The present subject matter relates generally to seals and in particular to flexible seals and a method of making flexible seals.
Seals used in joining parts, for example, in rockets, must be flexible but at the same time have sufficient mechanical strength to bear the stresses placed on the seal. The seals are generally made using a rigid component for reinforcement and a flexible component, with alternate layers of the two components attached to each other to form the seal. However, use of rigid components increases the weight of the seal even though it provides mechanical strength, which is a disadvantage in applications such as rockets where weight of parts is a critical factor.
The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and components where possible.
The present subject matter relates to flexible seals and a method of making flexible seals. Seals used in several applications, such as in rockets, need to be flexible and at the same time mechanically robust. Mechanical strength may be obtained by including rigid components in the seal. However, adding rigid components increases the weight of the seal, which becomes a concern in rockets, for example.
Typically, flexible seals employed in a thrust nozzle for rockets have laminates of alternate layers of rigid and elastomer material between the end rings and are sufficiently thick to contain enough elastomer to permit the required degree of angular displacement. To increase the load bearing capacity of the thick elastomer, it is divided into several thin layers, requiring an almost equal number of rigid shims. Since each shim conforms to the surface of an individually associated sphere having its own unique radius, therefore each shim is prefabricated from an individually identified mold. Thus, several molds are required for realizing all the shims of a particular flexible seal. In addition, thin reinforcement shims inherently lack the desired rigidity, dimensional stability, and are also susceptible to distortion due to the release of internal stresses after molding. Thick layers of elastomer and shims tend to have high shear stress in the flexible seal components.
Moreover, cured reinforced plastic shims have low lamination strength. In rocket applications, during the vectoring of a rocket nozzle, there is a possibility of the flexible seal being subjected to axial pull, which causes the layers of the cured reinforced plastic shim to be peeled, resulting in delamination. Furthermore, seals also need to be protected from thermal environments.
The weight of the seal is also a concern and has been reduced in different ways previously. For example, conventionally, some rigid shims used as reinforcement were replaced by flexible reinforcements or the rigid shims were eliminated. A significant reduction in seal weight was achieved by using a pliable fabric strip for the reinforcement, thus reducing the reinforcement structural requirement of the seal such that compressive circumferential loads are carried by the supporting structure. However, the usability of flexible reinforcing shims is limited depending on loading conditions and stiffness limitations. Generally, it is desired that the shims possess a certain degree of rigidity and be strong enough to be assembled to a thrust nozzle.
Some conventional processes have tried to overcome the above limitations of flexible seals using layering; however, the process needs to be done manually, requires skill, is laborious, and is time consuming. Furthermore, during layup in the green condition (i.e., prior to curing), while being installed in the mold for curing, the layers may slide, giving rise to folds in the cured component.
Reinforced plastic shims were also realized in conventional methods, where multiple quarter circles were cut or stamped from a pre-impregnated fabric sheet. During layup of such quarter circle plies in the female mold, a large number of joints occur in a layer. Also, the molding process is laborious, and the shims realized were defective and weak compared to those realized by continuous ply layup.
In another conventional method of making a flexible seal, each layer of reinforcement deposited on a layer of elastomer is formed by winding a resin pre-impregnated thread directly onto the underlying layer of elastomer. While the method has led to reduced operating and manufacturing costs, there is, however a need for reinforcing along the thickness, such that vectoring loads might not induce delamination within the thickness of the shim.
Thus, there is a need for realizing a reinforced plastic shim by a simple process, having sufficient thickness to be dimensionally stable and capable of overcoming delaminating and shear forces that are encountered during operation of the flexible seal.
The present subject matter relates to a method of making flexible seals and flexible seals produced therefrom. A rigid shim is obtained, wherein the rigid shim comprises a resin impregnated and cured fabric, the fabric having a three-dimensional structure comprising fibers disposed in three-dimensions. The method further includes stacking the rigid shims with intermediate elastomer layers to form alternate layers of intermediate elastomer and rigid shim; placing the stack of rigid shims and intermediate elastomer layers in a seal mold; and curing the intermediate elastomer layers to form the flexible seal with alternate rigid shims and elastormer layers. The flexible seal thus formed comprises a cured stack of layers of rigid shims alternating with elastomer layers, wherein the rigid shims comprise a cured resin-impregnated three-dimensional fabric having fibers disposed in three dimensions.
In one example, the rigid shim is prepared by impregnating a preform of the three-dimensional fabric with a resin and then curing it in a mold. In another example, the rigid shim is prepared by cutting a cloth impregnated with resin into strips and laying each of the strips around an inner circumference of a female mold in a layer by layer manner to form layered strips. The layered strips may be fastened together through the thickness of the layered strips to form a bundle of strips. The bundle of strips thus forms a fabric having three-dimensional structure. The female mold comprising the bundle of strips and a corresponding male mold may be placed in a mold base. The bundle of strips may be then cured to obtain the rigid shim.
The method of the present subject matter makes the process simpler than conventional processes and reduces the number of shims by using thick shims and elastomer layers, thereby reducing the number of shim molds to be used and thus, significantly reducing the manufacturing time and cost. The shims realized after demolding are rigid, dimensionally stable, and strong. The shims are not susceptible to distortion, warpage, and buckling. Fastening the layers of the strips together, for example by stitching, enables the cured shim to possess higher inter-laminar shear strength and makes it more tolerant to damage. A fiber roving passing across the thickness of the stack when fastening using stitching, binds the strips together to prevent delamination within the shim when the flexible seal is subjected to axial pull during nozzle vectoring. The present method also improves inter-laminar shear strength, whereby chances of delamination within the shim is eliminated when the seal is subjected to pressure and vectoring loads.
Reinforcement shims made according to the present subject matter are thus pliable to enable load transfer to the supporting structure. The shims are thick enough that they are dimensionally stable, strong, and stiff enough to bear the required compressive circumferential loads. An increase in the thickness of the rigid shim reduces the number of shims for a given elastomer thickness required for deflection. Correspondingly, the number of shim molding tools are also reduced. The shims are also free from delamination when the seal is subjected to pressure and vectoring loads. The shims may also extend beyond the resilient layers to form a protective heat and flame barrier, whereby the requirement of a thermal protective boot is eliminated, and weight of the flexible seal may be further reduced. In the absence of a thermal boot, the actuating load is also reduced and requires a small capacity actuator.
Aspects of the present subject matter are further described in conjunction with the appended figures. It should be noted that the description and figures merely illustrate the principles of the present subject matter. It will thus be appreciated that various arrangements that embody the principles of the present subject matter, although not explicitly described or shown herein, can be devised from the description and are included within its scope. Moreover, all statements herein reciting principles, aspects, and implementations of the present subject matter, as well as specific examples thereof, are intended to encompass equivalents thereof
The seal 100 may be prepared by obtaining rigid shims 110, stacking the rigid shims 110 with intermediate elastomer layers to form alternate layers of intermediate elastomer layers and rigid shims 110, placing the stack of rigid shims 110 and intermediate elastomer layers in a seal mold, and curing the intermediate elastomer layers to form the flexible seal 100. The flexible seal 100 thus formed comprises a cured stack of layers of rigid shims 110 alternating with elastomer layers 120, wherein the rigid shims 110 comprise a cured resin-impregnated three-dimensional fabric having fibers disposed in three dimensions.
The rigid shims 110 may have a surface corresponding to that of concentric spheres and similarly, the surface of elastomer layers 120 may correspond to concentric spheres. Thus, when the rigid shims 110 and elastomer layers 120 are stacked in alternate layers, the stack may form a part of a sphere.
The rigid shims 110 comprise a resin impregnated and cured fabric, the fabric having a three-dimensional structure comprising fibers disposed in three-dimensions. The rigid shims 110 may be prepared by various methods, for example, starting from a resin impregnated cloth from which a three-dimensional resin impregnated fabric structure is formed or starting from a preform having the three-dimensional fabric structure which is subsequently impregnated with a resin.
Once the desired number of rigid shims 110 is prepared, the rigid shims 110 and intermediate elastomer layers may be stacked alternately. The fore end ring 140 and aft end ring 130 may be grit-blasted and placed on two ends of the alternate layers of the intermediate elastomer layer and rigid shim 110. In an example, the rigid shims 110 may be buffed on the top and bottom surfaces, wiped with solvent, and coated with an adhesive. The adhesive may be, for example, Chemlok 205, Chemlok 220, or other such adhesive.
In another embodiment, the rigid shim 110 may be fabricated using a 3D preform.
The flexible seal 100 prepared using the methods described above comprises a cured stack of layers of rigid shims 110 alternating with elastomer layers 120. The rigid shim 110 comprises three-dimensional fabric impregnated with resin and cured. The three-dimensional fabric may comprise stitches running through the thickness direction and provide additional strength to the flexible seal 100 or may be woven or prepared as a three-dimensional fabric with high structural integrity.
Although the present subject matter is described in language specific to structural features, it is to be understood that the specific features and methods are disclosed as example embodiments for implementing the claimed subject matter.
In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.
Number | Date | Country | Kind |
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202011047359 | Oct 2020 | IN | national |
Filing Document | Filing Date | Country | Kind |
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PCT/IN2021/051011 | 10/25/2021 | WO |