Many types of closure means are known for sealing or plugging apertures in metal and other structures, e.g., tanks, drums, etc. The most commonly used are threaded screw-type means and force-fitting plugs. The efficiency of such closure means is directly related to the precision and tolerances used in sizing the mating surfaces. When the requirements on such devices are highly demanding, e.g., when it is desired to seal against the leakage or pressurized gaseous materials, extreme care must be taken in preparing the closure means and, often, in preparing the aperture itself as by threading or dressing the seating surfaces which will be contacted by a force-fitting plug.
Among the several types of force-fitting closure means presently in use are included so-called "expansion plugs." Expansion plugs are typically cup-shaped or disc-shaped. In either case the plug is seated by ramming it into place with a rod or similar means. In the case of the cup-type expansion plug, a plug which is slightly larger in radial dimension than the aperture is selected and such sealing as is obtained results from the spring effect of the resilient wall of the cup on the walls of the aperture. Substantial care must be exercised in ramming such plugs, because it is essential that the line of direction of the ramming force not to skew to the axis of the cup or else the plug will twist during seating. Such twisting would, of course, preclude achievement of an effective seal. In the case of the disc-type expansion plug, a deformed disc which has one surface which is generally convex and another surface which is generally concave, and which may be of a generally dished configuration, is inserted into an aperture having a radial dimension slightly larger than the radial dimension of the disc. Upon ramming, the disc flexes and such sealing as is obtained results from the radial forces which the flexed disc is capable of exerting. The recommended manner of setting such discs is to so size them such that when they are rammed they are capable of turning inside out, i.e., the concave surface becomes convex and vice versa, and then spring back to an almost flat position. In order to obtain such action, it is recognized that close tolerances must be achieved. In this regard, it is recommended that the angle between the cylindrical sides and the bottom of the counterbore in the aperture should be 95.degree. to 97.degree.. Such discs are subject to many difficulties, including the need to exercise very substantial care in preparing the seating surfaces in the aperture as by counterboring, the tendency of such plugs to self-release under high pressures, particularly when the plug is installed such that a concave surface faces the interior of the chamber to be sealed. Lastly, both types of expansion plugs are subject to the deficiency that a ramming force which has a jarring effect on the chamber to be sealed must be used. Such forces are often highly undesirable, since they cause vibrations which may loosen other fittings, disturb the contents of the chamber, or, in extreme cases, create risk of explosion.