Module 5 Lecture 3 Final
Transcript of Module 5 Lecture 3 Final
-
7/30/2019 Module 5 Lecture 3 Final
1/14
IIT, Bombay
Module5
Design for Reliability andQuality
-
7/30/2019 Module 5 Lecture 3 Final
2/14
IIT, Bombay
Lecture
3Design for Reliability
-
7/30/2019 Module 5 Lecture 3 Final
3/14
IIT, Bombay
Instructional Objectives
The objective of this lecture is to outline the basic concepts of risk, reliability and safety and the
methodologies to define reliability in a quantitative manner to aid to design for reliability.
Define Risk, Reliability and Safety
Riskis usually defined as the probability or the frequency of occurrence of a hazard or damage to
human, infrastructure, property or environment. Reliability refers to the ability of a product to
perform its specified function under service conditions. In other words, reliability can be
depicted as the probability that an item will perform appropriately for a specified time period
under a given service condition. For example, a reliability of 0.997 for a typical part implies that
there is aprobability of failure (an inverse of reliability) of 3 parts in every 1000 parts. Safety is
referred to the relative protection from exposure to hazards.
Risk Assessment
A product is considered safe if the risks associated with the product are assessed to be
acceptable. The risk assessment often involves the determination of the frequency of the
occurrence of a specified hazard and a conclusion on the acceptability of the hazard. Alternately,
the levels of risk can be classified as:
Tolerable RiskThis level of risk is usually accepted while a constant review of its cause and
ways to reduce the same is the must.
Acceptable Risk This level of risk is also acceptable and does not need immediate attention.
Unacceptable Risk: This level of risk is deemed to be unacceptable.
Quantify ReliabilityEngineering designs often neglect the stochastic nature of the material properties, the dimensions
of the components and the externally applied load and usually consider the same by applying a
factor of safety. However, a quantification of the uncertainty in design of parts is necessary in
critical applications like in space, aircraft and nuclear applications.
-
7/30/2019 Module 5 Lecture 3 Final
4/14
IIT, Bombay
For example, consider a structural member subjected to a static load experiences a stress and
exhibits the yield strength (of the material) as y . Figure 5.3.1 schematically depicts the nature
of variation of stress experienced and the material yield strength. As shown schematically in
Figure 5.3.1, the distribution of stress on the structural member can be envisaged as typicalGaussian in nature with a mean ( ) and standard deviation ( sd ). Similarly, the variation in the
yield strength of the material can also be envisaged as Gaussian with a presumed mean ( y ) and
standard deviation ( ysd ). If we subtract the stress distribution from the strength distribution, we
get a distribution as ( y ), which is always positive. Interesting to note, however, is the fact
that the frequency distribution of can overlap with that of y and can lead to a situation as
y
> , which will lead to failure. So, the probability of failure will be given as
)(PP yf >= (1)
and, the reliability will then be defined as
fP1R = (2)
The mean and the standard deviation of the distribution ( y ) can be easily determined
considering and y as two independent variables.
Figure 5.3.1 Schematic presentation of the distribution of stress on a typical structural member
and variation in the material yield strength
To avoid unforeseen failure as indicated above, the mechanical designs often undertake the
concept of a safety factorto reduce the probability of failure and hence, increase the reliability.
The safety factormay be estimated as the ratio of the mean capacity to the mean load or demand.
y
y
-
7/30/2019 Module 5 Lecture 3 Final
5/14
IIT, Bombay
Thus, reliability can be defined as the probability that a system, component, or device
will perform without failure for a specified period of time under a given operating conditions. If
R(t) and F(t) are respectively is the reliability and the probability of failure with respect to time t,
and are mutually exclusive, we can write
1)t(F)t(R =+ (3)
Furthermore, considering a typical test for reliability with 0N number of parts, and )t(NS and
)t(NF are respectively the number of parts surviving and failing till time t, we can write that
0FS N)t(N)t(N =+ (4)
We can write further that,
0
F
0
S
N
)t(N1)t(F1
N
)t(N)t(R === (5)
Hence, the instantaneous failure rate can be envisaged as the number of failures per unit time for
the total number of parts exposed during time t and given as,
)t(N
1
dt
)t(dN)t(h
0
F= (6)
In a statistical form, the instantaneous failure rate, h(t), can be envisaged as the probability that a
given test item has survived till time 1t and would fail between time interval 1t and 11 dtt + , and
can be expressed as
)tt|dtttt(P)t(R
)t(f
)t(F1
)t(f)t(h 1111 +==
= (7)
where f(t) refers to the typical nature of the statistical frequency distribution of time to failure.
Hence, f(t) can be written as
dt
)t(dR
dt
)]t(R1[d
dt
)t(dF)t(f
=
== (8)
Substituting equation (8) in (7), we can write,
=== t
0
dt)t(hexp)t(Rdt)t(h)t(R
)t(dR
)t(R
1
dt
)t(dR)t(h (9)
For a given constant value of failure rate, equation (9) can thus be used to estimate the reliability
of a test component. Figure 5.3.2 schematically shows the typical nature of the failure curves
realized for general components.
-
7/30/2019 Module 5 Lecture 3 Final
6/14
IIT, Bombay
Figure 5.3.2 Schematic presentation of the failure rate curve
The failure curve usually consists of three parts. The first part is a decreasing failure rate, known
as early failures that may occur due to design errors, manufacturing defects, or installations
errors. These early failures can be minimized by improving the production quality control, and
subjecting the parts to a proof test before their actual service with the customers. As time passes
the system stabilizes and these early failures leave the system leading to a constant failure rate in
the second part, which is also referred to as random failure zone. Here the failures occur because
of random overloads or random faults having no pattern at all. Mechanical components usually
do not exhibit a region of constant failure rate. After a stipulated long time, the mechanical
components and materials begin to age and wear out and this accelerates the failure rate leading
to an accelerated failure rate in the third (last) part. The failure curve is also referred to as
bathtub curve due to its shape. However, the usual nature of the distribution of mechanical
failures is slightly different from the electronic failures as shown in Figure 5.3.3.
-
7/30/2019 Module 5 Lecture 3 Final
7/14
IIT, Bombay
Figure 5.3.3 (a) Failure curve of a typical electronic equipment (b) Failure curve more typical
of mechanical equipment
With respect to the reliability of components, mean life of a part or component refers to
the average life a specified number of components which is measured over the entire span of
their life. The mean time to failure (MTTF)refers to the sum of survival time (up time) for all the
components put under test divided by the number of components, which are failed.
For example, assume the testing of 3 identical systems starting from time 0 until all of
them failed. The first system failed at 23 hours, the second failed at 26 hours and the third failed
at 20 hours. The MTTF is the average of the three failure times, which is 23 hours. The mean
time between failure (MTBF) refers to the mean or average time between two successive
component failures. MTBF is usually applied to a group of similar equipment, for example all
the pumps in a refinery. If there are 40 operating pumps with a total of 23 failures over one
month, the MTBF during that month is calculated as (40 / 23) or 1.74 months. For a single item,
MTBF is just the time period / number of failures. For example, if a pump fails thrice in one
month, the MTBF would be 1/3 = 0.33 months
System Reliability
The overall reliability of a typical mechanical or any system will be governed by the reliability of
the individual components. If the components are so arranged that the failure of any component
will lead to the total failure in the system, the same is envisaged to be in series. In contrast, a
-
7/30/2019 Module 5 Lecture 3 Final
8/14
IIT, Bombay
better arrangement of components would be one that would require failure of all components for
the total failure of the system. Such an arrangement exhibits parallel reliability. The system
reliability in series and parallel arrangement of components can be given as equations (10) and
(11), respectively.
nCBAsystem R........RRRR = (10)
)R1(........)R1()R1()R1(1R nCBAsystem = (11)
Requisiteness of Reliability of Product
There are a number of reasons why reliability is an essential attribute of a product.
Reputation
A companys reputation is very closely attached to the reliability of its products it produces. The
more reliable a product is, the more likely the company is to have a good reputation.
Customer satisfaction
A reliable product may not drastically affect customer satisfaction in a positive manner.
However, an unreliable product will definitely attract customer dissatisfaction Thus high
reliability is a quite essential requirement for customer satisfaction.
Warranty Costs
If a product fails to perform its desired function within the warranty period, the replacement andrepair costs will not only reduce the profits, but also gain unwanted negative attention.
Repeat business
A focused effort towards improved reliability shows existing customers that a manufacturer is
serious about its product, and committed to customer satisfaction. This type of attitude not only
has a positive impact on future business but also gives a competitive edge.
Cost Analysis
Companies may take reliability data and combine it with other cost information to illustrate the
cost-effectiveness of their products. This life-cycle cost analysis can prove that although the
initial cost of a product might be higher than those of its competitors product, the overall
lifetime cost is lower than that of a competitor's because their product requires fewer repairs or
less maintenance.
-
7/30/2019 Module 5 Lecture 3 Final
9/14
IIT, Bombay
Customer requirements
Demand from customers for an effective reliability program is ever increasing based on the
benefits of such programs.
Competitive advantage
Companies often publish the product reliability numbers to enhance competitiveness.
Difference between Quality and Reliability
Even though a product has a reliable design with all checks from the point of view of design for
quality, its reliability in service can be unsatisfactory that can be attributed to inappropriate
manufacturing process and / or the quality of the material used. So, even though the product has
a reliable design, it is effectively unreliable when fielded, which is actually the result of a
substandard manufacturing process and/or due to poor quality of material used for the
manufacturing of the product.
Design for Reliability
Design for reliability is a process which is performed during the design of the product so as to
ensure that the product is able to perform to a required level of reliability. Traditionally, the
reliability achieved is the outcome of the reliability that naturally accompanies with the design required for the product, and the reliability that come due to standard and historic practices followed by the design and
manufacturing units. ( like using standard well established components in critical areas )
The demand to achieve desired performance level in an efficient and optimized manner has
led to a growing movement towards increasing applications of design for reliability and its
spread to industries where it had not been used in the past. Previously design practices tend to
focus on mainly on functionality androbustness or product integrity. In developing the design
for the products, the following characteristics are usually missed: - (a) key failure modes and
failure rate of the product, (b) key failure mechanisms that may be present in the service
environment, (c) usable life of the product, (d) cost of maintenance required to maintain the
inherent reliability, (e) availability, and (f) rigorous testing.
-
7/30/2019 Module 5 Lecture 3 Final
10/14
IIT, Bombay
As to address all of the above issues, the process ofdesign for reliability has been developed
to ensure a predictable and reliable life of usual components. Some of the key elements for
design for reliability is as follows.
Concurrent Engineering
Concurrent engineering is a feature that ensures the design is not completed before reliability
requirements are identified and dealt with.
Configuration Design
The physical configuration is the key important characteristic that determines the reliability of an
asset. Depending on the severity of the product service and the maximum economic reliability of
available components present in the product, it may be necessary to build redundancy into some
locations.
Component Selection
The second important characteristic that determines reliability is the choice of components that
make the product. Components with better load bearing ability rather than cheap components
should be considered as better option.
Design and Build
It is possible to create a strong configuration and select robust components, and still produce a
product that is unreliable. There are design and assembly practices like use of protective
grommets at points of wear, use of strain relief at bends, or changes in direction that ensure theconfiguration and components deliver the desired reliability.
Verification and Performance Testing
The final assembled product may not always perform as expected. Interactions between dynamic
components can produce unexpected effects. As a result, it is necessary to verify that the
assembled product functions as expected. It is also essential to simulate the wear and tear that
represents an entire life using accelerated testing.
Customer Needs
The product must be designed not only based on functionality but also considering the customer
needs.
-
7/30/2019 Module 5 Lecture 3 Final
11/14
IIT, Bombay
Steps to ensure Design for Reliability
The activities towards design for reliability for a component should be spread over from the
conceptual design, embodiment design, detail design, actual production and service. For
example, the design for reliability activities in the conceptual, embodiment and detail design
stages involves
(a)the integration of the estimation of reliability requirement and the determination of likelyservice environment in the problem definition,
(b)investigation of redundancy and ensuring provision for accessibility for maintenanceduring configuration design,
(c)selection of reliable parts and components, establishment of failure modes and effects,estimations of likely failure rate and mean time between failures,
(d)building and testing of prototypes, and so on.Similar activities in theproduction andservice stages involve environment tests and checking for
quality assurance, collecting feedbacks on service failures and customers comments, ensuring
proper replacement of parts, etc.
The overall process starts with the conceptual design to identify the problem or objective, to
estimate the required level of reliability, and to carefully understands and consider all the factors
that make up the service environment. In the configuration design step of embodiment design the
physical arrangements of the various components present in the product are to be determined as
it critically affects the reliability. Also various redundancies are tested and it should be ensured
that the physical arrangement is good enough to allow access for maintenance. In the parametric
step of embodiment design, the selected components are to be tested for their reliability. Both
physical and computer models should be tested and subjected to the widest range of service
environmental conditions and various failure modes must be established. During detail design
the final specifications are finalized for manufacturing and testing the preproduction prototype
and the final productions designs are prepared. The work of the design department is not finishedeven after the designs are released to the production department. The production models are
given further environmental tests and these help to determine the quality assurance program and
the maintenance schedule. Once the product is put to service there is a constant feedback related
to field failures and mean time between failures that help the design department to redesign
efforts and follow-on products.
-
7/30/2019 Module 5 Lecture 3 Final
12/14
IIT, Bombay
Ways to improve Reliability by design
Various techniques are used by the design team to improve reliability. The work of the design
department is not finished even after the designs are released toReducing variability
Mechanical properties of engineering material exhibit variability. Fracture and fatigue properties
show greater variability that the yield strength and the tensile strength. Conservative design
values of material properties are essential so as to obtain design of a reliable product. Variability
in the material property had a huge impact on the probability of the failure of product hence
affecting the reliability of the product too. The probability of failure can be reduced if without
changing the mean value of the material properties if we could reduce the variability in the
properties.
Derating
The reliability of the product can be increased if their maximum operating conditions
(temperature, pressure, etc) are at values lower than their nameplate values. This reduces the
probability of failure and increases the reliability of the product.
Redundancy
One of the most efficient ways to increase reliability is with redundancy. Components that are
critical are duplicated such that two or more of them may exist in parallel to perform the same
function within the product thus increasing the reliability of the product. The existence of
parallel paths results in load sharing and each duplicate component is derated and has its life
increased by a longer than the average time. Another way to increase the redundancy is by
having a standby unit that cuts in and takes over when the current operating unit fails. The unit
should be provided by sensors and switching mechanisms to sense the failure and to place the
unit in service. The sensors and the switching mechanisms are the weak links in a standby
redundant systemDurability
The material selection and the design details should be finalized with the objective of producing
a product that is resistant to degradation from factors like corrosion, erosion, fatigue, wear, etc.
This usually requires selection of high performance material which can be expensive so as to
-
7/30/2019 Module 5 Lecture 3 Final
13/14
IIT, Bombay
increase service life and reduce the maintenance cost. Such decisions can be justified by using
techniques like life cycle costing and so on.
Ease of inspection
It is very essential that cracks or any other flaws are identified when they occur in the product.
Ideally it should be possible to perform various visual methods for detections of such cracks or
other flaws, but special design features may have to be provided in order to ensure this. The
product should be designed in such a way that it is capable for ready inspection.
Simplicity
Simplification of the component and the product reduces the chances for failure and errors and
increases the reliability.
Specificity
The greater the level of specificity, the higher will be the inherent reliability of the design.
Specifying standard components for the product increases the reliability. It means the component
being used has a history and their reliability is known.
Replacement
Whenever it is required to use components with high failure rates the design should specifically
take care for the ease of replacement of such component.
-
7/30/2019 Module 5 Lecture 3 Final
14/14
IIT, Bombay
Exercise
1. If a device has a failure rate of 5x10-6 failures/h, what is the reliability for an operating period
of 1000 h?
References
[1] 1. David M. Anderson and David M. Anderson, Design for Manufacturability andConcurrent Engineering, CIM Press, 2004.
[2] G Dieter, Engineering Design - a materials and processing approach, McGraw Hill, NY,2000.