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

（1） Inspection during operation

Bearings should be inspected by listening and the working temperature checked regularly in accordance with the regulations.

Listening inspection can be carried out by using a dedicated device, a stethoscope, or by touching one end of a wooden stick or screwdriver to the bearing housing and pressing the other end against the ear. Under normal circumstances, the bearing emits a humming sound. If there is a sharp clicking sound, it indicates that the lubricant is insufficient. If the sound is normal after replenishing the lubricant, it can be determined that the lack of lubricating oil is the cause. If there is an irregular knocking sound, it indicates that foreign objects have entered the bearing or the bearing has been damaged. In severe cases, the roller track may have already peeled off.

The working temperature check involves measuring the temperature on the bearing housing with a thermometer. 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 working conditions of the rolling mill, it should be stipulated that the bearings of the rolls should be thoroughly inspected and maintained at regular intervals. This can be carried out simultaneously with the change of rolls. Before the bearings are removed, the load area should be clearly marked. When reassembling, the outer ring should be rotated 90 degrees to change the load-bearing area of the outer ring.

When disassembling and inspecting, the bearings should be thoroughly cleaned and dried with dry compressed air. When inspecting, the inner and outer raceways, rolling elements and cages of the bearing should be carefully examined.

If there is dull wear on the raceways and rolling elements, but the degree is not very serious and does not exceed the standard during clearance testing, it can be regarded as normal wear and can continue to be used. If mirror-like bright wear occurs, it indicates insufficient lubrication. At this time, if the clearance detection does not exceed the standard, the lubrication can be strengthened and the bearing can continue to be used. If the clearance detection exceeds the standard, the bearing should be replaced. If there is a small amount of fatigue spalling (also known as fatigue pitting, usually called pitting) on the inner and outer raceways and rolling elements, it indicates that the fatigue life of the bearing has ended and it must be replaced.

If rust is found on the bearing parts during the inspection, it indicates that water or moisture has entered the bearing. It might be that the lubricating oil is not suitable, suggesting that there is a problem with the bearing housing seal. For oil mist and oil-gas lubrication, it is also possible that the compressed air contains water. If the rust is not severe, use an oilstone to grind and clean it. In severe cases, the bearing should be replaced.

Wear on the cage is usually caused by impurities entering the bearing or poor lubrication.

After the maintenance of bearings, for bearings lubricated with grease, the grease removed must not be refilled into the bearings. Instead, brand new and qualified grease must be used.

In addition, while disassembling and maintaining the bearings, the parts related to the bearing assembly should also be inspected to determine whether they meet the dimensional accuracy, surface finish and contact rate requirements specified in the drawings. In particular, check various indicators such as the wall thickness tolerance of the bearing housing, the cylindricity of the inner hole, the shape of the contact plane between the bearing housing and the archway, the contact rate, and the perpendicularity to the inner hole. Any problems found should be dealt with promptly.

If the above-mentioned regular inspection and maintenance work is carried out properly, it can ensure that the bearings always operate in the correct condition, promptly prevent the further deterioration of potential problems, and significantly extend the effective service life of the four-row cylindrical roller bearings used in rolling mills.

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

（1） The correct assembly relationship for four-row cylindrical roller bearings used in rolling mills has been determined

The correct assembly relationship for four-row cylindrical roller bearings used in rolling mills should be that each set of bearings should be assembled with a specific bearing housing and the fixed ends of one or several rolls, and fit in accordance with the specified clearance of the four-row cylindrical roller bearings for rolling mills. After the assembly of the bearings, bearing housings, accessories and rolls is completed, The parallelism between the working side of the bearing housing and the generatrix of the rolls should be measured and adjusted to ensure that the centerlines of the rolls are in the same vertical plane after the roll system is installed in the archway.

Since the inner rings of the four-row cylindrical roller bearings used in rolling mills can be separated, usually at least one spare roll system needs to be prepared for each rolling mill. Therefore, if strict specification requirements are not followed during the usage process, the actual production process often leads to the phenomenon of bearing assembly and use not following the original assembly relationship, especially when one set of bearings has a problem and the production task is busy Often, managers demand that this original pairing relationship be disrupted. This will seriously affect the correct assembly position of the rolls in the archway, and in severe cases, it will even impact the quality of the plate shape of the rolled material.

Once this fixed assembly relationship is broken, the already running-in fit between the rolling elements and the raceways is disrupted, which will cause severe uneven force distribution on the rolling elements and raceways. In mild cases, it may lead to bearing overheating, severe wear, and premature fatigue pitting. In severe cases, the rolling elements may break, damaging the raceways of the inner and outer rings of the bearing, resulting in the bearing burning out and seizing.

Therefore, the assembly relationship of the four-row cylindrical roller bearings used in rolling mills is not allowed to be damaged at will. Currently, some enterprises do not have a profound understanding of this point, and many damages to the four-row cylindrical roller bearings used in rolling mills are caused by this.

（2）Determination of bearing clearance

Bearing clearance is an important technical parameter for whether a bearing can work normally. 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 rolling load, rotational speed, lubrication, temperature rise, fit interference, and rotational accuracy need to be taken into consideration. The principle is that, under the condition that the working conditions permit, the smaller the bearing clearance, the more conducive it is to extending the service life of the rolling mill bearings. The rolling mill adopts standard four-row cylindrical roller bearings for rolling mills. Generally, the downstream clearance mostly selects standard clearances of C3 or C4 groups. For four-row cylindrical roller bearings used in high-precision rolling mills, the clearance needs to be jointly determined by the technicians of the bearing user and the bearing manufacturer based on the precision requirements of the rolling mill. A machining allowance should be reserved for the outer circle of the inner ring of the bearing. After the inner ring of the bearing is assembled onto the roll, the outer circle of the inner ring of the bearing should be ground together with the roll surface to the specified size to obtain a high-precision coaxiality tolerance.

During the use of rolling mill bearings, whether the bearing clearance is appropriate can be determined based on the extent of bearing damage.

If the bearing burns out not because the inner ring of the bearing slides on the roller diameter, or due to friction and poor lubrication of related parts causing overheating of the bearing, it can be considered that the working clearance of the rolling mill bearing is too small.

② If the cracking of the bearing rings and rollers is not due to the raw materials, bearing quality, excessive impact load or improper installation, it can be considered that the working clearance of the rolling mill bearing is too large, causing the load wrap Angle of the bearing to be too small (the ideal load wrap Angle is 120° to 150°), resulting in a large unit area strain pressure of the material and a large load on the rollers.

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

Reasonable and reliable fit accuracy is also a prerequisite for the normal use of rolling mill bearings. The inner rings of the four-row cylindrical roller bearings in the rolling mill roll system bear circumferential radial loads, so they must be in an interference fit with the roll neck. The outer ring is used to support concentrated loads and needs to be over-fitted with the box hole. Due to the fact that the matching bearings only bear axial loads and axial positioning functions, and are required to be easy to install and remove, the inner ring of the bearing and the roller neck are fitted with clearance, while the outer ring cannot bear radial forces and is fitted with a large clearance with the box hole.

3.2.8Strengthen the procurement and supply of bearings as well as their incoming inspection, 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 different technical levels of each manufacturer, the quality of the bearings they produce also varies greatly. Therefore, during the procurement process of bearings, it is necessary to conduct more investigations. Based on the actual usage conditions of bearings from different manufacturers, constantly explore and summarize to find a supplier that meets the usage requirements of one's own enterprise. At the same time, strengthen the inspection of bearings upon their entry into the factory to ensure the quality of the purchased bearings.

For standard bearings, acceptance can be carried out 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 in accordance with the contract and referring to the requirements of JB/T5389.1-2005 and the corresponding supporting standards, the following points should also be noted:

① Whether the bearing markings are clear and legible, and whether the factory name, year and other codes are qualified.

② Check if there are any scratches, abrasions, lathe tool marks, rust, black spots, material defects, etc. on the bearing surface. These external injuries often become stress concentration points, causing fatigue damage to the bearing.

③ Check if there are any missing parts in the bearing. It is not allowed for any of the rings, washers, bushings, retaining rings, seat rings, rolling elements, cages, sealing rings, etc. to be missing.

④ Check whether there are any mismatched parts in the bearings.

⑤ Check whether there is any mismatch in the specifications of the bearing parts.

3.2.9Strictly and scientifically formulate the production process to avoid bearing overload

As mentioned many times before, the characteristics of roller bearings are: harsh working environment, heavy working load, high unit pressure, large heat generation and short service life. It can be said that if bearings are used in accordance with the norms, they can definitely meet the normal scrapping standards. However, in the actual usage process, various overload phenomena often occur. Here, there are three factors: human factors, accident factors, and equipment factors. No matter what kind of overload it is, it will have a relatively significant impact on the bearings, and in severe cases, it may cause accidents.

When bearings operate under overload conditions, they usually experience abnormal wear and fatigue damage, and in severe cases, equipment accidents such as the fragmentation of rolling elements and the instantaneous destruction of bearings may occur.

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

① Stable unidirectional overload: For instance, additional loads are generated due to installation issues with the components of the rolling mill's drive system, changes in the roll diameter causing excessive deflection Angle of the drive joint of the drive system, and other reasons. This type of load is stable and unidirectional. This type of overload generally does not present a very large overload value, but it usually lasts for a long time and is not easy to be detected. It is only discovered when problems occur. For such overloads, they can be avoided by strengthening equipment inspection, maintenance and upkeep, and implementing standardized equipment management in accordance with equipment drawings, technical requirements and inspection and maintenance regulations. The key lies in enhancing the sense of responsibility of operators, maintenance and management personnel.

One reason is the technical proficiency of the process scheduling personnel, or their insufficient understanding of the technical and performance parameters of the rolling mill, which leads to an unreasonable rolling process arrangement and causes the rolling mill to be overloaded.

Secondly, in pursuit of output in a one-sided manner, the process arrangement of the processing rate of the major steps was deliberately carried out, with little consideration given to the load-bearing capacity of the rolling mill, resulting in overload of the rolling mill.

Thirdly, the operators of the rolling mill do not follow the process operation, secretly reduce the number of rolling passes during use, or change the reasonable rolling process, resulting in overload of the rolling mill.

Fourth, for rolling mills with a bending roll function, if the bending roll force is too large, the bending roll force and the deformation resistance of the rolled piece will jointly act on the rolling mill bearing, causing the bearing to overload.

All the above four situations cause the bearing to operate under the condition of cyclic overload for a long time.③ Instantaneous overload: During the rolling process. Due to the occurrence of process accidents, the bearing was instantaneously overloaded, such as causing the roller to wrap after the belt broke, and the section tolerance of the rolled strip blank was too large. For hot rolling mills, there are issues such as too low billet temperature and dead-end rolling, etc. Subject the rolling mill bearings to concentrated overload loads for a relatively short period of time, causing them to be damaged and fail.

In steel rolling machinery and equipment, the most common types of overload that occur in roll bearings are unidirectional overload or cyclic load overload. These overload effects often cause local fatigue cracks to emerge in the rolling elements, leading to fatigue failure of the bearings.

How to prevent the overload of four-row cylindrical roller bearings used in rolling mills is a key factor in improving their effective service life. From the actual usage situation, it is still quite common for processing enterprises to blindly pursue output, resulting in the long-term overload operation of four-row cylindrical roller bearings used in rolling mills. This issue still requires the managers of enterprises to enhance their awareness and strengthen management.

4 Scientific management of four-row cylindrical roller bearings for rolling mills

4.1 Strengthen technical training for employees and enhance their technical quality

Strengthen the technical training and skills exercise of the operators and maintenance personnel of the rolling mill, continuously improve their operation and maintenance skills, and make reasonable and standardized operation a voluntary behavior of the employees. First of all, the understanding level of the four-row cylindrical roller bearings used in the rolling mill among the operators and maintenance personnel of the rolling mill should be enhanced. They should be required to operate in a standardized manner and their awareness of standardized operation should be improved.

From the perspective of the operators of the rolling mill, effectively improving the operation skills of the rolling mill can effectively prevent accidents such as copper strip breakage, copper strip entanglement around the rolls, roll collision, and roll slippage.

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

From the perspective of process technicians, it is necessary to enhance their understanding of the technical performance of the rolling mill, rationally formulate the rolling process, and reasonably determine the tension of the strip and the force of the bending roll. Rolling accidents can easily cause the rolls to be overloaded or generate high temperatures instantaneously, leading to roll defects and simultaneously affecting the roll bearings. When these accidents occur, the rolls should be replaced immediately, ground, and the condition of the roll bearings should be inspected at the same time.

From the perspective of enterprise managers, it is necessary to properly balance the relationship between equipment and production, avoid the phenomenon of blindly pursuing output at the expense of equipment, and at the same time strengthen the management and education of employees.

4.2 Establish scientific and standardized management systems

(1) Establish standardized and scientific management systems for the operation, maintenance and monitoring of four-row cylindrical roller bearings used in rolling mills to extend their service life. Efforts should mainly be made in the following aspects:

① Lubrication aspect: How to keep the lubricating oil passage unobstructed, select the appropriate type of lubricant in accordance with technical specifications, specify the time and amount of lubricant addition, to ensure that the surface of the bearing rollers and the sliding surface between the rollers and the flange remain in a good lubricated state.

② Regarding sealing: It is stipulated that the sealing condition of the sealing parts should be inspected regularly to ensure that damaged sealing parts are replaced in a timely manner. Ensure the sealing performance of the bearing to prevent water and metal debris from entering the bearing and to prevent the leakage of bearing lubricant.

③ Inspection aspect: In light of 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. Regularly monitor and compare the sound with the normal one to promptly detect any abnormal conditions.

Lubricant monitoring: If the lubricant gets dirty, there will be worn particles or contaminants.

Temperature monitoring: When the temperature rises, abnormal operation will occur. The cause of the temperature increase should be identified in a timely manner to prevent the problem from escalating.

Establish records: They should be synchronized with the usage records of the rolls, recording the number of days the bearings are in online use, the amount of rolled sheet and strip materials, and the maintenance and monitoring status, etc., to strengthen the management of the operating conditions of the bearings.

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

The main precautions before installation should include:

Check if there are any quality issues with the bearings. Carefully inspect the surface of the bearing parts for any defects, such as indentations, burns, cracks, etc. It is strictly prohibited to install parts with quality problems.

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

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

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

Carefully inspect the bearing housing and the bearing and sealing parts of the roller neck for any defects such as scratches or burns.

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

② The main precautions during installation should include:

Avoid hard installation and impact. Ensure that the bearing is gently installed. Generally, the bearing and the seat hole have a small clearance fit. Under normal circumstances, the end face should be gently tapped and installed in the correct position. If the bearing is tilted due to forceful impact, it will be even more difficult to install, and the surface of the bearing housing hole may be damaged or even scrapped. It is recommended to use the special disassembly and assembly equipment for four-row cylindrical roller bearings used in rolling mills for assembly.

The inner ring of the bearing must be installed hot and the temperature must not exceed 120℃. Open flame heating is strictly prohibited when disassembling and assembling bearings.

If jamming occurs during assembly, the cause should be identified and eliminated in a timely manner. If problems such as deformation of parts are found, they should be repaired promptly. If necessary, request the modification of the drawings.

Pay attention to the assembly sequence of parts, assemble strictly in accordance with the drawings and technical requirements, and ensure reasonable assembly clearances.

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

During installation, pay attention to the installation of sealing parts to avoid damage to them. If any substandard sealing, gland 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 for maintenance, when reinstalling them, it should be specified that the load-bearing area of the outer ring of the bearing be adjusted and marked properly. For newly installed bearings, the load-bearing area should also be marked properly to facilitate the next adjustment.

③ The main precautions during disassembly should include:

When disassembling four-row cylindrical roller bearings, it is necessary to pay attention to protecting the parts and avoid the phenomenon of disassembling and damaging the parts. When disassembling the inner sleeve, it must be heated with a bearing induction heater and the temperature must not exceed 120℃. When it is difficult to disassemble due to sticking, it is strictly forbidden to cut by fire 熖 unless it is absolutely necessary. Otherwise, it will cause deterioration of the mating surface between the roller neck and the bearing and deformation of the roller.

When bearing damage is discovered, the first step should be to promptly identify the numbers of the frame, bearing housing and rolls, determine the cause and keep a record.

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

Four-row cylindrical roller bearings have more prominent advantages in rigidity than traditional oil film bearings. They have been used in the support rolls of cold rolling mills since the 1960s. After the 1980s, most newly-built rolling mills or cold rolling mills in China adopted four-row cylindrical roller bearings. In 1985, the Ministry of Machinery Industry issued the "Standard for the External Dimensions of Four-row Cylindrical Roller Bearings for Rolling Mills". In 1991, the Ministry of Machinery Industry issued the "Technical Conditions for Four-Row Cylindrical Roller Bearings for Rolling Mills" standard. In 1995, the Ministry of Machinery Industry merged the above two standards and issued the "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 internationally renowned manufacturers in the manufacturing of four-row cylindrical roller bearings for rolling mills. There are still many issues in terms of materials, processes and technologies that need to be explored and studied. There are many domestic manufacturers of four-row cylindrical roller bearings for rolling mills, with significant differences in quality and price. Moreover, the operation of rolling mills has obvious particularities. Therefore, there are many issues worth studying in the selection and procurement of four-row cylindrical roller bearings for rolling mills. How to purchase four-row cylindrical roller bearings for rolling mills with relatively high cost performance It is still necessary for each user manufacturer to make continuous comparisons and explorations.

Four-row cylindrical roller bearings for rolling mills are precision mechanical components. There are strict requirements for their storage and preservation, and the following aspects need to be noted:

① Warehouse temperature: All bearings are coated with anti-rust oil before leaving the factory. Both too low and too high temperatures can cause the anti-rust oil to deteriorate. It is more appropriate to control the warehouse temperature between 0 and 25 degrees Celsius.

② Warehouse humidity: Excessively high humidity can cause rust on bearings. The relative humidity in the warehouse should be maintained at 45% to 60%.

③ Warehouse environment: Bearings are best stored separately. When they must be stored together with other items, the other items stored together must not be chemical substances such as acids, alkalis, and salts.

④ Regular inspection: In accordance with the rust prevention regulations for bearing products, a regular inspection should be conducted every 10 to 12 months. If rust is found on the oil seal packaging, the oil seal packaging should be redone.

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

For bearings that have been removed from the rolling mill and still have practical value, or those that have been opened but not packaged, it is recommended to make mounting containers. After placing the bearings in them, add engine oil for soaking to prevent rust and facilitate their use.

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