Metal treatment – Stock – Ferrous
Reexamination Certificate
1998-06-17
2001-01-09
Ip, Sikyin (Department: 1742)
Metal treatment
Stock
Ferrous
C148S334000, C384S492000, C384S527000, C384S912000
Reexamination Certificate
active
06171414
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a rolling bearing, and more particularly to a rolling bearing used in a turbocharger of an automobile or the like.
BACKGROUND OF THE INVENTION
A rolling bearing used in a turbocharger of an automobile or the like is generally used under a severe condition in which the rolling bearing is subjected to cyclic shear stress under a bearing pressure, it is necessary to ensure a rolling fatigue life capable of withstanding the sear stress. In addition, with this type of rolling bearing, it is necessary to avoid the occurrence of wear attributable to the sliding between a rolling element and a race in operation, so that it is required that its wear resistance be excellent. For this reason, with conventional rolling bearings, a high-carbon chromium bearing steel (SUJ2) is used as the material of the rolling element and the material of the races, and the high-carbon chromium bearing steel is subjected to quenching and tempering treatment so as to ensure the rolling fatigue life and wear resistance.
In recent years, in conjunction with the tendency toward the higher load and higher speed of the machines using the rolling bearings, the working conditions of the rolling bearings have become increasingly severe, so that the rolling fatigue characteristic and the wear resistance characteristic in the present situation are becoming unsatisfactory. In particular, in the case of the rolling bearing used in a turbocharger of an automobile or the like, its average working temperature is very high at, e.g.., 150-200° C. (maximum working temperature: 300° C.), the rolling bearing while in use rotates at high speed, and the operation undergoes rapid acceleration and deceleration in a range of 0 to 180,000 r.p.m. Consequently, this results in a decline in the hardness of the high-carbon chromium bearing steel used as the material of the bearing, with the result that it has become impossible to satisfy the desired rolling fatigue characteristic and wear resistance.
In addition, under the aforementioned working environment at high temperature and high speed, it is also required that the dimensional stability of the races be excellent. To obtain such excellent dimensional stability, it is necessary to effect tempering treatment at a temperature higher than the working temperature of the bearing, so that it is necessary to use a heat resisting material which excels in strength at elevated temperature and makes it possible to obtain sufficient hardness even if tempering treatment at high temperature is carried out.
Accordingly, as a rolling bearing which excels in the hardness at elevated temperature and satisfies the rolling fatigue characteristic and the wear resistance characteristic, a rolling bearing using a heat resisting steel, such as M50, which contains large amounts of carbide formers (Cr, Mo, V, etc.) has been proposed (e.g., Japanese Utility Model Application Publication No. 8-9452).
In that rolling bearing, a heat resisting steel is used as the material of the bearing, and the heat resisting steel is subjected to tempering treatment at a predetermined temperature of 500° C. or thereabouts to allow fine carbides to be secondarily precipitated, thereby ensuring heat resistance. Thus, a decline and the like in the hardness in the use at high temperature are suppressed, and an attempt is thereby made to improve the rolling fatigue life and the wear resistance characteristic under a high-temperature high-speed environment.
With a turbocharger of an automobile, chances for performing the maintenance of oil are few as compared with other turbochargers, and a fine filter cannot be used for supplying lubricating oil to the bearing, thereby increasing the possibility that foreign matters incorporate in the lubricating oil. If the foreign matters enter between the rolling element and the race and bite into them, impressions are caused on the traveling portions of the bearings. As a result, there has been a problem in that the so-called early flaking of the impression starting-point type occurs in which an impression constitutes the starting point of flaking, resulting in the early breakage of the bearing. That is, although the presence of the residual austenite on the surface of the material is effective in preventing the early flaking of the impression starting-point type, in the conventional rolling bearing using the aforementioned heat resisting steel, the heat resistance is ensured by causing fine carbides to be secondarily precipitated, so that practically no residual austenite exists in the surface of the material. Hence, there has been a problem in that the conventional rolling bearing using the aforementioned heat resisting steel is inferior in durability, and causes early breakage of the bearing results.
In addition, if particularly hard foreign matters such as abrasive grains, among the aforementioned foreign matters, bite between the rolling element and the race, there are cases there the foreign matter sinks into the traveling portion of the race and becomes anchored therein. If the rolling element rolls on the traveling portion of the race in such a state, the concentration of strip local impressions occurs in the rolling element. The rolling element, in which such a concentration of strip impressions occurred, rotates in the bearing while changing its axis of rotation, so that a noise fault occurs.
SUMMARY OF THE INVENTION
The present invention has been devised in view of the above-described problems, and an object of the present invention is to provide a above-described problems, and an object of the present invention is to provide a rolling bearing which excels in the rolling fatigue life and the wear resistance characteristic even under the working conditions at high temperature and high speed, and does not cause a noise fault.
To prevent the occurrence of the aforementioned noise fault by avoiding the local concentration of impressions in the rolling element, it is considered desirable to use a ceramic material excellent in wear resistance. If a ceramic is used as the material of the rolling element, in a case where foreign matters have bitten between the rolling element and the race, impressions attributable to such biting one-sidedly occur in the race formed of a steel which is inferior in hardness, and the configuration of the impression becomes large and deep as compared with a case where a rolling element formed of an alloy steel material is used. If the configuration of the impression becomes large and deep, the concentration of stress at the edge of the impression also becomes large, and breakage due to the early flaking of the impression starting-point type is liable to occur.
Accordingly, as a result of conducting strenuous research and study, the present inventors obtained knowledge that, by subjecting an alloy steel material containing appropriate amounts of Si, Cr, and Mo incorporated therein to carbonitriding treatment, it is possible to improve the strength at elevated temperature of the matrix (martensite structure) of the material, and allow the residual austenite, which is effective in preventing the flaking of the impression starting-point type, to exist in the surface of the material.
The present invention has been devised on the basis of the above-described knowledge, and in accordance with the present invention there is provided a rolling bearing comprising: races including an inner race and an outer race formed of an alloy steel material; and a plurality of rolling elements formed of a ceramic material and interposed between the inner race and the outer race such that the rolling elements rotate freely, wherein at least one of the inner race and the outer race is formed of an alloy steel containing 0.7-1.5 wt. % Si, 0.5-2.0 wt. % Cr, and 0.5-2.0 wt. % Mo in a steel, and the surface of the alloy steel is subjected to carbonitriding treatment.
In accordance with the above-described structure, since the ceramic is used as the material of the rolling element, it becomes possible to avoid the occurrence of the lo
Koike Takaaki
Mitamura Nobuaki
Sekino Kazuo
Takahashi Yukio
Yamaguchi Mikio
Ip Sikyin
NSK Ltd.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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