Practical Applications of Journal
A hydrodynamic journal bearing operates effectively when it has a full fluid film
without any contact between the asperities of the journal and bearing surfaces.
However, under certain operating conditions, this bearing has limitations, and
unique designs are used to extend its application beyond these limits.
The first limitation of hydrodynamic bearings is that a certain minimum
speed is required to generate a full fluid film of sufficient thickness for complete
separation of the sliding surfaces. When the bearing operates below that speed,
there is only mixed or boundary lubrication, with direct contact between the
asperities. Even if the bearing is well designed and successfully operating at the
high-rated speed, it can be subjected to excessive friction and wear at low speed,
during starting and stopping of the machine. In particular, hydrodynamic bearings
undergo severe wear during start-up, when the journal accelerates from zero
speed, because static friction is higher than dynamic friction. In addition, there is
a limitation on the application of hydrodynamic bearings in machinery operating
at variable speed, because the bearing has high wear rate when the machine
operates in the low-speed range.
The second important limitation of hydrodynamic journal bearings is the
low stiffness to radial displacement of the journal, particularly under light loads
and high speed, when the eccentricity ratio, e, is low. Low stiffness rules out the
Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.
application of hydrodynamic bearings for precision applications, such as machine
tools and measurement machines. In addition, under dynamic loads, the low
stiffness of the hydrodynamic bearings can result in dynamic instability, referred
to as bearing whirl. It is important to prevent bearing whirl, which often causes
bearing failure. It is possible to demonstrate bearing whirl in a variable-speed
testing machine for journal bearings. When the speed is increa