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

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 Si₃N₄ 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.