A picture is worth a thousand words. An interface is worth a thousand pictures. Ben Shneiderman
In this article, I will outline how to evaluate an individual Design FMEA (DFMEA) against the FMEA Quality Objective for Interfaces.
What is an Interface in a DFMEA?
An interface is the point or surface where two parts or subsystems meet, and it can take various forms. There are four primary types of interfaces:
- Physical connection (e.g., brackets, bolts, clamps and various types of connectors).
- Material exchange (e.g., pneumatic fluids, hydraulic fluids or any other fluid or material exchange).
- Energy transfer (e.g., heat transfer, friction or motion transfer such as chain links or gears).
- Data exchange (e.g., computer inputs or outputs, wiring harnesses, electrical signals or any other types of information exchange).
Why are interfaces important in a DFMEA?
Based on empirical studies, interfaces contain fifty percent or more of the total failure modes in a system. Therefore, it is essential that any DFMEA carefully consider the interfaces between subsystems and components, in addition to the content of the subsystems and components themselves.
What is an example of an interface?
Using the bicycle example, let’s begin with the system hierarchy of a hand brake subsystem.
Certain components of the hand brake subsystem interface and interact with certain other components of the hand brake subsystem. Example is brake lever and brake cable. The interface is an adjustable fastener that connects the cable to the handle lever. Another example is the brake caliper and brake rotor. The caliper squeezes the rotor causing friction that slows or stops the bicycle wheel.
In some cases, the components also interface with items that are outside of the subsystem. Example is the brake lever, which interfaces with the handlebar, and also the hands of the rider.
What is an FMEA Block Diagram and how does it represent interfaces?
An FMEA Block diagram (or Boundary diagram) is a visual depiction of the entire system or design to clearly show the boundaries of the DFMEA (i.e., what is included and not included), the interfaces between the items and other information that can help to depict the scope of the analysis.
Making the scope visible ensures that the FMEA team agrees on the precise extent of the DFMEA, including interfaces. Elements of the diagram can map to corresponding elements of the DFMEA to help ensure that nothing is missed.
In the case of a System FMEA, the FMEA Block Diagram should visually show the interfaces between the various subsystems, as well as between the system and its users. For a Subsystem FMEA, the FMEA Block Diagram should visually show the interfaces between the various components.
An FMEA Block diagram is recommended as part of the preparation for every System or Design FMEA.
What is an example of an FMEA Block Diagram?
An example of a System Hierarchy and the corresponding FMEA Block diagram for a bicycle hand brake subsystem is shown next.
How is an FMEA Block Diagram used to address interfaces?
Important interfaces from the FMEA Block Diagram can be brought into the FMEA Function column as interface functions. This will provide traceability to ensure that no important interfaces are missed.
Here’s an example of Functions brought into the fictitious Hand Brake DFMEA:
What is the FMEA Quality Objective related to Interfaces?
FMEA Quality Objective 3: Interfaces is “For Design FMEAs, the FMEA scope includes integration and interface failure modes in both Block Diagram and analysis.”
How can you assess how well a DFMEA meets the Quality Objective for Interfaces?
There are three ways to assess how well a DFMEA meets this quality objective.
1. Does the FMEA Block Diagram clearly show all relevant interfaces within the scope of the FMEA, including those within the item being analyzed, as well as adjacent items?
2. Do the most important interfaces from the FMEA Block Diagram clearly trace to DFMEA Functions?
3. Do the interface functions get analyzed within the DFMEA?
What is an example of assessing Interfaces in a DFMEA?
Below is an example of evaluating FMEA Quality Objective # 3: Interfaces.
Tip
Ask the FMEA team how they addressed interfaces. This will help when evaluating Quality Objective 3.
Summary
When assessing how well a DFMEA meets the quality objective for interfaces, always review the FMEA Block Diagram for completeness and inclusion of interfaces, and trace the important interfaces through the DMEA analysis to ensure interface risk is addressed.
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