U.S. patent application number 15/953868 was filed with the patent office on 2018-08-16 for method for manufacturing endless metal belt, endless metal belt, and belt-type continuously variable transmission.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. The applicant listed for this patent is TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Koji NISHIDA.
Application Number | 20180231102 15/953868 |
Document ID | / |
Family ID | 50182755 |
Filed Date | 2018-08-16 |
United States Patent
Application |
20180231102 |
Kind Code |
A1 |
NISHIDA; Koji |
August 16, 2018 |
METHOD FOR MANUFACTURING ENDLESS METAL BELT, ENDLESS METAL BELT,
AND BELT-TYPE CONTINUOUSLY VARIABLE TRANSMISSION
Abstract
A method for manufacturing an endless metal belt used in a
belt-type continuously variable transmission, wherein a
stress-relief heat treatment is performed after the circumference
of a ring body has been adjusted, and aging/nitridation is
performed after the stress-relief heat treatment.
Inventors: |
NISHIDA; Koji; (Nisshin-shi,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
TOYOTA JIDOSHA KABUSHIKI KAISHA |
Aichi-ken |
|
JP |
|
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Aichi-ken
JP
|
Family ID: |
50182755 |
Appl. No.: |
15/953868 |
Filed: |
April 16, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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14418104 |
Jan 29, 2015 |
9982749 |
|
|
PCT/JP2012/072142 |
Aug 31, 2012 |
|
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15953868 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 1/30 20130101; F16G
5/16 20130101; C23C 8/26 20130101; C21D 9/0068 20130101; B21D 53/14
20130101; C23C 8/02 20130101; C23C 8/24 20130101 |
International
Class: |
F16G 5/16 20060101
F16G005/16; C23C 8/26 20060101 C23C008/26; C23C 8/24 20060101
C23C008/24; B21D 53/14 20060101 B21D053/14; C21D 9/00 20060101
C21D009/00; C21D 1/30 20060101 C21D001/30; C23C 8/02 20060101
C23C008/02 |
Claims
1. An endless metal belt to be used in a continuously variable
transmission, the endless metal belt being manufactured by an
endless metal belt manufacturing method including: performing a
stress-relief heat treatment of a ring body after the ring body is
subjected to a circumferential length adjusting work, and then,
without performing another circumferential length adjusting work,
performing an aging-nitriding treatment of the ring body after the
stress-relief heat treatment.
2. The endless metal belt according to claim 1, wherein the
circumferential length adjusting work is performed after the ring
body is subjected to rolling work.
3. The endless metal belt according to claim 1, wherein the
stress-relief heat treatment is performed on a plurality of ring
bodies in a lamination state, each of the ring bodies having been
subjected to the circumferential length adjusting work.
4. The endless metal belt according to claim 1, wherein the ring
body has residual stress almost equally accumulated on an outer
peripheral side and an inner peripheral side of the ring body.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This is a divisional of U.S. application Ser. No. 14/418,104
filed Jan. 29, 2015 (allowed), which is a national phase
application based on the PCT International Patent Application No.
PCT/JP2012/072142 filed on Aug. 31, 2012, the entire contents of
which are incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to a method for manufacturing
an endless metal belt forming a power transmission belt to be used
in a belt-type continuously variable transmission mounted in a
vehicle, the endless metal belt, and the belt-type continuously
variable transmission.
BACKGROUND ART
[0003] For instance, an endless metal belt to be used in a
belt-type continuously variable transmission mounted in a vehicle
and used to transmit drive power is composed of a plurality of
endless metal rings (e.g., nine rings) stacked or laminated in
close contact with each other, the endless metal rings each having
a circular-arc cross section and different circumferential lengths.
In general, as a material of an endless metal ring, there is so far
known maraging steel having superior strength characteristics.
Further, an endless metal ring is made of a steel-strip annular
member that has been subjected to rolling and thereafter to a
solution treatment, a circumferential length adjusting
(calibration), an aging-nitriding treatment in this order to
enhance fatigue strength (for example, see Patent Document 1).
RELATED ART DOCUMENTS
Patent Documents
[0004] Patent Document 1: Japanese Unexamined Patent Application
Publication (Translation of PCT Application) No. 2008-520437
SUMMARY OF INVENTION
Problems to be Solved by the Invention
[0005] However, the circumferential length adjusting (calibration)
is an expanding work to slightly expand a ring body by looping the
ring body over two rollers. After this work, the tensile residual
stress and the compressive residual stress are accumulated in the
ring body. When the circumferential length adjusting (calibration)
is performed before the aging-nitriding treatment as in the
technique of Patent Document 1, those residual stresses are not
released in a later aging-nitriding treatment and thus different
residual stresses are distributed between the outer peripheral side
and the inner peripheral side of the ring body.
[0006] FIG. 15 is a graph showing residual stress distribution in a
conventional ring body. This graph shows measurement results of
residual stress in an endless metal ring manufactured by undergoing
the steps in the conventional order by taking cross section of each
ring body before the nitriding treatment and after the nitriding
treatment by use of an X-ray stress measurement device to measure
the residual stress from an outer peripheral surface to an inner
peripheral surface. A horizontal axis represents distance (.mu.m)
from the outer peripheral surface in the ring body and a vertical
axis represents residual stress (MPa). In the vertical axis, a
minus side indicates compressive residual stress and a plus side
indicates a tensile residual stress. From the above graph, it is
found that before the nitriding treatment the compressive residual
stress F0 is accumulated on the outer peripheral side of the ring
body and a tensile residual stress G0 is accumulated on the inner
peripheral side. The compressive residual stress F0 on the outer
peripheral side gradually decreases from the outer peripheral
surface toward the vicinity of the center and becomes zero near the
center and in turn the tensile residual stress G0 on the inner
peripheral side gradually increases toward the inner peripheral
surface.
[0007] The above graph shows that, after the nitriding treatment
whereby a nitrided layer has been formed with a predetermined depth
(about 30 .mu.m) from each of the outer peripheral surface and the
inner peripheral surface of the ring body, the compressive residual
stresses E0 and D0 are imparted on or close to the surfaces on the
outer peripheral side and the inner peripheral side. However, the
temperature of the nitriding treatment is on the order of 400 to
500.degree. C. which is not so high as to melt alloy constituents
into solid solution. Thus, the compressive residual stress and the
tensile residual stress caused by the circumferential length
adjusting (calibration) remain as-is left (corresponding to A0).
Accordingly, as compared with a region (C0) where the nitrided
layer on the outer peripheral side is absent, or ends, the tensile
residual stress G0 previously accumulated by the circumferential
length adjusting (calibration) on the inner peripheral side is much
left near a region (B0) where the nitrided layer on the inner
peripheral side is absent, or ends. Thus, a portion (corresponding
to B0) in which the tensile residual stress is left more than in
other portions is likely to become a weakest portion apt to cause
fatigue fracture when the ring body is used in a continuously
variable transmission.
[0008] To reduce the tensile residual stress mentioned above, there
is known a method of subjecting the surface of the endless metal
ring to shot-peening work. This method further needs an additional
stress-relief treatment step, leading to an undesirable cost
increase.
[0009] The present invention has been made to solve the above
problems and has a purpose to provide an endless metal belt
manufacturing method, an endless metal belt, and a belt-type
continuously variable transmission, capable of reducing tensile
residual stress accumulated on an inner peripheral side by
circumferential length adjusting to thereby enhance fatigue
strength without adding a new stress-relief treatment step.
Means of Solving the Problems
[0010] (1) To achieve the above purpose, one aspect of the
invention provides a method for manufacturing an endless metal belt
to be used in a belt-type continuously variable transmission, the
method including: performing a stress-relief heat treatment of a
ring body after the ring body is subjected to a circumferential
length adjusting work, and performing an aging-nitriding treatment
of the ring body after the stress-relief heat treatment.
[0011] According to the above aspect, the ring body is subjected to
the stress-relief heat treatment after the circumferential length
adjusting work, and then to the aging-nitriding treatment after the
stress-relief heat treatment. Accordingly, the ring body having
therein little residual stress (the tensile residual stress and the
compressive residual stress) generated due to the circumferential
length adjusting work can be subjected to the aging-nitriding
treatment. Thus, the nitrided layer is formed with a predetermined
depth on each of the outer peripheral side and the inner peripheral
side of the ring body in which residual stress (the tensile
residual stress and the compressive residual stress) is hardly
left. The thus formed nitrided layers allow compressive residual
stress to be imparted nearly uniformly on the outer peripheral side
and the inner peripheral side of the ring body. In other words, the
magnitude of the compressive residual stress on the outer
peripheral surface of the ring body and the magnitude of the
compressive residual stress on the inner peripheral surface of the
ring body become almost equal. Further, the magnitude of the
tensile residual stress in a region on the outer peripheral side of
the ring body where the nitrided layer is absent and the tensile
residual stress in a region on the inner peripheral side of the
ring body where the nitrided layer is absent also become almost
equal. Thus, in the region on the inner peripheral side of the ring
body where the nitrided layer ends, there is formed no weakest
portion having the tensile residual stress left more than in other
portions. Since the ring body in which the tensile residual stress
and the compressive residual stress are hardly left is subjected to
the aging-nitriding treatment, the tensile residual stress
accumulated by the aging-nitriding treatment can be controlled to
be nearly uniform and minimum in the wall thickness direction of
the ring body.
[0012] (2) In the method for manufacturing an endless metal belt
described in (1), preferably, the circumferential length adjusting
work is performed after the ring body is subjected to rolling
work.
[0013] According to the above aspect, the ring body is subjected to
rolling work and then successively to the circumferential length
adjusting work. Thus, the residual stress (the compressive residual
stress and the tensile residual stress) by the rolling work and the
residual stress (the compressive residual stress and the tensile
residual stress) by the circumferential length adjusting work can
be simultaneously released by one stress-relief heat treatment. In
other words, various residual stresses accumulated in the ring body
until the end of the circumferential length adjusting work can be
simultaneously made almost zero in the single stress-relief heat
treatment. Therefore, there is no need to add another stress-relief
heat treatment. The stress-relief heat treatment is different
according to materials of the ring body. For instance, a material
such as maraging steel or precipitation hardening stainless steel
will undergo a solution treatment, or solutionizing. A material
such as austenite stainless steel will undergo stress-relief
annealing. A material such as carbon steel (quench-hardened steel)
will undergo a quenching treatment or both of a quenching treatment
and a tempering treatment.
[0014] (3) In the method for manufacturing an endless metal belt
described in (1) or (2), preferably, the stress-relief heat
treatment is performed on a plurality of ring bodies in a
lamination state, each of the ring bodies having been subjected to
the circumferential length adjusting work.
[0015] According to the above aspect, the stress-relief heat
treatment is performed on a laminated body of the plurality of ring
bodies each having been subjected to the circumferential length
adjusting. The entire laminated ring bodies can provide enhanced
rigidity, thereby enabling reducing the occurrence of undulation or
deformation of an edge face of each ring body during stress-relief
heat treatment. Therefore, when used in the belt continuously
variable transmission, the ring bodies can reduce the interference
between an edge face of the endless metal belt in its width
direction and neck portions of elements or the interference between
an edge face the endless metal belt in the width direction and wall
surfaces of a V-shaped groove of a pulley. Further, since the ring
bodies are laminated and then subjected to the stress-relief heat
treatment, a heat treatment furnace can be greatly reduced in size
and thus reduce facility cost and energy cost.
[0016] (4) To achieve the above purpose, another aspect of the
invention provides an endless metal belt manufactured by the method
for manufacturing an endless metal belt as described in any one of
(1) to (3).
[0017] According to the above aspect, it is possible to provide an
endless metal belt with high fatigue strength at low cost by
reducing tensile residual stress on the inner peripheral side
accumulated by the circumferential length adjusting.
[0018] (5) In the endless metal belt described in (4), preferably,
the ring body has residual stress almost equally accumulated on an
outer peripheral side and an inner peripheral side of the ring
body.
[0019] According to the above aspect, the ring body or bodies have
residual stress almost equally accumulated on the outer peripheral
side and the inner peripheral side. Thus, stress load in a region
where a nitrided layer on the inner peripheral side corresponding
to a conventional weakest portion is absent can be reduced, leading
to improved fatigue life. To be concrete, it is more preferable
that the magnitude of the compressive residual stress on the outer
peripheral surface of the ring body and the magnitude of the
compressive residual stress on the inner peripheral surface of the
ring body become nearly equal, and, the magnitude of the tensile
residual stress in a region on the outer peripheral side of the
ring body where a nitrided layer is absent and the magnitude of the
tensile residual stress in the region on the outer peripheral side
of the ring body where the nitrided layer is absent become nearly
equal. This configuration can made the tensile residual stress
accumulated in the ring body almost uniform and minimum in the wall
thickness direction of the ring body and can accumulate the
compressive residual stress with good balance on the outer
peripheral side and the inner peripheral side of the ring body to
further improve the fatigue life.
[0020] (6) To achieve the above purpose, still another aspect of
the invention provides a belt-type continuously variable
transmission including the endless metal belt as described in (4)
or (5).
[0021] According to this aspect, it is possible to reduce the
stress amplitude on the endless metal belt during use and thus
provide the continuously variable transmission with long fatigue
life at low cost.
Effects of the Invention
[0022] According to the invention, it is possible to provide a
method for manufacturing an endless metal belt capable of reducing
tensile residual stress on an inner peripheral side accumulated by
circumferential length adjusting without adding another
stress-relief treatment step to improve fatigue strength, the
endless metal belt, and a belt-type continuously variable
transmission.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a schematic vertical sectional view of a belt-type
continuously variable transmission according to the invention;
[0024] FIG. 2 is a perspective view showing a part of an endless
metal belt in a circumferential direction to be used in the
belt-type continuously variable transmission shown in FIG. 1;
[0025] FIG. 3 is a perspective view showing a part of an endless
metal ring forming the endless metal belt shown in FIG. 2;
[0026] FIG. 4 is a diagram of a first manufacturing process for the
endless metal belt according to the invention;
[0027] FIG. 5 is a diagram of a second manufacturing process for
the endless metal belt according to the invention;
[0028] FIG. 6 is a process chart showing manufacturing processes
for endless metal belts for comparison;
[0029] FIG. 7 is a perspective view of ring bodies subjected to a
stress-relief heat treatment in the first manufacturing
process;
[0030] FIG. 8 is a perspective view of ring bodies subjected to a
stress-relief heat treatment in the second manufacturing
process;
[0031] FIG. 9 is a perspective view of a carrying jig for conveying
laminated ring bodies during an aging-nitriding treatment in the
second manufacturing process;
[0032] FIG. 10 is a top view of the laminated ring bodies shown in
FIG. 9;
[0033] FIG. 11 is a graph showing residual stress distribution in a
ring body according to the invention;
[0034] FIG. 12 is a graph showing differences in residual stress
distribution in ring bodies between a conventional art and the
present invention;
[0035] FIG. 13 is a graph showing a warped amount of an edge face
of the ring body after the stress-relief heat treatment;
[0036] FIG. 14 is a graph showing fatigue life of an endless metal
belt; and
[0037] FIG. 15 is a graph showing residual stress distribution in a
conventional ring body.
MODE FOR CARRYING OUT THE INVENTION
[0038] A detailed description of a preferred embodiment of a method
for manufacturing an endless metal belt, the endless metal belt,
and a belt-type continuously variable transmission embodying the
present invention will now be given referring to the accompanying
drawings.
[0039] <Structure of a Belt-Type Continuously Variable
Transmission and an Endless Metal Belt>
[0040] The structure of the belt-type continuously variable
transmission and the endless metal belt to be used therein will be
first explained. FIG. 1 is a schematic vertical sectional view of
the belt-type continuously variable transmission according to the
invention. FIG. 2 is a perspective view showing a part of the
endless metal belt in a circumferential direction to be used in the
belt-type continuously variable transmission shown in FIG. 1. FIG.
3 is a perspective view showing a part of an endless metal ring
forming the endless metal belt shown in FIG. 2.
[0041] As shown in FIG. 1, a belt-type continuously variable
transmission 100 includes an input shaft J1 coupled to a drive
source, an output shaft J2 coupled to wheels, a drive pulley P1 and
a driven pulley P2 which are provided integral with the
corresponding input shaft J1 and output shaft J2 and are each
formed with a V-shaped groove changeable in width, and a power
transmission belt 10 wound in the V-shaped grooves of the drive
pulley P1 and the driven pulley P2 and configured to transmit drive
power. This continuously variable transmission 100 is configured to
change the width of each V-shaped groove of the drive pulley P1 and
the driven pulley P2, thereby changing the winding diameter of the
power transmission belt 10 to continuously change a relative
rotation speed ratio (V/W) of the input shaft J1 and the output
shaft J2.
[0042] As shown in FIG. 2, the power transmission belt 10 includes
two sets of endless metal belts 13 each of which is composed of a
plurality of endless metal rings 12 having different
circumferential lengths and being laminated in close contact in a
wall thickness direction, and a plurality of metal elements 11
having plate-like bodies arranged annularly in the circumferential
direction of the endless metal belts 13, 13. It is to be noted that
FIG. 2 shows three endless metal rings 12 in a lamination state for
simplifying the figure. The number of laminated endless metal rings
12 is however not limited thereto and may be nine or twelve.
[0043] On both sides of each metal element 11 in the width
direction, belt holding grooves 114 are formed nearly horizontally,
leaving a neck portion at the center. A lower end of the belt
holding grooves 114 forms a saddle portion 112 contacting with
inner peripheral surfaces of the endless metal belts 13.
Inclination surfaces of the saddle portions 112 inclining inward
and downward from the end faces of the saddle portions 112 in the
width direction are drive transmission portions 113 in friction
contact with the wall surfaces of each V-shaped groove of the drive
pulley P1 and the driven pulley P2 to transmit drive power. The
metal elements 11 transmit the drive power in such a manner that
each front-side metal element 11 in the rotation direction is
pushed by each rear-side metal element 11 in the rotation
direction. A boss 115 is formed on a front surface above each of
the neck portions. Each boss 115 is engaged in a recess not shown
formed in a back surface above each neck portion to prevent mutual
positional displacement of the continuously arranged metal elements
11. The metal elements 11 can be made of a high abrasion-resistant
steel material, for example, carbon tool steels (SK material).
[0044] As shown in FIG. 3, an outer peripheral surface 121 and an
inner peripheral surface 122 of each endless metal ring 12 are
formed in a crowning shape that is curved slightly upward at a
center in section. The radius of the crowing shape is for example
on the order of 800 mm. This crowning shape can facilitate keeping
the lamination state. Each edge face 123 of the endless metal ring
12 in the width direction is formed in a smooth shape. The edge
faces 123 in the width direction are formed by barrel polishing or
the like when burrs or the like are removed from a ring body cut
apart from a cylindrical body. The wall thickness t of each endless
metal ring 12 is substantially constant and for example can be on
the order of 180 to 190 .mu.m. In the present embodiment, the
material of the endless metal rings 12 is maraging steel that
necessarily contains iron, nickel, and molybdenum and also
appropriately contains, as needed, cobalt, titanium, aluminum, and
others. The content of nickel in the maraging steel is not limited
to 18 to 19 weight % and could be on the order of 20 to 25 weight
%. The content of molybdenum is preferably at least 3 weight % or
more.
[0045] <Method for Manufacturing the Endless Metal Belt>
[0046] The method for manufacturing the endless metal belt will be
explained below. This endless metal belt manufacturing method in
the present embodiment includes a method defined by a first
manufacturing process and a method defined by a second
manufacturing process.
[0047] (First Manufacturing Process)
[0048] A diagram of the first manufacturing process will be first
explained. FIG. 4 is the diagram of the first manufacturing process
for the endless metal belt according to the invention. As shown in
FIG. 4, the first manufacturing process for the endless metal belt
includes (a) a cylindrical body forming step, (b) a joining step,
(c) a first solutionizing (annealing) step, (d) a ring cutting
step, (e), a rolling step, (f) a circumferential length adjusting
step, (g) a second solutionizing step, (h) an aging-nitriding
treatment step, and (i) a laminating step. These steps are
performed in the order of (a) to (i).
[0049] (a) The cylindrical body forming step is a step of forming a
cylindrical body 1 having a predetermined length in an axial
direction and being open at both ends in the axial direction. In
this step, a strip-shaped maraging steel sheet Z is wound off from
a coil, cut into a sheet ZS of a predetermined size, and then bent
to bring opposite edges into abutment with each other. This bending
work may include a method using a roll or a die. The wall thickness
of the cylindrical body 1 is about 0.4 to 0.5 mm and the diameter
of the same is about 100 to 200 mm.
[0050] (b) The joining step is a step of joining the opposite edges
of the cylindrical body 1 brought in abutment. This joining method
may include a welding method by melting the opposite edges by
plasma welding, laser welding, or the like, a diffusion joining
method by removing oxidized film of the edges. In the present
embodiment, the plasma welding method is adopted. A welding device
2 is placed opposite a butting portion 14 of the cylindrical body
1, and either the cylindrical body 1 or a nozzle of the welding
device 2 is moved in the axial direction (an arrow F direction) to
perform butt welding. A weld portion 21 is formed to penetrate from
the outer peripheral surface to the inner peripheral surface of the
cylindrical body 1. When shrinkage or sinkage occurs at a boundary
between the weld portion 21 and a base material portion 22, it
leads to strength deterioration. Thus, welding conditions (spot
diameter, nozzle direction, welding speed, etc.) that cause no
shrinkage are selected.
[0051] (c) The first solutionizing (annealing) step is a step of
homogenizing the hardness of the cylindrical body 1 that was
partially hardened during the welding process in order to perform
nearly uniform rolling in the rolling step to be performed later.
In this solutionizing step, the welded cylindrical body 1 is put in
an axially upright posture on a mesh belt or the like and then
conveyed into a heat treatment furnace and subjected to the first
solutionizing. The first solutionizing is carried out by heating
the cylindrical body 1 to a temperature equal to or higher than a
temperature at which alloy constituents are dissolved in a solid
solution, and holding the cylindrical body 1 for a required time
and then cooling it. If the hardness of the weld portion is in a
hardness range corresponding to such a degree as to enable nearly
uniform rolling in the subsequent rolling step, the present step
can be skipped.
[0052] (d) The ring cutting step is a step of cutting the
cylindrical body 1 having homogenized hardness, in a direction
perpendicular to the axial direction into a plurality of annular
members 3 each having a predetermined length in the axial
direction. This cutting method may include a mechanical cutting
method by making a blade edge of a cutter sequentially dig into the
cylindrical body 1 in a circumferential direction, a thermally
cutting method by melt cutting by use of a laser or the like, etc.
Since irregularity (protrusions and depressions) such as burrs
generated in a cut area is likely to cause stress concentration
thereon during use, barrel polishing or the like is carried out to
remove such irregularity and smoothen the surface of each annular
member 3.
[0053] (e) The rolling step is a step of rolling each polished
annular member 3 to be extended to nearly a wall thickness usable
as an endless metal ring 12. The rolling method may include for
example a roller rolling method in which an annular member 3 is
wound over two opposed rollers, and a third roller is provided to
press the annular member 3 against one of the two rollers and move
in the circumferential direction to extend the circumferential
length of the annular member 3. The circumferential length of a
rolled ring body 4 is about 600 to 700 mm.
[0054] (f) The circumferential length adjusting step is a step of
adjusting the circumferential length of each rolled ring body 4 to
a predetermined circumferential length determined according to the
order of lamination prior to forming the endless metal belt 13 by
laminating a plurality of endless metal rings 12. The
circumferential length adjusting method may include for example a
roller adjusting method in which a ring body 4 is wound over two
opposed rollers and applied with tensile force while measuring a
distance of one of the roller from the other roller. Errors after
the circumferential length adjusting are controlled to about dozen
or so .mu.m to allow the ring bodies 4 to be laminated in close
contact relation.
[0055] (g) The second solutionizing step is a step of
recrystallizing metal texture of each ring body having the adjusted
circumferential length to restore crystal structure of the metal
texture deformed by the rolling work and the circumferential length
adjusting work, thereby relieving or removing processing strain and
internal stress (including residual stress). In this step, a second
solutionizing treatment is performed by locking the ring body after
the circumferential length adjusting in a posture vertically
extending in an axial direction on a carrying jig and conveyed into
a heat treatment furnace. The second solutionizing is performed by
heating the ring body to a temperature equal to or higher than a
temperature at which alloy constituents of the ring body are
dissolved in a solid solution, and holding the ring body for a
required time, and then cooling it. For example, the heating
temperature is about 820.degree. C. and the holding time is about 2
minutes.
[0056] (h) The aging-nitriding treatment step is a step of
subjecting each ring body having the adjusted circumferential
length and having undergone the second solutionizing to the aging
treatment to precipitate alloy elements to ensure predetermined
hardness and also to the nitriding treatment to form a nitrided
layer with a predetermined depth on a front surface side of the
ring body to impart compressive residual stress therein. The
aging-nitriding treatment is carried out in a continuous furnace.
In particular, to allow nitriding gas to uniformly diffuse over the
surface of each ring body, a plurality of the ring bodies are
locked on a special carrying jig capable of arranging the ring
bodies at intervals in the axial direction and passed through the
continuous furnace. A heat treatment history of each ring body is
managed to allow later checking.
[0057] (i) The laminating step is a step of laminating selected
ones of the ring bodies (endless metal rings 12) having been
subjected to the aging-nitriding treatment, the selected ones whose
circumferential lengths are larger from one on the inner peripheral
side toward another on the outer peripheral side, thereby forming
an endless metal belt 13. When this belt 13 is used as the power
transmission belt 10 in the belt-type continuously variable
transmission 100, the endless metal rings 12 forming the endless
metal belt 13 could not receive uniformly the stress exerted
thereon in case the endless metal rings 12 are not in close contact
with one another. Accordingly, the endless metal rings 12 have to
be laminated in close contact with one another almost uniformly
over the entire circumference.
[0058] (Second Manufacturing Process)
[0059] A diagram of the second manufacturing process will be
explained below. FIG. 5 is the diagram of the second manufacturing
process for the endless metal belt according to the invention. FIG.
6 is a process chart showing the manufacturing processes for
endless metal belts for comparison. FIG. 7 is a perspective view of
a ring body having been subjected to the stress-relief heat
treatment in the first manufacturing process. FIG. 8 is a
perspective view of an endless metal belt having been subjected to
the stress-relief heat treatment in the second manufacturing
process. FIG. 13 is a graph showing a warped amount of an edge face
of each ring body after the stress-relief heat treatment.
[0060] As shown in FIG. 5, the second manufacturing process for the
endless metal belt includes (a) a cylindrical body forming step,
(b) a joining step, (c) a first solutionizing (annealing) step, (d)
a ring cutting step, (e) a rolling step, (f) a circumferential
length adjusting step, (g) a laminating step, (h) a second
solutionizing step, and (i) an aging-nitriding treatment step.
These steps are performed in the order of (a) to (i). In the method
for manufacturing the endless metal belt according to present
invention, differences between the first manufacturing process and
the second manufacturing process are only in that the laminating
step of the ring body is performed after the aging-nitriding
treatment (the first manufacturing process) or the laminating step
of the ring body is performed after the circumferential length
adjusting (the second manufacturing process) as shown in FIG. 6.
The details in each step (a) to (i) are explained above in the
first manufacturing process and thus skipped hereinafter.
[0061] As shown in FIG. 6(b), in the first manufacturing process,
after rolling and circumferential length adjusting, each ring body
is subjected separately to the stress-relief heat treatment. Thus,
the ring body 5 having flat edge faces after the circumferential
length adjusting may be transformed to a ring body 6 whose edge
faces are undulated or deformed during stress-relief heat
treatment. On the other hand, as shown in FIG. 6(c), in the second
manufacturing process, after the rolling and the circumferential
length adjusting, a plurality of the ring bodies are laminated in
close contact with one another, thereby enhancing the rigidity of
the entire ring bodies, so that the laminated ring bodies undergo
the stress-relief heat treatment. Accordingly, as shown in FIG. 8,
the laminated ring bodies 7 after the circumferential length
adjusting could be produced as ring bodies 8 whose edge faces are
less likely to be undulated or deformed during the stress-relief
heat treatment. To be concrete, as shown in FIG. 13, when the ring
bodies are treated one by one (Process 1 of the present invention:
the first manufacturing process), the edge-face warped amounts of
the ring bodies subjected to the stress-relief heat treatment were
about 0.1 to 0.4 mm. In contrast, when the ring bodies are treated
in lamination form (Process 2 of the present invention: the second
manufacturing process), the edge-face warped amounts of the ring
bodies subjected to the stress-relief heat treatment were about
0.05 to 0.13 mm, with great reduction of undulation or deformation
during the stress-relief heat treatment.
[0062] In the aging-nitriding treatment step (i) in the second
manufacturing process shown in FIG. 5, the carrying jig is adapted
to uniformly diffuse nitriding gas over the laminated ring bodies
7, as explained below. FIG. 9 is a perspective view of the carrying
jig for conveying the laminated ring bodies during the
aging-nitriding treatment in the second manufacturing process. FIG.
10 is a top view of the laminated ring bodies shown in FIG. 9.
[0063] As shown in FIG. 9, for the aging-nitriding treatment in the
second manufacturing process, a carrying jig 80 for conveying the
laminated ring bodies 7 in a locked state includes a first support
roller 81, a second support roller 82, and a third support roller
83, respective axes C1, C2, and C3 being upright in a vertical
direction, a rectangular support table 84 pivotally supporting a
lower end of each of the support rollers, and a carrying rack 85
that is arranged adjacent to the support table 84 and extends in a
carrying direction. The first support roller 81 includes
cylindrical surfaces 812 contacting with the outer peripheral
surfaces of the laminated ring bodies 7 and flanges 811 contacting
with the edge faces of the laminated ring bodies 7. The flanges 811
are arranged more than one and spaced at predetermined intervals in
the axial direction. The second support roller 82 is provided
vertically and opposed to the first support roller 81 to clamp the
laminated ring bodies 7 from the inner peripheral side thereof. The
second support roller 82 has a cylindrical surface 822 contacting
with the inner peripheral surfaces of the laminated ring bodies 7.
The third support roller 83 is provided vertically in a position
symmetric to the second support roller 82 with respect to the axis
of the laminated ring bodies 7 and has cylindrical surfaces 832
contacting with the inner peripheral surfaces of the laminated ring
bodies 7. The third support roller 83 includes flanges 831 as many
as the flanges 811 of the first support roller 81 and at the same
level of the corresponding flanges 811. The diameter of the
cylindrical surfaces 832 of the third support roller 83 is smaller
than the diameter of the cylindrical surfaces 812 of the first
support roller 81 and the cylindrical surface 822 of the second
support roller 82. The first support roller 81 is provided at its
lower end with a gear 814 engaging with a straight gear 851 of the
carrying rack 85. The second support roller 82 is provided with at
its lower end with a gear 824 engaging with the gear 814.
[0064] As shown in FIGS. 9 and 10, when the support table 84 is
moved in the carrying direction (an arrow K direction), the first
support roller 81 is rotated in an arrow g direction, while the
second support roller 82 is rotated in an arrow h direction. The
laminated ring bodies 7 clamped between the first support roller 81
and the second support roller 82 are thus rotated in an arrow i
direction in association with rotation of the first support roller
81 and the second support roller 82. The third support roller 83 is
rotated in an arrow j direction in association with rotation of the
laminated ring bodies 7 in the arrow i direction.
[0065] At that time, each set of the laminated ring bodies 7 is
formed with gaps S between the ring bodies around one side (near
the third support roller 83) opposed to the other side clamped
between the first support roller 81 and the second support roller
82. The position of the gaps S moves in a circumferential direction
of the laminated ring bodies 7 as the support table 84 is moved in
the carrying direction (the arrow K direction). Accordingly, in
association with movement of the carrying jig, nitriding gas can be
uniformly diffused over each one of the laminated ring bodies
7.
[0066] <Residual Stress Distribution in the Ring Body and
Fatigue Life of the Endless Metal Belt>
[0067] Examination results of residual stress distribution in the
ring bodies manufactured by the aforementioned manufacturing
processes using the maraging steel of the following components will
be explained and further the mechanism of improving the fatigue
life of the endless metal belt will be explained. FIG. 11 is a
graph showing residual stress distribution in the ring bodies
according to the present invention. FIG. 12 is a graph showing
differences in residual stress distribution in ring bodies between
a conventional art and the present invention. FIG. 14 is a graph
showing fatigue life of the endless metal belt.
[0068] The alloy composition ratio (weight %) of maraging steel is
defined as below: nickel (Ni) is about 17 to 19%, cobalt (Co) is
about 7 to 13%, molybdenum (Mo) is about 3.5 to 4.5%, titanium (Ti)
is about 0.3 to 1.0%, aluminum (Al) is about 0.05 to 0.15%, and
carbon (C) is 0.03% or less.
[0069] (Examination Results of Residual Stress Distribution in the
Ring Bodies)
[0070] The graph shown in FIG. 11 is a graph showing measurement
results of residual stress in each of the endless metal rings
manufactured by the manufacturing method of the present invention,
by taking the cross section of each ring body before and after the
nitriding treatment by use of an X-ray stress measuring device to
measure the residual stress from the outer peripheral surface to
the inner peripheral surface. A horizontal axis represents
direction (.mu.m) from the outer peripheral surface in the ring
body and a vertical axis represents residual stress (MPa). In the
vertical axis, a minus side indicates compressive residual stress
and a plus side indicates tensile residual stress. From the above
graph, before the nitriding treatment, it appears that the
compressive residual stress is slightly accumulated on the outer
peripheral side of the ring body, while the tensile residual stress
is slightly accumulated on the inner peripheral side. However,
respective values of slight compressive residual stress and slight
tensile residual stress are apparent value appearing due to
limitations of the measuring device. Specifically, since the ring
bodies are each formed in a crowning shape protruding on the outer
peripheral side, when a sample is fixed in the measuring device,
the crowning is corrected to be flat. Accordingly, a measurement
result shows as if the compressive residual stress is slightly
accumulated on the outer peripheral side and the tensile residual
stress is slightly accumulated on the inner peripheral side. It is
however actually considered that the residual stress is nearly zero
from the outer peripheral side to the inner peripheral side. The
reason why the residual stress could be made zero from the outer
peripheral side to the inner peripheral side of the ring body is
that the stress-relief heat treatment (the second solutionizing)
performed before the nitriding treatment could restore the crystal
structure of metal texture deformed by the rolling work and the
circumferential length adjusting work and substantially perfectly
remove the processing strain and the internal stress (including the
residual stress).
[0071] From the graph shown in FIG. 11, after the nitriding
treatment, it is found that the nitrided layer is formed with a
predetermined depth (about 30 to 40 .mu.m) from each of the outer
peripheral surface and the inner peripheral surface of the ring
body in the nitriding treatment. This reveals that this formed
nitrided layer imparts nearly the same compressive residual
stresses E and D on or close to the surfaces on the outer
peripheral side and the inner peripheral side. Further, the tensile
residual stress A near the center in the wall thickness less varies
and is almost uniform in the wall thickness direction. As a result,
a value of the tensile residual stress in a region (C) where the
nitrided layer on the outer peripheral side is absent, or ends, and
a value of the tensile residual stress in a region (B) where the
nitrided layer on the inner peripheral side is absent, or ends, are
nearly equal to each other.
[0072] The graph shown in FIG. 12 is a graph showing measurement
results of residual stress in each of the endless metal rings
manufactured by the manufacturing method of the conventional art
and the manufacturing method of the present invention by taking the
cross section of each ring body after the nitriding treatment by
use of an X-ray stress measuring device to measure the residual
stress from the outer peripheral surface to the inner peripheral
surface. A horizontal axis represents direction (.mu.m) from the
outer peripheral surface in the ring body and a vertical axis
represents residual stress (MPa). In the vertical axis, a minus
side indicates compressive residual stress and a plus side
indicates tensile residual stress. In the above graph, by
comparison of the compressive residual stress on the outer
peripheral side in the ring body, that in the present invention is
smaller by about 180 MPa in approximately parallel in an arrow P
direction than that in the conventional ring body. By comparing a
difference in compressive residual stress between the outer
peripheral side and the inner peripheral surface, the conventional
ring body provides a large difference in compressive residual
stress between the outer peripheral surface and the inner
peripheral surface that a value on the inner peripheral surface is
lower by about 300 MPa than a value on the outer peripheral
surface, whereas the ring body of the present invention provides an
almost zero difference in compressive residual stress between the
outer peripheral surface and the inner peripheral surface that a
value on the inner peripheral surface is smaller by about 15 MPa
than a value on the outer peripheral surface. By comparing the
compressive residual stress in a region where the nitrided layer is
absent on the inner peripheral side, furthermore, the ring body of
the present invention is smaller by about 150 MPa in parallel in an
arrow Q direction than the conventional ring body. Consequently, in
the ring body of the present invention, the compressive residual
stress is almost equal between the outer peripheral side and the
inner peripheral side, and the tensile residual stress in the
region where the nitrided layer is absent on the inner peripheral
side greatly decreases than in the conventional ring body.
[0073] (Mechanism of Improving Fatigue Life of the Endless Metal
Belt)
[0074] According to the manufacturing method of the present
invention, the stress-relief heat treatment (the second
solutionizing) is conducted before the nitriding treatment.
Therefore, at a stage prior to conducting the nitriding treatment,
the crystal structure of metal texture deformed by the rolling work
and the circumferential length adjusting work can be restored, so
that the processing strain and the internal stress (including the
residual stress) can be substantially perfectly removed. Thus, the
residual stress in the ring body from the outer peripheral side to
the inner peripheral side can be made almost zero.
[0075] After the residual stress in the ring body from the outer
peripheral side to the inner peripheral side is made almost zero,
the nitriding treatment is performed. Thus, almost the same
residual stress is applied on or close to the surfaces on the outer
peripheral side and the inner peripheral side. The tensile residual
stress near the center in wall thickness less varies and is nearly
uniform in the wall thickness direction. As a result, a value of
the tensile residual stress in the region where the nitrided layer
on the inner peripheral side is absent and a value of the tensile
residual stress in the region where the nitrided layer on the inner
peripheral side is absent are almost equal. This greatly reduces a
portion which has locally increased stress amplitude and will be a
weakest portion apt to cause fatigue breakage when the endless
metal belt is used in the belt-type continuously variable
transmission.
[0076] In the present invention, furthermore, the ring body is
subjected to the stress-relief heat treatment (the second
solutionizing) in which the residual stress in the ring body from
the outer peripheral side to the inner peripheral side is made
almost zero and then subjected to the nitriding treatment.
Accordingly, it is unnecessary to add the tensile residual stress
previously accumulated by the rolling work and the circumferential
length adjusting work and thus possible to achieve uniformization
and minimization of tensile residual stress.
[0077] Since the nitrided layer is formed after the residual stress
of the ring body is made almost zero before the nitriding
treatment, as explained above, almost equal compressive residual
stress is imparted on or close to the surfaces on the outer
peripheral side and the inner peripheral side and the tensile
residual stress in the region where the nitrided layer is absent
can be uniformized and minimized. Consequently, when the endless
metal belt 13 is used in the belt-type continuously variable
transmission 100, the stress load is greatly smaller than the
conventional ring body, thereby enabling large improvement of
fatigue life (see FIG. 14).
[0078] The embodiment explained above may be changed in other
specific forms without departing from the essential characteristics
of the present invention. For instance, the present embodiment uses
maraging steel as the material of the ring body but is not limited
thereto. For example, precipitation hardening stainless steel,
austenite stainless steel, and carbon steel (quenched steel) may be
usable. In this case, in the second solutionizing step shown in
FIG. 4 or 5, the solution treatment is performed for the
precipitation hardening stainless steel, the stress-relief
annealing is conducted for austenite stainless steel, or the
quenching treatment or both of the quenching treatment and the
tempering treatment is performed for the carbon steel
(quench-hardened steel).
INDUSTRIAL APPLICABILITY
[0079] The present invention is available as the method for
manufacturing an endless metal belt forming a power transmission
belt to be used in a belt-type continuously variable transmission
mounted in a vehicle, the endless metal belt, and the belt-type
continuously variable transmission.
REFERENCE SIGNS LIST
[0080] 1: Cylindrical body [0081] 2: Welding device [0082] 3:
Annular member [0083] 4, 5, 6: Ring body [0084] 7, 8: Laminated
ring body [0085] 10: Transmission belt [0086] 11: Metal element
[0087] 12: Endless metal ring [0088] 13: Endless metal belt [0089]
14: Butting portion [0090] 21: Weld portion [0091] 100: Belt-type
continuously variable transmission
* * * * *