project & quality management quality management reliability

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  • Slide 1
  • Project & Quality Management Quality Management Reliability
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  • Reliability Management Why is it needed? Reliable operation of critical equipment Planning of maintenance activities Improved quality of an item
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  • Reliability Management Reliability management is concerned with performance and conformance over the expected life of the product the probability that a product or a piece of equipment performs its intended function for a stated period of time under specified operation conditions
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  • Definition of Reliability The definition has four important elements: Probability Time Performance Operating conditions
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  • Definition of Reliability Probability A value between 0 and 1 Precise meaning e.g. probability of 0.97 means that 97 of 100 items will still be working at stated time under stated conditions
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  • Definition of Reliability Performance Some criterion to define when product has failed e.g. bearing clearances in an engine or amount of emissions from a car
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  • Definition of Reliability Operating conditions These describe the operating conditions that correspond to the stated product life. e.g. for a car engine this might mean Speed Loading Effects of an expected amount of misuse such as over-revving and stalling.
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  • Reliability Measurement This is based on the Failure Rate i.e. Some products are scrapped when they fail e.g. hairdryer Others are repaired e.g. washing machine.
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  • Failure rate over the life of a product The failure rate is expected to vary over the life of a product Bathtub Curve A C D B
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  • Bathtub Curve A-B Early Failure Teething problems. Caused by design/material flaws B-C Constant Failure Lower than initial failure rate and more or less constant until end of life C-D End of life failure Failure rate rises again due to components reaching end of life
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  • Simplifying Assumption Exponential distribution of failure rate is assumed. This means that the failure rate remains constant over life of product Bathtub curve becomes a straight line Calculating Failure Rate
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  • Failure rate usually expressed by the Greek letter lambda ( ) The probability of a product surviving until time (t) is given by the following function: Reliability at time (t) = e is the exponential function
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  • Procedure To establish reliability of an item: Conduct a series of tests until a number of them fail. Calculate failure rate (Lambda). Calculate reliability for a given time using Reliability at time (t) = e- t
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  • Example Trial data shows that 105 items failed during a test with a total operating time of 1 million hours. (For all items i.e. both failed and passed). The failure rate
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  • Example Find the reliability of the product after 1000 hours i.e. (t) =1000 Reliability at 1000 hours: R (1000) = 0.9 Therefore the item has a 90% chance of surviving for 1000 hours


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