The present invention relates to a cylinder head gasket formed of at least three metal plates, and in particular, the cylinder head gasket is provided with partial resin coatings around a hole to be sealed.
A cylinder head gasket for sealing between a cylinder head and a cylinder block is generally formed of one or a plurality of metal plates, and includes holes for cylinder bores, oil holes, water holes, and bolt holes. Also, in order to seal combustion gases generated in the combustion chambers, oil circulating through an oil gallery and water in a water jacket, the gasket includes sealing means, such as beads and seal rings, for providing adequate sealing pressures for the respective holes to be sealed.
Especially, since high temperature and high pressure combustion gases are generated around the holes for the cylinder bores when the engine is operated, sealing around the holes for the cylinder bores is especially important. Therefore, the metal plate superior in mechanical strength and durability is used, and sealing means for forming main sealing portions are arranged around the holes for the cylinder bores to form seal lines.
The sealing means may be a full bead or half bead, or a combination thereof, which is arranged in various ways. Further, the surface pressure of the bead may be changed, and in order to protect the bead from creep relaxation, a stopper bead may be formed or metal shim may be laminated on a plate.
However, in the cylinder head gasket, the cylinder head and the cylinder block are formed of relatively soft aluminum alloy in order to make the weight of the engine light. Therefore, if the surface pressure of the bead is high, the cylinder block and cylinder head may be damaged when a force due to engine vibration and heat expansion is applied thereto. The combustion gas may leak through the damaged portion.
On the contrary, if the sealing surface pressure is insufficient, the combustion gas enters the oil hole and water hole to cause air inclusion, which also causes a trouble in circulation of oil and water.
It is, therefore, important to provide a sufficient sealing ability and to set a sealing surface pressure in order not to cause damage to the engine parts sandwiching the head gasket. Since the best sealing surface pressure which is determined by the contradictory demands is within a narrow range, it is required to have a structure of the cylinder head gasket such that the sealing surface pressure which is formed by processing is adjusted and set to generate a desired sealing surface pressure.
Conventionally, the adjustment of the sealing surface pressure, which is determined by the material of the metal plate or shape of the bead formed by the press, is made by a stopper, such as an auxiliary bead or metal shim. However, since the auxiliary bead is formed by press, accuracy is not great. Also, in view of the thickness and the material of the shim to be able to processing, the precise adjustment is difficult by the shim.
The present invention has been made to obviate these problems, and an object of the invention is to provide a cylinder head gasket, wherein a sealing surface pressure of a bead can be adjusted precisely to thereby set a best sealing surface pressure.
Another object of the invention is to provide a cylinder head gasket as stated above, wherein engine parts to which the gasket is installed are not damaged.
A further object of the invention is to provide a cylinder head gasket as stated above, wherein creep relaxation of the bead is prevented.
Further objects and advantages of the invention will be apparent from the following description of the invention.
A cylinder head gasket for an internal combustion engine of the invention basically comprises a first metal plate extending substantially throughout an entire area to be sealed and having a first hole corresponding to a hole of the engine, and second and third metal plates disposed to sandwich the first plate. The second and third metal plates include second and third holes formed therein corresponding to the first hole, and first and second beads formed in the respective second and third plates and extending toward the first metal plate. The gasket further includes a first resin coating deposited on at least one side of the first metal plate at an inner portion relative to the first bead, and a second resin coating deposited on at least one side of the first metal plate at an outer portion relative to the first bead without extending over an entire surface thereof. The second resin coating has a thickness at most equal to that of the first resin coating.
In the cylinder head gasket, since concave portions of the beads are directed to the engine parts, such as a cylinder head and a cylinder block, the sealing surface pressure at the beads is less than a case where top portions of the beads are directed to the engine parts. Thus, the applied force generated at the engine parts becomes small, so that the damages at the surfaces of the engine parts are not formed.
Also, the resin coatings which are arranged on both sides of the bead to form step coatings are softer than a metal shim, and the thickness thereof can be precisely made easily. Also, since the resin coatings are harder than a rubber coating, the resin coatings do not flow or break. Thus, precisely adjustment of the sealing surface pressure around the cylinder bore can be made easily by the adjustment of the thicknesses of the step coatings.
Since the step coatings do not flow or break, seal lines are formed at the step portions of the step coatings. Accordingly, the sealing ability is improved. Also, if the pressure applied to the bead is increased, the pressure is partly supported on both sides of the bead. Thus, flattening and creep relaxation of the bead are prevented.
In the gasket of the invention, the first resin coating and/or the second resin coating may be extended to be located within the width of the bead. In this structure, the resin step coatings are located within the width of the bead, and when the bead is compressed, one side of the resin coating near the top of the bead contacts, at first, a side of the bead to supplementarily support the bead. Thus, the sealing surface pressure may be increased slightly, as compared to a case where the step coating is formed outside the width of the bead.
Also, the maximum sealing surface pressure becomes greater as the step coatings extend close to the center of the width of the bead. Thus, the maximum sealing surface pressure can be adjusted precisely by not only the thicknesses of the step coatings but also the positions of the ends of the coatings at the sides of the beads.
Incidentally, if a metal shim or a thermally sprayed shim is used, it is impossible to adjust the thickness less than 30 xcexcm. Also, the processing for covering the side portions of the bead can not be made easily, so that the processing accuracy is bad and the processing cost increases.
In the gasket of the invention, the first and second resin coatings have the hardness of F to 6H in pencil hardness. If the resin coating is softer than F in pencil hardness, for example the coating is made of a rubber coating softer than the resin coating, the resin coating may flow or break by pressure and can not provide a step. Also, adjustment of the surface pressure can not be made, and since the step is not maintained, the seal lines can not be formed on both sides of the bead. If the resin layer is harder than 6H in pencil hardness, the sealing and absorbing abilities become bad. Therefore, the resin layer is formed to have the hardness of F to 6H in pencil hardness.
In the gasket of the invention, the first and second resin coatings may be formed of epoxy resin, phenol resin, phenoxy resin, fluoroplastics and polyamideimide, or combination thereof. Although the resin coatings may be formed of various resins, it is preferable to use epoxy resin, phenol resin, phenoxy resin, fluoroplastics and polyamideimide, or combination thereof in view of the hardness and easiness of the application. Especially, the phenoxy resin has heat resistance over 200xc2x0 C., and in view of the working ability and physical characteristics in coating, it is most preferable to use the phenoxy resin.
In the invention, the first and second resin coatings are applied by screen printing. Although the resin coatings may be formed by spraying using a mask, the resin coatings can be easily formed even in a complicated shape if the screen printing is used. Further, by multiple printings, the thickness of the resin coating can be easily changed. Thus, it is possible to easily form the resin coatings with adequate thickness according to the position thereof. In the multiple printing, the step resin coatings can be made with various shapes and thicknesses, and multiple structure with different materials can be made easily.
The gasket of the invention may have additional coatings, such as gum coatings on the entire surfaces of the gasket, in addition to the resin coatings explained above.