Bearing, as its name suggests, is to support the parts of the rotating shaft, or linear motion shaft. Its role is to provide support for the rotating body or linear back and forth motion body in the machinery. When other parts produce relative motion on the shaft, the bearing can maintain the center position of the shaft and control the movement.
Its kind is extremely complex, simple to divide, can be divided into two kinds, rolling bearing and sliding bearing. No matter what kind of bearing, its main function is to support the rotating body in motion. In order to ensure mechanical efficiency and rotation accuracy, it is required that the rotating body has a low friction coefficient in the process of movement, therefore, the smaller the vibration between the shaft and the bearing, the less wear, the lower the heat the better.
Taking rolling bearings as an example, when it starts to operate, the inner and outer rings of bearings and the bearing rolling bodies should bear the action of high frequency and variable stress. The contact area between the rolling bodies and the rings is very small, and the unit area should bear great pressure, generally up to 1500-5000N/mm^2. At the same time, the rolling bearing should also bear the effect of centrifugal force, and the centrifugal force will increase with the increase of the speed.
Under the action of such high frequency and variable stress, the surface of the ring or rolling body is easy to produce stress fatigue, and finally fatigue cracks are generated in the parts with low fatigue strength, forming fatigue spalling and making the bearing damaged and failed.The load and speed of the bearing are important factors to determine the life of the bearing,In addition to the stress extrusion and friction brought by the high-speed rotation process, bearings working under special conditions have special requirements, such as high temperature resistance, low temperature resistance, corrosion resistance and diamagnetism, and so on. The working conditions of bearings are very complex.
High-end equipment such as high-speed rail, large aircraft, heavy-duty weapons have the use of high-end bearings, and to meet the requirements of high-end bearings in precision, performance, life and reliability, the high quality and reliability of bearing materials is the decisive factor.In the four components of rolling bearing, in addition to the retainer, inner and outer ring, rolling body (ball, roller or needle) are composed of bearing steel, and bearing steel known as the “king of steel” title, is the steel production in the most stringent requirements.
Bearing steel also has many kinds, according to the different use requirements, the chemical composition is not the same, among which high carbon chromium bearing steel accounts for a large proportion, is the most commonly used steel manufacturing bearings and bearing parts.
The quality of bearing steel mainly depends on the following four factors: one is the content, shape, distribution and size of inclusions in steel; The content, shape, distribution and size of carbide in steel; Third, the center porosity and center segregation in steel; Fourth, the consistency of bearing steel product performance. These four factors can be summarized as purity and evenness indicators.
Among them, the purity requires the inclusion in the material to be as little as possible. The purity has a direct impact on the fatigue life of the bearing; The uniformity requires the inclusion and carbide particles in the material to be fine and dispersed, which will affect the deformation and microstructure uniformity of the bearing after heat treatment in the manufacturing.
To improve the purity of bearing steel, the first thing to do is to control the oxygen content in steel, steel making with ppm (per million) as a unit of oxygen content, generally speaking, 8 ppm steel belongs to the good steel, and high-end bearings need is 5 ppm of the top steel. In addition, titanium and other harmful elements in steel is easy to form multi-angular inclusions, will cause local stress concentration, fatigue cracks, should also try to avoid.
At present, with the improvement of the high purity steel smelting platform system, the purity of bearing steel has been greatly improved, the level of inclusion has been effectively controlled, the oxygen content of steel in developed countries has been controlled below 5ppm, so the content, distribution and size of carbide in steel has gradually become the main factor restricting the quality of bearing steel.