Creating a P-Diagram is an optional step when preparing a System or Subsystem FMEA. It is most useful when the item being analyzed is a complex system with many system interactions, operating conditions and design parameters as it helps the FMEA team to visually see these elements. While it can be a time-intensive step, it does provide great value in helping the FMEA team to understand and control the system and identify the outputs to the FMEA.
This topic discusses the typical elements in a P-Diagram and opportunities to share specific inputs and outputs with other relevant analyses (including the system hierarchy, FMEA analysis plans and the FMEA).
These are descriptions of the energy sources required for fulfilling the system functionality, such as speed, acceleration, input torque, etc. They can help the FMEA team to understand the nature of the system being analyzed and can use the same text as the items in the system hierarchy.
These are typically the system design parameters that the engineering team can change, such as a shaft diameter, stiffness, density, hardness, etc. and can be used to identify significant product characteristics. They can help the FMEA team to identify potential cause descriptions for the FMEA and can use the same text as the FMEA Causes.
These are things that can influence the design but that are not under the direct control of the engineer, such as those listed below. These factors, if not protected against, can make the design ineffective (i.e., the design is not robust against the expected noise factors). There are five types of noise factors:
The uncontrollable Piece to Piece variation in parts or manufacturing processes. They can use the same text as the Analysis Plan Assumptions, FMEA Causes and the PFD Worksheet Product Characteristics.
The Change Over Time, or the anticipated degradation of the system components or materials. They can help to identify the specific failure mode descriptions that are associated with the cause descriptions and can use the same text as the Conditions of Use and the FMEA Causes.
The Usage in the ways the customer uses the system, either intended or unintended. They can use the same text as the Conditions of Use.
The set of anticipated Environments that the system must operate within. They can use the same text as the Conditions of Use.
The System Interactions with other components.
These are the primary intended functional outputs of the system, such as output torque, speed, etc. They can be used an inputs to the function descriptions in the FMEA and can use the same text as the FMEA Functions.
These are any kind of inherent loss of energy transfer or other undesirable system outputs, such as exhaust gases, heat, vibration, leakage, unusual noise or bad odor. They can help the FMEA team as inputs to failure mode descriptions in the FMEA and can use the same text as the FMEA Failures.
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