Fiabilidad

Reliability and Life
An Introduction to Reliability Terminology, Modeling, Prediction, and MTBF
Summary
This article provides an introduction to reliability engineering terminology and concepts, such as reliability functions, failure rates, typical failure patterns, Weibull and exponential failure distributions, and Mean-Time-Between-Failure (MTBF). Life concepts of rolling element bearingsare explained. Examples and references for further reading are also provided.

GS02005 Gerard Schram and René Klerx SKF Engineering & Research Center 14 pages April 2003 SKF Reliability Systems @ptitudeXchange 5271 Viewridge Court San Diego, CA 92123 United States tel. +1 858 496 3554 fax +1 858 496 3555 email: info@aptitudexchange.com Internet: www.aptitudexchange.com

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Reliability and Life

Introduction
High reliability and maintainability levels are crucial for asset availability, which is the ultimate goal of maintenance practices. Often, the term reliability is (miss)used in qualitative sense without a sound understanding of failure rate patterns and real data. This article provides anintroduction to this field without creating a mathematical burden. Rudimentary terms and concepts are explained, followed with examples. For detailed treatment of the subject, refer to the various references.

includes the full period of time regardless of actual operation time. It is common for these two terms to be confused or misunderstood. Failure rates reported or collected from maintenancemanagement systems tend to be in terms of MTTF. Reliability: The probability that equipment, machinery or systems will perform their required functions satisfactorily under specific conditions within a certain time period. This could be measured by mean time between failure (MTBF) - the duration or probability of failure-free performance under stated conditions. The term reliability may also beused to denote the probability of success or success ratio. Often a distinction is made between inherent and operational reliability. Inherent reliability relates to a product reliability from design point-of-view. Operational reliability stands for the product being operational in the field. In the context of maintenance, we generally speak in terms of operational reliability. Failure rate: Thetotal number of failures within an item population, divided by the total time expended by that population during a particular measurement interval under stated conditions. Other formats include the number of failures per year, and in some cases, it is common to express failure rate as the number of failures per hour, or the number of failures of an item per unit time. Pump example (1): Assume 2 pumpsfailed out of a population of 16 pumps during three months. The failure rate is (2/16) / (0.25 year) = 0.5/year. Hazard rate: Hazard rate is the instantaneous speed of failure - being the instantaneous failure rate. Hazard rate is the ratio of failures that occur in an interval to the size of the population at the start of the interval, divided by the length of time. Of course, in case of 2Definitions
This article begins by defining common terminology. Mean Time Between Failures (MTBF): A basic measure of reliability for repairable items. Mean time between failures is calculated from the total accumulated operating time divided by the number of failures during the same period.
MTBF = Total Accumulated Operating Time Number of Failures

Mean Time Between Critical Failure (MTBCF): Ameasure of system reliability, which includes the effects of any fault tolerance that exist. The average time between failures that cause a loss of a system function defined as “critical” by the customer. Mean Time to Failure (MTTF): A basic measure of reliability for non-repairable items. Mean time to failure is calculated from the total elapsed time or operating time divided by the total...