Design for X: Enhancing Product Robustness & Reducing Failure Modes with FMEA & DfX, Slides of Computer-Aided Design for Engineers

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Design for X

OutLine  Design for X

• Trade-offs in Satisfaction
• Robust design
• Failure Modes & Effects Analysis
• Tolerance design Customer Needs (CN) Functional Requirements (FR) Design Parameters (DP) Process Variables (PV)

Design for X (DfX)

• During design, we often focus on the final product, and

not its manufacture.

• The Design For X (DfX) philosophy suggests that a design

be continually reviewed from the start to the end to find

ways to improve production and other non-functional

aspects

• Advantages of DfX techniques include
  • shorter production times
  • fewer production steps
  • smaller parts inventory
  • more standardized parts
  • simpler designs that are more likely to be robust
  • they can help when expertise is not available, or as

a way to re-examine traditional designs

• proven to be very successful over decades of application

The “X” in DfX

Failure Modes & Effects Analysis

• The FMEA Method seeks to systematically identify and correct potential product or process deficiencies before they occur
• The Process
  • Identify EVERY Way in Which Product Can FAIL; i.e., determine the Failure MODES
  • Analyze the CONSEQUENCES of Every Failure; i.e., determine the EFFECTS

Failure Modes & Effects Analysis

• After Completion of the FMEA Work to REDUCE - The NUMBER of Failure MODES - The SEVERITY of the EFFECTS
• Prioritize Risk Reduction using “Risk Priority No.”

FMEA Main Concepts

• Failure Mode : the “way” a part fails to perform
  • e.g. failure mode: hose leaks
• Effect : adverse consequence of failure mode
  • e.g. hose leak results in oil spills, refill costs
  • Effects can be severe or hardly noticeable.
• Cause : why it fails (or may fail)
  • e.g. poor hose manufacturing, improper pressure
  • Causes occur with some likelihood or probability
• Dectectability : the ability to discover the cause before the part is shipped from the factory. - e.g. conduct a pressure test to detect leaks?

FMEA Risk Metric  RPN

• Determine a rating for each mode of failure …. using a “risk priority number” (RPN) RPN = [Severity rating] x [Occurrence rating] x [Detection rating] RPN = (S)•(O)•(D)
• RPN will range from 1 to 1000
  • Large RPN  “ bad ”
  • Small RPN  “ good ”

RPN Calculation

• Step 2: determine potential effects of each failure mode
• Step 3: determine a severity ( S ) rating for each effect from the Severity rating table.
• Step 4: determine an occurrence ( O ) rating for each cause from the Occurrence rating table.
• Step 5: determine a detection ( D ) rating for each cause from the Detection rating table

Severity Rating Criteria

Severity (S) Rating Type of effects Description (^10) Catastrophic Causes injury to people, property and or the environment 9 Extremely Harmful Causes damage to product, property or environment 8 Very Harmful Causes damage to product 7 Harmful Major degradation of function 6 Moderate Causes partial malfunction of product 5 Significant Performance loss causes customer complaints 4 Annoying Loss of function is annoying, cannot be overcome 3 Minor Some loss of performance, but can be overcome 2 Insignificant Very little function degradation (^1) None No noticeable effects in function or harm to others

Detection Rating Criteria

Detection (D) Rating Detectability Description 10 Impossible Impossible to detect, or no inspection 9 Very rare 8 Rare 7 Possible Some chance of detecting, or 50% inspection 6 Quite possible 5 Somewhat likely 4 Likely Quite likely to detect, or 75% inspection 3 Quite likely 2 Almost certain (^1) Certain Will be detected, or 100% inspection

RPN Calculation & Reduction

• Step 6: calculate the risk priority number for each effect
• Step 7: prioritize or rank the failure modes for action
• Step 8: take action to eliminate the failure mode or reduce its severity
• Step 9: recalculate the risk priority number as failure modes are reduced or eliminated

RPN Example  Hose Failure

• Log-Splitter RPN & Remediation Severity (S) Occurrence (O) Detection (D) Failure mode Effects S Rating Causes O Rating controls tests D Rating RPN Recommended Action hydraulic hose leaks poor log splitting oil mess costs to refill 7 (harmful) weak spot 7 (1%) none 10 490 implement pressure test

Design for Safety Issues

• Injury
• Hazards
• Conditional Circumstances
• Legal Responsibilities
• Guidelines for Safe Products/Systems
• Safety Hierarchy
• Safe Design Principles