By Jude Iyinbor, CEng, VA III, Machinery Vibration Specialist
What pictures do sidebands paint when they show up in vibration spectral plots? This will largely depend on the pattern of occurrence. In this article we will discuss a list of common faults and what the sidebands generated look like.
Fault Recognition via Sidebands
In vibration analysis, sidebands are generated in many cases by the following types of issues:
- Bearing related issues
- Gear related Issues
- Electrical related issues
Bearing Related Issues:
When rolling element bearings deteriorate through a fault on the inner raceways, outer raceways, or the cage, there is sometimes the presence of sidebands in the spectral plot due to modulation of the running speed – 1x rpm (modulating frequency) around the bearing tones (center frequency). If as an illustration, there is a defect on the inner raceway (identified by the presence of Ball Pass Frequency Inner race – BPFI), this defect will move in and out of the bearing load zone at each rotation of the shaft (1x rpm). This results in amplitude modulation (fluctuations in amplitude) with a constant frequency as the amplitude level rises and falls as it goes in and out of the load zone. There could also be some harmonics due to the impacts felt every time the inner race defect is in the load zone. See Figure 1 for pictorial representation of what can be seen in some cases with BPFI faults in the frequency domain.
Gear Related Issues
When Gears rotate and mesh together, there are usually activities found at the Gear Mesh Frequency (GMF). If a tooth was worn or damaged on the pinion, it means that there will be a rising and falling of vibration as this damaged tooth comes into meshing activity. This will occur at each rotation of the pinion shaft with the damaged tooth (1x rpm of Pinion shaft). This causes amplitude modulation of the running speed of the Pinion shaft around the Gear Mesh Frequency. Any slight changes in speed may also cause frequency modulation (see Figure 2).
Electrical Related Issues
For electric motors, modulation can happen due to broken or damaged rotor bars. Rotor eccentricity can also show up as modulated signal with side bands in the frequency domain. In the case of damaged rotor bar, the fluctuation in amplitude is usually due to the slip that is generated around 1x rpm. Its harmonics, as the broken rotor bar goes in and out of the magnetic field, cause a rise and fall in vibration amplitude level. This slip is seen as Pole Pass Frequency (Number of Poles x Slip Frequency). The Pole Pass Frequency (FP) modulates around the 1x RPM and its harmonics seen in the lower frequency range. Most times it is possible to also see a visual individual presence of Pole Pass Frequency in the Low Frequency range (see Figure 3). For loose or open Rotor bar, Twice Electrical Line Frequency (2xLF) modulates around Rotor Bar Passing Frequency (RBPF = Number of Rotor Bars x 1x RPM) in the high frequency zone.
Questions?
Contact Jude Iyinbor, Machinery Vibration Specialist at 877-962-2400 to learn more about vibration monitoring and how it can be applied to your machine maintenance program.
