Patent ID: 8300355

Claim:
A fluid dynamic bearing comprising: a stationary shaft that comprises: a first end portion being relatively fixed to a base plate; a second end portion being provided with a flange portion; a first radial bearing surface being defined on an outer circumferential surface of the stationary shaft; a radial bearing member that comprises: a second radial bearing surface that faces the first radial bearing surface to have a first gap therebetween; a first thrust bearing surface being defined on a first end portion of the radial bearing member; and a communication hole that connects a second gap with a third gap formed between a second end portion of the radial bearing member and one end surface of the flange portion, the radial bearing member being supported rotatably with respect to the stationary shaft; a thrust bearing member that is relatively fixed to the base plate and comprising a second thrust bearing surface that faces the first thrust bearing surface to have the second gap therebetween, the second gap communicating with the first gap; a lubricating fluid that fills continuously the first gap, the second gap, the third gap and the communication hole; a radial bearing portion that comprises a radial dynamic pressure groove that is formed on one of the first radial bearing surface and the second radial bearing surface; a thrust bearing portion that comprises a thrust dynamic pressure groove that is formed on one of the first thrust bearing surface and the second thrust bearing surface; and an auxiliary bearing portion that comprises an auxiliary groove for raising pressure of the lubricating fluid in a direction from the flange portion toward the radial bearing portion, the auxiliary groove being formed on one of the first radial bearing surface and the second radial bearing surface at a position between the flange portion and the radial bearing portion, wherein the auxiliary groove is shallower in depth than the radial dynamic pressure groove, and wherein a size of the first gap in the auxiliary bearing portion is equal to or larger than a size of the first gap in the radial bearing portion.