An annular metal gasket that is called a metal C-ring gasket is used as a gasket that is used in environments such as a high-temperature environment, a plasma environment, and an ultra-vacuum environment in which a gasket made of rubber cannot be used for a variety of piping and mechanical equipment for nuclear facilities and semiconductor manufacturing equipment.
A metal gasket that is called a metal C-ring gasket is configured in the manner in which an inner ring made of a metal leaf spring or a coil spring is fitted inside a metal outer ring having a substantially C-shaped cross-section as disclosed in the Patent Literatures 1 and 2 for instance. Consequently, in the case in which the metal gasket is used, a tightening force larger than that for a gasket made of rubber is required, and a load in a tightening work and a fastening means such as a flange and a bolt are enlarged unfortunately.
Under such a background, this applicant has disclosed a metal gasket in which an inner ring is formed in a polygonal shape such as a square as an annular metal gasket in which a high sealing performance can be obtained by using a less tightening force (see Patent Literature 3).
FIG. 7 is a cross-sectional view showing a metal gasket that is disclosed in Patent Literature 3 describing a prior art. FIG. 8 is a cross-sectional view showing a state in which the metal gasket that is disclosed in Patent Literature 3 describing a prior art is mounted in a sealing groove.
As shown in FIG. 7, a metal gasket 100 of Patent Literature 3 is comprised of an outer ring 110 having a substantially C-shaped cross-section and having an opening 112 formed on a circumferential side and an inner ring 120 that is fitted inside the outer ring 110. As shown in FIG. 7, the inner ring 120 is formed so as to have a polygonal cross-section comprising a pair of an upper corner part 122a and a lower corner part 122b in a vertical direction of the cross section and a pair of an inner-circumference-side corner part 122c and an outer-circumference-side corner 122d in a horizontal direction of the cross section. The upper corner part 122a and the lower corner part 122b are fitted into the outer ring 110 so as to make contact with an inner side face of the outer ring 110.
As shown in FIG. 8, the metal gasket 100 of Patent Literature 3 is mounted in a sealing groove 134 that is formed in a flange 132, and is pressed in a vertical direction with a tightening force F by a flange 130 on the other side. The pair of the upper corner part 122a and the lower corner part 122b described above then strongly presses an inner side face of the outer ring 110 by a restoring force of the inner ring 120, and the upper side of the outer ring 110 and the flange 130 are partially strongly made contact with each other. In addition, the lower side of the outer ring 110 and the sealing groove 134 of the flange 132 are partially strongly made contact with each other. A space between the flange 130 and the flange 132 is sealed in such a manner that the contact part functions as a sealing part.
As described above, the metal gasket 100 of Patent Literature 3 describing a prior art is configured in such a manner that an inner side face of the outer ring 110 is pressed by the upper corner part 122a and the lower corner part 122b with a restoring force of the inner ring 120. Consequently, a high sealing performance can be obtained by a small tightening force as compared with a general metal gasket that is disclosed in Patent Literatures 1 and 2.
A relationship between a tightening force that is applied to a metal gasket and an amount of displacement of the metal gasket is preferably a relationship that is indicated by the line a of a graph in FIG. 9. FIG. 9 is a graph for illustrating a preferable relationship between a tightening force F and a displacement amount δ for a metal gasket. A relationship between a tightening force F and a displacement amount δ for a metal gasket 100 of Patent Literature 3 described above is generally approximated to the line a of the graph in FIG. 9. A vertical axis of the graph in FIG. 9 indicates a tightening force F and a horizontal axis of the graph in FIG. 9 indicates a displacement amount δ of the metal gasket.
As shown by the line a of the graph in FIG. 9, for a relationship between a tightening force F and a displacement amount δ for a metal gasket, it is preferable that an inclination of the line a of the graph indicates a rapid inclination E1 in the early stages.
This is because a settlement is required between a metal gasket and a flange face in order to obtain a certain level of sealing performance of a metal gasket and is it necessary that a large contact stress is applied to a sealing part of a metal gasket that comes into contact with a flange face in order to obtain the settlement. In order to apply a large contact stress to a sealing part of a metal gasket, it is preferable that an inclination E1 in the early stages of FIG. 9 indicates a rapid inclination in such a manner that a tightening force F is as large as possible to a displacement amount δ per unit.
In the case in which an inclination E1 in the early stages is gradual, it is necessary that a metal gasket is deformed greatly to apply a large contact stress to a sealing part of a metal gasket. In the case in which a tightening of a bolt is insufficient, a sufficient settlement cannot be obtained between a metal gasket and a flange face in some cases. The settlement that is said here is that a metal gasket that comes into contact with a flange face is deformed along minute concavity and convexity of the flange face and a gap between a metal gasket and a flange face is removed by bridging the concavity and convexity.
As shown by the line a of the graph of FIG. 9, for a relationship between a tightening force F and a displacement amount δ for a metal gasket, it is preferable that an inclination E2 of the line a of the graph indicates a gradual inclination after a tightening force F exceeds a predetermined tightening force F2.
This is because a metal gasket is used in the case in which a space between a flange 130 and a flange 132 is sealed as described above for instance in the case in which a space between two members is sealed. A tightening between flanges in this case is carried out until flanges come into contact with each other, that is, a metal touch of a flange face is carried out. That is, a tightening is carried out until the flanges 130 and 132 come into contact with each other in a state shown in FIG. 8 described above. In a state in which a tightening is completed, the metal gasket 100 is compressed by a displacement amount δ1.
As described above, a metal gasket is tightened until a metal touch of a flange face is carried out In the case of a metal gasket in which an inclination E2 of the line a of the graph indicates a rapid inclination, a tightening force F1 in the case of a metal touch is increased extremely and it is difficult to manage a tightening of a metal gasket.
In the case in which a management of a tightening of a metal gasket is carried out by a so-called displacement control in which a tightening is carried out until a displacement of a metal gasket becomes a predetermined displacement amount δ1, the following problems occur in some cases. In other words, an error occurs by a wave or a dimension tolerance of flange face and a sealing groove for a displacement amount of a metal touch. Therefore, in the case in which an inclination E2 of the line a of the graph indicates a rapid inclination, a tightening force is varied greatly only by a small error that occurs for a displacement amount of a metal touch. In the case in which a tightening force is decreased, a sealing performance may be deteriorated. In the case in which a tightening force is increased, a bolt is damaged or a tightening cannot be carried out until a metal touch unfortunately.
In the graph shown in FIG. 9, a symbol F0 indicates a tightening force that is required to obtain a predetermined sealing performance, and a symbol F2 indicates a tightening force in the case in which an inclination of the line a of the graph is changed from a rapid inclination E1 to gradual inclination E2. The tightening force F0 is smaller than a tightening force F1 in the case of a metal touch as a matter of course, and the tightening force F0 is smaller than a tightening force F2 described above. This is important to obtain a stable sealing performance for a metal gasket.