The present invention relates to a method for producing a crankshaft
by forging and more particularly to a method for producing a crankshaft by
hot forging.
BACKGROUND ART
[0002]
A reciprocating engine to be employed in a motor vehicle, a motorcycle,
an agricultural machine, a marine vessel or the like requires a crankshaft to
extract power by converting reciprocating motions of pistons to rotational
motion. There are two types of crankshafts: the type manufactured by die
forging and the type manufactured by casting. Especially when high
strength and high stiffness are required, forged crankshafts manufactured by
die forging are often employed.
[0003]
A forged crankshaft is generally produced by using a billet as a
starting material. The billet is circular or square in cross section, and the
cross-sectional area is constant throughout the length. A method for
producing a forged crankshaft includes a preforming step, a die forging step,
and a trimming step that are to be executed in this order. After the trimming
step, a coining step may be executed if needed. Typically, the preforming step
includes a rolling step and a bending step, and the die forging step includes a
rough forging step and a finish forging step.
[0004]
FIGS. lA to lF are schematic diagrams showing a conventional
method for producing a common forged crankshaft. A crankshaft 1 shown in
FIG. lF is a four-cylinder eight-counterweight crankshaft to be mounted in a
·f
four-cylinder engine. The crankshaft 1 includes five journals J1 to J5, four
pins P 1 to P 4, a front part Fr, a flange Fl, and eight crank arms (hereinafter
referred to simply as "arms") A1 to AS. The eight arms A1 to A8 connect the
journals J1 to J5 respectively to the pins P1 to P4. All of the eight arms A1 to
A8 have counterweights (hereinafter referred to simply as "weights") W1 to
W8, which are integrated with the armsA1 toA8, respectively.
[0005]
In the following paragraphs, when the journals J1 to J5, the pins P1 to
P4, the arms A1 to A8, and the weights W1 to W8 are each collectively
referred to, a reference character "J" is used for the journals, a reference
character "P" for the pins, a reference character "A" for the arms, and a
reference character "W" for the weights.
[0006]
In the production method shown in FIGS. 1A to 1F, the forged
crankshaft 1 is produced as follows. First, a billet 2 with a specified length as
shown in FIG. 1A is heated in a heating furnace (for example, an induction
heating furnace, a gas atmosphere heating furnace or the like), and the heated
billet undergoes a rolling step. In the rolling step, the billet 2 is rolled and
reduced, for example, by grooved rolls. This is to distribute the volume of the
billet 2 in the axial direction, and thereby, a rolled blank 3, which is an
in-process material, is obtained (see FIG. 1B). Next, in a bending step, the
rolled blank 3 is partly reduced from a direction perpendicular to the length
direction. This is to distribute the volume of the rolled blank 3, and thereby,
a bent blank 4, which is a next in-process material, is obtained (see FIG. 1C).
[0007]
Next, in a rough forging step, the bent blank 4 is reduced by a pair of
an upper die and a lower die, and thereby, a rough forged blank 5 is obtained
(see FIG. 1D). The rough forged blank 5 is roughly in the shape of a
crankshaft (finished product). In a finish forging step, the rough forged
blank 5 is reduced by a pair of an upper die and a lower die, and thereby, a
finish forged blank 6 is obtained (see FIG. 1E). The finish forged blank 6 has
a shape in agreement with the shape of the finished product, that is, the
crankshaft.
3
-j-
[OOOS]
During the rough forging and the finish forging, excess material flows
out through a space between the mutually facing parting faces of the dies,
which results in formation of flash. Accordingly, the rough forged blank 5
and the finish forged blank 6 have great flash B on the periphery.
[0009]
In a trimming step, for example, while the finish forged blank 6 is
nipped and held by a pair of dies, the finish forged blank 6 is punched by a
cutting die. Thereby, the flash B is removed from the finish forged blank 6,
and a forged blank with no flash is obtained. The forged blank with no flash
has substantially the same shape as the forged crankshaft 1 shown in FIG. 1F.
[0010]
In a coining step, main parts of the forged blank with no flash are
slightly pressed by dies from above and below so that the forged blank with no
flash can have the exact size and shape of the finished product. The main
parts of the forged blank with no flash are, for example, shaft parts such as
the journals J, the pins P, the front part Fr, the flange Fl and the like, and
further, the arms A and the weights W. In this way, the forged crankshaft 1
is produced.
[0011]
The production method shown in FIGS. 1A to 1F is applicable not only
to production of a four-cylinder eight-counterweight crankshaft as shown in
FIGS. 1F but also to production of any other crankshaft. For example, the
production method is applicable to a four-cylinder four-counterweight
crankshaft.
[0012]
In a four-cylinder four-counterweight crankshaft, only some of the
eight arms A1 to AS incorporate a weight W. For example, the front first arm
A1, the rearmost eighth arm AS and the central two arms (the fourth arm A4
and the fifth arm A5) incorporate a weight W. The other arms, namely, the
second, the third, the sixth and the seventh arms A2, A3, A6 and A 7 do not
have a weight. Such arms without a weight will hereinafter referred to as
"unweighted arms". These unweighted arms are oval.
·!·
[0013]
Other crankshafts, for example, crankshafts to be mounted in
three-cylinder engines, in·line six-cylinder engines, V-type six-cylinder
engines, eight-cylinder engines and others can be produced by the same
production method. It is noted that, when adjustment of the placement
angles of the pins is necessary, a twisting step is added after the trimming
step.
[0014]
In recent years, there has been a need for weight reduction of
reciprocating engines, particularly those for motor vehicles, in order to
improve the fuel economy. Accordingly, there is also an ever-increasing
demand for weight reduction of crankshafts to be mounted in reciprocating
engines. Techniques to reduce the weight of a crankshaft are disclosed in
Japanese Patent Application'Publication No. 2012-7726 (Patent Literature 1)
and Japanese Patent Application Publication No. 2010·230027 (Patent
Literature 2).
[0015]
Patent Literatures 1 and 2 teach an arm having a hole made in the
journal-facing surface and teach a method for producing a crankshaft with the
arm. The hole of the arm is made to lie on a straight line connecting the axis
of the journal and the axis of the pin (which will be hereinafter referred to as
an "arm centerline"), and the hole extends large and deep toward the pin.
This arm is reduced in weight by the weight corresponding to the volume of
the hole. The weight reduction of the arm leads to a weight reduction of the
weight paired with the arm, thereby resulting in a reduction in weight of the
whole forged crankshaft. Regarding the arm having a hole, in a region near
the pin, both side portions across the arm centerline are thick, which ensures
stiffness (torsional stiffness and bending stiffness).
[0016]
Forming a recessed portion in the journal-facing surface of the arm
while keeping both side portions of the arm thick as described above leads to
weight reduction and ensuring of stiffness.
[0017]
I
It is, however, difficult to produce such a forged crankshaft with such
uniquely shaped arms by a conventional production method. The reason is
as follows. When a recess is to be formed in the surface of an arm in the die
forging step, the draft of the die will become a reverse draft at the site of the
recess, and therefore, the forged blank will not be able to be removed from the
die.
[0018]
To avoid such situations, in the production methods disclosed in
Patent Literatures 1 and 2, the following process is carried out. In the die
forging step, the arm is shaped to be small with no recess formed in the
surface of the arm, and after the trimming step, a punch is pushed into the
surface of the arm so that the imprint made by the punch forms a recess.
[0019]
In the crankshaft sho'wn in FIG. 1F, all of the arms A and the weights
W integrated therewith have the same shape. Practically, however, the arms
A and the weights W integrated therewith may be different from one another
in shape as needed. Japanese Patent Application Publication No.
2007-71227 (Patent Literature 3) and Japanese Patent Application
Publication No. 2014-40856 (Patent Literature 4) disclose techniques for this.
[0020]
Patent Literature 3 discloses a four-cylinder eight-counterweight
crankshaft including a flywheel disposed at an end. In the crankshaft, the
arms incorporating a weight are different from one another in the thickness
and the center of gravity of the arm and in the mass of the weight.
Accordingly, it is possible to reduce the thicknesses of the arms that need to
have only low stiffness while ensuring the minimum necessary stiffness to
each of the arms, thereby resulting in a reduction in weight.
[0021]
Patent Literature 4 discloses a crankshaft for a multicylinder engine,
the crankshaft including a flywheel disposed at an end. In the crankshaft,
an arm that is less distant from the flywheel has higher bending stiffness and
higher torsional stiffness than an arm that is more distant from the flywheel.
Also, it is preferred that the arms are different from one another in the
I
bending stiffness and in the torsional stiffness. Accordingly, it is possible to
attain a reduction in weight while suppressing flexural vibration and
torsional vibration.
[0022]
In a case where the arms incorporating a weight have different arm
shapes and different weight shapes, what portion of the arm needs to have
high stiffness differs from arm to arm, depending on the shape. Specifically,
an arm may need to have high stiffness in the region near the pin, while
another arm may need to have high stiffness in the region near the journal.
CITATION LIST
PATENT LITERATURE
[0023]
Patent Literature 1:' Japanese Patent Application Publication No.
2012-7726
Patent Literature 2: Japanese Patent Application Publication No.
2010-230027
Patent Literature 3: Japanese Patent Application Publication No.
2007-71227
Patent Literature 4: Japanese Patent Application Publication No.
2014-40856
SUMMARY OF INVETION
TECHNICAL PROBLEMS
[0024]
The production methods disclosed in Patent Literatures 1 and 2 make
it possible to make a hole in the journal-facing surface of an arm while
keeping both side portions of the arm thick. Accordingly, the production
methods allow production of a forged crankshaft with a reduced weight and
assured stiffness.
[0025]
In the production methods, however, in making a hole, the surface of
an arm is strongly punched and is deformed entirely, and a great force is
-Inecessary
for the punching. Therefore, a special facility including a die for
supplying a great force to a punch is required, and it is also necessary to give
attention to the durability of the punch.
[0026]
It is an object of the present invention to provide a method for
producing a forged crankshaft with both a reduced weight and assured
stiffness by a simple process.
SOLUTION TO PROBLEMS
[0027]
A production method according to an embodiment of the present
invention is a method for producing a forged crankshaft including journals
serving as a center of rotation, pins decentered from the journals, and crank
arms connecting the journals and the pins. The forged crankshaft further
includes counterweights integrated with all or some of the crank arms. The
method includes a rough forging step of performing die forging to obtain a
rough forged blank with flash having a crankshaft shape, and a finish forging
step of applying die forging by use of a first pair of dies to the rough forged
blank to obtain a finish forged blank with flash. The rough forged blank
includes rough journals, rough pins, rough crank arms, and rough
counterweights, which correspond to the journals, the pins, the crank arms,
and the counterweights of the forged crankshaft, respectively. At least one of
the rough crank arms has, in the region adjacent rough pin, a first excess
portion protruding from an outer periphery of a side portion of the rough
crank arm. During the die forging in the finish forging step, the first excess
portion is deformed by the first pair of dies to bulge toward an adjacent one of
the rough journals.
ADVANTAGEOUS EFFECTS OF INVENTION
[0028]
In the forged crankshaft production method according to the present
invention, in the rough forging step, at least one of the rough crank arms is
formed to have, in a region near an adjacent rough pin, an excess portion
jprotruding
from an outer periphery of a side of the rough crank arm. In the
finish forging step, the excess portion is deformed by a pair of dies to bulge
toward an adjacent rough journal. This allows formation of a recess in the
journal-facing surface of the arm and an increase in the thickness of the side
portion of the arm in the region near the adjacent pin. Accordingly, the
forged crankshaft produced by this method has a reduced weight and assured
stiffness. Since the excess portion is deformed by die forging in the finish
forging step, the deformation can be carried out in a simple manner by use of
an existing equipment.
BRIEF DESCRIPTION OF DRAWINGS
[0029]
[FIG. lA] FIG. lA is a schematic diagram of a billet during a
conventional process of producing a common forged crankshaft.
[FIG. lB] FIG. lB is a schematic diagram of a rolled blank during the
conventional process of producing a common forged crankshaft.
[FIG. lC] FIG. lC is a schematic diagram of a bent blank during the
conventional process of producing a common forged crankshaft.
[FIG. lD] FIG. lD is a schematic diagram of a rough forged blank
during the conventional process of producing a common forged crankshaft.
[FIG. lE] FIG. lE is a schematic diagram of a finish forged blank
during the conventional process of producing a common forged crankshaft.
[FIG. lF] FIG. lF is a schematic diagram of a crankshaft during the
conventional process of producing a common forged crankshaft.
[FIG. 2A] FIG. 2A is a perspective view of an arm of a crankshaft
having a first exemplary configuration according to the present invention,
schematically showing the journal-facing surface of the arm.
[FIG. 2B] FIG. 2B is a diagram showing the journal-facing surface of
the arm shown in FIG. 2A.
[FIG. 2C] FIG. 2C is a diagram showing a side surface of the arm
shown in FIG. 2A.
[FIG. 2D] FIG. 2D is a sectional view along the line IID-IID in FIG.
2B.
9
!-
[FIG. 3A] FIG. 3A is a diagram showing the rough-journal-facing
surface of a rough arm of a rough forged blank for the crankshaft having the
first exemplary configuration.
[FIG. 3B] FIG. 3B is a diagram showing a side surface of the rough
arm shown in FIG. 3A.
[FIG. 3C] FIG. 3C is a sectional view along the line IIIC-IIIC in FIG.
3A.
[FIG. 4A] FIG. 4A is a perspective view of an arm of a crankshaft
having a second exemplary configuration according to the present invention,
schematically showing the pin-facing surface ofthe arm.
[FIG. 4B] FIG. 4B is a diagram showing the pin-facing surface of the
arm shown in FIG. 4A.
[FIG. 4C] FIG. 4C is a diagram showing a side surface of the arm
shown in FIG. 4A.
[FIG. 4D] FIG. 4D is a sectional view along the line IVD-IVD in FIG.
4B.
[FIG. 5A] FIG. 5A is a diagram showing the rough-pin-facing surface
of a rough arm of a rough forged blank for the crankshaft having the second
exemplary configuration.
[FIG. 5B] FIG. 5B is a diagram showing a side surface of the rough
arm shown in FIG. 5A.
[FIG. 5C] FIG. 5C is a sectional view along the line VC-VC in FIG. 5A.
[FIG. 6A] FIG. 6A is a diagram of an arm of a crankshaft having a
third exemplary configuration according to the present invention, showing the
pin-facing surface of the arm.
[FIG. 6B] FIG. 6B is a sectional view along the line VIB-VIB in FIG.
6A.
[FIG. 7A] FIG. 7A is a diagram showing the rough-pin-facing surface
of a rough arm of a rough forged blank for the crankshaft having the third
exemplary configuration.
[FIG. 7B] FIG. 6B is a sectional view along the line VIIB-VIIB in FIG.
7A.
[FIG. SA] FIG. SA is a sectional view of a rough forged blank at the
\O
-u{.
time when the rough forged blank has been placed in dies in an exemplary
process flow of the finish forging step according to the present invention.
[FIG. SB] FIG. 8B is a sectional view of the rough forged blank at the
time when reduction has been completed in the exemplary process flow of the
finish forging step according to the present invention.
[FIG. 9A] FIG. 9A is a view showing an example of the finish forging
step according to the present invention.
[FIG. 9B] FIG. 9B is a side view showing the example of the finish
forging step shown in FIG. 9A.
[FIG. 9C] FIG. 9C is a sectional view showing the example of the
finish forging step shown in FIG. 9A.
[FIG. lOA] FIG. lOA is a view showing another example of the finish
forging step according to the present invention.
[FIG. lOB] FIG. lOB is a side view showing the example of the finish
forging step shown in FIG. lOA.
[FIG. lOC] FIG. lOC is a sectional view showing the example of the
finish forging step shown in FIG. lOA.
DESCRIPTION OF EMBODIMENTS
[0030]
An embodiment of the present invention will hereinafter be described.
However, the present invention is not limited to the embodiment to be
described below.
[0031]
(Forged Crankshaft Production Method)
A production method according to the present invention is a method
for producing a forged crankshaft. The forged crankshaft includes journals
serving as a center of rotation, pins decentered from the journals, and crank
arms connecting the journals to the pins. The forged crankshaft further
includes counterweights integrated with all or some of the crank arms.
[0032]
The production method according to the present invention includes a
rough forging step and a finish forging step to be executed in this order. The
I I
-(
rough forging step is to obtain a rough forged blank with flash formed in a
crankshaft shape by die forging. The rough forged blank has roughly the
same shape as the crankshaft. The rough forged blank includes rough
journals, rough pins, rough crank arms and rough counterweights
corresponding to the journals, the pms, the crank arms and the
counterweights of the forged crankshaft, respectively. The rough forged
blank further includes flash and first excess portions which will be described
later.
[0033]
The finish forging step is to obtain a finish forged blank with flash by
applying die forging to the rough forged blank by use of a first pair of dies
(composed of a first upper die and a first lower die)_ At least one of the rough
crank arms has, in the region adjacent rough pin, a first excess portion
protruding from an outer periphery of a side portion of the rough crank arm.
During the die forging in the finish forging step, the first excess portion is
deformed by the first pair of dies to bulge toward the adjacent rough journaL
[0034]
All of the rough crank arms may include a first excess portion.
Alternatively, all or some of the rough crank arms incorporating a rough
counterweight may include a first excess portion. Also, only the rough crank
arms incorporating a rough counterweight may include a first excess portion.
Also, all or some of the crank arms not incorporating a rough counterweight
may include a first excess portion. Also, only the crank arms not
incorporating a rough counterweight may include a first excess portion.
[0035]
The rough crank arm has, in the region near an adjacent one of the
rough pins, first excess portions respectively protruding from outer
peripheries of two side portions of the rough crank arm. This structure
ensures stiffness of both side portions of the crank arm in the region near the
adjacent pin. Between the two first excess portions, an area As (area with a
recess) is present. The surface of the recess may be convex and bulge in the
center with respect to the width direction as will be described later.
[0036]
12-
-;£-
In the finish forging step, the first excess portion of the rough crank
arm may be deformed by the first pair of dies to bulge toward the rough
journal while a second die is in contact with the rough-journal-facing surface
of the rough crank arm at the portion between the first excess portion and the
flash. This prevents deformation of the surface of the recess (the surface on
the side toward which the first excess portion bulges), which will be described
later. When the rough crank arm has, in the region near the adjacent rough
pin, a first excess portion on only one of the two sides, the second die may be
pressed against only the side in which the first excess portion is provided,
bordered by the flash. When the rough crank arm has, in the region near the
rough pin, two first excess portions protruding respectively from the two sides,
a pair of dies may be used as the second die. When the rough crank arm has,
in the region near the rough pin, two first excess portions protruding
respectively from the two sides, alternatively, a two-pronged die may be used
as the second die. When a pair of dies is used as the second die, the dies are
placed to face each other across the flash and to contact the surface of the
rough crank arm. In the finish forging step, for example, while the dies in
the second pair face each other across the flash and contact the
rough-journal-facing surface of the rough crank arm at the portions between
the respective first excess portions and the flash, the first excess portions are
deformed by the first pair of dies to bulge toward the rough journal. When
the two-pronged die is used as the second die, the two edges of the die contact
the surface of the rough crank arm while nipping the flash. Then, while the
two-pronged die contacts the rough-journal-facing surface of the rough crank
arm at the portions between the respective first excess portions and the flash,
the first excess portions are deformed by the first pair of dies to bulge toward
the rough journal.
[0037]
In the finish forging step, the first excess portion may be deformed to
bulge toward the rough journal while the flash is being held by a holder.
Since the first excess portion is deformed by the first pair of dies, at the initial
stage of the finish forging step, the shape ofthe first excess portion does not fit
in the shape of the first pair of dies. Therefore, if the rough forged blank
-;I
having a first excess portion before being deformed is held by only the lower
first die, the rough forged blank will be in an unstable posture. This problem
is noticeable especially when the first excess portion is provided to only one of
the rough crank arms. By using the holder, it is possible to apply finish
forging to the rough forged blank while keeping the rough forged blank in a
stable posture. Specifically, finish forging can be performed with the flash of
the rough forged blank kept in a horizontal posture. Further, the rough
forged blank may be kept in the center between the upper die and the lower
die by the holder.
[0038]
The holder may move while keeping holding the lower surface of the
flash along with movements of the first pair of dies. The flash may move up
and down along with movements of the first pair of dies. In this case, it is
preferred that the holder moves (up and down) following the flash.
[0039]
Usually, a plurality of holders are used to hold the flash, and the lower
surface of the flash is held at a plurality of portions. In the finish forging step,
two or more (for example, three, four or more) holders may be used. Using
four or more holders allows the rough forged blank to be kept in a more stable
posture. The holders preferably hold the portions that are great in mass (for
example, the portions around the rough counterweights). The plurality of
holders may be located on different levels such that the rough forged blank at
the start of finish forging is in a stable posture. For example, in producing a
three-cylinder or six-cylinder crankshaft, the flash may not be formed on a
level. In this case, the holders shall be placed on levels as appropriate
according to the respective levels where the portions are to be kept.
[0040]
The holders are moved to and kept in proper positions to serve the
above function. There is no limit as to the mechanism for moving and
keeping the holders, and any conventional mechanism can be used. For
example, a hydraulic cylinder, an elastic member (spring) or the like can be
used to move and keep the holders. The holders may be moved and kept by a
conventional mechanism used for a knock-out pin. Alternatively, knock-out
pins may be used as the holders.
[0041]
-y
In the finish forging step, the deformation of the first excess portion
may be carried out by crushing or bending.
[0042]
At least one of the rough crank arms has, in the region near the
adjacent rough journal, a second excess portion protruding from an outer
periphery of a side portion of the rough crank arm. In this case, during the
die forging in the finish forging step, the second excess portion is deformed by
the first pair of dies to bulge toward the adjacent rough pin. All of the rough
crank arms may include a second excess portion. Alternatively, all or some of
the rough crank arms incorporating a rough counterweight may include a
second excess portion. Also, only the rough crank arms incorporating a
rough counterweight may include a second excess portion. Also, all or some
of the crank arms not incorporating a rough counterweight may include a
second excess portion. Also, only the crank arms not incorporating a rough
counterweight may include a second excess portion.
[0043]
In the finish forging step, the second excess portion may be deformed
to bulge toward the rough pin while a third die is in contact with the
rough-pin-facing surface of the rough crank arm at the portion between the
second excess portion and the flash. This keeps the rough forged blank in a
stable position during the finish forging step. This also prevents deformation
of the surface of the recess (the surface on the side toward which the second
excess portion bulges), which will be described later. When the rough crank
arm has, in the region near the adjacent rough journal, a second excess
portion on only one of the two sides, the third die may be pressed against only
the side in which the second excess portion is provided, bordered by the flash.
When the rough crank arm has, in the region near the adjacent rough journal,
two second excess portions protruding respectively from the two sides, a pair
of dies may be used as the third die. When the rough crank arm has, in the
region near the adjacent rough journal, two second excess portions protruding
respectively from the two sides, alternatively, a two-pronged die may be used
as the third die. When a pair of dies is used as the third die, the dies are
placed to face each other across the flash and to contact the surface of the
rough crank arm. In the finish forging step, for example, while the dies in
the third pair face each other across the flash and contact the rough-pin-facing
surface of the rough crank arm at the portions between the respective second
excess portions and the flash, the second excess portions are deformed by the
first pair of dies to bulge toward the adjacent rough journal. When a
two-pronged die is used as the third die, the two edges of the die contact the
surface of the rough crank arm while nipping the flash. Then, while the
two-pronged die contacts the rough-pin-facing surface of the rough crank arm
at the portions between the respective second excess portions and the flash,
the second excess portions are deformed by the first pair of dies to bulge
toward the rough pin.
[0044]
At least one of the rough crank arms may has, in the region near the
adjacent rough journal, two second excess portions protruding respectively
from the outer peripheries of the two side portions of the crank arm. This
structure ensures stiffness of both side portions of the crank arm in the region
near the adjacent journal. Between the two second excess portions, an area
At (area with a recess) is present. The surface of the recess may be convex
and bulge in the center with respect to the width direction as will be described
later.
[0045]
The rough crank arms having at least one second excess portion may
be the rough crank arms incorporating a rough counterweight.
[0046]
The rough crank arms having at least one second excess portion may
be the rough crank arms not incorporating a rough counterweight. In this
case, the rough crank arms not incorporating a rough counterweight may
have, in the region near the adjacent rough journal, second excess portions
respectively protruding from the outer peripheries of both side portions.
[0047]
In the finish forging step, the deformation of the second excess portion
lh
·rmay
be carried out by crushing or bending.
[0048]
The second die, the third die and the holder may be used either alone
or in any combination. For example, only the second die may be used, only
the third die may be used, or only the holder may be used. Alternatively, the
second die and the holder may be used in combination, the third die and the
holder may be used in combination, the second die and the third die may be
used in combination, or the second die, the third die and the holder may be
used in combination.
[0049]
The second die moves horizontally (in a direction perpendicular to the
moving direction of the first pair of dies) to come into contact with the rough
forged blank and nip the flash. The third die moves in the same way. When
the rough forged blank is moved up and down in the finish forging step, the
second die and the third die further moves along with the movement of the
rough forged blank. Specifically, when the rough forged blank is moved down
in the finish forging step, the second die and the third die move down along
with the downward movement of the rough forged blank. There is no limit as
to the mechanism for moving and keeping the second die and the third die,
and any conventional mechanism can be used. Each of the second die and
the third die may be provided with a horizontal movement mechanism and a
mechanism for moving the horizontal movement mechanism vertically (in the
up-down direction). For example, these mechanisms may employ a
mechanism used in a double-acting pressing machine.
[0050]
In an aspect, the present invention provides an exemplary method for
producing a forged crankshaft. The exemplary production method is
intended to produce a forged crankshaft including journals serving as a center
of rotation, pins decentered from the journals, and crank arms connecting the
journals to the pins. In the forged crankshaft, all or some of the crank arms
incorporate a counterweight. The exemplary production method includes a
rough forging step of performing die forging to obtain a rough forged blank
with flash having a crankshaft shape, a finish forging step of applying die
-Jjforging
to the rough forged blank by use of a pair of dies to obtain a finish
forged blank with flash. In the exemplary production method, in the rough
forging step, first excess portions are formed such that at least one of the
rough arms (rough crank arms) of the rough forged blank has, in a region near
the adjacent pin (rough pin), first excess portions protruding respectively from
outer peripheries of both side portions. Then, in the finish forging step, the
first excess portions are deformed by the first pair of dies, whereby both side
portions of each of these crank arms (rough crank arms) in the region near the
adjacent pin (rough pin) are thickened.
[0051]
The above paragraph describes a case where the rough forged blank
necessarily includes first excess portions. However, the production method
according to the present invention is applicable to a case where the rough
forged blank does not include' any first excess portions but include at least one
second excess portion. In this case, as mentioned above, during the die
forging in the finish forging step, the second excess portion is deformed by the
first pair of dies to bulge toward the adjacent rough pin.
[0052]
A forged crankshaft production method according an embodiment of
the present invention will hereinafter be described with reference to the
drawings. In the following paragraphs, descriptions of the matters already
described with reference to FIGS. lA to lF may not be repeated.
[0053]
1. Shape of Crankshaft
A forged crankshaft to be produced by a method according to the
present embodiment includes journals serving as a center of rotation, pins
decentered from the journals, and arms connecting the journals to the pins.
All or some of the arms incorporate a weight.
[0054]
In an aspect, the forged crankshaft to be produced by the method
includes a number of units (which are also referred to as "slots")
corresponding to the number of cylinders of the engine. One unit includes a
pin, and two arms located to hold the pin therebetween. On both sides of one
1<6
unit, two journals are located. '1\vo adjacent units are connected to each
other via a journal. In a crankshaft for a Vtype six-cylinder engine, two pins
and an arm therebetween define a small unit, and one small unit and two
arms on both sides thereof define a large unit. Further, journals are located
on both sides of the large unit.
[0055]
The forged crankshaft production method according to the present
embodiment is applicable to production of a four-cylinder eight-counterweight
crankshaft and production of a four-cylinder four-counterweight crankshaft,
for example. The forged crankshaft production method is applicable also to
production of other types of crankshafts, such as crankshafts to be mounted in
three-cylinder engines, in-line six-cylinder engines, V-type six-cylinder
engines, eight-cylinder engines and others.
[0056]
More specifically, the forged crankshaft production method according
to the present embodiment is applicable to production of a forged crankshaft
having a first exemplary configuration as shown in FIGS. 2A to 2D,
production of a forged crankshaft having a second exemplary configuration as
shown in FIGS. 4A to 4D, and production of a forged crankshaft having a third
exemplary configuration as shown in FIGS. 6A and 6B.
[0057]
In either of the crankshafts having the first, the second and the third
exemplary configurations, all of the arms, whether incorporating a weight or
not, have a recess in the journal-facing surface. As will be described later,
only some of the arms may have a recess in the journal-facing surface. In the
crankshaft having the second exemplary configuration, the arms
incorporating a weight further have a recess in the pin-facing surface. The
crankshaft having the third exemplary configuration includes arms
incorporating no weight. In the crankshaft having the third exemplary
configuration, the arms incorporating no weight have a recess in the
pin-facing surface in addition to the recess in the journal-facing surface.
[0058]
According to the present embodiment, in order to form recesses in the
-rarms,
the crankshaft production method includes a rough forging step to
obtain a rough forged blank, and a finish forging step to obtain a finish forged
blank from the rough forged blank. The arms of the rough forged blank have
first excess portions, and in the finish forging step, the first excess portions
are deformed to bulge toward the journals. The shapes of the arms of the
forged crankshaft (finished product) and of the rough forged blank will
hereinafter be described.
[0059]
FIGS. 2A to 2D are diagrams showing an arm of the crankshaft
having the first exemplary configuration according to the present invention.
FIG. 2A is a perspective view, FIG. 2B is a view showing the journal-facing
surface, and FIG. 2C is a side view. FIG. 2D is a sectional view along the line
IID-IID in FIG. 2B. FIGS. 2A to 2D show an arm of the finished crankshaft,
and the shape is attained, for example, by applying trimming to the finish
forged crankshaft. FIGS. 2A to 2D show an arm (incorporating a weight)
that is extracted from the crankshaft to represent the arms of the crankshaft,
and the other arms of the crankshaft are not shown. FIG. 2C is a view from
the direction indicated by the dashed arrow in FIG. 2B.
[0060]
As shown in FIGS. 2A to 2D, in the first exemplary configuration, each
of the arms A has a recess in the journal(J)-facing surface, in a region near the
adjacent pin P, in an area As inside of both side portions (two side portions) Aa
and Ab. The side portions Aa and Ab in the region near the pin P bulge
toward the journal J, and the thicknesses of the side portions (Aa, Ab) are
greater than the thickness of the recess. The side portions mean surfaces
and portions therearound at the edges of the arm A in the width direction (a
direction perpendicular to the decentering direction of the pins), that is, the
end portions of the arm A in the width direction. The area As is on the
surface opposite to the pin P, that is, on the journal(J)-facing surface.
[0061]
In the crankshaft having the first exemplary configuration, the side
portions Aa and Ab of each arm A are kept as thick as an arm without a recess.
Also, in terms of results, a recess is formed in the journal(J)-facing surface of
the arm A. Accordingly, the forged crankshaft having the first exemplary
configuration can achieve a reduction of weight by the recesses formed in the
arms A. Additionally, the maintained thicknesses of the side portions Aa and
Ab of the arm A ensure stiffness. In other words, the greater thicknesses of
the side portions Aa and Ab in the region near the adjacent pin P than the
thickness of the recess ensure stiffness.
[0062]
The sectional shape of the area As inside of the side portions Aa and
Ab (the bottom shape of the recess) is preferably convex and bulges in the
center with respect to the width direction as shown in FIG. 2D. In other
words, it is preferred that the thickness of the area As gradually decreases
with increasing distance from the center with respect to the width direction.
Since the bottom shape of the recess is convex and bulges in the center with
respect to the width directioh, the bending stiffness can be improved. It is
possible to form a recess with this bottom shape by making the sectional
shape of the central portion with respect to the width direction arc-shaped,
semi-elliptic or parabolic, for example.
[0063]
FIGS. 3A to 3C are diagrams showing a rough arm of a rough forged
blank for the crankshaft having the fust exemplary configuration. FIG. 3Ais
a view showing the rough-journal-facing surface. FIG. 3B is a side view.
FIG. 3C is a sectional view along the line nrc-nrc in FIG. 3A. FIGS. 3A to
3C show a rough arm (incorporating a rough weight W') extracted from the
rough forged blank having a crankshaft shape. FIG. 3B is a view from the
direction indicated by the dashed arrow in FIG. 3A.
[0064]
As shown in FIGS. 3A to 3C, each of the rough arms A' before the
finish forging step (after the rough forging step) has a surface shape in
agreement with the bottom shape of the recess after the finish forging step in
the rough-journal(J')-facing surface, in a region near the adjacent rough pin P',
in an area As' inside of the side portions Aa' and Ab'. The surface shape
extends smoothly to the side portions Aa' and Ab' in the region near the rough
pin P'. Accordingly, the thicknesses of the side portions Aa' and Ab' are
2J
smaller than those after the finish forging step.
[0065]
Each of the rough arms A' has, in the region near the adjacent rough
pin P', first excess portions Aaa and Aba on the outer peripheries of the side
portions Aa' and Ab'. The first excess portions Aaa and Aba protrude
respectively from the outer peripheries of the side portions Aa' and Ab' in the
region near the adjacent rough pin P'. The first excess portions Aaa and Aba
are plate-shaped, and extend along the outer peripheries of the side portions
Aa' and Ab' in the region near the adjacent rough pin P'. The thicknesses of
the first excess portions Aaa and Aba are substantially equal to or smaller
than the thicknesses of the bases thereof, that is, the side portions Aa' and Ab'.
[0066]
FIGS. 4A to 4D are diagrams showing an arm of the crankshaft
having the second exemplary' configuration according to the present invention.
FIG. 4Ais a perspective view, FIG. 4B is a view showing the pin-facing surface,
and FIG. 4C is a side view. FIG. 4D is a sectional view along the line
IVD-IVD in FIG. 4B. FIGS. 4A to 4D show one of the arms (incorporating a
weight) of the crankshaft. FIG. 4C is a view from the direction indicated by
the dashed arrow in FIG. 4B.
[0067]
In the second exemplary configuration, as in the first exemplary
configuration, each of the arms A has thick side portions in a region near the
adjacent pin P, and has a recess in the journal(J)-facing surface. Further, in
the second exemplary configuration, each of the arms A incorporating a weight
has a recess in the pin(P)-facing surface, in a region near the adjacent journal
J, in an area At inside of both side portions (two side portions) Ac and Ad, as
shown in FIGS. 4A to 4D. The side portions Ac and Ad in the region near the
journal J bulge toward the adjacent pin P, and the thicknesses of the side
portions Ac and Ad are greater than the thickness of the recess. The area At
is on the surface opposite to the journal J, that is, on the pin(P)-facing surface.
[0068]
In the crankshaft having the second exemplary configuration,
regarding all the arms A, the side portions in the region near the adjacent pin
-y{-
P are thick, and a recess is formed in the journal(J)-facing surface.
Regarding the arms A incorporating a weight, further, the side portions Ac
and Ad in the region near the adjacent journal J are kept as thick as those of
an arm A without a recess, and in terms of results, a recess is formed also in
the pin(P)-facing surface.
[0069]
Accordingly, the forged crankshaft having the second exemplary
configuration can achieve a reduction of weight by the recesses formed in the
respective journal(J)-facing surfaces of all the arms A and the recesses formed
in the respective pin(P)-facing surfaces of the arms A incorporating a weight.
Additionally, the maintained thicknesses of the side portions Aa and Ab in the
region near the adjacent pin P and the side portions Ac and Ad in the region
near the adjacent journal J ensure the stiffness. In other words, the greater
thicknesses of the side portions Aa and Ab in the region near the adjacent pin
P and the greater thicknesses of the side portions Ac and Ad in the region near
the adjacent journal J than the thicknesses of the recesses ensure stiffness.
[0070]
Regarding the arms A incorporating a weight, the sectional shape of
the area At inside of the side portions Ac and Ad (the bottom shape of the
recess) is preferably convex and bulges in the center with respect to the width
direction as shown in FIG. 4D. In other words, it is preferred that the
thickness of the area At gradually decreases with increasing distance from the
center with respect to the width direction. Since the bottom shape of the
recess is convex and bulges in the center with respect to the width direction,
the bending stiffness can be improved. It is possible to form a recess with
this bottom shape by making the sectional shape of the central portion with
respect to the width direction arc-shaped, semi -elliptic or parabolic, for
example.
[0071]
FIGS. 5A to 5C are diagrams showing a rough arm of a rough forged
blank for the crankshaft having the second exemplary configuration. FIG.
5A is a view showing the rough-pin-facing surface. FIG. 5B is a side view.
FIG. 5C is a sectional view along the line VC-VC in FIG. 5A. FIGS. 5A to 5C
show one of the rough arms (incorporating a rough weight) of the rough forged
blank. FIG. 5B is a view from the direction indicated by the dashed arrow in
FIG. 5A.
[0072]
In the second exemplary configuration, as in the first exemplary
configuration, each the rough arms A' before the finish forging step (after the
rough forging step) has a surface shape in agreement with the bottom shape of
the recess after the finish forging step, in the rough-journal(J')-facing surface,
in a region near the adjacent rough pin P', in an area As' inside of the side
portions Aa' and Ab'. Also, as in the first exemplary configuration, each of
the rough arms A' has, in the region near the adjacent rough pin P', first
excess portions Aaa and Aha on the outer peripheries of the side portions Aa'
and Ab'. The first excess portions Aaa and Aha protrude respectively from
the outer peripheries of the side portions Aa' and Ab' in the region near the
adjacent rough pin P'.
[0073]
Additionally, each of the rough arms A' incorporating a rough weight
W' has a surface shape in agreement with the bottom shape of the recess after
the finish forging step in the rough·pin(P')-facing surface, in a region near the
adjacent rough journal J', in an area At' inside of the side portions Ac' and Ad'.
The surface shape extends smoothly to the side portions Ac' and Ad' in the
region near the adjacent rough journal J'. Accordingly, the thicknesses of the
side portionsAc' and Ad' are smaller than those after the finish forging step.
[0074]
Each of the rough arms A' incorporating a rough weight W' further
has, in the region near the adjacent rough journal J', second excess portions
Aca and Ada on the outer peripheries of the side portions Ac' and Ad'. The
second excess portions Aca and Ada protrude respectively from the outer
peripheries of the side portions Ac' and Ad' in the region near the adjacent
rough journal J'. The second excess portionsAca and Ada shown in FIGS. 5A
to 5C are plate-shaped, and extend along the outer peripheries of the side
portions Ac' and Ad' in the region near the adjacent rough journal J'. The
thicknesses of the second excess portions Aca and Ada are substantially equal
to or smaller than the thicknesses of the bases thereof, that is, the side
portions Ac' and Ad'.
[0075]
FIGS. 6A and 6B are diagrams showing an arm of the crankshaft
having the third exemplary configuration according to the present invention.
FIG. 6Ais a view showing the pin-facing surface. FIG. 6B is a sectional view
along the line VIB-VIB in FIG. 6A. FIGS. 6Aand 6B show one of the arms of
the finished crankshaft, and the shape is attained, for example, by applying
trimming to the finish forged blank.
[0076]
The crankshaft having the third exemplary configuration includes a
plurality of arms, and only some of the arms incorporate a weight. FIGS. 6A
and 6B show one of the arms not incorporating a weight, that is, one of the
unweighted arms of the crankshaft.
[0077]
In the third exemplary configuration, as in the first exemplary
configuration, each of the arms, whether incorporating a weight or not, has
thick side portions in a region near the adjacent pin P and has a recess in the
journal-facing surface, though it is not shown in the drawings. In the third
exemplary configuration, as shown in FIGS. 6A and 6B, each of the
unweighted arms A further has a recess in the pin P-facing surface, in a region
near the adjacent journal J, in an area At inside of side portions Ac and Ad.
The side portions Ac and Ad in the region near the adjacent journal J bulge
toward the adjacent pin P, and the thicknesses of the side portions Ac and Ad
are greater than the thickness of the recess.
[0078]
In the crankshaft having the third exemplary configuration, regarding
all the arms A, the side portions Aa and Ab in the region near the adjacent pin
Pare thick, and a recess is formed in the journal J-facing surface. Regarding
the unweighted arms A, further, the side portions Ac and Ad in the region
near the adjacent journal J are kept as thick as those of an arm A without a
recess. Also, in terms of results, each of the unweighted arms A obtains a
recess in the pin P-facing surface.
[0079]
Accordingly, the forged crankshaft having the third exemplary
configuration can achieve a reduction of weight by the recesses formed in the
respective journal J-facing surfaces of all the arms A and the recesses formed
in the respective pin P-facing surfaces of the unweighted arms A.
Additionally, the maintained thicknesses of the side portions Aa and Ab in the
region near the adjacent pin P and the side portions Ac and Ad in the region
near the adjacent journal J ensure stiffness. In other words, the greater
thicknesses of the side portions Aa and Ab in the region near the pin adjacent
P and the greater thicknesses of the side portions Ac and Ad in the region near
the adjacent journal J than the thicknesses of the recesses ensure stiffness.
[0080]
Regarding the unweighted arms A, the sectional shape of the area At
inside of the side portions Ac and Ad (the bottom shape of the recess) is
preferably convex and bulges in the center with respect to the width direction
as shown in FIG. 6B. In other words, it is preferred that the thickness of the
area At gradually decreases with increasing distance from the center with
respect to the width direction. Since the bottom shape of the recess is convex
and bulges in the center with respect to the width direction, the bending
stiffness can be improved. It is possible to form a recess with this bottom
shape by making the sectional shape of the central portion with respect to the
width direction arc-shaped, semi-elliptic or parabolic, for example.
[0081]
FIGS. 7A and 7B are diagrams showing a rough arm of a rough forged
blank for the crankshaft having the third exemplary configuration. FIG. 7A
is a view showing the rough-pin-facing surface. FIG. 7B is a sectional view
along the line VIIB-VIIB in FIG. 7 A. FIGS. 7 A and 7B show one of the rough
unweighted arms of the rough forged blank having a crankshaft shape.
[0082]
In the third exemplary configuration, as in the first exemplary
configuration, each of the rough arms A' before the finish forging step (after
the rough forging step), whether incorporating a rough weight or not, has a
surface shape in agreement with the bottom shape of the recess after the
-~sfinish
forging step, in the rough-journal(J')-facing surface, in a region near the
adjacent rough pin P', in an area inside of the side portions, though it is not
shown in the drawings. Also, as in the first exemplary configuration, each of
the rough arms A' has, in the region near the adjacent rough pin P', first
excess portions Aaa and Aba on the outer peripheries of the side portions.
The first excess portions Aaa and Aha protrude respectively from the outer
peripheries of the side portions in the region near the adjacent rough pin P'.
[0083]
Additionally, each of the rough unweighted arms A' has a surface
shape in agreement with the bottom shape of the recess after the finish
forging step, in the rough-pin(P')·facing surface, in a region near the adjacent
rough journal J', in an area At' inside of the side portions Ac' and Ad'. The
surface shape extends smoothly to the side portions Ac' and Ad' in the region
near the adjacent rough journal J'. Accordingly, the thicknesses of the side
portions Ac' and Ad' are smaller than those after the finish forging step.
[0084]
Each of the unweighted rough arms A' further has, in the region near
the adjacent rough journal J', second excess portions Aca and Ada on the outer
peripheries of the side portions Ac' and Ad'. The second excess portions Aca
and Ada protrude respectively from the outer peripheries of the side portions
Ac' and Ad' in the region near the adjacent rough journal J'. The second
excess portions Aca and Ada are plate-shaped, and extend along the outer
peripheries of the side portions Ac' and Ad' in the region near the adjacent
rough journal J'. The thicknesses of the second excess portions Aca and Ada
are substantially equal to or smaller than the thicknesses of the bases thereof,
that is, the side portions Ac' and Ad'.
[0085]
2. Production Process of Forged Crankshaft
A forged crankshaft production method according to the present
embodiment includes a rough forging step and a finish forging step. Before
the rough forging step, for example, a preforming step may be executed as in a
conventional production process. After the finish forging step, for example, a
trimming step may be additionally executed as in a conventional production
-rprocess.
Further, a coining step may be executed after the trimming step, as
needed. When adjustment of placement angles of the pins is necessary, a
twisting step is executed after the trimming step. These steps are to apply
hot working and are to be executed sequentially. The forged crankshaft
production method according to the present embodiment may employ a
conventional production method m the parts other than the distinctive
features of the present invention. For example, the production process
according to the present invention may include at least one of the steps shown
in FIGS. lA to lF or one or more steps adapted from those shown in FIG. lA
to lF for the present invention as steps other than the distinctive features of
the present invention.
[0086]
The preforming step includes a rolling step and a bending step, for
example. In the rolling step and the bending step, the volume of a billet
(starting material) is distributed, whereby a bent blank is obtained.
[0087]
In the rough forging step, for example, the bent blank obtained by the
preforming step is die-forged, whereby a rough forged blank with flash is
obtained. The obtained rough forged blank is roughly in the shape of a
crankshaft as shown in FIGS. 3A to 3C. The rough forged blank includes
rough journals J', rough pins P', rough arms A' and others. Additionally, each
of the rough arms A' of the rough forged blank has, in a region near the
adjacent pin, first excess portions Aaa and Aba protruding from the outer
peripheries of side portions Aa' and Ab'. Accordingly, in the rough forging
step, dies that allow formation of such excess portions (first excess portions or
second excess portions, or first and second excess portions) are used for the die
forging.
[0088]
In the die forging in the rough forging step, the draft never becomes a
reverse draft. Specifically, the draft of the dies never becomes a reverse draft
at any portions corresponding to the bottom surfaces of the recesses to be
formed in the rough-journal(J')-facing surfaces of the rough arms A' (inner
areas As') and at any portions corresponding to the first excess portions Aaa
-fand
Aba. In other words, the rough forged blank having the above-described
shape can be produced by use of dies having no reverse draft. Therefore, the
rough forging by die forging can be carried out with no trouble, and a rough
forged blank as shown in FIGS. 3A to 3C can be obtained. Also, in producing
a rough forged blank as shown in FIGS. 5A to 5C and in producing a rough
forged blank as shown in FIGS. 7A and 7B, the draft never becomes a reverse
draft at any portions corresponding to the bottom surfaces of the recesses to be
formed in the rough-journal(P')-facing surfaces of the rough arms A' (inner
areas At') and at any portions corresponding to the second excess portions Aca
and Ada. In other words, the rough forged blank having the above-described
shape can be produced by use of dies having no reverse draft. Therefore, the
rough forging by die forging can be carried out with no trouble.
[0089]
In the finish forging step, the rough forged blank is die-forged by use
of a pair of dies. Then, a finish forged blank with flash is obtained. The
finish forged blank has a shape substantially in agreement with the finished
crankshaft. In the finish forging step, further, the first excess portions are
deformed by the pair of dies to bulge toward the rough journals. Thereby, the
thicknesses of the side portions of the rough arms in the region near the rough
pins are increased. The details of the finish forging step will be described
later.
[0090]
In the trimming step, for example, while the finish forged blank with
flash is held, the flash is punched out. Thereby, the flash is removed from the
finish forged blank, and then, a forged blank with no flash is obtained. In the
coining step, main parts of the forged blank with no flash may be slightly
pressed by dies from above and below so that the forged blank with no flash
can have the exact size and shape of a finished product. By the production
method according to the present invention, a forged crankshaft is obtained.
[0091]
3. Exemplary Process Flow of Finish Forging Step
FIGS. SA and 8B are sectional views showing an exemplary process
flow of the finish forging step of the crankshaft production method according
to the present invention. FIG. 8A shows the time when the rough forged
blank has been placed in a pair of dies, and FIG. 8B shows the time of
completion of reduction. FIGS. 8A and 8B are sectional views at a position
corresponding to the position indicated by the line IIIC-IIIC in FIG. 3A.
[0092]
In FIG. 8A, a rough forged blank 30 with flash, and a first pair of dies
10 are shown. The shape of the rough forged blank 30 with flash is the same
as the shape of the rough forged blank 30 with flash shown in FIGS. 3A to 3C.
The rough forged blank 30 has a crankshaft shape. FIG. 8A shows a rough
pin P' and two rough arms A' connected to the rough pin P', which are
extracted from the rough forged blank 30.
[0093]
The first pair of dies 10 includes an upper die (first upper die) 11 and a
lower die (first lower die) 12. The upper die 11 and the lower die 12 are
movable to separate from each other and movable to come close to each other.
In the die forging by use of the first pair of dies 10, while the rough forged
blank 30 is positioned between the upper die 11 and the lower die 12, the
upper die 11 and the lower die 12 are moved toward each other. Thereby, the
upper die 11 and the lower die 12 are pressed against the rough forged blank
30, and the rough forged blank 30 is reduced.
[0094]
The upper die 11 and the lower die 12 have impressions to shape the
rough forged blank 30 and to deform the protruding excess portions. The
impressions reflect a shape substantially in agreement with the shape of a
finished product, that is, the crankshaft. However, the impressions do not
reflect the shapes of some parts of the crankshaft.
[0095]
Specifically, when recesses are to be formed in the respective
journal-facing surfaces of the arms (see FIGS. 2A to 2D), the impressions of
the dies do not reflect the shapes of the recesses in the respective
journal-facing surfaces. When recesses are to be formed in the respective
pin-facing surfaces of the arms (see FIGS. 4A to 4D, 6A and 6B), the
impressions of the dies do not reflect the shapes of the recesses in the
-¥-
respective pin-facing surfaces. If the impressions reflect the sha,pes of these
recesses, the impressions will partly have a reverse draft.
[0096]
In the process flow using the first pair of dies 10, first, the upper die 11
and the lower die 12 are separated from each other. Then, as shown in FIG.
8A, the rough forged blank 30 with flash is placed between the upper die 11
and the lower die 12.
[0097]
Next, the upper die 11 and the lower die 12 are moved toward each
other. More specifically, the upper die 11 is moved to the bottom dead point.
Thereby, as shown in FIG. 8B, the upper die 11 and the lower die 12 are
pressed against the rough forged blank 30, and the rough forged blank 30 is
reduced by the upper die 11 and the lower die 12. By the reduction by use of
the first pair of dies 10, the' rough forged blank 30 is formed into a shape
substantially in agreement with the finished crankshaft. Then, a finish
forged blank 31 as shown in FIG. 8B is obtained. Each part of the finish
forged blank 31 shown in FIG. 8B has a shape substantially in agreement
with the shape of the corresponding part of the finished forged crankshaft.
Therefore, in FIG. 8B, each part of the finish forged blank 31 is denoted by the
same reference symbol used for the corresponding part of the finished forged
crankshaft.
[0098]
In the forged crankshaft production method according to the present
embodiment, further, the first excess portions Aaa and Aha are deformed by
the first pair of dies 10 during the process of die forging (during the reduction
by use of the first pair of dies 10). For example, the first excess portions Aaa
and Aha are crushed by the first pair of dies 10 or are bent toward the rough
journals along the first pair of dies 10. Thereby, the first excess portions Aaa
and Aha are formed into shapes along the first pair of dies 10 and are caused
to bulge toward the rough journals. Consequently, the thicknesses of the side
portions Aa' and Ab' of each of the rough arms A' in the region near the
adjacent rough pin P' are increased.
[0099]
31
-?-
In the rough forged blank 30, as shown in FIG. SA, each of the rough
arms A' has a surface shape in agreement with the bottom surface of the
recess in the rough-journal-facing surface, in the region near the adjacent
rough pin P', in the area As' inside of the side portions Aa' and Ab'. The
surface shape of the inner area As' is kept during the die forging. Meanwhile,
during the process of die forging, the side portions Aa' and Ab' in the region
near the adjacent rough pin P' are caused to bulge toward the adjacent rough
journal. Consequently, in the rough forged blank after undergoing the finish
forging (in a finish forged blank), the rough-journal-facing surface of each
rough arm A' has a recess in the area As inside of the side portions.
[0100]
The central portion of the area As is convex as shown in FIG. 8B.
The border between the flash B and another part in the central portion may
be outside the line connecting the respective edges of the side portions Aa and
Ab of the finish forged blank 31. This structure facilitates the trimming.
[0101]
Next, the upper die 11 and the lower die 12 are separated from each
other. More specifically, the upper die 11 is moved up to the top dead point.
In this state, the rough forged blank after undergoing the reduction (finish
forged blank) is taken out.
[0102]
In the forged crankshaft production method according to the present
embodiment, which includes the finish forging step, the thicknesses of the side
portions Aa and Ab of each arm A in the region near the adjacent pin P can be
increased in the finish forging step. Also, a recess is formed in the
journal(J)-facing surface of the arm A. According to the present invention,
the finish forging step can be carried out by use of a pair of dies which does not
have a reverse draft.
[0103]
In the steps after the finish forging step (for example, in the trimming
step and the coining step), the shape of the recess in the journal-facing surface
of each arm is maintained. When a recess is formed in the pin -facing surface
of any arm (see FIGS. 4A to 4D, 6A and 6B), the shape of the recess in the
pin-facing surface is also maintained. For example, if the impressions of the
dies used to hold the finish forged blank in the trimming step do not reflect
the shapes of the recess as the impressions of the dies used in the finish
forging step, the shapes of the recesses can be maintained in the trimming
step. Further, if the impressions of the dies used to hold the finish forged
blank in the coining step do not reflect the shapes of the recess as the
impressions of the dies used in the finish forging step, the shapes of the
recesses can be maintained in the coining step.
[0104]
Thus, the forged crankshaft production method according to the
present embodiment provides a finished product, wherein each of the arms A
has thickened side surfaces Aa and Ab in the region near the adjacent pin P
\.._
and has a recess in the journal-facing surface. Accordingly, the forged
crankshaft production method according to the present embodiment allows
production of a forged crankshaft with a reduced weight and assured stiffness.
[0105]
In the forged crankshaft production method according to the present
embodiment, in the forging step, the first excess portions Aaa and Aha are
deformed by the pair of dies used for the die forging, whereby the thicknesses
of the side portions of each rough arm in the region near the adjacent rough
pin are increased. Thus, any special tool, such as a punch or the like, is not
necessary for the deformation of the first excess portions, and the deformation
can be carried out in a simple manner by use of the existing equipment. Also,
it is not necessary to make any changes to the conventional production
process.
[0106]
The forged crankshaft production method according to the present
embodiment is applicable to production of the forged crankshaft having the
second exemplary configuration as shown in FIGS. 4A to 4D. In this case, in
the rough forged blank, each of the rough arms incorporating a rough weight
has second excess portions Aca and Ada as shown in FIGS. 5A to 5C. The
second excess portions Aca and Ada protrude respectively from the outer
peripheries of the side portions of the rough arm A' in the region near the
adjacent rough journal J'.
[0107]
1-
During the process of die forging in the finish forging step, not only
are the first excess portions Aaa and Aba deformed by the dies, but the second
excess portions Aca and Ada provided to each of the rough arms A'
incorporating a rough weight shall be also deformed by the dies to bulge
toward the adjacent rough pin P'. Thereby, regarding each of the rough arms
A' incorporating a rough weight, the thicknesses of the side portions Ac' and
Ad' in the region near the adjacent rough journal J' can be increased. The
deformation of the second excess portions may be performed, for example, by
crushing or bending. Consequently, a crankshaft with a more reduced
weight and assured stiffness as shown in FIGS. 4A to 4D can be obtained.
[0108]
The forged crankshaft production method according to the present
embodiment is applicable to the forged crankshaft having the third exemplary
configuration as shown in FIGS. 6A and 6B. In the forged crankshaft, only
some of the arms A incorporate a weight, and the other arms A are
unweighted arms which incorporate no weight.
[0109]
In this case, in the rough forging step, second excess portions Aca and
Ada are formed for each of the unweighted arms A' as shown in FIGS. 7A and
7B. The second excess portions Aca and Ada protrude respectively from the
outer peripheries of the side portions of each of the rough unweighted arms A'
in the region near the adjacent rough journal J'.
[0110]
During the process of die forging in the finish forging step, not only
are the first excess portions Aaa and Aba deformed by the dies, but the second
excess portions Aca and Ada provided to each of the rough unweighted arms A'
shall be also deformed by the dies to bulge toward the adjacent rough pin P'.
Thereby, regarding each of the rough unweighted arms A', the thicknesses of
the side portions Ac' and Ad' in the region near the adjacent rough journal J'
can be increased. The deformation of the second excess portions may be
performed, for example, by crushing or bending. Consequently, a crankshaft
.,.
with a more reduced weight and assured stiffness as shown in FIGS. 6A and
6B can be obtained.
[0111]
According to the present embodiment, the above-described second die
may be used in combination with the first pair of dies in the finish forging step.
Alternatively, in the finish forging step, the first pair of dies, the
above-described second die and the holder may be used in combination.
[0112]
An example of the finish forging step in which the first pair of dies, the
second die and the holder are used in combination is described with reference
to FIGS. 9A, 9B and 9C. In the example to be described below, the finish
forging is applied to a rough forged blank for a crankshaft having the first
exemplary configuration as shown in FIGS. 3A to 3C. However, the finish
forging by use of the second 'die and the holder in combination with the first
pair of dies is applicable also to other rough forged blanks (for example, rough
forged blanks for a crankshaft having the second exemplary configuration and
for a crankshaft having the third exemplary configuration). In the example
to be described below, both the second die and the holder are used in
combination with the first pair of dies. However, only the second die may be
used in combination with the first pair of dies.
[0113]
FIG. 9A is a view of a rough arm A' of the rough forged blank from the
adjacent rough journal J'. FIG. 9B is a view from the direction indicated by
the dashed arrow in FIG. 9A. FIG. 9C is a sectional view along the line
IXC-IXC in FIG. 9A. The matters already described with reference to FIGS.
8A and 8B may not be described repeatedly.
[0114]
In this example, the second die 20 is used. Further, the holder 23 is
used to hold the lower surface of the flash B. In this example, a plurality of
holders 23 are used. The drawings which will be referred to below show only
a part of the rough forged blank and accordingly show only one holder 23.
[0115]
First, the dies are placed as shown in FIGS. 9A to 9C. Specifically, a
·f·
rough forged blank 30 is placed between the upper die 11 and the lower die 12
of the first pair of dies 10. At this time, the lower surface of the flash B is
held by the plurality of holders 23. The plurality of holders 23 hold the rough
forged blank 30 such that the flash B is kept in a horizontal posture. Further,
a second pair of dies 20 is placed such that a second die 21 and a second die 22
face each other across the flash B. At this time, the second pair of dies 20 is
placed against the surface of the rough crank arm A' having the first excess
portions Aaa and Aba, in the portion opposite to the adjacent rough pin P'.
More specifically, the second pair of dies 20 is placed against the
rough-journal(J')-facing surface of the rough crank arm A', in the area As'
inside of the two side portions Aa' and Ab' in the region near the adjacent
rough pin P'. With this arrangement, the second pair of dies 20 prevents
deformation of the area As' during the finish forging step. In order to prevent
the deformation of the area As', the surfaces of the second pair of dies 20 to
contact the area As' preferably have shapes that fit in the inner area As', that
is, shapes proper for a tight connection with the area As'. In a case where the
rough arm A' has only one first excess portion in one side portion, only one of
the second dies may be placed against the surface, in the side where the first
excess portion is provided.
[0116]
There is no limit as to the order of placing the dies. In a preferred
example, the rough forged blank 30 placed between the upper die 11 and the
lower die 12 is held by the holders 23, and thereafter, the second pair of dies
20 is placed against the rough forged blank 30.
[0117]
Next, as has been described with reference to FIG. 8B, the upper die
11 and the lower die 12 are moved toward each other, and the rough forged
blank 30 is reduced by the upper die 11 and the lower die 12. Along with the
reduction by use of the first pair of dies 10, the rough forged blank 30 is
formed into a shape substantially in agreement with the shape of the finished
forged crankshaft. At this time, the first excess portions Aaa and Aba are
deformed to bulge toward the rough journal J' as described above.
[0118]
·f·
In a case where the upper die 11 is moved down to perform the finish
forging, the upper die 11 first comes into contact with the rough forged blank
30, and thereafter, the lower die 12 comes into contact with the rough forged
blank 30 along with the downward movement of the rough forged blank 30.
As the upper die 11 is moving further down, the rough forged blank 30 is
deformed. In this case, along with the movement of the first pair of dies 10
(downward movement of the upper die 11), the flash B moves down.
Accordingly, the holders 23 move down along with the downward movement of
the flash B while keeping holding the lower surface of the flash B. In other
words, the holders 23 moves following the flash B which changes its position
along with the movement of the first pair of dies 10. By use of the holders 23,
the finish forging can be performed while the rough forged blank 30 is kept in
a stable posture.
[0119]
The finish forging is carried out in the manner described above, and
thereby, a finish forged blank with flash is obtained. After the finish forging
step, the second pair of dies 20 and the holders 23 are retracted as needed,
and the finish forged blank is taken out.
[0120]
In a case where the rough forged blank includes second excess
portions, the third die may be used in combination with the first pair of dies in
the finish forging step. Alternatively, the second die and the third die may be
used in combination with the first pair of dies in the finish forging step. It is
also possible to use the second die, holder and the third die in combination
with the first pair of dies in the finish forging step.
[0121]
An example of the finish forging step in which the holder and the third
die are used in combination with the first pair of dies is described with
reference to FIGS. lOA, lOB and lOC. In the example to be described below,
the finish forging is applied to a rough forged blank for a crankshaft having
the second exemplary configuration as shown in FIGS. 5A to 5C. However,
the finish forging by use of the holder and the third die in combination with
the first pair of dies is applicable also to other rough forged blanks (for
.,.
example, a rough forged blank for a crankshaft having the third exemplary
configuration). In the example to be described below, both the holder and the
third die are used in combination with the first pair of dies. However, only
the third die may be used in combination with the first pair of dies. In either
case, the second die can be further used.
[0122]
FIG. lOA is a view of a rough·pin(P')-facing surface of a rough arm A'
ofthe rough forged blank. FIG. lOB is a view from the direction indicated by
the dashed arrow in FIG. lOA FIG. lOC is a sectional view along the line
XC-XC in FIG. lOA. The matters that have been described with reference to
FIGS. 8A and 8B may not be described repeatedly.
[0123]
In this example, the third die 35 is used. Further, a plurality of
holders 23 are used to hold the lower surface of the flash B. The drawings
which will be referred to below show only a part of the rough forged blank and
accordingly show only one holder 23.
[0124]
First, the dies are placed as shown in FIGS. lOA to lOC. Specifically,
a rough forged blank 30 is placed between the upper die 11 and the lower die
12 of the first pair of dies 10. At this time, the lower surface of the flash B is
held by the plurality of holders 23. Further, a third pair of dies 35 is placed
such that a third die 36 and a third die 37 face each other across the flash B.
At this time, the third pair of dies 35 is placed against the surface of the rough
crank arm A' having the second excess portions Aca and Ada, in the portion
opposite to the adjacent rough journal J'. More specifically, the third pair of
dies 35 is placed against the rough-pin(P')-facing surface of the rough crank
arm A', in the area At' inside of the two side portions Ac' and Ad' in the region
near the adjacent rough journal J'. With this arrangement, the third pair of
dies 35 prevents deformation of the inner area At' during the finish forging
step. In order to prevent the deformation of the area At', the surfaces of the
third pair of dies 35 to contact the area At' preferably have shapes that fit in
the area At', that is, shapes proper for a tight connection with the area At'. In
a case where the rough arm A' has only one second excess portion in one side
·I
portion, only one of the third dies may be placed against the surface, in the
side where the second excess portion is provided.
[0125]
There is no limit as to the order of placing the dies. In a preferred
example, the rough forged blank 30 placed between the upper die 11 and the
lower die 12 is held by the holders 23, and thereafter, the third dies 35 are
placed against the rough forged blank 30. In a case where the second dies 20
are used in combination with the third dies 35, the dies are arranged as shown
in FIGS. 9A to 9C.
[0126]
Next, as has been described with reference to FIG. 8B, the upper die
11 and the lower die 12 are moved toward each other, and the rough forged
blank 30 is reduced by the upper die 11 and the lower die 12. Along with the
reduction by use of the first pair of dies 10, the rough forged blank 30 is
formed into a shape substantially in agreement with the shape of the finished
forged crankshaft. At this time, the second excess portions Aca and Ada are
deformed to bulge toward the rough journal P' as described above. The
holders 23 move in the above-described manner.
[0127]
The finish forging is carried out in the manner described above, and
thereby, a finish forged blank with flash is obtained. In the finish forging
step of the production method according to the present invention, as described
above, the rough forged blank 30 can be shaped by one·time reduction by use
of the first pair of dies 10, and during the shaping of the rough forged blank 30,
the excess portions (first excess portions or second excess portions, or first and
second excess portions) can be deformed. After the finish forging step, the
third dies 35 and the holders 23 are retracted as needed, and the finish forged
blank is taken out.
[0128]
The first pair of dies 10, the second die 20, the holder 23 and the third
die 35 have shapes that do not block one another's movements and move in
such a manner not to block one another's movements. These dies are moved
in such a manner to allow the above-described operation. These dies may be
-rmoved
by a conventional mechanism. For example, the second die 20, the
holder 23 and the third die 35 may be moved by a mechanism used in a
conventional double-acting pressing machine.
[0129]
In either of the crankshafts having the first, the second and the third
configurations, all of the arms, whether incorporating a weight or not, have a
recess in the journal-facing surface, in a region near the adjacent pin. In a
crankshaft to be produced by the crankshaft production method according to
the present embodiment, some of the arms may have a recess in the
journal-facing surface, in a region near the adjacent pin. In other words, at
least one of the rough arms of a rough forged blank have first excess portions.
Which of the rough arms are to be provided with the first excess portions shall
be determined as appropriate according to the bending stiffness and the
torsional stiffness required ill each arm of the crankshaft and in what portions
the crankshaft is required to have high stiffness.
[0130]
As described above, an arm incorporating a weight (which will be
referred to as a "weighted arm") preferably has a recess in the pin-facing
surface, in a region near the adjacent journaL In a crankshaft including a
plurality of weighted arms, all of the weighted arms may have a recess in the
pin-facing surface, in a region near the adjacent journaL Alternatively, some
of the weighted arms may have a recess in the pin-facing surface. In other
words, at least one of the rough weighted arms of a rough forged blank may
have second excess portions. Which of the rough arms are to be provided
with the second excess portions shall be determined as appropriate according
to the bending stiffness and the torsional stiffness required in each arm of
the crankshaft and in what portions the crankshaft is required to have high
stiffness.
[0131]
As described above, an unweighted arm preferably has a recess in the
pin-facing surface, in a region near the adjacent journaL In a crankshaft
including a plurality of unweighted arms, all of the unweighted arms may
have a recess in the pin-facing surface, in a region near the adjacent journaL
-r-
Alternatively, some of the unweighted arms may have a recess in the
pin-facing surface. In other words, at least one of the rough unweighted
arms of a rough forged blank may have second excess portions. Which of the
rough arms are to be provided with the second excess portions shall be
determined as appropriate according to the bending stiffness and the torsional
stiffness required in each arm of the crankshaft and in what portions the
crankshaft is required to have high stiffness.
[0132]
The rough arm may have first excess portions on both sides in the
region near the adjacent rough pin as has been described in connection with
the first to the third exemplary configurations, or alternatively, the rough arm
may have a first excess portion only on one side in the region near the
adjacent rough pin. Even in a case where the rough arm has a first excess
portion only on one side in the region near the adjacent rough pin, the finish
forging step to cause the first excess portion to bulge toward the rough journal
provides an increase in the thickness of the side portion. This ensures
stiffness while reducing the weight. Which side of the rough arm is to be
provided with the first excess portion shall be determined as appropriate
according to the bending stiffness and the torsional stiffness required in the
arm and in what portion the arm is required to have high stiffness.
[0133]
In a case where the rough arm has a first excess portion only on one
side, it is preferred that the first excess portion is deformed by the first pair of
dies to bulge toward the rough journal while the second die is being pressed
against the rough-journal-facing surface of the rough arm, between the first
excess portion and the flash.
[0134]
In a case where a rough weighted arm has one or two second excess
portions, the rough weighted arm may have second excess portions on both
sides in the region near the adjacent rough journal as has been described in
connection with the second exemplary configuration. Alternatively, the
rough weighted arm may have a second excess portion on only one side in the
region near the rough journal. Even in a case where the rough arm has a
4-1
·f·
second excess portion only on one side in the region near the adjacent rough
pin, the finish forging step to cause the second excess portion to bulge toward
the rough pin provides an increase in the thickness of the side portion. This
ensures stiffness while reducing the weight. Which side of the rough arm is
to be provided with the second excess portion shall be determined as
appropriate according to the bending stiffness and the torsional stiffness
required in the arm and in what portion the arm is required to have high
stiffness. In a case where the rough arm has a second excess portion only on
one side, it is preferred that the second excess portion is deformed by the first
pair of dies to bulge toward the rough pin while the third die is being pressed
against the rough-pin-facing surface of the rough arm, between the second
excess portion and the flash.
[0135]
In a case where a rough unweighted arm has one or two second excess
portions, the rough unweighted arm may have second excess portions on both
sides in the region near the adjacent rough journal as has been described in
connection with the third exemplary configuration. Alternatively, the rough
unweighted arm may have a second excess portion on only one side in the
region near the rough journal. Even in a case where the rough arm has a
second excess portion only on one side in the region near the adjacent rough
pin, the finish forging step to cause the second excess portion to bulge toward
the rough pin provides an increase in the thickness of the side portion. This
ensures stiffness while reducing the weight. Which side of the rough arm is
to be provided with the second excess portion shall be determined as
appropriate according to the bending stiffness and the torsional stiffness
required in the arm and in what portion the arm is required to have high
stiffness. In a case where the rough arm has a second excess portion only on
one side, it is preferred that the second excess portion is deformed by the first
pair of dies to bulge toward the rough pin while the third die is being pressed
against the rough-pin-facing surface of the rough arm, between the second
excess portion and the flash.
INDUSTRIAL APPLICABILITY
-I-
[0136]
The present invention is efficiently utilized in a forged crankshaft
production method, for example, in a method for producing a forged
crankshaft to be mounted in a reciprocating engine.
LIST OF REFERENCE SYMBOLS
[0137]
1: forged crankshaft
J, J1 to J5: journal
P, P1 to P4: pin
Fr: front part
Fl: flange
A, A1 to AS: crank arm
W, W1 to ws: counterweight
J': rough journal
p•: rough pin
A': rough crank arm
w•: rough counterweight
Aa, Ab: side portion of an arm in a region near a pin
Aa', Ab': side portion of a rough arm in a region near a rough pin
Aaa, Aba: first excess portion
Ac, Ad: side portion of an arm in a region near a journal
Ac', Ad': side portion of a rough arm in a region near a rough journal
Aca, Ada: second excess portion
As: inner area of a journal-facing surface of an arm, inside of side
portions
As': inner area of a rough-journal-facing surface of a rough arm, inside
of side portions
At: inner area of a pin-facing surface of an arm, inside of side portions
At': inner area of a rough-pin-facing surface of a rough arm, inside of
side portions
B: flash
10: first pair of dies
11: upper die
12: lower die
20, 21, 22: second die
23: holder
so: rough forged blank
31: finish forged blank
35, 36, 37: third die

We claim:
1. A method for producing a forged crankshaft including journals serving
as a center of rotation, pins decentered from the journals, and crank arms
connecting the journals and the pins, wherein
the forged crankshatt further includes counterweights integrated with
all or some of the crank arms,
the method comprising:
a rough forging step of performing die forging to obtain a rough forged
blank with flash having a crankshaft shape; and
a finish forging step of applying die forging by use of a first pair of dies
to the rough forged blank to obtain a finish forged blank with flash, wherein:
the rough forged blank includes rough journals, rough pins, rough
crank arms, and rough cou~terweights, which correspond to the journals, the
pins, the crank arms, and the counterweights of the forged crankshaft,
respectively;
at least one of the rough crank arms has, in a region near an adjacent
one of the rough pins, a first excess portion protruding from an outer
periphery of a side portion of the rough crank arm; and
during the die forging in the finish forging step, the first excess
portion is deformed by the first pair of dies to bulge toward an adjacent one of
the rough journals.
2. The method for producing a forged crankshaft according to claim 1,
wherein
the rough crank arm has, in the region near the adjacent rough pin,
first excess portions respectively protruding from outer peripheries of two side
portions of the rough crank arm.
3. The method for producing a forged crankshaft according to claim 1 or
2, wherein
in the finish forging step, the first excess portion is deformed by the
first pair of dies to bulge toward the adjacent rough journal while a second die
·f·
is in contact with a rough-journal-facing surface of the rough crank arm at a
portion between the first excess portion and the flash.
4. The method for producing a forged crankshaft according to any one of
claims 1 to 3, wherein
in the finish forging step, the first excess portion is deformed by the
first pair of dies to bulge toward the adjacent rough journal while a holder is
holding a lower surface of the flash;
the holder moves along with a movement of the first pair of dies while
keeping holding the lower surface of the flash.
5. The method for producing a forged crankshaft according to any one of
claims 1 to 4, wherein
in the finish forging step, the first excess portion is deformed by
crushing or bending.
6. The method for producing a forged crankshaft according to any one of
claims 1 to 5, wherein
at least one of the rough crank arms has, in a region near an adjacent
one of the rough journals, a second excess portion protruding from an outer
periphery of a side portion of the rough crank arm; and
during the die forging of the rough forged blank in the finish forging
step, the second excess portion is deformed by the first pair of dies to bulge
toward the adjacent rough pin.
7. The method for producing a forged crankshaft according to claim 6,
wherein
in the finish forging step, the second excess portion is deformed by the
first pair of dies to bulge toward the adjacent rough pin while a third die is in
contact with a rough-pin-facing surface of the rough crank arm at a portion
between the second excess portion and the flash.
8. The method for producing a forged crankshaft according to claim 6 or
·~·
·f·
7, wherein
the rough crank arm has, in the regwn near the adjacent rough
journal, second excess portions respectively protruding from outer peripheries
of two side portions of the rough crank arm.
9. The method for producing a forged crankshaft according to any one of
claims 6 to 8, wherein
the rough crank arm having the second excess portion is the rough
crank arm incorporating a rough counterweight.
10. The method for producing a forged crankshaft according to any one of
claims 6 to 8, wherein
the rough crank arm having the second excess portion is the rough
I
crank arm not incorporating a rough counterweight.
11. The method for producing a forged crankshaft according to any one of
claims 6 to 10, wherein
in the finish forging step, the second excess portion is deformed by
crushing or bending.