Due to the high efficiency of up to 98 %, ball screw drives are almost exclusively used as feed axes in machine tools. The preload is defined by the design of the nut and influences the rigidity and dynamic behavior of the feed axis. High axis rigidity reduces positioning errors during machining due to the static and dynamic forces resulting from the process. The hysteresis behavior during the reversal movement of the axis is also positively influenced by a higher preload.
In addition to the advantages mentioned, high preloads also result in increased material fatigue and therefore a reduced nominal service life. The wear also leads to a reduction in the ball diameter and a resulting loss of rigidity of up to 22 %. This in turn has a negative effect on the positioning accuracy of the feed axis. When selecting the preload, a compromise must therefore be made between the positive effects of high preload and the negative effects of the resulting material fatigue.
For this reason, the Institute of Production Engineering and Machine Tools (IFW) at Leibniz University Hannover is developing and researching ways to preload ball screw drives during operation. Load-oriented preload adaptation has the potential to significantly increase the service life of a ball screw drives and at the same time minimize the loss of positioning accuracy. This has already been demonstrated by a previous IFW scientific study on preload adaptation in double nuts in the project "State-oriented preload adaptation in ball screws using hydraulic expansion cushions". In this project, the possibility of preload adaptation was created by means of an actuator between the double nut pairs. However, the actuator in the force flow results in a direction dependent loss of stiffness.
In the DFG project "Fundamentals of preload adaptation in ball screw drives with single nuts", the IFW is therefore currently researching a new type of preload adaptation. Similar to a hydraulic chuck, a cavity is created in the nut body and filled with hydraulic oil. The subject of current research is the radial deformation of the nut caused by an increase in pressure and the resulting increase in contact pressure in the ball-track contact. The resulting increase in preload offers the possibility of a load-appropriate preload adaptation, which can significantly increase the service life of a ball screw drive.
Contact:
For further information, please contact Julian Manthei, Institute of Production Engineering and Machine Tools at Leibniz Universität Hannover, on +49 511 762 18385 or by e-mail (manthei@ifw.uni-hannover.de).
