Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-10-17
2002-11-12
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S654000, C148S663000, C384S492000, C384S912000
Reexamination Certificate
active
06478893
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a rolling bearing and, more in particular, it relates to rolling bearings used in circumstances undergoing high temperature, large loads and large vibrations, for example, ball bearings for use in transmissions, hub units and engine auxiliaries (such as alternators, intermediate pulleys and solenoid clutches) of automobiles, guide roll bearings and backup roll bearings for use in iron and steel making, or rolling bearings suitable to roller bearings such as those used in railway vehicles, as well as a method of working rolling elements of them.
In the calculation for the life of rolling bearings, it has been known to determine a basic dynamic rating load of bearings while assuming that a probability for the occurrence of flaking in two objects in contact with each other (bearing ring and rolling element) such as a fixed ring and a rolling element, and a rotational ring and a rolling element is identical between both of them, and combining a basic load capacity of the rotational ring and the basic load capacity of the fixed ring (for example, in “Dynamic Load Capacity of Rolling Bearing Roller Bearing” written by Junzo Okamoto, printed from Seibunsha Co. Ltd.).
On the other hand, as one of countermeasures for improving the life of rolling bearings, cleanliness has been improved for the material of the bearing ring, and non-metal inclusions in steels have been decreased to greatly improve the rolling life in recent years (NSK Technical Journal, No. 652 (1992), Pages 1-8).
In the same manner, cleanliness has also been improved in the coil materials used for the rolling element to improve the reliability. However, the probability for the presence of non-metal inclusions at the central portion of the coil material tends to be higher compared withoursolling bearings not using the central portion of bar materials. Accordingly, in a circumstance where bearings undergo large loads and large vibrations, early flaking may sometimes occur in the rolling element.
As the prior art for improving the indentation resistance and the rolling life of the rolling element, there has been a technique, as described in Japanese Patent Examined Publication No. Hei 1-12812, of applying quenching and tempering to a rolling element and then applying a mechanical surface hardening treatment by air jet peening to cause plastic deformation to the surface layer thereby obtaining a large residual compressive stress layer to improve the fatigue life and improve the hardness thereby reducing the occurrence of surface flaws during handling of the rolling element.
Further, as a technique for increasing the life of steel balls for grease-sealed bearings, there has been reported, as disclosed in Japanese Patent Unexamined Publication No. Hei 3-173714, a technique of controlling the residual tensile strength in the direction of the thickness in the surface layer to 150 MPa at the maximum and, preferably, from 50 to 150 MPa in a case where a work hardening treatment is applied to the surface of a steel ball to result in a difference of hardness of HOURSC 1 or higher between the inside and the surface layer of the steel ball, thereby suppressing intrusion and accumulation of hydrogen into tensile stress exerting regions to prevent occurrence of fatigue cracking and flaking.
Further, as balls for use in ball bearings and a manufacturing method thereof, Japanese Patent Unexamined Publication No. Hei 6-264929 discloses a prior art of applying a tempering treatment after the surface hardening treatment for preventing aging deterioration of the accuracy on the surface of the ball thereby preventing aging deterioration of the acoustic characteristics of the ball bearings.
However, in a case of use in a working circumstance under large loads and large vibrations, since a sort of plastic working corresponding to contact fatigue such as mechanical surface hardening by air jet peening has already been applied to a rolling element as shown in Japanese Patent Examined Publication No. Hei 1-12812, excess plastic deformation of the surface layer proceeds in an accelerated manner in a working circumstance where the rolling element undergoes similar large loads and large vibrations and, as a result, this tends to cause early flaking.
Further, as disclosed in Japanese Patent Unexamined Publication No. Hei 3-173714, in the case of the technique of suppressing intrusion of hydrogen into the tensile stress exerting regions by controlling the maximum value of the residual tensile stress in the surface layer, to prevent occurrence of fatigue cracking and flaking, the maximum value of the residual tensile stress occurs near the position for the maximum shearing stress and, accordingly, propagation of cracks is further accelerated under large loads and large vibrations in a state where the maximum residual tensile stress of 150 MPa is loaded, so that the effect of extending the life can not be expected.
Further, in the technique disclosed in Japanese Patent Unexamined Publication No. Hei 6-264929, for coping with the acoustic problem caused by aging deterioration of the accuracy on the surface of a ball for use in HDD when used in a circumstance under small load and high speed rotation, rough grinding is applied after quenching/tempering treatment and, further, a tempering treatment is applied again after the surface hardening treatment followed by finish grinding. It takes no consideration for relieving the residual work strain which is most important for flaking under large loads and large vibrations that is the problem in the present invention, to still leave a room for improvement in this respect.
Furthermore, Plasticity and Working (Journal of Japanese Plastic Working Society), vol. 39, No. 446 (1988-3) shows, in “Several Problems for the Manufacture of Balls”, that grinding for correcting the shape of a steel ball (ball) and peening for improving the strength are required regarding the effect of the residual stress on the fatigue life but excess working may deteriorate the fatigue life, and states that the subject resides in the optimal peening.
The present invention has been accomplished for overcoming such drawbacks in the prior art and it is an object thereof to obtain a rolling element with improved resistance to rolling contact fatigue while keeping the hardness by devising a combination of a mechanical surface hardening treatment and a heat treatment to a rolling element of a rolling bearing and to provide a rolling bearing of long life even under large loads and large vibrations, by incorporating the rolling element described above.
More specifically, it intends to obtain a rolling element improved with the indentation resistance and rolling contact fatigue resistance by controlling the residual stress on the surface of a rolling element and, optionally, defining a relation with a residual stress value at a position for 2% diameter depth below the surface to optimize the residual stress distribution, thereby providing a rolling bearing having a long rolling life even under large loads and large vibrations.
SUMMARY OF THE INVENTION
Present invention relates to a rolling bearing capable of attaining such an object and a method of working a rolling element thereof.
A rolling bearing of the present invention has a feature in a rolling bearing which is used with a plurality of rolling elements being disposed between a fixed ring and a rotational ring, wherein the amount of residual austenite in the surface layer of the rolling element is from 0 to 15 vol %, a final residual compressive stress value is from −600 to −1200 MPa and a working strain relieving degree is from 10 to 60%.
That is, the rolling element is manufactured from a coil material and, in a case of the coil material, since a probability for the presence of non-metal inclusions in the central portion is higher than that in the bearing ring, flaking tends to occur particularly in the rolling element. In view of the above, it is effective for extending the life to prevent occurrence, particularly, of
Crowell & Moring LLP
NSK Ltd.
Yee Deborah
LandOfFree
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