New Information on Aviation Starter Bearings

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Aviation starter bearings have evolved significantly over the years. From 1955 to 1980, M50NiL bearing steel was widely used, and during this period, the rotational speed of aviation starter bearings increased steadily. The dn value (diameter-speed product) reached nearly 2.5 million. In the 1990s, the demand for high-speed and high-temperature rolling bearings in aviation starter systems became even greater. Despite the performance of existing bearing steels such as M50, 18-4-1, and 14Cr-4Mo, which are still effective at temperatures much higher than typical operating conditions, there remained a critical limitation: the tendency for hardened steel rings to crack under ultra-high speeds without warning. To address this issue, SKF’s MRC division, with support from the U.S. Air Force, conducted extensive research and eventually selected M50NiL as a superior material. M50NiL offers improved resistance to cracking compared to other high-temperature bearing materials. Its microstructure and fatigue strength are also exceptional. 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 microstructure and mechanical properties requires precise control of heat treatment. SKF invested significant resources into optimizing the quenching and tempering processes. Their research found that proper heat treatment can induce residual compressive stress near the raceway, which helps counteract circumferential stresses at high speeds, thereby improving bearing life. By using a specialized heat treatment process, SKF achieved a residual compressive stress that increased the hardening depth by three times compared to traditional methods. This enhanced process allowed the bearing to maintain a crack resistance value of 275–350 MPa-m^1/2 at a dn value of 3 million. For higher speeds or larger external defects, the crack resistance needs to reach around 700 MPa-m^1/2. To meet these demands, SKF developed a new process that enhances core resistance without compromising surface properties. This method involves cooling the workpiece to a specific temperature before re-heating and tempering the core while maintaining the surface's hardness. The core hardness is controlled between 30–45 HRC, ensuring toughness and strength. This advanced heat treatment has made M50NiL not only highly resistant to high temperatures but also capable of withstanding extreme mechanical loads. Additionally, surface treatments like nitriding (FCN) have proven beneficial, creating a high-pressure stress zone on the surface without forming carbides, thus improving corrosion and wear resistance. Another advantage of M50NiL is its weldability due to its low carbon content. This allows for the creation of unit bearings and composite components, reducing manufacturing costs. These features make M50NiL an ideal choice for modern aviation applications. Currently, bearings made from M50NiL are being tested or already used in 12 different aircraft launchers worldwide, with SKF leading the market in innovation and performance. In addition, ceramic materials are gaining attention for use in high-performance gas turbine engines. These engines can achieve speeds above Mach 3, with spindle bearings operating at extremely high temperatures—up to 800–900°C. Traditional high-temperature alloys struggle to meet these demands, but ceramics offer promising alternatives. SKF has explored various high-performance ceramics, including hot-pressed silicon nitride (Si₃N₄), known for its excellent high-temperature strength, hardness, and fatigue resistance. However, silicon nitride also has limitations, such as low tensile strength and poor crack arrest resistance. Other ceramic materials, like silicon carbide (SiC), titanium carbide (TiC), and silicon oxynitride (SiAlON), are being evaluated for their suitability in ball and raceway applications. SiC, for instance, exhibits excellent thermal conductivity and oxidation resistance, though its high modulus of elasticity poses challenges in contact stress management. Overall, the development of advanced materials like M50NiL and ceramics is driving the next generation of high-speed, high-temperature aviation bearings, ensuring safer and more reliable performance in demanding environments.

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Recommended Articles: - Common Questions About Rolling Bearings - How to Install Thrust Bearings - Understanding Bearing Speed Limits - Bearing Torque, Load, and Life Calculations - How to Determine if NTN Spherical Bearings Need Repair Link to this article: http:// Please cite China Bearing Network: http:// Previous: The Method of Bearing Heat Treatment in Foreign Machinery Industry (Part II) Next: Analysis of SKF Bearing Type Base Codes

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