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- Review the process—Use a process flowchart to identify
each process component.
- Brainstorm potential failure
modes—Review existing documentation and data for clues.
- List
potential effects of failure—There may be more than one for each
failure.
- Assign Severity rankings—Based on the severity of
the consequences of failure.
- Assign Occurrence
rankings—Based on how frequently the cause of the failure is likely
to occur.
- Assign Detection rankings—Based on the chances the
failure will be detected prior to the customer finding it.
- Calculate the RPN—Severity X Occurrence X Detection.
- Develop
the action plan—Define who will do what by when.
- Take
action—Implement the improvements identified by your PFMEA team.
-
Calculate the resulting RPN—Re-evaluate each of the
potential failures once improvements have been made and determine
the impact of the improvements.
Review
the process components and the intended function or functions of
those components.
- Use of a detailed flowchart of the process or a
traveler (or router) is a good starting point for reviewing the
process.
There are several reasons for reviewing the process
- First,
the review helps assure that all team members are familiar with the
process. This is especially important if you have team members who
do not work on the process on a daily basis.
- The second reason for
reviewing the process is to identify each of the main components of
the process and determine the function or functions of each of those
components.
- Finally, this review step will help assure that you
are studying all components of the process with the PFMEA.
Using
the process flowchart, label each component with a sequential
reference number.
- These reference numbers will be used throughout
the FMEA process.
- The marked-up flowchart will give you a powerful
visual to refer to throughout the PFMEA.
With the process
flowchart in hand, the PFMEA team members should familiarize
themselves with the process by physically walking through the
process. This is the time to assure everyone on the team understands
the basic process flow and the workings of the process components.
For each component, list its intended function or functions.
- The
function of the component is the value-adding role that component
performs or provides.
- Many components have more than one function.
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In Step 2, consider the
potential failure modes for each component and its corresponding
function.
- A potential failure mode represents any manner in which
the component or process step could fail to perform its intended
function or functions.
Using the list of components and related
functions generated in Step 1, as a team, brainstorm the potential
failure modes for each function.
- Don’t take shortcuts here; this
is the time to be thorough.
Prepare for the brainstorming session.
- Before you begin the brainstorming session, review documentation
for clues about potential failure modes.
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Determine the effects associated with each
failure mode. The effect is related directly to the ability of that
specific component to perform its intended function.
- An effect is
the impact a failure could make if it occurred.
- Some failures will
have an effect on the customers and others on the environment, the
facility, and even the process itself.
As with failure modes, use
descriptive and detailed terms to define effects.
- The effect
should be stated in terms meaningful to product or system
performance.
- If the effects are defined in general terms, it will
be difficult to identify (and reduce) true potential risks.
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Assign a severity ranking to each effect
that has been identified.
- The severity ranking is an estimate of
how serious an effect would be should it occur.
- To determine the
severity, consider the impact the effect would have on the customer,
on downstream operations, or on the employees operating the process.
The severity ranking is based on a relative scale ranging from 1
to 10.
- A “10” means the effect has a dangerously high severity
leading to a hazard without warning.
- Conversely, a severity
ranking of “1” means the severity is extremely low.
The ranking
scales (for severity, occurrence, and detection) are mission
critical for the success of a PFMEA because they establish the basis
for determining risk of one failure mode and effect relative to
another.
- The same ranking scales for PFMEAs should be used
consistently throughout your organization. This will make it
possible to compare the RPNs from different FMEAs to one another.
- See
FMEA Checklists
and Forms for an example PFMEA Severity Ranking
Scale.
The best way to customize a ranking scale is to start with
a standard, generic scale and then modify it to be more meaningful
to your organization.
- As you add examples specific to your
organization, consider adding several columns with each column
focused on a topic.
- One topic could provide descriptions of
severity levels for operational failures, another column for
customer satisfaction failures, and a third for environmental,
health, and safety issues.
- See
FMEA Checklists
and Forms for an examples of Custom PFMEA Ranking Scales. (Examples of custom scales for severity,
occurrence, and detection rankings are included in this Appendix.)
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Next, consider the potential
cause or failure mechanism for each failure mode; then assign an
occurrence ranking to each of those causes or failure mechanisms.
We need to know the potential cause to determine the occurrence
ranking because, just like the severity ranking is driven by the
effect, the occurrence ranking is a function of the cause. The
occurrence ranking is based on the likelihood, or frequency, that
the cause (or mechanism of failure) will occur.
If we know the
cause, we can better identify how frequently a specific mode of
failure will occur. How do you find the root cause?
- There are many
problem-finding and problem-solving methodologies.
- One of the
easiest to use is the 5-Whys technique.
- Once the cause is known,
capture data on the frequency of causes. Sources of data may be
scrap and rework reports, customer complaints, and equipment
maintenance records.
The occurrence ranking scale, like the
severity ranking, is on a relative scale from 1 to 10.
- An
occurrence ranking of “10” means the failure mode occurrence is very
high, and happens all of the time. Conversely, a “1” means the
probability of occurrence is remote.
- See
FMEA Checklists
and Forms for an example PFMEA Occurrence Ranking Scale.
Your
organization may need an occurrence ranking scale customized for a
low-volume, complex assembly process or a mixture of high-volume,
simple processes and low-volume, complex processes.
- Consider
customized occurrence ranking scales based on time-based,
event-based, or piece-based frequencies.
- See
FMEA Checklists
and Forms for
examples of Custom PFMEA Ranking Scales. (Examples of custom scales
for severity, occurrence, and detection rankings are included in
this Appendix.)
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To assign
detection rankings, identify the process or product related controls
in place for each failure mode and then assign a detection ranking
to each control. Detection rankings evaluate the current process
controls in place.
- A control can relate to the failure mode
itself, the cause (or mechanism) of failure, or the effects of a
failure mode.
- To make evaluating controls even more complex,
controls can either prevent a failure mode or cause from occurring
or detect a failure mode, cause of failure, or effect of failure
after it has occurred.
- Note that prevention controls cannot relate
to an effect. If failures are prevented, an effect (of failure)
cannot exist!
The Detection ranking scale, like the Severity and
Occurrence scales, is on a relative scale from 1 to 10.
- A
Detection ranking of “1” means the chance of detecting a failure is
certain.
- Conversely, a “10” means there is absolute certainty of
non-detection. This basically means that there are no controls in
place to prevent or detect.
- See
FMEA Checklists
and Forms for an example PFMEA Detection Ranking Scale.
Taking a lead from AIAG,
consider three different forms of Custom Detection Ranking options.
Custom examples for Mistake-Proofing, Gauging, and Manual Inspection
controls can be helpful to PFMEA teams.
- See
FMEA Checklists
and Forms for
examples of Custom PFMEA Ranking Scales. (Examples of custom scales
for severity, occurrence, and detection rankings are included in
this Appendix.)
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The RPN is the Risk
Priority Number. The RPN gives us a relative risk ranking. The
higher the RPN, the higher the potential risk.
The RPN is
calculated by multiplying the three rankings together. Multiply the
Severity ranking times the Occurrence ranking times the Detection
ranking. Calculate the RPN for each failure mode and effect.
- Editorial Note: The current FMEA Manual from AIAG suggests only
calculating the RPN for the highest effect ranking for each failure
mode. We do not agree with this suggestion; we believe that if this
suggestion is followed, it will be too easy to miss the need for
further improvement on a specific failure mode.
Since each of the
three relative ranking scales ranges from 1 to 10, the RPN will
always be between 1 and 1000. The higher the RPN, the higher the
relative risk. The RPN gives us an excellent tool to prioritize
focused improvement efforts.
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Taking action means reducing
the RPN. The RPN can be reduced by lowering any of the three
rankings (severity, occurrence, or detection) individually or in
combination with one another.
A reduction in the Severity ranking
for a PFMEA is often the most difficult. It usually requires a
physical modification to the process equipment or layout.
Reduction in the Occurrence ranking is accomplished by removing or
controlling the potential causes.
- Mistake-proofing tools are often
used to reduce the frequency of occurrence.
A reduction in the
Detection ranking can be accomplished by improving the process
controls in place.
- Adding process fail-safe shut-downs, alarm
signals (sensors or SPC), and validation practices including work
instructions, set-up procedures, calibration programs, and
preventative maintenance are all detection ranking improvement
approaches.
What is considered an acceptable RPN? The answer to
that question depends on the organization.
- For example, an
organization may decide any RPN above a maximum target of 200
presents an unacceptable risk and must be reduced. If so, then an
action plan identifying who will do what by when is needed.
There
are many tools to aid the PFMEA team in reducing the relative risk
of failure modes requiring action. Among the most powerful tools are
Mistake-Proofing, Statistical Process Control, and Design of
Experiments.
Mistake-Proofing (Poka Yoke)
- Techniques that can
make it impossible for a mistake to occur, reducing the Occurrence
ranking to 1.
- Especially important when the Severity ranking is
10.
Statistical Process Control (SPC)
- A statistical tool that
helps define the output of a process to determine the capability of
the process against the specification and then to maintain control
of the process in the future.
Design of Experiments (DOE)
- A
family of powerful statistical improvement techniques that can
identify the most critical variables in a process and the optimal
settings for these variables.
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The Action Plan
outlines what steps are needed to implement the solution, who will
do them, and when they will be completed.
A simple solution will
only need a Simple Action Plan while a complex solution needs more
thorough planning and documentation.
- Most Action Plans identified
during a PFMEA will be of the simple “who, what, & when” category.
Responsibilities and target completion dates for specific actions to
be taken are identified.
- Sometimes, the Action Plans can trigger a
fairly large-scale project. If that happens, conventional project
management tools such as PERT Charts and Gantt Charts will be needed
to keep the Action Plan on track.
- Most Action Plans identified
during a PFMEA will be of the simple “who, what, & when” category.
Responsibilities and target completion dates for specific actions to
be taken are identified.
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This step in a PFMEA confirms the action plan had the desired
results by calculating the resulting RPN.
To recalculate the RPN,
reassess the severity, occurrence, and detection rankings for the
failure modes after the action plan has been completed.
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