Abstract: There is a great difference between the working condition of 20-high rolling mill backing bearing and ordinary bearing, so there are high requirements for its performance and accuracy. This paper mainly analyzes the installation, use and maintenance methods of 20-high mill backing bearings.

Key words: 20-high mill; Backing bearing; maintenance

Bearing consumption has been high due to abnormal scrapping of bearings, affecting the cost of tons of steel, so it is very important to master the relevant maintenance methods of backing bearings. The back-up rolls of the 20-high rolling mill are usually of multi-support beam structure (that is, consisting of multiple cylinders rolled to bearings, saddles and mandrel). Because the 20-high mill adopts the tower roll system, the rolling force is transmitted by the work roll through the middle roll in a fan shape to the backing bearing of the outer support roll, and the backing bearing transfers the load to the frame evenly through the mandrel and the saddle, so the rolling mill stiffness is relatively large, and the roll deflection is small, which can roll high-precision strip. In addition, the dimensional accuracy of the backing bearing determines whether the rolling force distribution is uniform.

1 Structure and use conditions of backing bearings

There is a certain difference between backing bearings and ordinary bearings in the use conditions, for example, ordinary bearings are fixed on the outer ring, the inner ring rotates, and the back bearing is just the opposite; Secondly, the speed of the backing bearing is high, the speed is frequent, and the load is large. The use conditions of the backing bearing are as follows: first, the overall thickness of the outer ring and the thickness of the grinding layer are sufficient to ensure that the outer ring has a certain bearing capacity; Second, the outer ring of the backing bearing must have sufficient impact resistance, and quenching can be used to achieve this purpose; Third, the resistance of the inner ring; Fourth, to ensure the fatigue resistance of the rolling element, the performance needs to focus on the choice of materials and shape design.

2 Installation requirements for backing bearings

The dimensional accuracy of the backing bearing is extremely high, the error of the inner and outer rings is usually less than 5μm, and the maximum wall thickness point is marked with an arrow. In the process of installing the backing bearing, the first thing to do is to install the backing bearing of the same size on the same shaft, and keep the direction of the outer circular surface consistent. On the contrary, if the backing bearing is not in the same direction according to the installation standard, it will lead to abnormal vibration of the backing bearing during operation, which will aggravate the fatigue of the roller, not only seriously reduce its operating life, but also greatly affect the quality of the product.

During operation, the wear of each backing bearing is different, the main reasons are divided into two points, the first is that when the width of the rolled strip is narrower than the maximum rolling width of the rolling mill, the load borne by the middle of the roll backing bearing is greater than the load borne by the back bearing at both ends of the roll, and the wear is relatively serious; Second, it is because the external shaft of the 20-roll mill is much higher than the load to be borne by the internal shaft, which will directly increase the wear degree of the backing bearing of the external shaft. It can be concluded that in order to guarantee and improve the service life of the backing bearing, the position of the backing bearing should be replaced at regular intervals, and a record should be made at the same time. In general, the running time of the backing bearing reaches 400 hours to 1000 hours, and its position needs to be changed. In addition, because the inner ring of the backing bearing is in a static state on the mandrel, only a quarter of the torus is the load area. In summary, regularly rotating the inner ring to another load area can effectively improve the running life of the backing bearing.

3 Lubrication of backing bearings

The force principle of the 20-high mill is that the rolling force generated on the small-diameter work roll is passed through a series of larger rolls and finally transmitted to the frame by the backing bearing. Therefore, for the maintenance of backing bearings, lubrication plays an extremely important role and will be directly related to the service life of bearings. There are many lubricating effects of backing bearings, such as reducing the friction of related components, protecting the bearing surface from damage, removing foreign objects, and finally playing a cooling role on the bearing.

The lubrication system is divided into two parts: rolling mill process lubrication system and backing bearing lubrication system. First of all, according to the difference of rolling process, process cooling lubricants usually take two kinds of emulsion and mineral oil. The advantage of mineral oil is that it has good lubrication, can effectively reduce the rolling force, extend the service life of the roll, and also allow the strip to obtain good surface quality, but the disadvantage is that the cooling capacity is weak, and the economic cost is relatively high, usually used for small and medium-sized rolling mills. The other is the emulsion, the main advantage of this lubricant is the cooling effect is strong, usually used in large rolling mills or high-speed rolling mills. Because the emulsion has the characteristic of more water, and compared with oil, the heat capacity of water is more than twice, and the latent heat of evaporation is more than ten times, so the emulsion can absorb a lot of heat in a very short time.

3.1 Shared lubrication system

When the process lubrication system uses mineral oil in the process, in order to avoid product contamination, it is usually the same lubrication system as the backing bearing. According to the characteristics and application requirements of multi-roll mill, lubricating mineral oil with low viscosity characteristics is the best choice. However, at the same time, this mineral oil also has certain shortcomings, that is, it cannot provide more adequate bearing lubrication. In order to solve this problem, it is necessary to increase the lubricating oil flow. If the lubricating oil flow of the backing bearing is not sufficient, it may lead to deformation and damage of the bearing outer ring. In general, the filtration accuracy of mineral oil must be less than 5μm in order to meet the standard of use, but also to effectively extend the service life of the bearing. According to the work experience, the lubricating oil consumption range of different backing bearing diameter is summarized. For example, the diameter of the backing bearing is 110/mm~120/mm. The recommended lubricating oil consumption is 5/L· min-1 ~7/L·min-1. The diameter of the backing bearing is 160/mm~165/mm. The recommended lubricating oil dosage is 10/L· min-1 ~15/L·min-1. The backing diameter is 220/ mm~225/mm. The recommended oil dosage is 15/L· min-1 ~25/L·min-1.

3.2 Independent lubrication system

In general, when the process lubrication uses emulsion, the backing bearing must use an independent lubrication system. The main reason is that the moisture contained in the emulsion will have an erosion effect on the bearing. At present, the more common independent lubrication is oil mist lubrication and circulating thin lubrication. The oil mist lubrication system mainly includes oil water filter, pressure reducing valve, oil mist generator, air heater, oil heater and so on. Although the system is a reliable way, it is often subject to various constraints in the production operation, resulting in the actual effect is not ideal. For example, the atomization capacity of lubricating oil, atomization temperature and so on have higher requirements. According to practice, in many rolling mills introduced in our country, because the oil mist lubrication system can not bring the ideal effect, it has to be abandoned and changed to the same emulsion lubrication as the process lubrication. In addition, if the circulating thin oil lubrication system is used, the backing bearing of the integral sealing device needs to be used to prevent the emulsion from being contaminated.

4 Regrinding of backing bearings

Various damage will gradually occur during the operation of the backing bearing, such as flattening, pitting, etc. According to practical experience, the backing bearing needs to be reground and maintained after the running time of up to 3000 hours. Back bearing grinding is carried out on the roll grinder, and the operation is carried out according to the specification process, otherwise the bearing will be tempered during operation. In addition, the repair situation of the regrinding maintenance workpiece is determined by the hardness, material and roughness of the workpiece. According to the experiment, it is necessary to use ceramic alumina grinding wheel to grind the outer ring of the backing bearing, and the wheel sand can be used after strict grinding. The linear speed of the grinding wheel is controlled at 25m/s~30m/s, and the emulsion should be used for cooling during the grinding process. The main purpose of using the emulsion is to prevent the grinding surface from heating, but also to wash the grinding and keep it clean.

The chamfer and fillet parts of the outer ring of the backing bearing that need to be reground will be worn away a little bit when repaired, so the edge chamfer should be re-polished to avoid the stress concentration area during the application process, and the transition zone between the chamfer and the cylinder should be polished. We use professional micrometer to detect the outer ring regrinding, mark the measurement results, and determine the maximum wall thickness point according to the data display. In order to make the regrinding accuracy of the backing bearing within the controllable range, it is necessary to adopt advanced cutting technology and select suitable cutting tools. Several regrinding methods are introduced in detail. First, the backing bearing is grinded as a whole. In this method, the first step is to fix the grinding mandrel with the backing bearing, lock the cone bushing with the nut using the lock on the bearing, and expand the inner ring. The purpose of this method is to fully fill the internal gap.

The final step is to align the outer ring roller with the cage. The disadvantage of this regrinding method is that it takes a long time, the entire backing bearing needs to be removed and installed, and the measurement accuracy also takes a lot of time. Second, the outer ring grinding alone, this method is very common application, the main advantage is that the operation is simple. Grinding the outer ring device on the mandrel, using the method of hydraulic pressure to expand the mandrel, clamping the bearing outer ring, grinding operation can be carried out. Third, group grinding of the outer ring, the main advantage of this method is to ensure that the backing bearing is evenly stressed on the same support shaft. According to relevant literature, the outer ring is grinding in groups, in order to ensure accuracy, the grinding mandrel and the bearing outer ring must be closely matched, so the induction heating method needs to be taken to heat the outer ring, and then it can be installed to the mandrel. Similarly, when the outer ring is removed, induction heating is also required. It should be noted that in order to ensure that the outer ring can be smoothly installed and disassembled, it must be water-cooled to avoid the mandrel itself being heated.

5 Conclusion

To sum up, correct installation, reasonable lubrication, timely regrinding and careful maintenance are of great significance to ensure the accuracy of 20-high rolling mill backing bearings, prolong their service life and improve product accuracy.

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