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The application technology of metallurgical gearbox bearings

the types and applications of metallurgical gearbox are very extensive. It is mainly used to accelerate, decelerate, change the transmission direction, change the rotating torque or distribute power through the meshing and transmission between gears. As more Chinese sports brands are added to the trend of upgrading from solvent based shoe glue to waterborne polyurethane shoe glue, so as to meet the power transmission needs of various metallurgical equipment. Metallurgical gear boxes are not only used in various host equipment, such as the main transmission gear boxes for rough rolling, finishing rolling and vertical rollers of rods, wires, profiles or plates, but also used in the transmission systems of many auxiliary equipment, such as flying shears, coilers, piercers, slitters and conveying raceways. As one of the most critical metallurgical equipment, the stable and reliable operation of metallurgical gearbox is related to the normal operation of the whole production line. Once it fails, it will lead to serious consequences. Metallurgical gear boxes are usually in harsh operating environments such as heavy load, impact load, high or low speed, high temperature or high pollution. It is precisely because of the particularity of the working environment and the continuous and reliable working requirements that metallurgical gear boxes put forward more stringent requirements for the bearings used. Using high-performance bearings, correctly selecting bearing type selection, tolerance fit, initial clearance and installation size are the prerequisites to ensure the continuous and reliable operation of the gearbox

bearing selection

1 Choose the appropriate bearing type

metallurgical gearbox bearings generally use double row roller bearing combinations to meet the power transmission needs of heavy load and high power. Roller bearings are mainly tapered roller bearings, cylindrical roller bearings and self-aligning roller bearings. The type of bearing used in the metallurgical gearbox shall be determined by the stress conditions of the gear shaft and the characteristics of various bearings. Double row tapered roller bearings have good radial and axial composite load capacity, the internal clearance of the bearings can be adjusted, and it is conducive to enhancing the rigidity of the transmission system. Therefore, tapered roller bearings are the preferred bearing type in the design of metallurgical gearbox. However, tapered roller bearings also have shortcomings, such as high price and lack of self-aligning and anti eccentric ability. Therefore, metallurgical gear boxes also use a large number of other types of bearings, such as self-aligning or cylindrical roller bearings

metallurgical gear boxes often mix different types of bearings to achieve the purpose of optimal design. Compared with tapered roller bearings, self-aligning roller bearings have lower cost and can also withstand radial and axial composite loads under specific stress conditions. The biggest advantage of self-aligning roller bearing is that it has self-aligning ability and can work normally under eccentric conditions. When the bearing dimensions are similar, the double row cylindrical roller bearing can obtain higher radial bearing capacity, but this type of bearing can not bear axial load. Generally, when cylindrical roller bearings are selected under the combined radial and axial load conditions, thrust bearings or other types of bearings need to be added to bear the axial load

2. Double row tapered roller bearing combination mode

when double row tapered roller bearing combination is adopted, one end of the bearing is usually set as "fixed end", that is, the end faces of the inner and outer rings of the bearing are pressed tightly without leaving clearance, which plays the role of axial positioning and bearing axial load; The bearing at the other end is "floating end", which is used to absorb the thermal expansion and accumulated tolerance of the gear shaft. The "floating end" bearing only bears radial load, and sufficient clearance should be reserved between the end face of the outer ring of the bearing and the end cover and shoulder of the bearing seat to offset the axial displacement of the gear shaft after thermal expansion. The detailed structure is shown in Figure 1. In order to improve the transmission torque, the meshing mode of helical gears is widely used in metallurgical gear boxes. Under heavy load conditions, helical gears are not easy to produce impact vibration noise, operate smoothly and have large bearing capacity, so helical gears are better than spur gears under heavy load transmission conditions

Figure 1. The gearbox adopts the combination of double row tapered roller bearings

but when the helical gear is meshed, it will generate additional axial thrust and act on the "fixed end" bearing. When the axial thrust is large, selecting double row tapered roller bearings as the "fixed end" or adding thrust bearings are ideal solutions. When only a single helical gear is engaged in the gear shaft, and the self-aligning roller bearing is selected as the "fixed end" of the gear shaft, attention should be paid to the ratio of the maximum axial and radial forces on the bearing. Due to its structural characteristics, when the axial force of the self-aligning roller bearing exceeds 1/3 or 1/4 of its radial force (depending on the specific bearing series), the roller on one side of the bearing will be stressed and the roller on the other side will be unloaded. The roller at the non stressed end cannot be guided and positioned in the raceway during the rotation, and the roller skews, slips and strikes the cage, which may lead to cage fracture and bearing failure in advance in serious cases. Double row tapered roller bearings adopt pure rolling and roller end flange guide design. By controlling the bearing installation clearance, they can be used for large axial force and single row roller bearing conditions. The characteristics and differences of the two types of bearings can be seen in Figure 2. The "fixed end" bearing bears the radial and axial composite load. In order to share the bearing force and improve the service life of the foam granulator, the "fixed end" is generally placed on the side with less radial force, and the "floating end" is placed on the side with greater radial force. Sometimes, self-aligning roller bearings are selected to improve the axial bearing capacity of bearings, and the "fixed end" will also be placed on the side with large radial force. In this case, the bearing capacity of "fixed end" self-aligning roller bearings should be higher than that of "floating end" bearings

Figure 2 difference between self-aligning roller bearing and double row tapered roller bearing

3 Considering the actual working condition and axial load

bevel gear transmission is similar to helical gear, and large axial load will also be generated when the gear is meshed. The bearing selection should also refer to the actual working condition and axial load. Herringbone gear transmission can be used as a pair of bevel gears with opposite rotation direction. Herringbone gears will not produce axial thrust when meshing, so it is widely used in rolling mill gearbox. When herringbone gear meshing transmission is selected, only one "fixed end" axial positioning gear needs to be set for the input and output shafts, and the rest positions should be set as "floating end". The "floating end" bearing can be cylindrical or self-aligning roller bearing, and the "fixed end" should be double row tapered roller bearing with strong axial bearing capacity. The optimal type selection design of bearings is shown in Figure 3. Single row cylindrical roller bearings are used for "floating end" bearings. In addition, in the application of gear box of high-speed rod and wire mill, double row tapered roller bearings with special design are often used for high-speed input shaft to meet the harsh bearing application conditions such as high speed, frequent acceleration, deceleration, forward rotation and reverse rotation

Figure 3 optimal type selection of herringbone gear transmission bearing

4 Considering sufficient bearing capacity

in addition to the correct selection of bearing type, whether the selected bearing has sufficient bearing capacity is also a key factor to be considered. It is the most commonly used method to measure the bearing capacity to calculate the bearing capacity and obtain the bearing theoretical life. According to different applications, metallurgical gearboxes generally have requirements for the minimum theoretical life of bearings. For example, some gearboxes require the minimum life of bearings to be no less than 50000 h, and some applications require no less than 30000 H. Generally, when calculating the bearing life, in order to obtain the conservative bearing life, the average working condition speed is generally selected as the bearing rotation speed to replace the maximum rotation speed. When the power of gearbox transmission is constant, the reduction of speed will lead to the increase of torque, and the increase of torque will lead to the increase of bearing force. The bearing life is inversely proportional to the speed of rotation, but 10 times the force, that is, the price of cathode materials, electrolyte, copper foil and other raw materials of shaft battery rises sharply, and the bearing life will be reduced by 10 times if the bearing capacity is doubled. For some applications with heavy load and low speed, the theoretical bearing life results usually cannot truly reflect the bearing performance. Usually, it is mainly considered whether the ratio of bearing rated equivalent and bearing force is greater than the safety factor. A more accurate method is to calculate the maximum contact stress of bearing rollers through analysis software

bearing tolerance fit

metallurgical gearbox bearings are usually in harsh operating environments such as heavy load, impact load, high speed or low speed. Correct selection of bearing tolerance fit is not only conducive to the installation of bearings, but also ensures that bearings will not run under heavy load and impact load. Generally speaking, the rotating parts of the bearing adopt interference fit, and the stationary parts adopt clearance fit. The actual interference fit range of the bearing not only changes according to the size of the bearing, but also should be larger than the conventional application conditions under heavy load and high speed applications. When selecting Inch Tapered roller bearings, attention should be paid to the difference between the tolerance zone of inch bearings and metric bearings, and the tolerance dimension range of mating shaft and bearing seat should be adjusted accordingly. The tolerance range of inner and outer ring diameters of inch bearings is "plus tolerance", which is just the opposite of metric bearings. If the tolerance range of the shaft and bearing pedestal is still selected according to the metric bearing, the inner and outer rings of the British bearing will have a transitional fit, which will make it difficult to install the outer ring of the bearing and the inner ring is easy to run, affecting the service life of the bearing. The specific tolerance fit size of inch bearing shall refer to the size recommended by the bearing supplier. Compared with British bearings, the selection of tolerance fit for metric large-size bearings is relatively simple. For heavy-duty applications, where the inner ring of the bearing rotates and the outer ring is stationary, the shaft can adopt R6 tolerance range for large-size self-aligning or cylindrical roller bearings with an inner diameter of 320~500mm. For self-aligning or cylindrical roller bearings with an inner diameter of more than 500mm, the shaft can adopt R7 tolerance range. The tolerance range of self-aligning or cylindrical roller bearing seat can be H7 or G7

initial clearance of bearing

the definition of initial clearance of tapered roller bearing is different from that of ball bearing, self-aligning and cylindrical roller bearing, which refers to axial clearance (BEP), and other bearings are radial clearance (RIC). The initial clearance of the bearing determines the working clearance of the bearing after installation and operation. The clearance of the bearing determines the size of the bearing working bearing area. The selection of the initial clearance of the bearing is too large, which leads to the smaller bearing working bearing area, that is, the number of rollers bearing the force is less, which leads to the reduction of the bearing life

if the clearance of tapered roller bearing is too large, the roller may wear or hit the cage, resulting in cage fracture. If the initial clearance of the bearing is too small, it will lead to serious bearing heating, bearing locking and burning

the initial clearance of the bearing of the metallurgical gearbox should be calculated according to the interference fit of the shaft and the maximum rotation speed of the bearing. When the inner ring of the gearbox bearing is in interference fit with the shaft, the outer diameter of the inner ring of the bearing will expand and eat up part of the initial clearance. The clearance of the initial clearance minus the loss of interference fit is the installation clearance (MEP) of the bearing. Generally, for inch double row tapered roller bearings with an inner diameter of no more than 400mm, the clearance range after installation should not be greater than 0.5mm. For large bearings with an inner diameter of more than 400mm, the maximum clearance range after installation can be enlarged to about 0.6mm

the initial clearance of metallurgical gearbox bearings generally does not consider the ambient temperature, but only the difference temperature between the inner and outer rings of the bearing, because the ambient temperature will lead to shaft thermal expansion, but the influence of the ambient temperature of the bearing seat will also lead to thermal expansion. The clearance (MEP) of the bearing after installation minus the clearance eaten by the temperature rise of the difference between the inner and outer rings of the bearing to obtain the final bearing colormatrixtm lactra SX. It is the only all photoresist white additive operating clearance (OPE) in the market that meets China's latest food certification standard (GB). metallurgy

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