This Month's Theme is
Action Strategies to Reduce Severity Risk
Next month's theme will be action strategies to reduce occurrence
Every month in FMEA Corner, join Carl Carlson, a noted expert in the field of FMEAs and facilitation, as he addresses a different FMEA theme (based on his book Effective FMEAs) and also answers your questions.
Questions and answers are a great way to learn about FMEAs, for both experienced and less experienced FMEA practitioners. Please feel free to ask any question about any aspect of FMEAs. Send your questions to Carl.Carlson@EffectiveFMEAs.com, and your contact information will be kept anonymous. All questions will be answered, even if they are not featured in the FMEA Corner.
stra·te·gy [stra-tuh-jee, noun]
The Oxford English Dictionary defines "strategy" as "a plan of action or policy designed to achieve a major or overall aim."
What are "action strategies to reduce severity risk" and how are they used in an FMEA?
However beautiful the strategy, you should occasionally look at the results. - Winston Churchill
Severity risk is represented by the severity ranking. Using a severity scale of 1 to 10, high severity would typically mean 9s and 10s, at a minimum.
The FMEA team must review and fully understand all the high-severity issues so as to address them in its recommended actions to ensure those issues do not occur within the life of the product or manufacturing process.
There are at least four types of strategies that can be employed by an FMEA team to address high-severity risk:
- Design for fail-safe
- Design for fault tolerance
- Design for redundancy
- Design for early warning
When the FMEA team encounters a failure mode with an effect that is high severity, where possible, the FMEA team should recommend one or more action strategies that reduce the severity. Using one of these four strategies can accomplish that objective.
As can be seen from the titles of the strategies, all four are design-related. It is a fact that reducing severity risk requires a design modification. Directly below is a definition and example of "Design for fail-safe." (The four strategies are defined with multiple examples in the Effective FMEAs book.)
Design for fail-safe
A fail-safe design is one that, in the event of failure, responds in a way that will cause minimal harm to other devices or danger to personnel. Fail-safe does not mean that failure is improbable, but rather that a system’s design mitigates any unsafe consequences of failure. In FMEA language, fail-safe reduces the severity of the effect to a level that is safe.
Example: Laminated safety glass for windshields prevents injury from glass shards. The windshield can fail, but in a safe manner.
Multiple strategies to reduce risk
In most cases, reduction of a high-risk issue from an FMEA requires the use of multiple tasks. These are recorded in the Recommended Actions column.
Below is an example in Xfmea of the use of Recommended Actions to reduce severity risk, as well as risk from occurrence and detection. It is an excerpt from a fictional Design FMEA on a projector lamp.
The action "Install additional plastic shield on projector to ensure no injury to user if glass shatters" can reduce severity from 10 (potential for injury) to 8 (complete loss of performance).
Note the multiple actions to address this high-severity issue.
FMEA Tip of the Month
Always review high severity issues with management. Both management and the FMEA team must agree that everything possible has been done to prevent safety problems within the design life of the product or during the manufacturing process.
A fault-tolerant design is a design that enables a system to continue operation, possibly at a reduced level, rather than failing completely, when some part of the system fails. In FMEA language, fault-tolerance reduces the severity of the effect to a level that is consistent with performance degradation.
What design feature can be added to vehicle tires that incorporates fault tolerance? [Show/Hide Answer]
A passenger car can have "run-flat" tires, each of which contain a solid rubber core, allowing their use even if a tire is punctured. The punctured "run-flat" tire is effective for a limited time at a reduced speed.
I’ve always wanted to know about FMEAs
The important thing is not to stop questioning. - Albert Einstein
A reader submitted the following question to Carl Carlson. To submit your own question about any aspect of FMEA theory or application, e-mail Carl at Carl.Carlson@EffectiveFMEAs.com.
Perhaps a future Reliability Hotwire could examine FMEA and FMECA. From my research, it appears FMECA brings in another element of quantitative analysis. Is this correct? What are the true differences?
Carl: Simply stated, FMECA is FMEA with the added step of Criticality Analysis. Criticality Analysis can take two different forms: quantitative criticality and qualitative criticality.
Quantitative Criticality Analysis is a series of calculations to rank items and failure modes according to a defined formula.
- Calculate the expected failures for each item. [This is the number of failures estimated to occur based on the reliability/unreliability of the item at a given time.]
- Identify the mode ratio of unreliability for each potential failure mode. [This is the portion of the item’s unreliability (in terms of expected failures) attributable to each potential failure mode.]
- Rate the probability of loss that will result from each failure mode that will occur. [This is the probability that a failure of the item under analysis will cause a system failure.]
- Calculate the mode criticality for each potential failure mode. [Mode Criticality = Expected Failures (for the item) × Mode Ratio of Unreliability (for the failure mode) × Probability of Loss (for the failure mode).]
- Calculate the item criticality for each item. [Item Criticality = SUM of Mode Criticalities]
Qualitative Criticality Analysis does not involve the same rigorous calculations as Quantitative Criticality Analysis. To use Qualitative Criticality Analysis to evaluate risk and prioritize corrective actions:
- Rate the severity of the potential effects of failure. The severity ranking is determined using the unique severity scale for FMECA.
- Rate the likelihood of occurrence for each potential failure mode. The occurrence ranking is determined using the unique occurrence scale for FMECA.
- Compare failure modes using a criticality matrix. The criticality matrix identifies severity on the horizontal axis and occurrence on the vertical axis.
The procedure for FMECA, including examples, is covered more thoroughly in chapter 12 of my book, Effective FMEAs.
Let me know if you have any other questions.
About the Author
Carl S. Carlson is a consultant and instructor in the areas of FMEA, reliability program planning and other reliability engineering disciplines. He has 30 years of experience in reliability testing, engineering and management positions, and is currently supporting clients of ReliaSoft Corporation with reliability and FMEA training and consulting. Previous to ReliaSoft, he worked at General Motors, most recently senior manager for the Advanced Reliability Group. His responsibilities included FMEAs for North American operations, developing and implementing advanced reliability methods and managing teams of reliability engineers. Previous to General Motors, he worked as a Research and Development Engineer for Litton Systems, Inertial Navigation Division. Mr. Carlson co-chaired the cross-industry team that developed the commercial FMEA standard (SAE J1739, 2002 version), participated in the development of SAE JA 1000/1 Reliability Program Standard Implementation Guide, served for five years as Vice Chair for the SAE's G-11 Reliability Division and was a four-year member of the Reliability and Maintainability Symposium (RAMS) Advisory Board. He holds a B.S. in Mechanical Engineering from the University of Michigan and completed the 2-course Reliability Engineering sequence from the University of Maryland's Masters in Reliability Engineering program. He is a Senior Member of ASQ and a Certified Reliability Engineer.
Material for the FMEA tips, problems and solutions is excerpted from the book Effective FMEAs, published by John Wiley & Sons, ©2012. Information about the book Effective FMEAs, along with useful FMEA aids, links and checklists can be found on www.effectivefmeas.com.