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4S4S

Reliability

4S-2

Learning ObjectivesLearning Objectives

Define reliability Perform reliability computations Explain the purpose of redundancy in a

system

4S-3

ReliabilityReliability

Reliability: The ability of a product, part, or system to perform its intended function under a prescribed set of conditions

Failure: Situation in which a product, part, or system does not perform as intended

Normal operating conditions: The set of conditions under which an item’s reliability is specified

4S-4

Probability that the product or system will: Function when activated Function for a given length of time

Independent events Events whose occurrence or

nonoccurrence do not influence each other Redundancy

The use of backup components to increase reliability

Reliability Is a ProbabilityReliability Is a Probability

4S-5

Rule 1Rule 1

.90 .80

Lamp 1 Lamp 2

.90 x .80 = .72

4S-6

Rule 2Rule 2

.90

.80

.90 + (1-.90)*.80 = .98Lamp 1

Lamp 2 (backup)

4S-7

Rule 3Rule 3

.90

.80

.70

1-[(1-.90)*(1-.80)*(1-.70)] = .994

1 – P(all fail)

Lamp 1

Lamp 2 (backup for Lamp1)

Lamp 3 (backup for Lamp 2)

4S-8

Example 4S-1: ReliabilityExample 4S-1: Reliability

Determine the reliability of the system shown

.98 .90

.90 .92

.95

4S-9

Example 4S-1: SolutionExample 4S-1: Solution

The system can be reduced to a series of three components

.98 .90+.90(1-.90) .95+.92(1-.95)

.98 x .99 x .996 = .966

4S-10

Failure RateFailure RateFigure 4S.1

Few (random) failuresInfantmortality

Failures dueto wear-out

Time, T

Fai

lure

Rat

e

4S-11

Exponential DistributionExponential Distribution

Reliability = e -T/MTBF

1- e -T/MTBF

T Time

Figure 4S.2

4S-12

Normal DistributionNormal Distribution

Reliability

0 z

Figure 4S.3

4S-13

AvailabilityAvailability

The fraction of time a piece of equipment is expected to be available for operation

AvailabilityMTBF

MTBF MTR

MTBF = mean time between failuresMTR = mean time to repair

4S-14

Improving ReliabilityImproving Reliability

Component design Production/assembly techniques Testing Redundancy/backups Preventive maintenance procedures User education System design