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Life & MTBF

The vast majority of fan and blower failures occur because either the motor insulation or the motor bearings fail, with bearing failures being more predominant. Rotron fans and blowers use ball bearings exclusively, and the life of these bearings and lubricants are affected by temperature and speed. The typical dynamic load imposed by fans and blowers is such that bearing fatigue is not considered as contributing to the degradation of component life. Bearing failures occur more frequently than motor insulation breakdown failure, a ratio of 4:1 being typical.

The principle degradation mechanism of the grease in ball bearings, oxidation and evaporation, are of a chemical nature so the Arrhenius relationship that life will be halved for each 15C increase in temperature is approximately valid. This slope of the live vs. Temperature curve varies considerably, however, for each grease in different temperature ranges. For instance, it has been found that the slope is much steeper at temperature below 50C. Life tests run on a grease at certain specific temperatures provide a more reliable method of arriving at statistically valid life data and Rotron has an appreciable amount of this data available. Contact the Marketing Department for curves of life vs. Temperature for the specific blower of interest. This data is based on blowers running continuously at a constant temperature. The actual running life will be shorter if the unit is subjected to frequent start-stop cycles and/or temperature cycling.

The following definitions relating to operating life are in common usage:


L10LIFE is that life in hours beyond which 90% of a given population of blowers can be expected to survive. This expression has been used for many years by the AFBMA (Anti-Friction Bearing Manufacturers' Association) to express the fatigue life of rolling contact bearings. It is an indication of the number of "infant mortality" failures which might be encountered from a given population of blowers produced with normal manufacturing techniques.

Mean Time Between Failures
MTBF, or mean life, is, for an exponential failure rate, the time where 63.2% of a given population of a specific component has failed. Most electronic components exhibit a random failure pattern which is best characterized by this exponential distribution (i.e., constant) failure rate. Fractional horsepower motors of the type used in fans and blowers, however, exhibit a "wear-out" pattern of failure in which the failure rate is not constant but increases with age. This pattern is best described by the Weibull distribution. The MTBF can be approximated as that time when 50% of motors has failed.

Failure Rate

The failure rate is reciprocal of the MTBF, i.e.
FR = 1 / MTBF