Page 6 of catalogue Ball screws of SKF Linearsysteme GmbH

Recommendations for selection

Equivalent dynamic loads The loads acting on the screw can be calculated according to the laws of mechanics if the external forces (e.g. power transmission, work, rotary and linear inertia forces) are known or can be calculated. It is necessary to calculate the equivalent dynamic load. Radial and moment loads must be taken by linear bearing systems. It is extremely important to resolve these problems at the earliest conceptual stage. These forces are detrimental to the life and the expected performance of the screw. Fluctuating load When the load uctuates during the working cycle, it is necessary to calculate the equivalent dynamic load: this load is dened as that hypothetical load, constant in magnitude and direction, acting axially and centrally on the screw which, if applied, would have the same inuence on the screw life as the actual loads to which the screw is subjected. Additional loads due, for example to misalignment, uneven loading, shocks, and so on, must be taken in account. Their inuence on the nominal life of the screw is generally taken care of, consult SKF for advice.

Static load carrying capacity (Coa)
Ø
Load Co

Critical rotating speed for screw shafts
The shaft is equated to a cylinder, the diameter of which is the root diameter of the thread. The formulas use a parameter the value of which is dictated by the mounting of the screw shaft (whether it is simply supported or xed). As a rule the nut is not considered as a support of the screw shaft. Because of the potential inaccuracies in the mounting of the screw assembly, a safety factor of 0.80 is applied to the calculated critical speeds. Calculations which consider the nut as a support of the shaft, or reduce the safety factor, require practical tests and possibly an optimization of the design (1).

Rolling element

d
Ball Screws should be selected on the basis of the basic static load rating Coa instead of on bearing life when they are submitted to continuous or intermittent shock loads, while stationary or rotating at very low speed for short duration. The permissible load is determined by the permanent deformation caused by the load acting at the contact points. It is dened by ISO standards as the purely axially and centrally applied static load which will create, by calculation, a total (rolling element + thread surface) permanent deformation equal to 0.0001 of the diameter of the rolling element. A ball screw must be selected by its basic static load rating which must be, at least, equal to the product of the maximum axial static load applied and a safety factor “so”. The safety factor is selected in relation with past experience of similar applications and requirements of running smoothness and noise level (1).

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

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