Material characteristics and wear mechanism and the processing of ceramic bearing block.

Abstract: The paper proposes one kind of new support material, ceramics material, applied in the finishing process of bearing rings. And the respects such as the material behavior, friction - wear mechanism, the manufacture technology of the ceramics support material, and so on, are studied. The results show that the ceramics support material has an advantage over the traditional support materials for improving the processing accuracy and the surface quality of the bearing rings, and can eliminate the defection of support trace brought by the support material.

Key words: ceramics support; bearing rings; finishing process; applied research

1. Introduction

As a standard part, rolling bearings are widely used in aerospace, automobile, metallurgy, household appliances and other industries, and have a huge domestic and foreign market. In order to expand domestic demand and earn foreign exchange through export, domestic bearing manufacturing enterprises have begun to pay attention to the appearance quality of bearing products while trying to improve the internal accuracy of bearings. However, because the appearance quality of bearing products cannot meet the requirements of customers (the host country cannot effectively eliminate the bearing seal), they are unable to compete with world-renowned enterprises in the international market, causing huge economic losses to the country every year.

Centerless fixtures are widely used in the grinding and Superfinishing of bearing rings. Its characteristics are: high repeated positioning accuracy, no clamping deformation, convenient adjustment, easy loading and unloading of workpieces, radial runout of the spindle does not affect the machining accuracy, and it is convenient to realize automation. However, centerless fixtures are easy to leave support marks on the positioning surface or cause surface scratches during use. In order to eliminate support marks and surface scratches, the domestic bearing industry generally increases the light grinding process, which is bound to increase the bearing processing cost accordingly. At the same time, improper control of the light grinding process is also easy to cause damage to the processing accuracy of the ferrule. Therefore, it is necessary to find a new support material to replace the currently widely used centerless fixture support material. The characteristics of ceramic materials, such as high strength, high temperature resistance, wear resistance, corrosion resistance, good thermal stability, self-lubrication and highly accurate product size, make them have incomparable material characteristics compared with other traditional bearing ring grinding and superfinishing support materials. This paper will study the application technology of ceramic support materials in the Superfinishing of rings from the aspects of theory and experiment.

2. Material characteristics and wear mechanism

2.1 Defects of traditional supporting materials

At present, nylon and bakelite support are widely used in the Superfinishing of ferrule, and the ferrule is under the pressure and clamping force of oilstone. When the ferrule rotates rapidly, on the one hand, because the friction coefficient between nylon and bakelite and bearing steel is large, the friction is also large, which is easy to produce support marks on the positioning surface; On the other hand, due to the low hardness of nylon and bakelite, the particles falling off of wear debris and oilstone during superfinishing are easy to be embedded in nylon or bakelite under the pressure of oilstone, which is easy to cause scratches on the positioning surface of bearing ring. In addition, due to the low hardness and poor high temperature resistance of nylon and bakelite materials, serious wear and thermal deformation of support materials are easy to occur if there is insufficient lubrication and poor heat dissipation during processing.

2.2 Material characteristics of ceramic support

See Table 1 for the mechanical properties of ceramic materials at room temperature.

Table 1 normal temperature properties of HIP-Si₃N₄ ceramic material (20 ℃)

2.3 Wear mechanism

The super essential oil used in the super finishing of ferrule is mixed with kerosene, No. 20 engine oil, oleic acid and other additives, which plays the role of lubrication and cooling in the processing. In recent years, many basic studies have been carried out on the friction and wear properties and mechanism of silicon nitride ceramics under oil lubrication, and many useful results have been achieved. However, there is no literature report on the friction and wear mechanism of Si₃N₄- GCr15 steel in the state of 20 × oil lubrication. Therefore, the friction and wear properties of Si₃N₄- GCr15 steel friction pair under oil lubrication were experimentally studied on a friction testing machine, and the friction coefficient and wear amount of the friction pair were measured quantitatively, so as to provide a theoretical basis for the application of ceramic support in the Superfinishing of bearing rings. The experiment was carried out on a self-made friction testing machine by means of pin disk grinding. The ceramic sample block is 6mm fired by hot isostatic pressing (HIP) × 6mm × 10mm rectangular pin with surface roughness of 0.32 μ m。 The couple is GCr15 steel disc, with an outer diameter of 60mm and a thickness of 8mm. The material is quenched and tempered, with a hardness of 62HRCand a surface roughness of 0.32 μ m。 The experiment was carried out at room temperature for 1h. During the experiment, the pin is fixed, the disc is driven by the motor, and the speed is 500r/min. No. 20 engine oil flows into the experimental contact area by gravity. The average flow rate is 0.02L/min. The load is loaded with the help of the method code, and the friction force is uploaded to the strain gauge on the force arm through the experimental machine. After amplification and other signal processing, it is displayed with a digital meter. The calculation formula of volume wear w of ceramic pin is

W=L0-L1/L1 × 100%

Where, the measured data of l0 and L1 pins measured on the bearing height measuring instrument before and after the experiment

The experimental results show that the ceramic friction surface is polished under 20 # oil lubrication, the sliding friction coefficient of the friction pair is 0.09 ~ 0.17, the wear amount is less than 2%, and the friction coefficient of the friction pair decreases with the increase of load. Through analysis, the low friction coefficient is related to the anti-wear effect of the boundary adsorption film formed by the lubricating oil on the bare metal surface during the friction process. At the same time, the ceramic material has obvious porous structure (the porosity is in the range of 2.15% - 6.32%), and its pore diameter is mostly 0.33 ~ 1.52 μm, which is enough to make 20 # engine oil penetrate into it, and then form a thin protective oil film on the ceramic surface, which also helps to reduce the friction coefficient of the friction pair.

In addition, the hardness of ceramic materials is much higher than that of traditional support materials such as nylon and rubber. During superfinishing, the abrasive particles falling off from grinding and oilstone are not easy to be embedded into the support block under the pressure of oilstone to form abrasive wear, which greatly reduces the possibility of the support block scratching the positioning surface of the bearing sleeve.

3. Processing of ceramic bearing block

In recent years, a large number of experimental studies have been carried out on the precision machining technology of ceramic materials at home and abroad, and rich research results have been achieved. At present, the more mature precision machining technologies of ceramic materials mainly include cutting, grinding, grinding and polishing, EDM, ultrasonic machining, laser machining, etc. Based on the advantages and disadvantages of various processing technologies and considering the existing processing equipment of our university and cooperative units, we decided to adopt the processing method of diamond grinding wheel grinding or diamond grinding. Because the size of the abrasive grain of the resin bonded diamond wheel is an important factor affecting the surface roughness of ceramic grinding, and the improper selection of the feed rate and grinding depth of the wheel is easy to cause surface cracks and damage, so in the actual processing, we use the resin bonded diamond wheel with low particle size, strictly control the feed rate and cutting depth of the wheel, and test the dimensional accuracy of the processed ceramic block The geometric accuracy and surface roughness fully meet the technical requirements. In addition, we also tested the precision machining of ceramic blocks with diamond grinding paste on the grinder, and achieved success. However, the processing efficiency of this method is lower than that of diamond grinding.

Due to the high hardness of the ceramic support block, if the working surface of the ceramic support block and the supported surface of the ferrule are in point and line contact during the ultra-precision machining of the ferrule, the positioning surface of the workpiece will be scratched. Therefore, the surface shape of the ceramic support and the surface of the workpiece must meet the contact of the envelope surface, so as to prevent the ceramic support block from scratching the positioning surface of the bearing ferrule during ultra-precision machining.

4. Experimental analysis

4.1 Experimental process

(1) Experiment 1: 100 pieces of qualified products of 30203 tapered roller bearing outer rings processed in small batches and to be ultra-refined were randomly selected and divided into two groups, with 50 pieces in each group. Ceramic support and nylon support were respectively used on the same LZ211 machine tool for ultra-precision, Number each group of products after processing and measure the dimensional accuracy of bearing products after processing with a special bearing detector. At the same time, visually inspect the surface quality (whether there are scratches, whether there are support marks) and the wear of the support.

(2) Experiment 2: randomly select 100 qualified products from the 32218 tapered roller bearing inner rings processed in small batches to be ultra-refined, and divide them into two groups, with 50 pieces in each group. They are ultra-refined on the same 3MZ3310B machine tool with ceramic support and roller support respectively, Number each group of products after processing and measure the dimensional accuracy of bearing products after processing with a special bearing detector. At the same time, visually inspect the surface quality (whether there are scratches, whether there are support marks) and the wear of the support.

4.2 Analysis of experimental results

Through the comparative analysis of the recorded experimental data, it can be seen that compared with the traditional nylon and bakelite support materials, the dimensional accuracy of the ferrule after ultra-precision is not reduced, but slightly improved. The measured data can be calculated that the average taper deviation of the former is reduced by 3.85%, the average taper angle deviation is reduced by 76.9%, and the average wall thickness difference is basically the same. In addition, from the recorded surface quality of visual inspection, the workpiece surface with ceramic support and roller support has no scratches and support marks, while there are scratches and support marks with nylon support. Therefore, after using ceramic support, there is no need to add light grinding process to eliminate support marks and scratches, so as to avoid waste products caused by improper light grinding control and reduce processing costs.

According to the bearing wear records, the wear of ceramic bearings is very slight, while the wear of nylon and roller bearings is serious. Therefore, the use of ceramic bearings can effectively reduce the frequent downtime caused by the replacement of worn bearings, which can greatly improve the production efficiency of ultra-precision machining of bearing rings.

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