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Manual for Life Cost Based FMEA

1. Introduction

Failure occurs when one or more of the intended functions of a product are no longer fulfilled to the customer’s satisfaction. The most critical product failures are those that escape design reviews and in-house quality inspection and are found by the customer. The product may work for a while until its performance degrades to an unacceptable level or it may have not worked even before customer took possession of the product. The end results of failures which may lead to unsafe conditions or major losses of the main function are rated high in severity.

  • Traditional FMEA

Failure Modes and Effects Analysis (FMEA) is a tool widely used in the automotive,

aerospace, and electronics industries to identify, prioritize, and eliminate known potential failures, problems, and errors from systems under design, before the product is released (Stamatis, 1997). Several industrial FMEA standards such as those published by the Society of Automotive Engineers, US Department of Defense, and the Automotive Industry Action Group employ the Risk Priority Number (RPN) to measure risk and severity of failures. The Risk Priority Number (RPN) is a product of 3 indices: Occurrence (O), Severity (S), and Detection (D). In a traditional FMEA process design engineers typically analyze the “root cause” and “end-effects” of potential failures in a sub-system or component and assign penalty points through the

O, S, D values to each failure. The analysis is organized failure modes, which link the causes and effects of failures.

around

categories

called

A few actions are taken upon completing the FMEA worksheet. The RPN column generally will identify the high-risk areas. The idea of performing FMEA is to eliminate or reduce known and potential failures before they reach the customers. Thus, a plan of action must be in place for the next task. Not all failures can be resolved during the product development cycle, thus prioritization of actions must be made within the design group.

  • Limitations of traditional FMEA

One definition of detection difficulty (D) is how well the organization controls the development process. Another definition relates to the detectability of a particular failure in the product when it is in the hands of the customer. The former asks “What is the chance of catching the problem before we give it to the customer?” The latter asks “What is the chance of the customer catching the problem before the problem results in a catastrophic failure?” (Palady, 1995) These differing definitions confuse

the

FMEA

users

when

one

tries

to

determine

detection

difficulty.

Are

we

trying

to

measure how easy it is to detect where a failure has occurred or when it has Or are we trying to measure how easy or difficult it is to prevent failures?

occurred?

Ordinal scale variables are used to rank-order industries such as, hotels, restaurants, and movies (Note that a 4 star hotel is not necessarily twice as good as a 2 star hotel). Ordinal values preserve rank in a group of items, but the distance between the values cannot be measured since a distance function does not exist. Thus, the product or sum of ordinal variables loses its rank since each parameter has different scales. The RPN is a product of 3 independent ordinal variables, it can indicate that

FMEA MANUAL By S. Rhee and C.M. Spencer

3/26

January 2009

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