Metal treatment – Stock – Ferrous
Reexamination Certificate
2001-04-24
2004-08-03
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S319000, C148S906000, C384S492000
Reexamination Certificate
active
06770152
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a rolling bearing suitable to automobiles and industrial machines such as construction machines and iron and steel making machines and, particularly, suitable for the use in environments of severe lubrication conditions (for example, at high temperature).
The present invention also relates to a rolling bearing suitable, for example, to {circle around (1)} rotational supporting use in information equipment such as hard disk drives (HDD), video tape recorders (VTR) and digital audio tape recorders (DAT), {circle around (2)} supporting use for swinging portions of swing arms as constituent components such as of HDD and {circle around (3)} rotational supporting use for those equipments requiring quietness such as motors for blowers, motors for cleaners and vehicle turbo chargers.
Furthermore, this invention relates to a rolling bearing suitable to use undergoing fitting stress to an inner ring relative to a shaft and used at high temperature and lubrication with intrusion of obstacles.
BACKGROUND ART
Heretofore, high carbon chromium steels such as SUJ 2 have been mainly used as steel materials for rolling bearings but high speed steel materials such as AISI-M50 have been used in such uses as requiring high performance at high temperature (for example, bearings for use in aircraft jet engines). However, since the materials contain many alloy elements and require complicate heat treatment steps, the cost of the obtained bearings is expensive.
Japanese Published Unexamined Patent Application Hei 3-253542 discloses a steel containing Si and Mo having high softening resistance at high temperature more than SUJ 2 and containing an appropriate amount of Cr. This publication discloses that the fatigue life characteristic of rolling bearings at high temperature can be improved while keeping the manufacturing cost lower by the use of such steels.
Further, Japanese Published Examined Patent Application Hei 6-33441 describes bearing rings formed by using a steel containing, on the weight ratio, 0.95 to 1.10% of carbon, 1 to 2% of silicon or aluminum, 1.15% or less of manganese and 0.90 to 1.60% of chromium, with an oxygen content of 13 ppm or less, with the amount of residual austenite being restricted to 8% or less by conducting tempering at a temperature of 230° C. to 300° C. after hardening and with a hardness of HRC60 or more. Further, this invention intends to provide a rolling bearing ring having high dimensional stability and extended rolling life even in use at high temperature.
Further, Japanese Published Unexamined Patent Application Hei 10-68419 describes a rolling bearing in which carbides and/or carbonitrides with the maximum grain size of 5 &mgr;m or less are precipitated at the surface layer of bearing rings and/or rolling elements, and the surface hardness of the bearing ring and/or rolling element is Hv 600 or more and 700 or less at 300° C. According to this rolling bearing, while wear resistance at high temperature can be improved, manufacturing cost is increased since a carburization treatment or carbonitriding treatment is required during manufacture.
However, the rolling bearings disclosed in each of the publications still have a room for the improvement in view of the wear resistant at high temperature or manufacturing cost.
On the other hand, rolling bearings used for rotational support in those equipments requiring quietness as in {circle around (1)}-{circle around (3)} described above are required to have satisfactory acoustic characteristics at low torque (less noise). Therefore, bearing components such as inner rings, outer rings and rolling elements are finished at high dimensional accuracy. Further, the inner rings, outer rings and the rolling elements are formed of high carbon chromium bearing steels such as SUJ 2 or martensitic stainless steels such as SUS440C and then manufactured by applying hardening tempering and the hardness of the raceway surface is defined as HRC 58 to 64.
In recent years, since information equipments have been reduced in size and often adapted for portable use, they have more worry of undergoing impact shocks upon dropping or exposure to vibrations. Correspondingly, a worry of damages has also been increased for rolling bearings in the equipments. In small-sized ball bearings used for portable information equipments, since the contact ellipsis formed at the contact surface between the bearing ring and the rolling element is small, when impact load is applied, the contact portion suffers from permanent deformation to sometimes cause indentation at the raceway surface even when this is a relatively small impact load. As a result, there may be a worry for the deterioration of acoustic characteristics or occurrence of unevenness in the rotational torque.
Prior art for overcoming the problems can include, techniques as described in Japanese Published Unexamined Patent Application Hei 7-103241 and Japanese Published Unexamined Patent Application Hei 8-312651.
Japanese Published Unexamined Patent Application Hei 7-103241 describes that the amount of the residual austenite in the steel forming the raceway surface is decreased as low as 6% by volume or less to improve the indentation resistance of the raceway surface thereby avoiding permanent deformation of the raceway surface when impact load is applied to the rolling bearing. For example, after forming a bearing ring with SUJ 2, it is hardened at a hardening temperature of standard heat treatment (820 to 860° C.) and then subjected to sub zero treatment, or tempered at a relatively high temperature of 220 to 240° C. thereby decreasing the amount of the residual austenite as less as possible while maintaining required hardness for the raceway surface.
Japanese Published Unexamined Patent Application Hei 8-312651 discloses that a bearing ring is formed with usual bearing steel (high carbon chromium bearing steel such as of case hardened steel or SUJ 1-3) and then applied with a carbonitriding hardening treatment and tempering at a temperature of 350° C. or higher thereby reducing the amount of the residual austenite of steels forming the raceway surface to 0%, for improving the indentation resistance at the raceway surface.
It also discloses that the bearing ring is formed of a steel prepared by adding an element providing a tempering resistance and then applied with quench hardening and tempering at a temperature of 350° C. or higher to reduce the amount of the residual austenite of the steel forming the raceway surface to 0%. Further, it discloses that no indentation by contact with the raceway surface is formed to the rolling element, by making the rolling element with ceramic material.
However, in the prior art described above, there is still a room for the improvement in view of the acoustic characteristics when impact load is applied.
On the other hand, self alignment roller bearings used, for example, in paper making machine are sometimes used under high fitting stress in excess of 100 MPa (average stress along the cross section of an inner ring for the tension applied in the circumferential direction of the inner ring) in order to prevent creeping caused between a shaft and a bearing inner ring. In such a case, an inner ring having an inner diametrical portion fabricated into a tapered shape is press fit into a tapered shaft so as to facilitate application of fitting stress. As the inner ring of such a shape, those prepared by applying quenching and tempering treatment to fully cured steels such as high carbon chromium bearing steels (C: about 1 wt %, Cr: about 1.5 wt % content) are generally used.
Then, when the inner ring comprising the fully hardened steel is used under fitting stress in excess of 100 MPa, the inner ring may sometimes be cracked in the axial direction with non-metal inclusions present near the raceway surface as initiation points depending on the combination of the fitting stress and the rolling stress.
For preventing such cracking fracture of the inner ring, there is a general knowledge that improvement of the compre
Kiuchi Akihiro
Okita Shigeru
Takemura Hiromichi
Yoshikawa Tomonobu
Crowell & Moring LLP
NSK Ltd.
Yee Deborah
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