Once the product structure is clearly defined (Step 2), the next step in the AIAG-VDA DFMEA process is to understand what each part is supposed to do.
That’s the purpose of Function Analysis:
Define the intended function, performance requirement, and interaction of each component and interface.
This builds the foundation for the next step — Failure Analysis — because you can’t identify what might go wrong if you don’t know what’s supposed to go right.
🎯 Objective of Step 3 #
“Define the functions and associated performance requirements of each system, subsystem, component, and interface to enable effective failure analysis.”
🔍 What Is a Function in DFMEA? #
In DFMEA, a function is a task or behavior an element must perform to satisfy a requirement.
Each function should be linked with:
- A requirement or target (measurable)
- A unit of measure (if applicable)
- The operating condition or range
- The special characteristics, if applicable
🚗 Case Study Context: Electric Water Pump (EWP) #
Let’s apply Function Analysis to the 12V Electric Water Pump (EWP) structure we defined in the previous lesson.
📋 Sample Function-Requirement Table #
| Item | Function | Requirement/Target | Unit | Special Characteristic |
|---|---|---|---|---|
| EWP (System) | Deliver coolant flow | 20–120 L/min @ ΔP ≥ 20 kPa | L/min | ✔️ Safety |
| Impeller | Generate coolant flow | Rotate @ 4,000 RPM | RPM | – |
| Shaft | Transmit torque to impeller | No slip, max runout < 0.05 mm | mm | – |
| Bearings | Support rotation | Lifetime ≥ 10,000 hrs | hrs | – |
| Mechanical Seal | Prevent coolant leak | 0 ml/min leakage at 1 bar | ml/min | ✔️ Regulatory |
| BLDC Motor | Rotate rotor via PWM | RPM proportional to duty cycle | %/RPM | – |
| PCB (Controller) | Control motor based on PWM | Respond < 100ms delay | ms | – |
| Hall Sensor | Detect rotor position | Detect every 60° rotation | degrees | – |
| Electrical Connector | Interface with vehicle harness | Contact resistance < 10 mΩ | mΩ | – |
✅ Tip: Each function is ideally written as “do something to something” with a measurable outcome.
🔗 Interface Functions #
Don’t ignore interfaces! They are often the source of high-severity failures.
| Interface | Function | Requirement | Characteristic |
|---|---|---|---|
| Coolant ↔ Housing | Withstand coolant pressure | No leakage at 1.5 bar | ✔️ Safety |
| Connector ↔ Harness | Electrical contact | Vdrop < 0.2V @ 5A | – |
| ECU ↔ PCB | Transfer PWM signal | Valid duty cycle range 10–90% | – |
| Motor ↔ Shaft | Transmit torque | No slippage @ max torque 1.2 Nm | – |
🧩 Special Characteristics Mapping #
In DFMEA, some functions are tied to Special Characteristics (SC):
- 🔺 Safety (S) – Related to vehicle occupant safety
- 🔧 Regulatory (R) – Emissions, EMC, sealing, etc.
- 🎯 Critical to Quality (CTQ) – Customer experience, performance
| Function | Special Characteristic | Symbol |
|---|---|---|
| Deliver coolant flow | Safety | 🔺 |
| Prevent leak at seal | Regulatory | 🔧 |
| Reduce noise level | CTQ | 🎯 |
✅ These symbols should appear in your DFMEA form’s special characteristic column.
🖼️ P-Diagram (Parameter Diagram) #
The P-Diagram helps visualize:
- Inputs (control and noise)
- System under analysis
- Functions and outputs
- Error states
🎓 P-Diagram for EWP Example
| Category | EWP Example |
|---|---|
| Inputs – Control Factors | PWM duty from ECU, 12V power |
| Inputs – Noise Factors | Ambient temp, coolant contamination, voltage drop |
| System | Electric Water Pump assembly |
| Outputs | Coolant flow rate, pressure, RPM |
| Error States | No flow, leakage, EMC noise, high noise (NVH) |
📥 P-Diagram templates can be added to downloadable files for learners.
📑 Template: Function Analysis Worksheet (Excerpt) #
| Level | Item | Function | Requirement | Unit | SC |
|---|---|---|---|---|---|
| System | EWP | Deliver coolant flow | 20–120 L/min | L/min | 🔺 |
| Subsystem | PCB | Control motor | PWM duty to RPM map | % → RPM | – |
| Component | Impeller | Generate flow | Rotate @ 4000 RPM | RPM | – |
| Interface | Seal ↔ Shaft | Prevent leakage | 0 ml/min @ 1.5 bar | ml/min | 🔧 |
✅ This table will directly link into the next DFMEA columns: Function → Failure of Function
✅ Outputs of Step 3 #
By the end of Function Analysis, you will have:
- ✅ Defined functions and performance requirements for each structure element
- ✅ Identified and documented interface functions
- ✅ Marked special characteristics (S, R, CTQ)
- ✅ Created or updated your Function-Requirement Table
- ✅ Drafted a P-Diagram to visualize system behavior
🔗 Internal Linking Suggestions #
- Step 4: Failure Analysis – What Happens When Function Fails?
- Download: DFMEA Function-Requirement Table Template (Excel)
- Download: P-Diagram Template (PPT/Excel)
🧠 Pro Tip #
“Each function you define is a potential failure path. More detailed function analysis leads to more accurate risk assessment.”
🏁 Conclusion #
Function Analysis is the heart of DFMEA. Once you know what your system must do, it becomes easier to analyze how it might fail (Step 4), and how to reduce or detect those failures.
Now you’re ready to move forward to the Failure Analysis step, where you’ll identify:
- How functions can fail,
- What the effects are, and
- What causes lead to those failures.
