JP7073671B2 - Crank shaft - Google Patents

Description

The present invention relates to a crank shaft mounted on a reciprocating engine of an automobile, a motorcycle, an agricultural machine, a ship, or the like.

Reciprocating engines require a crank axle. This is to extract power by converting the reciprocating motion of the piston in the cylinder (cylinder) into rotary motion. Usually, a multi-cylinder engine is used for automobiles and the like.

1 and 2 are side views showing an example of a general crank shaft. The crank shaft 1 shown in FIGS. 1 and 2 is mounted on a 4-cylinder engine. The crank shaft 1 includes five journal portions J1 to J5, four pin portions P1 to P4, a front portion Fr, a flange portion Fl, and eight crank arm portions (hereinafter, also simply referred to as “arm portions”) A1 to A8. Be prepared. The eight arm portions A1 to A8 connect the journal portions J1 to J5 and the pin portions P1 to P4 corresponding to each of the eight arm portions A1 to A8.

In the crank shaft 1 shown in FIG. 1, all eight arm portions A1 to A8 integrally have counterweight portions (hereinafter, also simply referred to as “weight portions”) W1 to W8. This crank shaft 1 is referred to as a 4-cylinder-8 counterweight crank shaft.

In the following, when the journal parts J1 to J5, the pin parts P1 to P4, the arm parts A1 to A8, and the weight parts W1 to W8 are collectively referred to, the reference numerals are "J" for the journal part and "P" for the pin part. , "A" in the arm part and "W" in the weight part.

In the crank shaft 1 shown in FIG. 2, of the eight arm portions A, the first arm portion A1 at the head, the eighth arm portion A8 at the rear end, and the two arm portions A at the center (fourth arm portion A4 and the second arm portion A). The 5 arm portion A5) integrally has a weight portion W. The remaining second, third, sixth and seventh arm portions A2, A3, A6 and A7 do not have a weight portion. This crank shaft 1 is referred to as a 4-cylinder-4 counterweight crank shaft.

The journal portion J, the front portion Fr, and the flange portion Fl are arranged coaxially with the rotation center of the crank shaft 1. Each pin portion P is eccentrically arranged by a distance of half the piston stroke from the rotation center of the crank shaft 1. The journal portion J is supported by the engine block by a sliding bearing and serves as a rotation center axis. Each pin portion P is connected to a large end of a connecting rod (hereinafter, also referred to as "connecting rod") by a sliding bearing, and a piston is connected to a small end of the connecting rod. A pulley (not shown) for driving a timing belt, a fan belt, etc. is attached to the front portion Fr. A flywheel (not shown) is attached to the flange portion Fl.

From the viewpoint of improving fuel efficiency, the crank shaft is required to be lighter. A conventional technique for meeting this requirement is disclosed in, for example, Japanese Patent Application Laid-Open No. 2000-320531 (Patent Document 1). 3 and 4 are views showing an arm portion with a weight portion in the crank shaft disclosed in Patent Document 1. Of these views, FIG. 3 is a side view, and FIG. 4 is a front view when viewed from the pin portion side. With reference to FIGS. 3 and 4, the lightening portion 100 is formed on the surface of the arm portion A with the weight portion W on the pin portion P side of the arm portion A. The lightening portion 100 is formed over the entire width direction of the arm portion A and extends to the two side portions Aa of the arm portion A. The weight of the arm portion A is reduced by the lightening portion 100. As a result, the weight of the crank shaft can be reduced.

Japanese Unexamined Patent Publication No. 2000-320531

In recent years, there has been an increasing demand for weight reduction of crank shafts. However, in order to meet this demand, if the lightening portion formed in the arm portion with the weight portion is simply expanded in the region of the arm portion, the support rigidity of the arm portion may decrease. If the support rigidity of the arm portion is reduced, the crank shaft is likely to be elastically deformed, so that the journal portion may seize on the slide bearing, and the fuel efficiency may be deteriorated. In addition, the vibration characteristics may deteriorate. In short, the basic performance of the crank shaft may be impaired.

One object of the present invention is to provide a crank shaft capable of achieving weight reduction while maintaining basic performance.

The crank shaft according to the embodiment of the present invention includes a plurality of journal portions, a plurality of pin portions eccentric with respect to the plurality of journals, and a plurality of crank arm portions connecting the journal portions and the pin portions corresponding to each. .. One or more of the crank arm portions have a counterweight portion integrally. At least one of the crank arm portions having the counterweight portion is formed on the surface of the counterweight portion on the pin portion side, and has two lightening portions and two lightening portions, each of which extends to two side portions of the corresponding counterweight portion. It is provided with a rib portion adjacent to the lightening portion and provided along the center line of the crank arm portion.

Since the crank shaft according to the embodiment of the present invention has two lightening portions and rib portions formed in the counterweight portion, it is possible to realize weight reduction while maintaining basic performance.

FIG. 1 is a side view showing an example of a general crank shaft. FIG. 2 is a side view showing another example of a general crank shaft. FIG. 3 is a side view of an arm portion with a weight portion in the crank shaft disclosed in Patent Document 1. FIG. 4 is a front view of the arm portion with a weight portion shown in FIG. 3 when viewed from the pin portion side. FIG. 5 is a perspective view of the crank shaft assumed in the study step 1. FIG. 6 is a side view of the crank shaft shown in FIG. FIG. 7 is a side view of an arm portion with a weight portion in the crank shaft shown in FIG. FIG. 8 is a perspective view of the arm portion with a weight portion shown in FIG. 7. FIG. 9 is a cross-sectional view of an arm portion with a weight portion in line IX-IX of FIG. 7. FIG. 10 is a perspective view showing an example of a vibration mode appearing in the arm portion with a weight portion in the study step 1. FIG. 11 is a perspective view of the crank shaft assumed in the study step 2. FIG. 12 is a side view of the crank shaft shown in FIG. FIG. 13 is a side view of an arm portion with a weight portion in the crank shaft shown in FIG. FIG. 14 is a perspective view of the arm portion with a weight portion shown in FIG. FIG. 15 is a cross-sectional view of an arm portion with a weight portion in the line XV-XV of FIG. FIG. 16 is a graph summarizing the analysis results in the study step 2. FIG. 17 is a side view of an arm portion with a weight portion in the crank shaft of the present embodiment. FIG. 18 is a front view of the arm portion with a weight portion shown in FIG. 17 when viewed from the pin portion side.

In order to solve the above-mentioned problems, the present inventors have focused on the form of the lightening portion formed in the arm portion with the weight portion, and have made extensive studies. As a result, the following findings were obtained.

[Examination step 1]
In step 1, the lightening portion 100 formed in the arm portion A with the weight portion W shown in FIGS. 3 and 4 is not expanded in the region of the arm portion A, but is expanded in the region of the weight portion W. The form to be used was examined. This study was done by FEM analysis.

5 to 9 are diagrams showing the crank shaft assumed in the study step 1. Of these figures, FIG. 5 is a perspective view of the crank shaft, and FIG. 6 is a side view of the crank shaft. FIG. 7 is a side view of the arm portion with a weight portion on the crank shaft, FIG. 8 is a perspective view of the arm portion with a weight portion, and FIG. 9 is a perspective view of the arm portion with a weight portion in line IX-IX of FIG. It is a cross-sectional view. In addition, the cross section in line IX-IX of FIG. 7 shown in FIG. 9 is the position of the minimum width of the weight portion W in the region where the lightening portion 110 is formed in the cross section perpendicular to the center line of the arm portion. It is a cross section. In the present specification, the center line of the arm portion A is a straight line included in the surface including the axis Jc of the journal portion J and the axis Pc of the pin portion P, and is a straight line perpendicular to the axis Jc of the journal portion J. Means.

In the analysis model of the crank shaft 1 assumed in the study step 1 with reference to FIGS. 5 to 9, the lightening portion 110 is formed on the surface of the arm portion A with the weight portion W on the pin portion P side of the weight portion W. It is formed. The lightening portion 110 is formed over the entire width direction of the weight portion W and extends to two side portions Wa of the weight portion W. Further, the lightening portion 110 extends to the surface of the arm portion A on the pin portion P side, and also extends to the two side portions Aa of the arm portion A.

With reference to FIG. 7, when the arm portion A with the weight portion W provided with the lightening portion 110 is viewed from the side surface, the point JB closest to the weight portion W among the intersections between the outer circumference of the journal portion J and the arm portion A. The position L1 is the boundary between the arm portion A and the weight portion W. That is, the portion on the pin portion P side from the position L1 of the point JB becomes the arm portion A, and the portion opposite to the pin portion P side from the position L1 of the point JB becomes the weight portion W. In addition, in FIG. 5, FIG. 6, FIG. 7, and FIG. 9, a journal thrust is drawn between the journal portion J and the arm portion A according to the actual situation. In the engine, the contact between the journal thrust and the sliding bearing limits the axial movement of the journal portion J.

Under such conditions, with reference to FIGS. 6 and 7, when the arm portion A with the weight portion W provided with the lightening portion 110 is viewed from the side surface, it is opposite to the pin portion P side of the lightening portion 110. The side end (hereinafter, also referred to as “first end”) 110a is arranged at a position closer to the side opposite to the pin portion P side than the position L1 of the point JB, that is, at a position closer to the weight portion W. The first end 110a is arranged at a position closer to the pin portion P than the position L2 of the maximum width of the weight portion W. On the other hand, the end (hereinafter, also referred to as "second end") 110b of the lightening portion 110 on the pin portion P side is arranged at a position closer to the pin portion P than the position L1 of the point JB. More specifically, the second end 110b is arranged at a position closer to the pin portion P than the position L3 of the axial center Jc of the journal portion J.

In the arm portion A with the weight portion W in the study step 1, the lightening portion 110 exists in the region of the weight portion W. Further, the lightening portion 110 extends to the region of the arm portion A. Here, the width of the weight portion W is relatively large. In particular, since the planar shape of the weight portion W is generally fan-shaped, the maximum width of the weight portion W is much larger than the maximum width of the arm portion A. Therefore, the weight of the arm portion A with the weight portion W in the study step 1 is significantly reduced by the lightening portion 110 existing in the weight portion W. Further, the shape of the weight portion W has almost no effect on the support rigidity of the arm portion A with the weight portion W. Therefore, the support rigidity of the arm portion A with the weight portion W in the study step 1 is hardly reduced. In the present specification, the support rigidity means the deformation resistance of the arm portion A when a load is applied to the pin portion P.

However, in the arm portion A with the weight portion W in the study step 1, the lightening portion 110 is formed in the entire width direction of the weight portion W. In this case, while a large weight reduction can be expected, the natural frequency of the arm portion A with the weight portion W is significantly reduced. This is because the natural frequency is proportional to the overall rigidity of the arm portion A including the weight portion W.

In the study step 1, the natural frequency thereof was actually analyzed using the model of the arm portion A with the weight portion W shown in FIGS. 5 to 9. As a result, the natural frequency of the arm portion A with the weight portion W is significantly lower than the natural frequency of the arm portion A with the weight portion W in which the lightening portion 110 is not formed. Further, in the case of the arm portion A with the weight portion W in the examination step 1, a peculiar vibration mode appeared as the natural frequency decreased.

FIG. 10 is a perspective view showing an example of a vibration mode appearing in the arm portion with a weight portion in the study step 1. With reference to FIG. 10, in the case of the arm portion A with the weight portion W in the study step 1, the vibration in the primary vertical bending mode is mainly generated. In the vibration in the primary vertical bending mode, the two ends of the weight portion W swing in the front-rear direction in the same phase with the center line of the arm portion A as a fulcrum. The vibration of the weight portion W in the primary vertical bending mode causes the vibration of the crank shaft. Therefore, the crank shaft provided with the arm portion A with the weight portion W in the examination step 1 cannot maintain the basic performance.

[Examination step 2]
In step 2, based on the form of the arm portion with the weight portion in the study step 1, a form of suppressing the vibration of the weight portion W in the primary vertical bending mode was examined. This study was also performed by FEM analysis.

11 to 15 are diagrams showing the crank shaft assumed in the study step 2. Of these figures, FIG. 11 is a perspective view of the crank shaft, and FIG. 12 is a side view of the crank shaft. FIG. 13 is a side view of the arm portion with a weight portion on the crank shaft, FIG. 14 is a perspective view of the arm portion with a weight portion, and FIG. 15 is a perspective view of the arm portion with a weight portion in the line XV-XV of FIG. It is a cross-sectional view. In addition, the cross section in the line XV-XV of FIG. 13 shown in FIG. 15 is the position of the minimum width L of the weight portion W in the region where the lightening portion 10 is formed in the cross section perpendicular to the center line of the arm portion. It is a cross section of.

In the analysis model of the crank shaft 1 assumed in the study step 2 with reference to FIGS. 11 to 15, two lightening portions are formed on the surface of the arm portion A with the weight portion W on the pin portion P side of the weight portion W. 10 is formed. The two lightening portions 10 extend to the two side portions Wa of the weight portions W, each of which corresponds to the weight portion W. Further, the two lightening portions 10 extend to the surface of the arm portion A on the pin portion P side, and also extend to the two side portions Aa of the corresponding arm portion A. Further, a rib portion 11 is provided on the surface of the weight portion W on the pin portion P side along the center line of the arm portion A. The rib portion 11 is adjacent to the two lightening portions 10. The rib portion 11 also extends to the surface of the arm portion A on the pin portion P side. The width X of the rib portion 11 is constant along the center line of the arm portion A (see FIG. 15).

With reference to FIGS. 12 and 13, when the arm portion A with the weight portion W including the lightening portion 10 and the rib portion 11 is viewed from the side surface, the first end of the lightening portion 10 (the side opposite to the pin portion P side). (End) 10a is arranged at a position closer to the side opposite to the pin portion P side than the position L1 of the above point JB. The first end 10a is arranged at a position closer to the pin portion P than the position L2 of the maximum width of the weight portion W. On the other hand, the second end (end on the pin portion P side) 10b of the lightening portion 10 is arranged at a position closer to the pin portion P than the position L1 of the above point JB. More specifically, the second end 10b is arranged at a position closer to the pin portion P than the position L3 of the axial center Jc of the journal portion J.

In the arm portion A with the weight portion W in the study step 2, two lightening portions 10 exist in the region of the weight portion W. Further, those lightening portions 10 also exist in the region of the arm portion A. Here, the width of the weight portion W is relatively large. In particular, since the planar shape of the weight portion W is generally fan-shaped, the maximum width of the weight portion W is much larger than the maximum width of the arm portion A. Therefore, the weight of the arm portion A with the weight portion W in the examination step 2 is significantly reduced by the lightening portion 10 existing in the weight portion W. Further, the shape of the weight portion W has almost no effect on the support rigidity of the arm portion A with the weight portion W. Therefore, the support rigidity of the arm portion A with the weight portion W in the study step 2 is hardly reduced.

Further, in the arm portion A with the weight portion W in the examination step 2, the rib portion 11 exists in the region where the lightening portion 10 is formed. In this case, since the rigidity of the arm portion A with the weight portion W increases as compared with the arm portion A with the weight portion W described in the above-mentioned examination step 1, the natural frequency of the arm portion A with the weight portion W increases. Therefore, the vibration of the weight portion W in the primary vertical bending mode is suppressed. As a result, the vibration of the crank shaft is also suppressed. Therefore, the crank shaft provided with the arm portion A with the weight portion W in the examination step 2 can realize weight reduction while maintaining the basic performance.

In study step 2, the natural frequency thereof was actually analyzed using the model of the arm portion A with the weight portion W shown in FIGS. 11 to 15. At that time, the width X of the rib portion 11 was variously changed. For each width X of the rib portion 11, the natural frequency f of the arm portion A with the weight portion W and its mass m were obtained.

FIG. 16 is a graph summarizing the analysis results in the study step 2. The horizontal axis of FIG. 16 indicates the ratio X / L of the width X of the rib portion 11 to the minimum width L of the weight portion W in the region where the lightening portion 10 is formed as a percentage (%). Hereinafter, the ratio X / L is also referred to as a rib width ratio X / L. At the left end of the horizontal axis (rib width ratio X / L = 0%), the width X of the rib portion 11 is 0 (zero), there is no rib portion 11, and the lightening portion 10 exists in the entire width direction. That means. The larger the rib width ratio X / L, the higher the natural frequency f and the mass m.

On the other hand, the vertical axis of FIG. 16 is a ratio (%) of the ratio f / m of the natural frequency f to the mass m, with f / m when the width X of the rib portion 11 is 0 as a reference (100%). Indicated by. Hereinafter, the ratio of f / m is also referred to as a frequency increase rate f / m. When the frequency increase rate f / m is larger than 100%, the effect of installing the rib portion 11 is higher than that in the case where the rib portion 11 is not provided. This is because the degree of increase in the natural frequency due to the installation of the rib portion 11 is larger than the degree of increase in weight due to the installation of the rib portion 11.

The graph of FIG. 16 shows the following. When the rib width ratio X / L shown on the horizontal axis is 48% or more, the frequency increase rate f / m shown on the vertical axis is larger than 100%. Therefore, from the viewpoint of effectively obtaining the effect of installing the rib portion 11, the rib width ratio X / L is preferably 48% or more. More preferably, the rib width ratio X / L is 50% or more. Further, even if the rib width ratio X / L exceeds 80%, the frequency increase rate f / m is almost the same. Therefore, from the viewpoint of suppressing the increase in weight, the rib width ratio X / L is preferably 80% or less. More preferably, the rib width ratio X / L is 75% or less.

The crank shaft of the present invention has been completed based on the above findings.

The crank shaft according to the embodiment of the present invention includes a plurality of journal portions, a plurality of pin portions, and a plurality of crank arm portions. Multiple pins are eccentric to multiple journals. The plurality of crank arm portions connect the journal portion and the pin portion corresponding to each. One or more of the crank arm portions have a counterweight portion integrally. At least one of the crank arm portions having a counterweight portion includes two lightening portions and a rib portion. The two lightening portions are formed on the surface of the counterweight portion on the pin portion side. The two lightening portions extend to the two sides of the corresponding counterweight portions. The rib portion is adjacent to the two lightening portions. The rib portion is provided along the center line of the crank arm portion.

In the crank shaft according to the present embodiment, two lightening portions and rib portions are formed in the weight portion. The width of the weight portion is relatively large. Further, the shape of the weight portion has almost no effect on the support rigidity of the arm portion with the weight portion. On the other hand, if the lightening portion is formed in the entire width direction of the weight portion, the natural frequency of the arm portion with the weight portion decreases, so that the weight portion may cause peculiar vibration. The vibration of the weight portion causes the vibration of the crank shaft. On the other hand, by providing the rib portion in the weight portion, the natural frequency of the arm portion with the weight portion increases. Therefore, the peculiar vibration of the weight portion is suppressed. As a result, the vibration of the crank shaft is also suppressed. Therefore, the crank shaft according to the present embodiment can be reduced in weight while maintaining the basic performance.

In a typical example, the crank shaft of the present embodiment is a 4-cylinder-8 counterweight crank shaft or a 4-cylinder-4 counterweight crank shaft. However, the crank shaft of this embodiment is not limited to this type. For example, the crank shaft of the present embodiment may be a crank shaft for a 3-cylinder engine or a crank shaft for an in-line 6-cylinder engine.

The number of arm portions with weight portions on which the lightening portion and the rib portion are formed is not particularly limited. When the crank shaft has a plurality of arm portions with weight portions, a lightening portion and a rib portion may be formed on one arm portion with weight portions, or a lightening portion and a lightening portion and a rib portion may be formed on one or more arm portions with weight portions. The rib portion may be formed, or the lightening portion and the rib portion may be formed on all the arm portions with weight portions. From the viewpoint of maximally reducing the weight of the crank shaft, it is preferable that the lightening portion and the rib portion are formed on all the arm portions with weight portions.

In a typical example, the shape of the weight portion with the weight portion is symmetrical with respect to the center line of the arm portion. The shapes of the two lightening portions are also symmetrical with respect to the center line of the arm portion. However, the shapes of the two lightening portions may be asymmetric with respect to the center line of the arm portion. The shape of the weight portion with the weight portion may also be asymmetric with respect to the center line of the arm portion. The width of the rib portion may or may not be constant along the center line of the arm portion.

The lightening portion and the rib portion may be formed only on the weight portion of the weight portion and the arm portion, or may be formed on both the weight portion and the arm portion. From the viewpoint of maximally reducing the weight of the arm portion with the weight portion, it is preferable that the lightening portion and the rib portion are formed on both the weight portion and the arm portion.

In the crank shaft of the present embodiment, when the crank arm portion including the lightening portion and the rib portion is viewed from the side surface, the end (first end) of the lightening portion opposite to the pin portion side is the most of the weight portion. It is preferable that the position is closer to the pin portion than the significant position. In this case, in the weight portion, the weight of the portion far from the rotation center of the crank shaft (the axis of the journal portion) is secured. Therefore, the original function of the weight portion is not lost.

In the crank shaft of the present embodiment, when the crank arm portion including the lightening portion and the rib portion is viewed from the side surface, the end (second end) on the pin portion side of the lightening portion is from the position of the axis of the journal portion. Is also preferably arranged at a position closer to the pin portion. In this case, the weight of the arm portion with the weight portion can be further reduced. However, the second end of the lightening portion may be arranged at a position closer to the weight portion than the position of the axial center of the journal portion.

In the crank shaft of the present embodiment, the ratio of the width X of the rib portion to the minimum width L of the counterweight portion in the region where the lightening portion is formed is preferably 48% to 80%. In this case, the effect of installing the rib portion can be effectively obtained while suppressing the increase in weight to the maximum.

Hereinafter, a specific example of the crank shaft of the present embodiment will be described with reference to the drawings.

17 and 18 are views showing an arm portion with a weight portion in the crank shaft of the present embodiment. Of these views, FIG. 17 is a side view, and FIG. 18 is a front view when viewed from the pin portion side. The arm portion A with a weight portion W shown in FIGS. 17 and 18 is applied to all of the eight arm portions with a weight portion included in the crank shaft of, for example, a 4-cylinder-8 counterweight.

With reference to FIGS. 17 and 18, the arm portion A with the weight portion W is symmetrical with respect to the center line Ac of the arm portion A. Two lightening portions 10 are formed on the surfaces of both the weight portion W and the arm portion A on the pin portion P side. The two lightening portions 10 extend to the two side portions Wa of the weight portion W and the two side portions Aa of the arm portion A, each of which corresponds to the weight portion W. The shapes of the two lightening portions 10 are symmetrical with respect to the center line Ac of the arm portion A.

Further, a rib portion 11 is provided on the surface of both the weight portion W and the arm portion A on the pin portion P side along the center line Ac of the arm portion A. The rib portion 11 is adjacent to the two lightening portions 10. The width X of the rib portion 11 is constant along the center line Ac of the arm portion A. The rib width ratio X / L is 50%.

In the crank shaft of the present embodiment, two lightening portions and rib portions are formed on both the weight portion W and the arm portion A of the arm portion A with the weight portion W. Therefore, the weight of the arm portion A with the weight portion W can be reduced while ensuring the rigidity of the arm portion A with the weight portion W and its natural frequency. As a result, the crank shaft of the present embodiment can be reduced in weight while maintaining the basic performance.

In addition, the present invention is not limited to the above-described embodiment, and it goes without saying that various modifications can be made without departing from the spirit of the present invention.

The present invention can be effectively used for a crank shaft mounted on any reciprocating engine.

J, J1 to J5 Journal part Jc Journal part axis P, P1 to P4 Pin part Pc Pin part axis A, A1 to A8 Crank arm part Aa Crank arm part side part Ac Arm part center line W, W1 ~ W8 Counterweight part Wa Counterweight part side part 10 Lightening part 10a First end of lightening part 10b Second end of lightening part 11 Rib part

Claims (3)

A crank shaft including a plurality of journal portions, a plurality of pin portions eccentric to the plurality of journals, and a plurality of crank arm portions connecting the journal portion and the pin portion corresponding to each of the journal portions.
One or more of the crank arm portions has a counterweight portion integrally.
At least one of the crank arm portions having the counterweight portion is
Two lightening portions formed on the surface of the counterweight portion on the pin portion side and each extending to two side portions of the corresponding counterweight portion.
It is provided with a rib portion adjacent to the two lightening portions and provided along the center line of the crank arm portion.
The width of the rib portion is constant along the center line of the crank arm portion.
When the crank arm portion including the lightening portion and the rib portion is viewed from the side surface, the end of the lightening portion on the side opposite to the pin portion side is the outer circumference of the journal portion and the crank arm portion. It is arranged at a position closer to the counterweight portion than the position of the point closest to the counterweight portion among the intersections.
A crank shaft in which the ratio of the width X of the rib portion to the minimum width L of the counterweight portion in the region where the lightening portion is formed is 48% to 80% . The crank shaft according to claim 1.
When the crank arm portion including the lightening portion and the rib portion is viewed from the side surface,
The end of the lightening portion opposite to the pin portion side is a crank shaft arranged at a position closer to the pin portion than the position of the maximum width of the counterweight portion. The crank shaft according to claim 1 or 2.
When the crank arm portion including the lightening portion and the rib portion is viewed from the side surface,
The end of the lightening portion on the pin portion side is a crank shaft arranged at a position closer to the pin portion than the position of the axial center of the journal portion.

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