Crankshaft assembly for internal combustion engine

A crankshaft assembly for an internal combustion engine includes a crankshaft, an elastic member fixed to the crankshaft, and a flywheel fixed to the elastic member such that the flywheel is supported in an elastic relationship with the crankshaft. The elastic member has a rigidity in its rotating direction large enough to effectively transmit a driving power to a transmission through a clutch. On the other hand, the elastic member has a rigidity in an axial direction of the crankshaft small enough to shift a resonance frequency of a bending vibration out of a target frequency band of a forced vibration, while ensuring to prevent a failure of disengagement of the clutch.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a crankshaft assembly including a 
flywheel, for an internal combustion engine. More specifically, the 
present invention relates to a crankshaft assembly for an internal 
combustion engine, which can effectively shift a resonance frequency of a 
flexural or bending vibration of the crankshaft assembly out of a target 
frequency band of a forced vibration which results such as during 
acceleration of a vehicle so as to effectively prevent occurrence of a 
thick sound or noise in an engine room, while ensuring a quick response 
for clutch engaging and disengaging operations, and/or which can prevent 
occurrence of a fore and aft vibration of a vehicle floor at the time of 
engagement of the clutch. 
2. Description of the Background Art 
In a known crankshaft assembly for an internal combustion engine, a 
flywheel is directly connected to a crankshaft to use a mass of the 
flywheel mainly for reducing a torsional vibration generated in a rotating 
direction of the crankshaft assembly due to periodic torque fluctuation. 
However, the mass of the flywheel tends to generate a flexural or bending 
vibration in an axial direction of the crankshaft which causes a thick 
sound or noise in an engine room and thus in a vehicle compartment for an 
automotive vehicle, particularly at the time of the acceleration of the 
vehicle. 
Accordingly, there has been proposed a crankshaft assembly such as 
disclosed in Second Japanese Patent Publication No. 57-58542, wherein the 
flywheel is connected to the crankshaft through an elastic or flexible 
plate. The elastic plate has a rigidity in its rotating direction large 
enough for effectively transmitting the power between the crankshaft and a 
transmission through a clutch, while the elastic plate has a rigidity in 
the axial direction small enough for shifting a resonance frequency of the 
bending vibration out of a frequency band of a forced vibration which 
results during the most frequently used engine speed (4,000 rpm) so as to 
overcome the above-noted problem. 
However, the background art as mentioned above has the following problems. 
When the rigidity of the elastic plate in the axial direction (hereinafter 
referred to as "the axial rigidity") is too small, a clutch stroke for 
engaging and disengaging the clutch is likely to become larger, resulting 
in a delayed response of the clutch engaging and disengaging operations 
leading particularly to failure of the clutch disengagement which is 
likely to cause such as an engine stall. On the other hand, when the axial 
rigidity of the elastic plate is too large, the deviation of the resonance 
frequency of the bending vibration from the target frequency band of the 
forced vibration can not be ensured. 
Further, in the background art, when the flywheel is rotated, an axial 
run-out occurs on an engaging surface of the flywheel with a clutch facing 
of a clutch disc provided adjacent to the flywheel, due to a processing 
error and an assembling error of the elastic plate and the flywheel. 
Accordingly, when the clutch is engaged, a vibration is generated by a 
combination of the run-out of the engaging surface of the flywheel and the 
torque fluctuation of the engine, which is amplified by a vibration 
generated by the combustion in the engine cylinders and corresponding 
movements of associated members so as to cause a fore and aft vibration of 
the vehicle floor. Such vibration is uncomfortable for the driver and 
passengers in the vehicle compartment. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide a crankshaft 
assembly for an internal combustion engine that can eliminate the 
above-noted defects inherent in the background art. 
It is another object of the present invention to provide a crankshaft 
assembly for an internal combustion engine that can effectively shift a 
resonance frequency of a flexural or bending vibration of the crankshaft 
assembly out of a target frequency band of a forced vibration, 
particularly out of a target frequency band which results during 
acceleration of a vehicle so as to effectively prevent occurrence of a 
thick sound or noise in an engine room, while ensuring a quick response of 
the clutch engagement and disengagement operations so as to prevent 
particularly the failure of the clutch disengagement which is likely to 
cause such as an engine stall. 
It is still another object of the present invention to provide a crankshaft 
assembly for an internal combustion engine that can prevent occurrence of 
a fore and aft vibration of a vehicle floor at the time of the engagement 
of the clutch by effectively eliminating an axial run-out of an engaging 
surface of a flywheel with a clutch facing generated during rotation of 
the flywheel. 
To accomplish the above mentioned and other objects, according to one 
aspect of the present invention, a crankshaft assembly for an internal 
combustion engine comprises a crankshaft for transmitting a driving power 
to a transmission through a clutch, an elastic member fixed to the 
crankshaft, and a flywheel fixed to the elastic member such that the 
flywheel is supported in an elastic relationship with the crankshaft. 
The flywheel has an engageable surface at a side opposite to the elastic 
member in an axial direction of the crankshaft, and the engageable surface 
is engageable with an associated member of the clutch to receive a load 
therefrom in the axial direction when the engageable surface is engaged 
with the associated member of the clutch. 
The elastic member has a first predetermined rigidity in its rotating 
direction, the first predetermined rigidity being large enough to 
effectively transmit the driving power to the transmission through the 
clutch. On the other hand, the elastic member has a second predetermined 
rigidity in the axial direction, the second predetermined rigidity being 
small enough to shift a resonance frequency of a bending vibration out of 
a target frequency band of a forced vibration, while ensuring to prevent a 
failure of disengagement between the engageable surface of the flywheel 
and the associated member of the clutch. 
According to another aspect of the present invention, a method for forming 
a crankshaft assembly for an internal combustion engine comprises steps of 
fixing a flywheel to an elastic member to form a unit, assembling the unit 
onto the crankshaft with the elastic member mounted onto the crankshaft so 
as to support the flywheel in an elastic relationship with the crankshaft, 
and processing an engageable surface of the flywheel, which is engageable 
with an associated member of a clutch, based on an assembled condition 
between the elastic member and the crankshaft so as to minimize an axial 
run-out of the engageable surface. 
According to still another aspect of the present invention, a crankshaft 
assembly for an internal combustion engine comprises a crankshaft for 
transmitting a driving power to a transmission through a clutch, an 
elastic member fixed to the crankshaft, and a flywheel fixed to the 
elastic member such that the flywheel is supported in an elastic 
relationship with the crankshaft. 
The flywheel has an engageable surface at a side opposite to the elastic 
member in an axial direction of the crankshaft, and the engageable surface 
is engageable with an associated member of the clutch to control 
transmission of the driving power between the crankshaft and the 
transmission. 
The engageable surface is designed to have an axial run-out which is no 
more than 0.1 mm for ensuring a smooth engagement with the associated 
member of the clutch.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Now, a crankshaft assembly for an internal combustion engine according to 
preferred embodiments of the present invention will be described 
hereinbelow with reference to FIGS. 1 to 4. 
FIG. 1 shows a first preferred embodiment of the present invention. An 
engine crankshaft 1 is connected to pistons through respective connecting 
rods in a known manner for receiving the driving power therefrom. An 
elastic plate 2 substantially of a disc shape is fixed to one end of the 
crankshaft 1 by a plurality of bolts 3. The elastic plate 2 is formed at 
its outer peripheral edge portion with an axially extending section 2a to 
which a ring gear R is fixed. The ring gear R engages with pinion gears of 
an engine starter motor for transmitting the driving power from the engine 
starter motor to the crankshaft 1 when starting the engine. 
An annular reinforcing member 4 is disposed between the elastic plate 2 and 
heads of the bolts 3. The reinforcing member 4 is formed at its outer 
peripheral edge portion with a cylindrical section 4a extending in an 
axial direction of the crankshaft 1 and with a radially extending section 
4b. 
A flywheel body 5 of an annular shape is fixed to the elastic plate 2 at 
their respective outer peripheral edge portions 5a and 2b through a 
plurality of bolts 6 and corresponding reinforcing members 7 disposed 
between the elastic plate 2 and heads of the bolts 6. The annular flywheel 
body 5 has a stepped inner peripheral edge surface defining a mounting 
opening 5b for receiving the reinforcing member 4 therein. The stepped 
inner peripheral edge surface has a first section 5c extending axially, a 
second section 5d extending radially outward from the first section 5c and 
a third section 5e extending axially from the second section 5d. The axial 
section 4a of the reinforcing member 4 is in a slidable contact with the 
first section 5c of the flywheel body 5, and the radial section 4b of the 
reinforcing member 4 is spaced from the second section 5d of the flywheel 
body 5 by a predetermined distance for allowing an axial movement of the 
flywheel along with the elastic plate 2. A radially extending inner 
surface 5f of the flywheel facing the elastic plate 2 is spaced apart from 
the elastic plate 2 by a predetermined distance for ensuring an elasticity 
of the elastic plate 2. 
The flywheel body 5 further includes a radially extending surface 5g at a 
side axially opposite to the radial surface 5f or the elastic plate 2. The 
radial surface 5g is engageable with a clutch facing 8 of a clutch disc 9 
of a clutch in a known manner so as to control the transmission of the 
power between the crankshaft 1 and a transmission. 
A rigidity of the elastic plate 2 in its rotating direction (hereinafter 
referred to as "the circumferential rigidity") is set large enough for 
effectively transmitting the power between the crankshaft 1 and the 
transmission through the clutch, while a rigidity of the elastic plate 2 
in the axial direction (hereinafter referred to as "the axial rigidity") 
is set small enough for shifting a resonance frequency of the flexural or 
bending vibration out of a frequency band of a forced vibration which 
results during the acceleration of the engine. 
As described in the background art, when the axial rigidity of the elastic 
plate is too small, a clutch stroke for engaging and disengaging the 
clutch becomes larger, i.e. a clutch stroke loss gets larger, resulting in 
delayed response of the clutch engaging and disengaging operations leading 
particularly to the failure of the clutch disengagement which is likely to 
cause such as an engine stall. On the other hand, when the axial rigidity 
of the elastic plate is too large, the deviation of the resonance 
frequency of the bending vibration from the target frequency band of the 
forced vibration can not be attained. 
To overcome the above-noted problem, the axial rigidity of the elastic 
plate 2 in this embodiment is set to 600 kg/mm to 2200 kg/mm, wherein an 
axial displacement of the radial surface 5g of the flywheel 5 is no more 
than 1 mm when an axial load or force 600 kg to 2200 kg is applied to the 
radial surface 5g. By selecting a value of the axial rigidity of the 
elastic plate 2 within the foregoing range, not only is the failure of the 
clutch disengagement effectively prevented, but also the deviation of the 
resonance frequency of the bending vibration from the frequency band of 
the forced vibration, during the acceleration of the engine in this 
embodiment, is effectively attained so as to prevent generation of the 
thick sound or noise in the engine room. 
Specifically, it is confirmed that the failure of the clutch disengagement, 
i.e. the failure of the disengagement between the radial surface 5g of the 
flywheel and the clutch facing 8 of the clutch disc 9, happens when an 
axial displacement of the radial surface 5g at the time of engagement with 
the clutch facing 8 exceeds 5% of a normal clutch stroke (normally at 7 mm 
to 8 mm) fur engaging and disengaging the clutch. The normal clutch stroke 
is a distance between the radial surface 5g of the flywheel body 5 and the 
clutch facing 8 in a disengagement or released condition of the clutch. 
Accordingly, considering that an axial load applied to the flywheel body 5 
through the clutch facing 8 is normally at 150 kg to 200 kg, the lower 
limit value 600 kg/mm of the axial rigidity of the elastic plate is 
selected, wherein the axial displacement of the radial surface 5g is 
within 5% of the normal clutch stroke when applied with the axial load 150 
kg to 200 kg, as shown in TABLE 1. 
TABLE 1 
______________________________________ 
AXIAL 
AXIAL LOAD AXIAL RIGIDITY DISPLACEMENT 
______________________________________ 
150 kg 500 kg/mm 0.30 mm 
(3.8 to 4.3%) 
200 kg 500 kg/mm 0.40 mm 
(5.0 to 5.7%) 
150 kg 600 kg/mm 0.25 mm 
(3.1 to 3.6%) 
200 kg 600 kg/mm 0.33 mm 
(4.1 to 4.7%) 
150 kg 700 kg/mm 0.21 mm 
(2.6 to 3.0%) 
200 kg 700 kg/mm 0.29 mm 
(3.6 to 4.1%) 
______________________________________ 
(wherein, percentage denotes a rate of the axial displacement relative to 
the normal clutch stroke which is 7 to 8 mm) 
As seen from TABLE 1, the lower limit value 600 kg/mm of the axial rigidity 
of the elastic plate 2 ensures the axial displacement of the radial 
surface 5g of the flywheel body 5 within 5% of the normal clutch stroke, 
i.e. the axial displacement of the radial surface 5g is between 0.25 to 
0.33 mm or between 3.1 to 4.7% relative to the normal clutch stroke when 
applied with the normal axial load at 150 to 200 kg through the clutch 
facing 8, so that the failure of the clutch disengagement is effectively 
prevented. Naturally, the larger the axial rigidity of the elastic plate 
gets, the smaller the axial displacement of the flywheel gets. 
Now, the axial rigidity of the elastic plate 2 will be considered in view 
of shifting of the resonance frequency of the bending vibration out of a 
frequency band of a forced vibration which results during the acceleration 
of the engine where the sound or noise generated by the bending vibration 
is the most significant. It is confirmed that the sound or noise generated 
by the bending vibration is effectively reduced when the resonance 
frequency is shifted out of the frequency band of the forced vibration 
during the acceleration of the engine. 
FIG. 2 is a graph of bending vibration level versus frequency showing a 
result of experiments using various elastic plates having different axial 
rigidities. The frequency band of the forced vibration during the 
acceleration of the engine is 200 Hz to 500 Hz. In FIG. 2, a line Ao shows 
a relationship between the frequency and the bending vibration level 
without using the elastic plate, i.e. the flywheel is directly connected 
to the crankshaft. As can be seen, a resonance frequency of the line Ao is 
within 200 Hz to 500 Hz, which causes the sound or noise problem. A line 
A1 is derived by the elastic plate having the axial rigidity of 2200 
kg/mm, a line A2 is derived by the elastic plate having the axial rigidity 
of 1700 kg/mm, line A3 is derived by the elastic plate having the axial 
rigidity of 1200 kg/mm, and a line A4 is derived by the elastic plate 
having the axial rigidity of 1000 kg/mm. As can be seen, the resonance 
frequency of each of the lines A1 to A4 is shifted out of the frequency 
band 200 Hz to 500 Hz, and further, the vibration level of each of the 
lines A1 to A4 is considerably lower than the line Ao within the frequency 
band 200 Hz to 500 Hz. Though the line A1 has a vibration level higher 
than the line Ao around 200 Hz, this happens in a very small range of 
frequency. Accordingly, the value 2200 kg/mm is selected as an upper limit 
value of the axial rigidity of the elastic plate, and the value 1700 kg/mm 
is selected as a more preferable upper limit value of the axial rigidity. 
In light of the above, the axial rigidity of the elastic plate 2 in this 
embodiment is selected at 600 kg/mm to 2200 kg/mm, and preferably at 600 
kg/mm to 1700 kg/mm. 
As understood from the above description, this first embodiment, when the 
crankshaft 1 is rotated, the flywheel body 5 is ensured to rotate with the 
crankshaft 1 by means of the large circumferential rigidity of the elastic 
plate 2. When the clutch is engaged and the engine is accelerated, the 
driving power is transmitted to the transmission with a very low bending 
vibration level by means of the axial rigidity of the elastic plate being 
no more than 2200 kg/mm, so that the vehicle compartment can be kept 
quiet. On the other hand, when the clutch is disengaged, since the axial 
displacement of the flywheel is no more than 5% of the normal clutch 
stroke by means of the axial rigidity of the elastic plate being no less 
than 600 kg/mm, the failure of the disengagement of the clutch is 
effectively prevented. 
FIG. 3 shows a crankshaft assembly for an internal combustion engine 
according to a second embodiment of the present invention. In FIG. 3, the 
same or like parts or members are denoted by the same reference numerals. 
In the following description, explanations of those same or like members 
will be omitted to avoid redundant description. Further, though the clutch 
assembly is not shown in FIG. 3, the same clutch assembly including the 
clutch disc 9 and the clutch facing 8 is provided in the same manner as in 
FIG. 1. 
In FIG. 3, the crankshaft 1 includes a stepped end surface having a first 
section la extending radially inward from its outer peripheral edge, a 
second section 1b extending axially from the inward end of the first 
section 1a toward the clutch disc 9, and a third circular section 1c 
extending radially from the second section 1b. The elastic plate 2 is of 
an annular shape having a mounting opening at its center for receiving the 
second section 1b therethrough. The elastic plate 2 is fixed to the 
crankshaft 1 with its axially extending inward end 2c facing the second 
section of the crankshaft 1 and with its radially extending inward end 
portion 2d facing the first section of the crankshaft. The other structure 
is substantially the same as in FIG. 1. 
As mentioned in the background art, when the flywheel body 5 is rotated 
through the crankshaft 1, an axial run-out is generated on the radial 
surface 5g due to the processing error and the assembling error of the 
elastic plate 2 and the flywheel body 5 to cause the vibration when the 
clutch is engaged. The vibration further causes the fore and aft vibration 
of the vehicle floor. 
In order to overcome the above-noted problem, in this embodiment, the 
radial surface 5g is processed in a manner to make an amount of the axial 
run-out no more than 0.1 mm. Specifically, the processing of the radial 
surface 5g is performed in the following manner. 
The flywheel body 5 is first fixed to the elastic plate 2 by the bolts 6. 
Then, this unit is assembled to the crankshaft 1 with the axially 
extending inward end 2c of the elastic plate 2 facing the second section 
1b of the crankshaft 1 and with the radially extending inward end portion 
2d facing the first section 1a of the crankshaft. Then, the radial surface 
5g is processed based on the assembled condition between the axially 
extending inward end 2c and the second section 1b and/or between the 
radially extending inward end portion 2d and the first section 1a to make 
the axial run-out of the radial surface 5g no more than 0.1 mm. 
By using the above-noted manner, the radial surface 5g is easily and 
precisely processed to make the amount of the axial run-out no more than 
0.1 mm. 
FIG. 4 is a graph of axial run-out amount of flywheel (radial surface 5g) 
versus fore and aft vibration of vehicle floor showing a result of 
experiments. It is confirmed that the fore and aft vibration of the 
vehicle floor which does not give a uncomfortable feeling to a human body 
is normally no more than 0.1 G (gravitational acceleration). As can be 
seen from FIG. 4, a fore and aft vibration of the vehicle floor is 
substantially in direct proportion to an amount of the axial run-out of 
the radial surface 5g, and the fore and aft vibration becomes no more than 
0.1 G when the axial run-out becomes no more than 0.1 mm. Accordingly, by 
making the amount of the axial run-out no more than 0.1 mm as in this 
embodiment, the fore and aft vibration can be made no more than 0.1 G. 
As understood from the above description, in this second embodiment, when 
the crankshaft 1 is rotated, the flywheel body 5 is ensured to rotate with 
the crankshaft 1 by means of the large circumferential rigidity of the 
elastic plate 2. Since the amount of the axial run-out of the radial 
surface 5g is no more than 0.1 mm, the engagement between the radial 
surface 5g and the clutch facing 8 is performed quite smoothly, so that 
the fore and aft vibration does not exceed 0.1 G. Accordingly, the driving 
power is transmitted from the engine to the transmission without giving 
the uncomfortable feeling to the human body. 
It is to be appreciated that in this second embodiment, the axial rigidity 
of the elastic plate 2 is not necessarily selected at 600 kg/mm to 2200 
kg/mm. 
It is to be understood that the invention is not to be limited to the 
embodiments described above, and that various changes and modifications 
may be made without departing from the spirit and scope of the invention 
as defined in the appended claims.