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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 11, Gear Vibration

Section 6, Minimizing the Effect of a Beat on a FFT-Type Instrument

Fine tuning a vibration instrument to minimize the effect of a beat on the instrument is limited. For tuneable filters, it is difficult to eliminate this effect when the two frequencies that caused the beat are within 5 percent of each other. Many plants now have FFT's or FFT-type data collectors. Most of these are capable of measuring phase as well as providing spectra of amplitudes at various frequencies. Although there are differing instructions for different instruments, obtaining phase most often requires a reference strip of light-reflective tape attached to the shaft or other part of the rotor.

As the tape passes the light-sensitive phase pickup, a signal is given to the instrument. Phase is measured as the angle between the vibration "high spot" and the tape. (If the rotor cannot be shut down in order to attach the tape, a spot of special highly reflective white paint can be applied. A rubber eraser or small piece of plastic foam can be dipped into the paint and shot at the rotating part with a rubberband.)

The FFT can then be set up for "synchronous time averaging." With the phase mark signal originating only from the rotor being analyzed, the vibrations from other sources, over time, cancel themselves. Averages are obtained for anywhere from 30 seconds to several minutes. The closer the outside source frequency is to the frequency of the machine being analyzed, the longer it takes to get enough averages to eliminate the effects of the outside source. This method allows you to analyze or balance one rotor out of many that are mounted on the same structure, without the complications of beats. Specific setup details are obtained from the instrument manufacturer.


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