Page 193 - Steam Turbines Design, Applications, and Rerating

172 Chapter Ten

A variety of characteristics can be obtained for different numbers of
pads and different load orientations (Fig. 10.5). It should be noted that
there is no single best bearing for all rotors that can be assumed from
such comparisons. The complex interaction between stiffness and damp-
ing effects in the vertical and horizontal directions, plus significant
cross-coupling effects in fixed-arc bearings, must be evaluated by rotor
response calculations. In the design process, such calculations are made
for the rotor with different bearings and clearances. The final bearing
selection is then based on the results of the rotor dynamics analysis.

10.5 Refinements
As a consequence of laboratory bearing tests, several mathematical
options have been added to the basic analytical bearing model. A mod-
ified computer program is now capable of evaluating their effects. In
practice, they do not result in appreciable changes in calculated rotor
response for rotor-bearing support systems that are relatively insensi-
tive to unbalanced forces. In the case of more sensitive systems, how-
ever, the analysis may show significant changes. One of these effects
is associated with local deformation of the pad contact surfaces where
the concentrated compressive stresses are high. This hertzian effect
results in a decrease in the equivalent bearing stiffness values.
Another effect is due to slight thermal distortions of the pads, which
tend to produce the effects of preload. The oil temperature at the bab-
bitt surface of the pad is greater than at the back of the pad, which
effectively increases the bearing radius while the minimum bearing
clearance decreases. This introduces a small amount of preload that
can change the bearing characteristics.
Deliberate preload by design can cause significant changes in the
stiffness and damping coefficients. These changes must be evaluated
for specific applications to determine whether or not they are benefi-
cial. However, the general design philosophy is to produce a rotor-
bearing system that is not dependent on preload for success. One rea-
son for this is that it may be difficult to maintain the desired preload.
For example, slight bearing surface wear or wipes, or manual rework,
may inadvertently remove the preload and result in a vibration prob-
lem if the design is dependent on preload.
Other effects such as pad inertia and fluid inertia can also be con-
sidered. Pad inertia is not usually of practical significance. The pad size
is such that it readily tracks the shaft orbit and is not subject to flutter
or the introduction of cross-coupling effects. Fluid inertia is the carry-
over of momentum from one pad to the next. Its major effect is to sus-
tain an oil film wedge in the upper pads, which normally experience
little or no loading. Without this effect, the upper pads may theoreti-