阅读说明：本技术 滑动轴承、曲轴的支承结构 (Sliding bearing and crankshaft support structure ) 是由 后藤保明 于 2020-04-01 设计创作，主要内容包括：滑动轴承(40)由一组剖分轴承(41a、42a)构成。在各剖分轴承中,一方的轴承(42a)的周向上的中央位置(P2)的厚度尺寸(T2)比另一方的轴承(41a)的周向上的中央位置的厚度尺寸(T1)大。在上述另一方的轴承(41a)中,随着从周向的中央位置越朝向端面(51),厚度尺寸越小,由此端面(51)的厚度尺寸被设为第一厚度尺寸。在上述一方的轴承(42a)中,从周向的中央位置越朝向端面(61)则厚度尺寸越小,由此端面(61)的厚度尺寸被设为第二厚度尺寸。第一厚度尺寸与第二厚度尺寸相等。(The sliding bearing (40) is formed of a set of split bearings (41a, 42 a). In each split bearing, the thickness dimension (T2) of the circumferential central position (P2) of one bearing (42a) is larger than the thickness dimension (T1) of the circumferential central position of the other bearing (41 a). In the other bearing (41a), the thickness dimension decreases from the circumferential center position toward the end surface (51), and the thickness dimension of the end surface (51) is thereby set to the first thickness dimension. In the one bearing (42a), the thickness dimension decreases from the circumferential center position toward the end surface (61), and the thickness dimension of the end surface (61) is set to a second thickness dimension. The first thickness dimension is equal to the second thickness dimension.)

1. A plain bearing, characterized in that,

the sliding bearing (40) is configured to be provided with a first split bearing (41) and a second split bearing (42) which are semi-circular, is arranged on a cylinder block (20) in a state that end surfaces (51, 61) of the first split bearing and the second split bearing in the circumferential direction are mutually abutted, and supports a journal part (31) of a crankshaft (30),

In the slide bearing (40) described above,

the first split bearing and the second split bearing each have the same outer diameter dimension,

a thickness dimension (T2) of a central position in the circumferential direction of the second split bearing is larger than a thickness dimension (T1) of a central position in the circumferential direction of the first split bearing,

in the first split bearing, the thickness dimension of the end surface in the circumferential direction of the first split bearing becomes a first thickness dimension by decreasing the thickness dimension from the circumferential center position toward the circumferential end surface,

in the second split bearing, the thickness dimension decreases from the circumferential center position toward the circumferential end surface, whereby the thickness dimension of the circumferential end surface of the second split bearing becomes the second thickness dimension,

the first thickness dimension is equal to the second thickness dimension.

2. The plain bearing according to claim 1, wherein,

an arc surface along a first reference circle (C1) centered on a first center (O1) that is a point eccentric by a first offset amount (A1) from a reference point (O) that is a center point of a circle along an outer diameter of the sliding bearing toward a center position (P2) of the inner circumferential surface of the second split bearing in the circumferential direction is set as the inner circumferential surface of the first split bearing in the circumferential direction with the sliding bearing installed in the cylinder block, and a first reference distance from the first center to the center position of the inner circumferential surface of the first split bearing in the circumferential direction is set as a radius (R1),

An arc surface along a second reference circle (C2) centered on a second center (O2) that is a point eccentric by a second offset amount (A2) from the reference point toward a center position (P1) of the inner circumferential surface of the first split bearing in the circumferential direction and having a second reference distance from the second center to the center position of the inner circumferential surface of the second split bearing in the circumferential direction as a radius (R2) is set as the inner circumferential surface of the second split bearing in the circumferential direction in a state where the slide bearing is installed in the cylinder block,

the first thickness dimension is equal to the second thickness dimension by the first offset amount being smaller than the second offset amount and the first reference distance being shorter than the second reference distance.

3. A support structure of a crankshaft, characterized in that the support structure of the crankshaft is provided with the plain bearing according to claim 2, and the support structure of the crankshaft supports the crankshaft (30) having 3 or more of the journal portions (31),

in the support structure of the crankshaft, it is preferable that,

the first split bearing and the second split bearing are disposed vertically with respect to the journal portions in a bearing holding portion (38) of the cylinder block,

When 2 of the sliding bearings disposed at both ends are end bearings, and sliding bearings other than the end bearings are intermediate bearings, the first split bearing constituting the intermediate bearing and the second split bearing constituting the end bearings are disposed on the upper side with respect to the journal portion, and the second split bearing constituting the intermediate bearing and the first split bearing constituting the end bearings are disposed on the lower side with respect to the journal portion.

Technical Field

The present invention relates to a sliding bearing for supporting a crankshaft in a cylinder block and a support structure for a crankshaft provided with the sliding bearing.

Background

As such a support structure constituting an engine, there is known a support structure including: an annular sliding bearing provided corresponding to each journal portion of the crankshaft; and a cylinder block having a bearing holding portion that holds each bearing. Of the bearing holding portions, 2 bearing holding portions located at both ends in the direction in which the axial center of the crankshaft extends are set to have an axis line passing through the center position in the radial direction as a reference straight line. By reducing the amount of deviation between the center position of each bearing holding portion and the reference straight line, the center positions of the bearing holding portions can be made coaxial.

In the manufacturing process of an engine, when a cylinder head and a head gasket are assembled to a cylinder block, the cylinder block elastically bends and deforms along with the assembly. As a result, the center position of the bearing holding portion located at the middle in the direction in which the axis of the crankshaft extends in each bearing holding portion is greatly displaced from the reference straight line, and there is a possibility that the coaxiality of each bearing holding portion is impaired. In this case, the coaxiality of the bearings held by the bearing holding portions may be impaired, and the clearance between the inner circumferential surfaces of the bearings and the outer circumferential surface of the crankshaft may be shifted by an appropriate value.

As a technique for solving this problem, patent document 1 listed below discloses a support structure including a pair of split bearings having different thickness dimensions. Specifically, the split bearing having a large thickness dimension is disposed on the upper side and the split bearing having a smaller thickness dimension than the upper split bearing is disposed on the lower side at a position where the center position of the bearing holding portion is shifted upward with respect to the base alignment line. According to this support structure, the coaxiality of the bearing holding portions can be improved, and further, the coaxiality of the bearings can be improved.

Prior art documents

Patent document

Patent document 1: japanese patent No. 3906754

Disclosure of Invention

(problems to be solved by the invention)

Here, since the split bearings of one set constituting the bearing have different thickness dimensions, a step difference may occur on the inner circumferential surface side of the joint of the split bearings of one set due to the difference in thickness dimensions. The step may prevent smooth flow of the lubricating oil flowing between the crankshaft and the sliding bearing, and it may be difficult to form an oil film between the crankshaft and the sliding bearing.

The present disclosure has been made in view of the above problems, and an object thereof is to provide a sliding bearing and a support structure of a crankshaft including the sliding bearing, in which an oil film can be easily formed between the crankshaft and a bearing in a sliding bearing configured by a set of split bearings having different thickness dimensions.

(means for solving the problems)

The sliding bearing of the present disclosure is a sliding bearing which is configured by a first split bearing and a second split bearing of semicircular ring shapes, the first split bearing and the second split bearing being provided in a cylinder block in a state in which respective circumferential end surfaces of the first split bearing and the second split bearing are in contact with each other to support a journal portion of a crankshaft, wherein respective outer diameters of the first split bearing and the second split bearing are the same, a thickness dimension of a circumferential central position of the second split bearing is larger than a thickness dimension of a circumferential central position of the first split bearing, and the thickness dimension of the first split bearing is smaller toward the circumferential end surface from the circumferential central position, whereby the thickness dimension of the circumferential end surface of the first split bearing becomes the first thickness dimension, and the thickness dimension of the second split bearing is smaller toward the circumferential end surface from the circumferential central position, whereby a thickness dimension of a circumferential end surface of the second split bearing is a second thickness dimension, and the first thickness dimension is equal to the second thickness dimension.

In the present disclosure, a first thickness dimension, which is a thickness dimension of an end surface of the first split bearing in the circumferential direction, is equal to a second thickness dimension, which is a thickness dimension of an end surface of the second split bearing in the circumferential direction. Therefore, a step can be prevented from occurring on the inner peripheral surface side of the joint of each of the first and second split bearings. This makes it possible to smooth the flow of the lubricating oil between the sliding bearing and the crankshaft, and to appropriately form an oil film between the sliding bearing and the crankshaft.

Drawings

The above object, other objects, features and advantages of the present disclosure will become more apparent from the following detailed description with reference to the accompanying drawings.

Fig. 1 is a diagram showing an engine.

Fig. 2 is an exploded perspective view of a part of the engine.

Fig. 3 is a view of the support structure of the crankshaft as viewed from the axial direction of the crankshaft.

Fig. 4 is a view illustrating a structure of a split bearing.

Fig. 5 is a diagram illustrating the effect of improving the coaxiality.

Detailed Description

Hereinafter, an embodiment embodying the support structure of the crankshaft of the present disclosure will be described with reference to the drawings. The support structure constitutes an engine. In the present embodiment, a V-type 6-cylinder engine is used as the engine. The engine is mounted on, for example, an automobile.

First, the overall structure of engine 10 will be described with reference to fig. 1 to 3.

The engine 10 includes a cylinder block 20 and a crankshaft 30. The crankshaft 30 is formed of a material such as iron (e.g., cast iron). The crankshaft 30 has a plurality of journal portions and a plurality of crankpins arranged in a direction in which the axial center extends. Specifically, the crankshaft 30 is formed with first to fourth journal portions 31a to 31d, and the axial center of each journal portion 31a to 31d is disposed on the axial center of the crankshaft 30. In each of the journal portions 31a to 31d, the widths of the journal portions adjacent in the direction in which the axial center of the crankshaft 30 extends are the same. In addition, the intervals between the journal portions 31a to 31d adjacent to each other in the direction in which the axial center of the crankshaft 30 extends are the same.

The crankshaft 30 includes a crank pin 32 between the first journal portion 31a to the fourth journal portion 31 d. The crank pin 32 is disposed so as to be sandwiched between the first journal portion 31a to the fourth journal portion 31d in the crankshaft 30. In the present embodiment, since the engine 10 is a V-type 6 cylinder, each crank pin 32 is provided on the crankshaft 30 so as to be sandwiched between the first to fourth journal portions 31a to 31 d. Each crank pin 32 is disposed offset radially outward from the axial center of the crankshaft 30. A connecting rod, not shown, is rotatably coupled to each crank pin 32.

The cylinder block 20 is formed of, for example, aluminum. In the cylinder block 20, first to fourth cylinder-side mounting portions 21a to 21d corresponding to the first to fourth journal portions 31a to 31d are arranged in a line. In the cylinder-side mounting portions 21a to 21d, the intervals between adjacent cylinder-side mounting portions in the direction in which the axial center of the crankshaft 30 extends are the same.

The engine 10 includes first to fourth covers 33a to 33 d. In the present embodiment, the first to fourth covers 33a to 33d have the same shape. The center position in the direction extending along the axial center of the crankshaft 30 is the center between the second journal portion 31b and the third journal portion 31 c. With this center position as a reference, the first cover 33a and the fourth cover 33d are disposed at symmetrical positions, and the second cover 33b and the third cover 33c are disposed at symmetrical positions. The covers 33a to 33d are made of cast iron, for example.

The engine 10 includes a first bearing 40a, a second bearing 40b, a third bearing 40c, and a fourth bearing 40d that rotatably support the first to fourth journal portions 31a to 31 d. The first bearing 40a is a bearing for supporting the first journal portion 31a, and the second bearing 40b is a bearing for supporting the second journal portion 31 b. The third bearing 40c is a bearing for supporting the third journal portion 31c, and the fourth bearing 40d is a bearing for supporting the fourth journal portion 31 d. In the present embodiment, as the first to fourth bearings 40a to 40d, a sliding bearing composed of a pair of semicircular upper and lower side bearings is used. Each of the bearings 40a to 40d has a multilayer structure including an inner liner layer and a liner metal layer, for example. The thickness dimension of each of the upper bearings constituting the first and fourth bearings 40a and 40d and the lower bearings constituting the second and third bearings 40b and 40c, which will be described later, is smaller than the thickness dimension of each of the lower bearings constituting the first and fourth bearings 40a and 40d and the upper bearings constituting the second and third bearings 40b and 40 c. However, in fig. 1, for convenience, the thickness dimensions of the bearings 40a to 40d are set to be the same.

The support structure of each journal portion 31a to 31d will be described with reference to fig. 2 and 3. In the present embodiment, the support structures of the journal portions 31a to 31d are basically the same. Therefore, in fig. 2 and 3, the symbols a, b, c, and d among the reference numerals given to the respective members are omitted.

The cover 33 includes a semicircular arc-shaped receiving portion 34 and cover-side mounting portions 35 formed at both ends of the receiving portion 34. The inner peripheral side of the receiving portion 34 is a concave portion 36 recessed in a semicircular arc shape. Each cover-side mounting portion 35 is formed with a through hole 37.

A concave portion 22 recessed in a semicircular arc shape is formed in the cylinder-side mounting portion 21. The bearing holding portion 38 is formed by the recessed portion 22 of the cylinder-side mounting portion 21 and the recessed portion 36 of the receiving portion 34. That is, the bearing holding portion 38 is a hole formed by the concave portion 22 provided in the cylinder block 20 and the concave portion 36 provided in the cover 33. The bearing 40 is held by the bearing holding portion 38. Specifically, the screw 39 is screwed into the screw hole of the cylinder-side mounting portion 21 through the through hole 37 in a state where the outer peripheral surface of the upper bearing 42 abuts against the inner peripheral surface of the recessed portion 22 of the cylinder-side mounting portion 21 and the outer peripheral surface of the lower bearing 41 abuts against the inner peripheral surface of the recessed portion 36 of the receiving portion 34. Thereby, the cover 33 is fixed to the cylinder-side mounting portion 21, and the bearing 40 is held by the bearing holding portion 38.

The lower bearing 41 is a bearing arranged on the lower side of the pair of split bearings constituting the bearing 40, and the upper bearing 42 is a bearing arranged on the upper side of the pair of split bearings. The bearing 40 supports the shaft neck 31 in a state where the circumferential end surfaces of the upper bearing 42 and the lower bearing 41 are in contact with each other.

Returning to the description of fig. 1, the first bearing holding portion 38a that holds the first bearing 40a is formed by the recessed portion of the first cylinder-side mounting portion 21a and the recessed portion of the receiving portion of the first cover 33 a. The second bearing holding portion 38b that holds the second bearing 40b is formed by the concave portion of the second cylinder-side mounting portion 21b and the concave portion of the receiving portion of the second cover 33 b. The third bearing holding portion 38c that holds the third bearing 40c is formed by the concave portion of the third cylinder-side mounting portion 21c and the concave portion of the receiving portion of the third cover 33 c. A fourth bearing holding portion 38d for holding the fourth bearing 40d is formed by the concave portion of the fourth cylinder side mounting portion 21d and the concave portion of the receiving portion of the fourth cover 33 d.

Next, the detailed shape of the bearing 40 will be described. Hereinafter, the first bearing 40a will be described as an example. Fig. 4 is a view of the first bearing 40a held by the bearing holding portion 38 as viewed from the thickness direction of the first bearing 40a (the extending direction of the crankshaft). In fig. 4, the engine, the crankshaft, and the like are not shown for convenience of explanation.

The first lower bearing 41a and the first upper bearing 42a have the same outer diameter. Specifically, when the center of the outer diameter of the first bearing 40a is defined as the reference point O, the outer peripheral surface 50 of the first lower bearing 41a and the outer peripheral surface 60 of the first upper bearing 42a form an arc surface along a circle having the radius Rout with the reference point O as the center. In fig. 4, a straight line passing through an end surface 51 of the first lower bearing 41a in the circumferential direction and an end surface 61 of the first upper bearing 42a in the circumferential direction is defined as an x-axis. A line passing through the reference point O and the center positions P2, P1 of the inner circumferential surfaces 62, 52 in the circumferential direction of the first upper and lower bearings 42a, 41a is defined as the y-axis. The y-axis passes through the center positions of the outer circumferential surfaces 60 and 50 of the first upper and lower bearings 42a and 41a in the circumferential direction, in addition to the center positions P2 and P1 of the inner circumferential surfaces 62 and 52 of the first upper and lower bearings 42a and 41a in the circumferential direction.

The thickness dimension T2 at the circumferential center position of the first upper bearing 42a is larger than the thickness dimension T1 at the circumferential center position of the first lower bearing 41 a. The thickness dimension of the first lower bearing 41a decreases from the center position P1 of the inner circumferential surface 52 in the circumferential direction toward the end surface 51.

A point eccentric by the first offset amount a1 from the reference point O toward the center position P2 of the inner circumferential surface 62 in the circumferential direction of the first upper bearing 42a is defined as a first center O1. An arc surface along a first reference circle C1 is defined as the inner circumferential surface 52 of the first lower bearing 41a in the circumferential direction, and the first reference circle C1 is centered on the first center O1 and has a radius of a first reference distance R1 from the first center O1 to a central position P1 of the inner circumferential surface 52 of the first lower bearing 41a in the circumferential direction.

The thickness dimension of the first upper bearing 42a decreases from the center position P2 of the inner circumferential surface 62 in the circumferential direction toward the end surface 61. A point eccentric by the second offset amount a2 from the reference point O toward the center position P1 of the inner peripheral surface 52 of the first lower bearing 41a is defined as a second center O2. The second offset amount a2 is greater than the first offset amount a 1. An arc surface along the second reference circle C2 is defined as the inner circumferential surface 62 of the first upper bearing 42a in the circumferential direction, and the second reference circle C2 is centered on the second center O2 and has a radius of a second reference distance R2 from the second center O2 to a central position P2 of the inner circumferential surface 62 of the first upper bearing 42 a. The second reference distance R2 is longer than the first reference distance R1.

The first offset amount a1 is smaller than the second offset amount a2, and the first reference distance R1 is smaller than the second reference distance R2, whereby the first thickness dimension of the circumferential end surface 51 of the first lower bearing 41a is equal to the second thickness dimension of the circumferential end surface 61 of the first upper bearing 42 a.

Although not shown in fig. 4, a run-in relief portion is usually formed at an inner circumferential side circumferential end portion of the first lower bearing 41a and an inner circumferential side circumferential end portion of the first upper bearing 42 a. In this case, the depth of the first lower bearing 41a pushed into the relief is made equal to the depth of the first upper bearing 42a pushed into the relief, whereby the first thickness dimension of the first lower bearing 41a and the second thickness dimension of the first upper bearing 42a can be made equal to each other.

A first imaginary circle S1 having a radius centered on the reference point O and a distance from the reference point O to a center position P1 of the inner circumferential surface 52 in the circumferential direction of the first lower bearing 41a is indicated by a broken line. An intersection point between a straight line which is separated from the x-axis by a predetermined distance LA and is parallel to the x-axis and the outer peripheral surface 50 of the first lower bearing 41a is Q1. A distance between an intersection point with the first imaginary circle S1 and an intersection point with the inner peripheral surface 52 of the first lower bearing 41a in a straight line passing through the reference point O and the intersection point Q1 is the first oil spilling portion t 1.

A second imaginary circle S2 having a radius of a distance from the reference point O to the center position P2 of the inner circumferential surface 62 in the circumferential direction of the first upper bearing 42a and centered on the reference point O is indicated by a broken line. An intersection point between a straight line, which is separated from the x-axis by a predetermined distance LA and is parallel to the x-axis, and the outer peripheral surface 60 of the first upper bearing 42a is denoted by Q2. A distance between an intersection point with the second imaginary circle S2 and an intersection point with the inner peripheral surface 62 of the first upper bearing 42a in a straight line passing through the reference point O and the intersection point Q2 is the second oil spilling portion t 2. The second oil spilling portion t2 is larger than the first oil spilling portion t 1.

Next, the second to fourth bearings 40b to 40d will be described.

The second lower bearing 41b constituting the second bearing 40b, the third lower bearing 41c constituting the third bearing 40c, and the fourth upper bearing 42d constituting the fourth bearing 40d have the same shape as the first upper bearing 42a constituting the first bearing 40 a. The second upper bearing 42b constituting the second bearing 40b, the third upper bearing 42c constituting the third bearing 40c, and the fourth lower bearing 41d constituting the fourth bearing 40d have the same shape as the first lower bearing 41a constituting the first bearing 40 a. In the present embodiment, the first lower bearing 41a, the second upper bearing 42b, the third upper bearing 42c, and the fourth lower bearing 41d correspond to "first split bearings", and the first upper bearing 42a, the second lower bearing 41b, the third lower bearing 41c, and the fourth upper bearing 42d correspond to "second split bearings".

Next, the arrangement state of the first to fourth bearings 40a to 40d assembled to the engine 10 will be described. Fig. 5 (a) shows the positions of the bearings 40a to 40d before the completion of the engine 10, and fig. 5 (b) shows the positions of the bearings 40a to 40d after the completion of the engine 10. In fig. 5 a and 5 b, a line connecting the center positions of the inner diameters of the bearings 40a to 40d is indicated by a dashed-dotted line, and an axis passing through the center positions of the inner diameters of the first bearing holding portion 38a and the fourth bearing holding portion 38d is defined as a reference straight line L α (a dashed line in the drawing). "# 1", "# 2", "# 3", and "# 4" indicate the numbers of the first to fourth collar portions 31a to 31 d. The reference straight line L α is parallel to a direction extending along the axial center of the crankshaft 30.

In the first bearing 40a, the thickness dimension of the first upper bearing 42a is larger than the thickness dimension of the first lower bearing 41 a. In the second bearing 40b, the thickness dimension of the second lower bearing 41b is larger than the thickness dimension of the second upper bearing 42 b. In the third bearing 40c, the thickness dimension of the third lower bearing 41c is larger than the thickness dimension of the third upper bearing 42 c. In the fourth bearing 40d, the thickness dimension of the fourth upper bearing 42d is larger than the thickness dimension of the fourth lower bearing 41 d. In the present embodiment, as described above, the first upper bearing 42a, the second lower bearing 41b, the third lower bearing 41c, and the fourth upper bearing 42d have the same shape. The first lower bearing 41a, the second upper bearing 42b, the third upper bearing 42c, and the fourth lower bearing 41d have the same shape. The first and fourth bearings 40a and 40d correspond to "end bearings", and the second and third bearings 40b and 40c correspond to "intermediate bearings".

As shown in fig. 5 (a), before the engine 10 is completed, the thickness of the lower bearings 41b and 41c of the second and third bearings 40b and 40c is large, and therefore the inner diameter centers of the second and third bearings 40b and 40c are located above the inner diameter centers of the first and fourth bearings 40a and 40 d. Thereby, the line connecting the inner diameter centers of the bearings 40a to 40d is convex upward with respect to the reference straight line L α.

When the cylinder head and the head gasket are assembled to the cylinder block 20, elastic bending deformation occurs in the cylinder block 20 in association with the assembly. By this bending deformation, as shown in fig. 5 (b), the inner diameter centers of the first to fourth bearings 40a to 40d coincide with the reference straight line L α. As a result, the coaxiality of the first to fourth bearings 40a to 40d is not impaired.

In the engine 10, the load (for example, the load from the connecting rod) acting on the second and third lower bearings 41b and 41c constituting the second and third bearings 40b and 40c positioned in the middle among the split bearings constituting the first to fourth bearings 40a to 40d is larger than the load acting on the other split bearings. As a result, the temperature of the second and third lower bearings 41b and 41c may rise excessively. In the present embodiment, the second oil spilled portions t2 of the second and third lower bearings 41b and 41c are larger than the second oil spilled portions t2 of the first and fourth lower bearings 41a and 41 d. Therefore, the amount of lubricating oil supplied between the second and third lower bearings 41b and 41c and the second and third journal portions 31b and 31c can be increased, and an oil film can be formed between the second and third bearings 40b and 40c and the second and third journal portions 31b and 31c as appropriate. This can suppress an excessive temperature rise of the second and third lower bearings 41b and 41 c.

Next, the case where the first and second thickness dimensions are made equal will be described in more detail. Hereinafter, the first bearing 40a will be described as an example.

As shown in fig. 5 (a), β is the offset amount between the reference straight line L α and the center of the inner diameter of the second and third bearings 40b and 40c before completion of the engine. The offset β may be performed by, for example, experiments or simulation. Further, "Δ t" is set to β/2 ", and TS represents a reference plate thickness dimension of the first lower bearing 41a and the first upper bearing 42 a. In this case, referring to fig. 4, the thickness dimension T1 at the circumferential central position of the first lower bearing 41a and the thickness dimension T2 at the circumferential central position of the first upper bearing 42a are expressed by the following expressions (1) and (2).

T1＝TS-ΔT … (1)

T2＝TS+ΔT … (2)

Next, the first reference distance R1 and the first offset a1 are determined so that the first oil spilling portion t1 in each of the lower bearings 41a to 41d has a value that enables the oil film to be formed appropriately. The shape of the first oil spilling portion t1 is determined, thereby determining the first thickness dimension of the end surface 51 of each of the lower bearings 41a to 41 d. The first reference distance R1 may be performed by, for example, experiments or simulations.

Next, the second reference distance R2 and the second offset amount a2 are determined so that the dimension of the end surface 61 of each of the upper bearings 42a to 42d becomes the first thickness dimension. Specifically, a second reference circle C2 is determined which passes through the intersection point of the first reference circle C1 and the x-axis and the center position P2 of the inner peripheral surface 62 of the first upper bearing 42 a. At this time, the second offset amount a2 is made larger than the first offset amount a1, and the second reference distance R2 is made larger than the first reference distance R1. By determining the second reference circle C2, the second offset amount a2 and the second reference distance R2 are determined. As described above, the first reference circle C1 is a circle having the first center O1 as the center and the first reference distance R1 as the radius.

In the present embodiment described above, the following effects can be achieved.

The first lower bearing 41a and the first upper bearing 42a constituting the first bearing 40a have the same outer diameter. The thickness dimension T2 of the first upper bearing 42a at the circumferential center position is larger than the thickness dimension T1 of the first lower bearing 41a at the circumferential center position. The thickness dimension of the end surface 61 in the circumferential direction of the first upper bearing 42a is equal to the thickness dimension of the end surface 51 in the circumferential direction of the first lower bearing 41 a. This prevents a step from occurring on the inner circumferential surface side of the joint of the first upper and lower bearings 42a and 41 a. The same applies to the second to fourth bearings 40b to 40 d. Therefore, the flow of the lubricating oil between the first to fourth bearings 40a to 40d and the first to fourth journal portions 31a to 31d can be made smooth, and an oil film can be appropriately formed between the first to fourth bearings 40a to 40d and the first to fourth journal portions 31a to 31 d.

In each split bearing, the second oil spilling portions t2 of the second and third lower bearings 41b and 41c located midway in the direction in which the axis of the crankshaft 30 extends are larger than the second oil spilling portions t2 of the first and fourth lower bearings 41a and 41 d. Therefore, the amount of lubricating oil supplied between the second and third lower bearings 41b and 41c and the second and third journal portions 31b and 31c can be increased. This can suppress an excessive temperature rise of the second and third lower bearings 41b and 41c located in the middle, while minimizing the loss of the coaxiality of the crankshaft 30.

< other embodiments >

The above embodiment may be modified as follows.

As the engine 10, a 4-cylinder engine may be used. In this case, since 5 journals are provided on the crankshaft, 5 bearings are arranged in the engine 10 so as to correspond to the 5 journals. Of the 5 journal portions, the bearings corresponding to the 3 journal portions located at the center in the direction in which the axis of the crankshaft extends become intermediate bearings.

Here, in the case where 5 bearings are provided corresponding to 5 journal portions of the crankshaft, the oil spilling portion may be adjusted in accordance with the magnitude of the load acting on the lower bearing among the 3 intermediate bearings. In this case, of the intermediate bearings, the bearing that applies the largest load to the lower bearing (for example, the third bearing located at the center) may be determined such that the oil spill portion becomes the largest, and the first offset amount a1 and the first reference distance R1 may be determined.

(description of reference numerals)

20 … cylinder block, 30 … crankshaft, 31 … journal portion, 38 … bearing retaining portion, 40 … bearing, 41 … lower side bearing, 42 … upper side bearing.