Bias in hard disk drive rotary actuator pivot bearings: measurements and lubrication phenomena

Bias in disk drive rotary actuator bearings exhibits complex behavior related to the history of the actuator movement. This paper investigates the hysteresis component of bias present after seeks. Measurements of bias during full stroke actuator movements composed of multiple seeks of fixed length reveal that the magnitude of the hysteresis decreases as the lengths of component seeks increase up to a critical seek length. When the lengths of the component seek lengths are larger than the critical seek length, the magnitude of the hysteresis remains essentially constant. We explain these phenomena using a hypothesis based on bearing lubrication theory and the relationship between actuator velocity and seek length. We demonstrate a good empirical fit to the data which is strikingly similar from drive to drive and strongly supports our hypothesis.