Page 25 of Ball Screws by RBC Bearings

8 service life the service life of ball screws should be considered in terms of both wear life and fatigue life accuracy deterioration due to wear is largely influenced by operating conditions lubrication effectiveness etc 8.1 wear life normal maintenance lubrication and the use of contamination prevention devices such as bellows or telescopic covers will keep wear to a minimum the amount of wear is proportional to the number of cycles under load that the system experiences severe operating conditions such as high speed oscillation less than one turn or hostile dirty environments will shorten the wear life 8.2 fatigue life the fatigue life of the ball screw may be estimated from the basic dynamic rated load as is the case of rolling bearings 8.2.1 basic dynamic rated load ca the basic dynamic rated load is an axial load which allows 90 of a group of identical ball screws rotated under the same conditions to rotate without flaking for 1 million revolutions this basic dynamic rated load is shown in the table of dimensions for each series 8.2.2 fatigue life a calculation of fatigue life the fatigue life is generally expressed by the total number of revolutions the total rotation hours or total travel distance may also be used to express life the fatigue life is calculated as follows l l ca fa fw [ca fa x fw 3 x 106 where rated fatigue life revolutions basic dynamic rating lbs axial load lbs load factor [use 1.0~1.2 for smooth impact free rotation 1.2~1.5 for average conditions 1.5 for rotation with impact selection of an excessively long fatigue life for the application is not economical and results in a large size ball screw typical targets for fatigue life are machine tool industrial machinery automatic control systems measurement system 20,000 hours 10,000 hours 15,000 hours 15,000 hours 25

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