The present invention relates to a method for producing a crankshaft by
hot forging.
BACKGROUND ART
[0002]
Reciprocating engines for motor vehicles, motorcycles, agricultural
machines, marine vessels and the like require a crankshaft to extract power by
converting reciprocating motion of pistons to rotary motion. Crankshafts are
generally categorized into two types: those of the type produced by die forging
and those of the type produced by casting. In particular, in cases where high
strength and high stiffness àre required, the firstly mentioned forged
crankshafts, which are superior in those properties, are often employed.
[0003]
In general, forged crankshafts are produced by using, as a starting
material, a billet having a circular or square cross section and having a constant
cross-sectional area along the entire length. In a production process of a
forged crankshaft, the billet is subjected to the steps of preforming, die forging,
trimming and coining in order. Typically, the preforming step includes the
steps of roll forming and bending, and the die forging step includes the steps of
block forging and finish forging.
[0004]
FIGS. 1(a)-1(f) are views schemarically illustrating a typical
conventional process for producing a forged crankshaft. A crankshaft 1
illustrated in FIG. 1(f) is intended to be mounted in a 4-cylinder engine, and is
a 4-cylinder 8-countelweight crankshaft. The crankshaft 1 includes: five
journals Ji to J5; four crank pins pl top4; a front part Fr; a flange Fl; and eight
crank arms (hereinafter referred to simply as "arms") A1 to A8 that connect the
journals Jl to J5 and the crank pins p1 to p4 to each other. The eight crank
arms A1 to A8 have counterweights (hereinafter referred to simply as
"weights") wl to w8, respectively. The weights v/t to wg are integrally
formed with the arms A1 to 48, respectively.
[000s]
In the following paragraphs, when the journals J1 to J5, the crank pins
Pl to P4, the arms Al to 48, and the weights w1 to wg are each collectively
referred to, a reference character "J" is used for the journals, a reference
character "P" for the crank pins, a reference character "4" for the arms, and a
reference character "'w" for the weights. Also, a crank pin p and a pair of
arms A (including weights w) which connects with the crank pin p are
collectively referred to as a "throw".
[0006]
According to the production method shown in FIG. 1, the forged
crankshaft 1 is produced in the following manner. First, a billet 2 shown in
FIG. 1(a), which has been previously cut to a predetermined length, is heated
by a heating furnace (for example, an induction heater or a gas atmosphere
furnace) and then is subjected to roll forming. In the roll forming step, the
billef 2 is rolled and reduced in cross section by grooved rolls, for exarnple, to
distribute its volume in the longitudinal direction, whereby a rolled blank 3,
which is an intermediare marerial, is formed (see FIG. l(b)). In rhe bending
step, the rolled blank 3 obtained by roll forming is partially pressed in a
direction perpendicular to the longitudinal direction to distribute its volume,
whereby a bent blank 4, which is a secondary intermediate material, is formed
(see FIG. 1(c)).
[0007]
Then, in the block forging step, the bent blank 4 obtained by bending is
press forged with a pair of upper and lower dies, whereby a forged blank 5
having a general shape of a crankshaft (endproduct) is formed (see FIG. 1(d)).
Then, in the finish forging step, rhe block forged blank 5 obtained by block
forging is further press forged with a pair of upper and lower dies, whereby a
forged blank 6 having a shape in agreement with the shape of the crankshaft
(end product) is formed (see FIG. 1(e)). In the block forging and the finish
forging, excess material flows out fi-om between the mutually opposed parting
surfaces of the dies, theleby forming flash. Thus, the block forged blank 5
and the finish forged blank 6 have large flash (5a, 6a) around the shape of the
crankshaft.
[0008]
In the trimming step, the finish forged blank 6 with the flash 6a,
obtained by fînish forging, is held by dies from above and below, and the flash
6a is removed by a cutting die. In this manner, the forged crankshaft 1 is
obtained as shown in FIG. 1(f). In the coining step, principal parts of the
forged crankshaft l, from which the flash has been removed, are slightly
pressed with dies from above and below and corrected to the size and shape of
the end product. In this regard, the principal parts of the forged crankshaft 1
are, e.g., shaft parts such as the journals J, the crank pins p, the front part Fr
and the flange Fl, and in some cases the arms A and the weights w. In this
manner, the forged crankshaft I is produced.
l000el
The production process shown in FIGS. 1(a) to 1(f) is applicable not
only for producing a 4-cylinder 8-counterweight crankshaft as illustrated in
FIG. 1(Ð but also for producing various other types of crankshafts. For
example, the production process is applicable for producing a 4-cylinder
4-counterweight crankshaft. In a 4-cylinder 4-couterweight crankshaft, some
of the eight arms A have weights w. For example, among the eight arms A,
the leading first arm 41, the trailing eighth arm Ag, and the two central ar.ms
+
(the fourth arm A4 and the fîfth arm A5) have weights w Also, the same
production process can be applied for ploducing crankshafts that are to be
mounted in a 3-cylinder engine, an inline 6-cylinder engine, a V-type
6-cylinder engine, an 8-cylinder engine, and the like. It is noted that, when
adjustment of the placement angle of the crank pins is necessary, a twisting step
is added after the trimming step.
[0010]
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, which are a principal component of a
reciprocating engine. Conventional techniques intended for weight reduction
of a forged crankshaft include the following.
[0011]
Patent Literatures I and 2 each disclose an arm having a hollow portion
in the journal-side surface of the arm, and disclose a method for producing a
crankshaft having the arm. The hollow portion in the arm is positioned on a
line connecting the axis of the journal and the axis of the crank pin (the line
hereinafter being referred to as an "alm centerline"), and the hollow portion is
depressed greatly and deeply toward the crank pin. The arm is reduced in
weight by an amount corresponding to the volume of the hollow portion. The
weight reduction of the arm leads to weight reduction of the counterweight,
which forms a pair with the arm, and this in turn leads to weight reduction of
the forged crankshaft as a whole. Furtherrnore, each of the arms disclosed in
Patent Literatures 1 and 2 has sufficient stiffness (torsional rigidity and flexural
rigidity) because the side portions near the crank pin, between which the arm
centerline is interposed, have a large thickness.
l00r2l
By providing a recess in the journal-side surface of the arm while
ensuring a large thickness at the side portions of the arm as described above, it
I
is possible to achieve weight leduction in combination with sufficient stiffness.
[0013]
However, forged crankshafts having such a unique shape are difficult to
produce using conventional production methods. The reason is that, when the
recess in the surface of the arm is to be formed in the die forging step, a
situation will occur in which the draft of the die becomes a reverse draft at the
site of the recess and therefore the formed folged blank cannot be removed
from the die.
[0014]
To address such a situation, the production methods disclosed in Patent
Literatures 7 and2 are configured as follows: 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 stepr a punch is pressed into the surface of the arm so that the
mark made by the punch forms the recess.
[001s]
In the meantime, by the production process illustrated in FIGS. 1(a) to
1(f), unnecessary large flash, which is not apart of an end product, is generated,
thereby leading to yield reduction. Thus, in the technology for manufacture of
forged crankshafts, improving the yield by suppressing flash has been an object.
Conventional techniques intended to attain this object include the followings.
[0016]
For example, Patent Literature 3 discloses a technique for producing a
crankshaft comprising shaped journals and crank pins, and roughly shaped arms.
According to the technique, the used as a blank is a stepped round bar having
reduced diameter regions at portions to be forrned into journals and crank pins
of a crankshaft, and portions to be formed into a pair of journals between which
a crank pin intervenes ale held with dies.
[0017]
In this state, the dies are axially moved closer to each other to
compressively deforrn^ the round bar blank, and a punch is applied to the
portion to be formed into a crank pin in a direction perpendicular to the axial
direction, whereby the portion to be formed into a crank pin is pressed into an
eccentric position. This operation for pressing the portion to be formed into a
crank pin into an eccentric position is repeated in succession for all crank
throws. In this way, the journals and the crank pins are shaped, and the arms
are roughly shaped.
[0018]
Also, Patent Literature 4 discloses a technique for producing a
crankshaft comprising shaped journals and crank pins, and roughly shaped arms.
According to the technique, the used as a blank is a simple round bar. one of
the two ends of the round bar is held with a stationary die, and the other end
thereof is held with a movable die. Also, portions to be formed into journals
are held with journal dies, and por.tions to be formed into crank pins are held
with crank pin dies.
l001el
In this state, the movable die, the journal dies and the crank pin dies are
axially moved toward the stationary die to compressively deform the round bar
blank. At the same time, the crank pin dies are moved in an eccentric
. direction perpendicular to the axial direction to press the portions to be formed
into the crank pins into eccentric positions. In this way, the journals and the
crank pins are shaped, and the arms are roughly shaped.
[0020]
With both the techniques disclosed in Patent Literatures 3 and 4, no
flash is generated, and therefore, a significant improvement in yield can be
expected.
CITATION LIST
PATENT LITERATURE
[0021]
Patent Literature Japanese Patent Application Publication No.
2012-7726
Patent Literature
2010-230027
Patent Literature
2008- 155275
Patent Literature
2011-t6t496
2: Japanese Patent Application Publication No.
3: Japanese Patent Application Publication No.
Japanese Patent Application Publication No.
SUMMARY OF INVENTION
TECHNICAL PROBLEM
100221
with the techniques disclosed in Patent Literatures 1 and 2, it is
possible to make a recess in the journal-side surface of an arm while keeping
the both side portions of the arm thick. Thereby, it is possible to produce a
forged crankshaft having a reduced weight and sufficient stiffness.
[0023]
According to these techniques, however, a punch is applied to the
surface of the arm to deform the whole arm, thereby making a recess in the
surface of the arm, and a great force is needed for the application of a punch.
Therefore, a special mechanism for exerting a force for the application of a
punch is required, and maintenance of the punching performance must be
considered.
100241
With both the techniques disclosed in Patent Literatures 3 and 4, no
flash is generated, and therefore a significant improvement in yield can be
expected. However, neither of these documents discusses weight reduction of
a forged crankshaft, and these techniques do not meet the need for weight
reduction.
[002s]
It is an object of the present invention to provide a method for
producing a forged crankshaft that allows for a simple and higher-yield
production process ofa forged crankshaft having reduced weight and sufficient
stiffness.
SOLUTION TO PROBLEM
100261
A production method of a forged crankshaft according to an
embodiment of the present invention is a method for producing a forged
crankshaft which includes: journals that define a center of rotation; crank pins
that are eccentric with respect to the journals; and crank arms, each of the crank
arms connecting a corresponding one of the journals to a corråsponding one of
the crank pins. The production method includes: a preforming step of forming
a preformed blank with no flash, the preformed blank including a shape of the
crankshaft, wherein the crank arm has an excess projecting portion at an outer
periphery of each of side portions near the crank pin, the excess projecting
portion projecting from the outer periphery; a die forging step of forming a
forged blank with flash by pressing the preformed blank formed in the
preforming step with a pair of first dies; and a trimming step of rernoving the
flash from the forged blank formed in the die forging step. In the die forging
step, while a second die is abutted against an areain a surface of the crank arm
to hold the area, the surface being adjacent to the journal, the area not including
areas of the side portions, by the first dies the excess projecting portions of the
crank arm aÍe deformed so as to increase the side portions of the crank arm in
thickness.
100271
In the production method, the second die preferably has a guide groove
so that excess material flowing out to be formed into the flash during the
pressing in the die forging step is guided through the guide groove.
[0028]
In the production method, during the pressing in the die forging step,
I
the second die is preferably moved in a pressing direction such that the second
die is positioned in a center of a space between the paired first dies.
ADVANTAGEOUS EFFECTS OF INVENTION
[002e]
According to the present invention, in the preforming step, excess
projecting portions protruding locally from outer peripheries of side portions of
the arm are formed, and in the die forging step, the excess projecting portions
are deformed by the f,rrst dies to increase the side portions in thickness.
Thereby, it is possible to form a recess in the journal-side surface of the arm
while keeping the side portions of the arm thick. Accordingly, this allows for
production of a forged crankshaft having reduced weight and sufficient
stiffness.
[0030]
In the die forging step, the second die is abutted against the journal-side
surface of the arm except at least the both side portions and holds the
journal-side surface. using the second die prevents problems that may be
accompanied with the die forging and allows for a simple process requiring no
large force for formation of the recess in the arm. Also, the preformed blank
to be subjected to the die forging step already has the shape of the crankshaft
and has no flash, which leads to suppression of formation of flash in the die
forging step and an improvement in yield.
BRIEF DESCRIPTION OF DRAWINGS
[0031]
IFIG. 1l FIGS. 1(a) to 1(f) are views schematically illusrrating a typical
conventional process for producing a forged crankshaft, FIG. 1(a) showing a
billet, FIG. 1(b) showing a rolled blank, FIG. l(c) showing a bent blank, FIG.
1(d) showing a block forged blank, FIG. l(e) showing a finish forged blank,
and FIG. 1(f) showing a crankshaft.
lo
[FIG. 2] FIGS. 2(a) to 2(d) are views schemarically showing an
example of the shape of an arm of a crankshaft before subjected to forging
according to the present invention, FIG. 2(a) being a perspective view, FIG.
2(b) being a front view seen from the side of a journal, FIG. 2(c) being a top
view, and FIG. 2(d) being a sectional view along the line A-A.
[FIG. 3] FIGS. 3(a) to 3(d) are views schematically showing an
example of the shape of the arm of the crankshaft after subjected to forging
according to the present invention, FIG. 3(a) being a perspective view, FIG.
3(b) being a front view seen from the side of a journal, FIG. 3(c) being a top
view, and FIG. 3(d) being a sectional view along the line B-8.
[FIG. 4] FIGS. 4(a) to 4(c) are front views schematically illustrating an
example of movements of dies in a die forging step according to the present
invention, FIG. a(a) showing an early stage of the die forging step, FIG. 4(b)
showing an intermediate stage of the die forging step, and FIG. a(c) showing a
final stage of the die forging step.
[FIG. 5] FIGS. 5(a) and 5(b) are top views schematically showing an
example of arrangement of a second die in the die forging step according to the
present invention, FIG. 5(a) showing an early stage of the die forging step, and
FIG. 5(b) showing a f,inal srage of the die forging.
DESCRIPTION OF EMBODIMENTS
[0032]
A method for producing a forged crankshaft according to the present
invention will hereinafter be described with reference to the drawings.
[0033]
The method for producing a forged crankshaft according to the present
embodiment includes a preforming step, a die forging step and a trimming step.
All of the preforming step, the die forming step and the trimming step are hot
working.
[0034j
n
l. Shape ofArms of Crankshaft
FIGS. 2(a) to 2(f) are schematic views showing an example of the
shape of an arm of a crankshaft before subjected to forging according to the
present invention, FIG. 2(a) being a perspective view, FIG. 2(b) being a front
view seen from the side of a journal, FIG. 2(c) being a top view, and FIG. 2(d)
being a sectional view along the line A-A.
[003s]
FIGS. 3(a) to 3(d) are schematic views showing an example of the
shape of the arm of the crankshaft after subjected to forging according to the
present invention, FrG. 3(a) being a perspective view, FIG. 3(b) being a front
view seen from the side of a journal, FIG.3(c) being a top view, and FIG. 3(d)
being a sectional view along the line B-8.
[0036]
In FIGS. 2(a) to 2(d) and FIGS. 3(a) to 3(d), one arm (including a
weight) is illustrated as an example typical of the arms of the crankshaft, and
the other arms are not illustrated.
[0037]
with regard to the arm A after subjected to forging according to the
present embodiment, as illustrated in FIGS. 3(a) to 3(d), both sides bulge out
toward the journal J at portions (Aa, Ab) near the crank pin p, and accordingly,
the both side portions (Aa, Ab) are thicker. The arm A has a recess in the
surface on the side of the journal J, in an inner area As that is inward of the
both side portions (Aa, Ab). The side portions (Aa, Ab) of the arm A mean
side surfaces of the arm A and portions therearound. In other words, the side
portions (Aa, Ab) of the arm A mean end portions in the width direction of the
arm A (in a direction perpendicular to a plane including the axis of the journal J
and the axis of the crank pin p).
[0038]
Thus, the arm A after subjected to forging has thick side portions (Aa,
Ab) and a recess in the journal J-side surface. This shape of the arm A is
IY
maintained after the trimming step. According to the present embodiment, the
recess formed in the surface of the arm A allows for weight reduction of the
forged crankshaft. Moreover, the thick side portions (Aa, Ab) of the arm A
allow for ensuring of sufficient stiffness of the forged crankshaft.
[003e]
On the other hand, the arm A before subjected to forging, as illustrated
in FIGS. 2(a) to 2(d), has a recess in the journal J-side surface, in the inner area
As that is inward of the both side portions (Aa, Ab). The recess agrees with
the recess in the arm after subjected to forging (the arm of the end product) and
extends smoothly to the both side portions (Aa, Ab) of the arm A.
Accordingly, the both side portions (Aa, Ab) of the arm A before subjected to
forging are thinner than those after subjected to forging (of the end product).
[0040]
on the outside of the side portions (Aa, Ab) of the arm A, excess
projecting portions (Aaa, Aba) are formed. The excess projecting portions
(Aaa, Aba) project from the outer peripheries (side surfaces) of the side
portions (Aa, Ab). The excess projecting portions (Aa, Ab) shown in FIGS.
2(a) to 2(d) ate plate-like portions extending in the width direction and
stretching along the outer peripheries of the side portions (Aa, Ab). The
excess projecting portions (Aaa, Aba) have thicknesses nearly equal to or less
than those of the side portions (Aa, Ab) at the base thereof.
[0041]
2. Method for Producing Forged Crankshaft
According to the present embodiment, as mentioned above, the method
for producing a forged crankshaft includes the preforming step, the die forging
step and the trimming step, which are hot working steps to be sequentially
carried out. when adjustment of the placement angle of the crank pins is
necessary, a twisting step is carried out after the trimming step.
[0042]
In the preforming step, a billet used as a starting material is subjected
t3
to preforming, whereby a preformed blank having a general shape of a
crankshaft (end product) is formed. The preformed blank, which is an
intermediate product, has an excess volume so that the preformed blank can be
subjected to fînish forging, which is accompanied with formation of flash, in
the die forging step.
[0043]
The preforming step includes, for example, the steps of roiling and
multiple bending (commonly so-called, "preforming"). In the rolling step,
from the billet that is a starting material, a rolled blank that is an intermediate
material is formed. In the rolling step, the billet is subjected to roll forming
using grooved rolls, whereby the volume of the billet is distributed in the
longitudinal direction. In the subsequent bending step, the rolled blank is
partially pressed in a direction perpendicular to the longitudinal direction,
whereby the volume of the rolled blank is further distributed. By applying
such bending to the rolled blank repeatedly, a preformed blank having the
above-mentioned shape can be formed.
[0044]
In the preforming step, the technique disclosed in Patent Literature 3 or
4 may be used to form the preformed blank. Alternatively, cross rolling or
full-enclosed die forging may be adopted.
[004s]
Thus, the preformed blank obtained in the preforming step has the
general shape of the crankshaft (end product) as illustrated in FIG. 2. The
preform blank obtained in the preforming step has excess projecting portions
(Aaa, Aba) formed around the arm, and a recess formed in the journal J-side
surface of the arm. As described above, the excess projecting portions (Aaa,
Aba) are provided along the outer peripheries of the side portions (Aa, Ab) of
the arm, in the part of the arm near the crank pin R so as to project from the
outer peripheries. The recess is formed in the journal J-side surface of the
arm, in the inner area As inward from the side portions (Aa, Ab), and the shape
t¿t
,/
of the recess agrees with that after forging (that of the end product). Further,
the preformed blank has no flash.
100461
For the formation of a preformed blank in the preforming step, a mold
tool, such as a punch, dies or the like, is used. The mold tool has an
impression that reflects the shape of the arm A, speci{ically, the shapes of the
excess projecting portions and the recess in the area As. In any of the portions
corresponding to the excess projecting portions (Aaa, Aba) along the outer
peripheries of the arm and the portion corresponding to the recess in the surface
of the arm, the draft of the die is never a reverse draft. Accordingly, the
formation of a preformed blank can be done with no problem.
100471
Next, the process goes to the die forging step. In the die forging step,
as in a typical conventional die forging step (more specifically, a block forging
step and a finish forging step), a pair of first dies is used. In the production
method according to the present embodiment, additionally, a second die is used.
[0048]
FIGS. 4(a) to 4(c) are front views schematically illustrating an example
of movements of the dies in the die forging step according to the present
invention. FIG. a(a) shows an earry stage of the die forging step, FIG. 4(b)
shows an intermediate stage of the die forging step, and FIG. 4(c) shows a final
stage of the die forging step. FIGS. 4(a) to 4(c) show a preformed blank (31,
32), a pair of upper and lower first dies 10, and a second die 20.
[004e]
FIGS- 5(a) and 5(b) are top views schematically illustrating an
example of arrangement of the second die in the die forging step according to
the present invention. FIG. 5(a) shows an early stage of the die forging step,
and FIG. 5(b) shows a fïnal srage of rhe die forging step. FIGS. 5(a) and 5(b)
show the preformed blank (31,32) and the second die 20. In FIGS. 5(a) and
5(b), for the sake of simplicity of the drawings, the pair of first dies is not
fsr.
i:i-:::::::::li,;:¿tJ:ji;J:.i-i.: ì a 1 I :r,::;
shown, and the second die 20 is shown in section along the center plane of the
arm.
[00s0]
The upper die li and the lower die 12 of the first dies 10 have
impressions. The impressions reflect the shape of the crankshaft illustrated in
Fig. 3, except the recess in the area As of the arm A. Specifically, the
impressions reflect the shapes of the journal J and the crank pin p. Also, the
impressions reflect the shape of the armA, except the recess in the areaAs.
l00s 1l
The upper die 11 and the lower die 12 of the fîrst dies 10 are widely
open at the sites corresponding to the recess in the area As of the arm A so that
the second die 20 is accommodated at the open sites.
[00s2]
The second die 20 has an impression. The impression reflects the
shape of the journal J-side surface of the arm A, except at least the side
portions (Aa, Ab). The impression of the second die 20 illustrated in FIGS.
4(a) to 4(c) reflect the shape of the recess in the area As of the arm A.
[00s3]
The second die 20 is movable forward and backward to come in contact
with and separate from the journal J-side surface of the arm. This movement
of the second die 20 is effected by operation of a hydraulic cylinder or the like
connected to the second die 20.
[00s4]
Also, the second die 20 illustrated in FIGS. 4(a) to 4(c) is movable in a
pressing direction (vertical direction in Fig. 4) so as to be positioned in the
center of a space between the upper die 10 and the lower die 12of the first dies
10. A mechanism for this movement of the second die 20 may include, for
example, a holder (not shown in the drawings) holding the second die 20, a first
elastic component (e.g., a spring, not shown in the drawings), and a second
elastic component (e.g., a spring, not shown in the drawings). The first elastic
LL
component connects the lower die 12 and the holder, and the holder connected
to the lower die 12 is movable in the vertical direction. A fîrst end of the
second elastic component is connected to the upper die 11, and a second end
thereof is capable of coming in contact with the holder.
[00ss]
In this case, in an initial state, the upper die 11 and the lower die 12 are
separate from each other enough to keep the second end of the second elastic
component away from the holder. Accordingly, even when the upper die l1
and the lower die 12 come closer to each other, it is possible to maintain a
certain distance between the holder and the lower die 12. The upper die 11
and the lower die 12 come still closer to each other, and when the second die 20
is positioned in the center of the space between the upper die 11 and the lower
die 12, the second end of the second elastic component comes in contact with
the holder. When the upper die 11 and the lower dle 12 furthermore come
closer to each other, the first elastic component and the second elastic
component start compressing, and along with the compression of the first and
the second elastic components, the second die 20 moves down together with the
holder. The first elastic component and the second elastic component are
adjusted to compress by the same amount in this moment, and the second die 20
moves down while keeping its position in the center of the space between the
upper die 10 and the lower die 12.
l00s6l
The die forging step by use of the f,rrst dies 10 and the second die 20 are
carried out as follows. First, while the upper die 1l and the lower die 12 of
the first dies l0 are suff,rciently separate from each other, the preformed blank
is placed in the impression of the lower die 12. In this moment, the second die
20 is away from the preformed blank 31, and the recess in the journal J-side
surface (area As) of the arm A is not held.
[00s7]
Next, the second die 20 is moved forward to abut against the recess in
r+
r
the area As of the arm A as illustrated in FIGS. 4(a) and 5(a). Thereby, the
shape of the recess in the area As of the arm A is retained by the second die 20.
In this moment, the position of the second die20 in the pressing direction is not
in the center of the space between the paired f,rrst dies 10 but at a specified
distance from the lower die 12.
[00s8]
In this state, the upper die 11 is moved toward the lower die 12. In the
meantime, since the upper die 11 and the lower die 12 are sufficiently separate
from each other, the position of the second die 20 in the pressing direction
(vertical direction) is kept at the specified distance from the lower die 12.
More specifically, in a case of adopting the above-described mechanism
including the holder, the first elastic component and the second elastic
component, the second elastic component does not come in contact with the
second die 20, and therefore, the second die 20 is kept in the same position in
the pressing direction. The upper die 11 is moved further until the position of
the second die 20 in the pressing direction becomes the center of the space
between the paired first dies 10, and then, the positions of the first dies 10 and
the second die 20 become as shown in FIG. 4(b).
[00se]
By moving the upper die 11 further toward the lower die 12 from the
position shown in FIG. 4(b), the second die 20 is caused to start moving in the
pressing direction to keep its position in the center of the space between the
paired first dies 10. More specifically, in a case of adopting the
above-described mechanism including the holder, the first elastic component
and the second elastic component, the second elastic component comes in
contact with the second die 20, and both of the first elastic component and the
second elastic component start compressing. Along with the compression, the
second die 20 moves down. Since the first elastic component and the second
elastic component are adjusted so as to compress by the same amount for the
downward movement of the second die 20, the second die 20 moves down
IY
r
while keeping its position in the center of the space between the paired first
dies 10. Almost at the same time with the start of the movement of the second
die 20 in the pressing direction, the first dies l0 start pressing the preformed
blank 31.
[0060]
The upper die 1l is moved further to arrive at a pressing end position
(see FIG. 4(c)). In the meantime, along with the movement of the upper die
11, the second die 20 moves to keep its position substantially in the center of
the space between the paired first dies 10. When the upper die 11 arrives at
the pressing end position, the pressing of the preformed blank 31 is completed.
[0061]
In the process from the start of pressing to the end of pressing, the
preformed blank is pressed by the fîrst dies 10 and is formed into the shape
corresponding to the impressions of the upper die l l and the lower d,ie 12.
For example, the journals J and the crank pins p are formed on the preformed
blank' This formation along with the pressing is accompanied by formation of
flash 32a around the preformed blank.
[0062]
In the process of pressing, the second die 20 is abutted against the
recess in the area As of the arm A. Thus, the shape of the recess in the area As
of the arm A is maintained by the second die 20. It is preferred that the
second die 20 is movable in the vertical direction, for example, by the
above-described mechanism to keep its position in the center of the space
between the upper die 11 and the lower die 12 of the first dies 10. In this
regard, in the process of 'pressing, the arm centerline of the preformed blank
moves in the vertical direction, and specif,rcally, the arm centerline moves in
the vertical direction while being kept in the center of the space between the
upper die 11 and the lower dte 12 of the first dies 10. Thus, configuring the
second die 20 to be movable in the vertical direction permits the arm A and the
second die 20 to move vertically in the process of pressing while maintaining
tg
I
the positional relationship between the recess in the area As of the arm A and
the second die 20. Consequently, the shape of the recess in the area As of the
arm A is surely retained by the second die 20.
[0063]
The excess projecting portions (Aaa, Aba) ale formed around the outer
peripheries of the both side portions (Aa, Ab) of the arm A of the preformed
blank so as to protrude fiom the outer peripheries of the both side portions (Aa,
Ab). On the other hand, the impressions of the first dies 10 (rhe upper die 11
and the lower die 12) reflect the shape of the arm A except the r-ecess in the area
As, and more specifically reflect the side portions (Aa, Ab) of the arm A.
Accordingly, in the process of pressing, the impressions of the first dies l0 (the
upper die 11 and the lower die 12) are pressed to the excess projecting portions
(Aaa, Aba), whereby the excess projecting portions (Aaa; Aba) are bent or
crushed. Thus, the excess projecting portions (Aaa, Aba) are formed into the
shapes defined by the impressions of the first dies (the upper die i 1 and the
lower die 12). consequently, the journal J-side surface of the arm A bulges at
the side portions (Aa, Ab), and the side portions (Aa, Ab) increase in thickness.
[0064]
The obtained in this way is a forged blank, as shown in FIG. 3, of
which arm A has side portions (Aa, Ab) with increased thicknesses and has a
recess in the journal J-side surface.
t006sl
In the process ofpressing, the second die 20 is abutted against the area
As, wherein a recess is to be formed, of the journal J-side surface of the arm A
and holds the area As, and thereby, the shape of the recess in the area As is
stably formed.
[0066]
After completion of the pressing operation of the first dies 10, the
second die 20 is retracted frorn the arm A, and thereafter, the upper die 1l and
the lower dte 12 of the first dies 1CI are separated from each other. Then, the
La
i:
r
crankshaft (forged blank) is taken out.
[0067]
subsequently, in the trimming step, trimming is applied to the forged
blank with flash to remove the flash from the forged blank. Thereby, a
crankshaft is obtained. In this regard, the shapes of the main parts (for
example, the arms A, the journals J and the crank pins p) of the crankshaft
obtained in the forging step are maintained in the forged blank (crankshaft)
after subjected to the trimming step.
100681
Thus, by the production method according to the present embodiment,
it is possible to make a recess in the journal J-side surface of the arm A while
keeping the both side portions (Aa, Ab) of the arm A thick. Therefor.e, the
production method according to the present embodiment allows for production
of a forged crankshaft having a reduced weight and sufficient stiffness.
l006el
Also, in the production method according to present embodiment, the
excess projecting portions (Aaa, Aba) are formed so as to protrude locally from
the outer peripheries of the side portions (Aa, Ab) of the arm A, and the excess
projecting portions (Aaa, Aba) are deformed by pressing operation of the first
dies 10. The force required for the pressing operation of the first dies 10 is at
a comparable level with the force required for conventional forging.
Meanwhile, the second die 20 is abutted against the surface of the arm A. In
this regard, it is not necessary to push the second díe 20 further, and the force
required to support the second die 20 is small. Thus, the production method
according to the present embodiment requires no large force and can be carried
out in a simple manner.
[0070]
In the conventional production method as illustrated in FIG. l, press
forging is carried out by use of a pair of dies in a block forging step and in a
finish forging step, and the forged blanks obtained by the respective forging
2_t
!1.:; :J x::a:¡:1jti:,!:i!iâ:
steps have large flash. In the production method according to the present
embodiment, on the other hand, a preformed blank with no flash is obtained in
the preforming step, and a forged blank with flash is obtained in the die forging
step. However, the flash formed in the die forging step is small. As seen in
FIGS. 3(a) to 3(d) and FIG. 5(b), the second die is abutred againsr a porrion of
the forged blank 32 (the blank after subjected to die forging), the flash 32a is
not formed at the portion of the forged blank . Accordingly, the production
method according to the present embodiment allows for an improvement in
yield.
[0071]
In the production method according to the present embodiment, in the
die forging step, the blank is shaped while flash is formed, and excess material
that flows out of the impressions of the first dies 10 (the upper die 11 and the
lower die 12) to be formed into the flash may flow into a space between the
first dies 10 and the second die 20. This may cause breakage of the first dies
10 andlor the second die 20. Also, this may prevent the second die 20 from
moving forward and backward, which may result in stoppage of operation.
100721
In order to prevent these problems, the second die 20 preferably has a
guide groove 20a so that the excess material to be formed into the flash can be
guided through the guide groove 20a. For example, in the second die 20
illustrated in FIGS. 4 and 5, the guide groove 20a with a predetermined width is
formed such that the center of the width matches with the portion of the second
die 20 to be positioned in the center of the space between the upper die l1 and
the lower die 72 of the first dies 10. Thus, the excess material to be formed
into the flash around the blank is guided through the guide groove 20a without
flowing into the space between the first dies 10 (the upper die 11 and the lower
die 12) and the second die 20.
[0073]
The shape and the size of the guide groove 20a may be designed
L2_
according to the size of the flash to be formed. For example, the guide groove
20a may have a rectangular, ffapezoidal or semicircular cross-sectional shape.
100741
In the pressing operation in the die forging step, it is preferred that the
second die 20 is moved in the pressing direction so as to be positioned in the
center of the space between the upper and the lower dies of the first dies 10.
Thereby, the shapes of the recess to be formed in the area As of the arm A are
surely maintained by the second die 20, and the working accuracy in the area
against which the second die 20 abutted can be improved. As the mechanism
for moving the second die 20 in the pressing direction, the above-described
mechanism can be used, that is, the mechanism for moving the second die 20 in
the pressing direction may include a holde¡ a first elastic component and a
second elastic component.
[007s]
Meanwhile, in a crankshaft, stress concentration is likely to occur in
pin fillet portions that are joint portions between crank pins p and arms A. In
many cases, therefore, quenching by high-frequency induction heating is
applied to the pin fillet portions in order to improve the fatigue strength. In
this regard, pin top portions Ac of the respective arms Aare adjacent to thepin
fillet portions to be subjected to quenching, and unless the pin top portions Ac
are sufficiently thick, quenching cracks may occur therein.
[0076]
It is preferred that the pin top portions Ac of the arms A are formed into
thick shapes in the preforming step and that the thick shapes are maintained
even after the die forming step. Then, the produced crankshaft has
sufficiently thick pin top portions Ac. Alternatively, the pin top portions Ac
do not need to be formed into thick shapes in the preforming step, and the pin
top portions Ac may be formed into thick shapes in the die forging step. In
this case also, the produced crankshaft has suff,rciently thick pin top portions
Ac. By ensuring formation of thick pin top portions Ac, the resistance to
e_g
{
quenching cracks can be improved.
la077l
The production method according to the present embodiment is
intended not only for an arm having a weight integrated therewith as illustrated
in FIGS. 2(a) to 2(d) and 3(a) to 3(d) but also for an arm having no weight
integrated therewith. Specifically, in the above-described 4-cylinder
8-counterweight crankshaft, every arm of the crankshaft has a weight integrated
therewith. In this case, with regard to each and every one of the arms, excess
projecting portions may be formed on the both side portions, near a crank pin,
and the excess projecting portions may be deformed so as to increase the side
portions in thickness.
[0078]
In the above-mentioned 4-cylinder 4-couterweight crankshaft, some
arms of the crankshaft each have a weight integrated therewith. In this case,
only with regard to the arms having a weight, excess projecting portions may
be formed on the both side portions, near a crank pin, and the excess projecting
portions may be deformed so as to increase the side portions in thickness.
Alternatively, with regard to any of the arms, whether it has a weight integrated
therewith or no weight integrated therewith, excess projecting portions may be
formed on the both side portions, near a crank pin, and the excess projecting
portions may be deformed so as to increase the both side portions in thickness.
[007e]
The production process according to the present embodiment is
applicable not only to a crankshaft to be mounted in a 4-cylinder engine but
also to crankshafts that are to be mounted in a 3-cylinder engine, an inline
6-cylinder engine, a v-type 6-cylinder engine, an 8-cylinder engine, and the
like.
INDUSTRIAL APPLICABILITY
[0080]
2_+
The present invention is effectively used for manufacture of forged
crankshafts to be mounted in any kind of reciprocating engines.
REFERENCE SIGNS LIST
[0081]
1: forged crankshaft
J, Jl to J5: journal
P, Pl to P4: crank pin
Fr: front portion
Fl: flange
A, Al to A8: crank arm
W, Wl to V/8: counterweight
Aa, Ab: side portion of arm
Ac: pin top portion of arm
As: area of journal-side surface of arm, inward from side portions
Aaa, Aba: excess projecting portion
10: first dies
1 1: upper die
12: lower die
20: second die
20a: guide groove
31: blank before subjected to die forging
32:blank after subjected to die forging (forged blank)
32a: flash
z9

We claim:
l ' A method for producing a forged crankshaft, the crankshaft including:
joulnals that defìne a center of lotation; crank pins that are eccentric with
respect to the journals; and crank atms, each of the clank anns connecting a
colresponding one of the journals to a cor-responding one of the crank pins,
the rnethod cornprising:
a preforming step of for*ring a preformed blank with no flash, the
preformed blank including a shape of the crankshaft, wherein the crank arm has
an excess projecting portion at an outer periphery of each of side portions near
the clank pin, the excess projecting portion projecting from the outer periphery;
a die forging step of forming a forged blank with flash by pressing the
pleformed blank formed in the prefonning step with a pair of first dies; and
a trimming step of removing the flash from the forged blank formed in
the die forging step, wherein:
in the die forging step, while a second die is abutted against an area in a
surface of the crank arm to hold the area, the surface being adjacent to the
journal, the area not including areas of the side portions, by the fìrst dies the
excess projecting portions of the crank arm are deformed so as to increase the
side portions of the crank ann in thickness.
2- The method fo' producing a forged crankshaft according to claim 1,
wherein
the second die includes a guide groove so that excess material
out to be formed into the flash during the pressing in the die forging
guided through the guide groove.
3' The method for ploducing a forged crankshaft according to claim I or 2,
wherein
during the pressing in the die forging step, the second die is moved in a
flowing
step is
pressing direction such that the second die is positioned in a center of a space
between the paired first dies.