On the Whirl Dynamics of Hydrodynamic Bearing Spindle in Information Storage Systems

Hydrodynamic bearing (HDB) spindles currently being developed for information storage systems such as hard disk drives (HDDs) typically consist of two journal bearings with herringbone grooves. In this work, translational and conical motions of lightly loaded herringbone groove journal bearings are analyzed. For translational motion, the full eigenvalue solution for a typical HDB in a small form factor HDD may be simplified using scaling analysis. Perturbation equations for the stiffness and damping coefficients are derived from the Reynolds equation and solved for the lightly loaded herringbone groove bearing. The resulting analysis shows that the translational motion consists of a forward whirl mode and a backward whirl mode. The whirl frequencies of the two modes have the same magnitude but opposite sign. In addition, a simplified analysis shows that for a no-load herringbone groove journal bearing the in-line stiffness is not trivial while the cross coupling damping vanishes. On the other hand, for the conical mode eigenvalues, the same scaling analysis only applies when the bearing span is large. For typical HDB in HDDs, the conical whirl frequencies depend strongly on bearing and rotor geometries. Numerical calculations were carried out to demonstrate the validity and limitation of the theoretical analysis.