Understanding Sidebands in Vibration Analysis

Definition: What are Sidebands?

Sidebands are small frequency peaks that appear in an FFT spectrum at equal intervals on either side of a larger central frequency, known as the carrier frequency. The presence of sidebands is a definitive sign of modulation—a condition where one frequency is being “imprinted” onto another. The spacing of the sidebands is equal to the frequency of the modulating signal. Sidebands are one of the most powerful and definitive diagnostic tools in vibration analysis, especially for gearbox and bearing fault detection.

How are Sidebands Generated?

Sidebands are created when a primary vibration signal (the carrier) has its amplitude changed over time by a second, slower signal (the modulator). A classic example is a faulty gear tooth:

• The Gear Mesh Frequency (GMF) is the carrier frequency. This is a high frequency generated by the normal meshing of the gear teeth.
• A single cracked tooth on that gear will create a once-per-revolution impact. This impact modulates, or changes the amplitude of, the GMF signal every time the faulty tooth comes into mesh.
• The rotational speed of the gear is the modulating frequency.

The result in the FFT spectrum is a large peak at the GMF (the carrier) with smaller sideband peaks on either side, spaced at the gear’s rotational speed. This pattern definitively proves that a fault exists and that it is located on that specific gear.

The formula is: Sideband Frequency = Carrier Frequency ± (n × Modulating Frequency), where n=1, 2, 3…

Key Applications in Machine Diagnostics

1. Gearbox Diagnostics

This is the primary application for sideband analysis.

• Sidebands around GMF: If sidebands spaced at the running speed of a gear appear around its GMF, it indicates a fault on that gear (e.g., a cracked tooth, a worn tooth, or eccentricity).
• Sidebands around harmonics of GMF: Severe faults will often generate sidebands around the 2x and 3x GMF as well.
• Hunting Tooth Frequency: In complex gear sets, specific non-integer sidebands can pinpoint faults that only occur when two specific teeth on different gears come into contact.

2. Rolling Element Bearing Diagnostics

Sidebands are also critical for confirming bearing faults, especially inner race defects.

• A defect on the inner race is rotating, and as it moves in and out of the bearing’s load zone, the amplitude of the impacts it generates will change.
• This causes amplitude modulation of the inner race fault frequency (BPFI).
• The resulting spectrum shows a peak at BPFI with sidebands spaced at 1x the shaft’s rotational speed. Seeing this pattern is a very high-confidence indicator of an inner race defect.

3. Electric Motor Diagnostics

Problems with the rotor bars in an AC induction motor can cause sidebands to appear around the 1x running speed peak. These sidebands are spaced at the pole pass frequency, which is the slip frequency of the motor multiplied by the number of motor poles.

Analysis Considerations

To effectively use sideband analysis, a high-quality data is essential:

• High Resolution: A high-resolution FFT (e.g., 3200 or 6400 lines) is required to be able to clearly see the sideband peaks and accurately measure their spacing. With low resolution, the sidebands will be “smeared” together with the carrier peak.

– Trending: The number and amplitude of the sidebands are a good indicator of the severity of the fault. As a fault worsens, more sidebands will appear, and their amplitude will increase.

• Zoom FFT: This function on a data analyzer allows the analyst to “zoom in” on a specific frequency range with very high resolution to confirm the presence and spacing of sidebands.

When an analyst finds a clear sideband pattern, it raises the confidence of a fault diagnosis from “possible” to “highly probable.”

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