Industry News
Application and Maintenance of Sliding Bearings in Mine Elevators
Abstract: There are many types of bearings, which can be roughly divided into two categories: one is rolling bearings; One type is sliding bearings, which have been widely used in mine hoists due to their high load-bearing capacity, significant impact resistance and shock absorption effects, low manufacturing costs, and easy disassembly. Through the analysis of the bearing force and working principle of the sliding shaft of the main and auxiliary shaft gearbox, the cause of bearing damage was identified, and corresponding improvement measures were taken. Thus, strengthening the use and maintenance of bearings enables the reliable and stable operation of sliding bearings in large equipment, achieving the goal of extending the service life of sliding bearings.
Keywords: bearings; working principle; Reason for damage; improvement measures
1 Reasons for damage to sliding bearings:
1.1 Defects in the metallographic structure of the bearing itself
A、 Poor bonding between the lining and backing layers of the bearing: This may be due to poor soldering or poor temperature control when bonding the bearing with Babbitt alloy.
B、 There is gas present during tile hanging: it may be due to low pouring temperature or insufficient preheating of the bearing.
C、 Uneven internal grain of Babbitt alloy: The solidification temperature is too low, and the large grain size of tin antimony alloy leads to uneven internal grain of Babbitt alloy.
1.2 Assembly non-standard:
A、 The centerline of the bearing is not aligned with the centerline of the bearing seat, which leads to premature fatigue failure of the alloy layer.
B、 The bearing and bearing seat are not properly matched, resulting in sliding.
C、 Uneven or excessive bearing clearance.
1.3 Improper use:
A、 Foreign particles enter the bearing and scratch the bearing shell.
B、 Lubricating oil continuously oxidizes during use, producing acidic substances that corrode bearings.
C、 Insufficient viscosity of lubricating oil leads to a decrease in bearing capacity.
2. The working principle of sliding bearings:
Sliding bearings rely on the dynamic pressure of lubricating oil to support the rotating shaft in a suspended state and achieve liquid lubrication. In a stationary state, the main shaft is located at the Z-low point in the middle of the bearing due to its own weight, forming a wedge-shaped gap between the shaft and the bearing hole. When the shaft rotates, the lubricating oil rotates with the shaft due to its viscosity and adhesion to the shaft neck, squeezing towards the wedge-shaped gap, increasing the pressure in the wedge-shaped gap and establishing the starting pressure. The pressure in the wedge-shaped gap increases with the increase of the main shaft speed. When the pressure increases enough to balance the main shaft load (including the main shaft's own weight), the shaft is suspended in the bearing by the pressure oil, that is, the shaft is separated from the bearing by a lubricating oil, thus achieving liquid lubrication. When the sliding bearing works under liquid lubrication conditions, the center of the shaft is offset along the rotation direction and floats up.
3. The conditions for the formation of pressure oil film in sliding bearings:
The above analysis indicates that the dynamic pressure of sliding bearings increases with the increase of spindle speed; As the dynamic pressure increases, the center offset distance (eccentricity) between the shaft and the bearing hole also decreases, resulting in a decrease in the wedge angle, which in turn leads to a decrease in dynamic pressure. So, the necessary conditions for forming a stable load-bearing oil film are as follows:
3.1 Bearings need to have appropriate clearance, sufficient speed, and correct geometric shape.
3.2 Use lubricating oil of appropriate viscosity. Continuous and sufficient fuel supply is required.
3.3 The surface quality of the bearing joint surface is certain.
3.4 The bearing shell and journal should have a certain degree of coaxiality, and each journal should have a certain degree of cylindricity and roundness.
4. Friction of sliding bearings
Sliding bearings lubricated with lubricating oil may generally experience three types of friction between their sliding surfaces: boundary friction, liquid friction, and mixed friction
A. Boundary friction: Two sliding surfaces are separated by a boundary adsorption film formed by polar molecules in lubricating oil directly binding to the metal surface, or by an inorganic salt film formed by the chemical reaction between active substances such as sulfur, chlorine, and phosphorus and the metal surface. The friction coefficient is lower than dry friction, generally ranging from 0.1 to 0.3. The formation of boundary adsorption film or chemical film, as well as the load-bearing capacity of the boundary film, depends on the oiliness of the lubricating oil, and is independent of the viscosity of the lubricating oil.
B. Liquid friction: The two sliding surfaces are completely separated by a layer of lubricating oil (bearing oil film), and lubricating oil with high viscosity is easy to form a bearing oil film. Its frictional resistance is manifested as the internal frictional resistance of lubricating oil, therefore, the friction coefficient is very small, usually around 0.001-0.008, and only occurs in situations such as driving, parking, reverse rotation, rapid load changes, abnormal oil supply, etc., due to the destruction of the liquid friction state and the wear of the bearing.
C. Mixed friction: intermediate between boundary friction and liquid friction, therefore its friction coefficient has a large range of variation, usually around 0.01-0.1. Bearings with liquid friction lubrication conditions often exhibit mixed friction states during driving, parking, reverse rotation, or sudden changes in load, as well as abnormal oil supply.
5. Inspection and repair of sliding bearings
After a period of operation, the working surface of sliding bearings will inevitably experience a certain degree of wear. The bearing alloy may melt, strain, start, or even peel off, crack, etc. The back of the bearing will also wear and loosen at the joint due to long-term vibration. The above situation will lead to a decrease in rotational accuracy, an increase in frictional resistance, an acceleration of temperature rise, and even exceeding the allowable working temperature. The wear will accelerate sharply, and in severe cases, the bearing alloy may melt (burn) and lock with the spindle. Timely, reasonable, and high-quality maintenance of sliding bearings is crucial to ensure equipment safety, reliable operation, reduce maintenance costs, and shorten maintenance time.
5.1 Inspection of sliding bearings
A、 The main inspection contents of sliding bearings include:
① The wear condition and surface quality of bearing alloys, including bearing clearance, wear amount, and wear properties.
② The actual contact angle between the journal and the bearing shell.
③ The fitting quality of the outer circle of the bearing shell: including the axial stop, check for any looseness.
④ The quality condition of the journal: including the wear of the journal, including wear amount and roundness, cylindricity, and surface roughness.
⑤ The bonding quality between bearing alloy and tile shell: whether there are cracks and peeling phenomena between bearing alloy and tile shell; To provide a basis for repair and replacement.
B、 There are three methods for checking the bonding quality between bearing alloys and shells:
① Knocking method: Gently tap the bearing shell with a small hammer. If the sound is clear and there is no noise, it indicates good bonding. Conversely, if the sound is hoarse or there are aftershocks, it indicates peeling on the bonding surface.
② Oil immersion method: Soak the bearing shell in kerosene for about half an hour, remove and dry it, apply white powder to the joint between the bearing alloy and the shell, stop for a period of time, and check whether the white powder has any traces exposed by kerosene wetting. If there are wet traces, it indicates poor bonding quality.
③ Penetrant testing method: It uses a specialized dye penetrant for inspection. The penetrant consists of a cleaning agent, a penetrant, and a display agent. To use, first clean the area to be inspected with the cleaning agent and dry it, then spray the penetrant onto the cleaning area, let it stand for about 15 minutes, and then spray the display agent. If there is peeling at the inspection area, traces of penetrant color will seep out from the joint.
5.2 Repair of sliding bearings
The repair of sliding bearings can be divided into several steps, such as rough scraping, fine scraping, fine scraping, scraping the oil filling point, etc
① Rough scraping: When there are obvious machining marks on the workpiece or obvious strains and wear on the working surface of the repaired part, rough scraping should be carried out first. The rough scraping marks can be longer, and larger positive rake angles and larger scraper arc radii should be used to increase the scraping surface area and improve the scraping speed. When the contact point of the grinding reaches 4-6 points, rough scraping can be ended and fine scraping can be entered.
② Fine scraping: The actual working angle of the scraper should be smaller than that of coarse scraping. By shortening the cutting edge and using the part with a smaller radius of the scraper arc, the scraping point can become relatively small. The scraping direction should be changed every time it is scraped, and it is advisable to have a mesh pattern of 45 ° -60 °.
③ Fine scraping: further reducing the rake angle of the scraper, even using a negative rake angle, further shortening the length of the cutting edge to improve scraping accuracy and avoid dents. When precision scraping, attention should be paid to the light dropping of the blade and the quick lifting of the blade. Generally, only one scratch should be applied to each research point, and it should not be repeated and should always be done in a cross cutting manner. During the fine scraping process, all the bright spots of Z should be scraped off. A portion of the medium grit should be scraped off at the top, while the light and small grit should be left untouched. Regardless of rough scraping, fine scraping, or fine scraping, the application of the display agent should be uniform, the grinding pressure should be even and stable, and there should be no axial movement to avoid display distortion and false points. At the same time, the scraping plan for each related bearing should be considered comprehensively.
④ Scrape oil point:
A、 Scrape out oil storage points in an orderly and intersecting arrangement to store lubricating oil.
B、 Scratching quality of bearing shells: Scratching quality mainly includes three indicators: the number of corner points, the distribution quality of corner points, and the top clearance between the shaft neck and the bearing shell. In addition to the required contact points on the scraped surface every 25 × 25 square millimeters and the clearance between the shaft and the bearing shell, the distribution quality of contact points on each working surface of the bearing shell is also assessed.
6. Adjustment and measurement of bearing clearance
6.1 Bearing clearance: Bearing clearance is one of the important factors in forming oil wedges to obtain lubrication mechanisms. At the same time, it should ensure the rotational accuracy of the rotating parts and meet the requirements of rotational speed. The bearing clearance can be adjusted through the gasket at the dividing surface or directly obtained by scraping the upper tile.
6.2 Measurement of bearing clearance:
(1) The bearing clearance can be directly measured with a feeler gauge
(2) The top clearance of the bearing shell is usually measured using the lead wire pressing method, which involves placing several sections of lead wire with a diameter slightly larger than the clearance of the bearing shell along the axial direction, on the shaft neck and the four corner planes of the bearing shell. After tightening the nut evenly behind the bearing shell, tighten the middle plane, loosen the nut, remove the flattened lead wire, and measure its thickness in sequence (accurate to 0.01mm). The average thickness of the lead wire is the top clearance of the bearing.
In summary, through the above analysis and discussion of sliding bearings, the basic methods to solve the damage of sliding bearings have been obtained, which can further strengthen the use and maintenance of bearings and achieve the goal of extending the service life of sliding bearings.
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