Amplitude Modulation in Machine Vibration Signatures

Many machines produce vibration signatures which contain amplitude modulation, and as we have seen in the introduction to vibration section, amplitude modulation causes sidebands to occur in the vibration spectrum. Several types of machine problems can be diagnosed by detailed examination of these sidebands. Examples of machines that produce amplitude modulation are gearboxes, where the tooth mesh frequency is modulated by the turn speed of each gear, and rolling element bearings, where bearing tones can be modulated by turn speed or the fundamental train frequency of the bearing.

In the case of gearboxes, an eccentric gear or bent shaft will cause the tooth mesh tone to be stronger during the portion of the revolution of the gear where the radius is increasing -- the driven gear is actually being accelerated in its rotation during this time. The part of the rotation where the radius is decreasing places less force on the gear teeth, and the tooth mesh tone is less strong. (The tooth mesh tone is also frequency modulated at the same time, and this also creates sidebands in the spectrum, but for this discussion, we will only consider amplitude modulation). Any other defect in the gear, such as a cracked or spalled tooth, will also cause the tooth mesh tone to be irregular, and will result in modulation of the tone and consequent sidebands in the spectrum.

Since the gears in the gearbox usually rotate at different speeds, the amplitude modulation due to each gear will be at a different rate, and the resulting sidebands will be of different spacing. This allows the diagnosis of gearbox faults narrowed down to specific gears and/or shafts by analyzing the sideband patterns in the vibration spectra.

In bearings, modulation of the bearing tones occurs in several ways. If the inner race has a small defect such as a crack, this defect will move in and out of the bearing load zone at the rate of the shaft rpm. This assumes the inner race is rotating and that it is in a horizontal machine where gravity imparts a radial, rather than axial, force on the bearing. The bearing tone will be strongest when the defect is in the load zone, and weakest when it is out of the load zone. This means the inner race ball pass frequency will be amplitude modulated, and its spectrum will have sidebands spaced apart by the rpm of the race. In contrast to this, a fault in the outer race, which is stationary, will always be in the load zone of the bearing, and no modulation will occur, and no sidebands around the outer race frequency will be produced.

If a rolling element has a defect, this roller will move in and out of the load zone also, but will do so at the fundamental train frequency (FTF) rather than the rpm. This is because the rollers are migrating around in the bearing at the cage rpm. This condition will produce amplitude modulation of the ball spin frequency, and spectral sidebands will be spaced apart at the FTF.