Abstract: This paper introduces the characteristics and types of rolling mill roll bearings for non-ferrous metals, analyzes the installation key points and failure causes of the roll bearings; and proposes measures to extend their service life.

Key words: Roll bearing; Installation; Failure; Life expectancy

Roller bearings are one of the key components of the metal sheet strip rolling machines, used to support the rotating rollers and maintain their position in the frame. Roller bearings are easily damaged and worn out during the production process of enterprises. Due to the high price and long supply cycle of roller bearings, many enterprises include the consumption of roller bearings in their production and operation assessment indicators. Therefore, correctly using and maintaining roller bearings, improving their reliability and extending their lifespan, is crucial for reducing energy consumption, saving costs, increasing production capacity, and maximizing enterprise benefits.

1. Characteristics and Types of Roller Bearings

The most prominent feature of roller bearings is the high unit pressure, which is 2 to 5 times higher than that of general-purpose bearings. Moreover, the range of working speed changes is large, and the working environment is harsh. There are three types of roller bearings: sliding bearings, rolling bearings, and liquid friction bearings (commonly known as oil film bearings). Currently, sliding bearings are gradually being phased out and are only used in old-type rolling machines; liquid friction bearings have characteristics suitable for use under high-speed and heavy-load conditions and are widely used in steel rolling machines; rolling bearings have advantages such as low friction coefficient, high precision, safe movement, simple maintenance, and easy series production. In copper and aluminum sheet strip rolling machines, the most commonly used roller bearings are rolling bearings.

Rolling bearings for copper and aluminum sheet strip rolling machines mainly include three types: four-row short cylindrical roller bearings, four-row conical roller bearings, and ball bearings.

1.1 Four-row Short Cylindrical Roller Bearings

The cylindrical roller bearings for copper and aluminum sheet strip rolling machines are four-row short cylindrical roller bearings. This type of bearing has a low friction coefficient and a small cylindrical height; for a certain roll neck diameter, its size is small; for a certain roll body diameter, it can accommodate a larger roll neck diameter. This type of bearing has high bearing capacity and high running accuracy, and is suitable for working under high-speed and heavy-load conditions. However, this type of bearing can only withstand radial loads and cannot withstand axial loads; it must be used in conjunction with bearings that can withstand axial loads (such as deep groove ball bearings, etc.). Usually, the inner ring of this type of bearing is in a tight fit with the roll neck of the roller, and the outer ring is in a transitional fit with the bearing seat. When grinding the roller and disassembling it, the inner ring remains attached to the roller without separating, and the cylindrical rollers are on the outer ring.

1.2 Four-row Conical Roller Bearings

Four-row conical roller bearings are mainly suitable for bearing radial loads and combined radial and axial loads. Four-row conical roller bearings are separable bearings, and their inner ring assembly and outer ring can be installed separately, making disassembly convenient. Using four-row conical roller bearings as roller bearings in rolling machines has a relatively simple structure and is easy to maintain. However, under the same size conditions, the bearing capacity of this type of bearing is not as large as that of cylindrical roller bearings.

1.3 Ball Bearings

In copper and aluminum sheet strip rolling machines, when roller bearings use four-row short cylindrical bearings, special ball bearings that can withstand axial loads need to be set up. The main types of ball bearings used are single-row deep groove ball bearings, single-row radial thrust ball bearings, double-row radial thrust ball bearings, single-row paired installation angular contact ball bearings, single-row four-point contact ball bearings, and double-row angular contact ball bearings.

2. Installation of Roller Bearings

Correct installation of roller bearings is very important for improving their running accuracy and extending their service life. Many prematurely damaged bearings are not due to their own quality problems, but are largely caused by improper installation.

2.1 Pre-installation Preparation

Before installing the roller bearings, it is necessary to inspect all related components (including roll necks, bearing seats, thrust rings, etc.) for dimensional accuracy, geometric accuracy, surface roughness, etc. These parameters must meet technical requirements, and all relevant parts should be cleaned of iron filings, burrs, and sharp edges. The bearings and related components should be lubricated with oil, and the installation site environment should be kept clean and hygienic.

2.2 Installation of Inner Rings

The inner ring of the four-row cylindrical roller bearing has an interference fit with the roller neck. Before installation, the inner ring should be heated to about 90 to 100 degrees Celsius. Generally, an electromagnetic induction heater can be used for heating, or an oil bath can be employed to ensure uniform heating. It is strictly prohibited to use open flame heating. After the heating is completed, quickly insert the inner ring onto the roller neck. During the installation process, a copper rod can be used for tapping to ensure a tight fit between the inner ring and the retaining ring, etc.

2.3 Installation of the outer ring (including the bearing assembly)

The outer ring and the bearing housing usually have a transitional fit. During assembly, gently tap the outer ring into the bearing housing using a copper rod and position it close to the fixed end. When installing the outer ring, pay attention to whether there are marks on the end face of the outer ring and the end face of the bearing roller assembly component. If there are marks, the installation must maintain the consistency of the marks; if there are no marks, it should be installed in the order of the initial configuration when disassembling the shaft case.

2.4 Installation of the bearing housing onto the roller neck

Before installing the bearing housing onto the roller neck, use a fixture to align the center of the bearing housing with the center of the rolling mill. During installation, push and slightly rotate the bearing housing while moving it to prevent the inner ring from scratching the rollers. After installation, rotate it left and right to check its flexibility. If there is any jamming, remove it promptly, handle it appropriately, and then reinstall.

3 Failure of the rolling mill bearing

3.1 Contact fatigue failure

Contact fatigue failure refers to the material fatigue failure of the working surface of the bearing due to the action of alternating stress. The common form is fatigue spalling. It first occurs at the point with the maximum alternating shear stress below the contact surface, then expands to the surface to form different spalling shapes, such as point-like for pitting or pitting spalling, small sheet-like for shallow spalling, and as the spalling surface gradually expands and penetrates, it forms deep spalling.

3.2 Wear failure

Wear failure refers to the failure of the working surface of the bearing due to the relative sliding friction between the working surfaces. The continuous wear will cause the bearing parts to gradually deteriorate. Wear failure is one of the common failure modes of rolling mill bearings, and it usually includes abrasive wear and adhesive wear. Abrasive wear refers to the wear caused by the intrusion of foreign hard particles or metal chips between the working surfaces and the relative movement of the contact surfaces. Adhesive wear refers to the local deformation and micro-soldering phenomenon of the friction surface caused by the uneven force on the friction surface due to the microscopic protrusions or foreign substances on the friction surface, and in severe cases, the metal surface may melt locally.

3.3 Fracture failure

The main cause of fracture failure of the rolling mill bearing is the defect of the bearing itself and overloading. When the external load exceeds the strength limit of the material and causes the bearing and its components to fracture, it is called overloading fracture. The main cause of overloading is sudden failure or improper installation. When the micro-cracks, shrinkage cavities, bubbles, large foreign objects, overheated structure, and local burn damage defects in the bearing parts cause fracture due to impact overloading or severe vibration, it is called defect fracture. Generally speaking, the fracture of the rolling mill bearing is mostly overloading fracture.

3.4 Corrosion failure

The working environment of the rolling mill with non-ferrous metal sheet and strip rolling is relatively harsh. In the actual production process, it is inevitable to come into contact with water and corrosive media, which will cause rust and corrosion of the bearing. In addition, during operation, the bearing will also be affected by micro-current and static electricity, causing current corrosion of the bearing. Rust and corrosion will cause pits on the surface of the inner and outer rings and rollers, and as the rust and corrosion gradually expand, it will eventually cause the failure of the bearing. In addition, during operation, due to the influence of external and internal factors, the original clearance, accuracy, and temperature rise of the bearing may change, causing the bearing to fail.

4 Measures to extend the service life of the rolling mill bearing

4.1 Regularly adjust the bearing load area

Establish an usage record for the bearings, and regularly clean and inspect the bearings. Check the wear conditions of the working surfaces of the inner and outer rings and the surfaces of the rolling elements. Every 600-800 hours of operation, rotate the outer ring of the bearing by 120°-180° to change its force-bearing position, which can extend its service life.

4.2 Regularly adjust the assembly accuracy of the bearings

The assembly accuracy that has the greatest impact on the service life of the rolling mill bearings is the clearance. The size of the clearance directly determines the load distribution state inside the bearing, especially the change in the maximum rolling body load of the bearing. If the clearance is too large or too small, it will cause the rolling mill to shift or the bearing to burn out or get stuck. The internal clearance of the bearing often changes due to the influence of the fit, operating temperature, and deformation after bearing loading. Therefore, when disassembling the bearing housing, re-adjust the clearance reasonably to significantly increase its service life.

4.3 Reasonably select the lubrication method

The purpose of bearing lubrication is to form an oil film on the rolling surface and an elastic fluid dynamic lubricating film between the metal surfaces, reducing metal surface wear and increasing its service life. According to the speed and load conditions, rolling mill bearings can adopt lubrication methods such as thin oil lubrication, dry oil lubrication, oil mist lubrication, and oil-gas lubrication. When using dry oil lubrication, due to the flushing of the rolling mill oil, the dry oil is dissolved into the rolling mill oil, polluting the rolling mill oil, and thereby causing oil stains on the strip surface, affecting product quality. Therefore, it is rarely used. In addition, due to the disadvantages of complex working conditions in thin oil lubrication and the pollution of the environment in oil mist lubrication, oil-gas lubrication is widely used in the lubrication of rolling mill bearings in modern non-ferrous metal strip rolling mills. Oil-gas lubrication has the advantages of high lubrication efficiency, environmental friendliness, wide range of oil viscosity applicable, low operating and maintenance costs, low oil consumption, good monitoring performance, etc., and is particularly suitable for high-speed rolling mills.

4.4 Reasonably select the sealing method

As long as the sealing element and sealing method are selected well, the consumption of the rolling mill bearings is generally low. On the contrary, the consumption of the bearing will be very high. While choosing a reasonable sealing method and reliable sealing elements, domestic and foreign strip rolling mills have a development trend of using sealed bearings, that is, adding sealing elements at both ends and inner isolation rings of the bearing for another sealing of the bearing. Although this sealed bearing occupies a certain design space during use, due to good sealing, the bearing is always in a fully lubricated state and a clean working environment, significantly improving the service life of the bearing.

5 Conclusion

Rolling mill bearings, as one of the main key components of non-ferrous metal strip rolling mills, there are still unknown links in their usage that need to be further explored and improved. Through reasonable design selection, installation, and maintenance, effectively improve their operational reliability, significantly increase their service life, and continuously improve the operating efficiency of the enterprise, remains our persistent goal.

References

[1] Chen Yanbo, Zhao Hongliang, Weng Kangrong. Rolling Technology of Non-ferrous Metals. Beijing: Chemical Industry Press, 2007: 1-1.

[2] Chen Guoqing, Analysis of the Use of Cold Rolling Strip Rolling Rod Bearings. Baotou Steel Science & Technology, October 2005, Vol. 31, No. 5