After completing Structure Analysis (Step 2), the next step in the AIAG-VDA 7-Step FMEA methodology is Function Analysis.
👉 Function Analysis answers the question:
“What should each system, subsystem, component, or process step do, and how do they connect to each other?”
This step provides the link between structure and failure analysis by clearly defining functions and requirements.
Objectives of Function Analysis #
- Define functions for each element in the structure.
- Identify requirements/targets that make the functions measurable.
- Establish function chains (relationships between functions).
- Create a traceable map for moving into Failure Analysis.
📌 Without Function Analysis, failures cannot be defined properly.
What is a Function in FMEA? #
- A function describes what an item or process is intended to do.
- Functions must be written in action-oriented terms (e.g., “provide torque,” not just “torque”).
Examples of Functions
- DFMEA (Design):
- Motor winding → Generate electromagnetic field.
- Brake system → Provide controlled deceleration.
- PFMEA (Process):
- Welding step → Join two steel sheets.
- Painting process → Apply protective coating.
What is a Requirement in FMEA? #
- A requirement defines how well the function must be performed.
- Requirements must be specific, measurable, and verifiable.
Examples of Requirements
- Motor winding → Provide torque at 2000 ± 50 RPM under 12V supply.
- Brake system → Stop vehicle within 40 m from 100 km/h speed.
- Welding → Weld strength ≥ 5 kN shear load.
- Painting → Coating thickness 30 ± 5 microns.
📌 Functions without requirements are vague and make risk analysis ineffective.
Function Chains in FMEA #
Functions are rarely independent. They are linked in a function chain where the output of one becomes the input of another.
Example – Electric Motor Function Chain
- Battery → Inverter → Motor Stator → Rotor → Transmission → Wheels
- Each function depends on the previous element.
- A weak link in the chain leads to functional failure downstream.
📌 Function Chains help reveal cascading risks.
Visual Tools for Function Analysis #
- Function Net (AIAG-VDA)
- Shows relationships between functions.
- Example: Motor → Stator → Winding → Torque Output.
- Input–Output Diagrams
- Define what goes in and what should come out.
- Example: Process step “Welding” → Input = Two sheets, Output = Joined part.
Example – Function Analysis for PFMEA (Bolting Process) #
- Process Step: Apply torque with power tool.
- Function: Secure bolt to specified torque.
- Requirement: 100 ± 5 Nm torque.
- Function Chain:
- Operator picks bolt → Places in hole → Applies torque → Joint secured.
👉 Without a defined requirement (100 ± 5 Nm), the team cannot later define under-torque or over-torque as failure modes.
Common Mistakes in Function Analysis #
- Writing vague functions (e.g., “motor rotates” instead of “rotate at 2000 ± 50 RPM”).
- Not defining requirements/targets → makes failures impossible to evaluate.
- Ignoring interfaces between subsystems or process steps.
- Treating function analysis as a paperwork step instead of a thinking step.
Benefits of Function Analysis #
- Ensures functions are clearly defined and measurable.
- Provides traceability from structure → function → failure.
- Helps identify critical functions that impact safety and compliance.
- Strengthens cross-functional discussions → everyone agrees on what success looks like.
Case Study – DFMEA Function Analysis for Electric Motor #
- System: Electric Motor
- Subsystem: Stator
- Component: Winding
- Function: Carry current and generate magnetic field.
- Requirement: Current capacity up to 20A, Insulation withstand 150°C.
- Function Chain: Battery → ECU → Motor → Transmission.
👉 This sets the stage for Step 4: Failure Analysis, where the team can identify risks like open circuit, overheating, or short circuit.
Key Takeaways #
- Function Analysis = What each element should do, and how well.
- Must include functions + measurable requirements.
- Use function chains to connect subsystems and processes.
- A strong Function Analysis makes Failure Analysis systematic and complete.
Next Lesson #
👉 Continue with Lesson 3.4.1: Defining Functions & Requirements in FMEA
