Handbook of Turbomachinery Second Edition Revised - Ventech!

There is no fixed or absolute value for a satisfactory H-min. The value
depends on the size of the bearing, the nature of the application, the
operating conditions, the degree of reliability required, and other factors
including industrial practice. In no case, however, should the calculated Hmin
be smaller than the sum of the surface roughness of the two mating
surfaces.
Maximum Bearing Oil-Film Temperature
Knowing the value of the film maximum temperature T-max is as important
as knowing that of H-min. While too small an H-min can cause damage by
physical contact, excessive temperatures may cause failure by either
softening or melting of the bearing surface, and this can occur even when
there is an ample film thickness. T-max usually occurs ahead of the trailing
edge of the bearing pad due primarily to the presence of cool lubricant and
heat-transfer effects at the downstream boundary of the pad. In aligned
journal bearings, T-max occurs along the centerline.
It is common practice in industry to use the oil temperature rise, DT,
as a criterion of bearing performance, where AT is the difference between
the bulk temperatures of the oil discharging from the bearing and the oil
supply temperature. While monitoring DT may be helpful in spotting
sudden changes in bearing behavior, it is a poor indicator of the magnitude
of T-max.
Since the value of DT is determined by power loss and oil flow, raising
the oil inlet pressure or increasing the size of groove chamfers will reduce
DT.
Thus, while DT remains a useful overall indicator of the amount of
total heat generation and of any untoward changes in bearing behavior, it
cannot represent or replace the crucial quantity T-max. This can be obtained
only from an appropriate variable temperature analysis, and it has to be
viewed as an independent indicator of bearing safety and performance.
Bearing/Journal Dynamic Stability
The third important criterion of journal-bearing operation is stability.
Hydrodynamic forces may induce instabilities, even though there may be no
external stimuli. The nature of the bearing dynamics dictates that a journal
displacement in one direction produces resisting force Fx along x, as well as
a force Fy in the y direction (Fig. 61). These two resisting forces are
accompanied by two stiffness coefficients Kxx and Kxy, where the first
subscript refers to the direction of the displacement and the second subscript
to the direction of the resisting force. A similar situation arises with a
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