What should I do if the bearings of the rolling mill fail frequently? Inspection and maintenance are indispensable

Rolling mill bearings may be unfamiliar to everyone, and thus they are even less familiar with the faults of rolling mill bearings. How can we analyze the faults of rolling mill bearings and then carry out targeted methods for modification? Inspection and maintenance are very important for the use of rolling mill bearings.

What is a rolling mill bearing?

Rolling mill bearings refer to those used in the rolling industry, namely the rolling of non-ferrous metals, ferrous metals and non-metallic products. Bearings used at the roller diameter of the calendering roller system and on the roller. Four-row cylindrical roller bearings are usually used to bear radial loads, while thrust roller or thrust ball bearings, as well as angular contact ball or radial roller bearings, are used to bear axial loads. Nowadays, oil-gas lubrication, oil mist lubrication and grease are mostly used for lubrication and cooling.

The structural characteristics of rolling mill bearings

Rolling mill bearings typically use cylindrical roller bearings to bear radial loads, while deep groove ball bearings or angular contact ball bearings, or tapered roller bearings with radial or thrust designs to bear axial loads.

Four-row cylindrical roller bearings and six-row cylindrical roller bearings are almost all used in the roll necks, rollers and presses of steel rolling stands. Compared with other roller bearings, these bearings have lower friction. Because these bearings are usually installed on the roll neck with an interference fit, they are particularly suitable for applications in steel rolling mills with high rolling speeds. The low cross-section of these bearings allows for the use of relatively larger roll neck diameters compared to those of the rolls. Because it can accommodate a large number of rollers, its radial load capacity is very high. Multi-row cylindrical roller bearings can only withstand radial loads. Therefore, these bearings are installed together with deep groove ball bearings or angular contact ball bearings, or tapered roller bearings with radial or thrust designs, the latter of which bears the axial load. Four-row and six-row tapered roller bearings are designed in a separable manner, meaning that the bearing rings with integral flanges, rollers and cage assemblies can be installed separately from the separable bearing rings, or all bearing components can be installed separately. This simplifies the installation, maintenance and inspection of bearings to a considerable extent. The bearing can withstand a certain limit of axial displacement of the shaft relative to the bearing housing. Four-row cylindrical roller bearings have one cylindrical bore. Some sizes of bearings can also be supplied with conical bore. Bearings with conical bores can be adjusted during installation to achieve a certain radial internal clearance or a determined preload.

Fault Analysis of Rolling Mill bearings

The service life and reliability of bearings are closely related to their installation and design. Improper design and manufacture of bearing housings will lead to uneven force distribution on the bearings, reducing their lifespan. Bearing housings should have self-aligning properties to prevent the bearings from being subjected to eccentric loads due to the deflection and deformation of the rolls caused by burning. Due to the special process of the MPM continuous rolling mill in the hot-rolled seamless steel pipe plant, all the rolling mills are installed at a 45℃ incline. The on-site environment is under conditions such as high temperature, cooling water spray on the rolls, and large impact loads. The service life of imported bearings is generally about two years. If domestic bearings are used, the service life of the bearings will be further reduced, and faults will be more concentrated, seriously affecting production. According to statistical analysis, bearing failures are mainly caused by the following reasons: ① The on-site assembly accuracy and installation process of the bearing; ② The assembly of bearing seals is unreasonable; ③ Poor working conditions and inadequate lubrication.

1. Bearing design calculation

The service life of a bearing is inseparable from its reasonable design. When choosing a bearing, the equivalent load of the bearing must be calculated. Bearing loads can be roughly classified by direction into: radial loads, axial loads, and compound loads. According to the different natures during the load application process, it can be classified into three forms: pulse, constant and alternating. The rated load of a bearing is a fixed value of the bearing, which depends on the characteristics of the material used. The static rated load is the maximum allowable static load that the end of the rod can bear on its weakest section without reaching permanent deformation. The ideal static rated load value is obtained by conducting tensile stress tests on a large number of rod ends using appropriate materials under suitable ambient temperatures. The usage value is 80% of the test results, thus including a safety factor of 1.25. The dynamic rated load is used to determine the service life of the rod end spherical plain bearing under dynamic load conditions. Dynamic loads include those under swing, rotation and axial inclination.

2. Bearing assembly technology

The technical application characteristics of bearings, such as their precision and assembly clearance, directly affect their service life and reliability. Dimensional accuracy is a necessary item when installing bearings on shafts or bearing housings (boxes), referring to inner diameter, outer diameter, width, assembly width, chamfer dimensions and taper tolerances or allowable values. Shape accuracy refers to the allowable values of inner diameter variation, average inner diameter variation, outer diameter variation, and average outer diameter variation. Rotational accuracy refers to the deflection during rotation, which includes the allowable values of radial and axial deflection of the inner and outer rings, lateral deflection of the inner ring, and deflection of the outer diameter of the outer ring. The convexity of the inner ring, outer ring and rolling elements of the bearing is not on the same circumferential line, resulting in uneven force distribution at both ends of the rolling elements and local spalling. The main reason for the cracking of the small flange on the inner ring is that the width of the travel groove is too large. The depth of the overtravel groove is machined too deep, resulting in stress concentration areas. It is necessary to optimize the processing tolerance of parts and control the processing and assembly quality. The radial clearance of a rolling bearing refers to the amount of movement by which one ring remains stationary while the other ring moves from one extreme position to the other perpendicular to the bearing's axis. Whether the selection of bearing clearance is correct or not has a significant impact on mechanical operation accuracy, bearing life, frictional resistance, temperature rise, vibration and noise, etc. The on-site faults were statistically analyzed, and the factors affecting the quality of bearing spare parts included: surface spalling of the inner ring and rolling elements, and cracking of the small flange of the inner ring. The manufacturing and assembly deviation of the cage is too large. The assembly tolerance and clearance control are not precise.

Analysis of Maintenance of Rolling Mill Bearings

1. Inspection of bearings during operation

According to the requirements of the bearing usage environment, the bearings should be regularly inspected, observed and measured. Listening means using the specified equipment to stand against the axial end cover of the bearing and the radial shell of the bearing housing, with the other end pressed against the ear, to listen for any impact sounds or mechanical friction sounds during the operation of the bearing. Observation involves checking the operating environment, installation position, vibration offset, lubrication and other conditions to see if there are any adverse environments. Measurement involves using thermometers and vibration meters to quantitatively inspect the bearing housing.

2. Inspection of bearings in a stationary state

Check the tightness of the relevant spare parts, whether each component is in the correct position and whether there is any loosening, etc. It is necessary to frequently check whether the bearing housing is tightened and whether it is loose. In particular, the bearing housing on the operating side and the steel rolling frame must be axially fixed without any gap. The fit clearance between the bearing housing on the transmission side and the window of the steel rolling frame should be minimized as much as possible to reduce the impact of the runout of the transmission shaft on the bearing. In addition, the bearing lubricant can be tested to check if there are any impurities, iron oxide scale, water, etc. in the lubricating oil. Roughing mills are usually subjected to strong impact loads, causing significant vibration of the bearings and easy loss of lubricating grease. Therefore, it is required that the grease has strong adhesion, which can firmly adhere to the surface of the parts. Part of it lubricates the bearing on the raceway, while the rest is retained in the internal space of the bearing to play a sealing role. If external dirt invades the bearing housing, the grease near the outside will be contaminated first, causing wear on the surface of the bearing parts. As the amount of dirt increases, the wear surface will expand, and at the same time, cracks in the signal layer will gradually expand, eventually causing the ring to crack. In severe cases, related components such as the rolls and bearing housings may be scrapped.

3. Post-disassembly inspection of the bearings

After the rolls are replaced, a cleaning agent can be used to clean the bearings thoroughly, dry them with compressed air, and then inspect the rollers, cages and rollers. If there are dents, pitting or other phenomena on the rollers and raceways, replace them as the specific situation requires. At the same time, check the radial clearance of the axial seal box. After confirming that there are no errors, add lubricant for standby. In conclusion, the bearings of rolling mills have their unique application environments, characteristics and requirements. When the rolling force is relatively small, the cracking of the inner and outer rings of the bearing and the fragmentation of the rolling elements caused by excessive load are very rare. Therefore, special attention should be paid to the correct installation, inspection and maintenance of the bearing, and corresponding process regulations should be formulated to reduce the accident rate of bearing burning in the rolling mill. The factors influencing the service life of bearings are multi-faceted. Therefore, a comprehensive management system and process regulations should be formulated to achieve the goal of extending the service life of bearings and ensuring smooth production.

4. Maintenance, inspection and abnormal handling of rolling mill bearings

In order to maintain the original performance of the bearings in good condition for as long as possible, maintenance and inspection are necessary to prevent accidents before they occur, ensure the reliability of operation, and improve productivity and economy. Maintenance should be carried out regularly in accordance with the operation standards corresponding to the mechanical operating conditions. The content includes monitoring the operating status, replenishing or replacing the lubricant, and regular disassembly for inspection.

What should I do if the bearings of the rolling mill fail frequently? Inspection and maintenance are indispensable. The importance of maintaining and servicing bearings is beyond doubt. For the use of bearings, we must carry out regular inspections and maintenance. This not only improves the efficiency of bearing usage but also extends their service life.

Discussion on the Failure Analysis Process of Rolling Mill Bearings

Although the rotation of rolling bearings used in rolling mills is extremely reliable, improper installation, use or manufacturing can also cause early failure. Studying the early failure of bearings within their expected service life can prevent frequent early failures of bearings during use.

To reduce the early failure of rolling bearings and determine the causes of early failure of rolling bearings, based on the working conditions, it is first necessary to understand the alloy composition of the steel used to manufacture the bearing, the processing technology of the bearing from material input to finished product, and the precision grade of the bearing. According to the application scenarios, appropriately collect the rolling force, linear speed, types of rolled products, lubrication methods and effects, bearing sealing effects, shapes of rolled sheet materials and the quality of finished products after rolling of this bearing rolling mill. Based on the collected rolling mill data combined with all the bearing data, conduct a bearing failure analysis.

When a bearing fails to operate as expected, it can be considered that the bearing has failed. Generally, early failure occurs much earlier than the time when the bearing completely loses its function. Therefore, the research and analysis of early bearing failure are very necessary.

Early understanding before the supply of rolling mill bearings. For bearings, a new steel rolling environment designer needs to understand the previous usage conditions of the bearings. If possible, early investigations should include observing the basic characteristics of the parts that are compatible with the bearings before, during and after disassembly. These characteristics should be recorded in the form of text and photos for each component. So that the designers can design more practical rolling mill bearings based on the working conditions, and any improper aspects during the installation and disassembly process should be pointed out and corrected. A thorough understanding of the damage characteristics of the previously used bearings should be achieved, and early failure of the bearings should be avoided as much as possible through the data obtained in the early stage.

The installation and disassembly methods of bearings vary by type and are not elaborated here. No matter what method is adopted, care must be taken to avoid damaging the bearing. Violent means (such as hammering, prying with an iron rod, etc.) are not allowed. Even the slightest scratch marks can cause deviations in usage or future failure analysis. The dimensional tolerances of the bearing housing and rolls during installation can refer to the relevant national standards. After the bearing is damaged, the first site should be preserved for use in bearing failure analysis. If disassembly is urgently needed, photos of the damaged bearings and accessories should be taken as evidence. The removed waste bearings should be placed in the warehouse and kept in their original state without being damaged.

Failure mechanism

Mechanical damage Rolling mill bearing parts have good machining accuracy and surface roughness. Improper operation during installation and the appearance of scratches and abrasions on the rolling contact surface will change the surface stress state, thereby affecting the service life of the bearing. Especially when these scratches and indentations occur on the rolling surface, it is even more harmful. When the metal particles produced by scratches and abrasions move and embed into the raceway, they will have an impact on the bearing again.

The permanent indentations produced when the rolling elements are overloaded are called Brinell indentations. Such indentations can lead to bearing failure, for example; Bearing drops or improper installation, etc. Abnormal noises produced during the operation of bearings are all initial signals of the appearance of Brinell indentations.

When the bearing is subjected to vibration but the rolling elements do not rotate, the raceways will also be damaged. This kind of damage is called pseudo-Brinell indentation, which mainly occurs before and after the installation of the bearing. False Brinell indentation damage can be observed in the bearings of equipment that has been subjected to vibration during transportation and has been idle for a long time.

During the grinding process of bearing raceways, micro-vibrations on the raceway surface caused by machine tool vibration, unsharp dressing of grinding wheels, unreasonable configuration of the linear speed ratio between the workpiece and the grinding wheel, as well as improper handling of rough peaks and valleys, are all the root causes of early failure. This phenomenon will be manifested during the inspection of finished bearings.

Wear and tear damage   

Wear usually causes the gradual damage of the bearing parts of the rolling mill, and eventually leads to the loss of dimensional accuracy of the bearing parts and other related problems. The failure caused by wear does not mean that only the bearing needs to be replaced. Due to changes in clearance and fit, other states caused by wear may become the main failure mechanism, and wear may generate stress concentration sources that cause cracks.

Under appropriate lubrication conditions, the micro-protrusions on the surface of the bearing mating parts may undergo plastic deformation and become flat due to cold pressing. In this state, the bearing can achieve the expected service life. However, when the lubrication conditions deteriorate, metal-to-metal contact occurs on the rolling surfaces inside the bearing, leading to an increase in friction. This results in local deformation and frictional welding within the bearing. Under such alternating forces, the local frictional welding points can be torn off the body, increasing plastic deformation. As a result, surface pitting occurs, and there will be material transfer on the surface of another part. Whether this state deteriorates depends on the operating condition. Minor adhesion damage is called abrasion, while severe adhesion is called bite.

When hard particles are mixed between the contact surfaces of bearing parts and move relative to the contact surfaces, it will cause abrasive wear. Sharp abrasive particles can produce obvious furrows, which is also the root cause of early failure. These abrasive particles mainly come from inside the bearings or from within the machine system, such as: the purity of lubrication, the cleanliness during bearing installation, and the cleanliness of the bearings after cleaning before leaving the factory.

Crack failure

Cracks in bearing parts are caused by overload or cyclic load stress. Smelting and processing techniques can also lead to manufacturing cracks. Apart from cracks caused by inclusions and oxides, cracks resulting from smelting and processing techniques cannot proceed to subsequent processing procedures.

Cracks generated during the subsequent processing of bearings are usually related to heat treatment and grinding processes. If cracks exist in a certain manufacturing process and flow into the next process, they will promote rapid expansion in subsequent processing. However, there is one exception: if the cracks produced during the final grinding stage are not detected, they will be carried over to the operating equipment. Excluding the above situations, the crack problems usually encountered are mostly related to the installation and operation process, and are caused by installation scratches, abrasions and cyclic loads. The residual stress of the parts, the static stress caused by installation and the stress caused by external loads interact with each other, resulting in the stress state. Since the propagation direction of the crack is perpendicular to the direction of the applied stress, certain information can be obtained due to the propagation direction of the crack.

When the rolling mill bearings are taken off the machine for maintenance, the early failure sources of the bearings can be identified based on these crack damage information, thereby enabling the rolling mill bearings to have a longer service life.

There are many reasons for the failure of rolling mill bearings. It is not possible to increase the service life of rolling mill bearings by solving or explaining one or two reasons. If the service life of rolling mill bearings is to be improved, every factor link should be comprehensively considered. Control must be strengthened from aspects such as material selection, processing technology and manufacturing, transportation process, and installation operation status.