U.S. patent number RE37,967 [Application Number 08/869,844] was granted by the patent office on 2003-01-21 for antifriction bearing and alternator incorporating same for use in vehicles.
This patent grant is currently assigned to Denso Corporation, Koyo Seiko Co., Ltd.. Invention is credited to Yoshiki Fujita, Teruo Hoshino, Masayuki Kitamura, Kenzo Mitani, Hiroyuki Miyazaki, Sigenobu Nakamura, Tutomu Siga.
United States Patent |
RE37,967 |
Nakamura , et al. |
January 21, 2003 |
Antifriction bearing and alternator incorporating same for use in
vehicles
Abstract
An antifriction bearing includes a fixed ring which comprises a
steel containing up to about 10% of residual austenite. An
alternator for vehicles includes a stator mounted on a frame, a
rotor having its rotary shaft rotatably supported by a pair of
bearings on the frame and a drive pulley mounted on one end of the
shaft projecting outward from the frame. The outer ring of at least
the bearing toward the pulley comprises a steel containing up to
about 10% of residual austenite.
Inventors: |
Nakamura; Sigenobu (Anjo,
JP), Siga; Tutomu (Aichi-ken, JP), Mitani;
Kenzo (Obu, JP), Fujita; Yoshiki (Osaka,
JP), Kitamura; Masayuki (Osaka, JP),
Miyazaki; Hiroyuki (Osaka, JP), Hoshino; Teruo
(Osaka, JP) |
Assignee: |
Koyo Seiko Co., Ltd. (Osaka,
JP)
Denso Corporation (Kariya, JP)
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Family
ID: |
26476578 |
Appl.
No.: |
08/869,844 |
Filed: |
June 5, 1997 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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203972 |
Jun 8, 1988 |
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Reissue of: |
152687 |
Nov 16, 1993 |
05422524 |
Jun 6, 1995 |
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Foreign Application Priority Data
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Jun 10, 1987 [JP] |
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62-145483 |
Jun 10, 1987 [JP] |
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62-145484 |
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Current U.S.
Class: |
310/90;
29/898.14; 310/40R |
Current CPC
Class: |
F16C
33/64 (20130101); F16C 33/62 (20130101); H02K
5/1732 (20130101); F16C 2380/26 (20130101); F16C
2204/66 (20130101); Y10T 29/49709 (20150115); F16C
2204/62 (20130101) |
Current International
Class: |
F16C
33/62 (20060101); H02K 5/173 (20060101); F16C
035/00 (); H02K 005/16 () |
Field of
Search: |
;310/90,42
;29/898.14 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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362 807 |
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Jun 1981 |
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AT |
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1 361 553 |
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Jul 1974 |
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GB |
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56-34615 |
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Mar 1975 |
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JP |
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Other References
Okamoto et al., Effect of SI and Retained Austenite on the Rolling
Fatigue Life of Bearing Steel, Seititsu Kenkyu, No. 277, 1973.*
.
Tsushima et al., "Improvement of Rolling Contact Fatigue Life of
Carburized Tapered Rolling Bearings", SAE Technical Paper 860725,
Apr. 1986.* .
Patent Abstracts of Japan, unexamined applications, section C, vol.
12, No. 82 (c-481), Mar. 15, 1988. .
"Zwischenstufenumwandlung von Walzlagerstahlen," WTS 74 06 20, SKF
Kugellagerfabriken GmBH (Feb. 1974). .
E. Yamija et al., "Effects of Retained Austenite on the Rolling
Fatigue Life of Ball Bearing Steels,"translation of article
published in J. Japan Inst. Metals, vol. 36, p. 711 (1972). .
Handbook of Metals, Japan Metal Society, Maruzen Co., Ltd. (Rev.
4th Ed., 1982), sections 7.6.1-7.6.2..
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Primary Examiner: Tamai; Karl
Attorney, Agent or Firm: Jacobson Holman PLLC
Parent Case Text
More than one reissue application has been filed for the reissue of
U.S. Pat. No. 5,422,524, which is a division of application Ser.
No. 07/203,972 filed Jun. 8, 1988. The reissue applications are
application Ser. No. 08/869,844 (the present application), filed
Jun. 5, 1997, application Ser. No. 09/618,910 (a divisional reissue
of application Ser. No. 08/869,844), filed Jul. 18, 2000, and
10/040,627 (a continuation reissue of application Ser. No.
09/618,910), filed Jan. 9, 2002.
Claims
What is claimed is: .[.
1. An alternator for vehicles comprising a rotary shaft of a rotor
which is rotably supported by a pair of bearings,..each comprising
a fixed ring and a rotary ring, on a frame having a stator, and a
drive pulley which is mounted on one end of the rotary shaft
projecting outward from the frame, wherein the alternator comprises
at least the bearing directed toward the pulley comprising a fixed
ring comprising a steel containing up to about 10% of residual
austenite..]..[.
2. An alternator as defined in claim 1 wherein said steel
containing limited proportion of austenite has been made by
subjecting steel having a higher austenite content to a sub-zero
treatment..]..[.
3. An alternator as defined in claim 1 wherein said steel
containing limited proportion of austenite has been made by
subjecting steel having a higher austenite content to tempering at
a temperature of 250.degree. to 380.degree. C..]..[.
4. An alternator as defined in claim 1 wherein said steel
containing limited proportion of austenite has been made by
subjecting steel having a higher austenite content to a sub-zero
treatment and a subsequent tempering treatment at a temperature of
170.degree. to 230.degree. C..]..[.
5. An alternator as defined in claim 1 wherein said steel has been
subjected to carbonization hardening..]..[.
6. An alternator as defined in claim 1 wherein the amount of
residual austenite is up to 6%..]..Iadd.
7. An alternator for vehicles comprising a rotary shaft of a rotor
which is rotably supported by a pair of ball bearings, each
comprising a fixed ring and a rotary ring, on a frame having a
stator, and a drive pulley which is mounted on one end of the
rotary shaft projecting outward from the frame, the fixed ring
having a raceway, wherein the alternator comprises at least the
bearing directed toward the pulley comprising a fixed ring
comprising a steel containing up to about 3% of residual austenite,
and wherein said steel containing limited proportion of austenite
has been made by subjecting steel having a higher austenite content
to tempering at a temperature of 250.degree. to 380.degree. C.,
whereby the rolling fatigue life is improved by preventing
occurrence of partial structural changes or minute cracks
immediately under the raceway of the fixed ring caused by vibration
or impact..Iaddend..Iadd.
8. An alternator for vehicles comprising a rotary shaft of a rotor
which is rotatably supported by a pair of ball bearings, each
comprising a fixed ring and a rotary ring, on a frame having a
stator, and a drive pulley which is mounted on one end of the
rotary shaft projecting outward from the frame, the fixed ring
having a raceway, wherein the alternator comprises at least the
bearing directed toward the pulley comprising a fixed ring
comprising a steel containing up to about 3% of residual austenite,
and wherein said steel has been subjected to carburization
hardening, whereby the rolling fatigue life is improved by
preventing occurrence of partial structural changes or minute
cracks immediately under the raceway of the fixed ring caused by
vibration or impact..Iaddend.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an antifriction bearing for use in
an environment involving vibration or impact, and also to an
alternator incorporating the bearing for use in vehicles.
The bearing rings and rolling members of the antifriction bearing
(hereinafter referred to simply as the "bearing") are generally
subjected to a cyclic high-shear stress due to a rolling motion, so
that they are given high Rockwell hardness of HRC 58 to 64 by
hardening and tempering so as to retain increased strength against
rolling fatigue. It has been reported that there is the following
relationship between the hardness and the rolling fatigue life. The
life greatly shortens as the hardness decreases.
wherein LH: the life when the hardness varies fH: the hardness
factor p: a constant (3 for ball bearings, or 10/3 for roller
bearings) L: the life of the standard bearing Further,
fH=(HV/750).sup.2
wherein HV is Vickers hardness
Recently, however bearings are used for applications wherein the
intended rolling fatigue life is not available merely by assuring
the hardness.
With common bearings, the shearing stress due to the contact
between the bearing rings and the rolling members develops a crack
from an inclusion or the like, and the crack grows to cause
flaking. In the case where the inner ring is rotated, such flaking
occurs predominantly in the rotating ring, i.e., the inner ring.
Conversely, in the case of bearings which are used in an
environment involving vibration or impact, the vibration or impact
causes many minute cracks or changes in the structure immediately
under the raceway of the outer ring which is fixed, consequently
giving rise to flaking within a very short period of time to render
the bearing unserviceable.
This phenomenon appears attributable to the following reason. The
vibration or impact deforms the raceway and becomes more
pronounced, consequently causing greater microscropic strain in the
ring under the raceway.
The rolling fatigue life can be lengthened most easily by
increasing the size of the bearing to give an increased load
capacity. This achieves an advantage since a reduced stress value
will then result when the same load is applied. In actual use,
however, the vibration or impact load readily varies with the
structure around the bearing and mounting and operation conditions,
and it is impossible to meet the requirement of decreasing the size
and weight, so that the increase in the size of the bearing is not
a satisfactory solution.
High-carbon chrominum steel (such as JIS SUJ2 or SAE 52100) as
adjusted to the hardness of HRC 58 to 64 by the usual hardening and
tempering treatments as stated above is conventionally used for the
inner and ouster rings of the bearings for alternators for
vehicles.
In recent years, however, alternators are required to have a
smaller size, reduced weight and higher output to meet the need to
decrease the fuel cost of vehicles and increase various electrical
loads thereof. To fulfill this requirement, it has become practice
to use a greater pulley ratio and to rotate the alternator at a
high speed. Accordingly, the maximum speed of rotation is in excess
of 12000 r.p.m.
The problems involved in the high-speed rotation include the
slippage of the belt in connection with the external arrangement of
the alternator. This problem has been overcome by using a larger
number of belts under increased tension. On the other hand, the
problem associated with the internal arrangement of the alternator
is the need to render the bearing resistant to the high-speed
rotation and to the high tension involved. More specifically,
because the heat of agitation due to the high-speed rotation and
the increased frictional heat due to high tension shorten the life
of the grease used, the bearing must be adapted to overcome this
problem. Furthermore, the bearing must be rotatable at a high speed
without marked vibration that would result from the deformation of
the raceway due to high tension. Generally, the bearing is
rotatable at a high speed satisfactorily when reduced in size,
since the side reduction is effective for decreasing the amount of
heat generation.
Nevertheless, in the case where the bearing is subjected to high
tension as in the alternator, a reduced size leads to a decreased
load capacity to entail a shortened fatigue life, so that the
bearings in use are at least about 32 mm in outside diameter if
smallest.
Briefly, in assuring high-speed rotation under increased tension as
required for reducing the size and weight of the alternator and
increasing the output thereof, it is necessary to solve the
conflicting problems of inhibiting heat generation and taking a
countermeasure against the increased load while diminishing the
vibration, whereas difficulties are encountered with the convention
alternator described in overcoming these problems.
SUMMARY OF THE INVENTION
An object of the present invention is to a bearing which is adapted
to have a prolonged life in an environment of vibration or impact
without increasing the size of the bearing.
Another object of the invention is to provide an alternator which
is rotatable at an increased speed and is nevertheless operable
under the resulting increased tension, as required for reducing the
size and weight of the alternator and increasing the output
thereof.
As already described, we have found that the early flaking of the
outer ring of the bearing in use under vibration or impact is
attributable to many cracks or changes in structure which occur
immediately under the raceway under the dynamic action of excessive
stress due to the severe load of the vibration or impact, and
carried out repeated testing and research with attention directed
to the heat treatment of the outer ring to give the ring resistance
to cracks or changes in structure, whereby the present invention
has been accomplished.
Stated more specifically, the present invention provides a bearing
wherein the ring to be fixed comprises a steel up to about 10% in
the amount of residual austenite.
The amount of residual austenite in the fixed ring should be up to
about 10% for the following reason. The usual hardening-tempering
treatment of steel permits about 11 to about 14% of austenite to
remain on the average, and it is said that a somewhat higher
content of residual austenite leads to an improved rolling fatigue
life.
For example, C. Razim carburized steels such as 14NiCr.sub.14
(0.14% C, 0.46% Mn, 0.78% Cr and 3.67% Ni), 16MnCr5 and 20MoCr4 and
tested the steels for fatigue by contact with a roller with the
following conclusions (see C. Razim, Harterei Technische
Mitteilungen, 22(1967), Heft 4, S. 32). (1) The surface of the
steel in contact with the roller underwent plastic deformation due
to load stress. The width of contact therefore increased to result
in a lower contact surface pressure consequently improving the
pitting life. (2) Rotation bending fatigue test revealed that the
specimen containing 30 to 50% YR (residual austenite) was about 2
times the specimen of pure martensite in fatigue strength
improvement. (3) 14NiCr14 specimen with 50% YR was HV 550 in
hardness. The testing changed the surface hardness to HV 950. (4)
It was not apparent whether the testing converted the YR to
martensite. After the testing, a carbide was observed in the
structure microscopically.
J.P. Sheahan et al. carburized SAE 8620 steel under varying
conditions and subjected the steel to a pitting test with a roller
to ford that the specimens with a higher YR content were longer in
pitting life than those with a lesser content. (see J. P. Sheahan
and M. A. H. Howes, SAE 720268). A plastic flow and work hardening
are suggested as the reason. O. W. Mcmullan shares the same concept
as above, stating that the presence of YR is likely to mitigate the
load stress (see O. W. Mcmullan, Metal Progress (1962) April, p.
67).
According to R. A. Wilde, up to 10% of YR is not appropriate
because of excessive hardness. He states that the presence of a
proper amount of YR, which is optimally 10 to 25%, is useful for
mitigating the load stress (see R. A. Wilde, Research Center Eaton
and Towne Inc., (1967), Oct.).
Yajima et al., conducted a rolling fatigue test using bearing
steel, with the result that the pitting life improved with
increasing YR content (Yajima et al., The Japan Institute of
Metals, Symposium, 1972)
Since a detailed examination of the portion immediately below the
point of contact between the specimen and the steel ball indicated
that the test increased the hardness from HV 750 to about HV 1000
and that the X-ray diffraction line due to austenite almost
disappeared, they postulated that the result was due to the overall
effect of ausforming and stained induced transformation. Like
Yajima et al., Okamoto et al. carried out a rolling fatigue test
using bearing steel to investigate the influence of YR on pitting
life (see Okamoto et al., Seitetsu Kenkyu (Research on Iron
Making), 1973, No. 277, p. 82). They directed attention to the fact
that a specimen, containing YR, having a softer surface than the
one almost free from YR, underwent plastic deformation at the
surface to exhibit a reduction in the substantial surface pressure,
when the specimens were subjected to the same load, and compared
the tested specimens with the surface pressure corrected to find
that the specimens having a higher YR content exhibited a longer
pitting life than those with a lower YR content, as demonstrated by
Yajima et al. The reasons given for the result achieved by the
specimens with a higher YR are the function of a strain
concentrator which repeatedly absorbs stress to prevent the
occurrence and development of cracks, and hardening due to
convertion to martensite due to work as shown in FIG. 1.
However, when a large amount of residual austenite is present, the
structure of steel is unstable under vibration or impact, exhibits
lower strength as shown in FIG. 4 and is susceptible to plastic
deformation as already stated, with the result that the raceway
deforms to permit further pronounced vibration or impact. The
rolling frictional force also increases as shown in FIG. 5 to
result in increased stress and to produce increased strain in the
ring under the raceway. Alternatively, the structure is prone to a
change due to strain induced transformation, and the resulting
martensite structure, which is not tempered, is brittle.
Consequently, if the amount of residual austenite exceeds about
10%, the ring becomes susceptible to a local change in structure or
to cracking due to vibration or impact. The amount of residual
austenite is herein limited for the fixed ring based on the fording
that the bearing life is substantially dependent on the damage to
the fixed ring because the fixed ring, the loading region of which
is more definite, is subject to the influence of vibration or
impact more greatly. Preferably, the residual austenite content is
up to 6%.
The residual austenite content can be reduced by conducting a
sub-zero treatment between hardening and tempering. The sub-zero
treatment converts austenite to martensite to decrease the residual
austenite content.
The residual austenite content can be reduced also by adjusting at
least one of the hardening heating temperature, hardening cooling
rate and tempering temperature. For example, although tempering for
usual bearings is conducted at 150.degree. to 200.degree. C. to
obtain hardness of HRC 58 to 64, the tempering treatment, when
carried out at a higher temperature of 250.degree. to 380.degree.
C. reduces the residual austenite content, whereby the possible
change in structure or cracking can be precluded.
The high-temperature tempering at 250.degree. to 380.degree. C.
gives hardness of HRC 52 to 57 and is therefore likely to shorten
the usual flaking life under the common operating conditions,
whereas in the case where a vibration or impact load is involved,
the structural change or cracking can then be prevented as stated
above to result in a greatly lengthened service life.
The present treatment is conducted on the fixed ring, while the
rotatable ring is treated in the same manner as usual bearings. By
following this procedure, bearings are produced which, under the
common operating conditions, show no problems.
Thus, the fixed ring of the bearing of the present invention is
made of a steel having a residual austenite content of up to about
10%. Accordingly, even if subjected to vibration or impact, the
ring is less prone to deformation at its raceway, remains stable in
structure and is resistant to structural changes or cracking, with
the result that the bearing is usable for a prolonged period of
time in an environment involving vibration or impact without the
necessity of being made larger in size.
The present invention further provides an alternator wherein the
rotary shaft of a rotor is rotatably supported by a pair of
bearings on a frame having a stator, and a drive pulley is mounted
on one end of the rotary shaft projecting outward from the frame.
The alternator is characterized in that the outer ring of at least
the bearing toward the pulley comprises a steel up to about 10% in
the amount of residual austenite, so as to preclude the marked
vibration to be produced during high-speed rotation by the
deformation of the raceway due to high tension.
With the alternator of the present invention, the bearing outer
ring is reduced in residual austenite content as already described
and is thereby prevented from plastic deformation at the raceway,
whereby the raceway is prevented from caving in unevenly to assure
diminished vibration, a reduced frictional force and inhibited heat
evolution. This realizes a high-speed operation under increased
tension, making it possible to reduce the size and weight of the
alternator and increase the output thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the relationship between the amount of
residual austenite and the increase in hardness;
FIG. 2 is a graph showing the relationship between the tempering
temperature and the surface hardness when a sub-zero treatment is
conducted;
FIG. 3 is a view in vertical section showing an alternator
embodying the invention;
FIG. 4 is a graph showing the relationship between the amount of
residual austenite and the proof stress;
FIG. 5 is a graph showing the relationship between the amount of
residual austenite and the force of rolling friction; and
FIG. 6 is a graph showing variations in the vibration level with
the lapse of testing time.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The foregoing effects were substantiated by the following examples
of the invention wherein ball bearings were used.
The characteristics given below are required of bearing rings. (1)
High elastic limit since the ring is subjected locally to a high
contact stress. (2) Great rolling fatigue strength since a high
contact load is repeatedly applied to the ring. (3) High hardness.
(4) High abrasion resistance. (5) Least susceptibility to secular
changes. (6) Amenability to heat treatment with high stability.
Accordingly, generally used are high-carbon chromium bearing steels
such as JIS SUJ1, SUJ2 (equivalent to SAE52100), SUJ3, SUJ4 and
SUJ5, and carburized bearing steels such as JIS SCr415, SCr420,
SCM420, SNCM220, SNCM420 and SNCM815, SAE5120, SAE8620, SAE4320 and
SAE9310, among which SUJ2 is most widely used.
Five kinds of specimens were prepared in Comparative Example 1 and
Examples 1 to 4 as listed in Table 1 to substantiate the effects.
The residual austenite context was determined by X ray
diffractiometry at a position 0.1 mm radially outward from the
outer ring raceway.
TABLE 1 Material Heat Residual of outer treatment austenite content
Specimen ring of outer ring of outer ring (%) Comp SUJ2 Standard
hardening 11-14 Ex. 1 Tempering Example SUJ2 Standard hardening Up
to 3 1 Tempering at 350.degree. C. Example SUJ2 Standard hardening
7.9 2 Sub-zero at -70.degree. C. Tempering Example SAE5120
Carburization 5.7 3 hardening Sub-zero at -196.degree. C. Tempering
Example SAE5120 Carburization 9.8 4 hardening Sub-zero at
-60.degree. C. Tempering
COMPARATIVE EXAMPLE 1
As a reference for comparing the effects, an existing bearing was
used. The outer ring was made of SUJ2 and heated at a temperature
of 845.degree. C. for hardening. After hardening in oil, the ring
was tempered at 180.degree. C.
EXAMPLE 1
The outer ring was made of SUJ2 as in Comparative Example 1. The
ring was heated at 845.degree. C. for hardening, quenched in oil
and tempered at 350.degree. C.
EXAMPLE 2
The outer ring was prepared from SUJ2 as in Comparative Example 1.
The ring was heated at 845.degree. C. for hardening, quenched in
oil, then subjected to a sub-zero treatment at -70.degree. C. and
thereafter tempered at 200.degree. C.
EXAMPLE 3
The outer ring was prepared from SAE5120, subjected to
carburization hardening, then immersed in liquid nitrogen
(-196.degree. C.) and thereafter tempered at 210.degree. C.
EXAMPLE 4
The outer ring was prepared from SAE5120, subjected to
carburization hardening and then to a subzero treatment at
-60.degree. C. and thereafter tempered at 200.degree. C.
The sub-zero treatment affords higher hardness and lower toughness,
so that the ring was subsequently tempered at a higher temperature
than in the usual tempering process to give the ring the same
hardness as when no sub-zero treatment was conducted.
FIG. 2 shows the relationship between the tempering temperature of
the sub-zero treated product and the surface hardness thereof.
A tester having the specimen installed therein was placed on a
vibrating table, and the specimen was subjected to a vibration test
by applying a load and vibration thereto at the same time with the
inner ring held in rotation. The testing conditions were as
follows. Bearing load (static load)/load rating: 0.22 Speed of
inner ring: 8000 r.p.m. Calculated life (as above): 196 hours
Vibration acceleration: 10 G (on the vibrating table) Testing time:
500 hours
Each specimen was checked for the degree of fatigue in terms of the
time taken for flaking.
Table 2 shows the result. Since the specimens exhibited no
abnormalities except for the outer ring, the result is given only
for the outer ring.
TABLE 2 Specimen Duration of rotation until flaking Comp. Ex. 1 33
to 170 hours (n = 15) Flaking of outer ring Example 1 No flaking
for 500 hours (n = 6) Example 2 No flaking for 500 hours (n = 6)
Example 3 No flaking for 500 hours (n = 6) Example 4 No flaking for
500 hours (n = 6)
In Comparative Example 1, many cracks and structural changes were
observed immediately under the raceway of the outer ring after
testing, whereas neither cracking nor structural change was found
in Examples 1 and 3. Only a slight structural change was observed
in Examples 2 and 4.
During the testing, the bearing before flaking was removed from the
tester and was singly subjected to an axial load of 2.5 kgf with
the inner ring rotated at 1800 r.p.m. to measure variations with
time in the vibration acceleration of the outer side of the
bearing. Table 3 shows the result.
TABLE 3 Variations in vibration acceleration with time (G) After
After After After After Before 10 50 100 200 500 Specimen test
hours hours hours hours hours Comp. 0.07 0.23 0.40 0.51 -- -- Ex. 1
Ex. 1 0.05 0.06 0.09 0.12 0.14 0.16 Ex. 2 0.06 0.07 0.09 0.13 0.13
0.17 Ex. 3 0.06 0.08 0.10 0.13 0.14 0.16 Ex. 4 0.07 0.09 0.11 0.14
0.15 0.18
The result reveals that the vibration increased markedly in
Comparative Example 1 within a short period of time before flaking,
indicating a marked deformation of the raceway. In Examples 1 to 4,
unlike Comparative Example 1, there was little or no increase in
vibration even after a prolonged period of rotation tat, this
indicating that the raceway remained unchanged despite the
testing.
Tables 2 and 3 show that the bearings of Examples 1 to 4 of the
invention are greatly improved in life over the existing bearing of
Comparative Example 1.
Incidentally, the existing bearing of Comparative Example 1 was
installed in the tester in the same manner as above and tested for
rotation under a static load without giving any vibration using the
same conditions as above in respect of the bearing load and the
speed of inner ring. Even after rotation for 1500 hours, the
bearing was free of flaking with no cracking or structural change
observed immediately under the raceway. This indicates that the
bearing is operable without any trouble under the usual
conditions.
Next, the result achieved by an alternator will be described to
substantiate the effects of the invention. FIG. 3 shows the
construction of the alternator.
A pair of frames 10 and 11 forming the shell of the alternator are
each in the form of a bowl and are fastened to each other with
bolts and nuts. A stator 12 is fixedly fitted to the inner
peripheries of these frames 10 and 11 by a suitable method, as by a
press fit. The stator 12 is a known one comprising a stator core
12a and a stator coil 12b wound around the core.
The frames 10, 11 are centrally formed with hollow cylindrical
bearing boxes 10a, 11a, respectively, projecting inward. Radial
ball bearings 13, 14 are mounted in the boxes 10a, 11a,
respectively, and rotatably support a rotary shaft 15 thereon. A
pair of pawl-shaped pole cores 16a, 16b are mechanically fixed to
the shaft 15 so as to be positioned inside the stator 12. A rotor
coil 17 is clamped between these cores. The shaft 15, pole cores
16a, 16b and rotor coil 17 provide a known rotor 18.
Between the first bearing 13 and the pole core 16a of the rotor 18,
a collar 19 is fitted around the shaft 15. A pulley 20 positioned
outside the frames 10, 11 is fastened with a nut 21 to the end of
the shaft 15 projecting through the first bearing 13 out of the
frame 10. The shaft 15 is rotatable by an engine (not shown)
through the pulley 20.
Of the inner rings and the outer rings of the pair of bearings 13,
14, at least the outer ring of the bearing 13 adjacent to the
pulley 20 is made of a steel which is up to 10% in the amount of
residual austenite.
The residual austenite content is reduced to not higher than 10% by
the method already stated.
FIG. 5 shows the relationship between the amount of residual
austenite and the force of rolling friction. The ratio of rolling
frictional force plotted in FIG. 5 is 1 when the amount of residual
austenite is 10% in the case where the contact surface pressure is
250 kgf/mm.sup.2. FIG. 4 shows the relationship between the amount
of residual austenite and the proof stress. The proof stress
plotted in FIG. 4 is at the strain of 5.times.10.sup.-6. Austenite
is a structure of lower proof stress than martensite, so that the
raceway subjected to a load when the balls roll along, if having a
high austenite content, deforms to cause the balls to roll along a
recess to produce an increased frictional force. Conversely, a
reduction in the austenite content inhibits heat generation within
the bearing due to the high-speed operation of the alterator under
high tension, assuring improved endurance against seizure. Further
in the presence of a large amount of residual austenite, an
increased load due to high tension renders the raceway liable to
plastic deformation, with the result that vibration occurs every
time the balls roll along an unevenly recessed portion during
rotation. When the alternator is driven at a high speed, the
vibration becomes more pronounced, possibly causing the rotor to
interfere with the stator to result in locking. It is effective to
reduce the residual austenite content to preclude such plastic
deformation.
To substantiate the above effect achieved by reducing the residual
austenite content, examples are given below wherein radial ball
bearings were used.
First, four kinds of specimens were prepared in Comparative Example
2 and Examples 5 to 7 as bearings toward the pulley of the
alternator, using the material given in Table 4 for the inner and
outer rings. Specimens were also prepared as the bearings on the
other side (rear side) using the same material as in Comparative
Example 2 for the inner and outer rings.
COMPARATIVE EXAMPLE 2
As a reference for the comparison of the effect, an existing
bearing was used which was prepared from SUJ2 generally in use as a
bearing material. The hardening heating temperature was 845.degree.
C. Oil hardening was followed by tempering at 180.degree. C.
EXAMPLE 5
The material used was SUJ2 as in Comparative Example 2. Heating at
845.degree. C. for hardening was followed by oil quenching and then
by tempering at 350.degree. C.
EXAMPLE 6
The material used was SUJ2 as in Comparative Example 2. Heating at
845.degree. C. for hardening was followed by oil quenching, then by
a sub-zero treatment at -60.degree. C. and thereafter by tempering
at 200.degree. C.
EXAMPLE 7
The material used was SUJ2 as in Comparative Example 2. Heating at
845.degree. C. for hardening was followed by oil quenching, then by
a sub-zero treatment at -60.degree. C. and thereafter by tempering
at 200.degree. C.
TABLE 4 Residual austenite Specimen Material Heat treatment content
(%) Comp. SUJ2 Standard hardening 11-14 Ex. 2 Tempering Ex. 5 SUJ2
Standard hardening Up to 3 Tempering at 350.degree. C. Ex. 6 SUJ2
Standard hardening 9.7 Sub-zero at -60.degree. C. Tempering Ex. 7
SUJ2 Standard hardening 5.9 Sub-zero at -196.degree. C.
Tempering
The residual austenite content was determined by X-ray
diffractiometry over the depth of 0.2 mm from the bearing raceway
radially outward thereof. The bearing on the pulley side was of the
size being No. 6302 (42 mm in outside diameter), and the bearing on
the rear side was of the size bearing No. 6002 (32 mm in outside
diameter).
Each pair of specimens was incorporated into an alternator and
subjected to a high-speed high-tension test under the following
conditions.
TABLE 5 Specimen Duration of rotation until failure Comp. Ex. 2 980
to 1260 hours, n = 10 Example 5 No failure for 2500 hours (ther-
after discontinued) n = 5 Example 6 No failure for 2500 hours
(there- after discontinued) n = 5 Example 7 No failure for 2500
hours (there- after discontinued) n = 5
A failure occurred only in Comparative Example 2. More
specifically, the failure was seizure involving carbonization of
the grease and marked discoloration of the inner and outer rings
and the balls, and the retainer was broken to lock the rotatable
ring. The bearing on the pulley side only failed because this
bearing, which is close to the pulley, is subjected to a greater
momental load and therefore heated to a higher temperature than the
other bearing on the rear side.
Although no failure occurred in Examples 5 to 7, the grease was
checked for oxidation deterioration by infrared spectroscopic
analysis, which revealed almost no deterioration in Examples 5 and
7 but deterioration proceeding in Example 6 only.
In Comparative Example 2, the temperature of the inner and outer
rings was measured under the testing conditions to find that the
outer ring was 8 to 12.degree. C. higher than the inner ring in
temperature. This indicates the following. The inner ring is
connected to the rotor, which is driven at a higher speed than
conventionally and is therefore fully self-cooled by a fan effect
to lower the temperature of the inner ring to a level lower than in
the prior art, whereas the outer ring is mounted on the frames
having attached thereto the stator which evolves a larger amount of
heat due to a higher output, with the result that a larger amount
of heat is transferred from the stator to the outer ring to result
in a higher temperature than conventionally.
For illustrative purposes, FIG. 6 shows the result obtained by
measuring variations in the vibration level with the lapse of
testing time by a vibration acceleration sensor attached to the
frame. Although the specimens tested were found free of the failure
that the rotor interferes with the stator to lock the rotatable
ring, Comparative Example 2 exhibited a higher vibration level.
Presumably, this indicates that the higher residual austenite
content leads to greater plastic deformation.
Thus, when incorporating the bearings of Examples 5 to 7 containing
a reduced amount of residual austenite, the alternator can be
adapted for a high-speed operation under increased tension.
Incidentally, the conventional bearings for use in precision
machines or devices or the like include those subjected to the
sub-zero treatment in order to inhibit the dimensional variations
due to the decomposition of austenite. According to the present
invention, on the other band, attention is directed not to such
dimensional stability but to the characteristics of residual
austenite per se to provide the combination of an alternator and a
bearing which contains a reduced amount of residual austenite so as
to exhibit outstanding performance in an environment involving
vibration or impact. Consequently, the invention achieves the
entirely novel effect of making the alternator smaller in size,
lower in weight and higher in output.
Carburized materials such as SAE5120 are usable for the present
bearing to conduct the sub-zero treatment after carburization
hardening. In this case, unlike the use of SUJ2, additional
compressive residual stress is available which is advantageous to
fatigue life. Accordingly, such materials are useful for assuring
higher tension for rotation at a further increased speed as will be
apparent from the result of Examples 3 and 4 listed in Table 2 and
achieved with the ball bearings.
* * * * *