Page 8 of catalogue Ball screws of SKF Linearsysteme GmbH

Recommendations for selection

Preload torque: Internally preloaded Screws exhibit a torque due to this preload. This persists even when they are not externally loaded. Preload torque is measured when assembly is lubricated with ISO grade 64 oil. Starting torque: This is dened as the torque needed to overcome the following to start rotation: a) the total inertia of all moving parts accelerated by the energy source (including rotation and linear movement). b) the internal friction of the screw/nut assembly, bearing and associated guiding devices. In general, torque to overcome inertia (a) is greater than friction torque (b). The coefcient of friction of the high efciency screw when starting µs is estimated at up to double the dynamic coefcient µ, under normal conditions of use.

Static axial stiffness of a complete assembly
It is the ratio of the external axial load applied to the system and the axial displacement of the face of the nut in relation with the xed (anchored) end of the screw shaft. The inverse of the rigidity of the total system is equal to the sum of all the inverses of the rigidity of each of the Components (screw shaft, nut as mounted on the shaft, supporting bearing, supporting housings, etc…). Because of this, the rigidity of the total system is always less than the smallest individual rigidity. Nut rigidity When a preload is applied to a nut, rstly, the internal play is eliminated, then, the Hertzian elastic deformation increases as the preload is applied so that the overall rigidity increases. The theoretical deformation does not take into account machining inaccuracies, actual sharing of the load between the different contact surfaces, the elasticity of the nut and of the screw shaft. The practical stiffness values given in the catalogue are lower than the theoretical values for this reason. The rigidity values given in the SKF ball screw catalogue are individual practical values for the assembled nut. They are determined by SKF based on the value of the selected basic preload and an external load equal to twice this preload.

Elastic deformation of screw shaft This deformation is proportional to its length and inversely proportional to the square of the root diameter. According to the relative importance of the screw deformation (see rigidity of the total system), too large an increase in the preload of the nut and supporting bearings yields a limited increase of rigidity and notably increases the preload torque and therefore the running temperature. Consequently, the preload stated in the catalogue for each dimension is optimum and should not be increased.

Screw shaft buckling
The column loading of the screw shaft must be checked when it is submitted to compression loading (whether dynamically or statically). The maximum permissible compressive load is calculated using the Euler formulas. It is then multiplied by a safety factor of 3 to 5, depending on the application. The type of end mounting of the shaft is critical to select the proper coefcients to be used in the Euler formulas. When the screw shaft comprises a single diameter, the root diameter is used for the calculation. When the screw comprises different sections with various diameters, calculations becomes more complex (1).

Axial play and preload
Preloaded Nuts are subject to much less elastic deformation than non-preloaded nuts. Therefore they should be used whenever the accuracy of positioning under load is important. Preload is that force applied to a set of two half nuts to either press them together or push them apart with the purpose of eliminating backlash or increasing the rigidity or stiffness of the assembly. The preload is dened by the value of the preload torque (see under that heading in the previous paragrah). The torque depends on the type of nut and on the mode of preload (elastic or rigid).

Lead + Shift Nut

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

Lead

Screw

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