FMEA/PFMEA
Pages Appendix1 Appendix2 References
   

FAILURE MODE AND EFFECT ANALYSIS - METHODOLOGY

 

Failure mode and effect analysis (FMEA) is a systematic way of assuring that every conceivable potential failure of a design/process has been considered with the objective of minimising the probability of failure.

1 INTRODUCTION

  • Definition

  • Timing

  • Preparation

  • Method

2 OCCURRENCE

3 SEVERITY OF EFFECT

4 DETECTION

 

1 INTRODUCTION Definition Failure mode and effect analysis (FMEA) is a systematic way of assuring that every conceivable potential failure of a design/process has been considered. The object of using FMEA is to minimise the probability of failure. More precisely, IEEE Std 352-1975: Guide for General Principles of Reliability Analysis of Nuclear Power Generating Station Protection Systems, one of the definitive works on FMEA, defines the purposes of an FMEA as being to:

  • assist in selecting design alternatives with high reliability and high safety potential during early design phase
  • ensure that all conceivable failure modes and their effects on operational success of the system have been considered
  • list potential failures and identify the magnitude of their effects
  • develop early criteria for test planning and the design of the test and check-out systems
  • provide a basis for quantitative reliability and availability analyses
  • provide historical documentation for future reference to aid in analysis of field failures and consideration of design changes
  • provide input data for trade off studies
  • provide basis for establishing corrective action priorities
  • assist in the objective evaluation of design requirements related to redundancy,

failure detection systems, fail-safe characteristics and automatic and manual override .(IEEE Std 352-1975)

Timing The FMEA should be an integral part of the early design evaluation and should be periodically updated to reflect changes in design or application. An updated FMEA should be a major consideration in design reviews, inspections, or other major system review points in the program. During the design phase, an FMEA should be performed or updated at the following program stages:

  • Concept formulation or selection
  • Preliminary design or layout
  • Completion of detail part design
  • Design improvement programs

The FMEA may also be performed with limited design information in which case the basic questions to be answered by an FMEA are as follows:

  • How can each part conceivably fail?
  • What mechanisms might produce these modes of failure?
  • What could the effects be if these failures did occur?
  • Is the failure in the safe or unsafe direction?
  • How is the failure detected?
  • What inherent provisions are provided in the design to compensate for the failure?
 

Preparation Before undertaking an FMEA it is essential to undertake certain preparatory steps; the scope will depend on the complexity of the system/article being studied.

  • Definition of the system/article to be analyzed and its mission.
  • Description of the operation of the system.
  • Identification of failure categories.
  • Description of the environmental conditions.

Method Data is entered into a table (see below) under the following headings:

  • Part; each system component or part being analyzed is named (or referenced by other appropriate designator such as circuit reference
  • Function; brief note as to function of part.
  • Potential failure mode; this should cover ever way in which the part could fail and should include random and degradation failures. Ask 'How could it fail?' not 'Will it fail?'
  • Potential effect of failure; brief description of the consequences of failure.
  • Severity; see the section on severity below.
  • Potential causes of failure; what caused this failure mode.
  • Occurrence; see section on occurrence below for guidance.
  • How will the potential failure be detected? Some failures are obvious to the person using the subject of the FMEA, but if this is not the case, the means by which the failures can be detected should be listed.
  • Detection; see section on detection below for guidance.
  • Risk Priority Number (RPN) = Severity * Occurrence * Detection
  • Actions: Detail recommended actions

.Example failure mode and effect analysis table for ball-point pen The Occurrence is the assessment of the probability that the specific cause of the Failure mode will occur. It is part subjective, but the wording should describe the probability. Failure history is helpful in increasing the truth of the probability. Questions of the following type are helpful:

Part Function PotentialFailure Mode Potential effectsof failure SEVERITY Potentialcauses offailure OCCURRENCE How will thepotential failure be detected? DETECTION RPN Actions
Outer tube Provides grip for writer Hole gets blocked Vacuum on ink supply stops flow 7 Debris ingress into hole 3 Check clearance of hole 5 105 Make hole larger
Ink Provide writing medium Incorrect viscosity High flow 4 Too much solvent 2 QC on ink supply 4 32 Introduce more rigid QC
Ink Provide writing medium Incorrect viscosity Low flow 4 Too little solvent 2 QC on ink supply 3 24 No action required
 
  • What statistical data is available from previous or similar process designs?
  • Is the process a repeat of a previous design, or have there been some changes?
  • Is the process design completely new?
  • Has the environment in which the process is to operate changeable?
  • Have mathematical or engineering studies been used to predict failure

The Ranking and suggested criteria are: Severity is an assessment of the seriousness of the Effect and refers directly to the potential failure mode being studied. The Customer in process FMEA is both the internal and where appropriate, external Customer. The severity ranking is also an estimate of how difficult it will be for the subsequent operations to be carried out to its specification in Performance, Cost, and Time

Notional probability of failure Evaluated Failure Rates Cpk Rank
Remote: Failure is unlikely. No Failures ever associated with almost identical processes 1 in 1,500,000 >1.67 1
Very Low: Only Isolated Failures associated with almost identical processes 1 in 150,000 1.50 2
Low: Isolated Failures associated with similar processes 1 in 15,000 1.33 3
Moderate: Generally associated with processes similar to previous processes Failures, but not in 'major' proportions 1 in 2,000 1.17 4
1 in 400 1.00 5
1 in 80 0.83 6
High: Generally associated with processes similar to previous processes that have often failed 1 in 20 0.67 7
1 in 8 0.51 8
Very High: Failure is almost inevitable 1 in 3 0.33 9
1 in 2 <0.33 10
 

The Ranking and suggested criteria are:

Effect Criteria Severity of Effect Rank
None No Effect 1
Very Minor Minor disruption to production line A portion of the product may have to be reworked. Defect not noticed by average customers; cosmetic defects. 2
Minor Minor disruption to production line. A portion of the product may have to be reworked. Defect noticed by average customers; cosmetic defects. 3
Very Low Minor disruption to production line. The product may have to be sorted and reworked. Defect noticed by average customers; cosmetic defects. 4
Low Some disruption to product line. 100% of product may have to be reworked. Customer has some dissatisfaction. Item is fit for purpose but may have reduced levels of performance. 5
Moderate Some disruption to product line. A portion of the product may have to be scrapped. Customer has some dissatisfaction. Item is fit for purpose but may have reduced levels of performance. 6
High Some disruption to product line. Product may have to be sorted and a portion scrapped. Customer dissatisfied. Item is useable but at reduced levels of performance. 7
Very High Major disruption to production line. 100% of product may have to be scrapped. Loss of primary function. Item unusable. Customer very dissatisfied. 8
Hazard with warning May endanger machine or operator. Failure occurs with warning. The failure mode affects safe operation and involves noncompliance with regulations 9
Hazard without warning May endanger machine or operator Failure occurs without warning. The failure mode affects safe operation and involves noncompliance with regulations 10
 

This is an assessment of the probability that the proposed Process Controls will detect a potential cause of Failure or a Process weakness. Assume the Failure has occurred and then assess the ability of the Controls to prevent shipment of the part with that defect. Low Occurrence does not mean Low Detection - the Control should detect the Low Occurrence. Statistical sampling is an acceptable Control. Improving Product and/or Process design is the best strategy for reducing the Detection ranking - Improving means of Detection still requires improved designs with its subsequent improvement of the basic design. Higher rankings should question the method of the Control.

The Ranking and suggested criteria are:

Detection The likelihood the Controls will detect a Defect Rank
Almost Certain Current controls are almost certain to detect the Failure Mode. Reliable detection controls are known with similar processes. 1
Very High Very High likelihood the current controls will detect the Failure Mode. 2
High High likelihood that the current controls will detect the Failure Mode. 3
Moderately High Moderately high likelihood that the current controls will detect the Failure Mode. 4
Moderate Moderate likelihood that the current controls will detect the Failure Mode. 5
Low Low likelihood that the current controls will detect the Failure Mode 6
Very Low Very Low likelihood that the current controls will detect the Failure Mode 7
Remote Remote likelihood that the current controls will detect the Failure Mode 8
Very Remote Very Remote likelihood that the current controls will detect the Failure Mode 9
Almost Impossible No known controls available to detect the Failure Mode. 10





PFMEA Common Mistakes
  1. Applying thresholds.
  2. Not recognizing all potential failures.
  3. Failure to properly identify the customer.
  4. Misapplication of ranking scales.
  5. Confusing Failure Modes with Effects or Failure Modes with Causes.
  6. Failure to develop Recommended Actions or Recommended Actions that are
  7. neither actionable nor executable.
  8. Applying Occurrence and Detection too optimistically.
  9. Failure to accurately describe the Process Function Requirement.
  10. Failure to consider the "Potential" nature of Failure, Effect, Cause.
  11. Allowing the PFMEA to turn into a design review.

Top of page
 
Site Policy Contact Us Disclosure Policy Suggested Operating Environment