Peculiar, low-frequency, radial vibrations have been observed in various turbomachinery using tilt-pad journal bearings. Unlike discrete subsynchronous spikes that often indicate a serious problem, the vibrations are indiscrete and of low frequency and amplitude. The low level shaft indications have raised concern in witness tests of critical machinery, even in cases that comply with American Petroleum Institute (API) limits, owing to uncertainty regarding the cause and nature of the vibrations.
This paper presents shaft and pad vibration data from various tilt-pad journal bearing tests that were undertaken to investigate and better understand these subsynchronous indications. The vibration characteristics are defined and compared under the influence of speed, load, oil, flow, and bearing orientation. Results are presented for conventional and direct lube tilt-pad bearing designs, along with discussions of parameters and methods that were successful in reducing and eliminating these low level vibrations.
The test results indicate that the low-frequency shaft indications are caused by pad vibration. Hypotheses and analyses are presented and discussed in relation to the test observations.
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