The Scale from Remote to Certain
Some FMEA teams associate occurrence ranking with Failure Mode; others associate it with Effect. Still others connect associate it with Cause? Which is correct? This article discusses occurrence risk, including examples, and answers this question.
“Reality is not always probable, or likely.”
Jorge Luis Borges
Definition of “occurrence”
The Oxford English dictionary defines “occurrence” as “the fact or frequency of something happening.”
What is the definition of “occurrence” in an FMEA?
“Occurrence” is a ranking number associated with the likelihood that the failure mode and its associated cause will be present in the item being analyzed. For System and Design FMEAs, the occurrence ranking considers the likelihood of occurrence during the design life of the product. For Process FMEAs the occurrence ranking considers the likelihood of occurrence during production. It is based on the criteria from the corresponding occurrence scale. The occurrence ranking has a relative meaning rather than an absolute value and is determined without regard to the severity or likelihood of detection..
How is Occurrence risk assessed in FMEAs?
The team determines the likelihood of the failure mode / cause based on the above definition, and using the agreed upon occurrence scale, carefully reviews the criteria column to establish the occurrence ranking. This assessment of occurrence ranking should be as objective as possible, using past field history of similar items, previous test results, experience with similar systems, and other sources of information. There will always be a subjective element to this ranking, as the FMEA is done on new designs, design changes, and/or new applications. However, the FMEA team should endeavor to be as objective as possible, using the criteria from the occurrence scale to help determine the appropriate rank. If the assessment of occurrence ranking falls between discreet occurrence numbers on the scale (such as between 4 and 5) the team should use the higher number.
Is it the occurrence of the failure mode or the cause?
This question has been debated in the FMEA community. Most FMEA standards, such as AIAG, SAE J1739 and VDA, attribute the occurrence ranking to the cause of the failure. The reason for this has to do with the value of the risk assessment. If we have two different causes for “cable breaks”, such as “corrosion of cable wiring due to wrong material selected” and “fatigue cracks in cable wiring due to inadequate cable thickness”, it helps to differentiate the likelihood of occurrence of the two cause. The priority of corrective actions is aided by this differentiation.
What does an Occurrence Scale look like for Design FMEAs?
The following is an example of an occurrence scale for Design FMEAs. It is based on “Potential Failure Mode and Effects Analysis (FMEA) 4th Edition, 2008 Manual.”
What does an Occurrence Scale look like for Process FMEAs?
The following is an example of an occurrence scale for Process FMEAs. It is based on “Potential Failure Mode and Effects Analysis (FMEA) 4th Edition, 2008 Manual.”
What is an example of Occurrence in a Design FMEA?
[In this fictitious example, the occurrence ranking is assessed based on estimate there will be 1 in 2000 failures due to the cause identified in the FMEA.]
Item: Power steering pump
Function: Delivers hydraulic power for steering by transforming oil pressure at inlet ([xx] psi) into higher oil pressure at outlet [yy] psi during engine idle speed
Failure Mode: Inadequate outlet pressure (less than [yy] psi)
Effect (Local: Pump): Low pressure fluid goes to steering gear
Effect (Next level: Steering Subsystem): Increased friction at steering gear
Effect (End user): Increased steering effort with potential accident during steering maneuvers
Cause: Fluid incorrectly specified (viscosity too low)
What is an example of Occurrence in a Process FMEA?
[In this fictitious example, the occurrence ranking is assessed based on estimate there will be 1 in 100,000 failures due to the cause identified in the FMEA.]
Process Step: Induction harden shafts using induction hardening machine
Function: Induction harden shafts using induction-hardening machine ABC, with minimum hardness Brinell Hardness Number (BHN) “X”, according to specification #123.
Failure Mode: Shaft hardness less than BHN “X”
Effect (In plant): 100% scrap
Effect (End user): Shaft fractures with complete loss of performance
Effect (Assembly): Not noticeable during assembly
Severity: (Customer Effect): 8 (loss of primary function)
Severity: (Mfg/Assy Effect): 8 (major disruption)
Cause: Induction machine electrical voltage/current settings incorrect for part number
Note that prevention-type controls are considered input to the occurrence ranking. Some practitioners place the Prevention Controls column before the Occurrence column in the FMEA worksheet to facilitate this sequence.
Assessing the occurrence ranking is one of the most challenging aspects of FMEAs. The next article presents two problems that relate to this assessment, based on a fictitious case study, and highlights a common mistake.