How to conduct Failure Mode and Effect Analysis, FMEA
By Mahmoud Elhefnawi
What is FMEA?
The System FMEA is a team-oriented methodology for minimising the risk of the development and it stimulates the interdisciplinary co-operation between the affected areas already in a very early stage. Furthermore, it provides a documentation of knowledge within the company.
The Steps of Conducting of Process – FMEA Exercise:
Definition of system elements and system structure
The system consists of individual system elements (SE) that describe and subdivide the process layout. The system is organised in a tree diagram. This structure organises the individual SE’s into different hierarchical levels. Some SE’s are linked to others through interfaces. The functional structure of events is effectively described in this way. This way of structuring is important, especially for complex systems, to provide a good overview of the total system and to select the critical paths and interfaces which should be treated during the FMEA.
Function / Failure Analysis
The team considers all relevant system elements and the relevant potential failures, by means of the FMEA. The supposition is that a failure may, but should not, occur. Each unit under consideration may have one or more potential failures. Each failure, in turn, may lead to one or more effects.� Apart from that, each failure may result from one or several causes. All these aspects are recorded. Any existing or planned preventive/detection measures forming a normal part of the system are also recorded.
The next step is to record the estimated values of the occurrence probability “O”, the severity “S” and the probability of detection “D”, of each failure. The occurrence probability is estimated in view of the listed existing /planned prevention measures, and the detection probability is estimated in view of the listed existing/planned detection measures. The risk priority number (RPN) is the product (multiplication) of the three probabilities. The estimated O, S and D values range from 1 to 10, where 1 represents the smallest risk (probability) and 10 represents the highest risk (probability).
Determining actions to reduce the risks
Risks, which are unacceptably high, are recognised by the RPN values or the individual O, S and D values. You may define the RPN cut-off values over which an action is needed (150 will be fine). For these risky steps prevention and detection measures, responsible and due dates are determined.
The purpose of prevention and detection actions is:
- To reduce the severity of the failures (only possible through changing the concept or the design),
- To reduce the probability of occurrence of the failure causes (through preventive actions or design or process changes),
- To increase the probability of detection of the failures (through design or process changes, or improved failure detection methods).
Recording the FMEA structure and results
It is good idea to record the FMEA structure and results on an excel sheet,so it will be easy for further analysis.
After implementing all prevention and detection measures and in case that some process steps still have high RPNs (greater than the defined threshold ). Further improvement is required in order to eliminate the risk causes/effects exist in the process. This improvement is achieved through re-studying the risky process steps and setting new prevention and detection measures.