Stevenson9e_ch4s.ppt

4S

Reliability

McGraw-Hill/Irwin

Copyright © 2007 by The McGraw-Hill Companies, Inc. All rights reserved.

Learning Objectives  Define reliability  Perform reliability computations  Explain the purpose of redundancy in a system

4S-2

Reliability  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-3

Reliability is a Probability  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 4S-4

Rule 1

Lamp 1

.90

Lamp 2

.80

.90 x .80 = .72

4S-5

Rule 2

.80

Lamp 2 (backup)

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

.90 Lamp 1

4S-6

Rule 3 .70 Lamp 3 (backup for Lamp 2)

.80 Lamp 2 (backup for Lamp1)

1 – P(all fail) 1-[(1-.90)*(1-.80)*(1-.70)] = .994

.90 Lamp 1

4S-7

Example S-1 Reliability Determine the reliability of the system shown

.98

.90

.92

.90

.95

4S-8

Example 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-9

Failure Rate

Failure Rate

Figure 4S.1

Infant mortality

Failures due Few (random) failures to wear-out Time, T

4S-10

Exponential Distribution Figure 4S.2

Reliability = e -T/MTBF 1- e -T/MTBF T

Time

4S-11

Normal Distribution Figure 4S.3

Reliability

0

z

4S-12

Availability  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

4S-13

Improving Reliability       

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

4S-14