1 Introduction

The rolling mill is a key equipment in the production of copper strip materials. The four-row cylindrical roller bearings used in the rolling mill are an important component of the rolling mill and an essential guarantee element for achieving high-speed and high-precision rolling processes. Since the 1980s, the four-row cylindrical roller bearings used in rolling mills have been widely adopted in non-ferrous metal smelting enterprises. However, there are still many issues regarding their correct use and management in actual applications. The main problems are: how to increase the effective service life of the four-row cylindrical roller bearings used in rolling mills and how to reduce their impact on the accuracy of the rolled materials. This issue is particularly prominent for newly established copper strip processing enterprises, small copper strip processing enterprises, and enterprises with a rapid expansion of copper strip production but with insufficient management measures and technical strength. 

Due to the relatively weak technical strength of these enterprises and their shallow understanding of four-row cylindrical roller bearings for rolling mills, there are many problems in the installation, use and management of such bearings. This leads to a short service life of the bearings, frequent bearing-related issues affecting the rolled products, and significant impacts on the reliable startup rate of the equipment, the maintenance costs, and the labor intensity of the maintenance personnel. 

Therefore, it has become 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 experience in copper processing enterprises, at present, the main problems existing in the use and management of four-row cylindrical roller bearings for rolling mills are as follows.

The rolling mill is treated the same as ordinary bearings using four rows of cylindrical roller bearings, and no special management measures are implemented. 

② Randomly grinding the outer circle of the bearing inner ring will damage the bearing clearance. 

③ The bearing assembly was carried out in a rough and violent manner, with the parts being forcibly stretched, tilted, and forcefully pried apart. 

④ The lubrication is not standardized, especially for the dry oil lubrication method, where the over-lubrication is particularly severe. 

⑤ When used randomly with the rolls and bearing housings, the clearance accuracy will deteriorate. 

⑥ The bearings were not properly maintained and the usage method was incorrect. 

⑦ The newly installed bearings do not have the correct matching plan and clearance specifications. 

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

⑨ The equipment is severely overloaded, resulting in a decrease in the lifespan of the bearings.

2 Correct Understanding of Four-row Cylindrical Roller Bearings for Rolling Mills

The types of four-row cylindrical roller bearings used in rolling mills mainly include FC type (one inner ring with four rows of cylindrical rollers), FCD type (double inner rings with four rows of cylindrical rollers), and FCDP type (double inner rings with cylindrical rollers and flat retaining rings 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 lifespan. Compared with ordinary bearings, they have many different features. 

From an application perspective, the roller bearings need to have good lubrication and cooling conditions, and their radial dimensions should be as small as possible. The application scenarios of supporting rollers and working rollers vary greatly, and the types of bearings used and the lubrication methods are also different. 

The main application conditions that require special consideration for roller bearings are: 

① Speed: Here, not only the high speed is to be considered, but sometimes a slow speed can also be a problem. A slow speed makes it difficult to form an oil film. 

② High load capacity: Moreover, the rolling elements in multiple columns are not evenly loaded, and sometimes certain columns do not bear any load at all; 

③ Inner and outer ring eccentricity: This is mainly reflected in the bearing clearance, and it directly affects the accuracy of the rolled sheet metal strip; 

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

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

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

For certain special bearings, higher surface roughness is required, and the rollers need to have a special shape and use a special material formula, etc. Currently, 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 the design of the roller system structure, whether the configuration of the bearings is reasonable is the prerequisite for the rolling mill to meet the requirements of the rolled materials and for the normal operation of the rolling mill bearings. Currently, copper strip processing rolling mills are increasingly developing towards higher speed, higher load, higher precision and higher reliability. More and more four-row cylindrical roller bearings are used in combination with thrust bearings. This configuration has the following main advantages: 

When the cross-sectional dimensions are the same, its radial load-bearing capacity is the greatest. That is, for a given roller body diameter, it can accommodate the largest roller neck diameter. 

② Radial load and axial load are respectively borne by two types of bearings, each performing its own function, enabling them to adapt to higher rolling speeds. 

③ By using the thrust bearing alone to control the axial displacement, a smaller axial clearance can be achieved compared to double-row spherical roller bearings and four-row cylindrical roller bearings, which is beneficial for improving the accuracy of the sheet metal strip. 

④ The bearing accuracy can be easily improved and guaranteed, thereby enhancing its reliability and enabling the production 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 much more convenient. 

⑥ The inner ring of the bearing is closely fitted with the roller neck, preventing the occurrence of the problem where the inner ring runs out and causes the bearing to overheat. 

⑦ Maintenance and inspection are relatively easy; 

⑧ The overall cost of using bearings is low.

As mentioned above, due to the working characteristics of the rolling mill, compared with the general cylindrical roller bearings of the same inner diameter size, the special rolling mill bearings have the characteristics of smaller outer diameter size, wider width, greater bearing capacity, and lower limit rotational speed. According to the provisions of JB/T5389.1-2005, when the rolling mill bearing parts are manufactured using carbonized steel G20Cr2Ni4 and G20Cr2Ni4A in accordance with GB/T3203-1982, the depth of the carbonization layer and the quality of heat treatment should comply with the provisions of JB/T8881-2001; when the rolling mill bearing parts are manufactured using high-carbon chromium steel GCr15 and GCr15SiMn in accordance with GB/T18254-2002, the heat treatment quality should comply with the provisions of JB/T1255-2001. 

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

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

3 Proper Use of Four-row Cylindrical Roller Bearings for Rolling Mills

Due to the existence of the aforementioned nine issues during the use and management of the four-row cylindrical roller bearings in the rolling mill, the service life of the bearings is shortened, which has a significant impact on the production scale and cannot provide reliable guarantee for the accuracy of the rolled products. 

The lifespan of a rolling bearing usually refers to its fatigue life, which is the cumulative working hours or total revolutions until fatigue spalling occurs on the rolling surface of the bearing. Although the service life of rolling mill bearings is not typically specified, considering the reduction of production costs, it is still necessary to strive for the four-row cylindrical roller bearings used in rolling mills to have an as long as possible effective service time. From the actual usage of rolling mill four-row cylindrical roller bearings, apart from normal fatigue spalling, there are various forms of damage; common ones include wear, burning, and failure of lubricating grease. The manifestations of damage are that the bearing operates abnormally, the friction torque and temperature increase sharply, vibration and noise deteriorate, and even the bearing gets stuck and cannot rotate.

3.1 The damage forms and causes of four-row cylindrical roller bearings used in rolling mills

We need to understand the factors that affect the service life of bearings. First, we should analyze the types of bearing damage and identify the causes of the damage. 

Based on the actual usage situation of four-row cylindrical roller bearings for sheet and strip rolling machines, the main forms of damage that occur in the use of such bearings include the following: 

(1) Contact fatigue spalling: In the production of copper strip, the rolling machines are mostly reversible type. During the rolling process, the working rolls and supporting rolls are operated in reverse rotation, and the load undergoes repeated alternating effects, resulting in a high impact load. The bearings operate under heavy loads and high impact forces, and their fatigue life will be greatly affected. During use, fatigue spalling of the bearings is generally more likely to occur on the surface of the rollers. 

(2) Wear: The rolling bearing is not purely rolling. There is sliding between the rollers and the inner and outer raceways, as well as between the cage and the rollers. This sliding causes wear. After wear occurs, the surface quality of the rolling surface deteriorates, the rotational accuracy of the bearing decreases, the clearance increases, and even exceeds the allowable value. The vibration and noise also increase. 

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

After a certain period of use, wear and tear of the bearing is inevitable. If the bearing shows slight wear, it can still be used. However, when the precision of the rolled sheet material decreases and the size deviation becomes large, the bearing must be replaced. 

The contact fatigue spalling and wear of bearings are inevitable. Bearings cannot be used indefinitely. During use, efforts should be made to avoid high impact loads (this should be particularly noted in hot rolling machines). Therefore, improving lubrication can reduce wear, which is an important factor in extending the service life of bearings. 

(3) Burn damage: During the operation of the bearing, if the sliding friction parts within it experience a sudden increase in temperature, it indicates that the lubricating grease has failed. The surface structure of these parts changes, and in severe cases, adhesion and jamming occur, which is known as bearing burn damage. Burn damage is a transient phenomenon caused by a sharp increase in frictional heat. To prevent the occurrence of burn damage, the main approach is to improve the design of the bearing. The cage of the bearing should be made of metal with self-lubricating properties to reduce sliding friction, and the lubrication and cooling conditions should be improved to 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. Affected by the gradual oxidation of the base oil, the intrusion of impurities such as dust and water, the evaporation of the base oil or the leakage of the grease, the grease loses its normal lubrication performance, the friction temperature increases sharply, and the rolling surfaces suffer from burn damage. The manifestation of grease failure and damage is also burn damage, but it is caused by the gradual deterioration of the grease. The main factors affecting the lifespan of the grease include working temperature, bearing load, the composition of the grease and installation conditions, vibration, the content of dust and water, etc.

Based on the above several forms of bearing damage, several factors that affect the service life of bearings have been summarized. 

(1) Dynamic Load: Dynamic load is the primary factor affecting the service life of bearings. The supporting roller bearings are designed and selected based on the rolling force during the rolling process. During the rolling process, the unbalanced rolling force, vibration, and impact load are several times the normal load; especially when there are stuck materials, double rolling, missed reduction in process, foreign objects entering the rolling rolls, etc., it will cause tens to several tens of times of impact force. This impact force sometimes exceeds the maximum load value of the four-row cylindrical roller bearings used in the rolling mill significantly. It is the main culprit causing unreasonable damage to the four-row cylindrical roller bearings used in the rolling mill. Therefore, during the rolling process, operations should be standardized, and the above phenomena should be avoided to prevent sudden overload damage to the four-row cylindrical roller bearings used in the rolling mill and premature fatigue damage. 

(2) Bearing material: The chemical composition, smelting method, and heat treatment method of the bearing material are of vital importance to the lifespan of the bearing. The four-row cylindrical roller bearings used in rolling mills are subject to special requirements due to the influence of their working conditions. Continuous discussions need to be held with bearing manufacturers to continuously improve the bearing's material, hardness, and heat treatment method, in order to find out the suitable bearing material for the working conditions. 

(3) Installation and Sealing: The installation of the bearing has a significant impact on its lifespan. The installation of the bearing requires good concentricity, and the sealing device must be sufficient to prevent the intrusion of dust, particles, and moisture. The radial clearance during operation should be adjusted appropriately. If the clearance is too small, the friction temperature of the bearing will increase, causing burn damage to the roller path surface; if the clearance is too large, the bearing will experience severe vibration, inertia, and impact, resulting in high noise and reduced lifespan. 

(4) Fixed set of paired relationships: Since the four-row cylindrical roller bearings used in the rolling mill have separable inner rings, in actual use, one option is to select the appropriate bearing pairing relationship, or to grind the outer circle of the inner ring according to the pairing relationship to ensure the bearing clearance. Not only should the same set of bearings always remain paired, but also the assembly positions between the rolls and on the rolls should be fixed, and they must not be randomly paired for use. Additionally, the outer circle of the inner ring cannot be randomly ground to damage the bearing clearance, as this will seriously affect the force distribution of the rolling elements, accelerate the damage of the bearings, and also affect the quality of the strip material. 

(5) Lubrication: For four-row cylindrical roller bearings of the rolling mill that are lubricated with grease, the selection of the grease and the amount of grease used should be appropriate. Poor lubrication will cause early damage to the bearings. Reasonable filling of grease can reduce frictional heat, prevent excessive temperature rise, reduce wear, prevent rusting, and enhance sealing performance. However, excessive lubrication can also have adverse effects. Firstly, the heat dissipation becomes worse, especially for high-speed rolling mills. If too much grease is filled, it may be the main cause of bearing burnout. Secondly, it leads to waste. If the used grease cannot be completely removed when replacing the rolls, the impurities already present in the grease will often remain in the bearings, causing damage to the bearings. 

For four-row cylindrical roller bearings used in rolling mills that employ oil mist lubrication or oil-gas lubrication, ensuring the quality and quantity of the lubricant is of utmost importance. Failure to meet the lubrication requirements will cause significant damage to the bearings.

3.2 How to Use Four-row Cylindrical Roller Bearings for Rolling Mills Correctly

The correct use of four-row cylindrical roller bearings for rolling mills is of great significance for increasing strip production and reducing costs. It is essential to properly install and maintain the bearings to ensure that their service life can be controlled by us, so as to carry out regular replacements and avoid early damage to the bearings and the occurrence of unexpected accidents. Due to the working characteristics of the rolling mill, fatigue spalling and wear of the bearings during operation are inevitable. The purpose of 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 the production cost.

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

As mentioned earlier, the four-row cylindrical roller bearings used in rolling mills have many characteristics that are different from those of ordinary bearings. One cannot simply treat the four-row cylindrical roller bearings for rolling mills in the same way as ordinary bearings. The characteristics of rolling mills are: 

(1) Heavy load capacity, capable of bearing 2 to 5 times more than that of ordinary bearings; 

(2) The speed of the work varies greatly. 

(3) The working environment of the bearings is rather harsh, and lubricating fluids and debris from the rolling process can easily enter the interior of the bearings. 

(4) The rolling rolls are replaced more frequently. 

(5) Due to the high tolerance and plate shape accuracy requirements for copper strip materials, the rolling mill must have high rigidity and high precision. 

The four-row cylindrical roller bearings used in rolling mills have the following advantages: low friction coefficient, small radial size, high manufacturing accuracy, and the inner ring is detachable, which is convenient for disassembly and assembly. They are widely used. By using such bearings, the diameter of the roll neck of the rolling mill can be increased, and the bearing capacity and maximum rotational speed are higher than those of other bearings; however, they do not bear axial force and an auxiliary bearing for bearing axial force needs to be added. They are suitable for the working and supporting rolls of four-roll rolling mills.

3.2.2 Scientifically determine the bearing clearance and maintain a strict matching relationship

The radial clearance of the four-row cylindrical roller bearings used in rolling mills is achieved through the tolerance of the raceways. Currently, there are two methods for designing and implementing the radial clearance of the rolling mill's four-row cylindrical roller bearings: 

One method is the base shaft system approach, where the tolerance of the inner raceway dimensions is determined based on the range of the inner diameter size of the bearing. The radial clearance of different groups of this bearing is achieved by the variation of the size tolerance of the outer raceway (that is, by grinding the outer raceway to obtain the required radial clearance). This method is called the one-way tolerance method. This method is not very scientific. For bearings in the large clearance group, it is prone to cause changes in the diameter of the center circle of the rollers, and interference occurs between the rollers and the outer side edge of the cage pocket. 

Another method is the base hole system. In this method, the dimensional tolerances of the outer ring and the roller components are determined based on the size range of the inner diameter of the bearing. The radial clearance of different groups of this bearing is achieved by the variation of the dimensional tolerances of the inner ring raceways. That is, by using calculation data, the size of the inner ring raceways is ground to obtain the required radial clearance. This method is called the two-way tolerance method. This method is scientific and suitable for mass production. In JB/T5389.1—2005 "Rolling Bearings - Four-row Cylindrical Roller Bearings for Rolling Mills", the base hole system method is adopted. Most well-known foreign bearing manufacturers mostly use this method. 

In reality, there is another method for achieving different radial clearances of bearings, namely the mixed tolerance method. In "Table A6" (inner ring raceway diameter deviation) given in the JB/T5389.1—2005 standard, the inner ring raceway diameter deviation was originally provided 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 being used; otherwise, for bearings with a clearance greater than the basic group clearance, in order to achieve the radial clearance required by the finished bearing design, the size of the outer ring raceway has to be ground again, resulting in the realization of the bearing's radial clearance being achieved through the grinding of the raceways of the inner and outer rings of the bearing. Only when the inner diameters (inner re-rolling dimensions) of the bearing roller groups are the same and the design and implementation methods of the radial clearance are the same, can the interchangeability condition of the bearing inner ring be satisfied. 

The size of the radial clearance directly affects the load distribution within the bearing, especially influencing the variation of the load on the largest rolling element inside the bearing. Since the rolling mill requires changing the rolls, the bearing housing needs to be frequently disassembled and assembled. Therefore, for a four-row cylindrical roller bearing used in the rolling mill, it is inevitable to have a certain clearance. The clearance of the bearing changes due to the fit, operating temperature, and deformation after bearing 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 rolling roller should also be matched to avoid interference and affect the service life. 

When replacing the rolls, it is crucial to regularly adjust the force-bearing area of the outer ring of the four-row cylindrical roller bearings used in the rolling mill. If the outer ring of the bearing always works in one area, it will inevitably cause local wear on the inner circle of the outer ring, resulting in increased bearing clearance, reduced accuracy, and damage to the outer ring. The correct approach is to rotate the outer ring of the bearing by 90° each time the rolls are replaced and the bearings are repaired and maintained. This will ensure that the outer ring is evenly subjected to force and wears evenly, thereby maximizing the effective service life of the bearing. 

Establish a bearing usage record, specifying the pairing relationship between the bearings and the corresponding rolls and bearing housings, in order to ensure the clearance of the bearings. 

Regularly clean the bearings, inspect the working surfaces of the inner and outer raceways as well as the surfaces of the rolling elements for wear, and check the clearance. If the clearance exceeds the standard, replace the bearings. When using bearings, give priority to using the original same set of bearings. For the remaining parts, conduct careful measurements before selecting and combining them for use.

3.2.3 Standardize the assembly process of bearings to avoid rough handling.

The condition of the roller bearing housing is an important factor affecting the quality of the rolled products. Therefore, the bearing housing must be disassembled regularly and quickly so that it can be inspected, repaired and the bearings can be replaced. 

At present, the disassembly and installation of most enterprise rolling mill bearing housings are carried out by using cranes. Due to the heavy weight of the rolling mill and the bearing housing, as well as the high assembly accuracy of the bearings and the rolling mill, it is difficult to align the bearings and the rolling mill. Moreover, factors such as the poor stability and large swing of the crane also pose constraints. Using the crane operation method will result in extremely difficult disassembly and installation operations, and have the following drawbacks: 

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

(2) It is prone to mechanical jamming. Sometimes, even if the roller itself is lifted, it is still impossible to smoothly remove the bearings and the bearing seats from the roller; at the same time, the roller neck is more likely to be scratched, and the rolling elements of the bearing and the inner raceway are more likely to be damaged. 

(3) Poor safety and reliability, which can easily lead to personnel injuries and damage to the crane. 

Now, professional manufacturers have produced devices for disassembling and installing the working rolls, supporting rolls, bearings, and bearing housings of rolling mills. These devices have been gradually adopted in the steel industry and have achieved efficient, reliable and safe disassembly and installation of bearing housings. However, there is no precedent in the copper processing industry, and there is still a problem of understanding. 

Therefore, how to standardize the assembly process of roller bearings and adopt mechanized and specialized equipment for roller bearing assembly is a key issue that all enterprises need to consider. Using specialized equipment for disassembling and assembling roller bearings may involve a relatively large initial investment. However, considering aspects such as reducing labor costs, increasing the service life of the bearings, and improving labor productivity; and from a long-term usage perspective, it is undoubtedly beneficial.

3.2.4 Standardize lubrication to reduce bearing wear

The lubrication of bearings utilizes an oil film to separate the moving surfaces in relative motion from each other, preventing excessive wear and failure due to rough point contact. If the importance of bearing lubrication technology is highly regarded, it can ensure that the roller bearings maintain stable performance and rotational accuracy during the predetermined working life. 

During actual production, the following issues were found regarding the lubrication of the rolling mill bearings: 

(1) When the working rolls of the rolling machine are rotating at high speed, it is likely to damage the internal seal of the bearing housing, causing the lubricating oil to leak into the emulsified liquid system, polluting the emulsified liquid and affecting the surface quality of the rolled strip material. 

(2) Due to the pressurized spraying of the emulsion, the emulsion penetrates into the bearing housing of the rolling mill, causing the lubricating oil to emulsify and resulting in the failure of the lubrication function. This can easily lead to bearing damage and excessive consumption of the bearings. 

(3) The lubricant contains a large amount of water, causing the grease to soften and change color, resulting in the formation of block-like wear spots on the rolling surface of the bearing. 

(4) 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 machines, forced cooling is required. The bearing lubrication system needs to effectively address this issue. 

The failure analysis of the rolling mill bearings indicates that the normal operation of the bearings has never been interrupted due to fatigue failure caused by normal contact pressure. The failure of the bearings is mainly attributed to poor lubrication, the intrusion of mechanical impurities and moisture. Whether the moisture in the lubricant is dissolved or free, it will have a detrimental effect on the service life of the bearings. Water can erode the bearings, shorten their service life, and can also enter the microscopic cracks on the bearing raceway surface to cause corrosion and hydrogen embrittlement, accelerating the expansion of cracks and causing the bearings to peel off in large areas prematurely. 

The lubrication of rolling mill bearings is divided into grease lubrication and oil lubrication. The advantage of grease lubrication is 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-sized and simple four-roll rolling mills. In modern complex, high-precision, heavy-load, high-speed, high-temperature working conditions, rolling mills use oil lubrication. The methods of oil lubrication mainly include oil mist lubrication and oil-gas lubrication. 

(Lubricating oil lubrication) 

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

Lubricating grease is classified according to the type of thickener. There are various types such as lithium-based and sodium-based. The rolling mill bearings commonly use lithium-based lubricating grease. The characteristic of lithium-based lubricating grease is that it has good water resistance and a high drip point, and can be used in damp and water-contacting mechanical parts. 

Lubricating grease is classified into several grades based on its fluidity, namely the penetration value. The higher the penetration value, the softer the grease. 

The amount of grease to be filled should be one-third or one-half of the space in the bearing and the bearing housing. If too much grease is added, due to the heat generated by stirring, the grease may deteriorate or solidify, and it will also be unfavorable for the heat generated during rotation to be dissipated in time. At high speeds, it should only be filled to one-third or less. When the rotational speed is very low, to prevent foreign objects from entering the bearing, the entire space of the housing can be filled. 

The service life of grease is limited. Its lubrication performance gradually decreases during use, and wear also increases. Therefore, it must be replenished and replaced at regular intervals. The replenishment cycle of grease is related to the structure of the bearing, rotational speed, temperature and environment, and should be determined based on the specific working conditions of the enterprise. When replacing the grease, it should be noted that greases of different brands cannot be mixed. The mixture of greases with different types of thickening agents will damage the structure and consistency of the grease. If it is necessary to replace grease of different brands, the original grease in the bearing must be completely removed before adding new grease. 

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

The higher the rotational speed, the more appropriate it is to use a grease with a higher penetration value. 

② When the environmental temperature is low, use the type of grease with a larger penetration degree. 

③ When the environmental temperature is high, choose the grease with a higher pour point. 

④ It should have strong water resistance and should not use sodium-based lubricating oil as it is prone to emulsification. 

The cleanliness, corrosion resistance, moisture content and oil separation capacity of the fat should all be subject to the corresponding indicators. 

(2) Oil mist lubrication 

Oil mist lubrication is an efficient method of bearing lubrication. It uses compressed air as the power source to atomize the oil, generating a dry oil mist resembling smoke with particle sizes around 2 μm. This mist is then transported through pipelines to the lubrication points. Currently, oil mist lubrication systems are widely used in metallurgical enterprises for the lubrication of large, high-speed, and heavy-duty rolling bearings. 

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

The oil mist can be dispersed along with the compressed air to all the frictional areas that require lubrication. This results in a good and uniform lubrication effect. 

② Compressed air has a lower specific heat capacity and a higher flow rate, making it easy to remove the heat generated by friction. 

③ Significantly reduced the consumption of lubricating oil; 

④ Due to the presence of a certain pressure (2-3 KPa) in the oil mist, it can provide excellent sealing, preventing external impurities, moisture, etc. from entering the friction pair. 

However, there are also some drawbacks to oil mist lubrication. When choosing it, one should be aware of these: 

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

② A compressed air system must be provided. 

③ It is only suitable for lubricants with lower viscosity. The atomization rate is relatively low for lubricants with higher viscosity. 

④ Compared with oil and gas lubrication, it has poor adaptability in high-speed, high-temperature environments and in situations where the bearings are subjected to damage from contaminants, water, and organic chemicals in the flowing medium. 

⑤ The utilization rate of the lubricant is relatively low, approximately only about 60%. Compared with oil and gas lubrication, the fuel consumption is higher. The adjustment of the oil supply is poor, and it is impossible to supply oil at a fixed time and in a fixed amount. 

(3) Oil and Gas Lubrication 

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

Suitable for applications involving high speed, high temperature, heavy load, and where the bearings are exposed to contaminants, water and organic chemical hazards in the flowing medium. 

② Since the oil in the oil-gas lubrication system is not atomized but is merely transported to the lubrication area in the form of oil droplets by compressed air, the oil-gas lubrication system can deliver various types of lubricating oils without being limited by the viscosity of the oil. 

③ In oil-gas lubrication, the oil and gas can be mixed in the right proportions by adjusting the amounts of oil and compressed air to meet the requirements of each lubrication point. 

④ What is emitted into the atmosphere is merely air, and it causes no pollution to the environment. 

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

⑥ The monitoring system is complete and the degree of mechanical-electrical integration is high; 

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

Its drawback is that the initial investment is relatively large.

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

The condition of the rolling mill has a significant impact on the lifespan of the rolling mill bearings. The same bearing, when used in different working conditions of the rolling mill, will have a considerable difference in its service life. The working conditions of the rolling mill include: the load and the distribution of the load on the bearings, the rotational speed, sealing, lubrication, heat dissipation, working temperature, etc. 

In addition, the service life of the roller bearings is also highly dependent on the assembly state of the roller system in the rolling mill. If the roller system cannot be in the correct installation position as required by the design, it will result in 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 adjusting roller system and the arch frame, as well as the working state of the rolls, the following points should be particularly noted: 

Adjust the assembly accuracy of the rollers, regularly inspect and adjust the side sliding plates of the bearing housings of the working rollers and supporting rollers, ensuring that the axes of all rollers are parallel. Each set of roller bearing housings should be strictly controlled for use and not be used interchangeably at will, in order to reduce the additional axial force caused by the intersection of the rollers and at the same time avoid affecting the plate shape of the rolled material. 

② For the transmission system, especially the universal joint shafts, they should be replaced regularly. Otherwise, due to excessive total clearance, the system may cause oscillation and generate additional axial forces. 

③ The lifespan of the bearing is inseparable from the design of the bearing housing. If the design and manufacture of the bearing housing are improper, it will lead to uneven force distribution on the bearing, reducing its lifespan. The bearing housing should have the property of self-aligning to avoid the bearing being subjected to unbalanced load due to the bending deformation of the rolls. Before the roll system is installed in the frame, it is necessary to carefully check the contact situation between the bearing housing and the self-aligning bearings of the press-down mechanism, as well as the contact situation between the bearing housing and the self-aligning bearings of the base of the frame (or the adjustment mechanism of the rolling line). Avoid the occurrence of a stuck phenomenon in the self-aligning mechanism. 

④ Check whether the dimensions, geometric shapes, precision grades, tolerance ranges and designs of the spare parts related to the bearings are consistent. The main aspects here include the flatness, geometric shape, tolerance accuracy and contact condition of the discharge window of the rolling mill; the wall thickness difference of the bearing housing, which is a key dimension determining 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 window of the frame. Additionally, check whether the side clearance of the roller system after being installed in the frame meets the design requirements. If the clearance is too large, inevitable phenomena such as reversing and starting shock, and cross-rolling of the rolls will occur. 

⑤ Check whether the surface finish and hardness of the contact surfaces that mate with the bearings are within the specified range, and verify whether all the clearances and interference fits meet the design requirements, etc. 

⑥ The deflection of the rolling mill rolls has a significant impact on the load distribution of the rolling mill bearings. If the deflection of the rolls becomes too large after being subjected to force, the four rollers of the four-column cylindrical roller bearings used in the rolling mill will experience a situation where only half or only one column is subjected to force, which will seriously shorten the effective service life of the bearings. Therefore, it is emphasized that the load on the rolling mill should be uniform and reasonable, and it is extremely important to strictly prevent overloading. 

From the perspective of assembling four-row cylindrical roller bearings for the rolling mill with the bearing housing, the following points should be noted as the key points: 

Installation of the inner sleeve 

The inner rings of the four-row cylindrical roller bearings used in rolling mills should have an interference fit with the roll necks. During installation, they are usually heated using an induction heater or an oil bath. Flame baking heating is prohibited. The heating temperature should be controlled at 80℃ to 90℃, and should not exceed 120℃. Otherwise, it is very likely to cause the inner raceway of the bearing ring to anneal, affecting its hardness and wear resistance, resulting in a reduction in the effective service life of the bearing and premature failure. When using induction heating, after reaching the specified temperature, the inner ring should be immediately installed on the roll; when using an oil bath for heating, after the temperature reaches the specified value for 10 minutes, the bearing should be quickly removed from the oil and installed on the roll while hot. If necessary, a little pressure can be applied on the end face of the bearing ring with an installation tool to make the installation easier. After the bearing is installed on the roll, the inner ring must be pressed down immediately, and the gap between the end face of the inner ring and the positioning end face of the roll should be checked with a feeler gauge until it cools down. 

② Installation of the outer garment 

The outer sleeve of the four-row cylindrical roller bearing used for the rolling mill is in a transitional fit with the inner hole of the bearing housing. During assembly, the entire assembly consisting of the outer sleeve, rollers, and cage is gently inserted into the bearing housing using a copper rod, and then fixed against the inner side at the fixed end according to the specified assembly relationship as per the drawing. When installing the outer sleeve, attention should be paid to the markings on the end face and the end face of the cage. It should not be installed in the reverse direction. It should be inserted in the order of the initial state when the bearing housing was disassembled, to prevent the occurrence of the phenomenon where the rollers are subjected to uneven force and the bearing is burned. When installing the bearing, the bearing should be placed horizontally. After the bearing is installed, the force range should be marked to facilitate key inspection during roll replacement; in addition, it is convenient for identifying the force range of the outer ring in the future for adjustment. 

③ Proper assembly of bearing seals 

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

Therefore, the sealing of the bearing housing is of great importance. If the sealing is well handled in the rolling mill, the bearing consumption will be reduced. Therefore, when assembling the bearings and the bearing housing, special attention should be paid to the installation of the sealing components, and the assembly should be carried out strictly in accordance with the drawings. The bearing sealing components can consider using ordinary fluororubber frame seals. Not only are they inexpensive, but also, when used reasonably, they can achieve good results. The lip direction of the sealing rings on the axles in the two side labyrinth must face the outside of the bearing when installed, which can effectively prevent the splashing of rolling cooling liquid and tiny metal particles into the bearing. In addition, it is necessary to prevent the accidental damage to the sealing rings during installation.