1　Preface

For the rolling steel system, improving the effective operation rate of production is an important measure for "cost reduction and efficiency increase". The No. 5 bar rolling unit of Lingang Steel has continuously enhanced its production capacity as the rolling speed and equipment operation rate have increased. Due to the existence of unfavorable working conditions, high temperatures, and large amounts of dust on the rolling line, which affect the equipment, problems have gradually emerged during the operation of the rolling mill. The vibration of the rolling mill is significant, causing the connecting bolts of the pull rod sleeve of the rolling mill to loosen, and frequently resulting in damage to the rolling mill bearings, which requires replacement of the rolling mill. The service life of the rolling mill bearings is closely related to the quality of the bearings, as well as the quality of their installation and daily maintenance. Therefore, scientific and reasonable assembly and use, as well as strengthening daily maintenance management, can effectively increase the service life of the bearings and ensure the smooth operation of the rolling line.

2　Analysis of the Problems Existing in Rolling Machine Bearings

2.1 Problems encountered during the assembly process

(1) The roller manipulator and the small carts on both sides of the machine frame are not on the same horizontal plane, resulting in poor alignment. During the closing of the rolling mill, the inner sleeve of the rollers collides with the rolling elements of the bearings, causing the retaining rings of the bearings to be deformed and lose their ability to constrain the rolling elements. At the same time, the friction between the bearings and the inner sleeves causes the inner sleeves to be scratched, making the rollers less flexible during operation, which can also lead to bearing failures.

(2) After assembly, the height of the roller gaps on both sides of the rolling mill is not consistent or there is a misalignment phenomenon, causing uneven force on the rollers during operation, resulting in stress concentration in the bearings, which leads to bearing failures.

(3) The auxiliary bearings of the rolling mill are radial thrust bearings. Their characteristic is that they need to maintain a certain working clearance during operation. If the clearance is too large, it will cause the bearing's working area to decrease, increasing the stress on the rolling contact surface and reducing the motion accuracy; if the clearance is too small, it is prone to cause the bearing to heat up and increase the temperature rise, resulting in reduced bearing life, increased failure rate, and premature scrapping.

(4) When replacing the bearings, if a person uses a hammer to strike the outer ring of the bearing, the bearing will be subjected to concentrated stress and damaged. During the production process, an increase in the load on the bearings will also cause the bearings to burn out.

2.2 Problems with the lubrication system and seals

The lubrication of bearings utilizes an oil film to separate the moving surfaces from each other, preventing excessive wear and failure due to rough point contact. Maintaining an adequate amount of lubricating oil within the bearing housing is the key to ensuring the lubrication effect. Therefore, it can be said that lubrication is the foundation for the normal operation of bearings, and sealing is the guarantee for effective lubrication of the bearings. Thus, attention should be paid to: 

(1) When replacing the online rolling mill, the quick-connect joints must be cleaned thoroughly. Otherwise, after the oil pipes are connected, oil dirt and dust particles will be blown into the distributor, causing the oil passage to become blocked, resulting in insufficient oil supply to the rolling mill and causing the bearings to overheat. 

(2) Maintain the sealing condition of the bearings. Any defects such as irregular shapes, missing parts, or minor cracks in the sealing area will affect the sealing effect and have a significant impact on the bearings.

2.3 Problems existing during the power transmission process

The rolls are connected through the central cone sleeve and the universal shaft head to transmit the power from the reducer to the rolls. As the rotational diameter and moment of inertia of the reducer and the rolling machine increase, along with the large vibrations generated during rotation that are transmitted to the connecting shaft bracket and the rolling machine, the bouncing of the rolling machine will intensify, the internal holes of the rolls, the central cone sleeve and the flat head sleeve will wear more severely and the gaps will increase. This will impact the rolling machine bearings and subsequently affect the service life of the bearings.

2.4 The effects brought about by the rolling operation and the material type

The low heating temperature and large reduction amount of the steel billet have a significant impact on the bearings of the rolling mill. For instance, when rolling low-temperature steel, the deformation resistance during the rolling process increases, and the rolling force is transmitted from the rolls to the bearings of the rolling mill. Under the huge impact load of the outer ring, the bearings eventually break, leading to damage of the bearings. The No. 5 bar and No. 3 rolling mill of Lingang Steel have experienced multiple incidents of broken rolls and burning the rolling mill. Finally, by increasing the temperature and improving the process material parameters, the problem of broken rolls and burning the rolling mill was effectively solved.

3　Research on Improving the Bearing Scheme of Rolling Mills

3.1 Strengthen the assembly requirements for rolling mill bearings and enhance standardized and meticulous operations

(1) Before installing the rolling mill bearings, a detailed inspection of the relevant spare parts of the bearings should be conducted, including the roll neck, bearing housing, bearing rings and the bearing housing cover plate, etc. The surface of the mating parts should also be carefully inspected. Any burrs or sharp edges must be removed.

(2) During the assembly of the rolling mill bearings, the inner rings on the rolls should be carefully inspected and wiped.

(3) Re-measure and re-calibrate the hydraulic trolley, including horizontal and lateral alignment, to avoid mutual scraping during assembly. At the same time, strengthen the maintenance and repair of the roll changing mechanical hand to ensure the relative position and accuracy requirements of the trolley. Follow the assembly operation procedures strictly to ensure the assembly quality.

3.2 Improve the working conditions of the rolling mill bearings

(1) To reduce the wear of the center cone sleeve and the inner hole of the flat head, the center cone sleeve should be replaced regularly to ensure that the dimensions of the rolls, the center cone and the flat head are within a reasonable range of matching dimensions, thereby reducing the impact of unbalanced vibrations on the bearings of the rolling mill.

(2) Renovation of the lubrication pipes. The non-transparent pipes originally designed were modified, and an additional transparent section was added to facilitate on-site inspection, maintenance and repair of the oil and gas lubrication system.

(3) For distribution valves and oil-gas quick-change connectors with poor lubrication conditions, they should be replaced and inspected in a timely manner. During the process of changing the rollers, the oil circuit must be cleaned to ensure that the lubrication system has a smooth and clean operating environment.

3.3 Improve the precision requirements for bearing seals

Before assembling the bearings, it is necessary to carefully check the dimensional accuracy of the sealing groove at the sealing part and the roughness of the sealing surface to ensure that the sealing components do not fail. Special attention should be paid to the inspection of the sealing lip. Defective sealing components must not be installed in the bearing housing. For the material and hardness indicators of the bearing sealing components, they should be checked against the product certificate and delivery list, etc. Reasonable selection of the axial seals in the two static labyrinth chambers should be made to effectively prevent the splashing of cooling water and iron oxide scale and the overflow of lubricating oil.

3.4 Strengthen the intensity of daily inspections and patrols

The working environment of the bearing of the bar rolling machine is quite harsh. By regularly observing the radial outer shells of the bearing end covers and the bearing seats during operation, checking whether there are impact sounds and mechanical friction sounds, we can predict possible faults of the online rolling machine. If any abnormalities are found, the rolling machine should be replaced in advance. Strengthen the daily inspection of the operating environment, installation position, vibration offset, lubrication, bearing temperature, etc. of the bearings, and formulate corresponding inspection procedures to reduce the failure rate of the rolling machine bearings, and achieve the purpose of extending the service life of the bearings and ensuring the normal production of the rolling line.

3.5 Scientifically and reasonably organize the replacement of bearings

By using the scheduling report to track the production process of the rolling mill (both the upper and lower lines), the time during which the rolling mill's bearings are in contact with the steel can be calculated. Precisely calculate the amount of steel that the bearings come into contact with, and conduct regular offline inspections and replacements to prevent the bearings from burning out while in operation and to avoid affecting production.

4 Closing Remarks

The rolling system has high requirements for production continuity. The key points of equipment management should focus on precise assembly and process control, highlighting standardized operations, and conducting predictive maintenance for the equipment in advance. For rolling mills, regular maintenance is necessary, and good operating and usage habits should also be developed. Only in this way can the service life of the rolling mills be extended, the operation rate be improved, and better guarantees be provided for the stable operation of the enterprise's production.