Page 6 of catalogue Ground ball screws of SKF Linearsysteme GmbH

2 Recommendations Selection

Efficiency and back-driving
The performance of a screw is mainly dependant on the geometry of the contact surfaces and their finish 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 efficiency” is used to define the input torque required to transform the rotation of one member into the translation of the other. Conversely, the “indirect efficiency” is used to define the axial load required to transform the translation of one member into the rotation of the other one. It is used, also, to define the braking torque required to prevent that rotation. It is safe to consider that these Screws are reversible or back-driveable under almost all circumstances. It is therefore necessary to design a brake mechanism if backdriving is to be avoided (gear Reducers or brake).

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 at 100 rpm (without wipers) when assembly is lubricated with ISO grade 68 oil. Starting torque: This is defined 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 coefficient of friction of the high efficiency screw when starting μs is estimated at up to double the dynamic coefficient μ, under normal conditions of use.

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 defined 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).

Preload systems
Fig. 1
Lead + Shift Nut

PGFJ
Lead Screw Nut Lead

QGFL QGFE QGCL
Lead Screw Nut Nut Lead

PGFE PGCL
Lead Screw Lead

6