In view of the problem that the outer ring of large thin-walled tapered roller bearings is easily deformed during processing, the characteristics and causes of deformation during turning, heat treatment and grinding were analyzed. Taking the outer ring processing of L555249/L555210 large thin-walled tapered roller bearings as an example, the shape control processing technology of the main processes is proposed.
Keywords: rolling bearing; tapered roller bearing; outer ring; deformation; turning; heat treatment; grinding
Processing deformation of the outer ring of large thin-walled tapered roller bearings
The processing of the outer ring of large thin-walled tapered roller bearings generally requires forging, turning, heat treatment, grinding and other processes. Among them, turning, heat treatment and grinding are the key processes to control the dimensional accuracy, internal characteristics and surface morphology of the outer ring. Processing deformation in each process is inevitable.
Turning deformation
Turning is an important processing method for cutting and forming outer ring blanks or forgings. The turning deformation is the cutting deformation caused by the clamping deformation and unreasonable turning process, which is manifested as the single plane outer diameter variation and the average outer diameter variation of the outer ring out of tolerance.
Heat treatment deformation
Heat treatment is a key process to improve bearing contact fatigue strength, wear resistance, hardness and dimensional stability. Heat treatment deformation mainly manifests as changes in outer ring diameter and end face warpage, which are mainly caused by thermal stress and structural phase transformation stress in the quenching/tempering process. Influencing factors include quenching/tempering methods, process parameters, and the shaping effect of the quenching mold in the heat treatment equipment.
Grinding deformation
Grinding is a key process to improve the geometric accuracy and surface quality of the outer ring]. Grinding deformation is mainly caused by grinding heat, grinding force and insufficient rigidity of the workpiece. The influencing factors include grinding process, process parameters and support methods, etc. It is manifested in the deviation of the width of the ferrule end face and the variation of the outer diameter of a single plane out of tolerance.
Shape control processing technology
Taking the outer ring of L555249/L555210 large thin-walled tapered roller bearing as an example, its structure and key dimensional accuracy are shown in Figure 1, and the material is GCr15. The process allowances after turning the outer ring width, outer diameter and raceway diameter of conventional tapered roller bearings with an outer diameter of 315 to 400 mm are shown in Table 1. On this basis, the extra light and ultra-light series outer rings increase by 0.1 to 0.2 mm.
Fig.1 Structure of large size thin-walled tapered roller bearing outer ring
Tab.1 Process allowance of conventional tapered roller bearing outer ring after turning mm
| parameter | Process allowance |
| width | 0.5 |
| outer diameter | 0.6 |
| Raceway diameter | 0.9 |
Control processing deformation by optimizing key processes and improving tooling. The main shape control index requirements for turning, heat treatment and grinding are shown in Table 2.
Tab.2 Main shape control indexes for turning, heat treatment and grinding mm
| Craftsmanship | index | Require |
| turning | Single plane outer diameter variation VDsp | ≤0.100 |
| Average outer diameter variation VDmp | ≤0.100 | |
| heat treatment | Outer ring end warpage amount Cuc | ≤0.300 |
| Single plane outer diameter variation VDsp | ≤0.350 | |
| grinding | Outer ring width variation Vc | ≤0.016 |
| Single plane outer diameter variation VDsp | ≤0.040 | |
| Single plane outer raceway diameter variation VDep | ≤0.016 |
Turning
Original process and existing problems
The forging of the outer ring of large thin-walled tapered roller bearings usually uses two combined forgings. Traditional turning processing usually clamps one end of the combined forging and turns the other end. Generally, rough turning and finishing turning are performed. Rough turning uses a hard claw clamping tool. The soft claw clamping tool is used for fine turning. After turning, the forged piece is cut into 2 outer rings. Existing problems: 1) Rough turning produces large dimensional deviations, which will produce error reflections during fine turning; 2) The clamping method of the hard claw tip in rough turning and the smaller envelope surface of the soft claw in fine turning will cause the outer ring to Deformation.
Improvement measures
1) Add a secondary clamping and positioning reference process for turning; 2) Improve the clamping method of the rough turning hard claws and increase the contact area between the fine turning soft claws and the outer diameter surface of the outer ring.
Actual effect
The VDsp and VDmp of the outer ring before and after optimizing the turning process are shown in Figure 2: 1) Before optimization, only 20% of the VDsp of the outer ring met the turning requirements, and none of the VDmp met the turning requirements; 2) After optimization, 95% of the outer ring VDsp and VDmp meet the turning requirements. It shows that the optimized turning process can effectively control the deformation of the outer ring.
Fig.2 Main technical indexes of outer ring before and after optimization of turning process
Heat treatment
Original process and existing problems
The heat treatment of the outer ring of this large thin-walled tapered roller bearing usually uses salt quenching furnace heating and salt cooling quenching. The process curve is shown in Figure 3. The heat treatment quality such as quenching, tempering structure and hardness of the ferrule after processing by the original process can meet the requirements of ordinary products; however, large thin-walled outer rings are prone to large deformation, and generally the pressure end face warping and heat shaping are required after quenching. etc., which are labor-intensive, low efficiency, difficult to shape, and have a high scrap rate.
Fig.3 Diagram of heat treatment process curve
Improvement measures
1) The quenching process of the original salt quenching furnace is changed to the secondary oil cooling quenching process of the quenching machine; 2) The shaping process is changed from an independent shaping device to using a pressure quenching mold shaping in the secondary quenching process. The optimized heat treatment process curve is shown in Figure 4: First, the rotary hearth furnace is used for heating and insulation and primary oil cooling and quenching. Secondly, a quenching machine is used to bring the mold to secondary oil cooling and mold shaping. Then it is tempered in the tempering furnace, and finally additional tempering is performed in the air furnace. Secondary oil-cooling quenching with the mold is a key process for shaping. The pressure quenching mold is shown in Figure 5. It is a mold with upper and lower tapered structures.
Fig.4 Diagram of optimized heat treatment process curve
Fig.5 Structure diagram of pressure quenching mould
Actual effect
The CUC and VDsp of the outer ring before and after optimizing the heat treatment process are shown in Figure 6:1) Before optimization, only 45% of the CUC of the outer ring meets the heat treatment requirements, and none of the VDsp meets the heat treatment requirements. 2) After optimization, the CUC and VDsp of the outer ring meet the heat treatment requirements. It shows that the optimized heat treatment process can effectively control the deformation of the outer ring, and the heat treatment and shaping are continuous and tight, with low labor intensity.
Fig.6 Main technical indexes of outer ring before and after optimization of heat treatment process
Grinding
Original process and existing problems
The original process flow is: initial grinding of both end faces, outer diameter surface, and raceway → additional tempering → final grinding of both end faces and outer diameter face → fine grinding of raceway → final grinding of raceway → super-precision raceway. In the original process, the grinding force will deform the outer diameter surface when finely grinding the raceway, destroying the accuracy of the final grinding outer diameter surface as a benchmark. When the outer diameter surface is supported again and the raceway is ground, the deformation of the outer diameter surface will be reflected on the raceway, causing shape errors in the raceway, and the internal stress of the outer ring cannot be improved well.
Improvement measures
Keeping the outer ring clamping and supporting methods unchanged, adding fine grinding of both end faces, outer diameter surface and secondary additional tempering process, and adjusting the fine grinding raceway to the fine grinding of the outer diameter surface, the optimized grinding process The process is shown in Figure 7 (the dotted box is the process added to this article). Advantages: 1) The fine grinding process subdivides the grinding allowance of the initial grinding and final grinding processes, reduces deformation caused by grinding stress, improves grinding accuracy, and improves surface quality. 2) The secondary additional tempering process further eliminates the internal stress generated by the grinding force and grinding heat of the outer ring during the grinding process, allowing the workpiece to quickly reach a stable state.
Fig.7 Grinding process after optimization
Actual effect
The VCs, VDsp, and VDep of the outer ring before and after the optimization of the grinding process are shown in Figure 8: 1) Before optimization, only 45% of the VCs of the outer ring meet the grinding requirements, and only 40% of the VDsp and VDep of the outer ring meet the grinding requirements. 2) After optimization, the VCs, VDsp, and VDep of the outer ring all meet the grinding requirements. It shows that the optimized grinding process can effectively control the deformation of the outer ring.
Fig.8 Main technical indexes of outer ring before and after optimization of grinding process
Actual processing effect of finished product
The main technical indicators of the large thin-walled tapered roller bearing outer ring processed by shape control technology are shown in Table 3. All technical indicators meet the requirements.
Tab.3 Main technical indexes of large size thin-walled tapered roller bearing outer ring after shape control process
| Technical indicators | Numerical value/mm | Requirements/mm |
| Vcs | ≤0.011 | ≤0.016 |
| VDsp | ≤0.012 | ≤0.040 |
| VDmp | ≤0.010 | ≤0.030 |
| VDep | ≤0.010 | ≤0.030 |
Analysis of the improved process costs: 1) In terms of turning processing, the processing allowance has not changed. Tooling, fixtures, and clamping jaws can be improved in the turning workshop, and the cost is almost unchanged. 2) In terms of heat treatment, the outer ring thermal processing process time has not changed much, which increases the cost of the press quenching mold (about 5,000 yuan), but reduces the tooling and labor costs of the warpage of the press end face and the oval top shape. 3) In terms of grinding processing, the grinding allowance remains unchanged. Fine grinding and final grinding can be completed on the same grinding machine. The cost does not increase, except for the cost of secondary tempering. To sum up, on the basis of utilizing existing production equipment and processing capabilities, the increased cost after process optimization is small and insignificant compared with the cost of product scrapping caused by processing deformation before optimization.
Conclusion
In view of the problem that the outer ring of large thin-walled tapered roller bearings is easily deformed during the machining process, the turning, heat treatment and grinding processes were optimized respectively. The improved process effectively controlled the deformation of the outer ring of large thin-walled tapered roller bearings during the machining process. deformation, improving processing quality and reducing scrap rate.
