**High-Speed Spindle System of CNC Gold Cutting Machine (2)**
Home > Bearing Knowledge > High-speed spindle system of CNC gold cutting machine (2)
Source: Bearing Network | Time: March 7, 2013
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In recent years, with the development of ceramic cylindrical hybrid bearings and the smooth application of oil-gas lubrication, the high-speed performance of cylindrical roller bearings has been significantly improved. As a result, there are now more suitable options for medium-speed machining. The rear bearing of the electric spindle is typically selected as a cylindrical roller bearing that allows for greater relative movement between the inner and outer rings and can withstand higher radial loads (see Figure 4). This design enhances stiffness and reduces vibration, making it ideal for high-speed applications. For instance, Step-Tec in Switzerland produces high-speed electric spindles that use ceramic cylindrical hybrid bearings.
In addition to rolling bearings, several other types of bearings are used in high-speed electric spindles:
**Hydrostatic Bearings**: These offer high precision but have poor rigidity. Although they provide excellent reversal accuracy, they suffer from significant friction loss. Oil hydrostatic bearings experience turbulence in the oil chamber during high-speed rotation, leading to increased liquid collision forces, power loss, and heat generation. Therefore, they are rarely used in high-speed cutting machines. However, IBAG offers an oil-static bearing electric spindle capable of reaching 32,000 r/min, and FISCHER is developing a hydrostatic bearing electric spindle called Hydro-F, which can achieve up to 36,000 r/min (67 kW).
**Dynamic and Static Bearings**: These combine fluid dynamics and hydrostatic support to maintain the main shaft within an oil film. They offer high rigidity, precision, and damping, and are widely used in horizontal machining centers and high-speed grinding machines. However, when the spindle speed exceeds 50 m/s, the oil flow transitions from laminar to turbulent, causing excessive heat and power loss. Further increasing the speed remains a challenge in the industry.
**Magnetic Bearings**: These are high-performance bearings that use electromagnetic force to suspend the main shaft in the air. Active sensors continuously monitor the spindle's position and adjust the electromagnetic force in real-time, ensuring precise alignment. Magnetic bearings allow for controllable stiffness and damping, enabling active balancing. Their reversal accuracy can reach up to 0.1 μm, and their maximum linear speed can reach 200 m/s. Despite these advantages, magnetic bearings are expensive and not widely adopted in machine tools. Companies like GMN in Germany and IBAG in Switzerland offer magnetic bearing electric spindles, with IBAG producing models that reach up to 70,000 r/min.
The dynamic balance of the high-speed spindle is critical, as centrifugal force increases with the square of the spindle speed. Even small imbalances can cause significant vibrations, affecting machining quality. Professional electric spindle manufacturers perform strict dynamic balancing, often meeting ISO G0.4 standards. Additionally, FISCHER provides online active balancing systems to compensate for changes in tool imbalance during operation.
The connection between the spindle and the tool also plays a crucial role in processing quality. Traditional 7:24 solid shanks can lead to misalignment at high speeds due to centrifugal expansion. To address this, HSK tool holders have become popular in high-speed machining. HSK stands for "Hollow Shank Taper" and features a double-taper and face positioning system. Its short taper (1:10) and thin-walled cone ensure a tight fit under axial tension, eliminating gaps and improving rigidity. HSK holders are available in various types, such as A, B, C, D, E, and F, with Type A and E being the most commonly used.
Despite the progress in high-speed machining, the spindle speed has reached a relatively stable stage. Medium-speed spindles typically range from 20,000 to 60,000 r/min, while large ones operate between 8,000 and 24,000 r/min. Electric spindles are the primary choice, using rolling bearings with a dmh value usually below 250×10â´. Future developments may push this limit to 300×10â´ or higher, allowing for speeds up to 50,000 r/min. For even higher speeds, magnetic bearings remain the key solution, though cost and technical challenges must be addressed.
Currently, domestic electric spindle manufacturers are limited, and market demand is still largely met by imported products. However, China has made significant progress in research and development, and with government support, the future of domestic high-speed electric spindles looks promising.
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