The Seriousness of Consequences
Have you ever been in an FMEA meeting where the team did not agree on the severity rating? Understanding and correctly applying severity risk is an important part of FMEA application. This article discusses the subject of severity risk, including examples for design and process FMEAs, and offers a tip on what to do when the team does not agree on the severity risk rating.
“The only real mistake is the one from which we learn nothing.”
Definition of “severity”
The Macmillan dictionary defines “severity” as “the seriousness of something bad or unpleasant.”
What is the definition of “Severity” in an FMEA?
“Severity” is a ranking number associated with the most serious effect for a given failure mode, based on the criteria from a severity scale. It is a relative ranking within the scope of the specific FMEA and is determined without regard to the likelihood of occurrence or detection..
How is “Severity” assessed in FMEAs?
Having identified the most serious effect for the failure mode, the FMEA team assesses the severity ranking. This is the severity of the effect of the failure mode, not the severity of the failure mode itself.
Using the agreed-upon severity scale, the team carefully reviews the criteria column to make this judgment. If the effect is well defined, the severity is easily established by reviewing the severity scale criteria.
For Design FMEAs, the team assesses the severity of the end effect at system or end user.
For Process FMEAs, the team should consider the effect of the failure at the manufacturing or assembly level, as well as at the system or end user. The severity that is used in the Process FMEA is the highest of the two values.
What does a Severity Scale look like for Design FMEAs?
The following is an example of a severity scale for Design FMEAs. It is based on “Potential Failure Mode and Effects Analysis (FMEA) 4th Edition, 2008 Manual.”
What does a Severity Scale look like for Process FMEAs?
The following is an example of a severity scale for Process FMEAs. It is based on “Potential Failure Mode and Effects Analysis (FMEA) 4th Edition, 2008 Manual.”
What is an example of Severity in a Design FMEA?
[In this fictitious example, the Design FMEA team considers the severity of the end effect, using the criteria in the AIAG 4 severity scale, and enters in the FMEA worksheet.]
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
What is an example of Severity in a Process FMEA?
[In this fictitious example, the Process FMEA team considers the severity of product/customer effect along with severity of mfg/assy effect, using the criteria in the AIAG 4 severity scale, and enters the worst case in the FMEA worksheet.]
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)
Severity: 8 (entered in FMEA worksheet)
Tip 1: In the case of items that are redundant, and there is no detection or no warning that a redundant item has failed, the severity should be assessed as if all of the redundant items have failed.
Tip 2: If the effect is well defined, the severity is easily established by reviewing the severity scale criteria. Difficulty identifying the severity ranking is usually due to an improperly defined effect or inadequate severity scale criteria.
Is action always required on high-severity issues? What if severity is high (9 or 10 on a severity scale of 1 to 10), and the occurrence and detection rankings are both low? Is action still required? This problem, as well as a challenging problem involving fail-safe strategies, are the subject of the next problem-solution article.