A Closer Look at Sidebands in the FFT Spectrum

There are times when you will observe sidebands in the Fast Fourier Transform (FFT) spectrum. These sidebands are a common occurrence and are often linked to specific mechanical and electrical issues such as gear mesh frequencies, inner race bearing faults, and various electrical anomalies. Understanding their significance requires a deeper examination. Sidebands provide critical insights into the condition and performance of machinery components. They can indicate the presence of modulation caused by periodic forces or vibrations acting on the system. To fully grasp their implications, let’s dive into a specific example that illustrates how these sidebands manifest in the spectrum and what they signify about the health of the machinery.

An example of sidebands in the spectrum.

Peak and Sidebands Indicate Bearing Faults

In the provided spectrum, a distinct peak represents the inner race defect frequency, particularly noticeable when the outer race is fixed. This peak is flanked by sidebands occurring at the running speed of the shaft. As the bearing condition deteriorates, these sidebands become more pronounced, showing increased amplitudes. This increase in amplitude is a clear indication that the fault is worsening, causing more significant vibrations at harmonics of the defect frequency. These harmonics and sidebands can help diagnose the severity and progression of the fault, allowing for timely maintenance interventions. Thus, monitoring these spectral features is crucial for predictive maintenance and ensuring the longevity and reliability of machinery.

How Vibration Modulation Reveals Machinery Health

What is really going on though? Imagine the defect on the inner race of a bearing as it rotates. This defect, or fault, moves in and out of the load zone, creating two distinct frequencies that are closely related. The first frequency is the actual frequency of interest, which in this example, is caused by the defect on the inner race. The second frequency, known as the carrier frequency, is the shaft’s rotational speed, or shaft RPM. These frequencies interact because as the inner race defect enters and exits the load zone, it modulates the vibration signal at the carrier frequency.

As the shaft rotates, the defect on the inner race causes the vibration amplitude to increase when it moves into the load zone and to decrease when it exits the load zone. This modulation happens at the shaft RPM, meaning that for each full rotation of the shaft, the vibration signal varies in amplitude. When the defect is in the load zone, the bearing experiences higher amplitude vibrations. This is due to the increased force and stress, while outside the load zone, the vibrations are of lower amplitude. This pattern creates a characteristic vibration signature that can be analyzed to diagnose the condition of the bearing.

Modulation Patterns in Rotating Machinery

This concept is also applicable to other rotating machinery components, such as gears and electric motors. For instance, gears experience similar modulation effects when they engage and disengage, while electric motors can show modulation influenced by twice the line frequency. Recognizing these modulation patterns in the vibration waveform is crucial for diagnosing issues in rotating equipment. A deep understanding of how these frequencies interact and what they signify helps in accurately interpreting the condition of the machinery. This ultimately aids in more effective maintenance and prevention of unexpected failures.