FAILURE MODE AND EFFECT ANALYSIS - METHODOLOGY

  1. INTRODUCTION
  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:

(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:

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:

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.

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


Example failure mode and effect analysis table for ball-point pen

Part
Function
Potential Failure Mode
Potential effects of failure
SEVERITY
Potential causes of failure
OCCURRENCE
How will the potential 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

2 OCCURRENCE
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:

The Ranking and suggested criteria are:

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


3 SEVERITY
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
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


4 DETECTION
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