SKF aviation rolling bearing skills (1)
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**First, the latest developments in aviation engine bearings**
Since the 1950s, M50NiL bearing steel has been widely used in aviation applications. Over the years, the speed of these bearings increased steadily, with dn values reaching nearly 2.5 million. By the 1990s, high-speed and high-temperature conditions in aircraft engines placed even greater demands on rolling bearings. While existing bearing steels, such as M50, 18-4-1, and 14Cr-4Mo derivatives like CRB-7 and GB-42, performed well at elevated temperatures, they faced a critical issue—cracking under ultra-high speeds without warning.
To address this, SKF’s MRC Bearing Company, with support from the U.S. Air Force, conducted extensive research and eventually selected M50NiL. This material offers improved crack resistance, better microstructure, and higher fatigue strength compared to other high-temperature bearing materials. One key advantage is that M50NiL lacks large carbide particles, making it less prone to fatigue cracking.
Although M50NiL is easier to process than M50, achieving the desired microstructure and mechanical properties requires precise heat treatment. SKF invested significant resources into optimizing the quenching and tempering process. The company found that after proper heat treatment, M50NiL can generate residual compressive stress near the raceway, which helps counteract circumferential stress and extend bearing life.
Through advanced techniques, SKF achieved a hardening depth three times greater than traditional methods. When tested at dn=3 million, M50NiL demonstrated excellent crack arrest characteristics with a fracture toughness of 275–350 MPa·m¹/². To meet future demands, the required crack resistance must reach around 700 MPa·m¹/². SKF developed a specialized process to enhance core resistance without compromising surface properties, ensuring both durability and performance.
Additionally, surface treatments like nitriding (FCN) have proven beneficial for M50NiL. These treatments create a compressive stress zone in the carbide-free microstructure, improving corrosion resistance, wear resistance, and fatigue performance. Another advantage is the material’s low carbon content, which makes it weldable and suitable for joining components in unit bearings or composite structures, reducing overall costs.
Currently, bearings made from M50NiL are being tested or used in 12 different aircraft launchers worldwide, with SKF leading the industry.
**Second, ceramic materials for high-temperature turbine engines**
Ceramic materials offer a promising solution for next-generation gas turbine engines, where speeds can exceed Mach 3 and operating temperatures can reach up to 800–900°C. Traditional high-temperature alloys struggle to perform reliably at such extreme conditions, but ceramics, particularly hot-pressed silicon nitride (Si₃N₄), show great potential.
Silicon nitride exhibits excellent high-temperature strength, hardness, and contact properties. It also demonstrates superior rolling fatigue resistance when smooth enough. In 1984, SKF conducted long-term tests at temperatures above 500°C using solid lubricants, proving the feasibility of ceramic bearings in high-temperature environments.
However, silicon nitride has limitations, including low tensile strength, poor crack resistance, and a low thermal expansion coefficient. To overcome these challenges, SKF researchers are exploring alternatives like silicon carbide (SiC), titanium carbide (TiC), and silicon oxynitride (SiAlON). SiC, for example, has excellent thermal conductivity, oxidation resistance, and high purity, though its higher elastic modulus may lead to increased Hertzian contact stress. Adjustments in raceway curvature have been considered to mitigate this risk.
**Third, solid lubricants for high-temperature applications**
As aircraft engines become more powerful, traditional lubricants often fail under extreme temperatures. Solid lubricants such as oxides, sulfides, and fluorides rich in chemical elements have shown promise. Graphite and molybdenum disulfide, for instance, are effective due to their layered crystal structures that break down easily, allowing for smooth operation.
SKF experiments revealed that combining graphite with high-temperature additives can significantly improve oxidation resistance, extending the lifespan of bearing films. Other high-temperature solid lubricants, such as lead oxide (PbO), calcium fluoride (CaF₂/BaF₂), and bismuth molybdate compounds, are also being tested. The goal is to develop a high-performance solid lubricant that can withstand temperatures above 550°C while maintaining excellent lubricity.
In summary, advancements in bearing materials and lubrication technologies are critical for meeting the demands of modern aviation. With ongoing research, SKF continues to push the boundaries of what is possible in high-speed, high-temperature bearing applications.
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