Fluid Dynamic Bearing Device

Abstract

It is an object of the present invention to provide a bearing member having a highly accurate dynamic pressure generating portion stably at low cost. Dynamic pressure grooves ( 8 a 1 ) and ( 8 a 2 ) are formed as the dynamic pressure generating portions on an inner circumferential surface of the electroforming portion ( 10 ) by an electroforming process, and injection molding is performed by using a resin while inserting the electroforming portion ( 10 ), thereby forming a bearing member ( 8 ). A shaft member ( 2 ) is inserted into an inner circumference of the bearing member.

1 . A fluid dynamic bearing device, comprising:
 a bearing member; and   a shaft member inserted into an inner circumference of the bearing member,   wherein the bearing member is formed by injection molding in which an electroforming portion is inserted, and any one of an inner circumferential surface of the electroforming portion and an outer circumferential surface of the shaft member, which is opposed to the inner circumferential surface of the electroforming portion is provided with a dynamic pressure generating portion formed thereon.   
   
   
       2 . A fluid dynamic bearing device according to  claim 1 , wherein the dynamic pressure generating portion is formed on the inner circumferential surface of the electroforming portion. 
   
   
       3 . A fluid dynamic bearing device according to  claim 1 , wherein the dynamic pressure generating portion includes a plurality of dynamic pressure grooves. 
   
   
       4 . A fluid dynamic bearing device according to  claim 1 , wherein the dynamic pressure generating portion includes a plurality of circular arc surfaces. 
   
   
       5 . A fluid dynamic bearing device according to  claim 1 , wherein the electroforming portion is provided with a flange. 
   
   
       6 . A fluid dynamic bearing device according to  claim 5 , wherein an outer circumferential surface of the flange has a non-perfect circular shape. 
   
   
       7 . A fluid dynamic bearing device according to  claim 5 , wherein the flange is formed through plastic deformation of the electroforming portion. 
   
   
       8 . A fluid dynamic bearing device according to  claim 5 , wherein the flange is formed on at least one of both axial ends of a bearing surface. 
   
   
       9 . A fluid dynamic bearing device according to  claim 8 , wherein the electroforming portion has an axial length L 2  before the plastic deformation of the flange and an axial length L 1  after the plastic deformation of the flange, which satisfy a following relationship expressed as 0<A/L 1 ≦0.5 (where A=L 2 −L 1 ). 
   
   
       10 . A fluid dynamic bearing device according to  claim 1 , wherein:
 the bearing member is formed of a resin used as an injection molding material; and   a mold shrinkage factor of the resin is set within a range of 0.02% to 2.0% inclusive.   
   
   
       11 . A fluid dynamic bearing device according to  claim 1 , wherein the bearing member is formed of metal used as an injection molding material. 
   
   
       12 . A fluid dynamic bearing device according to  claim 1 , wherein the electroforming portion is provided with a thrust bearing surface formed thereon for supporting an end portion of the shaft member in a thrust direction. 
   
   
       13 . A fluid dynamic bearing device according to  claim 12 , wherein the thrust bearing surface comes into contact with and supports the shaft member in the thrust direction. 
   
   
       14 . A fluid dynamic bearing device according to  claim 12 , wherein any one of the thrust bearing surface and an end surface of the shaft member opposed to the thrust bearing surface is provided with a plurality of dynamic pressure grooves formed thereon. 
   
   
       15 . A fluid dynamic bearing device according to  claim 1 , wherein, the dynamic pressure generating portion has a cross section in a radius direction, in which a radius r 1  of a virtual circle inscribed to the inner circumferential surface of the electroforming portion is larger than a radius r 2  of a virtual circle circumscribed to the outer circumferential surface of the shaft member, and in which a sum of a circularity of the inner circumferential surface of the electroforming portion and a circularity of the outer circumferential surface of the shaft member is 4 μm or less. 
   
   
       16 . A fluid dynamic bearing device according to  claim 1 , wherein the shaft member is a master shaft used at a time of forming the electroforming portion. 
   
   
       17 . A fluid dynamic bearing device according to  claim 1 , wherein the shaft member is a member different from a master shaft used at a time of forming the electroforming portion. 
   
   
       18 . A fluid dynamic bearing device according to  claim 16 , comprising: a thrust bearing surface for supporting an end portion of the shaft member in a thrust direction; a forming portion in which the thrust bearing surface is formed, on one end of the master shaft; and a thrust bearing surface on another end of the master shaft. 
   
   
       19 . A motor, comprising the fluid dynamic bearing device according to  claim 1 . 
   
   
       20 . A method of manufacturing a bearing member, comprising the steps of:
 fabricating a master shaft having a forming portion corresponding to a shape of a dynamic pressure generating portion on an outer circumference of the master shaft;   forming an electroforming portion on the outer circumference of the master shaft including the forming portion;   performing injection molding while inserting the electroforming portion after the electroforming portion is formed; and   separating the master shaft and the electroforming portion from each other after the injection molding.   
   
   
       21 . A method of manufacturing the bearing member according to  claim 20 , wherein the master shaft and the electroforming portion are separated from each other by giving the master shaft and the electroforming portion a difference in thermal expansion amount. 
   
   
       22 . A method of manufacturing the bearing member according to  claim 20 , wherein, when the injection molding is performed while inserting the electroforming portion, the electroforming portion is plastically deformed by clamping a metal mold to form a flange on the electroforming portion.