U.S. patent application number 09/756217 was filed with the patent office on 2001-05-17 for rolling bearing.
Invention is credited to Kiuchi, Akihiro, Ohori, Manabu.
Application Number | 20010001172 09/756217 |
Document ID | / |
Family ID | 17968940 |
Filed Date | 2001-05-17 |
United States Patent
Application |
20010001172 |
Kind Code |
A1 |
Kiuchi, Akihiro ; et
al. |
May 17, 2001 |
Rolling bearing
Abstract
In view of the fact that, when backup roll bearings which have
been found satisfactory in the purity thereof in the sample
evaluation through observations using a microscope are checked many
times for the top surface layer portions of the raceway surfaces
thereof according to an ultrasonic detecting method, on quite rare
occasions, there can be found a bearing which contains one or more
inclusions each having a size of as large as several hundreds .mu.
m, and such inclusions give rise to occurrence of a short-life
bearing as a finished product, a range expressed by 2%Da
depth.times.raceway surface is used as a test piece volume and the
size of a non-metallic inclusion existing in the test piece volume
is restricted to a length less than 500 .mu.m, preferably, less
than 100 .mu.m. Here, the term "2%Da depth" means a depth up to 2%
of a rolling element mean diameter from the surface of an inner or
outer race of a bearing.
Inventors: |
Kiuchi, Akihiro; (Kanagawa,
JP) ; Ohori, Manabu; (Kanagawa, JP) |
Correspondence
Address: |
SUGHRUE, MION, ZINN,
MACPEAK & SEAS, PLLC
2100 PENNSYLVANIA AVENUE, N.W.
WASHINGTON
DC
20037-3213
US
|
Family ID: |
17968940 |
Appl. No.: |
09/756217 |
Filed: |
January 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09756217 |
Jan 9, 2001 |
|
|
|
09428792 |
Oct 28, 1999 |
|
|
|
Current U.S.
Class: |
73/593 ;
384/569 |
Current CPC
Class: |
G01N 2291/2634 20130101;
G01N 2291/2696 20130101; G01N 2291/044 20130101; F16C 33/58
20130101; G01M 13/045 20130101; G01N 29/348 20130101; Y10S 384/912
20130101; G01N 2291/0234 20130101; G01N 29/11 20130101; G01N 29/28
20130101; G01N 29/275 20130101; G01N 2291/0422 20130101 |
Class at
Publication: |
73/593 ;
384/569 |
International
Class: |
F16C 033/58 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 28, 1998 |
JP |
P. HEI. 10-307427 |
Claims
What is claimed is:
1. A rolling bearing comprising: at least one of an inner race and
an outer race; a plurality of rolling elements rollingly movable on
the inner or outer race, wherein a non-metallic inclusion, has a
length of less than 500 .mu.m within a volume of 2%Da multiplying
raceway surface area of at least one of said inner and outer race,
where said 2%Da depth expresses a depth which ranges from the
surfaces of the inner and outer races and rolling elements of the
bearing to 2% of the mean diameter of the rolling elements.
2. A rolling bearing as set forth in claim 1, wherein said
non-metallic inclusion has a length less than 100 .mu.m.
3. A rolling bearing as set forth in claim 1, wherein impurity
components within said volume defined by multiplying 2%Da by
raceway surface area of at least one of said inner are outer race
includes Oxygen 9 ppm or less and Sulfur 50 ppm or less.
4. A rolling bearing as set forth in claim 2, wherein impurity
components within said test piece volume defined by multiplying
2%Da by raceway surface of at least one of said inner are outer
race includes Oxygen 9 ppm or less and Sulfur 50 ppm or less.
5. An ultrasonic detecting method for detecting a bearing,
comprising the steps of: disposing a surface to be detected of a
bearing ring and an ultrasonic detection probe within an ultrasonic
transmission medium; transmitting an ultrasonic wave from said
ultrasonic detection probe to said surface to be detected of said
bearing ring; detecting a defect of said bearing ring in accordance
with an ultrasonic wave echo reflected from said surface to be
detected of said bearing ring; rotating said bearing ring, moving
said ultrasonic detection probe in the axial direction of said
bearing ring, and moving said bearing ring and said ultrasonic
detection probe relatively to each other; and transmitting an
ultrasonic wave within 2-30 MHz to the surface to be measured of
said ultrasonic detection probe to detecting the whole section of
the depth of said bearing ring up to the 2%Da depth from the
surface of said inner or outer race.
Description
BACKGROUND OF THE INVENTION
1. 1. Field of the Invention
2. The present invention relates to a rolling bearing and, in
particular, to a rolling bearing such as a bearing for iron and
steel represented by a roll neck bearing which, even under a severe
using condition of a high load and a high surface pressure, does
not produce an unexpected short life product but can be guaranteed
to have a stable life.
3. 2. Related art
4. Recently, a bearing using environment has been severer and
severer and, in a bearing to be used in iron and steel facilities,
a load and a surface pressure to be applied to the bearing have
been higher and higher. In such severe condition, as the bearing to
be used in iron and steel facilities, there is more and more
strongly required a bearing which not only can provide a long life
but also can stamp out a short-life bearing occurring unexpectedly.
The reason for this requirement is as follows: that is, in a
continuous production system in which the inspection and
maintenance of a production line are carried out every preset given
time, if there occurs a short-life bearing which comes short of the
given operating time, then the production line is caused to stop
within the operating time, which incurs great damage.
5. As one of factors that block the durability of the bearing,
there is found a defect or damage in the material of the bearing.
For steel material used to produce a bearing, recently, there has
been employed a method in which, after the steel material is rolled
into a steel bar, for example, in a steel making process, all steel
bars are checked for internal defects thereof using ultrasonic
waves or the like (see Special Steel, vol.46, No.6, page 31, edited
by Special Steel Club Co.). The main object of this inter-process
defect checking method is to detect a hole (a defect) existing in
the inside of the steel material, such as a macro-streak-flaw, a
blow hole, or an unpressed portion left in a rolling and forging
operation, or the like; and, this method uses a low frequency in
the range of 2-5 MHz for detection of the hole or defect. Thanks to
this method, there has been eliminated a large-sized defect of the
order of several mm. However, in the steel material that has been
rolled but left as it is, the crystal grain of the inside thereof
and the surface layer thereof are rough, thereby causing the noise
to become large, which in turn makes it impossible to detect the
defect with high accuracy.
6. On the other hand, it is known that a non-metallic inclusion
(intervening material) existing in the material of a bearing has a
great influence on the life of the bearing itself. For example, in
steel making facilities, a backup roll bearing is used under a high
load and in a well-lubricated condition given mainly by oil
lubrication; and, when the bearing is used in such lubrication
condition, it has been recently found that, if any non-metallic
inclusion exists in the vicinity of the surface layer portion of
the raceway surface of an inner or outer race of the bearing, then
such non-metallic inclusion causes a defect such as a crack or the
like in the bearing, thereby reducing the life of the bearing.
7. In order to avoid the above problem, recently, there have been
proposed many proposals each of which specifies the number of hard
inclusions (mainly, inclusions belonging to oxide-system materials
consisting mainly of Al.sup.2 O.sup.3, or inclusions belonging to
Ti-system materials consisting mainly of TiN) to thereby enhance
the purity of the bearing greatly and thus extend the life of the
bearing.
8. For example, according to Japanese Patent Publication No.
6-145883 of Heisei, there is disclosed a method in which
highly-purified steel including within nine pieces of
Al.sup.2O.sup.3 of 10 .mu.m or more existing and within nine pieces
of TiN of 5 m in an area to be checked of 320 mm.sup.2 is used to
thereby be able to guarantee the long life of the bearing. As
similar examples aiming at extending the life of the bearing by
limiting the number of the non-metallic inclusions, there are also
known Japanese Patent Publication No. 3-56640 of Heisei and
Japanese Patent Publication No. 7-109541 of Heisei respectively
filed by the present applicants, as well as Japanese Patent
Publication No. 5-117804 of Heisei, Japanese Patent Publication No.
6-192790 of Heisei, and the like.
9. Every one of the technologies disclosed in the above-cited
publications, when specifying the quantity of inclusions, observes
a very tiny given area of, for example, 320 mm.sup.2 or 165
mm.sup.2 by a microscope or the like and specifies the purity of
the steel material in accordance with the number of inclusions
detected in such given area.
10. However, although the purity of the steel material is enhanced
in the above-mentioned manner, all the products made of the steel
material are not inspected and verified for the number of
inclusions. In other words, every one of the above technologies,
simply by observing the very tiny given area of the bearing
material representatively, evaluates the purity of the whole
bearing and the bearing material.
11. The present inventors have taken up this problem and made every
effort to solve this problem. For example, when many raceway
surface layer portions of inner and outer races of bearings for
backup rolls used in a rolling mill were checked by an ultrasonic
detecting method, it has been found that, even in bearings the
purity of which was found satisfactory in a sample evaluation
through observations using a microscope, on quite rare occasions,
there can be found a bearing in which an inclusion of as large as
several hundreds .mu.m exists in the range from the raceway surface
of the inner or outer race of the bearing up to the depth of
several mm or so.
12. That is, even if a good result is obtained by checking the tiny
range of an area to be checked in the top surface of the raceway
surface of the inner or outer race of the bearing, it cannot be
always guaranteed that an inclusion of a large size is absent in
the bearing. Much less, in the case of a large-sized bearing such
as a backup roll bearing or the like, a relatively large load is
applied to the bearing because the area of the raceway surface
thereof is large, and the depth of the portion thereof to which a
stress is applied is thereby caused to increase. Therefore, simply
by accurately inspecting only the raceway surface of the bearing as
in the conventional method, it is difficult to eradicate a
short-life bearing which can occur unexpectedly.
SUMMARY OF THE INVENTION
13. The present invention has been made by directing our attention
to the fact that, when a bearing is formed of a steel material, the
existing position of a non-metallic inclusion in the bearing
constitutes an important factor for the long life of the bearing.
Accordingly, it is an object of the invention to provide a rolling
bearing which is guaranteed free from an internal defect in the
range from the rolling contact surface thereof, where its inner or
outer race and rolling elements are rollingly contacted with each
other, to a given depth and thus, even when it is used under a
severe environment of a high load and a high surface pressure as in
a bearing for iron and steel, can avoid a fear of occurrence of a
short-life bearing as a finished product to thereby provide a
long-life rolling bearing.
14. In attaining the above object, according to an aspect of the
present invention, there is provided a rolling bearing including at
least of an inner race, an outer race, and a plurality of rolling
elements rollingly movable on the inner or outer race, wherein a
non-metallic inclusion existing in a test piece, which has a length
of less than 500 .mu.m within 2%Da.times.raceway surface of said
inner or outer race.
15. Here, the term "2%Da depth" expresses a depth which ranges from
the surfaces of the inner and outer races and rolling elements of
the bearing to 2% of the mean diameter of the rolling elements.
16. In a rolling bearing according to the invention, at least as
material for the inner and outer races thereof, there can be used
steel material which contains, as impurity components, oxygen (O)
of 9 ppm or less and sulfur (S) of 0.005% by weight.
BRIEF DESCRIPTION OF DRAWINGS
17. FIG. 1 is a schematic view of an example of an ultrasonic
detective device which can be used in the invention; and,
18. FIG. 2 is a section view of the main portions of a life tester
which is used to test the life of a bearing.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
19. Now, description will be given below of the preferred
embodiments of a rolling bearing according to the invention with
reference to the accompanying drawings.
20. First Embodiment
21. When the present inventors have examined in detail a large
defective echo portion which can be detected very rarely in the
above-mentioned ultrasonic detecting method for detecting a defect
in the raceway surface of a bearing such as a backup roll bearing,
it has been found that a large non-metallic inclusion of as large
as several hundreds .mu.m can exist in the defective echo
portion.
22. Therefore, a large number of tests have been carried out on the
life of the present bearing to find out the correlation between the
strength of the defective echo portion, the length and size of an
inclusion detected, and the life of the bearing. As a result, if a
non-metallic inclusion having a length of 500 .mu.m or more exists
within the 2%Da depth range, then it has been found that the life
of the bearing is shortened extremely. Especially, if a
non-metallic inclusion having a length of 100 .mu.m or more does
not exist within the above-mentioned depth range, then it has been
found that a further stable life can be obtained.
23. In the present invention, the reason why the existing position
of the large inclusion is specified within 2%Da range is that a
depth, where a shearing stress generated when the outer and inner
races and rolling elements of the rolling bearing are rollingly
contacted with each other provides the largest value, is less than
2% of the mean diameter of the rolling element from the surface of
the rolling contact surface thereof, and also that the present
shearing stress acts on the large non-metallic inclusion to thereby
cause the bearing to break away.
24. Now, illustrations will be given below of the volumes of the
test pieces of the present invention and the prior art.
25. 1) In the case of 850RV1133 (a quadruple row cylindrical roller
bearing)
26. When there is used a roller bearing having the following
dimensions, that is, the inner race inside diameter; 850 mm: the
outer race outside diameter 1180 mm, the width 650 mm, and roller
diameter 80 mm, if the volumes of the test pieces of the outer and
inner races of the bearing up to the 2%Da depth thereof are
calculated, then the inner race provides a volume of approx.
3.1.times.10.sup.6 mm.sup.3 and the outer race provides a volume of
approx. 3.0.times.10.sup.6 mm.sup.3.
27. Thus, as the sum of the inner and outer races of the present
roller bearing, there is obtained a volume of 6.1.times.10.sup.6
mm.sup.3 in all.
28. 2) In the case of NU3336 (a cylindrical roller bearing)
29. When there is used a roller bearing having the following
dimensions, that is, the inner race inside diameter; 180 mm: the
outer race outside diameter 380 mm, the width 150 mm, and roller
diameter 48 mm, if the volumes of the test pieces of the outer and
inner races of the bearing up to the 2%Da depth thereof are
calculated, then the inner race provides a volume of approx.
1.1.times.10.sup.5 mm.sup.3 and the outer race provides a volume of
approx. 1.5.times.10.sup.5 mm.sup.3. Thus, similarly, as the sum of
the inner and outer races of the present roller bearing, there is
obtained a volume of 2.6.times.10.sup.5 mm.sup.3 in all.
30. On the other hand, if a single roller bearing according to the
above-mentioned prior art is sliced in the form of a cube having an
area of 320 mm.sup.2 and a depth of 10 .mu.m and the volume of the
present cubic slice is calculated, then there is obtained a volume
of approx. 3.2 mm.sup.3 which corresponds to one of two test pieces
of a single roller bearing according to the prior art. Thus,
similarly to the above, if this volume is doubled to find the
volume of the two test pieces of the single roller bearing
according to the prior art, then there is obtained a volume 6.2
mm.sup.3.
31. As can be clearly understood from the above examples, the test
piece volume for inspecting a non-metallic inclusion according to
the invention is larger by far than the prior art and, therefore,
the correlation between the test piece volume and bearing life is
similarly very higher in reliability, and the non-metallic
inclusion inspection or detection according to the invention can be
carried out in a non-destructive manner.
32. In the present invention, to detect the presence or absence of
a non-metallic inclusion within the test piece volume of
2%Da.times.raceway surface, preferably, the non-metallic inclusion
may be ultrasonically detected using a detection probe of 2-30 MHz
according to a surface wave detecting method or an angle incidence
detecting method. If the frequency of the detection probe is less
than 2 MHz, then there is a possibility that a non-metallic
inclusion having a length of 100 .mu.m cannot be detected.
33. On the other hand, if the frequency of the detection probe
exceeds 30 MHz, then the ultrasonic wave attenuates greatly within
the bearing to thereby be unable to detect up to the 2%Da depth. By
the way, it is said that the defect detecting limit in the
ultrasonic detecting method is 1/2of the wavelength; and, when
detecting a non-metallic inclusion of 100 .mu.m or more according
to an angle beam detecting method using a transversal wave (the
sound speed of a transversal wave through steel is 3230 m/s), a
frequency used is 16 MHz or higher.
34. Now, description will be given below of a comparison test which
was conducted on embodiments according to the invention and
comparison examples.
35. In the comparison test, as test pieces, there were prepared
cylindrical roller bearings of a type NU3336 respectively formed of
materials shown in Table 1, and the raceway surfaces of the inner
and outer races of these bearings were inspected for defects by an
ultrasonic detecting device.
1 TABLE 1 Non-metallic inclusion Steel 500 .mu.m 100 .mu.m Bearing
Life Division No. Kinds or more or more Test Embodiment 1 1 SUJ2 No
No 100 Hrs. or longer Embodiment 2 2 SCR420 No No 100 Hrs. or
longer Comparison 3 SCR420 Present Present 12 Hrs. example 1 (two)
(five) Comparison 4 SUJ2 Present Present 15 Hrs. example 1 (one)
(four) Comparison 5 SCR440 No Present 60 Hrs. example 1 (two)
36. Now, FIG. 1 is a schematic view of the ultrasonic detecting
device used in the comparison test. In FIG. 1, reference character
1 designates a water tank in which there is stored water used as an
ultrasonic wave transmission medium. Within the water tank 1, there
are disposed a bearing ring 2, which is a finished product of an
outer race (or an inner race) of a rolling bearing to be tested,
and an ultrasonic detecting probe 3 in such a manner that they are
respectively immersed in the water. As the ultrasonic detecting
probe 3, there is used a focusing type probe which is strong in
directivity and hard to be influenced by the curvature of the
bearing ring 2.
37. The bearing ring 2 is carried on two pulleys 4 disposed within
the water tank 1 and spaced apart from each other in the horizontal
direction, and a belt 7 is wound in an equilateral-triangle manner
around the two pulleys 4 and another pulley 6 fixed to the motor
shaft of a rotation drive motor 5.
38. The rotation drive motor 5 can be controlled through a motor
driving control amplifier 8 by a control unit 9 and, if the
rotation drive motor 5 is driven, then the bearing ring 2 carried
on the two pulleys 4 can be rotated at a given speed. By the way,
the control unit 9 is composed of a personal computer including
display means such as a CRT and the like.
39. The ultrasonic detecting probe 3 is mounted through a probe
mounting member 13 on an XY stage 12 supported by a linear guide
device 10 which is disposed in such a manner that it can be moved
along the axial direction of the bearing ring 2, and the ultrasonic
detecting probe 3 is so disposed as to be opposed to the raceway
surface of the bearing ring 2. Also, the ultrasonic detecting probe
3 not only transmits an ultrasonic pulse corresponding to a voltage
signal from an ultrasonic detecting device 14 to the inner
peripheral surface of the bearing ring 2 but also receives a
reflected echo of the ultrasonic pulse, converts the echo into a
voltage signal and transmits the voltage signal to the ultrasonic
detecting device 14.
40. The ultrasonic detecting device 14, in accordance with an
instruction from the control unit 9, transmits an instruction
signal composed of a voltage signal to the ultrasonic detecting
probe 3 and also transmits detection information, which is obtained
on the basis of its transmitted and received signals, to the
control unit 9; and, the control unit 9 displays the detection
information on an CRT.
41. The linear guide device 10 is capable of moving the ultrasonic
detecting probe 3 in the axial direction of the bearing ring 2
through a servo motor (not shown) which can be controlled by a
linear guide controller 16. The linear guide controller 16, if it
is detected that the bearing ring 2 has been rotated once
(360.degree.) by a rotary encoder 15 mounted on the outer
peripheral surface of the bearing ring 2, controls the servo motor
in accordance with an instruction from the control unit 9 to
thereby move the ultrasonic detecting probe 3 by a given dimension
in the axial direction of the bearing ring 2. As a result, the
whole raceway surface of the bearing ring 2 can be detected for
presence or absence of a defect.
42. The detection was carried out according to a water depth
detecting method under the following condition.
43. Detecting probe: Focusing type probe (vibrator diameter of 6
mm)
44. Frequency: 15 MHz
45. Also, in this detecting operation, the angle of refraction of
an ultrasonic wave entering the bearing ring 2 was set at an angle
of 30.degree., and the angle of refraction of an ultrasonic wave
entered was set at an angle of 5.degree.; that is, the detection
was carried out in such a manner that the test pieces could be
sufficiently detected for the defects thereof up to the 2%Da depth
thereof under these incidence conditions.
46. After the above detecting operation, a life test was conducted
on test pieces, that is, cylindrical roller bearings in which the
outer races having been found to be free from any defect of 100
microns or more as the result of the above detection are used in
combination with the inner races having been found to contain
therein large non-metallic inclusions of 500 microns or more and
100 microns or more shown in the test pieces No. 1-No. 5 of Table 1
as the result of the ultrasonic detection.
47. That is, the bearing life test was conducted under the
following conditions using a life tester.
48. FIG. 2 is a section view of the main portions of the life
tester used in the bearing test. An outer race 22 on which a
rolling element 21 of a bearing to be tested 20 was incorporated
into a housing 24, an inner race 23 was fitted with a rotary shaft
(roller) 25, a radial load Fr due to oil pressure was applied to
the bearing, and then, while rotating the rotary shaft 25, the life
test was conducted.
49. Bearing: Cylindrical roller bearing NU3336
50. Radial load: 800 KN (P/C=0.5)
51. Inner race rotation number: 1000 rpm
52. Lubrication: Grease
53. In the embodiments 1 and 2 shown in Table 1, although the steel
kinds thereof are respectively SUJ2 and SCR420 and thus are
different from each other, in the their respective test piece
volumes each consisting of 2%DA depth.times.raceway surface, not
only a non-metallic inclusion of 500 microns or more but also a
non-metallic inclusion of 100 microns or more were not found, and
no breakaway occurred in the bearings for the bearing life test
time exceeding 100 Hrs.
54. On the other hand, in the comparison example 1, a bearing, the
steel kind of which is SCR420, was ultrasonically detected for the
defect thereof and found that, on the raceway surface side of the
inner race thereof, there were contained two non-metallic
inclusions of 500 microns or more and five non-metallic inclusions
of 100 microns or more. After then, when a life test was conducted
on the present bearing as a test piece, there occurred a breakaway
in the bearing in the bearing life test time of 12 Hrs.
55. In the comparison example 2, a bearing, the steel kind of which
is SUJ2, was ultrasonically detected for the defect thereof and
found that, on the raceway surface side of the inner race thereof,
there were contained a non-metallic inclusion of 500 microns or
more and four non-metallic inclusions of 100 microns or more. After
then, when a life test was conducted on the present bearing as a
test piece, there occurred a breakaway in the bearing in the
bearing life test time of 15 Hrs.
56. In the comparison example 3, a bearing, the steel kind of which
is SCR440, was ultrasonically detected for the defect thereof and,
on the raceway surface side of the inner race thereof, there was
not found any non-metallic inclusion of 500 microns or more but
there were found two non-metallic inclusions of 100 microns or
more. After then, when a life test was conducted on the present
bearing as a test piece, the bearing was found to be improved in
life over the above-mentioned comparison examples 1 and 2, but
there occurred a breakaway in the bearing in the bearing life test
time of 60 Hrs.
57. From the above test results, to prevent occurrence of a
short-life bearing as a finished product, it is effective to
restrict the existence of a non-metallic inclusion of 500 microns
or more in the range of the 2%Da depth of the raceway surface of a
bearing and, especially, to provide the bearing with a long life,
it is preferred to restrict the existence of a non-metallic
inclusion of 100 microns or more in that range.
58. Second Embodiment
59. The present inventors also have checked the impurity components
of the steel materials of the bearings which had been found by the
ultrasonic detecting method that they contained therein one or more
large non-metallic inclusions each having a mean grain diameter of
100 microns or more and further one or more large non-metallic
inclusions each having a mean grain diameter of 500 microns or
more. As the results of our check, the present inventors have found
that, if the contents of oxygen O and sulfur S which are impurity
components contained in the steel material are restricted, then the
occurrence of the large non-metallic inclusions in the bearing can
be reduced.
60. Specifically, to restrict or reduce the existence of the large
non-metallic inclusions of 500 microns or more and further of 100
microns or more to thereby eliminate the occurrence of the
short-life product in the bearings, it has been found necessary to
reduce the contents of the impurity components, that is, reduce the
oxygen content down to 9 ppm or less and the sulfur content down to
0.005% by weight or less.
61. The reason for the above necessary condition is as follows:
62. That is, oxygen O exists in steel as inclusion components in
the form of oxide-system components such as Al.sub.2O.sub.3, CaO,
MgO and the like. Also, sulfur S exists as inclusion components in
the form of sulfide-system components such as MnS, CaS and the
like. It has been also found that, in steel, if these inclusions
collect together in a multiple manner, then they exist as
non-metallic inclusions each having a given length and a given
width, such as Al.sub.2O.sub.3--MgO--CaO,
Al.sub.2O.sub.3--MgO--CaO--MnS and the like.
63. In other words, if the contents of oxygen and sulfur which are
components forming the impurities are respectively controlled down
to 9 ppm or less and 0.005% by weight or less, then the occurrence
of the non-metallic inclusions each having a mean grain diameter of
500 microns or more and further of 500 microns or more can be
controlled, thereby being able to eliminate the occurrence of a
short-life bearing as a finished product.
64. To obtain steel which contains oxygen of 9 ppm or less and
sulfur of 0.005% by weight or less as impurity components thereof,
it is effective to employ a method in which, after a steel material
is resolved in an electric furnace and a blast furnace, the steel
material is treated according to a VAR process (a vacuum arm
re-resolution process).
65. Now, description will be given below of a comparison test
conducted on embodiments according to the invention and comparison
examples.
66. Similarly to the previously described first embodiment, as test
pieces, the components of a NU3336-type cylindrical roller bearing
were prepared using steel materials shown in Table 2.
2 TABLE 2 Number of inclusions Steel 500 .mu.m 100 .mu.m Bearing
Division No. Kinds S(%) O(ppm) or more or more Life Test Embodiment
3 6 SUJ2 0.003 9 Not found Not found 100 Hrs. or longer Embodiment
4 7 SCR420 0.005 7 Not found Not found 100 Hrs. or longer
Embodiment 5 8 SCR440 0.005 9 Not found Not found 100 Hrs. or
longer Comparison 9 SUJ2 0.015 11 three eight 10 Hrs. example 4
Comparison 10 SCR420 0.008 9 Not found two 83 Hrs. example 5
67. Further, as test pieces the impurity components of which are
limited, there were prepared bearings, that is, test pieces Nos.
6-10 respectively formed of steel materials containing impurity
components (S, O) shown in FIG. 2, by 200 s each. And, the raceway
surfaces of the inner and outer races of the thus obtained
bearings, similarly to the previously described case, were
respectively checked by the ultrasonic detecting device to confirm
as to whether the given defects (non-metallic inclusions) were
present or absent. By the way, similarly to the previously
described test pieces Nos. 1-5, for the outer races, there were
used steel materials which had been previously confirmed by the
ultrasonic detecting device that they did not contain any defect of
500 microns or more or any defect of 100 microns or more, only the
inner races were checked for the size and number of the
non-metallic inclusions thereof.
68. Referring here to a checking or evaluating method employed in
the above comparison test, all the test pieces of the respective
inner races (that is, 200 each test piece) were detected
ultrasonically under the same condition as in the first embodiment,
and were checked for the number of bearings in which the
non-metallic inclusions of 500 microns or more were found present
in the test piece volume range of 2%Da depth.times.raceway surface.
Also, there were confirmed the number of bearings in which the
non-metallic inclusions of 100 microns or more were found
present.
69. Next, in the case of the bearings in which the non-metallic
inclusions were found present, as a representative of them, the
bearing containing the largest inclusion was selected; and, in the
case of the bearings with no inclusion found, one of them was
selected at random. A life test was conducted on the two selected
bearings.
70. In the embodiments 3, 4 and 5 shown in Table 2, because the
steel kinds thereof were respectively SUJ2, SCR420 and SCR440 and
also because there were used steel materials the impurity
components S and O of which were respectively within the limit
range (S: 0.05% or less, O: 9 ppm or less) specified by the
invention, there was found no non-metallic inclusion of 500 microns
or more in the test piece volume of 2%Da depth.times.raceway
surface of each bearing as the finished product. Also, a life test
was conducted on the representative bearings of the respective
embodiments similarly to the first embodiment. As the results of
the life test, it was confirmed that, even for a life test time of
100 Hrs. or longer, no breakaway occurred in the bearings.
71. On the other hand, in the comparison example 4 in which the
impurity components S and O of the steel material thereof exceeded
the above-mentioned limit range, out of the bearings as the
finished products thereof, there were detected three bearings each
containing one or more non-metallic inclusions of 500 microns or
more in the test piece volume of 2%Da depth.times.raceway surface
of inner race thereof, and 8 bearings each containing one or more
non-metallic inclusions of 100 microns or more in the same test
piece volume. Among the bearings found containing one or more
non-metallic inclusions of 500 microns or more, the bearing
containing the largest non-metallic inclusion was subjected to a
life test. This life test showed that the bearing broke away in a
life test time of 10 Hrs., that is, the life of the bearing was
very short.
72. Next, in the comparison example 5 in which sulfur S of the
impurity components of the steel material thereof exceeded the
above limit range, there was found no bearing containing one or
more non-metallic inclusions of 500 microns or more in the test
piece volume of 2%Da depth.times.raceway surface of inner race
thereof, but there were found 2 bearings each containing one or
more non-metallic inclusions of 100 microns or more in the same
test piece volume. Out of the two bearings found containing one or
more non-metallic inclusions of 100 microns or more, the bearing
containing the largest non-metallic inclusion was subjected to a
life test. This life test showed that the bearing broke away in a
life test time of 83 Hrs., that is, the life of the bearing was
longer than that of the comparison example 4 but was much shorter
than the embodiments of the invention.
73. From the foregoing test results, it can be clearly understood
that use of a steel material containing, as impurity components
thereof, S of 0.05% or less and O of 9 ppm or less is able to
restrict the existence of large non-metallic inclusions each of 500
microns or more and 100 microns or more in a bearing as a finished
product, and thus can eliminate the occurrence of a short-life
bearing.
74. As has been described heretofore, according to the present
invention, there is employed, as a test piece volume range, a range
expressed by the product of a 2%Da depth, where a shearing stress
to be generated when the inner or outer race of a bearing and a
rolling element are rollingly contacted with each other provides
the largest value, and the raceway surface of the inner or outer
race of the bearing; and, using the test piece volume range, the
size and existence of non-metallic inclusions are restricted over
all the bearings as finished products. Thanks to this, differently
from the conventional purity restriction based on sample
evaluations, the present invention can provide an excellent effect
that, even under a severe using condition of a high load and a high
surface pressure, unexpected occurrence of a short-life bearing can
be prevented and thus the stable life of a bearing can be
guaranteed.
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