| Hydrodynamic bearing (HDB) spindles are being considered for disk-drive industry, because HDB spindles have significantly larger damping and lower acoustic noise. Currently, there are two types of design: rotating-hub design and rotating-shaft design. In rotating-hub design, the rotating hub (rotor) that carries the disks is mounted on a stationary shaft (stator) through either ball bearings or HDBs. This design is widely used in ball-bearing spindles. In rotating-shaft design, the rotating shaft and hub (rotor) are pressed into the stationary bearing sleeve and the base plate (stator). This design has fewer components and the bearing lubricant is less likely to leak. Therefore, this design is becoming dominant among HDB spindles.;The purpose of this research is to develop mathematical models predicting free and forced vibrations of both rotating-hub and rotating-shaft spindle systems, supported by HDBs. Equations of motion are derived through Lagrange equations and discretized in terms of spindle rocking, spindle translation, disk eigenmodes, and shaft eigenmodes. For free vibration, an eigenvalue analysis predicts natural frequencies, modal damping, and mode shapes of both spindle systems. The analysis indicates that the hub deformation is critical in the rotating-shaft design. Strain energy associated with the hub deformation needs to be considered in the model of rotating-shaft spindles in order to give accurate predictions of rocking mode frequencies. For forced vibration, use of Laplace transforms and Green's functions predicts transfer functions and transient response of the systems. Compared with disk-spindle systems with ball bearings, those with HDBs have a fundamental change in rocking vibration. Specifically, the rocking vibration consists of a pair of heavily damped rocking modes, a pair of half-speed whirls, and two pairs of lightly damped rocking modes. Also, the flexibility of the shaft significantly reduces the resonance frequencies of the rocking modes. Finally, frequency response functions and transient responses of both rotating-hub spindle system and rotating-shaft spindle system are predicted numerically and compared with experimental measurements to validate the mathematical models. The theoretical results agree well with experimental results, considering the fact that the major bearing properties are uncertain. |
