This paper discusses real cases on three different machines where subsynchronous vibration occurred suddenly at a frequency exactly equal to one-half $(12X)$ of the operational speed of 3600 rpm. In two cases, vibration amplitude increased from around 2 mils $(51 μm)$ pp to over 12 mils $(305 μm)$ pp, causing the machine to trip unexpectedly. The $12X$ vibration was even sustained during shutdown at speed below 3600 rpm after trip. Unlike other reported experimental results, shaft orbit measured by proximity probes was fairly circular and dominantly composed of the $12X$ component without significant components at multiple orders of the $12X$. The onset of the subsynchronous $12X$ vibration was sometimes triggered by changes in bearing lube oil temperature and supply pressure, which would typically be believed to be caused by oil whirl that could occur at close to the $12X$ frequency. An upward shift in shaft centerline plot was observed to occur prior to the onset of the subsynchronous vibration. The $12X$ vibration was successfully prevented by changing bearing clearance or adjustment of bearing oil temperature and supply pressure. Parametric excitation analysis on $12X$ vibration is also given in this paper. The main purpose of this paper, unlike previously published papers on analytical or experimental results, is to show shaft orbital and centerline plots as well as spectrum data due to the $12X$ vibration, including effects of operating conditions in real rotating machines.

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