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SKF aviation rolling bearing skills (1)
Source: China Bearing Network Time: 2018-01-13
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1. M50NiL bearing steel from 1955 to 1980 25 years; the speed of the aviation-initiated bearing bearing increased steadily; the dn value has reached nearly 2. 5 million. Entering the 1990s; the high-speed and high-temperature of the aviation-initiated machine put forward more for the rolling bearing High demand. However; existing bearing steel; even special high temperature bearing steel; such as M50; 18-4-1 and 14Cr-4Mo clan various derivative steel; such as CRB-7 and GB-42; It still works well at temperatures significantly higher than the current bearing bearings. But there is an important constraint; this is the easy cracking of hardened steel bearing rings under ultra-high speed conditions; There is no warning or less.
In order to find a bearing steel with the function of M50 bearing steel and better cracking; SKF's MRC bearing company carried out a series of research work with the support of the US Air Force; the end selected M50NiL.
M50NiL has improved resistance to cracking; compared with other high temperature hardened bearing materials; microscopic arrangement and fatigue strength are also very good. One of the reasons is that there is no large particle carbide in M50iL; thus; this steel is carbide The fatigue cracks incurred are not sensitive.
Although the exploration of M50NiL material is simpler than M50; and the data is more convenient for rolling and casting; but to obtain the required nucleation layer, core microstructure and certain data characteristics; it is necessary to accurately control the quenching and heat treatment process. In order to discuss the disposal method of M50NiL; SKF has put a lot of effort; invested a lot of money. MRC skills personnel believe that after heat treatment, this data can generate residual compressive stress at the vicinity of the raceway; The stress zone can offset the effect of circumferential stress; then improve the bearing life. Select SKF phase 奕 control process; can obtain higher compressive stress; and the hardening depth is three times higher than the traditional process. SKF used traditional methods M50NiL is used for overheating treatment; the obtained data has a cracking resistance value of 275~350MPa-m1/2; when the bearing transmission speed reaches dn=3 million; it has outstanding crack arresting characteristics. It is necessary to improve the bearing speed and/or occur more. Large external defects; crack resistance value must be close to 700MPa-m1/2. In order to improve core resistance; SKF developed a process; can obtain a heat-treated M50NiL without loss of appearance characteristics Specific core resistance. SKF researchers found that the process can also improve residual compressive stress; then further improve the function and reliability of the bearing. This process involves cooling the workpiece from the austenitizing temperature to the core and surface Markov a temperature between the temperatures at which the body ends are formed; then heating the workpiece to a higher temperature; and tempering the core before cooling and phase change of the surface layer (hardened layer). After selecting the appropriate core tempering cycle; The core can be heat treated to the required toughness and strength; it does not have much effect on the surface properties. It has been proven; according to the selected core tempering temperature; the core hardness should be controlled at 30~45HRC. This core resistance was once only available in CBS600 and Prowear53; today's KSF's thermal treatment process makes the M50NiL with outstanding high temperature resistance also have Song's core resistance. In addition, SKF researchers also found this appearance hardened. Steel also has other features. First, external treatment. Surface treatment such as nitriding iron (FCN) has a favorable effect on M50NiL; it can produce high-pressure stress zone in the appearance of carbide-free microscopic arrangement. >1000MPa). It is expected that this treatment method can improve the corrosion resistance, wear resistance and resistance to fatigue caused by appearance. Second, the weldable function. Because M50NiL has low carbon content; therefore; in the need of bearing and flange or other When similar components or materials are joined to make unit bearings and composite layout components; this information can be used to reduce costs.
Currently, bearings made with M50NiL are being tested or used on 12 different aircraft launchers in the world; SKF is a world leader.
2. The gas turbine engine powered by the ceramic material is extremely high in power; the aircraft speed can reach more than Mach 3. The operating conditions of the spindle bearing of the engine are very high; the spindle speed is estimated to exceed 30,000 rpm; the maximum temperature of the bearing is about 800~900 °C. It can be seen from the discussion; the operating temperature above 650 °C uses the high temperature alloy data; the longevity number is expected; the hope is confused; and the ceramic material brings the bearing operating temperature to significantly higher than 650 °C. Expectation.
SKF has been studied; selected a group of high-performance ceramic materials that can satisfy the needs of ultra-high temperature bearing operations; at high temperatures above 1100 °C; among these ceramic materials; one of the best functions; this is the development of the past ten years. Hot pressed silicon nitride or isobaric silicon nitride (Si3N4). Silicon nitride is an aspirational material; it has excellent high temperature strength and hardness; and favorable strength/component contact. When smooth enough It also has excellent anti-rolling fatigue characteristics. In 1984, SKF used a solid smoothing agent in the United States to conduct long-term experiments at temperatures above 500 °C.
However, silicon nitride also has defects; it contains low tensile strength; poor crack arrest resistance and thermal expansion coefficient is extremely low. Therefore, it is necessary to make and use ceramic bearings;
Currently, SKF researchers are identifying the suitability of other ceramic materials such as silicon carbide (SiC), titanium carbide (TiC) and silicon oxynitride (SiAlON) for ball and ferrule data; SKF has used silicon carbide Bearing test at 40,000 rpm. The advantageous function of silicon carbide as a high temperature bearing is excellent thermal conductivity, thermal diffusivity and oxidation resistance, and high purity of data (there is no influence due to impurities); First, the modulus of elasticity is high; about 50% higher than that of hot-pressed silicon nitride; this is considered to be a potential problem; because it has the risk of high-hertz contact stress. SKF researchers have tried to think about the adjusted raceway. The curvature ratio is used to reduce the impact of this; but doing so will incur aggravation of the conflict.
3. Solid smoothing agent because of the high temperature of the aircraft launching machine in the future; the bearing under such working conditions should be used to be smooth and smooth; the existing various compositions of smoothing agents are not enough. It is well known; At the limit of the promise; high-temperature, functionally stable solid compounds such as oxides, sulfides and fluorides rich in various chemical elements can be used.
Many common solid smoothing agents have become useful smoothing agents because their crystal lattices are simply broken; as is the case with graphite and molybdenum disulfide. In air, the temperature limit of graphite smoothness depends on its degree of oxidation. After the addition of oxidant or metal salt; can significantly improve the oxidation limit; and then improve the function of the bearing graphite film.
SKF's experiments indicate that silicon nitride is smoothed with graphite rich in high-temperature increasing agents; its appearance can form a conflict-resistant chemical film; it can reduce the collision coefficient of silicon nitride;
SKF has also studied other solid smoothing agents that can accept high temperatures above 550 ° C; their thermal stability is better than graphite and molybdenum disulfide. Currently; MRC Bearing Company is working on lead oxide (PbO), a total A calcium fluoride/calcium fluoride (CaF/BaF) mixture and a bismuth molybdenum compound were tested. A solid smoothing agent with high temperature resistance and excellent smoothness is expected to be available in the near future.
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