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Special Characteristics

by Dr Uwe Jarosch (Benteler Automobiltechnik GmbH)

The burden with Special Characteristics in the FMEA

How does FMEA help with specification of Special Characteristics? A proposed solution.

FMEAs describe technology. Special Characteristics (SC) are required in some industries, to achieve technological breakthroughs among other things. This article presents how the FMEA tool helps in the specification of SC with 3 additional rules.

Problem definition

Conflict area: IATF 16949 requires Special Characteristics (SC) and their pursuit in the course of risk management = FMEA

VDA+AIAG banish the SC from the design FMEA.

Some companies derive SC from the FMEA, but it does not work.

Then what seems to be the solution?

  • What is so special about the SC?
  • Technical standards as deciding criteria
  • Criteria for filter and FM specifications
  • Quality grades

Integration in FMEA templates.

Target setting

Clarification: What is so special about the Special Characteristics and how can the FMEA tool help in the determination?

Differentiation between risk declaration and the specification of technological characteristics.

Restriction of quality requirements to the technological necessity.

Approach

The customer provides the criticality.

Technological knowledge specifies focal points = main characteristics in the network of correlations.

The FMEA logic is supplemented by rules, from which then project related specifications for SC can follow.

The company’s experiences determine how high must be the quality requirement for SC.

Example

In a chassis component, the OEM and supplier agree that this welded joint is critical = SC-S or CC.

The welded joint in itself is a design element whose properties are described by approx. 20 product characteristics and possible error patterns. A similar number of process characteristics are involved for manufacturing this welded joint.

All 40 characteristics have a significance B = 10, since they contribute in the fault tree.
All characteristics have some influence on the result.

But which characteristics have an influence that justifies in designating them as Special Characteristics?

Engineering knowledge tells us that there are main characteristics, which when protected in the process, protect the main product characteristics and hence also the fulfilment of the function in total. -> Technologists specify the product and process characteristics that are the main characteristics.
If there are uncertainties, either “some more” can be selected or – even better – a statistical analysis can be made with the facts and figures, which are essential and yet correlated contributions. The limit values for this are a part of the company know-how just like the determination procedure. Thus, a basis for argumentation, which is also transferable, is created in the company.

3 rules describe the logic, with which a main characteristic becomes a special characteristic:

  1. A characteristic must be a main characteristic to be able to become a SC.
  2. The minimum values of significance are attained and interconnected in the fault tree of FMEA or the FTA.
  3. A Special Characteristic is prescribed at the next higher level.

The 3 rules shall make CC characteristics in the D-FMEA from the main technological characteristics for this application. The same 3 rules continue the chain in the P-FMEA to make the main characteristics of the process there into Special Characteristics in relation to the application.

The same technological correlations apply for another welded seam. Since this welded seam is not required as SC, there are no SCs here.

Process characteristics can influence several product characteristics. If none of the product characteristics is a main characteristic, these process characteristics would also not become an SC.
However, if only one of the influenced process characteristics is a main characteristic, then there will additionally be SCs even for the process characteristics.

Now the question of test requirements for these characteristics must still be asked.

The customer, just like the supplier, wants this welded joint to function safely over its service life.

Typical capabilities or 100% tests will – at least for most of the product characteristics of the welded seam - not be available. And will also not be seriously required. So, how good should be the characteristics to safely fulfil the functional expectations?

Therefore, a 5 step system of quality classes was developed, which reaches from zero error quality (= Q1) to the “normal” capability requirement cpk≥1.33 = Q2 to the occasional proof of the property (Q5).

For most of the main characteristics within a welded seam, destructive tests are conducted regularly, which are prepared as cuttings and measured. The dynamic of the welding process however inevitably leads to local fluctuations so the compliance of the specified tolerance generally satisfies all the concerned parties. It would correspond to an attributive analysis in regular sampling = Q4.

Result

Technological knowledge decides which characteristic becomes a Special Characteristic.

The prerequisite for a SC is that it should have a particularly strong influence in the effect chain – the process characteristic on the product characteristic, this in turn on the function of the product.

Only as many Special Characteristics as necessary and yet, as few as possible are specified. This gains acceptance.

The FMEA as a quality tool supports in capturing these correlations and in linking them using the error network.

Technological knowledge from templates should be used. Apart from the cause-effect relationships of the network, it should also show the main characteristics.

Software can apply or review the 3 rules in this network.

Ideally, the technological knowledge of the template FMEA prescribes not only the main characteristics, but also the typical quality requirements for these main characteristics. In this way, the test planning is kept as extensive as necessary and as simple as possible.

And thus, the FMEA also regains another bit of significance and reputation, as is desired by all those responsible for FMEA.

 

For more information on "Besondere Merkmale”, kindly refer to 14. FMEA Forum in Osnabrück on 27./28.03.2019.