Failure Mode and Effects Analysis, or FMEA, is a structured engineering method used to find possible failures before they happen. This cheat sheet helps students organize design or process risks, rank them, and choose actions that reduce the chance or impact of failure. It is useful for product design, manufacturing, robotics, electronics, and any system where safety, reliability, and quality matter.
The core of FMEA is identifying a failure mode, describing its effect, finding its cause, and rating the risk using severity, occurrence, and detection scores. The common risk priority number formula is RPN = Severity x Occurrence x Detection. High RPN values or high severity ratings guide engineers toward the most urgent corrective actions.
After improvements are made, the ratings should be updated to check whether risk was actually reduced.
Key Facts
- A failure mode is the specific way a part, process step, or system function could fail, such as cracks, jams, leaks, shorts, or incorrect assembly.
- An effect is the result of a failure mode on the user, system, product performance, safety, or next process step.
- A cause is the reason the failure mode could happen, such as weak material, poor alignment, overheating, software error, or operator mistake.
- Severity rates how serious the effect is, usually from 1 to 10, where 1 is minor and 10 is hazardous or system critical.
- Occurrence rates how likely the cause is to happen, usually from 1 to 10, where 1 is rare and 10 is very likely or frequent.
- Detection rates how likely current controls are to catch the problem before it reaches the user, usually from 1 to 10, where 1 is easy to detect and 10 is hard to detect.
- Risk Priority Number is calculated as RPN = Severity x Occurrence x Detection, so the maximum value on a 1 to 10 scale is 1000.
- Risk reduction actions should lower occurrence by preventing causes, lower detection by improving tests or inspections, or reduce severity by changing the design.
Vocabulary
- FMEA
- Failure Mode and Effects Analysis is a method for identifying possible failures, judging their risk, and planning actions to reduce that risk.
- Failure Mode
- A failure mode is the exact way a design feature, component, process step, or system function could fail.
- Effect
- An effect is what happens to the product, process, user, or system if the failure mode occurs.
- Cause
- A cause is the underlying reason a failure mode might happen.
- Current Controls
- Current controls are existing design features, inspections, tests, sensors, or procedures used to prevent or detect a failure.
- RPN
- Risk Priority Number is a score found by multiplying severity, occurrence, and detection to help prioritize risks.
Common Mistakes to Avoid
- Confusing failure mode with effect is wrong because the failure mode is how something fails, while the effect is what happens because of that failure.
- Giving a low severity score just because the failure is unlikely is wrong because severity measures impact only, not probability.
- Treating RPN as the only priority is wrong because a failure with very high severity may need action even if its RPN is not the highest.
- Writing vague actions like improve quality is wrong because an FMEA action should be specific, assigned to an owner, and connected to a cause or control.
- Forgetting to update ratings after corrective actions is wrong because the team cannot verify whether the action reduced occurrence, detection risk, or overall RPN.
Practice Questions
- 1 A motor mount failure has Severity = 8, Occurrence = 4, and Detection = 5. Calculate the RPN.
- 2 A process risk has RPN = 210 with Severity = 7 and Occurrence = 5. What is the Detection rating?
- 3 After adding a sensor, a team changes Detection from 8 to 3 while Severity = 9 and Occurrence = 2 stay the same. What was the old RPN and what is the new RPN?
- 4 A failure has Severity = 10 but a lower RPN than several minor failures. Explain why engineers may still choose to address the Severity = 10 failure first.