Module 13
Reliability
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Key Dimensions of Quality
• Performance – primary operating characteristics
• Features – “bells and whistles”
• Reliability – probability of operating for specific time
and conditions of use
• Conformance – degree to which characteristics match
standards
• Durability – amount of use before deterioration or
replacement
• Serviceability – speed, courtesy, and competence of
repair
• Aesthetics – look, feel, sound, taste, smell
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Key Dimensions of Service Quality
• Reliability – ability to provide what was promised
• Assurance – knowledge and courtesy of
employees and ability to convey trust
• Tangibles – physical facilities and appearance of
personnel
• Empathy – degree of caring and individual
attention
• Responsiveness – willingness to help customers
and provide prompt service
3
Reliability
• Generally defined as the ability of a
product or service to perform as expected
over time
• Formally defined as the probability that a
product, piece of equipment, or system
performs its intended function for a stated
period of time under specified operating
conditions
4
Maintainability
• The probability that a system or product
can be retained in, or one that has failed
can be restored to, operating condition in a
specified amount of time.
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Examples
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Types of Failures
• Functional failure – failure that occurs at
the start of product life due to
manufacturing or material detects
• Reliability failure – failure after some
period of use
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Types of Reliability
• Inherent reliability – predicted by
product design
• Achieved reliability – observed during
use
8
Reliability Measurement
• Failure rate (l) – number of failures per
unit time (e.g. # of operating hours)
• Alternative measures
– Mean time to failure
– Mean time between failures (1/ l)
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Cumulative Failure Rate Curve
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Average Failure Rate
11
Failure Rate Curve
(Bathtub Curve)
“Infant
mortality
period”
12
Reliability Function
• Probability density function of failures
f(t) = le-lt for t > 0
• Probability of failure from (0, T)
F(T) = 1 – e-lT
• Reliability function
R(T) = 1 – F(T) = e-lT
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Reliability Function (Example)
• So an item having a reliability of 0.97
over 100 hours of normal use, determine
the failure rate (l):
Reliability function: R(T) = e-lT
0.97 = e-100l  ln 0.97 = -100 l
l = -(ln 0.97)/100  0.0304/100
= 0.0003 failure rate (per hour)
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Series Systems
1
2
n
RS = R1 R2 ... Rn
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Series Systems (Example)
So a two component series system with failure rates
of 0.001 and 0.004 per hour:
Rs(T) = e-(0.004+0.001)T

e-0.005T
The probability of survival for 100 hours would be:
Rs(100) = e-0.005(100) 
e-0.5  0.6065 or 60.65%
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Parallel Systems
1
2
n
RS = 1 - (1 - R1)*(1 - R2)...*(1 - Rn)
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Series-Parallel Systems
C
RA
RB
A
B
RC
RD
D
C
RC
• Convert to equivalent series system
RA
RB
A
B
RD
C’
D
RC’ = 1 – (1-RC)(1-RC)
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Reliability Engineering
• Standardization (Certified Components)
• Redundancy (Series systems vs. Parallel)
• Physics of failure (Weathering)
• Reliability testing
– Accelerated life testing
• Burn-in
– Component Stress Testing
– Infant Mortality Periods
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Exam Review
• Demming’s 14 Points (Modules 3, 4)
• Juran’s Principles (Modules 3, 9/10)
• Definitions of Quality (Module 1)
• Key Dimensions of Quality (Module 4)
– Manufacturing & Service
• Continuous Improvement (Module 1, 2)
• Quality in Services (Module 2, 4, 13)
• Customer Needs (Module 4)
• Leadership (Mission/Vision) (Module 5)
• Process Management (Scope, Principles) (Module 7)
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Exam Review
• Performance Measures (COQ and ROQ) (Module 8)
• Capability Studies (Module 9/10)
– Variation
– Six Sigma
• Sampling & Measurement (R&R) (Module 11)
• Control Charts (Module 12)
– How to read & interpret
• Reliability & Maintainability (Module 13)
• Total Quality Management (Modules 1)
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Reliability Management
• Define customer performance requirements
• Determine important economic factors and
relationship with reliability requirements
• Define the environment and conditions of
product use
• Select components, designs, and vendors that
meet reliability and cost criteria
• Determine reliability requirements for machines
and equipment
• Analyze field reliability for improvement
24
Configuration Management
1. Establish approved baseline configurations
(designs)
2. Maintain control over all changes in the
baseline programs (change control)
3. Provide traceability of baselines and changes
(configuration accounting)
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Maintainability
• Maintainability is the totality of design
factors that allows maintenance to be
accomplished easily
• Preventive maintenance reduces the risk
of failure
• Corrective maintenance is the response to
failures
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Design Issues
• Access of parts for repair
• Modular construction and
standardization
• Diagnostic repair procedures and expert
systems
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Availability
• Operational availability
MTBF
AO =
MTBM  MDT
• Inherent availability
MTBF
AO =
MTBF  MTR
MTBM = mean time between
maintenance
MDT = mean down time
MTBF = mean time between
failures
MTR = mean time to repair
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