PFMEA

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PFMEA
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PFMEA or Process Failure Mode and Effects Analysis is a method that assigns quality risk levels to each task in a process. It is a prevention tool that helps eliminate potential defects before they occur. It is a modification of Failure Mode and Effects Analysis. PFMEA was designed for Six sigma, however it can be used in all quality improvement programs.

Steps of PFMEA

1. Setting scope of analysis

The scope of analysis is essential for the results. Probably there is no need to analyse parts that are well known from earlier products, unless they can be incompatible with new solutions. Also level of decomposition should be considered. Each analysis should be performed in well defined environment.

2. Functional decomposition

The object under investigation should be decomposed into elements. In case of process, it should be decomposed into tasks. For each task data should be collected, including:

  • number of failures,
  • types of failures,
  • known problems (e.g. analysis of literature),
  • limits of use (e.g. temperature, humidity).

3. Qualitative analysis

Qualitative analysis concerns possible failures, their causes and effects:

  • failure - what wrong can happen to the element under investigation,
  • cause - what has to happen to make the failure occur; there can be several causes and each should be analysed separately,
  • effect - what will happen if failure occur (not cause! - common mistake); there may be several short-term as well as long-term effects. Each should be analysed separately.

Set of failure + cause + effect is a failure mode.

4. Quantitative analysis

Quantitative analysis refers to three variables:

  • (P) Probability of occurrence (related to cause of failure),
  • (D) Detection difficulty (related to the failure itself),
  • (S) Severity (related to effect).

Product of those three variables gives overall rating of the analysed failure mode.

\(R = P \cdot D \cdot S\)

Each variable is rated in scale from 1 to 10, where 10 is maximum. The enterprise using FMEA should define own evaluation tables that help in assigning ranks.

5. Corrective and preventive actions in FMEA

There are three different ways of repair:

  • Reducing the probability,
  • Improve detection,
  • Reduce nuisance of failures.

The choice depends on the type and complexity of the failure and the product.

Probability, Detection and Severity scales

Probability scale

  1. <0.01 per 1000
  2. 0.1 per 1000
  3. 0.5 per 1000
  4. 1 per 1000
  5. 2 per 1000
  6. 5 per 1000
  7. 10 per 1000
  8. 20 per 1000
  9. 50 per 1000
  10. >100 per 1000

Detection scale

  1. Certain to detect
  2. Almost certain
  3. High
  4. Moderately high
  5. Moderate
  6. Low
  7. Very low
  8. Remote
  9. Very remote
  10. Impossible to detect

Severity scale

  1. None
  2. Very Minor
  3. Minor
  4. Very Low
  5. Low
  6. Moderate
  7. High
  8. Very High
  9. Hazardous, with warning
  10. Hazardous, without warning

Do not mistake detection with probability!

PFMEA Rules

  1. PFMEA is a teamwork, not one person.
  2. Analyse only critical processes. Do not waste resources on non-important ones.
  3. Include people with extensive experience, but also few less experienced employees. Younger employees don't know that something is impossible yet, and thus they can achieve it.
  4. PFMEA should be done at the place where the process occurs (Gemba).
  5. Collect data before PFMEA session
  6. Each change of process requires change in PFMEA. Decide when do you want to perform PFMEA session.
  7. Set limited time for analysis. Otherwise it can go too deep.

References

Author: Slawomir Wawak