessive offset occurs when the generator and turbine shafts are coupled together and are not
concentric. This can occur if the coupling bolts are a loose fit in the coupling. If excessive offset
is present, it usually requires realigning the shafts and reboring the coupling for oversized bolt
holes. On most couplings, there is also a register fit between the two shafts. If this is the case,
the register fit will have to be machined as well.
concentric. This can occur if the coupling bolts are a loose fit in the coupling. If excessive offset
is present, it usually requires realigning the shafts and reboring the coupling for oversized bolt
holes. On most couplings, there is also a register fit between the two shafts. If this is the case,
the register fit will have to be machined as well.
6.3 Static Runout Data
Static runout can be measured either of two ways. Both methods require rotating the shaft 90
degrees, four times. With Method I, described below, plumb readings are taken at each position.
Method II uses dial indicator readings. With either method, the shaft should be centered at the
upper guide bearing or, with an umbrella unit, at the guide bearing closest to the thrust bearing.
Before any readings are taken, it should be verified that the shaft is free. It may be necessary to
move the shaft off center to obtain a free shaft, especially if clearances are tight or the unit is
severely out of plumb. On spring loaded bearings where the springs are relatively soft (i.e., the
springs deflect significantly under just the weight of the unit), the shaft plumb may change if the
thrust runner is moved off the center of the thrust bearing. In these cases, it may be necessary to
shim the bridge to make it possible to obtain a full rotation with the shaft free and the thrust
block centered on the thrust bearing. It may take several shim moves before a full free rotation is
possible. Prior to each rotation, the shaft should be oiled and the shaft held in place snugly with
jacking bolts with bronze heads or, if the guide bearing is a segmented shoe type, four guide
bearings. This prevents excessive lateral movement, or "skating," of the thrust runner during the
rotation.
degrees, four times. With Method I, described below, plumb readings are taken at each position.
Method II uses dial indicator readings. With either method, the shaft should be centered at the
upper guide bearing or, with an umbrella unit, at the guide bearing closest to the thrust bearing.
Before any readings are taken, it should be verified that the shaft is free. It may be necessary to
move the shaft off center to obtain a free shaft, especially if clearances are tight or the unit is
severely out of plumb. On spring loaded bearings where the springs are relatively soft (i.e., the
springs deflect significantly under just the weight of the unit), the shaft plumb may change if the
thrust runner is moved off the center of the thrust bearing. In these cases, it may be necessary to
shim the bridge to make it possible to obtain a full rotation with the shaft free and the thrust
block centered on the thrust bearing. It may take several shim moves before a full free rotation is
possible. Prior to each rotation, the shaft should be oiled and the shaft held in place snugly with
jacking bolts with bronze heads or, if the guide bearing is a segmented shoe type, four guide
bearings. This prevents excessive lateral movement, or "skating," of the thrust runner during the
rotation.