Design (DFMEA) and Process (PFMEA) FMEA Failure Mode Identification

[YKT]

I've read somewhere that to identify the potential failure modes for FMEA (either design or process), one has to look at the following 4:
1) No function
2) partial function
3) intermittent function
4) unintended function

Can someone share the idea of how to identifying a unintended function in a process FMEA or a Design FMEA ?


Thanks

[Randy Stewart]

YKT,
These are generic statements to get you thinking.
Unintended function IMO is based on the related operations/function of the part. Lets say the process is the wiring of the electric window operating switch. One of the failure result or unintended function is that the door locks unlock due to improper wiring.

[YKT]

Thanks. Let me try this.
If I'm looking at painting process, which the process function is to apply a coat of paint evenly and cover the whole surface of the product, then

the no function = no paint at all
partial function = paint covering the whole body, but not even (smooth)
intermittent function = some patches are not cover by paint
unintended function = operator safety, i.e. painting operation release toxic fume, which endanger the operator life.

Am I right here ?

[Randy Stewart]

Don't place too much emphasis on verbiage YKT. If I was looking at the paint process I would probably break it down based on the desired output and amount of rework required to achieve the desired output:

Complete Failure -- no paint, all paint expended in the middle of the operation, uneven application (requires item to be striped and repainted) etc. (we have to address a similar situation with mastic being applied during door hemming operations).

Partial Failure -- Minor touch ups required (may be due to handling, etc.), drips, small areas of uneven application, etc.

You may also want to add robotic failures too if they're applicable.

I don't think I would address an "unintended function" in this operation unless you end up painting a wall of the booth instead of the part.
With the safety aspects, don't forget that the assumptions of an FMEA include that "ALL" equipment and material are present and correct. So you can include operator required safety equipment in setup.
We do have the responsibility to ensure the process is designed to keep the operator safe, so all equipment, toxicology review, MSDS reviews need to be completed up front. IMO they do not need to be addressed in the FMEA.

[Howard Atkins]

Here is the offical Ford definition


"Four types of Failure Modes occur. The first and second types apply often and are the most commonly seen and the third and fourth types are typically missed when performing the FMEA.

1. No Function: Process operation is totally non-functional or
inoperative.

2. Partial/Over Function/Degraded Over Time: Degraded
performance. Meets some of the specifications or some
combination of the specifications but does not fully comply with all
attributes or characteristics. This category includes over function.
A degraded function over time is not generally a Failure Mode type
in a Process FMEA.

3. Intermittent Function: Complies but loses some functionality or
becomes inoperative often due to external impacts such as
temperature, moisture and environmental. This Failure Mode
provides the condition of: on, suddenly off, recovered to on again
function or starts/stops/starts again series of events.

4. Unintended Function: This means that the interaction of several
elements whose independent performance is correct, adversely
impacts the product or process when synergy exists. Their
combination performance leads to an undesirable performance
and hence “unintended function.”
Each Failure Mode must have an associated function. A good check to discover “hidden” functions is to match all possible failures with the appropriate functions.

[howste]

Howard, just curious - where did you get the official Ford definitions from?

[Howard Atkins]

From the Ford FMEA training book.