U.S. patent application number 11/281461 was filed with the patent office on 2006-05-25 for power transmission shaft.
This patent application is currently assigned to TOYODA KOKI KABUSHIKI KAISHA. Invention is credited to Miwa Hokii, Kazuyuki Ichikawa, Isashi Kashiwagi, Koji Nishi, Toshiyuki Saito.
Application Number | 20060108026 11/281461 |
Document ID | / |
Family ID | 36459856 |
Filed Date | 2006-05-25 |
United States Patent
Application |
20060108026 |
Kind Code |
A1 |
Ichikawa; Kazuyuki ; et
al. |
May 25, 2006 |
Power transmission shaft
Abstract
A power transmission shaft which can be evaluated on torsional
fatigue strength based on a management indicator different from
compressive residual stress in an outer surface. In a power
transmission shaft having outer circumferential serrations 1a, 1b,
torsional fatigue strength is evaluated by using compressive
residual stress at a depth of 30 .mu.m (and 50 .mu.m) from an outer
surface of a rising portion of each concave of the outer
circumferential serrations 1a, 1b as a management indicator. A high
torsional fatigue strength can be secured by having a compressive
residual stress of 1150 MPa or more at a depth of 30 .mu.m (and 50
.mu.m) from the outer surface.
Inventors: |
Ichikawa; Kazuyuki;
(Okazaki-shi, JP) ; Kashiwagi; Isashi;
(Kariya-shi, JP) ; Hokii; Miwa; (Nagoya-shi,
JP) ; Nishi; Koji; (Anjo-shi, JP) ; Saito;
Toshiyuki; (Toyoake-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
TOYODA KOKI KABUSHIKI
KAISHA
Kariya-shi
JP
|
Family ID: |
36459856 |
Appl. No.: |
11/281461 |
Filed: |
November 18, 2005 |
Current U.S.
Class: |
148/320 |
Current CPC
Class: |
C21D 10/005 20130101;
F16D 1/116 20130101; C21D 9/28 20130101; C22C 38/00 20130101; F16D
2001/103 20130101; F16C 3/02 20130101; F16D 2300/10 20130101; C21D
7/06 20130101 |
Class at
Publication: |
148/320 |
International
Class: |
C22C 38/00 20060101
C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 19, 2004 |
JP |
2004-336715 |
Claims
1. A power transmission shaft having an axial notch on its outer
circumferential surface, said axial notch having an axial end
having a compressive residual stress of 1150 MPa or more at a depth
of 30 .mu.m from an outer surface.
2. The power transmission shaft according to claim 1, wherein said
axial end has a compressive residual stress of 1150 MPa or more at
a depth of 50 .mu.m from the outer surface.
3. The power transmission shaft according to claim 1, wherein said
compressive residual stress is imparted by shot peening comprising
pelting said axial end at a speed of 55 to 90 m/sec with shots
having an average particle radius of one-third to two-thirds of a
minimum curvature radius of said axial end and a higher hardness
than a surface hardness of said axial end before shot peening by 50
to 300 Hv.
4. The power transmission shaft according to claim 3, wherein
thermal treatment is applied to said axial end before said shot
peening.
5. The power transmission shaft according to claim 2, wherein said
compressive residual stress is imparted by shot peening comprising
pelting said axial end at a speed of 55 to 90 m/sec with shots
having an average particle radius of one-third to two-thirds of a
minimum curvature radius of said axial end and a higher hardness
than a surface hardness of said axial end before shot peening by 50
to 300 Hv.
6. The power transmission shaft according to claim 5, wherein
thermal treatment is applied to said axial end before said shot
peening.
7. The power transmission shaft according to claim 1, wherein said
axial notch is a serration.
8. The power transmission shaft according to claim 1, wherein said
power transmission shaft is a power transmission shaft for an
automobile to be connected to a universal coupling, and said axial
notch is a part to be connected to said universal coupling.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a power transmission shaft
used in an apparatus such as automobiles and industrial
machines.
[0003] 2. Description of the Related Art
[0004] A power transmission shaft used in an automobile or an
industrial machine has been requested to attain weight reduction
and strength enhancement. Regarding strength enhancement, static
strength and torsional fatigue strength enhancement has been
demanded. An example of power transmission shafts which aim to
enhance static strength and torsional fatigue strength is disclosed
in Japanese Unexamined Patent Publication No. 2003-307,211. In the
power transmission shaft disclosed in this publication, shot
peening is applied to parts to be improved in torsional fatigue
strength. Shot peening imparts compressive residual stress and
improves torsional fatigue strength.
[0005] Japanese Patent No. 3,374,667 discloses that torsional
fatigue strength can be improved securely by increasing compressive
residual stress in an outer surface to, for instance, 850 MPa or
more by shot peening. Furthermore, this Japanese patent discloses
that double shot peening is carried out in order to increase
compressive residual stress in the outer surface. The double shot
peening means applying shot peening twice under different
conditions. Namely, conventionally, shot peening has been applied
to increase torsional fatigue strength, and compressive residual
stress in an outer surface has been used as a management
indicator.
SUMMARY OF THE INVENTION
[0006] By the way, there have been demands for a further
enhancement of torsional fatigue strength and at the same time for
enhancement of torsional fatigue strength by a simpler process such
as applying shot peening once.
[0007] The present invention has been conceived under these
circumstances. It is an object of the present invention to provide
a power transmission shaft which can be evaluated on torsional
fatigue strength based on a management indicator different from
compressive residual stress in an outer surface.
[0008] The present inventors have earnestly studied and made a lot
of trials and errors in order to attain this object. As a result,
the present inventors have found that it is effective in improving
torsional fatigue strength to use, as a management indicator,
compressive residual stress not in an outer surface but in a part
at a predetermined depth from an outer surface and have completed
the present invention.
[0009] A power transmission shaft of the present invention is a
power transmission shaft having an axial notch on its outer
circumferential surface and the axial notch has an axial end having
a compressive residual stress of 1150 MPa or more at a depth of 30
.mu.m from an outer surface. It is more preferable that the axial
end has a compressive residual stress of 1150 MPa or more at a
depth of 50 .mu.m from the outer surface.
[0010] The present invention has the following advantages.
According to the power transmission shaft of the present invention,
compressive residual stress in a part at a depth of 30 .mu.m from
an outer surface is used as a first management indicator. The use
of compressive residual stress in a part at a depth of 30 .mu.m
from an outer surface as a management indicator permits appropriate
evaluation of torsional fatigue strength. Besides, torsional
fatigue strength can be improved sufficiently by having a
compressive residual stress of 1150 MPa or more in a part at a
depth of 30 .mu.m from an outer surface.
[0011] For example, when a power transmission shaft which has
compressive residual stress of 1200 MPa or more in an outer surface
and less than 1150 MPa in a part at a depth of 30 .mu.m from the
outer surface is compared in torsional fatigue strength with a
power transmission shaft which has a compressive residual stress of
less than 1200 MPa in an outer surface and 1150 MPa or more in a
part at a depth of 30 .mu.m from the outer surface, the latter has
been proven to have a higher torsional fatigue strength. This
example demonstrates that it is more appropriate to use, as a
management indicator, a compressive residual stress in a part at a
predetermined depth from an outer surface rather than a compressive
residual stress in an outer surface.
[0012] Moreover, according to the power transmission shaft of the
present invention, compressive residual stress in a part at a depth
of 50 .mu.m from an outer surface is used as a second management
indicator. The use of compressive residual stress in a part at a
depth of 50 .mu.m from an outer surface in addition to the above
first management indicator permits evaluation of a higher torsional
fatigue strength. Besides, torsional fatigue strength can be
improved further by having compressive residual stress of 1150 MPa
or more in parts at depths of 30 .mu.m and 50 .mu.m from an outer
surface.
[0013] Furthermore, the following advantages can also be obtained
by using compressive residual stress in a part at a predetermined
depth from an outer surface as a management indicator. In some
cases, shot peening is applied in order to impart compressive
residual stress to a power transmission shaft. When single shot
peening is applied, that is, shot peening is carried out once, a
part having a maximum compressive residual stress is not an outer
surface but a part at a depth of not less than 10 .mu.m, for
instance, from an outer surface. Therefore, even when compressive
residual stress in the outer surface is not so high, a sufficient
torsional fatigue stress can be secured by having a compressive
residual stress of 1150 MPa or more in a part at a predetermined
depth (e.g., 30 .mu.m, 50 .mu.m) from the outer surface.
[0014] On the other hand, if compressive residual stress in an
outer surface alone is used as a management indicator as before,
there arises a need to apply double shot peening, i.e., carry out
shot peening twice when compressive residual stress in the outer
surface after single shot peening is below a reference value.
Therefore, even when sufficient torsional fatigue strength is
secured, an appropriate judgment cannot be made on the base of the
conventional management indicator and, in some cases, double shot
peening is applied.
[0015] In contrast, the use of compressive residual stress in a
part at a predetermined depth from an outer surface as a management
indicator makes it possible to determine without faults cases where
single shot peening is sufficient and double shot peening is
unnecessary. Namely, according to the present invention, torsional
fatigue strength which is more than the conventional can be secured
by a simpler process than before (single shot peening, for
instance)
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Further objects and advantages of the present invention will
be apparent from the following description, reference being had to
the accompanying drawings.
[0017] FIG. 1 is an axial cross-sectional view of a drive shaft for
an automobile.
[0018] FIGS. 2(a) and 2(b) are partial cross-sectional views of an
outer circumferential serration 1a.
[0019] FIG. 3 is a graph showing a residual stress distribution by
depth from an outer surface of rising portions 1c of outer
circumferential serrations 1a, 1b.
[0020] FIG. 4 shows results of a torsional fatigue test.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] Hereinafter, a power transmission shaft, an axial notch, and
a process of imparting compressive residual stress according to an
embodiment of the present invention will be described in
detail.
(1) Power Transmission Shaft
[0022] The power transmission shaft according to the embodiment of
the present invention can be used in an apparatus such as
automobiles and industrial machines, as mentioned above. A typical
example of power transmission shafts used in automobiles is a power
transmission shaft for an automobile connected to a universal
joint. For instance, the power transmission shaft is an
intermediate shaft both ends of which are respectively connected to
universal couplings such as constant velocity joints. The power
transmission shaft is made of a metal material such as an
iron-based material which includes iron as a main component.
[0023] In the power transmission shaft, thermal treatment can be
usually applied to at least an axial notch. After the thermal
treatment is applied, compressive residual stress is imparted to an
axial end of the axial notch. This imparting of compressive
residual stress is conducted by shot peening, for instance, as will
be mentioned later. Namely, in the power transmission shaft,
thermal treatment is applied to the aforementioned axial notch,
before shot peening, for instance. Owing to the thermal treatment,
at least a sufficient static strength can be secured. Examples of
the thermal treatment include induction hardening, carburizing,
nitriding and flame hardening.
(2) Axial Notch
[0024] The axial notch according to the embodiment of the present
invention can be a concave portion when seen in an axial cross
section. This concave portion is formed, in some cases, on the
entire circumference and, in other cases, on a part of the
circumference. For example, when the power transmission shaft is an
automotive power transmission shaft to be connected to a universal
joint, an axial notch, which is a concave portion, formed on the
entire circumference is a part to be assembled with a boot for
covering the universal joint. An axial notch formed on a part of
the circumference is a part to be connected to or assembled with
the universal joint, such as an outer circumferential serration and
key grooves.
[0025] The power transmission shaft having such an axial notch has
a remarkably low torsional fatigue strength particularly at an
axial end of the axial notch. By imparting compressive residual
stress, torsional fatigue stress of the axial end of the axial
notch can be enforced. The term `the axial end of the axial notch`
as used herein means a portion rising from a bottom of a concave to
an outer circumferential surface. For instance, when the axial
notch is a concave formed on the entire circumference, the axial
end is each of the axial ends of the concave. When the axial notch
is a concave of a serration, formed on a part of the circumference,
the axial end is a rising portion of each concave of the
serration.
(Process of Imparting Compressive Residual Stress)
[0026] Imparting of compressive residual stress according to the
embodiment of the present invention can be carried out by shot
peening. For instance, this shot peening is a process of pelting
the aforementioned axial end at a speed of 55 to 90 m/sec with
shots having an average particle radius of one-third to two-thirds
of a minimum curvature radius of the abovementioned axial end and a
higher hardness than a surface hardness of the abovementioned axial
end before shot peening by 50 to 300 Hv. This shot peening can
securely impart a compressive residual stress of 1150 MPa or more
to a part at a predetermined depth from an outer surface.
[0027] When the axial end is a rising portion of each concave of a
serration, a part having the abovementioned minimum curvature
radius is each end of a concave bottom of the serration when seen
in a radial cross section of the rising portion of each concave of
the serration. The outer surface hardness before the shot peening
means an outer surface hardness of the power transmission shaft
before shot peening is applied.
[0028] By using shots having an average particle radius of not more
than two-thirds of a minimum curvature radius of the axial end, a
part having the minimum curvature radius can securely be pelted
with the shots, and accordingly compressive residual stress can
securely be imparted to this part. Moreover, by using shots having
an average particle radius of not less than one-third of a minimum
curvature radius of the axial end, a compressive residual stress of
1150 MPa or more can be imparted to a part at a predetermined depth
from an outer surface. It is more preferable that the average
particle radius of the shots is close to two-thirds of the minimum
curvature radius of the axial end. In this case, a high compressive
residual stress can be imparted. Namely, higher torsional fatigue
strength can be obtained.
[0029] Now, the present invention will be described in detail by
way of preferred embodiments.
(Structure of Automotive Drive Shaft)
[0030] An intermediate shaft used in a drive shaft for an
automobile will be described as a preferred embodiment of the power
transmission shaft of the present invention. First, the drive shaft
for an automobile will be described with reference to FIG. 1. FIG.
1 is an axial cross-sectional view of the drive shaft for an
automobile. As shown in FIG. 1, the drive shaft for an automobile
comprises an intermediate shaft 1, an inboard joint 2, an outboard
joint 3 and boots 4, 5.
(1.1) Intermediate Shaft 1
[0031] The intermediate shaft 1 is a power transmission shaft
formed of a solid rod. The intermediate shaft 1 transmits power
which has been input from a driving shaft side of the inboard joint
2 to a driven shaft side of the outboard joint 3. Formed on outer
circumferential surfaces of both ends of the intermediate shaft 1
are outer circumferential serrations 1a, 1b which are in parallel
with an axial direction.
[0032] Now, the outer circumferential serration 1a formed on an end
portion of the intermediate shaft 1 will be described in detail
with reference to FIGS. 2(a) and 2(b). FIG. 2(a) is an axial
cross-sectional view of the end portion having the outer
circumferential serration 1a. FIG. 2(b) is a cross-sectional view
taken along line B-B of FIG. 2(a), namely, an enlarged view of a
rising portion 1c of one concave of the outer circumferential
serration 1a.
[0033] As shown in FIG. 2(a), each concave of the outer
circumferential serration 1a formed on the intermediate shaft 1 has
the rising portion (an axial end) 1c on an axial center side of the
intermediate shaft 1 (the right side of FIG. 2). This rising
portion 1c is a slant portion which gradually rises from a concave
bottom, which forms the outer circumferential serration 1a, toward
the outer circumferential surface. As shown in FIG. 2(b), each
concave (axial notch) of the outer circumferential serration 1a
including the rising portion 1c has a radially cross sectional
shape comprising side portions constituted by involute curves and a
concave bottom. Of each concave, a part having a minimum curvature
radius is each end portion 1d of the concave bottom when seen in a
radial cross section. It is to be noted that the outer
circumferential serration 1b formed on the other end portion of the
intermediate shaft 1 has almost the same shape as the above
mentioned outer circumferential serration 1a formed on the one end
portion of the intermediate shaft 1.
[0034] Moreover, grooves (axial notches) 1e, 1f for attaching the
boots 4, 5 are formed near the outer circumferential serrations 1a,
1b on the axially center side of the intermediate shaft 1. These
grooves 1e, 1f for attaching the boots 4, 5 have concave shapes
when seen in an axial cross-sectional direction and are formed on
the entire circumference. Each of these grooves 1e, 1f is a portion
to be engaged with and fixed to one end of each of the boots 4, 5,
which will be mentioned later.
(1.2) Inboard Joint 2
[0035] The inboard joint 2 is a constant velocity joint such as a
slidable tripod joint. This inboard joint 2 is connected to a power
input side of the intermediate shaft 1, as shown in FIG. 1. This
inboard joint 2 comprises a tripod-type inner member 11,
tripod-type rollers 12 and a tripod-type outer member 13.
[0036] The tripod-type inner member 11 comprises a boss portion 21
and three trunnions 22. The boss portion 21 has a roughly
cylindrical shape and an inner circumferential serration 21a on its
inner circumferential surface. This inner circumferential serration
21a of the boss portion 21 is fitted on and engaged with the outer
circumferential serration 1a formed on the one end portion of the
intermediate shaft 1. Namely, the tripod-type inner member 11 is
integrally connected to the intermediate shaft 1. Each of the
trunnions 22 has a roughly cylindrical shape and is disposed so as
to extend from the outer circumference of the boss portion 21 in a
radially outward direction.
[0037] Each of the tripod-type rollers 12 has a roughly cylindrical
shape with an outer circumferential surface in a partially
spherical shape. Each of the tripod-type rollers 12 is disposed on
the outer circumferential surface of each of the trunnions 22 of
the tripod-type inner member 11 so as to be freely rotatable.
[0038] The tripod-type outer member 13 comprises a driving shaft
portion 31, and a tubular portion 32 which has a tubular shape with
a bottom and is formed integrally with the one end portion of the
driving shaft portion 31 so as to extend in a tubular shape. Formed
on an inner circumferential surface of the tubular portion 32 are
three guide grooves 32a which are in parallel with the axial
direction. The tripod-type rollers 12 are respectively engaged with
these guide grooves 32a in the circumferential direction and are
disposed in the guide grooves 32a so as to be freely slidable in
the axial direction. Moreover, formed on an outer circumferential
surface of the tubular portion 32 is a groove 32b to be engaged
with and fixed to the other end of the boot 4, which will be
mentioned later. The groove 32b for attaching the boot 4 has a
concave shape when seen in an axial cross section and is formed on
the entire circumference.
(1.3) Outboard Joint 3
[0039] The outboard joint 3 is a constant velocity joint such as a
fixed ball joint. This outboard joint 3 is connected to a power
output side of the intermediate shaft 1, as shown in FIG. 1. This
outboard joint 3 comprises a ball-type inner member 41, a ball-type
outer member 42, a cage 43 and balls 44.
[0040] The ball-type inner member 41 has a roughly cylindrical
shape. The outermost circumferential surface 41a of this ball-type
inner member 41 has uniformly circular arc shapes when seen in
axial cross sections, namely, has a partially spherical shape.
Formed on an outer circumferential surface of the ball-type inner
member 41 are inner ball guide grooves 41b which are in parallel
with the axial direction and comprise six circular arc concaves
disposed at even distances when seen in a radial cross section.
Formed on an inner circumferential surface of the ball-type inner
member 41 is an inner circumferential serration 41c. This inner
circumferential serration 41c of the ball-type inner member 41 is
fitted on and engaged with the outer circumferential serration 1b
formed on the other end portion of the intermediate shaft 1.
Namely, the ball-type inner member 41 is integrally connected to
the intermediate shaft 1.
[0041] The ball-type outer member 42 comprises a driven shaft 51
and a tubular portion 52 which has a tubular shape with a bottom
and is formed integrally with one end portion of the driven shaft
51 so as to extend in a tubular shape. The innermost
circumferential surface 52a of the tubular portion 52 has uniformly
circular arc shapes when seen in axial cross sections, namely, has
a partially spherical shape. Formed on an inner circumferential
surface of the tubular portion 52 are outer ball guide grooves 52b
which are in parallel with the axial direction and comprise six
circular arc concaves disposed at even distances when seen in a
radial cross section. Moreover, formed on an outer circumferential
surface of the tubular portion 52 is a groove 52c for attaching the
other end portion of the boot 5, which will be mentioned later.
This groove 52c for attaching the boot 5 has a concave shape when
seen in an axial cross section and is formed on the entire
circumference.
[0042] The cage 43 has a roughly cylindrical shape and is disposed
between the ball-type inner member 41 and the tubular portion 52 of
the ball-type outer member 42. An inner circumferential surface of
the cage 43 is formed in a shape corresponding to that of the
outermost circumferential surface 41a of the ball-type inner member
41. On the other hand, an outer circumferential surface of the cage
43 is formed in a shape corresponding to that of the innermost
circumferential surface 52a of the tubular portion 52 of the
ball-type outer member 42. Namely, the cage 43 is relatively
rotatable without making contact with the ball-type inner member 41
or the ball-type outer member 42. Moreover, the cage 43 has six
holes at even distances.
[0043] The balls 44 are respectively engaged in a circumferential
direction with the inner ball guide grooves 41b of the ball-type
inner member 41 and the outer ball guide grooves 52a of the tubular
portion 52 of the ball-type outer member 42 so as to be freely
rotatable. In addition, the balls 44 respectively penetrate the
circular holes of the cage 43. Namely, the balls 44 transmit
rotation of the ball-type inner member 41 to the ball-type outer
member 42.
(1.4) Boots 4, 5
[0044] The boot 4 has a bellows shape. One end portion of the boot
4 is fixed to the groove 1e of the intermediate shaft 1 for
attaching the boot 4, and the other end portion of the boot 4 is
fixed to the groove 32b of the inboard joint 2 for attaching the
boot 4. On the other hand, the boot 5 has a bellows shape. One end
portion of the boot 5 is fixed to the groove 1f of the intermediate
shaft 1 for attaching the boot 5, and the other end portion of the
boot 5 is fixed to the groove 52c of the outboard joint 3 for
attaching the boot 5.
2) Process of Producing the Intermediate Shaft 1
[0045] Now, a process of producing the above mentioned intermediate
shaft 1 will be described. First, the outer circumferential
serrations 1a, 1b and the grooves 1e, 1f for attaching the boots 4,
5 are formed on a raw material comprising a roughly rod-shape
steel. For example, the outer circumferential serrations 1a, 1b are
formed by rolling, and the grooves 1e, 1f for attaching the boots
4, 5 are formed by lathe turning.
[0046] Second, thermal treatment comprising induction hardening is
applied to the overall axial length of the intermediate shaft 1. In
parts having the outer circumferential serrations 1a, 1b and the
grooves 1e, 1f for attaching the boots 4, 5 of the intermediate
shaft 1, t/r.apprxeq.0.45 to 0.5, where r is the radius of the
shaft and t is the depth from the surface to be heat treated.
Third, shot peening is applied on the parts having the outer
circumferential serrations 1a, 1b and the grooves 1e, 1f for
attaching the boots 4, 5 of the intermediate shaft 1. The shot
peening applied on the outer circumferential serrations 1a, 1b will
be described in detail later. Induction hardening improves static
strength and fatigue strength, and additional application of shot
peening further enhances torsional fatigue strength.
(3) Shot Peening
[0047] Next, shot peening to be applied on the parts having the
outer circumferential serrations 1a, 1b of the intermediate shaft 1
has been conducted under a variety of conditions, and resultant
torsional fatigue strength in each case has been evaluated. The
shape and hardness of the intermediate shaft 1 before shot peening,
conditions of shot peening, compressive residual stress
distribution after shot peening, torsional fatigue test, torsional
fatigue strength evaluation results will be described
hereinafter.
(3.1) Shape and Hardness of the Intermediate Shaft 1 before Shot
Peening
[0048] First, the intermediate shaft 1 before shot peening will be
described. The outer circumferential serrations 1a, 1b of the
intermediate shaft 1 have a serration module of 1.05833. The rising
portions 1c of these serrations 1a, 1b have a minimum curvature
radius of 0.15 mm. As mentioned before, a part having the minimum
curvature radius is each end portion 1d of each concave bottom of
the serration 1a, as shown in FIG. 2(b). The rising portion 1c of
each concave of the serrations 1a, 1b have an outer surface
hardness of 700 Hv before shot peening. Namely, the outer surface
hardness before shot peening is controlled to 700 Hv by applying
thermal treatment before shot peening. The thermally treated
intermediate shaft 1 has a compressive residual stress of about 230
MPa in the outer surface, and as the depth from the outer surface
gets greater, the compressive residual stress gets gradually
smaller.
(3.2) Shot Peening Conditions
[0049] Next, shot peening conditions will be described. The shot
peening conditions were of six kinds from Case 1 to Case 6, as
shown in Table 1. As shown in Table 1, these six kinds of
conditions are different in the kind of shots and arc height. To be
concrete, these six kinds of conditions are different in the
material, average particle size (diameter) and average hardness of
shots, and arc height. TABLE-US-00001 TABLE 1 Case 1 Case 2 Case 3
Case 4 Case 5 Case 6 Shots Material steel amorphous steel steel
amorphous steel alloy alloy Average Particle Size (.mu.m) 200 200
100 100 50 50 Average Hardness (Hv) 789 925 789 560 925 789 Arc
Height (mmN) 0.51 0.515 0.295 0.415 0.19 0.17 Amount of Shots
(kg/min) 9.2 same as same as same as same as same as on the left on
the left on the left on the left on the left Shot Angle of Impact
(.degree.) 90 same as same as same as same as same as on the left
on the left on the left on the left on the left Pelting Distance
(mm) 150 same as same as same as same as same as on the left on the
left on the left on the left on the left Number of Workpiece
Revolutions (rpm) 30 same as same as same as same as same as on the
left on the left on the left on the left on the left Air Pressure
(MPa) 0.3 same as same as same as same as same as on the left on
the left on the left on the left on the left Injection Type Direct
same as same as same as same as same as Pressure on the left on the
left on the left on the left on the left Type Peening Time (sec) 10
same as same as same as same as same as on the left on the left on
the left on the left on the left Number of Peening Directions
(unit) 1 same as same as same as same as same as on the left on the
left on the left on the left on the left Coverage (%) not less same
as same as same as same as same as than 100 on the left on the left
on the left on the left on the left
[0050] As shown in Table 1, the material of shots of Cases 1, 3, 4,
6 is steel and that of Cases 2, 5 is an amorphous alloy. The
average particle size (diameter) of shots of Cases 1, 2 is 200
.mu.m, that of Cases 3, 4 is 100 .mu.m and that of Cases 5, 6 is 50
.mu.m. Namely, the average particle radius of shots of Cases 1, 2
is two-thirds of a minimum curvature radius of the rising portions
1c, that of Cases 3, 4 is one-third of the minimum curvature radius
of the rising portions 1c, and that of Cases 5, 6 is one-sixth of
the minimum curvature radius of the rising portions 1c.
[0051] The average hardness of shots of Cases 1, 3, 6 is 789 Hv,
that of Cases 2, 5 is 925 Hv, and that of Case 4 is 560 Hv. Namely,
the average hardness of shots of Cases 1, 3, 6 is higher than the
outer surface hardness before shot peening by 89 Hv, that of Cases
2, 5 is higher than the outer surface hardness before shot peening
by 225 Hv, and that of Case 5 is lower than the outer surface
hardness before shot peening by 140 Hv.
[0052] The amount of shots, shot angle of impact, pelting distance,
the number of workpiece revolutions, air pressure, injection type,
peening time, the number of peening directions, and coverage were
all kept constant, as shown in Table 1. Herein, the shot angle of
impact is an angle to the axis of the intermediate shaft 1. The
number of workpiece revolutions is the number of revolutions of the
intermediate shaft 1 as a workpiece to be shot peened. It is to be
noted that although the speed of pelting shots is determined based
on air pressure and an average particle size of shots, the speed of
pelting shots under all six kinds of conditions ranges from 55 to
90 m/sec.
(3.3) Compressive Residual Stress after Shot Peening
[0053] Measurement was conducted on compressive residual stress in
the rising portions 1c of the outer circumferential serrations 1a,
1b of the intermediate shaft 1 after the above shot peening. To be
concrete, the measurement was conducted on compressive residual
stress against depth from the outer surface of the rising portions
1c. The measurement of compressive residual stress was conducted by
digging the rising portions 1c from the outer surface toward the
axis center by electropolishing and measuring compressive residual
stress at each predetermined depth from the outer surface by an
X-ray stress analyzer. For comparison, measurement was also
conducted on compressive residual stress in the rising portions 1c
of the outer circumferential serrations 1a, 1b of the intermediate
shaft 1 before the above shot peening was applied.
[0054] Measurement results are shown in FIG. 3. FIG. 3 shows a
residual stress distribution by depth from the outer surface of the
rising portions 1c of the outer circumferential serrations 1a, 1b.
A negative sign of residual stress on the vertical axis means
compressive residual stress.
[0055] As shown in FIG. 3, Case 1 had a compressive residual stress
of 1115 MPa in the outer surface and a maximum compressive residual
stress of about 1370 MPa at a depth of about 45 .mu.m from the
outer surface. Case 2 had a compressive residual stress of 1046 MPa
in the outer surface and a maximum compressive residual stress of
about 1296 MPa at a depth of about 35 .mu.m from the outer surface.
Case 3 had a compressive residual stress of 1087 MPa in the outer
surface and a maximum compressive residual stress of about 1434 MPa
at a depth of about 17 .mu.m from the outer surface.
[0056] Case 4 had a compressive residual stress of 558 MPa in the
outer surface and a maximum compressive residual stress of about
1080 MPa at a depth of about 20 .mu.m from the outer surface. Case
5 had a compressive residual stress of 1417 MPa in the outer
surface and a maximum compressive residual stress of about 1449 MPa
at a depth of about 7 .mu.m from the outer surface. Case 6 had a
compressive residual stress of 1156 MPa in the outer surface and a
maximum compressive residual stress of about 1388 MPa at a depth of
about 9 .mu.m from the outer surface. The intermediate shaft 1
before shot peening had a compressive residual stress of 227 MPa in
the outer surface and had a smaller compressive residual stress as
the depth from the outer surface got greater.
(3.4) Torsional Fatigue Test
[0057] The following torsional fatigue test was conducted on the
intermediate shafts 1 which were subjected to the shot peening
under the abovementioned conditions and the intermediate shaft 1
which was not subjected to shot peening. The torsional fatigue test
was conducted by holding the outer circumferential serrations 1a,
1b of both the ends of the intermediate shaft 1 and applying a
torque of 700 Nm while alternating rotational directions. This
application of torque was continued until the intermediate shaft 1
caused failure. The number of times the direction of rotation
(torsion) for applying torque was changed was counted until the
intermediate shaft 1 caused failure. Of the intermediate shaft 1, a
part causing failure is generally the rising portion 1c of each
concave of the outer circumferential serrations 1a, 1b.
(3.5) Torsional Fatigue Strength Evaluation Results
[0058] The measurement results of the abovementioned torsional
fatigue test will be described with reference to FIG. 4. FIG. 4
shows measurement results of the torsional fatigue test. Namely,
FIG. 4 shows the number counted until the intermediate shaft 1
caused failure under each kind of shot peening conditions. As shown
in FIG. 4, Case 1 showed the counted number of 1,170,499, Case 2
showed that of 750,349, Case 3 showed that of 537,282, Case 4
showed that of 232,965, Case 5 showed that of 123,495, and Case 6
showed that of 104,044. The intermediate shaft 1 which was not shot
peened showed that of 138,427. It is to be noted that as the
counted number is greater, torsional fatigue strength is
higher.
[0059] Namely, as apparent from FIG. 4, the intermediate shafts 1
which were subjected to shot peening of Cases 1 to 3 had higher
compressive torsional strength than those which were subjected to
shot peening of Cases 4 to 6 or the intermediate shaft 1 which was
not subjected to shot peening.
[0060] Now, torsional fatigue strength will be evaluated with
reference to FIG. 3, which shows a compressive residual
distribution by depth from the outer surface of the rising portion
1c of each concave of the outer circumferential serrations 1a, 1b,
in addition to FIG. 4, which shows the measurement results of the
torsional fatigue test.
[0061] Shot peening of Cases 1, 2, which showed the highest and
second-highest torsional fatigue strength, imparted high
compressive residual stress to parts even at great depths from the
outer surface. Shot peening of Case 3, which showed the third
highest torsional fatigue strength, imparted a higher compressive
residual strength to parts even at great depths from the outer
surface than shot peening of other cases, although not as high as
shot peening of Cases 1, 2. It is to be noted that all of Cases 1
to 3 imparted compressive residual stress of about not less than
1000 MPa in the outer surface. On the other hand, shot peening of
Cases 4, 5, which didn't show very high torsional fatigue strength,
imparted high compressive residual stress of 1000 MPa or more in
the outer surface but low compressive residual stress at a depth of
30 .mu.m or more from the outer surface.
[0062] In more detailed analysis, a remarkable difference is seen
on compressive residual stress at depths of 30 .mu.m and 50 .mu.m
from the outer surface between Cases 1 to 3 and Cases 4 to 6.
Namely, Case 1 showed the compressive residual stress of about 1300
MPa both at a depth of 30 .mu.m and at a depth of 50 .mu.m. Case 2
showed the compressive residual stress of about 1250 MPa at a depth
of 30 .mu.m from the outer surface and about 1230 MPa at a depth of
50 .mu.m from the outer surface. Case 3 showed the compressive
residual stress of about 1230 MPa at a depth of 30 .mu.m from the
outer surface and about 400 MPa at a depth of 50 .mu.m from the
outer surface. On the other hand, Cases 4 to 6 showed the
compressive residual stress of not more than 1000 MPa at depths of
30 .mu.m and 50 .mu.m from the outer surface.
[0063] The above analysis has led to the following three findings.
First, when compressive residual stress at a depth of 30 .mu.m from
the outer surface is 1150 MPa or more, that is to say, a
compressive residual stress distribution curve passes below the
first reference point in FIG. 3, torsional fatigue strength is
high. Second, when compressive residual stress at a depth of 50
.mu.m from the outer surface is 1150 MPa or more in addition to the
above first finding, that is to say, a compressive residual stress
distribution curve passes below the second reference point in FIG.
3, torsional fatigue strength is much higher. Third, when
compressive residual stress in the outer surface is 1000 MPa or
more in addition to the above first and second findings, high
torsional fatigue strength is secured.
[0064] It can also be said from the above analysis that an
appropriate shot peening process for producing the intermediate
shaft 1 having a high torsional fatigue strength is to pelt parts
having the outer circumferential serrations 1a, 1b at a speed of 55
to 90 m/sec with shots having an average particle radius of one
third to two-thirds of a minimum curvature radius of the rising
portion 1c of each concave of the outer circumferential serrations
1a, 1b and a higher hardness than a surface hardness of the rising
portion 1c of each concave of the outer circumferential serrations
1a, 1b before shot peening by 50 to 300 Hv.
[0065] This invention may be practiced or embodied in still other
ways without departing from the spirit or essential characters
thereof. The preferred embodiment described herein is therefore
illustrative and not restrictive, the scope of the invention being
indicated by the appended claims and all variations which come
within the meaning of the claims are intended to be embraced
therein.
* * * * *