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How to choose the right rolling mill bearing for the steel mill? (Part Ⅱ)

Feb 17, 2023
After understanding the bearing and the corresponding use environment, we can analyze the cause of bearing failure. Through multiple comparisons and observations of rolling mill bearings, it is concluded that the following factors can cause bearing damage.

1 Radial load is too large
(1) Back-up roller bearings
Back-up roll bearings can withstand rolling forces up to 5000kN. Due to the difference in raw material plate shape (such as large longitudinal thickness difference, transverse wedge shape) and other reasons during rolling, in the case of a given reduction, it will cause frequent instantaneous large reductions. At this time, the rolling force will break through the design capacity, make the bearing run under overload, and its service life will be greatly reduced. In addition, due to the wedge shape of the raw material, there will be a large unilateral reduction, which will cause a large rolling force to be borne by the unilateral roll bearing, resulting in a fracture of the rolling body and damage to the cage, which shortens the life of the bearing.
(2) Working roller bearing
The radial force borne by the work roll bearing is generally roll bending force and tension. Due to the poor shape of the raw material, additional bending forces must be applied to the work rolls during rolling. The roll bending force is applied to the chock, causing the work roll to have a certain bending deformation, thereby rolling out a strip with a better shape. But at the same time, the application of bending force will cause uneven force on the bearing. The four-row cylindrical roller bearings are only stressed on the inner and outer two rows of rolling elements, the bearing load capacity is significantly reduced, and the inner and outer raceways of the bearing are prone to wear and cause bearing scrap.

2 Axial loads are ignored
Compared with the rolling force, the axial load acting on the work roll is often neglected, but it is also one of the main factors for the damage of the work roll bearing. It is generally considered that the axial force is 1%-2% of the rolling force, but in actual use, the axial force of the rolling mill is much greater than 5%, resulting in roll movement, axial fixing bolt breakage and rolling interruption, or bearing burnout And scrapped. There are many reasons for the formation of roll axial force, mainly including roll axis crossing, asymmetric rolling, rolling force deviation, rolling piece camber, wedge sensitivity, and the influence of transmission system, but the main cause of roll axial movement is It is the intersection between the rolls, especially the axis intersection of the drive rolls.

3 Poor sealing performance of the bearing seat
In order to absorb the heat generated by the rolling deformation and reduce the rolling load, cold rolling production using a large number of emulsion, the bearing seat is always in the emulsion surrounded by this liquid containing rolling micro dodgy metal particles, acidic substances, and other impurities. This harmful liquid and solid particles once in the bearing, not only affect the performance of the lubricant, and the destruction of its formed film but also directly cause abrasive wear, resulting in premature fatigue of the bearing and cause failure.

4 Influence of bearing material
Since the rolling bearing works with a small contact and bears a large load on the area, it is also very sensitive to small defects in the bearing steel material. Many bearing manufacturers often use GCr15 bearing steel when processing rolling mill bearings. However, GCr15 bearing steel has high impurity content, and relatively poor fatigue strength and impact resistance.
Therefore, CHG bearing manufacturing can provide carburized steel that has been vacuum smelted or electro slag remelted instead of GCr15 bearing steel to improve the bearing capacity, or use sulfurizing technology to improve wear resistance, thereby increasing the service life of the bearing.
In addition, lower bainite austempering works well in rolling mill bearing applications. After the high-carbon chromium bearing steel is quenched by lower bainite, its structure is composed of lower bainite, martensite, and residual carbide, among which bainite is irregular intersecting strips, the convex lens in space form, and bainite transformation It is a process related to the isothermal transformation time, which generally takes 5-6h. After isothermal quenching, the number of bainite increases with the extension of isothermal time. The lower bainite structure of high-carbon chromium bearing steel can improve the steel's industry proportion limit, yield strength, deformation strength, and reduction of area. Compared with the martensite structure after quenching, it has higher impact toughness, fracture toughness, and dimensional stability. The surface compressive stress is as high as -400-500MPa, which greatly reduces the tendency of quenching cracks. The surface pressure during grinding should be Offsetting part of the grinding stress reduces the overall stress level and greatly reduces grinding cracks. The prominent features of the lower bainite structure are impact toughness, fracture toughness, and wear resistance, and good dimensional stability. The surface stress state is compressive stress, so it is suitable for rolling mill bearings with large assembly interference, poor service conditions, and high rated load.

5 Influence of bearing clearance
The size of the radial clearance directly affects the load distribution state inside the bearing, especially the change of the maximum rolling element load in the bearing. Due to the frequent disassembly and assembly of the bearing seat, it is inevitable to have a certain clearance. The internal clearance of the bearing changes due to fit, operating temperature, and deformation of the bearing after loading. Too large or too small clearance will directly affect the life of the bearing. After repeated practice on site, the radial clearance of the cold rolling backup roll bearing (272752) is controlled within the range of 0.24mm≤σ≤0.30mm, and the radial clearance of the work roll bearing is controlled within the range of 0.09mm≤σ≤0.14mm. In addition, the clearance values ​​of the radial and axial bearings of the roll should also match each other to avoid mutual interference and affect the life.
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