The document discusses the concepts of reliability and maintainability in systems, emphasizing their importance in maintaining system capability while controlling costs. Reliability is defined as the probability that a product or service will perform as intended for a specified period, influenced by factors such as component reliability and design complexity. It also highlights the interrelationship between quality, reliability, and maintainability, aiming for customer satisfaction through effective product design.

1. Chapter - five
Reliability and Maintainability

2. Reliability – ability to provide what was promised
Assurance – knowledge and courtesy of employees
and ability to convey trust
Tangibles – physical facilities and appearance of
personnel
Empathy – degree of caring and individual attention
Responsiveness – willingness to help customers and
provide prompt service
Cont.….
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3. Key Dimensions of Quality
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4. Maintenance and Reliability
Maintenance and Reliability
 The objective of maintenance and reliability
is to maintain the capability of the system
while controlling costs
Maintenance is all activities involved in keeping a
system’s equipment in working order
Reliability is the probability that a machine will
function properly for a specified time.

5. What is reliability?
 Reliability is the probability that a product, service, or part
will perform as intended(without failure) for a specified
period of time under normal conditions.
 Failure – the termination of the ability of an item to
perform its required function.
 Failures may be sudden (non-predictable) or gradual
(predictable).
 They may also be partial or complete,
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6. Product Failure Rate (FR)
Product Failure Rate (FR)
Basic unit of measure for reliability
Basic unit of measure for reliability
FR(%) = x 100%
FR(%) = x 100%
Number of failures
Number of failures
Number of units tested
Number of units tested
FR(N) =
FR(N) =
Number of failures
Number of unit-hours of operating time
Mean time between failures
Mean time between failures
MTBF =
MTBF =
1
1
FR(N)
FR(N)

7. Failure Rate Example
Failure Rate Example
20 air conditioning units designed for use in NASA
20 air conditioning units designed for use in NASA space
shuttles operated for 1,000 hours One failed after 200 hours and
shuttles operated for 1,000 hours One failed after 200 hours and
one after 600 hours
one after 600 hours
FR(%) = (
FR(%) = (100%) = 10%
%) = 10%
2
2
20
20
FR(N) = = .000106 failure/unit hr.
2
2
20,000 - 1,200
MTBF = = 9,434 hrs.
1
.
.000106

8. Cont.d
 In the nineteenth and early twentieth-century:
 designs were less severely constrained by the cost and
schedule of delivery time pressures.
 Components were individually fabricated and there was
no mass production and standardization of parts.
 Thus, in many cases, high levels of reliability were
achieved as a result of over-design.
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9. Cont.
 Nowadays, modern technology has developed capacity to
design and manufacture equipment and systems of greater
capital cost, sophistication, complexity, and capacity.
• The consequences of low availability and high maintenance
cost of such systems led to the desire for:
high reliability,
high maintainability, and
low mean time to support (easily maintainable).
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10. Cont.
• This can be achieved by considering the above points in
designing products to meet customer specifications.
 As it is difficult to give guarantee for a product with 100
percent certainty to function properly, reliability can only
be maximized.
• Hence, for customer-oriented companies attaining a high
reliability is an important part of quality and they should
build this into their product design
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11. Cont.
• Reliability is a probability, a likelihood, or a chance.
• observed reliability is empirically defined as the ratio of
items which perform their function for the stated period to
the total number in the sample
• For example, a product with a 90 percent reliability has a
90 percent chance of functioning as intended. Or,
• the probability that the product will fail is 1 - .90 = .10, or
10 percent. This also means that 1 out of 10 products will
not function as expected.
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12. Cont.
 The reliability of a product is a direct function of the reliability
of its component parts.
 The probability that a garment will perform as intended is,
therefore, a function of the reliabilities of its fabric, sewing
thread, buttons, zip and other parts.
 If all the parts in a product must work for the product to
function, then the reliability of the system is computed as the
product of the reliabilities of the individual components:
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13. Cont.
Rs = (R1) (R2) (R3) . . . (Rn)
where Rs = reliability of the product or system.
R1... n = reliability of components 1 through n
• As the reliability value of a component is most of the time
less than one, the reliability of a system is lower than that
of individual components.
• This is because all the components in a series must
function for the product to work.
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14. Cont.
• If only one component doesn’t work, the entire product
doesn’t work.
• The more components a product has, the lower its
reliability.
• For example, a system with five components in series,
each with a reliability of .90, has a reliability of only (.90)
(.90)(.90)(.90)(.90) = 0.59.
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15. Cont.
Rs = Reliability of 1st
Component + (Reliability of 2nd
Component * Probability of needing 2nd
Component)
Where: Probability of needing 2nd
Component =
probability of the 1st
component failing
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16. Reliability can be increased by:
 Reducing Complexity: The fewer component parts and the
fewer types of material involved, the greater is the
likelihood of a reliable item.
• Modern equipment, so often condemned for its unreliability,
is frequently composed of thousands of component parts all
of which interact within various tolerances
 Duplication/replication: the use of additional, redundant,
parts
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17. Cont.
• But it adds capital cost, weight, Spares, Space,
preventive maintenance and power consumption
 Excess strength: Deliberate design to withstand
stresses higher than are anticipated will reduce failure
rates.
• Again this also incurs additional costs
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18. Quality and reliability
 Quality: conformance to specification
 Reliability: the probability of non-failure in a given period
 Maintainability: the probability that a failed item will be
restored to operational effectiveness within a given period
of time when the repair action is performed in accordance
with prescribed procedures.
 This, in turn, can be paraphrased as ‘The probability of
repair in a given time’.
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19. Cont.
 Quality and reliability are interrelated:
• A system cannot be reliable if it does not have high quality.
Is it true always? Likewise,
• a system cannot be of high quality if it is not reliable.
 Their goal is the same – to achieve customer satisfaction
• If a system is unreliable, it is unpredictable and if it is
unpredictable, it is not of high quality.
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20. Factors affecting reliability
 customer's reliability requirements evolve over time and
are not static – has a dynamic nature.
 Reliability is influenced by the market environment partly
as a result of the introduction of new products with new
features by competitors and partly as a result of new
legislatures
 So reliability can be maximized by reducing complexity,
duplication (spare production) and allowing excess
strength in designing
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21. Reliability and Cost
• There are three separate cost factors related to the
reliability of an item throughout its life –
Design & Development cost
Production cost
Maintenance & Repair cost
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22. Maintainability
Maintainability M(t) : the probability of repairing
a failed component or system in a specified
period of time.
Two kinds:
Preventive maintainability
Corrective maintainability

23. MAINTAINABILITY
 Mean Time Between Failures(MTBF) – Applies to repairable items.
 Mean Time To Failure(MTTF) – Applies to non-repairable items. Both
of these terms indicate the average time an item is expected to
function before failure.
 MTTR: Mean Time to Repair
 Availability = MTBF
____________
MTBF + MTTR
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24. Cont.
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25. Thank you!