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

Feb 14, 2023
Generally speaking, the design service life of the rolling mill bearings of strip mills is 3000-5000h, but in actual use, the service life of the bearings is far from reaching this goal. Steel manufacturers often suffer heavy economic losses due to premature failure of bearings, and among them, machine damage caused by the bonding of bearing inner rings and roll necks is even more costly. So, how to choose the right rolling mill bearing for the steel mill?

This article will be divided into two parts to conduct an in-depth discussion and analysis on the faults of roll bearings such as abnormal sound, vibration out of tolerance, precision drop, and sharp temperature rise in the actual use process, so as to propose various improvement measures to effectively improve the service life of the bearing.

First of all, we first analyze the use of rolling mill bearings, combining three aspects: the characteristics of cold rolling mills, the characteristics of rolling mill bearings, and the selection of bearing precision.
1 Characteristics of cold rolling mill
(1) Heavy load, 2-5 times higher than ordinary bearings;
(2) The working speed varies widely;
(3) The working environment of the bearing is relatively harsh, and sundries such as lubricating fluid and iron filings are easy to enter the interior of the bearing;
(4) Roll replacement is more frequent;
(5) High stiffness and high precision;
Therefore, cold rolling mill bearings should have the characteristics of high load capacity, high rotation accuracy, high limit speed, simple structure, and convenient assembly and disassembly.

Types and characteristics of commonly used roll bearings
(1) Self-aligning roller bearings: with self-aligning characteristics, used on backup rolls of small strip mills;
(2) Four-row tapered roller bearing: it can withstand large radial force and axial force, but it must be assembled strictly according to the serial number, and it is used for the backup roll of a large rolling mill;
(3) Multi-row cylindrical roller bearings: small friction coefficient, small radial dimension, high manufacturing precision, and the inner ring is detachable, easy to disassemble and assemble, and widely used. The use of this type of bearing can increase the roll neck size, and its load capacity and limit speed are higher than other bearings, but it does not bear axial force, and thrust bearings need to be added. It is suitable for the working and backup rolls of high-speed four-high rolling mills, and four-row cylindrical roller bearings or double-row cylindrical roller bearings are commonly used;
(4) Needle roller bearings: Large bearing capacity, can increase the size of the work roll neck, suitable for work roll transmission, and the rolling mill that transmits high torque.

3 Bearing precision selection
Due to the high precision of cold rolled strip steel, its longitudinal thickness difference is generally -0.03-0.10mm. Therefore, the bearing should use a higher level of precision. The bearing accuracy indicators that affect the accuracy of the rolled piece mainly include the variation of the inner diameter of a single plane △dmp and the radial runout of the inner diameter of the complete set of bearings △dia. Specific accuracy requirements can be proposed according to production needs. According to field experience, the inner diameter variation △dmp of cold rolled strip work roll bearing (672820) is controlled within the range of -0.050mm≤△dmp≤-0.065mm, and the radial runout of the inner diameter △kia is controlled within 0.006mm. The inner diameter variation △dmp of the rolling mill bearing (272752) is controlled within the range of -0.125mm≤△dmp ≤-0.160mm, and the radial runout of the inner diameter △kia is controlled within 0.10mm.

The above is a brief introduction to the commonly used types, characteristics, and working environment of cold rolling mill bearings. After having a corresponding understanding of cold rolling mill bearings, the next article will be announced for you: Why do cold rolling mill bearings frequently have a short service life and bearing failure, and our solutions to these problems?
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