JPH02120506A - Crank shaft for engine and its reinforcing treatment - Google Patents

Abstract

PURPOSE:To improve fatigue strength of the internal wall of an oil hole by forming a shot peening treatment layer on the interface, the internal wall of the oil hole, of the formed portion and the non-formed portion of a hardened layer. CONSTITUTION:In a crank pin 1 to which hardening is applied, compression residual stress due to hardening arises even in a hardened layer 3 inside an oil hole 2, on account of which the interface 4 of the hardened layer 3 and a non-hardened layer is subjected to tensile stress. In order to dissolve the tensile stress on the interface 4, a shot peening treatment layer 5 is formed by shot peening treatment of projecting a hard ball of a small dia. at this part under a preset high pressure. The projecting pressure of the hard ball in the shot peening treatment is adapted to be the maximum on the above-mentioned interface 4 and to decrease gradually with direction from the interface 4 to the inside of the oil hole. That results in continuously smaller distribution of the compression residual stress from the interface 4 to the inside of the oil hole.

Description

Detailed Description of the Invention (Industrial Field of Application) The present invention relates to an engine crankshaft and a method for strengthening the same, and more specifically relates to a method for forming a hardened layer on the surface of a crank pin and/or crank journal. The present invention relates to a crankshaft for an engine made by the above-mentioned method and a method for strengthening the same.

(Prior Art) A large torsional load is applied to the engine crankshaft pin and journal during operation, especially on the flywheel side, resulting in large torsional fatigue. For this reason,
It is known to subject the surfaces of the crank pin and crank journal to induction hardening for the purpose of increasing torsional fatigue strength.

An example of this is the method disclosed in Japanese Patent Laid-Open No. 58-153732. In this method, induction hardening is applied to the concave corners at the connection between the crank pin and the connecting portion and the concave corner at the connection between the crank journal and the connecting portion.

(Problem to be Solved by the Invention) However, when conducting a fatigue test on a crankshaft in which the crank pin and rank journal were subjected to induction hardening, it was found that the oil holes drilled in these bottles and journals were It has been found that cracks often occur from the periphery of the steel.

Recently, there has been a strong demand for smaller and lighter vehicle bodies, and as a result, the diameters of crankshaft bins and journals are also becoming smaller, so the occurrence of racks as described above is becoming a major problem.

The present invention was made in view of the above-mentioned problems, and its purpose is to improve the fatigue strength of the crank pin and crank journal as a whole against torsional loads, as well as to improve the fatigue strength of the inner wall of the oil hole, thereby improving the rank shaft and its reinforcement. The purpose is to provide a processing method.

(Means for Solving the Problem) The inventor of the present application researched the cause of cracks occurring around the oil hole of the crankshaft, and found that compressive residual stress occurs in the hardened portion of the crankshaft. There is compressive residual stress in the hardened area inside the opening end, but the unhardened area further inside, that is, the interface between the hardened layer formed area and the non-hardened layer area, has compressive residual stress. It was learned that the fatigue strength decreases due to stress, which causes the above-mentioned cracks, and the present invention was achieved based on this knowledge.

That is, the first invention of the present application provides a crankshaft for an engine in which a hardened layer is formed on the surface of a crank pin and/or crank journal in which an oil hole is previously formed. A shot peened layer is formed at the interface between the area where the injected layer is formed and the area where it is not formed.

In addition, in the method for strengthening a crankshaft of an engine according to the second invention of the present application, a hardened layer is formed on the surface of a crank pin and/or a crank journal in which an oil hole is drilled in advance, and the hardened layer is formed from the outside of the oil hole. Shot peening is performed by projecting small hard balls obliquely at high pressure toward the interface between the area where the layer is formed and the area where the layer is not formed.

(Function) When shot peening is performed by projecting a small hard ball under high pressure at an angle toward the interface between the inner wall of the oil hole where the hardened layer is formed and the area where the hardened layer is not formed, the interface becomes shot peened. As a result, the tensile stress that existed in that part is resolved and a compressive residual stress is applied.

(Example) Hereinafter, an engine crankshaft and a method for strengthening the same according to the present invention will be described with reference to the accompanying drawings.

FIG. 1 is a sectional view of a crank pin of a crankshaft taken along an oil hole, in which reference numeral 1 indicates a bottle of the crankshaft and reference numeral 2 indicates an oil hole.

The crank pin 1 is made of carbon steel, and has a hardened layer 3 formed on its surface by known induction hardening, and compressive residual stress acts on the hardened layer 3. The thickness of the hardened layer 3 is almost uniform over the entire circumference of the crank pin 1, but since the area around the opening of the oil hole 2 is an edge, the hardened layer 3 is not hardened in other parts. It is done slightly deeper than
Therefore, the hardened layer 3 is also slightly thicker than other parts.

In the crank pin that has been hardened in this way, compressive residual stress is generated in the hardened layer 3 inside the oil hole 2 due to the hardening, and therefore the hardened layer 3 and the non-hardened layer are The interface 4 is under tensile stress. In the present invention, in order to eliminate the tensile stress at this interface 4, a shot peening layer 5 is formed by shot peening treatment in which small-diameter hard balls are projected at a predetermined high pressure onto this portion.

In this shot peening process, the hard ball projection pressure is set to be maximum at the interface 4, and the hard ball projection pressure is gradually reduced from the interface 4 toward the inside of the oil hole. As a result, the compressive residual stress distribution from the interface 4 to the inside of the oil hole becomes continuously smaller.

In order to perform the shot peening treatment as described above, the interface 4 between the hardened layer 3 and the non-hardened layer on the inner peripheral wall of the oil hole 2 must be
By projecting a predetermined amount of small hard balls diagonally at high pressure toward the interface 4 so that the largest amount of hard balls hit the interface 4 most strongly, the strength and amount of hard balls hitting decrease as they move inward. good.

In addition, the diameter of the hard ball is optimally 0.3 mm to 0.4 mm.
If it is smaller than this, the resulting compressive stress will be small and the hard balls will wear out quickly, and if it is larger than this, problems such as clogging of the hard balls in the nozzle will occur.

The shot peening process is actually performed using a shot peening apparatus as shown in FIG.

In this device, a nozzle b is connected to a shot reservoir a having an appropriate amount of hard balls through a conduit C, and the conduit C is further connected to an air source.

On the other hand, the crank pin 1 or the crank journal is fixed to a fixing base d with a pin or the like so that the oil hole 2 is vertical. After positioning the nozzle b vertically just above the opening of the oil hole 2, pressurized air is forced into the nozzle b through the conduit C, and a certain amount of hard balls are ejected from the shot reservoir a and sent to the nozzle b. A hard ball is projected into the opening of the oil hole 2 by air pressure from the projection port e.

Incidentally, the projection port e is inclined at a certain angle in advance, so that when the nozzle b is located above the opening of the oil hole 2, the center of the projection port e is flush with the hardened layer 3 on the inner circumferential wall of the oil hole 2. It faces the interface 4 of the quenched layer, and when hard balls are projected, the hard balls appear especially large and strongly on this interface 4. At this time, the nozzle b rotates at a constant speed above the opening of the oil hole 2, and forms an annular shot peened layer 5 on the inner wall surface of the oil hole 2 centered on the interface 4.

After shot peening from one opening of the oil hole, next, apply shot peening to the other opening of the oil hole in the same manner as above to remove the quenched layer on the inner wall of the oil hole. An annular shot peened layer is formed on the inner wall surface centering on the interface with the input layer.

The hard balls projected into the oil hole are recovered and reused from the opening of the other oil hole that has not been subjected to shot peening.

Although the crank pin and its manufacturing method have been described above, the crank journal is also subjected to the same treatment as the crank pin, and a shot peening layer is formed on the inner wall of the oil hole.

After induction hardening the crank pin and crank journal as described above, shot peening was applied to the oil holes, and the fatigue strength of the crankshaft against torsional stress was tested.The results will be described below.

The crank pin and crank journal are made of carbon steel, and this carbon steel is made of CO,4[! %, S i (
1,42%, Mn 0.94%, P 0.01
548C of JIs containing 8%, SO, 04%
It is based on Cu O, 01%,
Contains N f O,02%, Cr O,12%, and further V O,06! It has been adjusted by adding %.

The test was conducted by comparing the fatigue strength of three types of crankshafts, two of which had only a hardened layer formed on the surface of the bottle and journal, and one type had a hardened layer with a thickness of 6 mm; The other one type was rated 4■. Also 3
For one of the crankshafts, a 6 mm hardened layer was formed on the bottle and journal, and the oil hole was subjected to T-shot peening as described in the previous example.

Induction hardening was performed at a frequency of 8.5 Kl (z, output of 150 KHz, heating time of 10 seconds, and cooling time of 15 seconds) to form a hardened layer of 6 mm.

In addition, regarding shot peening treatment, hard balls have a diameter of 0.
, 3 am, HRe (hardness) of 52 to 56, the diameter of the nozzle projection opening was 2.0 mm, the projection pressure was 6, Q
kg/cd, nozzle projection angle 10”
Projection was carried out for 1 minute at a nozzle rotation speed of 30 rpm and a hard ball projection amount of 500 g per minute.

The test involves applying a certain amount of twisting moment to the crankshaft and measuring the number of times it takes for cracks to form around the oil hole in the crankshaft.

The results are shown in Figure 3, where the vertical axis is the torsional moment, the horizontal axis is the number of times the torsional moment was applied, and the symbol Δ in the figure indicates the hardening layer applied only to the surfaces of the crankpin and crankshaft. ◇ indicates that the quenched layer is 6 mm thick, O indicates that the quenched layer is 6 mm thick as described above, and the oil hole is shot peened according to the present invention. This is what was done.

From this result, it was found that the fatigue strength of the oil holes subjected to shot peening treatment as in the present invention was significantly improved compared to those without shot peening treatment, and in the case of a torsional moment of 210 kg・m that actually occurs during operation. Looking at it, the number of repetitions is 10 for those without shot peening treatment.
Cracks occur around the oil holes between 6 and 107, but in the case of shot peening according to the present invention, no cracks were formed even when the number of repetitions exceeded 107.

(Effects) As described above, in the engine crankshaft according to the present invention, since the shot peening layer is formed on the inner wall of the oil hole at the interface between the part where the hardened layer is formed and the part where the hardened layer is not formed, This interface receives compressive stress due to shot peening, and the tensile stress that existed in that area is eliminated and compressive residual stress acts, greatly improving fatigue strength.

In addition, in the method of the present invention, shot peening is performed by projecting small hard balls obliquely at high pressure from the outside of the oil hole toward the interface between the hardened layer formed part and the non-hardened layer formed part. The treatment layer can be formed extremely easily, and the compressive residual stress caused by shot peening gradually decreases from the above interface toward the inside of the oil hole, making it difficult to form cracks in the oil hole and strengthening the crankshaft with improved fatigue strength. It can be a processing method.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a crankshaft bottle according to the present invention, and FIG. 2 is a schematic diagram showing a crank pin strengthening treatment method according to the present invention, with a portion enlarged. FIG. 3 is a graph showing the fatigue strength test results when a torsional moment is applied to the crankshaft according to the present invention.

Claims (2)

[Claims] (1) In an engine crankshaft in which a hardened layer is formed on the surface of a crank pin and/or crank journal in which an oil hole is previously drilled, the part on the inner wall of the oil hole where the hardened layer is formed. An engine crankshaft characterized in that a shot peening treatment layer is formed at the interface between the peening layer and the non-forming portion. (2) A hardened layer is formed on the surface of the crank pin and/or crank journal in which an oil hole is previously drilled, and directed from the outside of the oil hole toward the interface between the part where the hardened layer is formed and the part where it is not formed. A method for strengthening an engine crankshaft, which is characterized by performing shot peening treatment by projecting small hard balls diagonally at high pressure.