What is a rolling mill bearing?
Rolling mill bearings are used in the rolling industry, that is, rolling of non-ferrous metals, ferrous metals and non-metallic products. It is used for bearings at the roll diameter of the calender roll and on the drum. Usually four-row cylindrical roller bearings are used to withstand radial loads, and thrust roller or thrust ball bearings and radial angular contact ball or radial roller bearings are used to withstand axial loads. At present, oil and gas lubrication or oil mist lubrication and grease are mostly used for lubrication and cooling.
Influencing factors and preventive measures of rolling mill bearings
1.1 Assembly quality
(1) The life of the bearing is inseparable from the design of the bearing seat. If the bearing seat is improperly designed and manufactured, it will cause uneven bearing force and reduce the bearing life. The bearing seat should be self-aligning to avoid eccentric loading of the bearing due to the burning and deflection of the roll.
(2) Whether the size, geometry, accuracy grade, and tolerance range of spare parts related to the bearing are consistent with the design.
(3) Whether the smoothness and hardness of the contact surface matched with the bearing are within the specified range, whether all clearances and interference fits meet the design requirements, etc.
1.2 Installation of inner and outer jacket
1.2.1 Installation of inner sleeve
The inner sleeve and the roll neck of the four-row cylindrical roller bearing should be an interference fit. During installation, the temperature should be heated to 80-90 °C, and the temperature should not exceed 100 °C. Otherwise, the bearing ring raceway and rolling elements will be easily annealed, which will affect the hardness and resistance.
Abrasiveness, resulting in reduced bearing life and premature failure. When using the heating method to install the bearing, the bearing should be quickly removed from the oil after the oil temperature reaches the specified temperature for 10 minutes, and installed on the shaft while it is still hot. If necessary, a little pressure can be applied to the end face of the bearing inner ring with the installation tool, which makes it easier to install.
After the bearing is installed on the shaft, the inner ring must be pressed immediately until it cools. It is usually heated with induction heaters or oil, and the use of torches is prohibited. When the inner sleeve is installed on the roll neck, it is in close contact with the water retaining ring to prevent the water retaining ring from moving.
1.2.2 Installation of the jacket
The outer sleeve of the four-row cylindrical roller bearing is a transition fit with the inner hole of the bearing seat. When assembling, lightly drive the whole consisting of the jacket, rollers and cage into the bearing seat with a copper rod, and fix the end cover against the inner side.
When installing the outer casing, pay attention to the markings on the end face and the end face of the cage. It cannot be installed in reverse. It should be installed in the order of the initial state when the bearing is unpacked to prevent the bearing from burning due to uneven force on the rollers.
When installing the bearing, the bearing should be placed horizontally. After the bearing is installed, its force range should be marked for key inspection when the roller is replaced.
1.3 Proper assembly of bearing seals
Bearing seals can be considered to use ordinary fluororubber skeleton seals, which are not only cheap, but also can achieve good results with reasonable use. When the rolling mill is in a horizontal state, the lip of the shaft sealing ring in the static labyrinth on both sides must be installed toward the outside of the bearing, which can effectively prevent the splashing of cooling water and oxide scale; when the rolling mill is in a vertical state, the inside of the labyrinth on the transmission side The lip of the sealing ring is installed towards the inside of the bearing, which can effectively prevent the overflow of lubricating oil due to the downward gravity; the lip of the sealing ring in the labyrinth on the non-transmission side is also installed towards the outside of the bearing, which can effectively prevent the splashing of cooling water.
1.4 Reasonable selection of bearing grease
Since the commonly used grease for bearings is ordinary 3#MoS2 lithium-based grease, it has the disadvantages of high viscosity, low penetration, high temperature resistance and high speed resistance.
Moreover, it is easy to cause abnormal wear and burn out of the bearing due to the hardening of the grease and the failure to lubricate the bearing normally. In view of these disadvantages that are not conducive to the use of bearings, it is recommended to use extreme pressure lithium complex grease in consideration of the above factors.
1.5 Inspection of bearings
1.5.1 Inspection of bearings during operation
According to the requirements of the bearing use environment, the bearing should be regularly listened to, observed and measured. Listening is to use the specified instrument to face the radial housing of the bearing axial end cover and bearing seat, and listen closely to the other end to see if there is any impact sound and mechanical friction sound during the operation of the bearing; observation is to observe the operating environment, installation position, vibration deviation Check whether there is a bad environment; the measurement is to use a thermometer and a vibration meter to quantitatively detect the bearing seat.
Through the above methods, it can be determined whether the bearing is in a normal state during operation and the measures to be taken, the faults and preventive measures that the bearing is prone to occur during operation.
1.5.2 Inspection of bearings at rest
Check the tightening of the relevant spare parts, whether the parts are in the correct position, whether there is looseness, etc. Always check the bearing seat for tightness and looseness.
In particular, there must be no axial clearance between the operation side bearing seat and the rolling stand, and the matching gap between the transmission side bearing seat and the rolling stand window must be reduced as much as possible, so as to reduce the impact of the drive shaft runout on the bearing.
In addition, the bearing lubricant can be tested to check whether the lubricating oil enters the sundries, iron oxide scale, water, etc. Roughing mills are usually subjected to strong shock loads, the bearings vibrate greatly, and the grease is easily lost. Therefore, the grease is required to have strong adhesion, which can firmly adhere to the surface of the parts, part of which lubricates the bearing on the raceway, and the rest is kept in the inner space of the bearing for sealing.
If external dirt penetrates into the bearing seat, the first contamination is the grease near the outer side, which causes the surface of the bearing parts to wear. With the increase of dirt, the wear surface will increase, and at the same time, the cracks in the signal layer will gradually expand, and finally the ferrule will be cracked.
1.5.3 Inspection after dismantling of bearings
Due to the large rolling capacity of the single groove of the roughing mill, the roll changing cycle is long. After the roll is replaced, the cleaning agent can be used to clean the bearing, dry it with compressed air, and then inspect the roller, cage and rolling. The rollers and raceways have dents, pitting and other phenomena, which should be replaced according to the specific conditions. At the same time, check the radial clearance of the axial sealing box. After the check is correct, add lubricant for backup.
Rolling mill bearing classification
(1) Spherical roller bearings
The configuration of rolling mill bearings on the rolling mill is different. At that time, two sets of spherical roller bearings were mainly installed on the same roll neck in parallel. This configuration basically met the production conditions at that time, and the rolling speed could reach 600rpm.
However, as the speed increases, its shortcomings become more prominent: short bearing life, high consumption, low precision of finished products, serious roll neck wear, large axial movement of the roll, etc.
(2) Four-row cylindrical roller bearing + thrust bearing
The inner diameter of the cylindrical roller bearing is tightly fitted with the roll neck and can withstand radial force. It has the advantages of large load capacity, high limit speed, high precision, separable and interchangeable inner and outer rings, easy processing, low production cost, and convenient installation and disassembly.
The thrust bearing bears the axial force, and the specific structure type can be selected according to the characteristics of the rolling mill.
When the load is heavy and the speed is low, it is equipped with a thrust roller bearing to bear the thrust load with a small axial clearance. When the rolling speed is high, with angular contact ball bearing, not only the limit speed is high, but also the axial clearance can be strictly controlled during operation.
The rolls can be tightly guided in the axial direction and can bear the general axial load force. This type of bearing configuration not only has long bearing life, high reliability, but also has many advantages such as high precision and easy control of rolling products, so it is the most widely used, mostly used in wire rod rolling mills, sheet rolling mills, foil rolling mills, and double support rolling mills. Back-up rolls for cold rolling mills and hot rolling mills, etc.
(3) Four-row tapered roller bearings
Tapered roller bearings can bear both radial force and axial force, and no thrust bearing is required, so the main engine is more compact. The inner diameter of the tapered roller bearing adopts a loose fit with the roll neck, which is very convenient for installation and disassembly, but sometimes sliding creep may be caused due to the loose fit, so the inner diameter is often machined with a spiral oil groove.
This configuration type is still widely used, such as the work rolls of four-high hot rolling mills and cold rolling mills, billet mills, steel beam rolling mills and other occasions.
Four-row cylindrical roller bearings and six-row cylindrical roller bearings are almost exclusively used in roll necks, drums and rolling mills of rolling stands. These bearings have low friction compared to other roller bearings. Since these bearings are usually mounted on the roll neck with an interference fit, they are especially suitable for rolling mill applications with high rolling speeds.
The low cross-section of these bearings allows the use of relatively large roll neck diameters compared to roll diameters. The radial load capacity is very high due to the large number of rollers that can be accommodated.
Multi-row cylindrical roller bearings can only withstand radial loads. Therefore, these bearings are mounted with deep groove ball bearings or angular contact ball bearings, or tapered roller bearings of radial design or thrust design, which are subjected to axial loads. Four-row and six-row tapered roller bearings are of split design, that is, the bearing ring with integral flange and the roller and cage assembly can be installed separately from the split bearing ring, or all bearing components can be installed separately.
This considerably simplifies bearing installation, maintenance and inspection. The bearing can bear a certain limit of axial displacement of the shaft relative to the bearing seat. Four-row cylindrical roller bearings have a cylindrical bore, and some sizes are also available with a tapered bore. Bearings with tapered bores can be adjusted during installation to obtain a certain radial internal clearance or a certain preload.
Rolling mill bearing process
The design of rolling mill bearings, the burning loss of rolling mill bearings is related to many factors, which is generally normal. The burning loss of typical angular contact bearings of aluminum foil rolling mills is a worldwide problem. I personally think that lubrication is very important. The effect of oil mist lubrication Slightly better, there is also the issue of bearing installation clearance.
In addition, there are many factors such as the gap between the shaft end baffle and the bearing seat, and the load change of the bearing outer ring caused by the bending cylinder. . There are also bearing outer ring spring buffers and so on.
- Four-row cylindrical small roller bearings are generally used for the backup roll, and the inner ring is statically matched with the roller. Generally, after the taper sleeve is installed, the inner ring is installed, and the outer circle of the roller is finely ground based on the inner ring. The axial force generated during the rolling process needs to be supported by bearings. Generally, back-to-back double-row angular contact ball bearings, face-to-face double-row tapered roller bearings, thrust bearings and four-point angular contact ball bearings are used. , The domestic manufacturers of the tile shaft products are better, and the foreign manufacturers of the American iron mother are better.
- Four-row tapered roller bearings are generally used for work roll bearings. Because of their structure, the inner ring is in dynamic fit with the roll, that is to say, when the roll is regrind, the bearing is removed together with the bearing seat, which itself can bear a certain amount of shaft. Therefore, the axial force of some small rolling mills is borne by itself, while some larger or higher precision rolling mills generally use axial force bearings.