JPH10246330A - Piston for internal combustion engine - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence of crack and breakage in a pin boss part by adding compression stress to the vicinity of an inner end face of a pin boss as residual stress in a piston for internal combustion engine in which a small end part of a connecting rod is connected with the pin boss formed in a piston main body through a piston pin. SOLUTION: A piston 10 is integrally molded by using aluminum alloy and has a pair of pin bosses 22 in which a pin hole 23 which passes through a central part of a piston main body part 20 is formed in the central part thereof. It is connected with a small end part 24 at an upper end of a connecting rod 16 by a piston pin 21 which passes through the pin hole 23. That is, a bush 19 is fitted in a mounting hole 25 formed in the small end part 24, and then the piston pin 21 passes through it. In this case, a stress application part 30 due to the machining which crushes the unevenness on a surface by pressing a roll against an inner wall of the hole is provided in a stress concentration part A positioned in an upper face part of the pin boss 22 and at a position close to this stress concentration part A to prevent the occurrence of crack and breakage in a pin boss part.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a piston for an internal combustion engine, and more particularly to an internal combustion engine having a structure in which a piston body is connected and supported to a connecting rod by inserting a piston pin into a pin boss and a fitting hole of a connecting rod. Regarding pistons.

[0002]

2. Description of the Related Art In general, a piston in an internal combustion engine such as a diesel engine is connected and supported by inserting a piston pin into a pin boss formed in a piston body and a fitting hole formed in a small end of a connecting rod. It has a structure. By the way, on the pin boss formed on the conventional piston, the explosion pressure acting on the piston upper surface when the engine is operating acts as concentrated stress on the pin boss upper surface on the piston upper surface side, and the surface pressure on the pin boss upper surface becomes excessive, and in the worst case, May cause crack breakage.

That is, since the conventional pin boss portion is machined substantially parallel to the upper surface of the piston, when an explosion pressure is applied to the upper surface of the piston during operation of the engine, the piston pin is subjected to considerable bending and elliptical deformation. As a result, excessive stress is concentrated on the inner wall surface of the pin boss, and there is a problem in that crack breakage occurs in the pin boss portion. Therefore, as a means for preventing cracks, for example, Japanese Utility Model Laid-Open No. 61-5354
As disclosed in Japanese Unexamined Patent Publication (Kokai) No. 1 (Kokai) No. 1 (KOKAI), there has been proposed an internal combustion engine piston in which the inner wall surface of a pin boss is formed in an arc shape toward the center of the piston.

[0004]

Problems to be Solved by the Invention
According to the internal combustion engine piston disclosed in Japanese Patent No. 3541, the inner wall surface of the pin boss is cut in an arc shape toward the center of the piston, so that the explosion pressure acts on the piston and the piston pin is deformed. The pin comes into close contact with the inner wall surface of the pin boss processed into an arc shape. As a result, the concentrated stress is averaged, so that it is possible to prevent the pin boss from being cracked.

[0005] However, with the above-described piston structure, the workability of the piston body is reduced. That is, as is well known, the pin boss is a hole formed through the piston body.
Therefore, it is very difficult to form the inner wall surface of the pin boss, which is a hole, in an arc toward the center of the piston. Therefore, there is a problem that an arc having a desired curvature cannot be formed with high accuracy.

As a result, when the curvature of the arc becomes large, the concentrated stress cannot be sufficiently averaged, and an excessive stress is generated on the inner wall surface of the pin boss. Also,
Conversely, if the curvature of the arc becomes smaller, a gap is created between the pin boss and the piston pin even when the explosion pressure is applied, and the area receiving the stress becomes smaller, resulting in a reduced concentrated stress. It cannot be averaged sufficiently.

The present invention has been made in view of the above points, and a piston for an internal combustion engine in which a crack is prevented from being generated in a pin boss portion by applying a compressive stress as a residual stress in the vicinity of an inner end face of the pin boss. The purpose is to provide.

[0008]

Means for Solving the Problems In order to solve the above problems, the present invention is characterized by taking the following means. According to the first aspect of the present invention, the piston body is connected to the connecting rod by inserting a piston pin into a pin boss formed in the piston body and a fitting hole formed in a small end portion of the connecting rod. An internal combustion engine piston configured to be supported is characterized in that a compression stress is added as a residual stress in the vicinity of the inner end face of the pin boss.

According to a second aspect of the present invention, in the piston for an internal combustion engine according to the first aspect, the compressive stress is applied by using shot peening. Each of the above means operates as follows.

According to the first aspect of the present invention, the compressive stress is applied as a residual stress in the vicinity of the inner end face of the pin boss, so that the tensile stress in the vicinity of the inner end face of the pin boss can be reduced. Can be prevented from occurring. Further, since it is not necessary to cut the pin boss into an arc shape or the like, the workability is improved, and thus the productivity of the piston for the internal combustion engine can be improved.

According to the second aspect of the present invention, since the compressive stress is applied by using shot peening, the compressive stress can be reliably applied to the inner end surface of the pin boss having the hole structure. Further, even if the pin boss has a tapered surface, it is possible to reliably apply a compressive stress to the inner end surface of the pin boss.

[0012]

Next, an embodiment of the present invention will be described with reference to the drawings. 1 to 4 show a piston 10 for an internal combustion engine (hereinafter simply referred to as a piston) according to a first embodiment of the present invention. FIG. 1 is a cross-sectional view taken along the axis of the pin boss of the piston 10, FIG. 2 is a cross-sectional view taken along a direction perpendicular to the axis of the pin boss of the piston 10, and FIG.
FIG. 4 is an exploded perspective view showing an attachment structure of the piston 10 to the connecting rod, and FIG. 4 is a partially cutaway perspective view of the piston 10.

The piston 10 is a member integrally cast from a lightweight alloy such as an aluminum alloy. This piston 10
A piston head 12 is provided at an upper end portion of the piston main body 20 on the combustion chamber side. A plurality of annular ring grooves 14 are formed on the outer peripheral surface of the piston head 12. By mounting a piston ring (not shown) in the ring groove 14, the piston 10 can slide in the cylinder while maintaining airtightness with the combustion chamber. Further, a pair of skirts 18 are provided below the piston head 12 so as to face each other. The skirt 18 is formed so as to extend downward along the outer periphery of the piston head 12.

On the other hand, a pair of pin bosses 22 is formed at the center of the piston body 20, and a space 17 in which a connecting rod 16 described later is interposed is formed between the pair of pin bosses 22. I have. Further, the pin boss 22 is formed with a pin hole 23 penetrating the central portion thereof. The piston pin 21 is inserted into the pin hole 23 as described later.

In this embodiment, the inner diameter of the pin hole 23 is the same over the entire range.
May be formed so that the inner diameter of the is gradually increased from the inside toward the outside. Thus, by forming the pin hole 23 into a shape having a tapered surface that gradually expands from the inside to the outside,
As in the above-described prior art, the concentrated stress can be averaged, so that the occurrence of crack breakage can be prevented.

The piston 10 having the above structure is attached to a connecting rod 16. Hereinafter, a structure for attaching the piston 10 to the connecting rod 16 will be described. The connecting rod 16 has a small end 24 at an upper portion thereof, and a mounting hole 25 is formed in the small end 24. After the bush 19 is fitted into the mounting hole 25, the piston pin 21 is inserted therethrough. Also,
A large end 26 is provided below the connecting rod 16. This large end 26 is a bearing cab 28
And a bearing for a crankpin (not shown) provided on the crankshaft. The bearing cab 28 is fastened to the large end 26 by a bolt 29.

The piston 10 is connected to a connecting rod 16
First, the small end 24 of the connecting rod 16 is interposed in the space 17 formed in the piston 10. In this interposed state, the positioning is performed so that the mounting hole 25 of the small end portion 24 and the pin hole 23 are coaxial. Then, the piston pin 21 is fitted into the small end portion 24 and the pin boss 22 while maintaining this state. This allows
The piston 10 is attached to a connecting rod 16.

In the above configuration, when an explosion pressure acts on the piston head 12 during operation of the engine, the piston 10 is urged downward in the cylinder bore, and this explosion pressure is transmitted to the connecting rod 16 via the piston pin 21. . Further, since the connecting rod 16 is connected to the crankshaft, the crankshaft is rotationally urged by the explosion pressure, whereby the explosion pressure can be converted into the rotational force of the crankshaft.

Here, attention is paid to the relationship between various forces acting on the piston pin 21 when the explosion pressure acts on the upper surface of the piston head 12 during the operation of the engine as described above. Now, assuming that the explosion pressure is F1 (indicated by an arrow in FIG. 1), the explosion pressure F1 is applied to the piston pin 21 via the pair of pin bosses 22. At this time, each pin boss 22
An applied force F2 (F2 ≒ F1 / 2) that is approximately half of the explosion pressure F1
(Indicated by an arrow in FIG. 1), the both sides of the piston pin 21 are pressed downward. The center of the piston pin 21 is also connected to the connecting rod 16, and the center of the piston pin 21 is connected to the connecting rod 16.
The reaction force F3 (F3 ≒ F1) acts upward.

As described above, when the explosion pressure acts on the upper surface of the piston head 12, the explosion pressure F2 acts on both sides of the piston pin 21 downward and the reaction force F3 acts on the central portion. The bending deformation of the piston pin 21 occurs as shown by a dashed line in FIG. 1 (the state of the deformation is exaggerated in the figure for easy understanding). For this reason, stress concentrates on the upper surface portion of each pin boss 22, especially on the inner end portion of the upper surface of the pin boss 22 (the position indicated by arrow A in FIG. 1; hereinafter, referred to as a stress concentration portion). A often caused crack damage.

Therefore, in this embodiment, a structure is adopted in which a compressive stress is applied as a residual stress in the vicinity of the inner end face of the pin boss 22. Hereinafter, a portion where the compressive stress is added as the residual stress is referred to as a stress applying portion 30, and the stress applying portion 30 is shown in a matte pattern in the figure. As shown in FIGS. 1 and 2, in this embodiment, the stress applying unit 30 is provided at a stress concentration portion A located on the upper surface of the pin boss 22 and at a position close to the stress concentration portion A. In order to form the stress applying section 30 on the pin boss 22, it is necessary to apply a compressive stress to the pin boss 22 from inside the pin hole 23. As described above, a method of applying a compressive stress to the pin boss 22 from the inside of the pin hole 23 is as follows.
For example, roll burnishing is conceivable.

This roll burnishing is used for finishing the inner surface of a hole, and is a method of pressing a roll against the inner wall of the hole to crush the unevenness of the surface, thereby improving the surface roughness. As described above, since the roll burnishing is a process of pressing the inner wall of the hole by the roll, a compressive stress can be applied to the inner wall of the hole as a residual stress by the pressing force.

When the diameter of the pin hole 23 formed in the pin boss 22 is uniform as in this embodiment, a desired compressive stress can be applied to the pin boss 22 by roll burnishing. However, in a configuration in which the inner diameter of the pin hole 23 is formed so as to gradually expand from the inside to the outside to average the concentrated stress, roll burnishing cannot be used. This is because the roll burnishing is a processing method applicable only to holes having the same diameter.

Therefore, the pin boss 2 is inserted from inside the pin hole 23.
Shot peening can be considered as another method of applying a compressive stress to No. 2. Shot peening is a method of applying a compressive stress by accelerating a shot (steel grain) by pneumatic or centrifugal force and colliding it with a processing surface. In the shot peening according to the present embodiment, for example, a shotgun in the form of a nozzle capable of injecting a shot from the tip by air pressure is used, and this shotgun is inserted into the pin hole 23 and located on the upper surface of the pin boss 22. A method of applying a compressive stress by colliding a shot with the stress concentration portion A and a position close to the stress concentration portion A was applied.

As described above, by using shot peening as a means for applying the compressive stress as a residual stress, the compressive stress can be easily applied to a desired portion. It can be easily controlled by appropriately selecting the type of shot, the injection time, and the like. Further, since compressive stress can be applied regardless of the shape of the portion to be processed, compressive stress can be reliably and easily applied to the pin boss 22 having the above-described pin hole 23 having a tapered surface.

Next, the effect of providing the stress concentration portion A located on the upper surface portion of the pin boss 22 as described above and the stress application portion 30 at a position close to the stress concentration portion A will be described with reference to FIG. Will be explained. FIG.
FIG. 2 shows a stress fatigue limit diagram of the present invention. In the figure, the vertical axis represents the stress amplitude, and the horizontal axis represents the average stress. In particular, on the horizontal axis, the portion on the right side of the origin O is the average tensile stress, and the portion on the left side of the origin O is the average compressive stress. The solid line in FIG. 3 is the stress fatigue limit line of the piston 10. Therefore, by setting the tensile and compressive stresses in the region surrounded by the solid line, it is possible to suppress the occurrence of cracks in the piston 10. In particular, since the tensile stress is generally smaller than the compressive stress, by setting the tensile stress in the above range, it is possible to suppress the occurrence of cracks in the piston 10.

Therefore, attention is first paid to the tensile stress characteristics of the conventional piston without the stress applying section 30. In a piston having no stress applying unit 30, in other words, a structure in which a compressive stress is not added as a residual stress, a magnitude of a tensile stress (hereinafter, referred to as a tensile strength) that can withstand a stress applied by an explosion pressure. It is indicated at the origin O ₁ as the length E1 of a line indicated by a chain line in the drawing as a starting point.

On the other hand, the piston 10 according to the present embodiment
In the configuration in which the stress applying unit 30 is provided as described above, the magnitude of the tensile strength is represented by a two-dot chain line starting from a point O ₂ shifted to the left (compressive stress side) from the origin O ₁ . Is shown as length E2. The difference Δσ between the starting points O ₁ and O ₂ corresponds to the magnitude of the compressive stress (residual stress) applied to the piston 10 by providing the stress applying unit 30.

Here, the magnitude of the conventional tensile strength E
1 and the magnitude E2 of the tensile strength of the piston 10 according to the present embodiment, the tensile strength E2 of the piston 10 according to the present embodiment is larger than the conventional tensile strength E1. (E2> E1). Therefore, the result shown in FIG. 5 proves that the piston 10 according to the present embodiment can suppress the generation of cracks in the piston 10 when the explosion pressure is applied, as compared with the conventional piston. .

FIG. 6 shows a piston 10A according to a second embodiment of the present invention. 6, the same components as those of the piston 10 according to the first embodiment described with reference to FIGS. 1 to 4 are denoted by the same reference numerals, and description thereof will be omitted. The piston 10A according to the present embodiment is characterized in that a tapered surface 31 is formed in a stress concentration portion A located on the upper surface of the pin boss 22. As described above, when the piston pin 21 is deformed by the application of the explosion pressure to the piston head 12 of the piston 10A as described above, substantially the entire surface of the deformed piston pin 21 becomes the tapered surface 31. The contact angle is selected. Thereby, when the explosion pressure is applied, the piston pin 21 explosion comes into close contact with the inner wall surface of the pin boss 22, and the stress is averaged, so that it is possible to prevent the piston 10A from cracking.

Also in this embodiment, the pin boss 22
Concentrated portion A located on the upper surface of
The stress applying unit 30 is formed by performing shot peening at a position close to. Therefore, it is possible to prevent the piston 10A from being cracked for the same reason as described in the first embodiment. on the other hand,
The present embodiment is also characterized in that the above-described tapered surface 31 is formed by using shot peening.
That is, in the present embodiment, the formation of the tapered surface 31 is
0 is formed at the same time. As mentioned above,
In shot peening, the strength of the shot can be controlled by appropriately selecting the air pressure, the type of shot, the injection time, and the like. In addition, the shot ejection position can be easily determined by adjusting the insertion depth of the shot gun into the pin hole 23. Therefore, the tapered surface 3
At the formation position 1, the tapered surface 31 can be easily formed by, for example, injecting a shot strongly.

In the above-described embodiment, an example in which the present invention is applied to a piston for an internal combustion engine has been described. However, the present invention can be widely applied to a bearing structure in which a shaft portion receives stress.

[0033]

According to the present invention as described above, the following various effects can be realized. According to the first aspect of the present invention, it is possible to reduce the tensile stress in the vicinity of the inner end surface of the pin boss, thereby preventing the pin boss from being cracked. Further, since it is not necessary to cut the pin boss into an arc shape or the like, the productivity of the piston for the internal combustion engine can be improved.

According to the second aspect of the present invention, the compressive stress can be reliably applied to the inner end face of the pin boss having the hole structure, and even if the pin boss has a tapered surface, Compressive stress can be reliably applied to the inner end surface of the pin boss.

[Brief description of the drawings]

FIG. 1 is a sectional view taken along an axis of a pin boss of a piston according to a first embodiment of the present invention.

FIG. 2 is a cross-sectional view taken along a direction perpendicular to an axis of a pin boss of a piston according to the first embodiment of the present invention.

FIG. 3 is an exploded perspective view showing a structure for attaching a piston to a connecting rod according to the first embodiment of the present invention.

FIG. 4 is a partially cutaway perspective view of a piston according to the first embodiment of the present invention.

FIG. 5 is a stress fatigue limit diagram of the piston according to the first embodiment of the present invention.

FIG. 6 is a sectional view taken along a direction perpendicular to an axis of a pin boss of a piston according to a second embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Piston 12 Piston head 16 Connecting rod 19 Bush 20 Piston main body 22 Pin boss 23 Pin hole 24 Small end 25 Mounting hole 26 Large end 28 Bearing cap 30 Stress applying part 31 Tapered surface

Claims (2)

[Claims] A pin boss formed on a piston body;
An internal combustion engine piston configured to connect and support the piston body to the connecting rod by inserting a piston pin into a fitting hole formed at a small end of the connecting rod, wherein the vicinity of the inner end surface of the pin boss A piston for an internal combustion engine, wherein a compression stress is added as a residual stress. 2. The piston for an internal combustion engine according to claim 1, wherein said compressive stress is applied by using shot peening.