Abstract: According to the characteristics of rolling mill bearing and the actual operation in production, from the design, material selection, technology and use of each link, analyze the various factors affecting the life of bearing, and take measures to improve the rolling mill bearing life, reduce bearing loss, improve production efficiency, reduce production costs.

Key words: rolling bearing; Rolling mill bearing; Life span; Reliability

At present, rolling bearings are widely used in the support of the roll (especially the work roll) of the plate mill in the world, and the work roll of the plate mill generally uses four rows of tapered or cylindrical roller bearings. Due to the limitation of roll size, the radial space of bearing seat and bearing is small, the design of bearings is restricted, especially the four-row cylindrical roller bearings used as work rollers, although they bear heavy loads, but can only be designed for light series, is the wearing part of the rolling mill (1). In the production consumption, the consumption of bearings only accounts for about 5% of the total consumption, but the impact on the output of rolling steel is up to 20%. Therefore, as a key part of the rolling mill, the failure of bearings in the maintenance cycle leads to production suspension, which is not tolerated by the manufacturer. This paper discusses how to take measures in design, material selection, manufacture and application to improve the service life of rolling mill bearings.

1. Bearing design

If the life of the bearing is represented by the working time, the life of the roller bearing with 90% reliability is

Where: Ln is the calculation life of the bearing, h; n is bearing speed, r/min; Pr is the radial equivalent dynamic load of the bearing, N; Cr is the basic radial rated dynamic load of the bearing, N; K is the material coefficient (K =3 for vacuum degassed steel).

It can be seen from the above equation that the main factors affecting the bearing life are Cr and K.

According to ISO281, the rated dynamic load of roller bearings is

Where, fe is the structural correction coefficient; a2 is material correction coefficient; Lwe is the effective length of the roller; i is the number of scroll columns; Z is the number of rollers in a single column; α is the contact Angle; Dwe is the average diameter of the rolling body.



When designing bearings, the higher the Cr value is, the longer the life of bearings will be. Among the size parameters Lwe, Z and Dwe that determine the Cr value of bearings, Dwe has the greatest influence. At the same time, fe, K and a2 also play a key role in the reliability and life of bearings.

(1) Among bearing dimensions and parameters, the increase of Dwe has the greatest impact on Cr value. Increasing Dwe value should be the first choice in the design pursuit, but the strength of bearing inner and outer rings should meet the requirements. This is an important way to improve the bearing life. However, as bearings are the basic parts of various machinery, the use conditions of all kinds of bearings differ greatly, and the thinning of bearing rings brings some problems to bearing manufacturing, so the selection of strength coefficient must be careful, and it should be improved step by step from the analysis of use and processing conditions.

(2) A slight difference in fe in bearing design will cause changes in the rated dynamic load of the whole set of bearings. If the roller or raceway on the structure has stress concentration, its rated load will decrease, but if the stress distribution along the whole length of the roller is uniform and the stress concentration is small, its rated load will increase.

(3) From equations (1) and (2), it can be seen that K and a2 are important to bearing life. In the rolling process, the bearing of plate rolling mill is subject to the action of alternating load and the impact load is also great, so carburized bearing steel is generally selected. As the carburized bearing steel belongs to high alloy steel, the brittle oxides of impurities (especially alumina) will become the source of fatigue cracks, resulting in premature fatigue fracture of bearings. Electroslag remelting steel 20Cr2Ni4A with low oxygen content, fine oxide and good dispersion should be used. Bearing material is an important factor affecting bearing life and reliability.

2. The influence of processing technology and processing equipment

Because the bearing of medium plate rolling mill is subject to variable impact load, and has the characteristics of high speed, heavy load and poor working environment, it is required that the parts of rolling mill bearing should not have weak parts. In order to improve the manufacturing level of rolling mill bearings, some manufacturers have introduced the international advanced level of large bearing processing equipment and testing equipment. However, during the communication and visit, it was found that there were the following problems in individual manufacturers that needed to be improved.

(1) The material selection of some bearing manufacturing units is not standard. In order to reduce the cost, GCr15SiMn and GCr18Mo are used to replace carburized steel. Although the bearing hardness can meet the requirements, the toughness is lower than carburized steel, the rated dynamic load of the bearing is reduced, and the service life is reduced.

(2) Carburizing process is a special process in heavy bearing manufacturing. In order to reduce costs, some manufacturers use high temperature and strong permeability and reduce grinding processing retention, so that the bearing surface appears coarse mesh carbide and soft points, the grain coarsing of the infiltration layer, resulting in premature fatigue spalling and failure of the bearing. The average diameter of carbides is refined from 1.14μm to 0.6μm, and the life of carbides is increased by 4 times.

(3) Individual manufacturers use expanded grinding machine processing range, or use cars instead of grinding, resulting in the bearing surface roughness is out of tolerance, reduce the lubrication ability, resulting in adhesive wear. In addition, due to the low precision of processing equipment, the processed products can not guarantee the uniform force of four rows of rollers. According to equations (1) and (2), the influence degree of uneven force on bearing life of four rows of rollers can be calculated, as shown in Table 1.

It can be seen that even if the high quality ESR steel 20Cr2Ni4A is used, if the bearing process is not reasonable and the processing accuracy does not meet the requirements, the bearing life will not only not be improved, but also reduced.

3 The influence of rolling mill service condition

The influence of rolling mill service condition mainly includes: load and load distribution on the bearing, rolling speed, lubrication and sealing, temperature rise and heat dissipation.

(1) If the surface of the mill arch is too large, the size of the bearing box is out of whack and the wall thickness is inconsistent, cross rolls will occur, there will be mutual force between the two rolls, and the roller will produce axial force and uneven force, which will greatly reduce the fatigue life of the bearing. The calculated fatigue life is improved when the thrust caused by cross roll decreases from 5% to 0.5%. Therefore, the axis of rotation of the upper and lower work rolls and the axis of rotation of the upper and lower support rolls should be kept in the same vertical plane, and the axis of rotation of each roll is parallel to each other. Regular inspection and maintenance of the archway. Bearing boxes should be used in groups, the same group of bearing boxes together maintenance, so that the bearing box axis coaxiality is not more than 1'.

(2) Lubrication has become one of the key factors to improve the bearing life. Choosing the right lubricating oil is the basis to ensure that the bearing is in good condition.

According to the oil film coefficient formula, the minimum oil film thickness hmin is determined, which is related to the minimum oil film coefficient λ and the surface roughness values δ1 and δ2 of the two contact parts. According to the experimental study, when the minimum oil film coefficient λ≧3, the bearing is in complete elastohydrodynamic lubrication.

Select the appropriate lubricating oil viscosity η0 according to the Dowson-Higginson minimum oil film calculation formula.

4 Closing Remarks

(1) The working condition and characteristics of the bearing of plate rolling mill require it to have high comprehensive mechanical properties. The selection of high-quality steel is the premise of reasonable and optimized selection of bearing design parameters in limited space, and is the guarantee of improving its rated life.

(2) Adopt advanced processing equipment (large numerical control processing equipment), improve the processing technology and method, improve the size and position accuracy and internal quality of bearings, improve the reliability and life of bearings in plate rolling mill, and eliminate the shutdown caused by abnormal damage.

(3) Strengthen the daily maintenance of the rolling mill to ensure that the technical parameters of the rolling mill are within the required range; Appropriate high-quality lubricating oil and reasonable lubrication mode are selected, and bearings are strictly sealed to ensure that bearings are in a complete state of elastohydrodynamic lubrication, so as to improve the life of bearings.

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