New Information on Aviation Starter Bearings
Source: Bearing Network | Time: December 16, 2013
Aviation starter bearings have seen significant advancements over the years. From 1955 to 1980, M50NiL bearing steel was widely used, and during this period, the rotational speed of aviation starter bearings steadily increased, with the dn value reaching nearly 2.5 million. In the 1990s, as aircraft engines required higher speeds and temperatures, the demand for high-performance rolling bearings grew significantly.
Despite the development of high-temperature bearing steels such as M50, 18-4-1, and 14Cr-4Mo, along with their derivatives like CRB-7 and GB-42, these materials still performed well at temperatures far beyond current standards. However, a critical limitation remained: the risk of cracking in hardened steel rings under ultra-high-speed conditions, often without warning.
To address this issue, SKF's MRC bearing division, supported by the U.S. Air Force, conducted extensive research and eventually selected M50NiL. This material offers improved resistance to cracking compared to other high-temperature bearing steels, with excellent microstructure and fatigue strength. One key advantage is the absence of large carbide particles, which reduces sensitivity to fatigue cracks caused by carbides.
Although M50NiL is easier to process than M50, achieving the desired core microstructure and properties requires precise control of heat treatment. SKF invested heavily in optimizing the process, leading to the development of a method that introduces residual compressive stress near the raceway. This stress helps counteract circumferential stress at high dn values, thereby extending bearing life.
Using advanced heat treatment techniques, SKF achieved a crack resistance value of 275–350 MPa-m¹/² for M50NiL, which performs exceptionally well at dn = 3 million. To meet future demands, the crack resistance must reach around 700 MPa-m¹/². SKF developed a specialized process to enhance core resistance without compromising surface characteristics. This involves controlled cooling and tempering to achieve optimal hardness (30–45 HRC) while maintaining surface integrity.
Additionally, surface treatments like nitriding (FCN) improve wear and corrosion resistance, creating a high-pressure stress zone on the surface. M50NiL also exhibits good weldability due to its low carbon content, allowing it to be combined with other components, reducing costs and improving design flexibility.
Currently, bearings made from M50NiL are being tested or used on 12 different aircraft launchers worldwide, with SKF leading the market.
In another development, ceramic materials are being explored for use in gas turbine engines. These engines can operate at extremely high speeds, exceeding Mach 3, with spindle bearings operating at over 30,000 rpm and temperatures up to 800–900°C. Traditional high-temperature alloys struggle to perform reliably above 650°C, but ceramics offer the potential for much higher operating temperatures.
SKF has identified high-performance ceramics such as hot-pressed silicon nitride (Si₃N₄) as promising candidates. Silicon nitride is known for its high-temperature strength, hardness, and anti-fatigue properties. Long-term tests conducted in the 1980s demonstrated its performance at temperatures above 500°C.
However, silicon nitride has limitations, including low tensile strength, poor crack arrest resistance, and a low thermal expansion coefficient. Researchers are exploring alternatives like silicon carbide (SiC), titanium carbide (TiC), and silicon oxynitride (SiAlON). SiC shows excellent thermal conductivity and oxidation resistance, though its high modulus of elasticity may increase contact stress. Adjustments in raceway curvature are being considered to mitigate this challenge.
As technology continues to evolve, the development of advanced materials and heat treatment processes will play a crucial role in enhancing the performance and reliability of aviation bearings.
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