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 testing machine for journal bearings. When the speed is increased, it reaches the critical whirl speed, where noise and severe vibrations are generated. In a rotating system of a rotor supported by two hydrodynamic journal bearings, the stiffness of the shaft combines with that of the hydrodynamic journal bearings (similar to the stiffness of two springs in series). This stiffness and the distributed mass of the rotor determine the natural frequencies, also referred to as the critical speeds of the rotor system. Whenever the force on the bearing oscillates at a frequency close to one of the critical speeds, bearing instability results (similar to resonance in dynamic systems), which often causes bearing failure. An example of an oscillating force is the centrifugal force due to imbalance in the rotor and shaft unit.
HYDRODYNAMIC BEARING WHIRL
In addition to resonance near the critical speeds of the rotor system, there is a failure of the oil ﬁlm in hydrodynamic journal bearings under certain dynamic conditions. The stiffness of long hydrodynamic bearings is not similar to that of a spring support. The bearing reaction force increases with the radial displacement, o–o1 , of the journal center (or eccentricity, e). However, the reaction force is not in the same direction as the displacement. There is a component of cross-stiffness, namely, a reaction-force component in a direction perpendicular to that of the displacement. In fact, the bearing force based on the Sommerfeld solution is only in the normal direction to the radial displacement of the journal center. The cross-stiffness of hydrodynamic bearings causes the effect of the halffrequency whirl; namely, the journal bearing loses its load capacity when the external load oscillates at a frequency equal to about half of the journal rotation speed. It is possible to demonstrate this effect by computer simulation of the trajectory of the journal center of a long bearing under external oscillating force....