Abstract: Combined with the structural characteristics and working conditions of rolling mill bearings, this paper introduces the precautions and relevant requirements in the assembly, use and maintenance of bearings, and analyzes the faults that are prone to occur during the operation of bearings and their preventive measures.
1 Introduction
Roller bearings are used to support the rotating rolls and keep the rolls in the correct position in the frame. Roller bearings should have the characteristics of small friction coefficient, sufficient strength and rigidity, long life, and easy roll replacement.
2 Analysis of rolling mill working conditions and damage causes
The rough rolling mill of Handan Steel Wire and Bar Plant is a two-roll horizontal rolling mill with harsh working conditions, high temperature and large dust. When the rolled piece passes through, the surface temperature drops rapidly due to the influence of cooling water, and the temperature is maintained at 70-90℃, while the internal temperature is still very high due to lack of cooling. Dust in the steel rolling process sometimes enters the rolling mill, resulting in a high damage rate of the rolling mill, and the damage of the rolling mill is mainly caused by bearing damage. Bearing damage accounts for more than 60% of the damage of the rolling mill. After a long investigation, the author found that many bearing damages are not caused by the quality of the bearing itself, but by the assembly quality and incorrect use of the bearing. It can be seen that the correct use method, reasonable assembly and maintenance of bearings are the key to extending the service life of bearings.
3 Structural analysis of four-row cylindrical roller bearings
Four-row cylindrical roller bearings are single inner sleeve and double outer sleeve comb-shaped cage bearings, which are mainly composed of outer sleeve, inner sleeve, rolling element and cage. This type of bearing has large load capacity, high limit speed and high rotation accuracy. The inner sleeve and outer sleeve are separable and interchangeable, and the geometric shape is simpler than other bearings. The rolling element and raceway of the bearing are both cylindrical, so a higher processing accuracy can be obtained, thereby ensuring the assembly accuracy of the bearing.
4 Factors affecting assembly quality and installation
4.1 Factors affecting 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 lead to uneven force on the bearing and reduce the life of the bearing. The bearing seat should be self-aligning to avoid unbalanced load on the bearing due to roll deflection.
(2) Whether the size, geometry, accuracy grade and tolerance range of bearing-related spare parts are consistent with the design.
(3) Whether the finish and hardness of the contact surface with the bearing are within the specified range, whether all clearances and interference fits meet the design requirements, etc.
4.2 Installation of the inner sleeve
The inner sleeve of the four-row cylindrical roller bearing should be interference fit with the roll neck. When installing, heat it to 100-120℃, usually with an induction heater or engine oil. Do not use a cutting torch to heat it. When the inner sleeve is installed on the roll neck, make it in close contact with the water retaining ring to prevent the water retaining ring from moving.
4.3 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, use a copper rod to gently drive the outer sleeve, roller, and cage into the bearing seat, and fix the end cover tightly on the inside. When installing the outer sleeve, pay attention to the markings on the end face and the cage end face. Do not install them upside down. Install them in the order of the initial state when the bearing package is unpacked to prevent the bearing from burning due to uneven force on the rollers. When installing the bearing, the bearing seat should be placed horizontally. After the bearing is installed, its force range should be marked for key inspection when replacing the rollers.
5 Bearing failures and preventive measures
5.1 Inspection of bearings during operation
According to the requirements of the process regulations, the bearings should be listened to, observed and measured regularly. Listening means using a specified instrument with one end facing the axial end cover of the bearing and the radial shell of the bearing seat, and the other end close to the ear to listen to whether there is any impact sound and mechanical friction sound during the operation of the bearing; observation means observing the operating environment, installation position, vibration offset, lubrication and other conditions to see if there is an unfavorable environment; measurement means using a thermometer and a vibration meter to perform quantitative testing on the bearing seat.
Through the above methods, it can be determined whether the bearing is in normal condition during operation and the corresponding measures to be taken. The common faults of bearings in operation and the preventive measures are shown in Table 1.
5.2 Inspection of bearings at rest
Check the fastening of related spare parts, whether each component is in the correct position, whether there is any looseness, etc. For the rough rolling mill of the wire rod plant, since the red billet needs to be turned 90 degrees, some rolls are subjected to large axial forces. Therefore, it is necessary to frequently check whether the roller bearing seat has been tightened and whether it is loose. In particular, there must be axial gap-free fixation between the operating side bearing seat and the rolling mill frame, and the matching clearance between the transmission side bearing seat and the rolling mill frame window should be reduced as much as possible to reduce the impact of the transmission shaft runout on the bearing. In addition, the bearing lubricant can be tested to check whether the lubricating oil has sundries, iron oxide, water, etc. The rough rolling mill is usually subjected to strong impact loads, the bearing vibrates greatly, and the grease is easy to lose. Therefore, the grease is required to have strong adhesion and can adhere firmly to the surface of the parts. A part of it lubricates the bearing on the raceway, and the rest is stored in the internal space of the bearing to play a sealing role. If external dirt invades the bearing seat, the first thing to be contaminated is the grease near the outside, causing wear on the surface of the bearing parts. As the dirt increases, the wear surface will increase, and cracks will form and gradually expand, eventually causing the rings to crack. In severe cases, the rollers, bearing seats and other related components will be scrapped.
5.3 Inspection of bearings after disassembly
Since the roughing mill has a large rolling volume per groove, the roller replacement cycle is long. After the roller is replaced, you can use a cleaning agent to clean the bearing, blow it dry with compressed air, and then check the rollers, cages, and raceways. If there are dents, pitting, etc. on the rollers and raceways, replace them according to the specific situation. At the same time, check the axial seal and radial clearance. After checking, add lubricant for standby.
6 Conclusion
After the correct installation, inspection, and maintenance of bearings were incorporated into relevant regulations, the accident rate of bearing burnout in rolling mills has been reduced. There are many factors that affect the service life of bearings, and a comprehensive management system and requirements should be formulated to better achieve the purpose of extending the service life of bearings.
