Rolling element used for toroidal-type continuously variable transmission and method for producing same

The invention relates to a power roller used for a toroidal-type continuously variable transmission. This power roller has on its first side a groove for receiving a bearing, and the surface of the groove (bearing surface) has a carbon concentration which is not lower than that of the back surface of a second side opposed to the first side. The power roller has a hardened outer layer having first and second portions which are respectively defined by the bearing surface and the back surface. The first portion of the hardened outer layer has an effective depth which is not shallower than that of the second portion thereof. The power roller is prepared by the sequential steps of surface-hardening, quenching, and grinding. The surface-hardening process is one of carburizing and carbonitriding. The grinding includes the sequential steps of: (1) grinding at least one of first and second outer portions a quenched preform of the power roller to remove the same, such that there is obtained a datum that is at least one of the surface of the groove and the inner flat surface which are respectively exposed by removing the first and second outer portions; and (2) grinding a third outer portion of the quenched preform, based on the datum, thereby to remove the third outer portion. The bearing surface of the power roller becomes high in carbon concentration, and the first portion of the hardened outer layer of the power roller becomes sufficiently deep in effective thickness.

The contents of Japanese Patent Application No. 8-342006, with a filing 
date of Dec. 20, 1996, are hereby incorporated by reference. 
BACKGROUND OF THE INVENTION 
The present invention relates in general to a toroidal-type continuously 
variable transmission used in vehicles such as automobiles, and more 
particularly to a power roller of a rolling element used for the 
transmission, and a method for producing the power roller. This rolling 
element of the transmission is defined as being a combination of the power 
roller, an input side disk and an output disk, as will be clarified 
hereinafter. 
A conventional toroidal-type continuously variable transmission is, as 
shown, for example, in FIG. 6, constructed so that an input side disk 11 
and an output side disk 12 are coaxially disposed so as to be opposed to 
each other inside a housing (not shown in FIG. 6). An input shaft 13 
passes through the shaft center of the toroidal transmission section 
having the input side disk and the output side disk. A loading cam 14 is 
disposed on an end of the input shaft 13. The loading cam 14 transmits the 
motive power (rotational force) of the input shaft 13 to the input side 
disk 11 through a cam roller 15. 
The input side disk 11 and the output side disk 12, having substantially 
the same shape, are disposed so as to be symmetrical, and formed so as to 
be substantially semicircular in section as viewed in the axial direction 
with both opposed surfaces thereof taken into view. A pair of power 
rollers 16 and 17 that transmit motion are disposed so as to be in contact 
with the input side disk 11 and the output side disk 12, respectively, 
within a toroidal cavity formed by the toroidal surfaces of the input side 
disk 11 and the output side disk 12. Reference numeral 23 designates 
thrust ball bearings. In this case, the power rollers 16 and 17 are 
pivotably attached to trunnions 20 and 21 through pivots 18 and 19, and 
pivotably supported with a pivot A as the center, the pivot A serving as 
the center of the toroidal surface of the input side disk 11 and the 
output side disk 12. The surfaces of contact among the input side disk 11, 
the output side disk 12, and the power rollers 16 and 17 are supplied with 
a lubricating oil whose viscous frictional resistance is large, so that 
the motive power applied to the input side disk 11 is transmitted to the 
output side disk 12 through the lubricating oil film and the power rollers 
16 and 17. 
The input side disk 11 and the output side disk 12 are independent of the 
input shaft 13 (not being directly affected by the motive power of the 
input shaft 13) through needles 25. An output shaft 24 is attached to the 
output side disk 12. The output shaft extends in parallel with the input 
shaft 13 and is rotatably supported by the housing through an angular 
bearing 22. 
In this toroidal-type continuously variable transmission, the motive power 
of the input shaft 13 is transmitted to the loading cam 14. When the 
loading cam 14 is rotated by the transmission of the motive power, this 
rotational power is transmitted to the input side disk 11 through the cam 
roller 15, which in turn causes the input side disk 11 to rotate. The 
motive power generated by the rotation of the input side disk 11 is 
transmitted to the output side disk 12 through the power rollers 16 and 
17. The output side disk 12 rotates integrally with the output shaft 24. 
At the time of changing the speed, the two trunnions 20 and 21 are slightly 
moved toward the pivot A. That is, the axial movement of the trunnions 20 
and 21 releases the intersection between the rotating shaft of the power 
rollers 16 and 17 and the shafts of the input side disk 11 and the output 
side disk 12. As a result, the power rollers 16 and 17 oscillates over the 
surfaces of the input side disk 11 and the output side disk 12, thereby 
changing the speed ratio to either accelerate or decelerate the motor 
vehicle. 
Such a toroidal-type continuously variable transmission is disclosed, for 
example, in U.S. Pat. No. 5,556,348 corresponding to Japanese Patent 
Unexamined Publication No. 7-71555. In this patent '348, effective 
carburized depths of the input side disk, the output side disk, and the 
power roller are limited to fall in a range of from 2.0 mm to 4.0 mm. As 
conventional examples of the above-mentioned input side disk, output side 
disk, and power rollers, those using AISI 52100 (equivalent of a high 
carbon chromium bearing steel having a symbol of SUJ 2 according to 
Japanese Industrial Standard (JIS) G 4805, 1970) are known (see NASA 
Technical Note, NASA TN D-8362, Dec. 1976). 
When the above-mentioned toroidal-type continuously variable transmission 
is driven, the bearing surface of the power roller receives a high load 
from the input side disk and the output side disk. Furthermore, the power 
roller rotates at a high speed, while engaging with both the input side 
disk and the output side disk. Thus, the bearing surface of the power 
roller is subjected to high temperature and high pressure, and therefore 
tends to flake due to the rolling contact fatigue. In other words, when 
the power roller is in a rolling contact with the input side disk and the 
output side disk under high temperature and high pressure, heat generated 
by the rolling contact tends to lower the hardness of an outer layer of 
the power roller. With this, the outer layer lowers in fatigue strength 
and thus tends to flake. In view of this, the outer layer of the power 
roller is required to have a sufficient strength or hardness at high 
temperature, that is, temper hardness. The power roller may be subjected 
to a hardening process, that is, a carburizing or carbonitriding process, 
in order to harden the surface of the same. The strength of the power 
roller at high temperature is greatly affected by the carbon or carbon and 
nitrogen concentrations of the outer layer of the same which has been 
subjected to the hardening process. In fact, when the outer layer of the 
power roller is low in carbon or carbon and nitrogen concentrations, this 
outer layer becomes low in high-temperature strength. With this, the power 
roller becomes insufficient in rolling fatigue life. Furthermore, if each 
outer layer of the power rollers varies to a great extent in carbon or 
carbon and nitrogen concentrations, the outer layer will have a wide 
variation in high-temperature strength and thus in rolling fatigue life. 
This is a disadvantage in the production of power roller in an industrial 
scale. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a rolling 
element used for a toroidal-type continuously variable transmission, which 
rolling element has a power roller having a bearing surface superior in 
durability, particularly in rolling fatigue strength, under high 
temperature and high surface stress. 
It is another object of the present invention to provide a method for 
producing such rolling element. 
According to the present invention, there is provided a rolling element 
used for a toroidal-type continuously variable transmission. This rolling 
element includes an input side disk adapted to be disposed on an input 
shaft of the transmission; an output side disk adapted to be disposed on 
an output shaft of the transmission; and a power roller for transmitting 
motive power of the input shaft to the output shaft, while engaging both 
the input side disk and the output side disk. 
According to the present invention, the power roller is prepared by a 
method including the sequential steps of (a) subjecting a preform of the 
power roller to a surface-hardening process which is one of a carburizing 
process and a carbonitriding process, thereby to prepare a 
surface-hardened preform; and (b) subjecting the surface-hardened preform 
to a grinding process, thereby to prepare the power roller. 
According to the present invention, the power roller has first and second 
sides which are opposed to each other in an axial direction thereof. This 
first side has a groove for receiving a bearing, and a surface of the 
groove (bearing surface) has a carbon concentration which is not lower 
than that of a surface of the second side (back surface). The power roller 
prepared by the above method has a hardened outer layer having first and 
second portions which are disposed on the first and second sides 
respectively. The first portion of the hardened outer layer is defined as 
having an exterior boundary which is the bearing surface, and the second 
portion of the hardened outer layer is defined as having an exterior 
boundary which is the back surface. The first portion of the hardened 
outer layer has an effective depth which is not shallower than that of the 
second portion thereof. 
In the invention, the above method may further include a step of quenching 
the surface-hardened preform, between the steps (a) and (b). The grinding 
process of the step (b) may include the sequential steps of (1) grinding 
at least one of first and second outer portions of the surface-hardened 
preform to remove the same, such that there is obtained a datum that is at 
least one of the bearing surface and an inner flat surface which are 
respectively exposed by removing the first and second outer portions, the 
inner flat surface being positioned closer to a center of the power roller 
in a radial direction thereof than the groove is; and (2) grinding a third 
outer portion of the surface-hardened preform, based on the datum, thereby 
to remove the third outer portion. 
According to the present invention, there is further provided a 
toroidal-type continuously variable transmission having the 
above-mentioned input side disk, output side disk, and power roller. 
By virtue of the present invention, the bearing surface of the power roller 
becomes high in carbon concentration, and the bearing portion (i.e., the 
first portion) of the hardened outer layer of the power roller becomes 
sufficiently deep in effective thickness. Therefore, the bearing surface 
becomes superior in durability, particularly in rolling fatigue strength, 
under high temperature and high surface stress.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Since the basic construction of a toroidal-type continuously variable 
transmission of the present invention is substantially the same as that of 
the above-mentioned conventional one, the description thereof will be 
omitted from the following. 
As stated above, a rolling element of a toroidal-type continuously variable 
transmission of the present invention is herein defined as being a 
combination of the input side disk, the output side disk, and the power 
roller. FIG. 7 shows the whole construction of a toroidal-type 
continuously variable transmission, to which the present invention can be 
applied. As is shown in FIG. 6, each power roller 16 or 17 has a groove 
for receiving a part of a thrust ball bearing 23. In order to improve the 
surface of the groove (bearing surface) in rolling fatigue life under high 
temperature and high surface stress, the material of the power roller is 
required to be superior in high temperature strength, that is, hardness 
under high temperature (temper hardness). To meet this requirement, a 
preform of the power roller may be subjected to a surface-hardening 
process, that is, a carburizing or carbonitriding process. Regarding the 
carburizing process, as the carbon concentration of the outer layer of the 
power roller becomes higher, this outer layer improves in high temperature 
strength and thus in rolling fatigue life. Similarly, regarding the 
carbonitriding process, as the carbon and nitrogen concentrations of the 
outer layer become higher, this outer layer improves in those. In the 
carburizing process, carbon diffuses from the surface of the power roller 
toward the interior thereof Therefore, after the carburizing process, the 
carbon concentration of the power roller is the highest at its surface and 
decreases toward a deeper position thereof. Similarly, after the 
carbonitriding process, the carbon and nitrogen concentrations thereof are 
the highest at its surface and decrease toward that. 
In the invention, as shown in FIG. 1, the surface-hardened preform 30 of 
the power roller, which has a configuration as shown by a chain line in 
FIG. 1, is subjected to a grinding process, thereby to remove the outer 
peripheral portion 32 of the preform 30. The carbon or carbon and nitrogen 
concentrations at the surface of the power roller 34 vary with the 
thickness of this outer peripheral portion 32 to be removed by the 
grinding process. In fact, in case that the thickness of this outer 
peripheral-portion 32 is too much, the carbon or carbon and nitrogen 
concentrations at the surface of the power roller 34 become too low. 
Furthermore, the dimensions of each power roller 34 after the grinding 
process are always substantially constant. Therefore, the thickness 44 of 
the outer peripheral portion 32 on the first side 36 (i.e., the side of 
the bearing surface) increases with the decrease of the thickness 46 of 
the outer peripheral portion 32 on the second side 38 (i.e., the side of 
the back surface) which is opposed to the first side 36. It is necessary 
to make the bearing surface 40 of the power roller 34 have high carbon or 
carbon and nitrogen concentrations. With this, the bearing surface 40 
becomes superior in rolling fatigue life under high temperature and high 
surface stress. In contrast, the back surface 42 of the power roller 34 is 
brought into contact with a needle bearing or sliding bearing, under a far 
less severe condition than that of the bearing surface 40. Therefore, the 
back surface 42 of the power roller 34 may have lower carbon or carbon and 
nitrogen concentrations by a certain degree(s) than those of the bearing 
surface 40, in order to make the back surface 42 superior in durability. 
Thus, the bearing surface 40 of the power roller 34 is maintained high in 
carbon or carbon and nitrogen concentrations by making the thickness 44 of 
the outer peripheral portion 32 on the first side 36 not thicker than the 
thickness 46 of the outer peripheral portion 32 on the second side 38. 
With this, the bearing surface 40 of the power roller 34 will have a 
carbon concentration which is not lower than that of the back surface 42 
thereof. Thus, the rolling fatigue strength of the bearing surface 40 does 
not decrease so much by the grinding process, thereby to improve the 
bearing surface in durability. 
As is seen from FIG. 1, the power roller 34 of the invention has an inner 
flat surface 48 which is positioned closer to the center B of the power 
roller 34 in a radial direction thereof than the groove is. In the 
invention, it is optional to make the carbon concentration of the bearing 
surface 40 substantially the same as that of the inner flat surface 48. 
With this, the carbon concentration of the bearing surface 40 can easily 
be estimated from that of the inner flat surface 48 by measuring the 
latter, for example, by emission spectrochemical analysis, without 
destruction of the bearing surface 40. Thus, quality of each power roller 
can easily be checked by this estimation with respect to the carbon 
concentration of the bearing surface. With this, it becomes possible to 
make the variation of the carbon concentration thereof small and thus the 
variation of durability thereof small. 
As stated above, the grinding process of the invention may include the 
following first and second sequential steps. The first step is a step of 
grinding at least one of first and second outer portions 50, 52 of the 
surface-hardened preform 32 to remove at least one of these first and 
second outer portions 50, 52. This first portion 50 is defined as being 
such that the bearing surface 40 will be exposed by the removal of the 
first portion 50, and the second portion 52 is defined as being such that 
the inner flat surface 48 will be exposed by the removal of the second 
portion 52. Herein, at least one of the bearing surface 40 and the inner 
flat surface 48, which has been exposed by the first step, is defined as a 
datum. The second step of the grinding process is a step of grinding a 
third outer portion 54 of the surface-hardened preform 32, based on the 
datum, thereby to remove the third outer portion 54. In other words, the 
thickness 46 of the third outer portion 54 to be removed by the second 
step is determined, based on at least one of the thickness 44 of the first 
outer portion 50 and the thickness 56 of the second outer portion 52. The 
third outer portion 54 may be defined as being such that the back surface 
42 of the power roller 34 will be exposed by the removal of the third 
outer portion 54. According to this grinding process, the thickness of at 
least one of the first and second outer portions 50, 52 can substantially 
be decreased to a substantially constant value. For example, the first and 
second steps of the grinding process may be conducted in a manner that the 
thickness 44 of the first outer portion 50, which causes the bearing 
surface 40 to be exposed, is not thicker than that 46 of the third outer 
portion 54, which causes the back surface 42 to be exposed. Thus, the 
carbon concentration of at least one of the bearing surface 40 and the 
inner flat surface 48 can substantially be increased to a substantially 
constant value. This contributes to the improvement of the bearing surface 
40 of the power roller 34 in durability. In contrast to the present 
invention, if the third outer portion 54 is removed prior to the removal 
of at least one of the first and second outer portions 50, 52, the carbon 
concentration of at least one of the bearing surface 40 and the inner flat 
surface 48 may vary to a great extent, depending on the thickness 46 of 
the third outer portion 54. In the invention, the first step of the 
grinding process may be conducted by grinding both the first and second 
outer portions 50, 52 in a manner that the thickness 44 of the first outer 
portion 50 is substantially the same as that 56 of the second outer 
portion 52. With this, for example, the carbon concentration of the 
bearing surface 40 becomes substantially the same as that of the inner 
flat surface 48. With this, as stated above, the carbon concentration of 
the bearing surface 40 can easily be estimated from that of the inner flat 
surface 48 by measuring the latter, without destruction of the bearing 
surface 40. 
In the invention, the quenching of the surface-hardened preform may be 
conducted by a press quenching between the surface-hardening and grinding 
processes (see FIGS. 3 and 4). With this, quenching strain can 
substantially be reduced to a substantially constant value. This also 
contributes to the improvement of the bearing surface 40 of the power 
roller 34 in durability. Furthermore, the quenching may be an oil 
quenching, too. 
The following nonlimitative examples are illustrative of the present 
invention. 
EXAMPLE 1 
In this example, each power roller used for a toroidal-type continuously 
variable transmission was prepared as follows. At first, a preform of the 
power roller was prepared from a hollow cylindrical blister steel having a 
chemical composition of 0.2 wt % of C, 0.25 wt % of Si, 0.8 wt % of Mn, 
1.1 wt % of Cr, 0.15 wt % of Mo, 0.015 wt % of P, 0.009 wt % of S, and the 
balance of Fe and impurities. Then, as shown in FIG. 2, this preform was 
subjected to a carburizing process, thereby to prepare a first 
surface-hardened preform of which hardened layer had an effective depth of 
from 1.0 to 2.0 mm. The carbon concentration of the surface of the first 
preform was 1.1 wt %, as shown in Table. Then, as shown in FIG. 2, the 
first surface-hardened preform was subjected to an oil quenching, thereby 
to prepare a second surface-hardened preform 30. Then, the second 
surface-hardened preform 30 was subjected to a grinding process. In the 
grinding process, as is seen from FIG. 1, at first, a first outer portion 
50 of the second surface-hardened preform 30 was removed, thereby to 
expose the bearing surface 40, such that the carbon concentration of the 
bearing surface 40 was in a range shown in Table. Then, a second outer 
portion 52 of the second preform 30 was removed by grinding the same, 
thereby to expose the inner flat surface 48, such that the carbon 
concentration of the inner flat surface 48 was in a range shown in Table. 
The thickness of the second outer portion 52 of each second preform 30 was 
not regulated to be the same as that of the first outer portion 50 
thereof. Thus, the former was slightly different from the latter. After 
the removal of the first outer portion 50, a third outer portion 54 of the 
second preform 30 was removed, based on the bearing surface 40 as a datum, 
to expose the back surface 42. In other words, the thickness 46 of the 
third outer portion 54 to be removed was determined, based on the 
thickness 44 of the first outer portion 50. The grinding process was 
conducted such that the obtained power roller 34 had predetermined 
constant dimensions. The carbon concentration of the back surface 42 was 
in a range shown in Table. 
EXAMPLE 2 
In this example, Example 1 was repeated except in that the thickness 56 of 
the second outer portion 52 of each second preform 32 was regulated in the 
grinding process to be substantially the same as that 44 of the first 
outer portion 50 thereof. Thus, the carbon concentration of the bearing 
surface 40 of each power roller 34 was substantially the same as that of 
the inner flat surface 48 thereof, as shown in Table. 
EXAMPLE 3 
In this example, Example 2 was repeated except in that the oil quenching 
was replaced with a press quenching. In fact, some samples of the first 
surface-hardened preforms were subjected to a first press quenching, as 
shown in FIG. 3. In the first press quenching 60, the first 
surface-hardened preform 62 was pressed between a die 64 and a punch 66, 
while a quenching oil was allowed to flow along grooves 68 provided on 
pressing portions 70 of the punch 66 and grooves (not shown) provided on 
pressing portions 70 of the die 64. These grooves of each pressing portion 
70 of the punch 66 and the die 64 had a configuration shown in a 
magnification of the pressing portion 70 of the punch 66 of FIG. 3. 
Furthermore, the other samples of the first surface-hardened preforms 62 
were subjected to a second press quenching 80, as shown in FIG. 4. The 
second press quenching was the same as the first press quenching, except 
in that a punch 82 having a different configuration was used. In fact, 
this punch 82 had pressing portions 84 each being provided with grooves 86 
along which a quenching oil was allowed to flow. These grooves 86 of each 
pressing portion 84 of the punch 82 and the die 64 also had a 
configuration shown in a magnification of the pressing portion 84 of the 
punch 82 of FIG. 4. FIG. 5 shows a second surface-hardened preform 30 
prepared by each of the first and second press quenchings 60, 80 shown in 
FIGS. 3 and 4. 
COMATIVE EXAMPLE 
In this comparative example, Example 1 was repeated except in that the 
grinding process was modified as follows. In the grinding process, as is 
seen from FIG. 1, at first, a third outer portion 54 of the second 
surface-hardened preform 30 was removed, thereby to expose the back 
surface 42, such that the carbon concentration of the back surface 42 was 
in a range shown in Table. Then, a first outer portion 50 of the second 
preform 30 was removed, based on a datum of the back surface 42, to expose 
the bearing surface 40. After the removal of the third outer portion 54, a 
second outer portion 52 of the second preform 30 was removed, thereby to 
expose the inner flat surface 48, such that the carbon concentration of 
the inner flat surface 48 was in a range shown in Table. The thickness 56 
of the second outer portion 52 of each second preform 30 was not regulated 
to be the same as that 44 of the first outer portion 50 thereof. Thus, the 
former 56 was slightly different from the latter 44. 
TABLE 
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Surface 
Carbon Conc. 
Carbon Conc. 
Carbon Conc. 
Carbon Conc. 
of Bearing 
of Back 
of Inner Flat 
Datum in 
after Surface- 
Surface after 
Surface after 
Surface after 
Surface-hardening Process & 
Grinding 
hardening 
Grinding 
Grinding 
Grinding 
Quenching Process Process (wt %) 
Process (wt %) 
Process (wt %) 
Process (wt 
__________________________________________________________________________ 
%) 
Example 1 
Carburizing & Oil Quenching 
Bearing Surface 
1.1 0.8-1.0 
0.5-0.8 
0.75-0.95 
Example 2 
Carburizing & Oil Quenching 
Bearing Surface 
1.1 0.8-1.0 
0.5-0.8 
0.8-1.0 
Example 3 
Carburizing & Press Quenching 
Bearing Surface 
1.1 0.95-1.05 
0.6-0.75 
0.95-1.05 
Com. Ex. 
Carburizing & Oil Quenching 
Back Surface 
1.1 0.5-0.8 
0.8-1.0 
0.55-0.8 
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