The terms "natural frequency," "resonance" and "critical speed" are used as synonymous, but they are not exactly the same concept. It's also common when the analyst doesn't know the source of high vibrations; the diagnosed fault is resonance.
"Critical speed" is when the shaft itself is resonant, usually changing its deflection shape. We usually say, "the machine is passing through its critical speed." However, we can't refer to "critical speed" when talking about a foundation, pipe, column, or non-rotating component. Well-trained vibration analysts consider it. However, vibration analysis becomes more common (which is great!), and we start calling analysts to colleagues who wrote an exam and got a certificate; it's common now to mix the terms to refer to mix the terms "natural frequency," "resonance" and "critical speed".
If any of the many vibration sources creates a vibration frequency equal or nearly equal to the natural frequency, that part will resonate. Let's see what natural frequency is.
Every physical system has its natural frequencies. These are the frequencies at which the system will tend to vibrate when subjected to certain dynamic forces. The natural frequencies are dependent on the mass and stiffness distribution in the system. Then, resonance is the condition in which a dynamic force drives a system to vibrate at its natural frequency. When a mechanical system is in resonance, a small force can produce a large vibration response.
Experienced vibration analysts know that a well-balanced rotor vibration produces a large vibration if its RPM is near the mechanical system's natural frequency where it is working (the machine). The solution could be not only balancing the rotor. Instead, other solutions could be running the rotor at another RPM, changing the system's stiffness, or damping it.
The imbalance is definitively not the only vibrations' source that can cause resonance. Vibrations from misalignment, gearmesh, bearings, electrical issues, or even vibrations transmitted from other machines can cause resonance if they are near the system's natural frequency.
The reality is more complicated than this. Every machine component, a span of steel, concrete, pipes' segment, and concrete bases have its natural frequencies. A non-rotating part can generate a resonance condition, and it won't be easy to detect doing a regular vibration analysis. In these cases, resonance is diagnosed by discarding other causes of high vibration. However, plotting mode shapes is the right way to determine whether a part is resonant.
Other ways to test resonance are:
- Start-up and shut-down test
- Impact tests
- Use braces to change the resonant frequency temporarily
- Use of variable speed vibration shaker
Power-MI faults catalog includes resonance as a vibration analysis fault. For more information on this topic, click here.
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