**Comparison of Rolling Bearing Characteristics for Main Shafts** | Bearing Type | Tapered Roller Bearings | Single Row Cylindrical Roller Bearings | Double Row Cylindrical Roller Bearings | Angular Contact Ball Bearings (Pair or Group) | Angular Contact Ball Bearings | |------------------------------------|--------------------------|----------------------------------------|----------------------------------------|-----------------------------------------------|-------------------------------| | **Load Capacity - Radial** | □ | □ | △ | □ | □ | | **Load Capacity - Axial** | □ | ← | ☓ | ☓ | ←←← | | **Axial Load Direction** | ○ | ☓ | ☓ | □ | □ | | **High-Speed Performance** | □ | □ | △ | ○ | □ | | **High Precision** | □ | □ | □ | ○ | ○ | | **Low Noise and Torque** | □ | △ | △ | ○ | □ | | **Rigidity** | ○ | □ | ○ | △ | □ | | **Notes** | 2 sets in opposite use | N(NN) type, NU(NNU) type | Contact angles: C(15°C), AC(25°C), 60°C| 2 sets or more combined use | Not applicable | **Material Performance Comparison: Silicon Nitride (Si3N4) vs. Bearing Steel (GCr15)** | Property | GCr15 | Si3N4 | Ratio (Si3N4 / GCr15) | |----------------------------------|---------------------------|--------------------------|------------------------| | Specific Heat Resistance (°C) | 180 | 800 | 4.44 | | Density (g/cm³) | 7.8 | 3.2 | 0.41 | | Linear Expansion Coefficient | 12.5×10⁻⁶/°C | 3.2×10⁻⁶/°C | 0.26 | | Elastic Modulus (MPa) | 20.8×10⁴ | 31.4×10⁴ | 1.52 | | Vickers Hardness (HV) | 700–800 | 1300–1800 | – | | Poisson’s Ratio | 0.3 | 0.26 | 0.87 | | Thermal Conductivity (W/m·K) | 4.9–50.2 | 29.3 | – | | Corrosion Resistance | – | Good | – | | Magnetic Properties | Strong | Non-magnetic | – | | Electrical Conductivity | Conductive | Insulating | – | **Development of High-Speed Spindle Bearings** To meet the demands of high-speed machine tools, spindle bearings must be highly rigid and capable of operating at high speeds. Angular contact ball bearings are typically the best choice due to their superior high-speed performance, followed by cylindrical roller bearings (see Table 1). As speed increases, a suitable lubrication system becomes essential. The dmN value, which represents the bearing's high-speed capability (dm = pitch circle diameter in mm, N = rotational speed in rpm), is an important metric. Under grease lubrication, the dmN value is generally below 50×10⁴. With oil and gas lubrication, this value can exceed 100×10⁴. Ceramic ball angular contact bearings have further pushed the limit, achieving dmN values over 200×10⁴. By the 1990s, jet lubrication allowed dmN values to reach up to 300×10⁴. **High-Speed Spindle Bearing Technology** Angular contact ball bearings are widely used in high-speed spindles due to their contact angle, which enhances axial rigidity. However, larger contact angles also increase gyroscopic and spin slip, leading to higher heat generation. To improve speed performance, engineers reduce ball size, adjust the curvature coefficient, and increase the number of balls to enhance rigidity and reduce internal load. **Ceramic Ball Angular Contact Bearings** Hybrid ceramic bearings using silicon nitride (Si3N4) as the rolling elements offer significant advantages over traditional steel balls. They are lighter, reducing centrifugal force and improving thermal management. Their low thermal expansion minimizes preloading, and their high hardness improves rigidity. These properties allow ceramic bearings to achieve about 25–35% higher speeds and three times the life of steel bearings. New hybrid bearings with stainless steel inner rings have been developed to further reduce preload caused by centrifugal forces. This helps maintain stability at high speeds. Ceramic inner ring bearings also offer improved rigidity and reduced heat generation, enabling even higher speeds. However, care must be taken to avoid excessive mating stress between the ceramic bearing and metal spindle. **Lubrication for High-Speed Spindle Bearings** As spindle speeds increase, traditional grease lubrication becomes insufficient. Oil and gas or oil mist lubrication methods are preferred for higher dmN values, offering better temperature control and allowing higher speeds. However, these systems require more complex equipment and consume more energy. Jet lubrication, while effective, is often too costly for widespread use. At major machine tool exhibitions, such as the Tokyo International Machine Tool Expo (1996) and Osaka International Machine Tool Expo (1998), over 85% of exhibited CNC machines used ceramic rolling elements, oil and gas lubrication, or oil mist systems, indicating a clear trend toward high-speed, high-performance spindle technology. **Conclusion** Increasing spindle bearing speed is key to achieving high-efficiency machining. Ceramic materials significantly enhance dmN values, making them ideal for high-speed applications. While grease lubrication remains common, oil and gas or oil mist lubrication is becoming the standard for high-speed spindles. Future developments will likely focus on optimizing these systems for even greater performance and reliability.

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