Use of FMEA in design
Contenido
AMFE puede ofrecer un enfoque analítico al gestionar los modos de fallos potenciales y sus causas asociadas. Al tener en cuenta posibles fallos en el diseño de seguridad, coste, rendimiento, calidad o resistencia, un ingeniero puede obtener una gran cantidad de información sobre como alterar los procesos de fabricación para evitar estos fallos.
AMFE otorga una herramienta sencilla para determinar qué riesgo es el más importante, y por lo tanto qué acción es necesaria para prevenir el problema antes de que ocurra. El desarrollo de estas especificaciones asegura que el producto cumplirá los requisitos definidos.
Previous work
The process for conducting an FMEA is linear. It is developed in three main phases in which the appropriate actions must be defined. But before starting an FMEA it is important to complete some preliminary work to ensure what information about the resistance and history of the product is included in the analysis.
A resistance analysis can be obtained through an interface of matrices, limit diagrams and parameter diagrams. Many failures are due to interaction with other systems and parts, since engineers generally tend to focus only on what they directly control.
To begin, it is necessary to describe the system and its function, since a good understanding of it simplifies its analysis. In this way an engineer can check which uses of the system are suitable and which are not. It is important to consider both intended and unintended uses. Unintended uses are a type of hostile environment.
Next, a block diagram of the system must be created. This diagram offers an overview of the main components or steps in the process, and how these are related to each other. This is called logical relationships, around which an FMEA can be developed. Creating a coding system to identify the different parts or processes is highly recommended and useful. The block diagram must always be included with the FMEA.
Before starting the FMEA, a worksheet must be created with the requirements and containing important information about the system such as revision date or name of the components. In this worksheet all items or functions or the title must be listed in a logical way, based on block diagrams.
Step 1: Gravity
Determine all failure modes based on functional requirements and their effects. Examples of failure modes are: electrical short circuits, corrosion or deformation.
It is important to note that a failure in one component can lead to a failure in another component. The failure mode should be listed in technical terms and by function. Thus, the final effect of each failure mode must be taken into account. A failure effect is defined as the result of a failure mode in the system function perceived by the user. Therefore it is necessary to record in writing these effects as the user will see or experience them. Examples of failure effects are: poor performance, noise, and damage to a user. Each effect receives a severity number (S) ranging from 1 (no danger) to 10 (critical). These numbers will help engineers prioritize failure modes and their effects. If the severity of an effect is a 9 or 10, consideration should be given to changing the design by removing the failure mode or protecting the user from its effect. A grade 9 or 10 is reserved for those effects that would cause harm to the user.
Step 2: Occurrence or Frequency
In this step it is necessary to observe the cause of the failure and determine how frequently it occurs. This can be achieved by observing similar products or processes and documenting their failures. The cause of a failure is seen as a weak point in the design. All potential failure mode causes must be identified and documented using technical terminology. Examples of causes are: erroneous algorithms, excessive voltage, or improper operating conditions.
A failure mode receives a probability number (O) that can range from 1 to 10. Actions must be taken if the incidence is high (>4 for non-safety-related failures and >1 when the severity number in step 1 is 9 or 10). This step is known as the detailed development of the FMEA process. The incidence can also be defined as a percentage. If a non-security issue has an incidence of less than 1% it can be given a figure of 1; depending on the product and user specifications.
Step 3: Detection Capability (Reverse)
When the appropriate actions have been determined, it is necessary to check their efficiency and perform a design verification. The appropriate inspection method must be selected. First, an engineer must observe the current system controls that prevent failure modes or detect them before they reach the consumer.
Subsequently, testing, analysis and monitoring techniques that have been used in similar systems to detect failures must be identified.
From these controls, an engineer can learn how likely faults are to occur and how to detect them. Each combination of the previous two steps receives a detection number (D). This number represents the ability of planned tests and inspections to eliminate defects and detect failure modes.
After these three basic steps, the risk priority numbers known as (RPN) are calculated.
Risk priority numbers
Risk priority numbers are not an important part of the selection criteria for a failure mode action plan. They are rather a helpful parameter in the evaluation of these actions.
After evaluating the severity, incidence and detectability the risk priority numbers can be calculated by multiplying these three numbers: NPR = S x O x D.
This must be done for the entire process or design. Once it is calculated, it is easy to determine the areas that should be of greatest concern. Failure modes that have a higher risk priority number should be those that receive the highest priority for developing corrective actions. This means that it is not always the failure modes with the highest severity numbers that need to be fixed first. There may be less serious failures, but they occur more often and are less detectable.
After assigning these values, a series of actions with an objective are recommended, responsibilities are distributed and implementation dates are defined. These actions may include specific inspections, testing, quality tests, redesign, etc. After actions are implemented in the design or process, the risk priority number should be checked again to confirm improvements. These tests are usually represented graphically for easy visualization. Whenever changes are made to a process or design, the FMEA must be updated.
Some obvious but important points should be noted:.
Note: You cannot "Minimize failure severity" since severity measures the severity of the effect (a fact). For example, if the effect of a failure is "possible death of a user", the severity is "10" - whether or not the frequency of the failure is minimized.