Manual for Life Cost Based FMEA
likelihood of detection by the relevant company design review and testing procedures program (see Table 4). Column A reflects the chance of catching a failure before it goes to the customers. Column B reflects the organization’s controls, systems and maturity level of its quality and reliability programs.
Table 4. Detection criteria [Palady, 1995]
Detection Difficulty Impossible to detect Remote detection Very slight detection Slight detection Low detection Medium detection Moderate detection Good detection High chance of detection Certain to detect
Criteria No known techniques available Only unproven or unreliable technique available Proving durability tests on products with system components installed Tests on product with prototypes with system components installed Tests on similar system components Tests on preproduction system components Tests on early prototype system elements Simulation and modeling in early stage Proven computer analysis available in early design stage Proven detection methods available in concept stage
Rating 10 9 8 7 6 5 4 3 2 1
Occurrence, Severity, and Detection ratings can be customized to the user’s needs and
type of industry.
2.9. Risk Priority Number (RPN)
We define the Risk Priority Number (RPN) as follows:
(Occurrence) × (Severity) × (Detection)
Larger RPNs indicate the need for corrective action or failure resolution. Give special attention to the effect and its causes when the severity rating is high regardless of the RPN. You should construct a Pareto chart (Crosby, 1969) of RPNs vs. causes or failure modes to clearly summarize the FMEA. Note that each discrete failure scenario (i.e. mode, cause, and effect) should have its own associated Occurrence, Severity and Detection values, and therefore a distinct RPN number.
Actions Recommended to Reduce RPN (see Table 1) is a list of corrective actions and failure resolutions. Recommendations could include, in the order of priority:
1) Design solutions to eliminate the failure mode or reduce its likelihood, including: functional redundancies and error proofing the assembly, installation and usage.
2) Actions to reduce the severity of the failure mode in terms of its impact on
the user, performance, and other systems 3) Developing means of detecting causes
of failure modes
manufacturing including: inspection, testing, and error proofing. 4) Tests to provide more information data to assess Probability and Severity
5) Providing diagnostics to easily identify the failure mode or cause during manufacturing or operation.
FMEA MANUAL By S. Rhee and C.M. Spencer