55 CHAPTER 4 FAILURE MODE AND EFFECTS ANALYSIS (FMEA) – CASE STUDY FMEA is a proactive analysis tool, allowing engineers to anticipate failure modes even before they happen, or even before a new product or process is released. It also helps the engineer to prevent the negative effects of these failure modes from reaching the customer, primarily by eliminating their causes and increasing the chances of detecting them before they can do any damage. The actions generated by a good FMEA cycle will also translate to better yield, quality, reliability and of course greater customer satisfaction. FMEA was being used around for a very long time. Before any documented format was developed, most of the inventors and process experts would try to anticipate what could go wrong with a design or process before it was developed. The trial and error alternative was both costly and time consuming. FMEA was formally introduced in the late 1940’s with the introduction of the military standard 1629. Being used for aerospace/rocket development, the FMEA was helpful in avoiding errors on small sample sizes of costly rocket technology. FMEA was encouraged in the 1960’s for space product development and served well on getting a man on the moon. Ford Motor Company reintroduced FMEA in the late 1970’s for safety and regulatory consideration. Ford Motor Company has used FMEA effectively for production improvement as well as design improvement.
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CHAPTER 4
FAILURE MODE AND EFFECTS ANALYSIS (FMEA) –
CASE STUDY
FMEA is a proactive analysis tool, allowing engineers to anticipate
failure modes even before they happen, or even before a new product or
process is released. It also helps the engineer to prevent the negative effects of
these failure modes from reaching the customer, primarily by eliminating
their causes and increasing the chances of detecting them before they can do
any damage. The actions generated by a good FMEA cycle will also translate
to better yield, quality, reliability and of course greater customer satisfaction.
FMEA was being used around for a very long time. Before any
documented format was developed, most of the inventors and process experts
would try to anticipate what could go wrong with a design or process before it
was developed. The trial and error alternative was both costly and time
consuming. FMEA was formally introduced in the late 1940’s with the
introduction of the military standard 1629. Being used for aerospace/rocket
development, the FMEA was helpful in avoiding errors on small sample sizes
of costly rocket technology.
FMEA was encouraged in the 1960’s for space product
development and served well on getting a man on the moon. Ford Motor
Company reintroduced FMEA in the late 1970’s for safety and regulatory
consideration. Ford Motor Company has used FMEA effectively for
production improvement as well as design improvement.
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The output of an FMEA cycle is the FMEA table, which documents
how vulnerable a product or process is to its potential failure modes. The
FMEA table also shows the level of risk attached to each potential failure
mode, and the corrective actions needed (or already completed) to make the
product or process more robust. The FMEA table generally consists of 16 to
17 columns, with each column corresponding to a piece of information
required by FMEA.
4.1 PURPOSE OF FMEA
The purpose of FMEA is to identify the different failures and
modes of failure that can occur at the component, subsystem and system
levels and to evaluate the consequences of these failures. FMEA is not a
problem solver. It is used with problem solving tools. FMEA can be described
as a systematic group of activities intended
(i) to recognise and evaluate the potential failure of a product/
process and the effects of the failure.
(ii) to identify action that could eliminate or reduce the chance
of such potential failure occurring
(iii) to document the entire process.
4.2 FMEA PREREQUISITES
The prerequisites of FMEA are given below.
(i) Select proper team and organise members effectively.
(ii) Select team for each product/services, process/system
(iii) Create a ranking system
(iv) Agree on format for FMEA
(v) Define the customer need
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(vi) Design/process requirement
(vii) Develop the process flow chart
4.3 UPDATING FMEA TABLE
FMEA table is to be updated when
(i) a new product or process is being designed or introduced.
(ii) a critical change in the operating conditions of the product or
process occurs.
(iii) the product or process itself undergoes a change
(iv) a new regulation that affects the product or process
(v) customer complaints about the product or process are
received
(vi) an error in the FMEA table is discovered or new information
that affects its contents comes to light.
4.4 BENEFITS OF FMEA
FMEA is designed to assist the engineer to improve the quality and
reliability of design. Properly used FMEA provides the engineer several
benefits and they are given below.
(i) Improves product/process reliability and quality
(ii) Increases customer satisfaction
(iii) Helps for early identification and elimination of potential
product/process failure
(iv) Prioritises product/process deficiencies
(v) Captures engineering/organisation knowledge
(vi) Emphasises problem prevention
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(vii) Documents risk and actions taken to reduce risk
(viii) Provides focus for improved testing and development
(ix) Minimises late changes and associated cost
(x) Serves as a catalyst for teamwork and idea exchange
between functions.
4.5 KEY TERMS USED IN FMEA
(i) Criticality
Criticality rating is the mathematical product of severity and
occurrence ratings. This number is used to place priority on items that require
additional quality planning.
(ii) Critical characteristics
Critical characteristics are the special characteristics defined by
Ford Motor Company that affect customers’ safety and/or could result in non-
compliance with government regulations and thus require special controls to
ensure 100% compliance.
(iii) Causes
A particular element of the design or process results in a failure
mode, due to a cause.
(iv) Failure mode
Failure modes are sometimes described as categories of failure. A
potential failure mode describes the way in which a product or process could
fail to perform its function (design intent or performances requirement) as
described by the needs, wants and expectations of internal and external
customers.
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(v) Severity
Severity (S) is an assessment of how serious the effect of the
potential failure mode is. A rating of 1 to 10 is chosen based on the severity.
The severity ratings are given in the Table A2.1 in Appendix 2.
(vi) Occurrence
Occurrence (O) is an assessment of the likelihood that a particular
cause will happen and result in failure mode during the life and use of a
product. Occurrence rating is given from 1 to 10 which is to be chosen as
given in the Table A2.2.
(vii) Detection
Detection (D) is an assessment of the likelihood that the current
control (design and process) will detect the causes of failure mode or the
failure mode itself, thus preventing it from reaching the customer. Detection
rating of 1 to 10 is to be chosen as given in Table A2.3 in Appendix 2.
(viii) Current control
Current control (design and process) are the mechanisms that
prevent the causes of failure mode from occurring, or which detect the failure
before it reaches the customer.
(ix) Risk Priority Number (RPN)
The RPN is the mathematical product of the Severity (S),
Occurrence (O) and Detection (D).
RPN = S x O x D
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4.6 CASE STUDY ON FMEA
FMEA was carried out at an industry which is a leading
manufacturer of Engine Valves. situated at Chennai, Tamilnadu, South India.
4.6.1 Process Flow Chart
The process flow chart of an engine exhaust valve is shown in
Figure A2.1. FMEA was carried out for the friction welding process of the
engine exhaust valves.
4.6.2 Friction Welding
Friction welding employs the heat produced due to friction welding
and pressure to accomplish the fusion of materials. The basic operation is
carried out by rotating one component and made to be in contact with the
secondary component which is held stationary. Axial pressure is applied
during the rotation which aids in generating heat. This process of pressurising
and heating creates a bond at the interface of the two mating parts. It is
significant that the metal at the interface does not melt, but rather becomes
plastic. Welding heat is obtained at the joint by rotating one part against the
other at a constant or varied RPM, with an axial force applied to the mating
components. Energy is provided to this joint from a continuously running
prime mover, directly connected to the machine spindle. This energy source is
infinite with respect to time, and is supplied to the interface until the proper
total heat is obtained. When this point is reached, the rotating member is
stopped and a forging load is applied to the parts to be joined. Figure 4.1
shows the various components of a friction welding machine. The photograph
of the friction welding machine that was used in this study is shown in
Figure 4.2.
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Figure 4.1 Components of a Friction Welding Machine
Figure 4.2 Friction Welding Machine
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4.6.2.1 Study of Key variables and process parameters
The variables in friction welding process can essentially be divided
into machine related variables and non-machine related variables. Non-
machine variables will include the material type to be welded and the part
configuration and size. These variables like in any other welding process will
determine the selection of welding parameters. The machine variables include
friction and forge pressures, speed of rotation. The rotational speed and the
pressure will control the ultimate quality of the weld.
The rotational speed and pressure affect both the width and the
shape of the heat-affected zone (HAZ). High pressures tend to compress the
HAZ, especially at the center, heavily work the interfacial material and cause
a notch effect at the junction. Higher speeds tend to increase the width of the
HAZ and also the grain size. Subsequent use of high pressure forging after
spindle stop is used to work the structure and refine the grain size. The
parameters that can be selected and controlled will optimise the metallurgical
condition.
The process parameters of the friction welding process are soft
force, soft force time, upset force, friction force, upset time, burn off,
permissible shrinkage limit, slide home position, clamp open position and
lube cycle.
4.6.3 Failure Report
Failure reports of the various types of exhaust valves in passenger
car line for two months have been collected and given in TableA2.4 and
shown in Figure 4.3.
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Figure 4.3 Various failure modes
From the failure report it was observed that the majority of the
failures are due to runout. Part Number 40767 has got the highest failure
percentage. The prominent failure modes are weld crack and low tensile
strength and part number 40767 has a production volume of more than 98%
and hence part number 40767 was chosen for carrying out FMEA. Table 4.1
gives the specifications of the parts to be welded.
Table 4.1 Specifications of the parts to be welded
Compositionin %Structure
C Si Mn Cr Ni P
Dimensions inmm
Before FrictionWelding
FinalDimensions in mm
Head Austenite 0.4To
0.55
1To2
<0.6 7.5To9.5
<0.6 <0.03 Dia- 6.175Length-125
Stem Martensite 0.15To
0.25
0.75To
1.25
1.5 20To22
10.5To
12.5
<0.04 Dia- 6.175Length-63
Dia- 6.175±0.025
Length-181
4.6.4 Failure modes and detection
(i) Low tensile strength: The valve has to withstand a
minimum tensile strength of 700N/mm2. Tensile testing is
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used to detect the tensile strength of the valves. For every lot
of 1000 numbers, 7 valves are subjected to tensile testing.
(ii) Runout: Runout is the ‘out of roundness’ of valves. A
tolerance limit of +0.04 mm is set as threshold and the valve
which exceeds the threshold is scraped. Runout is detected
in the process by the operator by random checking. The
defective valves that pass through friction welding are
detected in centre less grinding process by 100% inspection.
Both the inspections are carried out by v-block and dial
gauge assembly.
(iii) Weld crack: Ultrasonic testing is used to detect the weld
crack in the weld zone. The rejected valves are kept
separately and examined again. The valves which show
discontinuation are scraped.
(iv) Overall bar length: The bar length for the valve should be
181 mm after friction welding. The length is randomly
checked by the operator. The defective valves which pass
through friction welding are detected during loading of valve
in upsetting machine.
4.6.5 Estimation of Severity Ranking
The severity of each failure can be estimated by interpreting the
failure mode and effect diagram with the suggested severity evaluation. The
failure mode and effect diagram is given in Figure 4.4. The estimation of
severity ranking was done using Table A2.1 and is given in Table 4.2.