DESIGN FAILURE MODES
AND EFFECTS ANALYSIS
(DFMEA)
PRESENTED BY
Denis J. Devos BA, BESc, MEng, PEng
Devos Associates Inc.
University of Western Ontario
October 17 2012
Devos Associates
Advisors to the Automotive Industry
1
Setting the Stage
• Design FMEA
Effects and Severity
Causes and Probability
Controls and Effectiveness
• Automotive S,O,D guidelines
• Special Characteristics
Devos Associates
Advisors to the Automotive Industry
2
Introduction
•
In today’s competitive environment, quality goods
and services is not enough
•
We must anticipate failure and nonconformance,
and prevent these events before they occur.
Studies of vehicle warrantee
campaigns have shown that
more disciplined use of the
DFMEA would have prevented
many of these campaigns
Devos Associates
Advisors to the Automotive Industry
3
Lets Recap
Potential Failure Mode
Potential Effects of the Failure
Severity
Potential Causes/Mechanisms
of the Failure
Occurrence
Current Design Controls
Prevention, Detection
Detection
Where S, O, D, numbers are unknown,
estimate high".
(also called “fear of the unknown!”)
Devos Associates
Advisors to the Automotive Industry
4
Start the FMEA early
• As soon as concepts are developed
• Start from an existing DFMEA, and focus on
differences
• Continually update as the design evolves
• Completed before drawings are released
DFMEA becomes an
input to Process FMEA
Devos Associates
Advisors to the Automotive Industry
5
Define the
Subsystem
Reliability
Tools/Analysis
Design
Testing
RISK
Assessment
Devos Associates
Advisors to the Automotive Industry
6
What is an FMEA?
• A tool to focus discussion within a team
• A graphical approach to collecting data
• A logical flow:
Failure
Effect
Cause
Control
• A repository for corporate learning about a
process throughout the life cycle
Devos Associates
Advisors to the Automotive Industry
7
The FMEA Team
The lead design engineer is responsible to
assemble a team from all disciplines (often a Core
Team and Support Team):
•
•
•
•
•
Design engineers
Process engineers
Testing engineers
Purchasing
Suppliers
•
•
•
•
•
QA
Sales
Field Service
Maintenance
Customers
The DFMEA is a catalyst to promote discussion
and exchange of ideas among all stakeholders
Devos Associates
Advisors to the Automotive Industry
8
Customer of the FMEA
Design FMEA
• Design engineers, other system design teams
• Manufacturing Process Engineers
Process FMEA
• End User
• Downstream manufacturing operations
• Service and maintenance operations
Devos Associates
Advisors to the Automotive Industry
9
Consider a Refrigerator as a System
System – Refrigerator
Subsystem
• Compressor Subsystem
• Insulation
• Door Subsystem
• Ice-Maker Subsystem
Components (ice maker)
• Plastic bucket in the door
• Plastic tube
• Plastic push handle
Devos Associates
Advisors to the Automotive Industry
10
Column 1 – Process Function,
Requirements
• Part Number, name and description of the
•
process step
Include the function of the item which
meets the design intent. Include
information relating to the environment
where the item operates
Devos Associates
Advisors to the Automotive Industry
11
Column 1 – Process Function, Reqt’s
• Describe in terms that can be measured
• Answer the question “what is this item
supposed to do?”
• Functions:
• Measureable, can be verified/validated
• Includes additional constraints or design
parameters such as reliability specs,
servicing specs, weight, size, location
Devos Associates
Advisors to the Automotive Industry
12
Process Function Requirements
• Functions:
• Describe functional intent or engineering
requirement
• Representation of all wants, needs,
requirements, spoken and unspoken, for
all customers and systems.
Remember: Functions cannot
FAIL if they do not have
measurable specifications
Devos Associates
Advisors to the Automotive Industry
13
Process Function Requirements
• Describe in terms that can be measured
• Answer the question “what is this item
supposed to do?”
• Written as Verb-Noun-Measureable
“Store fluid, six litres, with zero leaks”
“Conduct current, 10 amps”
“Open door with 100 Newtons of effort”
Devos Associates
Advisors to the Automotive Industry
14
Process Function Requirements
• (Gasket) Seal the door flush with zero leaks
• (Door Stop) Stop the door at 120° open
• (Levelling Feet) Level the Refrigerator to a
front-to-back differential of 2 inches and a
side-to-side differential of 1 inch.
• (compressor) Cool the compartment to 1° C
fully packed with 1.5 Litre water bottles in an
ambient of 45° C, 80% RH continuously for 10
years.
Devos Associates
Advisors to the Automotive Industry
15
Column 2 – Potential Failure Mode
• How could this component, subsystem fail
•
•
•
to meet the design intent?
Describe the potential nonconformances
only for that function.
A mode here can be the cause of a
downstream process error, or the result of
an upstream process error
Potential failure modes can occur only
under certain operating conditions, and/or
certain customer situations
Devos Associates
Advisors to the Automotive Industry
16
Column 2 – Potential Failure Mode
• How can the process fail to meet the intent
of the “Functions” column?
• Typical Examples
•
•
•
•
No Function
Partial/Over Function/Degraded over time
Intermittent Function
Unintended Function
Devos Associates
Advisors to the Automotive Industry
17
Example
Jack for changing a flat tire:
Raise vehicle to X feet above the ground, within Y
minutes, using Z maximum force, under all weather
conditions
No Function
Does not raise the vehicle
Partial/Over/Degraded
Does not raise to X, takes longer
than Y, uses more than Z force,
Intermittent function
Inoperable in the rain
Unintended function
None Known
Devos Associates
Advisors to the Automotive Industry
18
Example
Windshield Wipers:
Return to the rest position and remain after being
switched off, within ± 5mm from the rest position
measured from the mid-point of the blade
No Function
Switch doesn’t turn off the wiper
Wipers don’t remain in rest position
Partial/Over/Degraded
Wipers rest off-location
Wipers drift their position over time
Intermittent function
Wipers out of position below 0°C
Unintended function
Wipers turn off when actuating turn signal
Devos Associates
Advisors to the Automotive Industry
19
Example
Refrigerator Light:
Light turns off when the door is closed, for 10 years
No Function
Door doesn’t engage the switch
Switch does not turn off the light
Partial/Over/Degraded
Switch wears out before 10 years
Door partially engages the switch
Intermittent function
Switch intermittently turns off the light
Unintended function
Bulb overheats, melts the plastic cover
Devos Associates
Advisors to the Automotive Industry
20
Column 3– Potential Effects of Failure
• Describe the failure as perceived by the customer
• State if the failure impacts on safety or product
•
regulations
Express the effects in terms of the specific
component or sub-system being analyzed
A part can fracture, causing the
assembly to vibrate, resulting in
intermittent operation. This could
cause degradation over time and
premature failure.
Devos Associates
Advisors to the Automotive Industry
21
Potential Effects of Failure
Ask: “If this failure mode happens, what will be the
effect onF.”
•
•
•
•
•
•
Operation, function, of the items sub-components?
Operation, function, of the assembly
Operation, function, of the sub-system or system
Operation, safety, drive-ability of the vehicle
What will the customer see and experience?
Compliance with government regulations?
Devos Associates
Advisors to the Automotive Industry
22
Potential Effects of the Failure
•
•
•
Effect on the (internal or external) customer.
Assess if the failure could result in
noncompliance with safety, other regulations
Remember relationships – a failure mode
could negatively impact subsequent
operations
Describe these modes
in terms of what the
customer perceives.
Devos Associates
Advisors to the Automotive Industry
23
Sentencing Technique
leads to
FAILURE
MODE
CAUSE
could
result in
EFFECT
due to
Remember: Relate cause back to
failure mode, not back to effect
Devos Associates
Advisors to the Automotive Industry
24
Example – Cold Air Leaking from the Door
Q: What could “cold air leaking from the door” result in?
A: Increased energy consumption (effect)
Q: What could “cold air leaking from the door” be due to?
A: A kink or twist in the door seal
“Cold air leaking from the door” can result in “increased
energy consumption” (effect)
“Cold air leaking from the door” can be due to “a kink or
twist in the door seal” (cause)
Devos Associates
Advisors to the Automotive Industry
25
Column 4 – Severity of the Failure
• How serious is the effect of the failure.
• Use a 1 to 10 scale
• Use the voice of the customer to get as
•
•
accurate an assessment as possible
Design teams should customize its definitions
of severity to best suit their needs, as long as
consistency is achieved
Severity can only be reduced through design
changes. (process redesign is rare)
Devos Associates
Advisors to the Automotive Industry
26
Column 5 - Classification
• Used to identify Special Characteristics which
may require additional design controls and
validation testing, (and manufacturing controls)
• When Severity is 9 or 10, a potential Critical
Characteristic exists. Enter “pCC” here
• Used to communicate Team assessment to the
Product Designers for additional consideration
and inclusion in design documentation
Devos Associates
Advisors to the Automotive Industry
27
Column 6 -Potential Causes of Failure
•
•
•
•
•
What could cause the failure mode?
What circumstances could cause the failure?
How could the item fail to meet specifications?
How could the item not deliver its function?
How could item interactions be incompatible,
mismatched or unsynchronized?
Devos Associates
Advisors to the Automotive Industry
28
Design Deficiency 1: The product is
manufactured properly, but poor design
•
•
•
•
•
•
•
Material specification unsuitable for application
Material hardness specified too low
Specified lubricant too thick
Actual stresses higher than design loads
Specified torque is too low, too high
Inadequate design life assumption
Excessive heat, vibration, noise
Devos Associates
Advisors to the Automotive Industry
29
Design Deficiency 2: The product design
leads to poor manufacturing
•
•
Is orientation, alignment important to function?
•
Are engineering tolerances compatible with
manufacturing capabilities?
Can components be assembled upside-down
or backwards?
Design-for-Assembly
Design-for-Manufacturability
Devos Associates
Advisors to the Automotive Industry
30
Design Deficiency 2: The product design
leads to poor manufacturing
Examples:
•
Specifying heat-treatment such that some lots
of steel cannot be properly machined
•
Symmetrical Design that allows a part to be
installed backwards or upside down
•
Wrong fastener used because the design is
too similar to a more standard fastener
Devos Associates
Advisors to the Automotive Industry
31
Column 7 – Probability of Occurrence
•
What is the service history, field experience with
similar components, subsystems, systems?
•
Is the component a carry-over or similar to a past
design?
•
•
•
•
Is the part very new or different from the past?
Has the application of the part changed?
Are there environmental, customer use changes?
Have reliability studies been used to estimate the
expected failure rates for the application?
Devos Associates
Advisors to the Automotive Industry
32
Overlaying Probabilities
Often, a failure event does not necessarily cause
the failure mode.
Example: leaving the refrigerator door open may
lead to the overheating of the compressor, if it’s a
hot summer day (more than 30°C)
Two Probabilities: Leave the door open – 1%
It happens to be a hot summer day – 10%
What is the probability of the “failure mode”?
It is usually simpler to assume 1-to-1 correlation
Devos Associates
Advisors to the Automotive Industry
33
Column 7 – Probability of Occurrence
•
•
Use a consistent ranking chart
•
Be careful assigning numbers 1,2,3. The
difference between 1 and 3 is a factor of 50.
•
When Severity is between 5-8, and
Occurrence is between 4-8, a potential
Significant Characteristic exists. Use “pSC”
The group decides the number, or rounds up
to the next higher number.
Devos Associates
Advisors to the Automotive Industry
34
Column 8-9 - Current Design Controls
Prevention Design Controls
•
Product Design features that prevent the
Cause/Mechanism or Failure Mode from
occurring or reduce the rate of Occurrence
Detection Design Controls
•
Product Testing and analysis to qualify a
product before it is released to production
Devos Associates
Advisors to the Automotive Industry
35
Column 8 – Prevention Design Controls
Controls to prevent the failure, or lessen the
probability of occurrence
• Asymmetry in part design features
• Special grades of raw materials, etc.
• Redundancy
• Vibration dampening materials
• Lubrication, feedback control systems
• One-part designs, combined functions
• Design Standards/Guides, Best Practices
Devos Associates
Advisors to the Automotive Industry
36
Column 9 – Detection Design Controls
Controls to detect the failure during testing (DV
controls), or mitigate the failure if it occurs in use
• Design Reviews
• Reliability Analysis
• Simulation
• Lab Testing, Accelerated Life Testing
• Field Testing
• Failsafe designs – pressure relief valves
• Feedback, faults, alarms
Devos Associates
Advisors to the Automotive Industry
37
Watch Out!
If a potential cause is overlooked, a
product with a design deficiency may
go into production!
Look at all Failure Modes as a way of
detecting overlooked causes
Devos Associates
Advisors to the Automotive Industry
38
Column 10 – Effectiveness of Detection
Consider the effectiveness of analyses and tests:
• Design Analysis methods
• Simulation, FEA
• Tolerance stack-up studies
• Material Studies
• Team design reviews
• Design Development Tests
• Experiments, Lab Testing
• Prototype Testing
• Durability, life cycle tests
Devos Associates
Advisors to the Automotive Industry
39
(continued)
• Experience with similar designs
• Number of samples to be tested
• Statistically significant samples sizes
• One prototype, because of cost and complexity
• Timeliness of testing & evaluation
• Early in the concept stage
• At prototype stage
• Just prior to engineering
design freeze
Devos Associates
Advisors to the Automotive Industry
40
Lets Recap
Potential Failure Mode
Potential Effects of the Failure
Severity
Potential Causes/Mechanisms
of the Failure
Occurrence
Current Design Controls
Prevention, Detection
Detection
Where S, O, D, numbers are unknown,
estimate high".
(also called “fear of the unknown!”)
Devos Associates
Advisors to the Automotive Industry
41
Column 13 – Responsibility & Timing
•
It is important that recommended actions
always be expressed as specific tasks, and
assigned to individuals with a specific time line.
• The Design Team Leader or Product
Engineer
must ensure that all actions have been
implemented and properly addressed.
•
Reconvene the team, and recalculate the new
RPN resultant from the process improvements.
Devos Associates
Advisors to the Automotive Industry
42
FMEA Follow-Up
•
Team leader should ensure that all Recommended
Actions are properly assigned, executed, and
closed-out.
•
After completion, FMEA should be compared to
objectives for process, product, and organization.
•
Results can be incorporated with APQP team sign
off, or Management Review.
Devos Associates
Advisors to the Automotive Industry
43
Appendix:
Special Characteristics
Devos Associates
Advisors to the Automotive Industry
44
Introduction
•
All products and processes have characteristics
that need to be controlled.
•
Some Special Characteristics require extra
attention and control to prevent nonconformance
•
Special Characteristics have high impact upon:
• Product & Process Safety
• Compliance with Regulations
• Customer Satisfaction
Devos Associates
Advisors to the Automotive Industry
45
Special Characteristics - Symbols
•
•
Characteristics are Special or not
Special Characteristics can be considered in two
categories:
•
•
•
Safety or Regulatory – Critical Characteristics (CC)
Critical to Function and
Customer Satisfaction – Significant Characteristics (SC)
Special Characteristics are assigned symbols and
deployed down through drawings and other documents
Devos Associates
Advisors to the Automotive Industry
46
EXAMPLE
Effect
Classifi Indicates
-cation
Safety/
Regulatory
Customer
Satisfaction
CC
Mfg./
Assembly
CC
Impact on
Design CC
Mfg. /
Assembly
SC
Impact on
Design SC
Mfg. /
Assembly
SC
Critical
Characteristic
Significant
Characteristic
(blank) Not a Special
Characteristic
Devos Associates
Advisors to the Automotive Industry
DFMEA
Actions
Required
Criteria
Sev: 9 - 10 Special
Controls
Sev: 5 – 8 Special
Occur: 4-10 Controls
Sev: 5 – 8 Process
Occur: 4-10 Controls
Sev: 9 - 10 Process
Controls
Routine
control
47
Critical Characteristics - CC
Critical Characteristics - CC
•
•
Impact upon safe use and function
Impact compliance with governmental regulations
Design Controls
Process Controls
•
•
•
•
•
•
•
•
•
•
•
Special Materials
Safety Factors
Fail-safes
Special Analysis
Special Testing
Devos Associates
Advisors to the Automotive Industry
Special Tooling
Materials Inspections
Product Testing
Product Inspections
Special Torques
Assembly Sequences
48
Significant Characteristics - SC
Significant Characteristics - SC
•
•
•
Critical to customer satisfaction
Characteristics sensitive to manufacturing variation
Special manufacturing controls required to
assure compliance
Design and Process Controls
as for Critical Characteristics
Devos Associates
Advisors to the Automotive Industry
49
Significant Characteristics –
Two Approaches
• Significant to Customer Satisfaction
• Significant Sensitivity to Variation
σ
Devos Associates
Advisors to the Automotive Industry
Design For Six Sigma
50
“Not Significant” Characteristics
Features are robust to
manufacturing variation
Customer Dissatisfaction remains
about constant over the design
tolerances
X
Tolerance
Devos Associates
Advisors to the Automotive Industry
Target
Value
Tolerance
51
Taguchi Loss Function
There is an increasing “loss to society” as
deviation from a target value increases
Loss increases to
the right and left
of the target
Devos Associates
Advisors to the Automotive Industry
52
Loss Function – Consider a Train
Schedule
Increasing “Loss
X
Train Leaves earlier:
people arrive on-time
and miss the train
Devos Associates
Advisors to the Automotive Industry
to Society”
Published
Train
Departure
Time
Train Leaves later: no
one misses the train,
but train arrives late
and misses connections
53
Significant Characteristics
Features are sensitive to
manufacturing variation
Customer Dissatisfaction increases
sharply as features depart from the
designed target values
X
Tolerance
Devos Associates
Advisors to the Automotive Industry
Target
Value
Tolerance
54
Special Controls
•
Every effort must be made to eliminate Special
Characteristics through design actions to improve
product robustness.
•
Special Characteristics are confirmed after all
design/process alternatives are exhausted and
when Special Controls have been identified
Devos Associates
Advisors to the Automotive Industry
55