Jewel Bearings for Micro Disk Drives

Current trends in the disk drive industry mandate ever-increasing data storage density. While the overall size of disk drives gets smaller, the total memory capacity has been increasing. The areal density of disk-drives is increasing at annual rate of 60% and is expected to reach 10 Gbit/in^2 by the turn of the century [1.1]. To achieve this goal, the linear track density must exceed 25,000 tracks per inch, which corresponds to tracks that lie on a 1 um pitch. Having narrower tracks means more stringent position control of the actuator arm. With the trend towards smaller and smaller form factor disk drives, the frictional effects in the ball bearing become critical, as friction limits the ability to position the arm accurately. Therefore, it is important to understand the mechanics of the bearing. A well-known model is the Dahl's model, developed by Dahl to account for the dry friction in ball bearing. In a typical assembly, the outer race is stationary and the inner race rotates about the bearing axis, as shown in Fig. 2. As the inner race rotates, the rolling and spinning resistances slowly build up through a small angle of rotation and approach the steady state value asymptotically [1.2]. Because of this dynamics, the frictional torque in ball bearings can be dependent upon position as well as velocity. Detailed analysis of the ball bearings is done in [1.2]