Abstract: This paper introduces the characteristics and common fault phenomena of cold-rolled roll bearings, and focuses on studying the causes of their faults.

Key words: Cold-rolled roll bearings Cause of damage; Fatigue damage

Introduction: Roll bearings are quite important in the production of cold rolling mills. They can withstand heavy loads, operate in harsh environments, and have good performance. However, precisely because of this, coupled with the fact that it is prone to water ingress and difficult to disassemble and assemble, the bearings are prone to damage, which affects cold-rolled products. Therefore, it is extremely necessary to analyze the cause of its damage.

1 Characteristics and Fault Phenomena of Cold-rolled Roll Bearings

Compared with ordinary bearings, cold-rolled roll bearings can bear heavier loads, usually reaching 2 to 5 times the ordinary standard. During operation, their running speed can vary over a wide range [1]. Under normal circumstances, cold-rolled roll bearings need to operate in relatively harsh environments, so impurities such as iron oxide chips can easily enter them and cause damage.

During operation, cold-rolled rolling bearings may encounter certain faults, mainly manifested as follows: Firstly, during the disassembly and cleaning process, fatigue damage is often observed at the inner and outer raceways, which can cause color changes, reduced hardness, accompanied by cracks, and severe damage to the rolling elements, etc. Secondly, during operation, the temperature at the bearing part rises and a distinct noise can be heard. In severe cases, phenomena such as smoke and jamming may occur.

2 Analysis of the Causes of Bearing Damage in Cold Rolling Rolls

（1）Fatigue damage to the inner and outer raceways

Under long-term use, the inner and outer rings of the bearing will wear out. If the lubrication effect is poor, the degree of surface wear will increase, and in severe cases, fatigue damage will occur. On this basis, the fine materials on the surface will constantly fall off. Over time, they will accumulate between the rolling elements and the raceways, increasing the stress and making it highly concentrated, which leads to the formation of cracks. As the cracks continue to expand, the surface will fall off and a pitted surface will appear. This cycle repeats, eventually causing serious damage.

The main reasons for poor lubrication of the bearing include the following aspects: First, the labyrinth seal component composed of the water retaining ring and the bearing housing is worn, resulting in a large gap between them. This may be due to assembly problems, such as the bearing housing not being precisely positioned, causing friction with the labyrinth seal of the water retaining ring. Or, during the rolling process, the bearing was affected and damaged, causing the internal water-blocking ring and the bearing housing to shift, resulting in friction at the sealing area. Additionally, it is also possible that the inner ring is not tightened in the axial direction, thus preventing the water-blocking ring from being fixed in its original position. Secondly, due to long-term use, the sealing rubber fails and the elastic modulus of the spring decreases, resulting in an inability to achieve a good sealing effect. In this case, the rolling emulsion will leak and flow into the bearing housing, diluting the lubricating oil and losing its original lubricating effect. This situation often occurs in the lower roll bearing.

（2）The inner and outer rings become hot and discolored, and their hardness decreases

This phenomenon is rather serious. Due to the significant temperature rise of the bearing, the fit clearance between the inner ring and the roller shaft increases, causing relative sliding. At the same time, relative sliding also occurs between the outer ring and the bearing housing. Under the above effects, the temperature of the bearing further rises. In this repetitive cycle, the inner and outer rings become hot and discolored, and their hardness decreases. In addition, the axial fastening capacity decreases. For instance, if the shaft end bolts of the inner ring of the bearing become loose or the bolts are damaged due to excessive axial force, the inner ring may run away.

(3) The most severe fault

The temperature at the bearing part rises and a distinct noise can be heard. In severe cases, smoke and jamming may occur. Such problems are the most serious in bearings.

The materials used for the inner and outer rings of the bearing are relatively thin, with high hardness and low softness. They are wear-resistant and pressure-resistant, but have very poor resistance to bending and shearing. Their use must be in conjunction with roller shafts and bearing housings to enhance the overall performance. The concentrated force must be shared by the roller shaft and the bearing housing. Once the inner ring and the roller shaft as well as the outer ring and the bearing housing cannot fit well, it is extremely easy for the inner and outer rings to bear the strong force alone and break. Therefore, strict regulations must be made on the dimensions, shapes and other indicators of the mounting shaft and bearing housing, and it is essential to ensure that they meet the requirements. Under normal circumstances, problems often occur in roller shafts and bearing housings that have been in use for a long time.

During operation, the cold-rolled strip moves at a relatively high speed, which may lead to deviation, strip breakage or exceeding the maximum load. If the overload protection capability is weak, under the strong impact force, it may cause severe damage to the bearing, resulting in running of the inner and outer rings. In severe cases, problems such as cracking and jamming may follow, and the rolling shaft and bearing housing may be severely damaged. Its specifications and shapes change, the surface is worn, and then grooves appear or severe deformation occurs.

During the assembly process of the shaft, if it is not carefully inspected, problems will be difficult to detect. Or, if problems are known but necessary measures are not taken to improve them before installation and application, at this time, due to the strong impact force generated during rolling, the possibility of the inner and outer rings of the bearing cracking will increase, and other components such as rolling elements and support brackets will be damaged.

Damage to the above-mentioned bearings or poor lubrication can cause a significant increase in bearing temperature. An accurate judgment can be made by human touch.

(4) Excessive radial force

The bearing of the support roller can withstand considerable rolling force. However, due to reasons such as material shape, under certain pressure, it is prone to sudden high pressure. In this case, the rolling force will increase significantly, and the bearing force will exceed the maximum range. If such operation is carried out for a long time, its service life will be adversely affected. In addition, if the material is wedge-shaped, the force on one side will also exceed the standard. Excessive force on one side may cause the rolling elements to break, the support frame to be damaged, and the service life of the bearing to be reduced.

The bending roller force and tension are the radial forces that the working roller shaft often experiences. Due to the shape of the raw materials, additional bending force must be applied during the rolling process. When the bending roller force is applied to the bearing housing, the working shaft will bend, so as to roll out the strip with a good plate shape. However, the bending roller force can also cause the bearing to be subjected to uneven force, with the force mainly concentrated on the inner and outer rolling elements. At this time, the load-bearing capacity of the bearing is severely reduced, and the inner and outer raceways thus wear out.

(5) Excessive axial force

In addition to the rolling force, the working roller shaft is also subjected to axial force, which can affect the working roller shaft. However, this force is often overlooked. Theoretically, the magnitude of the axial force is 1% to 2% of the rolling force. However, in reality, this force can reach or even far exceed 5% of the rolling force. As a result, the rolls will move, the bolts used for axial fixation will break, causing work interruption. In severe cases, the bearings will be directly burned out [2]. There are many reasons for the increase in axial force, such as the intersection of rolling axes, rolling asymmetry, and unstable operation of the transmission system. However, the most significant reason is the intersection between rolls, especially the intersection of the axes of the transmission rolls.

3 Conclusion

Compared with ordinary bearings, the performance of cold-rolled roll bearings is better. However, during the usage process, it will be damaged to varying degrees. Therefore, the causes of the damage should be carefully analyzed, and the best solution should be found through exploration to ensure the quality of cold-rolled products.

[References]

[1] Li Xiangyang, Ma Jia, Cui Junli Analysis of the Causes of Bearing Damage in Vertical Grinding [J]. Cement, 2019,000 (009): 42-44.

[2] Sun Yujie A Brief Discussion on the Management of Cold Rolling Rolls during Service [J]. Metal World, 2019,203 (03):14-17