Abstract:
A rolling bearing composed of a combination of bearing wings formed with a steel material and rolling elements formed with a ceramic material, wherein the residual austenite amount of the steel material forming the rolling rings is controlled to a specific value. By the construction, at rolling of the rolling elements formed with a relatively hard ceramic material, the occurrence of the surface scratches of the rolling elements can be restrained, which results in restraining the vibrations and noises caused from the rolling bearing.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a rolling bearing, and more particularly to a rolling bearing suitably used for, for example, information instruments such as a hard disc device (HDD), a video tape recorder (VTR), a laser printer, etc. 
     BACKGROUND OF THE INVENTION 
     Hitherto, as, for example, a roller bearing for supporting a spindle motor of hard disc devices, a roller bearing wherein the bearing ring and rolling element are formed with a ball bearing steel, etc., has become the main current. 
     In the above-described hard disc device, to prevent the occurrence of writing errors and reading errors, the improvement of the acoustic characteristics such as vibrations, noises, etc., has been required. 
     Also, because these information instruments are frequently placed near users at a relatively calm place in an office, a home, etc., it is strongly desired to reduce the generation of noises, particularly, having the frequency of the audible range of a human. 
     Under such circumstances, to quiet a rolling bearing for supporting spindle motor, it has been carried out to reduce the shape errors of each constituting element as small as possible, such as, for example, to improve the out of roundness of inner and outer rings and to equalize the sizes of all rolling elements used. 
     In addition, when the constituting elements of a rolling bearing are composed of a ball bearing steel, generally a heat treatment such as hardening, tempering, etc., is applied to the elements to control such that the residual austenite amount becomes about 10%. 
     Now, in the above-described conventional counterplan, there is a definite limit and thus further improvement has become necessary. Also, under the circumstance that the lubricating condition is severe, a rolling bearing is liable to generate heat as well as fine impressions, cracks, peelings, etc., are liable to form on the rolling elements and the bearing rings, whereby the caustic characteristics such as vibrations, noises, etc., are reduced. 
     Also, as the result of investigations, it has been confirmed that one of the causes of generating noises of a rolling bearing is the fine scratches formed on, in particular, rolling elements. On the other hand, recently, there are rolling elements formed with a ceramic material which is hard to be scratched. When the rolling element is formed with a ceramic material, the generation of noises by scratches can be restrained as well as because the material of the rolling element differs from the material of a bearing ring (steel-made), the effect of preventing sticking by coagulation of them by the inferior lubricant can be obtained. 
     As described above, the formation of rolling elements with a ceramic material is effective for improving the acoustic characteristics of a ball bearing but by the inventor&#39;s further investigations. It has been found that there remains the following point to be solved. 
     That is, when a rolling element is formed with a ceramic, because the rigidity of the rolling element is increased and the rolling element is hard to deform, the contact area of the rolling element and the bearing ring becomes small and the stress applied to the orbital surface becomes large. Accordingly, when the bearing ring is formed with a conventional ball bearing steel wherein the residual austenite amount is about 10%, the contact portion with the bearing element is liable to cause a permanent deformation and impressions form to generate vibrations and noises to deteriorate the caustic characteristics. Also, when a stress applied to the orbital surface becomes large, there is a problem that austenite is liable to cause a martensite transformation. Furthermore, in the case of using a ball bearing steel wherein the residual austenite amount is about 10%, there is a problem that the amount of the generated heat by the martensite transformation caused at raising the bearing temperature becomes large. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a rolling bearing excellent in the caustic characteristics such as vibrations, noises, etc. 
     Other object of the present invention is to provide a rolling bearing capable of stably and continuously giving excellent caustic characteristics for a long period of time even under the circumstances of severe lubricating condition. 
     Still other objects, the features, and the merits of the present invention will become clear from the following descriptions. 
     That is, a 1st aspect of the present invention is a rolling bearing comprising a bearing ring formed with a steel material and a rolling element formed with a ceramic material, wherein the residual austenite amount of the steel material forming the bearing ring is not more than 8%. 
     Furthermore, a 2nd aspect of the present invention in a preferred embodiment is a rolling bearing of the 1st aspect wherein the residual austenite amount of the steel material forming the bearing ring is from 0.05% to 6%. 
     In addition, if the residual austenite amount (γ R ) is less than 0.05%, the crack proceeding speed becomes fast caused by greatly lowering the tenacity to lower the life. On the other hand, if the residual austenite amount exceeds 8%, the permanent deformation amount of the orbital surface by a load (a static load and an impact load) from outside is increased and scratches and impressions are liable to form. From the view point, the critical value of the residual austenite (γ R ) is specified as described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects as well as advantages of the present invention will become clear by the following description of preferred embodiments of the present invention with reference to the accompanying drawings, wherein: 
     FIG. 1 is a cross-sectional view of the upper half of a preferred embodiment of the rolling nearing of the present invention, 
     FIG. 2 is a graph showing the change of the vibration of the rolling bearing of FIG. 1 with the passage of time, and 
     FIG. 3 is a cross-sectional view showing an using example of the rolling bearing of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Then, the preferred example of the rolling bearing of the present invention is explained by referring to the accompanying drawings. 
     First, a preferred example of the rolling bearing of the present invention is explained by referring to FIG. 1 to FIG.  3 . 
     The rolling bearing A according to the preferred example of the present invention is a deep groove ball bearing. The rolling bearing A is constituted by an inner ring  1 , an outer ring  2 , plural rolling elements  3 , and a retainer  4 . Also, the inner ring  1  and the outer ring  2  are formed with a metal material, the rolling elements  3  are formed with a ceramic material, and the retainer  4  is formed with a synthetic resin material, etc. 
     Practically, the inner ring  1  and the outer ring  2  are prepared by, after shaping the forms using a ball bearing steel (JIS Standard SUJ 2), applying a series of heat treatments such as a hardening treatment, a sub-zero treatment, a tempering treatment, whereby the residual austenite amount (γ R ) is controlled to become from 0.05 to 6%, and thereafter, by applying thereto a finishing treatment such as polishing, etc. The above-described hardening treatment is the treatment of holding for several tens minutes at 850° C. and thereafter oil-cooling. The sub-zero treatment is the treatment of holding for from several tens minutes to several hours at −70° C. and thereafter air-cooling. And the tempering treatment is the treatment of holding for several tens minutes to several hours at 220° C. and thereafter air-cooling. For example, when in the hardening treatment, after holding for 10 minutes at about 850° C., oil-cooled, in the sub-zero treatment, after holding for 1 hour at −70° C., air-cooled, and in the tempering treatment, after holding for 1 hour at 220° C., air-cooled, the residual austenite amount (γ R ) can be controlled to 2%. The region of the residual austenite is a surface portion, for example, is the portion from the surface to a depth of about 10 μm, and also the surface hardness is established to be from 60 to 64 in HRC. In addition, as the material for the inner ring 1 and the outer ring  2 , in addition to the above-described material, JIS standard SUS440 and various ball bearing steels obtained by improving the above-described JIS standard SUJ2 may be used. 
     The rolling elements  3  can be formed with ceramics made up of silicon nitride (Si 3 N 4 ) as the main body and using yttria (Y 2 O 3 ) and alumina (Al 2 O 3 ), and further aluminum nitride (AlN), titanium oxide (TiO 2 ), and spinel (MgAl 2 O 4 ) as sintering aids, and also can be formed with ceramics using alumina (Al 2 O 3 ), siliconcarbide (SiC), zirconia (ZrO 2 ) aluminumnitride (AlN)), etc. 
     Practically, it is preferred that the rolling elements 3 are formed with a ceramic made up of from 1.5 to 5.5% weight yttria (Y 2 O 3 ), from 1 to 2% by weight aluminum nitride (AlN), from 2 to 4.5% by weight alumina (Al 2 O 3 ), from 0.5 to 1.0% by weight titanium oxide (TiO 2 ), and rest being silicon nitride (Si 3 O 4 ). Also, when the rolling element 3 is a ball having a diameter of 2 mm, it is preferred that the precision is JIS Standard B1501-G3 as shown in or higher following Table 1. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Surface 
                 Diameter 
               
               
                   
                 Diameter 
                   
                 roughness 
                 difference 
               
               
                   
                 unequal 
                 Sphericity 
                 Ra 
                 of rots 
               
               
                 Class 
                 Largest 
                 Largest 
                 Largest 
                 Largest 
               
               
                   
               
             
             
               
                 3 
                 0.03 
                 0.03 
                 0.002 
                 0.05 
               
               
                 5 
                 0.08 
                 0.08 
                 0.004 
                 0.14 
               
               
                   
               
             
          
         
       
     
     In the above table, the unit is μm. 
     The retainer 4 can be formed with a general polyamide resin (nylon 66) or other thermoplastic resins having a heat resistance, for example, fluorine-based resins such as polytetrafluoroethylene (PTFE), ethylene tetrafluoroethylene (ETFE), etc., and engineering plastics such as polyether ether ketone (PEEK), polyphenylene sulfide (PPS), polyether sulfone (PES), nylon 46, etc. In addition, in the use under a low-temperature circumstance at about room temperature and under a right-load condition, polypropylene (PP) or polyethylene (PE) can be used. Also, as the form of the retainer  4 , in addition to the crown form shown in FIG. 1, a wave form, etc., is optionally used. 
     In the above-described rolling bearing A according to the preferred practical example of the present invention, the orbital surfaces of the inner ring  1  and the outer ring  2  have a proper hardness and also a proper tenacity and have a stable structure that the residual austenite is hard to cause a martensite transformation. Also, the work scratches on the surface of the rolling element  3  are less than those of a steel-made rolling element and the rolling movement of the rolling elements  3  becomes smooth. 
     In particular, when the rolling elements  3  are formed with a ceramic, the contact surface pressure becomes large but by constructing the inner ring  1  and the outer ring  2  as described above, fine impressions, cracks, peelings, etc., are hard to form on the inner and outer rings  1  and  2 , whereby the occurrence of the surface scratches with the passage of time is restrained. 
     Moreover, because the material of the inner and outer rings  1  and  2  differs from the material of the rolling elements  3 , even in the circumstance of severe lubricating condition and the circumstance that the rolling elements  3  become rolling contact with slipping, the surface scratches of the inner and outer rings  1  and  2  and the rolling elements  3  by sticking by aggregation as the conventional case of using a same metallic material for these parts become hard to occur. That is, the scratch-retraining effect of the inner and outer rings  1  and  2  and the rolling elements  3  is greatly improved as compared with the conventional cases and also the dimensional changes of the inner and outer rings  1  and  2  are retrained, whereby the rolling movement of the rolling elements  3  can be smoothly maintained. 
     From the above-described improvements, the acoustic characteristics such as vibrations and noises of the rolling bearing A can be stabilized for a long period of time at a low value. 
     The acoustic characteristics of the rolling bearing A practically determined are explained using FIG.  2 . Although not shown in the figure, the test was carried out in the state that an axis is support in the inside wall of an outer cylinder via a pair of two sample bearings. As the above-described bearings, the roller bearings described above as the preferred practical example were used and as a comparative example, rolling bearings wherein the inner and outer rings and the rolling elements were formed with a ball bearing steel which was subjected to a hardening treatment and a tempering treatment were used. 
     The sample bearing is call no. 695 of JIS Standard, the pressurization is 1.5 kgf, and the outer ring rotation is 72—rpm. The lubricating state is an oil dropped lubrication (1 mg, viscosity 14 cst at 40° C. 
     The acoustic value is almost same at the initial value in the preferred example of the present invention and the comparative example but in the case of the comparative example, the acoustic value rapidly increased after 800 hours and in the case of the preferred example of the present invention, the test was stopped after 1400 hours but the value is almost same as the initial value. As described above, in the preferred example of the present invention, the acoustic characteristics such as vibrations and noises can be stabilized for a long period of time at a low value. 
     The rolling bearing A according to the preferred example of the present invention described above can be used for supporting a spindle motor of a hard disc device as shown in FIG.  3 . In FIG. 3, the numeral 50 shows a spindle motor for HDD and a rotor hub  51  to which a discs  53  are fixed is rotatably supported via a pair of rolling bearings A, A to a motor spindle  52  to which a stator  51  is fixed. 
     In this case, an excellent calmness can be maintained such that the rotary shaking of the motor spindle  52  can be reduced as small as possible and the occurrence of the vibrations and the noises of the rolling bearings A, A can be restrained. Thus, the rolling bearing of the present invention can largely contribute to the prevention of the occurrences of the writing errors of information to the disc  53  and the reading errors of information from the disc  53 . 
     In addition, the rolling bearing of present invention can be utilized for not only a spindle motor for HDD but also a video tape recorder and a laser bean printer, the support of the rotary portions by the rolling bearing is stabilized, and thus, the rolling bearings of the present invention can contribute to the improvement of the performance. 
     While there has been described what is at present considered to be preferred embodiment of this invention, it will be understood that various modifications may be made therein, and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of this invention.