Failure mode: A statement of fact describing what would happen when a system, a part, or a process has potentially failed to meet the designer specification intent or performance requirements. The cause might be a design flaw or a change in the product that prevents it from functioning properly.
Effect: A description of what the end user will experience or notice. The users might be line operators, the next department to receive the parts, or the customers.
Cause: The reason why a failure occurred.
Severity(SEV): How significant is the impact of the effects to the customers (internal or external)?
Occurrence (OCC): How likely is the cause of the effect to occur?
Detection (DET): How likely will the current system detect the cause of the failure mode?
Risk priority number (RPN): A numerical calculation of the relative risk of a particular failure mode, obtained by multiplying the severity, occurrence and detection numbers of each failure listed in the FMEA chart.
Figure 1.8 Failure mode and effect analysis(FMEA) chart
All items with an RPN that exceeds 120 should be investigated first. An item that could cause a safety-related failure, a field recall, or one with a high customers requirement should be considered critical and dealt with promptly.
FMEA is an excellent tool for investigating potential failures m products or processes. It could lead directly to improving the design or manufacturing quality, especially when priori- lzing w lc parts or processes to work on first. Ideally, it should be used for all parts of the process，product, or system. In practice, a methodology such as QFD should be established to prioritize which elements are to be analyzed using FMEA.
FMEA is a good example of using tools to identify and prioritize quality problems in design and manufacturing. It is another tool to guide the enterprise on where to start quality improvements on the road to six sigma. Some of the benefits of FMEA projects are:
• Establish priorities as to which of the failure items should be improved first
• Identify potential failure modes for each item
• List the types, risks, and causes of failures, and the effects these failures might have
• Calculate a risk priority number, and then use the same number to benchmark improvement in design or manufacturing
• Encourage the planning of a proposed corrective action
• Establish an ordered list of current controls
• List completed quality actions and who performed them
• Document improvements to the process or design