1 introduction

In recent years, in the metallurgical bar line production industry, the high rigidity mill with short stress line is generally welcomed by the rolling mills because of its advantages of less investment, quick effect, convenient installation and adjustment, easy operation, less rolling waste, light weight and so on. However, rolling mill bearing failure accidents occur frequently, as many as 22 rolling mills per month, and it is difficult to replace, and the economic loss is large. This paper studies and analyzes the failure phenomenon and puts forward practical and effective improvement methods.

2 characteristics and assembly structure of rolling mill bearing

The rolling mill adopts two sets of four row short cylindrical bearings and one set of deep groove ball bearings. Among them, four rows of short cylindrical bearings, long life, large bearing capacity, but can only bear radial force, can not bear axial force, so the use of double row deep groove ball bearings to bear axial force. The outer rings of the four rows of short cylindrical bearings can be released freely, so that the inner rings can be set on the roller neck in advance, and the outer rings can be loaded into the bearing seat first. The bearing seat is pushed onto the roller neck to coordinate with the inner ring, and the assembly of the bearing seat and roller becomes the self-assembly of the bearing itself, as shown in Figure 1. FIG. 1 The left side of the roll is assembled with fixed end bearings. Four rows of cylindrical roller bearings bear the main rolling force, and the inner ring and roll neck adopt interference fit. Axial fixation of rolling roller is realized by deep groove ball bearings which bear all axial load and part of radial load during rolling.

FIG. 1 The right side of the roll is the assembly of the driving end bearing. The driving end has only one set of four-row cylindrical roller bearing to bear the rolling force during rolling. There is no deep groove ball bearing, and there is no axial locking device between the roll and the bearing seat. At the same time, four rows of cylindrical roller bearings have small internal oil storage space. Its drive end retaining ring, half ring and four rows of cylindrical roller bearing inner ring in axial clearance fit.

On the right of Figure 1 are the cardan shaft and the connecting shaft bracket. The cardan shaft connects the motor reducer and the shaft roller, and the connecting shaft bracket is used to lift the cardan shaft and reduce the vibration of the cardan shaft.

3 Research and analysis of rolling mill bearing failure

The main failure parts of rolling mill bearings are two sets of four-row cylindrical roller bearings, of which the four-row cylindrical roller bearings on the right drive end are more frequent, and the accident probability is about 5∶ 1 compared with the fixed end on the left. The main failure modes of bearings are as follows: the roller neck rotates relative to the inner ring of four rows of cylindrical roller bearings, resulting in the roller neck damage or fracture; Four rows of cylindrical roller bearing inner ring, outer ring, rolling body locked, rolling body embedded inside and outside the ring; Part of the outer ring rotates in the bearing seat, causing damage to the inner and outer end covers on both sides of the outer ring and the inner holes of the bearing seat.

3.1 Research and analysis of roll cooling

When the work roll, the cooling water should be rolling heat away in time, but the unreasonable point arrangement for cooling water, cooling water flow is too small, a variety of reasons, such as water nozzle clogging roll cooling effect is not ideal, so make the roll bearings in the local area produces 170 ℃ or higher, grease will soon become a liquid, lubricating oil in large quantities leak out through the seals, The sharp decline in lubrication conditions leads to a further increase in the bearing temperature. The rolling mill bearing is in a high temperature environment for a long time, which not only easily causes the rapid drying of grease in the bearing and causes the bearing failure, but also easily causes the bearing fatigue damage and shortens the bearing life. Therefore, in the production of rolling mill, the management of roll water cooling is also the first content to improve bearing failure.

3.2 Bearing seal research and analysis

The roller bearing seat works in the environment where a large amount of cooling water, rolling oil and oxide mixture splash around, and the sealing ring works under bad conditions. Frequent and rapid roller replacement can also cause damage to the sealing ring in the bearing loading and unloading process. Dissection of used roller bearings found that: lubricants contain a lot of water, grease softening, discoloration, individual bearings like cleaning in water.

3.2.1 Bearing selection analysis

On the basis of maintaining the original sealing at both ends of the bearing seat, bearing with seal ring can be selected. Compared with bearing without seal ring, bearing with seal ring has the advantages of longer service life, significantly reducing the consumption of lubricating oil, and extending a certain period of maintenance interval.

3.2.2 Improvement analysis of sealing structure

The sealing structure is divided into three parts:

The first part is non-contact labyrinth seal, radial bearing end is static labyrinth, roller end is dynamic labyrinth. The length of convex rings of static labyrinth and dynamic labyrinth should be lengthened appropriately to prevent cooling water and oxide sheet from flowing into roller bearings.

The second part is two back to back skeleton oil seal, skeleton using fluorine rubber skeleton, not only low price, but also reasonable use can achieve good results. When the rolling mill is in horizontal state, the lip direction of the shaft sealing ring in the static labyrinth on both sides must be installed outside the bearing, which can effectively prevent the splashing of cooling water and oxide iron sheet; When the rolling mill is in vertical state, the lip of the sealing ring inside the labyrinth on the drive side is installed in the bearing, which can effectively prevent the overflow of lubricating oil due to gravity downward; The lip of the inner labyrinth seal ring on the non-drive side is also installed outside the bearing, which can effectively prevent the splashing of cooling water.

The third part is waterproof cover, in the labyrinth sealed static maze, moving maze set on the waterproof cover made of cyanobutadiene rubber, set on the static maze is static waterproof cover, set on the moving maze is moving waterproof cover. In this way, the moving waterproof sleeve rotates with the moving maze, while the static waterproof sleeve and static maze are still, and the dynamic waterproof sleeve and static waterproof sleeve contact surface is coated with thousands of oil when installed, in order to prevent the roller cooling water and rolled pieces of oxide sheet flow into the labyrinth seal.

3.3 Bearing lubrication research and analysis

3.3.1 Analysis of grease Storage Space

Traditional bearing grease storage space is small, mainly depends on the clearance between the rolling body inside the bearing. The outer end cover can be extended to the outside to increase the oil storage space and prolong the use time of grease.

3.2.2 Grease analysis

Because the grease commonly used in bearings is common lithium grease, it has the disadvantages of high viscosity, low coning degree, inresistance to high temperature and high speed. After opening the rolling mill, the grease dry plate of bearings and heat jam phenomenon are serious. Moreover, it is easy to occur abnormal wear and burn bearing accidents due to grease compaction and abnormal lubrication of bearings.

In order to reduce the cost, there are bearings using domestic roller special grease, urea based grease, low price. Through the comparison of the characteristics of compound lithium grease and urea grease, it can be seen that the compound lithium grease is suitable for heavy load, low speed, high temperature, no need to refueling halfway occasions. Urea - based grease is suitable for high speed, light load, concentrated lubrication. The finishing mill of high-speed bar production line has heavy load and low rotation speed (high 1200r/min). It is refueled at one time when the mill is assembled. After 3 ~ 7 days of rolling, it is off line without refueling. See Table 1. At the same time, considering that the transmission end of ￠350 mill of high speed bar production line has small oil storage space and less oil storage, which also aggravates poor lubrication, it is suggested to use extreme pressure compound lithium base grease.

Bearing lubrication using dry oil lubrication mode, the original manual lubrication mode, laborious and time-consuming, if the operator's sense of responsibility is not strong, often cause the roller bearing oil leakage, leading to bearing failure. Can be changed to automatic centralized lubrication mode, the effect will be better.

The cooling effect of oil lubrication is strong, and can take away dirt and moisture from the bearing. The lubrication methods of roller bearing using oil lubrication are pressure oil lubrication, oil spray lubrication, oil mist lubrication and oil and gas lubrication. Pressure oil lubrication is an effective lubrication mode of roller bearing under conventional speed. Oil injection lubrication is to spray the lubricating oil into the bearing interior at a certain pressure through the nozzle mounted on the bearing side for lubrication, generally used in high-speed roller bearings, or pressure oil lubrication can not meet the cooling requirements of the occasion; Spray and oil-gas lubrication is to spray dry compressed air containing oil mist into the bearing interior for lubrication. The advantage is very little oil consumption, due to the action of air, the cooling effect is very strong. It is mainly used for large roll bearings with high rolling speed and high rolling accuracy, or for roll bearings that are not often disassembled in the bearing box.

3.4 Research and analysis on vibration of rolling mill bracket and cardan shaft

Bracket, universal shaft vibration, poor transmission. When the bracket overheats and the swing is large, the mill will burn the bearings. However, at the end of the universal shaft, the vibration is larger and the roller bearing failure phenomenon is more. In terms of accident probability, the transmission end is much larger than the operating end. It can be concluded that the vibration of bracket and universal shaft has great influence on bearing burning accident of rolling mill. When roll bearing by vibration have wear, or clearance and precision changes will cause the forced vibration of rolling mill, thus affecting mill production, safety operation, product quality, if excessive vibration in production to cause a decline in rolling mill running performance or failure, sometimes the product surface quality fell sharply, with vibration lines.

As for the bracket, the improvement measures can be taken as follows: first, repair the spring balance device of the bracket to reduce the impact of the vibration of the bracket on the rolling mill. In addition, positioning sleeve and positioning ring are installed in the roller sleeve to improve the smoothness of transmission and reduce the vibration of the bracket obviously.

As for the universal shaft, the improvement measures can be taken as follows: on the one hand, the dynamic balance universal shaft is used to eliminate the influence of the swing of the universal shaft itself on the rolling mill drive, and the dynamic balance test grade is 6.3; on the other hand, the assembly of the universal shaft must be strictly required to ensure the assembly accuracy. Thus, on the one hand, the influence of universal shaft vibration on the rolling mill can be reduced; On the other hand, it can improve the service life of cardan shaft and reduce the service cost.

3.5 Bearing seat structure and bearing assembly research and analysis

3.5.1 Bearing seat structure analysis

Bearing life is inseparable from the design of the bearing seat. If the bearing seat is not designed and manufactured properly, it will lead to uneven bearing force and reduce bearing life. The bearing seat should have centrality adjustment to avoid biased bearing due to roll burning and bending deformation. The size, geometry, precision grade and tolerance range of spare parts related to bearings are consistent with the design. Whether the smoothness and hardness of the contact surface matched with the bearing are within the specified range, and whether all the clearance and interference fit amount meet the design requirements, etc.

3.5.2 Bearing assembly analysis

The roller bearing with clearance fit is mainly used to facilitate the replacement of the work roll. The bearing adopts clearance fit on the journal and in the bearing seat. According to the size of the inner diameter, the clearance range of the inner ring fit is from 0. 5 mm to zero. 6 mm. Due to loose fit, the bearing inner ring and roll neck will move relative when rolling mill is working. Therefore, in order to prevent wear on the roller neck, the roller neck should have certain hardness requirements. In addition, because the edge of the bearing inner ring will be dented under long-term huge pressure, it will be difficult to disassemble and assemble the bearing. Therefore, in the surface of the roller neck and the corresponding part of the interval ring should be driven out of the ring groove, and the width of the groove should be larger than the width of the interval ring, to ensure that the disassembly time of the bearing ring is not stuck in the groove. When the radial clearance of the bearing changes after working for a period of time, the interval ring between the inner ring of the bearing can be repaired to adjust it. Bearing outer ring positioning is achieved by the increase and decrease of adjusting gaskets between the gland and the bearing seat, bearing inner ring is tightened by the pressing ring, the pressing ring is fixed together with the roller neck by keys. The axial position of the clamping ring is adjusted by groove nut. When the slotted nut is in position, lock it to the half ring with a locating block to prevent the nut from moving.

4 bearing failure improvement

FIG. 2 is a schematic diagram of improved roll bearing assembly. In terms of structure, the roller side of the bearing seat is changed into a dynamic and static labyrinth ring, at the same time, a waterproof sleeve is set on the labyrinth ring, the skeleton seal is increased between the two labyrinth rings, and the bearing seal is good. If you want to play a better sealing effect, you can also choose four rows of cylindrical roller bearings with seals; Lengthening the end cover of the bearing seat on both sides increases the storage space of grease and reduces the frequency of oil injection; Add a positioning sleeve to the joint of journal and universal shaft to improve the smoothness of transmission.

In terms of daily management and maintenance, check the cooling system water quantity and nozzle blockage regularly; Lubricating grease can not be considered short-term benefits, need to use more reliable extreme pressure lithium compound grease; Lubrication mode should be changed from manual to centralized automatic lubrication, if conditions permit grease lubrication can be changed to oil lubrication; Bearing seat manufacturing and assembly must be in strict accordance with the requirements of manufacturing and assembly accuracy; Bearing assembly is strictly controlled, and it must be checked and cleaned regularly to avoid the breeding of internal impurities in bearings; Standardize the operation of steel rolling, strengthen the management of operation procedures, check the leakage and make up for the deficiency, and prevent the future.

5 conclusion

According to the research and analysis results, the number of rolling mill bearing failures decreases month by month after strengthening management and standardized operation, and the annual cost reduction and maintenance cost is estimated to be millions of yuan.