Patent ID: 7237956

Claim:
A hydrodynamic bearing comprising: a sleeve having a bearing hole at the nearly central portion thereof, a shaft rotatably inserted into said bearing hole of said sleeve, and a nearly disc-shaped flange secured to one end of said shaft, one face of said flange opposing to the end face of said sleeve and the other face thereof opposing to a thrust plate provided to hermetically seal a region including said end face of said sleeve, wherein herringbone-shaped first and second dynamic pressure generation grooves are provided on at least one of the inner circumferential face of said sleeve and the outer circumferential face of said shaft so as to be arranged in the direction along said shaft, herringbone-shaped third dynamic pressure generation grooves are provided on at least one of the mutually opposed faces of said flange and said thrust plate, said first, second and third dynamic pressure generation grooves are filled with oil having a kinematic viscosity of 4 cSt or more at 40° C. of temperature, one of said sleeve and said shaft is secured to a base and the other is secured to a rotatable hub rotor, and where an outside diameter of the herringbone pattern of said third dynamic pressure generation groove is designated as d 1 o , an inside diameter thereof is designated as d 1 i , a diameter of the turn-back part of the herringbone pattern is designated as d 1 m , the value of the diameter d 1 m being in the range of 1 mm or more and 10 mm or less, wherein a diameter where the oil pressure generated by said third dynamic pressure generation grooves in the direction from the outer circumference to the inner circumference of said flange equals the oil pressure generated in the direction from the inner circumference to the outer circumference thereof is designated as dsy and is represented by: dsy ={( d 1 i 2 +d 1 o 2 )/2} 1/2 , the diameter d 1 m of the turn-back part is determined so that the value obtained by subtracting the diameter d 1 m from the diameter dsy, (dsy−d 1 m ), is in the range of 0.05 mm or more and 0.8 mm or less.