The working principle, structure, inspection and maintenance of sliding bearing for mining heavy machinery and determination and adjustment of sliding bearing clearance.
Abstract: by analyzing the lubrication mechanism of sliding bearing of mining heavy machinery, this paper determines the key points of sliding bearing management and maintenance, expounds the necessary means and methods of sliding bearing monitoring and maintenance, analyzes the process of sliding bearing scraping and various disadvantages and causes, and summarizes the systematic method of sliding bearing management and maintenance.
Key words: Sliding bearing; Scraping and grinding; Gap adjustment
1. Working principle and structure of sliding bearing
1.1 Sliding bearings rely on the dynamic pressure of lubricating oil to support the rotating shaft in the suspended state to realize liquid lubrication.
The establishment of liquid friction between journal and bearing requires three stages. (1) Stationary stage (as shown in Fig. 1 (a)): in the stationary state, the journal is in contact with the lowest point of the bearing. Since the journal has not yet rotated, there is no friction, but a wedge-shaped gap is formed between the journal and the bearing. (2) Starting stage (as shown in Fig. 1 (b)): when the journal starts to operate, the journal climbs up along the right side of the inner wall of the bearing. At the same time, the lubricating oil rotates with the shaft due to its viscosity and adhesion to the journal, squeezing into the wedge gap to form dynamic pressure, which forms a resultant force to the left to push the journal to the left. (3) Stable stage (as shown in Fig. 1 (c)): with the increase of journal speed, the dynamic pressure of wedge clearance further increases. When the pressure increases enough to balance the journal load, the journal is suspended by pressure oil in the bearing, so as to realize liquid friction. When the working speed of the machine is reached, the journal runs stably to the left of the bearing.
Fig. 1 liquid friction stage 3 (set static downward)
The oil layer between journal and bearing is usually called oil film, and the minimum oil film is the clearance necessary to realize liquid friction. When the minimum oil film thickness is insufficient, if the working surface roughness of journal and bearing is too low or the rotation axis of journal deviates, the minimum oil film thickness will be damaged and the hydrodynamic lubrication between journal and bearing cannot be realized.
Under the condition of liquid friction, the center of the journal will shift and float along the rotation direction. At this time, the center distance between the journal and the bearing hole - eccentricity is obviously less than that in the stationary stage. With the increase of journal speed, the eccentricity will decrease, so the wedge clearance will further increase, and the increase of wedge clearance reduces the dynamic pressure effect - resulting in the decrease of dynamic pressure. Therefore, the bearing capacity of the journal also decreases. The analysis shows that the dynamic pressure of the sliding bearing increases with the increase of the journal speed. With the increase of the dynamic pressure, the eccentricity between the journal and the bearing decreases, so the wedge gap increases, resulting in the decrease of the dynamic pressure. Therefore, the following conditions are required to push out the liquid friction of the sliding bearing:
(1) With proper wedge clearance, sufficient speed and correct geometry. (2) Lubricating oil of appropriate viscosity, continuous and sufficient supply. (3) Journal and bearing have certain machining accuracy and coaxiality.
1.2 Structure and materials of sliding bearing
According to the structure, the radial pressure sliding bearing of mine hoist is usually divided into integral type and split type. The bearing materials are generally tin based Babbitt alloy and lead based Babbitt alloy. It has the advantages of high wear reduction, oil adsorption, plasticity and low friction coefficient. It is suitable for high-speed and heavy-duty environment.
2 Inspection and maintenance of sliding bearing
2.1 Inspection means and methods of sliding bearing in operation
(1) Online temperature monitoring: when the clearance of sliding bearing is too small and the bearing is rolled, the temperature usually increases. PT100 thermal resistance is installed with the help of the temperature measuring hole of the sliding bearing, which is monitored online through the standard interface of PC when the temperature is over, the alarm will be given automatically to avoid serious tile grinding and holding accidents.
(2) Online vibration monitoring: when the clearance of sliding bearing is too large and the alloy is peeling off, the amplitude is usually increased. The displacement sensor is installed on the sliding bearing pad seat, and the amplitude is monitored online through the standard interface of PC when the limit is exceeded, the alarm will be given automatically, which can be handled in time to avoid subsequent mechanical accidents caused by pad falling off.
(3) Regular ferrographic analysis: ferrographic analysis technology can measure the concentration of wear debris in lubricating oil and judge the shape, size and composition of wear debris. If the sliding bearing is poorly lubricated or the oil film is damaged during startup and shutdown, oxidized Babbitt alloy can be produced. With the help of ferrography microscope composed of double light microscope and reader, it will be found that it is black with blue and red spots. The number of particles indicates the wear degree of sliding bearing. With the help of ferrography analysis technology, we can regularly grasp the working conditions of sliding bearings and determine the maintenance measures and maintenance time.
2.2 Main contents of sliding bearing maintenance
(1) Wear condition and surface quality of bearing alloy: including bearing clearance, wear amount and wear property. (2) Actual contact angle between journal and bearing. (3) Fit quality of bearing outer circle. (4) Wear condition of Journal: including wear amount, roundness, cylindricity and surface roughness. (5) Whether the combination of bearing alloy and bearing shell has cracks and peeling.
2.3 Common inspection methods for the combination quality of bearing alloy and bearing shell
(1) Knocking method: tap the bearing bush with a small hammer. If the sound is clear and clear without noise, it indicates good bonding. If the sound is hoarse or the hand feels aftershock, it indicates peeling on the bonding surface.
(2) Oil immersion method: immerse the bearing bush in kerosene for about half an hour, take it out and wipe it dry, apply chalk powder at the joint between the bearing alloy and the shell, and stop for a period of time. If the chalk powder is wet with kerosene to expose the trace, it indicates that the bonding quality is poor.
(3) Penetrant flaw detection method: the flaw detection agent is composed of cleaning agent, penetrant and display agent. When using, first clean the parts to be inspected with cleaning agent and wipe them dry, then spray the cleaned parts with penetrant, and spray the display agent for about 15min. If there is shelling at the inspection part, the trace of penetrant color will seep out at the joint.
2.4 Repair of sliding bearing
There is no serious peeling between the bearing alloy and the bearing shell, the wear of each part has not exceeded the limit, and the bearing bush without serious defects on the surface of the bearing alloy can be repaired by scraping and grinding.
2.4.1 Scraping and grinding tools and their grinding
(1) Scraping tool. There are mainly triangular scraper and snakehead scraper. The triangular scraper should leave 2 ~ 3mm edges and sharpen three edges. The snakehead scraper mainly sharpens the circular arc surfaces on both sides and two planes.
(2) Auxiliary material - display agent. The display agent is usually mixed with red lead powder and engine oil. When used for rough scraping, the red lead powder should be thinned to facilitate coating, and the display speed is fast and the display point is large, which is conducive to improving the scraping efficiency; During fine scraping, thicker display agent shall be used to truly display the size of the contact point. Generally, it is not suitable to apply display agent again during final scraping. During scraping, the display agent can be applied to the scraped workpiece or the lapping workpiece. The former shows no flash at the research point, which is easy to observe and see clearly. The latter has only the high point coloring of the workpiece, and the grinding point appears dim, but it also reduces the number of times of coating the display agent.
2.4.2 Scraping steps and requirements of sliding bearing
(1) Rough scraping: when the workpiece has obvious machining knife marks or the working surface of the repair part has obvious scratch and wear, it shall be scraped first. The knife marks of rough scraping can be longer, and the parts with larger positive rake angle and larger arc radius of the scraper shall be used to increase the scraping area and improve the scraping speed. When the contact point of butt grinding reaches 4 ~ 6 points, the rough scraping can be ended and enter the fine scraping.
(2) Fine scraping: the actual working angle of the scraper shall be less than that of the coarse scraping. The scraping point shall be relatively small by shortening the blade trace and using the part with small arc radius of the scraper. The scraping direction shall be changed every time, and it is appropriate to have 45 ° ~ 60 ° reticulated state.
(3) Fine scraping: further reduce the rake angle of the scraper, or even adopt negative rake angle, and further shorten the length of the knife trace, so as to improve the scraping accuracy and avoid dents. During fine scraping, pay attention to the light falling of the knife and the rapid lifting of the knife. Generally, only one knife shall be scraped on each grinding point, which shall not be repeated, and shall be carried out cross all the time. During fine scraping, the largest and brightest grinding points shall be scraped off. A part of the medium grinding point shall be scraped off at the top, and the light and small grinding point shall not be scraped.
(4) Scraping oil filling points: scrape out the oil filling points according to the cross evenly distributed and orderly arrangement to store lubricating oil.
(5) Scraping and lapping quality of bearing: scraping and lapping quality mainly includes three indexes: contact points, distribution quality of contact points, contact angle and top side clearance between journal and bearing bush, that is, in addition to the requirement that every 25 ×25mm2 the contact points on the area and the gap between shaft and bearing bush, the distribution quality of contact points on the working surface of each bearing bush shall be evaluated at the same time. The contact angle is generally controlled within 90 °～ 120 °. Generally, the contact points in the middle section of the bearing bush are scratched thinly, and the contact points at both ends of the bearing bush are scratched densely, which can make the middle clearance of the bearing bush slightly larger, so as to improve the lubrication. At the same time, the two ends of the bearing bush are closely matched, so that the bearing bush will not leak oil.
(6) Disadvantages and prevention methods in scraping (Table 1)
(7) Size of oil groove: generally, the shape of oil groove can be divided into axial “linear” groove, oblique “cross” groove and radial “King shaped” groove. The diameter of oil hole is generally 0.02 times the diameter of journal plus 4mm. The width of oil groove should be slightly wider than the diameter of oil hole.
Table 1 disadvantages and prevention methods in scraping
1. Scraper tilt
2. Too much force
3. The cutting edge is too curved
1. When scraping, hold the knife stably without deflection
2. Reduce stress
3. Proper arc trimming of cutting edge
1. Scraping is carried out in one direction
2. Parallel edge during edge scraping
3. The blade extends out of the workpiece too much
1. Cross scraping
2. The scraper is at an angle of 45 ° to the edge
3. The blade extending out of the workpiece shall not exceed 1% of the tool width Π four
1. The edge is not sharp
2. Notch or crack on the cutting edge
1. Sharpening of cutting edge
2. Grinding edge
3. Determination and adjustment of sliding bearing clearance
3.1 Determination of radial clearance of sliding bearing
(1) The radial clearance of sliding bearing is usually selected according to the table of quality standard for maintenance of mechanical and electrical equipment in coal mine, and the side clearance is generally half of the top clearance.
(2) The axial clearance of sliding bearing is generally ≤ 2mm, but the minimum is not lower than the possible thermal expansion elongation of journal.
3.2 Measurement of sliding bearing clearance
(1) When measuring the axial clearance of sliding bearing, move the journal to the extreme position, measure with feeler gauge or dial indicator, and add the two extreme data to obtain the axial clearance.
(2) When measuring the side clearance of sliding bearing, the feeler gauge shall be used for measurement, and the insertion length shall be 30 ° arc length.
(3) When measuring the top clearance of sliding bearing, feeler gauge or lead wire pressing method can be used. The result of lead wire pressing method is more accurate. The diameter of lead wire shall be 1.5 ~ 2 times of the specified top clearance, the length shall be 30 ~ 40mm, and the lead wire shall be soft. During the measurement, the lead wire is coated with a little grease and placed at the position shown in Figure 2. Cover the bearing cover, tighten the studs evenly, loosen the bolts, measure the thickness of each section of lead wire and calculate the top clearance.
A1= (a1+b1)/2 A2= (a2+b2)/2
Top clearance =(( c1-A1) + (c2-A2))/2
Fig. 2 lead wire layout of lead pressing method
3.3 adjustment of sliding bearing clearance
When the top clearance of sliding bearing exceeds the limit value, the thickness of pad opening adjusting gasket shall be reduced for adjustment. At the same time, the contact angle between journal and lower bearing pad shall be checked. When the contact angle exceeds 120 °, the lower bearing pad must be scraped.
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