A reader asks whether sleeve bearings can take the place of more expensive tilting pad bearings in a high-speed compressor application.

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EXPERT NO. 1
The answer is that every application
needs to be separately analyzed.
In real machinery, steady-state conditions
do not exist. There is relative motion
between the fluid film bearing and
the shaft as in starting, in stopping or
changing the bearing load suddenly.
Changes in bearing loads may result
from meshing gears, engaging clutches,
etc., or may be periodic as in reciprocating
machinery. In addition, rotating
loads superimposed on unidirectional
loads result from shaft unbalance. The
other issue is that whirl of the shaft
can result in half-frequency whirl and
loss of the hydrodynamic film thickness
separating the shaft from the bearing
and failure of the operating system.

My guess is that the original design using
the tilting pad bearing was based on
inhibiting whirl in the bearing system as
a real or anticipated problem. My recommendation
is if the system is working
and not causing a problem by using
the tilting pad bearing, continue to use
it. However, if a straight sleeve bearing
is to be substituted in the application,
the entire system needs to be analyzed
including a heat balance if the bearing
behavior is to be predicted with any accuracy.
In summary, you cannot substitute
one bearing type for another without
performing an engineering design
analysis and parametrically testing the
new design.