Failure Mode and Effects Analysis (FMEA)


                Failure mode and effects analysis (FMEA) is a team – based methodology for identifying potential problems with the new or existing designs. It is the first step of a system reliability study. FMEA is the core task in reliability engineering, safety engineering, and quality engineering. FMEA involves reviewing as many components, assemblies, and subsystem as possible to identify failure modes, and their causes and effects. In order to determine the components of the process that are most in need of change, FMEA includes the following steps:

1.      Steps in the process.
2.      Failure modes (What could go wrong?)
3.      Failure causes (Why would the failure happen?)
4.      Failure effects (What would be the consequences of each failure?)

            Three factors that are considered in developing a FMEA:

1.      The severity of the failure.
2.      The probability of occurrence of failure.
3.      The likelihood of detecting the failure in either design or manufacturing, before the product is used by the customer.

            Different types of FMEA analysis:
1.      Functional,
2.      Design, and
3.      Process FMEA.

            Major benefits derived from a properly implemented FMEA are as follows:
1.      It provides documentation in selecting design with high probability of successful operation and safety.
2.      A uniform documentation method of assessing potential failure mechanism, failure modes and their impact on system operation, resulting in a list of failure modes ranked according to seriousness of their system impact and likelihood of occurrence.
3.      Early identification of single failure points (SFPS) and system interface problems which may be critical to mission success and/or safety.
4.      An effective method for evaluating the effect of proposed changes t the design and/or operational procedures on mission success and safety.
5.      A basis for in – flight troubleshooting procedures and for locating performance monitoring and fault – detection devices.
6.      Criteria for early planning of tests.


Example of FMEA analysis for car tire:

Keywords:
SEV – severity
OCC – occurrence
DET – Detection
RPN – Risk Priority Number

Function or Process Step
Failure Type
Potential Impact
SEV
Potential Causes
OCC
Detection Mode
DET
RPN
Briefly outline function, step or item being analyzed
Describe what has gone wrong
What is the impact on the key output variables or internal requirements?
How severe is the effect to the customer?
What causes the key input to go wrong?
How frequently is this likely to occur?
What are the existing controls that either prevent the failure from occurring or detect it should it occur?
How easy is it to detect?
Risk priority number
Tire function: support weight of car, traction, comfort
Flat tire
Stops car journey, driver and passengers stranded
10
Puncture
2
Tire checks before journey. While driving, steering pulls to one side, excess noise
3
60

Recommended Actions
Responsibility
Target Date
Action Taken
SEV
OCC
DET
RPN
What are the actions for reducing the occurrence of the cause or improving the detection?
Who is responsible for the recommended action?
What is the target date for the recommended action?
What were the actions implemented? Now recalculate the RPN to see if the action has reduced the risk.
Carry spare tire and appropriate tools to change tire
Car owner
From immediate effect
Spare tire and appropriate tools permanently carried in trunk
4
2
3
24

Criteria for FMEA Analysis
            FMEA is analyzed based on three criteria:
1.      Severity effect on customer.
2.      Occurrence of failure.
3.      Easy to detect.
            It is then ranked from 1 (low) to 10 (high) for each criterion.
Table 1: Severity, Occurrence and Detection Ratings
Description
Low Number
High Number
Severity
Severity ranking encompasses what is important to the industry, company or customers (e.g., safety standards, environment, legal, production continuity, scrap, loss of business, damaged reputation)
Low impact
High impact
Occurrence
Rank the probability of a failure occuring during the expected lifetime of the product or service
Not likely to occur
Inevitable
Detection
Rank the probability of the problem being detected and acted upon before it has happened
Very likely to be detected
Not likely to be detected

Formula for RPN is:

RPN = severity x occurrence x detection

 

 

 

 

Setting Priorities

 

            After failure has been identified, the FMEA list of failure is adjusted in descending RPN order. This can ensure that the corrective action can be focused.

 

Making Corrective Action
            Once the priorities have been agreed, it’s time to generate appropriate corrective actions for reducing the occurrence of the failure modes. When the corrective actions have been done, it is better to rescore and reassess the severity, probability of occurrence and likelihood of detection for the top failure modes. This enables to determine the effectiveness of the corrective action taken.