A flat plane crankshaft for an in-line four cylinder engine includes eight crank arms. A fourth crank arm and a fifth crank arm are respectively provided with counter weights. Each of a width of the fourth crank arm and a width of the fifth crank arm is configured to be smaller than a width of a second crank arm. Each of a width of the third crank arm and a width of the sixth crank arm is configured to be greater than the width of the second crank arm.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2014-252275 filed on Dec. 12, 2014 including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a crankshaft for an in-line four cylinder engine.

2. Description of Related Art

A crankshaft for an in-line four cylinder engine includes: crank journals serving as a rotational center of the crankshaft; crank pins disposed at positions eccentric to the corresponding crank journals, and to which connecting rods are coupled; and crank arms that couple the respective crank journals to the corresponding crank pins.

Insufficient rigidity of crankshafts causes vibrations or the like during rotation thereof; therefore, partial reinforcement is often provided to the crankshafts so as to secure sufficient rigidity. An example of such partial reinforcement for securing the rigidity may include a crankshaft described in Japanese Patent Application Publication No. 2012-172797. In the crankshaft described in JP 2012-172797 A, in order to suppress generation of torsional vibrations at the crank arms near a flywheel because of vibrations of the flywheel disposed at a shaft end of the crankshaft, the crank arms near the flywheel are provided with partial reinforced portions so as to enhance rigidity of the crank arms.

SUMMARY OF THE INVENTION

It is possible to secure rigidity of the crankshaft by providing the crankshaft with partial reinforcement. Unfortunately, simply providing such reinforcement rather results in increase in weight of the crankshaft. Increase in weight of the crankshaft causes increase in moment of inertia of the crankshaft, so that inconvenience, such as deterioration of fuel efficiency and increase in weight of an engine, is caused. Hence, ensuring rigidity and maximum reduction in weight are both desired in designing of crankshafts.

The present invention provides a crankshaft having a light weight and a high rigidity.

The crankshaft for solving the above problems is a flat plane crankshaft for an in-line four cylinder engine including: five crank journals; four crank pins; and eight crank arms. A fourth crank arm that is a crank arm at a fourth position counted from one of both shaft ends of the crankshaft and a fifth crank arm that is a crank arm at a fifth position counted from the shaft end are respectively provided with counter weights.

If the width of each crank arm is defined to be a maximum value of a dimension of the crank arm between a central axis of the crank journal and a central axis of the crank pin in a direction orthogonal to a line segment vertically extending through the central axis of the crank journal and the central axis of the crank pin if the crank arm is viewed from the shaft end of the crankshaft, each width of the fourth crank arm and the fifth crank arm is configured to be smaller than a width of a second crank arm that is a crank arm at a second position counted from the shaft end. Each width of a third crank arm that is a crank arm at a third position counted from the shaft end and a sixth crank arm that is a crank arm at a sixth position counted from the shaft end is configured to be greater than the width of the second crank arm.

The above described “width” in the aforementioned configuration excludes a dimension of a projecting portion (e.g., boss portion or the like where a stamp or the like is provided), which is a portion projecting from the crank arm, and affects less influence on the rigidity and weight of this crank arm.

The crank arm may be provided with a counter weight on an opposite side to the corresponding crank pin relative to the corresponding journal. As the crank arm has a smaller weight so as to decrease a rotation moment of this crank arm, it is possible to reduce more weight and more dimension of the counter weight disposed to this crank arm.

Each width of the fourth crank arm and the fifth crank arm that are provided with the counter weights is configured to be smaller than the width of the second crank arm. Hence, compared with the case of configuring each width of the fourth crank arm and the fifth crank arm to be equal to the width of the second crank arm, it is possible to reduce each weight of the fourth crank arm and the fifth crank arm. In each of the fourth crank arm and the fifth crank arm, the weight of the crank arm between the corresponding crank journal and the corresponding crank pin becomes smaller, and thus as aforementioned, it is possible to reduce each weight of the counter weights respectively provided to the fourth crank arm and the fifth crank arm. By configuring each width of the fourth crank arm and the fifth crank arm to be smaller than the width of the second crank arm, it is possible to reduce not only each weight of the fourth crank arm and the fifth crank arm, but also each weight of the counter weights thereof, thereby effectively reducing the weight of the crankshaft.

If each width of the fourth crank arm and the fifth crank arm is configured to be smaller than the width of the second crank arm, rigidity of the fourth crank arm and the fifth crank arm becomes deteriorated.

To cope with this, in the crankshaft for an in-line four cylinder engine, the third crank arm that is a crank arm at the third position counted from the shaft end is fixed to the crank pin fixed to the fourth crank arm, and the third crank arm and the fourth crank arm serve as a pair of crank arms that support the crank pin. Similarly, the sixth crank arm that is a crank arm at the sixth position counted from the shaft end is fixed to the crank pin fixed to the fifth crank arm, and the fifth crank arm and the sixth crank arm also serve as a pair of crank arms that support the crank pin.

In the above configuration, the width of the third crank arm paired with the fourth crank arm is configured to be greater than the width of the second crank arm. Accordingly, compared with the case of configuring the width of the third crank arm to be equal to the width of the second crank arm, the third crank arm has a greater rigidity. Even if the rigidity of the fourth crank arm becomes decreased, the rigidity of the third crank arm becomes enhanced, thus suppressing deterioration of overall rigidity of the pair of crank arms. Accordingly, it is possible to suppress deterioration of the rigidity of the crankshaft owing to reduction in weight of the fourth crank arm.

Similarly, in the above configuration, the width of the sixth crank arm paired with the fifth crank arm is configured to be greater than the width of the second crank arm. Accordingly, compared with the case of configuring the width of the sixth crank arm to be equal to the width of the second crank arm, the sixth crank arm has a greater rigidity. Even if the rigidity of the fifth crank arm becomes decreased, the rigidity of the sixth crank arm becomes enhanced, thus suppressing deterioration of overall rigidity of the pair of crank arms. Accordingly, it is possible to suppress deterioration of the rigidity of the crankshaft owing to reduction in weight of the fifth crank arm.

According to the above configuration, it is possible to reduce the weight of the crankshaft as well as suppress deterioration of the rigidity of the crankshaft owing to reduction in weight thereof; therefore it is possible to attain a crankshaft having a light weight and a high rigidity.

On the other hand, as the width of the crank arm is decreased, the rigidity of this crank arm becomes deteriorated. It should be noted that the rigidity of the crank arm tends to become significantly deteriorated if the width of the crank arm becomes smaller than the diameter of the corresponding crank journal. To cope with this, in the aforementioned crankshaft, it is preferable that each width of the fourth crank arm and the fifth crank arm is configured to be greater than the diameter of the crank journal disposed between the fourth crank arm and the fifth crank arm.

According to the above configuration, it is possible to suppress significant deterioration of the rigidity of the crank arms owing to decrease in width of the crank arms. In the aforementioned crankshaft, it is preferable that both a ratio of the width of the third crank arm relative to the width of the fourth crank arm, and a ratio of the width of the sixth crank arm relative to the width of the fifth crank arm are configured to be an identical ratio within a range of 1.03 to 1.26.

According to the above configuration, compared with the case of configuring the width of the third crank arm, the width of the fourth crank arm, the width of the fifth crank arm, and the width of the sixth crank arm to be all the same, it is possible to reduce the weight of the crankshaft while maintaining the rigidity of the crankshaft to be equivalent to the comparative case.

In the above configuration, the ratio of the width of the third crank arm relative to the width of the fourth crank arm denotes a ratio represented by (H3/H4) if the width of the fourth crank arm is defined as H4, and the width of the third crank arm is defined as H3. Similarly, the ratio of the width of the sixth crank arm relative to the width of the fifth crank arm denotes a ratio represented by (H6/H5) if the width of the fifth crank arm is defined as H5, and the width of the sixth crank arm is defined as H6.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, one embodiment in which a crankshaft is embodied will be described with reference toFIG. 1toFIG. 4. As shown inFIG. 1, a crankshaft1of the present embodiment is a flat plane crankshaft for an in-line four cylinder engine. The crankshaft is made of a manganese steel that is one of alloy steels for machine structure use, and is produced by forging.

A pulley or the like used for driving a timing belt or a fan belt is attached to a shaft end4of the crankshaft1on the left side of the drawing, for example. A flywheel averaging a shaft torque of an engine so as to attain smooth rotation is attached to a shaft end5of the crankshaft1on the right side of the drawing.

The crankshaft1is provided with five crank journals (hereinafter, referred to as journals) serving as a rotational center of the crankshaft1. Hereinafter, a journal at a first position counted from the shaft end4is referred to as a first journal J1, a journal at a second position counted from the shaft end4is referred to as a second journal J2. A journal at a third position counted from the shaft end4is referred to as a third journal J3, a journal at a fourth position counted from the shaft end4is referred to as a fourth journal J4, and a journal at a fifth position counted from the shaft end4is referred to as a fifth journal J5. Each of the journals J1to J5has the same shaft diameter. Respective central axes of the journals J1to J5are coaxial, and coincide with the rotational central axis of the crankshaft1. The rotational central axis of the above configured crankshaft1is referred to as a journal central axis JL, hereinafter.

Between every two adjacent journals J1to J5, a crank pin to which a connecting rod is coupled is disposed at an eccentric position to the two adjacent journals. Hereinafter, a crank pin at a first position counted from the shaft end4is referred to as a first crank pin CP1, a crank pin at a second position counted from the shaft end4is referred to as a second crank pin CP2, a crank pin at a third position counted from the shaft end4is referred to as a third crank pin CP3, and a crank pin at a fourth position counted from the shaft end4is referred to as a fourth crank pin CP4. Each of the crank pins CP1to CP4has the same shaft diameter.

Since the crankshaft1is a flat plane crankshaft, the first crank pin CP1and the fourth crank pin CP4have the same arrangement phase in a rotational direction of the crankshaft1. Hence, a central axis CS of the first crank pin CP1and a central axis CS of the fourth crank pin CP4are coaxial. In the rotational direction of the crankshaft1, the second crank pin CP2has an arrangement phase deviating from the arrangement phase of the first crank pin CP1by 180°, and the third crank pin CP3has the same arrangement phase as the arrangement phase of the second crank pin CP2. Accordingly, a central axis CS of the second crank pin CP2and a central axis CS of the third crank pin CP3are coaxial.

It is assumed that respective four cylinders of the in-line four cylinder engine are referred to as a first cylinder, a second cylinder, a third cylinder, and a fourth cylinder in the arrangement order thereof, a connecting rod provided to the first cylinder of the engine is coupled to the first crank pin CP1, a connecting rod provided to the second cylinder of the engine is coupled to the second crank pin CP2. A connecting rod provided to the third cylinder of the engine is coupled to the third crank pin CP3, and a connecting rod provided to the fourth cylinder of the engine is coupled to the fourth crank pin CP4.

The respective journals J1to J5are coupled to the corresponding crank pins CP1to CP4through eight crank arms. Hereinafter, a crank arm at a first position counted from the shaft end4is referred to as a first crank arm A1, a crank arm at a second position counted from the shaft end4is referred to as a second crank arm A2, a crank arm at a third position counted from the shaft end4is referred to as a third crank arm A3, and a crank arm at a fourth position counted from the shaft end4is referred to as a fourth crank arm A4. A crank arm at a fifth position counted from the shaft end4is referred to as a fifth crank arm A5, a crank arm at a sixth position counted from the shaft end4is referred to as a sixth crank arm A6, a crank arm at a seventh position counted from the shaft end4is referred to as a seventh crank arm A7, and a crank arm at an eighth position counted from the shaft end4is referred to as an eighth crank arm A8.

The first journal J1and the first crank pin CP1are coupled to each other through the first crank arm A1. The first crank pin CP1and the second journal J2are coupled to each other through the second crank arm A2. The second journal J2and the second crank pin CP2are coupled to each other through the third crank arm A3. The second crank pin CP2and the third journal J3are coupled to each other through the fourth crank arm A4. The third journal J3and the third crank pin CP3are coupled to each other through the fifth crank arm A5. The third crank pin CP3and the fourth journal J4are coupled to each other through the sixth crank arm A6. The fourth journal J4and the fourth crank pin CP4are coupled to each other through the seventh crank arm A7. The fourth crank pin CP4and the fifth journal J5are coupled to each other through the eighth crank arm A8.

The second crank arm A2, the third crank arm A3, and the fourth crank arm A4have respective different shapes. The second crank arm A2and the seventh crank arm A7are formed in the same shape. The third crank arm A3and the sixth crank arm A6are formed in the same shape. The fourth crank arm A4and the fifth crank arm A5are formed in the same shape.

Based on results of tests and simulations previously conducted, each shape of the second crank arm A2and the seventh crank arm A7is so defined as to have weight as small as possible while securing various rigidity properties, such as flexural rigidity and torsional rigidity, which are required in each crank arm.

Each crank arm except for the third crank arm A3and the sixth crank arm A6is provided with a counter weight on an opposite side to the corresponding crank pin relative to the corresponding journal. These counter weights are weights for restoring balance to a rotation inertia force generated by movements of pistons and the connecting rods, and these counter weights are so provided as to reduce loads acting on the respective journals. Hereinafter, a counter weight provided to the first crank arm A1is referred to as a first counter weight CW1, a counter weight provided to the second crank arm A2is referred to as a second counter weight CW2, and a counter weight provided to the fourth crank arm A4is referred to as a fourth counter weight CW4. A counter weight provided to the fifth crank arm A5is referred to as a fifth counter weight CW5, a counter weight provided to the seventh crank arm A7is referred to as a seventh counter weight CW7, and a counter weight provided to the eighth crank arm A8is referred to as an eighth counter weight CW8.

The fourth counter weight CW4and the fifth counter weight CW5are formed in the same shape. In the present embodiment, the first counter weight CW1, the second counter weight CW2, the seventh counter weight CW7, and the eighth counter weight CW8have respective different shapes. However, each shape of the first counter weight CW1, the second counter weight CW2, the seventh counter weight CW7, and the eighth counter weight CW8may be appropriately changed.

FIG. 2shows a front view of each crank arm as viewed from the shaft end4of the crankshaft1. More specifically,FIG. 2(D2) shows a front view of the second crank arm A2as viewed from the shaft end4.FIG. 2(D3) shows a front view of the third crank arm A3as viewed from the shaft end4.FIG. 2(D4) shows a front view of the fourth crank arm A4as viewed from the shaft end4.

As shown inFIG. 2, hereinafter, if one of the crank arms is viewed from the shaft end4of the crankshaft1, a width of the concerned crank arm is defined to be a maximum value of a dimension of the crank arm within a range R between the journal central axis JL and the central axis CS of the crank pin in a direction orthogonal to a line segment CL vertically extending through the journal central axis JL and the central axis CS of the crank pin.

Hence, as shown inFIG. 2(2D), if the second crank arm A2is viewed from the shaft end4of the crankshaft1, a width H2of the second crank arm A2is defined to be a maximum value of a dimension of the second crank arm A2in the range R between the journal central axis JL and the central axis CS of the first crank pin CP1in the direction orthogonal to the line segment CL vertically extending through the journal central axis JL and the central axis CS of the first crank pin CP1. As aforementioned, the second crank arm A2and the seventh crank arm A7have the same shape, and thus a width H7of the seventh crank arm A7and the width H2of the second crank arm A2are the same.

As shown inFIG. 2(D3), if the third crank arm A3is viewed from the shaft end4of the crankshaft1, a width H3of the third crank arm A3is defined to be a maximum value of a dimension of the third crank arm A3in the range R between the journal central axis JL and the central axis CS of the second crank pin CP2in the direction orthogonal to the line segment CL vertically extending through the journal central axis JL and the central axis CS of the second crank pin CP2. As aforementioned, the third crank arm A3and the sixth crank arm A6have the same shape, and thus a width H6of the sixth crank arm A6and the width H3of the third crank arm A3are the same.

Furthermore, as shown inFIG. 2(D4), if the fourth crank arm A4is viewed from the shaft end4of the crankshaft1, a width H4of the fourth crank arm A4is defined to be a maximum value of a dimension of the fourth crank arm A4in the range R between the journal central axis JL and the central axis CS of the second crank pin CP2in the direction orthogonal to the line segment CL vertically extending through the journal central axis JL and the central axis CS of the second crank pin CP2. As aforementioned, the fourth crank arm A4and the fifth crank arm A5have the same shape, and thus a width H5of the fifth crank arm A5and the width H4of the fourth crank arm A4are the same. As indicated by a two-dot chain line inFIG. 2(D4), each aforementioned width of the crank arms excludes a dimension of a projecting portion B (e.g., boss portion or the like where a stamp or the like is provided), which is a portion projecting from the crank arm, and affects less influence on the rigidity and weight of the crank arm.

With respect to each width of the crank arms as above defined, in the present embodiment, each of the width H4of the fourth crank arm A4and the width H5of the fifth crank arm A5is configured to be smaller than the width H2of the second crank arm A2. As shown inFIG. 2(D4), each of the width H4of the fourth crank arm A4and the width H5of the fifth crank arm A5is configured to be greater than a diameter T of the third journal J3disposed between the fourth crank arm A4and the fifth crank arm A5.

Each of the width H3of the third crank arm A3and the width H6of the sixth crank arm A6is configured to be greater than the width H2of the second crank arm A2. Both a ratio of the width H3of the third crank arm A3relative to the width H4of the fourth crank arm A4, that is, a ratio represented by (H3/H4), and a ratio of the width H6of the sixth crank arm A6relative to the width H5of the fifth crank arm A5, that is, a ratio represented by (H6/H5) are defined to be an identical ratio within a range of 1.03 to 1.26.

As aforementioned, the crankshaft1is configured to include the crank arms having different widths, and thus the following effects are attained. Specifically, as the weight of the crank arm of interest is reduced so as to decrease a rotation moment of the crank arm, it is possible to reduce more weight and more dimension of the counter weight disposed to the crank arm. Each of the width H4of the fourth crank arm A4provided with the fourth counter weight CW4and the width H5of the fifth crank arm A5provided with the fifth counter weight CW5is configured to be smaller than the width H2of the second crank arm A2. Hence, compared with the case of configuring each of the width H4of the fourth crank arm A4and the width H5of the fifth crank arm A5to be equal to the width H2of the second crank arm A2, it is possible to reduce each weight of the fourth crank arm A4and the fifth crank arm A5.

As shown inFIG. 2, the width of the crank arm of interest in the range R between the journal central axis JL and the central axis CS of the crank pin is decreased; therefore, in the fourth crank arm A4, the weight between the third journal J3and the second crank pin CP2becomes reduced. Accordingly, as aforementioned, the weight of the fourth counter weight CW4disposed to the fourth crank arm A4can also be reduced. Similarly, in the fifth crank arm A5, the weight between the third journal J3and the third crank pin CP3is reduced. Accordingly, as aforementioned, the weight of the fifth counter weight CW5disposed to the fifth crank arm A5can also be reduced.

By configuring each of the width H4of the fourth crank arm A4and the width H5of the fifth crank arm A5to be smaller than the width H2of the second crank arm A2, it is possible to reduce not only each weight of the fourth crank arm A4and the fifth crank arm A5, but also each weight of the counter weights disposed to the respective crank arms. Accordingly, it is possible to effectively reduce the weight of the crankshaft1.

If each width of the fourth crank arm A4and the fifth crank arm A5is configured to be smaller than the width of the second crank arm A2, rigidity of the fourth crank arm A4and the fifth crank arm A5becomes deteriorated compared with the case of configuring each width of the fourth crank arm A4and the fifth crank arm A5to be equal to the width H2of the second crank arm A2.

To cope with this, in the crankshaft1, the third crank arm A3is fixed to the second crank pin CP2that is fixed to the fourth crank arm A4, and the third crank arm A3and the fourth crank arm A4serve as a pair of crank arms that support the second crank pin CP2. Similarly, the sixth crank arm A6is fixed to the third crank pin CP3that is fixed to the fifth crank arm A5, and the fifth crank arm A5and the sixth crank arm A6serve as a pair of crank arms that support the crank pin CP3.

The width H3of the third crank arm A3paired with the fourth crank arm A4is configured to be greater than the width H2of the second crank arm A2. Hence, compared with the case of configuring the width H3of the third crank arm A3to be equal to the width H2of the second crank arm A2, the third crank arm A3has a greater rigidity. Even if the rigidity of the fourth crank arm A4becomes decreased, the rigidity of the third crank arm A3becomes enhanced, thus suppressing deterioration of overall rigidity of the pair of crank arms. Accordingly, it is possible to suppress deterioration of the rigidity of the crankshaft1owing to reduction in weight of the fourth crank arm A4.

Similarly, the width H6of the sixth crank arm A6paired with the fifth crank arm A5is configured to be greater than the width H2of the second crank arm A2. Hence, compared with the case of configuring the width H6of the sixth crank arm A6to be equal to the width H2of the second crank arm A2, the sixth crank arm A6has a greater rigidity. Even if the rigidity of the fifth crank arm A5becomes decreased, the rigidity of the sixth crank arm A6becomes enhanced, thus suppressing deterioration of overall rigidity of the pair of crank arms. Accordingly, it is possible to suppress deterioration of the rigidity of the crankshaft1owing to reduction in weight of the fifth crank arm A5.

Meanwhile, as the width of the crank arm of interest is decreased, the rigidity of the crank arm is decreased. It should be noted that the rigidity of the crank arm tends to become significantly decreased if the width of the crank arm becomes smaller than the diameter of the corresponding journal. To cope with this, as aforementioned, in the crankshaft1according to the present embodiment, each of the width H4of the fourth crank arm A4and the width H5of the fifth crank arm A5is configured to be greater than the diameter T of the third journal J3disposed between the fourth crank arm A4and the fifth crank arm A5. Accordingly, it is possible to suppress significant deterioration of the rigidity of the fourth crank arm A4and the fifth crank arm A5owing to configuring each width of the fourth crank arm A4and the fifth crank arm A5to be smaller than the width of the second crank arm A2.

FIG. 3shows a test result regarding change in weight of the crankshaft1by a single cylinder if the ratio of the width H3of the third crank arm A3relative to the width H4of the fourth crank arm A4(H3/H4) is changed under a condition to ensure rigidity of the crankshaft equivalent to rigidity of the crankshaft obtained by configuring each of the width H4of the fourth crank arm A4and the width H3of the third crank arm A3to be equal to the width H2of the second crank arm A2. “Asymmetricity” as shown inFIG. 3denotes the aforementioned ratio of the width H3of the third crank arm A3relative to the width H4of the fourth crank arm A4. “Weight by single cylinder” as shown in the graph ofFIG. 3denotes a weight of the crankshaft1corresponding to the second cylinder, more specifically, a total value of respective weights of the third crank arm A3, the fourth crank arm A4, the fourth counter weight CW4, and the second crank pin CP2. “Comparative weight W1” as shown in the graph ofFIG. 3is a comparative example relative to the crankshaft1of the present embodiment, and corresponds to a “weight by single cylinder” in which the “asymmetricity is “1”, that is,” the fourth crank arm A4and the third crank arm A3have the same shape, and the width H4of the fourth crank arm A4and the width H3of the third crank arm A3are the same.

As shown inFIG. 3, if the asymmetricity was in a range of more than “1” to not more than “1.14”, as the asymmetricity became greater, the “weight by single cylinder” became smaller than the comparative weight W1. If the asymmetricity became more than “1.14”, the “weight by single cylinder” having been on a decreasing trend reversely became increased, and if the asymmetricity further became increased to exceed the vicinity of “1.29”, the “weight by single cylinder” became greater than the comparative weight W1.

The fourth crank arm A4and the fifth crank arm A5had the same shape, and the third crank arm A3and the sixth crank arm A6had the same shape as well. Accordingly, the test result as shown inFIG. 3was equivalent to change in weight of the crankshaft1by a single cylinder in the case of changing the ratio (H6/H5) of the width H6of the sixth crank arm A6relative to the width H5of the fifth crank arm A5. The “weight by single cylinder” in this case is a weight of the crankshaft1corresponding to the third cylinder, more specifically, a total value of the respective weights of the fifth crank arm A5, the sixth crank arm A6, the fifth counter weight CW5, and the third crank pin CP3.

Considering the test result as shown inFIG. 3, in the crankshaft1of the present embodiment, both the ratio (H3/H4) of the width H3of the third crank arm A3relative to the width H4of the fourth crank arm A4, and the ratio (H6/H5) of the width H6of the sixth crank arm A6relative to the width H5of the fifth crank arm A5are configured to be an identical ratio within the range of 1.03 to 1.26. Compared with the case of configuring the respective widths of the third crank arm, the fourth crank arm, the fifth crank arm, and the sixth crank arm to be all the same, it is possible to reduce the weight of the crankshaft1while maintaining the rigidity of the crankshaft1to be equivalent to the comparative case.

The third journal J3of the crankshaft1is fixed to the fourth crank arm A4and the fifth crank arm A5. The second crank pin CP2is fixed to the fourth crank arm A4, and the third crank pin CP3is fixed to the fifth crank arm A5. Hence, both a load acting on the second crank pin CP2and a load acting on the third crank pin CP3act on the third journal J3. As aforementioned, in the flat plane crankshaft1, the crank pin CP2and the crank pin CP3have the same arrangement phase in the rotational direction of the crankshaft1. Hence, two loads act on the third journal J3in the same direction.

Consequently, as shown inFIG. 4, in the flat plane crankshaft1, the load acting on the third journal J3becomes greater than a load acting on each of the other journals. In order to reduce the greater load acting on the third journal J3as far as possible, the fourth counter weight CW4provided to the fourth crank arm A4, and the fifth counter weight CW5provided to the fifth crank arm A5are configured to be greater than the counter weight of each of the other crank arms.

A diameter of a starting material of the crankshaft1before forging is defined depending on a maximum dimension in the radial direction of the crankshaft1after forging; therefore, the dimensions of the fourth counter weight CW4and the fifth counter weight CW5often become a factor to determine the diameter of the starting material of the crankshaft1.

In the light of this point, as aforementioned, in the crankshaft1of the present embodiment, by reducing the weight of each of the fourth crank arm A4and the fifth crank arm A5, it is possible to reduce each weight of the fourth counter weight CW4and the fifth counter weight CW5. Hence, it is possible to decrease the dimensions of the fourth counter weight CW4and the fifth counter weight CW5. Accordingly, the diameter of the starting material of the crankshaft1before forging can be decreased, thereby reducing manufacturing cost of the crankshaft1.

According to the aforementioned present embodiment, the following advantageous effects can be attained. (1) Each of the width H4of the fourth crank arm A4and the width H5of the fifth crank arm A5is configured to be smaller than the width H2of the second crank arm A2. Each of the width H3of the third crank arm A3and the width H6of the sixth crank arm A6is configured to be greater than the width H2of the second crank arm A2. Accordingly, it is possible to reduce the weight of the crankshaft1as well as suppress deterioration of the rigidity of the crankshaft1owing to the reduction in weight thereof, thereby obtaining the crankshaft1having a light weight and a high rigidity.

(2) Each of the width H4of the fourth crank arm A4and the width H5of the fifth crank arm A5is configured to be greater than the diameter T of the third journal J3disposed between the fourth crank arm A4and the fifth crank arm A5. Accordingly, it is possible to suppress significant deterioration of each rigidity of the fourth crank arm A4and the fifth crank arm A5owing to decreasing each width of the fourth crank arm A4and the fifth crank arm A5.

(3) Both the ratio (H3/H4) of the width H3of the third crank arm A3relative to the width H4of the fourth crank arm A4, and the ratio (H6/H5) of the width H6of the sixth crank arm A6relative to the width H5of the fifth crank arm A5are configured to be an identical ratio within the range of 1.03 to 1.26. Accordingly, compared with the case of configuring the widths of the third crank arm A3to the sixth crank arm A6to be all the same, it is possible to reduce the weight of the crankshaft1while maintaining the rigidity of the crankshaft1to be equivalent to that of the comparative case.

(4) In the flat plane crankshaft1, it is possible to reduce each dimension of the fourth counter weight CW4disposed to the fourth crank arm A4and the fifth counter weight CW5disposed to the fifth crank arm A5, thus attaining reduction in manufacturing cost of the crankshaft1.

The present embodiment may be changed and carried out as follows. —Each of the width H4of the fourth crank arm A4and the width H5of the fifth crank arm A5may be configured to be not more than the diameter T of the third journal J3disposed between the fourth crank arm A4and the fifth crank arm A5. In this case, it is also possible to attain the advantageous effects except for the above (2).Both the ratio (H3/H4) of the width H3of the third crank arm A3relative to the width H4of the fourth crank arm A4, and the ratio (H6/H5) of the width H6of the sixth crank arm A6relative to the width H5of the fifth crank arm A5are configured to be the same ratio, and the ratios may also be configured to be a different ratio.Each of the ratio (H3/H4) of the width H3of the third crank arm A3relative to the width H4of the fourth crank arm A4, and the ratio (H6/H5) of the width H6of the sixth crank arm A6relative to the width H5of the fifth crank arm A5is configured to be a value within the range of 1.03 to 1.26. Other than this configuration, each of the ratios may be configured to be a value out of the range of 1.03 to 1.26 as far as it is possible to reduce the weight of the crankshaft1as well as suppress deterioration of the rigidity of the crankshaft1owing to the reduction in weight thereof.It may be configured to provide each of the third crank arm A3and the sixth crank arm A6with a crank weight small enough not to offset the weight reduction effect attained by decreasing each width of the fourth crank arm A4and the fifth crank arm A5.

The crankshaft1is formed of a manganese steel, but may also be formed of another alloy steel for machine structure use. In such a case, the following alloy steels may be employed: a chromium steel, a manganese chromium steel, a vanadium steel, a leaded free cutting steel, and a sulfur free cutting steel. In these alloy steels for machine structure use, it is preferable to use an alloy steel having a carbon content of 0.5% or less. The crankshaft1may also be formed of a carbon steel for machine structure use.The crankshaft1is formed by using a forging, and may also be formed by using a casting.