Interesting Rotor Dynamics
Observations on
Oil Whirl and Whip
Lin Liu,
Zhuang Li,
Suri
Ganeriwala
SpectraQuest Inc., 8201 Hermitage Road, Richmond, VA 23228
Published: April 2006
Abstract
In this study, the effects of load on oil whirl and whip were
studied by using a rotor dynamics simulator with fluid film
bearings. Rotor displacement during machine run up and coast down
under different loading conditions were measured, analyzed and
presented. Besides the oil whirl and whip introduced in traditional
textbooks, harmonics of oil whip are observed. Some other vibration
components associated with oil whip are also observed
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Introduction
Oil whirl is a common problem with
journal bearings used on machines equipped with pressure lubrication
systems operating at relatively high speeds.
If the shaft is moved off center due to load, eccentricity, or
imbalance, then the clearance on one side of the bearing will be
greater than that on the other side, as shown in Fig. 1.
Figure 1 Shaft off-center in
Journal Bearing
As the lubricant rotates at less
than 50% of shaft speed, it must squeeze through the narrow area
where the shaft is closest to the bearing. The average speed of the
lubricant increases inside the gap and slows down when it leaves the
gap. Such a speeding up and slowing down process creates turbulence
on both sides of the gap, and a vortex develops in the high-pressure
lubricant zone.
The shaft that rides on the oil vortex performs
much like a surfboard that rides the surface of a wave. The
so-called oil whirl, whose frequency is somewhat less than half of
the shaft rotational speed, causes instability. The oil whirl stays
proportional to the shaft frequency and drops out when shaft RPM
drops below the instability threshold.
The oil whirl frequency approaches the first
critical speed of the shaft as the shaft exceeds more than two times
its first critical speed, creating a resonant condition called oil
whip. The oil whip frequency remains constant at the first critical
speed of the shaft and drops out when shaft frequency drops below
two times its first critical speed. Both the phenomena can be severe
and result in a penetration of the lubrication film. When this
happens, the shaft impacts against the bearing and serious damage
may happen.
In this study, a series of tests were carried out on a rotor
dynamic simulator with fluid film journal bearings to observe the
oil whirl and whip phenomena.
Figure 2. Waterfall Plot for Shaft with One thin
Disk (dB scale)
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