Do you know of a relatively easy method to determine if a machinery vibration issue might be resonance related?
While life-limiting structural resonance issues are relatively rare, a single resonance problem can cause more plant downtime, maintenance cost, and reputational damage than numerous more common vibration issues like imbalance or misalignment. Identifying whether a machinery issue is resonance-related is critical, and this blog presents an easy method to help you make that determination.
Typically, vibration amplitude versus frequency data (FFT plot) is displayed on a linear scale (as shown in Figure 1). However, to distinguish between a high-force issue (such as imbalance) and a more complex resonance problem, you can plot the vibration amplitude on a logarithmic scale (Figure 2). Most vibration analyzers offer this feature, making it a simple yet effective method—often referred to as a "poor man's impact test"—to assess whether resonance is the root cause.
If a natural frequency is close to an excitation frequency (like the 1x rpm or 60 Hz, as shown in Figure 2), it suggests that the issue is more likely resonance-related (represented by a broadband peak highlighted by red lines). In such cases, rebalancing the rotor to reduce the excitation source might only provide a temporary fix. A more permanent solution would involve modifying the stiffness of the machinery system to increase the separation margin between the excitation frequency and the structural natural frequency or improving damping within the system.
However, if the structural natural frequency occurs at a different frequency—say, at 80 Hz—while the high vibration is at 1x rpm (60 Hz), then it is more likely that the cause is a force such as unbalance, rather than structural resonance. In this case, addressing the resonance issue may not be necessary, and a simpler solution, such as rebalancing the rotor, could resolve the problem.
Figure 1. Vibration data is typically plotted on a linear scale.
Figure 2. Plotting with the y-axis on a logarithmic scale can help determine if a problem is related to natural frequency excitation (resonance). If resonance is suspected, the next step should be to perform an experimental modal analysis (i.e., impact test). The results, plotted as a Frequency Response Function (FRF), can confirm natural frequencies and provide important damping data for further analysis.
