Rolling mill bearings are important components in the rolling mill stand. Their main functions are to support the rotating rolls, withstand the rolling force transmitted by the rolls, and maintain the correct position of the rolls in the stand . The working conditions of rolling mill bearings are very harsh, and they are affected by factors such as high temperature, heavy load, water and dust. Although the consumption of bearings in production only accounts for about 5% of the total consumption, the impact on the output of rolled steel is as high as 20%. Therefore, special attention must be paid to the various uses and performance indicators of rolling mill bearings.
Analysis of factors affecting rolling mill bearing failure
Rolling mill bearings are precision parts with large loads and poor working environments . Common failure forms of rolling mill bearings include fatigue peeling, wear, corrosion, etc. of the working surface. The reasons for failure are complicated and often cross-expressed in one or more forms. There are many factors that are difficult to quantitatively express and directly relate to bearing failure. From the fishbone diagram (Figure 1) of the factors affecting the service life of rolling mill bearings, it can be seen that they are mainly divided into two aspects: internal factors and external factors. Internal factors include bearing materials, structural design, processing and manufacturing, etc., and external factors include bearing speed, load conditions, lubrication, cooling, etc.
1) Raw materials
The metallurgical quality of materials fundamentally affects the service life of rolling mill bearings . Material defects such as pores, looseness, carbide accumulation, inclusions, etc. need to be strictly controlled. Especially when inclusions exist, cracks first form and expand at the junction of inclusions and matrix, leading to early fatigue spalling .
Poor material cleanliness will destroy the continuity of the metal matrix and reduce the plasticity, toughness and fatigue performance of the material. Under the action of alternating stress, stress concentration occurs near the inclusions, causing pitting corrosion. When the pitting corrosion expands, fatigue spalling will form. The damage in the later stage of fatigue spalling is fatigue fragmentation.
2) Heat treatment
Heat treatment of bearing steel is very important, and a reasonable heat treatment process is the key to ensuring the performance of the material. Unqualified heat treatment metallographic structure, non-dense and uneven structure, too deep decarburization layer, too much residual austenite, excessive stress, etc., will cause the strength of the material to be unqualified. Low hardness is prone to wear or indentation, and high hardness is prone to cracking.
After quenching and low-temperature tempering, the structure of high-carbon bearing steel will change into undissolved carbides, acicular martensite and residual austenite. The undissolved carbide content and carbide morphology distribution, the size of acicular martensite and residual austenite will affect the apparent performance of the bearing. The lower the undissolved carbide content of the bearing steel, the higher the hardness of the bearing steel. The reason is that the less the undissolved carbide content, the higher the carbon concentration of the martensite matrix will be, and the higher the hardness will be.
The small amount of undissolved carbides in the quenched bearing steel helps to improve the wear resistance of the bearing and also helps to obtain fine-grained cryptocrystalline martensite, thereby improving the toughness and fatigue resistance of the bearing ; the particle size of the carbide also has a great influence on the life of the bearing. The carbide particles of the bearing steel are less than 0.6 μm, and its service life will be significantly improved. The carbide particles of high-quality bearing steel are much smaller than those of general bearing steel, and the distribution of carbide particles is more uniform and will not appear in a banded distribution; the network carbide distribution will affect the connection between the matrix grains, thereby reducing the fatigue limit of the bearing. When the stress between the rolling element and the raceway exceeds the fatigue limit, cracks will gradually appear, thereby shortening the bearing life.
The characteristics of bainite structure will increase the proportional limit, bending strength, yield strength and cross-sectional reduction rate of carbon-chromium bearing steel, improve the toughness of bearing steel, enhance the ability of bearings to withstand impact force, breaking force and friction force, and also help to maintain good bearing size.
3) Loading conditions
The bearings of rolling mills are heavy. For example, the radial bearings of rolling mills are multi-row bearings. The design idea of multi-row bearings is to rely on multi-row rolling elements to evenly load and thus enhance the bearing capacity. However, in the actual use of rolling mills, the load of multi-row rolling elements cannot be completely even and may even have large deviations. The main factors causing the deviations include the deviations from the design and manufacturing of the bearings themselves, the installation accuracy of the bearing seats, and the wear of the rolling mill components. At the same time, the bearing load distribution is also affected by the rolling axial force and overturning moment. This causes the load borne by each row of rolling elements to deform under the condition that the equivalent load borne by the overall bearing remains unchanged, which eventually leads to eccentric load. Once the eccentric load is formed, it will continue to increase with continued use until the load borne by a row of rolling elements exceeds the ultimate load-bearing capacity of the rolling element, resulting in local overload. In addition to causing unbalanced load distribution of rollers in each row, it will also cause the single-row rollers to tilt, causing stress to concentrate locally, resulting in rolling element sliding. Once the rolling conditions change, contact and slippage will occur between the rolling elements and the inner and outer rings, causing the bearing to heat up and fail.
4) Processing
Chamfers, oil grooves, and grinding wheel overtravel grooves have sharp corners or are in the wrong position, which will form stress concentration areas; parts processing accuracy is poor, geometric accuracy, form and position tolerances, and surface roughness are poor, and there are high points such as deformation, mechanical damage, and rough peaks, which increase the stress on the contact surface and unevenly distribute the bearing load, affecting the formation and protection of the bearing lubricating oil film and causing surface fatigue peeling.
5) Installation
Establish clear installation procedures, use appropriate tools, avoid improper force, and improve installation accuracy; adjust the bearing installation clearance appropriately; improve the coaxiality of the bearing seat hole and reduce the inclination and intersection of the center line.
6) Lubrication
The long-term and reliable use of bearings is inseparable from the guarantee of lubrication quality. Rolling mill bearings are subject to multiple frictions during normal operation. The outer ring raceway load zone is the most serious part of the friction. There will definitely be radial clearance during the operation of the bearing, and there will be problems with the load zone and the non-load zone. In the non-load zone, the roller is in a semi-rolling and semi-sliding state. When the roller enters the load zone, the roller speed will suddenly increase. During the sudden increase in speed, the roller and the raceway will rub violently, and at the same time, they will also bear the impact load from the steel rolling process. In this case, if the bearing is poorly lubricated, the roughness of the part surface will continue to increase, which will cause the wear to gradually increase, and the pressure on the unit surface of the roller will continue to increase. At the same time, during the operation of the bearing, there will be sliding friction between the roller and the raceway, the roller and the cage, and the cage and the inner and outer rings, and this sliding friction will increase with the increase of load. The existence of sliding friction will cause relative creeping (creeping) between the various parts of the bearing, resulting in wear. Therefore, it is necessary to maintain good lubrication between the various parts of the bearing. The lubricating oil film can well isolate the contact surfaces between the various parts to avoid direct friction contact between metal and metal. At the same time, good lubrication can also play a good role in heat dissipation, which can reduce the friction heat during operation. In addition, it is necessary to choose the right grease and the correct relubrication method and cycle.
7) Sealing
Sealing performance is the key to the performance of rolling mill bearings. The success of bearing operation depends on its sealing ability, that is, the ability to retain grease in the bearing and keep dirt out. Bearings used in rolling equipment are usually contaminated by production cooling water and iron oxide. If the bearing is not sealed well, the grease will be contaminated by water, which will reduce the fatigue resistance of the bearing material and cause cracks. When rolling steel, iron oxide will destroy the lubrication conditions inside the bearing, and friction and wear will occur on the bearing surface, causing the bearing to wear. Therefore, ideal bearing sealing can effectively increase the service life of the bearing, while also reducing the probability of sudden bearing damage affecting production.
For example, a waterproof rubber pad can be installed on the upper part of the vertical roller box , and a waterproof sealing ring can be installed on the bearing cover ; a fluororubber skeleton can be used to seal the bearing to prevent cooling water or iron oxide from entering the bearing, thereby extending the service life of the bearing; commonly used sealing methods include combined sealing design and narrow sealing design .
summary
The service life of rolling mill bearings is affected by both internal and external factors of the bearings. In addition to selecting reliable bearings, the bearing installation accuracy must be improved. In addition, the quality of lubricating oil must be strictly controlled and the lubrication method must be reasonable during daily maintenance. At the same time, the sealing degree of the bearings must be improved. Improving the service life of rolling mill bearings is one of the key tasks of maintenance personnel. Only by being familiar with the performance of bearings, understanding the operating conditions, and mastering the maintenance knowledge can we effectively grasp the focus of work in daily bearing maintenance work, improve the operating quality of rolling mill equipment, and thus ensure the smooth development of production.
