Non-contact measurement method of mechanical stiffness for high revolutionary speed precision fluid dynamic bearing rotors

Mechanical properties of rotors supported by fluid dynamic bearings (FDBs) cannot be directly measured because most of the mechanical properties of FDBs become active only when the rotors are in revolutionary motion. A typical example is the measurement of mechanical stiffness of the rotors that are used for hard disk drives. A popular measurement method so-called pitch modes requires the rotor to be loaded with disks and rotated at a particular revolutionary speed to maintain a certain mechanical stiffness for the pitch mode measurement. Although the measured pitch modes could represent the mechanical stiffness of the rotors, this approach is rather ad hoc. In this article an attempt is made for eliminating the dummy loads and the mechanical contacts by using hydrodynamic force induced excitations. Applying a compressed airstream to a rotating FDB that generates radial shift of the rotor, the proposed method determines mechanical stiffness of the rotor by relating the air pressure and the radial displacement. A whole design process including numerical validation and component calibration is explained and a series of hardware verification testing is also performed.