There's been a steady evolution from
breakdown maintenance through to proactive maintenance. |
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RCM embodies
the best aspects of the four main maintenance
philosophies |
Maintenance
philosophies and practices have been around for many
years - the objective of these philosophies being the
economic consideration of optimizing plant availability
in industry. There has been a steady evolution from
breakdown maintenance to predictive maintenance to
proactive maintenance in the quest to keep industrial
equipment operating economically, efficiently and safely.
Breakdown
maintenance |
Thirty years ago it
may have been sufficient to adopt a breakdown maintenance
philosophy whereby plant management would allow equipment
to run to destruction and simply replace it at failure.
In today’s economic climate this is no longer
viable. Not only does the cost of repair or replacement
have to be borne in the mind, but the cost of down-time,
lost production and safety must be considered. In the
majority of cases, it does not make economic sense to
manage a plant in this way.
Preventive
maintenance |
With the increase in
the cost of machinery, spares and labour, and the cost to
industry of lost or poor production, breakdown
maintenance evolved into preventive maintenance in the
late sixties and early seventies.
The objective of
preventive maintenance was to organize a time-based
schedule for service and overhaul of essential equipment.
By using preventive maintenance it was hoped to eliminate
or drastically reduce component failure and down-time,
improving productivity and profitability.
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The cost of maintenance must be
balanced against the cost of breakdown. |
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Predictive
maintenance |
Preventive
maintenance was a great step in the right direction, but
could only take maintenance philosophy so far, in that
equipment was being checked, serviced and repaired on a
regular basis and that history files were being kept on
the equipment. One of the problems with preventive
maintenance is that it is possible to overmaintain
equipment; the cost of maintenance must be balanced
against the cost of breakdown. This led to a shift from
preventive to predictive maintenance.
The concept of predictive
maintenance is to carry on with the planned preventive
maintenance, but at the same time, conduct as many
non-destructive test on the items of plant to determine
their mechanical integrity. Basically, this philosophy
asks the question, "What is the point of fixing
something that isn’t broken?" Oil analysis is
probably the cheapest and easiest form of predictive
maintenance to implement, but other techniques include
vibration analysis, thermographic imaging, motor current
analysis, balancing and alignment. It may be found that
some failure modes cannot be detected by oil analysis on
certain equipment, for example on non-lubricated
components such as electric motors. In these cases other
techniques must be employed.
The bar chart below shows
the maintenance costs for general industrial rotating
machinery, comparing reactive (breakdown), preventive and
predictive maintenance philosophies.
Studies have shown that a
successful preventive maintenance program can provide a
30% reduction in maintenance costs over a breakdown
maintenance program.
One important aspect of a
predictive maintenance program is that the plant is not
overhauled unnecessarily.
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Proactive maintenance looks at root
cause failure analysis. |
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Contrary to popular belief, there
is not a strong relationship between operating age and
reliability. Once a piece of equipment has
"bedded-in" and established a reliable
operating mode, overhaul will disturb this mode and
research has shown that the overhauled component will not
last as long as the new component. If the predictive
maintenance techniques employed do not indicate a
problem, then don’t disturb a stable and reliable
operating mode. With
the introduction of predictive maintenance and accurate
record keeping in machine history files, certain failure
modes can be determined and this leads directly to a
proactive maintenance philosophy.
Proactive
maintenance |
Proactive
maintenance looks at root cause failure analysis (RCFA)
to determine the cause of certain common failures in
industrial equipment. If the cause of failure can be
determined, can that cause be eliminated? Whereas
previous maintenance philosophies have looked at
predicting the failure of a component and taking action
before the failure actually occurs, proactive maintenance
looks at the root cause of failure and aims to eliminate
the cause so that, in turn, that failure mode is also
eliminated.
A good example of using
RCFA in proactive maintenance is the influence of
particulate contamination in the failure of fluid power
systems. Research has shown that 70% - 85% of hydraulic
component failures are due to particulate contamination,
with up to 90% of these failures due to abrasive wear.
Predictive maintenance has
established the cause of the majority of hydraulic system
failures; the object of the proactive maintenance
philosophy is to eliminate that cause by keeping
hydraulic fluids as clean as is practically possible. By
regularly checking the cleanliness of hydraulic fluids
and taking steps to clean contaminated fluids, a large
number of hydraulic component failures can be prevented.
Oil analysis forms an
important part of proactive maintenance. In the example
cited above, oil analysis is used to determine the
cleanliness levels of lubricants so that dirty oils can
be either cleaned or changed when unacceptable levels of
contamination are detected. Although oil analysis is
strictly part of predictive maintenance, it can also be
instrumental in RCFA and can lead to good proactive
maintenance practices. For example, oil analysis could be
used to predict bearing failure in an automotive
application. If the engine can be dismantled before
actual failure then, firstly, subsequent damage can be
avoided and, secondly, it is hoped that the cause of the
bearing failure can be determined and eliminated. This is
particularly useful when one considers that certain
failure modes, at the point of failure, can destroy the
evidence of the cause of failure. In this way oil
analysis can form an integral part of proactive
maintenance.
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RCM encompasses all the best aspects
of the four maintenance philosophies. |
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Reliability-centered
maintenance (RCM) |
A lot of large
industrial organizations have adopted the concept of RCM
which encompasses all the best aspects of the four basic
maintenance philosophies discussed above. Even breakdown
maintenance has its benefits in certain applications
(replacing a burnt out light bulb, for example).
The introduction of RCM to
a company or organization is not something that can be
carried out overnight and the benefits are not likely to
be immediately quantifiable. What is most important is
that the company must have a maintenance philosophy or
mission statement and a top-down management commitment to
that mission statement. Only then can an RCM system be
developed or expanded from existing maintenance systems.
Management must be involved at all levels with regard to
decision-making in the maintenance department before an
effective maintenance philosophy can be implemented.
General acceptance of an
RCM group by the workforce can sometimes be one of the
biggest hurdles to overcome because the maintenance
planning department is frequently seen as a "police
force" checking up on the quality of work carried
out by the artisans in the plant.
However, once an RCM
program has been instituted, developed and receives the
full support of management and the workforce, then the
company can look forward to dramatically increased
reliability, productivity and safety as well as the
ultimate benefit, increased profitability.
References |
- W.M. Needleman - Filtration
for Wear Control.
- F Pardue, K Piety, R
Moore - Elements of Reliability Based
Maintenance.
- FJ Hudachek, VR Dodd
- Progress and Payout of a Machinery
Surveillance and Diagnostic Program.
- R Owen - Strategic
Maintenance is Essential.
- MN James - Mechanical
Technology (May 1994).
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