U.S. patent application number 11/355046 was filed with the patent office on 2006-06-22 for method of forming bearing ring resistant to season cracking.
This patent application is currently assigned to Koyo Seiko Co., Ltd.. Invention is credited to Shinichirou Kashiwagi, Kazutoshi Toda, Daisaku Tomita.
Application Number | 20060130333 11/355046 |
Document ID | / |
Family ID | 26623894 |
Filed Date | 2006-06-22 |
United States Patent
Application |
20060130333 |
Kind Code |
A1 |
Toda; Kazutoshi ; et
al. |
June 22, 2006 |
Method of forming bearing ring resistant to season cracking
Abstract
A method sets a retained austenite amount of an edge of a
bearing ring of a bearing to between 3% or more and 20% or less.
The method includes exposing the bearing ring to an environment
having a temperature in the range of 850.degree. C. to 930.degree.
C., an ammonia concentration, and a carbon concentration CP in a
range of 0.9 to 1.1%, for a period of not more than 5 hours. The
ammonia concentration of the environment is in a range of 4 to 7
CFH. The temperature of the environment is then reduced to a
temperature in a range of 800.degree. C. to 830.degree. C. for a
second period of at least 30 minutes. The bearing ring is next
quenched in oil and then heat treated in an environment set to a
temperature of 160.degree. C. to 200.degree. C. and then air
cooled.
Inventors: |
Toda; Kazutoshi;
(Tomdabaya-shi, JP) ; Kashiwagi; Shinichirou;
(Yao-shi, JP) ; Tomita; Daisaku; (Osaka-shi,
JP) |
Correspondence
Address: |
JORDAN AND HAMBURG LLP
122 EAST 42ND STREET
SUITE 4000
NEW YORK
NY
10168
US
|
Assignee: |
Koyo Seiko Co., Ltd.
Osaka-shi
JP
|
Family ID: |
26623894 |
Appl. No.: |
11/355046 |
Filed: |
February 15, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10267501 |
Oct 9, 2002 |
7001078 |
|
|
11355046 |
Feb 15, 2006 |
|
|
|
Current U.S.
Class: |
29/898.13 ;
148/906 |
Current CPC
Class: |
F16C 19/186 20130101;
B60B 27/0094 20130101; F16C 2326/02 20130101; Y10T 29/49707
20150115; F16C 43/04 20130101; B60B 27/0084 20130101; B60B 27/00
20130101; B60B 27/0005 20130101 |
Class at
Publication: |
029/898.13 ;
148/906 |
International
Class: |
B21D 53/10 20060101
B21D053/10 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 15, 2001 |
JP |
P2001-316536 |
Oct 8, 2002 |
JP |
P2002-294785 |
Claims
1. A method of producing a bearing ring with a retained austenite
concentration of 3% to 20% in an edge surface, comprising: forming
a bearing ring of carbon steel; and exposing the bearing ring to an
environment having a temperature in the range of 850.degree. C. to
930.degree. C. an ammonia concentration, and a carbon concentration
CP in a range of 0.9 to less than 1.1%, for a period of not more
than 5 hours.
2. The method of claim 1 further comprising setting the ammonia
concentration of the environment in a range of 4 to 7 CFH.
3. The method of claim 2 further comprising reducing the
temperature of the environment to a temperature in a range of
800.degree. C. to 830.degree. C. for a second period of at least 30
minutes after the period of not more than 5 hours is expired.
4. The method of claim 3 further comprising removing the bearing
ring from the environment at the end of the second period and
quenching the bearing ring in oil at a temperature in a range of
60.degree. C. to 100.degree. C.
5. The method of claim 4 further comprising tempering the bearing
ring after the quenching in an environment set to a temperature of
160.degree. C. to 200.degree. C.
6. The method of claim 5 further comprising air cooling the bearing
ring after the quenching.
7. The method of claim 6 wherein the carbon steel is high carbon
chromium steel corresponding to Japanese Industrial Standards
SUJ2.
8. The method of claim 6 wherein the carbon steel is machine
structural use steel corresponding to Japanese Industrial Standards
S55C.
9. The method of claim 1 further comprising setting an ammonia
concentration of the environment is in the range of 4 to less than
7 CFH.
10. The method of claim 9 further comprising reducing the
temperature of the environment to a temperature in a range of
800.degree. C. to 830.degree. C. for a second period of at least 30
minutes after the period of not more than 5 hours is expired.
11. The method of claim 10 further comprising removing the bearing
ring from the environment at the end of the second period and
quenching the bearing ring in oil at a temperature in a range of
60.degree. C. to 100.degree. C.
12. The method of claim 11 further comprising tempering the bearing
ring after the quenching in an environment set to a temperature in
a range of 160.degree. C. to 200.degree. C.
13. The method of claim 12 further comprising air cooling the
bearing ring after the quenching.
14. The method of claim 13 wherein the carbon steel is high carbon
chromium steel corresponding to Japanese Industrial Standards
SUJ2.
15. The method of claim 13 wherein the carbon steel is machine
structural use steel corresponding to Japanese Industrial Standards
S55C.
16. A method of producing a bearing ring with a retained austenite
concentration of 3% to 20% in an edge surface, comprising: forming
a bearing ring of carbon steel; and exposing the bearing ring to an
environment having a temperature in the range of 850.degree. C. to
930.degree. C., an ammonia concentration of the environment in the
range of 4 to less than 7 CFH, and a carbon concentration for a
period of not more than 5 hours.
17. The method of claim 16 further comprising setting the carbon
concentration CP in a range of 0.9 to less than 1.1%,
18. The method of claim 17 further comprising reducing the
temperature of the environment to a temperature in a range of
800.degree. C. to 830.degree. C. for a second period of at least 30
minutes after the period of not more than 5 hours is expired.
19. The method of claim 18 further comprising removing the bearing
ring from the environment at the end of the second period and
quenching the bearing ring in oil at a temperature in a range of
60.degree. C. to 100.degree. C.
20. The method of claim 19 further comprising tempering the bearing
ring after the quenching in an environment set to a temperature in
a range of 160.degree. C. to 200.degree. C.
21. The method of claim 20 further comprising air cooling the
bearing ring after the quenching.
22. The method of claim 21 wherein the carbon steel is high carbon
chromium steel corresponding to Japanese Industrial Standards
SUJ2.
23. The method of claim 21 wherein the carbon steel is machine
structural use steel corresponding to Japanese Industrial Standards
S55C.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to a rolling bearing unit having a
rolling bearing attached to the outer periphery of a shaft body
such as a vehicle-use hub unit.
[0002] The vehicle-use hub unit, in general, has a hub wheel and a
double row rolling bearing. The rolling bearing is mounted on the
outer periphery of the shaft body of a hub wheel so as not to slip
off the shaft body.
[0003] The shaft body of the hub wheel has on its free end side a
cylindrical portion used for preventing the bearing from slipping
off the shaft body. The cylindrical portion is bent and deformed
outward in a radial direction using a caulking jig to be caulked
onto an axial outer end face of an inner ring of the bearing.
Thereby the cylindrical portion forms a caulking portion. The
caulking portion prevents the bearing from slipping off the hub
wheel. At the same time, a pre-load is applied from the caulking
portion to the inner ring of the bearing. Carbon steel is used as
the material of the inner ring of the bearing.
[0004] In order to enhance the life of the bearing, the inner ring
is heat-treated to increase the strength of a raceway surface
thereof.
SUMMARY OF THE INVENTION
[0005] When the rolling bearing unit is left for the purpose of
storage or the like with the inner ring caulked onto the shaft body
of the hub wheel, a phenomenon in which the inner ring is cracked
and broken (hereinafter, referred to as season cracking) may be
generated.
[0006] Accordingly, it is a main object of the invention to provide
a rolling bearing unit preventing the generation of season cracking
in the edge of an inner ring of a rolling bearing.
[0007] Other objects, features and advantages of the invention will
be apparent from the following description.
[0008] In one aspect of the invention, this is accomplished by
providing a rolling bearing unit comprising a shaft body, an inner
ring and a fastening member. The inner ring is mounted on an outer
peripheral surface of the shaft body. The inner ring is formed of
carbon steel. The surface of the inner ring is heat treated and has
a surface layer in which retained austenite exists. The fastening
member is fastened to an axial outer end face of the inner ring for
applying a tensile stress in an outer peripheral direction of the
inner ring. The inner ring has an edge on an outside in a radial
direction thereof. The surface layer is removed from the edge of
the inner ring.
[0009] According to the rolling bearing unit of the invention, the
surface layer, which causes season cracking, is removed from the
edge of the inner ring. Therefore, the influence of a volume
increase in the transformation of retained austenite into
martensite, is reduced. As a result, the generation of the season
cracking on the edge of the inner ring due to an increase in a
tensile strength is prevented.
[0010] Preferably, the surface layer is removed from edge of the
inner ring by polishing or turning. More preferably, an amount of
the retained austenite existing in the edge of the inner ring is
set between 3% or more and 20% or less by removing the surface
layer.
BRIEF DESCRIPTION OF THE DRAWINGS FIGURE.
[0011] These and other objects as well as advantages of the
invention will become clear by the following description of
preferred embodiments of the invention with reference to the
accompanying drawings, wherein:
[0012] FIG. 1 is a sectional view showing a rolling bearing unit
comprising a hub unit for a driving wheel according to an
embodiment of the invention;
[0013] FIG. 2 is a sectional view showing an inner ring of the
rolling bearing unit according to the embodiment of the
invention;
[0014] FIG. 3 is a sectional view showing an inner ring of a
rolling bearing unit;
[0015] FIG. 4 is a view illustrating a surface layer generated on
the surface of the inner ring of the rolling bearing unit; and
[0016] FIG. 5 is a sectional view showing an inner ring of a
rolling bearing unit according to another embodiment of the
invention.
[0017] In all these figures, like components are indicated by the
same numerals.
DETAILED DESCRIPTION
[0018] Referring now to the drawings, FIG. 1 illustrates a rolling
bearing unit comprising a vehicle-use hub unit for a driving wheel
according to a preferred embodiment of the invention. A hub wheel 1
on which a wheel is mounted has a shaft body 2 as a rotatable
shaft. A double row rolling bearing 3, which is specifically an
angular ball bearing as one example of rolling bearings, is pressed
and fitted onto the outer peripheral surface of the shaft body 2
from the vehicle inner side.
[0019] The rolling bearing 3 has an inner ring 32. The inner ring
32 is attached to an annular groove 2a in the outer peripheral
surface of the vehicle inner side of the shaft body 2. The inner
ring 32 has an inner ring raceway 32d, while the shaft body 2 has
an inner ring raceway 2b on the outer peripheral surface in the
middle in an axial direction thereof. The inner ring raceway 2b
constitutes a pair of inner ring raceways in the axial direction
together with the inner ring raceway 32d of the inner ring 32.
Moreover, the shaft body 2 has an outward radial flange 2c on the
outer peripheral surface of the vehicle outer side thereof.
[0020] A tire wheel and a brake disk, not shown, are attached to a
side of the vehicle outer side of the flange 2c. Furthermore, the
rolling bearing 3 has an outer ring 33. The outer ring 33 is fixed
to a vehicle body through a steering knuckle, not shown, and is
provided with outer ring raceways 38 and 39 in two rows in the
axial direction on the inner peripheral surface thereof.
[0021] The outer ring 33 is situated coaxially on the outside in
the radial direction with respect to the inner ring 32. The rolling
bearing 3 includes; balls 34 and 35 in two rows in the axial
direction interposed between the inner ring raceways 32d and 2b and
outer ring raceways 38 and 39; cages 36 and 37 in two rows in the
axial direction which hold the balls 34 and 35; and seal rings 7
and 8 for sealing both ends in the axial direction of the rolling
bearing 3.
[0022] A shaft end of the vehicle inner side of the shaft body 2 is
bent and deformed outward in a radial direction and is caulked onto
an outer end face 32a on the vehicle inner side of the inner ring
32, thereby forming a caulking portion 4. The caulking portion 4 as
a fastening member applies an appropriate pre-load to the balls 34
and 35 and prevents the rolling bearing 3 from slipping from the
shaft body 2.
[0023] With reference to FIG. 2, carbon steel such as high carbon
chromium steel (Japanese Industrial Standards SUJ2, bearing steel)
or carbon steels for machine structural use (Japanese Industrial
Standards S55C) is used for the material of the inner ring 32.
[0024] In order to increase the surface strength of the raceway 32d
of the inner ring 32 to enhance the life of the bearing, the inner
ring 32 is heat treated in a process for manufacturing the inner
ring 32. The heat treatment is carried out in a slight carburizing
atmosphere in order to prevent decarburization. Or the heat
treatment is carried out in a carbonitriding atmosphere in which a
small amount of NH.sub.3 (ammonia gas) is added to an atmospheric
gas.
[0025] As a result of an examination of season cracking, the
following matters are considered. With reference to FIGS. 4 and 5,
description will be given to the phenomenon in which the season
cracking is generated. Numeral 32 denotes an inner ring and numeral
2 denotes a shaft body of a hub wheel. The end of the shaft body 2
is caulked onto an outer end face 32a on the end in the axial
direction of the inner ring 32, thereby forming a caulking portion
4. A tensile stress in a circumferential direction is applied from
the caulking portion 4 to a portion of the inner ring 32 in the
vicinity of the caulking portion 4. Since the outer end face 32a of
the inner ring 32 is held by the caulking portion 4, the tensile
stress thereof converges on an outer diameter side edge 32b of the
inner ring 32.
[0026] After the heat treatment of the inner ring, a surface layer
L having an amount of retained austenite larger than other portions
of the inner ring 32 is formed on the surface of the inner ring 32
as shown in FIG. 4.
[0027] An outer diameter side edge 32b of the outer end face 32a of
the inner ring 32 is constituted by the outer end face 32a of the
inner ring 32 and an outer peripheral surface 32c in a shoulder
portion of the inner ring. In the surface layer L of the outer
diameter side edge 32b, the retained austenite amount becomes
larger than that of the other surface layer due to heating and
carbon diffusion from the outer end face 32a and the outer
peripheral surface 32c in the inner ring 32 during the heat
treatment of the inner ring 32.
[0028] After the heat treatment, the retained austenite in the
surface layer L is transformed into martensite, so that the volume
of the surface layer L is increased.
[0029] Together with the tensile stress applied in the outer
diameter side edge 32b of the inner ring 32, the increase in the
volume of the surface layer L by the martensite results in an
increase in a tensile stress. As a result the season cracking may
be caused over the diameter side edge 32b of the inner ring 32.
[0030] As described above, it is considered that the retained
austenite amount in the outer diameter side edge 32b of the inner
ring 32 causes the generation of the season cracking
phenomenon.
[0031] Thus, in order to prevent the season cracking, this
invention proposes a rolling bearing unit capable of preventing the
season cracking phenomenon by decreasing the retained austenite
amount in the outer diameter side edge 32b of the inner ring
32.
[0032] Hereinafter, description will be given of a method of
decreasing the retained austenite amount in the outer diameter side
edge 32b of the inner ring 32.
[0033] According to a first method, in view of a point that the
retained austenite amount in the surface layer L gradually
decreases from the surface of the inner ring to the inside, the
surface layer L in the outer diameter side edge 32b of the inner
ring 32 is removed until a predetermined depth by polishing or
turning, thereby decreasing the retained austenite amount in the
outer diameter side edge 32b.
[0034] Accordingly, in the outer diameter side edge 32b, the
surface layer L is removed such that the retained austenite amount
becomes between 3% or more and 20% or less in the range a in which
at least tensile stress converges, thereby reducing the percentage
of generation of the season cracking in the inner ring 32.
[0035] A concrete range a is determined as follows.
[0036] The range a in a radius r, in which the surface layer is
removed from the edge 32b, satisfies the following expression:
(.phi.B-.phi.A)/2.gtoreq.r.gtoreq.D
[0037] wherein .phi.A represents an outer diameter of the caulking
portion 4, .phi.B represents an outer diameter of the outer
peripheral surface 32c in the shoulder portion of the inner ring
32, r represents a radius from the edge 32b of the inner ring 32 in
the vicinity of the caulking portion 4, and D (mm) represents a
numeric value which is almost equal to the thickness of the surface
layer in which an amount of retained austenite which generates on
the surface of the inner ring 32 is much more than that of other
portions of the inner ring (for example, 20% or more). The
thickness of the surface layer is preferably 0.5 mm.
[0038] By removing the surface layer present within the range
satisfying the expression by the radius r from the edge 32b of the
inner ring 32, thus, retained austenite amount on the edge 32b is
between 3% or more and 20% or less, preferably between 5% or more
and 15% or less, more preferably between 5% or more and 10% or
less.
[0039] When the retained austenite amount was equal to or less than
20%, the season cracking was not generated after the passage of the
days. When the retained austenite amount was 23%, the season
cracking was generated on a tenth day. When the retained austenite
amount was 30%, the season cracking was generated on a seventh
day.
[0040] According to the rolling bearing unit thus constituted, the
retained austenite amount is set between 3% or more and 20% or less
within the range of the radius r satisfying the expression from the
edge 32b of the inner ring 32 so that the influence of a volume
dilatation can be reduced in the transformation of the retained
austenite into martensite. As a result, it is possible to prevent
the season cracking from being generated on the edge 32b of the
inner ring 32 due to an increase in a tensile stress caused by the
caulking portion 4. Consequently, the reliability of the bearing
can be enhanced.
[0041] The invention is not restricted to the embodiment in which
the edge 32b of the inner ring 32 is subjected to the polishing or
the turning to set the retained austenite amount in the edge 32b to
be between 3% or more and 20% or less but the retained austenite
amount in the edge 32b may be set between 3% or more and 20% or
less by the optimization of heat-treating conditions in the process
for manufacturing the inner ring 32.
[0042] Conventional heat-treating conditions are as follows. More
specifically, the inner ring 32 is put in a heat treatment furnace.
At this time, the internal temperature of the heat treating furnace
is set to 850.degree. C. to 930.degree. C. The heat treating time
for the inner ring 32 is set to five hours. A carbon concentration
CP (CarbonPotential) in the heat treatment furnace is set to 1.1 to
1.4%, and an ammonia concentration is set to 7 to 11 CFH. The CFH
represents ft.sup.3/H (cubic foot per hour). Then, the internal
temperature of the heat treatment furnace is dropped to 800.degree.
C. to 830.degree. C. and the temperature dropping state is kept for
30 minutes. After that the inner ring 32 is taken out of the heat
treatment furnace and is then thrown into the oil kept at
temperatures of 60.degree. C. to 100.degree. C. to be quenched (oil
cooling). After the oil cooling, the inner ring 32 is tempered
again for two hours in a heat treatment furnace having an internal
temperature of 160.degree. C. to 200.degree. C. Subsequently, the
inner ring 32 is taken out of the heat treating furnace and is air
cooled.
[0043] In the invention, the conventional heat-treating conditions
for the inner ring 32 are improved. More specifically, the inner
ring 32 is put in the heat treatment furnace. At this time, the
internal temperature of the heat treating furnace is set to
850.degree. C. to 930.degree. C. The heat treating time of the
inner ring 32 is set to five hours. A carbon concentration CP
(CarbonPotential) in the heat treating furnace is set to 0.9 to
1.1%, and an ammonia concentration is set to 4 to 7 CFH. Then, the
internal temperature of the heat treatment furnace is dropped to
800.degree. C. to 830.degree. C. The temperature dropping state is
held for 30 minutes. Thereafter the inner ring 32 is then taken out
of the heat treatment furnace and is thereafter put into oil kept
at temperatures 60.degree. C. to 100.degree. C. to be quenched (oil
cooling). After the oil cooling, the inner ring 32 is tempered
again for two hours in a heat treatment furnace having an internal
temperature of 160.degree. C. to 200.degree. C. Subsequently, the
inner ring 32 is taken out of the heat treating furnace and is air
cooled.
[0044] In the invention, thus, the carbon concentration and the
Ammonia concentration in the heat-treating conditions are improved
so that the retained austenite amount in the edge 32b can be set
between 3% or more and 20% or less without polishing or turning the
edge 32b of the inner ring 32.
[0045] The invention is not restricted to the inner ring 32 having
the structure described in the embodiment. For example, as shown in
FIG. 5, a step 32e having a diameter reduced is provided on the
vehicle inner side end of the shoulder portion of the inner ring
32, and the step 32e is set to be the installation space of a
rotating speed sensor and the vehicle inner side end of the step
32e is set to be the edge 32b. The retained austenite amount within
the range of the radius r satisfying the expression may be set
between 3% or more and 20% or less in the edge 32b.
[0046] In the invention, a fastening member is not restricted to
the caulking portion 4 in the embodiment but may be a nut screwed
to the shaft body 2 and fastened to an outer end face in the axial
direction of the inner ring 32. In brief, the shaft body 2 includes
a fastening member which is fastened to the outer end face in the
axial direction of the inner ring 32 and applies a tensile stress
in the outer peripheral direction of the inner ring 32.
[0047] The invention is not restricted to the hub unit comprising a
combination of the hub wheel and the angular ball bearing in the
embodiment but the hub wheel or the like may be set to be the shaft
body and a rolling bearing such as an angular ball bearing may be
applied to various rolling bearing units provided on the outer
periphery of the shaft body.
* * * * *