Friction gear transmission systems or components – Friction gear includes idler engaging facing concave surfaces – Toroidal
Reexamination Certificate
2001-09-28
2004-11-09
Joyce, William C. (Department: 3682)
Friction gear transmission systems or components
Friction gear includes idler engaging facing concave surfaces
Toroidal
C476S046000, C384S492000, C384S912000
Reexamination Certificate
active
06814685
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a rolling element for use in a rolling bearing or a toroidal continuously variable transmission (CVT) for automobiles and a process for producing the rolling element. More specifically, this invention relates to a rolling element for the CVT which is improved in rolling-fatigue strength by preventing short-life peeling or flaking due to hydrogen-induced embrittlement that will be caused when hydrogen generated by decomposition of a lubricating oil during rolling of the rolling element penetrates into the metal material of the rolling element, and a process for producing the rolling element.
U.S. Pat. No. 5,556,348 discloses a toroidal CVT which includes input and output disks and a power roller disposed between the input and output disks. The input and output disks and the power roller are subjected to carburizing and grinding to improve fatigue fracture lives of traction surfaces of the input and output disks and power roller.
U.S. Pat. No. 6,051,080 discloses a power roller for a toroidal CVT which is improved in durability by carburizing and in rolling-fatigue strength of the bearing surface receiving balls by grinding. In addition, Japanese Patent Application First Publication No. 2000-291757 discloses a power roller for a toroidal CVT in which residual compression stress layers are formed on the bearing surfaces receiving balls by shot-peening. This technique contemplates to reduce the contact surface pressure produced when the balls roll on the bearing surfaces, to thereby restrain deterioration of fatigue life of the power roller.
However, these earlier techniques do not disclose positive suppression of the above-described short-life flaking due to the hydrogen-induced embrittlement. U.S. Pat. No. 5,510,974 discloses a grease-sealed bearing aiming at suppressing hydrogen infiltration into races of the bearing. The races have triiron tetroxide layers on the bearing surfaces which are formed by blackening treatment to thereby restrain the occurrence of flaking on the bearing surfaces and improve lives of the bearing.
Japanese Patent Application First Publication No. 6-313434 discloses a corrosion resistant rolling bearing in which a nickel plating layer is formed on a surface of at least one of an inner race, an outer race, rolling members and a retainer for the rolling members. This technique contemplates to improve corrosion resistance under the severe corrosive environment such as salt water spraying and enhance the plating ability.
SUMMARY OF THE INVENTION
In general, the toroidal CVT includes an input disk, an output disk and power rollers contacted with the input and output disks via a lubricating oil. Each of the disks has a traction surface contacted with a traction surface of an inner race of each power roller. Rotation of the input disk is transmitted to the output disk by the traction drive produced between the traction surfaces of the disks and power rollers. When the toroidal CVT is driven, a high loading force is applied to the traction surfaces of the input and output disks and the traction surfaces of the power rollers. This will cause a high contact surface pressure exerted on bearing surfaces of the inner and outer races of each power roller which are in rolling contact with rolling members such as steel balls. At this time, the maximum contact surface pressure may reach more than 3 GPa. Further, traction force and radial load are applied onto the bearing surfaces of the races of the power roller when the rolling members roll on the bearing surfaces. This may cause microscopic metal-to-metal contact between the bearing surfaces and the rolling members or increase rolling-friction resistance generated therebetween, whereby tangential force applied onto the bearing surfaces will become large so that rolling-fatigue lives of the races will be lowered.
In addition, it is known that a grease-lubricating bearing tends to be affected by the tribochemical reaction caused between the grease and the bearing surfaces of the races which are in contact with a plurality of rolling members. The tribochemical reaction will be promoted by a catalytic action of the neo-surface that is newly produced on the bearing surface by the microscopic metal-to-metal contact between the bearing surfaces and the rolling members. This will cause chemical decomposition of the grease, resulting in the production of hydrogen. The hydrogen produced will infiltrate into the metal structure of the races to thereby deteriorate the rolling-fatigue lives thereof.
In order to eliminate the above-described problem of the grease-lubricating bearing, there has been proposed the blackening treatment as disclosed in the above-described earlier technique. In the blackening treatment, the races are immersed in a caustic soda solution heated at a temperature of 130° C.-160° C. This will make adverse influence on working environment and therefore it is industrially undesirable. Further, the triiron tetroxide layers formed by the blackening treatment will not sufficiently remain on the bearing surfaces under the severe conditions such as high temperature and high contact surface pressure. Therefore, it will not be assured to suppress the hydrogen infiltration into the metal structure of the races.
There is a demand to solve the above-described problems in the earlier techniques. An object of the present invention is to provide a rolling element for a continuously variable transmission (CVT) which is free from the neo-surface production caused by the microscopic metal-to-metal contact and therefore suppresses the hydrogen infiltration into the metal structure of the rolling element, by forming a protection coat capable of preventing hydrogen from infiltrating therethrough into the metal structure of the rolling element. Specifically, the object of the present invention is to provide the rolling element that can be prevented from suffering from the short-life flaking due to the hydrogen-induced embrittlement which is caused by infiltration of the hydrogen generated by chemical decomposition of a lubricating oil upon rolling of the rolling element, into the metal structure of the rolling element, so as to be improved in rolling-fatigue life. It is another object of the present invention to provide a process for producing the rolling element using a relatively simple surface treatment. Still other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.
According to one aspect of the present invention, there is provided a rolling element for a continuously variable transmission, including input and output disks and a power roller interposed between the input and output disks, the power roller including an inner race, an outer race and a plurality of rolling members interposed between the inner and outer races, the input and output disks and the inner race having rolling contact surfaces coming into rolling contact with each other via lubricating oil, the inner and outer races having rolling contact surfaces coming into rolling contact with the rolling members via lubricating oil, the rolling element comprising:
a nickel-based coat formed on at least one of the rolling contact surfaces, the nickel-based coat having a thickness ranging from 0.1 to 20 &mgr;m.
The nickel-based coat of the rolling element according to the invention can be formed by a relatively simple surface treatment. With the formation of the nickel-based coat, the rolling element can be free from the microscopic metal-to-metal contact and be sufficiently prevented from suffering from hydrogen infiltration into the metal structure of the rolling element. The rolling element can be significantly improved in the rolling-fatigue life, and the excellent property of the coat can be maintained for a long period. If the thickness of the coat is less than 0.1 &mgr;m, the effects of reducing the microscopic metal-to-metal contact and suppressing the hydrogen infiltration into the metal structure of the rolling el
Hirai Eiji
Kino Nobuo
Okuhata Mitsuhiro
Otani Keizo
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