Abstract: The working characteristics of roll bearing are briefly described, the factors affecting the service life of roll bearing are analyzed, and the application of roll bearing has certain guiding effect on the consumption reduction of bearing in hot strip rolling mill.
Key words: hot strip mill; Roll bearing; Service life; consumption
1.Introduction
The hot mill used by Panzhiang Great Wall Special Steel Co. LTD welding pipe wire mill is narrow strip hot mill, and the main rolling bearing is rolling bearing. The bearing capacity of bearings is represented by the anti-plastic deformation amount at the contact point of rolling body and raceway at static time, and the anti-fatigue spalling ability of materials on the working surface of bearing raceway and rolling body at dynamic time. Therefore, the bearing capacity is an important basis for selecting and calculating the service life of bearings.
3.1 Correlation between working speed (RPM), equivalent dynamic load P and rated dynamic load C of rolling bearings and service life of bearings
According to the general calculation formula for the service life of rolling bearings, the correlation between the rated dynamic load C, equivalent dynamic load P and working speed n and the service life of bearings is as follows:
It can be seen from the above formula that, when selecting the bearing type, the bearing with a larger rated dynamic load C should be selected as far as possible to improve the service life of the bearing. The working speed n is determined by the design and is not easy to change. The equivalent dynamic load P is more affected by the actual use factors and is difficult to control.
3.2. Application of radial clearance of rolling bearings and bearing service life
Because the size of the radial clearance of the rolling bearing directly affects the load distribution inside the bearing, especially the change of the maximum rolling load inside the bearing, the radial clearance of the bearing is an important factor affecting the service life of the bearing.
The radial clearance of the bearing can be positive or negative. When the radial clearance of the bearing is about negative, that is, when the load rate is 0.7 ~ 0.8, the service life of the bearing is *** long. When the load rate is less than 0.5, because the load area in the bearing decreases, the load of the large rolling element increases, so the service life of the bearing decreases obviously. When the load rate is greater than 0.8, the bearing works in the condition of too large negative clearance, the bearing heat is large, and the service life declines more obviously.
3.3. Application of rolling bearing working temperature and bearing service life
The system is mainly reflected in the reduction of allowable dynamic load C value. The higher the working temperature is, the lower the allowable dynamic load C value is, thus the lower the service life of bearings.
3.4 Relationship between hardness of rolling bearing materials and service life of bearings
The influence of hardness of working surface material of rolling bearing on the service life of bearing is mainly reflected in the decrease of rated dynamic load C of rolling bearing. The hardness of material surface of bearing components is generally HRC58-64. If the hardness of material surface of bearing components is lower than the above range, the rated dynamic load C has a downward trend.
3.5 Steel smelting method and bearing life
Non-metallic inclusions in steel have adverse effects on contact fatigue, and fatigue cracks are easy to occur between oxides, inclusions and material matrix which are not easy to plastic deformation under the surface of metal under the action of external alternating stress. The more non-metallic inclusions in steel, the lower the fatigue life of bearing. To reduce the non-metallic inclusions in steel, it is necessary to adopt an effective smelting method, and it is better to use vacuum smelting several times. When the steel has been smelted many times, although the fatigue life of the bearing has been improved, the cost of the steel is too expensive. Therefore, for important bearing materials, steel after vacuum remelting or vacuum degassing refining is often used, at which time the service life of bearings is enough to meet its performance requirements.
3.6 Correlation between residual stress on bearing material surface and bearing service life
Bearing elements in the process of heat treatment, such as improper treatment will produce large thermal stress; In the grinding process, if the cooling is not sufficient or the grinding amount is too large, grinding stress will also occur; When the bearing is fitted with the shaft housing, the fit stress will also occur because the fit interference is too large. Even harmful defects on the working surface of bearings can cause changes in residual stress on the surface of materials. In general, these additional residual stresses generated by external factors on the surface of materials are normal stresses. (tensile stress)
A bearing that has been processed normally and used correctly should have residual negative stress (compressive stress) on its working surface.
The residual compressive stress on the surface of the material can restrain the generation of fatigue cracks and the expansion rate, so as to prolong the fatigue life of the bearing. On the contrary, the residual tensile stress will reduce the fatigue life of bearings.
3.7 Influence of lubrication on the service life of bearings
Practice has proved that: when the lubricating oil is highly clean, that is, when the lubrication state of the bearing is at the best level, the rolling bearing is fatigue resistant. But the bearing is in mixed friction state in most cases, that is to say, the rolling working surface of the rolling bearing, although there is enough lubricating oil, but can not form a full oil film state, the more impurities contained in the lubricating oil, the more mixed friction, the worse the lubrication state, the lower the service life of the bearing.
3.8 Bearing processing technology and bearing service life
The service life of bearings is not only based on the chemical composition of steel, but also closely related to the manufacturing process of bearings.
3.8.1 Fiber streamline to forging ratio
The distribution and fatigue life of steel fiber streamer on rolling contact surface are very important. Studies have shown that when the fiber flow direction is perpendicular to the contact surface, the life is generally low, that is, the fatigue failure of the material often occurs on the surface of the fiber outcrop. When the fiber flow is parallel to the contact surface, the life is best. Bearing blank in the forging, when the forging is larger, can make the material lattice fine, non-metallic inclusion fracture, grain flow is consistent, so the fatigue life of the material is higher. Considering the fiber flow distribution of the material, the ball pole and the ring parts of the steel ball and the ring turned by the rod are not favorable to the fatigue life, while the ring blank with rolling channel has a longer fatigue life.
3.8.2 Correct heat treatment process
General bearing steel after quenching and tempering the metallographic organization shall be the fine needle and more than 80% of the fine granular martensite, 5% ~ 10% of the retained austenite and 7% did not dissolve carbide. When the carbon content of coarse particles carbide or martensite is too low after heat treatment of bearing steel, the fatigue life of the material will decrease. When the carbon content of martensite increases (more than 0.5%), brittleness of martensite occurs. The content of residual austenite can improve the fatigue resistance of the material, but the unstable residual austenite will affect the geometric stability of the bearing.
3.8.3 Rationality of mechanical processing technology
Grinding and superfinishing are the key points in the final machining of bearing components, which have an important influence on the service performance of bearings. The rationality of the roughness of the working surface of the bearing is the condition to ensure good lubrication. Too large roughness will cause difficulties in the formation of oil film, which will lead to the increase of tangential force and shorten the service life. In the grinding process, if the cooling is insufficient, the grinding feed is too large, and the temperature rise in the grinding area is too high, there will be secondary fire layer on the material surface, which will lead to the decline of the bearing fatigue life.
3.9 Installation accuracy and bearing service life
The decrease of installation accuracy, especially the excessive inclination of roller bearing in installation, will seriously affect the normal distribution of bearing internal load, cause harmful stress concentration, and lead to the obvious decline of bearing service life.
4. Use of roller bearings
4.1 Two-roll bearing and back-up roll bearing
Bearing speed, rated dynamic load and equivalent load are the basic parameters for calculating bearing life. For rolling mill bearings, because of their special working conditions, the selection parameters are limited, that is, the equivalent load the bearing can bear should be less than 30% of its rated dynamic load value, otherwise the bearing load is too heavy.
The rated life of rolling mill bearing is proportional to the 10/3 power of the bearing load ratio. Under the same size, the rated dynamic load value of four-row cylindrical roller bearing is more than 15% higher than that of four-row tapered roller bearing, so the rated life of the former is 60% higher than that of the latter. The selection of two-roll bearings and back-up roller bearings plays an important role in narrow strip rolling mills, and the consumption of two-roll bearings and back-up roller bearings has been large in the past few years. In recent two years, the selection of four-row cylindrical roller bearings with large load capacity has achieved obvious results, and the consumption of tons of steel bearings has decreased significantly. In addition, because the size of the load Angle is inversely proportional to the bearing clearance, in order to make the four row cylindrical roller bearings have a larger load capacity, the bearing with smaller clearance is selected.
The precision grade of rolling mill bearing also affects the service life. Due to the limitation of rolling mill speed, the dynamic pressure lubricating oil film of two-roll bearing and back-up roll bearing is generally thin. Therefore, bearings with high precision grade can maintain the continuity of oil film better and lubricity better because of the low raceway surface roughness. Therefore, P6 precision bearings are selected.
If the bearing seat is not designed and manufactured properly, it will lead to uneven bearing force and reduce the bearing life. The requirements for the bearing seat are: the bearing seat load area should have enough wall thickness, in general, should not be less than 1/3 of the bearing diameter. Bearing seat should have the ability to adjust the center, to avoid the roller deflection deformation caused by the bearing by off-load. The adjusted frame window liner should be taken as the base surface, and the axis of the bearing seat hole should be kept parallel to it. Check whether the cylindricity of the bearing seat hole meets the requirements when installing the bearing each time to ensure that the bearing has a better stress state.
4.2 Work roll bearing
Most of the strong impact load on the work roll of the hot strip mill is transferred to the support roll, while a part of the axial impact acts on the work roll bearing. The main reason for the axial impact force is that the window base lining plate is improperly adjusted, resulting in the two roll axes are not parallel, the greater the roller axis Angle, the greater the axial force generated during rolling. The axial impact force will not only damage the positioning bearing of the work roll, but also sometimes make the bearing seat gland deformation, and even break the fastening bolt. Therefore, it is necessary to adjust the base lining plate, which is an important measure to reduce the axial impact force and protect the work roll bearing.
For strip mill, the larger the roll's axial channeling momentum, the larger the axial impact force. Therefore, it is necessary to check whether the working roller bearing seat has been pressed and whether it is loose. In particular, there must be no axial clearance fixation between the operating side bearing seat and the frame window, so as to increase the rigidity as far as possible to reduce the impact on the bearing, and at the same time to minimize the matching clearance between the transmission side bearing seat and the frame window, so as to slow the impact of the transmission shaft runout on the bearing.
In order to increase the axial load capacity of work roller bearings, four rows of tapered roller bearings are used.
4.3 Lubrication and sealing of bearings
Lithium base, aluminum base and polyurea grease are ideal lubricants for rolling mill bearing. For low speed and heavy duty narrow strip mill, in order to form dynamic pressure oil film on bearing raceway, these grease base oils are required to have a high viscosity, generally not less than 320mm² / s. Grease should contain extreme pressure additives, at a certain temperature and bearing raceway synthesis of extreme pressure film, low speed can play a role in reducing wear. Therefore, lithium molybdenum disulfide grease is used.
Hot strip mill is usually subjected to strong impact load, bearing vibration is large, easy to lose grease. Therefore, grease is required to have strong adhesion, can be firmly attached to the surface of parts, part of the raceway lubrication bearing, the rest of the preserved in the bearing internal space to seal.
Poor seal is also a major cause of rolling mill bearing damage. If the external dirt intrudes into the bearing seat, the grease near the outside is polluted first, so that the bearing parts surface wear, with the increase of dirt, the wear surface will continue to expand, along the circumference of the formation of cracks and gradually expand, finally make the ring crack. When serious, it will scrap roll, bearing seat and other related parts. An oil seal device is installed for this purpose.
5.Calculation of service life of bearing
The roller bearing used is a roller bearing, and the formula for calculating its rated fatigue life is:
In actual use, lubrication effect and rolling roller axial channeling have great influence on the effective service life of bearings.
6.Closing remarks
After nearly two years of research and practice on the service life of roll bearing, the service life of roll bearing is greatly improved, the consumption of tonnage steel of roll bearing is greatly reduced, and the production cost of narrow strip steel is reduced.
【 Reference 】
[1] Huang Huaqing. Steel rolling machinery. Beijing: Metallurgical Industry Press,1979, 70-83
[2] Mechanical Design Manual (Volume 1, Volume 2). Beijing: Chemical Industry Press, 1987:962 ~ 1092,1474 ~ 1490
[3] Xu Hao. Manual of Mechanical Design (Volume 4). Beijing: China Machine Press,1991,28 ~ 3,28 ~ 75
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