Dynamics of the oil-air interface in hard disk drive bearings

Practically all hard disk drives (HDD) built today contain disks supported on fluid dynamic bearings (FDB) because they offer quiet operation and unmatched non-repeatable run-out (NRRO). To operate FDBs with reliability and robustness one must control an exceedingly small amount of oil over a wide range of operating conditions. In particular, the behavior of the oil-air interface (OAI) must be predictable and consistent. This paper presents a linearized analysis of the OAI valid when the flow in the fluid film obeys Reynolds equation and the kinematic boundary conditions at the free surface. It suggests that more grooves reduce the instability. The results establish the occurrence of air intrusion into the oil, which in its late stages may lead to tip streaming, a capillary instability in which minute air bubbles are released into the oil.