A study on the development of a new conceptual automatic variable preload system for a spindle bearing

In machine-tool spindle systems, rolling bearings are generally the most widely used type of bearing, offering high stiffness and a large load capacity. The performance of bearings is greatly affected by the applied preload. This paper suggests a new automatic variable preload system that uses an eccentric mass device applied to the bearings of a spindle system. A spindle sustains centrifugal force when rotating. The eccentric mass device converts the radial direction force caused by the centrifugal force into axial force using an eccentric mass device. The eccentric mass device applies a small amount of force to the bearing when the spindle rotates at a low speed and applies large force to the bearing when the spindle rotates at a high speed. In this design, the device maintains a large preload at low speeds and lowers the preload at high speeds. Depending on the increase in the spindle rpm, the force that the eccentric mass device delivers to the axial direction will also increase. In addition, the preload applied to the bearings will be reduced. A finite element analysis was conducted to predict the shape of the eccentric mass device and changes of the preload. Based on the analysis results, a prototype was fabricated. According to the results of experiments conducted on the prototype, it was confirmed that the automatic variable preload device with the suggested structure operated satisfactorily. Also, vibration and noise of the prototype were measured and analyzed. The approach suggested in this study is expected to allow reduced manufacturing and operating costs, as the complex devices for hydraulic pressure or electricity required in existing variable preload devices can be eliminated.