1 Introduction

Rolling mills are key equipment in the production of copper sheet and strip materials. Four-row cylindrical roller bearings for rolling mills are important components of rolling mills and are crucial guarantee elements for achieving high-speed and high-precision rolling processes. Since the 1980s, four-row cylindrical roller bearings for rolling mills have been widely used in non-ferrous metallurgical enterprises. However, there are still many issues regarding their correct use and management in practical applications. The main points are: how to increase the effective service life of four-row cylindrical roller bearings used in rolling mills and how to reduce their impact on the accuracy of rolled materials. This is particularly prominent for newly established copper strip and sheet processing enterprises, small copper strip and sheet processing enterprises, and enterprises that have rapidly expanded in copper strip and sheet but whose various management measures and technical strength cannot keep up.

Due to the relatively weak technical strength of these enterprises, their understanding of four-row cylindrical roller bearings for rolling mills is rather superficial. There are many problems in the installation, use and management of four-row cylindrical roller bearings for rolling mills. This leads to a short service life of bearings, and the rolled products are often affected by bearing problems, which greatly impacts the reliable operating rate of equipment, equipment maintenance costs, and the labor intensity of maintenance personnel.

Therefore, it is increasingly urgent to enhance the correct understanding of four-row cylindrical roller bearings used in rolling mills and to strengthen their rational use and management. Based on the summary of practical working experience in copper processing enterprises, there are mainly the following problems in the current use and management of four-row cylindrical roller bearings for rolling mills.

① Four-row cylindrical roller bearings for rolling mills should be treated equally with ordinary bearings without any special management measures.

② Grinding the inner ring and outer circle of the bearing at will will damage the bearing clearance.

③ Rough and brutal assembly of bearings, tilting, pulling, and rough prying and smashing.

④ Improper lubrication, especially for dry oil lubrication, leads to severe overlubrication.

⑤ When used in random pairs with rolls and bearing housings, the clearance accuracy will decrease.

⑥ Inadequate maintenance of bearings and improper usage methods.

⑦ The newly installed bearings do not have the correct pairing planning and clearance regulations.

⑧ The inspection and acceptance of bearings upon their entry into the factory are not scientific.

⑨ The equipment is severely overloaded, reducing the service life of the bearings.

2. Correctly understand the four-row cylindrical roller bearings used in rolling mills

The main types of four-row cylindrical roller bearings used in rolling mills include FC type (one inner ring four-row cylindrical roller bearing), FCD type (double inner ring four-row cylindrical roller bearing), and FCDP type (double inner ring four-row cylindrical roller bearing with a flat retaining ring on the outer ring).

The working characteristics of roller bearings are harsh working environment, heavy working load, high unit pressure, large heat generation and short service life. Compared with ordinary bearings, it has many different features.

From the perspective of application, the bearings of the rolls should have good lubrication and cooling conditions, and the radial dimensions should be as small as possible. The application situations of the support rolls and the work rolls vary greatly, and the types of bearings and lubrication used are also different.

The main application conditions that need to be particularly considered for roll bearings are

① Rotational speed: Here, not only the maximum rotational speed should be considered, but sometimes a slow rotational speed can also be a problem. A slow rotational speed makes it difficult to form a lubricating oil film.

② Large force: Moreover, the force on multiple rolling elements is uneven, and sometimes even some columns are not subjected to force at all.

③ Eccentricity of inner and outer rings: This is mainly reflected in the bearing clearance, which directly affects the accuracy of rolled sheet and strip materials.

④ Axial force: For four-row cylindrical roller bearings used in rolling mills, since they cannot withstand axial force, it should be considered separately.

⑤ High temperature: This is a factor that may lead to poor lubrication.

From the perspective of bearing manufacturing, the material should possess properties such as high load-bearing capacity, low friction coefficient, impact resistance, and the ability to absorb hard particles entering the bearing. For large-sized roller bearings, once the size exceeds a certain range, the materials, processes, manufacturing, etc. will be different. For instance, for certain grades of fully quenched steel, if the size is too large, it is not so easy to achieve full quenching.

For some special bearings, higher surface roughness is required, rollers need special shapes, special material formulas, and so on. At present, there are still many problems in the manufacturing of large bearings by domestic manufacturers. The roller bearings for important equipment still need to be imported.

From the perspective of roll system structure design, whether the configuration of bearings is reasonable is the prerequisite for whether the rolling mill can meet the requirements of the rolled materials and for the normal use of the rolling mill bearings. At present, copper plate and strip processing rolling mills are increasingly developing towards high speed, high load, high precision and high reliability. More and more four-row cylindrical roller bearings are used in combination with thrust bearings. This configuration mainly has the following advantages:

① In the case of the same profile size, its radial bearing capacity is the largest, that is, for a certain roller diameter, it can accommodate the largest roller neck diameter;

② Radial and axial loads are respectively borne by two types of bearings, each exerting its own characteristics, which can adapt to higher rolling speeds.

③ By using the thrust bearing alone to control the axial movement, a smaller axial clearance can be achieved compared to double-row spherical roller bearings and four-row tapered roller bearings, which is conducive to improving the precision of sheet and strip materials.

④ The precision of bearings is easy to improve and ensure, making them more reliable and enabling the rolling of products with higher precision.

⑤ The inner and outer rings of the bearing can be separated, making the installation and disassembly of the rolls more convenient.

⑥ The inner ring of the bearing fits tightly with the roller neck, avoiding the phenomenon of the inner ring running and burning the bearing.

⑦ Easy to maintain and inspect;

⑧ The comprehensive cost of using bearings is low.

As mentioned above, due to the working characteristics of the rolling mill, compared with the special bearings for rolling mills, the common cylindrical roller bearings with the same inner diameter have the characteristics of smaller outer diameter, wider width, greater load-carrying capacity and lower limiting speed. According to the provisions of JB/T5389.1-2005, when the bearing parts of the rolling mill are made of carburized steel G20Cr2Ni4 and G20Cr2Ni4A that comply with the provisions of GB/T3203-1982, the depth of the carburized layer and the quality of heat treatment should conform to the provisions of JB/T8881-2001. When the bearing parts of the rolling mill are made of high-carbon chromium steel GCr15 and GCr15SiMn in accordance with the provisions of GB/T18254-2002, the quality of heat treatment shall comply with the provisions of JB/T1255-2001.

In addition, based on the working characteristics of the rolling mill and environmental factors, the service life of the rolling mill bearings is generally not specified.

Furthermore, for high-precision sheet and strip rolling mills, to ensure the reliability of the roll system accuracy, when selecting four-row cylindrical roller bearings for the rolling mill, a certain allowance should be left for the outer circle size of the inner ring of the bearing. After the inner ring of the bearing is assembled onto the roll, it should be ground together with the roll to the required size to ensure the correct radial clearance. A relatively high coaxiality between the inner diameter and outer circle of the bearing and the roller surface can be obtained, which is conducive to ensuring the accuracy of the processed sheet and strip materials.

The main types of four-row cylindrical roller bearings used in rolling mills include FC type (one inner ring four-row cylindrical roller bearing), FCD type (double inner ring four-row cylindrical roller bearing), and FCDP type (double inner ring four-row cylindrical roller bearing with a flat retaining ring on the outer ring).

The working characteristics of roller bearings are harsh working environment, heavy working load, high unit pressure, large heat generation and short service life. Compared with ordinary bearings, it has many different features.

From the perspective of application, the bearings of the rolls should have good lubrication and cooling conditions, and the radial dimensions should be as small as possible. The application situations of the support rolls and the work rolls vary greatly, and the types of bearings and lubrication used are also different.

The main application conditions that need to be particularly considered for roll bearings are

① Rotational speed: Here, not only the maximum rotational speed should be considered, but sometimes a slow rotational speed can also be a problem. A slow rotational speed makes it difficult to form a lubricating oil film.

② Large force: Moreover, the force on multiple rolling elements is uneven, and sometimes even some columns are not subjected to force at all.

③ Eccentricity of inner and outer rings: This is mainly reflected in the bearing clearance, which directly affects the accuracy of rolled sheet and strip materials.

④ Axial force: For four-row cylindrical roller bearings used in rolling mills, since they cannot withstand axial force, it should be considered separately.

⑤ High temperature: This is a factor that may lead to poor lubrication.

From the perspective of bearing manufacturing, the material should possess properties such as high load-bearing capacity, low friction coefficient, impact resistance, and the ability to absorb hard particles entering the bearing. For large-sized roller bearings, once the size exceeds a certain range, the materials, processes, manufacturing, etc. will be different. For instance, for certain grades of fully quenched steel, if the size is too large, it is not so easy to achieve full quenching.

For some special bearings, higher surface roughness is required, rollers need special shapes, special material formulas, and so on. At present, there are still many problems in the manufacturing of large bearings by domestic manufacturers. The roller bearings for important equipment still need to be imported.

From the perspective of roll system structure design, whether the configuration of bearings is reasonable is the prerequisite for whether the rolling mill can meet the requirements of the rolled materials and for the normal use of the rolling mill bearings. At present, copper plate and strip processing rolling mills are increasingly developing towards high speed, high load, high precision and high reliability. More and more four-row cylindrical roller bearings are used in combination with thrust bearings. This configuration mainly has the following advantages:

① In the case of the same profile size, its radial bearing capacity is the largest, that is, for a certain roller diameter, it can accommodate the largest roller neck diameter;

② Radial and axial loads are respectively borne by two types of bearings, each exerting its own characteristics, which can adapt to higher rolling speeds.

③ By using the thrust bearing alone to control the axial movement, a smaller axial clearance can be achieved compared to double-row spherical roller bearings and four-row tapered roller bearings, which is conducive to improving the precision of sheet and strip materials.

④ The precision of bearings is easy to improve and ensure, making them more reliable and enabling the rolling of products with higher precision.

⑤ The inner and outer rings of the bearing can be separated, making the installation and disassembly of the rolls more convenient.

⑥ The inner ring of the bearing fits tightly with the roller neck, avoiding the phenomenon of the inner ring running and burning the bearing.

⑦ Easy to maintain and inspect;

⑧ The comprehensive cost of using bearings is low.

As mentioned above, due to the working characteristics of the rolling mill, compared with the special bearings for rolling mills, the common cylindrical roller bearings with the same inner diameter have the characteristics of smaller outer diameter, wider width, greater load-carrying capacity and lower limiting speed. According to the provisions of JB/T5389.1-2005, when the bearing parts of the rolling mill are made of carburized steel G20Cr2Ni4 and G20Cr2Ni4A that comply with the provisions of GB/T3203-1982, the depth of the carburized layer and the quality of heat treatment should conform to the provisions of JB/T8881-2001. When the bearing parts of the rolling mill are made of high-carbon chromium steel GCr15 and GCr15SiMn in accordance with the provisions of GB/T18254-2002, the quality of heat treatment shall comply with the provisions of JB/T1255-2001.

In addition, based on the working characteristics of the rolling mill and environmental factors, the service life of the rolling mill bearings is generally not specified.

Furthermore, for high-precision sheet and strip rolling mills, to ensure the reliability of the roll system accuracy, when selecting four-row cylindrical roller bearings for the rolling mill, a certain allowance should be left for the outer circle size of the inner ring of the bearing. After the inner ring of the bearing is assembled onto the roll, it should be ground together with the roll to the required size to ensure the correct radial clearance. A relatively high coaxiality between the inner diameter and outer circle of the bearing and the roller surface can be obtained, which is conducive to ensuring the accuracy of the processed sheet and strip materials.

3. Use the four-row cylindrical roller bearings for rolling mills correctly

Due to the nine problems mentioned above in the use and management of four-row cylindrical roller bearings for rolling mills, the service life of the bearings is short, which has a significant impact on the production scale and cannot provide reliable guarantees for the precision of rolled products.

Generally, the service life of a rolling bearing refers to its fatigue life, which is the cumulative working hours or total number of revolutions until fatigue spalling occurs on the rolling surface of the bearing. Although the service life of rolling mill bearings is generally not specified, from the perspective of reducing production costs, it is still necessary to pursue the longest possible effective service time of the four-row cylindrical roller bearings used in rolling mills. From the actual usage of four-row cylindrical roller bearings for rolling mills, in addition to normal fatigue spalling, there are also various forms of damage. There are often wear and tear, burns, and grease failure. The manifestations of damage are that the bearing operates inflexibly, the frictional torque and temperature increase sharply, the vibration and noise deteriorate, and even the bearing gets stuck and cannot rotate.

3.1 Damage forms and causes of four-row cylindrical roller bearings for rolling mills

To master the factors that affect the service life of bearings, we should first analyze the forms of bearing damage and identify the causes of the damage.

Based on the actual usage of four-row cylindrical roller bearings for sheet and strip rolling mills, the main forms of damage existing in the use of four-row cylindrical roller bearings for rolling mills are as follows:

(1) Contact fatigue spalling: In the production of copper sheet and strip, the rolling mills are basically reversible. During the rolling process, the working rolls and support rolls rotate in both forward and reverse directions, with repeated alternating loads and large impact loads. Bearings operate under heavy loads and high impact forces, and their fatigue life will be greatly affected. During use, fatigue spalling of bearings usually tends to occur on the surface of rollers.

(2) Wear: Rolling bearings do not roll purely. There is sliding between the rollers and the raceways of the inner and outer rings, as well as between the cage and the rollers. Sliding causes wear. After wear, the surface quality of the rolling surface deteriorates, the rotational accuracy of the bearing decreases, the clearance increases, and may even exceed the allowable value. Vibration and noise also increase.

To reduce sliding wear, it is necessary to provide good lubrication to form an oil film between the sliding surfaces of metals and prevent direct contact between metals. To this end, efforts should be made to improve lubrication so that the occurrence of significant wear lags behind the fatigue life.

After being used to a certain extent, wear and tear is inevitable for bearings. Bearings that are slightly worn can still be used. However, once the precision of the rolled sheet and strip decreases and the dimensional deviation becomes large, the bearings must be replaced.

Contact fatigue spalling and wear of bearings are inevitable. Bearings cannot be used permanently. During use, high impact loads should be avoided as much as possible (this point should be paid more attention to in hot rolling mills). Therefore, improving lubrication can reduce wear and is an important factor in extending the service life of bearings.

(3) Burn damage: When a bearing is in operation, if the temperature between the sliding friction parts rises sharply, it indicates that the grease has failed and the surface structure has changed. In severe cases, adhesion and jamming may occur, which is known as bearing burn. Burns are an instantaneous phenomenon caused by a sharp increase in frictional heat. To prevent the occurrence of burning, the main approach is to improve the design of the bearing. The bearing cage should be made of metal with self-lubricating properties to reduce sliding friction, improve lubrication and cooling conditions, and ensure the supply of lubricant.

(4) Grease failure: For dry oil lubricated roller systems, the performance of the grease gradually changes during the operation of the bearings. This is mainly due to the repeated shearing of the grease, which causes changes in its structure. Due to the gradual oxidation of the base oil, the invasion of impurities such as dust and moisture, the evaporation of the base oil or the leakage of grease, etc., the lubricating grease loses its normal lubricating performance, the friction temperature increases sharply, and the rolling surface is burned. The manifestation of grease failure and damage is also burn, but it is caused by the gradual deterioration of the grease. The main factors affecting the service life of grease include working temperature, bearing load, composition of grease and installation conditions, vibration, dust, moisture content, etc.

Based on the above several forms of bearing damage, several factors affecting the service life of bearings are summarized.

(1) Dynamic load: Dynamic load is the main factor affecting the service life of bearings. When designing and selecting the bearings of support rolls, they are determined based on the rolling force during the rolling process. During the rolling process, the load caused by unbalanced rolling force, vibration and impact is several times the normal load. Especially when material jamming, stacking, process reduction, or foreign objects entering the rolls occur, it can cause an impact force of ten to dozens of times. This impact force sometimes far exceeds the maximum load value of the four-row cylindrical roller bearings used in the rolling mill. It is the main culprit for the unreasonable damage of the four-row cylindrical roller bearings used in the rolling mill. Therefore, standardized operation should be carried out during rolling to eliminate the above phenomena. Avoid sudden overload damage and premature fatigue damage of four-row cylindrical roller bearings used in rolling mills.

(2) Bearing materials: The chemical composition, smelting methods, and heat treatment methods of bearing materials have a crucial impact on the service life of bearings. Due to the influence of working conditions, four-row cylindrical roller bearings used in rolling mills have special requirements for bearing manufacturing. It is necessary to constantly discuss with bearing manufacturers, continuously improve the material, hardness, heat treatment methods, etc. of the bearings, and find the suitable roller bearing materials for the working conditions.

(3) Installation and sealing: The installation of bearings has a significant impact on their service life. The installation of bearings requires good concentricity, and the sealing device should be sufficient to prevent the intrusion of dust, particles and moisture. The working radial clearance should be adjusted appropriately. If the clearance is too small, the friction temperature of the bearing will increase, and the surface of the roller conveyor will be burned. If the clearance is too large, the bearing will suffer from severe vibration, inertia and shock, generate a lot of noise and have a shortened service life.

(4) Fixed set matching relationship: As the four-row cylindrical roller bearings used in rolling mills have detachable inner rings, in actual use, one is to select an appropriate bearing matching relationship or grind the outer circle of the inner ring according to the matching relationship to ensure the bearing clearance. Not only should the same set of bearings always be matched, but also their assembly positions with the rolls and on the rolls should be fixed. They must not be used in pairs at will. In addition, the inner ring and outer circle should not be ground at will, which will damage the bearing clearance. This will seriously affect the force condition of the rolling elements, accelerate the bearing damage, and at the same time affect the quality of the strip material.

(5) Lubrication: For four-row cylindrical roller bearings used in rolling mills lubricated with grease, the selection of grease should be appropriate and the amount of grease should be proper. Poor lubrication will cause premature damage to the bearings. A reasonable amount of grease filling can reduce frictional heat generation, prevent excessive temperature rise, reduce wear, prevent rust, and enhance sealing performance. However, over-lubrication can also have adverse effects. Firstly, the heat dissipation will deteriorate, especially for high-speed rolling mills. Excessive grease filling may be the main cause of bearing burnout. Secondly, it leads to waste. If the used grease is not thoroughly removed when replacing the rolls, the impurities already present in the grease will often remain in the bearings, causing damage to them.

For four-row cylindrical roller bearings used in rolling mills with oil mist lubrication or oil-gas lubrication, ensuring the quality and content of the lubricant is of vital importance. Once the lubrication requirements are not met, it will cause significant damage to the bearings.

3.2 How to Correctly Use four-row cylindrical roller bearings for rolling mills

The correct use of four-row cylindrical roller bearings for rolling mills is of great significance for increasing the output of strip materials and reducing costs. We should do a good job in the installation and maintenance of bearings to keep track of their service life, so as to carry out regular replacement, avoid early damage of bearings and prevent sudden accidents. Due to the working characteristics of the rolling mill, fatigue spalling and wear of the bearings during operation are inevitable. The purpose of correctly using four-row cylindrical roller bearings for rolling mills is to extend their effective service life as much as possible and reduce the proportion of bearing replacement costs in production costs.

3.2.1 Correctly understand the four-row cylindrical roller bearings used in rolling mills and avoid one-sided understanding

As mentioned earlier, four-row cylindrical roller bearings for rolling mills have many characteristics compared with ordinary bearings. They cannot be simply treated in the same way as ordinary bearings. The characteristics of rolling mills are:

(1) Heavy load, 2 to 5 times higher than that of ordinary bearings;

(2) The range of variation in working speed is large;

(3) The working environment of bearings is relatively harsh, and rolling lubricating fluid, debris and other impurities are prone to enter the interior of the bearings.

(4) The rolls are replaced frequently;

(5) Due to the high tolerance and plate shape accuracy requirements of copper sheet and strip materials, the rolling mill has high rigidity and high precision.

Four-row cylindrical roller bearings for rolling mills feature a low coefficient of friction, small radial dimensions, high manufacturing precision, and detachable inner rings for easy disassembly and assembly. They are increasingly widely used. The adoption of this type of bearing can increase the size of the roller neck, and its load-bearing capacity and limiting speed are higher than those of other bearings. However, it does not bear axial force, and auxiliary bearings for carrying axial force need to be added. It is applicable to the working and supporting rolls of four-high rolling mills.

3.2.2 Scientifically determine the bearing clearance and strictly enforce the pairing relationship

The radial clearance of four-row cylindrical roller bearings used in rolling mills is achieved by the tolerance of the raceways. At present, there are two methods for the design and implementation of radial clearance for four-row cylindrical roller bearings used in rolling mills:

One method is the base shaft method, which determines the dimensional tolerance of the inner ring raceway according to the inner diameter range of the bearing. The radial clearance of different groups of this bearing is achieved by the variation of the dimensional tolerance of the outer ring raceway (that is, by grinding the outer ring raceway to obtain the required radial clearance). This method is called the one-way tolerance method. This method is not scientific enough. For bearings with large clearance groups, it is prone to cause changes in the diameter of the roller center circle and interference between the rollers and the outer side of the cage pocket.

Another method is the base hole system method, which determines the dimensional tolerance of the outer ring and roller assembly according to the inner diameter range of the bearing. The radial clearance of different groups of this bearing is achieved by the dimensional tolerance variation of the inner ring raceway, that is, the required radial clearance is obtained by grinding the inner ring raceway size based on the calculated data. This method is called the bidirectional tolerance method. This method is scientific and suitable for mass production. In JB/T5389.1-2005 "Four-Row Cylindrical Roller Bearings for Rolling Mills", the base hole manufacturing method is adopted. Most well-known foreign bearing manufacturers use this method.

In reality, there is another method to achieve radial clearance in different groups of bearings, namely the mixed tolerance method. In the "Table A6" (Inner Ring Raceway Diameter deviation) provided in the JB/T5389.1-2005 standard, the inner ring raceway diameter deviation was originally given based on the basic group clearance of the bearing. If the designed bearing clearance is not the basic group clearance, it should be corrected before use. Otherwise, for bearings with a clearance greater than the basic group, in order to achieve the radial clearance required by the finished bearing design, the dimensions of the outer raceway have to be ground again, resulting in the realization of the radial clearance of the bearing being achieved by grinding the raceways of the inner and outer rings of the bearing. Only when the inner diameters (inner circular dimensions) of the bearing roller sets are the same and the design and implementation methods of the radial clearance are the same, can the conditions for the interchangeability of the inner rings of the bearings be met.

The size of radial clearance directly affects the load distribution state inside the bearing, especially influencing the variation of the maximum rolling element load within the bearing. Due to the need to change rolls in the rolling mill, the bearing housing needs to be disassembled and reassembled frequently. Therefore, the four-row cylindrical roller bearings used in the rolling mill inevitably have a certain amount of clearance. The clearance of the bearing changes due to the fit, operating temperature, and deformation of the bearing after loading. Excessive or insufficient clearance will directly affect the service life of the bearing. In addition, the clearance values of the radial and axial bearings of the rolls should also match each other to avoid mutual interference and affect the service life.

When changing the rolls, it is very important to regularly adjust the force-bearing area of the outer ring of the four-row cylindrical roller bearing used in the rolling mill. If the outer ring of the bearing always operates in one area, it will inevitably cause local wear of the inner circle of the outer ring, resulting in increased bearing clearance, reduced accuracy, and damage to the outer ring. The correct approach is that each time the rolls are replaced and the bearings are maintained and repaired, the outer ring of the bearing should be rotated 90° to change the force-bearing position, so that the outer ring of the bearing is evenly stressed and worn, thereby extending the effective service life of the bearing to the greatest extent possible.

Establish a bearing usage file and specify the pairing usage relationship between the bearing and the corresponding rolls and bearing housings to ensure the clearance of the bearing.

Regularly clean the bearings, inspect the wear of the working surfaces of the inner and outer raceways and the surface of the rolling elements, check the clearance, and replace them if it exceeds the standard. When using bearings, it is advisable to give priority to the original set of bearings. The remaining parts should be carefully measured before being selected and combined for use.

3.2.3 Standardize the bearing assembly process and avoid rough handling

The condition of the roller bearing housing is an important factor affecting the quality of rolled products. Therefore, the bearing housing must be disassembled regularly and quickly to facilitate the inspection, maintenance and replacement of the bearings.

At present, the disassembly and installation of roller bearing housings in the vast majority of enterprises are carried out by cranes. Due to the heavy weight of the rolls and bearing housings themselves, the assembly accuracy between the bearings and the rolls is high, and the bearings and rolls should not be centered. In addition, it is restricted by factors such as poor driving stability and large swaying. The operation mode of using cranes will lead to very difficult disassembly and installation operations, and has the following defects:

(1) The operation is troublesome, laborious, time-consuming, with low work efficiency and high labor intensity for workers.

(2) Mechanical jamming is prone to occur. Sometimes, even if the rolls themselves are lifted, the bearings and bearing housings cannot be smoothly removed from the rolls. At the same time, it makes the roller neck of the roll more prone to scratches and more likely to cause damage to the rolling elements of the bearing and the inner raceway.

(3) Poor safety and reliability, which can easily cause personal injury and damage to cranes.

Nowadays, there are already professional manufacturers producing disassembly and installation devices for the work rolls, support rolls, bearings and bearing housings of rolling mills. These devices have been gradually promoted in the steel industry, achieving efficient, reliable and safe disassembly and installation of bearing housings. However, there is no precedent for its application in the copper processing industry yet, and there is still a problem of understanding here.

Therefore, how to standardize the assembly process of roller bearings and adopt mechanized and specialized roller bearing assembly equipment is a key issue that each enterprise should consider. The adoption of specialized roller bearing disassembly and assembly equipment may require a relatively large one-time investment, but considering several aspects such as reducing labor costs, extending the service life of bearings, and enhancing labor productivity; From the perspective of long-term use, it should be of great benefit.

3.2.4 Standardize lubrication to reduce bearing wear

The lubrication of bearings is achieved by using an oil film to separate the relatively moving rolling surfaces from each other, preventing excessive wear and failure due to rough point contact. If great importance is attached to the lubrication technology of bearings, it can ensure that the bearings of the rolls maintain stable performance and rotational accuracy during the predetermined working life.

In actual production, the following problems have been found in the lubrication of roll bearings:

When the work rolls of the rolling mill are running at high speed, the internal seal of the bearing housing is prone to damage, causing the grease to leak into the emulsion system, contaminating the emulsion and affecting the surface quality of the rolled strip.

(2) Due to the pressurized injection of the emulsion, it seeps into the bearing housing of the rolls, causing emulsification of the grease and failure of the lubrication function. This can easily lead to bearing burnout and excessive bearing consumption.

(3) The lubricant contains a large amount of water, causing the grease to soften and discolor, resulting in blocky corrosion points on the rolling surface of the bearing.

(4) A portion of the heat generated by the bearings during operation needs to be absorbed and dissipated by the system itself. For high-speed and heavy-load rolling mills, forced heat dissipation is required, and the bearing lubrication system needs to effectively address this issue.

The failure analysis of the roller bearings shows that the normal operation of the roller bearings has never been interrupted due to fatigue failure caused by normal contact pressure. The failure of the bearings is mainly caused by poor lubrication, mechanical impurities and the intrusion of moisture. Whether the water in the lubricant is dissolved or free, it will have a harmful effect on the service life of the bearing. Water can erode bearings, shorten their service life, and also enter the microscopic cracks on the surface of the bearing raceways, causing corrosion and hydrogen embrittlement, accelerating the expansion of cracks and leading to large-scale spalling of the bearings prematurely.

The lubrication of rolling mill bearings is divided into grease lubrication and oil lubrication. The advantages of grease lubrication are that the lubrication facilities are simple, the grease is not easy to leak, and it has a certain ability to prevent water, air and other impurities from entering the bearings. Therefore, grease lubrication is generally widely used in small and simple four-high rolling mills. Rolling mills operating under modern complex, high-precision, heavy-load, high-speed and high-temperature conditions use oil lubrication. The main methods of oil lubrication include oil mist lubrication and oil-gas lubrication.

(1) Lubrication with grease

Grease is composed of base oil, thickener and additive. The viscosity of the base oil plays a significant role in the lubricating performance of grease. The composition of the thickener has a considerable impact on the performance of the grease, especially its temperature characteristics, water resistance, and oil separation. Additives are mainly used to enhance the anti-vaporization, anti-rust, and extreme pressure properties of grease.

Grease is classified according to the type of thickener, including lithium-based, nano-based and many others. Lithium-based grease is commonly used for rolling mill bearings. The characteristics of lithium-based grease are good water resistance and a high dropping point, which can be used in mechanical parts that are damp and in contact with water.

Grease is classified into several grades according to its fluidity, that is, penetration. The larger the penetration value, the softer the grease.

The filling amount of grease should be one-third and one-half of the space between the bearing and the bearing housing respectively. If too much grease is added, due to the heat generated by stirring, the grease will deteriorate or vaporize, and it is also not conducive to the timely dissipation of the heat generated during rotation. At high speeds, it should only be filled to one third or less. When the rotational speed is very low, to prevent external foreign objects from entering the bearing, the housing space can be filled up.

The service life of grease is limited. Its lubricating performance gradually decreases and wear increases during use. Therefore, it must be replenished and replaced at regular intervals. The replenishment cycle of grease is related to the structure, speed, temperature and environment of the bearing, and should be determined according to the specific working conditions of the enterprise. When replacing grease, it should be noted that greases of different grades must not be mixed. Mixing greases containing different types of thickeners will damage the structure and consistency of the grease. If it is necessary to replace grease of different grades, the original grease in the bearing should be completely removed before adding new grease.

The purpose of bearing lubrication is to form an oil film on the rolling and sliding surfaces and an elastic hydrodynamic lubricating oil film between the metal surfaces. When the lubricant is selected correctly, is clean enough, and the rotational speed, temperature and load are appropriate, the service life of the bearing will far exceed the calculated life. Therefore, the reasonable selection of the lubricant is of vital importance to the service life of the roller bearing. The selection of grease should follow the following points:

The higher the rotational speed, the more penetrating grease should be used.

② When the ambient temperature is low, use a lipid with a high penetration rate.

③ When the ambient temperature is high, choose grease with a higher dropping point.

④ It should have strong water resistance. Sodium-based lipids should not be used as they are prone to emulsification.

The cleanliness, anti-corrosion property, water content and oil content of the grease should all be noted as indicators.

(2) Oil mist lubrication

Oil mist lubrication is an efficient method for lubricating bearings. Oil mist lubrication uses compressed air as the power source to atomize the oil, generating dry oil mist with a particle size of about 2μm, similar to smoke, which is then transported to the lubrication area through pipelines. At present, in metallurgical enterprises, oil mist lubrication devices are mostly used for lubricating large, high-speed and heavy-load rolling bearings.

Compared with other lubrication methods, oil mist lubrication has many unique advantages

The oil mist can disperse with the compressed air to all the friction parts that need lubrication. It can achieve a good and uniform lubrication effect.

② Compressed air has a low specific heat and a high flow rate, making it very easy to carry away the heat generated by friction.

③ Significantly reduced the consumption of lubricating oil;

④ Due to the oil mist having a certain pressure (2 to 3KPa), it can play a good sealing role, preventing external impurities, moisture, etc. from invading the friction pair.

However, oil mist lubrication also has some disadvantages. When choosing it, attention should be paid:

In the discharged compressed air, there are 20% to 50% suspended oil particles, which pollute the environment and are detrimental to the health of operators. Therefore, a ventilation and mist exhaust device needs to be added.

② A set of compressed air system must be available;

③ It is only suitable for lubricating oils with lower viscosity and has a relatively low atomization rate for lubricants with higher viscosity.

Compared with oil-gas lubrication, it has poor adaptability in high-speed, high-temperature conditions and in situations where bearings are exposed to dirt, water and fluids with organic chemical hazards.

⑤ The utilization rate of lubricants is relatively low, approximately only around 60%. Compared with oil-gas lubrication, it consumes more oil. The adjustability of oil supply is poor, and it is impossible to supply oil at regular intervals and in fixed quantities.

(3) Oil-gas lubrication

Oil-gas lubrication utilizes the continuous action of compressed air to drive the lubricating oil to flow continuously along the inner wall of the pipeline and form a vortex of oil-gas mixture (the oil and gas do not truly blend or atomize), which is introduced into the lubrication point in the form of fine oil droplets. The compressed air is continuously supplied, while the oil is intermittently supplied. Compared with oil mist lubrication, it not only has the lubrication and cooling functions of oil mist lubrication, but also has the following advantages:

① It is suitable for high-speed, high-temperature, heavy-load applications and situations where bearings are exposed to dirt, water and organic chemically hazardous fluids.

② Since the oil in oil-gas lubrication is not atomized but is delivered to the lubrication point in the form of oil droplets by compressed air, the oil-gas lubrication system can transport lubricating oils of various properties and is not limited by the viscosity of the lubricating oil.

In oil-gas lubrication, the proportion of oil and air can be adjusted to meet the requirements of each lubrication point by regulating the oil volume and the compressed air volume.

④ Only air is discharged into the atmosphere, causing no pollution to the environment.

⑤ The pressure in the lubrication chamber is determined by the compressed air. The high pressure inside the chamber is extremely beneficial for preventing dust and other impurities from entering.

⑥ Complete monitoring means and a high degree of mechanical and electrical integration;

⑦ The system has few moving parts, operates reliably and requires little maintenance.

Its drawback is that the one-time investment is relatively large.

3.2.5 Strengthen the installation and management of the roller system to ensure that it is in a normal working condition

The operating conditions of the rolling mill have a significant impact on the service life of the roll bearings. The same bearings, when used in rolling mills under different operating conditions, have very different service lives. The working conditions of the rolling mill include: load and the distribution state of the load on the bearing, rotational speed, sealing, lubrication, heat dissipation, working temperature, etc.

In addition, the service life of the roll bearings is also closely related to the assembly state of the roll system in the rolling mill. If the roll system cannot be installed in the correct position as required by the design, it will cause significant uneven load distribution and additional loads, which will seriously affect the effective service life of the bearings.

Therefore, in terms of adjusting the assembly relationship between the roller system and the archway as well as the working state of the rolls, the following points should be noted as the key points:

Adjust the assembly accuracy of the rollers, regularly inspect and adjust the side sliding plates of the bearing seats of the working rollers and support rollers, and ensure that the axes of each roller are parallel. The bearing seats of each set of rollers should be strictly controlled in use and must not be used interchangeably at will to reduce the additional axial force caused by the crossing of rolls and at the same time affect the shape of the rolled sheet.

For the transmission system, especially the universal joint shaft, it should be replaced regularly; otherwise, due to excessive total clearance, it will cause swinging and generate additional axial force.

The lifespan of a bearing is inseparable from the design of its housing. If the bearing housing is improperly designed and manufactured, it will lead to uneven force distribution on the bearing and reduce its service life. The bearing housing should have self-aligning properties to prevent the bearing from being subjected to eccentric loads due to the deflection and deformation of the rolls. Before installing the roller system into the archway, carefully inspect the contact condition between the bearing housing and the self-aligning bearings of the lowering mechanism, as well as between the bearing housing and the archway base (or the rolling line adjustment mechanism), to avoid jamming of the self-aligning mechanism.

④ Check whether the dimensions, geometries, precision grades, tolerance ranges of the spare parts related to the bearings are consistent with the design. Here, the main aspects include the flatness, geometric shape, tolerance accuracy and contact condition of the rolling mill's discharge window. The wall thickness difference of the bearing housing is a key dimension that determines the coaxiality of the bearing. As well as the flatness, geometric shape, tolerance accuracy and contact condition of the mating surface between the bearing housing and the archway window. Another aspect to consider is whether the side clearance of the roller system after being installed in the archway meets the design requirements. If the clearance is too large, it will inevitably lead to situations such as reversing and starting impact, as well as roller crossing.

⑤ Check whether the smoothness and hardness of the contact surface that fits with the bearing are within the specified range, and whether all clearances and interference fits meet the design requirements, etc.

The deflection of the rolls has a significant impact on the load distribution of the roll bearings. If the deflection of the rolls is too large after being subjected to force, only half or just one row of the four rows of cylindrical roller bearings used in the rolling mill will be subjected to force, which will seriously shorten the effective service life of the bearings. Therefore, it is very important to emphasize the uniform and reasonable load of the rolling mill and strictly prevent overloading.

From the perspective of the assembly of four-row cylindrical roller bearings and bearing housings for rolling mills, the following key points should be noted:

① Installation of the inner sleeve

The inner ring and the roller neck of the four-row cylindrical roller bearing used in the rolling mill should have an interference fit. During installation, induction heaters or oil baths are usually used for heating. Flame baking heating is strictly prohibited. The heating temperature should be controlled at 80℃ to 90℃ and should not exceed 120℃. Otherwise, it is very likely to cause annealing of the raceway of the inner ring of the bearing, affecting its hardness and wear resistance. This leads to a reduction in the effective service life of the bearings and their premature scrapping. When induction heating is carried out, the inner ring should be immediately installed on the rolls after reaching the specified temperature. When using oil bath heating, after the temperature reaches the specified temperature for 10 minutes, the bearing should be quickly removed from the oil and installed on the roller while still hot. When necessary, a little pressure can be applied to the end face of the inner ring of the bearing with an installation tool, which makes installation easier. After the bearing is installed on the roller, the inner ring must be pressed down immediately. Use a feeler gauge to check the gap between the end face of the inner ring and the end face of the roller diameter positioning until it cools down.

② Installation of the coat

The outer sleeve of the four-row cylindrical roller bearing for rolling mills and the inner hole of the bearing housing are in a transitional fit. When assembling, gently drive the entire assembly consisting of the outer jacket, rollers and cage into the bearing housing with a copper rod, and tightly adhere to the inner fixed end in accordance with the assembly relationship specified in the drawing. When installing the outer jacket, pay attention to the markings on the end face and the end face of the cage. Do not install them in reverse. They should be installed in the order of the initial state when opening the bearing packaging to prevent the bearing from burning out due to uneven force on the rollers. When installing bearings, they should be placed horizontally. After the bearings are installed, their force range should be marked for key inspection when changing rollers. In addition, it is convenient for identification when adjusting the force-bearing area of the outer ring in the future.

③ Reasonable assembly of bearing seals

To absorb the heat generated by the deformation of the rolled piece and reduce the rolling load, a large amount of emulsion is used in the rolling production. The bearing housing is always surrounded by the emulsion, which contains tiny metal particles, acidic substances and other impurities produced during the rolling process. Once these harmful liquids and solid particles enter the bearings, they not only affect the performance of the lubricant and destroy the formed oil film, but also directly cause abrasive wear, leading to premature fatigue and failure of the bearings. In addition, regardless of the lubrication method adopted, it is required that the inner cavity of the bearing housing remain sealed. This is not only to prevent the leakage of lubricating oil but also to ensure the sealing of the inner cavity, effectively guaranteeing the pressure of oil mist and oil-gas lubrication and ensuring the lubrication effect.

Therefore, the sealing of the bearing housing is of great significance. A well-sealed rolling mill will consume less bearing. Therefore, when assembling the bearing and the bearing housing, special attention should be paid to the installation of the sealing parts, and the assembly should be carried out strictly in accordance with the provisions of the drawings. For bearing seals, ordinary fluororubber skeleton seals can be considered. Not only are they low in price, but they can also achieve good results when used reasonably. The lip direction of the shaft sealing ring in the labyrinth on both sides must be installed outward towards the bearing, which can effectively prevent the splashing of rolling coolant and tiny metal particles. In addition, it is necessary to prevent human damage to the sealing ring during installation.

To be continued