In Wikipedia, the bearing is defined as a device to
permit constrained relative motion between two parts, typically
rotation or linear movement.1 Fluid bearings use a thin layer fluid
to support the bearing load so that there is no metal-to-metal
contact when the shaft rotates, which is the able to reduce wear.
The portion of the rotating shaft at the bearing is often called the
journal. The working mechanism of fluid film bearing was
discovered more then 100 years ago. “Without relative motion or a
converging clearance, no pressure or load capacity will be
developed. It is the pressure in the lubricant film that carries the
external load and separates the solid surfaces.” 2 Figure 1
illustrates a typical hydrodynamic bearing where the fluid is pumped
in through an orifice. When the shaft spins with an angular velocity
of ω, the fluid is dragged into a convergent clearance. Due
to gravity, eccentricity, manufacturing imperfection, misalignment,
unbalance, and other factors, the rotor cannot always be perfectly
centered. Therefore, the fluid forms a wedge (h1 is larger
than h0 in Fig. 1) which generates pressure to support the
applied load. By assuming velocity continuity, the fluid velocity on
the surface of the journal is the same as the velocity of the
journal (V) at the contact point, and is zero on the surface
of the bearing. The fluid velocity distribution through the
thickness is normally not linear, as shown in Fig.1. This is
presumably caused by fluid loss due to end leakage. So the fluid
flows not only in the radial direction following the rotating of the
shaft, but also in the axial direction. As a result, the average
velocity of the fluid is slightly less than V/2.
As the fluid friction
is proportional to the viscosity as well as the velocity gradient,
an increase in the running speed will increase the heat generation.
On the other hand, lubricant’s viscosity is very sensitive to
temperature. In the meanwhile, the dynamic parameters, namely
stiffness and damping, are strongly affected by the fluid viscosity.
Consequently, when a machine is speeding up, an increase in
temperature will decrease the bearing stiffness as well as damping,
which will further influence the dynamic behavior of the rotor
system.
Bearing clearance is
also an important parameter in rotor dynamics. For a cylindrical
bearing, the diametral clearance is defined as the difference
between the bearing and shaft diameters. The eccentric ratio, which
is inversely proportional to the clearance, has a significant effect
on the dynamic characteristics of a rotor system. In addition, a
change in the clearance will change the fluid film and the shear
stress, which will further affect the energy dissipation caused by
the shear deformation. Therefore, the clearance has quite
complicated effects on the performance on a rotor system. In this
study experiments have been carried out to investigate the effects
of clearance on the bearing damping.
Figure 1. Schematic of a hydrodynamic bearing and
the fluid velocity distribution.
Figure 2. Rotor Dynamics Simulator
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