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Preload and unloading load of machine tool spindle bearing

by:JNSN     2021-12-10
The proper preload of the machine tool spindle bearing can improve the rotation accuracy of the bearing, increase the rigidity, reduce the vibration and noise, suppress the temperature rise and prolong the service life. The preload of the main shaft bearing is divided into three types: light, medium and heavy. Its value has been determined by the bearing manufacturer. If the user has special requirements, it can be customized with the bearing manufacturer through consultation. High-speed machine tool spindle bearings generally use light and medium preloads, because under normal circumstances, the preload increases, and the temperature rise also increases. Figure 3 shows the relationship curve of bearing temperature rise with load under the same operating conditions. Figure 3 shows that at the initial stage of preload, as the preload increases, the internal contact state of the bearing tends to stabilize and the vibration decreases. When the preload reaches a certain level, continue to increase the preload, the friction inside the bearing will increase, and the temperature rise will increase accordingly. It shows that there is an optimal preload value for the assembled bearing. Under normal circumstances, because the inner ring rotates and the outer ring is fixed, the outer ring radiates part of the heat through the bearing seat or box. Therefore, the working temperature of the inner ring is higher than that of the outer ring. When the bearing and its environment are not cooled, and there is no external heating source, the temperature difference between the inner and outer rings of the machine tool spindle bearing is 5-10 ℃. Analyze the influence of preload on bearing performance. Although the preload will increase the friction and heating of the bearing and shorten the life when the preload is too large, the large preload will help the bearing rigidity and critical speed increase, and prevent the occurrence of resonance. Therefore, when determining the optimal preload, the bearing assembly method, operating speed, system rigidity and life should be considered comprehensively, and the final determination can be made through repeated trials. Figure 4 shows the displacement-load relationship curve of the double angular contact ball bearing with back-to-back (DB) configuration (positioning preload). In the figure, Fa0 is the preload and Fa is the external axial load. It can be seen from the figure on the right that as Fa increases, the axial displacement δaA of the inner ring of bearing A gradually increases, while the axial displacement δaB of the inner ring of bearing B gradually decreases. When Fa increases to a certain value (assumed to be Famax) When, then δaB u003d 0. At this time, bearing B is in the unloaded state, that is, the inner and outer rings of bearing B and the steel ball are in a state of separation, δaA u003d δa0A + δa0B (1)    If it is assumed that the structures of bearings A and B and their main parameters are exactly the same, there should be δa0A u003d δa0B (2)  Substitute (2) into (1), δaA u003d 2δa0A (3)  According to the Hertz elastic contact theory, the axial deformation δa of a point contact ball bearing under the action of an external axial load Fa is δa u003d cF2/ In equation 3a (4), c is a constant determined by bearing material, type, structure and size. Substitute δaA u003d cF2/ 3amax and δa0A u003d cF2/ 3a0 into equation (3), and then Famax u003d 2.83 Fa0 is the unloaded load or shaft The limit value of the forward load Fa shall not exceed 2.83 times of the bearing preload. In the same way, the unloading load of other common assembly types of bearings can be calculated, and the results are shown in Table 1.
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