Not unlike chemical elements, the world of vibration analysis is also built on patterns. There are unifying commonalities in mechanical systems, such as rotating shafts, bearings, blades, gears, etc. Sources of vibration create measurable response amplitudes, repeating rates of occurrence (or frequency response) and sometimes induce overall structural motions (phase response), each providing clues to the underlying machinery fault.
The amplitude component tells the analyst that a measurement may be "out-of-family" with groups of similar machine "types" or "classes." Frequencies are generated in the FFT spectrum, providing patterns that can be related to the design or function of the machine (e.g., rolling element bearings, gear teeth, turbine blades, etc.). Sets of frequencies can indicate normal operation or the onset of mechanical faults or defects. Phase analysis is a diagnostic tool that allows the analyst to sift through faults that have similar appearance in the spectrum and cannot be distinguished individually.
Thus far, this information should be secondhand to the seasoned vibration analyst. However, when the faults are grouped directionally and according to frequency content, a significant amount of information unfolds in our one-page table format. Instead of searching for sample spectra in a book or on a wall chart for something that looks similar to the measurement spectrum from your machine, we can now logically define the fault from a different direction. The result is a useful tool designed to help the analyst narrow down the numerous possibilities when faced with a difficult machinery vibration signature.
Terminology & Groupings
A review of terms is required as we walk through the structure of the Vibration Analysis Periodic Table. The groupings by column contain the dominant vibration faults by frequency content. The column headers are shown on the full table in Figure 1.
The Synchronous Group The synchronous grouping in Figure 2 includes faults that generate a predominant 1x rpm response in the spectrum. There are many faults that fall into this category, including some faults that may start as a synchronous fault and, if left unchecked, may deteriorate into another group.
For our purposes, the synchronous faults start with an elevated 1x rpm response and hold this pattern (save increasing amplitudes). The synchronous group is a small select group that is narrowly defined in the first column on the table. This group includes unbalance, eccentricity, mechanical looseness type A, gear tooth faults and belt drive misalignment problems.
The second column is also representative of synchronous response, but often may include an additional harmonic as well as the 1x rpm peak. This effect can be related to the severity of the fault and may change with overall fatigue in the machine. However, we will see that some of the other categories will also, on occasion, overlap into adjacent groupings.
The Harmonic Group Frequency content that is considered harmonic will include (you guessed it) harmonic or integer multiples of the 1x rpm rotating speed (see Figure 3). As noted previously, there can be a bit of overlap with the synchronous group, however, the harmonic group can include a single harmonic or dozens of harmonics of the fundamental frequency.
The second column on the table includes faults that have typically elevated 1x rpm and a single second harmonic. This group includes coupling misalignment (offset and angular), bent or bowed rotors, and cocked rolling element bearings. The expanded group of faults is found in columns three and four of the table. These faults include gear meshing harmonics, blade passing, rotor bar passing and mechanical looseness (types B and C) signatures.
Sub-Harmonic/Sub-Synchronous The sub-harmonic or sub-synchronous table grouping (see Figure 4) generates frequency content below the 1x rpm synchronous rotor speed or the fundamental order of the fault. The fault can be an integer fraction of rotating speed or non-synchronous with respect to this speed. The group includes mechanical looseness types B and C, rotor/stator rub events, belt drive frequencies, gear tooth repeat problems (assembly phase and hunting tooth), oil whirl and oil whip instabilities, flow turbulence/cavitation problems, electrical pole passing frequency and rolling element bearing cage (train) frequency.
This grouping includes overlap from harmonic and non-synchronous 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 The non-synchronous group shown in Figure 5, somewhat overlaps the sub-synchronous group. This grouping of faults requires that the fault frequency is NOT 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 electromagnetic field theory.
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 creates clearances and eccentricities that ensure the instability point (whirl) is non-synchronous.
Flow-related problems create random energy and broad-band 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.
The Modulation/Sidebands Group The modulation group (see Figure 6) 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 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 (see Figure 7).
Whenever modulation is involved in the vibration signature, the severity of the problem is typically 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 over time.
The Directional Response Pattern
A secondary useful pattern in the Vibration Analysis Periodic Table can be found in the directional groupings inherent in the fault. Vibration amplitude response can present itself in various directions, but there are preferred directional responses in many fault signatures. A side note here will remind the analyst that measurements in multiple directions require making a directionality assessment.
The table is color-coded for the dominant direction of the vibratory response. It may not be casually apparent, but this concept of directional screening is very useful in reducing the likelihood of potential fault sources.
The Radial Response Group The radial response group shown in Figure 8 is a powerful tool because out of the 35 basic faults presented on the table, only one third have a dominant radial preference. In horizontally mounted machines, the mechanical looseness types A and B signatures will most often induce response only in the vertical direction. Likewise, rolling element bearing faults are best detected in the vertical measurement direction in the vicinity of the bearing load zone. The remaining faults in this group can be detected in either the vertical or horizontal (radial) directions.
The Axial Response Group The axial response directional fault group in Figure 9 is an even smaller group than the radial faults on the Vibration Analysis Periodic Table. This grouping includes only five truly axial faults and another three that can be predominantly axial based on design (gears) or by fault severity (bent shaft and overhung rotor unbalance).
Remember, if we are analyzing measurement signatures, we have already narrowed down the fault based on frequency content. If the data indicates that the remaining possibilities also include a predominantly axial response, the final group is reduced very quickly.
The Axial and/or Radial Group This group includes faults that are either axially or radially inclined are covered in Figure 10. The set defines 18 potential faults. Again, at this point, the analyst has already screened the measurement by frequency response. Additional knowledge that the fault is NOT purely "radial" or purely "axial" will eliminate several possible fault sources.
An Effective Screening Tool
Hopefully at this juncture, the effectiveness of the Vibration Analysis Periodic Table as a screening tool is becoming obvious. To this point, we have used the frequency and directional response category groupings to eliminate many potential faults, but note that we have NOT looked at an example spectrum on a wall chart or reference book.
Let's face it, it's not likely that our unique machinery problem is neatly duplicated in a book. Even if it were, with the number of variables involved, it's unlikely that we would be able to find it! The "hunt and peck" method of analysis is not an efficient use of the analyst's time.
Additional Table Resources
We are not finished with the periodic table just yet. You may note that the table includes additional information within the colored blocks that defines each vibration problem. The upper left-hand corner notes the frequency content category with a letter: S, H, SS, NS, or M (see Figure 11).
Figure 11: Icon detail information
The lower left-hand corner provides a reference page number for the Vibration Fault Guide (VFG) (See Figures 12a-c). As we narrow down the possibilities, we can turn to the VFG for additional information and distinguishing aspects of the potential machinery fault.
Figure 12a: Vibration Fault Guide (VGF) - belt drive misalignment
Figure 12b: VGF - gear meshing frequency
Figure 12c: VGF - oil whirl instability
The upper right hand corner includes a symbol for the appropriate diagnostic test that can be performed to provide insight to the potential fault.
The first five icons shown in Figure 13 represent the following diagnostic tests:
Figure 13: Diagnostic test icon references
Phase Analysis - Traditionally used to distinguish faults with identical frequency response signatures. The chart includes 10 potential faults where a phase analysis may be appropriate.
Time Waveform Analysis - Time waveform is essential as the singular method of detection for gear tooth fault problems. It is also used for corroborating evidence in looseness and alignment problems, as well as rub events and beat frequency problems.
Orbit Analysis - Considered essential in the analysis of fluid film (journal) bearings to detect instabilities and loading issues.
Ultrasonic Spectrum Analysis - Most commonly used to detect early rolling element bearing fatigue and lubrication problems. This tool is helpful in gear train problems as well.
Impact Natural Frequency Testing - Defines structural natural frequencies, resonance margin, damping and mode shapes.
The final symbol in Figure 13 is found in the lower right-hand corner of some select faults on the table. The symbol indicates that there is a formula, calculation, or table that can provide additional insight into the fault.
The Vibration Analysis Periodic Table concept has been in work for over a year since the publication of this article. The construction has been peer reviewed in several venues, however, with wider exposure, it is likely that readers will notice a lacking element or limitation that needs to be addressed. These critiques are welcome.
The long-term vision for the Vibration Analysis Periodic Table is for it to become a self-guided computer training platform where the analyst can drill down into each fault and experience machinery information, animations and case histories at will. The concept is currently being implemented into all of the training courses at Full Spectrum Diagnostics.
Dan Ambre, P.E., is a Mechanical Engineer and founder of Full Spectrum Diagnostics, PLLC, a full-service Predictive Maintenance Consulting company. Dan is also a Certified Software Training representative for Vibrant Technology, Inc., the creators of ME'scope VES software tools. Full Spectrum Diagnostics' ODS and Modal Analysis training targets the In-Plant Vibration Analyst. www.fullspec.net