What does "Class" mean in an FMEA?

ISO_Man

Involved In Discussions
#12
I found the following description.

For Automotive applications the following applies:

Critical Characteristic is a product feature, dimension or note that reasonably anticipated variation could directly affect compliance with government regulations or safe operation of the equipment. Any of those characteristics that affect a failure mode with a severity of 9 or 10 must be reviewed to determine if they should become a critical characteristic.

Significant Characteristic is a feature, dimension, or note that reasonably anticipated variation could affect principal fit, function, durability, customer satisfaction, or manufacturability. Any of those characteristics that affect a failure mode with a severity of 5 to 8 and an occurrence of 4 or higher must be reviewed to determine if they should become a significant characteristic

So it's just a way of marking something that you've already categorized as a high severity? It seems a bit redundant to me.
 

GRP

Involved In Discussions
#13
So it's just a way of marking something that you've already categorized as a high severity? It seems a bit redundant to me.
If you are in the automotive industry, special characteristics are not redundant and are addressed in IATF16949.

The organization shall use a multidisciplinary approach to establish, document, and implement its process(es) to identify special characteristics, including those determined by the customer and the risk analysis performed by the organization, and shall include the following: ...


Also check the text below:

FMEA Mistake #7: Improper Use Of “Class” Column
The seventh mistake a company can make is improper use of the Class column. The Class column or Special Characteristic designation was created to identify issues that expose the company to unacceptable safety and/or financial risk. Because of the unacceptable risk level any line of the Process FMEA with a class symbol, assuming it has been properly determined, MUST be worked on.

This is from a company which sells software and services, which I do not intend to promote. But if you search the net for 10 FMEA mistakes you might find some interesting opinions.
 

AndyN

A problem shared...
Staff member
Super Moderator
#14
It seems a bit redundant to me.
Not really. Let's not forget that the PFMEA is simply a "stab in the dark" at what could go wrong. If there's been some history with similar products/processes (and who hasn't encountered THAT?) it's simply a way to indicate a specific failure mode and flag it for that category. It might SEEM redundant, but when these can run to pages, so why not assist in prioritization?
 

John Predmore

Involved In Discussions
#15
So it's just a way of marking something that you've already categorized as a high severity?
The CC and SC classes both identify failures arising from "reasonably anticipated variation". If the steering wheel falls off my car while I am driving, that would be a high severity failure mode, but the wheel falling off is hopefully not arising from reasonably anticipated variation. If you reasonably foresee steering wheels falling off due to manufacturing variation, then absolutely, you need to flag that scenario in your quality planning for special care, regardless of how low the Occurrence rating in the RPN is. The CC and SC class failure modes deserve special attention, such as 100% in-process inspection, SPC or Design for Six Sigma to address the reasonably anticipated variation.
 
Last edited:

GRP

Involved In Discussions
#16
The CC and SC classes both identify failures arising from "reasonably anticipated variation". .
Where does this "reasonably anticipated variation" come from? I think this must be specific to a certain organization.

In broad terms, special characteristics are a way of highlighting characteristics and ensuring a proper follow up is given throughout the development. Special characteristics must be addressed through drawings, risk analysis, control plan, WI. Each organization will have a certain documented process for identifying and implementing them (which can be a good or bad process, there seems to be no given criteria).

For the sake of clarity, I would add that CC and SC are symbols related to Ford, as far as I know. There are all sorts of symbols beyond CC and SC. It depends on who your client is.

Miner shared a definition of CC and SC above, which are types of special characteristics. Here is the general definition from Ford:

Special Characteristics are those product or process characteristics (CC, SC, OS, and HI) that affect vehicle or process safety, compliance with government regulations, customer satisfaction, or process operation. Special Characteristics require Quality Planning actions that must be addressed in a Control Plan.
 
#17
So it's just a way of marking something that you've already categorized as a high severity? It seems a bit redundant to me.
It goes further in the data stream: those characteristics must be shared with other design teams, with manufacturing and suppliers and they have to consider them in their DFMEAs or PFMEAs. In supplier's DFMEA case, when you audit/assess their DFMEA maturity, you have to be sure they have in place robust risks mitigation actions and they have cascaded these to their manufacturing or sub-suppliers. For manufacturing, the impact is even more serious: for critical characteristics they need to implement 100% in-process inspection means (meaning higher production costs and delay in the process) and for special characteristics, they need to have in place robust measuring methods and devices to check in batches those characteristics.
They might need to create side measurement benches or implement automatic check (which is not cheap), their process takt might increase with several seconds, etc.

E.g. for a car door, besides the geometrical features (dimensions, GD&T, flushness, etc) the colour is really important and has to match the colour of surrounding parts, and it is highlighted by the designer as critical characteristic - the supplier or manufacturing have to implement a 100% colour check (there are nowadays laser handhelds capable to check subtle differences in nuances).

In my experience, those characteristics are the main disagreement point during the assessing/agreement discussions with suppliers or manufacturing, because of the costs implications.
 

Top Bottom