The speed of rolling mills in a domestic steel plant has been increased, resulting in the high-temperature rise of four-row cylindrical roller bearings on the backup roll. In view of this problem, the friction torque and calorific value of the bearing before and after speed increase are calculated and compared. It shows that the influence of speed on friction torque is not obvious, and it is approximately proportional to the heat value. The temperature rise of the bearing is optimized. The main measures taken are to reduce the contact area of sliding parts, add a cooling oil circuit, reduce the roughness of contact surface, optimize the bearing oil hole, and improve the heat dissipation effect. A simplified calculation method for the contact between roller end face and rib is proposed. After application, the temperature rise of the optimized bearing is obviously slowed down and the service life is improved.
With the continuous promotion of supply-side structural reform and unprecedented severe environmental protection pressure, a large number of steel mills have been listed in the ranks of de production capacity. However, domestic large steel mills are in short supply for the time being. Therefore, the rolling speed has been increased to achieve the purpose of increasing efficiency. The speed of four-row cylindrical roller bearing for the backup roll of 1250 cold rolling line in a steel plant is increased from 197 R / min to 257 R / min under the same rolling force and lubrication mode. After increasing the speed, the bearing temperature rises too high many times and the alarm stops. According to incomplete statistics, this model has been used on about 200 lines in nearly 20 domestic steel mills, and the market utilization rate is very high, which is of certain value for its optimal design. The structure of the four-row cylindrical roller bearing is shown in Fig. 1. The overall dimension is Φ 690 × Φ 980 × 750, the material is G20Cr2Ni4A, the cage is welded by column, the accuracy Grade is P5, the bearing capacity Cr is 20700kN, cor is 56500kN.
1. Impact of speed increase
1.1 variation of friction torque
The temperature rise of the bearing mainly comes from the friction inside the bearing during the working process. There are many formulas for calculating the friction moment of bearings, and the Harris TA formula is used here.
For Formula: m is the total friction distance, Nmm; M0 is the friction distance of bearing under no load, M1 is the friction distance caused by load, Nmm; F0 and F1 are empirical coefficients; ν is the kinematic viscosity of lubricating oil, mm2 / S (the viscosity of base oil of lubricating grease); n is the speed of bearing, R / min; P is the equivalent load, N; Dpw is the pitch diameter, mm.
In catalog, the parameter values are: F0 = 2, F1 = 0.0003, ν = 12mm2 / s, n = 197r / min before speed increase, 257r / min after speed increase, DPW = 836mm, the maximum rolling force under application condition is about 1000 tons, P = 5 × 106n. The calculation results are shown in Table 1.
It can be seen from the above table that when the speed increases by 30.46%, the friction torque M0 of the bearing under no-load increases by 19.39%, and the friction torque M1 caused by load does not change. However, due to the large load, M1 accounts for a large proportion of the total friction torque, and the total friction torque increases by only 0.32%. Obviously, the bearing belongs to the low-speed and heavy-duty condition. At this time, the load is the main factor causing the bearing friction torque, and the speed change has little effect on the total friction distance change of the bearing.
1.2 change of bearing calorific value
The calculation formula of bearing calorific value is as follows:
Where q is the calorific value, W. The friction torque and speed are substituted into the calculation, and the results are shown in Table 2.
It can be seen from the above calculation that the total friction torque of the bearing increases by 0.32%, while the calorific value of the bearing increases by 30.87%. Due to the small change of the friction torque, the calorific value (increased by 30.87%) and the rotational speed (increased by 30.46%) approximately increase in proportion. The results also show that although the bearing heating comes from various internal rolling sliding friction, it is not accurate to understand that only reducing the bearing friction torque can solve the problem of bearing heating. In this case, it can be seen that the bearing heating is mainly related to the load and speed.
2. Optimization design of rolling mill bearing
From the above analysis, it can be seen that the heat output of the bearing increases more, and measures should be taken to let out the heat. The heat transfer modes of the bearing are mainly heat conduction, heat convection, and heat radiation. The calculation of bearing heating efficiency and heat dissipation efficiency is very complicated. It can be seen from the relevant calculation equations that the main parameters affecting the heat dissipation efficiency are contacted stress, sliding speed, oil film-related parameters, and contact area. Therefore, in order to meet the operation requirements after the change of working conditions, the optimization design idea is as follows:
1) The sliding part reduces the contact area;
2) The sliding part is provided with a cooling oil circuit;
3) Reduce the roughness of the contact surface and optimize the processing texture;
4) Optimize the bearing oil hole, increase the number and diameter.
2.1 optimization of bearing pitch circle size
The heat value of the circle can be adjusted from the heat generation equation only. This equation is not based on the internal contact of the bearing. It can be seen that reducing DPW is beneficial to reducing friction torque. In particular, M0 is positively correlated with the third power of pitch circle diameter, which changes greatly.
In addition, friction heat will also be generated when the rolling element passes through the lubricant in the bearing cavity in the revolution. The calculation equation is as follows:
In this formula, hrdrag is the friction heating rate; ω m is the revolution speed of roller, rad / S; FV is the viscous traction force, N; Z is the number of rollers; J is the conversion constant from nm / s to W. It can be seen that the friction heating rate is directly proportional to the pitch diameter and the revolution speed of the roller. The heating rate of the inner cavity lubricant to the roller increases in direct proportion after the speed increase, which indirectly indicates that the more lubricants, the better.
In conclusion, the internal structure of the bearing is optimized to reduce the pitch circle size of the bearing. The pitch diameter is also related to the bearing load and life, and the reduction is limited.
2.2 optimize the contact between the ring flange and rolling element
The cylindrical roller bearing mainly bears the radial load and also bears the axial load depending on the ring flange. In the contact surface, there is sliding friction between the roller end face and the rib due to the speed difference. If the sliding at both ends of the roller is different, the greater the friction force, the roller will even skew in the working process. The geometry of the roller end face and ring flange has a significant influence on the sliding friction and oil film formation between them. It is generally considered that the friction effect of point contact is the best compared with that of surface contact. In order to improve the contact state between the roller end face and the rib, the roller end face adopts the ball base surface, and the ring rib adopts the inclined rib. Through theoretical calculation, the position of the contact point between the center of the roller spherical base surface and the ring rib is controlled, so as to achieve the best lubrication state. The calculation is as follows.
In Figure 2, h is the height of the rib, H1 is the height of the rib with no oil hole size, a is the midpoint, R is the arc of roller end face, the contact angle is α, and S is the maximum clearance. There is a relationship in Fig. 2A
Where DW is the roller diameter, mm. When the roller diameter and rib height are known, the value of roller end face R can be determined by determining the angle α. The contact point calculated by the equation is actually the middle point of the flange, including the size of the oil groove, and the more accurate calculation should exclude the size of the oil groove, the midpoint of point H1. Therefore, it should be amended as follows:
Force on flange:
In order to ensure uniform force, the contact clearance between roller end face and rib edge should be greater than or equal to 0. The compression of steel steel point contact is as follows:
In the formula, η δ coefficient can be found in table [4]; Σ ρ is the main curvature sum function, and its calculation equation is as follows:
According to the geometric relationship in Fig. 2B, the maximum gap is as follows:
δ shall be ≤ s. The values of α and R can be obtained from equation (5) ~ (10), and the axial force FA of roller can be simplified as that the total axial force of bearing is evenly distributed to each roller. In fact, according to experience, α is generally between 10 'and 30'. When the working condition of bearing is low speed and heavy load, large deflection angle should be taken to form oil film. Compared with the plane contact, the oil film is easier to form around the point contact. In the sliding process, the oil film can take away the heat. It should be noted that the algorithm is not accurate, more accurate algorithm should use the relevant theory of EHL. For engineering practice, the algorithm is simple and practical, and can roughly calculate the value of angle α. Moreover, it is difficult to accurately control a certain fixed value between 10 '- 30' in the current machining accuracy. Within a certain tolerance range, the above algorithm can be considered to be correct.
2.3 optimize the contact surface of the middle retaining ring
There is a large area of contact between the middle retaining ring and the outer ring and the roller end face. The lower half of the middle retaining ring is designed as an inclined rib and an oil groove is designed. It can reduce the sliding area and increase the cooling oil way.
2.4 optimize cage structure
The post welding retainer is still used. In the process of bearing work, the support is used to guide and centralize the roller to prevent the roller from skewing, so the contact surface between the strut and the roller strut hole will produce impact and sliding friction. In order to improve the contact state between the support surface and the roller strut hole in the process of rotation, and reduce the friction between them, the roller strut hole is finely reamed to improve the surface roughness registration of the roller strut hole and increase the stability of the roller operation. This measure is also to prevent the strut and the roller not to fit well, the roller will shake or skew, so that the roller will produce extra sliding friction in the raceway, and improve the force and friction of the rib.
At the same time, leave a slope at both ends of the roller strut hole or do large chamfering treatment, which can reduce the contact area between the strut and the roller hole, and reduce the shear stress of the roller to the strut; at the same time, control the tolerance of the diameter of the pillar hole on the washer, the tolerance of the distance between two adjacent pillar holes in the circumferential direction, and the welding quality of the pillar head, so as to ensure the assembly accuracy of the roller and the support.
2.4 optimize raceway roughness
The roughness of the working surface has a great influence on wear resistance. The better the surface quality, the more conducive to the formation of oil film, so as to reduce the friction coefficient, reduce friction heating, and also slow down the wear of the raceway surface. Under the heavy load condition, the bearing bears a large radial load, which easily leads to high contact stress on the working face. If the roughness of the working surface is not good, the wave crest and trough are like sharp corner notch and crack, which are sensitive to stress concentration, thus affecting the fatigue strength of parts. The results show that the roughness peak height parameter has the most obvious influence on pressure distribution and oil film thickness. With the increase of roughness peak height, the number and amplitude of pressure peaks increase, while the minimum oil film thickness decreases. When the wavelength is small, the small change of the peak height will cause a sharp increase in the maximum temperature rise of the oil film. When the wavelength is large, the maximum temperature rise of oil film is not sensitive to the change of peak height. It can be seen from the related research that the influence of surface roughness on oil film formation and temperature rise is very complex.
In this case, the raceway of the ferrule is superfinishing. It can not only reduce the surface roughness but also form better texture, fully improve the elastohydrodynamic lubrication characteristics of the raceway, reduce the rolling sliding friction, and reduce the temperature rise. Using high-end equipment 1.6m magerle superfinishing machine, the raceway roughness can reach below Ra0.2. At the same time, the super precision of the raceway can also form the convex profile, which can significantly improve the contact stress of the raceway.
3. Optimization effect
Through the above optimization measures, the optimized bearing has been installed in the steel plant for trial use, and the service condition of the bearing has been tracked and recorded. Under the working conditions of the maximum speed of 250r / min and the maximum rolling force of about 1000t, up to now (it has been used for 5 months), there is no over-temperature phenomenon of the bearing. The optimized bearing meets the working condition after speed increases.
4. Conclusion
Increasing speed and increasing efficiency has become the development trend of the iron and steel industry in the future. The design of a four-row cylindrical roller bearing should also be developed in the direction of reducing temperature rise. The measures taken are to reduce the rolling sliding friction of the contact surface on the one hand and to study the effective measures of bearing heat dissipation on the other hand. At present, the theory of bearing heating and heat dissipation still needs more in-depth and systematic research, and the relevant theory should be actively transformed into practice in engineering applications, especially in the stage of bearing development and design.
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