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.
func·tion [fuhngk-shuhn, noun]
In an FMEA, a "function" is what the item or process is intended to do, usually to a given standard of performance or requirement. For Design FMEAs, this is the primary purpose or design intent of the item. For Process FMEAs, this is the primary purpose of the manufacturing or assembly operation; wording should consider “Do this [operation] to this [the part] with this [the tooling]” along with any needed requirement. There can be many functions for each item or operation.
FMEA Tip of the Month
For Design FMEAs, the function statement needs to include the standard of performance. Many FMEA practitioners omit this important element of the function description. For example, an FMEA team might identify one of the primary functions of a power steering pump as “delivers hydraulic power for steering.” However, without including a standard of performance, it becomes more difficult to properly identify the failure modes, and as a result, important failure modes can be missed. A better statement of function would be “Delivers hydraulic power for steering by transforming oil pressure at inlet ([xx] psi) into higher oil pressure at outlet [yy] psi during engine idle speed.” Including the standard of performance in the function statement enables the FMEA team to more easily identify the correct failure modes. For Process FMEAs, the function is usually stated in terms of Do this [operation], to this [part], with this [tooling], with the addition of applicable standard of performance. There can be many functions for each item being analyzed, although the FMEA team should limit function descriptions to the primary functions.
In an FMEA, which of the following is true about a “function”? [Show/Hide Answers]
1. A “function” is what the item is intended to do, and can be listed with or without respect to a 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.
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.)
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.)
Scenario: You are preparing to lead a series of FMEAs on the all-terrain bicycle project. The “All-Terrain Bicycle Functional/Technical Specifications” includes the following verbiage:
Problem 1: The front suspension has a requirement: “Should withstand g-force acceleration to 3g, above which it is considered abusive.” Should this requirement be part of the Suspension Subsystem Design FMEA, and if so, how would it be used? [Show/Hide Answer]
Answer: During the ground rules and assumptions portion of FMEA preparation, the team will need to consider whether abuse of the all-terrain bicycle will be within the scope of the analysis. The specification clearly says that g-forces above 3g are considered abusive. The team may wish to ignore any g-forces above 3g. In this case, the 3g limit would carry over to the function column of the Suspension Subsystem Design FMEA as one of the primary functions, such as “The suspension subsystem should be able to safely and reliably withstand g-forces up to 3g while riding on all surfaces, environments and maneuvers, according to the customer usage document ABC.” However, if the FMEA team decides to address g-forces above the 3g limit (abusive) for safety or any other reason, they will need to document this decision (with approvals as needed) and generate the appropriate function verbiage.
Problem 2: The gears have a requirement: “Ease of pedaling—should be able to move bicycle with 5 Nm torque on first gear.” Should this requirement be part of the Pedal-sprocket-derailleur Subsystem Design FMEA, and if so, how would it be used? [Show/Hide Answer]
Answer: The ease of pedaling requirement should be used in the function column of the Pedal-sprocket-derailleur Subsystem Design FMEA as one of the primary functions. For example, one of the functions might be “The pedal-sprocket-derailleur subsystem should allow the rider to easily pedal the bicycle forward in first gear applying 5 Nm of torque.”
Problem 3: What type of requirement seems to be missing from the All-Terrain Bicycle Functional/Technical Specifications? [Show/Hide Answer]
Answer: The requirements in the document cover subsystem usage, subsystem-to-user interface, and subsystem-to-environment interface. Missing are requirements related to subsystem-to-subsystem interfaces. Example might be “Handle Bar Subsystem interfaces with the Hand Brake Subsystem to provide a user adjustable brake lever mechanism that remains fixed in orientation under all operating conditions.” Subsystem-to-subsystem interface functions typically are included in the System FMEA.
Something I’ve always wanted to know about FMEAs
“A prudent question is one-half of wisdom.” Francis Bacon
Beginning next month, and continuing each month, one FMEA question/answer will be highlighted in the “Something I’ve always wanted to know about FMEAs” section. HotWire readers are encouraged to send FMEA questions to Carl.Carlson@EffectiveFMEAs.com. Questions can be on any FMEA topic, with any level of difficulty, from beginner to advanced. Question submitters can either provide name and company, or (if requested) can be kept anonymous. Don’t hesitate to ask any question on the subject of FMEA theory or application.
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 35 years of experience in reliability testing, engineering and management positions, and is currently supporting clients from a wide variety of industries, including clients of HBM Prenscia. Previously, 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.
Selected material for FMEA Corner articles 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. Carl Carlson can be reached at firstname.lastname@example.org.