clearance. If the shaft isn't plumb and centered, the clearances can be calculated based on a final plot of
the shaft and turbine bearing centerline, similar to the plot in figure 15, after the center of runout is plumb.
Sleeve type journal bearings usually are a tight fit in the upper and lower bridges so that the position of the
shaft is not critical.
While the turbine bearing bore should be concentric with its shell, there can be some deviation. If there is
any doubt as to whether the bearing bore is concentric to its shell or if the housing is not concentric to the
bearing bore, as may be the case with a doweled bearing, the turbine guide bearing should be installed at
this point. With the turbine guide bearing in place, a free shaft will no longer be possible; but for
installing the bearings, the shaft will simply be used as a reference and a free shaft is no longer important.
The most common methods of securing the turbine bearing in its housing are employing a tight fit
between the bearing and housing, employing a tapered fit between the bearing and housing, or using
dowels. A bearing with a tight fit is lowered into place and the flange is bolted tight to the bearing
housing. The tight fit between the bearing shell and housing prevents any lateral movement. There may
also be dowels in the flange to prevent angular movement.
The bearing with the tapered fit is somewhat self centering. As the bearing shell is lowered into the
housing, the taper centers the bearing and holds it in place. Bolts in the bearing flange are used to hold the
bearing in place but there is always a gap between the flange and the housing. When installing a bearing
with a tapered fit, it is important to keep the bearing level. There is usually a machined surface on top of
the bearing that is suitable for a precision level. The flange bolts should be tightened so that the bearing
shell is tight in the housing but not so tight that the clearances are reduced. When tightening the flange
bolts, it is important to frequently check the clearances on the bearing. This will provide an indication of
the level of bearing and whether it is being driven too far into the fit.
The bearings that use dowels normally have some clearance between the bearing shell and the housing.
Several dowels are used to prevent lateral movement of the bearing. To install, the bearing is lowered into
place, the dowels are installed, and then the flange bolts are tightened.
Once the turbine bearing is in place, the shaft should be centered in the bore, either with jacking bolts or
with shims. A set of plumb readings are then taken to verify the position of shaft and to determine how
far to move the top of the shaft to make it plumb. With the shaft plumb, the generator guide bearings can
be installed. The design of adjustable shoe guide bearings varies, but most use jack bolts or adjustment
screws that also act as pivot points for the shoes. The adjustment screw is used to set the bearing
clearance. Because the bearing segments are free to pivot in any direction, setting the clearances
accurately can be very challenging. Feeler gauges should extend all the way through the bearing when
taking readings to prevent a false high reading at the top of the bearing when the bottom is tight against
the shaft. To provide proper lubrication, the radius of tilting pad bearings may be machined to a larger
value than the shaft radius plus the design clearance. Because of this, clearances measured at the edges of
the bearing will be larger than the clearance at the center. The design clearance or the specified clearance
on the drawings refers to the clearance in the center of the bearing, so feeler gauge readings should be
taken directly in front of the pivot point.
The installation of sleeve type journal bearings is usually straightforward. The sleeve type bearings are
usually a tight fit in the bridge, or they are doweled in place. Installation consists of bolting the bearings
in place and checking the centers with feeler gauges. Checking the centers is critical. In some instances,
what was thought to be a tight fit actually has considerable clearance, allowing the bearing to be installed