Metal treatment – Stock – Carburized or nitrided
Reexamination Certificate
2001-03-27
2002-08-27
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Carburized or nitrided
C148S906000, C384S492000, C384S625000
Reexamination Certificate
active
06440233
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rolling bearing which is suited to use for supporting rotation in machinery requiring silence in {circle around (1)} use for supporting rotation of information machinery such as hard disc drive (HDD), video tape recorder (VTR), digital audio tape recorder (DAT), {circle around (2)} use for supporting a swinging part of a swing arm as a composing part of HDD, and {circle around (3)} motors for fans or cleaners, or turbocharger for vehicle.
2. Description of the Related Art
The rolling bearing for the above usage is required to be low torque and good acoustic characteristic (low noise), and therefore composing parts of bearings as an inner ring, an outer ring or rolling elements are finish-processed at dimensionally high precision. The inner, outer rings and the rolling elements are fabricated with high carbon chromium bearing steels as SUJ2 or martensite based stainless steels as SUS440C, and then performed with hardening—tempering, so that hardness of a raceway surface is to be HRC58 to 64.
Information machinery has recently been miniaturized and often portably served, and accordingly so much exposed to risks of receiving shocks by dropping or vibrations comparing with occasions in a past. Accompanied therewith, the rolling bearings within the machinery are much probable to be injured. In a small sized ball bearing employed in portable information machinery, since a contact ellipse generated in a contacting face between a bearing ring and rolling elements is small, if adding an impact load, the contacting part is permanently deformed even if the impact load is relatively small, and the raceway surface is probably impressed with depression (impression or indentation), resulting in deteriorating an acoustic characteristic or causing irregularity in rotation torque.
As examples of the conventional arts for solving the above problem, technologies described in JP-A-7-103241 and JP-A-8-312651 may be enumerated.
According to JP-A-7-103241, the amount of a residual austenite in a steel serving as the raceway surface is decreased to be 6 vol % or less so as to heighten an impression resistance in the raceway surface, whereby the raceway surface is prevented from a permanent deformation when the rolling bearing receives the impact load. For example, the bearing ring is formed with SUJ2, and thereafter subjected to a hardening at standard temperature therefor (820 to 860° C.), and to a sub-zero treatment or a tempering treatment at relatively high temperature as 220 to 240° C., thereby reducing the amount of the residual austenite as low as possible while keeping a hardness requisite to the raceway surface.
In JP-A-8-312651, for heightening the impression resistance in the raceway surface, it is described to form the bearing ring with an ordinary bearing steel (a case-hardening steel, or high carbon chromium bearding steels as SUJL to 3), then perform a carbonitriding hardening treatment and the tempering at 350° C. or higher, thereby decreasing the amount of the residual austenite to be 0% in a steel for forming the raceway surface. It is further described to form the bearing ring with a steel which is added to SUJ2 with an element for imparting resistance against a temper softening, and perform a quench hardening and the tempering at 350° C. or higher, thereby decreasing the amount of the residual austenite to be 0% in a steel forming the raceway surface. It is also set forth to make the rolling elements of a ceramic, thereby to avoid the rolling elements from the impression by contacting with the raceway surface.
However, the above conventional technologies do not pay attention to the acoustic characteristic when being exposed to vibration, and a room is still left for also improving the acoustic characteristic when receiving impact load. That is, there is a problem that, in the rolling bearing within the machinery, fine vibrations or fluctuations given to the machinery cause fretting (a phenomenon where two faces to be contacted repeatedly cause a relative fine sliding, and thus become worn) in the contacting faces of the rolling elements and the bearing ring to deteriorate the acoustic characteristic. However, the above publications make no reference thereto.
SUMMARY OF THE INVENTION
The invention has been realized in view of problems involved with the conventional art, and accordingly it is an object of the invention to provide a rolling bearing suited to a small sized and portable information machinery, and excellent in the acoustic characteristic.
For solving the above problems, the rolling bearing according to the invention which is provided, as a bearing ring, with an outer ring and an inner ring or a shaft when a raceway surface of the inner ring is formed in the shaft, is characterized in that, of the inner ring (or the shaft) and an outer ring, at least the inner ring (or the shaft) is fabricated with a bearing steel of carbon being 0.6 wt % or higher and has a carbonitrided layer in the raceway face thereof, and a multiple Gd of an impression resistance expressed with the following formula (1) is 1.6 or more by nitrogen content percentage (wt %): Np at a position (a position of 2% Da depth) to be from the surface of the raceway to a side of a core portion by the amount of a dimension corresponding to 2% of a diameter of a rolling element and by Vickers hardness (Hv): H
0.2
at said position of 2% Da depth.
Gd
=(
H
0.2
/500)+2·
Np
(1).
In the rolling bearing of the invention which is provided, as the bearing ring, with the outer ring and the inner ring or the shaft that the raceway surface of the inner ring is formed in the shaft, it is preferable that at least the inner ring (or the shaft) of the inner ring (or the shaft) and an outer ring is fabricated with a bearing steel of carbon being 0.6 wt % or higher, has a carbonitrided layer in the raceway face thereof, and at the position (the position of 2% Da depth) from the surface of the raceway to the side of the core portion by the amount of a dimension corresponding to 2% of the diameter of the rolling element, the amount of a residual austenite is 1.0 vol % or lower, the hardness at the above-mentioned position (the position of 2% Da depth) is HRC58 or more in Rockwell hardness (Hv653 or more in Vickers hardness), the nitrogen content at the above-mentioned position is 0.05 wt % or higher, and the multiple Gd of the impression resistance expressed with the following formula (1) by the nitrogen content percentage (wt %): Np and by Vickers hardness (Hv): H
0.2
at the above-mentioned position is 1.6 or more.
Gd
=(
H
0.2
/500)+2·
Np
(1).
Inventors found that even if the carbonitrided layer formed in the raceway surface has the same hardness, the higher the nitrogen content in the carbonitrided layer, the better the impression resistance. A book entitled “Elastic coefficient of metal materials” (October 1980) issued by Japan Machinery Society, mentions at page 11 “it is considered that with respect to the elastic coefficient of an alloy in solid solution, in case a solute atom is an interstitial atom, crystal lattice is largely disturbed and the elastic coefficient is inevitably lowered.” As mentioned, it is assumed that even if the hardness is the same, the higher the nitrogen content, the lower the elastic coefficient in the carbonitrided layer. That is, although the carbonitrided layer of the same hardness is formed in the raceway surface, if nitrogen content is higher in the carbonitriding layer, an elastic deformability is higher in the raceway surface, so that the contacting surface pressure with the rolling elements is small and the impression is less to occur.
The invention specifies the nitrogen content to be 0.05 wt % or higher in a position where a maximum shearing stress in a depth direction of the carbonitrided layer is added (such a position to be from the raceway surface to the side of the core portion by the amount of the dimension corresponding to 2% of the diameter of th
Okita Shigeru
Tanaka Susumu
Yoshikawa Tomonobu
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