临城方言故事会视频:10 Steps to Conduct a DFMEA

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December 30, 2011

10 Steps to Conduct a DFMEA

10 Steps to Conduct a DFMEA
Step 1
Review the design—Use a blueprint or schematic of the design/product to identify each component and interface.
Step 2
Brainstorm potential failure modes—Review existing documentation and data for clues.
Step 3
List potential effects of failure—There may be more than one for each failure.
Step 4
Assign Severity rankings—Based on the severity of the consequences of failure.
Step 5
Assign Occurrence rankings—Based on how frequently the cause of the failure is likely to occur.
Step 6
Assign Detection rankings—Based on the chances the failure will be detected prior to the customer finding it.
Step 7
Calculate the RPN—Severity x Occurrence x Detection.
Step 8
Develop the action plan—Define who will do what by when.
Step 9
Take action—Implement the improvements identified by your DFMEA team.
Step 10
Calculate the resulting RPN—Re-evaluate each of the potential failures once improvements have been made and determine their impact on the RPNs.
Step 1: Review the Design
Reasons for the review:
Help assure all team members are familiar with the product and its design. Identify each of the main components of the design and determine the function or functions of those components and interfaces between them. Make sure you are studying all components defined in the scope of the DFMEA.
Use a print or schematic for the review.
Add Reference Numbers to each component and interface.
Try out a prototype or sample.
Invite a subject matter expert to answer questions. Document the function(s) of each component and interface.
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Step 2: Brainstorm Potential Failure Modes
Consider potential failure modes for each component and interface.
A potential failure mode represents any manner in which the product component could fail to perform its intended function or functions. Remember that many components will have more than one failure mode. Document each one. Do not leave out a potential failure mode because it rarely happens. Don’t take shortcuts here; this is the time to be thorough.
Prepare for the brainstorming activity.
Before you begin the brainstorming session, review documentation for clues about potential failure modes. Use customer complaints, warranty reports, and reports that identify things that have gone wrong, such as hold tag reports, scrap, damage, and rework, as inputs for the brainstorming activity. Additionally, consider what may happen to the product under difficult usage conditions and how the product might fail when it interacts with other products.
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Step 3: List Potential Effects of Failure
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 should it occur. Some failures will have an effect on customers; others on the environment, the process the product will be made on, and even the product itself.
The effect should be stated in terms meaningful to product performance. If the effects are defined in general terms, it will be difficult to identify (and reduce) true potential risks.
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Step 4: Assign Severity Rankings
The ranking scales are mission critical for the success of a DFMEA because they establish the basis for determining risk of one failure mode and effect relative to another.
The same ranking scales for DFMEAs should be used consistently throughout an organization. This will make it possible to compare the RPNs from different FMEAs to one another.
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 scales provide a relative, not an absolute, scale. SeeFMEA Checklists and Forms for an example DFMEA 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.
By adding organization-specific examples to the ranking definitions, DFMEA teams will have an easier time using the scales. The use of examples saves teams time and improves the consistency of rankings from team to team. 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 customer satisfaction failures and another for environmental, health, and safety issues. However, remember that each row should reflect the same relative impact, or severity, on the organization or customer. SeeFMEA Checklists and Forms for an example of Custom DFMEA Ranking Scales. (Examples of custom scales for severity, occurrence, and detection rankings are included in this Appendix.)
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Step 5: Assign Occurrence Rankings
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.
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; it happens all of the time. Conversely, a “1” means the probability of occurrence is remote. SeeFMEA Checklists and Forms for an example DFMEA Occurrence Ranking Scale.
Your organization may need to customize the occurrence ranking scale to apply to different levels or complexities of design. It is difficult to use the same scale for a modular design, a complex design, and a custom design.
Some organizations develop three different occurrence ranking options (time-based, event-based, and piece-based) and select the option that applies to the design or product. SeeFMEA Checklists and Forms for an examples of Custom DFMEA Ranking Scales. (Examples of custom scales for severity, occurrence, and detection rankings are included in this Appendix.)
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Step 6: Assign Detection Rankings
To assign detection rankings, consider the design or product-related controls already in place for each failure mode and then assign a detection ranking to each control.
Think of the detection ranking as an evaluation of the ability of the design controls to prevent or detect the mechanism of failure.
Prevention controls are always preferred over detection controls.
Prevention controls prevent the cause or mechanism of failure or the failure mode itself from occurring; they generally impact the frequency of occurrence. Prevention controls come in different forms and levels of effectiveness. Detection controls detect the cause, the mechanism of failure, or the failure mode itself after the failure has occurred BUT before the product is released from the design stage.
A detection ranking of “1” means the chance of detecting a failure is almost certain. Conversely, a “10” means the detection of a failure or mechanism of failure is absolutely uncertain.
See Appendix 6 for the “Standard” (AIAG) DFMEA Detection Ranking Scale.
To provide DFMEA teams with meaningful examples of Design Controls, consider adding examples tied to the Detection Ranking scale for design related topics such as:
Design Rules DFA/DFM (design for assembly and design for manufacturability) Issues Simulation and Verification Testing
SeeFMEA Checklists and Forms for examples of Custom DFMEA Ranking Scales. (Examples of custom scales for severity, occurrence, and detection rankings are included in this Appendix.)
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Step 7: Calculate the RPN
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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Step 8: Develop the Action Plan
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 DFMEA is often the most difficult to attain. It usually requires a design change. Reduction in the Occurrence Ranking is accomplished by removing or controlling the potential causes or mechanisms of failure. And a reduction in the Detection Ranking is accomplished by adding or improving prevention or detection controls.
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 DFMEA team in reducing the relative risk of those failure modes requiring action. The following recaps some of the most powerful action tools for DFMEAs.
Design of Experiments (DOE)
A family of powerful statistical improvement techniques that can identify the most critical variables in a design and the optimal settings for those variables.
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.
Design for Assembly and Design for Manufacturability (DFA/DFM)
Techniques that help simplify assembly and manufacturing by modularizing product sub-assemblies, reducing components, and standardizing components.
Simulations
Simulation approaches include pre-production prototypes, computer models, accelerated life tests, and value-engineering analyses.
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Step 9: Take Action
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 DFMEA 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.
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Step 10: Recalculate the Resulting RPN
This step in a DFMEA 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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