Basic Steps of Applying
Reliability Centered Maintenance (RCM) Part II
Although there is a great deal of variation in
the application of Reliability Centered Maintenance (RCM), most procedures
include some or all of the seven steps shown below:
- Prepare for the Analysis
- Select the Equipment to Be Analyzed
- Identify Functions
- Identify Functional Failures
- Identify and Evaluate (Categorize) the
Effects of Failure
- Identify the Causes of Failure
- Select Maintenance Tasks
If we were to group the seven steps into three
major blocks, these blocks would be:
DEFINE (Steps 1, 2 and 3)
ANALYZE (Steps 4, 5 and 6)
ACT (Step 7)
Reliability HotWire discussed the DEFINE stage. In this article, we
examine the ANALYZE stage.
Identify Functional Failures
A functional failure is defined as the inability of an asset to fulfill
one or more intended function(s) to a standard of performance that is
acceptable to the user of the asset. Functional failures may include:
Complete failure to perform a function
Poor performance of a function
Over-performance of a function
Performing an unintended function
We need to keep in mind that the focus of RCM
is on preventing loss of function, not equipment; we should therefore avoid
talking about equipment in these failure statements.
Also, remember that functional failures do not have to be simple definitions
or a single statement. A function can have multiple failures. These should
be distinguished, because in later steps, these different failures will
relate to different failure modes, which may be due to different causes and
have different effects that need to be dealt with differently. All
functional failures are, so to speak, not created equal.
Furthermore, keep in mind that failure definitions of an asset are closely
related to its operating context. Therefore, we should generalize neither
about functions of identical assets nor about their functional failures, in
case the assets operate under different contexts or are used under different
requirements and expectations.
Here are a few examples of functional failures (listed with the function
1. To maintain
discharge flow of 500 gpm +/- 10%.
Unable to discharge at all.
B. Fluid flow restricted.
C. Fails to contain the fluid.
D. Discharge flow exceeds 505 gpm.
E. Discharge flow drops below 495 gmp.
Note that the performance limits that have been identified for the function
may provide a guide to the functional failure description.
Identify and Evaluate (Categorize) the Effects of Failure
Failure effects analysis is concerned with what happens when a failure mode
occurs. Revealing the effects of failure involves asking questions such as:
What will be observed when the failure occurs?
What is the impact on operations/production?
What is the impact on the environment/safety?
What physical change will occur to the equipment or adjacent equipment?
What alarms or indications will be observed?
Effects can be defined at three different levels:
Local Effect - What is observed at the individual component?
Next Level Effect What is observed at the sub-system level?
End Effect What will be observed at the system level?
example, if you run out of gas in your car, then:
Local Effect: Fuel injectors fail to supply gas to the engine.
Next Level Effect: Engine stops working.
End Effect: Car stops, you are late to work.
Many RCM references contain logic diagrams that can be used to evaluate and
categorize the effects of failure. For example, the following logic diagram
is provided as an example in the SAE JA1012 "Guide to the
Reliability-Centered Maintenance (RCM) Standard." (Other published logic
diagrams may consist of 3 or 5 questions and 4 or 5 failure effect
Going down the above questions tree yields the appropriate classification of
a failure effect.
Identify the Causes of Failure (Failure Modes)
The cause of failure (failure mode) represents the specific cause of the
functional failure at the actionable level (i.e. the level at which
it will be possible to apply a maintenance strategy to address the potential
failure). Identifying causes (failure modes) is of paramount importance. It
is a time- and effort-intensive step, but it is well worth it! The
day-to-day issues of maintenance are mostly managed at the failure mode
level (e.g. work orders issued to deal with specific failure modes,
maintenance plans designed to deal with failure modes, product recalls due
to a certain unexpected failure mode or frequent failure mode, emergency
design or maintenance meetings triggered because of an occurrence of a
failure mode, etc.) Extensive discussions about failure mode identification
in this step of the RCM process will have a great beneficial impact on the
success of the RCM project. It is what could make a difference between a
reactive and a proactive maintenance management plan.
following is an example of failure mode identification:
1. To deliver oil from tank T1 to tank
T2 at not less than 1000L/min.
| A. Oil
B. No oil delivered.
C. Deliver less than 1000L/min.
A.2. Screws in the joints not tightened
A.3. Screws broke.
B.1. Impeller comes adrift.
B.2. Impeller jammed by foreign objects.
B.3. Motor burned out.
B.4. Inlet valve jammed.
B.5. Bearing seized.
C.1. Impeller worn.
C.2. Partially blocked suction line.
C.3. Motor slowing down.
C.4. Inlet valve partially clogged.
causes of failure (failure modes) can be described to almost any level of
detail. Different levels are appropriate in different situations. In some
situations, it might be sufficient to state the direct reason of failure
(for example: tire puncture) while in other situations, drilling down to the
molecular level is needed. The SAE JA1012 guideline presents a useful
demonstration of the many levels of detail that can be used to describe
failure modes. For example:
|Pump set fails.
Because: Pump fails.
Because: Impeller fails.
Because: Impeller came adrift.
Because: Mounting nut undone.
Because: Nut not tightened correctly.
The guideline recommends that "failure modes should be described in enough
detail for it to be possible to select an appropriate failure management
policy, but not in so much detail that excessive amounts of time are wasted
on the analysis process itself." It is a tough balance to strike. It is for
you to decide the required level of the root cause investigation. A common
rule is to stop at the level beyond which it becomes impossible for the
organization to define a failure management policy.
If you plan to perform statistical data
analyses, another crucial piece of information that needs to be collected in
this step is the time dimension. The exact times at which failure modes have
occurred (or times between occurrences) need to be recorded (along with
other information such as conditions in which the asset was used). Such data
will be central to life data analysis and assessment of probability of
occurrence. It is what will be used to quantify the "R" (reliability) in
RCM. Failure modes that have not occurred but are considered to be real
possibilities present a challenging aspect of the RCM process. More judgment
calls and/or other reliability analysis techniques (such as accelerated
testing) would be needed in this case.
NOTE: Many RCM practitioners choose to
identify the Failure Mode prior to describing and categorizing the Effects
of failure. The analysis logic can be applied the same way, regardless of
which sequence is used.
The article summarized the "analysis" or "investigative" steps in the RCM
process, in which functional failures, the failure modes and their effect
and causes are explored. A future article will tackle the failures head-on
and discuss different alternatives for addressing failures.
ATA MSG-3 "Operator/Manufacturer Scheduled Maintenance Development," updated
in March 2003.
Moubray, John, Reliability-Centered Maintenance, Industrial Press,
Inc., New York City, NY, 1997.
Nowlan, F. Stanley and Howard F. Heap, Reliability-Centered Maintenance.
Issued in December, 1978.
SAE JA1012 "A Guide to the Reliability-Centered Maintenance (RCM) Standard,"
issued in January 2002.
Smith, Anthony, Hinchcliffe, Glenn R., RCM - Gateway to World Class
Maintenance, Elsevier Inc, Burlington, MA, 2004.