Maintenance can Yield More than a 10-Fold Savings Over Conventional Predictive/Preventive
C. Fitch, P.E.
Plainly stated, the growing cost of maintenance is a serious business problem.
According to DuPont, "maintenance is the largest single controllable expenditure
in a plant: in many companies it often exceeds annual net profit." One
major U.S. automotive manufacturer has a maintenance staff of between 15,000
and 18,000, all plants combined. They say "85% to 90% is crisis work" (breakdown).
While preventive maintenance, when well implemented, has been shown to
produce savings in excess of 25 percent, beyond that its benefit quickly
approaches a point of diminishing return. According to a Forbes Magazine
study, one out of every three dollars spent on preventive maintenance is
wasted. A major overhaul facility reports that "60 percent of hydraulic
pumps sent in for rebuild had nothing wrong with them." These inefficiencies
are the result of maintenance performed in accordance with a schedule (guess
work) as opposed to the machine's true condition and need.
Most recently, predictive maintenance (also known as condition monitoring)
has been leading the way to additional savings over preventive maintenance.
The use of real time or portable instruments such as vibration monitors,
thermography, ferrography, etc. has been effective at recognizing the symptoms
of impending machine failure. The major benefit is the availability of
an earlier warning, from a few hours to a few days, which reduces the number
of breakdown "catastrophic" failures.
Predictive maintenance is usually implemented concurrently with preventive
maintenance and targets both the warning signs of impending failure and
the recognition of small failures that begin the chain reaction that leads
to big failures (i.e., damage control).
"Life Extension" Maintenance
Proactive maintenance has now received worldwide attention as the single
most important means of achieving savings unsurpassed by conventional maintenance
techniques. The approach supplants the maintenance philosophy of "failure
reactive" with "failure proactive" by avoiding the underlying conditions
that lead to machine faults and degradation. Unlike predictive/preventive
maintenance, proactive maintenance commissions corrective actions aimed
at failure root causes, not just symptoms. Its central theme is to extend
the life of mechanical machinery as opposed to (1) making repairs when
often nothing is broken, (2) accommodating failure as routine and normal,
or (3) preempting crises failure maintenance in favor of scheduled failure
While the root causes of failure are many, or at least presumed to be,
it is generally accepted that 10 percent of the causes of failure are responsible
for 90 percent of the occurrences. Most often, the symptoms of failure
mask the root cause or they are presumed themselves to be the cause. For
example, a sudden bearing failure is often blamed on poor quality or a
bad lubricant. The root cause, on the other hand, is often contamination
in the lubricant (bad filter) or faulty installation of the bearing.
When a machine is well designed and well manufactured, the causes of failure
can generally be reduced to machine misapplication or contamination. And,
among these two, contamination is clearly the most common and serious failure
culprit. A great deal of laboratory proof and field confirmation now are
available to support this fact. Therefore, the logical first-approach to
proactive maintenance is the implementation of rigorous contamination control
programs for lubrication fluids, hydraulic fluids, coolants, air, and fuel.
The appropriateness and veracity of this maintenance strategy is emphasized
A. According to the
bearings division of TRW, "contamination is the number one cause of bearing
damage that leads to premature removal."
B. Machine Design
Magazine reports that "less than 10 percent of all rolling-element bearings
reach the fatigue limit because contamination usually causes wear or spalling
failure far earlier."
C. According to Caterpillar,
"dirt and contamination are by far the number one cause of hydraulic system
failures." J. I. Case states that "one thing holds true about hydraulic
systems: the systems must be kept clean -- spotlessly clean -- in order
to achieve the productivity they're capable of."
D. Protractive studies
by the U.S. Navy show that the cost of contamination on marine and aviation
equipment per operating hour exceeds 60 percent of the cost of fuel per
hour on the same equipment.
Institute of Technology states that "six to seven percent of the gross
national product ($240 Billion) is required just to repair the damage caused
by mechanical wear." Wear occurs as a result of contamination.
F. Oklahoma State
University reports that when fluid is maintained 10 times cleaner hydraulic
pump life can be extended by 50 times.
Medicine Parallel to Maintenance Strategies
The human body offers many parallels to machine maintenance. In fact, from
good observation of advances in human medicine we can gain excellent insight
to effective strategies in the maintenance of machinery.
Most machinery are fluid dependent systems, just like the human body. Fluids
such as lubricants, hydraulic fluids, coolants, fuels, and air bring contaminants
into the system and transport the contaminants within the system. The abnormal
presence of contamination in a system can be described as an incipient
failure, meaning that, while the machine is not currently experiencing
loss of performance or component degradation, the conditions that lead
to failure and shortened service life are present and untenable. High contaminant
levels are similar to living with high cholesterol and high blood pressure:
more sooner than later you die. And similar to cholesterol, high contamination
is a correctable condition.
of Contaminant-Induced Failures
There are many types of contaminant-induced failures in machinery. The
most common are wear, stiction, seizure, erosion, and corrosion. Contaminants
involved include solid particles, moisture, air, chemicals, and other materials
foreign to the system. However, of the failure types, abrasive wear, caused
by solid particles, is substantially the most serious. According to the
Vickers division of Trinova/Aeroquip, "abrasive wear accounts for about
90% of failures due to contamination." This abrasive wear is the result
of particles (too small to be seen) that cut and plow rolling and sliding
The rate at which contamination enters the fluids of hydraulic and lubricating
machinery is typically greatly underestimated and understated. Likewise,
the effectiveness of filters at removing fluid contaminants in field systems
is greatly overstated. According to a study of hydraulic equipment at Oklahoma
State University, "it has been demonstrated that apparent ingression rates
of 10 million to 100 million particles greater than 10 microns (per minute)
characterize field systems".
Hence, the filter, if existent, is challenged with the formidable task
of removing particles from the fluid at the same rate at which they are
entering (ingression). Tests by machinery manufacturers show that filters
have great difficulty achieving this task in the field, where they are
subjected to conditions of frequent and large changes in temperature, fluid
viscosity, pressure, and flow (surges), plus the effects of shock, vibration,
and fatigue. Other common problems are filter bypass valves that get stuck
open, damaged or missing filter gaskets, and filters that are installed
backwards or crooked. Accordingly, the spoils and vagaries of field-oriented
situations are many. As a result, fluid contaminant levels must be frequently
monitored to verify filter performance and to provide the essential "feedback"
that gives integrity to a contamination control program.
When it comes to proactive contamination control maintenance, the Japanese
may be the global leaders. They have clearly taken a "do-it, don't-just-talk-about-it"
approach. Evidence of this comes from reports by two of the world's largest
steel mills, Nippon Steel and Kawasaki Steel, both in Japan:
A. After Nippon Steel
implemented a hydraulic system contamination control program plant-wide,
involving both improved filtration and rigorous fluid cleanliness monitoring,
pump replacement frequencies were reduced to one fifth and the cumulative
frequency of all tribological failures (i.e., failures relating to wear
and contamination) was reduced to one tenth.
B. Likewise, Kawasaki
Steel, not to be outdone, implemented a similar contamination control program
and achieved an almost unbelievable 97% reduction in hydraulic component
failures. Such claims as these spurred the British Hydromechanics Research
Association (BHRA) and the U.S. Navy to conduct their own controlled studies
to substantiate benefits of proactive contamination control maintenance:
The BHRA study covered a three-year
period and was based on the carefully monitored field experience of 117
hydraulic machines evenly spread across eight categories (i.e., injection
molding, machine tools, material handling, mobile/construction, marine,
metal working, test stands, and miscellaneous). The results of the study
showed a dramatic relationship between fluid contamination levels and service
life. Improved system cleanliness achieved extended actual mean time between
failures (MTBF) from 10 to 50 times, depending on cleanliness.
A study by the Naval Air Development
Center in Warminster, Pennsylvania performed on aircraft hydraulic pumps
showed nearly a 4-fold wear-life extension with a 66 percent improvement
in filtration and a 13-fold wear-life extension with a 93 percent improvement
According to the Bearing Division of TRW, "contamination is the number
one cause of bearing damage... the amount of damage caused by solid contaminants
passing between the rolling and sliding surfaces of an anti-friction bearing
is proportional to the size and concentration of the contaminants." Unlike
subsurface-originated damage commonly associated with fatigue, contamination
causes surface-originated damage to bearings. This contaminant-induced
wear reduces bearing life to as little as five percent of its rated life,
according to Japanese researchers. Other in-plant studies are just as resounding:
The contamination control program
reported by Nippon Steel included lubricating systems involving both journal
and roller bearings. Over the study's three-year period they successfully
achieved a 50 percent reduction in bearing purchases plant-wide.
International Paper Company
reported nearly a 90 percent reduction in bearing failures in just six
months after they implemented improved filtration and contamination control
in their Pine Bluff paper mill.
According to the post-sales
research of a well known manufacturer of high-duty thrust and journal bearings,
"dirt has been responsible for 85 percent or more of their customers' troubles."
This appears to conclude that 85 percent of problems and failures with
bearings can be eliminated if contaminant levels are reduced and controlled.
Engine and Gas Turbine Maintenance Savings
The benefits associated with the proactive contamination control of diesel
engine lube oils are great. Historically, there have been many misconceptions
regarding the influence of contamination on engine service life. Hence,
filters with very poor efficiencies have been and still frequently are
specified for engine lube oils. However, from a number of important new
field and lab studies we can now conclude that lube oil contamination is
the primary cause of engine wear that begins what is referred to as the
chain-reaction to failure.
In diesel engines, high local stresses associated with sliding contact
wear result in abrasive removal of material surfaces. When loads are concentrated
on the effective area of a small particle, the resulting surface stresses
can be greater than 500,000 psi, far beyond the elastic limit of substrate
materials. Oil film thicknesses, between which particles can reach and
attack surfaces, are typically in the 10-micron range. This explains why,
according to a wear study by Cummins Engine, particles smaller than 10
microns generated about 3.5 times more wear (rods, rings and main bearings)
than particles greater than 10 microns. Other important well documented
studies are described below:
Pall Corporation, in participation
with Detroit Diesel Allison (DDA), investigated the influence of improved
lube oil cleanliness on the performance and reliability of 150-ton diesel
trucks operating in an open pit mine. The study revealed substantial reductions
in wear metal concentrations.
AC Delco Division of General
Motors also tested DDA engines and found an eight-fold improvement in wear
rates and engine life with lower lube oil contaminant levels. In a related
study on both diesel and automotive engines, General Motors reports, "compared
to a 40-micron filter, engine wear was reduced by 50% with 30 micron filtration.
Likewise, wear was reduced by 70% with 15 micron filtration."
A study conducted by the supermarket
chain Albertson's Inc. on a series of over-the-road Cummins tractor engines
found markedly reduced wear rates with greater lube oil cleanliness. After
analyzing six engines having 600,000 operating miles, Albertson's reports,
"engine crankshaft journals showed only 0.0005 inches of wear. The rod
and main bearings hadn't even worn through to the copper layer. Compression-ring
and oil-ring wear were negligible."
An independent European university
study, as published in Lubrication Engineering Magazine, reports a reduction
in diesel engine wear by a factor of 14 when better lube oil cleanliness
is maintained. The study also equates the resulting friction reduction
with a 5 percent increase in fuel economy.
In reference to gas turbine engines, the U.S. Department of Defense states
that "approximately 30 percent of all engine failures are caused by metal
particulate contamination in lubricating oil systems." More precise studies,
if conducted, would likely prove the true percentage to be much higher.
After all, the wear processes and failures of gas turbines, by design,
should be very similar to diesel engine and bearings failures, as previously
reported and well documented.
It is interesting to note that currently an estimated 25 to 50 million
lube oil samples are analyzed by commercial and in-house fluid analysis
labs in the United States each year. Yet, despite the fact that contamination
is the largest contributor to engine failure, fewer than 5 percent of these
labs do particle counting on lube oil samples. Wear metal analysis and
elemental analysis are too often confused as being indicative of actual
particle sizes and concentrations in lube oils. Only accurate particle
counting devices can determine this.
to Implementing Proactive Contamination Control Maintenance
Contamination control, being the bedrock of proactive maintenance, can
be implemented in three simple steps:
Using the Contaminant Life Index,
establish the target fluid cleanliness levels for each machine and fluid
Select and install filtration
equipment (or upgrade current filter rating) and contaminant exclusion
techniques to achieve target cleanliness levels.
Monitor fluid cleanliness at
regular intervals to verify that targets are achieved. Adjust filtration
and contaminant exclusion techniques, as required, to stabilize target
A thorough explanation for implementing each of these steps can be found
in the book Fluid Contamination Control, by Dr. Ernest C. Fitch, which
can be obtained by contacting the author of the paper.
It is important to note that a common myth among people responsible for
machine maintenance is the belief that the incremental costs outweigh the
benefits of achieving improved fluid cleanliness. These costs are assumed
to be associated with the addition or upgrading of filters and/or the more
frequent changing of fluids. While it is not the intent of this article
to detail the host of techniques for implementing fluid contamination control,
it should be noted that if program origination costs are required, they
are generally very quickly absorbed by maintenance cost savings. Beyond
origination costs, incremental operating costs to maintain improved fluid
cleanliness would be expected only for certain high contaminant-ingression
applications, typically less than 10 percent of the cases. Otherwise, savings
usually outweigh costs by great margins.
Generally speaking, fluids and lubricants have indefinite life when protected
from excessive heat, moisture, air, and particles. As these are all considered
contaminants, their control should be a part of the contamination control
program. In fact, some power generation lube oils have achieved service
life in excess of ten thyears. Referring to the Nippon Steel report, they
state that the influence of rigid contamination control practices contributed
to a reduction of oil 83 consumption of 83 percent. Pall Corporation claims
that by improving fluid cleanliness, oil change intervals can be extended
by a factor of 2 or more.
Due to significantly lower wear rates (particle generation), Pall also
claims filter change intervals can be extended up to a factor of 2. This
can be extended further by taking steps to restrict the entry of contaminants
into the fluid. Additional savings can be achieved by routine monitoring
of fluid contaminant levels in order to time filter changes at optimum
Monitoring is Essential to Successful Contamination Control
Unassailably, fluid contaminant monitoring is the operative element to
achieving the goal of extended machine life. Machine contaminant levels,
as effected by ingression and filtration, are extremely dynamic. And, it
is not unusual for levels to vary two or three orders of magnitude over
a period of days or even hours. Accordingly, contaminant monitoring closes
the loop by providing the essential feedback and therefore control. Flying
an airplane in a storm without an altimeter, or navigating a ship at sea
without a direction finder, or driving a car a cross-country without a
fuel gage are some of the analogies that could be used for attempting maintenance
Fluid contaminant monitoring can be accomplished in the field or plant
by extracting samples of fluid into bottles for lab analysis or by portable
instruments used right at the machine. Recently there has been a trend
away from bottle sampling and lab analysis for routine contaminant monitoring
due to the associated higher cost, reduced accuracy, and time delay. In
its place has been the use of portable monitors that receive fluids directly
out of machines for on-the-spot analysis.
One instrument, sold by Diagnetics, called digital Contam-Alert (dCA),
is battery operated and extremely lightweight. It consists of a sensor
attached by cable to a hand-held computer. During a test, the sensor is
placed momentarily on a special diagnostic port permanently installed on
the machine. A small sample of fluid under pressure passes into the sensor
and after a minute or two the particle count is displayed on the computer
The unit can be used with a variety of different fluids, such as lube oils,
hydraulic fluids, transmission fluids, gear oils, and coolants. After each
test the handle on the sensor is depressed, which expels the sample, making
it immediately ready for reuse. Particle count data can be easily stored
in the computer, tagged to machine I.D., the date, and user comments. Later,
the data can be printed out with a portable printer or it can be down-loaded
to a desk-top personal computer.
Use of the portable contaminant monitor provides easy in-the-plant or in-the-field
proactive or predictive maintenance. Maintenance operators can simply walk
from machine to machine checking fluid contaminant levels and compare them
to target baselines. Maintenance work orders can then be issued to correct
of Contaminant Monitoring to Typical Predictive Maintenance Techniques
Outside of its usefulness as a proactive maintenance tool, contaminant
monitoring can be equally effective as a first-alert to impending machine
failure, i.e., predictive maintenance. When a machine failure is in progress
there is a precipitous generation of wear debris resulting in an abnormal
presence of particles in the fluids. This chain-reaction of few particles
generating more and more particles is an incontestable indication of progressive
failure. Using portable contaminant monitors, distinct shifts in contaminant
levels can be easily recognized, usually in plenty of time to schedule
This technique has a number of advantages over other predictive maintenance
According to the text "Hanbuch der Schadenverhutung," 63% of compressor
failures and 78% of turbine failures do not cause a change in vibration.
Further, in attempts to detect centrifugal compressor failures using vibration
monitoring, Chevron reports, "Many thrust bearing failures occur instantaneously,
allowing only seconds from the first indication of trouble to internal
contact of rotating and stationary parts." They further state, "the vibration
orbits have always 'blossomed' just prior to sudden catastrophic failure,
exceeding the shutdown limit in both the X and Y directions."
Wear occurs well ahead of the generation of aberrant vibration signals
in most rotating machinery. The resultant accelerated particle levels in
the lube oil is therefore the earliest sign of impending failure. Further,
there are many types of equipment where the vibration signals are far too
complex to monitor without highly sophisticated computer software to decipher
the signature. So far, for instance, attempts to use vibration monitoring
on hydraulic equipment have not been particularly successful.
Ferrography describes the process of depositing ferromagnetic particles
on a laboratory slide and then viewing them under a microscope. Its use
is limited to laboratory analysis from sample bottles due to its lack of
portability. More often, owing to the high cost of analytical ferrography
equipment, samples are sent to commercial labs where results can take several
days to several weeks. Also, analytical ferrography is not a quantitative
technique and does not assess the presence of non-magnetic particles, such
as aluminum, brass, copper, and chromium. Analytical ferrography can, however,
be very useful as a supplemental tool to localize faults and interpret
wear processes, once the initial indication is given by contaminant monitoring.
Spectrographic analysis has been used since World War II to establish and
quantify the presence of wear metals and additives in lube oils and hydraulic
fluids. There have been many conflicting studies regarding the usefulness
and accuracy of spectrographic analysis. The doubters state that the technique
cannot detect particles greater than 10 microns and no quantitative data
regarding particle size and count can be determined. One study published
in Lubrication Engineering Magazine involved over 150 used oil samples
taken from industrial gear boxes, compressors, power transmissions, and
hydraulic systems. It concluded that:
"High contamination levels in
these systems contribute to higher levels of wear, accelerate the process
of wear, and results in premature failure."
"By the time wear metals analysis
alone [as opposed to contaminant monitoring] indicates an increase in wear,
the abrasive process may be irreversible and the system may in fact be
at the point of catastrophic failure."
"It is interesting to note that
spectroscopic wear metal analysis results DID NOT CHANGE significantly
[despite greatly improved filtration], however an overall reduction in
total wear was achieved after several months of monitoring the system."
Still, another study showed that "spectrographic analysis did not predict
the failure of oil-wetted components on aircraft." Amazingly, after analyzing
an oil sample taken from an electric generator in another report, the spectrographic
results indicated "no major problems." In fact, the sample had been taken
from the engine AFTER catastrophic failure, a point at which exorbitant
wear metal levels should have been detected.
It has been stated that the fundamental purpose for contamination control
and contaminant monitoring is to achieve greatly extended mean time between
failures (MTBF), not damage control. However, when anomalous conditions
are present, as first measured contamimetrically, further analysis using
ferrography or vibration can identify the source of the problem.
For each application, three baselines are established. The first baseline
is routine contaminant monitoring, which serves the major system monitoring
requirements. It establishes the target cleanliness level, within which
the desired extended machine life can be accomplished. The second baseline
is set above the first on the contaminant level scale and represents abnormal
conditions requiring further analysis. The author prefers ferrography as
the means to localize and explain the source of the contamination.
In the example, a cylinder wiper seal failure is shown. This type of failure
has no immediate operational performance effect but does result in abnormally
high particle ingression. Once corrected, contaminant levels return below
the first baseline, to normal. The second movement past the first baseline
was determined to be a spent filter, which was replaced. In both cases,
ferrography failed to confirm unusual wear debris levels, directing the
trouble shooting elsewhere.
In some systems where vibration monitoring can be used, a third baseline
is established. If contaminant levels proceed into this region and ferrography
confirms abrasive or abnormal wear, then vibration analysis can be employed
as a damage control technique. Other methods such as volumetric analysis
or spectrograph analysis may be helpful as well. Once the problem component
is identified, maintenance can be readily scheduled.
This system monitoring hierarchy should be customized to appropriate user
and application requirements. It is designed to serve the combined needs
of proactive and predictive maintenance to achieve the maximum savings
possible. As a guide, the key to effective implementation is 90 percent
planning and 10 percent doing. The author plans a follow-up article to
fully describe the implementation strategy.
Proactive maintenance is presented as an important means to cure failure
root causes and extend machine life. Fluid contamination control is established
as an essential technique to implementing proactive maintenance. Substantial
savings are cydraulic, bearing, engine, and gas turbine aponfirmed based
on case studies involving hplications. Numerous examples of 10-fold maintenance
cost improvements are given.
As opposed to traditional predictive maintenance, contaminant monitoring
is cited as being key to achieving contamination control and proactive
maintenance. A comparison of contaminant monitoring to other predictive
techniques is discussed. It is concluded that contaminant monitoring offers
the preferred "first defense" against mechanical failure, followed by ferrography
and vibration monitoring.
Finally, it seems inevitable that future machinery include on-board contaminant
sensors for real-time proactive maintenance and condition control. Expert
system software combined with strategically located sensors and transducers
(e.g., pressure, temperature, vibration, viscosity, wear debris, and moisture)
will provide comprehensive machine health monitoring for the most sophisticated
future machine applications.
This story reprinted
courtesy of Noria Corporation.
to the Oil Analysis Reference Articles Index
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