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Manual for Life Cost Based FMEA

2.1. Function

Function is the specific behavior (e.g., hold vacuum) intended by the designers. Use the functional analysis to list major sub-functions or use customer requirements (Voice Of the Customers= VOC) in this column. Discrepancies from the intended function or requirement are the result of a failure in the system. We recommend organizing the FMEA worksheet first by listing the functions of the device in the leftmost column.

2.2. Potential Failure Mode

It describes the departure from the intended function or requirement. For function- based FMEA, interpret failure modes as a sub-function occurring improperly or not at all. Potential failure modes can be considered in any of the following four categories:

  • 1.

    No Function: There is a complete absence of the intended function.

  • 2.

    Partial/degraded function: The item does not meet some of the required


  • 3.

    Intermittent function: The item performs a function intermittently.

  • 4.

    Unintended function: Another function (behavior) is performed which was

unintended in the original design.

It is important to note that even if certain failures have never occurred for a similar device, it should be listed on the FMEA worksheet if the failure is physically possible.

2.3. Cause of Failure

It describes why the desired requirement fails. Consider the needed conditions for each customer requirement. Also, ask yourself “what has to happen for the function to occur properly?” Then, list the possible causes of failure due to components’ behaviors, usage conditions (especially regarding human interaction), operating environment, and interfaces with other systems.

2.4. Root Cause of Failure

To find the root cause of failure may, in many cases require detailed analysis of the failure mode. You may need to identify root causes by using by other techniques such as “Five Why’s” (ask why succeeding failures happen five times until a root cause is determined) or the Ishikawa Fishbone Root Cause Analysis (organize root causes around man, machine, material, method, measurement, environment) (Ishikawa, 1985).

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


January 2009

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