Why is the contact stress of hybrid ceramic bearings large, but its life is longer than that of ordinary bearings?

Hybrid ceramic bearings are suitable for harsh working conditions such as poor lubrication and serious pollution [2-3]. Due to the inherently higher stiffness of the ceramic material, hybrid ceramic bearings have a slightly smaller Hertzian contact area, resulting in greater contact and subsurface stresses than an all-steel bearing of the same size. In theory, this leads to a shortened fatigue life of the bearing. However, it has been observed that in some specific applications, hybrid bearings have a longer service life. How to explain this abnormal phenomenon? How to model it? This paper not only answers these questions, but also shows that a generic bearing life model can simulate and explain field observations well. The SKF Generic Bearing Life Model for hybrid bearings is now available.

The ring material of hybrid bearings is bearing steel and the rolling element material is bearing-grade silicon nitride (Si3N4) (fig. 1). Silicon nitride is a ceramic (i.e., non-metallic) material characterized by high hardness, high modulus of elasticity, high temperature and chemical resistance, low density, low electrical conductivity, and low ductility. Because silicon nitride ceramic materials are excellent insulators, hybrid ceramic bearings can be used in AC and DC motors and generators to form effective insulation between the casing and the shaft. Compared to all-steel bearings, hybrid bearings perform well in conditions of poor lubrication and the presence of solid contaminants, but contact stresses are higher due to the higher stiffness of the ceramic rolling elements under the same load.

Figure 1 Hybrid ball and roller bearings are available in various size segments

In addition, hybrid bearings provide higher speed capability and, in most cases, a longer life than an all-steel bearing of the same size in the same operating conditions. Hybrid bearings also perform extremely well under conditions of high acceleration, vibration or pendulum motion. High-speed applications such as machine tool spindles and turbochargers require special bearing designs, special materials and lubrication systems. This trend is expected to continue and many other modern industrial applications will recognize and take advantage of the unique capabilities of hybrid bearings.

The use of ceramics as a bearing material was first proposed in the 1960s to manufacture extreme temperature bearings for aerospace applications. Fully dense hot-pressed silicon nitride has been demonstrated to have the best rolling contact fatigue (RCF) resistance compared to other ceramic materials through component rolling contact fatigue testing. Significant differences in fatigue resistance were also observed between production lots of seemingly identical ceramic balls. In the 1980s, Lorösch et al. (1980) [4] performed fatigue life tests on hybrid ceramic angular contact ball bearings. Using the highest quality ceramic balls, they found that the rolling contact fatigue resistance of hybrid ceramic bearings was comparable to that of all-steel bearings under the same load, despite a 12% increase in contact stress. However, additional tests on a second batch of ceramic balls found a lower fatigue life, suggesting that the quality of the ceramic balls is critical to fatigue life performance.

The quality and reliability of ceramic rolling elements has improved significantly over the past few years. Thanks to the introduction of non-destructive testing (NDE) methods and continuous advancements in the purification and sintering of silicon nitride materials, engineers were finally able to develop ceramic balls with reliable rolling contact fatigue resistance (Galbato et al., 1992)[ 5]. Because of this, the use of hybrid ceramic bearings in high-speed machine tool spindles increased significantly in the 1990s, greatly contributing to the high-speed performance and accuracy of these mechanical components (Cundill, 1993) [6]. Figure 2 shows the improvement in the fatigue strength of ceramics (Cundill, 1990) [7], which is
attributed to the improvement of the hot-press sintering process throughout the manufacturing process and the continuous reduction of porosity and surface defects (which can now be controlled by non-destructive testing).

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