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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 11, Gear Vibration

Section 4, Minimizing the Effect of a Beat on a Tuneable Filter-Type Instrument

Previously in this textbook, the word resonance was used to indicate the resonance or resonant frequency of a mechanical part such as part of the machine's structure or of its rotor. However, electrical/ electronic circuitry, such as in radio or TV sets can also be tuned to specific frequencies. Similarly, when the tuneable vibration instrument is tuned to a specific frequency, the "electronic" resonance curve looks similar to that of a mechanical resonance curve. As in a mechanical system, it can also have a relatively wide resonance range or a relatively narrow range, as shown in the diagrams.

Assume that the vibration frequency for which the instrument is tuned coincides with the peak of its electronic resonance curve. In Fig. 1, notice that when properly tuned for the same vibration amplitude, both the broad and sharp (narrow) filter settings pick up the same amount of the vibration and give the same readings.

In Fig. 2, the magnified sections of both resonance curves is tuned to frequency A, but the actual frequency of vibration being measured is frequency B. The tuning frequency error is the same for both curves. The broad filter will still pick up almost all of the vibration, giving only a negligible error in amplitude reading. However, the sharp or narrow filter will give a much larger amplitude error. Also notice that with the broad filter, the actual vibration frequency B crosses the curve on a relatively horizontal and flat part of the peak as compared to that of the sharp filter. The broad filter produces very little error in indicated phase angle, whereas the sharp filter gives a much larger error.

When a beat is not present, it is best to use a broad filter as slight errors in tuning will give minimal, erroneous phase and amplitude readings. But if a beat is present, the broad filter will result in picking up the most interference from the unwanted vibration that originated from the other frequency source. Therefore, for beats, minimize the effect of a beat on the instrument's indicated phase and amplitude by using a sharp or narrow filter setting. However, if using a sharp filter, the instrument has to be very carefully tuned. It should also be retuned each time the machine is stopped and restarted.

Sometimes certain types of machinery do not reach exactly the same operating speed each time. Often when balancing electric motors, for example, the motor increases its speed slightly as the vibration is decreased. The few seconds that it takes to retune the filter on each run, prevents the error (especially phase) that makes the final precision balancing time consuming. This is even more important for variable speed machines, such as turbines and internal combustion engines.

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