Abstract:Roll bearing is a part of rolling mill which is easy to wear and damage. In rolling mill production, roll bearing not only ensures the smoothness of roll rotation, but also plays a role in limiting roll position and avoiding roll deviation. Therefore, bearing wear and damage will directly affect the rolling accuracy and rolling mill operation, resulting in defective products or production stagnation and other problems. Based on the production practice, this paper analyzes the causes of roll bearing failure of universal rolling mill, and puts forward the relevant measures of daily maintenance. 

Key words: universal rolling mill; Roll bearing; Wear failure; Reason analysis; Routine maintenance

With the improvement of production capacity of rolling production line and the continuous improvement of users' requirements for rolling product quality, roll bearing, as an important part related to the good and stable operation of rolling mill equipment, its use and maintenance problems are becoming increasingly prominent. Rolling mill bearing is the main consumable and wearing part in universal rolling mill. In rolling mill production, roll bearing not only bears the huge rolling force transmitted by rolling piece to roll, but also bears the friction brought by rolling roll rotation. Therefore, the wear problem of roll bearing is more serious. The wear and damage of the roll bearing will lead to the axial and radial movement of the roll, resulting in a large deviation of rolling precision, and even the stagnation of rolling equipment, which will affect the production. In view of the common failure of roll bearings in rolling mill operation, it is very necessary to strengthen the maintenance of roll bearings, which plays an important role in improving the production efficiency and product quality of universal rolling mill and reducing the production cost.

1 The working characteristics of roll bearings

1. Basic composition of universal rolling mill bearing system

The roll system of universal rolling mill is composed of a pair of horizontal rolls and an opposite roll, and there are two bearing seats on both sides of each roll. The roll bearing is arranged on the inner diameter of the bearing seat on both sides of the roll and is matched with the rolling roll head, which mainly supports the roll and assists the roll rotation. The four-row ball roller bearing has small radial size, good mechanical strength and impact resistance, which can keep the roll stable and smooth rotation under the condition of great rolling force. A set of double-row tapered thrust bearings is installed on the non-driving side of the roll to withstand the rolling roll axial force and prevent the rolling roll from moving. The roller driving end of the universal finishing mill is equipped with a set of single-row cylindrical roller external bearings, which are used to withstand the bending force of the roll; The vertical roller bearings are double-row tapered roller bearings mounted back-to-back.

2. Working characteristics of universal rolling mill roll bearing

Compared with bearings of general mechanical equipment, roll bearings mainly have the following working characteristics:

(1) The force is large. The roll bearing and bearing seat are the supporting mechanism of the roll. In the rolling process, the reaction force of the rolled part on the roll will be transferred to the bearing seat and bearing, so the rolling force is very large. Due to the limited size of the bearing seat, it can not be greater than the minimum diameter of the roll body, and the length of the roll neck is short, so the unit load on the bearing is very large, and the unit pressure under it can be as high as 2000 ~ 4800MP, which is several times that of ordinary bearings; pv value is 3 to 20 times that of ordinary bearings. Therefore, roller bearings are required to have extremely high strength.

(2) Fast running speed. The outlet speed of the universal rolling mill can reach tens of meters per second, and the lubrication effect and wear resistance of the bearing are very high requirements at the high roll running speed.

(3) Bad working environment. First of all, roll bearings need to withstand high operating temperature in rolling production; Secondly, there are many impurities such as sewage and oxide scale in the rolling environment. If these impurities enter the bearing, the bearing lubrication will be polluted and the bearing wear will be aggravated. Therefore, under the harsh working conditions of rolling production, the bearing is required to have good lubricity and high sealing.

In summary, the basic requirements for roll bearings are high strength, large bearing capacity, small friction coefficient, impact resistance, and can adapt to work at different speeds; In terms of structure, the radial size should be as small as possible, with good lubrication conditions, cooling conditions and sealing.

2 Roll bearing failure form and cause analysis

Bearings belong to the wearing parts and consumption parts of the rolling mill, after a certain service cycle will be due to serious wear and failure to scrap, but if the lubrication and maintenance is not in place, it will speed up the wear of the bearing, reduce its service life, so that it fails in advance. The main reasons for bearing failure are poor lubrication, impurity pollution and improper assembly, and these three reasons account for more than 70% of bearing failure reasons. Several typical failure forms of roll bearings and their causes are analyzed as follows:

1. Typical fatigue damage. The main performance is that the bearing raceway and the surface of the rolling body are fractured and deformed, and the material is spalling, which will cause the roll to run poorly, the roll appears relatively large axial or radial channeling, and the equipment vibration is obvious; When rolling, sometimes accompanied by sharp metal friction sound. The reason is that the bearing reaches the designed service life and fatigue wear occurs. When the bearing reaches its designed service life, it must be replaced. If the daily lubrication maintenance is improper, overload use or poor assembly, the bearing does not reach the design service life will appear fatigue damage, at this time should be replaced in advance according to the use of the new parts.

2. Corrosion marks appear. The main performance is that the bearing surface appears reddish-brown rust, there is slag in the raceway, ball and cage, the vibration and wear increase when the equipment is running, the preload decreases or the clearance increases. The intrusion of water or other aggressive media is the root cause of bearing corrosion, and it is also related to the poor lubrication seal of bearings. The solution measures should first reduce the contact between water, dirt and bearings, and do not store corrosive substances in the surrounding environment; In addition, it is necessary to improve the sealing of bearings by using moisture resistant lubricating oil and sealed bearings.

3. External particulate pollution. The adhesion of external particles often produces dents or spalling in the bearing area of the bearing jacket, and some dents also have convex edges, which makes the bearing surface rough; If the particles with high external hardness enter the bearing raceway or ball, it will not only pollute the lubricating oil, but also lead to the increase of the wear of the ball and raceway during the bearing operation, and make the raceway indentation. The intrusion of external particles may be brought into the bearing assembly or the injection of lubricating oil, or it may be caused by seal wear during the operation of the equipment. Prevention and control measures: First, the surrounding environment and assembly tools should be clean in the bearing assembly link, and the bearing packaging can be opened before installation; Before lubrication and maintenance of the equipment, it is necessary to ensure the cleanliness of the lubricating oil. If there is a possibility of pollution, the lubricating oil should be filtered and then injected. The mill work place should be away from grinding, and the bearing seal system should be checked and replaced regularly.

4. Overheat damage. Under high working temperature, overheating damage is also one of the typical failure causes of roll bearings. The external performance is that the bearing rolling body, ring and cage turn blue and fade, the bearing rolling body and ring have plastic deformation, and the bearing may be stuck under the action of thermal expansion. The increase of roll bearing speed and the increase of rolling load will cause the overheating phenomenon of the bearing, especially in the case of the reduction of bearing working clearance, high-speed equipment, overload operation and poor lubrication, the bearing is easy to overheat damage, and even may be scrapped directly. Prevention and control measures: first, appropriate adjustment and increase bearing free clearance, reduce running resistance; The second is to improve the lubrication state of the bearing, avoid over-lubrication and lubricant poverty, and ensure the flexible rotation of the bearing; The third is to avoid overspeed and overload operation of equipment, and reduce the impact of rolling parts and rolls on bearing alternating load; Fourth, in the case of an external heat source, ensure uniform and slow heating.

5. Bearing slip. The bearing surface appears pitting and creep occurs. The reason is that the bearing mating free clearance is too large, the friction between the ball and the raceway is insufficient; In the low bearing zone, the ball slips on the raceway and accelerates when entering the bearing zone. The equipment runs fast and slow, rapidly changing speed. Prevention and control measures: appropriate adjustment to reduce bearing clearance; The use of low bearing capacity of bearings, equipment operation stage to ensure sufficient load; Improve the lubricant.

6. Poor lubrication. The performance is pressure polishing in the high bearing area and dark gray raceway in the low bearing area. Causes of poor lubrication: first, the use of insufficient lubricants, poor quality; Second, the operating temperature of the equipment is too high, resulting in reduced lubricant adhesion and lubrication failure; The third is the intrusion of water and external impurities, due to the bearing end seal is not strict, the cooling descaling water, oxide sheet and other impurities due to the relative rotation of the bearing seat end cover and the rotating part support ring, it is easy to enter the bearing seat and the bearing clearance space, and then cause the emulsification and deterioration of the lubricating oil and the viscosity decline, the bearing wear is aggravated, and the serious will lead to the burning roll accident. Prevention and control measures: improve the quality of lubricants, use high-viscosity lubricants; Ensure adequate supply of lubricant and smooth application; Improve the waterproof performance of the sealing system.

7. Scratches. The raceway of tapered roller and cylindrical roller bearings has edge uplift on the axis, and the scratches are different in depth, and the scratches are usually distributed according to the distance between rollers. The main reason is that the inner and outer rings are not coaxial when the bearing is installed. For this problem, we should mainly start with the equipment assembly link, rationally use the installation tool, strictly follow the step process, and rotate the bearing during installation if conditions exist to determine whether the inner and outer rings of the bearing are well matched and aligned.

8. Axial fracture of bearing ring. The axial fracture of the bearing ring can occur during the running of the bearing or during the disassembly of the bearing. The reason is that the bearing ring is too tight with the shaft or the force is not uniform, long-term operation is easy to lead to the bearing ring tear; In addition, it may also be caused by uneven external thermal stress. Prevention and control measures: improve bearing lubrication quality; Reasonably adjust the bearing ring position to avoid grinding with the surrounding parts; Improve the quality of the ring, using carburized steel bearing with better strength and wear resistance.

3 Roll bearing use and maintenance measures

1. Precautions before use. Do not open the packaging before the bearing is installed and used to avoid contamination of the bearing by external environmental impurities; Handling should be handled lightly during loading and unloading to avoid severe collision and friction resulting in bearing damage. Bearing parts should be stored in a dry, non-corrosive place, long-term storage beyond the rust period is not used, should take the necessary cleaning and rust treatment.

2. Improve the lubrication conditions during the use of bearings. First, it is necessary to ensure that the oil circuit is smooth, so that the bearing can get sufficient lubricant supply. When assembling rolls, the bearing lubrication should be carefully checked, the reasons for poor lubrication should be analyzed, dredging and fastening measures should be taken, and the damaged tubing should be replaced if it cannot be repaired. The second is to choose high-quality lubricating oil, according to the use of roller bearing conditions should be used high temperature, water resistant lithium based grease, timely quantitative grease lubrication, but also according to the seasonal temperature changes should choose the appropriate grease label; When the roll is assembled and replaced, remove the impurities contained in the bearing seat and add an appropriate amount of clean grease.

3. Strengthen the daily inspection and maintenance of bearing systems. Clean grease should be re-added after the bearing seat is off the line, and the grease should be spilt from the outlet until the new grease is added. If there are foreign bodies such as iron powder in the discharged old grease, it must be decomposed and checked; If it is found to contain more water, the sealing ring should be decomposed and checked to ensure that there is no foreign matter or abrasion on the lip of the sealing ring, the thrust ring and the sliding surface of the supplementary ring. In addition, the bearing should be regularly inspected, the bearing should be removed from the seat, cleaned, inspected and replaced.

4. On-line monitoring of roller bearing temperature in use. In the past, the damage of roller bearings is based on the size of the motor current, when the current exceeds the requirements, often the damage of the bearing has been very serious or scrapped, and the roll change will also affect the production. In order to detect bearing wear in time, the method of online monitoring bearing temperature can be adopted. The temperature measuring gun is used to measure bearing temperature regularly during production. When the temperature is abnormal, it is timely reported to the production dispatch, and the roll change is prepared in advance.

4  Conclusion

Roll bearing is the key part of rolling mill production, and it is also easy to be damaged under bad working conditions. In this regard, we should take preventive measures against abnormal situations in the use of roll bearings and strengthen the daily maintenance of bearings in order to reduce the occurrence of bearing failure problems, extend the service life of bearings, so as to ensure production and reduce cost consumption.

【 Reference 】

[1] LI Ji. Failure Cause Analysis and Countermeasures of Roll Bearing [J]. Lianyuan Steel Technology and Management, 2017 (4) : 48-51.

[2] Tang Lijuan, Wang Demi. Maintenance and Failure Analysis of Roll Bearing in Universal Rolling Mill [J]. Metallurgical Equipment, 2011 (S1) : 158-160.