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Overview
The main reasons why
PMO is faster than RCM are summarized in figure 2
below. The points are discussed in detail later in the
paper.
1. Insignificant
failure modes are not analyzed by PMO whereas RCM
analyses all likely failure modes.
2. Using PMO, many
failure modes can be rolled up and analyzed together
whereas with RCM, failure modes are analyzed
separately.
3. With PMO, a detailed
functional analysis is an optional step. The function
of the equipment is completed as part of Consequence
Evaluation because a consequence of any failure is a
loss of function by definition.
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Figure
2
Comparison of the costs,
time and benefits of RCM compared with PMO
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How and why failure
mode analysis of insignificant failures is avoided by
PMO.
The
equipment design and the way it is operated determine
the type and likelihood of failure modes.
In the context of maintenance analysis, failure
modes can be broken into categories based on the
following:
·
their
likelihood,
·
their
consequences, and
·
the
practicality and feasibility of preventing or
predicting them.
This point is illustrated in
Figure 3.
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Figure
3
Considerations required for
maintenance analysis.
Likelihood
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Consequences
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PM
Feasibility
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High
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Hazard
|
Feasible
|
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Medium
|
High
Cost
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Not
Feasible
|
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Low
|
Low
Cost
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A
PM program is targeted at the conditions
listed in the gray areas of the table.
These comprise the minority of
failures, as the intent of design is to
engineer out hazards and high costs failures
particularly when their likelihood is high.
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The focus of good
equipment design is to ensure high levels of
reliability, maintainability and operability. This
means eliminating high likelihood and high consequence
failures.
It is therefore, not
surprising that when reviewing the complete set of
likely failure modes using RCM analysis, that by far
the greatest number of outcomes, or recommendations,
are No Scheduled Maintenance. This is to say that for
the failure modes left in the design in question,
either:
- Their likelihood is
very low,
- There is no
technically feasible predictive or preventive
maintenance task known to manage them, or
- The task that is
known costs more to do than the cost of the cost
of unexpected failure. The less critical the
equipment is to productive capacity, the more
likely that the cost of the maintenance outweighs
the costs of the failure over a given life cycle.
In the author’s
experience, full RCM analysis of equipment shows that,
on average, about 80% of failure modes result with the
policy of No Scheduled Maintenance¹
. This information is presented in Figure
4. This number rises with electronic equipment
such as a Programmable Logic Controller (PLC) and
falls with equipment that has a high number of moving
parts such as a conveyor.
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¹
This figure will vary markedly with
some equipment having a 50% return. The other variable
is the propensity for teams to “black box” and
by-pass certain parts of the system because, from
experience, these items are known to have few or no
failure modes that are preventable or predictable or
are hidden. Whilst reducing the ratio of No Scheduled
Maintenance outcomes, such bypassing streamlines the
RCM process and therefore is a non conformance to the
standard. |
