Page 194 - Steam Turbines Design, Applications, and Rerating

Rotor Dynamics Technology 173

cally nose-dive because of the moment caused by shear forces along the
babbitt surface.

10.6 Bearing Support Considerations
The dynamic characteristics of the structure that supports the bear-
ings can have an appreciable effect on rotor response, depending pri-
marily on the interaction between the oil film coefficients and the
support stiffnesses. The support theoretically includes all of the struc-
tures installed between the oil film and the earth. Part of this is a func-
tion of the turbine design; the other major part is in the foundation
design.
The bearing itself must be aligned to the journal and held firmly. For
fixed-arc bearings, this is accomplished by the use of spherical bearing
liner seats and an interference fit between the bearing liner and the
bearing bracket. The purpose of the spherical seat is to obtain initial
alignment. Once this is done, the bearing is clamped in place by the
interference fit and cannot change its position. Tilting-pad bearings
that have pads with spherical pivots do not require spherical liner
seats for alignment, since the pads may tilt in any direction. An inter-
ference fit is still provided between the bearing liner and the bearing
bracket to prevent looseness that may result in vibration at multiples
of the running speed.
The turbine casing support at the thrust bearing end of the turbine
is usually a flex-plate design as shown in Figs. 1.5 and 8.6. It accom-
modates axial thermal expansions by bending but is stiff in the verti-
cal and transverse directions. Another type of support occasionally
used in special applications is the sliding support. It is free to slide axi-
ally and is keyed to restrict transverse motion. Its stiffness is some-
what variable, depending on the static loading and the key fit.
Impedance test measurements indicate that it is comparable to the
flex-plate design in stiffness.
The bearing support at the drive end of condensing turbines includes
the exhaust casing wall and the casing support feet. The support feet
are free to move transversely to accommodate casing thermal expan-
sions, and they are keyed to resist axial movement. Considerable
design effort has been made in the modern exhaust casings to achieve
increased stiffness in the transmission path from the drive end bear-
ing, through the back wall, and into the casing support feet. Modern
noncondensing turbine casings are supported at the drive end by trun-
nions. The bearing itself is directly supported by a pedestal that pro-
vides a stiff connection to the foundation.
One of the important reasons for maximizing stiffness between the
bearings and the foundation is that it optimizes the inherent damping