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|First of all let's start out with a definition for failure mode - it is the way in which a
failure is observed. It describes the way the failure occurs, and its impact on
equipment operation. A failure mode deals with the present, whereas a failure
cause happened in the past and a failure effect deals with the future (see the
FMEA definitions page).
If you are somewhat new to FMEA, then you may be a bit unsure on how to come
up with or postulate failure modes for your system. Obviously, failure modes are a
key ingredient to a Failure Mode and Effects Analysis, so it is important to have a
good understanding of just what they are and how they are normally derived.
|Aircraft Carrier USS Nimitz
U.S. Navy photo by Mass Communication
Specialist 3rd Class Samuel Souvannason
The diagram to the right may be
helpful in visualizing the
relationship between a failure
mode and its causes and effects.
As mentioned above, the failure
mode happens in the present
and it describes the way in which
the failure is observed. It is due
to the failure cause and it may
result in the failure effect.
Let's look at a common example that is often used in the automotive industry, say a
headlamp assembly. Obviously, its main function is to illuminate the road, so that
we can see while driving at night. The headlamp assembly itself, can be broken
down further into subassemblies or components that perform various functions.
The trimmers allow the headlamp to be adjusted vertically or horizontally and
maintain their settings.
So, if we use one of the trimmers for our
example the function would be to adjust the
headlight beam in both directions. When
identifying failure modes associated with a
function normally we look at common
types: loss of function, partial function,
intermittent function, or unintended
function. On some types of functions we
may also want to consider some timing
issues. For example, we might
have premature operation or failure to operate at the proper time. We may even
want to consider a failure to stop operating at the proper time. This can be
especially true if we are working with sophisticated electronic equipment that has
sensitive output signals and timing.
Anyway, getting back to our headlamp example and the function of the trimmers. In
this case some examples of failure modes might be:
For the loss of function it could be no adjustment.
For the partial function it could be beam adjustment only in one direction - up, but
Similarly, if the function worked intermittently or there was some unintended function
that may occur then failure modes could be identified to cover these situations. The
exact wording that is used for the failure mode descriptions will vary from one
analyst to another, but try to be fairly concise and as specific as possible. Also, try
to maintain a consistent approach throughout your analysis to avoid confusion. This
way when someone is reviewing your analysis, or report, your terminology and
descriptions are pretty uniform, and failure modes and effects are not described
one way in part of the analysis and then drastically different elsewhere.
Hopefully, this short example provides you with some insight on how to identify or
postulate failure modes as part of your FMEA / FMECA. If you have laid the initial
groundwork and partitioned your system and functions out properly, then identifying
applicable failure modes is not that difficult.