EPEN Technology Knowledge: Repair Technology of Machine Tool Plain Bearing

  • Based on the common spindle bearing structure in general machine tools and taking various bearings as clues, this paper describes the repair methods adopted for various spindle components in China. In order to restore the original performance or accuracy of the machine tool through more reasonable repair technology, and improve or improve the structural performance of the spindle.

 We should fully grasp the characteristics of spindle structure and take corresponding technical measures when repairing machine tools.

 

 

 

Abstract: Based on the common spindle bearing structure in general machine tools and taking various bearings as clues, this paper describes the repair methods adopted for various spindle components in China. In order to restore the original performance or accuracy of the machine tool through more reasonable repair technology, and improve or improve the structural performance of the spindle.

 

Key words: Machine tool maintenance; Spindle adjustment; Plain bearing; Single oil wedge plain bearing

 

Repair of machine tool spindle parts spindle parts is an important part of machine tools. The rotation accuracy, stiffness, temperature rise, vibration resistance and other properties of spindle components have a great impact on ensuring the machining accuracy and roughness of the machine tool. The main factor is the structural type of spindle. The structural performance of spindle components depends on various factors, such as the bearing type and support point adopted by the spindle, the size proportion of spindle components and the position of transmission parts (gears, pulleys,...), the material of spindle and its heat treatment, the manufacturing accuracy and surface quality of spindle and bearing (box) holes, as well as the lubrication and heat dissipation of spindle components. The bearing structure adopted by the spindle components can be roughly divided into two types: plain bearing and rolling bearing. This paper focuses on the analysis and discussion of the maintenance technology of plain bearing.

 

1. Repair, assembly and adjustment of plain bearing

In traditional machine tools, due to the small speed change range of the spindle and the low maximum speed, plain bearing is more used as the spindle bearing. On all kinds of grinding machines, although the spindle speed of the grinding wheel frame is high, there are many plain bearings as the spindle bearing of the grinding wheel frame because it does not need to change speed frequently. Therefore, the application of plain bearing in machine tool can be briefly summarized into two extreme states: one is used as unimportant or low speed and low load spindle bearing. The other is used as the spindle bearing of high precision, high speed and high load machine tools. Plain bearings are usually divided into dynamic pressure type (dynamic pressure bearing) and static pressure type (static pressure bearing) according to their friction properties when the shaft runs in the plain bearing under working conditions and the conditions under which the lubricating oil forms oil film pressure. In the former, the lubricating oil is brought into the bearing clearance when the main shaft rotates, and the oil film pressure is established to balance the external load. The latter relies on the external oil pump to input the pressure oil between the main shaft and the bearing clearance to support the load. In recent years, with the progress of technology, a bearing integrating the characteristics of dynamic and static pressure bearing has been developed, which is called dynamic and static pressure bearing. When the main shaft rotates, the lubricating oil is brought into the wedge oil gap formed between the main shaft and the bearing. As the gap gradually narrows, the oil pressure increases and the main shaft is lifted to form an oil wedge. According to the form of oil wedge, dynamic pressure bearing can be divided into single oil wedge plain (dynamic pressure) bearing and multi oil wedge plain (dynamic pressure) bearing.

 

Both plain bearings must have eccentricity between the spindle and the bearing  (or clearance), there is dynamic pressure effect to form oil wedge, and the position of oil wedge changes with the direction of load. Due to the poor rigidity of the oil film, the rotation accuracy of the main shaft is low. When the main shaft rotates at high speed, the bearing bush can swing freely on the spherical support screw, so that the lubricating oil forms an oil wedge between the main shaft and the bearing bush to generate oil film pressure to support the main shaft load. The integral deformation bearing is composed of five bearing shells connected with each other by five steel sheets with a thickness of 0.5 ~ 0.7mm. The five long ribs on the back of the bearing are arc-shaped, and the radius of curvature of the arc is less than the radius of the inner hole of the box (or bushing), so the bearing is in contact with the inner hole of the box (or bushing) with five lines. And each long rib is not in the middle (symmetrical) position of each bearing bush, but deviates an angle in the rotation direction of the main shaft to form an oil wedge. Due to the back of the bearing  (i.e. circular arc back) the bus is a 1:20 cone, so when the bearing moves axially in the inner hole of the box (or bushing), the bearing can produce elastic deformation to adjust the gap between the main shaft and the bearing, and the Archimedes curve oil groove is pre-processed in its inner hole; A wedge is formed between the main shaft and the bearing to generate oil film pressure when the main shaft rotates.

 

2. Single oil wedge plain bearing

When the single oil wedge plain bearing relies on the rotation of the main shaft, the lubricating oil is brought into the gap between the main shaft and the bearing to produce dynamic pressure effect and form an oil wedge.

 

1Inner column outer tapered plain bearing

The spindle of grinding wheel frame of domestic m7120 horizontal shaft rectangular table surface grinder adopts inner column outer conical plain bearing. The inner hole of this bearing is cylindrical and the outer circle is conical. Many axial grooves are opened on the outer return surface of the single shaft for uniform contraction when adjusting the clearance. The number and distribution of grooves are closely related to the uniform deformation and roundness of the bearing inner hole. In addition, there is an open dovetail groove in the axial direction of the bearing, and there is a lifting screw in the groove. When the bearing clearance is adjusted, the screw is tightened to force the outer circle of the bearing close to the inner wall of the bushing to make up for the uneven shrinkage of the bearing. The fatal disadvantage of this bearing is that when adjusting the clearance between the main shaft and the bearing, the bearing must move axially in the box hole to force the shrinkage of the inner hole of the bearing. In addition, due to poor lubrication, the main shaft journal and bearing bore will be seriously worn, and even hold the shaft. Generally, the following steps should be taken to repair this type of spindle with complex structure: (1) repair the roundness, taper, surface roughness and coaxiality of the worn main shaft journal. It is generally believed that the quality of the spindle repaired by ultra-precision grinding is better than that repaired by grinding. (2) Check and repair the inner conical hole of the box with precision manufactured cone inspection rod (sleeve) to make it meet the requirements of shaping tolerance. (3) Taking the inner taper hole of the box (or bushing) as the benchmark, grind the outer cone of the bearing so that the contact rate between the outer cone of the bearing and the inner taper hole of the box is not less than 80%. During fine grinding, the bearing is sleeved on the mandrel whose outer diameter is equal to the diameter of the main shaft after grinding, and the inner hole of the bearing must be adjusted to slide fit with the mandrel diameter in a free state. (4) After the outer cone of grinding wheel main shaft and sleeve bearing is repaired, the inner hole of bearing can be scraped and grinded. During scraping and grinding, a grinding rod is used to replace the main shaft, and the size of the grinding rod is manufactured according to the actual size of the repaired main shaft and the size of the inner hole of the support. After the scraping and grinding of the bearing inner hole meets the requirements of the contact point, adjust the size of the bearing inner hole to a slightly tight plain fit with the grinding rod. At this time, the bearing will not be removed and will be cleaned directly in the box. Then grind the bearing with chromium oxide as abrasive to eliminate the knife marks and burrs of scraping inch. It is precisely because the structure of this type of bearing is aging and difficult to deform or deform unevenly during adjustment that people try to work hard on the deformation groove of the bearing, or deepen and widen the size of the deformation groove; Or insert bamboo chips with equal size into each deformation groove to force the bearing to deform evenly; or open more axial grooves to improve the deformation uniformity, which will actually become the basis of multi watt deformation bearing in the future.

 

2Inner cone outer cylindrical plain bearing

The grinding wheel frame spindle of domestic M7120A horizontal shaft rectangular table surface grinder adopts the inner cone and outer cylindrical plain bearing. The front and rear bearings of the spindle with this structure are supported by the plain bearing of the inner cone and outer column. The main shaft journal is a cone of 130, and the radial clearance of the bearing is 0.005mm, which is guaranteed by washer 1. The axial load of the main shaft is borne by the thrust washer 3, the spherical washer 4 and the spring. The pressure of the main shaft system is lubricated. The inner hole of the sleeve 2 is processed into a groove with an eccentricity of 0.8mm at the place where the thrust washer 3 is installed. The stored oil is squeezed into the long hole by the thrust washer 3 through the eccentric groove, pressed into the front and rear bearings respectively, and then flows back from the groove of the sleeve 2 to form an automatic circulation. When the main shaft rotates, an oil film is generated, and the thickness of the oil film is greater than 0.005mm (radial), push the main shaft forward to make the washer leave the end face of the front bearing to reduce the friction and heating of the end face. The problems of this structure are: the bearing is a single oil wedge and the axial drift is large. The position of oil groove is unreasonable, the circulation of lubricating oil in the bearing is poor, the oil film stiffness is low, and the bearing temperature rises. To solve these problems, in the improved structure, an oil wedge is formed on the inner hole of the bearing to improve the forced lubrication and heat dissipation conditions of the main shaft.

 

3. Conclusion

Therefore, when repairing machine tools, we should fully grasp the characteristics of spindle structure and take corresponding technical measures, so as to obtain better results and create greater economic benefits for enterprises.

 

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