Finding actual or potential defects



Definitions

Failure Modes are actual or potential defects in a process or item.  Generally these are limited to defects which affect the customer.

Failure Mode Effect Analysis (FMEA) analyzes the potential failure modes of a process, and classifies these modes by severity or the types of effects on the system.

Timing and Requirements

FMEA is predictive: the analysis should begin in the product's design stage, in subsequent re-design stages, or when manufacturing processes are being revised.

The analysis does need historic data:

  • How have existing components failed?
  • How have similar products failed?
  • How has this product failed before this re-design or before this process improvement?

FMEA needs pre-work: in particular, the product must have enough documentation to let the review team know the valued functions, the components, how the components work together.  The more known, the better the analysis can be.

Leading by Example

This may be a familiar example.  Consider the bathroom toilet with a tank to store clean water for flushing.  A hollow metal float is attached to a lever which controls both a drain stopper and a cut-off valve.  The user flushes by moving the lever to lift the stopper; this causes rapid drainage.  This also opens the cut-off valve so fresh water begins to slowly refill the tank.  When the tank's water level goes down, the float drops to permit the stopper to drop and stop the drainage.  When the float rises high enough, then the cut-off valve stops refilling the tank.

Part of the worksheet might look like this:

Function Failure
Mode
Effects Severity
Rating
Cause(s)

Refill tank

Tank refills forever.
Level is high.

Wastes water.
Noise.
Possible overflow onto floor.

5

Leaky float does not rise.
Float detached from lever.

Refill tank

Tank refills forever.
Level is low.

Wastes water.
Noise.

4

Stopper is not water-tight.

Refill tank

Tank remains empty

Cannot flush toilet next time.

2

Plumbing leading to toilet.
Cut-off valve is stuck.

 

Details

Tha Failure Mode Effect Analysis is, of course, more complex – even for a simple example like the one above.

An worksheet might include:

  • Function: each function (which the customer values) is listed.
  • Failure Mode: each failure mode (which affects the function which the customer values), for each function, is listed.
  • Effects: what the customer would notice.
  • Severity Rating: a rating from 1 (minimal) to 10 (extremely high).  Values 9 and 10 are generally reserved for "life-and-death" issues, health issues, or other issues which may lead to lawsuits or criminal charges.
  • Cause(s): what would cause this failure?
  • Occurrence Ranking: from 1 (very infrequent) to 10 (very frequent).  (Often there will be rules to raise the Occurrence Ranking if the Severity Rating is 9 or 10 – just to be sure that this failure is not lost in the later analysis).
  • Detection Rating: based on the likelihood that a failure would be detected by the manufacturer, before the customer would be affected: from 1 (very likely to be detected) to 10 (would escape detection by the manufacturer).
  • Risk Priority Number (RPN): the product of Severity Rating X Occurrence Ranking X Detection Rating.
  • Recommended Actions
  • Responsibility
  • Target Date
  • Action Taken

Some organizations use ratings from 1 to 4 rather than 1 to 10.  Regardless of the ratings scale, the goal is to determine the RPN for each failure in order to prioritize the remediation efforts.

Summary

The risk-based analysis is very important in achieving higher quality by working toward:

  • Higher reliability, as measured by fewer breakdowns
  • Lower total cost of ownership, due to fewer breakdowns and targeting the failures with the more severe consequences
  • Greater safety, since the more dangerous failures are given more importance

By Oskar Olofsson



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