Spherical roller bearings, which have two rows of rollers, are primarily designed to withstand radial loads while also being able to bear axial loads in any direction. They have a high radial load capacity, making them particularly suitable for heavy or vibrating loads, but they cannot withstand pure axial loads.

These types of bearings have a spherical outer ring raceway, which provides excellent self-aligning performance and can compensate for misalignment errors. Spherical roller bearings are widely used in industries such as steel, papermaking, shipping, power generation, and in machinery such as steelmaking converters, continuous casting machines, paper machines, elevators, gearboxes, and mining and heavy machinery.

Now, let me share an application example of a series of solutions implemented after a bearing failure in the main reducer of a hot rolling production line in a steel plant.

 Original Bearing Structure and Operating Conditions

The fixed-end bearing on the high-speed shaft of the reducer used a 241 series CA-type spherical roller bearing. The bearing consists of an outer ring, inner ring, rolling elements, cage, and fixed center retaining edge. It used a separate steel cage with a lifespan of about 4 months, and the failure mode was fracture of the solid steel cage bottom subassembly. 

Failure Analysis

After analysis, the reasons for premature bearing failure are as follows:

(1) Due to the adoption of helical gear transmission in the reducer, the bearing installed on the fixed end of the transmission shaft experiences a significant axial load. However, the design intention and application of 241 series spherical roller bearings mainly focus on bearing radial loads, while their axial load-carrying capacity is relatively weak.

(2) Single-row loading occurred. When the clearance of this type of bearing is greater than 0, and the ratio of axial load to radial load (Fa/Fr) exceeds the tangent of the contact angle (α), a single-row loading condition occurs theoretically. In the case of single-row loading, the unloaded row of rollers may slip, leading to damage to the bearing during operation.

(3) Single-row loading causes a difference in the rotational speed of the rollers on both sides, resulting in different operating speeds of the cage. The loaded row has a higher rotational speed, imposing higher demands on the cage's performance.

(4) Significant vibration and large impact loads were present during operation, subjecting the cage to substantial impact forces that it was unable to withstand.

Solution

To address the above operating conditions without changing the bearing's outer dimensions (due to installation limitations) and load-carrying capacity, the following design optimization solutions were implemented:

(1) Replace the brass cage with a split-type carbon steel cage made of 20 steel, with increased dimensions to enhance its strength and load-carrying capacity.

(2) Increase the cage tooth width. Two options were considered: the first option was to keep the number of rollers the same while reducing their diameter, and the second option was to keep the roller diameter unchanged while reducing the number of rollers. Due to the complex loading conditions of the bearing, reducing the diameter of the rollers to ensure sufficient load-carrying capacity is not feasible. Therefore, the second option was chosen, reducing the number of rollers to 20.

(3) Reduce the roller length from the original 128 mm to 120 mm to increase the beam width of the cage. As the rollers are shortened, attention should be paid to the dimension of the cage's outer diameter locking groove, denoted as L1 in Figure 3. Improper selection of L1 can cause the rollers to easily disengage from the cage during assembly. In this case, with L = 105 mm and α = 13°, L1 was determined to be 90 mm, which proved suitable through installation testing.

(4) Replace the fixed center retaining edge with a movable center retaining ring. The main difference between the movable center retaining ring and the fixed center retaining edge lies in their operation during bearing rotation. The fixed center retaining edge cannot move axially, but it guides the rollers effectively. When subjected to axial loads, it cannot adjust the load on the two rows of rollers, leading to single-row loading or stress concentration. On the other hand, the movable center retaining ring can move axially and can adjust the load on both rows of rollers, ensuring uniform load distribution and avoiding stress concentration. If there is no movable center retaining ring, this function is assumed by the cage.

(5) The outer dimensions of the bearing and the wall thickness of the collar remain unchanged to ensure proper installation. Other technical requirements comply with current national and industry standards, such as dimensional tolerances, heat treatment, non-destructive testing, and inspection methods.

After the improvement, the bearing's lifespan increased significantly, allowing it to be used for more than a year. This greatly reduced production downtime and bearing replacement costs.