EFFECT OF THE HYDRODYNAMIC BEARING ON ROTOR STATOR CONTACT IN A RING-TYPE ULTRASONIC MOTOR

A hybrid numerical analysis that includes the hydrodynamic bearing effect and elastic contact in a ring-type ultrasonic motor is presented. The two-dimensional time-dependent compressible Reynolds equation is solved numerically by a second order time accurate, noniterative, factored implicit finite difference algorithm. The rotor deformation is described by a one dimensional Green's function obtained by calculating the actual rotor deformation due to a normal point load using a finite element elastic analysis code. The contact problem is solved by an iteration method so that the contact condition and the hydrodynamic bearing condition are satisfied simultaneously. The results show that the hydrodynamic bearing effect, especially the squeeze effect, is significant for ultrasonic frequency contact of the rotor and stator. Surface roughness, contact area, and normal vibrating speed of the stator are important parameters in the hydrodynamic bearing. The disagreement between the friction coefficient needed in the numerical analysis and the experimentally measured one in the previous study, which did not include the air bearing, is settled as well.