KYOCM Technology Knowledge: Discussion on technology of optimization of bearing raceway working properties

Abstract: By analyzing the key processes of bearing ring, the principle of delaying raceway failure is discussed, at the same time some methods of solving problem on technology is put forward.

Keywords: bearing raceway; mechanical properties; failure; contact stress

1. Preface

As an important basic element, rolling bearing (hereinafter referred to as bearing) has been widely used in all mechanical equipment. Therefore, the performance of bearings directly affects the key performance of the overall mechanical equipment to a large extent, such as the accuracy of processing equipment, the speed and safety of transportation tools, and even the reliability and stability of combat weapons, all of which are directly related to the performance of bearings. Generally, the main indicators of bearing performance are: bearing capacity, rotation accuracy and working life, and one of the key factors determining these performance indicators mainly depends on the processing quality of the bearing working surface.

2. Significance of optimizing the working performance of bearing raceways

The working surface of the bearing includes the inner and outer race raceways or raceways (hereinafter referred to as raceways) surface and the surface of the rolling element in contact with the raceway. Because the processing technology of the rolling element is very different from that of the inner and outer rings, the processing of the rolling element is not discussed here.

2.1 requirements for mechanical properties of bearing parts

The main function of bearing is to guide movement and transmit load, so its working surface should have good comprehensive mechanical properties, such as sufficient strength and hardness, high contact fatigue strength, good wear resistance, high elastic limit and impact toughness, to meet the use requirements.

2.2 main failure modes of bearings

Facts have proved that under the normal conditions of installation, fitting, lubrication, sealing and maintenance, the vast majority of bearings are due to the repeated action of alternating stress, and the materials on the working surface are flaked off, resulting in bearing failure. In addition, friction and wear are inevitable during the operation of the bearing, and a long time may also lead to bearing failure. Therefore, fatigue and wear should be the normal failure form of bearings.

2.3 purpose of optimizing the working performance of bearing raceway

Targeted improvement of a certain mechanical property of bearing materials will improve the service performance of the bearing as a whole. For example, improving the contact fatigue strength of the bearing raceway surface will significantly prolong the working life under the original working conditions, or improve the reliability and stability of the bearing products within the original calculated life limit. Prolonging the service life of bearings is essentially to delay the occurrence of fatigue and wear.

2.4 feasibility analysis of optimizing the working performance of bearing raceway

On the premise that the current geometric parameter design of bearings is almost perfect and the quality of existing materials is relatively stable, if we want to make a significant improvement in the service performance of bearing products, we can only carry out analysis, research and experiment in the production process, that is, the process from raw materials to products. To realize this process, two aspects should be fully considered.

First, the product has an ideal geometric shape, including the dimensional accuracy, geometric accuracy and surface quality of each part of the product. For this reason, all bearing manufacturers and related industries have made great efforts. For example, a large number of advanced processing equipment such as CNC lathes and high-precision grinders are used, but the products produced in this way can only be said to have excellent external quality, which is specifically manifested in the fact that the rotation accuracy and overall dimension meet the product requirements of a certain level.

Second, the product has ideal service performance, that is, the bearing capacity, service life and other internal quality of bearing products, as well as the stability and reliability shown in the process of realizing product performance.

3. Process discussion on optimizing the working performance of bearing raceway

Today, with the improvement of bearing technology, the production process of each manufacturer for a certain type of product is almost the same. In order to obtain the ideal internal quality of bearing materials, we should focus on the analysis of the key processes in the process.

3.1 forging

The forging process is the process of transforming the bar into the blank of bearing ring. In this process, if the deformation rate of the workpiece is maximized and the deformation times are increased as much as possible, the grain of the material will be further refined, the structure will be more uniform, and the material defects will be weakened, so as to improve the comprehensive mechanical properties of the material.

In general, due to the limitations of technology and equipment, it is impossible to forge repeatedly during the blank forming process. Therefore, the simplest way to improve the material deformation rate is to try to use small-diameter bars. When the blanking weight of a certain specification ferrule is fixed, the smaller the diameter of the material section, the greater the material deformation during upsetting.

3.2 rolling and expansion

The significance of rolling and expanding the ferrule blank is not only to obtain the neat shape of the blank, but also to obtain the fiber structure in the circumferential direction of the ferrule, as shown in Figure 1.

During the rolling process of the ferrule, the grains are elongated in the circumferential direction to form the fiber direction, as shown in Figure 2. At this time, the anisotropy of the grain is the most obvious, and the tensile strength in the circumferential direction and the compressive and impact resistance in the diameter direction of the ferrule are the best. Therefore, in the forming process of ferrule blank, if the process equipment conditions allow, we should try to improve the milling forming ratio, so as to obtain a more dense and uniform fiber structure.

3.3 the raceway cannot be soft ground

Soft grinding can improve the geometric accuracy of machined parts, so it is widely used on a certain surface of the ferrule, such as the reference plane of the ferrule, the outer surface of the outer ring, etc.

However, soft grinding may also bring fatal harm to bearing rings. This is because the top angle of the abrasive particles is much sharper than the tool tip arc of the turning tool, that is, the radius of the bottom of the wear mark on the surface of the ferrule is much smaller than that during turning. During quenching, the tensile stress on the surface of the ferrule will be concentrated at the bottom of the wear mark, thus becoming a factor for the initiation of cracks (including quenching cracks and grinding cracks).

Figure 1 Schematic diagram of fiber tissue trend after ferrule rolling

Fig. 2 Schematic diagram of grain deformation of metal materials under rolling

3.4 adopt precision grinding for the raceway

Precision grinding, also known as low roughness grinding, is to use a small dressing lead and dressing feed to finely trim the grinding wheel, so that the surface of the grinding wheel produces many equal height micro edges. These micro edges cut the metal with a small grinding amount, and the machined surface forms a low roughness surface under the action of fine cutting, friction and polishing of the micro edges. See Figure 3 for the relationship between the dressing lead of the grinding wheel and the surface roughness value.

Fig. 3 Relationship between dressing lead and surface roughness

This process should be carried out after the fine grinding of the raceway. The purpose is to cut a very thin layer of metal on the raceway surface with a small cutting amount, so that the residual micro bulges in the previous process are further ground flat, as shown in Figure 4, so as to improve the contact rate between the raceway and the rolling element and reduce the contact stress.

4. Conclusion

To improve the performance of bearings, there is still great potential to be tapped in the process. The process is not only the forming process of products, but also the process of shaping product performance.

a. surface before precision grinding         b. surface after precision grinding

Figure 4 Schematic diagram before and after precision grinding of raceway

More about KYOCM Needle Roller Bearing:

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