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The classification of rolling mill bearings and the reasons affecting the service life of rolling mill bearings

The classification of rolling mill bearings and the reasons affecting the service life of rolling mill bearings

2021-05-08

Rolling mill bearings are used in the calendering industry, namely the calendering of nonferrous metals, ferrous metals, and nonmetallic products. Used for bearing at roll diameter and roller of calendering roll system. Usually, four rows of cylindrical roller bearings are used to bear radial load, thrust roller or thrust ball bearings and centripetal angular contact ball or centripetal roller bearings are used to bear axial load. Now, most of them are lubricated by oil and gas lubrication or oil mist lubrication and grease.


Rolling mill bearing classification

(1) Self-aligning roller bearings

Rolling mill bearings have different configuration types on the rolling mill. At that time, two sets of self-aligning roller bearings were mainly installed on the same roller neck. This configuration basically meets the production conditions at that time, and the rolling speed can reach 600rpm.But with the increase of speed, its shortcomings are more and more prominent: bearing life is short, consumption is large, the finished product accuracy is low, the roller neckwear is serious, the rolling roll axial movement is large, etc..

(2) Four rows of cylindrical roller bearings + thrust bearings

The inner diameter of the cylindrical roller bearing and the roller neck adopts tight matching, bearing radial force, has the advantages of large load capacity, high limit speed, high precision, inner and outer rings can be separated and interchangeable, easy processing, low production cost, convenient installation, and disassembly, etc. Thrust bearing bearing bearing axial force, the specific structure can be selected according to the characteristics of the mill.

Under heavy load and low speed, with thrust roller bearing, with a small axial clearance to bear the thrust load. When the rolling speed is high, with angular contact ball bearings, not only the limit speed is high, but also the axial clearance can be strictly controlled when working. The roller can be guided by tight axial direction and can withstand general axial load. This type of bearing configuration not only has the advantages of long bearing life, high reliability, but also has the advantages of high rolling accuracy, easy control, and so on, so it is the most widely used at present, and is mostly used in wire rolling mill, sheet rolling mill, foil rolling mill, double supporting roll rolling mill, cold rolling mill, and hot rolling mill.

(3) four rows of tapered roller bearings

Tapered roller bearings can withstand both radial and axial forces without the need for thrust bearings, making the main engine more compact. The inner diameter of the tapered roller bearing and the roller neck adopt loose fit, which is very convenient for installation and disassembly, but sometimes it will cause sliding creep due to loose fit, so the inner diameter is often processed with a spiral oil groove. This type of configuration is still widely used, such as four-high hot and cold rolling mill work roll, billet machine, steel beam rolling mill, and other occasions of the roll.

Four rows of cylindrical roller bearings and six rows of cylindrical roller bearings are almost all used in the roll neck, roller, and rolling machine of the steel rolling frame. These bearings have low friction compared with other roller bearings. Because these bearings are usually mounted on the roll neck with interference fit, they are particularly suitable for rolling mill applications with high rolling speed. The low cross-section of these bearings allows the use of a relatively large roll neck diameter compared to the roll diameter. Due to a large number of rollers, the radial load capacity is very high.

Multi-row cylindrical roller bearings can only bear radial loads.These bearings are therefore installed with deep groove ball bearings or angular contact ball bearings, or tapered roller bearings of radial or thrust design, which bear the axial load. Four-row and six-row tapered roller bearings are of a separate design, that is, the bearing rings with integral flanges and the roller and cage assembly may be mounted separately from the separate bearing rings, or all bearing parts may be mounted separately.

This simplifies bearing installation, maintenance, and inspection to a considerable extent. The bearing can withstand a certain limit of axial displacement of the shaft relative to the bearing seat. Four-row cylindrical roller bearings with a cylindrical hole, some sizes of bearings can also be supplied with tapered holes. Bearings with tapered holes can be adjusted during installation to obtain a certain radial internal clearance or to determine preload.

In principle, the lubrication of roller bearings is basically the same as that of other rolling bearings, but the working conditions of roller bearings are relatively bad, so whether their working performance can be effectively played depends to a large extent on the lubrication of bearings.The main lubrication methods for roller bearings are grease lubrication and oil lubrication.

(1) grease lubrication and sealing function, sealing structure and lubrication facilities are simple, easy to add grease, so as long as the working conditions allow, roller bearings are generally grease lubrication. The cooling effect of oil lubrication is strong, and it can take away dirt and moisture from the bearing. The lubricating methods of oil lubrication for roller bearings include pressure oil supply lubrication, oil injection lubrication, oil mist lubrication, and oil and gas lubrication.

(2) Pressure oil supply lubrication is the most effective lubrication method for roller bearings under conventional rotating speed. Oil injection lubrication is to inject the lubricating oil into the bearing at a certain pressure through the nozzle installed on one side of the bearing for lubrication. It is generally used in high-speed roller bearings or occasions where the pressure oil supply lubrication can not meet the cooling requirements.

(3) Spray lubrication is to spray dry compressed air containing oil mist into the bearing interior for lubrication. The amount of oil is less. Due to the effect of air, the cooling effect is very strong. Both pressure oil supply lubrication and oil injection lubrication need to be equipped with inlet and outlet tubing, lubrication pump, oil storage device, and sometimes lubricating oil cooler. Therefore, the cost is higher, and generally, roller bearings are less used.

Reasons affecting the service life of rolling mill bearings

Rolling mill bearing is an important part of the rolling mill, in the process of rolling mill operation, the bearing supports the roll, bears the rolling force of the roll and maintains the correct position of the roll, the quality of the rolling mill bearing and the length of service life directly affect the reliability of the rolling mill operation. There are many factors affecting the service life of rolling mill bearings, such as bearing material, structural design, manufacturing precision, installation and sealing, moisture change, cooling and a series of internal and external factors that will affect the service life of the rolling mill bearings.

The bad working condition is the main cause of premature failure of the rolling mill bearing, as conditions mainly including the load-bearing operating conditions and their distribution, lubrication, sealing, speed transmission, working temperature, cooling condition, such as bearing quality and the use of parts of the same circumstances, different working condition, its service life will there is a big gap.

1. The impact of carbide on bearing life

High carbon steel bearing into after quenching and low-temperature tempering processing organization becomes not dissolve carbide, acicular martensite and retained austenite, not soluble content of carbide and carbide morphology size distribution and acicular martensite and residual austenite experience affect the apparent performance of bearing, bearing steel not dissolved carbon content is lower, then the higher the hardness of the bearing steel, its reason lies in not dissolved carbon content is less, The carbon concentration of martensite matrix will increase, and the hardness will be higher.

The presence of a small amount of undissolved carbides in the quenched bearing steel is helpful to improve the wear resistance of the bearing and to obtain fine grained cryptocrystalline martensite, so as to improve the toughness and fatigue strength of the bearing. The carbide particle size has a great influence on the bearing life. The carbide particle size of bearing steel is less than 0.6um, and its service life will be significantly improved. The carbide particle size of high-quality bearing steel is much lower than that of general bearing steel, and the distribution of carbide particles is more uniform, and will not be presented in a zonal distribution. The network carbide distribution will affect the connection between the matrix grains, so as to reduce the fatigue resistance limit of bearings. When the stress between the rolling element and the raceway exceeds the fatigue limit, cracks will gradually occur, thus shortening the life of bearings.

2. Influence of bainite on bearing life

The characteristics of bainite structure will improve the proportional limit, bending strength, yield strength, and reduction of a section of carbon chromium bearing steel, improve the toughness of bearing steel, enhance the ability to bear impact force, fracture force and friction force, and also contribute to the good retention of bearing size.

3. Influence of load conditions on bearing life

There are mainly the following kinds of bearings used on rolling mills, such as multi-cylindrical roller bearings, four-row tapered bearings and double-row spherical roller bearings. Without considering the quality of the bearings themselves, the life of the bearings used under working conditions is mainly determined by the load of the bearings.

Because GaXian mill rolling force increases unceasingly, in roll neck can only select multiple columns bearing, multiple columns bearing design idea is to rely on more columns to uniform roller bearing, in turn, enhance the bearing capacity, but in the actual process of a rolling mill using multiple columns of rolling-element bearing can't completely uniform can appear even bigger deviation, The main factors causing the deviation include the deviation from the design and manufacture of the bearing itself, the installation accuracy of the bearing seat and the wear of the rolling mill components. At the same time, the bearing load distribution is also affected by the rolling force, axial force and roll bending force.

This will inevitably cause the bearing overall equivalent load under the same conditions, every column on the roller load-deformation, eventually led to the partial load and partial load once formed will be increasing with continued use, until the amount of a column on the roller load exceeds the limit bearing capacity of a roller which can lead to partial overload fracture.

Partial load is one of the serious influence of bearing life, in addition to the above analysis of the causes of the columns roller load distribution imbalance, can also lead to a single roller tilt, caused to local stress concentration, lead to sliding roller phenomenon occurs, once rolling conditions change, bearing and roller contact occurs between the internal and external ring sliding, lead-bearing heating further damage.


4. Influence of moistening and changing quality on bearing life

The bearing can be reliable for a long time use without lubrication quality assurance, rolling mill bearings in normal operation when subject to various friction, the outer ring raceway load area is under the worst parts of internal friction, the oil-bearing operation will exist in the process of the radial clearance, roller rolling will only in the problem of the load area, this area will not load in the rolling and sliding state, The roller from the non-load area into the load area of the process, the speed of the roller will suddenly increase, in the process of the speed of the sudden increase, the roller and the raceway will be intense friction, but also bear and from the process of steel rolling impact load.

In this case if the poor lubrication of bearing, the parts surface roughness will continue to increase, which will lead to increase, gradually wear on the surface of the roller unit pressure is increasing, at the same time bearing in the process of operation, operation rolling element and raceway, rolling element and cage, cage and have the sliding friction between inner and outer ring, And the sliding friction will increase as the load increases, the existence of the sliding friction can cause relatively crawling occurs between bearing parts, in order to reduce the bearing parts relative to climb to the result of wear and tear, you must keep good lubrication between the bearing parts, the lubricating film can be good isolation between the various components of the interface, Avoid direct frictional contact between metals. At the same time, good lubrication will also play a good role in heat dissipation, can play a role in reducing the friction heat transfer in operation.

5. Influence of sealing quality on bearing life

Due to the harsh operating environment of the rolling mill bearing, the probability of contamination will increase during the operation of the bearing. Therefore, it is necessary to keep the bearing seal well in order to avoid polluting the grease. Usually, the bearings used in steel rolling equipment are mainly contaminated by production cooling water and oxide sheet.

After the grease is polluted by water, the fatigue resistance of bearing materials will be reduced and cracks will occur. If the grease is polluted by oxide sheets, the situation is even worse. The oxide sheet will destroy the lubrication conditions inside the bearing, and the bearing surface will suffer from frictional wear.

By cooling water and iron oxide pollution after bearing, in the process of using ring will be worsening, as they grow in quantity of pollutants, will ultimately result in the crack of the ring, bearing damage, therefore, the ideal bearing seal can effectively improve the service life of the bearing, but also reduces the bearing damage probability events affect production.

rolling mill bearing

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