Vibration Problems
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25 Years of Troubleshooting Vibration Problems
by Nelson Baxter, P.E.
Abstract
Exactly 43 case histories of vibration problems on a broad variety of machines is the topic of this paper. As the title suggests, it offers insight into 25 years of firsthand experience troubleshooting vibration problems. Though the number of case histories is large (43), each write-up is brief and to the point focusing on the problem, type of data collected, key analysis points, conclusions from the data, and finally solutions. Another thing that makes this paper unique is some of the data collected to solve the problem(s) is not vibration data, but instead is either current data or sound measurements. Due to its format, this paper could be read over the course of a few days, week, etc as one could read one or more case histories at a time. Both the beginner and experienced vibration analyst could easily benefit from this compilation of real case histories and lessons learned.
PREVIEW
“PART I- SPECIAL CASES
CASE 1- ¼ HZ VIBRATION IN HYDROELECTRIC DAM.
During the operation of a hydro-electric turbine, the whole structure would begin to shake in a certain load range. The problem was identified as Rheingan’s influence. Rheinghan’s influence is caused by spiral vortex filaments that rotate at a speed lower than the turbine RPM. The load range at which the Rheingan’s influence occurred varied in this case, depending upon the up and down stream water level. The problem was that the vibration supervisory system did not register any excessive levels during the transient, so the operators who were located at a remote location, did not know what load ranges to avoid. The spectra shown in Figure 1 were taken from shaft proximity probes while the problem was present.
The above plots taken with a spectrum analyzer that was DC coupled showed that there was no difficulty in detecting the vibration. The reason that the supervisory did not detect the 15 cpm vibration caused by the Rheinghan effect was that its AC coupling capacitor filtered out the vibration. Since nothing can be done to prevent the Rheinghan effect, modifications to the supervisory system are being considered to allow the operators to at least avoid the unstable load ranges.
CASE NO. 2- VIBRATION OF STEEL STRIP IN STEEL MILL CAUSED BY INDUCTION FURNACE.
EQUIPMENT: An induction furnace in a steel mill was used to heat and diffuse galvanize into the steel strips.
SYMPTOMS: During the induction process, a loud high pitch frequency would radiate from the steel plate. When the sound would begin, vertical stripes would also appear on the plating. The stripes were causing the steel to be rejected by the customer of the steel mill.
TEST DATA AND OBSERVATIONS: An FFT analyzer was set up to determine the frequency of the sound, along with the vibration on the induction furnace and the current being supplied to the induction coils. The frequency being detected in all three cases was at 7250 cycles/second. This frequency corresponded to the operating frequency of the induction furnace. To determine if a change in frequency would have an effect on the problem, the furnace frequency was increased to 9000 Hz. The stripes did not disappear at the higher frequency, but merely moved closer together.
CONCLUSIONS AND RECOMMENDATIONS: It was determined that the induction furnace was exciting the natural frequencies of the plate, creating standing waves, which resulted in the stripes being formed as the galvanize flowed to the nodes.”
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