Practical Applications of Journal Bearings
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