NSK company can provide precision angular joint angular ball bearings in 2, 3 or 4 row paired combined bearings. When the angular contact ball bearing model is used at the fixed end of the spindle, there are generally two-row combination (DB), three-row combination (DBD) or four-row combination (DBB). However, the pre-tightening force distribution of each bearing is uneven in the three-row combination mode, so the most suitable pre-tightening force setting range is small and not suitable for high-speed rotation. In production, the combined bearings are produced together in a pair, and the pre-set pre-tightening force is reached after being combined with each other. The dimensional tolerances of the inner and outer diameters of the combined bearing are controlled within 1/3 of the allowable tolerance range. Back-to-back combination DB: able to bear radial load and axial load in two directions. Due to the large distance between the action points, the rigidity is very high when there is a moment load. However, when the bearing seat has poor accuracy, installation errors (misalignment), etc., because of the high moment rigidity, its internal load increases, which is prone to damage such as early spalling. Face-to-face combination DF: The distance between the acting points is small, so the ability to withstand moment loads is poor. However, it is possible to suppress an increase in the internal load caused by the inclination of the installation. Therefore, this type of combination is often used in situations such as poor bearing seat accuracy, low shaft rigidity, and large shaft bending caused by load. Parallel combination DT: Although it can only bear radial load and axial load in one direction, its axial rigidity is twice that of single row, so it is suitable for large loads in one direction. The load directions of DB back-to-back combination and DT parallel combination are shown in Figure 4.2. 3-row combination DBD: can bear radial load and axial load in 2 directions. However, the preload of the bearing is not evenly distributed, and the internal load of the single-row bearing is twice that of the two-row side bearing. During high-speed rotation, the internal load on the single-row side bearing increases, which may cause bearing damage. Therefore, the DBD combination is not suitable for high-speed rotation. The internal preload distribution of the DBD combination is shown in Figure 4.3. 4-row combination DBB: It can bear radial load and axial load in 2 directions. In the case of the same internal clearance, its preload and stiffness are about twice that of the DB combination, and the ultimate axial load that can be withstood is also relatively large. The internal preload distribution of the DBB combination is shown in Figure 4.4. Figure 4.3 Distribution of internal preload of DBD combination Figure 4.2 Load direction of back-to-back combination and parallel combination DB DB DF DT250N 250N 500N preload u003d 500N Figure 4.4 Distribution of internal preload of DBB combination 250N 250N.