U.S. patent application number 12/216683 was filed with the patent office on 2008-11-13 for rolling bearings.
Invention is credited to Kikuo Maeda, Yukio Matsubara.
Application Number | 20080276458 12/216683 |
Document ID | / |
Family ID | 32821496 |
Filed Date | 2008-11-13 |
United States Patent
Application |
20080276458 |
Kind Code |
A1 |
Maeda; Kikuo ; et
al. |
November 13, 2008 |
Rolling bearings
Abstract
An inexpensive rolling bearing is proposed which can be used
stably for a long time where the lubricating conditions are severe
or bending stresses act. Inner and outer rings of rolling bearings
mounted in a rocker arm of an automobile are made of high-carbon
chrome bearing steel. They are subjected to heat treatment in which
after carbonitriding, high-temperature tempering is carried out.
Then they are induction hardened to impart resistance to material
quality change and a compressive stress of not less than 200 MPa to
the surface layer, thereby markedly improving the rolling contact
fatigue life and the tension-compression fatigue strength while
keeping the material cost as before. Thus, this bearing can be used
stably for a long time even though it is a full complement type
bearing, lubricating conditions tend to worsen and the outer ring
is repeatedly subjected to bending stress from the cam.
Inventors: |
Maeda; Kikuo; (Mie, JP)
; Matsubara; Yukio; (Mie, JP) |
Correspondence
Address: |
WENDEROTH, LIND & PONACK, L.L.P.
2033 K STREET N. W., SUITE 800
WASHINGTON
DC
20006-1021
US
|
Family ID: |
32821496 |
Appl. No.: |
12/216683 |
Filed: |
July 9, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10807230 |
Mar 24, 2004 |
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12216683 |
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Current U.S.
Class: |
29/898.06 |
Current CPC
Class: |
F16C 33/64 20130101;
F16C 2360/18 20130101; Y10T 29/49679 20150115; F01L 1/181 20130101;
F16C 33/62 20130101; F01L 2301/00 20200501; F16C 2240/18 20130101;
F01L 2303/00 20200501; F01L 2305/00 20200501 |
Class at
Publication: |
29/898.06 |
International
Class: |
B21D 53/10 20060101
B21D053/10 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 26, 2003 |
JP |
2003-085309 |
Claims
1.-6. (canceled)
7. A method of manufacturing a rolling bearing, said method
comprising: forming the inner and outer rings from a chrome bearing
steel, a carburized steel or a carbon steel for mechanical
structures; carbonitriding at least one of an inner ring and an
outer ring, followed by oil quenching and then induction hardening
the at least one of the inner and outer rings, such that a surface
layer of the at least one of the inner and outer rings has a
compressive strength of not less than 200 MPa and a tempering
hardness at 500.degree. C. of not less than Hv 550; and mounting a
plurality of rolling elements between the inner and outer
rings.
8. The method of manufacturing a rolling bearing in accordance with
claim 7 further comprising after said carbonitriding and before
said induction hardening, tempering the at least one of the inner
and outer rings.
9. The method of manufacturing a rolling bearing in accordance with
claim 7, wherein the tempering is carried out at a temperature of
600.degree. C.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to rolling bearings used for
applications where lubricating conditions are severe or bending
stress acts, such as bearings for rocker arms of automobiles.
[0002] In recent years, in machines such as automobiles, for lower
fuel consumption or freedom from maintenance, bearings are often
operated under harsher lubricating conditions than before, e.g.
with lubricating oil having low viscosity or used for a long period
of time without renewing lubricating oil. In bearings used for
applications where lubricating conditions are severe, a suitable
oil film sometimes does not form between bearing rings and rolling
elements, so that due to surface heat buildup or metallic contact,
required life is not met.
[0003] In particular, in full complement roller bearings, since the
amount of lubricating oil supplied into between the bearing rings
and the rollers tends to be insufficient, peeling from the surface
of a bearing ring tends to occur. For example, in bearings for
automotive rocker arms as shown in FIG. 1, which shows an
embodiment of the present invention, or in needle bearings, the
working life often shortens markedly because peeling develops in
the inner ring (or shaft integrally formed with the inner
ring).
[0004] On the other hand, because, rolling bearings are anticipated
to be used at higher loads as driving performance of machinery
improves, it is expected that problems will develop regarding
cracking of bearings. In particular, because bearings that receive
loads, not enclosed in housings, such as bearings for rocker arms,
or bearings enclosed in a thin housing, are exposed to bending
stress repeatedly during use, the possibility is high that cracks
due to fatigue may pose a problem.
[0005] To solve problems that arise due to increasingly harsher use
conditions of rolling bearings as described above, if attempts are
made to cope with such problems by changing the bearing material,
the material cost would increase markedly. Thus, measures that will
incur little cost increase are required by improving heat treatment
procedures.
[0006] One known measure from the heat treatment aspect is
carbonitriding treatment. Carbonitriding is a technique in which
nitrogen and carbon are infiltrated by diffusion through the
surface of a steel material. It imparts a resistance to material
quality change (resistance to micro-structural change and hardness
softening) to the surface layer, thereby prolonging the life of the
material. Its application has expanded to inner and outer rings of
bearings made of bearing steel, carburized steel and carbon steel
for machine structures, which are known as ordinary materials for
bearing.
[0007] But since carbonitriding is a technique of diffusing through
the material surface, a long time is taken to obtain a required
carbo-nitrided depth. Thus, carbo-nitrided materials still have
problems that the structure of the surface layer tends to be rough,
or the crystal grains of the entire material tend to be large, thus
lowering the fatigue strength.
[0008] Further, regarding bearings for rocker arms, JP patent
publication 2002-194438 discloses a method of improving the wear
resistance by changing carbon concentration distributions on the
outer peripheral surface and the rolling surfaces to prevent wear
of the outer ring due to contact with the mating cam. It also
discloses a method of controlling the carbon concentration and
hardness of the surface layer of a shaft formed integral with the
inner ring (shaft subjected to induction hardening after
carburizing or carbonitriding) and the hardness of its interior.
But there have been no techniques which have addressed the rolling
life or resistance to cracking.
[0009] An object of this invention is to provide inexpensive
rolling bearings which can be used stably for a long time even for
applications where the lubricating conditions are severe or ones
where bending stresses act.
SUMMARY OF THE INVENTION
[0010] According to this invention, there is provided a rolling
bearing comprising an inner ring and an outer ring made of a
bearing steel, a carburized steel or a carbon steel for mechanical
structures, and a plurality of rolling elements mounted between the
inner ring and the outer ring. A compressive stress of not less
than 200 MPa is imparted to the surface layer of at least one of
the inner ring and the outer ring by heat treatment including
carbonitriding and induction hardening.
[0011] That is, by subjecting at least the inner ring or the outer
ring to induction hardening after carbonitriding, and imparting
resistance to material quality change and a high compressive stress
of not less than 200 MPa to the surface, it is possible to improve
resistance to damage to the surface while keeping the material cost
unchanged compared to the prior art bearing. Thus, extended rolling
contact fatigue life and improved fatigue strength are achieved
simultaneously.
[0012] The bearing ring to be subjected to the heat treatment
preferably has a tempering hardness at 500.degree. C. of not less
than Hv 550 in the surface layer thereof from the viewpoint of the
rolling fatigue life, and has a prior austenite grain diameter of
not less than Gc 10 in the surface layer in view of the fatigue
strength.
[0013] Further, if high-temperature tempering is carried out
between carbonitriding and induction hardening, the entire bearing
rings will have low hardness when tempered, so that it is possible
to further increase the compressive stress in the surface layer
which is imparted by the subsequent induction hardening. Thus, it
is possible to further improve the rolling contact fatigue life and
fatigue strength.
[0014] This invention can be effectively applied to rolling
bearings in which the rolling elements are rollers that are
arranged in a full complement arrangement.
[0015] This invention has a substantial effect for bearing
applications for a rocker arm of an automobile, which are used
under severe lubricating conditions and in which bending stresses
act repeatedly during use.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Other features and objects of the present invention will
become apparent from the following description made with reference
to the accompanying drawings, in which:
[0017] FIG. 1 is a front sectional view showing how the rolling
bearing of an embodiment is mounted in a rocker arm.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0018] With reference to FIG. 1, the embodiment of this invention
will be described. FIG. 1 shows the rolling bearing 1 of the
embodiment mounted in a rocker arm 4 disposed between an engine
valve 2 of an automobile and a cam 3 for opening and closing the
valve 2. The rolling bearing 1 comprises an inner ring 5 fitted on
a fixed shaft 4a of the rocker arm 4, an outer ring 6 facing the
cam 3, and a plurality of rollers 7 arranged between the bearing
rings 5 and 6 in a full state. The rocker arm 4 has its central
portion mounted on a pivotable rocker shaft 8, and has one end
thereof coupled to the engine valve 2 so as to be biased downwardly
in the figure by a valve spring 9, thereby pressing the outer
periphery of the outer ring 6 of the bearing 1, which is mounted at
the other end, against the cam 3. Thus, as the cam 3 rotates about
its axis together with its camshaft 10, the rocker arm 4 will pivot
through the bearing 1, so that the valve 2 moves up and down. That
is, valve 2 closes and opens.
[0019] The inner ring 5, outer ring 6 and rollers 7 of the rolling
bearing 1 are all made of high-carbon chrome bearing steel. During
heat treatment, the rollers 7 are subjected to ordinary
carbonitriding only, while the inner and outer rings 5 and 6 are
subjected to carbonitriding, tempering at high temperature, and
then induction hardening. By this heat treatment, a compressive
stress not less than 200 MPa is imparted to the surface layers of
the bearing rings 5 and 6, and the tempering hardness of the
surface layer at 500.degree. C. will be not less than Hv 550. As a
result, the rolling contact fatigue life and the
tension-compression fatigue strength markedly improve. By
controlling the prior austenite grain diameter in the surface layer
as fine, as not less than Gc 10, by properly setting the
temperature during induction hardening, the outer ring 6 has higher
tension-compression fatigue strength than the inner ring 5.
[0020] Thus, in spite of the fact that this bearing 1 is a full
complement type bearing, is used under lubricating conditions that
can easily deteriorate and is exposed to bending stress and contact
stress repeatedly, no premature surface peeling or cracks will
develop in the bearing rings 5 and 6. Thus, the bearing 1 can be
stably used for a long period, and it can withstand increased
harshness in lubricating and loading conditions to improve the
performance of automobiles.
[0021] In the above embodiment, for both the inner and outer rings,
a compressive stress not less than 200 MPa is imparted to their
surface layers by subjecting both of them to heat treatment in
which high-temperature tempering and induction heating are carried
out after carbonitriding. But only one of them may be subjected to
such heat treatment. According to applications and use conditions
of the bearing, high-temperature tempering among three heat
treatment steps may be omitted.
[0022] The material of the bearing rings is not limited to bearing
steel, but carburized steel and carbon steel for mechanical
structures may also be used.
[0023] Next, experiments conducted to confirm the performance of
the bearing according to the present invention will be
described.
(1) Rolling Contact Fatigue Life Comparison Experiment
[0024] In this experiment, the rolling fatigue life for full
complement needle roller bearings in which the inner ring is
integral with the shaft, was examined. For the experiment, seven
bearings having inner rings, different in material and heat
treatment, were prepared including comparative examples. Each
bearing had an inner ring (shaft) of 14.64 mm outer
diameter.times.17.3 mm wide, an outer ring of 18.64 mm inner
diameter.times.24 mm outer diameter.times.6.9 mm wide, and 26
rollers of 2 mm outer diameter.times.6.8 mm long. The materials and
heat treatments of the inner rings (shafts) are shown in Table 1
together with the results of the experiment. The outer rings and
rollers were made of high-carbon chrome bearing steel and were
subjected to normal carbonitriding.
[0025] Among the heat treatments for the inner rings (shafts) shown
in Table 1, the carbonitriding for bearing steels of examples (No.
1, No. 3) was carried out in an atmosphere having a carbon
potential of 1.1-1.2% and an ammonia concentration of 5%[[,]] at a
temperature of 850.degree. C. for 90 minutes to form a
carbonitrided layer having a depth of about 0.3 mm, and then oil
quenching was carried out. The carbon concentration in the surface
layer after carbonitriding was 1.05-1.1% and the nitrogen
concentration was about 0.25%.
[0026] On the other hand, for carbonitriding of carburized steel
(No. 2, No. 4), in the heating time of 400 minutes at 920.degree.
C., for the first 250 minutes, only carburization was carried out
in an atmosphere having the carbon potential of 1.0%, and for the
last 150 minutes, carbonitriding was carried out in an atmosphere
having the carbon potential of 0.8% and the ammonia concentration
of 7%. Thus forming a carburized and hardened layer having a depth
of about 1.5 mm and a carbonitrided layer having a depth of about
0.3 mm, and then oil quenching was carried out. After the
treatment, the carbon concentration in the surface layer was about
1.0% and the nitrogen concentration was about 0.35%.
[0027] In the induction hardening (No. 1-4) after the
carbonitriding for each example, a hardened layer having a depth of
about 1 mm was formed. When high-temperature tempering was carried
out between carbonitriding and induction hardening (No. 3 and No.
4), the tempering temperature was 600.degree. C.
[0028] In contrast, among comparative examples, for standard heat
treatment for bearing steel (No. 5), after heating at 850.degree.
C. for 45 minutes in an atmosphere having the carbon potential of
0.9%, oil quenching was carried out. Conditions of carbonitriding
for bearing steel (No. 6) were the same as those of examples (No. 2
and No. 4). In the standard carburizing for carburized steel (No.
7), in the heating time of 400 minutes at 920.degree. C., for the
first 250 minutes, carburizing in an atmosphere having the carbon
potential of 1.0%, and for the last 150 minutes, diffusing in an
atmosphere having the carbon potential of 0.8% was carried out, and
then oil quenching was carried out.
[0029] These bearings were set in an outer ring rotating type life
tester and the time elapsed until peeling occurred in the surface
layer of each inner ring (shaft) was measured under the following
conditions:
(Test Conditions)
[0030] Load: 2.58 kN (30% of basic dynamic rated load)
[0031] Outer ring rotating speed: 7000 rpm
[0032] Lubricating oil: engine oil (10W-30)
[0033] Lubricating oil temperature: 100.degree. C.
[0034] The time taken until surface layer peeling occurred in the
inner ring (shaft) of each bearing was converted to a ratio with
reference to the standard heat-treated article of bearing steel
(No. 5), and was shown in Table 1 as the rolling fatigue life
ratio.
[0035] As is apparent from Table 1, in each example, in the surface
layer of the inner ring (shaft), a compressive stress of not less
than 200 MPa and a 500.degree. C. tempering hardness of not less
than Hv 550 were ensured. Thus, the rolling contact fatigue life
was 3.5 times that of the standard article (No. 5) or over.
Further, among the examples, those in which high-temperature
tempering was carried out between carbonitriding and induction
hardening (No. 3 and No. 4) were high in compressive stress and
long in life compared with those having the same steel but no
high-temperature tempering (No. 1 and No. 2).
(2) Tension-Compression Fatigue Life Comparison Experiment
[0036] In this experiment, a total of seven kinds (four each) of
ring test specimens that differed in material and heat treatment
were prepared. Dimensions of each test piece were 45 mm inner
diameter.times.60 mm outer diameter.times.15 mm wide. The material
and heat treatment are shown in Table 2 together with the results
of the experiment. The contents of the heat treatment for each of
the examples and comparative examples are the same as those
described about Table 1.
[0037] Each of these test pieces was set on a ring rotating type
fatigue life tester, and loads were applied repeatedly while
rotating under the following test conditions. The number of
loadings was repeated until cracks developed and were measured.
(Test Conditions)
[0038] Load: 9.8 kN
[0039] Load speed: 8000 cpm
[0040] Lubricating oil: turbine oil (VG68)
[0041] The number of loadings repeated until each test piece
cracked was converted to a ratio with reference to the standard
heat-treated article of bearing steel (No. 5), and shown in Table 2
as a tension-compression fatigue life ratio. The numerical values
in Table 2 are average values for four test pieces of each
kind.
[0042] As is apparent from Table 2, each example showed the
compressive stress in the surface layer of not less than 200 MPa,
the 500.degree. C. tempering hardness of not less than Hv 550, and
the prior austenite grain diameter of not less than Gc 10. This
shows that the tension-compression fatigue life markedly improved
compared to the standard article (No. 5).
[0043] As described above, according to this invention, by
subjecting at least one of the inner ring and the outer ring of the
rolling bearing to induction hardening after carbonitriding,
resistance to material quality change and a high compressive stress
of not less than 200 MPa are imparted. Thus, it is possible to
markedly improve the rolling fatigue life and the crack fatigue
strength with no increase in the cost of the bearing material.
[0044] Thus, the rolling bearing to which this invention is applied
can be used stably for a long time even in applications where
lubricating conditions are severe or in applications where bending
stresses act. Thus it is possible to cope with severer lubricating
and load conditions expected in the future.
TABLE-US-00001 TABLE 1 Tempering Rolling Compressive hardness
contact Hardness stress at 500.degree. C. fatigue No. Material Heat
treatment (Hv) (Mpa) (Hv) life ratio Ex. 1 Bearing Carbonitriding +
770 230 600 3.5 steel induction hardening 2 Carburized
Carbonitriding + 750 450 620 3.8 steel induction hardening 3
Bearing Carbonitriding + high 850 370 610 4.6 steel temperature
tempering + induction hardening 4 Carburized Carbonitriding + high
830 520 630 5.0 steel temperature tempering + induction hardening
Comp. Ex. 5 Bearing Standard heat 740 0 470 1.0 steel treatment 6
Bearing Carbonitriding 780 120 580 1.9 steel 7 Carburized Standard
carburizing 720 290 480 1.3 steel
TABLE-US-00002 TABLE 2 Hardness Tempering Prior Tension- at surface
Compressive hardness austenite compression Heat layer stress at
500.degree. C. grain fatigue life No. Material treatment (Hv) (Mpa)
(Hv) size (Gc) ratio Ex. 1 Bearing Carbonitriding + 780 200 610 11
3.7 steel induction hardening 2 Carburized Carbonitriding + 770 420
600 10 4.9 steel induction hardening 3 Bearing Carbonitriding + 840
350 580 11 4.0 steel high temperature tempering + induction
hardening 4 Carburized Carbonitriding + 820 590 610 10 5.3 steel
high temperature tempering + induction hardening Comp. Ex. 5
Bearing Standard heat 740 50 470 9~10 1.0 steel treatment 6 Bearing
Carbonitriding 760 120 590 8~9 2.2 steel 7 Carburized Standard 720
340 470 4~8 2.8 steel carburizing
* * * * *