During the production process of the large H-type steel rolling mill, the thrust bearing plays a crucial role. Its lubrication condition directly affects the stable operation of the rolling mill and the quality of the products. The original oil supply system was affected by insufficient air pressure and oil quantity distribution, resulting in poor lubrication effect of the thrust bearing and frequent malfunctions. Therefore, it is necessary to carry out a separate oil supply renovation for it. Given the particularity of the rolling process, the axial force fluctuation will be relatively large, causing the thrust bearing to remain in a harsh alternating load condition for a long time. The traditional rolling mill lubrication scheme usually incorporates the gearbox transmission gears and various bearings into the same set of concentrated lubrication system with thin oil.
The centralized lubrication system must simultaneously meet the requirements of gear meshing cooling, radial bearing lubrication, and thrust bearing lubrication. The oil pressure required for gear meshing is relatively low, and there are significant differences in the resistance along the pipeline at each lubrication point. This results in the difficulty of achieving balanced system back pressure. It was found that the contaminants were not transmitted from the hydraulic pump direction (P to A), but were generated by the internal wear and damage of the actuating element (hydraulic cylinder). [1] The thrust bearing is at the end of the oil circuit. The actual supply volume is usually lower than the designed value, making it difficult to form a stable bearing oil film, which further aggravates the wear and heating situation.
When the gears are in high-speed and heavy-load meshing, the tooth surfaces of the transmission gears will continuously undergo sliding friction, and metal chips will be generated accordingly. These tiny hard particles will suspend in the lubricating oil and spread throughout the entire system with the circulation of the oil. Currently, most of the existing centralized lubrication systems mainly focus on overall oil supply, and it is difficult to carry out specialized high-precision filtration work for precision components such as thrust bearings. This makes it easy for tiny contaminants to break through the defense line.
The oil tank of the lubrication system has an adequate capacity. The design of the cooler needs to take into account the thermal balance of gear meshing. There will be a certain delay in responding to the local high heat load of the thrust bearing. During the rolling operation, a high intensity is maintained continuously. The return oil temperature remains at a high level, causing the inlet oil temperature to exceed the specified range. The heat generated by the bearing cannot be effectively dissipated, thereby forming a "heat accumulation" effect. For the bending and curvature of the steel formed during the process from the rolling operation to the cooling bed, the straightening treatment of the finished product can be carried out using a roller straightening machine or a pressure straightening machine. [2]
The thermal balance calculation of the large rolling mill gearbox is generally carried out based on the overall heat source. When the gears mesh, heat is generated, and the oil temperature will also rise accordingly. Although the cooler can perform the regulation of the overall system temperature, if the thrust bearing is in a high-temperature area, its inlet oil temperature will still be restricted by the total return oil temperature. During the steel rolling process, due to the uneven temperature or structure of the rolled piece, the axial force of the rolls may change. The upper and lower horizontal rolls may experience axial displacement due to this and "misalignment" may occur, resulting in a decline in product quality. [3]
The thrust bearings of the large H-type steel rolling mill are mainly used to withstand the axial force of the rolls, ensuring the stable axial position of the rolls during the rolling process. They reduce friction between the shaft and the bearings through the supporting effect of the oil film, thereby minimizing wear.
The original oil supply system adopted a centralized oil supply method. The main oil pump extracted oil from the oil tank, filtered and cooled it, and then distributed it to various lubrication points of the main bearings. Subsequently, it was redistributed to the thrust bearings through the main bearing distributor. However, this oil supply method has some problems, such as unstable oil supply pressure and uneven oil flow distribution, resulting in too low oil flow at the thrust bearings, causing excessively high bearing temperatures and making it difficult to ensure the oil film thickness, which affected the normal operation of the thrust bearings. High operating temperatures are a very unfavorable factor for the use of bearings. Therefore, the principle for selecting the material of the bearing should be high surface hardness and wear resistance, and it is required to have good internal toughness. [4]
The screw pump unit and the oil tank form an independent circulation unit. The double-tube filter and the plate-type cooler also participate in it. The traditional control unit independently fulfills the closed-loop control functions of position and speed in this typical position control. [5] The pressure-flow regulating valve group operates in coordination with the pipeline accessories. The oil pump draws the oil from the independent oil tank and performs purification work through a high-precision filter. The flow regulating valve precisely distributes the oil, allowing it to enter the thrust bearing friction pair, thereby removing heat and wear particles. After that, the oil flows through the return pipeline back to the independent oil tank, forming a closed-loop circulation.
The load characteristics of the thrust bearing play a decisive role in the system design. The variable pump and the proportional relief valve form the control loop. The solution for the minimum oil film thickness of the bearing was carried out through theoretical calculations. This thickness plays a decisive role in the required supply of oil.
The independent oil supply system adopts a dual-level control method of "oil suction filtration and high-pressure pipeline fine filtration", which is different from the extensive filtration mode used in the centralized system. The high-pressure pipeline is equipped with a double-tube switching filter configuration. Its filtration accuracy is determined based on the clearance of the bearing moving pair and is capable of effectively intercepting micrometer-sized particles. This ensures that the cleanliness of the oil entering the bearing meets the requirements of the servo system, and completely eliminates the potential problem of abrasive wear.
The plate heat exchanger adopts an independent configuration method. Temperature sensors and electric regulating valves are installed in the oil supply pipeline, and the PID regulation algorithm for the cooling water flow rate is adopted. Based on the real-time detection of the supply oil temperature, automatic control can be achieved. The lubrication temperature of the thrust bearing is stabilized by an independent temperature control mechanism, which is not affected by the overall oil temperature fluctuation of the gearbox, ensuring the bearing's load-bearing stiffness. This optimization procedure can save energy without changing the original equipment, significantly reducing the production costs of the enterprise and enhancing the market competitiveness of the products. It has further application and promotion value. [6]
To ensure the stable operation of the system, the design of the safety interlock protection mechanism must be carried out comprehensively and meticulously. Electrical contact pressure gauges and flow switches should be configured in the oil supply pipeline. If the system pressure drops below the minimum value required for the formation of the bearing oil film, or if the flow rate drops sharply, an audible and visual alarm will be triggered immediately, and the main motor of the rolling mill will be linked to reduce the rolling speed or stop operation. This can prevent the bearing from suffering from oil shortage damage and promotes the innovation of maintenance methods. The full digitalization of the control system and the digital assignment of the control system speed reference values through computer networks have greatly improved the control accuracy, anti-interference ability and reliability of the system. [8]
Design an independent oil supply system, separating the lubrication for the main bearings from that for the thrust bearings. Each part will be supplied with oil independently to provide stable and reliable lubrication for the thrust bearings. This system can precisely control the oil supply pressure, flow rate and oil temperature according to the working requirements of the thrust bearings, ensuring that the thrust bearings are in a good lubrication state.
Independent oil pump: Select an oil pump with appropriate flow rate and pressure to supply oil specifically to the thrust bearing, ensuring the stability of the oil supply.
Filtering device: Install a high-precision filter to remove impurities from the oil, preventing them from entering the thrust bearing, which could affect the lubrication effect and cause wear.
Cooling device: Equipped with an efficient cooler to control the oil temperature within an appropriate range, preventing the oil from becoming too hot which would cause a decrease in viscosity and thereby affect the formation of the oil film.
Pressure and flow control components: Install pressure regulating valves and flow regulating valves to precisely adjust the oil supply pressure and flow according to the actual requirements of the thrust bearing.
Fuel tank: Install a separate fuel tank to store sufficient lubricating oil and ensure good sealing and heat dissipation performance.
An advanced automated control system is adopted, with the pressure, flow rate, oil temperature and other parameters of the individual oil supply system connected to the operation console. The operators can conduct real-time monitoring and control. When the parameters exceed the set range, the system can automatically alarm and take corresponding measures, such as adjusting the valve opening or starting the backup equipment. Both the upper and lower horizontal rollers, as well as the left and right vertical rollers, are controlled by electrical drive. The axial position is controlled by hydraulic servo valves. [7]
According to the design plan, the independent oil pump, filtration device, cooling device, pressure and flow control components and other equipment should be installed at the designated positions and the pipelines and circuits should be connected properly. During the installation process, operations must be carried out strictly in accordance with the equipment installation manual and relevant standards to ensure the installation quality.
Reasonably arrange the oil supply pipelines. The overall length of the pipelines on the finishing mill frame should cover the position of the frame without any joints. Use stainless steel materials to ensure smooth oil flow without leakage. At the same time, protective treatment should be carried out for the ground pipelines to prevent oil leakage caused by damage to the pipelines.
Connect the electrical circuits properly and ensure that all equipment can operate normally. Debug the control system, set the alarm values and control ranges of the parameters, and check whether the automatic regulation and protection functions of the system are functioning properly.
Before the system is officially put into use, a trial run should be conducted. Observe whether the parameters of the separate oil supply system are stable, whether the lubrication condition of the thrust bearing has been improved, such as whether the oil film thickness is appropriate, whether the oil temperature is normal, and whether the wear condition of the bearing has decreased, etc. Based on the results of the trial run, further adjustments and optimizations should be made to the system.
After the modification, the oil film thickness of the thrust bearing was effectively guaranteed, the oil temperature remained stable within an appropriate range, the friction coefficient significantly decreased, reducing the wear of the bearing and increasing its service life. The characteristics of oil-gas lubrication is that compressed air is used as the power to convey the lubricating oil, which mixes with the two media to form a two-phase oil-gas mixed flow, and then is supplied to the lubrication point to lubricate the equipment. [9]
Due to the improved lubrication condition of the thrust bearing, the failure rate has significantly decreased, reducing the downtime of the equipment, increasing the operating rate of the rolling mill, and providing a guarantee for the continuous production of H-shaped steel.
By extending the service life of the thrust bearings, the equipment maintenance costs and the expenses for replacing the bearings were reduced. At the same time, the production efficiency was improved, the product output and quality were increased, bringing significant economic benefits to the enterprise. We actively promoted the policy of "production to drive advancement", guiding it from the policy perspective, and expanding the proportion and scope of the application of steel structures. [10]
The retrofit of the thrust bearings of the large H-type steel rolling mill with independent oil supply is a very necessary and effective measure. By designing and implementing an independent oil supply system, the problems existing in the original oil supply system were solved, the lubrication effect and reliability of the thrust bearings were improved, the equipment failure rate was reduced, and good economic and social benefits were achieved. This retrofit scheme also has certain reference value for other similar rolling mill equipment.
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[10] Gao Haijian, Liu Weihua, Jin Shengping. Current Production Status of Hot-Rolled H-Section Steel Rolling Mills in China and Problems in Their Promotion and Application [C]// China Metallurgical Society. 1999 China Iron and Steel Annual Conference Proceedings (Part II). Maanshan Iron and Steel Co., Ltd.; Maanshan Iron and Steel Co., Ltd.; Maanshan Iron and Steel Design Institute, 1999: 331-335.
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