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What is FMEA?
Failure modes and effects analysis (FMEA) is a systematic process to evaluate failure modes and causes associated with the design and manufacturing processes of a new product. It is somewhat similar to the potential problem analysis (PPA) phase of the Kepner-Tregoe program. Here is a list of activities for a FMEA:
1. Determine potential failure modes of each component or subassembly and causes associated with the designing and manufacturing of a product.
2. Identify actions which could be eliminate or reduce the chance of a potential failure occurring.
3. Document the process and give each mode a numeric rating for frequency of occurrence, criticality, and probability of detection.
4. Multiply these three numbers together to obtain the risk priority number (RPN), which is used to guide the design effort to the most critical problems first.
Two aspects of FMEA are particularly important: a team approach and timeliness. The team approach is vital because the broader the expertise that is brought to bear on making and assigning values to the failure mode list, the more effective the FMEA will be.
Timeliness is important because FMEA is primarily a preventive tool, which can help steer design decisions between alternatives before failure modes are designed-in, rather than redesigning after the failure occurs. FMEA is equally applicable to hardware or software, to components or systems.
Comparison to FTA
Another similar process is fault tree analysis (FTA). While FMEA is a bottom-up approach, FTA is top-down. FTA starts with the assumption of a system failure mode, and then works down through the system block diagram to look for possible causes of that mode.
Thus, FTA requires fairly complete, detailed information about the system, and is most effective after the system is well-defined. (FTA could be performed, in a limited way, on alternative system concepts; this could be used to help decide the best of several alternatives.) A separate FTA must be performed for each system failure mode.
FTA and FMEA are complimentary. Whenever possible, both should be used. For practical reasons, FTA should be limited to the really serious system-level failure modes, such as those involving safety or permanent system damage. FMEA can be used at the component, subassembly, and module level, to help optimize those modules. There are excellent discussions and examples of FTA in References 2 and 4, and it will not be discussed further in this column.
Benefits of FMEA
The RPN calculated by FMEA allows prioritization of the failure mode list, guiding design effort to the most critical areas first. It also provides a documentary record of the failure prevention efforts of the design team, which is helpful to management in gauging the quality and extent of the effort, to production in solving problems which occur despite these efforts, and to future projects which can benefit from all the work and thinking that went into the failure mode and cause lists.
Eliminating potential failure modes has both short term and long term benefits. The short term benefit is most often recognized because it represents savings of the costs of repair, retest, and downtime, which are objectively accountable. The long term benefit is much more difficult to measure, since it relates to the customer satisfaction or dissatisfaction with the product, and perception of its quality.
FMEA supports the design process by:
- Aiding in the objective evaluation of alternatives during design
- Increasing the probability that potential failure modes and their effects on system operation have been considered during design
- Providing additional information to aid in the planning of thorough and efficient test programs
- Developing a list of potential failure modes ranked according to their probable effect on the customer, thus establishing a priority system for design and test
- Providing an open, documented format for recommending and tracking risk-reducing actions
- Identifying known and potential failure modes which might otherwise be overlooked
- Exposing and documenting the ways a system can fail, and the effects of such failures
- Detecting primary but often minor failures which may cause serious secondary failures or consequent damage
- Detecting areas where "fail safe" or "fail soft" features are needed
- Providing a fresh viewpoint in understanding a system's functions
The uses of a FMEA report include:
- A formal record of the safety and reliability analysis and planning, to satisfy customers or regulatory agencies
- Evidence in litigation involving safety or reliability
- Design of diagnostic routines or built-in tests
- A basis for creating trouble-shooting procedures
- A means to consider and prevent manufacturing defects
- Problem follow-up and corrective action tracking
- A future reference to aid in analyzing field failures, evaluating design changes, or developing improved designs
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