Customers are placing increased demands on companies for high quality, reliable products.
The increasing capabilities and functionality of many products are making it more difficult
for manufacturers to maintain the quality and reliability. Traditionally, reliability has been
achieved through extensive testing and use of techniques such as probabilistic reliability modeling.
These are techniques done in the late stages of development. The challenge is to design in quality
and reliability early in the development cycle.
Failure Modes and Effects Analysis (FMEA) is methodology for analyzing potential reliability
problems early in the development cycle where it is easier to take actions to overcome these issues,
thereby enhancing reliability through design. FMEA is used to identify potential failure modes,
determine their effect on the operation of the product, and identify actions to mitigate the
failures. A crucial step is anticipating what might go wrong with a product. While anticipating
every failure mode is not possible, the development team should formulate as extensive a list
of potential failure modes as possible.
The early and consistent use of FMEAs in the design process allows the engineer to design
out failures and produce reliable, safe, and customer pleasing products. FMEAs also capture
historical information for use in future product improvement.
Types of FMEA's
There are several types of FMEAs, some are used much more often than others.
FMEAs should always be done whenever failures would mean potential harm or injury to the
user of the end item being designed. The types of FMEA are:
- System - focuses on global system functions
- Design - focuses on components and subsystems
- Process - focuses on manufacturing and assembly processes
- Service - focuses on service functions
- Software - focuses on software functions
Historically, engineers have done a good job of evaluating the functions and the form
of products and processes in the design phase. They have not always done so well at designing
in reliability and quality. Often the engineer uses safety factors as a way of making sure that the
design will work and protected the user against product or process failure. As described in
a recent article:
"A large safety factor does not necessarily translate into a reliable
product. Instead, it often leads to an overdesigned product with reliability problems."
Failure Analysis Beats Murphey's Law
, September 1993
FMEA's provide the engineer with a tool that can assist in providing reliable, safe, and
customer pleasing products and processes. Since FMEA help the engineer identify potential
product or process failures, they can use it to:
- Develop product or process requirements that minimize the likelihood of those failures.
- Evaluate the requirements obtained from the customer or other participants in the
design process to ensure that those requirements do not introduce potential failures.
- Identify design characteristics that contribute to failures and design them out of the
system or at least minimize the resulting effects.
- Develop methods and procedures to develop and test the product/process to ensure that the
failures have been successfully eliminated.
- Track and manage potential risks in the design. Tracking the risks contributes to the
development of corporate memory and the success of future products as well.
- Ensure that any failures that could occur will not injure or seriously impact the customer
of the product/process.
Benefits of FMEA
FMEA is designed to assist the engineer improve the quality and reliability of design.
Properly used the FMEA provides the engineer several benefits. Among others, these
- Improve product/process reliability and quality
- Increase customer satisfaction
- Early identification and elimination of potential product/process failure modes
- Prioritize product/process deficiencies
- Capture engineering/organization knowledge
- Emphasizes problem prevention
- Documents risk and actions taken to reduce risk
- Provide focus for improved testing and development
- Minimizes late changes and associated cost
- Catalyst for teamwork and idea exchange between functions
The FMEA is a living document. Throughout the product development cycle change and
updates are made to the product and process. These changes can and often do introduce
new failure modes. It is therefore important to review and/or update the FMEA when:
- A new product or process is being initiated (at the beginning of the cycle).
- Changes are made to the operating conditions the product or process is expected to function in.
- A change is made to either the product or process design. The product and process are inter-related. When the product design is changed the process is impacted and vice-versa.
- New regulations are instituted.
- Customer feedback indicates problems in the product or process.
The process for conducting an FMEA is straightforward. The basic steps are outlined below.
- Describe the product/process and its function. An understanding of the product or
process under consideration is important to have clearly articulated. This understanding
simplifies the process of analysis by helping the engineer identify those product/process
uses that fall within the intended function and which ones fall outside. It is important to
consider both intentional and unintentional uses since product failure often ends in litigation,
which can be costly and time consuming.
- Create a Block Diagram of the product or process. A block diagram of the product/process
should be developed. This diagram shows major components or process steps as blocks connected
together by lines that indicate how the components or steps are related. The diagram shows the
logical relationships of components and establishes a structure around which the FMEA can be
developed. Establish a Coding System to identify system elements. The block diagram should
always be included with the FMEA form.
- Complete the header on the FMEA Form worksheet:
Product/System, Subsys./Assy., Component, Design Lead, Prepared By,
Date, Revision (letter or number), and Revision Date. Modify these
headings as needed.
- Use the diagram prepared above to begin listing
items or functions. If items are components, list them in a logical
manner under their subsystem/assembly based on the block diagram.
- Identify Failure Modes. A failure mode is defined as the manner in which a component,
subsystem, system, process, etc. could potentially fail to meet the design intent. Examples
of potential failure modes include:
- Hydrogen embrittlement
- Electrical Short or Open
- Torque Fatigue
- A failure mode in one component can serve as the cause of a failure mode in another
component. Each failure should be listed in technical terms. Failure modes should be
listed for function of each component or process step. At this point the failure mode
should be identified whether or not the failure is likely to occur. Looking at similar
products or processes and the failures that have been documented for them is an excellent
- Describe the effects of those failure modes. For each failure mode identified the
engineer should determine what the ultimate effect will be. A failure effect is defined
as the result of a failure mode on the function of the product/process as perceived by the
customer. They should be described in terms of what the customer might see or experience
should the identified failure mode occur. Keep in mind the internal as well as the external
customer. Examples of failure effects include:
- Injury to the user
- Inoperability of the product or process
- Improper appearance of the product or process
- Degraded performance
Establish a numerical ranking for the severity of the effect. A common industry
standard scale uses 1 to represent no effect and 10 to indicate very severe with failure
affecting system operation and safety without warning. The intent of the ranking is to
help the analyst determine whether a failure would be a minor nuisance or a catastrophic
occurrence to the customer. This enables the engineer to prioritize the failures and address
the real big issues first.
- Identify the causes for each failure mode. A failure cause is defined as a design
weakness that may result in a failure. The potential causes for each failure mode should
be identified and documented. The causes should be listed in technical terms and not in
terms of symptoms. Examples of potential causes include:
- Improper torque applied
- Improper operating conditions
- Erroneous algorithms
- Improper alignment
- Excessive loading
- Excessive voltage
- Enter the Probability factor. A numerical weight
should be assigned to each cause that indicates how likely that cause is
(probability of the cause occuring). A common industry standard scale
uses 1 to represent not likely and 10 to indicate inevitable.
- Identify Current Controls (design or process). Current Controls (design or process)
are the mechanisms that prevent the cause of the failure mode from occurring or which
detect the failure before it reaches the Customer. The engineer should now identify
testing, analysis, monitoring, and other techniques that can or have been used on the
same or similar products/processes to detect failures. Each of these controls should
be assessed to determine how well it is expected to identify or detect failure modes.
After a new product or process has been in use previously undetected or unidentified
failure modes may appear. The FMEA should then be updated and plans made to address
those failures to eliminate them from the product/process.
- Determine the likelihood of Detection. Detection
is an assessment of the likelihood that the Current Controls (design and
process) will detect the Cause of the Failure Mode or the Failure Mode
itself, thus preventing it from reaching the Customer. Based on the
Current Controls, consider the likelihood of Detection using the
following table for guidance.
- Review Risk Priority Numbers (RPN). The Risk Priority Number is a mathematical
product of the numerical Severity, Probability, and Detection ratings:
RPN = (Severity) x (Probability) x (Detection)
The RPN is used to prioritize items than require additional quality planning
- Determine Recommended Action(s) to address
potential failures that have a high RPN. These actions could include
specific inspection, testing or quality procedures; selection of
different components or materials; de-rating; limiting environmental
stresses or operating range; redesign of the item to avoid the failure
mode; monitoring mechanisms; performing preventative maintenance; and
inclusion of back-up systems or redundancy.
- Assign Responsibility and a Target Completion
Date for these actions. This makes responsibility clear-cut and
- Indicate Actions Taken. After these actions have
been taken, re-assess the severity, probability and detection and review
the revised RPN's. Are any further actions required?
- Update the FMEA as the design or process changes, the assessment changes or new
information becomes known.