Module 3 : Product Quality Improvement

Lecture 3 : What is Design FMEA?
 
Current Design Controls

In order to improve the occurrence rating for the particular failure mode, the design control must be employed. Current Design control indicates the state of control that will be able to detect the occurance of a failure or minimize the failure chances.

Detection (D)
This is a relative measure of assessment of the ability of the design control to detect either a potential cause/mechanism or the subsequent failure mode before the component goes to end/next user. Typically, in order to achieve a lower detection rating, design control must be improved. A reference for rating in detection phase is given in Table 3.3.   
Table 3.3 Rating of Likelihood of Detection in Design FMEA


Rankings of likelihood of dectection by Design Control for Design FMEA

Effect

Criteria : severity of Effect

Ranking

Absolutely Impossible  

Design control will not and / or cannot detected a potential cause / mechanism and subsequent failure mode : or there is no design control

10

Very remote

Very remote chance the design control will detected a potential cause /mechanism subsequent failure mode

9

Remote

Remote chance the design control will detect a potential cause / mechanism and subsequent failure mode.

8

Very low

Very low chance the design control will detect a potential cause /mechanism and subsequent and failure mode

 7

Low

Low chance the design control will detected a potential cause / mechanism and subsequent failure mode

6

Moderate

Moderate chance the design control will detect a potential cause / mechanism and subsequent failure  mode

5

Moderate highly

Moderately high chance the design control will detect a potential cause / mechanism and subsequent failure  mode

 4

High

High chance the design control will detect a potential cause / mechanism and subsequent failure  mode

3

Very High

Very high chance the design control will detect a potential cause /mechanism and subsequent failure  mode

 2

Almost
certain

Design control will almost certainly detect a potential cause / mechanism and subsequent failure  mode

 1

Risk Priority Number (RPN)

The Risk Priority Number is the product of the severity (S), occurrence (0), and detection (P) rankings. This product may be viewed as a relative measure of the design risk. Values for the RPN can range from 1 to 1000, with 1 being the smallest design risk possible. This value is then used to rank order the various causes of failure in the design. For causes with a relatively high RPN, the engineering team must make efforts to take corrective action to reduce the RPN. Any score above 50 may be considered as cutoff to eliminate/minimize the impact of a particular cause.  However, because a certain concern has a relatively low RPN (<50), the FMEA team should not overlook the concern and neglect an effort to reduce the RPN. This is especially true when the severity of a concern is high. In such case(s), a low RPN may be extremely misleading, not placing enough importance on a concern where the level of severity may be disastrous. In general, the purpose of the RPN is to rank the various causes on the record. However, every cause should be given full priority by the team, and the team should look for every method available to reduce the RPN.