Charge generation and bleed-off in spindle motors with ceramic ball bearings

• Published in: IEEE transactions on magnetics Vol. 39; no. 5; pp. 2435 - 2437
• Main Authors: Klaassen, K.B., van Peppen, J.C.L.
• Format: Journal Article Conference Proceeding
• Language: English
• Published: New York, NY IEEE 01.09.2003; Institute of Electrical and Electronics Engineers; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Acoustic emission; Applied sciences; Ball bearings; Ceramics; Charge pumps; Current measurement; Disk drives; Electronics; Exact sciences and technology; Hard disks; Magnetic and optical mass memories; Magnetism; Other magnetic recording and storage devices (including tapes, disks, and drums); Rotors; Seals; Stators; Storage and reproduction of information; Voltage; Electrical engineering; Disk drive; Ferromagnetic fluid; electrostatic damage; Stator; electrical over-stress; Silicon nitride; Theoretical study; Experimental study; Rotor; Ball bearing; spindle motors; hard disk drives; Magnetic disk; Ceramic ball bearings; Electric motor; Hard disk; Ceramic materials; Electrostatics; Magnetic recording
• ISSN: 0018-9464; 1941-0069
• DOI: 10.1109/TMAG.2003.816429

Spindle motors with hybrid ceramic bearings are well suited for high revolution per minute disk drives due to their high rigidity and low acoustic emission. The rolling elements are nonconductive Si/sub 3/N/sub 4/ precision ground balls. They allow buildup of a voltage between the rotor and stator of the motor. This voltage also appears across the head-disk interface. If large enough, this voltage will damage the drive. This paper analyzes the charge pump mechanism (bearings) responsible for the voltage build up. It also examines the charge bleed-off path (ferro-fluid seal), which holds down the maximum voltage that can appear. An electrical model is given for the ferro-fluid seal that is dynamically as well as statically correct. This model is verified by comparing it to measured static and dynamic current-voltage curves and to the voltage step response of the seal. By a proper design of the ferro-fluid seal, its conductivity can be held low enough to limit the motor voltages to safe levels. This makes the use of these hybrid ceramic bearings in today's hard disk drives attractive.