Quality & Reliability Quality & Reliability Quality & Reliability Quality is a snapshot at the start of product life and reliability is a motion picture of the day-by-day operation. Time zero defects are manufacturing mistakes that escaped final test. The additional defects that appear over time are "reliability defects" or reliability fallout. The quality level might be described by a single fraction defective. To describe reliability fallout a probability model that describes the fraction fallout over time is needed. This is known as the life distribution model. Quality & Reliability • Reliability (for non-repairable items) can be defined as “the probability that an item will perform a defined function without failure under stated conditions for a stated period of time.” • One must grasp the concept of probabilities in order to understand the concept of reliability. The numerical values of both reliability and unreliability are expressed as a probability from 0 to 1 and have no units. • The Unreliability, F(t), of a component or system is defined as the probability that the component or system experiences the first failure or has failed one or more times during the time interval zero to time t, given that it was operating or repaired to a like new condition at time zero. • Relation between Reliability & Un-reliability: • R(t) + F(t) = 1 • or • Un-Reliability F(t) = 1 – R(t) Difference between quality and reliability • Quality is today, while reliability is the future. • It is not possible to improve the performance of a system by using low quality components, whereas it is certainly possible to improve the performance of a system by using components with low reliability. • When once the product is accepted by the customer and put into working, either by itself or as a part of a bigger assembly, length of the useful service indicates the quality of performance, which is called reliability. • Reliability depends on numerous factors and majority of them are random. Reliability cannot be measured by means of any instrument. The reliability of the components of complex equipment depends on the method of manufacturing, the environmental conditions in which the equipment performs and so on. In view of these considerations, the equipment’s reliability is closely related to many uncertainty factors. Survival probability • In reliability, the survival probability is the proportion of units that survive beyond a specified time. These estimates of survival probabilities are frequently referred to as reliability estimates. Use these values to determine whether your product meets reliability requirements or to compare the reliability of two or more designs. • For example, A Zipper(Garments accessories) manufacturer is studying the reliability of a component part in an accelerated life test. The survival probability at 1 year is 0.197736. This means that at 1 year , approximately 19.77% of these parts will have not yet failed. Hazard Rate • The hazard or failure rate is the rate at which the population survivors at any given instant are "falling over the cliff“. • Hazard rate refers to the rate of death for an item of a given age, and is also known as the failure rate. It is part of a larger equation called the hazard function, which analyzes the likelihood that something will survive to a certain point in time based on its survival to an earlier time.In other words, it is the likelihood that if something survives to one moment, it will also survive to the next. Hazard rate only applies to those items which cannot be repaired. Hazard Rate • The hazard rate for any time can be determined using the following equation: h(t) = f(t) / R(t) • f(t) is the probability density function, or the probability that the value (failure or death) will fall in a specified interval (for example, a specific year). • R(t) is the survival function, or the probability that something will survive past a certain time (t). • The hazard rate cannot be negative, and it is necessary to have a set "lifetime" on which to model the equation. THE END
