md. imrul kaes - reliability 2013-5-13

7/30/2019 Md. Imrul Kaes - Reliability 2013-5-13

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RELIABILITY


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DEFINITION

The ability of an item to perform a required

function under stated condition for a period of

time or the probability that an item will continue

to perform its intended function without failure

for a specified period of time under statedconditions.

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MAJOR IMPORTANCEOF RELIABILITY:

Reputation: a companys reputation is closelyrelated to the reliability of the product. The morereliable a product is, the more likely thecompany is to have a favorable reputation.

Customer satisfaction: while a reliable productmay not dramatically affect customersatisfaction in a positive manner, an unreliableproduct will negatively affect customersatisfaction.

Warranty Cost: if a product fails to perform itsfunction within the warranty period, thereplacement and the repair costs will negativelyaffect profits.

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GOAL:

The goal of reliability engineering is to evaluate

the inherent reliability of a product and pinpoint

the potential areas for reliability improvement.

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RELIABILITY REDEFINED

High quality does not mean high reliability

Components can pass all quality control

checks, but when they are assembled, theinteraction effect may decrease the long term

operational capability of the assembly. Similarly,

when too many technically sophisticated

components are assembled, it may lose

reliability up to a certain degree.

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System: Input Transformation Process -

Output

Series System: where all components are

assembled functionally in series.

Parallel System: where all components are

placed functionally in parallel.

Combination System: a combination of series

and parallel systems.

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SERIES SYSTEM

Here the system will work if all the componentswork simultaneously.

Rs = 0.99 x 0.98 x 0.96 = 0.93 = 93%

System reliability is lower than the lowestreliability of individual component.

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PARALLEL SYSTEM

Reliability of a system can be increased by placing

components in parallel. Thus, a system consisting

of n components in parallel will fail only if all

components fail simultaneously.

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B1

B2A

B3

C


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If A, B, C, D are in parallel connection and their failure

probability is F(A), F(B), F(C), F(D) correspondingly.

So The failure probability of the system

F = F(A) x F(B) x F(C) x F(D)

So, the reliability of the system R = 1-F

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WHENRELIABILITYCHANGESWITHTIME5

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Failure Rare (): On an average how many

components fail per hour.

MTBF (1/ ): On an average how many hours taken

by a component to be failed.

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F(t) = 1- R(t)


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Failure rate () of a system which has a series of components:

= 1+ 2+..+ n

MTBF (1/ ) of a system which has parallel - connectedcomponents:

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Reliability of the components are B1 = B2 = B3 =

0.75 and A = C = 0.97. These reliabilities are given

for 100 hours. Find out MTBF of the system per

10,000 hours.

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B1

B2A

B3

C

md. imrul kaes - reliability 2013-5-13

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