Reliability Srategy

1. Implementing the
Reliability Strategy
Prof. Charlton S. Inao
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Unit IX

2. The importance of reliability
Electrical, electronic and Mechanical equipment is used in a
number of fields — in industry for the control of processes, in
computers, in medical electronics, atomic energy, in weapon
systems, defence equipments, communications, navigation at sea
and in the air, and in many other fields.
It is essential that this equipment should operate reliably under all
the conditions in which it is used. In the air navigation, military and
atomic energy fields, for instance, failure could result in a dangerous
situation.
Very complicated systems, involving large numbers of separate
units, such as avionic and aerospace electronic systems are coming
into use more and more. These systems are extremely complex and
use a large number of component parts. As each individual part is
liable to failure, the overall reliability will decrease unless the
reliability of each component part can be improved.
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3. Mechanical reliability
The well-reported failures, such as the Space Shuttle Challenger, Chernobyl
nuclear accidents, and the Bhopal gas escape, emphasize vividly the necessity for
mechanical reliability.
Buildings, bridges, transit systems. railways, automotive systems, robots, offshore
structures, oil pipe lines and tanks, steam turbine plates, roller bearings, etc., all
have their particular modes of failure affecting their reliability.
There are a number of common modes of mechanical failures, which are worth
listing, e.g. with structures:
(1)Corrosion failures
(2) Fatigue failures
(3) Wear failures
(4) Fretting failures
(5) Creep failures
(6) Impact failures
These may be considered the main failure modes, but there are of course many
others, such as ductile rupture, thermal shock, galling, brinelling, spalling,
radiation damage, etc.
A ‘failure’ is any inability of a part or equipment to carry out its
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specified function.

4. Reliability Engineering
• Reliability engineering is an engineering field that deals
with the study, evaluation, and life-cycle
management of reliability: the ability of a system or
component to perform its required functions under
stated conditions for a specified period of time
• Reliability engineering is a sub-discipline within systems
engineering. Reliability is often measured
as probability of failure, frequency of failures, or in terms
of availability, a probability derived from reliability and
maintainability. Maintainability and maintenance are
often important parts of reliability engineering.

5. Well-publicized system failures such as those listed below may have
also contributed to more serious consideration of reliability in product
design
• Space Shuttle Challenger Disaster:
This debacle occurred in 1986, in which all crew
members lost their lives. The main reason for this
disaster was design defects.
• Chernobyl Nuclear Reactor Explosion:
This disaster also occurred in 1986, in the former
Soviet Union, in which 31 lives were lost. This
debacle was also the result of design defects.
• Point Pleasant Bridge Disaster:
This bridge located on the West Virginia/ Ohio border
collapsed in 1967. The disaster resulted in the loss
of 46 lives and its basic cause was the metal fatigue
of a critical eye bar.

6. RELIABILITY SPECIALIZED AND
APPLICATION AREAS
• Mechanical reliability
This is concerned with the reliability of mechanical
items. Many textbooks and other publications have
appeared on this topic.
Example:
 Critical mechanical component assessment
 Shaft strength
 Selection of flexible couplings and transmission brakes
 Gear life assessment; screening of belt drives
 Assessment of bearing life, load ratings of slider bearings and shaft
sealing devices
 Bolt loading and lubrication systems

7. • Software reliability.
This is an important emerging area of reliability as
the use of computers is increasing at an alarming
rate.
• Human reliability.
In the past, many times systems have failed not due
to technical faults but due to human error. The
first book on the topic appeared in 1986
• Reliability optimization.
This is concerned with the reliability optimization of
engineering systems
• Reliability growth.
This is basically concerned with monitoring
reliability growth of engineering systems during
their design and development

8. • Structural reliability.
This is concerned with the reliability of
engineering structures, in particular civil
engineering
• Power system reliability.
This is a well-developed area and is basically
concerned with the application of
reliability principles to conventional power
system related problems. Many books on
the subject have appeared over the years
including a vast number of other
publications

9. • Robot reliability and safety.
This is an emerging new area of the application
of basic reliability and safety principles to robot
associated problems.
• Life cycle costing.
This is an important subject that is directly
related to reliability. In particular, when
estimating the ownership cost of the product,
the knowledge regarding its failure rate is
essential.
• Maintainability.
This is closely coupled to reliability and is
concerned with the maintaining aspect of the
product.

10. TERMS AND DEFINITIONS
• Reliability: This is the probability that an item will
carry out its assigned mission satisfactorily for the
stated time period when used under the specified
conditions.
• Failure: This is the inability of an item to function
within the initially defined guidelines.
• Downtime: This is the time period during which the
item is not in a condition to carry out its stated
mission.
• Maintainability: This is the probability that a failed
item will be repaired to its satisfactory working state.
• Redundancy :This is the existence of more than one
means for accomplishing a defined function.
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11. Active redundancy: This is a type of redundancy when all redundant
items are operating simultaneously.
Availability: This is the probability that an item is available for
application or use when needed.
Useful life: This is the length of time an item operates within an
acceptable level of failure rate.
Mission time: This is the time during which the item is performing its
specified operating condition.
Human error: This is the failure to perform a given task (or the
performance of a forbidden action) that could lead to disruption of
scheduled operations or result in damage to property/equipment.
Human reliability: This is the probability of completing a job/task
successfully by humans at any required stage in the system operation
within a defined minimum time limit (if the time requirement is
specified).
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12. MEAN TIME BETWEEN FAILURES (MTBF): The mean exposure
time between consecutive failures of a component. This applies to
repairable items, and means that if an item fails, say 5 times over
a period of use totaling 1000hours, the MTBF would be 1000/5 or
200hours.
MEAN TIME BETWEEN MAINTENANCE (MTBM): The average
time between all maintenance events that cause downtime, both
preventative and corrective maintenance, and also includes any
associated logistics delay time.
MEAN TIME TO FAILURE (MTTF): Mean Time To Failure (MTTF): It
is the average time that elapses until a failure occurs. MTTF is
commonly found for non repairable items such as fuses or bulbs,
etc.
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13. GENERAL RELIABILITY ANALYSIS RELATED
FORMULAS
Evaluating the left-hand side of Equation (6) yields
t
  
From Equation (7), we get
t
 ( t )dt
The above equation is the general expression for the
reliability function. Thus, it can be used to obtain
reliability of an item when its times to failure follow any
known statistical distribution, for example, exponential,
Rayleigh,Weibull, and gamma distributions.
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ln R(t) (t)dt...(7)
0 
R(t) 
e 0
...(8)
 

14. GENERAL RELIABILITY ANALYSIS RELATED FORMULAS
Mean time to failure: This can be obtained by using any of the
following three formulas:
MTTF  E t 
tf t dt
( ) ( ) ...(9)
or
MTTF R t dt
( ) .............(10)

or
where:
MTTF is the item mean time to failure,
E(t) is the expected value,
s is the Laplace transform variable,
R(s) is the Laplace transform for the reliability function, R (t).
is the failure rate
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...(11)
1
( )
0
0
0

 





MTTF LimitR s
s


15. Mean time between failure MTBF
where MTBF stands for mean operating time between failures.
MTBF should be confined to the case of repairable items with
constant failure rate
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GENERAL RELIABILITY ANALYSIS RELATED FORMULAS
1

MTBF
is the failure rate 

16. Bathtub Hazard Rate Curve
• Bathtub hazard rate curve is a well known concept to
represent failure behavior of various engineering
items/products because the failure rate of these items
changes with time.
• Its name stem from its shape resembling a bathtub as shown
in Figure 1.
• Three distinct regions of the curve are identified in the figure:
burn-in region(early failures),
useful life region, and
wear-out region.
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17. • These regions denote three phases that a newly
manufactured product passes through during its
life span.
• During the burn-in region/period, the product
hazard rate (i.e., time dependent failure rate)
decreases and some of the reasons for the
occurrence of failures during this period are poor
workmanship, substandard parts and materials,
poor quality control, poor manufacturing
methods, …….
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18. incorrect installation and start-up human error, inadequate
debugging, incorrect packaging, inadequate processes, and
poor handling methods. Other names used for the “burn-in
region” are “debugging region,” “infant mortality region,” and
“break-in region.”
• During the useful life region, the product hazard rate remains
constant and the failures occur randomly or unpredictably.
Some of the reasons for their occurrence are undetectable
defects, abuse, low safety factors, higher random stress than
expected, unavoidable conditions, and human errors.
• During the wear-out region, the product hazard rate increases
and some of the reasons for the occurrence of “wear-out
region” failures are as follows: Poor maintenance, Wear due to
friction, Wear due to aging, Corrosion and creep, Wrong
overhaul practices, and Short designed-in life of the product.
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19. Figure 1: Bathtub hazard rate curve.
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21. Example 1 :
• Assume that a railway engine’s constant failure rate λ is 0.0002
failures per hour. Calculate the engine’s mean time to failure.
1
1
MTTF  
Thus, the railway engine’s expected time to failure is 5000 h.
• Assume that the failure rate of an automobile is 0.0004 failures/h.
Calculate the automobile reliability for a 15-h mission and mean
time to failure.
Using the given data in Equation
R t e
( ) ...(8)
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5000h
0.0002
λ
(0.0004)(15)

e
0.994
( )
0







e
t
t dt
t


Example 2 :

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Similarly, inserting the specified data for the automobile failure
rate into Equation MTTF, we get
MTTF R t dt
( ) .............(10)

0

MTTF 
e dt


MTTF e dt
h
t
t
1
0.0004
2,500
..
..
0
(0.0004)
0









Thus, the reliability and mean time to failure of the automobile
are 0.994 and 2,500 h, respectively.

23. Definition of Maintainability
Maintainability is a measure of the speed with which
loss of performance is detected, diagnosed and made
good.
Maintainability is the probability that a unit or system
will be restored to specified conditions within a given
period when maintenance action is taken in accordance
with prescribed procedures and resources.
It is a characteristic of the design and installation of the
unit or system.
The ‘availability’ or time an equipment is functioning
correctly while in use depends both on reliability and on
maintainability.
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24. Definition of Availability
Availability. Availability is defined as the percentage of
time that a system is available to perform its required
function(s).
It is measured in a variety of ways, but it is principally
a function of downtime.
Availability can be used to describe a component or
system but it is most useful when describing the nature
of a system of components working together. Because it
is a fraction of time spent in the “available” state, the
value can never exceed the bounds of 0 < A < 1. Thus,
availability will most often be written as a decimal, as
in 0.99999, as a percentage, as in 99.999%,
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25. Availability
• Availability
This is the probability that an item is available for
application or use when needed.
Maintainability together with reliability
determine the availability of a machinery
system. Availability is influenced by the time
demand made by preventive and corrective
maintenance measures.
Availability(A) is measured by:
A= MTBF/MTBF + MTTR

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Quality and reliability
The quality of a device is the degree of performance to
applicable specification and workmanship standards.
What is the difference between Quality and Reliability?
Quality means good performance and longevity.
Quality of any manufactured product is determined by its design,
the materials from which it is made and the processes used in its
manufacture.
Quality control measures performance and its variations from
specimen to specimen by statistical methods to determine
whether production satisfies the design requirements.
Quality of a product is determined by conformity and reliability.
In Reliability it matters how long a product will maintain its
original characteristics when in operation.

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Analytical Techniques and Methods in Reliability
 Built-in test (BIT) (Testability analysis)
 Failure mode and effects analysis (FMEA)
 Reliability Hazard analysis
 Reliability Block Diagram analysis
 Fault tree analysis
 Root cause analysis

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
Accelerated Testing

Reliability Growth analysis

Weibull analysis

Thermal analysis by Finite Element Analysis (FEA) and /
or Measurement

Thermal induced, shock and vibration fatigue analysis
by FEA and / or Measurement

Electromagnetic analysis

Statistical interference

Predictive and preventive maintenance: Reliability
Centered Maintenance (RCM) analysis

Human error analysis

Operational Hazard analysis
Results are presented during the system design reviews and logistics reviews.
Reliability is just one requirement among many system design requirements.

29. KEY POINTS
• Reliability is a measure of uncertainty and therefore
estimating reliability means using statistics and
probability theory
• Reliability is quality over time
• Reliability must be designed into a product or service
• Most important aspect of reliability is to identify cause
of failure and eliminate in design if possible otherwise
identify ways of accommodation
• Reliability is defined as the ability of an item to
perform a required function without failure under stated
conditions for a stated period of time
• The costs of unreliability can be damaging to a
company
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30. Steps in Implementation
1. Arrange for schedules to be in corporated in
relevant work plans
2. Identify the training needs in discussion with
relevant personnel
3. Assist personnel to develop required skills for
inspections and servicing within scope and
authority.
4. Collect data/information with performance
indicators
5. Recommend improvements to reliability
strategy in accordance with procedures.
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31. The End
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