The present invention relates to rolling bearings that are to be assembled into hard disk drives (HDD), video tape recorders (VTR), digital audio tape recorders (DAT) or the like, in particular, to support rotating spindles at high speed thereinto or to support swing arms for the HDD or the like.
FIG. 1 shows a spindle motor that is to be assembled into a HDD on a computer for causing a hard disk driving shaft 1 to rotate at high speed. The shaft 1 and a housing 2 have a pair of ball bearings 3 as rolling bearings provided between the outer circumference of the shaft 1 and the inner circumference of the housing 2 in such a way that the shaft 1 is supported to rotate freely inside the housing 2. Each ball bearing 3 includes a steel inner race 5 having an inner raceway 4 on the outer circumference, a steel outer race .7 having an outer raceway 6 on the inner circumference, and a plurality of steel balls 8 as rolling elements provided rollingly between the inner raceway 4 and the outer raceway 6. All balls 8 are provided with a preload to insure that they may not be fluctuated during the rotation of the shaft 1.
FIGS. 2 and 3 show the structures of other spindle motors for use in HDDS. The structure shown in FIG. 2 has a hub which is integrated with an outer race 7a having a plurality of outer raceways 6 on the inner circumference, and inner raceways 4 which are formed on the outer circumference of a shaft 1 and that of an inner race 5 fitted around the shaft
1. The structure shown in FIG. 3 has a plurality of inner raceways 4 which are formed on the outer circumference of a shaft 1, as well as a plurality of outer raceways 6 formed on the inner circumference of an outer race 7b.
FIG. 20 shows the structure of the HDD into which the a swing arm bearing 30 is assembled with a spindle motor bearing 20 of a type shown in FIGS. 1-3 on the left side of FIG. 20. A magnetic disk 22 is rotated by the spindle motor 21. A pair of reading heads 23 are positioned on the magnetic disk 22 for reading recorded data in the magnetic disk 22. A swing arm 24 is provided for positioning the reading heads 23 to an accessing point within an available area on the magnetic disk 22. The swing arm 24 includes arm rods 28 coupling the reading heads 23 at ends thereof, and a swing arm shaft 27 supported by a swing arm bearing 30. The swing arm 24 is rotated by a coil (not shown) while being controlled. Thus the reading heads 23 can moves radially all over the available area on the magnetic disk 22. The swing arm bearing 30 supports the swing arm 24 rotated at lower revolutions than the spindle motor 21 supported by the spindle motor bearing 20.
In these conventional structures, the members provided with the inner raceways 4 and the outer raceways 6 (namely, inner races 5 and outer races 7 in FIG. 1; outer race 7a, shaft 1 and inner race 5 in FIG. 2; or shaft 1 and outer race 7b in FIG. 3) have been formed of a high-carbon chromium bearing steel SUJ2 (JIS G4805) by hardening at 820.degree.-860.degree. C. and subsequently tempering at 160.degree.-200.degree. C. As a result of these heat treatments, the races are rendered to have a Rockwell hardness of H.sub.R C 58-64 and they contain retained austenite (YR) in an amount of 8-14 vol%.
For use in parts which require sufficient corrosion resistance, the races have occasionally been formed of stainless steels such as SUS 440C (JIS G4303) and 13Cr-based martensitic stainless steel, which are first hardened at temperatures around 1050.degree. C., then subjected to a subzero treatment, followed by tempering at about 150.degree.-200.degree. C. The races formed from these stainless steels have a Rockwell hardness of H.sub.R C 57-62 and contain retained austenite (.gamma..sub.R) in an amount of 8-12 vol%. In particular, such stainless steels have been used in the swing-arm bearings to support the reading heads in the HDDs, as shown in FIG. 20.
Unexamined Japanese Patent Publication No. Hei. 5195069 teaches a technique of shot-peening the surface of a bearing steel. According to the disclosure, the technique is capable of increasing a hardness of the bearing steel to the Vickers hardness of H.sub.v 850-950 (equal to H.sub.R C 65.5-68 in terms of Rockwell hardness).sup.1. Therefore, if races or rolling elements are produced by the disclosed technique, impressions are not easily formed on the surfaces of the races even if the lubricant is contaminated with foreign matter. In addition, the races may not wear rapidly. These contribute to a longer life of the bearing.
The permanent (plastic) deformation that occurs to the raceways (both inner raceways 4 and outer raceways 6) in the ball bearing and the rolling surfaces of balls 8 are conventionally defined in terms of the basic static load rating CO. Accordingly, it has been proposed that a deleterious permanent deformation occurs in the raceways and the rolling surfaces if the maximum contact pressure between the two parts exceeds 4000 MPa.
Among various kinds of rolling bearings, small-sized ball bearings which are used in HDDs and VTRs have high precision such as JIS Class 5 or better in dimensional or rotating precision. The ball bearings which are required to rotate with small torque must satisfy further strict requirements in acoustic and noise performance. With such small-sized ball bearings with high precision, there has been a serious problem in that they experience acoustic deterioration (increase in noise level) due to an extremely small permanent deformation that occurs in raceways or rolling surfaces under much smaller loads (e.g. impact load) than 4000 Mpa, or the value of maximum contact pressure specified by the basic static load rating C.sub.0, supra. Similarly, the permanent deformation affects adversely the swing arm bearing 30 as shown in FIG. 20.
Ball bearings 3 (see FIGS. 1-3) used as such small-sized, high-precision ball bearings are commonly designed in such a way that balls 8 assembled between the inner raceway 4 and the outer raceway 6 have a diameter D.sub.w (see FIG. 1) of no more than 3 mm and are spaced on the circumference of a pitch circle having a diameter D.sub.PW (also see FIG. 1) of no more than 11 mm. However, with the recent trend toward smaller HDDs and VTRs, the size of ball bearings 3 to be assembled into these equipment is also decreasing. With the ball bearings 3 assembled in such small equipment, they are exposed to more accidents of impact application primarily due to the increased possibility of drop of the portable equipment. Even if the intensity of an applied impact is relatively small, the raceways or rolling surfaces of the bearings undergoes permanent deformation, which can cause various problems of deterioration in the performance of the equipment which incorporates the ball bearings, as exemplified by acoustic deterioration and irregular rotating torques.
The permanent deformation of the raceways or rolling surfaces which leads to deterioration in the performance of the equipment incorporating ball bearings is known to occur if the retained austenite in the steel forming the races or rolling elements has low yield stress. In order to prevent this permanent deformation problem originating from the retained austenite, it has previously been proposed that the amount of retained austenite in the steel forming races of a rolling bearing should be reduced to 6 vol% and less This approach is effective in making the raceways hard to deform permanently under impact load and offers the following advantages.
(1) When the races are to be made from SUJ2, the amount of retained austenite can substantially be reduced to 0% by tempering the steel at about 240.degree. C. and the impact resistance of the races (their ability to withstand impact loads without permanent deformation) can be improved remarkably. If the process described in Unexamined Japanese Patent Publication No. Hei. 5-195069 is employed, the amount of retained austenite is reduced to 10 vol% or less and the hardness is increased to H.sub.V 850-950. Therefore, an impression resistance is improved. PA1 (2) In the case of SUS 440C and 13Cr-based martensite stainless steel, tempering at temperatures exceeding 500.degree. C. is capable of reducing the amount of retained austenite to 6 vol% and below. Therefore, an impact resistance is improved. PA1 (3) Bearing materials of secondary hardening type steel M50 (designation AMS 6490 or 6491; AMS is a standard in SAE) and high-speed tool steel SXH4 (JIS G4403), which are conventionally used to make races, have high surface hardness and exhibit superior impact resistance.
However the races described under (1)-(3) have their own problems. The races of (1) and (3) types are low in Cr content, so that the races cannot achieve satisfactory corrosion resistance depending on the operating conditions. For example, in the case of a ball bearing to be used in HDD, since an adhesive is used to fix the races, the internal surfaces of inner races or the external surfaces of outer races are completely degreased. Accordingly, in order to prevent those internal or external surfaces from rusting after 1s assembling into the bearing, the races must be made of materials which have satisfactory corrosion resistance. However, the races described under (1) and (3) are not suitable for this purpose.
The race described under (2) has satisfactory corrosion resistance but, on the other hand, it is not highly durable. Tempering at temperatures in excess of 500.degree. C. reduces the surface hardness to H.sub.R C 56 and below. As a result, the rolling fatigue life and wear resistance of the races decrease to shorten the life of the ball bearing incorporating those races. In addition, the raceways are prone to be damaged on the assembly line of bearings to cause inconveniences such as increasing poor acoustic performance that may occur during the process of manufacture.