Accrued Operating Time

the length of time a piece, of equipment functions or the volume of work it does during a certain time interval. Examples include 24-hour accrued operating time, monthly accrued operating time, accrued operating time to first failure, accrued operating time between failures, and accrued operating time between overhauls. Accrued operating time represents a major index of reliability.

Accrued operating time may be measured in hours and minutes, cubic meters, hectares, kilometers, tons, or cycles. Such a measure depends on the technical characteristics of the equipment and the conditions in which it is used. For example, the 24-hour accrued operating time of an excavator, expressed in cubic meters of earth moved, depends on such factors as the length of time the excavator can work, the characteristics of the soil, and the volume of the scoop. Because operating time is influenced by such factors as temperature and humidity or differences in the structure and strength of the parts of the equipment, it can be considered a random quantity.

Accrued operating time may be expressed as average accrued operating time until first failure for equipment that cannot be repaired and as average accrued operating time between failures or accrued operating time per failure for equipment that can be repaired. During the planning stage, average accrued operating time until first failure or accrued operating time per failure is calculated on the basis of preassembly reliability characteristics of the equipment’s components. During use of the equipment these indexes are further determined by the methods of mathematical statistics using data about accrued operating times of similar equipment.