Metallic gasket

Abstract: A metallic gasket (100) of a structure in which a bead (2) is formed to a base plate having at least one elastic metallic sheet, wherein a shim (24) is secured to a flat portion (1a) of the base plate on the concave side of the bead at a position displaced from the center of the bead toward a bore hole (5) of the base plate, with the ends of the base plate and the shim (24) being aligned on the side of the bore hole, and the shim is extended into the bead with a contour matching the concave surface of the bead.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention concerns a metallic gasket disposed between joining faces of a cylinder head and a cylinder block that constitute an internal combustion engine to prevent combustion gas, cooling water, lubricant or the like from leaking. Description of the Prior Art In an internal combustion engine, a gasket is interposed between a cylinder block and a cylinder head to provide a seal between them. The materials of the gasket for a gasoline engine use a mixture of asbestos and rubber, having a spiked steel sheet embedded therein. A bore side peripheral edge covered with a stainless steel sheet and peripheral edge of bolt holes covered with a copper sheet, respectively, are used as a countermeasure for a gap between the seal and the joining faces. Further, as for a diesel engine, a material comprising asbestos, rubber and a wire gauze embedded therein is used, as a countermeasure for a gap at the edges and the joining faces as described above and a fire ring is further engaged along the edge of the bore. Such gaskets are referred to as soft gaskets. However since asbestos is used in gaskets of this type, the use has become restricted, particularly, in view of environmental problems. As far as the inventor knows, synthetic fibers are used instead of asbestos in a gasoline engine. Further, for a diesel engine, a structure comprising a steel laminate including a steel sheet base plate formed with a minute bead, a stainless steel subplate stacked thereon and a steel wire core disposed to the bore peripheral edge is used. The subplate is folded back to cover the wire on the inside and extends along the upper surface of the edge of the bore holes. A baked rubber seal ring is disposed at the bolt holes. The soft gaskets are not satisfactory for providing reduced thickness, endurance to cope with decreasing distance between bores, engine weight and size reduction, material recycling, heat resistance, high performance, cost reduction and freedom of design. Eventually the demand for metallic gaskets will increase in the field of automobile industry. By the way, in a metallic gasket, a bead is formed along the periphery of holes, in particular, at the periphery of combustion chamber holes (hereinafter referred to as a bore hole). When a metallic gasket is tightened between a cylinder head and a cylinder block, the bead is elastically deformed to function as a seal. Further, a stopper is formed, together with the bead, at the periphery of the bore hole in the metallic gasket. The stopper suppresses vibrations at the joining face area between the cylinder block and the cylinder head caused by the operation of the engine, so that it also functions as a sealing member to prevent fatigue failure on account of engine vibrations. In the metallic gasket, a complete sealing effect is intended by attaining a primary seal with the stopper and a secondary seal with the bead compensated with the durability by the stopper. The stopper may be disposed by folding back a sub-plate described above so as to cover the inner circumferential surface of the bore hole, or it may be formed by arc plasma spray on a gasket base plate or as a metal shim. In any case, the stopper must have a thickness corresponding to a gap in the engine, that is, a gap between the joined surfaces of the cylinder block and the cylinder head. Of recent, with the performance improvement owing to the progress in the engine technology, fuel economy and the size and weight reduction, the distance between bores has been narrowed. In addition, the combustion temperature and the explosive pressure have increased due to the adoption of super-chargers. Accordingly, with the prior art, the width of the stopper can not be increased and it is difficult to impede the bead vibrations. Further, as the engine temperature becomes higher, the pressure exerted on the gasket surface increases because of the combustion expansion, and dent into the aluminum cylinder head is caused when a narrow shim stopper is used. Further, complete sealing has become difficult. A double seal with a stopper primary seal and a bead secondary seal can not be obtained. In addition, since the shim stopper is not fixed to the base plate, it sometimes slides or becomes detached. This invention has been accomplished in view of the foregoing situation and it is an object thereof to provide a metallic gasket with a shim even if a distance between bores is narrow and also capable of adjusting the bead spring force. As an example of the metallic gasket in the relevant prior art, the inventor has accomplished invention as disclosed in Japanese Patent Publication Hei 2 (1990)-58502 (JP-B2-2-58502) and, further, is well aware of Japanese Patent Laid-Open Sho 62 (1987)-155376 (JP-A-62-155376) and Japanese Patent Publications Hei 3 (1991)-20626 (JP-B2-3-20626) and Hei 4 (1993)-40540 (JP-B2-4-40540) as the prior art. EP-A-0 486 817 discloses a metallic gasket in accordance with the pre-characterising part of claim 1. SUMMARY OF THE INVENTION The present invention provides a metallic gasket comprising: at least one base plate having at least one elastic sheet; at least one bore hole, a flat portion disposed around the bore hole, and a bead concentrically disposed around the flat portion formed in the elastic sheet; and a shim secured to the flat portion, a bore hole side end face of the shim being aligned with a bore hole side end face of the base plate; characterised in that the shim extends into the bead with a contour of the shim matching and extending along a concave surface of the bead. The shim extends into the bead with a matching concave contour in order to compensate the spring constant of the bead. Then, the extended width of the shim varies from 110 to about 200% of the width of the flat portion depending on the attaching position of the shim. That is, the extension width of the shim is reduced near bolt holes, while the width is increased between the bolt holes. This is because the spring constant of the bead can be small near the bolt holes since the tightening surface pressure is large and the gap is small at the joint area of the cylinder block and cylinder head. However, the spring constant of the bead has to be increased in the portion between bolt holes since the tightening surface pressure is smaller and the gap is larger. Thus, the shim is disposed with a contour matching the concave surface of the bead, and the shim extension into the bead is made larger between the bolt holes where the surface pressure is the lowest, and it is reduced near the bolt holes where the surface pressure is high, thereby making the spring force uniform. However, according to the experience of the inventor, in a case where the flat portion of the base plate to which the shim is attached is extremely narrow as in a case of an engine with a narrow inter-bore gap, the shim width has to be narrowed correspondingly and, if the extension of the shim into the bead is such that the width of the shim is smaller than about 110% of the width of the flat portion, it is impossible to prevent a dent formed by the shim on the engine. That is, unless the extension of the shim into the bead is at least about 10% of the flat portion of the base plate, it is so narrow that a dent is formed on the engine. On the other hand, if the extension of the shim into the bead is such that the width of the shim exceeds about 200% of the width of the flat portion, it causes undesirable interference of the shim a narrow inter-bore gap engine. Further, the shim can be applied with a case in which the flat portion near the bolt hole has to be very narrow. This can be accomplished by fixing the shim to the base plate so that the shim edge extends into the bead and a flat portion is secured to the flat portion of the base plate. Then, the thickness of the shim is usually made less than the thickness of the base plate, and it can be conformed to an uneven gap at the joint of the cylinder head and the cylinder block. Further, another base plate may be disposed to the base plate described above with the recess of the bead of another base plate facing the recess of the bead of the base plate described to form a structure wherein the shim is sandwiched between the two base plates, so that it can cope with engine temperature deformation. In addition to the constitution described above, if an intermediate plate is placed between the shimmed base plate and the base plate, it can enhance the sealing effect of the base plate and also function as a spacer. Then, in addition to the constitution described above, the combined state of the shim and the base plate may be turned over and another base plate is stacked thereover with the recesses of the beads being in the same direction. The amount of spring restoration is reduced and the spring rigidity is increased. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a plan view of a typical embodiment of a metallic gasket of the present invention; Fig. 2(a) is a cross sectional view taken along line A-A in Fig. 1. Fig. 2(b) is a cross sectional view taken along line B-B in Fig. 1. Fig. 2(c) is a cross sectional view illustrating a preferred embodiment of a shim edge which protrudes into a bead; Fig. 3 is a cross sectional view of a preferred embodiment; Fig. 4 is another cross sectional view of a preferred embodiment; Fig. 5 is a cross sectional view of a different preferred embodiment. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Example 1 As shown in Fig. 1, a metallic gasket, which is entirely depicted as 100, comprises an elastic metallic sheet (hereinafter referred to as a base plate) 1. A bore hole (for combustion chamber bore) 5 corresponds to cylinder bore number of a cylinder block not shown. A bead 2 is formed in the base plate 1 along a periphery of the bore hole and a circle 21 shown by a chain line represents the crest of bead 2. A shim 24 is an annular plate having an inner diameter equal to the diameter of the bore hole 5 in the base plate 1 and the inner diameter aligned with the bore hole 5. As shown in Fig. 1, the base plate 1 has bolt holes 29 at a position determined by dividing the circumference of the bore hole 5 into even four parts. Oil holes 30 is situated near two of four bolt holes 29 and a cocoon-shaped water hole 31 is formed between the bolt holes 29. As can be seen from Fig. 1, the outer diameter of the shim 24 is smaller close to the bolt holes 29 and is larger at a portion between the bolt holes 29. In this embodiment, a portion of the shim 24 protruding into the bead 2 extends to the maximum, the crest line 21, while a portion thereof protrudes slightly into the edge of the bead 2 where it is close to the bolt hole. That is, in this embodiment, assume the width of a flat portion 1a of the base plate 1 as 100%, the width of the protruding portion of the shim 24 that extends as far as the crest line 21 of the bead 2 is 200%, and the width of the slightly protruding portion thereof is 110%. As described above, the outer diametrical shape of the shim 24 is not true circular while the bore hole 5 is a true circle. The shim diameter is smaller at portions 26-1, 26-2, 26-3 and 26-4 near the bolt holes 29 and the diameter is larger at portions 25-1, 25-2, 25-3 and 25-4 corresponding to the intermediate area between the bolt holes 29. Fig. 2(a) shows the intermediate area between the bolt holes 29, taken along line A-A in Fig. 1. In this portion, the shim 24 extends as far as the crest of the bead 2, and the width of the shim 24 is depicted by W 1 . Fig. 2(b) shows a portion near the bolt hole 29, taken along line B-B in Fig. 1, in which the shim 24 extends slightly into bead 2, and the width of the shim 24 is depicted by W 2 . Then, as shown in Fig. 2, it is apparent that the width or extending amount of the shim 24 has a relationship: W 1 > W 2 for the area between the bolt holes 29 and the portion near the bolt holes. The figure shows that the shape for the outer edge of the shim varies along the circumferential edge thereof. By the way, as shown in Fig. 2(a) and (b), the thickness T 2 of the shim 24 is about one-half T 1 of the base plate. Since the shim 24 is used to reinforce base plate 1, it is preferred to keep the thickness thereof as necessary for the reinforcement and make it as thin as possible so that it will conform to the joint area between the cylinder block and the cylinder head. Fig. 2(a) and (b) show an embodiment in which an edge 34a of the shim 24 protruding into the bead and a flat portion is stacked and secured to the base plate and is aligned with the edge 34b of the base plate 1 which is contiguous from a flat portion 1a on the side of the bore hole 5 to the bead 2. In a case where the flat portion near the bolt hole 29 is extremely narrow as shown in Fig. 1, the border edge 34a of the shim 24 may be displaced from the edge portion 34b of the base plate 1 to the bead 2 so as to overhang in the bead 2 as shown in Fig. 2(c). In this way, the flat portion of the shim 24 is secured to the flat portion 1a of the bore hole 5 base plate as shown by 27 in Fig. 2 by bonding or welding. In this case, the end of the flat portion 1a of the base plate 1 near the bore 5 and the end of the flat portion of the shim 24 have to be aligned. Thus, when the shim 24 is appended to the base plate 1, insufficient or uneven surface pressure caused by a gap variation at the joint area between the cylinder head and the cylinder block can be eliminated. Necessary spring constant can be attained and effectively ensures a seal without over tightening the shim 24 to cause a dent in the cylinder head. In this constitution, since the flat portion of the shim 24 covers the edge 34b of the base plate 1 on the side of the bore hole 5, the pressure exerted on the surface at the edge becomes highest to improve the sealing effect. Example 2 Fig. 3 shows another embodiment. In this embodiment, two base plate 1-1 and 1-2 are joined with the recesses of the beads 2-1 and 2-2 facing each other, a shim 24 is secured at a flat portion 1a of the base plate 1-1, another base plate 1-2 is appended to the lower surface of the shim 24, so that the shim 24 is sandwiched between the two base plate 1-1 and 1-2. The drawing shows an intermediate position between the bolt holes 29 shown in Fig. 1, in which the extended shim 24 reaches the crest of the bead 2-1. This embodiment is effective in coping with engine deformation due to the temperature. Example 3 Fig. 4 shows an addition to the constitution of the embodiment shown in Fig. 3. An intermediate plate 33 is interposed between a shim 24 and a base plate 1-2. This can provide an effective seal and also function as a spacer. Example 4 Fig. 5 shows an addition to the constitution of the embodiment shown in Fig. 3, the combined state of a shim 24 and a base plate 1-1 is turned over, and a base plate 1-2 is stacked thereover such that the recesses of the beads 2 are in the same direction. This embodiment is suitable to reduce the restoration force and increase the spring rigidity. As has been described above, it can be understood that the spring constant can be varied by controlling the width of the shim and the surface pressure of the edge at the bore hole can be increased on the concave side of the bead and the sealing effect can further be improved by this invention.

Claims: A metallic gasket (100) comprising: at least one base plate having at least one elastic sheet (1;1-1); at least one bore hole (5), a flat portion (1a) disposed around the bore hole (5), and a bead (2) concentrically disposed around the flat portion (1a) formed in the elastic sheet (1,1-1); and a shim (24) secured to the flat portion (1a), a bore hole side end face of the shim (24) being aligned with a bore hole side end face of the base plate (1,1-1); characterised in that the shim (24) extends into the bead (2) with a contour of the shim matching and extending along a concave surface of the bead. A metallic gasket as claimed in claim 1, wherein the width of the shim (24) varies from 110 to 200% of a width of the flat portion (1a) of the base plate (1;1-1) depending on the position of the shim with respect to the base plate. A metallic gasket as claimed in claim 1 or 2, wherein the shim (24) is secured to the flat portion of the base plate (1;1-1) such that a border edge (34a) between the portion extended into the bead and a flat portion of the shim is disposed at a position in the bead (2) so as to overhang in the bead. A metallic gasket as claimed in any of claims 1 to 3, wherein the thickness (T 2 ) of the shim (24) is less than the thickness (T 1 ) of the base plate (1). A metallic gasket as claimed in any of claims 1 to 4, wherein another base plate (1-2) is disposed adjacent to the one base plate (1-1), with a concave portion of a bead (2-2) of the other base plate (1-2) facing the concave of the bead (2-1) of the one base plate (1-1), and the shim (24) is sandwiched between the base plate (1-1,1-2). A metallic gasket as claimed in any of claims 1 to 4, wherein another base plate (1-2) is disposed adjacent to the one base plate (1-1), with a concave portion (2-2) of the other base plate (1-2) facing the concave of the bead (2-1) of the one base plate (1-1), and an intermediate plate (33) is disposed between the other base plate (1-2) and the shim (24), so that the shim (24) and the intermediate plate (33) are sandwiched between the base plates (1-1,1-2). A metallic gasket as claimed in any of claims 1 to 4, wherein the shim (24) is secured to a flat portion (1a) ofthe one base plate (1-1) on a concave side of the bead (2-1) at a position displaced from the center of the bead (2-1) toward a bore hole of the one base plate, with the ends of the shim (24) and the one base plate (1-1) at the bore hole being aligned, the shim (24) extending into and matching a concave contour surface of the bead (2-1), and another base plate (1-2) is disposed adjacent to the one base plate (1-1), with the concave of the bead (2-1) of the one base plate (1-1) being aligned with a bead (2-2) of the other base plate (1-2) when the other base plate (1-2) is placed over the shim (24).

Inventors: UETA KOSAKU, UETA, KOSAKU, UETA, KOSAKU C/O JAPAN METAL GASKET CO., LTD.
Applicants: NIHON METAL GASKET, JAPAN METAL GASKET CO., LTD.
Application year: 1993
Main classification: F16J15
IPC codes: F16J15