Understanding Sidebands in Vibration Analysis

Devices

Portable balancer & Vibration analyzer Balanset-1A

€1,975.00 + VAT (if applicable)

Parts

Vibration sensor

€90.00 + VAT (if applicable)

Parts

Optical Sensor (Laser Tachometer)

€124.00 + VAT (if applicable)

Devices

Balanset-4

€6,803.00 + VAT (if applicable)

Parts

Magnetic Stand Insize-60-kgf

€46.00 + VAT (if applicable)

Parts

Reflective tape

€10.00 + VAT (if applicable)

Devices

Dynamic balancer “Balanset-1A” OEM

€1,735.00 + VAT (if applicable)

Sidebands are small frequency peaks that appear in an FFT spectrum at equal intervals on either side of a larger central peak known as the carrier frequency. Their presence is a definitive sign of modulation — a condition in which one signal is being “imprinted” onto another — and the spacing between the sidebands equals the frequency of the modulating signal. Because that spacing points directly at the rotating element responsible, sidebands are among the most powerful and definitive diagnostic patterns in vibration analysis, particularly for gearbox and bearing fault detection.

1. What Sidebands Are: Modulation in the Spectrum

Modulation is a familiar idea from radio, and the mechanism in a gearbox is the same. A steady high-frequency tone (the carrier) has its strength varied by a slower repeating event (the modulator); in the spectrum, that variation does not blur the carrier peak — it splits energy off into symmetric satellite peaks. The carrier itself is usually a forced vibration generated by normal operation, while the modulator is the once-per-revolution rhythm of a faulty component. Recognising the pattern is what separates a confident diagnosis from a guess.

2. How Sidebands Are Generated

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

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

The result in the FFT spectrum is a large peak at the GMF (the carrier) flanked by smaller sideband peaks spaced at the gear’s rotational speed. This pattern proves not only that a fault exists but that it is located on that specific gear. The relationship is captured by a simple formula:

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

The family of peaks above and below the carrier therefore forms an evenly spaced comb, and counting the spacing in hertz — then converting it to rpm — tells the analyst exactly which shaft is misbehaving.

3. Key Applications in Machine Diagnostics

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, they indicate a fault on that gear — a cracked tooth, a worn tooth, or eccentricity.
• Sidebands around harmonics of GMF: severe faults will often generate sidebands around the 2× and 3× GMF as well, so the comb pattern repeats around each harmonic.
• Hunting Tooth Frequency: in complex gear sets, specific non-integer sidebands at the hunting tooth frequency can pinpoint a fault that only occurs when two particular teeth on different gears come into contact.

Rolling-Element Bearing Diagnostics

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

• A defect on the inner race rotates with the shaft, and as it moves in and out of the bearing’s load zone the amplitude of the impacts it generates rises and falls.
• This produces amplitude modulation of the inner-race fault frequency, BPFI.
• The resulting spectrum shows a peak at BPFI with sidebands spaced at 1× the shaft’s rotational speed. Seeing this pattern is a very high-confidence indicator of an inner-race defect — and it is one reason envelope analysis is so effective at demodulating these signals.

Electric-Motor Diagnostics

Problems with the rotor bars in an AC induction motor can cause sidebands to appear around the 1× running-speed peak. These sidebands are spaced at the pole pass frequency — the slip frequency of the motor multiplied by the number of motor poles — and are a classic signature of broken rotor bars.

4. Analysis Considerations

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

• High resolution: a high-resolution FFT (for example 3200 or 6400 lines) is required to see the sideband peaks clearly and measure their spacing accurately. With low resolution the sidebands are “smeared” together with the carrier peak. The relationship between line count, span, and resolution can be checked with an FFT resolution calculator.
• Trending: the number and amplitude of the sidebands are a good indicator of fault severity. As a fault worsens, more sidebands appear and their amplitude grows, so logging them over time through trend analysis tracks the deterioration.
• Zoom FFT: the Zoom FFT function on an analyser lets the analyst magnify a narrow frequency range at very high resolution to confirm the presence and spacing of sidebands.

5. Reading the Spacing: From Pattern to Diagnosis

The diagnostic power of a sideband family lies in its arithmetic. Because the spacing equals the modulating frequency, an analyst can work backwards from the comb to the culprit: spacing at 1× shaft speed implicates that shaft; spacing at the slip-related pole-pass frequency implicates the motor’s electrical condition; non-integer spacing implicates a specific tooth pairing. Measuring the gear-mesh frequency and its expected sideband structure in advance — for instance with a dedicated gear mesh frequency calculator — lets the analyst predict exactly where to look before opening the spectrum.

In the field these patterns are captured with a portable spectrum analyser carried from machine to machine. An instrument such as the Balanset-1A measures the vibration spectrum on a running machine at high enough resolution to resolve the sideband comb around a gear-mesh or bearing fault frequency, so an engineer can confirm the diagnosis on site; and when the same survey reveals that the dominant problem is simple unbalance rather than a tooth or race defect, the instrument moves straight into field balancing to correct it.

When an analyst finds a clear, symmetric sideband pattern at the expected spacing, the confidence of the diagnosis rises from “possible” to “highly probable” — which is precisely why sidebands are treated as one of the most trustworthy fingerprints in the field.

← Back to Main Index