This document defines reliability and discusses how it can be calculated and improved. It defines reliability as the ability of a system to perform its intended function under normal conditions. Reliability is calculated as a probability. Redundancy, or adding backup components, increases overall system reliability. Several rules for calculating the reliability of systems with multiple components are presented. Availability is defined as the fraction of time a system is operational versus down for repairs. Methods to improve reliability discussed include component design, production techniques, testing, redundancy, maintenance procedures, user education, and system design.

1. McGraw-Hill/Irwin Copyright © 2010 by The McGraw-Hill Companies, Inc. All rights reserved.
4S4S
Reliability

2. 4S-2
Learning ObjectivesLearning Objectives
 Define reliability
 Perform reliability computations
 Explain the purpose of redundancy in a
system

3. 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

4. 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

5. 4S-5
Rule 1Rule 1
.90 .80
Lamp 1 Lamp 2
.90 x .80 = .72

6. 4S-6
Rule 2Rule 2
.90
.80
.90 + (1-.90)*.80 = .98
Lamp 1
Lamp 2 (backup)

7. 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)

8. 4S-8
Example 4S-1: ReliabilityExample 4S-1: Reliability
Determine the reliability of the system shown
.98 .90
.90 .92
.95

9. 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

10. 4S-10
Failure RateFailure Rate
Figure 4S.1
Few (random) failures
Infant
mortality
Failures due
to wear-out
Time, T
FailureRate

11. 4S-11
Exponential DistributionExponential Distribution
Reliability = e -T/MTBF
1- e -T/MTBF
T Time
Figure 4S.2

12. 4S-12
Normal DistributionNormal Distribution
Reliability
0 z
Figure 4S.3

13. 4S-13
AvailabilityAvailability
 The fraction of time a piece of equipment
is expected to be available for operation
Availability
MTBF
MTBF MTR
=
+
MTBF = mean time between failures
MTR = mean time to repair

14. 4S-14
Improving ReliabilityImproving Reliability
 Component design
 Production/assembly techniques
 Testing
 Redundancy/backups
 Preventive maintenance procedures
 User education
 System design