Prediction of air-oil interface location in tied-shaft type fluid dynamic bearings

Abstract

Tied-shaft type Fluid dynamic bearings (FDB) are widely used for small, high precision motor applications such as hard disk drives. This bearing contains two or more air-oil interfaces that cause the meniscus location to change as the bearing rotates from the stationary condition. To prevent oil shortage or leakage at the seal of the FDB, designing an internal geometry of the FDB that can maintain the airoil interface location at a suitable range is critical. This study proposes a system-level prediction model for the air-oil interface location in the taper seal element of the FDB in operating conditions. The FDB elements, grooves and a circulation hole are represented as pumps and flow resistance. With these representations, the FDB can be analyzed with a lumped parameter model. In addition, a hydrostatic equilibrium condition that incorporates capillary force at the meniscus in the taper seal element is used. The location of the meniscus is determined by the pressure difference between the upper and lower seal that is generated by the hydrodynamic effect inside the FDB in operating conditions. The characteristics of the FDB elements are obtained from CFD analysis and are used to solve five equations that predict the system-level performance characteristics: Upper/lower meniscus height, flow rate, pressure and flying height. With the proposed prediction model, the effects of the upper/lower chamfer location and the inclination angle of the circulation hole are investigated. These results confirm that an oil shortage occurs if the upper chamfer radius is below 1.5 mm, and the upper/lower chamfer radii have a bigger effect on the meniscus location than the inclination angle of the circulation hole.