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@ReliaSoft.com, and your contact information will be kept anonymous. All questions will be answered, even if they are not featured in the FMEA Corner.
occurrence [uh-kur-uhns, noun]
In an FMEA, Occurrence is a ranking number associated with the likelihood that the failure mode and its associated cause will be present in the item being analyzed. For System and Design FMEAs, the occurrence ranking considers the likelihood of occurrence during the design life of the product. For Process FMEAs, the occurrence ranking considers the likelihood of occurrence during production. It is based on the criteria from the corresponding occurrence scale. The occurrence ranking has a relative meaning rather than an absolute value and is determined without regard to the severity or likelihood of detection.
FMEA Tip of the Month
Tip 1: The assessment of the occurrence ranking should be as objective as possible, using past field history of similar items, previous test results, experience with similar systems and other sources of information.
Tip 2: Note that prevention-type controls are considered as inputs to the occurrence ranking. Some practitioners place the Prevention Controls column before the Cause column in the FMEA worksheet to facilitate this sequence.
You are performing a Design FMEA. Using the occurrence scale shown below, select an appropriate occurrence ranking for a failure mode/cause that has a 1 in 500 likelihood within the design life of the product.
Answer: An occurrence likelihood of 1 in 500 translates into a Quantitative Value of 0.002. Therefore, using this scale, the occurrence ranking would be 5.
Scenario: You have been asked to perform a Design FMEA on the hand brake subsystem of a new version of the All-Terrain bicycle. One failure mode is "Insufficient friction delivered between brake pad assembly and wheel rim to safely stop bicycle in required distance." The effect is "Bicycle wheel does not slow down when the brake lever is pulled potentially resulting in accident" and one potential cause is "Cable Binds due to inadequate lubrication."
Problem: You are trying to determine the occurrence ranking. The field representative on your FMEA team says they have actual data that shows there has been one accident that resulted from this failure mode, out of 10,000 users. Since this amounts to a 0.0002 likelihood of an accident, the team has determined that an occurrence ranking of 2 would be appropriate, based on the scale they were using. Was this the correct method to determine the occurrence ranking? Why? [Show/Hide Answer]
Answer: Unfortunately, this is not the correct method for determining the occurrence ranking. Occurrence is a ranking number associated with the likelihood that the failure mode and its associated cause will be present in the item being analyzed. The team has identified the occurrence of the effect, not the occurrence of the failure mode and its associated cause. The team needs to assess the likelihood of the cause, "Cable Binds due to inadequate lubrication" during the life of the All-Terrain bicycle.
Problem: In the scenario described in the Intermediate Problem above, the team needs to correctly assess the likelihood of the cause, "Cable Binds due to inadequate lubrication" during the life of the All-Terrain bicycle. What are three sources of information that the team could use to help determine the occurrence ranking for this cause? [Show/Hide Answer]
Answer: The following is a partial list of potential sources of information that the FMEA team could use:
Warranty data from the previous version of the All-Terrain bicycle.
Tech-support phone records for the previous version of the All-Terrain bicycle.
Surveys of actual All-Terrain bicycle customers regarding failures they have experienced during the life of their product.
Interview with brake cable design engineer, asking subjective assessment of likelihood of this particular cause, given the current design of the brake cable and lubrication.
Bicycle durability testing for the previous version of the All-Terrain bicycle.
"Change point" analysis of the specific changes (material, geometry, customer usage, environment, etc.) that are planned for the hand brake subsystem, specifically focusing on brake cable and lubrication.
Review of the prevention-type controls for the cause that is being analyzed. Recall that prevention-type controls are considered as inputs to the occurrence ranking.
Correctly assessing the occurrence ranking is one of the more challenging aspects of FMEA. Teams can use a variety of sources of information to make this assessment.
I’ve always wanted to know about FMEAs
The important thing is not
to stop questioning. - Albert Einstein
A HotWire 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@ReliaSoft.com.
Thank you for giving back to the Reliability community and helping to progress Reliability training. I purchased a personal copy of "Effective FMEAs: Achieving Safe, Reliable, and Economic Products and Processes Using Failure Mode and Effects Analysis" and it resides on my desk as a handy reference.
I read the Reliability Hotwire article for recommended actions. After taking the quiz, I had a question about the advanced problem. Is the action "Review results of function test…" redundant with the detection control? In other words, is the recommended action double dipping on the detection control?
Carl: You made a very good observation. By definition, controls are the methods that are already in place or currently planned. Therefore, the bench test identified in the controls will happen with or without the recommended action. The action is merely to review the results, and as you pointed out, it is redundant. It does not add value to the power steering pump design.
Some companies have used the action column of the Design FMEA to verify the results of testing. That is redundant, in my opinion, and does not support the important role of Design FMEA, which is to improve the product design and, secondarily, improve the effectiveness of testing.
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.