In Step 3: Function Analysis of the AIAG-VDA 7-Step FMEA approach, the first sub-step is defining functions and requirements.
👉 This step answers two key questions:
- What is the item supposed to do? (Function)
- How well should it do it? (Requirement)
Defining functions and requirements properly is critical, because they form the foundation for identifying failures in Step 4 (Failure Analysis).
What is a Function in FMEA? #
A function describes the intended purpose or task of a system, subsystem, component, or process step.
📌 Functions should be written in clear, action-oriented terms.
Examples of Functions
- DFMEA (Design FMEA):
- Brake system → Provide vehicle deceleration.
- Motor winding → Generate magnetic field.
- PFMEA (Process FMEA):
- Welding process → Join two metal sheets.
- Painting process → Apply protective coating.
👉 Weak Function: “Motor rotation”
👉 Strong Function: “Provide rotation at specified torque and speed.”
What is a Requirement in FMEA? #
A requirement is the measurable criterion that determines if the function is being performed correctly.
📌 Requirements must be:
- Specific – Clearly state what is expected.
- Measurable – Define numerical or testable criteria.
- Verifiable – Able to be checked through testing or inspection.
Examples of Requirements
- Motor Function: Provide rotation.
- Requirement: 2000 ± 50 RPM at 12V supply.
- Brake Function: Provide vehicle deceleration.
- Requirement: Stop from 100 km/h within 40 m.
- Welding Function: Join two metal sheets.
- Requirement: Weld strength ≥ 5 kN shear load.
👉 Weak Requirement: “Strong enough weld.”
👉 Strong Requirement: “Weld must withstand 5 kN shear load.”
Why Functions & Requirements Must Be Defined Together #
- Functions show intent (what should happen).
- Requirements show criteria (how success is measured).
- Without requirements, teams cannot identify when a function has failed.
📌 Example – Bolting Process (PFMEA):
- Function: Secure bolt in assembly.
- Requirement: Torque 100 ± 5 Nm.
- Failure = Under-torque (<95 Nm) or Over-torque (>105 Nm).
Industry Examples – Functions & Requirements #
DFMEA – Electric Motor Stator
- Function: Generate magnetic flux.
- Requirement: Maintain flux density at 1.2 T ± 0.1.
PFMEA – Painting Process
- Function: Apply corrosion protection coating.
- Requirement: 30 ± 5 microns coating thickness.
FMEA-MSR – ABS Braking System
- Function: Monitor wheel speed.
- Requirement: Detect slip ratio within 20 ms.
Best Practices for Writing Functions & Requirements #
- Always use action verbs in functions (e.g., provide, generate, deliver, apply).
- Ensure requirements are quantifiable (numbers, tolerances, time limits).
- Define both primary and secondary functions.
- Link requirements to customer and regulatory specifications.
- Review definitions with the cross-functional team for clarity.
Common Mistakes to Avoid #
- Writing vague functions (“Motor rotates” instead of measurable criteria).
- Using subjective requirements (“good strength” instead of “≥ 5 kN shear load”).
- Ignoring secondary functions (comfort, durability, appearance).
- Not linking functions/requirements to customer-specific requirements (CSRs).
Case Study – Seatbelt DFMEA #
- Function: Restrain passenger in case of crash.
- Requirement: Withstand 10 kN tensile force for 5 seconds.
- Failure Mode (Step 4): Seatbelt breaks under 6 kN force.
👉 Because the function & requirement were defined clearly, the team could identify the failure and take preventive actions (material upgrade).
Key Takeaways #
- Function = What the item/process should do.
- Requirement = How well it should do it (measurable criteria).
- Functions must always be paired with requirements for effective FMEA.
- Clear functions & requirements → enable accurate failure analysis.
Next Lesson #
👉 Continue with Lesson 3.4.2: How to Write Measurable Requirements in FMEA