SKF aviation rolling bearing skills (1)
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**First, the New Information of the Aviation Engine Bearing**
From 1955 to 1980, M50NiL bearing steel was widely used for 25 years. During this time, the speed of aviation bearings increased steadily, and the dn value reached nearly 2.5 million. In the 1990s, high-speed and high-temperature demands from aviation machines led to more challenges for rolling bearings.
Despite existing bearing steels such as M50, 18-4-1, and 14Cr-4Mo, along with derivatives like CRB-7 and GB-42, they still performed well at temperatures much higher than typical bearing conditions. However, a major constraint was that hardened steel rings could crack under ultra-high speeds without warning or minimal signs.
To address this, SKF’s MRC bearing company, with support from the US Air Force, conducted extensive research and eventually selected M50NiL. This material offers improved resistance to cracking compared to other high-temperature bearing materials, with excellent microstructure and fatigue strength. One key advantage is the absence of large carbide particles, making it less sensitive to fatigue cracks caused by carbides.
Although M50NiL is easier to process than M50, achieving the desired core structure and properties requires precise control over quenching and heat treatment. SKF invested heavily in optimizing the process, leading to better residual compressive stress around the raceway, which helps counteract circumferential stress and improve bearing life.
Using SKF’s advanced heat treatment process, the hardening depth can be three times greater than traditional methods. This results in a crack resistance value of 275–350 MPa-m¹/² when the bearing reaches a dn value of 3 million. To meet future demands, the crack resistance must reach close to 700 MPa-m¹/². SKF developed a specialized process to enhance core resistance while maintaining surface properties.
The process involves cooling the workpiece to a specific temperature between austenitizing and phase transformation, followed by reheating and tempering the core before surface hardening. By selecting the right tempering cycle, the core hardness can be controlled between 30–45 HRC, ensuring both toughness and strength.
This technology has made M50NiL not only suitable for high-temperature environments but also capable of withstanding extreme mechanical loads. Additionally, surface treatments like nitriding (FCN) have been found beneficial, creating a high-pressure stress zone that improves corrosion resistance, wear resistance, and fatigue performance.
M50NiL also has good weldability due to its low carbon content, making it ideal for joining components in unit bearings and composite layouts. Bearings made with M50NiL are currently being tested or used on 12 different aircraft launchers worldwide, solidifying SKF's leadership in this field.
**Second, Ceramic Materials in Gas Turbine Engines**
Ceramic materials offer extremely high power output, enabling aircraft speeds beyond Mach 3. The operating conditions for spindle bearings in these engines are extreme, with speeds exceeding 30,000 rpm and temperatures reaching up to 800–900°C. Traditional high-temperature alloys struggle to meet these demands, but ceramics allow bearings to operate at significantly higher temperatures.
SKF has researched and selected high-performance ceramic materials, such as hot-pressed silicon nitride (Si₃N₄), which has excellent high-temperature strength, hardness, and contact properties. It also exhibits superior rolling fatigue resistance when smooth enough. In 1984, SKF conducted long-term tests using solid lubricants at temperatures above 500°C.
However, silicon nitride has limitations, including low tensile strength, poor crack arrest resistance, and a very low thermal expansion coefficient. These factors require careful design and application of ceramic bearings.
Currently, SKF is exploring other ceramics like silicon carbide (SiC), titanium carbide (TiC), and silicon oxynitride (SiAlON) for use in ball and ring applications. Silicon carbide, for instance, offers excellent thermal conductivity, oxidation resistance, and high purity. Its modulus of elasticity is about 50% higher than that of silicon nitride, which may increase Hertzian contact stress. Researchers are working on adjusting the curvature ratio to mitigate this issue.
**Third, Solid Lubricants for High-Temperature Applications**
As aircraft engines become hotter, traditional lubricants are no longer sufficient. High-temperature solid lubricants such as oxides, sulfides, and fluorides are increasingly being used. Graphite and molybdenum disulfide are common examples, but their performance depends on oxidation resistance.
Adding oxidants or metal salts can significantly improve graphite’s oxidation limit and enhance its effectiveness. SKF experiments show that combining silicon nitride with graphite-based additives forms a protective chemical film, reducing friction and improving performance.
Other high-temperature solid lubricants, such as lead oxide (PbO), calcium fluoride (CaFâ‚‚/BaFâ‚‚), and bismuth molybdate, are also being tested. These materials offer better thermal stability than graphite and molybdenum disulfide. A new high-temperature lubricant with excellent performance is expected soon.
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