Page 5 of catalogue Ball screws of SKF Linearsysteme GmbH

Recommendations for selection

Recommendations for selection
Only basic selection parameters are included. To make the very best selection of a ball screw, the designer should specify such critical parameters as the load profile, the linear or rotational speed, the rates of acceleration and deceleration, the cycle rate, the environment, the required life, the lead accuracy, the stiffness, and any other special requirement. If in doubt, please consult an SKF ball screw specialist before placing an order.

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Basic dynamic load rating (Ca)
The dynamic rating is used to compute the fatigue life of ball Screws. It is the axial load constant in magnitude and direction, and acting centrally under which the nominal life (as dened by ISO) reaches one million revolutions.

Nominal fatigue life L10 The nominal life of a ball screw is the number of revolutions (or the number of operating hours at a given constant speed) which the ball screw is capable of enduring before the rst sign of fatigue (aking, spalling) occurs on one of the rolling surfaces. It is however evident from both laboratory tests and practical experience that seemingly identical ball screws operating under identical conditions have different lives, hence the notion of nominal life.

It is, in accordance with ISO denition, the life achieved or exceeded by 90 % of a sufciently large group of apparently identical ball screws, working in identical conditions (alignment, axial and centrally applied load, speed, acceleration, lubrication, temperature and cleanliness). Service life The actual life achieved by a specic ball screw before it fails is known as “service life”. Failure is not only by fatigue (aking or spalling); but also by inadequate lubrication and wear; wear of the recirculation system, corrosion, contamination, and, more generally, by loss of the functional characteristics required by the application. Experience acquired with similar applications will help to select the proper screw to obtain the required service life. One must also take into account structural requirements such as the strength of screw ends and nut attachments, due to the loads applied on these elements in service. To attain L10 life performance a mean working load of up to 80 % of Ca and a stroke higher than 4 leads are permitted.

Life test bench

(1) SKF can help you to define this value in relation with the actual conditions of service.

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