Transcript Chapter 1

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
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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
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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
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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
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Failure Rate Curve
(Bathtub Curve)
“Infant
mortality
period”
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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
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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
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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
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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
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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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