Interesting Rotor Dynamics Observations on Oil Whirl and Whip

Lin Liu, Zhuang Li & Suri Ganeriwala
SpectraQuest Inc., 8227 Hermitage Road, Richmond, VA 23228
Published: April, 01 2006

Abstract

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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

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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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