I. Correct understanding of four-row cylindrical roller bearings for rolling Mills

The main types of four-row cylindrical roller bearings for rolling mills include FC type (single inner ring), FCD type (double inner ring), and FCDP type (double inner ring with flat retaining ring). Its working environment is harsh, with heavy loads, high unit pressure, large heat generation, and a relatively short service life, which is significantly different from that of ordinary bearings.

From the perspective of application, the roller bearings need to have good lubrication and cooling conditions, and the radial dimensions should be as small as possible. Rolls at different positions (such as support rolls and work rolls) have significant differences in usage conditions and bearing selection. The main application conditions include:

Rotational speed: It is necessary to take into account both the maximum rotational speed and the oil film formation capability during low-speed operation.

High load: Uneven force distribution is likely to occur among multiple rows of rollers, and even some rows may not bear the load.

Eccentricity of the inner and outer rings: It affects the clearance and, in turn, the precision of the sheet and strip material.

Axial force: This type of bearing does not bear axial force and needs to be used in conjunction with thrust bearings.

High temperature: It may cause lubrication failure and requires special attention.

In terms of manufacturing, the bearing materials need to have high load-carrying capacity, low friction coefficient, impact resistance and good foreign matter tolerance. Large-sized bearings have special requirements in terms of materials and processes. Currently, there are still certain shortcomings in the manufacturing of large-sized high-end bearings in China, and the bearings used in key equipment still rely on imports.

From the perspective of the roller system structure design, a reasonable bearing configuration is the prerequisite for ensuring the performance of the rolling mill and the normal use of the bearings. With the development of aluminum sheet and strip rolling mills towards high speed, high load, high precision and high reliability, the configuration mode of "four-row cylindrical roller bearings + thrust bearings" is becoming increasingly popular. Its advantages include:

The radial load-bearing capacity is the largest under the same cross-sectional dimensions.

The radial and axial loads are respectively borne by different bearings, adapting to higher rolling speeds.

Axial clearance control is more precise, which helps improve product accuracy.

The bearing has high precision and good reliability.

The inner and outer rings can be separated, facilitating the installation and disassembly of the rolls.

The inner ring fits tightly with the roller neck to prevent burning damage caused by ring running.

Maintenance and inspection are simple and convenient, and the comprehensive usage cost is low.

According to the JB/T 5389.1-2005 standard, the bearing parts of the rolling mill are made of carburized steel G20Cr2Ni4(A) or high-carbon chromium bearing steel GCr15(SiMn), and comply with the corresponding heat treatment specifications. Given the working characteristics of the rolling mill, such bearings generally do not have a fixed service life set. For high-precision plate and strip rolling mills, to ensure the accuracy of the roll system, the outer circle of the inner ring of the bearing is often reserved with machining allowance. After being assembled onto the roll, it is ground together with the roll body to the required size to ensure radial clearance and coaxiality.

Ii. Correct use of four-row cylindrical roller bearings for rolling mills

The above-mentioned problems existing in the current use and management have led to short bearing life, poor production stability and inability to guarantee product accuracy. Although rolling mill bearings do not have a fixed service life, enterprises should still strive to extend their effective service time to reduce production costs. In actual use, apart from normal fatigue spalling, common damage forms also include wear, burning and grease failure, which are manifested as poor rotation, sharp increase in temperature rise, abnormal vibration and noise, and even jamming.

2.1 Analysis of Damage Forms and Causes

Contact fatigue spalling

Most aluminum sheet and strip rolling mills are reversible rolling. The bearings operate under heavy loads, high impacts and alternating stresses, and fatigue spalling is common on the surface of the rollers.

(2) Wear

There is rolling and sliding friction inside the bearing. Poor lubrication will accelerate wear, resulting in increased clearance and loss of accuracy. Slight wear and tear can still maintain its use, but once the accuracy of the plate and strip is affected, it needs to be replaced immediately.

(3) Burns

Due to the sharp increase in frictional heat, lubrication fails, the surface structure changes, and even adhesion and jamming occur. Improving bearing design, adopting self-lubricating cages, and enhancing cooling and lubrication are the key to prevention.

(4) Grease failure

Grease gradually fails under the influence of factors such as shearing, oxidation, contamination and leakage, causing frictional temperature rise and surface burning.

The main factors affecting the service life of bearings include

Dynamic load: Impact load is the main cause of abnormal damage to bearings. Standardized operation is required to avoid abnormal conditions such as material jamming and overlapping rolling.

Bearing materials: The material composition and heat treatment process have a significant impact on the service life. They should be optimized in collaboration with the manufacturer in combination with the working conditions.

Installation and sealing: Good concentricity, effective sealing and reasonable clearance are the basis for ensuring service life.

Pairing relationship: Maintain a fixed pairing of the bearing with the rolls and bearing housing, avoid random grinding of the inner ring, and ensure uniform force distribution.

Lubrication: Select appropriate lubricants and oiling methods, avoid excessive or insufficient oiling, and prevent impurities from entering.

2.2 Key Points for Correct Usage

2.2.1 Establish a correct understanding

Rolling mill bearings have the characteristics of heavy load (2 to 5 times that of ordinary bearings), wide speed range, harsh environment, frequent roll replacement and high precision requirements. It features a low coefficient of friction, compact radial dimensions, high precision, and convenient installation and disassembly. It is suitable for the work rolls and support rolls of four-high rolling mills and needs to be used in conjunction with thrust bearings to bear the axial force.

2.2.2 Scientifically set the clearance and strictly manage the pairing

The design and implementation of radial clearance for rolling mill bearings mainly include two methods: the base shaft system and the base hole system. JB/T 5389.1-2005 recommends the base hole system, which is conducive to batch production and quality control. The size of the clearance directly affects the internal load distribution of the bearing and the maximum load of the rolling elements. It needs to be determined comprehensively based on factors such as fit, temperature rise and deformation. At the same time, the clearance of the radial and axial bearings should be matched to avoid mutual interference.

When changing the rollers, the outer ring of the bearing should be rotated 90° regularly to balance the wear and extend the service life. Establish a bearing usage file and fix its pairing relationship with the rolls and bearing housings. Regularly clean and inspect, determine whether to replace based on the clearance standard, and give priority to using the original complete set of bearings.

2.2.3 Standardize the assembly process

At present, most enterprises use cranes for the disassembly and assembly of bearing housings, which has problems such as difficult centering, low efficiency, and easy damage to the roller neck and bearings. Professional disassembly and assembly equipment has been promoted in the steel industry. Copper processing enterprises should gradually introduce it to achieve safe, efficient and precise operations. Although the initial investment is relatively high, the long-term benefits are significant.

2.2.4 Standardize lubrication management

Lubrication aims to isolate the metal surface through an oil film to reduce wear. Common practical problems include seal damage leading to grease contamination of emulsions, emulsion intrusion causing lubrication failure, and moisture causing rust and hydrogen embrittlement, etc. The main lubrication methods include grease lubrication and oil lubrication (oil mist, oil vapor).

Grease lubrication: Suitable for small and simple rolling mills. Lithium-based grease and other lubricating greases with good water resistance should be selected. The penetration and filling amount should be determined according to the rotational speed, temperature and environment (generally 1/3 to 1/2 of the space). Regular replenishment and replacement should be carried out. Mixing different grades is strictly prohibited.

Oil mist lubrication: Lubricating oil is atomized by compressed air and delivered to the lubrication point. The advantages are uniform lubrication, good cooling effect, low oil consumption and certain sealing performance. The drawback is that the exhaust gas contains oil mist and needs to be treated. It relies on a compressed air system and is only suitable for low-viscosity oil, with a relatively low utilization rate.

Oil-gas lubrication: Mix oil droplets with compressed air for transportation without atomization. It is suitable for high-speed, high-temperature, heavy-load and multi-pollution occasions, and can be applied to various viscosity oil products. The oil volume and gas volume are adjustable. It is environmentally friendly, with high internal cavity pressure and strong anti-pollution performance, but the investment is relatively large.

2.2.5 Strengthen the installation management of roller systems

The operating conditions of the rolling mill and the assembly quality of the roll system directly affect the service life of the bearings. The following aspects should be noted:

Regularly check and adjust the parallelism of each roller axis. The bearing housing should be dedicated to avoid mutual matching.

Replace the worn parts of the transmission system in a timely manner to reduce the additional axial force caused by clearance.

The bearing housing should have a self-aligning function to avoid uneven loading.

Check the dimensions, positional tolerances and fit conditions of related components such as the archway window and bearing housing;

Control the deflection of the rolls to avoid overloading which may cause uneven force distribution on the roller train.

Standard assembly of bearings and bearing housings:

Inner ring installation: Induction or oil bath heating (≤120℃) is adopted. Flame heating is strictly prohibited. After installation, press tightly until cooled.

Outer ring installation: Tap the seat gently, pay attention to the end face marking, maintain the original sequence, and record the force-bearing area.

Sealing assembly: Select appropriate sealing parts (such as fluororubber skeleton seals), install the lip direction correctly, and prevent coolant and impurities from entering.

Through systematic management, standardized operations and continuous optimization, the service life and reliability of four-row cylindrical roller bearings for rolling mills can be significantly enhanced, providing a solid guarantee for the production of high-quality copper sheet and strip.