The continuous expansion of the mining industry and the introduction of large mechanical equipment in mine operations have promoted the improvement of mine efficiency. Many large hoist motors use rolling bearings, which can better serve the application of mining. However, the sliding bearing hoist motor is also often used in mining, which can extend the service life of the sliding bearing and prolong the service life of the hoist motor. Of course, the maintenance of sliding bearings is difficult and constrained by various factors, resulting in negative effects on the operation of hoist motors in coal mining. Therefore, it is very important to maintain and repair the sliding bearings of large hoist motors.
1. Conditions for liquid friction of sliding bearings
The lubrication form of sliding bearings is liquid, and the conditions for its formation are relatively strict. It relies on the dynamic pressure of lubricating oil to support the rotating shaft in a suspended state, which forms liquid lubrication and becomes the friction condition of the bearing. The thickness of liquid lubricating oil is more important. If the lubrication amount is small, it will increase the roughness of the friction surface between the shaft diameter and the bearing, causing the axis of rotation of the shaft diameter to shift. Mild cases may affect the sliding of the bearing, while severe cases may damage the thickness of the Z small oil film, leading to lubrication failure between the shaft diameter and the bearing.
Under the friction conditions of liquid lubrication, sliding bearings can push the rotation direction of the journal and form a floating eccentricity. Eccentricity is the center distance between the journal and the bearing hole that is smaller than the static environment. It is related to the speed of the bearing. The faster the bearing speed, the smaller the eccentricity will be. At this time, a dynamic pressure effect is generated in the bearing, which can widen the wedge-shaped clearance and reduce the sliding and bearing capacity of the bearing journal. Therefore, the dynamic pressure of sliding bearings is directly proportional to the operating speed of the journal, and the dynamic pressure effect is inversely proportional to the eccentricity. This summarizes the conditions for liquid friction in sliding bearings: firstly, a wedge-shaped clearance and sufficient operating speed; Secondly, sufficient and sustained lubricating oil; Thirdly, the coaxiality between the journal bearings.
2. Structure and materials of sliding bearings
The structure of dynamic pressure sliding bearings for mining hoist motors can be divided into two types: integral and split. And bearing materials are also divided into two categories: tin based Babbitt alloys and lead based Babbitt alloys. These two materials have the advantage of low friction coefficient, which can increase the adsorption of lubricating oil, thereby reducing the friction of bearings and improving the overload capacity of the hoist motor.
3. Maintenance of sliding bearings for large-scale mining hoists
3.1 Maintenance process of sliding bearings for large-scale mining hoists
Based on practical experience, there are seven main aspects to the maintenance process of sliding bearings for large-scale mining hoists: suspending the skip at the right time, then locking and cutting off the drum and motor power supply respectively, and measuring the gap size between the motor rotor and stator; Secondly, remove the coupling housing and measure the gap size between its end faces; Thirdly, remove the stator casing of the motor from the slip ring, motor bearing cover, oil pipeline, and bearing pad, and then measure the bearing clearance and shaft level; Fourthly, magnetic particle testing is used to detect the wear condition of the shaft circumferential surface at the bearing seat, and then processing is carried out based on the results; Fifth, remove the bearing shell and inspect its wear and correction, then clean the bearing seat; Sixth, install the bearing pad and motor rotor in place according to relevant specifications, and then measure whether the shaft levelness meets the standard. After meeting the standard, install the bearing upper tile and measure the distance between the upper and lower tiles to ensure compliance. If it is not compliant, adjust it accordingly; Seventh, reset and install the lift components removed during the above maintenance one by one, and restore the lifting system. After that, perform debugging to ensure its normal operation.
3.2 Maintenance of sliding bearings during operation
3.2.1 Temperature detection
The common problem with running bearings is that the temperature is too high, causing the clearance of the sliding bearing to be reduced, which is not conducive to the operation of the sliding bearing. At this time, a heating resistor is added to the temperature measuring hole of the sliding bearing, and a thermal resistor is used to detect the online temperature of the sliding bearing. If the temperature is too high, the thermal resistor will automatically transmit an alarm function, which will be reflected to the main interface through the PC standard interface to remind the timely detection of high bearing temperature faults and reduce serious bearing failures such as rolling and holding.
3.2.2 Detection of Operational Vibration
The excessive amplitude of sliding bearings directly affects the clearance of sliding bearings and the peeling of bearing alloy materials, resulting in operational failures. At this time, displacement sensors are used to detect the online amplitude of the sliding bearing, and a PC standard machine is installed on the bearing seat of the sliding bearing to automatically alarm the bearing, remind timely repair of amplitude faults, and reduce the possibility of bearing detachment.
3.2.3 Regular Ferrographic Analysis
Iron spectrum analysis can detect the lubrication level of lubricating oil and the friction debris generated by bearing friction, and determine the size, shape, and composition of the friction debris, which directly affects the performance of bearing materials, especially poor lubrication can have a significant impact on Babbitt alloy. Usually, ferrography analysis can detect the degree of wear of sliding bearings. When blue or red spots appear, it indicates insufficient lubrication and severe friction, which will remind the level of bearing maintenance.
3.3 Maintenance content of sliding bearings
The integrity of bearing material properties, the degree of friction and surface quality performance of alloys, and the size of bearing clearance during maintenance; Secondly, the actual contact position and angle between the journal and the bearing during operation; Thirdly, the fitting ratio and quality of the outer circle of the bearing; Fourth, the wear amount, wear roundness, wear surface thickness, and wear cylindricity of the bearing; Fifth, cracks and peeling of bearing alloys and bearing shells, as well as the degree of bonding between alloys and shells; Sixth, after installing the bearing pads, ensure that the rotor of the motor is in place, measure the horizontal distance of the bearings, and adjust the clearance of the bearing pads; Seventh, the reset status of the dismantled hoist components, and the debugging and operation of the restored hoisting system.
3.4 Maintenance of bearing alloys and bearing shells
Strike method: Strike the bearing shell and identify the peeling situation of the shell based on the striking sound. If the striking sound is clear and clean, it indicates that there is no peeling situation of the shell. If there is a hoarse or obvious amplitude of the striking sound, it indicates that the surface of the shell is worn and there is a possibility of peeling; Secondly, oil immersion method: Apply powder on the surface of the bearing and the joint of the bearing shell, soak the bearing shell in kerosene, and take it out every half an hour to observe whether there are any wet marks of kerosene immersion on the powder. Wet marks are a warning of poor bonding quality; Thirdly, penetrant testing method: Flaw detection agent is a testing agent composed of cleaning agent, penetrant, and display agent. Firstly, the bearing is wiped dry with the cleaning agent. After the penetrant is sprayed into the bearing and left to stand for half an hour, the display agent is sprayed. If there is a color change of the display agent at the joint between the bearing and the bearing, it indicates poor peeling.
3.5 Scratching and Grinding Repair of Sliding Bearings
Through the above tests, there are no abnormalities in the peeling of the bearing alloy and shell, and there are no serious defects in the bearing shell, which can be repaired by scraping to ensure the operation of the sliding bearing; Selection of scraping and grinding tools: choose triangular scraper or snake head scraper, which can soak oil stones to polish the scraping surface; Secondly, auxiliary materials: Display agents are commonly used auxiliary materials that can display the contact between the scraping surface and the grinding surface. As the blending material of the display agent is red lead powder and engine oil, it can react to the contact point and judge the display situation based on the flashing of the needle grinding point; Thirdly, the scraping steps of sliding bearings: Firstly, rough scraping is used to scrape the scraped surface with the longer cutting edge to improve the contact point of scraping. Next is fine scraping, where the working angle of the scraper is smaller than that of coarse scraping, shortening the cutting edge to reduce the scraping point and conforming to the scraping direction. Once again, it's fine scraping. The further reduction of the working angle improves the accuracy of scraping and shortens the cutting edge, scraping off the shiny parts of the grinding points and leaving light and smaller grinding points; Fourthly, the scraping quality of bearings: Important reference indicators for scraping quality include the number of contact points, contact angle, top clearance of the journal bearing, etc., and ensure the distribution quality of contact points on the working surface of the bearing. In addition, the range of contact angle is compressed to between 90 degrees and 120 degrees, which can improve the clearance of the bearing and enhance the lubrication of the sliding bearing.
4. Conclusion
In summary, the maintenance of sliding bearings in large mining hoist motors is a daily work in coal mine production. Sliding bearings are an important component of hoist motors. Understanding the operating mechanism of the system, solving common problems with sliding bearings in a targeted manner, and improving the application value of sliding bearings.
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