The intersection of the axes of the four roll rolling mill generates significant axial force, which exceeds the rated load of the bearing and leads to frequent bearing damage. By adjusting the thickness of the backing plate and keeping the axis of the roller system parallel, the axial force is reduced and the service life of the bearing is improved.

1. Introduction

Jinan Iron and Steel Co., Ltd. Medium Plate Plant (referred to as Jinan Iron and Steel Medium Plate Plant) uses four row cylindrical roller bearings and double row tapered roller bearings for the work roll and support roll of the 2500 four high rolling mill. Due to the lack of use of bending rolls to adjust the roll shape during four roll rolling, the radial force on the cylindrical bearing of the work roll is mainly due to overbalance force, rather than bearing rolling force. The roll is replaced every 2-3 shifts and daily maintenance is carried out on the bearing. Therefore, the work roller bearing has a relatively small bearing capacity compared to the supporting roller shaft, a good working environment, a long service life, and generally rarely damages. However, in recent times, the tapered roller bearings of the work rolls have been frequently damaged, which has affected the normal operation of production.

Observing the condition of the roll system during steel plate rolling, it can be observed that the axial movement of the work roll bearing seat occurs. When the rolled piece is rolled towards the outlet direction, the roll and bearing seat move towards the non transmission end of the rolling mill, i.e. the operating side. When rolling towards the inlet direction, the roll moves towards the opposite direction, i.e. the transmission end, and sometimes abnormal sounds similar to dry friction can be heard. Most of the damaged bearings are upper work roller tapered roller bearings, and there are also a few cylindrical bearings. The damaged bearings are all located on the operating side. The damaged bearing cone ball is broken, some bearing outer rings are broken, and some cylindrical roller bearings have some broken beads, causing the cage to scatter. Due to the breakage of the ball, the roller neck and bearing seat are locked, making disassembly difficult. There are many impurities in the bearing, and the lubrication is poor. Sometimes, the flat key that drives the adjustment ring to rotate with the roller is cut off. The surface temperature of the roller is slightly higher than usual, reaching 80 ℃ measured with an infrared thermometer.

2. Analysis of the causes of bearing damage

2.1 Generation of axial force on rolling rolls

Analyzing the damaged bearings, it can be found that the main characteristics of the damage are ball breakage and surface spalling. Because the bearings were previously used well, quality issues with the bearings were ruled out. Although the ambient temperature is slightly higher, due to sufficient lubrication and less load, it does not cause roller breakage. Due to installation errors and wear on the positioning mating surface, the axis of the work roll and the axis of the support roll may not be parallel, resulting in a certain angle between the axes of the two rolls α， As shown in Figure 1.

The work roll is an active roll that drives the support roll to rotate, decomposing the linear velocity on the surface of the work roll along the axis of the support roll and perpendicular to the axis to obtain the component along the axis of the support roll ν' For:

ν'=ν sin α                    (1)

Equation (1) indicates that there is a trend of relative movement of the work roll along the axis of the support roll due to the uneven movement of the two roll axes, which causes axial shear deformation of the contact zone between the two rolls and forms axial shear stress. The sum of shear stresses on the entire contact strip is the axial force acting on the work roll and support roll, that is, the axial static friction force between the work roll and support roll.

2.2 Force analysis of conical shaft

Figure 1 Schematic diagram of roller axis intersection

When the axis of the rolling rolls intersects, the working roll system is subjected to the rolling force P, and the contact surface is a narrow strip. On a narrow strip, the two rollers have a relative sliding speed along the axis of the support roller ν 1。

The axial force X1 on the support roller is:

X1=∫ τ dA=f（P N）  （2）

In the equation τ— Shear stress between contact surfaces;

A - Narrow band area of the contact surface between the support roller and the work roller;

P - Rolling force of the rolling mill;

F - maximum static friction coefficient, taken as 0.1;

N - Balance force of the work roll.

Due to the use of four balanced hydraulic cylinders:

N=4 × p × π × (D/2) ²     (3)

In the formula, p-equilibrium pressure;

D - Diameter of the piston in the oil chamber of the balance hydraulic cylinder.

Using equations (2) and (3) with P of 2000t, p of 12MPa, and D of 600mm, the axial force X1 is 3316.48kN. For a four high rolling mill, due to the strong fluidity of metal in the hot state, the contact surface between the work roll and the slab is not prone to axial friction. The axial friction force suffered by the work roll is mainly caused by the relative axial sliding between the work roll and the support roll. Therefore, the axial force X2 applied to the work roller is:

X2=X1cos α

When the intersection is at one end, the axial force reaches its maximum due to α Smaller, therefore α Approximately E/L (E is the offset distance on one side of the horizontal plane of the roller axis, measured as 3mm, and L is the center distance between the two bearing seats of 3600mm). Therefore, X2 is approximately 3316kN. The basic rated dynamic load of the 10979/500 bearing is 1760kN, indicating that the axial force acting on the bearing exceeds the basic rated dynamic load of the bearing. Due to the axial positioning of the work roll on the operating side and the free bearing seat at the other end, the operating side bearing bears a large axial force, resulting in a decrease in bearing service life.

2.3 Impact of axial force on the stress condition of roller system components

The axial friction force causes the roller to have a tendency to rotate around another straight line perpendicular to the axis, causing the bearing to be biased. The horizontal direction generates a significant additional radial force, increasing the load on cylindrical roller bearings. In order to maintain the stability of the work roll, a four high rolling mill is generally designed with an offset between the centerline of the work roll and the centerline of the support roll, so that the horizontal support reaction force of the stand on the work roll bearing seat is greater than zero and the direction of force remains unchanged. Due to the presence of additional radial forces, the impact of the frame on the horizontal support reaction of the bearing seat is reduced, and the working roller system will be in an unstable state. This state will reduce rolling accuracy, exacerbate wear of the roller and bearing seat slide plate, which is also the reason for the increase in roller temperature.

3. Improvement measures

In order to reduce the impact on the bearing during movement, the severely worn guide plate of the lower work roll bearing seat and its matched upper work roll groove are welded and repaired to restore axial fitting accuracy. The positioning accuracy of the lower work roll and support roll can be maintained by timely replacement of the inner sliding plate of the lower support bearing seat and the sliding plate of the lower work roll bearing seat. When the sliding plate of the memorial archway boss of the rolling mill is worn, if it needs to be stopped for replacement, it will take a long time. In order to improve the operation rate of the rolling mill, the method of adding a backing plate inside the sliding plate of the upper work roll bearing seat is adopted to compensate for the wear of the sliding plate and ensure the positioning accuracy. The adjustment of the roller axis is shown in Figure 2.