What is Reliability?

Reliability is the probability that a device, system, or process will perform its prescribed duty without failure for a given time when operated correctly in a specified environment.

Why Reliability?

Reliability has two broad ranges of meanings: 1) qualitatively-operating without failure for long periods of time just as the advertisements for sale suggest, and 2) quantitatively-where life is predictable long and measureable in test to assure satisfactory field conditions are achieved to meet customer requirements. Reliability is concerned with failure-free operation for periods of time, whereas quality is concerned with avoiding non-conformances at a specified time prior to shipment thus reliability measures a dynamic situation but quality measures a static situation. As in physics, statics is easier to understand and calculate than dynamics which involves higher levels of math and greater mental capabilities for comprehension.

When Reliability?

Reliability is expected for new equipment to start, run, and continue to function for long periods of time without failure. Reliability is also expected when the equipment is dormant and called to duty. Reliability is also expected upon service or restoration and resumption of long life. Reliability is designed into the system by up-front activities, and reliability is sustained by careful operation of the system along with careful nurturing of the system with sustaining maintenance activities. Reliability always terminates in a failure and the roots of failure can be due to design, fabrication, installation, operation, maintenance (repair and period servicing), and management of the system-in short there are many ways and means to kill the system but few ways to keep is operating without failure.

Where Reliability?

The adage says the proof of the pudding is in the eating; and for reliability, the proof of the system is in the long failure free interval. Reliability tools are used from stem to stern to demonstrate high reliability (the absence of failures for long periods of time) by use of many tools such as:

• reliability acceptance test to demonstrate long life,
• reliability analysis to compute the expected results,
• reliability and maintainability the mathematical tasks which predict the expected results from the elements,
• reliability apportionment to allocate life issues in a top-down manner to meet an overall reliability goal,
• reliability assessment determines the achieved level of reliability of an existing system using data gathered during test or use
• reliability assurance implements planned management and technical measures to provide confidence that a reliability target is obtained and maintained,
• reliability block diagrams to graphically and mathematically calculate reliability results prior to building a system,
• reliability-centered maintenance is the systematic approach to identify preventive support and service according to a set of procedures to reduce and avoid failures,
• reliability confidence limits demonstrate the limits for reliability within a given confidence limit,
• reliability control is the coordination and direction of system dependability through design activities and management planning,
• reliability critical item identification whereby failure significantly affects system safety/cost or operational success or maintenance/logistics support costs,
• reliability data is the basic age-to-failure data as life unit information relating to the time-to-failure when organized by probability distributions,
• reliability degradation which incurs loss of the failure-free performance due to poor workmanship or bad parts or improper operation or abuse or inadequate maintenance,
• reliability design practices are a series of trade-off-tools to meet or beat the design specification for reliability,
• reliability development/growth test are the evaluations to disclose deficiencies and verify corrective actions to prevent reoccurrence of the failures to achieve the design specifications and sustain reliability growth toward longer times between failure,
• reliability estimates are life values used prior to statistical experimentation with the end products to make predictions or assessments, or stress analysis evaluations,
• reliability function is the graphical representation of life characteristics plotted against operating time,
• reliability growth achievement is the systematic improvements of a item/systems dependability by removing failure mechanisms through corrective actions to eliminate deficiencies and flaws often achieved by means of test-analyze and fix,
• reliability growth models (Crow-AMSAA) measures the reliability growth by means of log-log plots of cumulative failures on the Y-axis and cumulative time on the X-axis to demonstrate with statistics that failures are coming more slowly and reliability goals have been achieved,
• reliability guarantee is the commitment by suppliers to provide a given meant time between replacements or to maintenance and overhauls intervals for equipment,
• reliability improvement is the identification of failure modes and effects having a critical impact on the system failure potential of the design along with the systematic removal of the failures to produce long life without failures,
• reliability index is the ratio of the mean reliability level achieved to the acceptable level specified in the design as a figure of merit,
• reliability measurement is failure free endurance assessment activity for making decisions about reliability and demonstrating compliance,
• reliability mission is the mission time for demonstrating failure free performance,
• reliability prediction is the process of quantitatively assessing whether a proposed or existing deign meets a specified life requirement,
• reliability prediction functions estimate the life characteristics for setting goals and evaluating the design benchmarks and needs,
• reliability prediction limitations describes the shortcomings in life values by analytical methods
• reliability prediction requirements describes life assumptions, environmental data, and failure rates for the design,
• reliability prediction summary is a report providing conclusions and recommendations based upon an reliability assessment analysis
• reliability program at the activities to organize and achieve a system to insure reliability goals are achieved and deficient areas shored-up,
• reliability program plan is the formal written definition of the specific tasks to fulfill the reliability requirements<
• reliability qualification test (RQT) is an evaluation conducted under specified conditions using items representative of the approved product configuration,
• reliability quantitative elements are the life characteristics and factors considered in predicting and measuring reliability performance,
• reliability requirements are the numerical values representing a specified failure-free life or dependability performance characteristic,
• reliability sequential tests are evaluations of the number of failures and the time required to reach a decision based on the accumulated results of the reliability tests,
• reliability tasks describe the activities required to achieve a reliability program,
• reliability tests are the formal evaluation to determine a product's longevity for the failure-free interval or stability relative to time/usage,

and finally

• reliability with repair is the failure-free performance achieved by redundancy with permitted online repairs without interrupting equipment operation.

These definitions are written by H. Paul Barringer

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