Page 7 of catalogue Ball screws of SKF Linearsysteme GmbH

Recommendations for selection

Permissible speed limit
The permissible speed limit is that speed which a screw cannot reliably exceed at any time. It is generally the limiting speed of the recirculation system in the nut. It is expressed as the product of the rpm and the nominal diameter of the screw shaft (in mm). The speed limits quoted in this catalogue are the maximum speeds that may be applied through very short periods and in optimized running conditions of alignment, light external load and preload with monitored lubrication. Running a screw continuously at the permissible speed limit may lead to a reduction of the calculated life of the nut mechanism.

Lubrication
The lubrication of Screws rotating at high speed must be properly considered in quantity and quality.

Efficiency and back-driving
The performance of a screw is mainly dependant on the geometry of the contact surfaces and their nish as well as the helix angle of the thread. It is, also, dependant on the working conditions of the screw (load, speed, lubrication, preload, alignment, etc…). The “direct efciency” is used to dene the input torque required to transform the rotation of one member into the translation of the other. Conversely, the “indirect efciency” is used to dene the axial load required to transform the translation of one member into the rotation of the other one. It is used, also, to dene the braking torque required to prevent that rotation. It is safe to consider that these screws are reversible or backdriveable under almost all circumstances. It is therefore necessary to design a brake mechanism if backdriving is to be avoided (gear Reducers or brake).

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High speed associated with high load requires a large input torque and yields a relatively short nominal life (1). In the case of high acceleration and deceleration, it is recommended to either work under a nominal external load or to apply a light preload to the nut to avoid internal sliding during reversal. The value of preload of screws submitted to high velocity must be that preload which ensures that the rolling elements do not slide (1). Too high a preload will create unacceptable increases of the internal temperature.

The volume, spread and frequency of the application of the lubricant (oil or grease) must be properly selected and monitored. At high speed the lubricant spread on the surface of the screw shaft may be thrown off by centrifugal forces. It is important to monitor this phenomenon during the rst run at high speed and possibly adapt the frequency of relubrication or the ow of lubricant, or select a lubricant with a different viscosity. Monitoring the steady temperature reached by the nut permits the frequency of relubrication or the oil ow rate to be optimized.

Friction torque Tf

> Back driving torque Tr

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

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