The roller bearings of the rolling mill are an important component of the rolling mill. Their characteristics include high unit pressure and a wide range of working speeds. As a result, the temperature of the bearings is unstable. Therefore, the bearings need to have a high load-bearing capacity. In the design process of the rolling mill, how to correctly select the roller bearings is a very important task.

The roller bearings used in plate and strip rolling mills mainly include two types: sliding bearings and rolling bearings.

This article focuses on discussing the selection of rolling bearings in the design process and the problems encountered during installation, use and maintenance.

I. Selection of Rolling Bearings

1. Types of Radial Bearings

Rolling bearings have the advantages of low friction coefficient, high precision, ease of series production, and batch production capability. Therefore, most of the bearings used for the rolls of plate rolling mills are of the rolling type.

(1) Compared with double-row spherical roller bearings and conical roller bearings, multi-row cylindrical roller bearings can withstand the largest external load under the same size of external dimensions, and the working speed they can adapt to is much higher than the former two. However, they cannot withstand axial loads. When using such bearings, an additional separate thrust bearing must be added to bear the axial load, so that multi-row cylindrical roller bearings can fully bear the radial load.

(2) Four-row conical roller bearings can withstand both radial and axial loads because the rollers of four-row conical roller bearings have an angle with the horizontal during installation.

(3) Double-row spherical roller bearings should also have a certain angle with the horizontal during installation, which enables them to withstand both radial and small axial loads and have the characteristic of automatic alignment.

2. Thrust Bearings

When the roller bearings of the rolling mill use multi-row cylindrical roller bearings, needle bearings, and hydrodynamic bearings, a thrust bearing needs to be installed to bear the axial force

II. Selection and Calculation of Rolling Mill Bearings

The bearing load is the fundamental factor determining the lifespan of the bearing. Therefore, reducing the load on the bearing or enhancing its carrying capacity becomes the core of the problem. According to the bearing lifespan calculation formula: Lh = (C/P)^(10/3), it can be known that the equivalent load P of the bearing and the lifespan Lh of the bearing have an inverse proportional relationship of 10/3 power. When using four-row conical roller bearings and double-row radial spherical roller bearings, the equivalent dynamic load calculation formula is: P = Fr + Y1Fa (Fa/ Fr ≤ e) P = 0.67Fr + Y2Fa (Fa/ Fr > e) When using multi-row cylindrical roller bearings and needle roller bearings, the axial load Fa of the rolling mill rolls is taken as zero, and the axial load is borne by the dedicated thrust bearings. At this time, the equivalent dynamic load calculation formula is P = Fa. Where: P - equivalent dynamic load, KN; Fr - radial load, KN; Fa - axial load, KN; e - load distribution coefficient; Y1, Y2 - axial load coefficients;

1. Radial load Fr of the working roll bearing

For the radial force of the working roll bearing, in addition to the rolling force, it also includes tension, bending roll force. Among them, the rolling force and tension depend on the rolling process, and the bending roll force is an external radial load, mainly used to compensate for the plate shape and original quality error or operation-induced problems of the slab.

2. Axial load Fa of the rolling mill roll

For the axial load Fa of the rolling mill rolls using multi-row cylindrical roller bearings, needle roller bearings and hydrodynamic bearings, it is borne by the thrust bearings that are matched with them. For the rolling mill rolls using four-row conical roller bearings, it can bear the combined load of radial and axial. Generally, the axial load is 0.5 - 2% of the rolling force.

III. Installation of Roll Bearings

1. When installing multi-row cylindrical roller bearings, in order to maintain high accuracy of coaxiality, the inner ring of the bearing is placed in the oil groove and heated to achieve hot press assembly of the inner hole of the inner ring and the roller diameter of the rolling mill. After the assembly is completed, the final size of the rolling path of the inner ring of the bearing is ground based on the rolling mill roller as the reference to improve the radial rotational accuracy.

2. When installing four-row conical roller bearings, when installing the four-row conical roller bearings, first install the entire set of bearings into the bearing housing, and then install the roller neck of the rolling mill into the inner hole of the bearing. Therefore, the roller neck and the inner hole of the bearing can only adopt a clearance fit. At the same time, oil grooves should be opened on the surface of the inner hole of the bearing, the end face of the inner ring, the surface of the roller neck of the rolling mill, the axial inner retaining ring end face, and the surface of the rolling mill roller neck.

3. When installing double-row radial spherical roller bearings, the inner hole of the bearing and the shaft are fitted with a clearance fit.

IV. Methods for Correct Use and Maintenance of Rolling Mill Bearings

1. When the cumulative working time of the bearing reaches one-third of its service life, a comprehensive inspection and cleaning must be carried out, and the internal clearance of the bearing should be measured. If it is greater than twice the original clearance, adjustment is necessary. After cleaning the outer ring of the bearing, rotate the outer ring by 90°, and then reassemble after replacing the loaded area.

2. For the bearing housing of the rolling mill, regular inspections should be conducted. If it does not meet the original design requirements in terms of size, it should be replaced promptly.

3. Strictly follow the maintenance procedures for cleaning the bearings.

V. Some Measures to Enhance the Service Life of Rolling Mill Bearings

Generally, the designed service life of the working roller bearings of a plate and strip rolling machine is 3,000 to 5,000 hours. However, in reality, this target value is rarely achieved. To avoid or reduce the damage to the bearings, some necessary measures should be taken to increase the service life of the rolling roller bearings:

1. Reasonably use the bending force of the rolls

The hydraulic bending of the working rolls of the rolling machine should strictly follow the operating regulations to significantly reduce the radial load on the bearings and extend their service life.

2. Improve the bearing capacity of the rolling rollers

Since the rolling rollers are in a limited space position, some bearings need to withstand large radial and axial combined loads for high-speed operation. The inner diameter, outer diameter, width, and internal structure of the bearings must be very compact. When designing, bearings made of bearing steel with high wear resistance, such as carburized steel, should be selected.

3. Improve the lubrication conditions of the rolling roller bearings

Most plate and strip rolling machines currently use lubrication methods such as thin oil, dry oil, and oil mist. However, compared with the above three lubrication methods, the oil-gas lubrication method is superior to the others. The oil-gas lubrication has the following advantages:

(1) Oil-gas lubrication can accurately measure the supply of oil, unaffected by the viscosity of the lubricating oil. The viscosity of the lubricating oil directly affects the lubrication effect and the service life of the bearings. Oil-gas lubrication can accurately supply lubricating oil with a viscosity as high as 7,000 mm2/s.

(2) Good sealing performance

Seals at both ends of the oil-gas lubrication bearing housing must be tight to improve dust prevention and prevent liquid from seeping in, improving the sealing performance.

(3) No pollution

The oil in the oil-gas lubrication does not get atomized and contaminated. The strength of the exhaust gas from the bearing housing is significantly improved, and it also has a cooling effect.

(4) Convenient control and operation

In the oil-gas lubrication system, the distribution of oil and gas is controlled by monitoring devices. The transmission efficiency is high, and the oil consumption is economical. The importance of bearings in the rolling machine production has been proven by practice. Correct selection of installation and maintenance will definitely increase the operation rate and product output, thereby improving the production efficiency and economic benefits of the enterprise.