Six most common mistakes in FMEA analysis 

SIx most common mistakes in FMEA

We have collected the knowledge of our specialists about mistakes encountered while working on the FMEA analysis and we have compiled a list of the six most common mistakes. In the article below each error is described in detail and explains how not to make this mistake and how to do the FMEA (Failure Mode and Effects Analysis) analysis correctly. 

1st mistake
Entering into the PFMEA analysis as work elements/4M BOM materials (constituting part of the product). 

1st mistake

The premise of PFMEA is that the input materials are good.  This means that the materials which we get from the supplier comply with the requirements and are not considered a factor of mistake. 

But there is an exception! The incoming materials which will be part of our product we can write to FMEA as 4M if we have a bad experience with the supplier.

How not to make this mistake? 

The materials that can be entered into the work elements are only those that we use to manufacture the product, for its processing and their quality may effect on the occurrence of defects. Such materials are, for example, grease, coolant, etc.  

 

2nd mistake
Wrong defined phase functions.

2nd mistake

Often product requirements are described imprecisely, for example by using terms such as: good, correct, ok. 

How not to make this mistake? 

We define the functions of the phase precisely, as detailed as possible. The correct wording will be e.g. hole diameter 5mm, hole diameter according to the specification, width according to the drawing. The wording will also be correct: no scratches or no mechanical damage. 

 

3rd mistake
Several product characteristics for one phase function.
 

3rd

 

In a correct FMEA analysis, one product characteristic should be assigned to one phase function. If more than one product characteristic fits to given function, it means that the requirement is not described in sufficient detail. This mistake is results of the second mistake that is described above. 

How not to make this mistake? 

Remember to add only one product characteristic to one function. If more than one characteristic fits, back in the analysis to the level of the phase function and define it in more detail so that only one characteristic would fit.

For example: For the function “dimensions according to specification” I match the characteristics of the product: length and width. In this case, the functions should be broken down into two: length according to specification and width according to specification.

You can also use the appropriate software (e.g. PQ-FMEA), which makes it impossible to add several product characteristics to one function. 

 

4th mistake
Defining indirect causes.

4th mistake

Often the causes include too far-reaching failure causes (the so-called causes of causes), which do not constitute the first WHY. Doing that this way leads to that we are unable to properly prevent the real cause from which the failure directly arises. 

How not to make this mistake? 

Remember when you defining the causes take only int account immediate, primary causes their failure occurs.  

The causes must relate to 4M and constitute the first WHY. 

When defining the cause, we can use the functions of work elements (of course, as long as they are well defined), the cause of which is a contradiction. 

For example: 

  • Work element function – operator: drilling 2 holes. 
  • Incorrect cause: wrong instruction. 
  • Correct cause: operator has drilled more / less than 2 holes.

By correcting the instruction, we can “hit” only in one out of many sources. Although remember that this next WHY or Ishikawa diagram may be helpful in choosing the optimal solution to the problem. 

 

5th mistake
No distinction between failure mode control and failure cause control in the FMEA form.
 

5th mistake

This is a significant mistake, especially if we ignore that it is a detection. 

Failure mode controls affect all its causes, not one as it is with failure cause control. This results in defect checks being assigned to all causes within a given defect we control, and thus gives us the opportunity to lower the risk for all causes of a given fault.

How not to make this mistake? 

When defining the control, consider whether it is a failure cause control or a failure mode control and mark it in the form (this is of course our proposal, not a requirement) by entering before the control: 

  • FM (failure mode) as control related to the failure. 
  • FC (failure cause) as a control related to the cause. 

For example: “[FM] Visual inspection by operator (8)”. 

When the control involve defect, add this control to each cause within the controlled failure. 

The functionality of marking the failure mode control is also available in our PQ-FMEA program, in which, after entering a given control, you can mark that it is a failure mode control and then the program will automatically assign the control to all failure causes. 

 

6th mistake
Wrong defining influence on planned prevention and detection actions.
 

6th mistake

Another problem that occurs quite often is that after adding a planned prevention action (in step 6 – Optimization), not only the OCC value (occurrence), but also the DET value (detection) is lowered and after adding a detection action, it is not only lowered DET value but also OCC value.  

An attempt to protect against this mistake can be seen in the changes that have occurred in the latest AIAG & VDA manual. 

  • AIAG ed.4 – one field for planned preventive and detection actions. 
  • AIAG & VDA ed.1 – two separate fields for prevention and detection actions. 

How not to make this mistake? 

Planned prevention actions are actions to prevent the occurrence of a given cause, and if they are effective, they influence the occurrence (OCC) of this cause. 

Planned detection actions are actions detecting a given failure or cause, they are therefore assigned as cause (FC) or failure (FM) checks and have an impact on detection (DET). 

The exception is when OCC = 1 GIVES US DET = 1
This is only when OCC = 1 means Poka-Yoke, and situations where there is no possibility of a given cause, and thus it loses its control. 

Why control something that physically has no chance of rising? 

 

An example of a correctly performed analysis can be found https://pq-fmea.com/files/ in the analysis examples.