Slewing bearing, also known as turntable bearing and rotary bearing, is a kind of large bearing that can withstand combined loads, and can also bear relatively large axial loads, radial loads, and overturning moments. However, the slewing bearing itself has a relatively large volume, while the cross-sectional area of its parts is relatively small. In addition, in order to meet the requirements of actual work, precision machining is required. In the processing of slewing bearings, the machining steps are relatively long, deformation is likely to occur, and the machining difficulty is relatively great. This paper adopts corresponding process measures for each supporting ring and outer ring of the slewing bearing and analyzes the technological process, aiming to provide a strong reference for improving the actual quality of slewing bearings in practical production and machining.

Keywords: slewing bearing; machining process; quenching; deformation

Based on the characteristic that slewing bearings can withstand combined loads, they have been widely applied in China’s lifting and transportation industry. Slewing bearings are fully used in some port machinery, ship production and transportation, as well as other devices requiring large-scale slewing motion. The actual load-carrying capacity of a slewing bearing directly determines its actual quality in use, and the main factors affecting slewing bearings lie in the depth and uniformity of the raceway hardened layer, the raceway curvature radius, and the raceway contact angle. This article studies the various production processes of slewing bearings, laying a foundation for optimizing the existing slewing bearing machining process.

1. Difficulty of Slewing Bearing Machining

The reasonable machining of slewing bearings requires a large investment of manpower and material resources. First of all, the low machining efficiency of slewing bearings is the main problem in the current machining process. In order to meet the requirements of raceway shape, profiling, templates, and forming tools need to be used in coordination during the machining process. Furthermore, the machining process of slewing bearings places relatively high requirements on machining workers. Workers need to possess professional expertise. Long labor time, high labor intensity, and certain operational risks are characteristics of slewing bearing machining. Finally, machining accuracy is also one of the important factors to be solved in slewing bearing machining. With manual machining methods, the raceway surface presents a serrated irregular pattern, which cannot ensure the curvature ratio of the raceway, thereby making the hardness layer and surface hardness distribution after quenching uneven and unable to meet actual technical requirements.

2. Machining Process of Slewing Bearings

2.1 Actual Selection in Blank Forging

In the machining of slewing bearings, the supporting ring, outer ring, and fixing ring should be machined separately. Among them, the supporting ring and fixing ring belong to thin-walled annular parts. If single-piece blank forging is adopted, it will cause a large accumulation of blank allowance. In order to prevent thermal deformation during the machining of such thin-walled annular parts, it is necessary to ensure an increase in the allowance for quenching and tempering. In the machining of the fixing ring and supporting ring, since their inner and outer circular dimensions are relatively close, through reasonable analysis of the two, large forgings can be forged to reduce the difficulty of machining. In actual machining work, the fixing ring and supporting ring can be forged as one integral piece, so that the blank forging in the slewing bearing consists of two pieces, namely the outer ring and the combined forging composed of the fixing ring and supporting ring.

2.2 Rough Machining of the Blank

Rough machining of the blank is an important measure to shape the blank and is also an important procedure before quenching and tempering. An important step in rough machining is to separate the fixing ring and the supporting ring, ensuring that the work of the two does not conflict with each other. However, when separating the fixing ring and supporting ring, full consideration should be given to the allowance between them to ensure sufficient allowance. During the heat treatment process, the blank tends to expand when heated. If the fixing ring and supporting ring are not separated, then in the later semi-finishing process, the hardness after quenching and tempering will be too high, and the corresponding turning process and cutting process of the large ring will be relatively difficult, making errors or inadequate processing likely to occur. Looking at the whole technological process of slewing bearing manufacturing, they may first remain unseparated and be separated only after quenching and tempering is completed. This is because the amount of deformation is relatively small during the quenching and tempering process, and separating them after quenching and tempering is beneficial for better heat treatment, reducing the degree of thermal deformation of the parts during heat treatment, thereby ensuring the actual quality of the heat treatment procedure.

From the above analysis, it can be concluded that the fixing ring and supporting ring may not be separated before heat treatment, thereby ensuring that the machining allowance in the rough machining process is relatively small and the quality is guaranteed.

2.3 Quenching Treatment

In the machining of slewing bearings, the thickness of the hardened layer of the fixing ring and supporting ring is required to be controlled within 3–5 mm, and after surface quenching, the hardness should reach HRC 55–60. Because the slewing bearing itself has relatively large part diameters and relatively small cross-sectional dimensions, surface quenching is carried out after finish machining to the required size. If it is found that the deformation of the part itself is relatively large, it can only be scrapped. This method of final surface quenching poses a relatively great threat of deformation to the part itself and results in relatively great part loss. If quenching is carried out first, it is necessary to leave an additional 3–5 mm allowance on the original hardened layer to ensure that the part can be reasonably machined later. For cases where the allowance is smaller than the deformation amount, the part will inevitably be unable to meet the relevant later process requirements and must be scrapped. When the allowance is greater than the deformation amount, although the part can be machined, the machining difficulty is relatively great and the time consumed is relatively long. Practice has shown that when semi-finishing the part, the allowance on the quenched surface (raceway surface) should be controlled at 2 mm, and the hardened layer thickness should be maintained at 5–7 mm. During the quenching process, multi-point support can be used to ensure that the amount of deformation can be controlled within 1.5 mm, leaving a 2 mm allowance. After quenching and tempering, the hardness of the part should first be inspected. After ensuring that the hardness of the part meets the relevant standard requirements, the supporting ring, fixing ring, and outer ring should be separately inspected to ensure that the deformation amount of the supporting ring, fixing ring, and outer ring is less than or equal to 1 mm, with a minimum controlled at 0.70 mm, and it must not exceed the allowance. During re-machining, more supports should also be used, the allowance on the quenched surface should be controlled at 1.5 mm, and the more support points there are, the allowance can be further reduced (it may be reduced to about 1 mm).

2.4 Finish Machining of Slewing Bearings

During the turning process, since the hardness of the slewing bearing is between HRC 59–60, special tools (special alloy tools) should first be used for initial turning. The turning method should adopt low speed and small feed cutting. On the basis of ensuring fine machining of the part, attention should also be paid to the reasonable protection of the tool, so as to avoid tool damage and large machining errors caused by excessively high turning speed. When turning reaches an allowance of 0.02 mm, a bowl-shaped grinding wheel is used to grind the part. After grinding to the required size, a polishing abrasive belt is used for polishing, ensuring that after the size reaches the requirement, the roughness accuracy can also meet the relevant requirements planned in the drawing, thereby laying a solid foundation for manufacturing high-quality slewing bearings.

3. Matters Requiring Attention in the Slewing Bearing Machining Process

First, in order to ensure the actual quality of slewing bearings, it is necessary to ensure the quality of the materials put into use. The quality of 50Mn steel can be improved and enhanced. The content of chemical components in the material should be well controlled, and the mixing of non-metallic materials should be reduced, thereby improving the quality of slewing bearings.

Second, excessive blank machining allowance will also lead to greater cutting stress, resulting in increased quenching deformation. To improve the precision of slewing bearings and the machining precision of parts, stress relief annealing should be carried out after rough machining.

Third, the design of the slewing bearing should be improved by adopting the advanced steel-ball arc raceway design. Supported by the process technology of this steel-ball arc raceway, the part can be directly polished after quenching is nearly completed, and can meet the relevant design requirements without grinding. This can effectively prevent cracks in the raceway.

Fourth, the depth of the hardened layer should be increased. Increasing the depth of the hardened layer can effectively ensure the wear resistance of the raceway surface of the slewing bearing, and also has a corresponding influence on the raceway strength of the slewing bearing. In actual machining work, the depth of the hardened layer can be enhanced (up to 3–4 mm) by increasing the heating power of the medium-frequency quenching unit and reducing the heating speed, thereby improving the actual strength of the raceway.

Fifth, the dimensional stability of the slewing bearing should be improved. For some ground parts, low-temperature tempering must be carried out to avoid dimensional deviation or cracks in the slewing bearing itself caused by grinding stress. Flame quenching on the raceway surface has lower cost, lower energy demand, and guaranteed hardened layer depth. Because of its own advantages in various aspects, this advanced flame-quenching process can be promoted and applied in the machining process of slewing bearings. When flame quenching is used, no cracks are found in the raceway; therefore, this technology can be put into practice.

4. Conclusion

In summary, for the analysis of the slewing bearing machining process, it is first necessary to analyze the possible problems encountered during the slewing bearing machining process as well as the hidden dangers existing in the machining process, and then analyze the actual machining process. In the process of analyzing the machining process, more attention should be paid to the matters requiring attention in the machining process, so as to study the possible problems that may occur in different machining processes. According to the causes of the problems, relevant measures for optimizing the slewing bearing machining process should be proposed, the traditional foundation should be continuously improved and optimized, and modern technology should be integrated into the slewing bearing machining process, so as to ensure the actual quality of slewing bearing machining.