Operations management by stevenson 9th ch4s

Learning Objectives Define reliability  Perform reliability computations  Explain the purpose of redundancy in a system...  Reliability: The ability of a product, part, or system to

Reliability

Learning Objectives

 Define reliability

 Perform reliability computations

 Explain the purpose of redundancy in a system

 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

 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

 The use of backup components to increase reliability

Reliability is a Probability

Rule 1

Lamp 1 Lamp 2

.90 x 80 = 72

Rule 2

.90

.80

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

Lamp 2 (backup)

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)

Example S-1 Reliability

Determine the reliability of the system shown

.95

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

Failure Rate

Figure 4S.1

Few (random) failures

Infant mortality

Failures due

to wear-out Time, T

Exponential Distribution

Reliability = e -T/MTBF

1- e -T/MTBF

Figure 4S.2

Normal Distribution

Reliability

Figure 4S.3

 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

Improving Reliability

 Component design

 Production/assembly techniques

 Testing

 Redundancy/backups

 Preventive maintenance procedures

 User education

 System design