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Case 1: Electric Power Steering System DFMEA

System Background

The Electric Power Steering (EPS) system provides steering assist via an electric motor, replacing traditional hydraulic assist. This case analyzes the DFMEA for the EPS controller module.

Structure Analysis

Level Element Function
System (Higher Level) Electric Power Steering System Provide appropriate steering assist based on driver steering intent
Focus Element EPS Electronic Control Unit (ECU) Receive sensor signals, calculate and output assist motor control commands
Lower Level Torque Sensor Interface Circuit Acquire torque sensor signal, convert to digital input for MCU

Failure Analysis Example

Failure Effect (Higher) Failure Mode (Focus) Failure Cause (Lower) S O D AP
Sudden loss of steering assist; driver must apply greater force ECU incorrectly shuts off assist motor output Torque sensor interface circuit signal loss (cold solder joint) 9 3 4 M
Steering assist in wrong direction; vehicle may deviate from intended path ECU outputs reversed assist command Torque sensor signal polarity reversed (PCB routing error) 10 2 2 M
Inconsistent steering feel; driver perceives abnormality ECU assist gain deviation is excessive ADC reference voltage drift (capacitor value shift) 6 4 5 M

Improvement Actions

  • Add torque sensor signal plausibility checks (range check, rate-of-change check)
  • Design redundant sensor channel with cross-validation
  • Add critical signal polarity dedicated review to PCB design review
  • Increase PCBA end-of-line functional test coverage

Case 2: Engine Cylinder Head Bolt Tightening PFMEA

Operation Background

Engine assembly line cylinder head bolt tightening operation using a multi-spindle servo tightening system to torque ten M12 cylinder head bolts in a specified sequence using angle-controlled method.

Structure Analysis

Level Element Function / Requirement
Process Item Engine Cylinder Head Assembly Ensure cylinder head seal integrity, withstand combustion pressure
Process Step OP30 - Cylinder Head Bolt Tightening Tighten per specification (Step 1: 60 Nm, Step 2: +90 deg +/- 5 deg)
4M Element Tightening spindle accuracy / Bolt lubrication / Tightening program Spindle repeatability +/-1%, bolt friction coefficient 0.10-0.14

Failure Analysis Example

Failure Effect Failure Mode Failure Cause (4M) S O D AP
Cylinder head seal failure, coolant leakage, engine overheating Final bolt angle below lower specification limit Machine: Tightening spindle sensor drift, not calibrated 8 3 3 L
Bolt fracture, cylinder head loses preload, severe failure Bolt over-tightened (exceeds angle upper limit) Method: Tightening program parameters set incorrectly 9 2 2 M
Cylinder head distortion, uneven gasket seal, oil consumption Incorrect tightening sequence Man: Operator manual intervention without resetting auto sequence 7 3 4 M
Same as above Inconsistent bolt clamping force Material: Bolt surface coating uneven, friction coefficient out of spec 7 5 6 H

Improvement Actions

  • For bolt friction coefficient issue (AP=H):
    • Supplier to add 100% bolt coating friction coefficient inspection
    • Increase incoming inspection frequency (5 pieces per lot for friction test)
    • Add torque-angle curve monitoring window to tightening program
  • For tightening sequence issue (AP=M): Add PLC program interlock; manual override requires restart from beginning
  • For over-tightening issue (AP=M): Tightening program parameter changes require dual confirmation and error-proof verification

Case 3: Door Weatherstrip Installation PFMEA

Operation Background

Vehicle final assembly line manual installation of EPDM door weatherstrip along the door frame channel.

Failure Analysis

Failure Effect Failure Mode Failure Cause Prevention Control Detection Control AP
Increased wind noise, customer complaint Weatherstrip partially dislodged from channel Operator did not follow specified starting point and direction Work instruction + onboarding training Post-installation visual inspection M
Water leakage, interior damage Excessive gap at weatherstrip corner joint Weatherstrip cut length deviation too large Cutting tool length stop fixture Door closing force test + water spray test M
Abnormal door closing effort, poor customer perception Weatherstrip lifts after installation Weatherstrip hardness out of spec (cold weather stiffening) Incoming material hardness inspection Door closing force test L

Summary and Best Practices

FMEA Writing Guidelines

  • Failure modes should describe the negation of the function
  • Failure causes should be specific to 4M elements
  • Controls should describe specific methods, not vague statements
  • Improvement actions should have clear owners and deadlines
  • Ratings should be based on team consensus, not individual judgment

Common Mistakes to Avoid

  • Using defect descriptions as failure modes (e.g., "scratch" should be "surface roughness out of spec")
  • Failure causes too vague (e.g., "improper operation" needs to be specific)
  • Confusing prevention controls with detection controls
  • Focusing only on RPN value while ignoring high-severity items
  • Not updating FMEA after completion (not treating it as a living document)