Low Frequency Shaft Vibration Tests and Analyses

DeCamillo Sa, Cloud CHb, Byrne JMb and He Mb

a Kingsbury, Inc. , 10385 Drummond Road Philadelphia. PA 19154, USA.
b ERG Machinery Consulting. LLC, 703 Highland Avenue. Charlottesville, Virginia 22903. USA.

Tilt pad journal bearings are well known for overcoming vibration limitations of fixed geometry bearings. There are, however, other vibration phenomena associated with tilt pad bearings that are topics of past and present research. Over the past few years, unusual, low-frequency, radial vibrations have been observed in different types of machines using tilt pad journal bearings. The vibrations are not like sharp, subsynchronous spikes that often indicate a serious problem. They are of low frequency and amplitude, and fluctuate randomly. These low level vibrations have raised concern in acceptance tests of critical machinery, even in cases that comply with industry specifications, because their cause and nature are unknown.

This paper presents shaft and pad vibration data from tilt pad journal bearing tests that were performed to investigate and better understand these subsynchronous indications. The low frequency vibrations are compared under the influence of speed, load, oil flow, and bearing orientation. Results are presented for conventional and direct lube tilt pad bearings, along with discussions of parameters and methods that were successful in reducing and eliminating these low level vibrations.

Theoretical analyses are also presented and discussed in relation to the test observations. The analyses use an algorithm that accounts for effects when there is insufficient flow for the oil films, and the resulting full matrix dynamic coefficients are used to examine the frequency response characteristics of each pad in relation to test measured pad vibration. The model correlates well with test results, thereby providing a useful tool to assess the effects of multiple factors and complex pad/shaft interactions that influence these low level vibrations.

The intention is that this information will be of value to researchers, analysts, and other personnel involved with design and prediction of hydrodynamic bearings in high-speed machinery.

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