Reliability is defined as the probability that a device will perform its required function under stated condition for a specific period of time.
F.R., F.I.T., MTBF, MTTF, MTTD, MTTR are reliability acronysms of methods for estimations based on a compilation of data for predictiong the failure rate of a component or system.
MTBF or MTTF estimates should be made during the development of the product to establish the probability of failures. With the reliability report at hand, the engineer can improve product quality by determining the better reliable components for his product.
MTBF: estimates the average time between two consecutive failures of repairable equipment. It estimates the rate of random failures, excluding systematic failures due to design errors (e.g. software errors), manufacturing defects (produced in early life) or wears at the end of life product. The MTBF is mostly a measure of the complexity of a device, the more components, the worse the estimate could be.
MTTF: is a basic measure of reliability used for non-repairable parts or systems. It represents the length of time that an item is expected to last in operation until it fails. Its value is calculated by looking at a large number of the same kind of items over an extended period and seeing what is their mean time to failure.
MTTR: describes the average time required to examine the failure of the equipment, to replace the damaged part, and to place the device into service again.
MTTD: in a system breakdown, MTTD refers to the average time from the start of the issue and the time it takes to identify the problem.
F.R: Failure rate refers to how frequently a piece fails. It is the anticipated failure in one million hours of operation.
F.I.T.: Failure in time represents another expression of the predicted failures in one billion hours of operation. This term is employed by the semiconductor and the electromechanical industries.
Methods of Analysis
The MIL-STD-217 and Telcordia SR-332 are the broad MTBF prediction methods. A specific description of the end-use environment is provided by each method to help estimate the failure rate data for the product.
Analysis by MIL-HDBK-217 includes two methods of predicting the reliability:Part Count estimations, and Part Stress estimation.
Part Count Estimation
The parts count analysis is a simple approach to determine the reliability by estimating the failure rate for each part in the device. It is performed early in the design stage to assess the reliability of the product before the manufacturing process.
Part Stress Estimation
Parts stress analysis is carried at the following stage of development and usually provides a lower and more reliable MTBF figure. Unlike the parts count analysis, parts stress takes into account more information on each component, such as the product environment, electrical stress, component quality factor, and temperature factor.
Components like batteries or fans that would require attention during the life of the system, can be managed in two possible ways:
a) Corrective maintenance: critical components are replaced after a failure. In this viewpoint, the MTBF will be seen by the battery life, resulting in a low MTBF figure.
b) Preventive maintenance: critical components are replaced before they fail. In this case, those components likely do not affect the MTBF value.
At the end of the MTBF calculation, a report will be provided which can be used by the marketing and sales department, or issued directly to your customers, when required.
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