Tooth), Oil Whirl & Oil Whip instabilities, Flow Turbulence / Cavitation problems, Electrical Pole Passing Frequency, and Rolling Element Bearing Cage (Train) Frequency.
The Modulation / Sidebands Group
This grouping includes overlap from Harmonic and Nonsynchronous groups and can include additional frequency content. However; the analyst should remember the unique “Sub-Synchronous” aspect of these faults that can eliminate other potential sources. The Non-Synchronous Group
Figure 6.0
Figure 5.0
The Non-Synchronous group overlaps the subsynchronous group somewhat. This grouping of faults requires that the fault frequency NOT to be a multiple or whole fraction of the fundamental rotor speed or even a function of that speed. All of the sub-synchronous faults in this category are also non-synchronous faults. These fault frequencies are created from geometric quantities in bearing design, belt diameters, piping design, or created from electro-magnetic field theory.
The Modulation group includes faults that are more commonly distinguished by their “sideband” sets. Many rolling element bearing faults tend to generate sidebands in later failure stages. Electro-erosion in rolling element bearings will generate “haystacks” of peaks related to the defect frequencies in the bearing. Barring faults tend to create sidebands surrounding a paper roll natural frequency. The center frequency can be related to the diameters of the rolls in nip, their alignment, or eccentricity ratios. The Multiple Indication Group
All Rolling Element Bearing faults (including the Cage, Element Spin and Raceway frequencies) are always defined as non-synchronous. The geometry in the design of journal-type bearings create clearances and eccentricities that ensure the instability point (whirl) is non-synchronous. Flow related problems create random energy and broadband frequency responses that are not related to the rotor speed. The AC and DC Motor Electrical faults are added to this group as well as the Natural Frequency fault series. Specialty faults such as “barring” or “corrugation” problems in paper rolls and film production are related to roll diameters, alignment or structural natural frequencies. The “fluting” or “electro-erosion” fault is related to the already noted non-synchronous rolling element bearing signature.
Figure 7.0
Several faults are highlighted with dotted lines and linked to other areas of the table. These are faults that can be described by another category and/or by modulation signatures alone. This is the Multiple Indication Group. Whenever modulation is involved in the vibration signature, the severity of the problem is typical related to the number of sideband sets found in the frequency spectrum, or the amount of amplitude pulsation noted in the Time Waveform. Either indicator will allow trending of the deterioration included in the fault with time .