RELIABILITY. BPT2423 – STATISTICAL PROCESS CONTROL. CHAPTER OUTLINE. Fundamental Aspects Product Life Cycle Curve Measures of Reliability Failure Rate, Mean Life and Availability Calculating System Reliability Reliability Engineers. LESSON OUTCOMES.
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RELIABILITY BPT2423 – STATISTICAL PROCESS CONTROL
CHAPTER OUTLINE • Fundamental Aspects • Product Life Cycle Curve • Measures of Reliability • Failure Rate, Mean Life and Availability • Calculating System Reliability • Reliability Engineers
LESSON OUTCOMES • 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
FUNDAMENTAL ASPECTS • 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.
FUNDAMENTAL ASPECTS • 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: • Numerical value • Intended function • Life • Environmental conditions
LIFE-HISTORY CURVE Life cycle of a product is commonly broken down into 3 phases: • 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
LIFE-HISTORY CURVE • Chance failure • 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 • Wear-out • 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)
MEASURES OF RELIABILITY • 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
MEASURES OF RELIABILITY Several different types of test exist to judge the reliability of a product : • 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 • 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 • Sequential Test - relies on the accumulated results of the tests
MEASURES OF RELIABILITY Determine Failure Rate, λ Determine Average Life, θ or Determine Availability
MEASURES OF RELIABILITY 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 θ = 1 / λ = 1 / 0.00197 = 507.6 hours = 2 / 1017 = 0.00197
SYSTEM RELIABILITY 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
SYSTEM RELIABILITY Reliability in Parallel Reliability in Redundant Systems and Backup Components r1 rb
SYSTEM RELIABILITY Example: Calculation :
SYSTEM RELIABILITY Exercise : A paranoid citizen has installed the home alert system shown below. What is the overall system reliability?