It is important to recognize sidebands phenomenon in machinery analysis. Sidebands are caused by amplitude modulation between two signals. They can be found in machines which have gearbox, rolling element bearing, and other certain fault conditions.
Sidebands can be recognized by carefully examining evenly spaced peaks in the spectrum (frequency domain), centered on one peak. The center peak is called carrier frequency and the sidebands are the modulation frequency.
An example of modulation frequency is gear misalignment. The modulation occurs in a gearbox because two frequencies are tied together. The two frequencies are the shaft turning speed and gear mesh frequency. Note that gear mesh frequency is the number of teeth on a gear multiplies by the speed of its shaft. Since the shaft is misaligned, the forces of gears vary as the shaft turns. It produces varying amplitude. The frequency modulation is the shaft speed of one of the gears. Examples of carrier frequencies include:
- Gearmesh frequency
- Bearing inner race frequency
- Rotor bar pass frequency
Examples of sideband frequencies include:
- Running speed
- Fundamental cage frequency in bearing
- Pole pass frequency