If a Tree Falls in a Forest . . .
Why do you think FMEA procedure requires effects to be taken to the system or end user? Why not describe the consequence only at the local level. If a bolt in a complex system fails, the parts that the bolt was clamping together may come apart. Isn’t that enough?
“Wisdom consists of the anticipation of consequences.”
Definition of “Effect”
The Oxford English dictionary defines “Effect” as “a change which is a consequence of an action or other cause.”
What is the definition of “Effect” in an FMEA?
An “Effect” is the consequence of the failure on the system or end user. Depending on the ground rules for the analysis, the team may define a single description of the effect on the top-level system and/or end user, or three levels of effects:
Local effect: The consequence of the failure on the item or adjacent items
Next-higher level effect: The consequence of the failure on the next-higher level assembly
End effect: The consequence of the failure on the top-level system and/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 effect of the failure at the manufacturing or assembly level can manifest at the next operation, subsequent operations, the dealer or distributor, as well as the end customer.
Since “An Effect is the consequence of the failure on the system or end user,” it is important to understand the concept of failure mode in an FMEA. Be sure to read the article “Understanding Failure Modes.”
How are Effects identified in FMEAs?
For each failure mode, the FMEA team identifies the effect(s) of failure, following the definition above. There can be more than one effect for each failure mode. However, in most applications the FMEA team will use the most serious of the end effects for the analysis.
For Design FMEAs, the team can be asked questions such as “what is the consequence of the failure on the system or end user”, or other variations of this question.
Why system or end user? Because the severity ranking will be based on matching the effect description to the severity scale criteria. Analyzing whether or not there is a potential safety problem, or loss or degradation of performance, requires understanding the effect at the product or user level.
For Process FMEAs, the team can be asked questions such as “what is the consequence of the failure on the manufacturing plant”, or other variations of this question. Process FMEA teams consider both the effect of the failure at the manufacturing or assembly level, as well as at the system or end user.
What is an example of a Design FMEA Effect
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 a Process FMEA Effect
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): Potential shaft fracture with complete loss of performance
Depending on the FMEA standard used, Effects may include local, next level and end effect, or it may include only the end effect. If the standard being used by the team only requires end effect, it is still a good idea to include the verbiage in the end effect column tracing the local and next level effects to the end effect. This will be helpful when the team decides on corrective actions for high severity issues, and shows due care in the analysis. However done, the team should always arrive at the end effect of the failure mode on the user or system. End user effects should reflect what the user might notice or experience. They should clearly state if the effect of a failure mode could potentially affect safety or non-compliance to regulations, when applicable.
The loss of the aft cargo door on the American Airlines DC-10 aircraft will be the focus of the next article. Readers will be asked to identify the effect of the door latch-pin failure. Give it a try!