Transcript RELIABILITY - Ahmad Nazif Bin Noor Kamar

BPT2423 – STATISTICAL PROCESS CONTROL
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Fundamental Aspects
Product Life Cycle Curve
Measures of Reliability
 Failure Rate, Mean Life and Availability
 Calculating System Reliability
Reliability Engineers
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Known the importance of system reliability
Identify on how performance during the life of
a product, process or system is affected by its
design and configuration
Able to compute the reliability of systems
including systems in series, parallel and hybrid
combinations
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Reliability (quality over the long term) is the ability of a
product to perform its intended function over a period
of time and under prescribed environmental conditions
Reliability of component dependent on many factors:
 quality of research performed at its conception
 the original design and any subsequent design
changes
 the complexity of the design
 the manufacturing processes
 the handling received during shipping
 the environment surrounding its use
 the end users and etc.
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Reliability testing enables a company to better
comprehend how their products will perform under
normal usage as well as extreme or unexpected
situations
Reliability programs provide information about product
performance by systematically studying the product
Four factors associated with reliability:
1. Numerical value
2. Intended function
3. Life
4. Environmental conditions
Life cycle of a product is commonly broken down into 3
phases:
1. Early failure (infant mortality)
 phase is characterized by failures occurring very quickly
after the product has been produced or put into use by the
consumer
 the curve during this phase is exponential with the number
of failures decreasing the longer the product is in use
 some early failures are due to inappropriate or inadequate
materials, marginal components, incorrect installation or
poor manufacturing techniques
2. Chance failure
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During the failure, portion of a product’s useful life, failures
occur randomly
May be due to inadequate or insufficient design margins
Misapplication or misuse of the product by the consumer
can lead to product failure – eg.: overstressing
3. Wear-out
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Failures increase in number until few, if any of the product
are left
Due to a variety of causes such as related to actual product
function or cosmetic (scratched, dented, discolorations,
misalignments and interference between components)
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Overall system reliability depends on the individual
reliabilities associated with the parts, components and
subassemblies
Reliability values are sought to determine the
performance of a product, reveal any recurring patterns
of failure and the underlying causes of those failures
Reliability test determine what failed, how it failed and
the number of hours, cycles, actuations or stresses it was
able to bear before failure
If result known, decisions can be made concerning
product reliability expectations, corrective action steps,
maintenance procedures and cost of repair/replacement
Several different types of test exist to judge the reliability
of a product :
1. Failure-terminated test
Ended when a predetermined number of failures occur
within the sample being tested
 Decision concerning whether or not the product is
acceptable is based on the number of products that have
failed during the test
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2.
Time-terminated test
Concluded when an established number of hours is reached
 Product is accepted on the basis of how many products
failed before reaching the time limit
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3.
Sequential Test - relies on the accumulated results of the
tests
Determine Failure Rate, λ
Determine Average Life, θ
or
Determine Availability
Example :
Determine the failure rate for a 90-hour test of 12 items
where 2 items fail at 45 and 72 hours, respectively. What
is the mean life of the product?
λ =
2
45 + 72 + (10) 90
= 2 / 1017
= 0.00197
θ = 1/λ
= 1 / 0.00197
= 507.6 hours
Reliability is the probability that a product will not fail during a
particular time period.
where :
s – those units performing satisfactorily
n – total number of units being tested
Reliability in Series
Reliability in Parallel
Reliability in Redundant Systems and Backup Components
r1
rb
Example:
Calculation :
Exercise :
A paranoid citizen has installed the home alert system shown below.
What is the overall system reliability?