Condition Monitoring, Troubleshooting and Reliability in Rotating Machinery

12Is Vibration Beating on Machinery a Problem?

By Robert X. Perez and Andrew P. Conkey

What is Vibration Beating?

When two vibration signals of nearly equal frequencies are generated by a machine or group of machines, a beat vibration frequency can be produced. The interaction between the two sinusoidal signals composed of similar frequencies results in an alternating constructive and destructive interference pattern (see Figure 12.1a), which causes the composite wave to periodically increase and then decrease in amplitude (see Figure 12.1b) regularly over time. The frequency of the variation in the composite wave is called the beat frequency, which is why this phenomenon is called beating. The count of beats per second is equivalent to the difference in frequencies of the two waves is called beat frequency. Therefore, the beat frequency is calculated as follows:

In this formula, f_b is the beat frequency, f₁ is the frequency of the first vibration signal, and f₂ is the frequency of the second vibration signal (see Figure 12.2).

Beat phenomena can be found on a number of process machines found in the industry. Here are some common machinery situations where beat phenomena may be seen:

• On two poles, induction electric motors where 2x and 2x-line amplitudes can be significant. The absence of beats in the vibration signal may indicate there is only a mechanical problem. In contrast, ...