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For industries like power generation and petro-chemical,
vibration analysis has historically been the technique of choice for monitoring
the condition of large, critical pieces of rotating equipment. Conversely,
the fleet industries have relied upon oil analysis to make effective maintenance
decisions. It is common for industries such as primary metals, pulp &
paper, etc., to use both techniques. In general, vibration analysis and
oil analysis are the most effective techniques for monitoring the health
of machinery. The two techniques are natural allies due to the complementary
nature of their respective strengths. Unfortunately, the two techniques
are rarely combined to form an effective union. |
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| Browse the . . . |
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| where you'll find books on
oil analysis and vibration analysis. |
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| Vibration analysis activities typically reside
in the condition monitoring or vibration monitoring group, while oil analysis
usually resides with the lubrication team. Making matters worse, the oil
analysis program usually consists of submitting occasional samples to a
laboratory in exchange for results that frequently look more like chemistry
than machine condition monitoring. And, too often, oil analysis is used
to schedule oil changes while equipment condition assessments are left
primarily to vibration analysis. |
| This is changing in many organizations. For
example, the Palo Verde Nuclear Generating Station in Arizona made a dramatic
change in their approach to condition monitoring. They combined vibration
analysis and oil analysis into a common group, brought their oil analysis
on-site and began working as a team. Their results have been remarkable.
In an assessment of bearing defects detected by technology, they found
that oil analysis was responsible for 40% of the defects found, vibration
analysis was responsible for 33%, and both techniques converged on the
remaining 27% of the defects found. The loss of either technology would
have reduced their detection resolution and their ability to control the
root causes of machine failure. |
| In research conducted at Monash University,
Melborne, Australia, the correlation between oil analysis and vibration
analysis was found to be generally very good. However, there are instances
when one technique indicates a fault while the other shows no change or
even a contradictory result. For example, in applications where sliding
wear is prevalent, one might detect increasing rates of wear generation
and decreasing rates of vibration. This is caused by what the researchers
termed a ?lapping? effect. Essentially, the sliding wear slowly hones the
surfaces smooth, reducing the overall vibrations until the point at which
looseness and mechanical vibration are induced. The effect is intensified
by the presence of abrasive dirt. |
| Conversely, the Australian researchers found
that 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 falls to the bottom of the
sump, never finding its way into a sample bottle until it is oxidized and
leeches into the oil, a process that could take months. The Australian
researchers concluded that both techniques are required to effectively
monitor and diagnose the condition of plant machinery because each technique
evaluates different and complimentary symptoms. |
| An example in which both techniques are required
to effectively solve a problem is the case of a gearbox with increasing
vibration at the gear mesh frequency. Inspection of the particle count
and ferrous percentage revealed an increase in both categories, improving
confidence that a problem existed. It was not until the oil?s viscosity
trend was assessed, however, that the true nature of the problem was revealed.
A drop in viscosity from 220 cSt at 40°C to 70 cSt at 40°C was
observed. A review of the work history showed that the oil was changed
two weeks earlier. In all likelihood, the oil change was performed using
the wrong oil leading to the wear and vibration. Without the combination
of condition monitoring technologies, the root of the problem may have
gone undetected. |
| In general, we can make the following conclusions
about combining oil analysis and vibration analysis in detecting and analyzing
machine faults: 1. Both techniques are required to control the root causes
of machine failure. 2. Often, one technique serves as the leading indicator
of machine failure while the other serves as the confirming indicator.
3. Oil analysis is generally stronger in detecting failures in gearboxes,
hydraulic systems and reciprocating equipment. 4. Vibration analysis is
generally stronger in detecting failures in high-speed journal bearing
systems. 5. Vibration analysis is often better at localizing the point
of failure depending on the application. 6. Oil analysis is often stronger
in determining which wear mechanism is inducing failure. 7. Both techniques
are required to effectively determine the root cause of failure. 8. Correlation
between oil analysis and vibration analysis is very good, but there are
contrary instances. |
| In conclusion, oil analysis and vibration
analysis are natural allies in achieving machine reliability. They offer
complementary strengths in controlling the root causes of machine failure
and in identifying and understanding the nature of abnormal conditions.
Success depends on making changes in the organization to foster the development
of condition monitoring and machine diagnostic generalists in lieu of technology
specialists. A carpenter goes to the site with all the tools necessary
to complete the job. While it may be possible to cut a board with the claw
of a hammer, the carpenter is more likely to draw his saw, a more effective
tool for the task. We in condition monitoring must view technologies as
enabling tools. We need the right tools in our bag to complete the job
of ensuring machine reliability. |
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CASE
STUDY: Bearing Failure From Contaminated Oil Missed by Vibration Analysis
Ref: Johnson, Maxwell, Arizona Public Service.
Troyer, Drew, Enteract Conference 1998
Mathew, J., Stecki, J.S., Comparison of Vibration
and Direct Reading Ferrographic Techniques in Application to High-Speed
Gears Operating Under Steady and Varying Load Conditions, 1986 |
