40542873 Operations Management Stevenson Ch04s Ppt
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McGraw-Hill/Irwin Copyright © 2007 by The McGraw-Hill Companies, Inc. All rights reserved.
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 3 Rule 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 S-1 ReliabilityExample S-1 Reliability
Determine the reliability of the system shown
.98 .90
.90 .92
.95
4S-9
Example S-1 SolutionExample S-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
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