Onset of 12X Vibration and Its Prevention

Abstract

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.