Handbook of Turbomachinery Second Edition Revised - Ventech!

In a steam turbine, a thrust bearing is used to locate the rotor axially
with respect to the stationary parts and at the same time carry any axial
steam loads that may be imposed on the rotor. Journal bearings are used to
carry the weight of the rotors and any transverse steam loads imposed on
the rotor, and also to locate the rotor radially with respect to the stationary
parts.
A thin film of lubricant, which provides the necessary load support
function, separates the journal and bearing surfaces. The function of the
lubrication supply system is to provide a continuous supply of oil into the
clearance space between the bearing and the rotating journal. Once the oil is
within the clearance space, actual separation of the journal surface from the
bearing surface is achieved by formation of the load-supporting oil film via
hydrodynamic action (Fig. 52).
The oil film develops because oil adheres to the journal and when the
journal is rotated, it pulls some oil with it. Since the radius of the journal is
smaller than the radius of the bearing, the surfaces form a converging wedge
and the clearance between them will be a minimum at one circumferential
position. When oil is pulled from a position with greater clearance into the
position of minimum clearance, oil pressure within the film becomes
established. The film pressure increases to equal the journal loads and push
the journal away from the bearing.
The two main criteria of bearing performance are the coefficient of
friction and the minimum thickness of the oil film. When full film
lubrication is achieved, the rotor glides along without any metal-to-metal
contact because the oil film supports the journal. Without full film
lubrication, metal-to-metal contact of the rotating journal with the bearing
occurs and results in a marked increase in frictional resistance. While the
resulting increase of drive torque required to overcome the increased
frictional resistance is of very little concern, the side effects of increased
bearing temperature and wear of the bearing and/or journal surfaces are of
major concern.
A closer look at the operating geometry and fluid-film pressure
distribution of the journal bearing in Fig. 52 shows that the journal center,
0 0 , is displaced radically from the bearing center, 0. The resulting line of
centers is not in line with the direction of the applied radial load but is at an
angle to it. Choosing the angular location of maximum film thickness as a
reference (o ¼ 0), the minimum film thickness will be at o ¼ 180. The solidline
pressure distribution is typical for incompressible fluid-film journal
bearings. In this case, positive pressures are generated within the converging
film thickness zone with peak pressure obtained just before the location of
minimum film thickness. The pressure is reduced to ambient pressure just
after the location of minimum film thickness. Beyond this point of return to
Copyright © 2003 Marcel Dekker, Inc.