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We will see later how the RCM process uses these categories as the basis
of a strategic framework for maintenance decision-making. By forcing a
structured review of the consequences of each failure mode in terms of
the above categories, it integrates the operational, environmental and
safety objectives of the maintenance function. |
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This helps to bring safety and the environment into the mainstream of maintenance
management. |
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The consequence evaluation process also shifts emphasis away from the idea
that all failures are bad and must be prevented. In so doing, it focuses
attention on the maintenance activities which have most effect on the performance
of the organization, and diverts energy away from those which have little
or no effect. It also encourages us to think more broadly about different
ways of managing failure, rather than to concentrate only on failure prevention.
Failure management techniques are divided into two categories: |
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proactive tasks: these
are tasks undertaken before a failure occurs, in order to prevent the item
from getting into a failed state. They embrace what is traditionally known
as 'predictive' and 'preventive' maintenance, although we will see later
that RCM uses the terms scheduled restoration scheduled discard and on-condition
maintenance
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default actions: these
deal with the failed state, and are chosen when it is not possible to identify
an effective proactive task. Default actions include failure-finding, redesign
and run-to-failure.
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The consequence evaluation process is discussed again briefly later this
chapter, and in much more detail in Chapter 5 of the book. The next section
of this chapter looks at proactive tasks in more detail. |
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Many people still believe that the best way to optimize plant availability
is to do some kind of proactive maintenance on a routine basis. Second
Generation wisdom suggested that this should consist of overhauls of component
replacements at fixed intervals. Figure 1.4 illustrates the fixed |
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Figure 1.4 is based on the assumption that most items operate reliably
for a period of time, and then wear out. Classical thinking suggests that
extensive records about failure will enable us to determine this life and
so make plans to take preventive action shortly before the item is due
to fail in future. |
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This model is true for certain types of simple equipment, and for some
complex items with dominant failure modes. In particular, wear-out char-acteristics
are often found where equipment comes into direct contact with the product.
Age-related failures are also often associated with fatigue, corrosion,
abrasion and evaporation. |
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However, equipment in general is far more complex than it was twenty years
ago. This has led to startling changes in the patterns of failure, as shown
in Figure 1.5. The graphs show conditional probability of failure against
operating age for a variety of electrical and mechanical items. |
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Pattern A is the well-known bathtub curve. It begins with a high incidence
of failure (known as infant mortality) followed by a constant or gradually
increasing conditional probability of failure, then by a wear-out zone.
Pattern B shows constant or slowly increasing conditional prob-ability
of failure, ending in a wear-out zone (the same as Figure 1.4). |
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Pattern C shows slowly increasing conditional probability of failure, but
there is no identifiable wear-out age. Pattern D shows low conditional
probability of failure when the item is new or just out of the shop, then
a rapid increase to a constant level, while pattern E shows a constant
conditional probability of failure at all ages (random failure). Pattern
F starts with high infant mortality, which drops eventually to a constant
or very slowly increasing conditional probability of failure. |
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Studies done on civil aircraft showed that 4% of the items conformed to
pattern A, 2% to B, 5% to C, 7% to D, 14% to E and no fewer than 68% to
pattern F. (The number of times these patterns occur in aircraft is not
necessarily the same as in industry. But there is no doubt that as assets
become more complex, we see more and more of patterns E and F.) |
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These findings contradict the belief that there is always a connection
between reliability and operating age. This belief led to the idea that
the more often an item is overhauled, the less likely it is to fail. Nowadays,
this is seldom true. Unless there is a dominant age-related failure mode,
age limits do little or nothing to improve the reliability of complex items.
In fact scheduled overhauls can actually increase overall failure rates
by introducing infant mortality into otherwise stable systems. |
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An awareness of these facts has led some organizations to abandon the idea
of proactive maintenance altogether. In fact, this can be the right thing
to do for failures with minor consequences. But when the failure consequences
are significant, something must be done to prevent or predict the failures,
or at least to reduce the consequences. |
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This brings us back to the question of proactive tasks. As mentioned earlier,
RCM divides proactive tasks into three categories, as follows: |
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scheduled restoration tasks
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scheduled discard tasks
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scheduled on-condition tasks.
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| Scheduled restoration
and scheduled discard tasks |
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Scheduled restoration entails remanufacturing a component or overhauling
an assembly at or before a specified age limit, regardless of its condition
at the time. Similarly, scheduled discard entails discarding an item at
or before a specified life limit, regardless of its condition at the time. |
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Collectively, these two types of tasks are now generally known as preventive
maintenance. They used to be by far the most widely used form of proactive
maintenance. However for the reasons discussed above, they are much less
widely used than they were twenty years ago. |
| On-condition tasks |
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The continuing need to prevent certain types of failure, and the growing
inability of classical techniques to do so, are behind the growth of new
types of failure management. The majority of these techniques rely on the
fact that most failures give some warning of the fact that they are about
to occur. These warnings are known as potential failures, and are defined
as identifiable physical conditions which indicate that a functional failure
is about to occur or is in the process of occurring. |
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The new techniques are used to detect potential failures so that action
can be taken to avoid the consequences which could occur if they degenerate
into functional failures. They are called on-condition tasks because items
are left in service on the condition that they continue to meet desired
performance standards. (On-condition maintenance includes predictive maintenance,
condition-based maintenance and condition monitoring.) Used appropriately,
on-condition tasks are a very good way of managing failures, but they can
also be an expensive waste of time. RCM enables decisions in this area
to be made with particular confidence. |
| Default Actions |
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RCM recognizes three major categories of default actions, as follows: |
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failure-finding: Failure-finding
tasks entail checking hidden functions periodically to determine whether
they have failed (whereas condition-based tasks entail checking if something
is failing).
|
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redesign: redesign entails
making any one-off change to the built-in capability of a system. This
includes modifications to the hardware and also covers once-off changes
to procedures.
|
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no scheduled maintenance:
as the name implies, this default entails making no effort to anticipate
or prevent failure modes to which it is applied, and so those failures
are simply allowed to occur and then repaired. This default is also called
run-to-failure.
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| The RCM Task Selection
Process |
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A great strength of RCM is the way it provides simple, precise and easily
understood criteria for deciding which (if any) of the proactive tasks
is technically feasible in any context, and if so for deciding how often
they should be done and who should do them. These criteria are discussed
in more detail in Chapters 6 and 7 of the book. |
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Whether or not a proactive task is technically feasible is governed by
the technical characteristics of the task and of the failure which it is
meant to prevent. Whether it is worth doing is governed by how well it
deals with the consequences of the failure. If a proactive task cannot
be found which is both technically feasible and worth doing, then suitable
default action must be taken. The essence of the task selection process
is as follows: |
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for hidden failures, a proactive
task is worth doing if it reduces the risk of the multiple failure associated
with that function to an acceptably low level. If such a task cannot be
found then a scheduled failure-finding task must be performed. If a suitable
failure-finding task cannot be found, then the secondary default decision
is that the item may have to be re-designed (depending on the consequences
of the multiple failure).
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for failures with safety or
environmental consequences, a proactive task is only worth doing if it
reduces the risk of that failure on its own to a very low level indeed,
if it does not eliminate it altogether. If a task cannot be found which
reduces the risk of the failure to an acceptably low level, the item must
be redesigned or the process must be changed.
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if the failure has operational
consequences, a proactive task is only worth doing if the total cost of
doing it over a period of time is less than the cost of the operational
consequences and the cost of repair over the same period. In other words,
the task must be justified on economic grounds. If it is not justified,
the initial default decision is no scheduled maintenance. (If this occurs
and the operational consequences are still unacceptable then the secondary
default decision is again redesign).
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if a failure has non-operational
consequences a proactive task is only worth doing if the cost of the task
over a period of time is less than the cost of repair over the same period.
So these tasks must also be justified on economic grounds. If it is not
justified, the initial default decision is again no scheduled maintenance,
and if the repair costs are too high, the secondary default decision is
once again redesign.
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This approach means that proactive tasks are only specified for failures
which really need them, which in turn leads to substantial reductions in
routine workloads. This routine work also means that the remaining tasks
are more I likely to be done properly. This together with the elimination
of counterproductive tasks leads to more effective maintenance. Compare
this with the traditional approach to the development of maintenance policies.
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