A case of successful optimization and customization of spherical roller bearings

Aug 30, 2021

One of Cooper Rolling mill plant client's production line adopts helical gear transmission, and the bearing adopts 241 series spherical roller bearings. The dimension is 480*790*308mm. It is a large bearing. During the use of this reducer, the bearing life is short and easy to damage. 

Original bearing structure and operating conditions

The fixed end bearing of the high-speed shaft of this reducer uses 241 series CA type spherical roller bearings. The bearing consists of an outer ring, an inner ring, rolling elements, CA cage and a fixed middle rib. In the meantime, the plant use spherical roller bearing with a split steel cage, with a life of about 4 months, and the failure mode was the fracture of the bottom of the steel cage.

Failure analysis

After analysis, it is believed that failure reasons are as follows:

1）Since the reducer adopts helical gear transmission, the bearing is installed at the fixed end of the drive shaft, and the bearing bears a relatively large axial force. The 241 series of spherical roller bearings are designed for the original purpose and applications, mainly to withstand large radial forces, but the axial load carrying capacity is relatively weak.

2）There has been a single row loading situation. The clearance of this type of bearing is bigger than 0 and the ratio of axial force to radial force is large to a certain extent, and the following situations occur: Fa/Fr＞tanα, where: Fa is the axial force; Fr is the radial force; α is the contact angle . At this time, in theory, there will be a single column loading situation. When the single row of rollers is loaded, the unloaded rollers will slip, which will cause the risk of damage to the bearing during operation.

3）Single row load makes the roller speed difference on both sides, the running speed of the cage is different, and the loaded row speed is higher, which has higher requirements for the performance of the cage.

4) Due operation, the vibration is large and there is a large impact load, so that the cage can withstand a large impact force, and the cage is not enough to bear the vibration and impact force.

Solution

In view of the above working conditions, without changing the bearing dimensions (installation position limitation) and bearing capacity, increase the thickness of the cage to enhance its bearing capacity. The specific design optimization plan is as follows: 

(1) The split carbon steel cage is used to replace the brass cage. The material is 20 steel. The relevant size of the cage is increased to enhance its strength and bearing capacity.

(2) Increase the tooth width of the cage, keep the roller diameter constant, and reduce the number of rollers. Due to the complicated bearing forces in the working conditions, in order to ensure that the rollers have sufficient bearing capacity, the solution of reducing their diameters is not advisable. It is determined that the number of rollers is reduced to 20.

(3) The roller length is reduced from the original 128 mm to 120 mm to increase the cage beam width. Due to the shortening of the rollers, attention should be paid to the L1 dimension of the outer diameter of the cage. If it is not selected properly, the rollers will easily fall out of the cage during assembly. In this example, L = 105mm, α = 13°, L1 is 90 mm, and the value is appropriate after installation and testing 

(4) Cancel the fixed middle retaining edge and adopt the movable middle retaining ring. The main difference between the movable retaining ring and the fixed retaining rib is: When the bearing is running, the fixed retaining rib cannot be moved axially, but it guides the rollers well. When bearing the axial load, the load of the two rows of rollers cannot be adjusted, which will easily cause the single row of rollers to be stressed or cause stress concentration. During the movement, the retaining ring can move axially to compensate the left and right. When the bearing is bearing the axial load, the load of the two rows of rollers can be adjusted to make it evenly loaded and avoid stress concentration. If there is no movable retaining ring, its function is assumed by the cage.

(5) The outer dimensions of the bearing and the wall thickness of the ring remain unchanged to ensure installation. Other technical requirements are in line with current national and industry standards, such as shape and position tolerances, heat treatment, flaw detection and testing methods.

The improved bearing has been installed and used for more than one year, and there is no failure at present.