U.S. patent application number 11/819523 was filed with the patent office on 2007-11-01 for power transmission shaft.
This patent application is currently assigned to NTN Corporation. Invention is credited to Katsuyuki Ikei, Hisaaki Kura, Kazuya Wakita.
Application Number | 20070251606 11/819523 |
Document ID | / |
Family ID | 26615792 |
Filed Date | 2007-11-01 |
United States Patent
Application |
20070251606 |
Kind Code |
A1 |
Wakita; Kazuya ; et
al. |
November 1, 2007 |
Power transmission shaft
Abstract
A power transmission shaft having an improved strength and
ensuring a stable torsion fatigue strength. The power transmission
shaft has coupling members respectively provided on the opposite
ends of an pipe part made of a steel material. The steel material
includes 0.30-0.45% by weight of C, 0.05-0.35% by weight of Si,
1.0-2.0% by weight of Mn, 0.05% by weight or less of Al, 0.01% by
weight or less of S, and the remainder (iron Fe and unavoidable
impurities). The pipe part has an electro-unite portion that
extends in the axial direction. The electro-unite portion and
neighborhood thereof are hardened so as to have a Rockwell hardness
HRC of 45 or more. Also, another power transmission shaft has
coupling members integrally formed on opposite ends thereof. In
addition, the shaft is formed from a steel element tube by a
plastic working. The shaft has an inner diametrical surface which
is subjected to a high-frequency induction hardening and tempering
treatment to make the surface portion hardness of the inner
diametrical surface to have a Rockwell hardness HRC of 35 or
more.
Inventors: |
Wakita; Kazuya;
(Shizuoka-ken, JP) ; Ikei; Katsuyuki;
(Shizuoka-ken, JP) ; Kura; Hisaaki; (Shizuoka-ken,
JP) |
Correspondence
Address: |
ARENT FOX PLLC
1050 CONNECTICUT AVENUE, N.W.
SUITE 400
WASHINGTON
DC
20036
US
|
Assignee: |
NTN Corporation
|
Family ID: |
26615792 |
Appl. No.: |
11/819523 |
Filed: |
June 28, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10153865 |
May 24, 2002 |
7252721 |
|
|
11819523 |
Jun 28, 2007 |
|
|
|
Current U.S.
Class: |
148/400 |
Current CPC
Class: |
Y10S 148/909 20130101;
B21K 1/063 20130101; F16C 2326/06 20130101; C21D 9/28 20130101;
F16C 3/02 20130101; B21K 1/12 20130101; F16C 2204/62 20130101; Y10T
428/12965 20150115 |
Class at
Publication: |
148/400 |
International
Class: |
C22C 28/00 20060101
C22C028/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 28, 2001 |
JP |
2001-158691 |
May 28, 2001 |
JP |
2001-158766 |
Claims
1. A power transmission shaft integrally with coupling members
integrally formed on opposite ends thereof, which is formed from a
steel element tube by a plastic working, comprising an inner
diametrical surface which is subjected to a hardening
treatment.
2. The power transmission shaft according to claim 1, wherein the
hardening treatment is a high-frequency induction hardening and
tempering treatment.
3. The power transmission shaft according to claim 1 or 2, wherein
the hardened layer formed by the hardening treatment extends from
an outer diametrical surface to the inner diametrical surface.
4. The power transmission shaft according to claim 3, wherein the
surface portion hardness of the inner diametrical surface is a
Rockwell hardness HRC of 35 or more.
5. The power transmission shaft according to claim 3, wherein a
predetermined residual compression stress is applied on the outer
diametrical surface.
6. The power transmission shaft according to claim 5, wherein the
residual compression stress is applied by a shot peening
treatment.
7. The power transmission shaft according to claim 5, wherein the
residual compression stress is 750 MPa or more.
8. The power transmission shaft according to claim 6, wherein the
residual compression stress is 750 MPa or more.
Description
[0001] This is a Divisional Application which claims the benefit of
Pending U.S. patent application Ser. No. 10/153,865, filed May 24,
2002 which also claims the benefit of priority from Japanese Patent
Application Nos. 2001-158691 filed May 28, 2001; 2001-158766 filed
May 28, 2001. The disclosures of the prior applications are hereby
incorporated herein in their entirety by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a power transmission shaft
to be used as, for example, a drive shaft or a propeller shaft,
which constitutes a part of a power transmission system in an
automobile.
[0004] 2. Description of the Related Art
[0005] In general, there are several kinds of transmission shafts
that constitute a power transmission system of an automobile. The
shafts include a drive shaft for connecting between an engine and a
wheel-bearing device, a propeller shaft for transmitting power from
a transmission to reduction gears, and so on. Each of these shafts
has a coupling member such as a spline on the shaft-end. The power
transmission shafts may be broadly classified in the group of solid
shafts made of solid bars and the other group of hollow shafts made
of steel pipes or the like, according to their basic
structures.
[0006] Conventionally, solid shafts have been used as power
transmission shafts for automobiles. In recent years, for
responding to the needs for higher function of automobiles, the
sound insulating properties of a cabin to keep quiet, and the like,
there are increasing demands of providing a power transmission
shaft with various kinds of characteristic features, such as light
weight, compactness, and comfortability against NVH (noise,
vibration, and harshness), in addition to strength and durability.
In addition, there is also required to improve the torsional
rigidity of shafts for increasing the controllability and direct
feeling of automobile at the time of start. In this case, for
improving the torsional rigidity, there is an idea of increasing
the diameter of the shaft. However, it will effect an increase in
costs because of increasing the weight and the cutting amount of a
coupling portion. In addition to the above demands, there is a need
for adjusting the natural frequency of automobile for avoiding the
noise produced by a resonance between vibrations of an engine and a
shaft while the automobile runs. For adjusting the natural
frequency, there is an idea of attaching a dumper or the like on
the power transmission shaft. However, it will lead to an increase
in costs because of increasing the number of structural components
and the number of assembling steps in the manufacturing
process.
[0007] As a consequence of considering the above demands in terms
of functions, there is an increasing tendency to make greater use
of hollow shafts instead of the solid shafts. The hollow shafts can
be broadly divided into integral-type and joined-type. The
integral-type hollow shaft comprises a middle pipe part having the
largest outer diameter and shaft parts integrally formed on the
opposite ends of the pipe part. The shafts parts are made of the
same material as that of the middle pipe part and a coupling
portion such as a spline is formed on the outer periphery of each
shaft-end. On the other hand, the joined-type hollow shaft
comprises a pipe part and shaft parts. These parts are shaped
separately and are then joined together using friction pressure
welding, electric welding, or the like.
[0008] Comparing with the solid shaft, the integral-type or
joined-type hollow shaft has a reduced section modulus, while the
maximum shear-stress thereof operative to the hollow shaft is
large. Therefore, there is a possibility of a decrease in the shear
strength of the hollow shaft.
[0009] In some cases, an electro-resistance-welded tube having a
wall thickness with an extremely high accuracy and an extremely
stable strength is used as a power transmission hollow shaft. The
welded tube is comprised of two or more pipe parts. The pipe parts
are made of a steel material having a good dimensional accuracy and
a good finishing accuracy and are butt-joined in a straight line
using electric resistance welding. Therefore, the welded portion of
an electro-unite part of the welded pipe, which extends in the
axial direction, tends to be broken, leading to a decrease in the
strength of the power transmission shaft.
[0010] In addition, the integral-type hollow shaft for power
transmission is typically formed by, for example, a swaging in
which the diameter of an element tube is reduced by stamping in the
radial direction thereof at high speed, while rotating the tube
around the axis; or a press working in which the diameter of an
element tube is reduced by inserting the element tube into a die.
The hollow shaft formed by such a plastic working of the swaging or
the like may have a plastic flow of the raw material into the inner
radial at the time of reducing the diameter of the element tube.
Thus, there is a tendency in which the inner radial surface of the
hollow shaft become wrinkled. Such a wrinkle may become the origin
of breakage, causing a decrease in the strength of the power
transmission shaft.
SUMMARY OF THE INVENTION
[0011] An object of the present invention is to provide a power
transmission shaft allowing an improvement in the strength and
allowing a stable torsion fatigue strength.
[0012] As technical means for attaining the above object, a first
aspect of the present invention is to provide a power transmission
shaft comprising coupling members respectively provided on opposite
ends of a pipe part made of a steel material, wherein the steel
material includes 0.30-0.45% by weight of carbon (C), 0.05-0.35% by
weight of silicon (Si), 1.0-2.0% by weight of manganese (Mn), 0.05%
by weight or less of aluminum (Al), 0.01% by weight or less of
sulfur (S), and the remainder, iron (Fe) and unavoidable
impurities, and the pipe part has an electro-unite portion that
extends in an axial direction, the electro-unite portion and
neighborhood thereof being hardened by a hardening treatment so as
to have a Rockwell hardness HRC of 45 or more. Here, the hardening
treatment may be preferably a high-frequency induction hardening
and tempering treatment. Here, the term "neighborhood of the
electro-unite portion" means that a portion within 5 mm far from
the middle to the opposite ends in the circumferential direction of
the electro-unite portion.
[0013] In this embodiment, a steel material in which the amount of
each of the above components (C, Si, Mn, Al, and S) is defined in
the above range is used and its electro-unite portion and
neighborhood thereof are hardened so that the Rockwell hardness HRC
thereof can be 45 or over. Therefore, the hardness of the pipe to
be required as a power transmission shaft can be satisfied. Such a
hardness makes sure of a stable torsion fatigue strength, providing
a power transmission shaft of an elongated useful life and a high
reliability. In addition, an electric-resistance welded tube is
used as a steel pipe to be used as the subject pipe for stably
ensuring the shaft strength. Thus, the power transmission shaft can
be hardly broken at an electro-unite portion thereof, preventing a
decrease in the strength of the pipe.
[0014] A second aspect of the present invention is to provide a
power transmission shaft with coupling members integrally formed on
opposite ends thereof, which is formed from a steel element tube by
a plastic working, comprising an inner diametrical surface which is
subjected to a hardening treatment. Preferably, the hardening
treatment may be a high-frequency induction hardening and tempering
treatment. The hardening treatment on the inner diametrical surface
can be performed by arranging a coil for high-frequency induction
heating on the inner diametrical side of the power transmission
shaft. Alternatively, the hardening treatment from the outer
diametrical surface can be performed by arranging such a coil for
high-frequency induction heating on the outer diametrical side of
the power transmission shaft. In the hardening treatment with
high-frequency induction hardening and tempering, the
surface-portion hardness of the inner diametrical surface is a
Rockwell hardness HRC of 35 or more. Here, the term "surface
portion" means that, for example, about one fourth of the wall
thickness of the power transmission shaft.
[0015] According to the present invention, as described above, the
inner diametrical surface is subjected to the hardening treatment,
so that it becomes possible to ensure a hardness to be required for
the power transmission shaft. In addition, such a resulting
hardness allows to prevent the generation of wrinkle on the inner
diametrical surface by a plastic working to be effected as an
origin of breakage. As a result, the power transmission shaft that
ensures a stable torsion fatigue strength and having a high
reliability and a long useful life can be obtained.
[0016] Furthermore, by applying a predetermined residual
compression stress on the outer diametrical surface of the power
transmission shaft, the residual compression stress increases. As a
result, it becomes possible to further increase the torsion fatigue
strength. Such a residual compression stress can be easily applied
by a shot peening treatment. In addition, the residual compression
stress may be preferably 750 MPa or more.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] In the accompanying drawings:
[0018] FIG. 1 is a partially cross-sectional front view of a power
transmission shaft as one of preferred embodiments of the present
invention, which is provided as an integral-type hollow shaft;
[0019] FIG. 2 is a partially cross-sectional front view of a power
transmission shaft as another preferred embodiment of the present
invention, which is provide as a joined-type hollow shaft;
[0020] FIG. 3 is a radial cross-sectional view of an electro-unite
portion of a pipe (i.e., an electric-resistance welded tube);
[0021] FIG. 4 is a table of the results of an examination of
torsion fatigue strength;
[0022] FIG. 5 is a table of the results of an examination of
torsion fatigue strength; and
[0023] FIG. 6 is a table of the results of an examination of
torsion fatigue strength in the presence or absence of shot
peening.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] FIG. 1 shows a power transmission shaft as one of preferred
embodiments of the present invention. The power transmission shaft
1 is provided as an integral-type hollow shaft. That is, the power
transmission shaft 1 comprises: a middle pipe part 1a having a
largest diameter portion, compared with the others; and axial parts
1b provided on the opposite ends of the middle pipe part 1a in
which a coupling portion such as a spline is formed on the outer
periphery of each end portion of the axial parts 1b. These parts
1a, 1b are integrally shaped from the same element tube.
[0025] Referring now to FIG. 2, there is shown a power transmission
shaft as an alternative preferred embodiment of the present
invention. In this embodiment, the power transmission shaft 2 may
be provided as a joined-type hollow shaft. That is, this shaft is
fabricated by joining a pipe part 2a and axial parts 2b by welding
such as friction pressure welding, or the like. In this case, it is
noted that these parts 2a, 2b are separately formed. FIG. 3 is a
cross sectional view for illustrating the pipe part 1a of the power
transmission shaft 1 shown in FIG. 1 or the pipe part 2a of the
power transmission shaft 2 shown in FIG. 2. For simplified
description because of commonly observed structural features in
FIG. 1 and FIG. 2, we will describe the structure shown in FIG. 3
in accordance with the embodiment shown in FIG. 1.
[0026] In this case, the power transmission shaft 1 is prepared
from an electric-resistance welded tube having a wall thickness
with an excellent accuracy. In this electric-resistance welded
tube, each of pipes is prepared by shaping a plate having an
excellent dimensional accuracy and finishing accuracy into a pipe,
and the thus-obtained pipes are butt-joined in a straight line
using electric resistance welding. Thus, the power transmission
shaft 1 has an electro-unite portion 3 as welded portions formed in
the axial direction (see FIG. 3).
[0027] The power transmission shaft 1 is made of a steel material,
which includes 0.30-0.45% by weight of carbon (C), 0.05-0.35% by
weight of silicon (Si), 1.0-2.0% by weight of manganese (Mn), 0.05%
by weight or less of aluminum (Al), 0.01% by weight or less of
sulfur (S), and the remainder, iron (Fe) and unavoidable
impurities, and the electro-unite portion 3 and neighborhood
thereof are hardened so as to have a Rockwell hardness HRC of 45 or
more.
[0028] Such a hardening treatment can be realized by performing a
high-frequency induction hardening and tempering treatment on the
power transmission shaft 1. Hatching portions shown in FIGS. 1 and
2 are regions subjected to the high-frequency induction hardening
and tempering, indicating a bake-out state. The power transmission
shaft 1 is fabricated using a steel material comprising the above
components, so that the hardening treatment can be allowed to
provide the electro-unite portion 3 and neighborhood thereof with a
Rockwell hardness HRC of 45 or more. Consequently, the strength of
the pipe itself increases, while making sure of a stable torsion
fatigue strength.
[0029] Here, in general, it is known in the art that a torsion
fatigue strength of the power transmission shaft 1 is substantially
depended on the hardness, and also the hardness is depended on the
composition of the steel material. That is, there is the need to
adjust the amount of each component because an element that defines
the hardness after quench hardening is carbon (C), while other
elements (e.g., Si and Mn) may be effective to define the hardness
after quench hardening in the depth direction.
[0030] Carbon (C) is an essential element for obtaining a desired
torsion fatigue strength of the power transmission shaft 1. For
obtaining the predetermined hardness after the application of heat,
the amount of carbon (C) should be 0.3% by weight or more. Further,
if it is more than 0.45% by weight, the hardness of the steel
material becomes excess. Therefore, the machinability of the steel
material becomes decreased. Thus, the upper limit for the amount of
carbon to be contained is 0.45% by weight. A small amount silica
(Si) is required as a deoxidizer for the steel material in addition
to be required to ensure the effect of induction hardening on the
steel material. If the amount of Si is less than 0.05% by weight,
such an effect is not sufficient. If it is more than 0.35% by
weight, the machinability of the steel material becomes
substantially decreased. Thus, the upper limit for the amount of Si
to be contained is 0.35% by weight. The addition amount of
manganese (Mn) should be 1.0% by weight or more to ensure the
effect of induction hardening on the steel material. If 2.0% by
weight or more of Mn is added in the steel material, then the
machinability of the steel material becomes substantially
decreased. Thus, the upper limit for the amount of Mn to be
contained is 2.0% by weight. Aluminum (Al) is added as a deoxidizer
of the steel material. It is preferable that the content of Al be
minimized because of the cleanness of the steel material. Thus, the
amount of Al may be 0.05% weight or less. Furthermore, sulfur (S)
depresses the deformability of the steel material at the time of
cold working. If the amount of S is more than 0.01% by weight, such
a decrease in the deformability becomes excess. Thus, the amount of
S may be 0.01% by weight or less.
[0031] For complementing the induction-hardening acceptability of
the steel material, the steel material may include 0.1-0.35% by
weight of chromium (Cr) and 0.0005-0.005% by weight of boron (B).
Alternatively, at least one of Cr and B may be included. If the
content of Cr is less than 0.1% by weight, the effect of
complementing the induction-hardening acceptability of the steel
material becomes decreased. On the other hand, more than 0.35% by
weight of Cr leads to an increase in cost of the steel material. In
addition, if the content of B is less than 0.0005% by weight, the
effect of complementing the induction-hardening acceptability of
the steel material becomes decreased. On the other hand, more than
0.005% by weight of B does not influence on the effects of the
induction hardening on the steel material.
[0032] After the high-frequency induction hardening and tempering
treatment, a shot peening treatment may be performed on the whole
circumference of the power transmission shaft 1. A torsion fatigue
strength of the shaft 1 can be further increased by increasing a
residual compression stress on the surface of the power
transmission shaft 1. Here, the term "shot peening treatment" means
that the stress on the surface of a target metal is uniformed by
throwing small steel particles with great force exerted by
compressed air or centrifugal force onto the surface of the
metal.
[0033] The inventors of the present invention conducted the
evaluations of torsion fatigue strength with respect to eight power
transmission shafts (sample Nos. 1 to 8), where each of the samples
have its own contents of C, Si, Mn, S, Al, Cr, and B in its
composition, and its own Rockwell hardness HRC at and around the
electro-unite portion. The results of the evaluations are listed in
the table shown in FIG. 4.
[0034] For any portion except the electro-unite portion, the
hardening and tempering treatment was performed such that the
surface hardness of the outer diameter portion would have a
hardness distribution of 50 or more in Rockwell hardness HRC. The
hardness of neighborhood of the electro-unite portion is a result
of converting a Vickers hardness measured at a location of 2 mm
from the inner diametrical side to a Rockwell hardness. In this
experiment, in a state in which both ends of the power transmission
shaft 1 was being supported, one end of the power transmission
shaft 1 was fixed, while a load torque was applied on the other
end. For interpretation of results, the lower limit of the strength
of the solid shaft having the same axial part diameter was used as
a standard on which a judgment could be based. The power
transmission shaft that endured 400,000 times or more of the
repeated torque application was accepted.
[0035] As shown in the results listed in the table of FIG. 4, each
of the samples (sample Nos. 4 to 8) that came up to the standard
with respect to torsion fatigue strength was a power transmission
shaft 1 constructed of a steel material including 0.30-0.45% by
weight of carbon (C), 0.05-0.35% by weight of silicon (Si),
1.0-2.0% by weight of manganese (Mn), 0.05% by weight or less of
aluminum (Al), 0.01% by weight or less of sulfur (S), and the
electro-unite portion and neighborhood thereof are hardened so as
to have a Rockwell hardness HRC of 45 or more.
[0036] As a power transmission shaft 1, other kinds of tubes except
the electric-resistance welded tube can be used, for example as
follows.
[0037] The power transmission shaft 1 is typically formed by a
plastic working, for example, a swaging in which the diameter of an
element tube is reduced by stamping in the radial direction thereof
at high speed, while rotating the tube around the axis, or the
like. The power transmission shaft 1 formed by such a plastic
working of the swaging or the like may have a plastic flow of the
raw material into the inner diametrical side at the time of
reducing the diameter of the element tube. Thus, there is a
tendency in which the inner diametrical surface of the hollow shaft
become wrinkled.
[0038] According to the present embodiment, therefore, the inner
diametrical surface 1c of the power transmission shaft 1 is
hardened by the high-frequency induction hardening and tempering
treatment. In other words, the hardening treatment is performed on
the inner diametrical surface 1c of the power transmission shaft 1
by placing a coil for high frequency induction heating on the outer
diametrical side of the power transmission shaft 1. The hardening
treatment can be performed from the outer diametrical surface 1d of
the power transmission shaft 1 to the inner diametrical surface 1c
by the high-frequency induction hardening and tempering treatment
through the entire wall thickness (a hatching portion shown in FIG.
1 is a region subjected to the high-frequency induction hardening
and tempering, indicating a bake-out state). Therefore, the
hardness of the surface portion of the inner diametrical surface 1c
of the power transmission shaft 1 is brought to a Rockwell hardness
HRC of 35 or more. Here, the term "surface portion" means, for
example, a portion corresponding to almost one fourth of the wall
thickness of the power transmission shaft 1.
[0039] As described above, the inner diametrical surface 1c of the
power transmission shaft 1 is hardened using the high-frequency
induction hardening and tempering treatment and the hardness of the
surface portion of the inner diametrical surface 1c is brought to a
Rockwell hardness HRC of 35 or more. Therefore, at the time of
manufacturing the power transmission shaft 1 by plastic working,
the wrinkle caused on the inner diametrical surface 1c at the time
of manufacturing the power transmission shaft by plastic working is
hardly brought into the origin of breakage, causing an increase in
the strength of the power transmission shaft itself in addition to
ensure a stable torsion fatigue strength.
[0040] In this embodiment, by the way, a coil for high frequency
induction heating is arranged on the outer diametrical side of the
power transmission shaft 1 to allow the hardening treatment from
the outer diametrical side of the power transmission shaft 1.
According to the present invention, however, it is not limited to
such an arrangement. The coil for high frequency induction heating
may be arranged on the inner diametrical side of the power
transmission shaft 1 to allow the hardening treatment from the
inner diametrical side of the power transmission shaft 1.
[0041] In addition, if a predetermined residual compression stress
is applied on the outer diametrical surface 1d of the power
transmission shaft 1, it becomes possible to further increase the
torsion fatigue strength of the power transmission shaft 1 by means
of an increase in the residual compression stress. A residual
compression stress may be applied by two-stage shot peening
treatment and may be then reached to 750 MPa or more.
[0042] In the first stage of the shot peening treatment, a high
residual compression stress is applied to exert an influence upon
the surface of the metal, deeply. For this purpose, such a shot
peening treatment should be performed under the conditions that
each of particles to be shot has a hardness HV of 750 or more, and
a particle size of 0.5-1.0 mm, and is shot at a speed of 60
m/second or more. If the particle size of the shot particle is more
than 1 mm, the surface of the power transmission shaft 1 becomes
rough and a fatigue strength thereof becomes decreased.
[0043] In the second stage of the shot peening treatment, on the
other hand, shot particles smaller than those of the first stage
are used to increase the residual compression stress and maximum
surface hardness of the surface of the power transmission shaft 1
to improve the surface roughness. For this purpose, therefore, each
of the particles has a hardness HV of 750 or more and a diameter of
0.1-0.5 mm, which is smaller than that of the first stage. Thus,
the shot peening treatment using smaller particles allows a
residual compression stress of 750 MPa or more on the surface of
the power transmission shaft 1. The reason of defining such a
stress to 750 MPa or more is that the residual compression stress
of the power transmission shaft on which a residual compression
stress is not applied by the shot peening treatment, or the like is
750 MPa or less. In other words, the application of 750 MPa or more
residual compression stress allows a further increase in the
torsion fatigue strength.
[0044] The present inventors evaluated a torsion fatigue strength
of each of nine power transmission shafts (samples). The power
transmission shafts have the same inner and outer diameters, and
they were subjected to the high-frequency induction hardening and
tempering treatment under different conditions so that each of them
has a surface portion hardness (Rockwell hardness HRC) of the inner
diametrical surface different from one another, while a Rockwell
hardness HRC of the surface portion hardness of the outer
diametrical surface was 50 or more. The results of the evaluations
are listed in the table shown in FIG. 5.
[0045] The hardness of the surface portion of the inner diametrical
surface 1c is a result of measuring the hardness of a portion at
almost 0.5 mm from the inner diametrical surface by a Vickers
hardness measuring device and converting the measured hardness into
a Rockwell hardness. In this test, in a state in which both ends of
the power transmission shaft 1 was being supported, one end of the
power transmission shaft 1 was fixed, while a load torque (.+-.1.0
kN.quadrature.m and .+-.1.2 kN.quadrature.m) was applied on the
other end thereof. In the table, the breakage origin of "out"
indicates the outer diametrical side origin, while "in" indicates
the inner diametrical side origin. For interpretation of results,
the lower limit of the strength of the solid shaft having the same
axial part diameter was used as a standard on which a judgment
could be based. When the load torque was .+-.1.0 kN.quadrature.m,
the power transmission shaft that endured 400,000 times or more of
the repeated torque application was accepted. When the load torque
was .+-.1.2 kN.quadrature.m, the power transmission shaft that
endured 100,000 times or more of the repeated torque application
was accepted.
[0046] As is evident from the results shown in FIG. 5, in the case
of the power transmission shaft 1 having the inner diametrical
surface 1c with the surface portion hardness of 35 or more in
Rockwell hardness HRC, a wrinkle generated on the inner diametrical
surface 1c does not become an origin of breakage, so that such a
shaft 1 is accepted with respect to the torsion fatigue
strength.
[0047] Furthermore, the present inventors evaluated a torsion
fatigue strength of each of the power transmission shafts 1
(samples) having the same inner and outer diameters. They were
subjected to the high-frequency induction hardening and tempering
treatments under the same conditions in the presence or absence of
a shot peening treatment, respectively. The results of the test are
listed in the table shown in FIG. 6. Regarding the residual
compression stress of the outer diametrical surface of the power
transmission shaft 1, any sample was extracted and was then
subjected to the measurement for one with or without the shot
peening treatment. In this test, in a state in which both ends of
the power transmission shaft 1 was being supported, one end of the
power transmission shaft 1 was fixed, while a load torque (0-1.3
kN.quadrature.m) was applied on the other end thereof.
[0048] As is evident from the evaluation results shown in FIG. 6,
the power transmission shaft 1 subjected to the shot peening
treatment is advantageous in an increase in the torsion fatigue
strength, compared with one without being subjected to the shot
peening treatment. In addition, the power transmission shaft 1
without being subjected to the shot peening treatment has the
surface with a residual compression stress of 750 MPa at maximum.
Therefore, it is preferable to provide the surface of the power
transmission shaft 1 with a residual compression stress of 750 MPa
or more by subjecting to the shot peening treatment.
* * * * *