NSK can provide precision angular ball bearings with 2-row, 3-row or 4-row paired combined bearings. When the angular contact ball bearing type is used on the fixed end of the main shaft, there are generally 2-row combination (DB), 3-row combination (DBD) or 4-row combination (DBB). However, in the case of the 3-row combination, the distribution of the preload force on each bearing is not uniform, so the optimum preload force setting range is small, and it is not suitable for high-speed rotation. During production, the combined bearing is produced in a pair, and the pre-set preload force is achieved after combining with each other. The dimensional tolerance of the inner and outer diameters of the combined bearing is controlled within 1/3 of the allowable tolerance range. Back-to-back combination DB: able to withstand radial loads and axial loads in both directions. Due to the large distance of the action point, the stiffness is very high when there is a moment load. However, when the bearing seat has poor accuracy, installation error (misalignment), etc., due to the high moment stiffness, the internal load increases, and damage such as early peeling is prone to occur. Face-to-face combination DF: The distance between the action points is small, so the ability to bear moment loads is poor. However, an increase in the internal load due to installation inclination or the like can be suppressed. Therefore, in the case of poor bearing seat accuracy, low shaft stiffness, and large bending of the shaft caused by load, this type of combination is often used. Parallel combination DT: Although it can only bear radial load and axial load in one direction, its axial stiffness value is twice that of single row, so it is suitable for the case of large load in one direction. The load directions for DB back-to-back and DT side-by-side combinations are shown in Figure 4.2. 3-row combined DBD: can withstand radial load and axial load in two directions. However, the preload distribution of the bearing is uneven, 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, so the DBD combination is not suitable for high-speed rotation. The distribution of the internal preload for the DBD combination is shown in Figure 4.3. 4-row combination DBB: can bear radial load and axial load in 2 directions. In the case of the same internal clearance, its preload and stiffness are about 2 times that of the DB combination, and the ultimate axial load that can be endured is also relatively large. The distribution of the internal preload for the DBB combination is shown in Figure 4.4. Figure 4.3 Distribution of internal preload force of DBD combination Figure 4.2 Load direction of back-to-back combination and parallel combination DB DB DF DT250N 250N 500N Preload force u003d 500N Figure 4.4 Distribution of internal preload force of DBB combination 250N 250N.