WO2009084779A1

WO2009084779A1 - Cam shaft of light weight using thermal expansion coefficient

Info

Publication number: WO2009084779A1
Application number: PCT/KR2008/003062
Authority: WO
Inventors: Shae Kwang Kim
Original assignee: Korea Institute Of Industrial Technology
Priority date: 2007-12-27
Filing date: 2008-05-30
Publication date: 2009-07-09

Abstract

The invention relates to a lightweight camshaft accountable for difference of thermal expansion coefficients. The lightweight camshaft comprises a cam part of iron and a shaft part of lightweight metal for mounting the cam part. The lightweight camshaft comprises a contraction reinforcing part formed on each of the cam pieces to reinforce the strength of bonding between the cam piece of iron and the shaft part of lightweight metal so as to protect the bonding strength against weakening due to the contraction in response to a difference of the thermal expansion coefficients (i.e. shrinkage). A reinforcing joint part is formed at the shaft part by injecting molten lightweight metal into the contraction reinforcing part so that the cam pieces and the shaft part may be integrally combined. Therefore the weakening of the bond strength between the cam pieces and the shaft part could be structurally prohibited.

Description

[DESCRIPTION]

[Invention Title]

CAM SHAFT OF LIGHT WEIGHT USING THERMAL EXPANSION COEFFICIENT

[Technical Field]

The present invention relates to a lightweight camshaft applied to a vehicle engine, and more particularly, to a lightweight camshaft accountable for difference of the thermal expansion coefficients, wherein the cam parts as separate cam pieces of iron and the shaft part (journal part) which is formed by injecting lightweight metal such as aluminum alloys or heat resistant magnesium alloys molten by aluminum casting, are combined in the mold, so that the weakening of the bond strength between the cam pieces and the shaft part, which may occur due to the difference of the thermal expansion coefficient in response to the difference of the materials, could be structurally prohibited, wherein the cam parts and the shaft part is made from lightweight metal so as to decrease weight, and wherein the combining structure and forming process of the bonding region between cam pieces and shaft part has altered for structurally prohibiting the weakening of the bond strength.

[Background Art] As is well known, a camshaft which is an essential component in an engine power system transmits power to a locker arm, a tappet, and a push rod sequentially in order to open and close a suction valve and an exhaust valve through a repetitive rotational movement, so that the valves are opened and closed in periodical accurate timing.

In particular, a camshaft for use in an automobile engine receives a rotational force transmitted from a crankshaft and controls opening and closing times of a suction valve and an exhaust valve for a fuel combustion chamber. That is, the camshaft should be used under an adverse environment such as high-speed rotation, rotational bending fatigue and lubricational wear of 750 ~ 4500 RPM, at low-temperature and high-temperature of -15 ~ 120 °C .

Usually, a camshaft is installed between a cylinder block and a cylinder head, and has valve cams arranged as many as the number of valves in order to open and close the valves. That is, the valve cams as may as the number of the suction valve and the exhaust valve in the engine, are supported to the lateral portion of upper portion of a crank case by bearings and in parallel with a crankshaft at a shaft portion on which the valve cams are arranged at a correct position and angle. An eccentric cam driving a fuel pump and a helical gear driving a distributor or an oil pump are mounted on the camshaft.

Meanwhile, even if the curves of the surfaces of the valve cams including the rods vary slightly only, an open/close period of left of each valve varies, to thus make a big influence upon an engine performance. Thus, it is preferable that the camshaft should be designed so that wear of the surfaces of the valve cams or bending of the camshaft can be prevented even in the case of a long-time use.

The camshaft is coaxially installed together with a timing gear in order to link with a crankshaft by a timing belt. Accordingly, the rotational force of the crankshaft can be transmitted to the camshaft. According to rotation of the camshaft, locker arms closely attached to the surfaces of the valve cams open and close a number of suction and exhaust valves and make respective devices linked with the valves.

Meanwhile, a conventional camshaft used in an internal combustion engine is made of chilled cast iron, but the cast iron made camshaft cannot meet the requirement for pursuing high performance and lightweight of an engine continuously. That is, in the case of the conventional camshaft, the cams and shaft portion are made of an identical material. The conventional camshaft is formed of an integral rod shape. In this case, according to an annealing method, a cast iron mold is made of a special steel forging method.

As described above, in the case of a camshaft fabricated by a conventional annealing method for a cast iron casting, the surfaces of rods in cams which are rubbed against locker arms or tappets by friction need to be hardened. Accordingly, heat treatment should be undergone through various kinds of methods, such as chill casting, re-melt and chill casting, high-frequency heating of spheroidal graphite casting, austempering, etc.

The cast iron casting camshaft weighs automobile fuel consumption since the weight of the camshaft weighs at the situation which requires enhancement of fuel efficiency, high speed output, and high power output, and increases the number of valves for the high speed and the high power output. However, since gaps between cams are narrow in this case, inferiorities due to the defects of casting often occur, and a wear-resistant property and corrosion-resistant property of the cam surfaces may be lowered. Also, the cast iron casting camshaft has a high inferiority ratio in view of a cast iron casting manufacturing industry which belongs to a representative evading industry. Since it is also difficult to automate the cast iron casting manufacturing apparatus, the productivity thereof becomes low.

Thus, a new camshaft manufacturing method of manufacturing a camshaft need to be developed, in order to supplement defects of the conventional cast iron casting camshaft, optimize engine performance, countermeasure multiple valves according to a lightweight engine, and supply a camshaft having an excellent wear-resistant property.

Meanwhile, a modular method of manufacturing camshaft has been proposed to meet the need. The modular method was a diffusion joining hollow camshaft manufacturing method or a mechanical modular camshaft manufacturing method.

The diffusion joining hollow camshaft manufacturing method is a metal joining method in which cams among camshaft components are formed of metal powder, sintered at 1000°C as a primary preliminary sintering, modularly sintered in a hollow tube with a jig, and the tube and sintered cams are joined by diffusion with each other, and in which a journal dam, a front piece, and a rear piece are joined. The mechanical modular camshaft manufacturing method is a method of assembling and fixing camshaft components in a hollow tube by a mechanical friction pressing and fixing method, or a method of injection molding, fixing and assembling camshaft components between other components by use of a plastic material.

The hollow camshaft has a merit of accomplishing a lightweight camshaft of approximate 30% in comparison with the conventional cast iron casting camshaft, but its manufacturing process is complicated and there are many variables of manufacturing conditions, to thereby cause a problem in mass-producing products regularly. That is, Toring-Ton(a U.S. company), Presta (a European company) partly produce camshafts using a modular method and Nippon Piston Ring (NPR) (a Japanese company) produces modular camshafts.

However, since Toring-Ton employs a diffusion method and Presta adopts an injection method, various kinds of problems are exposed that a joining or binding force is low thereby causing a cam to secede, a wear-resistant property is remarkably lowered according to a heat treatment condition, and it is not possible to produce a camshaft unless the thickness of a cam lobe is 5mm or more.

In particular, modular camshafts produced by NPR are hollow sintering camshafts which combine a wear-resistant property enhanced sintering cams with lightweight steel tubes, in order to meet requirements which the conventional cast iron casting camshafts could not meet.

That is, the modular camshafts using sintering cams using the powder metal is hollow by steel tube shafts. This can be applied to a cam whose width is narrow to thus make it difficult to produce a compact cam by a casting method or forging method. Accordingly, the modular camshafts have a degree of freedom of designing shapes of components and selecting materials of the components, and freely designing a gap between cams, in addition to realization of lightweight cams whose weights are considerably reduced.

PFCl, PFC2 and PFC4 which are widely used as the materials of sintering cams in the hollow sintering camshafts, show results of excellent pitching-resistance and wear-resistance under high surface pressure, since a Cr-based high hardness carbide is uniformly dispersed in a bainite textile structure. Thus, the hollow sintering camshaft has several merits that an investment cost is inexpensive, the product is greenness, the process is simplified, error rate is reduced. Also, the hollow sintering camshaft has other merits that a high durability is secured due to formation of composite carbide, and the product can be made lighter by 30% or more than cast iron due to the hollow type.

As described above, the hollow sintering camshaft has some more merits that a degree of freedom in designing is large in addition to the merit of lightweight. Thus, as an engine is recently formed of a multiplicity of valves and made into a compact design, cams should be disposed proximately, and made into a high performance. As a result, the hollow sintering camshaft is widely employed. However, since the hollow sintering camshaft should be assembled so that cams are combined on a shaft portion while the cams are in phases, it is difficult to assemble the cams on the shaft portion.

In addition, in the case of the currently available camshaft, a shaft portion does not only play a role of a structural member which supports cams and transmits a rotational force, but also plays a role of an oil path through which engine oil is transmitted to a journal portion. However, an extruding material steel tube is used as the camshaft, it is limited to reduce size of the inner diameter of the tube. For example, in the case that the outer diameter is 26 mm, it is nearly impossible to draw the inner diameter down to 13 mm or less. In this case, the camshaft makes a loss of an effect of a lightweight product.

Thus, an oil supply from an oil fan positioned at the lower end of an engine to a camshaft is delayed during cold start. A wear proceeds due to no supply of oil. The wear affects mostly on the lifetime of the camshaft.

Meanwhile, it has recently attempted a reinforced output by making engine driving components light, and reducing a moment of inertia to thereby realize a high RPM(Revolutions Per Minute), in order to obtain a high output and a high efficiency of the engine. Accordingly, the hollow sintering camshaft using powder cams and a steel tube has been developed as described above from the cast iron camshaft which is a main stream until beginning of the past 1990s. Therefore, a high performance camshaft having a more lightweight feature, capable of overcoming wear during cold start, and having a price competition capability, need to be developed in the future.

To meet the need, a patent application (Korea application number 10-2004-0071282) titled as "Lightweight Aluminum Camshaft and Manufacturing Method Thereof" has been filed by the present applicant. That is, it discloses a method of manufacturing a lightweight aluminum camshaft having a cam portion having cams the number of which is same as that of valves in order to open and close suction and exhaust valves in an engine, and a shaft portion on which the cam portion is mounted, wherein the lightweight aluminum camshaft is manufactured by the steps of: separately fabricating the cam portion with cam pieces; inserting the cam pieces into a mold; and injecting molten aluminum alloy into the mold through an aluminum casting method, wherein the cam pieces and the shaft portion called as a journal portion are formed in an integral form.

According to the above method, a lightweight aluminum camshaft which can enhance an output from an engine and realize a low fuel efficiency through enhancement of engine durability due to minimization of no- oil- supply wear occurring during cold start in existing engine, reduction of an inertia of moment due to the lightweight camshaft, and high RPM of the engine, could be provided.

However, the lightweight aluminum camshaft manufactured by the above method has a weak bond region problem at the bonding interface between the shaft portion and the cam pieces, due to the fact that cam pieces was combined to the aluminum shaft portion by inserting and casting. In another words, in the process of the combining the cam pieces of iron with the shaft portion of aluminum by inserting and casting, their bonding strength could be weaken due to their different thermal expansion coefficients (i.e. shrinkage) according to their different materials.

[Disclosure]

[Technical Problem]

To solve the above problems, it is an object of the present invention to provide a new high-performance lightweight aluminum or heat resistant magnesium camshaft and to provide more lighter weighed camshaft than the hollow sintering camshaft, wherein the lightweight camshaft comprises a cam part and a shaft part for mounting the cam part, and the cam part is produced as separate cam pieces of iron which are inserted into a mold, to manufacture a lightweight camshaft for automobile, the cam pieces are integrally molded with the shaft part (journal part) which is formed by injecting lightweight metal such as aluminum alloys or heat resistant magnesium alloys molten by aluminum casting into the mold, so that the weakening of the bond strength between the cam pieces and the shaft part, which may occur due to the difference of the thermal expansion coefficient in response to the difference of the materials, could be prohibited, and the shape and structure of the bonding regions between the cam pieces and the shaft part is altered so that the bonding strength could be mechanically enhanced.

And it is an another object of the present invention to provide a lightweight camshaft accountable for difference of thermal expansion coefficients, wherein both of the cam part and the shaft part is not only made of lightweight metal so as to decrease their weight, but also the cam part and the shaft part is made into an integral body so as to prohibit the weakening of the bonding strength of it.

[Technical Solution]

To accomplish the above object, the present invention provides a lightweight camshaft accountable for difference of thermal expansion coefficients, which comprises a cam part and a shaft part for mounting the cam part, wherein the cam part is produced as separate cam pieces of iron which are inserted into a mold so that the cam pieces are integrally molded with the shaft part which is formed by injecting molten lightweight metal into the mold, characterized by comprising : a contraction reinforcing part formed on each of the cam pieces to reinforce the strength of bonding between the cam piece of iron and the shaft part of aluminum alloys or heat resistant magnesium alloys that is lightweight metal, so as to protect the bonding strength against weakening due to the contraction in response to difference of the thermal expansion coefficients (i.e. shrinkage); and a reinforcing joint part formed at the shaft part by injecting molten lightweight metal into the contraction reinforcing part so that the cam pieces and the shaft part may be integrally combined.

In particular, each of the joint portions has a shape of reinforce-rib that protrudes from both sides of each of the reinforcing joint part to encompass sides of each of the cam pieces 2a, so that the bonding strength could be increased at the bonding region between the cam pieces and the shaft part when the shaft part is formed integrally with the cam pieces in the mold.

And, the contraction reinforcing part on a cam piece is formed as a plurality of round holes that penetrate the cam piece at adjacent area around a shaft hole formed in the center of the cam piece, and the reinforcing joint part of the shaft part is formed by injecting molten lightweight metal into the contraction reinforcing part in shape of round holes so as to be integrally combined. As a result, the combined structure has enhanced mechanical structure accountable for the shrinkage characteristics due to the difference of the thermal expansion coefficients between the cam pieces and the shaft part.

According to another embodiment of the invention, the contraction reinforcing part on a cam piece is formed as a plurality of long grooves that are recessed in the inside wall of a shaft hole formed in the center of the cam piece, each of the long groove's bottom is enlarged to its both sides, and the reinforcing joint part of the shaft part is formed by injecting molten lightweight metal into the contraction reinforcing part in shape of long grooves so as to be integrally combined.

In other words, the contraction reinforcing part of the cam piece and the reinforcing joint part of the shaft part have the shape corresponding to each other. The shapes of the contraction reinforcing part and the reinforcing joint part could be any form, provided that the shapes might compensate the shrinkage difference at the bonding regions due to the difference of thermal expansion coefficients, that is, the difference of shrinkage between the cam pieces of iron and the shaft part of lightweight metal such as aluminum alloys or heat resistant magnesium alloys.

In particular, the lightweight camshaft accountable for difference of thermal expansion coefficients according to the present invention may be formed using any one of the casting methods that comprise die-casting method, squeeze casting method, and semi-solid casting method, when the shaft part is formed integrally with the cam pieces by inserting the cam pieces of iron into the mold and by injecting molten lightweight metal, the aluminum alloys or heat resistant magnesium alloys into the mold,

And, as an enhanced embodiment, the lightweight camshaft accountable for difference of thermal expansion coefficients according to the present invention may have a liner layer that is coated on the surface of the cam part, with thickness of 300 ~ βOOμm, and hardness of 300 ~ 1,000Hv by any one of the methods thai comprise thermal spray, vapor deposition, anodizing, electroplating, and ion nitration.

In another embodiment, the lightweight camshaft accountable for difference of thermal expansion coefficients according to the present invention may have a liner layer that is coated on the surface of the shaft part, with thickness of 300 ~ 600μm, and hardness of 300 ~ 1,000Hv by any one of the methods that comprise thermal spray, vapor deposition, anodizing, electroplating, and ion nitration. And, the cam part and the shaft part are made of lightweight metal with different thermal extension coefficient, and formed at the same time by any one of the casting methods that comprise cold chamber die casting method, gravitational casting method, low-pressure casting method, squeeze casting method, and semi-solid casting method.

And, each of the liner layers of the surfaces of the cam part 2 and the shaft part 3 is coated by the thermal spray method with any one chosen from the group that comprises ceramics of AI2O3, AlN, MgO and Mg3N2, or coated by the electroplating method with metal carbides of WC or Cr3C2, metal oxides of Al2θ3+ Tiθ2, cemet(ceramic+ metal) of TiC, TiN, or TiCN, or a metal compound comprising Fe, or may be coated by any one vapor deposition method of PVD(Physical Vapor Deposition), CVTXChemical Vapor Deposition), TRD(Thermo Reactive Deposition and Diffusion) and PCVD(Plasma Chemical Vapor Deposition) with any one chosen from titanic metal compound such as TiCN, TiAlN, TiAlCN, TiAlON and TiAlSiCNO, or a combination thereof.

In another embodiment, each of the liner layers of the surfaces of the cam part and the shaft part may be coated by the Anodizing method that comprises Keronite method, most outer surface of each liner layer may be formed by the electroplating method with any one of ultra-hardness metal that comprises Cr, W, Ni and Co, or an alloy thereof, and may be coated by the ion-nitration method with AlN or Mg3N2.

In addition, each of the liner layers of the surfaces of the cam part and the shaft part may be coated by the electroplated iron or alloy of iron, and then by surface processing, deposition, nitration or carbonization of the electroplated iron or alloy of iron, in particular, the electroplated iron or alloy of iron of each of the liner layers of the surfaces of the cam part and the shaft part may be coated by any one method chosen from PVD(Physical Vapor Deposition), CVTXChemical Vapor Deposition), TRD(Thermo Reactive Deposition and Diffusion) and PCVD(Plasma Chemical Vapor Deposition) with any one chosen from TiCN, TiAlN, TiAlCN, TiAlON and TiAlSiCNO, or a combination thereof.

[Advantageous Effects]

As mentioned above, according to the lightweight camshaft accountable for the thermal expansion characteristics, which comprises a cam part and a shaft part, wherein the cam part is produced as separate cam pieces of iron and are integrally molded with the shaft part which is formed by injecting molten lightweight metal such as aluminum alloy or heat resistant magnesium alloy. It can be seen that the lightweight camshaft according to the present invention becomes lighter substantially by 30% or so, in comparison with the conventional hollow sintering camshaft. The lightweight camshaft according to the present invention may be inexpensive and have high durability and hardness.

In particular, in order to prohibit the weakening of the bonding force due to the different thermal expansion coefficients, also known as shrinkages between the cam pieces and the shaft part made of different materials, the lightweight camshaft according to the present invention has the contraction reinforcing part on the cam piece and the reinforcing joint part on the shaft part. So the bonding force may be enhanced by adapting the shape of the bonding region between the cam piece and the shaft part. In addition, the lightweight camshaft according to the present invention also has the various advantages that could be accomplished by the fabrication method of lightweight aluminum camshaft described in the previous Korea application number 10-2004-71282 filed by the present applicant.

And the lightweight camshaft according to the present invention has the bonding structure accountable for the different thermal expansion coefficients between composition elements. The lightweight camshaft according to the present invention can provide advantages such as enhancement of the engine durability due to minimizing no-oil supply wear, lowering the moment of inertia due to the lighter camshaft, and enhancement of the engine power and lower gasoline mileage due to the high-RPM of the engine.

In addition, the lightweight camshaft according to the present invention may mechanically and physically prohibit the weakening of the bonding strength, that might occur due to different thermal expansion characteristics. The lightweight camshaft according to the present invention may enhance wear-resistance and corrosion-resistance. And in the lightweight camshaft according to the present invention, the cam part and the shaft part is formed into a body and made of lightweight metals, the fuel efficiency could be enhanced due to the lowering weight of the camshaft, as well as the weakening of the bonding force between the cam part and the shaft part could be prohibited mechanically and physically.

[Description of Drawings]

Figs.l and 2 show a perspective view and a sectional view of combined state of a lightweight aluminum camshaft according to the present invention. Fig.3 shows a perspective view of combined state of the main part of the lightweight metal camshaft combined with a cam piece and a shaft part having different thermal expansion coefficients.

Fig.4 and Fig.5 show exploded perspective views of combining structure of a cam piece and a shaft part as a first and a second embodiments of the present invention.

Figs.6 to 8 successively show the manufacture process for forming the lightweight aluminum camshaft, wherein Fig.6 shows a state of the cam pieces injected into a mold, Fig.7 shows a state after injecting lightweight metal, and Fig.8 shows a taken out camshaft from the mold after injecting. Figs.9 and 10 show sectional views of A-A line and B-B line of Fig.l.

[Best Mode]

The preferred embodiments of the invention will hereinafter be described in conjunction with the appended drawings provided to illustrate and not to limit the invention.

The present invention employs a manufacturing method of a lightweight camshaft filed before as a patent application by the present applicant as mentioned above, which shall be described together with description for the present invention. The present invention relates a camshaft comprising the bonding structure for reinforcing bonding strength between elements that are made of different materials, as well as relates a lightweight camshaft accountable for different thermal expansion characteristics between the shaft part and the cam part of the camshaft.

First referring to Fig.l and Fig.2, a lightweight camshaft 1 according to an basic embodiment of the present invention, like conventional cast-iron camshaft or hollow-sintered camshaft, comprises a cam part 2 having elements arranged as many as the number of valves to open and close an intake-exhaust valve of an engine and a shaft part or a journal part 3 for mounting the cam part. According to the present invention, the shaft part 3 of the camshaft 1 can be made of aluminum alloys of heat resistant magnesium alloys so that the camshaft 1 could be relatively more lighter than the conventional hollow-sintered camshaft.

To manufacture the structure of lightweight camshaft accountable for the different thermal expansion coefficients between elements according to the present invention, the cam part 2 is separately fabricated with cam pieces 2a by sintering material powder cam fabrication method using conventionally well-known metal powder. This separately fabricated cam pieces 2a are inserted as inserts into a mold 4. Thereafter, molten lightweight metal such as aluminum alloys or heat resistant magnesium alloys is injected into the mold 4 using an aluminum casting method, thereby the cam pieces 2a is integrally bonded with the shaft part 3.

Here, the cam pieces 2a forming the cam part 2 are separately fabricated in the form of a sintering material powder cam or a forging cam by the conventionally well-known technology. Accordingly, it has been confirmed that the cam pieces 2a have the features such as a wear-resistant property and a corrosion-proof property which are known to have been already possessed by the conventional camshaft. In particular, according to the present invention the shaft part 3 of the lightweight camshaft 1 may be basically made of aluminum alloy among lightweight metals, but may be optionally made of lightweight metals comprising heat resistant magnesium alloys which could enforce bonding strength between the cam part 2 of iron having different thermal expansion characteristics with it.

As described above, it is preferable that the cam pieces 2a in the lightweight camshaft 1 according to the present invention are fabricated using a powder cam fabrication method using powder metal which is known as more preferable in view of a wear-resistant property and a corrosion-proof property, but is not limited thereto. The cam pieces 2a can be of course fabricated in the form of a forging cam by well-known forging other that the powder cam fabrication method.

During the bonding process that the shaft part 3 of molten lightweight metal alloy is injected to the cam pieces 2a of iron, the lightweight camshaft 1 according to the present invention, as shown as Figs.6a to 6c, has an altered shape from the previous camshaft shape which had been introduced by the previous application so that weakening of the bonding strength should be prohibited due to the different shrinkage at the bonding regions because of the different thermal expansion coefficients, That is, the present invention suggests an altered structure of the bonding region between the cam pieces 2a of iron and the shaft part 3 of lightweight metal alloy so as to compensate each other for taking account for the shrinkage difference.

As shown in Fig.4 of a first embodiment and Fig.5 of a second embodiment, a contraction reinforcing part 7a or 7b is formed on each of the cam pieces 2a to reinforce the strength of bonding between the cam piece 2a of iron and the shaft part 3 of aluminum alloys or heat resistant magnesium alloys that is lightweight metal, so as to protect the bonding strength against weakening due to the contraction in response to difference of the thermal expansion coefficients (i.e. shrinkage), and a reinforcing joint part 8a or 8b is formed at the shaft part 3 by injecting molten lightweight metal into the contraction reinforcing part 7a or 7b so that the cam pieces 2a and the shaft part 3 may be integrally combined.

Particularly, in the first embodiment of Fig.4, the contraction reinforcing part 7a on a cam piece 2a is formed as a plurality of long grooves that are recessed in the inside wall of a shaft hole 9a formed in the center of the cam piece 2a, each of the long groove's bottom is enlarged to its both sides, and the reinforcing joint part 8a of the shaft part 3 is formed by injecting molten lightweight metal into the contraction reinforcing part 7a in shape of long grooves so as to be integrally combined. Here, at least one or more of the contraction reinforcing part 7a in shape of long groove may be formed at spaced positions around on the inside wall of the shaft hole 9a.

According to such a bonding structure, lightweight metal such as aluminum alloys or heat resistant magnesium alloys, which is to form reinforcing joint part 8a of the shaft part 3, is injected into and integrally combined with the contraction reinforcing part 7a in shape of long recessed and bottom-enlarged grooves on the inside wall of the cam pieces 2a of iron. Hence, even if the bonding strength between the shaft part 3 of lightweight metal that is injected and solidified and the cam pieces 2a may be weaken due to shrinkage, the weakening of the bonding strength could be prohibited structurely and mechanically because that the combined structure of the long groove type contraction reinforcing part 7a and the injected and inserted reinforcing joint part 8a into it could strongly grip each other no matter how much different shrinkages the parts 7a and 8a have.

And, the second embodiment shown in Fig.5 is an altered embodiment compared with the first embodiment shown in Fig.4. In this embodiment, the contraction reinforcing part 7b on a cam piece 2a is formed as a plurality of small round holes not as the long grooves. That is, the contraction reinforcing part 7b is formed as a plurality of relatively small round holes that penetrate the cam piece 2a at adjacent area around a shaft hole 9a formed in the center of the cam piece 2a. The reinforcing joint part 8b is formed by injecting molten lightweight metal into the contraction reinforcing part 7b in shape of round holes so as to be integrally combined. As a result, the combined structure of the round hole type contraction reinforcing part 7b and the injected and inserted reinforcing joint part 8b into it could strongly grip each other no matter how much different thermal expansion coefficients the parts 7b and 81) have.

According to the bonding structure accountable for the different thermal expansion coefficients between different elements, the reinforcing joint part 8b of lightweight metal is injected into and combinded with round holes of the contraction reinforcing part 7b of metal(iron). Hence, even if the bonding strength between the shaft part 3 of lightweight metal that is injected and solidified and the cam pieces 2a may be weaken due to shrinkage, the weakening of the bonding strength could be prohibited structurely and mechanically because that the combined structure of the contraction reinforcing part 7b and the reinforcing joint part 8b could strongly grip each other no matter how much different shrinkages the parts 7b and 8b have. And this bonding structure could be more stronger that the shape structure of the first embodiment. Here, it is preferred that the contraction reinforcing part 7b of round holes are positioned at adjacent area encompassing the shaft hole 9a formed in the center of the cam piece 2a.

In addition, as shown in the first and second embodiments, the contraction reinforcing part 7a or 7b of the cam piece 2a of iron and the reinforcing joint part 8a or 8b of the shaft part 3 of lightweight metal such as aluminum alloys or heat resistant magnesium alloys have the shape corresponding to each other. The shapes of the contraction reinforcing part 7a or 7b and the reinforcing joint part 8a or 8b could be any form, provided that the shapes might compensate the shrinkage difference at the bonding regions due to the difference of thermal expansion coefficients, that is, the difference of shrinkage between the cam pieces 2a of iron and the shaft part 3 of lightweight metal.

Meanwhile, joint portions 3a may be added to the camshaft, wherein each of the joint portions 3a has a shape of reinforce-rib that encompasses sides of each of the cam pieces 2a and has a diameter more larger than that of the shaft part 3, so that the bonding strength could be increased at the bonding region between the cam pieces 2a and the shaft part 3 when the shaft part 3 is formed integrally with the cam pieces 2a in the mold 4.

That is, the joint portion 3a has a structure encompassing the sides of the cam piece 2a, so that the joint portion 3a could act to protect the reinforcing joint part 8a or 8b of the shaft part 3 from being exposed, as well as act to protect the cam piece 2a from moving along the shaft part 3 resulted in enforcing the bonding strength

The present invention employs the manufacturing method of lightweight camshaft filed before by the present applicant to combine the cam part 2 and the shaft part 3 designed as mentioned above. The present invention may use a casting method such as a diecasting method which is relatively easier and more precise than a conventional sintering process. According the conventional sintering process, a hollow steel tube shaft and a sintering material powder cam may be combined by diffusion in a sintering furnace. Accordingly, the present invention has a merit of further simplifying the camshaft fabricating process.

Detailed features of a method fabricating a lightweight camshaft for automobile accountable for different thermal expansion coefficients and a lightweight camshaft made by said method according the present invention, will be described below. The detailed processes largely include a material preparation process, a casting process, an oil hole working process, and a finish process.

1. material preparation process

As described above, the material preparation process is a step where the cam pieces 2a corresponding to the cam part 2 in the camshaft 1 are separately fabricated and prepared, but the separately fabricated and prepared cam pieces 2a are not inserted into a mold 4 as inserts as shown in Fig.6a. Here, the separately fabricated and prepared cam pieces 2a may be formed and sintered in the form of a powder cam using powder metal through a hollow sintering camshaft fabrication method among the comventional modular camshaft fabrication methods.

In particular, since only the cam pieces 2a are separately sintered and fabricated in a sintering furnace in the present invention, an amount of the powder metal being charged into the sintering furnace can be increased in comparison with the case that both powder cams and steel tubes are charged into the sintering furnace and sintered and joined as the conventional hollow sintering camshaft is fabricated. Accordingly, the present invention has a merit of enhancing a charging efficiency of the sintering furnace. However, other than the powder cam, also forged cam pieces 2a formed by forging process can be used.

Here, it is preferable that the separately fabricated cam pieces are fabricated using metal or alloy which can be cold-formed or heat treated. The chemical components and mechanical features of the quality of the material are illustrated in the following Tables 1 and 2, respectively.

[Table 1]

[Table 2]

2. casting process

First, the cam pieces 2a which have been prepared through the material preparation process, are inserted as inserts into a mold 4 which is designed to meet the structure of a camshaft, as shown in Fig.6b. Here, the cam pieces 2a are sequentially inserted into the mold 4 as inserts based on positions which have been set according to a disposition structure of the cam part 2 at the portion to be formed into the shaft part 3 in the mold 4.

In this manner, after the cam pieces 2a are inserted into the mold 4 as inserts, lightweight metal such as aluminum alloy or heat resistant magnesium alloy is injected in a molten state according to a casting method. Here, it is preferable that a casting method using diecasting equipment of a cold chamber diecasting type is used as the casting method. It is also preferable that atmosphere and vacuum or horizontal and vertical diecasting equipment is used as the diecasting equipment according to shapes and features required thereof.

In particular, it is preferable that a casting pressure according to the diecasting method which is called an injection pressure ranges between 60~l,300kgf/cnf. It is preferable that diecasting velocity which is called an injection velocity ranges between 0.5-3. Om/s. In addition, it is preferable that a diecasting molten metal temperature ranges between 600~800°C . Of course, a gravitational casting method, a squeeze casting method, or a semisolid casting method can be used as the aluminum casting method, in addition to the general diecasting method. Here, it is preferable that a molten metal temperature is maintained between 600-800 ^°C even in the case of the gravitational casting method.

Also, it is preferable that the injection pressure at the squeeze casting method corresponding to the pressurized casting ranges between 200~l,600kgf/cuf, the injection velocity ranges between 0.5-2.0m/s, and the molten metal temperature ranges between 600-800 °C . It is preferable that the injection pressure at the semisolid casting method ranges between 50~500kgf/cin\ the injection velocity ranges between 0.5— 2.0m/s, and the semisolid molten metal temperature ranges between 450~650^°C .

Under the above-described conditions, molten aluminum alloy (or heat resistant magnesium alloy) is injected into the mold 4, and then the cam pieces 2a and the shaft part 3 are integrally formed to thus complete casting. The final aluminum camshaft 1 is taken out from the mold 4 as shown in Fig.6c, to thereby complete the casting process.

Here, in the result of reviewing the mechanical features of the test product for the lightweight camshaft 1 taken out from the mold 4 in this manner, a tensile strength of the product is 200~350MPa, the proof stress is (0.2%) 180-30OMPa, and the elongation ratio is 1-15%.

In particular, the mold 4 has a structure that a joint portion 3a can be cast of aluminum alloy (or heat resistant magnesium alloy), in which the joint portion 3a whose diameter is larger than that of the shaft part 3 and which is extended and protruded from the side surfaces of the each cam piece 2a, is provided in the binder portion of the cam pieces 2a and the shaft part 3, in order to enhance the bonding strength, when the cam pieces 2a and the shaft part 3 are combined together. That is, if the lightweight camshaft is cast in the form that the cam pieces 2a and the shaft part 3 are joined together through the casting process, the joint portion 3a having a larger diameter than that of the shaft part 3 is provided in the side surfaces of the cam pieces 2a.

The joint portion 3a has a structure that cannot be formed through the hollow sintering camshaft fabrication method in which powder cams are sintered and joined at the conventional steel tube, but can be formed through an casting forming fabrication method using lightweight metal camshaft accountable for the different thermal expansion coefficients according to the present invention. That is, the joint portion 3a in the lightweight camshaft according to the present invention, may be embodied to have a structure encompassing the side surfaces of the cam pieces 2a in the mold 4, and has a structural feature that heightens the bonding strength between the cam pieces 2a and the shaft part 3.

3. oil hole working process After the lightweight camshaft 1 is casting-formed through the die casting method, front peices and oil holes 5 necessary for the structure (or function) of the camshaft 1 are formed. In particular, the lightweight camshaft 1 taken out through the casting process has a structure of a filled camshaft. Accordingly, if the center of the shaft part 3 in the camshaft 1 is made to be a hollow portion 6 at the same time when the oil holes 5 are worked through a drill work in the oil hole working process, the filled camshaft can be finally altered in the form of a hollow camshaft. As a result, a lighter camshaft 1 can be proviced.

Of course, in addition to the drill work process, a core process may be used to form the oil holes 5, wherein cores (not shown) in form of oil holes 5 are inserted in the mold 4, then molten lightweight metal is injected to form the shaft part 3, and then the cores are taken out from the mold 4.

Meanwhile, in the case that a steel tube is used in the conventional hollow sintering camshaft as described above, the diameter of the hollow tube cannot be worked into a predetermined size of less in view of a steel tube feature fabricated through a drawing work. Thus, an oil supply from an oil fan to a camshaft which are located in the lower end of an engine is delayed at the time of a cold-start due to the problem of an oil filling time, and wear due to no oil supply can proceed. In order to solve this problem, a capacity of an oil pump should be increased. However, in the case of the lightweight camshaft 1 according to the present invention, the oil holes 5 and the hollow portion 6 should be worked through a separate drill work. As a result, the present invention has a merit of working the diameter of the hollow portion 6 into a size meeting a lightweight condition and an optimal condition necessary for an oil filling time based on oil supply. 4. finish process

After the oil holes 5 are worked in the casting-formed camshaft 1, the journal part is roughly ground and then precisely ground through a well-known common technique in order to perform a more precise finish. The cam part 2 composed of a plurality of cam pieces 2a is also finished more precisely.

A final product of a lightweight aluminum camshaft is finally produced through the finish process such as the rough grinding and the precise grinding. Of course, a particular heat treatment can be performed in the finish process, in order to enhance the feature such as durability and to improve the quality of the surface of the cam part.

[Mode for Invention]

Meanwhile, as shown in Figs. 7a and 7b, the camshaft according to the present invention may have a liner layer 10a or 10b. The liner layer 10a or 10b is formed by coating with various methods on each surfaces of the cam pieces 2a of the cam part 2 and the shaft part 3, so that wear-resistance and corrosion-resistance could be enhanced as well as weakening of the bonding strength, that might occur due to different thermal expansion characteristics, could mechanically and physically be prohibited.

Figs.7a and 7b show sectional views of A-A line and B-B line for the cam part 2 and the shaft part 3 of the camshaft according to the present invention. A liner layer 10a or 10b having a predetermined thickness may be coated on the shaft part 3 as well as the cam part 2 that comprises the cam pieces 2a separately made in same number of valves to open and close the intake-exhaust valves of an engine. Here, the cam part 2 and the shaft part 3 may be made of lightweight metals with different thermal extension coefficients, and may be formed into a body at the same time by any one of the casting methods that comprise cold chamber die casting method, gravitational casting method, low-pressure casting method, squeeze casting method, and semi-solid casting method.

In particular, the liner layer 10a formed on the cam part 2, which enhances wear-resistance and corrosion-resistance as to open and close the intake -exhaust valves of the engine, may be formed with thickness of lμm ~ 10mm, preferably 300 ~ 600μm, and hardness of 300 ~ 1,000Hv by any chosen one of the methods that comprise thermal spray, vapor deposition, anodizing, electroplating, and ion nitration.

For example, in case that the liner layer 10a is formed by the thermal spray method, it is formed with any one chosen from the group that comprises ceramics of AI2O3, AlN, MgO and Mg3N2. Or the liner layer 10a may be formed on the cam part 2 by the electroplating method with metal carbides of WC or Cr3C2, metal oxides of Al2θ3+ Tiθ2, cemet(ceramic+ metal) of TiC, TiN, or TiCN, or a metal compound comprising Fe.

And in case that the liner layer 10a is formed by the vapor deposition method, it may be formed by any one vapor deposition method of PVTXPhysical Vapor Deposition), CVD(Chemical Vapor Deposition), TRD(Thermo Reactive Deposition and Diffusion) and PCVD(Plasma Chemical Vapor Deposition) with any one chosen from the group that comprises TiCN, TiAlN, TiAlCN, TiAlON and TiAlSiCNO, or a combination thereof.

And, in case that the liner layer 10a is formed by the anodizing method, the aluminum alloy or the heat resistant magnesium alloy comprising the cam part 2 is coated by the anodizing technology that comprises Keronite etc. Meanwhile, in case that the liner layer 10a is formed by the electroplating method, the surfaces of the aluminum alloy or the heat resistant magnesium alloy comprising the cam part 2 may be coated with any one of ultra-hardness metal that comprises Cr, W, Ni and Co, or an alloy thereof.

And, in case that the liner layer 10a is formed by the ion nitration method, the surfaces of the aluminum alloy or the heat resistant magnesium alloy comprising the cam part 2 may be coated with AlN or Mg3N2.

And, the liner layer 10a may be formed by the electroplated iron or alloy of iron, and then by surface processing, deposition, nitration or carbonization of the electroplated iron or alloy of iron, wherein the deposition may be any one method chosen from PVD(Physical Vapor Deposition), CVD(Chemical Vapor Deposition), TRD(Thermo Reactive Deposition and Diffusion) and PCVD(Plasma Chemical Vapor Deposition) with any one chosen from TiCN, TiAlN, TiAlCN, TiAlON and TiAlSiCNO, or a combination thereof.

As described above, the camshaft 1 according to the present invention has a cam part 2 and a shaft part 3 formed into a body of same lightweight metals but of different thermal expansion coefficients, such as aluminum alloy or heat resistant magnesium. So the weakening of the bonding strength, that might occur due to different thermal expansion characteristics, could mechanically and physically be prohibited. In addition, the cam part 2 of the camshaft 1 may be made of the lightweight metals such as aluminum alloy or heat resistant magnesium, so that the lighter camshaft 1 could be provided, as well as weakening of wear-resistance and corrosion-resistance could be prohibited.

And the camshaft 1 has liner layers 10a on the cam pieces 2a comprising the cam part 2 and liner layers 10b on the shaft part 3. In particular, the liner layer 10b of the shaft part 3 may be formed at same time as the liner layer 10a of the cam part with same method. The liner layer 10b may additionally enhance the mechanical characteristics of the shaft part 3 as the liner layer 10b may solidify the surface of the shaft part 3.

As such, the camshaft 1 according to the present invention has a cam part 2 and a shaft part 3 formed into a body of same lightweight metals but of different thermal expansion coefficients, such as aluminum alloy or heat resistant magnesium. It may be formed into a body using any one of the casting methods that comprise die-casting method, squeeze casting method, and semi-solid casting method. The cam part 2 and the shaft part may have liner layers 10a and 10b on their surfaces, with thickness of lμm ~ 10mm, preferably 300 ~ 600μm, and hardness of 300 ^" 1,000Hv by any one of the methods that comprise thermal spray, vapor deposition, anodizing, electroplating, and ion nitration, so that wear-resistance and corrosion-resistance could be enhanced as well as bonding strength between the cam part 2 and the shaft part 3 comprising the camshaft 1 could be enforced. [Industrial Applicability]

The present invention relates to a lightweight camshaft applied to a vehicle engine, the lightweight camshaft accountable for difference of thermal expansion coefficients may becomes lighter substantially by 30% or so, in comparison with the conventional hollow sintering camshaft. The lightweight camshaft according to the present invention may be inexpensive and have high durability and hardness. The lightweight camshaft according to the present invention has the contraction reinforcing part on the cam piece and the reinforcing joint part on the shaft part. So the bonding force may be enhanced by adapting the shape of the bonding region between the campiece and the shaft part. The lightweight camshaft according to the present invention can provide advantages such as enhancement of the engine durability due to minimizing no -oil supply wear, lowering the moment of inertia due to the lighter camshaft, and enhancement of the engine power and lower gasoline mileage due to the high-RPM of the engine.

Claims

[CLAIMS]

[Claim 1]

A lightweight camshaft accountable for difference of thermal expansion coefficients, which comprises a cam part 2 and a shaft part 3 for mounting the cam part 2, wherein the cam part 2 is produced as separate cam pieces 2a of iron which are inserted into a mold 4 so that the cam pieces 2a are integrally molded with the shaft part 3 which is formed by injecting molten lightweight metal into the mold, characterized by comprising

a contraction reinforcing part 7a or 7b formed on each of the cam pieces 2a to reinforce the strength of bonding between the cam piece 2a of iron and the shaft part 3 of aluminum alloys or heat resistant magnesium alloys that is lightweight metal, so as to protect the bonding strength against weakening due to the contraction in response to difference of the thermal expansion coefficients; and a reinforcing joint part 8a or 8b formed at the shaft part 3 by injecting molten lightweight metal into the contraction reinforcing part 7a or 7b so that the cam pieces 2a and the shaft part 3 may be integrally combined.

[Claim 2]

The lightweight camshaft accountable for difference of thermal expansion coefficients as claimed in claim 1, characterized in that the shaft part 3 has joint portions 3a, wherein each of the joint portions 3a has a shape of reinforce-rib that protrudes from both sides of each of the reinforcing joint part 8a or 8b to encompass sides of each of the cam pieces 2a, so that the bonding strength could be increased at the bonding region between the cam pieces 2a and the shaft part 3 when the shaft part 3 is formed integrally with the cam pieces 2a in the mold 4.

[Claim 31

The lightweight camshaft accountable for difference of thermal expansion coefficients as claimed in claim 1, characterized in that the contraction reinforcing part 7b on a cam piece 2a is formed as a plurality of round holes that penetrate the cam piece 2a at adjacent area around a shaft hole 9a formed in the center of the cam piece 2a, and the reinforcing joint part 8b of the shaft part 3 is formed by injecting molten lightweight metal into the contraction reinforcing part 7b in shape of round holes so as to be integrally combined.

[Claim 4]

The lightweight camshaft accountable for difference of thermal expansion coefficients as claimed in claim 1, characterized in that the contraction reinforcing part 7a on a cam piece 2a is formed as a plurality of long grooves that are recessed in the inside wall of a shaft hole 9a formed in the center of the cam piece 2a, each of the long groove's bottom is enlarged to its both sides, and the reinforcing joint part 8a of the shaft part 3 is formed by injecting molten lightweight metal into the contraction reinforcing part 7a in shape of long grooves so as to be integrally combined.

[Claim 5]

The lightweight camshaft accountable for difference of thermal expansion coefficients as claimed in claim 1, characterized in that, when the shaft part 3 is formed integrally with the cam pieces 2a by inserting the cam pieces 2a of iron into the mold and by injecting molten lightweight metal, the aluminum alloys or heat resistant magnesium alloys into the mold, any one of the casting methods that comprise die-casting method, squeeze casting method, and semi-solid casting method is applied.

[Claim 6]

The lightweight camshaft accountable for difference of thermal expansion coefficients as claimed in claim 1, characterized in that a liner layer 10a is coated on the surface of the cam part 2, with thickness of 30 0 ~ 600j[M, and hardness of 300 ~ 1,000Hv by any one of the methods that comprise thermal spray, vapor deposition, anodizing, electroplating, and ion nitration.

[Claim 7]

The lightweight camshaft accountable for difference of thermal expansion coefficients as claimed in claim 1, characterized in that a liner layer 10b is coated on the surface of the shaft part 3, with thickness of 30 0 ~ 600_/zm, and hardness of 300 ~ 1,000Hv by any one of the methods that comprise thermal spray, vapor deposition, anodizing, electroplating, and ion nitration.

[Claim 8]

The lightweight camshaft accountable for difference of thermal expansion coefficients as claimed in claim 1, characterized in that the cam part 2 and the shaft part 3 are made of lightweight metals with different thermal extension coefficients, and formed at the same time by any one of the casting methods that comprise cold chamber die casting method, gravitational casting method, low-pressure casting method, squeeze casting method, and semi-solid casting method.

[Claim 9]

The lightweight camshaft accountable for difference of thermal expansion coefficients as claimed in claim 6 or claim 7, characterized in that each of the liner layers 10a and 10b of the surfaces of the cam part 2 and the shaft part 3 is coated by the thermal spray method with any one chosen from the group that comprises ceramics of AI2O3, AlN, MgO and Mg3N2, or coated by the electroplating method with metal carbides of WC or Cr3C2, metal oxides of Al2θ3+ Tiθ2, cemet(ceramic+ metal) of TiC, TiN, or TiCN, or a metal compound comprising Fe.

[Claim 10]

The lightweight camshaft accountable for difference of thermal expansion coefficients as claimed in claim 6 or claim 7, characterized in that each of the liner layers 10a and 10b of the surfaces of the cam part 2 and the shaft part 3 is coated by any one vapor deposition method of PVD(Physical Vapor Deposition), CVD(Chemical Vapor Deposition), TRD(Thermo Reactive Deposition and Diffusion) and PCVD(Plasma Chemical Vapor Deposition) with any one chosen from titanic metal compound such as TiCN, TiAlN, TiAlCN, TiAlON and TiAlSiCNO, or a combination thereof.

[Claim 11]

The lightweight camshaft accountable for difference of thermal expansion coefficients as claimed in claim 6 or claim 7, characterized in that each of the liner layers 10a and 10b of the surfaces of the cam part 2 and the shaft part 3 is coated by the Anodizing method that comprises Keronite method.

[Claim 12]

The lightweight camshaft accountable for difference of thermal expansion coefficients as claimed in claim 6 or claim 7, characterized in that most outer surface of each liner layer 10a or 10b of the surfaces of the cam part 2 and the shaft part 3 is formed by the electroplating method with any one of ultra-hardness metal that comprises Cr, W, Ni and Co, or an alloy thereof.

[Claim 13]

The lightweight camshaft accountable for difference of thermal expansion coefficients as claimed in claim 6 or claim 7, characterized in that each of the liner layers 10a and 10b of the surfaces of the cam part 2 and the shaft part 3 is coated by the ion-nitration method with AlN or Mg3N2.

[Claim 14]

The lightweight camshaft accountable for difference of thermal expansion coefficients as claimed in claim 6 or claim 7, characterized in that each of the liner layers 10a and 10b of the surfaces of the cam part 2 and the shaft part 3 is coated by the electroplated iron or alloy of iron, and then by surface processing, deposition, nitration or carbonization of the electroplated iron or alloy of iron. [Claim 15]

The lightweight camshaft accountable for difference of thermal expansion coefficients as claimed in claim 14, characterized in that the electroplated iron or alloy of iron of each of the liner layers 10a and 10b of the surfaces of the cam part 2 and the shaft part 3 is coated by any one method chosen from PVD(Physical Vapor Deposition), CVD(Chemical Vapor Deposition), TRD(Thermo Reactive Deposition and Diffusion) and PCVD(Plasma Chemical Vapor Deposition) with any one chosen from TiCN, TiAlN, TiAlCN, TiAlON and TiAlSiCNO, or a combination thereof.