FMEA Q and A
In this question, a reader asks how to define system-level functions, and how they are different from subsystem and component functions.
“A prudent question is one-half of wisdom.”
How can you define a system level function? The DFMEA team tends to drill down functions to subsystem and component functions when we conduct system level DFMEA. It would be endless and low efficiency, and should be handled by subsystem/component DFMEA. So could you please share some of your experience regarding how to define system level functions, subsystem functions and component functions?
Answer to Reader’s Question
Staying within the boundaries of the analysis is important in order to avoid overly long FMEAs. This takes good preparation (FMEA block diagrams, etc.) and good facilitation. This concept can be demonstrated using excerpts from fictitious FMEAs on an All-Terrain Bicycle: System FMEA on entire bicycle, a subsystem Design FMEA on the handbrake subsystem and a component design FMEA on the brake cable.
The facilitator must keep the team focused on both the scope and the level of the analysis. The team can drill down by doing lower levels of analyses. Notice the functions have increasing detail as you move to lower levels of analysis.
The following definition of System FMEA will help to clarify the appropriate functions at the system level.
System FMEA is the highest-level analysis of an entire system, made up of various subsystems. The focus is on system-related deficiencies, including system safety, system integration, interfaces or interactions between subsystems or with other systems, interactions with the surrounding environment, human interaction, service and other issues that could cause the overall system not to work as intended. In a System FMEA, the focus is on functions and relationships that are unique to the system as a whole (i.e., do not exist at lower levels). The System level FMEA includes failure modes associated with interfaces and interactions in addition to considering single-point failures (where a single component failure can result in complete failure of the entire system). Some practitioners separate out human interaction and service into their own respective FMEAs.
An example from a fictitious bicycle System FMEA, hand brake Subsystem FMEA and brake cable Design FMEA will help to illustrate.
One function from the fictitious bicycle System FMEA:
The bicycle must provide safe and reliable transportation, including safe stopping distances and safe operation under all customer usage conditions as defined in the All-Terrain technical specification.
One function from the fictitious bicycle hand brake Subsystem FMEA:
Provides the correct level of friction between brake pad assembly and wheel rim to safely stop bicycle in the required distance, under all operating conditions.
One function from the fictitious bicycle brake cable Design FMEA:
Provides adjustable and calibrated movement between the brake lever and brake caliper, under specified conditions of use and operating environment.
Notice how the high-level system function is decomposed to subsystem function, and further decomposed to the component function.
System functions should be described at a high level, and be unique to the system as a whole (i.e., do not exist at lower levels), with the addition of system interaction and interface functions (including interfaces between subsystems).
All types of FMEA functions are thoroughly described, including examples and application information, in chapter 6 of my book Effective FMEAs.
Do you make this mistake when defining your failure modes? The FMEA Definitions and Concepts Series continues with, “Understanding FMEA Failure Modes: Essential Elements.” The article will provide theory and practical examples of defining failure modes, and highlight a mistake that some practitioners make.