The Wear and failure form of the grinding-carriage spindle and bearing bush and the repair of grinding wheel carrier spindle and sliding bearing.
Abstract: in the modern manufacturing industry, grinding machines are developing towards high precision, high speed and heavy grinding. The adoption of correct repair technology and reasonable assembly and maintenance technology is of great significance to ensure the accuracy of the grinding-carriage spindle and sliding bearing system, prolong the service life of equipment, reduce the loss caused by physical wear of equipment, reduce enterprise costs and improve enterprise economic benefits. However, the assembly and repair of grinding wheel frame spindle and sliding bearing is a technical work with strong technicality and great difficulty in repair and assembly in the maintenance technology of metal cutting machine tool. After years of production practice, the author has accumulated rich experience in this regard, mastered this technology, and achieved good results in practice. This paper introduces the author’s assembly and repair experience accumulated over the years.
Key words: Grinding-carriage Spindle, spindle, sliding bearing, assembly and repairing,grinding machine
In modern manufacturing, grinding machines are developing towards high precision, high speed and heavy grinding. The requirements for the design, manufacturing accuracy and assembly and repair accuracy of the grinding-carriage spindle and sliding bearing of grinding machines are higher and higher. Through experiments, we know that the assembly accuracy of the spindle and sliding bearing of the grinding wheel frame has a great impact on the grinding quality of the grinder. If the manufacturing accuracy and installation accuracy of the grinding-carriage spindle and bearing bush are ideal, its hydrodynamic lubrication can be guaranteed. If we do not pay attention to its characteristics and requirements and are careless in assembly and repair, it will affect the normal use of the equipment, cause adverse consequences such as shaft holding and bearing burning, and shorten the service life of the equipment.
1. Wear and failure form of the grinding-carriage spindle and bearing bush
The main shaft and bearing bush of grinding wheel frame of grinder mainly transmit speed and torque and bear load in work. Therefore, friction and wear will inevitably occur, and even faults such as holding shaft and burning bearing will occur. There are various reasons for various faults, mainly (1) the manufacturing accuracy and quality of the grinding-carriage spindle and bearing bush do not meet the specified technical requirements, and the bearing bush materials have sand holes, pores, looseness, etc. (2) the assembly clearance between spindle and bearing bush is small, the front and rear bearings of spindle, bearings and bearing installation holes are not coaxial Cause vibration; (3) the main shaft, bearing bush and related parts are not cleaned thoroughly; (4) the main shaft and bearing bush are not lubricated fully; (5) fatigue wear; (6) cavitation; (7) the lubricating oil used is not clean or the brand is incorrect; (8) the static and dynamic balance of grinding wheel main shaft and related parts is not done well; (9) there is no unloading device on the main shaft pulley, the belt tension is too large, etc.
Due to these reasons, the the grinding-carriage spindle and bearing bush will appear dry friction, wear, scratch, roughening, threading, tearing off, bite wear, or vibration, reduce accuracy, aggravate wear and reduce the service life of the equipment.
2. Repairing of Grinding-carriage Spindle
The spindle of grinding wheel frame is the key component of grinding machine, so when the spindle of grinding wheel breaks down, its repairability and economic value after repair must be considered. When repairing or replacing a new spindle, it is necessary to comprehensively consider the nature of the spindle failure, the location of the failure and the troubleshooting methods, combined with the accuracy, structural characteristics, material and process performance of the spindle, including the process and technical requirements of heat treatment, manufacturing conditions, manufacturing cycle and the process performance requirements of maintenance, and replace a new spindle when necessary.
Whether replacing a new spindle or repairing a faulty spindle, before replacement or repair, it is necessary to strictly check whether the material and surface hardness of the new spindle and the old spindle meet the technical requirements of the drawing and whether there are micro cracks. This is a very important key inspection item. But not all spindles require high hardness, which is related to the speed and load of the spindle. The hardness of the the grinding-carriage spindle with high speed and medium and high load is generally required to be 56 ~ 62Hrc. The hardness of the the grinding-carriage spindle with medium and low speed and medium and light load is not high, generally only 52 ~ 55HRC, and some can be even lower.
According to the practical experience of on-site repair, the loss of precision of the grinding-carriage spindle of grinder is mainly due to the wear and out of tolerance of sliding bearing. Under the condition of ensuring hardness, strength and stiffness, the main shaft journal with less wear can generally be repaired by grinding, grinding, polishing, fine grinding and super fine grinding to improve the surface roughness. During fine grinding, it shall be determined according to the physical wear of the journal. If the journal has slight scratch, deep scratch or galling, tearing and wear, or the journal bus is not straight and the shape and position tolerance is out of tolerance, it can be subject to fine grinding or super fine grinding, and finally ground and polished.
When grinding the spindle, the spindle accuracy must be restored by removing the minimum amount of the machined surface on the premise of ensuring the geometric accuracy of the spindle. The less the cutting amount, the better. Generally, it shall not exceed 0.1 ~ 0.15mm. Otherwise, the radius of curvature of the the grinding-carriage spindle will be less than that of the inner surface of the bearing bush, resulting in excessive clearance between the shaft and the bearing bush. If 20Cr and 38crmoaia are used for the grinding-carriage spindle, the thickness of carburizing and nitriding layer must be considered during heat treatment, and the grinding amount must be paid attention to, so as to prevent the carburizing or nitriding layer from being ground off and losing its hardness.
In addition, the correct assembly relationship between the main shaft and its matching parts or components shall also be considered, such as the tolerance fit of gear sleeve hole, worm hole, motor rotor hole, fan hole, flange hole, oil slinger hole, washer, gasket, etc. matched with the outer surface of the main shaft. And try to reach or restore the nominal size of the original parts or components. Otherwise, the spindle rotation accuracy will be affected.
For the main shaft with a large amount of wear or that has exceeded the adjustment range of the bearing bush after many overhauls, it should be considered to replace the new main shaft, or adopt different repair methods according to the wear of the journal, such as metal spraying method or chromium plating method. Before chromium plating, the old shaft must be rounded, vibration surfacing method to avoid thermal stress or insert method, etc. Finally, machining shall be carried out to restore the geometric accuracy and nominal size of the original spindle as far as possible. Of course, we should also take into account the economy and workmanship after repair, the length of manufacturing and repair cycle, the distance and quantity of purchased goods and other factors to optimize an economic and reasonable scheme.
The technical conditions of the main shaft are designed and formulated according to the function and working conditions of the main shaft, and the accuracy of the technical conditions is determined based on the front and rear journal supported in the bearing hole. Therefore, when checking and repairing the spindle and various accuracy, special attention must be paid to the front and rear bearing journals as the benchmark. The positioning accuracy of the two journals is located by taking one end of the central hole of the main shaft as the conical hole, because it is not only the benchmark of machining and measurement, but also the benchmark of assembly and repair. Therefore, when the spindle center hole or conical hole often loses the correct geometric accuracy due to wear or collision, it must be repaired on the high-precision center hole grinder to ensure that the technical requirements on the drawing are met. Attention should be paid to the accuracy of the central hole of the grinding wheel frame spindle of the grinder, especially when replacing the spindle or repairing the spindle, if there is no good central hole, even on the high-precision grinder, high-precision products cannot be processed.
The coaxiality between the outer circle of the cone of the grinding wheel flange installed on the spindle and the outer circle of the spindle bearing must be maintained. Therefore, when repairing the old spindle and polishing the worn parts of the two journals, the outer circle of the flange cone of the grinding wheel installed on the spindle should also be polished.
If the threads at both ends of the the grinding-carriage spindle are worn or bruised, the coaxiality requirements between the axis line of the thread surface and the axis line of the supporting journal must be ensured during repair, otherwise the axis line of the spindle and its related fasteners, pulley, flange and other axis lines will be skewed and lose balance, resulting in deflection torque and radial runout of the spindle. The threads at both ends of the repaired main shaft must be equipped with nuts according to the repaired thread size specifications. At the same time, the perpendicularity requirements between the end face of the nut and the axis line of the main shaft must be ensured, otherwise the refitting and maintenance will seriously affect the assembly accuracy of the main shaft.
If the keyway on the main shaft is worn, it can be solved by repairing welding or enlarging the keyway, preparing a new key and machining the hardness that can be achieved. During repair welding, attention shall be paid to the influence of thermal stress generated by welding and the reduction of hardness due to local annealing.
To sum up, the main part of the main shaft is the repair of two supporting journal, central hole, thread and keyway. The coaxiality of the two supporting journals is to ensure the coaxiality of the front and rear bearings, which is also the key to the repair of the grinding wheel frame spindle.
3. Repair of sliding bearing - scraping and grinding
The maintenance of various bearings is actually the process of scraping, grinding, assembly and adjustment. Whether it is three long bearing shells, three short bearing shells, five long bearing shells, five short bearing shells, two half bearing shells, cylindrical bearings and end face thrust bearings, their scraping and grinding have many commonalities.
In the scraping and grinding work of sliding bearing, there are two places worthy of attention: one is the matching of bearing, bearing seat and body shell, and the other is the matching of bearing inner hole and main shaft. The importance of the latter is well known, but sometimes the former is ignored. The bearing, bearing seat and body shell must be in good contact and ensure that the inner holes of the front and rear bearings are coaxial. Otherwise, even if the bearing bush is scraped well, there is still a risk of holding the shaft and biting the brake. If the front and rear bearings adopt short three pad bearings, since the back of the bearing bush is not in direct contact with the body shell, it can be self-positioned. Therefore, even if the front and rear bearings are slightly skewed with different shafts, as long as they can reach the geometric accuracy range of the machine tool and have little impact on the working conditions of the bearing.
After disassembling the bearing, if there is serious wire flow on the main shaft and bearing bush, i.e. roughening, stringing, etc., the main shaft must be finely ground. After fine grinding of the spindle, assemble the spindle and make dynamic balance. Do not rush to install the balanced spindle into the grinding head, because it is inconvenient to scrape it.
When disassembling the bearing, carefully mark the position of the parts, so that the parts will not be misplaced during scraping and assembly in the future, because most of these parts are single matched and cannot be interchanged.
Before scraping and grinding, the main shaft shall be cleaned with gasoline, and the indicator blue oil or red lead powder must be thin and uniform, otherwise false scraping points will appear.
Close the bearing bush on the main shaft for matching, grinding and scraping points. During rough scraping, the points shall be scraped out. Generally, the main shaft shall be placed horizontally, and about 12 scraping points within 25mm2 are appropriate. During fine scraping, about 18 points within 25mm2 lining range is appropriate. Whether it is rough scraping or fine scraping, the scraping points must be uniform. It should be noted that the sharp corners and burrs in the groove or edge of the bearing must be repaired.
If the bearing has no bite brake, or there are no serious silk flow and wear marks, after fine scraping of the bearing, the display agent, copper chips or Babbitt alloy chips can be cleaned, and then the polishing work can be carried out. Put the main shaft into the second "Fensheng sand brake" of the first two flashes of the second issue of the year of Jidong. Okadaikawa stayed in the pseudo shell and injected green oil oxidation path and a small amount of kerosene between the bearing and the main shaft for polishing. During polishing, the main shaft shall move slightly towards both ends, and the distance shall not exceed the length of the main journal.
The roughness of the spindle will deteriorate after the matching research and polishing. Put the spindle on the lathe, hold the metallographic sandpaper by hand, rotate and move left and right to polish the spindle. The special thick glass plate for roughening can also be coated with green oil and kerosene. While rotating, move the thick glass plate left and right, and polish the main shaft journal until it meets the technical requirements.
In the above process, pay strict attention to the cleaning work, and note that there shall be no hair, iron filings, copper filings, Babbitt alloy filings and other sundries between the contact surfaces, otherwise the matching accuracy will be changed and false scraping and grinding points will appear.
4. Pay attention to the following items when assembling the Grinding-carriage Spindle
(1) The the grinding-carriage spindle, rotor and fan must be subject to dynamic balance test on the dynamic balancing machine, which is required to reach level 1 and at least level 2. If the balance accuracy is checked on h010 dynamic balancing machine, the balance value shall reach 1 ~ 2 grids (1 grid is 1um) and shall not exceed 2 grids.
(2) Adjust the fit clearance between the grinding-carriage spindle and bearing. Change the built-in or motor rolling bearing or the rolling bearing at the rear end of the main shaft to national standard level D (old standard level a).
(3) For static balance of grinding wheel, it is better to balance until it stops at all points, and it is required to stop at least 8 points. For the new grinding wheel, it must be balanced twice, that is, the initial balance before trimming and the balance again after rough trimming. Pay attention to dressing the grinding wheel evenly and slowly and use coolant.
(4) Rubber washer and anti-vibration cushion block shall be added at the screw of non -built-in motor base.
(5) The assembly clearance between the the grinding-carriage spindle and the bearing bush should not be too large, and the spindle should not float in the bearing bush.
(6) Ensure good contact between the main shaft and the bearing bush, that is, good adhesion, and ensure the coaxiality of the front and rear bearings. At the same time, the coaxiality between the outer circle of the flange cone equipped with the grinding wheel and the outer circle of the main shaft bearing shall meet the requirements.
5. Commissioning after adjustment
(1) First, check whether the rotation direction of the spindle is correct. This must not be overlooked, otherwise all previous efforts will be wasted.
(2) Rotate the main shaft by hand and make sure that the main shaft is fully lubricated before jogging and starting. If looseness and tightness are found during hand rotation, it indicates that the bearing is not scraped properly and false points appear, so the bearing bush needs to be scraped again.
(3) Empty vehicle test. During commissioning, special attention shall be paid to the heavily scraped bearing bush. The bearing clearance shall not be too small (about 0.1mm). After 1 ~ 2h, it shall be removed for inspection. If there are hard points, they must be repaired. If there are roughening and wire lifting, they shall also be repaired. Repeat this method until the temperature rise is appropriate and the gap required in the manual is adjusted
(4) If the temperature rise of the bearing does not exceed 30 ℃, the oil temperature is usually 35 ~ 40 ℃ and the outlet temperature of the bearing oil film is 55 ~ 60 ℃ after running in for 4 ~ 8h without load, the test run grinding can be carried out with load.
(5) During operation, pay attention to the temperature and operation of the sliding bearing to prevent the melting of the bearing alloy, that is, to prevent the occurrence of tile burning accident. Generally, the temperature of the bearing shall not exceed 60 ℃.
If it is found that the bearing temperature is too high, do not stop immediately. Take effective measures in time, such as increasing the amount of lubricating oil, forced ventilation or cooling with water. Stop the machine after the bearing temperature drops. Disassemble the bearing and find out the cause until the fault is eliminated.
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In order to enhance the anti-seizure property of the sliding bearing while making the content of Pb contained in the Sn-based overlay layer smaller or while using no Pb if possible, there is provided a multi-layer sliding bearing comprising a back metal layer, a bearing alloy layer, an intermediate plating layer and a Sn-based overlay layer, the Sn-based overlay layer containing reinforcing metal and/or inorganic particles, the content of the reinforcing metal and/or inorganic particles being varied step-wise or successively in a direction of a thickness of the overlay layer so that the content in a thickness-wise central portion of said overlay layer is relatively high and so that another content in a surface portion of the overlay layer is zero or smaller than the content in the central portion.