This paper briefly explains the working characteristics of roll bearings, analyzes the factors affecting the service life of roll bearings, and explains the use of roll bearings, which has a certain guiding role in reducing the consumption of narrow strip hot rolling mill bearings.
Keywords: narrow strip hot rolling mill; roll bearing; service life; consumption
Introduction
The load-carrying capacity of the bearing is manifested in the static state as the resistance to plastic deformation at the contact point between the rolling element and the raceway, and in the dynamic state as the ability of the material on the working surface of the bearing raceway and rolling element to resist fatigue spalling. Therefore, the load-carrying capacity of the bearing is Size is an important basis for selecting and calculating bearing service life.
The damage phenomena of rolling bearings include: peeling, wear, fracture, temperature rise, abnormal sound, vibration out of tolerance, precision reduction and other forms. Most of the causes of these damages are improper use and poor maintenance of the bearings. There are also problems with the manufacturing and design of the bearings. If improper use factors and defects in the bearing processing design are eliminated, it can be considered that: during the rotation process , the alternating contact stress endured by the rolling surface material of the bearing is the main cause of bearing damage. The main form of bearing damage is material peeling caused by rolling fatigue. During the entire use of the bearing, regardless of whether it is used reasonably or not, rolling fatigue is unavoidable. Therefore, the fatigue spalling damage of the rolling working surface material of the bearing can be narrowly referred to as the end of the life of the rolling bearing.
The bearing life in a broad sense can be understood as: when the technical performance parameters such as noise, vibration, and precision of the bearing exceed the initial value and cannot meet the needs of mechanical equipment, it can be called the termination of the “noise life” and “precision life” of the bearing. For sealed bearings filled with grease, when the bearing cannot operate normally due to grease failure, it can also be called the end of “grease life”.
Due to the large consumption of hot rolling mill bearings, studying the service life of the bearings is of great significance to reduce bearing consumption and reduce the production cost of narrow strip steel.
Working characteristics of roller bearings
Roller bearings are important components in the working base of rolling mills. Their characteristics are:
(1) Large working load: Since the diameter of the roll body should ensure strength, the outer dimensions of the bearing seat should not be larger than the minimum diameter of the roll body. Therefore, the outer dimensions of the roll bearing are limited and the unit load is large. Usually the unit pressure of roller bearings is 2 to 4 times higher than that of general-purpose bearings, or even higher.
(2) There is a big difference in operating speed: the speed from JP1 to JP6 differs several times.
(3) The working environment is harsh: During rolling, the rolls are cooled with water and there is spattering of iron oxide scale.
Factors affecting the service life of roller bearings
The relationship between the operating speed of the rolling bearing (rev/min), the equivalent dynamic load P, the rated dynamic load C of the bearing and the service life of the bearing
According to the general calculation formula for the service life of rolling bearings, the relationship between the rated dynamic load C, equivalent dynamic load P and operating speed n of the bearing and the service life of the bearing is:
It can be seen from the above formula that when selecting a bearing model, a bearing with a larger rated dynamic load C should be selected as much as possible to increase the service life of the bearing. The operating speed n is determined during design and is not easy to change. The equivalent dynamic load P is affected by the actual There are many factors affecting the use and it is difficult to control.
The relationship between the radial clearance of rolling bearings and the service life of the bearings
Since the size of the radial clearance of the rolling bearing directly affects the internal load distribution of the bearing, especially the change of the maximum rolling element load in 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 bearing radial clearance is approximately negative, that is, when the load rate is 0.7 to 0.8, the bearing has the longest service life. When the load factor is less than 0.5, the service life of the bearing is significantly reduced due to the reduction in the load area within the bearing and the increase in the maximum rolling element load. When the load rate is greater than 0.8, the bearing operates with excessive negative clearance, the bearing generates a lot of heat, and the service life decreases more significantly.
The relationship between the operating temperature of rolling bearings and the service life of the bearings
The relationship is mainly reflected in the reduction of the C value of the allowable dynamic load. The higher the operating temperature, the lower the C value of the allowable dynamic load, and the lower the service life of the bearing.
Relationship between the hardness of rolling bearing materials and bearing service life
The impact of the hardness of the working surface material of the rolling bearing on the service life of the bearing is mainly reflected in the decrease in the C value of the rated dynamic load of the rolling bearing. The material surface hardness of the bearing components is generally HRC58-64. If the material surface hardness of the bearing components is lower than the above range, then The rated dynamic load C has a downward trend.
The relationship between steel melting method and bearing life
Non-metallic inclusions in steel will have an adverse effect on contact fatigue. Fatigue cracks are prone to occur under the action of external alternating stress between the oxides and inclusions under the metal surface that are not prone to plastic deformation and the material matrix. The greater the number of non-metallic inclusions in the steel, the lower the fatigue life of the bearing. To reduce non-metallic inclusions in steel, effective smelting methods must be adopted, and vacuum smelting multiple times is better. When the steel is smelted multiple times, although the fatigue life of the bearing is improved, the cost of the steel appears to be too expensive. Therefore, for important bearing materials, steel materials that have been vacuum remelted once or vacuum degassed and refined are often used. At this time, the service life of the bearing is sufficient to meet its performance requirements.
The relationship between the residual stress on the surface of the bearing material and the service life of the bearing
During the heat treatment process, if the bearing components are not handled properly, large thermal stress will be generated; during the grinding process, if the cooling is insufficient or the grinding amount is too large, grinding stress will also be generated; when the bearing and shaft shell are matched, due to If the fit interference is too large, fit stress will also occur; even harmful defects on the working surface of the bearing will cause changes in residual stress on the surface of the material. Generally speaking, these additional residual stresses on the material surface caused by external factors are normal stresses. (tensile stress)
A bearing that has undergone normal hot and cold processing and is used correctly should have residual negative stress (compressive stress) on its working surface.
The residual compressive stress on the surface of the material can inhibit the generation and expansion rate of fatigue cracks, thereby extending the fatigue life of the bearing. In contrast, residual tensile stress will reduce the fatigue life of the bearing.
The relationship between lubrication and bearing service life
Practice has proved that when the lubricating oil is highly clean, that is, when the lubrication condition of the bearing is at its best, the rolling bearing is fatigue-resistant. However, bearings are in a mixed friction state in most cases. That is to say, the rolling working surface of the rolling bearing, although there is enough lubricating oil, cannot form a full oil film state. The more impurities in the lubricating oil, the more mixed friction, and the lubrication state The worse it is, the lower the service life of the bearing.
The relationship between bearing processing technology and bearing service life
The service life of bearings is not only based on the chemical composition of the steel, but is also closely related to the bearing manufacturing process.
Fiber streamlines and forging ratio
The distribution of steel fiber streamlines on the rolling contact surface has a great relationship with fatigue life. Research shows that when the fiber flow direction is perpendicular to the contact surface, the service life is generally lower, that is, the fatigue damage of the material often occurs where there are fiber outcrops on the surface. When the fiber flow direction is parallel to the contact surface, the lifespan is optimal. When the bearing blank is forged, when the forging pressure is relatively large, the material lattice can be refined, non-metallic inclusions can be broken, and the grain flow direction is consistent, so the fatigue life of the material is higher. Considering the fiber streamline distribution of the material, the spherical pole and annulus parts of the steel ball and the ferrules turned from bar stock are detrimental to the fatigue life, while the ferrule blanks with rolled channels have a longer fatigue life.
Correct heat treatment process
After quenching and tempering, the metallographic structure of general bearing steel should be more than 80% of fine needle-like and fine-grained martensite, 5% to 10% of retained austenite and about 7% of undissolved carbides. composition. When the bearing steel is heat treated and coarse-grained carbides appear or the carbon content of martensite is too low, the fatigue life of the material will decrease. When the carbon content of martensite increases (more than 0.5%), martensite brittleness will appear. The content of retained austenite can improve the fatigue resistance of the material, but unstable retained austenite will affect the geometric and dimensional stability of the bearing.
Rationality of mechanical processing technology
Grinding and super-precision grinding in mechanical processing are the key to the final processing of bearing components and have an important impact on the performance of the bearing. The rationality of the roughness of the bearing working surface is a condition for ensuring good lubrication. Excessive roughness will cause difficulty in the formation of oil film, resulting in increased tangential force and shortened service life. During 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, a secondary fire layer will appear on the surface of the material, which will lead to a decrease in the fatigue life of the bearing.
The relationship between installation accuracy and bearing service life
The decrease in installation accuracy, especially the excessive inclination of the roller bearing during installation, will seriously affect the normal distribution of the internal load of the bearing, causing harmful stress concentration, resulting in a significant reduction in the service life of the bearing.
Use of roller bearings
Two-roller bearings and back-up roller bearings
The bearing’s rotational speed, rated dynamic load and equivalent load are the basic parameters for calculating bearing life. For rolling mill bearings, due to their special working conditions, the selection parameters are limited, that is, the equivalent load that the bearing can bear should be less than 30% of its rated dynamic load value, otherwise the bearing load will be too heavy.
The rated life of rolling mill bearings is proportional to the 10/3 power of the bearing load ratio. Under the same external dimensions, the rated dynamic load value of four-row cylindrical roller bearings is more than 15% higher than that of four-row tapered roller bearings. Therefore, the rated life of the former is 60% higher than that of the latter. The selection of two-roller bearings and backup-roller bearings plays a decisive role in narrow-strip steel rolling mills, and the consumption of two-roller bearings and backup-roller bearings has been relatively large in the past few years. In the past two years, four-row cylindrical bearings with larger load capacity have been selected. Roller bearings have achieved significant results, and the consumption of tons of steel bearings has dropped significantly. In addition, since the load angle is inversely proportional to the bearing clearance, in order to make the four-row cylindrical roller bearing have a larger load capacity, a bearing with a smaller clearance is selected.
The accuracy grade of rolling mill bearings also has an impact on service life. Due to the limited rotation speed of the rolling mill, the dynamic pressure lubricating oil film of the second-roller bearing and the back-up roller bearing is generally thin. Therefore, high-precision grade bearings can maintain better continuity of the oil film due to their lower raceway surface roughness. , lubrication is more ideal. Therefore, P6 precision bearings were selected.
If the bearing seat is improperly designed and manufactured, it will cause uneven stress on the bearing and reduce the bearing life. The requirements for the bearing seat are: sufficient wall thickness should be left in the load area of the bearing seat. Generally, it should not be less than 1/3 of the bearing diameter. The bearing seat should be self-aligning to prevent the bearing from being unbalanced due to roll deflection and deformation. The adjusted frame window lining should be used as the base surface, and the axis of the bearing seat hole should be kept parallel to it. Every time you install a bearing, check whether the cylindricity of the bearing seat hole meets the requirements to ensure that the bearing is in a good stress state.
Work roll bearings
Most of the strong impact load on the work roll of the narrow strip hot rolling mill is transmitted to the support roll, while part of the axial impact acts on the work roll bearing. The main reason for the axial impact force is improper adjustment of the window base lining, which causes the axes of the two rollers to be non-parallel. The greater the angle between the roller axes, the greater the axial force generated during steel rolling. The axial impact force will not only damage the positioning bearing of the work roll, but sometimes also deform the bearing seat gland and even break the fastening bolts. For this reason, the base lining plate must be adjusted, which is an important measure to reduce the axial impact force and protect the work roll bearings.
For strip mills, the greater the axial movement of the roll, the greater the axial impact force. Therefore, it is necessary to always check whether the work roll bearing seat is pressed tightly and whether it is loose. In particular, the operating side bearing seat and the frame window must be fixed axially without any gap, and the rigidity must be increased as much as possible to reduce the impact on the bearing. At the same time, the matching gap between the transmission side bearing seat and the frame window must be minimized to slow down the impact. The impact of transmission shaft runout on bearings.
In order to increase the axial load capacity of the work roll bearing, four rows of tapered roller bearings are used.
Bearing lubrication and sealing
Lithium-based, aluminum-based, and polyurea-based greases are all ideal lubricants for rolling mill bearings. For low-speed, heavy-duty narrow-strip steel rolling mills, in order to form a dynamic pressure oil film on the bearing raceway, these grease base oils are required to have a higher viscosity, generally not less than 320 mm²/s. The grease should contain extreme pressure additives, which can form an extreme pressure film with the bearing raceway at a certain temperature, which can reduce friction at low speeds. Therefore, lithium molybdenum disulfide-based grease lubricant is used.
Narrow-strip steel hot rolling mills are usually subject to strong impact loads, the bearings vibrate greatly, and grease is easily lost. Therefore, the grease is required to have strong adhesion and can be firmly adhered to the surface of the part. Part of it is used to lubricate the bearing on the raceway, and the rest is kept in the internal space of the bearing for sealing.
Poor sealing is also a major cause of damage to rolling mill bearings. If external dirt invades the bearing seat, the first thing to be contaminated is the grease near the outside, causing wear on the surface of the bearing parts. As the dirt increases, the wear surface will continue to expand, cracks will form along the circumferential direction and gradually expand, eventually causing the bearing parts to wear out. The ferrule is cracked. In severe cases, rollers, bearing seats and other related parts will be scrapped. An oil seal device is installed for this purpose.
Bearing service life calculation
The roll bearing used is a roller bearing, and its rated fatigue life calculation formula is:
In the formula, Lh-rated fatigue life (hours); fh-life coefficient. According to relevant information, fh=1-3 can be found
In actual use, the lubrication effect and the axial movement of the roller have the greatest impact on the effective service life of the bearing.
Conclusion
After nearly two years of research and practice on the service life of roll bearings, the service life of roll bearings has been greatly improved, the consumption of tons of steel for roll bearings has been greatly reduced, and the production cost of narrow strip steel has been reduced.
