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One Vibration Analysis Expert Shares His Views About the Importance of Oil Analysis
Why would a person totally immersed in the vibration analysis field for over 20 years now recommend expanding condition monitoring programs under his watchcare to include the oil analysis technology? Not meant to be a technical presentation on the topic of oil analysis, this insightful article provides an overall view of the relative strengths and weaknesses of oil analysis and vibration analysis and discusses how powerful these tools are when combined in today’s proactive condition-based monitoring programs.
Today’s condition monitoring programs, at least those that wish to advance toward true “Reliability-Centered Maintenance” (RCM), must incorporate more than one technology into their diagnostic tool kits. No longer can an organization expect to achieve its reliability goals while “putting all their eggs into one basket.” Historically, most industrial condition monitoring programs included only vibration analysis. And most of these programs experienced at least moderate success, particularly if their condition monitoring teams received the professional training that is vital to achieving proficiency in the application of vibration analysis technology. Many engineers and managers felt that vibration analysis alone was sufficient to achieve their reliability objectives.
At one time, the author of this paper was among those who felt that virtually every machine type could be effectively evaluated using vibration analysis alone. The belief was that if every available vibration analysis tool was employed, (i.e. FFT, time wave-form, synchronous time averaging, true order tracking, phase analysis, amplitude demodulation, stress wave analysis, operating deflection shape analysis, modal analysis, etc.) vibration analysis alone could “tackle the whole job” of condition-based maintenance. Since then, however, it has become evident that an integrated approach to condition monitoring yields a higher degree of success.
Used alone, vibration analysis detects some problems very early and eventually reveals the condition of many machines with a high degree of success. Figure 1 illustrates the application of vibration analysis to monitor the progression of about 80% of the rolling element bearing failures (ref. 1). Figure 1 identifies four bearing defect frequencies that reveal faults on the inner race, outer race, cage or rolling elements of the bearing. The four variables that define the equations in Figure 1 must be known to accurately calculate the bearing’s defect frequencies. Most vibration analysis software provides the information for specific bearing makes and models, allowing the analyst to differentiate between faults coming from one bearing versus those coming from an adjacent bearing. Each of the four failure stages through which a rolling element bearing typically passes are all shown on Figure 1.
Figure 2 details, including photographs, the four stages of bearing failure. Importantly, note that during the initial failure stage (Stage 1) when only incipient wear is present, there is typically no indication in vibration spectra (advanced analysis techniques like amplitude demodulation and / or stress wave analysis techniques can often detect Stage 1 bearing problems). When a bearing reaches failure stage 1, it normally has approximately 10 to 20% remaining life. When wear progresses to Stage 2, further damage appears on the bearing which is evidenced by the appearance of bearing component natural frequencies in vibration spectra as shown in Figure 2. At stage 2, a bearing ordinarily has about 5 to 10% of remaining life. At stage 3, the bearing typically has only 1 to 5% of its expected life remaining. Stage 3 is the point at which the previously discussed fault frequencies usually begin to appear. A bearing must be replaced before it progresses to failure Stage 4 where the remaining life typically ranges from only 1 hour to 1%.
Vibration analysis technology allows the experienced analyst to detect a wide range of mechanical and electrical problems within rotating machinery and their components such as bearings and gears (ref. 1). However, despite determined efforts by the author and numerous colleagues in the vibration analysis field, certain machine types for the most part still cannot be adequately evaluated by vibration analysis alone (at least to the depth desired). These machines include reciprocating air compressors, internal combustion engines, greased motor operated valves, presses, piston type hydraulic pumps, etc. And, where vibration analysis can effectively evaluate the condition of the machinery, adding oil analysis provides a much more complete picture. Oil analysis has effectively detected certain problems within rotating machines before they are evident with vibration analysis - particularly on multi-stage gearboxes, plain bearings, rotary screw air compressors, roots blowers and on certain rolling element bearings which might be located at a distance from an accelerometer mounting location.
Editor's Comment: The links to the stages below serve to facilitate an understanding of how oil analysis correlates to Jim Berry’s four stages of a bearing failure as detected with vibration analysis.
During the mid-1970’s, vibration analysis technology truly began making inroads in condition monitoring with the development of the FFT spectrum analyzer. Later, in the early 1980’s, when FFT analysis became available in portable, hand-held data collectors, and condition monitoring software was developed to handle the data acquired by the new data collectors, a virtual explosion began. Thousands of plants, that had previously not performed condition monitoring, began to perform vibration analysis. While the tremendous growth and popularity of vibration analysis produced great benefit by spreading condition monitoring throughout a significant portion of industry, hindsight now shows it caused one major problem - a concentrated focus on just one condition monitoring technology. In fact, almost a decade passed before this almost singular focus on vibration analysis would be broken to allow the deployment of an array of other powerful “machinery health assurance weapons” that would allow the same plants to finally begin to approach proactive RCM. These additional tools include oil analysis, thermographic analysis, ultrasonic analysis, motor current analysis, stress wave analysis, and others.
The focus of this paper is to describe how the integration of just one of these tools - oil analysis - with vibration analysis has greatly enhanced the reliability and effectiveness of condition monitoring programs. Actually, oil analysis technology has been around for many years. The problem was that numerous condition monitoring teams were either not aware of oil analysis or, if plants did have personnel assigned to perform oil analysis, these people in most cases did not interface with the vibration analysis condition monitoring teams on the same plant site. The decade of the 1990’s has fortunately seen a great shift in this trend. At least some plants have seen the wisdom in adding oil analysis to vibration monitoring to improve their machine condition monitoring programs. Likewise, several vibration condition monitoring vendors have begun to expand their offerings to incorporate oil analysis products, services and data management. With the combined offering, the analyst sees a more complete picture of the operating condition of the machinery under his watchcare, and is better positioned to make more effective decisions and recommendations.
One comprehensive study at a nuclear plant beginning in 1994 clearly showed how the integration of oil analysis with vibration analysis could widen the depth and breadth of a plant condition monitoring program (refs. 2 and 3). Table 1, taken from ref. 2, provides a meaningful comparison of the relative strengths and weaknesses of oil analysis and vibration analysis. And, it provides insight into how the results of one technology complement those of the other. As described in Table 1, when oil analysis and vibration analysis are “married” within a program, the weaknesses in one technology can be overcome by the strengths in the other. For example, while oil analysis cannot detect resonance, vibration analysis is very adept at doing so. Conversely, vibration analysis has only mixed success in detecting wear of oil lubricated journal bearings, where oil analysis is very adept at both detecting the wear and in assessing the severity, thereby supporting the important decision whether or not the machine should continue to operate. Also, when both technologies pinpoint the same problem, the diagnosis and follow-up recommendations are rarely inaccurate. The authors of ref. 2 stated “Our experience shows that a strong, up-to-date vibration program can be improved by closely integrating it with a strong oil analysis program. The combined program becomes more than the sum of the parts”.
|Condition||Lube Program||Vibe Program||Correlation|
|Oil Lubricated Antifriction Bearings||Strength||Strength||Lubrication analysis will detect / can detect an infant failure condition. Vibration provides strong late failure state information.|
|Oil Lubricated Journal / Thrust Bearings||Strength||Mixed||Wear debris will generate in the oil prior to a rub or looseness condition.|
|Machine Unbalance||Not Applicaple||Strength||Vibration program can detect an unbalance condition. Lube analysis will eventually see the effect of increased bearing load.|
|Water in Oil||Strength||Not Applicable||Water can lead to a rapid failure. It is unlikely that a random monthly vibe scan would detect the anomaly.|
|Greased Bearings||Mixed||Strength||It makes economic sense to rely on vibration monitoring for routine greased bearing analysis. Many lube labs do not have enough experience with greased bearings to provide reliable information.|
|Greased Motor Operated Valves||Mixed||Weakness||Accuators are an important machinery in the nuclear industry. Grease samples can be readily tested; it can be difficult to obtain a representative sample. It can be hard to find these valves operating, making it difficult to monitor with vibration techniques.|
|Shaft Cracks||Not Applicable||Strength||Vibration analysis can be very effective in monitoring a cracked shaft.|
|Gear Wear||Strength||Strength||Vibration techniques can predict which gear. Lube analysis can predict the type of failure mode.|
|Alignment||Not Applicable||Strength||Vibration program can detect a misalignment condition. Lube analysis will eventually see the effect of increased / improper bearing load.|
|Lubricant Condition Monitoring||Strength||Not Applicable||The lubricant can be a significant cause of failure.|
|Resonance||Not Applicable||Strength||Vibration program can detect a resonance condition. Lube analysis will eventually see the effect.|
|Root Cause Analysis||Strength||Strength||Best when both programs work together.|
Source: “Integration of Lubrication and Vibration Analysis Technologies;” by Bryan Johnson and Howard Maxwell; Pale Verde Nuclear Generating station (ref. 2).
The complement between the two technologies continues. For example while vibration analysis can pinpoint which gear has a problem, oil analysis can identify the type of failure mode. Also, oil analysis can detect stage 1 rolling element bearing defects where, as previously discussed, vibration analysis typically can’t until the failure reaches stage 2 (see Figures 1 and 2).
Having the information from both technologies facilitates the process of determining a problem’s root cause. In doing so, the program is elevated to a more proactive capability. In fact, a condition monitoring program is not truly effective until it has put in place a “Root Cause of Failure” analysis process to continually identify the failure / problem source(s), allowing proper corrective actions to be taken which can prevent the problem(s) from recurring.
Building the Case
A review of some of the data presently available reveals several important facts about the need to integrate oil analysis and vibration analysis:
Early Detection of Rolling Element Bearing Problems - Oil analysis is typically more adept in detecting early bearing failure conditions. When both technologies detect faults, problem diagnosis and its assessment is rarely incorrect (ref. 2).
Effect of Integrating Oil and Vibration Analysis - Integrating oil and vibration analysis can allow early detection and trending of numerous problems to which a machine can be subjected. “Detecting the faults is the first step in the diagnostic process. Early fault detection yields benefits in diagnostic time, avoidance of unplanned down-time, elimination of chain reaction failures, and improved precision of maintenance actions.” Often, stopping a machine and repairing a single component can prevent this problem component from adversely impacting adjacent machine parts, thereby avoiding costly (and potentially catastrophic) failure (ref. 5).
Root Cause Failure Analysis - “Both oil analysis and vibration analysis are required to effectively determine failure root cause. Confidence in maintenance and operations decisions is substantially improved when both methods are employed” (ref. 5).
Condition of Lubricating Fluid - “The life of the machinery is in the lube.” Oil analysis is required to assess the quality of this “life blood,” no matter what the type of machine it might be (ref. 4).
In running condition monitoring programs at Technical Associates of Charlotte over the past 18 years, we have found integration of oil analysis with vibration analysis has significantly improved the effectiveness of our programs. We encourage our clients to allow us to employ oil analysis in their program, particularly on gearboxes, large machines outfitted with plain bearings, compressors, etc. Our experience has led us to make the following conclusions about oil analysis:
1) The leading indicator of gear problems is oil analysis. In one case, a wear problem initially detected and trended by lube analysis was not detected by vibration for approximately 6 months; it then trended in both technologies for approximately another 18 months until it was decided corrective actions were necessary.
2) Oil analysis is effective on large motors outfitted with plain bearings (particularly on motors greater than approximately 1000 HP). Oil analysis has proven to be a more reliable tool than vibration analysis for detecting sleeve bearing wear on many machine types. Alternatively, vibration analysis is still the tool of choice to detect other plain bearing problems including oil whirl and oil whip.
3) We have employed oil analysis to verify the presence and severity of faults in large centrifugal air compressors that have problems with impeller pinions and impeller bearings. In one case, the results of oil analysis convinced a client to shut down a compressor that had fairly extensive gear problems. Although the client saw the results of vibration data, he did not truly grasp the problem’s severity. When shown the results of the compressor’s lube oil analysis, revealing extensive gear wear metals suspended in the oil, he was convinced to bring the machine down before a costly failure occurred.
Since adding oil analysis to our “condition monitoring arsenal” at Technical Associates, we have attempted to employ the following policy with condition monitoring clients: If a gearbox is considered critical, oil analysis should be considered mandatory. It is often difficult to clearly differentiate between actual gear wear versus gear tooth shape (profile) or tooth orientation problems (i.e., tooth misalignment, eccentricity and / or excessive backlash) with vibration analysis. Plus, in some gearboxes, oil analysis differentiates the wear from gears versus the wear from bearings within the same machine. Research performed at Monash University in Victoria, Australia unveiled some important findings on the condition monitoring of evaluation of gears (ref. 6):
“In applications where sliding wear is prevalent, one might detect increasing rates of wear generation and decreasing rates of vibration. This is caused by a ‘lapping’ effect where essentially the sliding wear slowly machines the surfaces smooth, reducing the vibration until the point at which mechanical failure is induced. All the while, the machine’s surface is being worn away. Conversely, vibration analysis very effectively identifies the presence of a fractured gear tooth, but because the size of the debris generated is so large, wear particle analysis is ineffective. The debris simply falls to the bottom of the sump and never finds its way into a sample until it is oxidized allowing dissolved metals to leach into the oil. The process could take months and still yield very marginal results. Therefore, both oil and vibration analysis techniques are required to effectively monitor and diagnose the condition of plant machinery because each technique evaluates different and complementary symptoms”.
an Integrated Condition Monitoring Program
The plant can incorporate oil analysis into its condition monitoring program by collecting oil samples and sending them off-site for analysis, by employing on-site instruments for oil analysis, or with a combination of both. The laboratory approach does not require a substantial investment in instruments and manpower training. And it provides sophisticated, in-depth analysis of the lube by experienced (full-time) analysts / technicians. However, this approach also has several drawbacks. First, there is the delay in time between collection of the oil sample and receipt of the oil analysis results, which, in some cases, could potentially put a critical machine at risk. Next, there is the issue of “ownership” of the oil analysis program by the persons who must “live with the machinery”. These persons have intimate knowledge of its operation and are directly affected when the machinery is not “feeling well”. Certain “in-shop” oil analysis equipment can provide rapid answers (in less than one hour) to confirm wear problems (ref. 4). On-site analysis does, however, require some investment. Management should review its options carefully before proceeding.
After selecting the best strategy for the individual technologies, the condition monitoring team must be organized to handle multiple technologies. Over the past decade, the author has had the opportunity to audit many condition monitoring programs located throughout the United States. In so doing, we have developed a documented audit instrument that allows us to:
A. Evaluate the knowledge
and experience of the condition monitoring team members
B. Assess the condition monitoring training these individuals have received (a very important factor)
C. Evaluate condition monitoring instruments and software tools currently at the plant
D. Appraise the effectiveness of each of the condition monitoring technologies implemented.
- After completing this extensive program audit process, certain “common threads” have emerged from those programs that the audit process has found to be the most successful:
- The entire condition monitoring team performing all technologies has been brought together in one common area allowing much information transfer and improving the accuracy / reliability of diagnostic calls as well as root cause analysis.
- All condition monitoring personnel report to a single plant program manager who himself directly reports to plant management (providing him the ear of both maintenance and production plant management).
- All condition monitoring personnel are “cross trained” in at least one other condition monitoring technology giving them greater confidence and understanding of the other technology.
- All condition monitoring personnel work full time in their field (they may occasionally assist in performing certain corrective actions, but are not expected to do this on a regular basis).
All condition monitoring personnel receive formal training in their areas of expertise at least one week per year to keep them up to date and to further advance their knowledge which is of immediate benefit to the plant. Audits through the years have proven there is a direct correlation of program effectiveness to the quantity and quality of continuing training condition monitoring team members receive.
- Is vibration analysis a powerful condition monitoring tool? You’d better believe it!
- Is oil analysis a powerful condition monitoring tool? Ditto.
- Does one technology “fill the gaps” left open by the other? Yes.
- In other cases, does it improve the confidence and credibility of the analyst if both tools diagnose problems on a critical machine? Absolutely.
- Only one question remains - If you have only employed one of these powerful tools in your own program to date, why not significantly enhance the effectiveness of your program by adding its “complementary cousin” to your program? You and your plant management will be glad you did.
1) Berry, J. E. (1997) “Tracking of Rolling Element Bearing Failure Stages Using Vibration and High Frequency Enveloping and Demodulation Spectral Techniques”; Analysis II - Concentrated Vibration Signature Analysis and Related Condition Monitoring Techniques (2nd Edition), Technical Associates of Charlotte, P.C.: Charlotte, NC.
2) Johnson, B and H.
Maxwell “Integration of Lubrication and Vibration Analysis Technologies”;
Palo Verde Nuclear
3) Maxwell, H. and B Johnson (1997) “Vibration and Lube Oil Analysis in an Integrated Predictive Maintenance Program”, Vibration Institute Proceedings; June 17-19, 1997.
4) Garvey, R. (1994) “Case Histories and Cost Savings: Using In-Shop Oil Analysis for Industrial Plant Applications”, Application Paper, Computational Systems, Inc.: Knoxville, TN.
5) Troyer (1998) “Effective Integration of Vibration Analysis and Oil Analysis”; Maintenance Technology Magazine, November, pages 17-21.
6) Mathew, J. and J.
Stecki (1986) “Comparison of Vibration and Direct Reading Ferrographic
Techniques in Application to High-Speed Gears Operating Under Steady and Varying
Load Conditions”; Proceedings from the 41st Annual Meeting of the Society
of Tribologists and Lubrication Engineers, Toronto, Ontario,
Canada, May 12-15, pages 646-653.
7) Bern, A. (1997) “Integrating Oil Analysis with Entek For Windows”; Proceedings from Enteract ‘97; April 28-30, Cincinnati, OH, USA.
Figures 1 and 2 are excerpts taken from Technical Associates’ “Illustrated Vibration Diagnostic Chart.” To obtain the complete 5-page chart, contact Technical Associates at 704-333-9011 or refer to information regarding the diagnostic chart from the Technical Associates’ web site “www.technicalassociates.net.”