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5. Determine the required bearing life
Bearing life is quoted in millions of revolutions but it may be more convenient to think in terms of years or hours first, then calculate the required bearing life as the product of speed (rpm) and the desired life (mins) divided by 106. The type of machine and its nominal duty will determine the recommended operating hours required as shown in the following table.

CLASS OF MACHINE OPERATING HOURS
Intermittent; domestic machines, instruments 300 - 3000
Machines used for short periods: hand tools, construction machines, lifting tackle 3000 - 8000
Machines requiring high reliability during short periods: lifts, cranes 8000 - 12000
Machines used for 8 hours/day with partial utilisation: gears, electric motors 10000 - 25000
Fully utilised machines operated for 8 hours/day: machine tools, fans, conveyor belts 20000 - 30000
Machines required for continuous use: hoists, pumps, compressors, rolling mills 40000 - 50000

A service factor may be applied if the transmisson involves a significant dynamic load unless the magnitude of load fluctuations are known and can be taken into account. Use the following table if the magnitude of the fluctuations is unknown.

TRANSMISSION TYPE SERVlCE FACTOR
Precision Gears 1.05 - 1.1
Commercial 4.1 - 1.3
Toothed Belts 1.2 - 1.6
Flat Belts 2.5 - 4.0
Vee Belts 2.0 - 2.5

NB: These factors given for transmission systems take into account the pre-tensions, dynamic effects and imbalance of rotating elements. Further information may be found in manufacturers' literature.

The resultant "life" is taken as L10, the rating life defined by the ISO as that exceeded by 90% of a group of apparently identical bearings before noticable evidence of fatigue develops.

If more than 90% reliability is required then L10 should be multiplied by a factor from the following table (see ISO 281/1) to give an adjusted value L10a = a1L10,

% RELIABILITY 90 95 96 97 98 99
L10 L5 L4 L3 L2 L1
a1 1.0 0.62 0.53 0.44 0.33 0.21

Check the adjusted value is exceeds the required life.