Abstract:
A production method for a fluid dynamic pressure sintered bearing includes: preparing a sintered bearing having a porosity of 8 to 20 vol % as a material; and controlling at least one of an overall length, an outer diameter, and an inner diameter of the sintered bearing by repressing the sintered bearing. The production method further includes: forming grooves for generating a fluid dynamic pressure on a bearing surface of the sintered bearing by performing repressing and plastic working on the sintered bearing; and sealing pores exposed on the bearing surface by infiltrating a resin into at least the pores; and barreling entire surface of the sintered bearing by magnetic barreling or electromagnetic barreling.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a production method for a fluid dynamic pressure bearing composed of a sintered compact. In particular, the present invention relates to a production method for a fluid dynamic pressure sintered bearing which is desirably used for compact driving motors of various information devices, for example, the driving motors being driving sources of disc drive devices (which read and write information from and to a magnetic disc or an optical disc such a CD or a DVD) or polygon motors of laser printers.  
         [0003]     2. Description of Related Art  
         [0004]     In these kinds of compact driving motors of various information devices, not only rotational performances of high speed and high precision but also mass production, low cost, and low noise are required to be improved. Whether these properties are good or not depends on a bearing which supports a shaft. In recent years, the above fluid dynamic pressure sintered bearing has been widely used as a bearing which can meet the above requirements. In the fluid dynamic pressure sintered bearing composed of a sintered compact, an oil film composed of lubricating oil is formed in a small gap between a shaft and the bearing, and the oil film is compressed by rotating the shaft, so that the shaft is supported with high stiffness. This fluid dynamic pressure sintered bearing is known as a non-contact type bearing. Grooves for generating a fluid dynamic pressure are formed on bearing surfaces (an inner peripheral surface and an end surface) on which the shaft slides, so that the generated fluid dynamic pressure can be effectively obtained. The grooves may be herringbone grooves or spiral grooves (see Japanese Unexamined Patent Application Publication No. 2003-262217).  
         [0005]     The fluid dynamic pressure sintered bearing has pores. For example, a fluid dynamic pressure sintered bearing for compact motors has a porosity of about 15 vol %. When the pores exist on a bearing surface, lubricating oil infiltrates the pores of the bearing, the pressure of oil film is decreased, so that effects of fluid dynamic pressure are decreased. In order to solve this problem, the sealing of at least the pores on the bearing surface is desirable and the decrease of fluid dynamic pressure is thereby prevented. The sealing uses mechanical impacting (for example, shot blasting or sand blasting) or infiltration of suitable resin into pores (see Japanese Unexamined Patent Application Publication No. Hei 11-62948).  
         [0006]     However, in the sealing by the mechanical impacting, grooves formed for generating a fluid dynamic pressure may wear, an inner peripheral surface shape of the bearing may be deformed, and it is difficult to sufficiently seal the pores. Due to these, the fluid dynamic pressure is inevitably decreased to some degree. On the other hand, when the resin infiltrates the pores, the sealing condition is good. However, it is difficult to sufficiently remove the resin on the surface by water washing, and the resin inevitably remains thereon. Due to this, size precision of the bearing is deteriorated. When plastic working by repressing (sizing) after the resin infiltration is performed so that grooves for generating a fluid dynamic pressure are formed, the resin is adhered to a male die of sizing core or the like. In the case of repressing by using the male die to which the resin adheres, size precision of the bearing is deteriorated and deformation thereof occurs. Since the fluid dynamic pressure sintered bearing is porous, even when ejection of green compact, which corresponds to the fluid dynamic pressure sintered bearing, from die is performed after compacting in the die, a spring back is small and transfer properties of grooves are good. However, when the fluid dynamic pressure sintered bearing which has the infiltrated resin is repressed, a spring back becomes large due to decrease of pores, and the amount of the spring back is uneven.  
       SUMMARY OF THE INVENTION  
       [0007]     Therefore, an object of the present invention is to provide a production method for a fluid dynamic pressure sintered bearing, which can prevent change of the bearing in size after repressing and thereby can improve size precision of the bearing, thereby improving rotational performances of high speed and high precision and bearing properties of low noise.  
         [0008]     According to one aspect of the present invention, a production method for a fluid dynamic pressure sintered bearing includes: preparing a sintered bearing having a porosity of 8 to 20 vol % as a material; and controlling at least one of an overall length, an outer diameter, and an inner diameter of the sintered bearing by repressing the sintered bearing. The production method further includes: forming grooves for generating a fluid dynamic pressure on a bearing surface of the sintered bearing by performing repressing and plastic working on the sintered bearing; and sealing pores exposed on the bearing surface by infiltrating a resin into at least the pores; and barreling entire surface of the sintered bearing by magnetic barreling or electromagnetic barreling.  
         [0009]     In the production method of the present invention, the fluid dynamic pressure sintered bearing is obtained as follows. That is, both the size control of the whole sintered bearing and the formation of the grooves on the bearing surface are performed by the repressing. Next, the pores exposed on the bearing surface are sealed by the resin infiltration. Finally, the sintered bearing is subjected to the magnetic barreling or the electromagnetic barreling. Thus, the fluid dynamic pressure sintered bearing is obtained in the above process order. In this production method, since the sealing of the pores is first performed by the resin infiltration, in comparison with a case sealing is performed by mechanical impacting (for example, shot blasting), deformation of the grooves, the inner peripheral surface supporting the shaft, and the like can be prevented, and decrease in the fluid dynamic pressure can be prevented by the sufficiently sealing of the pores.  
         [0010]     When the pores are sealed by the resin infiltration in the conventional technique, the resin remaining on the surface of the sintered bearing after water washing of resin causes decrease in size precision. In contrast, in the production method of the present invention, since the sintered bearing is subjected to the barreling after the resin infiltration and the entire surface of the sintered bearing is polished, the resin remaining on the surface is removed by the barreling, so that size precision of the sintered bearing can be secured. Since the resin infiltration is performed after the all repressing, adhesion of the resin to the male die for the repressing can be prevented, so that deterioration of size precision, which may be caused by the adhesion, can be prevented. Since in the repressing, the sintered bearing is simply composed of sintered compact having the unsealed pores, the spring back after the repressing of the sintered bearing and the ejection of the sintered bearing from the die is maintained to be small and the transfer properties of the fluid dynamic pressure grooves onto the sintered bearing are maintained to be good.  
         [0011]     The barreling of the present invention is limited to magnetic barreling or electromagnetic barreling. In these barreling, in a barrel (vessel) having a spatial magnetic field generated therein, works are agitated together with plural fine media, and the media give impacts to the works, so that a fine burr and irregularity existing on surfaces of the works are removed and the surfaces of the works are thereby flat and smooth. In particular, these barreling are typical methods which are desirably used for final finishing of works having complex shapes. In particular, when these barreling are used for the bearing of the present invention, these barreling can give impact effects of the media to the inner peripheral surface of the bearing without damaging the shapes of the fluid dynamic pressure grooves formed on the bearing. In the production method of the present invention, by the barreling finally on the bearing, the resin, which remains on the surface after the cleaning of the bearing in the above manner, is removed, so that the bearing surface becomes clean. In addiction, the remaining pores are closed by plastic flow due to the impacts of media, thereby being completely sealed.  
         [0012]     Since the entire surface of the sintered bearing, which includes the inner peripheral surface, is cleaned by the barreling, for example, resin coating for improving the sealing effects can be desirably performed. For example, a resin coating layer can be formed on the entire surface which includes the inner peripheral surface. The resin coating layer can be composed of a fluororesin and have a thickness of 5 μm or less. Since the entire surface is oil-repellent to the lubricating oil by the resin coating, even when the pores remain inside the bearing, the infiltration of the lubricating oil into the bearing can be prevented, so that fluid dynamic pressure effects can be more improved.  
         [0013]     In the production method of the present invention, both the size control of the whole sintered bearing and the formation of the fluid dynamic pressure grooves on the bearing surface are performed by the repressing. Next, the pores are sealed by the resin infiltration, and the sintered bearing is subjected to the magnetic barreling or the electromagnetic barreling. By performing the processing in the above process order, the pores can be sufficiently sealed and the decrease of fluid dynamic pressure can be prevented effectively. In addition, since the size precision can be improved, bearing performances (for example, rotational performance of high speed and high precision, and low noise) can be improved. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1  is a longitudinal cross sectional view showing a fluid dynamic pressure sintered bearing produced by a production method of an embodiment according to the present invention.  
         [0015]      FIG. 2  is a plan view showing the fluid dynamic pressure bearing shown in  FIG. 1 .  
         [0016]      FIG. 3  is a cross sectional view showing the fluid dynamic pressure bearing shown in  FIG. 1 .  
         [0017]      FIG. 4  is a diagram showing a process order of the production method of the embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0018]     An embodiment of the present invention will be described hereinafter with reference to the drawings.  
         [0019]      FIG. 1  is a longitudinal cross sectional view showing a fluid dynamic pressure sintered bearing  1  (hereinafter simply referred to “bearing”). The bearing  1  is cylindrical and is produced by a production method of the embodiment according to the present invention.  FIG. 2  is a plan view showing the bearing  1  shown in  FIG. 1 .  FIG. 3  is a cross sectional view showing the bearing  1  shown in  FIG. 1 . The bearing  1  is a compact bearing which is desirably used for spindle motors of magnetic record disc drive devices. For example, the bearing  1  has an outer diameter of about 5 to 6 mm and an inner diameter (that is, a diameter of shaft hole  11 ) of about 2 to 3 mm. The bearing  1  is composed of a sintered compact obtained by sintering a green compact formed by compacting a raw metal powder. The bearing  1  has a porosity of 8 to 20 vol %. In practical use of the bearing  1 , lubricating oil infiltrates pores of the bearing, so that an oil-impregnated sintered bearing is obtained.  
         [0020]     The bearing  1  rotatably supports a shaft (denoted by reference numeral  2  shown in  FIG. 3 ) which is inserted into the shaft hole  11  of the bearing  1 . In this case, the shaft  2  has a shaft body, which is inserted into the shaft hole  11 , and a flange which is formed on the shaft body. In  FIG. 1 , the shaft body  2  is inserted into the shaft hole  11  from the upside, and the flange faces an upper surface of the bearing  1 . A radial load of the shaft  2  is supported by an inner peripheral surface  13  of the bearing  1 , and a thrust load of the shaft  2  is supported by an upper surface  12  of the bearing  1 .  
         [0021]     As shown in  FIG. 2 , on the upper surface  12  of the bearing  1 , plural spiral grooves  14  are formed at equal intervals in one circumferential direction. The spiral grooves  14  extend so as to inwardly curve toward a rotation direction R of the shaft  2 . In  FIG. 2 , the spiral grooves  14  are shown by using hatched lines so as to be distinguished from the upper surface  12 . End portions on peripheral sides of the spiral grooves  14  open to a peripheral surface. End portions on inner peripheral sides of the spiral grooves  14  do not open to an inner peripheral surface  13  of the shaft hole  11  so as to close. The number of the spiral grooves  14  is about 10 (for example,  12  in  FIG. 2 ). The maximum depth of the spiral groove  14  is about 10 to 20 μm.  
         [0022]     As shown in  FIG. 3 , plural separation grooves  15  are formed at equal intervals in a circumferential direction on the inner peripheral surface  13  of the shaft hole  11  of the bearing  1 . The separation grooves  15  are semi-circular arcs in cross section, and straightly extend from one end surface of the bearing  1  to the other end surface thereof in an axial direction. Eccentric grooves  16  are formed between the respective separation grooves  15  of the inner peripheral surface  13 . Centers of the eccentric grooves  16  are eccentric with respect to an axial center P of the outer diameter of the bearing  1 . The eccentric grooves  16  are inwardly biased toward one rotation direction of the shaft  2  shown by an arrow R. In this case, as shown in the drawings, the number of the separation grooves  15  is 5, and the number of the eccentric grooves  16  is 5. These numbers are desirably 3 to 6.  
         [0023]     A small gap between the inner surfaces of the eccentric grooves  16  and a peripheral circumferential surface of the shaft  2  is wedge-shaped in cross section so as to be gradually narrower and smaller in the rotation direction of the shaft  2 . In this case, the separation groove  15  has a width corresponding to an angle ? of 8 to 20 degrees in the circumferential direction having the axial center P as a center as shown in  FIG. 3 . The separation groove  15  has a maximum depth of about 0.10 mm.  
         [0024]     A bearing gap of radial side is formed between the inner peripheral surface  13  of the bearing  1  and the peripheral surface of the shaft body of the shaft  2  inserted into the shaft hole  11 . A bearing gap of thrust side is formed between the upper end surface  12  of the bearing  1  and the flange of the shaft  2 . Lubricating oil is supplied into the bearing gaps. For example, the bearing gap of radial side has a width of about 1 to 3 μm, and the bearing gap of thrust side has a width of about 5 to 10 μm.  
         [0025]     In the bearing  1 , when the shaft  2  inserted into the shaft hole  11  is rotated in the arrow R direction as shown in  FIGS. 2 and 3 , the lubricating oil is exuded to the respective separation grooves  15  of the inner peripheral surface  13  and is held therein. The lubricating oil held therein is efficiently moved by the shaft  2 , and enters into the wedge-shaped small gap between the eccentric groove  16  and the shaft  2 , so that an oil film is formed. The lubricating oil entering the small gap flows to the narrower and smaller side thereof, and it thereby is under high pressure due to the wedge effect, so that a high radial fluid dynamic pressure is generated. Portions under high pressure in the oil film are generated at equal intervals in the peripheral direction in accordance with the eccentric grooves  16 . As a result, the radial load of the shaft  2  is supported in a well-balanced manner to have high stiffness.  
         [0026]     On the other hand, the lubricating oil is exuded to the respective spiral grooves  14  formed on the upper end surface  12  of the bearing  1 , and is held therein. One portion of the lubricating oil held therein is moved from the respective spiral grooves  14  by the rotation of the shaft  2 , so that an oil film thereof is formed between the upper end surface  11  and the flange. The lubricating oil held in each spiral groove  14  flows from the peripheral side of each spiral groove  14  to the inner peripheral side thereof, so that a thrust fluid dynamic pressure is generated and it is highest at an end portion on the inner peripheral side thereof. The flange receives the thrust fluid dynamic pressure, so that the shaft  2  is floated by small amount. As a result, the thrust load of the shaft  2  is supported with high stiffness in a well-balanced manner.  
         [0027]     Next, a production method for the above bearing  1  of the embodiment according to the present invention will be explained.  FIG. 4  is a diagram showing a process order of the production method. 
    1. Compacting of Raw Powder and Sintering    
 
         [0029]     First, a raw powder of a metal powder is compacted, so that a green compact, which has a near net shape corresponding to the bearing  1 , is obtained. The green compact is provided in a sintering furnace and is sintered therein, so that a sintering compact having a porosity of 8 to 20 vol % is obtained as a material. 
    2. Size Control by Repressing and Forming of Fluid Dynamic Pressure Groove    
 
         [0031]     Next, the obtained sintered bearing is set in a die having a predetermined shape and it is repressed therein, so that an outer diameter, an inner diameter, and axial direction length (height) of the sintered bearing are controlled with a required size precision. By using a core having protrusions corresponding to the above spiral grooves  14 , spiral grooves  14  are transferred and formed on one end surface (the above upper end surface  12 ) of the sintered bearing which has the controlled size. By using a sizing core having protrusions corresponding to the above separation grooves  15  and the above eccentric grooves  16 , separation grooves  15  and eccentric grooves  16  are transferred and formed on an inner peripheral surface  13  of the sintered bearing. 
    3. Infiltration of Resin    
 
         [0033]     The sintered bearing which has the spiral grooves  14  formed on the other end surface and the separation grooves  15  and the eccentric grooves  16  formed on the inner peripheral surface  13  is immersed in a resin solution in vacuum condition. Next, the sintered bearing is opened to the air. A resin solution is infiltrated into pores of the sintered bearing by pressure difference between vacuum and the air. An anaerobic adhesive which is mainly composed of polyglycol dimethacrylate is desirably used as the resin for the infiltration. The resin is cured by heating after being infiltrated into the sintered bearing. Since the infiltrated resin is adhered so as to cover the entire surface of the sintered bearing, the resin on the entire surface including the inner peripheral surface  13  is removed by water washing before the resin is cured. 
    4. Barreling    
 
         [0035]     The sintered bearing, of which the pores are sealed by the resin infiltration, is subjected to magnetic barreling or electromagnetic barreling. Fine stainless pin having a diameter of about 0.5 mm is desirably used as media for the barreling. Many media give impacts to the surface of the sintered bearing by the magnetic barreling or the electromagnetic barreling. As a result, the entire surface of the sintered bearing, which includes the end surface having the spiral grooves  14  formed thereon and the inner peripheral surface  13  having the separation grooves  15  and eccentric grooves  16  formed thereon, is subjected to barreling by the media. And the resin, which is adhered to the surface of the sintered bearing and cannot be removed by the water washing, is completely removed, and the surface thereof becomes clean.  
         [0036]     When the infiltrated resin is the anaerobic adhesive, the volume of the resin expands in the curing by the heating, and the resin is exuded to the surface of the sintered bearing, so that the small amount of the resin may remain on the surface of the sintered bearing. When the pores are filled with the resin and the sintered bearing is cooled to room temperature after the curing, the store of the resin contracts, so that the small amount of the pores remains. Thus, when the resin remains on the surface or, in contrast, the pores remains thereon, the surface of the sintered bearing is subjected to the barreling, so that the remaining resin is removed or the remaining pores are closed by plastic flow due to the impacts of media and completely sealed. 
    5. Resin Coating    
 
         [0038]     The entire surface of the sintered bearing, which was subjected to the barreling, is covered with a resin by the following coating method, so that a resin coating layer is formed thereon. In the coating method, the sintered bearing is immersed in a resin solution for coating or a resin resolution for coating is sprayed onto the entire surface of the sintered bearing. A material of the resin may be acrylic one or epoxy one. The material of the resin is desirably composed of fluororesin which is quick-drying and is superior in oil repellency. In order not to influence on size precision of the sintered bearing, the coating layer has a thickness of 5 μm or less, and desirably has a thickness of about 1 μm.  
         [0039]     In the production method of the fluid dynamic pressure sintered bearing of the embodiment according to the present invention, the size control of the whole sintered bearing is performed by the repressing. Next, the formation of the spiral grooves  14  on the upper surface  12  and the formation of the separation grooves  15  and the eccentric grooves  16  on the inner peripheral surface  13  are performed by the repressing. After that, the sealing of the pores by the resin infiltration, the barreling (magnetic barreling or electromagnetic barreling), and the resin coating are performed in this process order. As a result, the fluid dynamic pressure sintered bearing is obtained.  
         [0040]     In this method, since the sealing of the pores is performed by the resin infiltration, in comparison with a case sealing is performed by mechanical impacting (for example, shot blasting), deformation of the spiral grooves  14 , the separation grooves  15 , the eccentric grooves  16 , the inner peripheral surface  13  supporting the shaft  2 , and the like can be prevented, and decrease in the fluid dynamic pressure can be prevented by the sufficiently sealing of the pores. Since the resin infiltration is performed after the all repressing, adhesion of the resin to the male die for the repressing can be prevented, so that deterioration of size precision of the sintered bearing, which may be caused by the adhesion, can be prevented. Since in the repressing, the sintered bearing is simply composed of sintered compact having the unsealed pores, the spring back after the repressing of the sintered bearing and the ejection of the sintered bearing from the die is maintained to be small, and the transfer properties of the fluid dynamic pressure grooves onto the sintered bearing are maintained to be good. Since the sintered bearing is subjected to the barreling after the resin infiltration and the entire surface of the sintered bearing is polished, the resin remaining on the surface or the pores remaining thereon are removed by the barreling, so that the size precision can be maintained. Due to these, in the obtained fluid dynamic pressure sintered bearing, the pores can be sufficiently sealed and the decrease of fluid dynamic pressure can be prevented. In addition, since the size precision can be improved, bearing performances (for example, rotational performance of high speed and high precision, and low noise) can be improved.  
         [0041]     Since the entire surface of the sintered bearing is cleaned by the barreling, the resin coating layer can be formed to be good. In this kind of fluid dynamic pressure sintered bearing for motors, a Fe—Cu based metal material is often used therefor from a view point of the time of initial running-in and the strength, but this material easily rusts in the air. However, since the resin coating layer is formed on the surface of the sintered bearing, the water repellent effects can be improved, and the rusting can be effectively prevented.