How to improve the precision of China made bearings?

High precision bearings belongs to high performance bearing. The high performance we mentioned mainly requires high speed and high load capacity, long service life and reliability, low vibration noise and low friction torque, and can withstand some extreme construction conditions (such as high and low temperature, corrosion), etc. High performance bearings are mainly used in aviation and aerospace, as well as high-speed railway, precision machine tools and high-end automobiles. High precision bearings generally involve “high technology” and “high quality”.

After more than 60 years of reform and opening up, China’s bearing industry is growing. At present, there are no significant deficiencies in the types, varieties and specifications of bearing products, basically with a relatively complete product line. However, the product structure is not very balanced. The production capacity of middle and low-end ordinary products has been in excess, while the market of high-end products is still dominated by foreign famous brand products. So far, There are significant breakthrough has been made in some bearings technology just like the railway bearings.

For the current situation of the technical quality level of China’s bearing products, we can summarize the situation that the quality is not as good as that of the top quality and the high-end bearing is not as skilled as the SKF, FAG, INA, NSK, NTN brand.

Ordinary bearings are relatively mature in technology, and there is basically no restriction of entry threshold. No matter large enterprises or small enterprises are able to carry out production of the bearings, so the competition is mainly based on price, so domestic bearings constitute the main body of the market whether in the domestic market or in the export of foreign exports. However, even for some ordinary bearings with low technical content, there is still a big gap between domestic bearings and foreign famous brand products in terms of their own accuracy consistency, performance stability, life reliability and other quality levels. In the current transformation and upgrading tasks of our national bearing industry, the most urgent need is to carry out the product Adjust the structure. Among them, the focus is to complete the strategic layout in the field of high-precision bearings, so that they can achieve effective breakthroughs in the short term, and occupy the main position at the highest point of technology. This is not only the basic demand for the sustainable development of the bearing industry, but also the objective requirement for the national economy and the security of national defense. The key technology of high-precision bearing manufacturing is that the bearing is a kind of precision mechanical product, so the technology is the key point in the bearing manufacturing process.

1. Equipment is the prerequisite for the implementation of Technology

For example, grinding is the most critical final machining step in bearing ring manufacturing. The main equipments used are special grinding machine and super finishing machine. First of all, we must meet the basic requirements of high precision and high efficiency. In the past, the bearing grinder was mainly a semi-automatic and automatic machine controlled by electro-hydraulic. With the development trend of technology progress, CNC machine tools dominate the bearing grinder and super finishing machine tools. The CNC grinding machine composed of automatic production line is not only suitable for the mass continuous production of ordinary bearings, but also very flexible, and can cooperate with the production of a variety of special bearings. Therefore, CNC has become the main choice for many bearing enterprises to upgrade the processing equipment technology. Some types of bearing rings have simple geometric structure and low precision requirements, so it is OK to adopt PLC economic numerical control system; some types of bearing rings have high requirements, such as the raceway must be processed into the logarithmic shape of “convexity”. General or high-end CNC system shall be used to meet the requirements.

2. The bottleneck restricting the processing quality of high-precision bearings

For high-end bearings, in addition to the gap between raw materials and heat treatment, the accuracy and related problems are mainly reflected in the consistency of parts grinding, especially in batch processing.

(1) A typical example of precision bearings is angular contact ball bearings for machine tool spindles. For example, P4 or higher bearing rings have a large difference in machining accuracy. A good company usually has a one-time treatment qualification rate of about 80%. The matching rate was 60%. Some types of bearings (such as tapered roller bearings of wind turbine shafts) are also difficult to maintain quality above P4.

(2) The high-precision grade I and grade 0 rollers used for cylindrical roller bearings and taper roller bearings with P4 accuracy and above cannot be mass produced, and the processing quality of grade II or III rollers will fluctuate.

(3) The dimensional stability of parts can not be well controlled and guaranteed. Grinding stresses and deformations are too large and machining defects such as grinding burns and cracks occur sometimes.

3.Key technologies and measures to break the bottleneck

(1) Use machine tools with high accuracy, stability and reliability. For some bearing rings with complex shapes, it is necessary to use the grinder with full-function CNC system to directly call the function so that it can be controlled and interpolated in time; for internal surface grinding, the high-speed motorized spindle is used, while for external surface grinding, the high-speed motorized hydrostatic grinding wheel spindle is mainly used to complete the high-speed grinding; for semi closed loop and closed-loop servo system, it is necessary to use the grinder with high-speed control and interpolation It can control and complete precise micro feed and compensation. The feed resolution can imagine 0.25 μ m or reach 0.1 μ m, and the accuracy of repeated positioning can meet 1 μ m or reach 0.5 μ m. It uses high-precision, rigidity, wear-resistant ball screws, guide rails and other moving parts. It uses a bed with sufficient aging stability and relatively good vibration absorption effect, which is higher The precision grinder needs to use granite bed with strong vibration attenuation ability and relatively low thermal expansion coefficient.

(2) The active measurement method is used for grinding dimension control, with control accuracy of 3 μm or more to 1 μM. The servo motor is used to drive and control the wheel dresser, reduce the interval of wheel dressing, and improve the quality of wheel dressing; or the diamond wheel and CBN wheel with good shape retention are used to ensure the consistency of shape accuracy in batch machining.

(3) Through forging, finishing or precision cold rolling, the grinding allowance can be reduced, and additional tempering and cold treatment can be fully carried out between each working procedure to eliminate grinding stress and avoid excessive feed to produce defects such as grinding burns and cracks, so as to improve machining accuracy and surface quality.

(4) Pay attention to improve the manufacturing level of key parts such as rolling element and cage. Especially for high-precision cylindrical roller and tapered roller, the use of automatic production line. High precision rollers must be completely stable and aged.

(5) Realize the control of the environment condition of precision and ultra precision machining, and give up the long-term practice of the open workshop or general air-conditioning plant of bearing production. According to the requirements of constant temperature and humidity, vibration isolation and cleaning, the environmental facilities in the processing area shall be constructed or reconstructed strictly.

(6) Fully develop the characteristics of “Mastering and inheriting” in the production of precision parts, strengthen the skill training and experience accumulation of operators, strictly enforce the process flow and process discipline, strengthen the quality assurance system, match the “software” with the “hardware”, and improve the performance of excellent hardware.

In addition to the relatively low overall technical level, the two main problems of China bearing in the field of high-precision bearings are raw materials and manufacturing technology. In the use of manufacturing technology, equipment and technology are required to have a certain breakthrough, and also need to use sophisticated equipment and advanced technology to guarantee the high accuracy and retention of high-end bearings, so that their own high-performance and stability, as well as long life and reliability and other characteristics can be guaranteed and realized. The well-known Bearing Companies in the world regard equipment and technology as one of their own “technical treasures”, which also shows that manufacturing technology is the core competitiveness of enterprises for development. At present, our country’s bearing industry is actively undergoing transformation and upgrading, so we must focus on manufacturing technology as the main direction of attack, and complete key breakthroughs in the field of high-end bearings, hoping to make our country develop from a big bearing country to a strong bearing country as soon as possible.