A hydraulic dynamic bearing has a rotor comprised of a flanged shaft
having a thrust ring portion and a cylinder portion, a hub attached to
the flanged shaft, and a rotor magnet attached to an inner peripheral
face of a skirt portion of the hub. A stator rotatably supports the rotor
and comprises a bearing sleeve to which are attached stator coils, and a
stator base plate that supports the sleeve. A gap exists between
confronting surfaces of the flanged shaft and the bearing sleeve, and
lubricating oil is sealed in the gap. Thrust dynamic pressure generating
grooves are formed in one or both faces of the thrust ring, and the
grooves are in the form of a herringbone pattern having radial inner
groove sections and outer groove sections. The depth of the inner groove
sections is shallower than that of the outer groove sections, and the
depths of both groove sections are constant. The optimum inner-to-outer
groove depth ratio is 0.6 to 0.7.