U.S. patent application number 13/821636 was filed with the patent office on 2013-06-27 for gear.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is Keisuke Kadota, Masahiko Mitsubayashi, Hideyuki Sakaue, Hiroyoshi Tawa. Invention is credited to Keisuke Kadota, Masahiko Mitsubayashi, Hideyuki Sakaue, Hiroyoshi Tawa.
Application Number | 20130160899 13/821636 |
Document ID | / |
Family ID | 45810254 |
Filed Date | 2013-06-27 |
United States Patent
Application |
20130160899 |
Kind Code |
A1 |
Tawa; Hiroyoshi ; et
al. |
June 27, 2013 |
GEAR
Abstract
The problem of the present invention involves providing a gear
that has high tooth-root bending strength and for which there is no
chipping of the tips of the teeth. Accordingly, the surface of the
gear is carburized and the gear is strengthened by imparting
residual stress, with the residual stress in the region with a
surface depth of 5 .mu.m to 20 .mu.m being -1000 MPa or less, and
the residual stress in the region with a surface depth of 50 .mu.m
to 150 .mu.m being -1000 MPa or greater.
Inventors: |
Tawa; Hiroyoshi;
(Okazaki-shi, JP) ; Mitsubayashi; Masahiko;
(Nagoya-shi, JP) ; Sakaue; Hideyuki; (Aichi-gun,
JP) ; Kadota; Keisuke; (Toyota-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Tawa; Hiroyoshi
Mitsubayashi; Masahiko
Sakaue; Hideyuki
Kadota; Keisuke |
Okazaki-shi
Nagoya-shi
Aichi-gun
Toyota-shi |
|
JP
JP
JP
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Toyota-shi
JP
|
Family ID: |
45810254 |
Appl. No.: |
13/821636 |
Filed: |
September 9, 2010 |
PCT Filed: |
September 9, 2010 |
PCT NO: |
PCT/JP2010/065482 |
371 Date: |
March 8, 2013 |
Current U.S.
Class: |
148/316 |
Current CPC
Class: |
Y10T 29/49462 20150115;
F16H 55/17 20130101; F16H 55/06 20130101; B24C 1/10 20130101; B23P
9/00 20130101 |
Class at
Publication: |
148/316 |
International
Class: |
B23P 9/00 20060101
B23P009/00 |
Claims
1. A gear including a carburized surface and strengthened by
application of residual stress to the gear, wherein the residual
stress in a region at a depth of 5 .mu.m or more but 20 .mu.m or
less from the surface is -1400 MPa or less, so that when a tooth of
the gear is subjected to repeated stress, the repeated stress is
canceled out by compressive stress to prevent surface cracks in the
surface of a tooth root and improve a strengthening rate of the
gear, the residual stress in a region at a depth of 50 .mu.m or
more but 150 .mu.m or less from the surface is -1000 MPa or more to
prevent inner cracks and restrain tooth-tip chipping, and the
residual stress in a region at a depth of more than 150 .mu.m but
230 .mu.m or less is -800 MPa or less so that the residual stress
cancels out hertz stress caused by contact of the teeth of the
gears.
2. The gear according to claim 1, wherein the region at a depth of
50 .mu.m or more but 150 .mu.m or less from the surface is
subjected to a shot peening treatment using shot particles, and the
region at a depth of 5 .mu.m or more but 20 .mu.m or less from the
surface is subjected to a shot peening treatment using shot
particles with a smaller particle diameter and a larger hardness
than said shot particles.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a 371 national phase application of
PCT/JP2010/065482 filed on 9 Sep. 2010, the entire contents of
which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a gear strengthened by
applying residual stress to the gear having a carburized
surface.
BACKGROUND OF THE INVENTION
[0003] Patent Document 1 discloses, for the purpose of easily
improving the fatigue strength of mechanical parts or components, a
carburizing treatment process on a gear, a nitriding treatment
process on the gear, a first shot peening process for shot peening
the gear using shot particles with a shot particle diameter of 0.8
mm, and a second shot peening process for shot peening the gear
using shot particles with a particle diameter of 0.1 mm.
[0004] Patent Document 2 discloses, for the purpose of modifying
the surface of alloy steel for mechanical structure, a first shot
peening process for shot peening using shot particles with a shot
particle diameter of 0.6 mm, which is conducted after a vacuum
carburizing treatment and a heating and rapid cooling treatment for
ultrahigh-speed and short time, and a second shot peening process
for shot peening using shot particles with a shot particle diameter
of 0.08 mm.
[0005] In the techniques of Patent Documents 1 and 2, the residual
stress in a region located at a depth of 20 .mu.m or less from the
surface is adjusted to -1400 MPa to increase tooth-root bending
fatigue strength. That is, the stress (compressive stress) of -1400
MPa or less is left in the region at a depth of 20 .mu.m or less
from the surface. When repeated stress on teeth acts as a large
repeated stress on the surface(s) of a tooth or teeth root(s), this
repeated stress is canceled out by the residual stress. Thus,
fatigue strength is increased.
RELATED ART DOCUMENTS
Patent Documents
[0006] Patent Document 1: JP-A-2007-262506
[0007] Patent Document 2: JP-A-2002-030344
SUMMARY OF INVENTION
Problems to be Solved by the Invention
[0008] However, the conventional gears cause the following
problems. Firstly, gear strength depends on fatigue fracture of
tooth surface caused by contact surface pressure in addition to the
tooth-root bending fatigue strength. However, Patent Documents 1
and 2 fail to consider a problem with the contact pressure fatigue
strength.
[0009] Secondly, when the region at a depth of 20 .mu.m or less
from the surface is applied with higher residual stress in order to
enhance the tooth-root bending fatigue strength, this leads to a
problem with tooth-tip chipping.
[0010] To be concrete, FIG. 8 shows a CAE analysis result of a gear
when a shot peening treatment test (1) which is conventionally
performed to increase the tooth-root bending fatigue fracture. The
analysis result in FIG. 8 is obtained in a simulation analysis
performed in such a manner that a gear is carburized to a hardness
of HV750 and shot particles each having a particle diameter of 0.8
mm and a hardness of HV 800 are caused to impact or shot peen the
gear so that shot dents overlap one another by three-quarters of
the particle diameter. FIG. 12 shows a distribution diagram of
residual stress in a gear in the shot peening treatment test
(1).
[0011] As shown in FIG. 8, it is found that a region P1 located at
a depth T of 100 .mu.m from the surface P has a strain at maximum
level S1. Strain level is larger toward S1 and smaller toward S6.
It is further revealed that a region P2 located at a depth of about
50 .mu.m from the surface P and a region P3 located at a depth of
about 150 .mu.m from the surface P (from an upper side in the
figure) each has a strain at a level S3 or more.
[0012] As shown in FIG. 12, a solid line E indicates that the
residual stress in regions E2 to E3 located at a depth of 50 .mu.m
or more but 150 .mu.m or less from the surface P is larger than
that in other regions. Furthermore, a peak position E1 also exists
in the region at a depth of 50 .mu.m or more but 150 .mu.m or less
from the surface P.
[0013] In addition, a tooth-tip cross section of a gear immediately
before a tooth tip is chipped is checked by a micrograph shown in
FIG. 9. This shows, as traced in FIG. 10, that inner cracks Q occur
in a region located at a depth of 100 .mu.m from the surface U
corresponding to the region P1 having a S1-level strain. This also
reveals that an inner crack Q1 near a region located at a depth of
100 .mu.m from the surface U has a wide width. This reason why the
width of the inner crack Q1 near the region at a depth of 100 .mu.m
from the surface U is wide is conceivably in that the strain in the
regions P2 and P3 surrounding the region P1 shown in FIG. 8 has a
strain at level S3 or more and thus the inner crack is widened.
[0014] As shown in FIG. 13, in the shot peening treatment test (1),
cracks occur in five of eight gears.
[0015] Specifically, in the conventional shot peening treatment
test (1), it is found that excessively increasing the residual
stress to enhance the fatigue strength of tooth-root bending caused
strain in the region located at a depth of 50 .mu.m or more but 150
.mu.m or less from the surface, resulting in the occurrence of
inner cracks therein.
[0016] On the other hand, the inventors of the present invention
performed a shot peening treatment test (2) to prevent the inner
cracks by reducing the residual stress as indicated by a solid line
D in FIG. 11 in the region at a depth of 50 .mu.m or more but 150
.mu.m or less from the surface. The shot peening treatment test (2)
shown in FIG. 11 is conducted using shot particles with particle
diameter of 0.8 mm and hardness of HV580. In this shot peening
treatment test (2), the residual stress in regions D2 to D3 at a
depth of 50 .mu.m or more but 150 .mu.m or less from the surface
could be reduced as compared with that in the shot peening
treatment test (1) shown in FIG. 12. Further, a peak position D1
could also be made lower than the peak position E1.
[0017] Therefore, as shown in FIG. 13, inner cracks did not occur
in all eight gears in the shot peening treatment test (2).
[0018] In the shot peening treatment test (2), the residual stress
in the region at a depth of 50 .mu.m or more but 150 .mu.m or less
from the surface could be reduced, whereas the residual stress
became small in a region at a depth of 20 .mu.m or less from the
surface, which needs sufficient tooth-root bending strength, and a
region at a depth of more than 160 .mu.m but 230 .mu.m or less from
the surface, which needs sufficient strength to fatigue fracture of
tooth surface by contact surface pressure. Thus, a problem with
insufficient strength occurs.
[0019] As above, all of the tooth-root bending fatigue strength and
the tooth-surface fatigue strength by contact surface pressure
could not be increased.
[0020] The present invention has been made to solve the above
problems and has a purpose to provide a gear configured with high
tooth-root bending strength and without causing tooth-tip
chipping.
Means of Solving the Problems
[0021] To achieve the above purpose, one aspect of a gear of the
invention provides the following configuration.
[0022] (1) In a gear including a carburized surface and
strengthened by application of residual stress to the gear, the
residual stress in a region at a depth of 5 .mu.m or more but 20
.mu.m or less from the surface is -1400 MPa or less so that when a
tooth of the gear is subjected to repeated stress, the repeated
stress is canceled out by compressive stress to prevent surface
cracks in the surface of a tooth root and improve a strengthening
rate of the gear, the residual stress in a region at a depth of 50
.mu.m or more but 150 .mu.m or less from the surface is -1000 MPa
or more to prevent inner cracks and restrain tooth-tip chipping,
and the residual stress in a region at a depth of more than 150
.mu.m but 230 .mu.m or less is -800 MPa or less so that the
residual stress cancels out hertz stress caused by contact of the
teeth of the gears.
[0023] (2) In the gear described in (1), preferably, the region at
a depth of 50 .mu.m or more but 150 .mu.m or less from the surface
is subjected to a shot peening treatment using shot particles, and
the region at a depth of 5 .mu.m or more but 20 .mu.m or less from
the surface is subjected to a shot peening treatment using shot
particles with a smaller particle diameter and a larger hardness
than said shot particles.
Effects of the Invention
[0024] Operations and effects of the gear according to the present
invention will be explained below.
[0025] (1) In the gear strengthened by applying residual stress to
the gear having a carburized surface, the residual stress in the
region at a depth of 5 .mu.m or more but 20 .mu.m or less from the
surface is -1400 MPa or less so that when a tooth of the gear is
subjected to repeated stress, the repeated stress is canceled out
by compressive stress to prevent surface cracks in the surface of a
tooth root and improve a strengthening rate of the gear. Further,
the residual stress in the region at a depth of 50 .mu.m or more
but 150 .mu.m or less from the surface is -1000 MPa or more to
prevent inner cracks and restrain tooth-tip chipping, and the
residual stress in a region at a depth of more than 150 .mu.m but
230 .mu.m or less is -500 MPa or less so that the residual stress
cancels out hertz stress caused by contact of the teeth of the
gears. To be concrete, the residual stress in the region at a depth
of from 5 .mu.m to 20 .mu.m from the surface is -1400 MPa or less
(1400 MPa or more in terms of compressive stress) for the fatigue
fracture of a tooth root (dedendum). When the teeth are subjected
to repeated loading (stress), therefore, this stress is canceled
out by the compressive stress. Thus, no cracks occur in the surface
of a tooth root. This can prevent tooth-root fatigue fracture.
[0026] In addition, since the residual stress in the region at a
depth of from 50 .mu.m to 150 .mu.m from the surface is adjusted to
-1000 MPa or more, no inner cracks occur. Thus, the occurrence of
tooth-tip chipping can be avoided.
[0027] (2) The region at a depth of 50 .mu.m or more but 150 .mu.m
or less from the surface is subjected to a shot peening treatment
using shot particles, and the region at a depth of 5 .mu.m or more
but 20 .mu.m or less from the surface is subjected to a shot
peening treatment using shot particles with a smaller particle
diameter and a larger hardness than said shot particles so that the
residual stress in the region at a depth of 50 .mu.m or more but
150 .mu.m or less from the surface is adjusted to -1000 MPa or
more. Thus, no cracks occur and the occurrence of tooth-tip
chipping can be prevented.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a distribution diagram of residual stress after a
shot peening B-process and a shot peening C-process are
performed;
[0029] FIG. 2 is a distribution diagram of residual stress in a
gear in the shot peening B-process;
[0030] FIG. 3 is a distribution diagram of residual stress in a
gear in the shot peening C-process;
[0031] FIG. 4 is a micrograph with a scale showing a photographed
cross section of a tooth tip of a gear having been subjected to the
shot peening B-process and the shot peening C-process;
[0032] FIG. 5 is a diagram corresponding to the micrograph shown in
FIG. 4;
[0033] FIG. 6 is a distribution diagram of residual stress in a
gear subjected to a shot peening A-process and the shot peening
C-process;
[0034] FIG. 7 is a distribution diagram of residual stress in a
gear in the shot peening A-process;
[0035] FIG. 8 is a distribution diagram of strain in CAE analysis
on a cross section of a tooth tip of a gear subjected to a
conventional shot peening;
[0036] FIG. 9 is a micrograph with a scale showing a cross section
of the gear tooth-tip in FIG. 8;
[0037] FIG. 10 is a diagram corresponding to the micrograph shown
in FIG. 9;
[0038] FIG. 11 is a distribution diagram of residual stress in a
gear in a shot peening treatment test (1);
[0039] FIG. 12 is a distribution diagram of residual stress in a
gear in a shot peening treatment test (2); and
[0040] FIG. 13 is a table showing experimental results of tooth
chipping.
DETAILED DESCRIPTION
[0041] A detailed description of a preferred embodiment of a gear
embodying the present invention and a method for manufacturing the
gear will now be given referring to the accompanying drawings.
First Embodiment
[0042] A manufacturing process in which shot peening is performed
on a gear in a first embodiment will be explained.
[0043] Before a shot peening treatment, a gear which is a
mechanical part is subjected to a carburizing treatment. In the
carburizing treatment, when a carburized material is held at a
temperature equal to or higher than a transformation point, carbon
enters from the surface of the gear. When this is quenched, only a
carburized portion is hardened. The carburizing treatment is
similar to a conventional art and hence is not explained herein. In
some cases, a nitriding treatment is performed in addition to the
carburizing treatment.
[0044] (Shot Peening Treatment)
[0045] A shot peening treatment is performed on the gear subjected
to the carburizing treatment. This shot peening treatment, which is
an essential feature of the invention, is explained in detail
below.
[0046] The shot peening treatment is confirmed to have an advantage
that remarkably increases fatigue strength, and is widely used. For
instance, high pressure air or high pressure water jet is used to
accelerate shot particles so that the shot particles are injected
through a nozzle to impact or shot peen the part, thereby
generating compression residual stress in the part.
[0047] The shot peening treatment is an effective treatment because
it can control a depth of residual stress and a peak value
according to particle diameter and hardness of shot particles and
also provide a wide controllable range. This shot peening treatment
can control the residual stress for a depth ranging from a top
surface to 400 .mu.m and the residual stress for a peak value
ranging -800 MPa to -1600 MPa.
[0048] To be concrete, increasing the hardness of shot particles
can increase a value of residual stress to be generated. When the
particle diameter of the shot particles is set to be large, a peak
of the residual stress can be adjusted to a deep position from the
surface of a part. On the other hand, when the particle diameter of
the shot particles is set to be small, a peak of the residual
stress can be adjusted to a shallow position from the surface of a
part.
[0049] In the first embodiment, the shot peening process is carried
out in the order of the shot peening B-treatment process and the
shot peening C-treatment process. The shot peening treatment is
conducted, beginning with a deeper area from the surface.
[0050] (Shot Peening B-Treatment Process)
[0051] In the shot peening B-treatment, residual stress is applied
to a region at a depth of 50 .mu.m or more but 150 .mu.m or less
from the surface.
[0052] FIG. 2 is a distribution diagram of the residual stress in
the shot peening B-treatment. The residual stress was measured with
an X-ray stress analyzer (Rigaku Corporation) (residual stress
measurements in other distribution diagrams were similarly
conducted). In the present embodiment, in the shot peening
B-treatment, the residual stress in a region at a depth of 50 .mu.m
or more but 150 .mu.m or less from the surface is adjusted to -1000
MPa or more to prevent chipping of a tooth tip. When the residual
stress in the region at a depth of 50 .mu.m or more but 150 .mu.m
or less from the surface can be adjusted to -1000 MPa or more, a
residual stress of -1000 MPa or less that may cause inner cracks
does not exist. Therefore, because of the absence of inner cracks,
it is possible to prevent tooth-tip chipping.
[0053] In the shot peening B-treatment, the particle diameter of
shot particles is set to 1 mm. Since the particle diameter of short
particles is set to 1 mm, as shown in FIG. 2, the peak position of
residual stress can be adjusted to a position at 50 .mu.m or more
but 150 .mu.m or less below the surface.
[0054] The hardness of shot particles is set to HV450. With this
hardness HV450 of shot particles, the residual stress can be
adjusted to a value, -1000 MPa or more, as shown in FIG. 2. To be
specific, as shown in FIG. 2, the residual stress is indicated by a
solid line B. This solid line B shows that the residual stress is
-1000 MPa at a depth of 80 .mu.m from the surface, which is a peak
position indicated by B1. The residual stress in a position B2 at a
depth of 50 .mu.m from the surface is -900 MPa and the residual
stress in a position B2 at a depth of 150 .mu.m from the surface is
-850 MPa. Therefore, in a region at a depth of 50 .mu.m or more but
150 .mu.m or less from the surface, the residual stress ranges from
-850 MPa to -1000 MPa.
[0055] As above, when the particle diameter of shot particles is
set to 1 mm and the hardness is set to HV450, the region at a depth
of 50 .mu.m or more but 150 .mu.m or less from the surface can have
a residual stress of -1000 MPa or more.
[0056] Although the particle diameter is set to 1 mm in the present
embodiment, it is experimentally confirmed that any particle
diameter of 0.8 mm or more but 1.2 mm or less with a hardness of
HV450 can achieve the residual stress of -1000 MPa or more in the
region at a depth of 50 .mu.m or more but 150 .mu.m or less from
the surface.
[0057] (Shot Peening C-Treatment Process)
[0058] In the shot peening C-treatment, residual stress is applied
to a region at a depth of 5 .mu.m or more but 20 .mu.m or less from
the surface. This is because the region at a depth of 5 .mu.m or
more but 20 .mu.m or less is subjected to tooth-root bending
fatigue and thus needs improved strength. FIG. 3 is a distribution
diagram of residual stress in the shot peening C-treatment.
[0059] In the present embodiment, in the shot peening C-treatment,
the residual stress in the region at a depth of 5 .mu.m or more but
20 .mu.m or less from the surface is adjusted to -1500 MPa or less
in order to improve the strength of a portion which is likely to be
subjected to tooth-root bending fatigue stress. Since the residual
stress in the region at a depth of 5 .mu.m or more but 20 .mu.m or
less from the surface can be adjusted to -1500 MPa or less, when
repeated loading (stress) is imparted on a tooth or teeth, the
stress is canceled out by compressive stress. This can prevent the
occurrence of cracks in the surface(s) of a tooth or teeth root(s)
and avoid tooth-root fatigue fracture. In the present embodiment,
of course, the residual stress adjusted to -1500 MPa or less also
includes a residual stress of -1000 MPa or less.
[0060] In the shot peening C-treatment, the particle diameter of
shot particles is set to 0.2 mm. With the particle diameter of shot
particles set to 0.2 mm, as shown in FIG. 4, a peak of residual
stress can be adjusted to a position at a depth of 5 .mu.m or more
but 20 .mu.m or less.
[0061] The hardness of shot particles is also set to HV800. With
the hardness of shot particles set to HV800, as shown in FIG. 3,
the residual stress can be increased to a value, -1400 MPa or
less.
[0062] To be concrete, as shown in FIG. 3, the residual stress is
indicated by a solid line C. This solid line C shows that residual
stress is -1600 MPa at a depth of 15 .mu.m from the surface, which
is a peak position indicated by C1. The residual stress in a
position C2 at a depth of 5 .mu.m from the surface is -1500 MPa and
the residual stress in a position C3 at a depth of 20 .mu.m from
the surface is -1400 MPa. Therefore, in the region at a depth of 5
.mu.m or more but 20 .mu.m or less from the surface, which needs
residual stress enough to prevent tooth-root bending fatigue, the
residual stress can be kept at a high residual stress of -1400 MPa
to -1600 MPa.
[0063] As above, when the particle diameter of shot particles is
set to 0.2 mm and the hardness is set to HV800, the region at a
depth of 5 .mu.m or more but 20 .mu.m or less from the surface can
have a residual stress of -1400 MPa or less.
[0064] Although the particle diameter is set to 0.2 mm in the
present embodiment, it is experimentally confirmed that any
particle diameter of 0.2 mm or more but 0.3 mm or less with a
hardness of HV800 can achieve the residual stress of -1400 MPa or
less in the region at a depth of 5 .mu.m or more but 20 .mu.m or
less from the surface.
[0065] (Advantageous Effects of Gear Subjected to Shot Peening
B-Treatment and Shot Peening C-Treatment)
[0066] FIG. 1 shows a distribution diagram of residual stress in a
gear subjected to the shot peening B-treatment and the shot peening
C-treatment in the present embodiment. In FIG. 1, a solid line A
indicates residual stress in a gear in the present embodiment, and
a broken line E3 indicates residual stress in a gear subjected to
the conventional shot peening treatment test (1) conducted to
increase tooth-root bending fatigue strength of the gear as shown
in FIG. 12. The solid line A is depicted as a combination of the
distribution diagrams obtained when the shot peening B-treatment
and the shot peening C-treatment are performed.
[0067] As shown in FIG. 1, in the solid line A, the residual stress
is -1600 MPa or less at A1, which is likely to be subjected to
tooth-root bending fatigue, corresponding to a depth of 5 .mu.m or
more but 20 .mu.m or less from the surface. This residual stress is
larger by -650 MPa or more than a residual stress of -950 MPa at E1
of the broken line E corresponding to the depth of 5 .mu.m or more
but 20 .mu.m or less from the surface.
[0068] Accordingly, the residual stress in the region at a depth of
5 .mu.m or more but 20 .mu.m or less from the surface is -1400 MPa
or less (1400 MPa or more in terms of compressive stress) with
respect to tooth-root fatigue stress. When repeated loading
(stress) is imparted on a tooth or teeth, the stress is canceled
out by the compressive stress. This can prevent the occurrence of
inner cracks in the surface of a tooth root and avoid tooth-root
fatigue fracture.
[0069] As shown in FIG. 1, in the solid line A, a region A2 to A3,
which may cause inner cracks, corresponding to the depth of 50
.mu.m or more but 150 .mu.m or less from the surface has a residual
stress of -1000 MPa or more. This residual stress is not so large
as that in the region at a depth of 50 .mu.m or more but 150 .mu.m
or less from the surface indicated by the broken line E. Thus, the
region A2 to A3 is lower in strength than the broken line E.
However, the residual stress of -1000 MPa or more is not so large
but is sufficient to keep the strength of a gear. Therefore, the
gear strength can be sufficiently maintained.
[0070] In the case where a residual stress of -1500 MPa or less
exits as in a region E2 to E3 of the broken line E, inner cracks
occur, leading to tooth-tip chipping. In the region A2 to A3,
having a residual stress of -1000 MPa or more, no inner cracks
occur and hence tooth-tip chipping is not caused.
[0071] FIG. 4 is a micrograph with a scale of a photographed cross
section of a tooth tip of a gear 1 having been subjected to the
shot peening B-treatment and the shot peening C-treatment. FIG. 5
is a diagram corresponding to the micrograph shown in FIG. 4.
[0072] As shown in FIG. 5, in a region R1 located at a depth of 100
.mu.m from the surface R of the gear 1 subjected to the shot
peening B-treatment and the shot peening C-treatment, such inner
cracks Q as shown in FIG. 10 did not occur.
[0073] According to the present embodiment in which the shot
peening B-treatment and the shot peening C-treatment are performed,
furthermore, inner cracks did not occur in all of eight gears as
shown in FIG. 13. Accordingly, the occurrence of inner cracks can
be prevented and thus tooth-tip chipping can be avoided.
Second Embodiment
[0074] A gear and a method for manufacturing the gear in a second
embodiment is substantially identical to the gear and the gear
manufacturing method in the first embodiment excepting that a shot
peening A-treatment is performed in addition to the shot peening
B-treatment and the shot peening C-treatment. The second embodiment
is therefore explained about residual stress of a final gear having
been subjected to the shot peening B-treatment and the shot peening
C-treatment. Other explanations are thus omitted. The shot peening
A-treatment is carried out before the shot peening B-treatment and
the shot peening C-treatment. In the second embodiment, therefore,
the shot peening process is performed in the order of the shot
peening A-treatment, the shot peening B-treatment, and the shot
peening C-treatment.
[0075] The second embodiment, omitting other explanations, can
provide the same operations and effects as those in the first
embodiment.
[0076] (Shot Peening A-Treatment)
[0077] In the shot peening A-treatment, residual stress is applied
to a region at a depth of more than 160 .mu.m but 230 .mu.m or less
from the surface. This is because the region corresponding to the
depth of more than 160 .mu.m but 230 .mu.m or less from the surface
is likely to be subjected to contact pressure fatigue during gear
operation and thus needs improved strength. FIG. 7 shows a
distribution diagram of residual stress in the shot peening
A-treatment.
[0078] In the shot peening A-treatment, the particle diameter of
shot particles is set to 2 mm. With the particle diameter of shot
particles set to 2 mm, a peak of the residual stress can be
adjusted to a position at a depth of more than 160 .mu.m but 230
.mu.m or less from the surface as shown in FIG. 7.
[0079] The hardness of shot particles is set to HV700. With the
hardness of shot particles set to HV700, the residual stress of in
the region corresponding to the depth of more than 160 .mu.m but
230 .mu.m or less from the surface can be increased to a value,
-1200 MPa or less, as shown in FIG. 7.
[0080] As above, when the particle diameter of shot particles is
set to 2 mm and the hardness is set to HV700, the region at a depth
of more than 160 .mu.m but 230 .mu.m or less from the surface can
have a residual stress of -1000 MPa or less.
[0081] To be concrete, as shown in FIG. 7, the residual stress is
indicated by a solid line F. This solid line F shows that the
residual stress is -1300 MPa at a depth of more than 160 .mu.m but
230 .mu.m or less from the surface, which is a peak position
indicated by F1.
[0082] The residual stress in a position F2 at a depth of 160 .mu.m
from the surface is -1000 MPa and the residual stress in a position
F3 at a depth of 230 .mu.m from the surface is -1050 MPa.
Therefore, in the region at a depth of more than 160 .mu.m but 230
.mu.m or less from the surface, which needs residual stress enough
to prevent tooth-root bending fatigue, the residual stress can be
kept at a high residual stress of -1000 MPa to -1300 MPa.
[0083] Accordingly, when the particle diameter of shot particles is
set to 2 mm and the hardness is set to HV700, the region at a depth
of more than 160 .mu.m but 230 .mu.m or less from the surface can
have a residual stress of -1200 MPa or less.
[0084] Furthermore, the residual stress in a position at a depth of
150 .mu.m from the surface is -900 MPa and the residual stress in a
position F3 at a depth of 230 .mu.m from the surface is -1050 MPa.
Thus, in the region at a depth of more than 150 .mu.m but 230 .mu.m
or less from the surface, needing residual stress enough to prevent
tooth-root bending fatigue, the residual stress can be kept at a
high residual stress of -900 MPa to -1300 MPa.
[0085] In the present embodiment, the region at a depth of more
than 150 .mu.m but 230 .mu.m or less from the surface is adjusted
to -900 MPa or less. However, the present applicants experimentally
confirmed that even when the region at a depth of more than 150
.mu.m but 230 .mu.m or less from the surface is adjusted to -500
MPa, sufficient contact pressure fatigue strength can be
maintained.
[0086] (Advantageous Effects of Gear Subjected to Shot Peening
A-Treatment through Shot Peening C-Treatment)
[0087] FIG. 6 is a distribution diagram of residual stress in a
gear subjected to the shot peening A-treatment through the shot
peening C-treatment.
[0088] In FIG. 6, a solid line X indicates residual stress in a
gear in the present embodiment, a first broken line Y indicates
bending load stress, and a second broken line Z indicates load
stress to contact or surface pressure. The solid line X is depicted
as a combination of distribution diagrams obtained when the shot
peening A-treatment through the shot peening C-treatment are
performed.
[0089] The residual stress of bending load stress at Y1 of the
first broken line Y, located at a depth of 5 .mu.m or more but 20
.mu.m or less from the surface, is -1000 MPa. On the other hand,
the residual stress at X1 of the solid line X, located at a depth
of 5 .mu.m or more but 20 .mu.m or less from the surface,
corresponding to Y1, is -1500 MPa. Accordingly, after the
aforementioned shot peening A-treatment is performed, the residual
stress in a portion of the gear at a depth of 5 .mu.m or more but
20 .mu.m or less from the surface is larger by -500 MPa than the
bending load stress.
[0090] Therefore, the residual stress in the region at a depth of 5
.mu.m or more but 20 .mu.m or less from the surface is -1400 MPa or
less (1400 MPa or more in terms of compressive stress) with respect
to tooth-root fatigue fracture. Thus, when repeated loading
(stress) is imparted on a tooth or teeth, this stress is canceled
out by the compressive stress. This can prevent the occurrence of
cracks in the surface of a tooth root and avoid tooth-root fatigue
fracture.
[0091] Furthermore, even when the residual stress of bending load
stress at Y1 at a depth of 5 .mu.m or more but 20 .mu.m or less
from the surface is -1000 MPa or less, the residual stress is not
smaller than the load stress to bending. Thus, when the teeth are
subjected to repeated loading (stress), this stress is canceled out
by the compressive stress. This can prevent the occurrence of
cracks in the surface of a tooth root and thus avoid tooth-root
fatigue fracture.
[0092] As shown in FIG. 6, the residual stress of load stress to
contact pressure at a maximum point Z1 of the second broken line Z
in a range of the depth of more than 160 .mu.m but 230 .mu.m or
less from the surface is -1000 MPa or less. On the other hand, the
residual stress at X4 of the corresponding solid line X at a depth
of more than 160 .mu.m but 230 .mu.m or less from the surface is
-1000 MPa or less. Accordingly, when the aforementioned shot
peening A-treatment is performed, the residual stress in a portion
of the gear at a depth of more than 160 .mu.m but 230 .mu.m or less
from the surface is not smaller than the load stress to contact
pressure.
[0093] Accordingly, the hertz stress (a maximum value thereof in a
gear exists near at a depth of 200 .mu.m from the surface) that
occurs by contact between teeth surfaces can be canceled out. This
can improve the contact pressure fatigue strength.
[0094] In the present embodiment, the residual stress of the second
broken line Z1 is adjusted to -1000 MPa. On the other hand, the
size of load stress to contact pressure depends on the size and
others of a gear. The present applicants therefore carried out
experiments on various-sized gears and experimentally confirmed
that the contact pressure fatigue strength could be kept if the
residual stress of load stress to contact pressure is -500 MPa or
less at the depth of more than 150 .mu.m but 230 .mu.m from the
surface.
[0095] As shown in FIG. 6, the load stress of bending load stress
at Y2 of the first broken line Y in a region at a depth of 50
.mu.or more from the surface is -600 MPa or less. The load stress
of surface contact at Z2 of the second broken line Z in a region at
a depth of 50 .mu.m or more from the surface is -200 MPa or less.
On the other hand, the residual stress at X2 of the solid line X in
a region at a depth of 50 .mu.m or more from the surface is -1000
MPa or more.
[0096] The bending load stress at Y3 of the first broken line Y in
a region at a depth of 150 .mu.m or more from the surface is -450
MPa or more. The load stress of surface contact at Z3 of the second
broken line Z in a region at a depth of 150 .mu.m or more from the
surface is -700 MPa or more. On the other hand, the residual stress
at X3 of the solid line X in a region at a depth of 50 .mu.m or
more from the surface is -1000 MPa or more.
[0097] Accordingly, in the region at a depth of 50 .mu.m or more
but 150 .mu.m or less, the residual stress is adjusted to -1000 MPa
or more as shown by the solid line X. The residual stress indicated
by the solid line X is larger than the bending load stress
indicated by the first broken line Y and the load stress of contact
pressure indicated by the second broken line Z.
[0098] In the region corresponding to the depth of 50 .mu.m or more
but 150 .mu.m or less, where cracks are most likely to occur, from
the surface, it has a low probability of occurrence of inner cracks
and further it is possible to prevent the occurrence of tooth-tip
chipping.
[0099] The gear of the present invention and the gear manufacturing
method are not limited to the above embodiments and may be embodied
in other specific forms without departing from the essential
characteristics thereof.
[0100] For instance, the above embodiments perform the shot peening
treatment, but may also adopt other techniques such as wet blast,
ultrasonic shot, and heavy working.
[0101] Since the residual stress in the region at a depth of more
than 150 .mu.m but 300 .mu.m or less from the surface is not
increased uniformly from a peak of the residual stress in the
region at a depth of 150 .mu.m or less from the surface, the hertz
stress (a maximum value thereof in a gear exists near at a depth of
200 .mu.m from the surface) that occurs by contact between teeth
surfaces can be canceled out. This can improve the contact pressure
fatigue strength.
[0102] In the method for manufacturing the gear strengthened in
such a manner that the residual stress is applied by the shot
peening to the gear having a surface subjected to the carburizing
treatment, there is included the shot peening C-treatment process
to shot peen the gear by use of shot particles with a particle
diameter of 0.3 mm or less so that the residual stress in the
region at a depth of 5 .mu.m or more but 20 .mu.m or less from the
surface is -1000 MPa or less. Since the residual stress is -1000
MPa or less (1000 MPa or more in terms of compressive stress) in
the region at a depth of 5 .mu.m or more but 20 .mu.m or less from
the surface with respect to the tooth-root fatigue fracture. When
the tooth or teeth are subjected to repeated load (stress), the
stress is canceled out by the compressive stress. Thus, no cracks
occur in the surface of the tooth root. This can prevent the
tooth-root fatigue fracture.
[0103] To obtain a residual stress of -1000 MPa or more in the
region at a depth of 50 .mu.m or more but 150 .mu.m or less from
the surface, the shot peening B-treatment process is carried out to
shot peen the gear by use of shot particles with a particle
diameter of 0.8 mm or more but 1.2 mm or less and the shot peening
C-treatment process is conducted following the shot peening
B-treatment process. Since the residual stress in the region at a
depth of 50 .mu.m or more but 150 .mu.m or less from the surface is
-1000 MPa or more, no inner cracks occur and thus the generation of
tooth-tip chipping can be prevented.
[0104] To obtain a residual stress of -1200 MPa or less in the
region at a depth of more than 150 .mu.m but 230 .mu.m or less from
the surface, the shot peening A-treatment process is carried out to
shot peen the gear by use of shot particles with a particle
diameter of 1.5 mm or more and the shot peening C-treatment process
is conducted following the shot peening A-treatment process.
Accordingly, the hertz stress (a maximum value thereof in a gear
exists near at a depth of 200 .mu.m from the surface) that occurs
by contact between teeth surfaces can be canceled out. This can
improve the contact pressure fatigue strength.
[0105] Since the shot peening B-treatment process is carried out
after the shot peening A-treatment process, the residual stress in
the region at a depth of 5 .mu.m or more but 20 .mu.m or less from
the surface is -1000 MPa or less (1000 MPa in terms of compressive
stress) with respect to the teeth root fatigue fracture. When the
tooth or teeth are subjected to repeated load (stress), this stress
is canceled out by the compressive stress. No cracks therefore
occur in the surfaces of a tooth root. This can prevent tooth-root
fatigue fracture.
[0106] Furthermore, since the residual stress in the region at a
depth of 50 .mu.m or more but 150 .mu.m or less from the surface is
-1000 MPa or more, no cracks occurs and thus the generation of
tooth-tip chipping can be prevented.
[0107] Since the hertz stress (a maximum value thereof in a gear
exists near at a depth of 200 .mu.m from the surface) that occurs
by contact between teeth surfaces can be canceled out, the contact
pressure fatigue strength can be improved.
REFERENCE SIGNS LIST
[0108] R Surface
* * * * *