3.2.6Strengthen the maintenance and upkeep of bearings, and try to extend their effective service life as much as possible.

Regular inspections of the bearings and checks of their operating temperatures should be conducted in accordance with the regulations. 

The inspection can be carried out by using dedicated equipment to monitor the stethoscope, or by placing one end of a wooden stick or screwdriver against the bearing housing and the other end against the ear for monitoring. Under normal circumstances, the bearing makes a buzzing sound. If there is a sharp clicking sound, it indicates that the lubricant is lacking. If the sound returns to normal after adding lubricant, it can be determined that it was caused by a lack of lubricating oil. If there is an irregular knocking sound, it means that a foreign object has entered the bearing or the bearing has been damaged. In severe cases, the roller track may have peeled off. 

The inspection of working temperature involves using a thermometer to measure the temperature on the bearing housing. If the temperature rise is too high, it may be due to too small internal clearance or inaccurate bearing positioning.

（2） Inspection and maintenance after disassembly

According to the different operating conditions of the rolling mill, it should be stipulated that the rolling mill bearings should be thoroughly inspected and maintained every certain period of time. This can be done simultaneously with the replacement of the rolling mill rolls. Before the bearings are removed, mark the load area clearly. During reassembly, the outer ring should be rotated by 90° to change the bearing's load-bearing area. 

During the disassembly and inspection process, the bearings should be thoroughly cleaned and then dried with dry compressed air. During the inspection, the inner and outer raceways, rolling elements, and cage of the bearings should be carefully examined. 

If there is dull and unreflective wear on the raceways and rolling elements, the extent is not very serious and it does not exceed the limit during the clearance detection. This can be regarded as normal wear and can be used further. If there is mirror-like shiny wear, it indicates insufficient lubrication. If the clearance detection does not exceed the limit at this time, the lubrication can be strengthened and it can be used further. If the clearance detection exceeds the limit at this time, the bearing should be replaced. If there is a small amount of fatigue spalling (also called fatigue pitting, usually called pitted areas) on the inner and outer raceways and rolling elements, it indicates that the fatigue life of the bearing has reached its limit and it must be replaced. 

If rust is found on the bearing parts during inspection, it indicates that water or moisture has entered the bearing, which may be due to an inappropriate lubricant, suggesting a problem with the bearing housing seal. For oil mist and oil-air lubrication, it could also be that the compressed air contains water. If the rust is not severe, it can be cleaned with an oilstone for grinding. In severe cases, the bearing should be replaced. 

The wear on the cage is usually caused by impurities entering the bearing or due to poor lubrication. 

After bearing maintenance, for lubricated bearings using grease, the removed grease must not be reinserted into the bearing; instead, a completely new and qualified lubricant must be used for replacement. 

In addition, during the disassembly and maintenance of the bearings, the components related to the assembly of the bearings should also be inspected to determine whether they meet the dimensional accuracy, surface finish, and contact rate requirements specified in the drawings; especially the wall thickness tolerance of the bearing housing, the cylindricality of the inner hole, the shape of the contact plane between the bearing housing and the frame, the contact rate, and the perpendicularity to the inner hole, etc. All indicators should be checked. Any problems found should be dealt with promptly. 

If the aforementioned regular inspection and maintenance work is carried out properly, it can ensure that the bearings always operate in a correct state, effectively preventing the further deterioration of potential problems and significantly extending the effective service life of the four-row cylindrical roller bearings used in the rolling mill.

3.2.7The correct matching relationship and clearance determination of four-row cylindrical roller bearings for rolling mills

（1） Determination of the correct assembly relationship for four-row cylindrical roller bearings used in rolling mills

The correct assembly relationship for the four-row cylindrical roller bearings used in the rolling mill should be that each set of bearings should be assembled with a specific bearing housing, one or several fixed ends of the rolls, in accordance with the specified clearance for the rolling mill's four-row cylindrical roller bearings. After the bearings, bearing housings, and associated components, as well as the rolls are assembled, the parallelism between the working side of the bearing housing and the roll axis should be measured and adjusted to ensure that after the roll system is installed in the frame, the center line of the rolls is within the same vertical plane. 

Since the inner rings of the four-row cylindrical roller bearings used in the rolling mill can be separated, usually one rolling mill needs to prepare at least one set of spare roller systems. If strict specifications are not strictly followed during use, in actual production, there often occurs the phenomenon of not assembling the bearings according to the original assembly relationship. Especially when one of the bearings fails and the production task is busy, the manager often requires to break this original pairing relationship. This will seriously affect the correct assembly position of the rolling rods in the frame, and even affect the plate shape quality of the rolled material. 

When this fixed assembly relationship is disrupted, it will break the already well-adjusted matching relationship between the rolling elements and the raceways. This will result in severe uneven force distribution on the rolling elements and raceways. In the mild cases, it will cause the bearings to overheat and suffer severe wear, and lead to premature fatigue pitting; in the severe cases, the rolling elements will break, damaging the raceways of the inner and outer rings of the bearing, causing the bearing to burn out and get stuck. 

Therefore, the assembly relationship of using four-row cylindrical roller bearings in the rolling mill is not something that can be casually disrupted. Currently, some enterprises have not fully understood this point, and many cases of damage to the four-row cylindrical roller bearings in the rolling mills are attributed to this.

（2）Determination of bearing clearance

The bearing clearance is an important technical parameter for the normal operation of the bearing. It directly affects the load distribution, friction, temperature rise, and service life of the bearing. Due to the harsh working conditions of the rolling mill bearings, factors such as the rolling load, rotational speed, lubrication, temperature rise, fit clearance, and rotational accuracy need to be considered. In principle, the smaller the bearing clearance, the more conducive it is to extending the service life of the rolling mill bearings. Rolling mills typically use standard rolling mill four-row cylindrical roller bearings. In general, the clearance is usually selected from the C3 or C4 standard clearances. For high-precision rolling mill four-row cylindrical roller bearings, the clearance needs to be determined jointly by the technical personnel of the bearing manufacturer and the bearing manufacturer based on the precision requirements of the rolling mill. Processing allowances are reserved for the inner and outer rings of the bearing. After the inner ring of the bearing is assembled onto the rolling mill roll, the outer circle of the bearing is ground together with the roll surface to the specified size to achieve high precision coaxiality tolerance. 

During the operation of the rolling mill bearings, the appropriateness of the bearing clearance can be determined based on the condition of the bearing damage. 

If the bearing burns out, and it is not due to the inner ring of the bearing sliding along the roller diameter, or due to the friction between related parts and poor lubrication causing the bearing to overheat, then it can be concluded that the working clearance of the rolling mill bearing is too small. 

② The bearing rings and rollers have fractured. If it is not due to reasons such as poor raw material quality, poor bearing quality, excessive impact load, or improper installation, it can be concluded that the working clearance of the rolling mill bearings is too large, resulting in a too small load angle of the bearing (the ideal load angle is 120° to 150°), causing a high stress on the material per unit area and a high load on the rollers. 

In addition, when using the bearings from different manufacturers interchangeably, it is essential to ensure that the selection standards for the bearing clearance are the same (that is, whether the outer diameter and tolerance of the inner circle of the bearing components are consistent, whether the inner raceway dimensions and tolerance values are the same, and whether the clearance is the same). Otherwise, the bearings cannot be interchanged. This is another reason why the above emphasizes that the bearings must have a fixed assembly relationship. 

Reasonable and reliable alignment accuracy is also a prerequisite for the normal operation of the rolling mill bearings. The inner rings of the four columns of cylindrical roller bearings in the rolling mill roll system bear the circumferential radial loads, so they must be in interference fit with the roll neck; the outer rings are to support concentrated loads and need to be in transition fit with the box holes; the matching bearings only bear axial loads and axial positioning functions, and require convenient installation and removal. Therefore, the inner rings of the bearings are in clearance fit with the roll neck, and the outer rings cannot bear radial forces and have a large clearance fit with the box holes.

3.2.8Strengthen the procurement and supply of bearings as well as the incoming inspection at the factory, and continuously accumulate usage experience.

At present, there are many bearing manufacturers in our country that produce four-row cylindrical roller bearings for rolling mills. Due to the varying technical levels of each manufacturer, the quality of the bearings produced also varies significantly. Therefore, during the procurement process of bearings, it is necessary to conduct more investigations. Based on the actual usage conditions of the bearings from different manufacturers, continuous exploration and summarization should be carried out to find a supply manufacturer that meets the usage requirements of the enterprise. At the same time, the inspection of bearings upon entry to the factory should be strengthened to ensure the quality of the purchased bearings. 

Due to the particularity of the four-row cylindrical roller bearings used in the rolling mill, although the standard JB/T5389.1—2005 "Rolling Bearings - Four-row Cylindrical Roller Bearings for Rolling Mills" has stipulated the corresponding requirements, in actual use, there are often many enterprises that, based on the special requirements of the equipment, sign non-standard bearing contracts with the bearing manufacturers. 

For standard bearings, they can be accepted through inspection in accordance with the requirements of JB/T5389.1—2005 and the corresponding supporting standards. 

For non-standard bearings, in addition to conducting factory acceptance based on the contract and in accordance with the requirements of JB/T5389.1—2005 and the corresponding supporting standards, the following points should also be noted: 

Is the marking on the bearing legible and recognizable? Are the factory name, year code, etc. all in compliance with the standards? 

② Are there any scratches, abrasions, tool marks, rust, black spots, material defects, etc. on the surface of the bearing? These external damages often become stress concentration points, leading to fatigue failure of the bearing. 

③ Are there any missing parts in the bearings? Any component such as the raceway, washer, bushing, retaining ring, seat ring, rolling element, cage, or sealing ring must not be missing. 

④ Check for any mismatched parts in the bearings. 

⑤ Check for any mismatch in the specifications of the bearing components.

3.2.9Strictly formulate the production process scientifically to avoid overloading of the bearings.

The characteristics of the roller bearing have been mentioned several times before: the working environment is harsh, the working load is heavy, the unit pressure is high, and the heat generation is large, and the service life is short. It can be said that if the bearings are used in accordance with the specifications, they can completely meet the normal scrapping standards. However, in actual usage, various overloading phenomena often occur. These are mainly caused by human factors, accident factors, and equipment factors. No matter what kind of overload it is, it will have a significant impact on the bearings, and in severe cases, it can even cause accidents. 

When bearings operate under overload conditions, they usually suffer from abnormal wear and fatigue damage, and even result in the fragmentation of rolling elements and the instantaneous failure of the equipment. 

The common forms of overload for rolling mill bearings are as follows: 

① Stable unidirectional overload: For instance, problems with the installation of components in the rolling mill's transmission system, changes in the diameter of the rolls causing excessive deflection of the transmission components, etc., can result in additional loads. Such loads are stable and unidirectional. Such overloads generally do not have very high overload values, but they last for a relatively long time and are not easy to be detected. They will only be discovered when problems arise. To deal with such overloads, by strengthening equipment maintenance, repair, and upkeep, and implementing standardized management of the equipment according to the equipment drawings, technical requirements, and maintenance and repair procedures, it is possible to avoid them. The key lies in enhancing the responsibility of operators, maintenance personnel, and management personnel. 

② Circulating load overload: The arrangement of the rolling process is unreasonable, resulting in an unreasonable distribution of processing rates for each rolling pass, and uneven distribution of bearing loads among the rolling passes. This causes the bearings to operate under a large load for a long time. There are four reasons for this. 

One possible reason is the issue with the technical proficiency of the process planning personnel. Or, they may not have a thorough understanding of the technical and performance parameters of the rolling mill, resulting in an unreasonable rolling process and causing the rolling mill to overwork. 

Secondly, in an attempt to solely focus on increasing production volume, the processing rate of the rolling mill was deliberately arranged in a certain way, and the bearing capacity of the rolling mill was not given much consideration, resulting in the rolling mill being overloaded. 

Thirdly, the operators of the rolling machines do not follow the process procedures. They deliberately reduce the number of rolling passes during operation, or change the reasonable rolling process, which leads to overloading of the rolling machines. 

Fourth, there are rolling mills with bending roll function. If the bending force is too high, the bending force and the deformation resistance of the rolled piece will act together on the rolling mill bearings, causing the bearings to be overloaded. 

All of the above four situations result in the bearings working under long-term cyclic overload conditions. 

③ Instantaneous Overload: During the rolling process. Due to the occurrence of process accidents, the bearings experience instantaneous overload, such as when the strip breaks causing the roller to get tangled, or when the tolerance of the rolled strip section is too large; for a hot rolling machine, the material temperature is too low, or the overlap rolling process, etc. This causes the rolling machine bearings to bear concentrated overload loads for a short period of time, resulting in damage and failure of the bearings. 

In steel rolling machinery, the most common type of overload in rolling bearing components is unidirectional overload or cyclic load overload. These overloads often cause local fatigue cracks to form on the rolling elements, leading to fatigue failure of the bearings. 

How to prevent the overload of the four-row cylindrical roller bearings used in the rolling mill is a key factor for enhancing their effective service life. From the actual usage situation, processing enterprises' one-sided pursuit of production output has led to a relatively frequent phenomenon of the four-row cylindrical roller bearings used in the rolling mill operating under overload for a long time. This issue requires the managers of the enterprises to enhance their awareness and strengthen management.

4 Scientifically managed four-row cylindrical roller bearings for rolling mills

4.1 Strengthen employee technical training to enhance their technical proficiency

Strengthen the technical training and skill development of the operators and maintenance personnel of the rolling machines, continuously enhance their operational and maintenance skills, and make rational and standardized operations become the conscious behavior of the workers. First of all, it is necessary to improve the understanding level of the operators and maintenance personnel regarding the four-row cylindrical roller bearings used in the rolling machines. Change the requirement for them to operate in a standardized manner into I will operate in a standardized manner, and enhance their awareness of standardized operation. 

From the perspective of the rolling machine operators, it is necessary to effectively enhance their operational skills to effectively prevent accidents such as copper strip breakage, copper strip entanglement with the rolls, roll collision, and roll slipping. 

From the perspective of the rolling mill maintenance personnel, it is necessary to assemble the four-row cylindrical roller bearings for the rolling mill in a standardized and correct manner, ensuring that they operate in the structural state required by the design; and avoiding any damage to the bearings during the assembly process. 

From the perspective of process technicians, it is necessary to enhance their understanding of the technical performance of the rolling mill, formulate reasonable rolling processes, determine the tension of the strip material and the bending force of the rolls. Rolling accidents can easily cause the rolls to overload or generate high temperatures instantaneously, resulting in roll defects and affecting the rolls' bearings. In such cases, the rolls should be immediately replaced, and the rolls should be ground. At the same time, the condition of the roll bearings should be inspected. 

From the perspective of enterprise managers, it is necessary to properly balance the relationship between equipment and production, avoid the situation where the sole focus is on increasing output at the expense of equipment, and at the same time strengthen the management and education of employees.

4.2Establish scientific and standardized management systems

（1） Establish a standardized and scientific management system for the operation, maintenance and monitoring of the four-row cylindrical roller bearings used in the rolling mill, in order to extend their service life. The following aspects should be mainly considered:

① Lubrication aspect: How to keep the lubrication pipeline unobstructed? Select the type of lubricant according to the technical specifications, and stipulate the time and amount of lubricant addition to ensure that the surface of the bearing rollers and the sliding surfaces between the rollers and the retaining edges maintain a good lubrication state. 

② Sealing aspect: Specify the regular inspection time for the sealing condition of the sealing components to enable timely replacement of damaged sealing components. Ensure the sealing performance of the bearings to prevent water and metal debris from entering the bearings and to prevent the leakage of bearing lubricants. 

③ In terms of testing: Based on the actual situation of the enterprise, actively carry out monitoring of the operating conditions of the bearings, including: 

Noise monitoring: When the roller bearings are operating normally, there should be a smooth humming sound; regular monitoring should be conducted and compared with the normal sound to promptly detect any abnormalities. 

Lubricant monitoring: If the lubricant becomes dirty, there will be wear particles or contaminants. 

Temperature monitoring: When the temperature rises, abnormal operation will occur. It is necessary to promptly identify the cause of the temperature increase and prevent the problem from escalating. 

Establish records: These records should be synchronized with the usage records of the rollers. They should include the number of days the bearings were in use, the amount of rolled sheet materials, and the maintenance monitoring status, etc. This will enhance the management of the bearing operation conditions.

（2）Assembly and management of four-row cylindrical roller bearings for rolling mills

① Pre-installation precautions mainly include:

Check whether the bearings have any quality issues; carefully inspect the surface of the bearing parts for any defects, such as indentations, burns, cracks, etc.; strictly prohibit the installation of parts with quality problems. 

Ensure that the installation site is clean and strictly prevent any foreign objects from entering the bearing interior. 

Make sure the bearings are clean. Do not install bearings that have not been cleaned properly. 

Ensure the quality of the seal. Carefully check whether the model, specification, and size of the seal parts are appropriate, whether there are any defects or quality issues, whether the related accessories are complete, and whether the fit is reasonable. 

Carefully inspect the bearing housing, the bearing parts of the roll neck, and the sealing parts for any defects such as pitting or burning. 

For the newly assembled roller system, all the parts' dimensions and surface quality should be carefully inspected. 

② The main points to note during installation should include: 

Avoid hard installation and impact. It is necessary to ensure that the bearing is installed gently. Generally, the bearing and the seat hole have a small clearance fit. Under normal circumstances, it is recommended to gently tap the end face to install it in the correct position; if it is impacted forcefully, the bearing may tilt and be more difficult to install, and it may even damage the surface of the bearing seat hole or even cause it to be scrapped. It is recommended to use the special assembly equipment for four-row cylindrical roller bearings designed for rolling mills for assembly. 

The inner sleeve of the bearing must be installed by heating, and the temperature must not exceed 120℃. It is strictly prohibited to use open flames for heating when disassembling or assembling the bearing. 

During the assembly process, if any jamming occurs, promptly identify the cause and eliminate it. If any problems such as part deformation are found, they should be repaired in a timely manner. If necessary, request the revision of the drawings. 

Pay attention to the assembly sequence of the parts. Assemble strictly in accordance with the drawings and technical requirements to ensure reasonable assembly clearance. 

For bearings lubricated with grease, strictly inject the required amount of clean grease as per the specifications, and be careful not to overdo it. 

During the installation process, pay attention to the installation of the sealing components. Avoid any damage to the sealing components during the installation. If any substandard sealing, pressure cover, or other parts are found, they must be strictly repaired or scrapped to ensure that the assembly quality meets the technical requirements. 

For the bearings that have been disassembled and repaired, when reinstalling them, it is necessary to specify the adjustment of the bearing outer ring's load-bearing area and make proper markings. For newly installed bearings, the load-bearing area should also be marked to facilitate future adjustments. 

③ The main precautions during disassembly should include: 

When disassembling the four-row cylindrical roller bearings, care should be taken to protect the parts and no damage to the parts should occur during the disassembly process. When disassembling the inner sleeve, the bearing induction heater must be used for heating, and the temperature must not exceed 120℃. If disassembly is difficult due to adhesion, and it is not an absolutely necessary measure, burning and cutting with fire shall not be used. Otherwise, the surface of the bearing's mating part with the roller neck and the deformation of the rolling mill roll will be caused. 

When a bearing damage issue is discovered, the first step is to promptly determine the serial numbers of the frame, bearing housing and rolls, identify the cause and make a detailed record.

4.3 Strengthen the procurement, storage and management of four-row cylindrical roller bearings for rolling mills

The four-row cylindrical roller bearings have more prominent advantages in terms of rigidity compared to the traditional oil film bearings. They have been used in the supporting rollers of cold rolling mills since 1960. After the 1980s, most newly-built or updated cold rolling mills in China adopted four-row cylindrical roller bearings. In 1985, the Ministry of Machinery Industry issued the "Form Dimensions of Four-Row Cylindrical Roller Bearings for Rolling Mills" standard. In 1991, the Ministry of Machinery Industry issued the "Technical Conditions for Rolling Bearings - Four-Row Cylindrical Roller Bearings for Rolling Mills" standard. In 1995, the Ministry of Machinery Industry merged these two standards and issued the "Rolling Bearings - Four-Row Cylindrical Roller Bearings for Rolling Mills" standard. In 2005, it was revised and reissued. 

At present, there is still a certain gap between China and international renowned manufacturers in the production of four-row cylindrical roller bearings for rolling mills. There are still many issues related to materials, processes, and technologies that need to be explored and studied. There are quite a number of domestic manufacturers producing four-row cylindrical roller bearings for rolling mills, and there is a significant difference in quality and price. The operation of rolling mills also has obvious special characteristics, so there are many issues worthy of study in the selection and procurement of rolling mill four-row cylindrical roller bearings. How to purchase rolling mill four-row cylindrical roller bearings with a higher cost-performance ratio still requires continuous comparison and exploration by each user manufacturer. 

The four-row cylindrical roller bearings used in rolling mills are precision mechanical components. There are strict requirements for their storage and preservation, and the following aspects need to be paid attention to: 

① Warehouse temperature: Before the bearings are manufactured, they are all coated with anti-rust oil. If the temperature is too low or too high, the anti-rust oil will deteriorate. It is more appropriate to keep the warehouse temperature within the range of 0 to 25 degrees Celsius. 

② Warehouse humidity: Excessive humidity can cause rusting of the bearings. The relative humidity of the warehouse should be maintained at 45% to 60%. 

③ Warehouse environment: Bearings should be stored separately. If they must be stored together with other items, the other items stored together must not be chemical substances such as acids, alkalis, or salts. 

④ Regular Inspection: According to the rust prevention regulations for bearing products, a regular inspection should be conducted every 10 to 12 months. If any rust is found on the oil seal packaging, the oil seal should be re-packaged. 

In addition, the bearings should be placed on shelves. During the winter heating season in the north, they should be kept away from the heating pipes to prevent the anti-rust oil from drying out too quickly due to heat. 

For bearings that have been removed from the rolling mill and still have usable value, or bearings that have been opened but without packaging, it is recommended to make a container to hold the bearings. After placing the bearings in the container, add engine oil for soaking to prevent rusting and make it easier to access.