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.
de·tec·tion [di-tek-shuhn, noun]
In an FMEA, Detection is a ranking number associated with the best control from the list of detection-type controls, based on the criteria from the detection scale. The detection ranking considers the likelihood of detection of the failure mode/cause, according to defined criteria. Detection is a relative ranking within the scope of the specific FMEA and is determined without regard to the severity or likelihood of occurrence.
FMEA Tip of the Month
The most common misunderstanding or misapplication of the detection scale is to confuse or commingle the three types of detection risk:
Likelihood of detection by the identified controls – specifically, what is the likelihood that the current detection-type control will be able to discover the failure mode or its cause (remote, low, moderate, high, etc.).
Timing of the opportunity for detection – specifically, what is the timing of the current detection-type control (prior to design freeze, post design freeze, in service, etc.).
Type of test used to detect the cause of the problem – what is the quality of test method used to detect the failure mode or its cause (degradation test, test to failure, pass/fail test, etc.).
The detection scale must clearly identify which of the three types of detection risk is being assessed by the individual criteria of the scale.
You are doing a Design FMEA and it is time to identify the detection ranking for a given failure mode/cause. One of your team members asks you to clarify the definition of detection. Identify which of the following are true or false responses to this question? [Show/Hide Answers]
1. The detection ranking considers the likelihood that the current detection-type design controls will detect the failure mode/cause.
2. The detection ranking is associated with prevention-type design controls.
3. Detection is the likelihood that the effect of the failure mode will manifest sometime during the product life cycle.
(False. Detection is related to the failure mode/cause, not the effect.)
4. The detection ranking is associated with detection-type design controls.
October Beginner’s Solution
In an FMEA, which of the following is true about a “function”? (Select all that apply)
1. A “function” is what the item is intended to do, and can be listed with or without respect to any standard of performance. (False. A function description needs to include the standard of performance. It is the function statement including the standard of performance that allows the FMEA team to determine the failure modes.)
2. A “function” is what the item is intended to do, usually to a given standard of performance. (True)
3. There is always one function for each item in an FMEA. (False. There can be many functions for an item.)
4. The function description in an FMEA must include the consequence or impact on the end user. (False. An effect must include the consequence or impact on the end user, not a function.)
Problem: You are doing a Design FMEA on the hand brake subsystem of an all-terrain bicycle. Review the following excerpt from this FMEA and determine the error in the detection ranking. [Show/Hide Answer]
Answer: The team has assigned a detection ranking of "2" for the cause "Cable Binds due to inadequate lubrication." Remember, the definition of detection is "a ranking number associated with the best control from the list of detection-type controls, based on the criteria from the detection scale." Since there is no detection-type control for this cause, the detection risk would be very high.
Problem: Company X is developing a next generation sub-sea drilling system. Although it is important to detect failure modes and their causes before the new system goes into operation, it is even more important that failures be detected once the system is operating, so that mitigating action can be taken to avoid a potential catastrophe. How can a detection scale be configured to assess detection risk during operation? [Show/Hide Answer]
Answer: In-service detection techniques can be designed in to system operations. An example is a warning system in a nuclear power plant in which sensors detect an emerging problem, alerting personnel who can then prevent the problem or avert it before an accident or serious consequence occurs.
It is possible to define criteria for the detection scale that assesses the likelihood of the monitoring-type control to detect the problem during system operation. The nature of the application should determine the specific criteria of this unique detection scale.
An example of an in-service detection scale is below. This scale can be configured to the unique circumstances of the operations being assessed.
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.
I do not introduce special characteristics for components in Process FMEA because we have to consider that the components that we are buying have to be according with drawing specifications and we are not influencing it in our process.
Those special characteristics are treated in Design FMEA and Control Plan.
Could you tell me your opinion about this?
Carl: This is an excellent question.
You are correct that you are not influencing the specifications of a purchased part during your assembly process. In many cases, Process FMEA teams assume that supplier components are in accordance with drawing specifications. However, there are exceptions to this assumption, which I will outline with an example.
Let's say company A manufactures fuel delivery subassemblies, and they purchase fuel line quick-connects from supplier B. Let's further say that company A is conducting a Process FMEA on their assembly process for the fuel subassemblies. Here's the general rule about incoming parts:
Process FMEAs typically assume the design requirements are correct and incoming parts and materials to an operation meet design intent. In addition, the PFMEA team may wish to consider an exception when historical data indicates incoming part quality issues: incoming parts or materials may have variation and do not necessarily meet engineering requirements.
With regard to the quick-connect incoming parts from supplier B, company A now has a choice. They can assume the parts they receive from supplier B meet specifications. However, they can make an exception to that assumption. If the quick-connect device is critical to their fuel subassembly and they have reason to believe that the device may not meet requirements, they can consider this exception: incoming parts or materials may have variation and do not necessarily meet engineering requirements. Historical data for quick-connects or fuel sub-assembly risk analysis may indicate this exception. If the fuel subassembly Process FMEA team wishes to make this exception and not assume the incoming quick-connect device meets specifications, they may need to add a step or two to their manufacturing process, or modify existing steps.
Let's trace this progression from the viewpoint of special characteristics.
One possible special product characteristic for supplier B's quick-connect is "inside diameter of the connecting device." Supplier B would make this a special product characteristic, and identify associated special process characteristic(s) that are needed to control the inside diameter during the quick-connect manufacturing process. As you point out, those special characteristics are treated in the supplier's DFMEA and Control Plan. The final quick-connect product should meet dimensional and performance specifications before shipping to company A for use in the fuel system assembly. If company A is concerned about this special product characteristic (inside diameter of connecting device), and if they choose to make an exception to the assumptions about this incoming part, they can consider changes to their assembly process to accommodate this exception. Changes might include measuring the inside diameter, or modifying the process controls to detect and accommodate inside diameter variation. They can also recommend action that improves the quality of supplier B's quick-connect device. In addition, they can request that the design team consider modifications to the fuel subassembly design to be more robust to anticipated variation in quick-connect inside diameter variation.
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.