In Step 3: Function Analysis of the AIAG-VDA 7-Step FMEA approach, defining individual functions and requirements is not enough.
👉 We must also understand how these functions connect and interact with each other.
This is where the concepts of the Function Net and the Function Chain are applied.
- Function Net: A network that shows how functions interact.
- Function Chain: A linear sequence where the output of one function becomes the input of the next.
Both are critical for identifying cascading risks and ensuring traceability in FMEA.
What is a Function Chain? #
A Function Chain shows the logical sequence of functions in a system or process.
- Each function’s output becomes the input of the next function.
- Failures in one function can propagate down the chain.
Example – Electric Vehicle Powertrain
- Battery → Supplies DC power.
- Inverter → Converts DC to AC.
- Motor → Converts electrical energy to mechanical rotation.
- Transmission → Transfers torque to wheels.
- Wheels → Move the vehicle.
📌 If the inverter fails, the entire function chain breaks, even though the motor and transmission are fine.
What is a Function Net? #
A Function Net is a network representation of functions and their interdependencies. Unlike a linear chain, a function net shows parallel and cross-functional interactions.
Example – Electric Motor Function Net
- Battery → Inverter → Motor Stator
- Cooling System → Motor Housing
- ECU → Motor Control
Here, functions are interconnected:
- The motor depends not only on the inverter but also on cooling and ECU.
- Failures in any of these connected functions can cause motor failure.
Why Function Nets & Chains are Important in FMEA #
- Traceability: Provides a clear path from system-level functions down to component-level.
- Cascading Risks: Helps identify failures that spread across multiple subsystems.
- Completeness: Ensures no function or interface is overlooked.
- Basis for Failure Analysis: Each function in the chain/net becomes an anchor point for identifying potential failures.
Example – PFMEA Function Chain (Welding Process) #
- Load components into fixture
- Position components correctly
- Apply electrode pressure
- Apply welding current
- Release and unload
👉 If positioning fails, all downstream functions (pressure, current, weld quality) are affected.
Example – DFMEA Function Net (ABS Braking System) #
- Wheel Speed Sensors → ECU (signal input)
- ECU → Hydraulic Modulator (control signal)
- ECU → Warning Light (feedback to driver)
👉 If the ECU fails, multiple connected functions (hydraulic control, driver warning) are impacted at once.
Best Practices for Function Nets & Chains #
- Always start with a structure tree (system → subsystem → component).
- Define primary and secondary functions for each element.
- Use arrows to show direction of flow (energy, signal, material).
- In PFMEA, link each process step in sequence (function chain).
- In DFMEA, map system interactions (function net).
Common Mistakes to Avoid #
- Treating functions as isolated (no connections shown).
- Forgetting interfaces between subsystems (where most failures occur).
- Overcomplicating diagrams (too much detail → loss of clarity).
- Not updating nets/chains when design or process changes.
Case Study – Function Net for Electric Motor Cooling #
- Function: Motor generates torque.
- Supporting Function: Cooling system removes heat.
- Function Net: Motor ↔ Cooling System ↔ ECU control.
👉 Without cooling, the motor overheats → torque function fails → ECU triggers fault.
This function net shows that risks are not just inside the motor but across systems.
Key Takeaways #
- Function Chain = Linear sequence of functions.
- Function Net = Network of interconnected functions.
- Both tools ensure traceability, completeness, and risk visibility in FMEA.
- They are essential bridges between Structure Analysis (Step 2) and Failure Analysis (Step 4).
Next Lesson #
👉 Continue with Lesson 3.5: Step 4 – Failure Analysis (Failure Modes, Effects, Causes)
