Preload and unloading of machine tool spindle bearings

Appropriate preload of machine tool spindle bearings can improve the bearing's rotation accuracy, increase rigidity, reduce vibration and noise, suppress temperature rise and prolong service life. The preload of the main shaft bearing is divided into three types: light, medium and heavy. The value has been determined by the bearing manufacturer. If the user has special requirements, it can be negotiated with the bearing manufacturer and customized. High-speed machine tool spindle bearings generally use two types of preload, light and medium, because under normal circumstances, the increase in preload will increase the temperature rise. Figure 3 shows the relationship between the bearing temperature rise and the load under the same operating conditions. Figure 3 shows that at the initial stage of preload, with the increase of preload, the internal contact state of the bearing tends to be stable and the vibration decreases. When the preload reaches a certain level, continue to increase the preload, the friction inside the bearing intensifies, and the temperature rise increases. Indicates that there is an optimal preload value for the assembled bearing. Under normal circumstances, because the bearing rotates on the inner ring, the outer ring is fixed, and the outer ring dissipates part of the heat through the bearing seat or the box. Therefore, the operating 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 to 10 °C. The influence of preload on bearing performance is analyzed. Although the preload is too large, the friction and heating of the bearing will be increased and the service life will be shortened, but the large preload is beneficial to the increase of bearing rigidity and critical speed, and to prevent the occurrence of resonance. Therefore, when determining the optimal preload, factors such as bearing assembly method, working speed, system rigidity and life should be comprehensively considered, and the final determination can be made after repeated tests. Figure 4 shows the displacement-load relationship for a double angular contact ball bearing in a 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 with the increase of Fa, the axial displacement aA of the inner ring of bearing A gradually increases, while the axial displacement aB of the inner ring of bearing B gradually decreases. When Fa increases to a certain value (assuming Famax) , then aB u003d 0. At this time, the bearing B is in the unloaded state, that is, the inner and outer rings of the bearing B and the steel balls are in a state of detachment. u003d a0B (2) Substitute (2) into (1), there is aA u003d 2a0A (3) According to the Hertzian elastic contact theory, the axial deformation a of the point contact ball bearing under the action of the external axial load Fa is a u003d cF2/ 3a (4) where c is a constant determined by bearing material, type, structure and size, etc. Substitute aA u003d cF2/ 3amax and a0A u003d cF2/ 3a0 into (3) to obtain Famax u003d 2. 83 Fa0 is the unloaded load or shaft The limit value of the load Fa shall not exceed 2.83 times 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.