Sleeve for hydrodynamic bearing device, hydrodynamic bearing device and spindle motor using the same, and method for manufacturing sleeve

Abstract

A bearing stiffness of a sintered metal sleeve is prevented from lowering. A sleeve includes an inner section formed of metal powder for sintering and a resin for impregnation, and a surface deformation section which covers a surface of the inner section and is formed by shot blast process. Since the surface deformation section is formed by the shot blast process, the number of pores formed between the metal powder for sintering near the surface can be reduced. In this way, a supporting pressure at a bearing portion can be prevented from being released out through the pores, and the bearing stiffness can be prevented from lowering.

1 . A manufacturing method of a sleeve for a hydrodynamic bearing device, the method comprising the steps of:
 forming a primary compact form metal powder for sintering;   sintering the primary compact;   sizing the sintered primary compact to form a secondary compact; and   contacting the secondary compact with a high-temperature steam after the sizing step.   
     
     
         2 . A manufacturing method of a sleeve for a hydrodynamic bearing device according to  claim 1 , the method further comprising the step of: finishing a surface of the secondary compact. 
     
     
         3 . A manufacturing method of a sleeve for a hydrodynamic bearing device according to  claim 1 , the method further comprising the step of: removing at least a part of an iron oxide film formed on a surface of the secondary compact. 
     
     
         4 . A manufacturing method of a sleeve for a hydrodynamic bearing device according to  claim 2 , wherein the primary compact or the secondary compact is treated with nonelectrolytic nickel plating process or DLC film coating process in the surface finishing. 
     
     
         5 . A manufacturing method of a sleeve for a hydrodynamic bearing device according to  claim 1 , wherein an average density of a portion of the metal powder for sintering of the secondary compact is 6.8 g/cm 3  or higher. 
     
     
         6 . A manufacturing method of a sleeve for a hydrodynamic bearing device according to  claim 1 , wherein: the primary compact includes a tubular sleeve main body and a tubular projection projecting from the sleeve main body in an axial direction; and a rate of change in a dimension of the tubular projection is larger than a rate of change in a dimension of the sleeve main body in the sizing process. 
     
     
         7 . A manufacturing method of a sleeve for a hydrodynamic bearing device according to  claim 1 , comprising: a sleeve; a shaft inserted into a bearing hole of the sleeve so as to be relatively rotatable; and at least one radial bearing having hydrodynamic grooves formed on at least one of an outer peripheral surface of the shaft and an inner peripheral surface of the sleeve, wherein a volume density of a portion of the metal powder for sintering of the secondary compact is 85% or higher. 
     
     
         8 . A manufacturing method of a sleeve for a hydrodynamic bearing device according to  claim 1 , wherein: the sleeve is brought into contact with a high-temperature steam at an atmospheric temperature within the range of 600° C. to 700° C. for 15 to 50 minutes in the steam process. 
     
     
         9 . A manufacturing method of a sleeve for a hydrodynamic bearing device according to  claim 1 , wherein: the sleeve is brought into contact with a high-temperature steam at an atmospheric temperature within the range of 400° C. to 700° C. for 25 to 80 minutes in the steam process. 
     
     
         10 . A manufacturing method of a sleeve (for a hydrodynamic bearing device) according to  claim 1 , wherein the metal powder for sintering includes pure iron or an iron based powder having a volume density of at least 80% iron. 
     
     
         11 . A manufacturing method of a sleeve for hydrodynamic bearing device according to  claim 10 , wherein the contacting step includes arranging a layer including an oxide on a surface of the secondary compact. 
     
     
         12 . A manufacturing method of a sleeve for hydrodynamic bearing device according to  claim 1 , wherein the secondary compact includes thereon a Fe 3 O 4  film including a thickness of approximately 2 μm to approximately 10 μm after the contacting step.