A failure that could have been foreseen
A new bracket goes into production. After a few weeks, the customer finds brackets where a press-fit bush has worked loose. The investigation finds that the bore diameter depends on the temperature of the die, which nobody measures. Looking back, the team agrees: everything needed to foresee this was known before the start of production. Nobody had asked the question systematically.
That systematic questioning is what an FMEA does.
What an FMEA is
Failure Mode and Effect Analysis is a team method for working through a design or a process and asking, element by element: how could this fail, what would happen if it did, and why might it happen? It is done during product and process development, when changes are still cheap.
The main types are:
- Design FMEA (DFMEA): failures of the product design, for example a part that is too weak or a tolerance that does not ensure function.
- Process FMEA (PFMEA): failures in manufacturing and assembly, for example a wrong torque or a missing part.
- FMEA for monitoring and system response (FMEA-MSR): introduced in the AIAG & VDA handbook for systems that detect faults in the field and react, such as vehicle diagnostics.
The chain: failure mode, effect, cause
At the heart of an FMEA is a chain of three elements:
- Failure mode: how the product or process can fail. The bush is loose in the bore.
- Effect: what the customer or the next step notices. The bracket rattles and wears; in the worst case the bush falls out.
- Cause: why the failure occurs. Bore oversize due to die temperature.
Each element is rated on a scale from 1 to 10:
- Severity (S) of the effect
- Occurrence (O) of the cause, given the current prevention controls
- Detection (D): how likely the current detection controls are to find the cause or the failure before the part leaves
From ratings to priorities
Traditionally, the three ratings were multiplied into a Risk Priority Number, RPN = S × O × D, and the highest RPNs were treated first. The problem is that very different risks can share the same number: a severity of 10 with low occurrence can have the same RPN as a minor nuisance that happens often.
The AIAG & VDA FMEA Handbook (1st edition, 2019) replaces this with Action Priority (AP): a table that assigns each combination of S, O and D a priority of high, medium or low, giving most weight to severity, then occurrence, then detection.
Example. For the loose bush: S = 8 (loss of function), O = 6 (the cause occurs from time to time), D = 7 (no measurement of bore diameter). RPN = 8 × 6 × 7 = 336. With high severity and moderate occurrence, Action Priority would also be high: action is required. The ratings are illustrative.
The seven steps
The AIAG & VDA handbook organises the work in seven steps:
- Planning and preparation
- Structure analysis
- Function analysis
- Failure analysis
- Risk analysis
- Optimisation
- Results documentation
Steps 2 and 3 break the system into elements and describe what each element must do. Step 4 builds the failure chains, step 5 rates them, and step 6 defines and follows up the actions.
From FMEA to production
The analysis is only useful if it changes something. The preferred actions reduce occurrence, by changing the design or the process. Where that is not possible, detection is improved. In a PFMEA, the controls end up in the control plan: which characteristic is checked, how, with which gauge, how often, and what to do if it fails.
In the example, the action could be to control die temperature and to measure the bore on a sample of parts each shift, plotted on a control chart. The gauge used for that check should itself be verified with a measurement system analysis. In this way the FMEA becomes part of quality assurance rather than a document filed for the customer.
Common pitfalls
- Done too late. An FMEA written after the design is frozen documents risks instead of reducing them.
- Done alone. The method depends on a cross-functional team with design, production and quality knowledge.
- Never updated. Complaints, scrap and process changes should flow back into the analysis. An FMEA is a living document.
- Ratings negotiated down. Lowering a detection rating without a real new control only hides the risk.
In short
An FMEA asks what can go wrong before it does. It links failures to effects and causes, rates the risks, and turns the highest priorities into actions and controls that are actually used on the shop floor.
