DFMEA Function Analysis

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.

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🎯 Objective of Step 3 #

“Define the functions and associated performance requirements of each system, subsystem, component, and interface to enable effective failure analysis.”

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🔍 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

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🚗 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.

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📋 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.

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🔗 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	–

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🧩 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.

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🖼️ 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.

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📑 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

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✅ 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

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🔗 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)

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🧠 Pro Tip #

“Each function you define is a potential failure path. More detailed function analysis leads to more accurate risk assessment.”

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🏁 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.