Research status of convexity machining technology and research prospect of convexity machining technology.

Abstract: the research progress of roller bearing crown machining technology is discussed from the aspects of crown grinding and ultra-precision grinding. It is considered that: the forming method and mechanism of crown grinding are basically clear, and the key technology lies in the motion accuracy control of dresser. The forming mechanism and law of crown superfinishing are not completely clear, especially in the process of roller crown superfinishing, the contact state between oilstone and roller, the forming mechanism and grinding principle of crown need to be further studied. The stability of roller motion in roller crown machining also needs deep research.

Key words: roller bearing; convexity processing; research progress

In the early cylindrical and tapered roller bearings, the roller and the inner and outer raceways were designed and processed into straight plain cylindrical or conical surfaces, which were in a straight-line contact state, and it was expected to obtain a uniform contact stress distribution, but these bearings often suffered premature fatigue damage at the end of the roller or on both sides of the raceway. Both theoretical analysis and experimental research show that this contact state has edge effect at the end of the roller, causing stress concentration and significantly affecting the service life of the bearing. Concentration should be stress to eliminate edges. The concept of modifying the straight prime line of rollers and raceways is proposed. Because the straight prime line becomes a convex curve after modification, it is called convexity curve, and thus forms the convexity technology of roller bearings.

The emergence and development of convexity technology has had a significant impact on the improvement of the performance of roller bearings, and has become an important technology of roller bearings. However, due to the complexity of the convexity problem, there are still many problems that have not been solved. Convexity technology mainly includes three aspects: convexity design, processing and detection technology. The following describes the research status of roller bearing crown machining technology, and prospects the future research direction.

1. Research status of convexity machining technology

1.1 Processing technology of raceway crown

The grinding of raceway convexity adopts cutting machining, and the method of forming convexity is forming method. Therefore, the shape accuracy of convexity and the size of convexity measurement mainly depend on the dressing accuracy of grinding wheel profile. The curve motion accuracy control of grinding wheel dressing system is the key. It mainly involves precision guide rail and precision numerical control technology. The outer ring raceway convexity can also be processed with straight profile grinding wheel, but the form of convexity can only be hyperbolic. This processing method requires that the rotation axes of the grinding wheel and the ferrule are not in the same plane, and the intersection angle of the two axes directly affects the convexity of the outer raceway.

The superfinishing machining methods of raceway crown include through type and cutting type. The basic principle of forming crown is to make the accumulated super-finish at both ends of raceway greater than that in the middle. The through type uses relatively narrow oilstone, which oscillates at high frequency and small amplitude, and makes reciprocating motion in the full width range along the width of the raceway. The reciprocating motion of the oilstone is a variable speed motion, and its motion law determines the difference of the superfinishing time of the oilstone at each position of the raceway width, so the accumulated super-finish is also different. Sometimes the frequency of small amplitude oscillation of the oilstone also changes according to a certain law with the processing position of the oilstone], so that the super-finish at both ends of the raceway is greater than the middle. Therefore, the shape accuracy of convexity and the size of convexity measure mainly depend on the law of reciprocating motion and the change law of oscillation frequency. Unfortunately, there is no public literature to report the quantitative relationship between this change law and convexity. The cutting type adopts special-shaped wide oilstone which is slightly larger than the width of the raceway. The thickness of special-shaped oilstone gradually decreases from both ends to the middle, so the ultra-precision at both ends of the raceway is larger than that in the middle. The quantitative relationship between the thickness change law of special-shaped oilstone and its convexity has not been reported.

1.2 Roller crown processing technology

1.2.1 Roller crown grinding technology

The grinding of roller crown generally adopts the machining method of centerless penetration, and it can also adopt the cutting type machining. The machining method of centerless penetration has the advantage of high machining efficiency. The method of forming convexity curve is the generation method. The shape and measurement of convexity depend on the attitude of the roller. In the grinding of the through track and the contour shape of the grinding wheel working surface, the roller crown surface forms a pair of conjugate surfaces with the grinding wheel working surface on the one hand, and a pair of conjugate surfaces with the guide wheel working surface on the other hand. According to the principle of conjugate surface, when the convexity of the roller is determined, and under the condition that the penetration trajectory of the roller has an envelope, determine a kind of trajectory curve, and then the corresponding grinding wheel working surface contour and guide wheel working surface contour can be determined. The trajectory curve with envelope is not unique. It is of great significance to reasonably select the through trajectory curve of the roller for the grinding of crown. On the one hand, the selection should make the guide wheel and grinding wheel working surface of the conjugate as simple as possible, on the other hand, it is conducive to the stability of the through motion of the roller. Document 4 reports the design of the trajectory curve and the corresponding grinding wheel and guide wheel contour curve in arc crown grinding, while other crown curves have not been reported.

The convexity shape accuracy and convexity measurement of the roller after centerless through grinding are related to the grinding and the dressing accuracy of the contour shape of the grinding wheel working surface, and also to the stability of the roller through motion. The cylindrical roller convexity grinding wheel and the guide wheel working surface contour are trimmed by the machine tool's own dresser, which is relatively difficult and easy to realize. The working surface of taper roller crown grinding guide wheel is a complex spiral contour, which needs a special grinder for grinding, so the shape accuracy control is difficult and the cost is high.

The method of cutting in machining to form crown curve is forming method, and its crown mainly depends on the contour of grinding wheel. This processing method is low efficiency but high precision, suitable for large precision rollers.

1.2.2 Roller crown superfinishing Technology

The Superfinishing of roller crown adopts the processing method of centerless penetration, and a row of oilstones of various types are pressed on the top of a row of rollers for high-frequency oscillation for grinding. It belongs to multi station processing, sometimes up to 10 stations. Supported and driven by a pair of rotating guide rollers, and guided by the shape of the guide roller, the roller passes under the oilstone while rotating with a certain track and attitude. Finish the superfinishing process of one roller within the time period from the oilstone entering the first station to leaving the last station.

Although the processing method of crown superfinishing is similar to grinding, the forming mechanism of roller crown curve is much more complex than grinding. This is because although the roller crown surface and guide roller surface can be regarded as a pair of conjugate surfaces, considering the following factors, it is difficult to regard the roller crown surface and the working surface of oilstone as a pair of conjugate surfaces.

(l) The oilstone is pressed on the roller with a certain pressure, and the oilstone at each station can independently and elastically yield or automatically supplement the roller as a whole, unlike the fixed position of the grinding wheel relative to the roller in grinding.

(2) The working surface of the oilstone is a concave surface with a high degree of coincidence with the shape of the roller surface, and its contact with the roller may be scattered multi-point contact; In grinding, the contact between grinding wheel and roller is point or line contact.

(3) The formation process of roller crown is accompanied by significant wear and deformation of the working surface of the oilstone. For the Superfinishing of most rollers, compared with the removal rate of workpiece material, the wear rate of the stone is roughly one order of magnitude. Depending on the performance of the oilstone and the workpiece material, the wear rate of the oilstone is sometimes greater than the removal rate of the workpiece material, Therefore, the wear and deformation speed of the oilstone working face is almost as fast as the forming speed of the roller crown, which cannot be ignored. In the through grinding, the wear of the grinding wheel can be ignored during the period from the grinding of a single roller to the end of the grinding.

(4) The deformation process of oilstone working face may have a direct interaction with the forming process of roller crown, and this action mechanism is not available in generation grinding. Because the wear rate of roller and oilstone is close, the crown of roller is relatively small in value, generally 2~15um. In the grinding process, the contact between roller and oilstone is generally convex to concave, and the degree of fitting is quite high, but the shape of convex and concave will never be completely consistent. Therefore, the change of roller crown shape and oilstone working surface shape may significantly affect the contact state; On the contrary, the change of contact state will cause the change of the surface shape of rollers and oilstones.

Literature (5~7) reported the influence of experience based superfinishing process parameters on crown, but considering the above complex factors, there is no report on the in-depth study of the formation mechanism of roller superfinishing crown curve. It is precisely because the formation mechanism of the crown curve of roller superfinishing has not been clearly studied, so it is very difficult to correctly select the running path of the roller. According to the principle of conjugate surface, this directly affects the design of the guide roller shape of superfinishing. In the process of roller penetration, the roller shape and the guide roller shape form a pair of conjugate surfaces). The existing research on the design of guide roll shape has insufficient understanding of the formation mechanism of roller superfinishing crown curve. Therefore, in engineering practice, the design of guide roll shape, especially the design of non-circular arc crown roller superfinishing guide roll, is highly dependent on experience.

The cylindrical roller crown superfinishing guide roller is a complex convex surface, and the conical roller crown superfinishing guide roller is a complex spiral surface. The guide roller is large in size and made of high hardness and wear-resistant materials, so it is difficult to process. Special machine tools are needed for the machining of tapered roller crown superfine grinding guide rollers.

2. Research prospect of convexity machining technology

To sum up, the forming method and mechanism of crown grinding have been basically clear. The key to improve the crown grinding accuracy is to improve the dressing accuracy of the contour of the grinding wheel and guide wheel working surface from the aspect of machine tool motion accuracy control. In addition, in the non-circular arc full crown grinding of rollers, the formation of crown will lead to the contact between rollers and guide wheel working surface and cutter plate into theoretical point contact, Reduce the support stability of rollers under the action of grinding force, the stability of roller penetration and rotation motion will decline, which will affect the crown shape accuracy and roundness after roller grinding. Therefore, it is necessary to study the stability of roller motion in crown grinding.

Stability of roller motion in crown grinding. The convexity forming mechanism and law of convexity superfinishing are not completely clear. The convexity forming problem of raceway superfinishing is relatively simple, because there is only one oilstone, and the raceway width is also very limited. The forming mechanism and law of roller superfinishing crown involve not only subtle and complex contact state changes between oilstone and roller, but also large penetration length. Multi position oilstone combination and complex spiral or non-spiral surface, so it is necessary to further study in order to improve the design method of super precision grinding guide roll shape and optimize the process parameters. In addition, in the cylindrical roller crown superfinishing, there is always point contact between the roller and the convex surface guide roller, and the support stability of the guide roller to the roller is poor. Under the action of oilstone pressure and oscillating grinding friction, the stability of the roller penetration and rotation movement decreases, which affects the crown shape accuracy and roundness of the roller after superfinishing. Therefore, it is also necessary to study.

3. Conclusion

Grinding and superfinishing are the main processes of convexity machining. The forming method and mechanism of grinding crown are basically clear. The key technology lies in the motion accuracy control of the dresser; The forming mechanism and law of super precision grinding crown are not completely clear, especially the forming mechanism and law of roller super precision grinding crown are very complex, which need to be further studied. The stability of roller motion in roller crown machining also needs deep research.

More about ITS Spherical Roller Bearing:

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Spherical roller bearings have two rows of symmetrical rollers, a common sphered outer ring raceway and two inner ring race ways inclined at an angle to the bearing axis. The central point of the sphere in the outer ring raceway is at the bearing axis.

There are several features of spherical roller bearing. First, accommodate misalignment Spherical roller bearings are self-aligning like self-aligning ball bearings or CARB bearings. Secondly, high load carrying capacity. Spherical roller bearings are designed to accommodate both heavy radial loads and axial loads in both directions. Thirdly, the rollers are manufactured to such tight dimensional and geometrical tolerances that they are practically identical in a roller set the symmetrical rollers self-adjust, providing optimal load distribution along the roller length and together with the special profile prevent stress peaks at the roller ends.