Machine Tool Main Spindle Bearings with “Air Cooling
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-38- NTN TECHNICAL REVIEW No.80（2012） [ Technical Article ] Machine Tool Main Spindle Bearings with “Air Cooling Spacer” 1. Introduction Machine tools are used for machining parts of automobiles, aircrafts, and construction equipment. Therefore, machine tools are called “mother machines,” and are under constant pressure to increase machining efficiency and improve quality of machined products. In order to improve machining efficiency, increase of rotational speed of the main spindles is effective. However, when the bearings run in high speed, the centrifugal force on the rolling elements will increase. Therefore, lightweight ceramics are used for rolling elements to reduce this centrifugal force. 1) Also, for quality improvement of machined products, bearings are required to improve rotational precision and increase shaft rigidity. For improving rotational precision, it is effective to increase the number of rolling elements and improve dimensional precision for inner/outer rings. On the other hand, for higher rigidity of bearings, it is effective to increase the size of rolling elements and raise the pressurized amount. These improvements tend to increase temperature in the bearings; therefore, it is difficult to achieve high speed and high rigidity at the same time. However, for recent machining centers and field-assisted machining, both high speed and high rigidity are desired for space saving and process integration. Technology for fulfilling high speed and high rigidity has been realized by supplying coolant in the spindle (core-cooling main spindle) and a device to adjust running bearing pressurization; however, these methods require complex main spindle designs. 2) To cope with this issue, NTN is developing air cooling spacer technology , which will be able to decrease the operating temperature of bearings by applying cooling air onto the outside diameter of inner ring spacer . This technology can be used to achieve high speed or high rigidity of machine tools, or both, without complicating the main spindle design. In this paper, the performance of this air cooling spacer is introduced. 2. Structure of bearings with air cooling spacer Fig. 1 shows the structure of bearings with air cooling spacer. The outer ring spacer installed in the center of a back-to-back (DB) arrangement is provided with environment friendly nozzles, which are air cooling nozzles, independent from the air oil supplying nozzle for lubricating bearings. The air cooling nozzles are offset from the spindle center at three locations on the circumference of the ring, pointing toward the direction of rotation. Room temperature air injected from the air cooling nozzles passes between the inner ring spacer and outer ring spacer, and through the bearings swirling toward the direction of the rotation to cool off the bearings. **Machining Tool/Aerospace Engineering Dept., Industrial Machinery Division **Advanced Technology R&D Center High speed and high rigidity performances are always required for high precision bearings for machine tool main spindles. However, since both high speed and high rigidity increase operating temperature, it has been very difficult to achieve high speed and high rigidity performance at the same time. In this paper new “air cooling spacer” technology, which will be able to decrease the operating temperature of bearings by applying cooling air onto the outside diameter of inner ring spacer located, is introduced. Yuushi ONDA* Mamoru MIZUTANI* Masatsugu MORI**
Machine Tool Main Spindle Bearings with “Air Cooling
