Design FMEA for a component - Should I make the following assumptions?

david316

Involved In Discussions
#1
Hello,

When doing a design FMEA for a component should you make the following assumptions?

-The component meets its specifications (e.g. for a plastic part its meets the dimensions and tolerances specified on the drawing)
-The component is suitably designed to meet its intended function.

For example, assuming we are looking at a simple plastic component of a system, would it be appropriate to state something like below as a cause of a failure mode
-Dimension x tolerances are to wide
-Dimension x is to small

Thanks
 
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Miner

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Staff member
Admin
#2
When doing a Design FMEA, you assume that the product will be manufactured exactly as designed. However, you may consider whether the dimension or the tolerance on the dimension was designed too small or too wide. For example, if a failure mode is bending or breaking, a potential cause might be that the dimension was designed too thin. Or, another failure mode might be the assembled dimension is too large, and the failure cause was the tolerance on a component was too wide allowing an assembly stackup too wide.

You may also consider whether the design will create manufacturability issues (e.g., mold draft is too shallow).
 

david316

Involved In Discussions
#3
Great. Thanks very much. That is helpful. Using your example, if we identified the design was to thin which would be a suitable risk control be:

-Design must be x thick
-Design must be of a thickness that will with stand a bend force of y over 100 cycles.

Thanks
 

Chennaiite

Never-say-die
Trusted Information Resource
#4
Great. Thanks very much. That is helpful. Using your example, if we identified the design was to thin which would be a suitable risk control be:

-Design must be x thick
-Design must be of a thickness that will with stand a bend force of y over 100 cycles.

Thanks
Suitable Risk control for "thin" design can be one or more of the following, in my imagination;

Preventive control :
Design calculation used to fix "thickness" specification, proven design in similar part/application, CAE analysis, "thickness" fixed based on technical Design Standard;
Detection Control :
Endurance testing, Load test or Any other tests to detect Bending or Breaking in the above example;
 

Miner

Forum Moderator
Staff member
Admin
#5
Chennaiite gave a very good response to your question. I'll just add a few more examples.

Preventative: Finite element analysis, Monte Carlo simulations, derating of components.
Detection: any type of physical testing, demonstration testing
 

david316

Involved In Discussions
#7
Actually, one other question because I am a little confused about how detection controls get used. Using the above example for a design FMEA, if I have one of the following

Detection Control :
Endurance testing, Load test or Any other tests to detect Bending or Breaking in the above example;

Does this mean that during development I conduct this test to see if the current design bends/breaks and if it does I fix the problem? It is doesn't bend/break do I assume the design is adequate? I'm a little confused on how you use a detection control as part of a design FMEA.
 

Chennaiite

Never-say-die
Trusted Information Resource
#8
Does this mean that during development I conduct this test to see if the current design bends/breaks and if it does I fix the problem? It is doesn't bend/break do I assume the design is adequate? I'm a little confused on how you use a detection control as part of a design FMEA.
Simple answer is - Yes.
Let me add some quick points for consideration :
If one or more of the Preventive controls can give you confidence in the design, it is your discretion to take a technical judgment to forgo physical testing. Example, Structural analysis can be performed in your PC using Finite Element Analysis tool. But correlation factor between the analysis tool and physical testing is the point of consideration;
Or If it is a proven design with minimal & insignificant Specification changes, may be physical testing is not required;
 
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