A METHOD OF FASHIONING AN ANNULAR GASKET WITH A FRONTAL SEALING ACTION AND THE GASKET THUS OBTAINED
Description
The present invention relates to a method by which, to fashion an annular gasket with a frontal sealing action, and to the gasket obtained by way of such a method. Reference is made in particular to flat gaskets consisting in a ring formed by pairing together spiral wound strips of dissimilar materials (for example, steel with asbestos, steel with graphite, or others) . In operation, the ring is compressed frontally between two mutually opposed sealing surfaces serving to isolate two environments one from another, one of which containing a fluid under pressure. The steel strips exhibit a convex profile directed toward the pressurized environment . Among the possible uses for a gasket of this type, one example may be found in the butterfly shut-off valves installed conventionally on large diameter pipelines. In particular, the gasket according to the invention is suitable for application to the moving element of such a valve: this consists in a
disc rotatable through one quarter turn, to which the gasket is mounted in such a way as to impinge on a pack of movable rings fitted to the disc and interacting with the seat of the valve to create a lateral seal. In a valve of this type, the gasket must be compressed in order to guarantee a sealing action, although not over-compressed to the point of disallowing the sliding movement of the movable rings, without which the valve will not be able to close properly.
Conventional gaskets of the type in question are fashioned currently by a method that consists in coiling a continuous strip of thin steel around a cylindrical former of predetermined diameter. The initial coil is fixed in position by spot-welding, whereupon the metal strip is coiled together with a second continuous strip, in this instance graphite, in such a way as to build up a plurality of single coils. The final coil is made with the metal strip only, and secured in place by welding.
Viewed in a typical section on a radial plane, the gasket appears as a plurality of alternating metal and graphite layers arranged one alongside another in the radial direction. Each such layer exhibits a convex profile directed toward the lateral surface
of the gasket, on which the high pressure fluid impinges.
By virtue of the particular shape of its component layers, the gasket undergoes a deformation brought about by the very pressure of the fluid the valve is designed to shut off, which improves its frontal sealing action. In short, the selfsame pressure rise existing in one of the environments isolated by the valve helps to press the gasket against the surfaces with which it creates a seal.
Nonetheless, gaskets of the conventional type in question betray certain drawbacks, a first among which is that they are not "bidirectional", i.e. the seal is effective only as long as the pressure rise takes place on one particular side of a valve isolating two environments; if not, or in effect, if the pressure rise takes place on the opposite side, then leaks will occur. In practice therefore, a butterfly valve fitted with this type of gasket is able to shut off a direct flow of fluid, without leaks, in one direction only.
In addition, gaskets of this conventional design require frequent replacement, principally by reason of the fact that the frontal sealing surfaces are subject to relatively rapid wear.
The object of the present invention is to overcome the aforementioned drawbacks inherent in the prior art, by providing an annular gasket with a frontal sealing action which is of simple design, durable, and bidirectional.
A further object of the present invention is to set forth a simple and inexpensive method of fashioning such a gasket. The stated objects and others besides are realized in a method and an annular gasket as disclosed and as characterized by the appended claims: the method comprises the step of forming a continuous strip of sheet steel in such a manner as to create at least one first longitudinal portion affording a convex profile presented by one side of the strip and at least one second longitudinal portion which affords a convex profile presented by the opposite side; viewed in a typical section on a radial plane, the annular gasket appears substantially as a plurality of alternating metal and graphite layers arranged one alongside another in the radial direction, and shaped in such a way that each will present convex profiles facing in a radial direction both toward the hollow centre and toward the periphery of the gasket.
A preferred embodiment of the invention will now be described in detail, by way purely of example, with the aid of the accompanying drawings, in which:
- fig 1 illustrates a portion of the annular gasket according to the invention, viewed in perspective and in section on a radial plane;
- fig 2 is a further perspective illustrating a length of strip obtained by the forming step of the method according to the invention. A possible version of the method disclosed might comprise the following sequence of steps: a) preparing a strip or ribbon of sheet steel of predetermined width, typically 5.5 mm; b) forming the strip in such a way as to obtain a first longitudinal portion, aligned on the median axis of the strip, and two second longitudinal portions disposed on either side of and joined to the first, the two second longitudinal portions appearing as grooves with convex faces occupying one side of the strip, and the first longitudinal portion appearing likewise as a groove of which the convex face occupies the opposite side of the strip (fig 2 of the accompanying drawings shows a length of strip, denoted 2 in its entirety, produced by the forming step) ;
c) looping one end of the formed strip 2 around a cylindrical support of diameter identical to the internal diameter of the gasket to be fashioned; this step consists in coiling the strip around the support initially through a limited number of turns (typically 2.75), having secured the first complete single coil by spot-welding; d) coiling the strip further, together with a strip of graphite or other soft material of width greater than that of the metal βtrip prior to being formed, in such a way that the two edges of the graphite strip project laterally beyond the metal strip on either side, the effect of the coiling action being ultimately to impress the graphite strip likewise with portions of convex/concave profile matched to the corresponding portions of the metal strip; in short, the softer graphite strip is caused by the harder metal strip to undergo a plastic deformation during the coiling operation; e) coiling the metal strip still further through a predetermined number of turns (typically 3.25), without the graphite strip, and securing the final turn by spot-welding; f) pressing the projecting portions of the coiled graphite strip in such a way as to create two flat
frontal surfaces, respectively on opposite sides, which are destined to perform a sealing action; as a result of the pressing operation, the graphite material acquires specific physical and mechanical properties, including greater density and therefore surface hardness.
Referring to the drawings, 1 denotes an annular gasket with a frontal sealing action, fashioned by the method as described above, which comprises a plurality of annular layers 8 of thin strip metal, one overlaying another in the radial direction, alternated with layers 9 of graphite or other soft material. The individual layers 8 and 9 are shaped in such a way as to exhibit a profile composed of a central first portion 3 and two lateral second portions 4, all appearing essentially as grooves of which the convex faces are directed respectively toward the periphery and toward the centre of the gasket 1. The gasket 1 is faced frontally on either side by two respective layers 7 consisting of the graphite material, in this instance pressed and therefore of higher density than that occupying the core, each of which defines a flat frontal sealing surface 6. The gasket further comprises a predetermined number
of internal and external layers of the metal strip which are coiled without any interfacing layers of graphite. The first or innermost and the last or outermost coil are secured to the relative adjacent coil by spot-welds.
The two frontal surfaces 6 afford a sealing action intended to isolate two environments converging on the internal and external lateral surfaces of the gasket . In the event that a higher pressure in one environment should i pijige on the internal lateral surface of the gasket, the second portions 4 will be stressed by the resulting force and the gasket 1 compressed against the frontal sealing surfaces. If the external lateral surface is that exposed to the higher pressure environment, then it will be the first portion 3 which deforms and causes the gasket to bear against the sealing surfaces in the same manner. By incorporating the layers 7 of pressed graphite, the gasket 1 is rendered especially resistant to wear.