Abstract:
Systems and methods associated with a fluid-activated shaft seal are provided. The system includes a housing and a seal disposed relative to the housing. The method reduces flow of a fluid between the shaft and the housing. The method involves installing the housing in an opening relative to the shaft and locating the seal relative to the housing. The seal includes a gel material that expands when exposed to a fluid. The seal reduces flow of the fluid through a gap between the housing and the shaft. Advantages of the system and method include reduced wear to the seal due to friction because the seal does not engage the shaft in the absence of a fluid. The seal engages the shaft in the presence of a fluid. Sealing quality is improved by increasing the pressure differential acting on the gel material. In some embodiments, the seal includes a lip portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. Application Ser. No. 60/629,911, which was filed on Nov. 22, 2004, and the contents of which are hereby incorporated herein by reference. 
     
    
     GOVERNMENT SUPPORT  
       [0002]     Portions of the concepts described herein were made or developed with government support under Contract Number N65538-04-M-0157. The federal government may have certain rights in such concepts. 
     
    
     TECHNICAL FIELD  
       [0003]     The invention generally relates to seals, and, in particular, to fluid-activated shaft seals incorporating a gel material.  
       BACKGROUND  
       [0004]     Many industrial applications require a shaft to pass from one area of a structure through a wall to another area of the structure. The shaft generally passes through an opening in the wall and moves relative to the position of the wall. For example, the shaft can translate, rotate, or move in some combination of translation and rotation (e.g., twisting, bending, or stretching) about an axis of the shaft through the surface. A spatial clearance generally exists between the shaft and the opening in the wall to facilitate this movement.  
         [0005]     Some applications that involve moving shafts require fluid isolation between the separate areas of the structure so that a leak or contamination in one area of the structure does not migrate or flow to an adjacent area of the structure via the clearance between the shaft and the opening in the wall. For example, a propulsion shaft that extends along the length of the hull of a ship passes through several bulkheads that separate different compartments of the ship. Preventing a water leak in one compartment from advancing past a bulkhead into the next compartment along the shaft is critical in preventing the hull from filling with water and sinking the ship. Shaft seals are typically used to limit the flow of fluids from one bulkhead to the next during, for example, rotation of the shaft relative to the shaft opening in the ship structure.  
         [0006]     An example of a conventional shaft seal designed to rotate as the shaft rotates during shaft operation is the ND-type shaft seal sold by Wartsila-Lips, Inc. of Poulsbo, Wash. The ND-type seal includes an o-ring positioned at a rubber molding-shaft interface, such that both the molding and the o-ring spin as the shaft spins. A pressure differential across the o-ring generated by an unequal amount of pressure on either side of a bulkhead causes the rubber molding to deform and press against the housing, which is positioned about the opening. Rotation of the molding is hindered when the molding presses against the housing. The stationary molding creates a seal against the housing and forces the o-ring into contact with the shaft. The o-ring also stops spinning and creates a fluid seal with respect to the shaft.  
         [0007]     One drawback of the ND-type shaft seal is that contact between the sealing components and the shaft while the sealing components are dry leads to premature failure of the sealing components caused by the associated friction-induced wear on the o-ring and the rubber molding. Current shipbuilding specifications generally require a pressure differential between the opposing surfaces of the sealing components of about ⅓ psig (pounds per square inch gauge or about 234.1 kg/m 2 ), or roughly 8-9 inches (about 203-229 mm) of water before the seal engages the shaft. The presence of water assists in creating a water-tight interface between the shaft and the sealing components (e.g., the molding and the o-ring). The ND-type seals have activated (e.g., contacted the shaft) in the presence of as little as 0.6-0.9 inches (about 15.2-22.9 mm) of water. The sealing components contact the shaft while the sealing components are relatively dry and lead to premature wear. Additionally, a pressure differential insufficient to generate a fluid-tight seal develops between opposing sealing components when the sealing components are activated in the presence of relatively small quantities of water. When a fluid-tight seal does not develop, leakage rates associated with the shaft and sealing components can be in excess of shipbuilder specifications.  
         [0008]     For example, shipbuilder specifications generally require self-activating bulkhead shaft seals with a maximum leakage rate of 0.5 U.S. pint/hour (about 0.065 ml/s). Self-activating shaft seals generally do not require human operation (e.g., adjustment of the seals) after installation with respect to a shaft and during a leak. For the DDG-type destroyer, the maximum leakage rate permitted under the shipbuilder specifications is 1 U.S. pint/minute (about 0.1314 ml/s). Activation of the seal in the presence of a relatively low pressure differential (e.g., premature activation caused by relatively low water levels discussed above) causes accelerated wear of the seal components. Accelerated wear of a seal leads to premature failure of the seal and noncompliance with shipbuilder specifications.  
         [0009]     Hence, there is a need for self-activating shaft seals that do not prematurely activate in the presence of relatively small amounts of fluid. There also is a need for shaft seals designed to resist premature wear. There also is a need for shaft seals whose design can be scaled to effectively seal shafts of a variety of diameters. For example, there is a need for shaft seal designs for relatively small diameter shafts and relatively large diameter shafts, both of which can be found on destroyers or various industrial applications.  
       SUMMARY  
       [0010]     The concepts described herein address these and other issues associated with current shaft seal designs by associating the seal with a gel material that expands when exposed to a fluid. In one embodiment, seal designs employing such materials generally are not in contact with the moving shaft in absence of a fluid. Separation between the seal and the shaft prolongs the life of the seal by reducing the effects of friction on the seal in the absence of a fluid. When the gel material expands, the seal is deformed and urged into contact with the shaft. Various features and advantages associated with such seals are described below.  
         [0011]     The invention, in one aspect, features a system that includes a housing and a seal disposed relative to the housing. The seal includes a gel material that expands when exposed to a fluid for reducing a flow of the fluid through a gap or clearance between the housing and a shaft. The seal includes a lip portion and expansion of the gel material urges the lip portion into contact with the shaft.  
         [0012]     In some embodiments, the seal reduces the flow area associated with the gap between the housing and the shaft. In some embodiments, the seal is urged into contact with the shaft by the expansion of the gel material and substantially eliminates the flow area (e.g., by closing the gap). In some embodiments, the gel material is adapted to contract in the absence of a fluid for at least one of forming or increasing the gap between the seal and the shaft. In one embodiment, after fluid is removed from the compartment, the gel material dries out and contracts to permit the seal to deform back to the original shape the seal occupied before exposure to the fluid. In some embodiments, expansion of the gel material is a reversible process or effect. In some embodiments, the shaft passes through a bulkhead (e.g., a propeller shaft that passes through the hull of a vessel). In some embodiments, the gel material is formed by embedding gel particles into an open cell foam. In some embodiments, the open cell foam is a hard open cell foam or a soft open cell foam. In some embodiments, the open cell foam is shaped to fit within a pocket of the seal.  
         [0013]     In some embodiments, the seal includes a lip seal. The seal or the lip seal can include an elastomeric material, a natural rubber material, or a synthetic rubber material. In some embodiments, the seal includes a low-wear material to reduce friction between the seal and the shaft. The low-wear material can be a Teflon® material, for example, molded, bonded, coated, or otherwise secured or applied to the seal. The low-wear material reduces friction between the shaft and the seal that may occur during long-term contact between the shaft and the seal. In some embodiments, the system includes an alignment ring adapted to align the seal relative to the housing, the shaft, or both. In some embodiments, the alignment ring includes a contact surface with respect to the shaft, and a low-wear material is used to reduce friction between the alignment ring and the shaft. In some embodiments, the low-wear material comprises a Teflon® material. In some embodiments, the low-wear material is secured with respect to the seal by at least one of bonding, seating, fitting, coupling, or any combination of these. In some embodiments, the seal captures the low-wear material, for example, through a groove, a pocket, interference fit (e.g., a diametral interference fit), or a friction fit. The alignment ring can be formed from a low-wear or a low-friction material, for example, a Teflon® material.  
         [0014]     In some embodiments, the system includes a second seal that comprises a second gel material that expands when exposed to the fluid and reduces the flow of the fluid through the gap between the housing and the shaft. In some embodiments, the seal reduces the flow of the fluid through the gap between the housing and the shaft along a first direction and the second seal reduces a second flow between the housing and the shaft along a second direction. In some embodiments, the second direction is substantially opposite the first direction.  
         [0015]     In some embodiments, the seal is a backup seal. The seal can reduce the flow of the fluid between the housing and the shaft during motion of the shaft (e.g., rotation, translation, or any combination of these) relative to the housing. In some embodiments, a pressure differential is maintained across the seal when the seal is exposed to the fluid. The pressure differential can improve the performance of the seal, for example, by interacting with the gel material and causing the gel material to expand.  
         [0016]     The invention, in another aspect, features a method for reducing a flow of a fluid between a shaft and a housing. The method involves installing the housing in an opening relative to a shaft. The method also involves locating a seal relative to the housing. The seal comprises a gel material that expands when exposed to the fluid and reduces a flow of the fluid through a gap between the housing and the shaft.  
         [0017]     In some embodiments, the method involves installing at least one alignment ring adapted to align the seal relative to the housing, the shaft, or both. In some embodiments, the method involves coupling an alignment ring to the seal for aligning the housing relative to the seal. In some embodiments, the method involves installing a second seal comprising a second gel material that expands when exposed to the fluid reduces a second flow of the fluid through the gap between the housing and the shaft. In some embodiments, the method involves installing a second alignment ring adapted to align the seal relative to the housing, the shaft, or both. In some embodiments, the method involves securing a low-wear material with respect to the seal, and securing includes at least of bonding, seating, fitting, coupling, or any combination of these. In some embodiments, securing includes the seal capturing the low-wear material, for example, through a groove, a pocket, interference fit (e.g., a diametral interference fit), or a friction fit.  
         [0018]     The invention, in another aspect, features a system that includes a housing disposed relative to a shaft, and a means for reducing a gap between the housing and the shaft in response to exposure to a fluid. The means for reducing the gap includes a fluid-responsive gel material.  
         [0019]     The invention, in another aspect, features a system that includes a housing and a seal disposed relative to the housing. The seal includes a gel material that expands when exposed to a fluid for reducing a flow of the fluid through a gap between the housing and a structure.  
         [0020]     In some embodiments, a portion of the seal is not in contact with the structure in the absence of fluid, for example, to reduce wear associated with friction on components of the system such as the seal. In some embodiments, a portion of the seal contacts the structure when the seal is exposed to the fluid. In some embodiments, the seal includes a lip portion and expansion of the gel material urges the lip portion into contact with the structure.  
         [0021]     In other embodiments of the invention, any of the aspects above can include one or more of the above features. One embodiment of the invention can provide all of the above features and advantages.  
         [0022]     These and other features will be more fully understood by reference to the following description and drawings, which are illustrative and not necessarily to scale. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a cross-sectional view of a portion of a system that reduces a flow of a fluid between a shaft and a housing that embodies the invention.  
         [0024]      FIG. 2  is an exploded perspective view of a system that reduces a flow of a fluid between a shaft and a housing that embodies the invention.  
         [0025]      FIG. 3  is a cross-sectional perspective view of a lip seal including a pocket designed to contain a gel material, according to an illustrative embodiment of the invention.  
         [0026]      FIG. 4  is a cross-sectional view of a system, according to an illustrative embodiment of the invention.  
         [0027]      FIG. 5  is a cross-sectional perspective view of a back-to-back lip seal for use in the system of  FIG. 4 .  
         [0028]      FIG. 6  is a graphical representation of leak rate data for a system that embodies the invention.  
         [0029]      FIG. 7  is a cross-sectional perspective view of a seal according to an illustrative embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0030]      FIG. 1  is a cross-sectional view of a system  100  that reduces a flow of a fluid between a shaft and a housing that embodies the invention. The system  100  is designed for installation and use with a shaft  102  that defines a longitudinal axis A. The system  100  includes a housing  104  coupled to an extensive portion  106  that defines one or more through-holes  108 . The system  100  can be secured or mounted fixedly with respect to the shaft  102 , for example, by passing one or more bolts through the through-holes  108  to corresponding holes (not shown) on a support surface or bulkhead (not shown). In some embodiments, the bolts thread directly into the corresponding holes. In some embodiments, the corresponding holes are instead through-holes, and the system  100  is secured by threading the bolts securely to one or more nuts on the opposite side of the support surface or bulkhead (e.g., in a different area of the structure).  
         [0031]     The system  100  features two connection plates  110   a  and  110   b  disposed within the housing  104 . The connection plates  110   a  and  110   b  define two surfaces  112   a  and  112   b . Surface  112   a  of connection plate  110   a  faces an interior surface  114   b  of the housing  104 . Surface  112   b  of connection plate  110   b  faces an interior surface  114   a  of the housing  104 . The system  100  includes an alignment ring  116 . The alignment ring  116  is designed to fit relatively between the surfaces  112   a  and  112   b  of the connection plates  110   a  and  110   b . The combination of the connection plates  110   a  and  110   b  and the alignment ring  116  are designed to fit relatively loosely between the surfaces  114   a  and  114   b  of the housing  104 . The connection plates  110   a  and  110   b  define annular grooves  117 . The alignment ring  116  defines corresponding annular grooves (not shown) for housing one or more o-rings  118 . In some embodiments, the annular grooves of the alignment ring  116  cooperate with the annular grooves  117  of the connection plates  110   a  and  110   b  to form a substantially continuous annular groove for seating the  0 -rings  118 . In some embodiments, the alignment ring  116  is manufactured from a low-wear or low-friction material, for example, a Teflon® material.  
         [0032]     The combined thickness of the connection plates  110   a  and  110   b  and the alignment ring  116  is slightly less than distance  119   a  of the housing  104  (i.e., the distance along the X-axis between the surfaces  114   a  and  114   b ). A slight clearance  119   b  exists between the housing  104  and the combination of the alignment ring  116  and the connection plates  110   a  and  110   b . The o-rings  118  provide a seal between the combination of the connection plates  110   a  and  110   b  and the alignment ring  116  and the housing  104  to hinder fluid from entering an interior region  114   c  of the housing  104 . Additionally, the  0 -rings  118  allow the connection plates  110   a  and  110   b  and the alignment ring  116  to flex or move relative to the shaft  102  to accommodate, for example, translations of the shaft  102  along the Y-axis without overstressing the housing  104  (e.g., the extensive portion  106 , the through-holes  108 , or the surface to which the housing  104  is secured). In some embodiments, the shaft  102  moves with respect to the housing  104  (e.g., a propeller shaft on a ship). The shaft  102  can move by translation or rotation or some combination of translation and rotation (e.g., twisting) with respect to the housing  104 . In some embodiments, the shaft  102  is stationary with respect to the housing  104 . Examples of stationary shafts include conduits, pipes, electrical cables, and other structures that pass through a surface.  
         [0033]     The system  100  also features two seals  120   a  and  120   b  positioned relative to the shaft  102 . The seals  120   a  and  120   b  are positioned in abutting relation to the connection plates  110   a  and  110   b  and the alignment ring  116 . In some embodiments, the seals  120   a  and  120   b  are coupled to the alignment ring  116 , for example, by bonding the seals  120   a  and  120   b  to the alignment ring  116 . In some embodiments, the seals  120   a  and  120   b  and the alignment ring  116  are a unitary structure formed from the same material. A ring  122  is disposed between the shaft  102  and a bottom face  124  of the alignment ring  116 .  
         [0034]     In some embodiments, the ring  122  is made from a Teflon® material or other low-wear or low-friction material. During assembly and in operation, the ring  122  can be used to align the various components of the system  100 . For example, the ring  122  can align the alignment ring  116  and the connection plates  110   a  and  110   b  with respect to the housing  104  and the shaft  102 . In some embodiments, the ring  122  is not included in the system  100 . In some embodiments, the ring  122  is coupled to the shaft  102  such that the ring  122  moves (e.g., rotates or translates) as the shaft  102  moves. In some embodiments, a gap between the ring  122  and the shaft  102  is sufficiently small to reduce an air or fluid flow through the gap. In some embodiments, the ring  122  or other low-wear material is secured with respect to the seals  120   a  and  120   b , for example, by bonding, seating, fitting, coupling, or some combination of these. The ring  122  or other low-wear material can be captured by the seals  120   a  and  120   b , for example, by a groove (not shown), a pocket (not shown), or other types of fits, such as a diametral interference fit or a friction fit.  
         [0035]     In this embodiment, the ring  122  has a thickness of about 0.1 mm (along the Y-axis), approximately equal to the radial clearance  130  between the lips  126   a  and  126   b  of each of the seals  120   a  and  120   b  relative to the shaft  102 . Some embodiments feature a radial clearance  130  between the lips  126   a  and  126   b  and the shaft  102  that is greater than the thickness of the ring  122 . In some embodiments, the clearance  132  between the shaft-facing surface of the lips  126   a  and  126   b  of the seals  120   a  and  120   b  and the connection plate-facing surface of the seals  120   a  and  120   b  is about 25 mm. In some embodiments, the clearance  132  is approximately 12.7 mm. Alternate geometries and dimensions are contemplated and within the scope of the invention.  
         [0036]     The seals  120   a  and  120   b  each define a pocket  128   a  and  128   b , respectively. The pockets  128   a  and  128   b  are filled with a gel material (also referred to herein as a gel). In some embodiments, the gel material is a hydrogel, an acrylamide gel (e.g., a “smart” gel), or other gels that expand in the presence of a fluid. In some embodiments, the seals  120   a  and  120   b  comprise a pliable or bendable material, for example, an elastomeric material, a natural rubber material, or a synthetic rubber material.  
         [0037]     In some embodiments, the seal comprises a material having a hardness of about Shore A60. In general, the gel material expands when exposed to a fluid and reacts against the interior surfaces of the pockets  128   a  and  128   b . Because the seals  120   a  and  120   b  are pliable, as the gel material expands, the lips  126   a  and  126   b  of the seals  120   a  and  120   b  are urged into contact with the shaft  102  to form a sealing arrangement. In this manner, the lips  126   a  and  126   b  reduce or eliminate the flow area associated with the clearance  130  between the lips  126   a  and  126   b  and the shaft  102 . In some embodiments, the seals  120   a  and  120   b  include a low-wear material, for example a Teflon® material to reduce friction between the seals  120   a  and  120   b  and the shaft  102 . For example, the low-wear material can be secured or applied (e.g., molded, bonded, or surface coated) to the lips  126   a  and  126   b.    
         [0038]     Tests have shown that in one embodiment, the gel material can produce more than about 37 psi (about 26,010 kg/m 2 ) of pressure against the lips  126   a  and  126   b  in forcing the lips  126   a  and  126   b  into contact with the shaft  102 .  
         [0039]     In this embodiment, the system  100  defines a first compartment I and a second compartment II that are generally not in fluid communication with each other. Compartment I is separated from compartment II by, for example, a surface (e.g., a bulkhead of a ship) coupled to the housing  104 . Exemplary operation of the system  100  occurs when a fluid leak occurs in a first compartment I. A pressure differential develops between the first compartment I and a second compartment II. The fluid tends to migrate from the first compartment I to the second compartment II via the clearance  130  between the seals  120   a  and  120   b  and the shaft  102 .  
         [0040]     In some embodiments, the fluid contacts the gel material disposed in the pocket  128   b  (e.g., by splashing or inundation), causing the gel material to expand and react against the inside surface of the pocket  128   b  to urge the lip  126   b  into contact with the shaft  102 . In some embodiments, the gel material expands and urges the lip  126   b  into contact with the shaft  102  within several seconds after exposure of the gel material to the fluid. The system  100  can be used with a variety of naturally-occurring or synthetic fluids, for example, freshwater, salt water, acids, alkaline fluids, blood or other biological fluids, and oils. In general, the type of fluid that is present influences the choice of gel material used in the system  100 . Gel materials can respond (e.g., expand and contract) based, in part, on properties of the fluid.  
         [0041]     Generally, the seal  120   b  is not activated (i.e., the lip  126   b  does not move towards the shaft  102 ) until fluid is present because the gel material does not expand in the absence of a fluid. When the lip  126   b  is in contact with the shaft  102 , the clearance  130  or gap between the lip  126   b  and the shaft  102  is reduced, which reduces the flow area through which the migrating fluid passes. The pressure differential between the first compartment I and the second compartment II is increased as the lip  126   b  approaches or comes into contact with the shaft  102 . In some embodiments, the ability of the lip  126   b  to reduce the flow through the clearance  130  increases as the pressure differential between the first compartment I and the second compartment II increases. The o-rings  118  hinder fluid from migrating from the first compartment I to the second compartment II through the housing  104  by sealing the combination of the alignment ring  116  and the connection plates  110   a  and  110   b  with respect to the inside surfaces  114   a  and  114   b  of the housing  104 .  
         [0042]     In some embodiments, the pressure differential that occurs when the lip  126   b  engages the shaft  102  also operates on the gel material disposed in the pocket  128   a  of the seal  120   a  that faces the second compartment II. The pressure differential causes the gel material in the pocket  128   a  to expand, and force the lip  126   a  to also engage the shaft  102 . In some embodiments, the lip  126   a  of the seal  120   a  forms a backup seal to hinder fluid from entering the second compartment II if the seal  120   b  (e.g., the lip  126   b ) facing the first compartment I fails (e.g., due to wear). In some embodiments, some fluid migrates from the first compartment I to the second compartment II and contacts or wets the gel material disposed in the pocket  128   a . When the gel material in the pocket  128   a  is exposed to the fluid, the lip  126   a  of the seal  120   a  moves towards and/or into contact with the shaft  102 . In this manner, fluid in the second compartment II is hindered from migrating back to the first compartment I by the operation of the seal  120   a.    
         [0043]     After the seals  120   a  and  120   b  have been activated by exposure to a fluid, the lips  126   a  and  126   b  remain in contact with the shaft  102  until the fluid is removed and the seals  120   a  and  120   b  are dried (e.g., by the application of heat). In some embodiments, the seals  120   a  and  120   b  are dried by exposure to air. The gel material contracts in the absence of a fluid, for example, when the gel material is dried. As the gel material contracts, the seals  120   a  and  120   b  deform back to the original shape that the seals  120   a  and  120   b  occupied prior to exposure to the fluid (e.g., prior to the leak). The system  100  can be used for multiple leaks without being replaced because expansion of the gel material is a reversible effect (e.g., by contraction of the gel material).  
         [0044]     Generally, a gel is a colloid material in which dispersed particles couple to the dispersion medium to form a semi-solid material. Gels are sometimes classified according to the dispersion medium associated with the gel. For example, the dispersion medium for a hydrogel is water, and the dispersion medium for an acrylamide gel is acetone.  
         [0045]     Gel materials used in some embodiments of the invention are generated by immersing polymer strands (e.g., gel particles) into a solution (e.g., water or acetone). In some embodiments, gel particles are suspended in the dispersion medium. In some embodiments, a gel changes volume (e.g., expands or contracts) in response to the environment to which the gel is exposed. The change in volume of the gel material is directly proportional to a property of the gel material known as Osmotic pressure. Osmotic pressure depends on a variety of factors, for example, temperature, ionization of the dispersion medium, concentration of the solution (e.g., of acetone), or the external forces (e.g., pressure) acting on the gel material. Generally, the gel seeks to occupy the state with lowest thermodynamic energy (e.g., lowest Osmotic pressure). In the absence of external forces (e.g., Osmotic pressure of 0 N/m 2 ), the gel collapses unto itself. For example, some acrylamide gels (also called “smart gels”) can reduce in volume by a factor of about 1,000 when exposed to appropriate external conditions sufficient to cause a phase change in the smart gel.  
         [0046]     In general, environmental changes affect a gel material by causing the gel to undergo a reversible volumetric change. Such environmental changes include changes in temperature, pH of the fluid, ionic strength of the fluid, light, and electromagnetic fields in the presence of the gel. The environmental changes generally cause the volume of the gel material to expand or contract in response to changes in the environment. The range of environmental values over which the gel material experiences volumetric change can be referred to as the environmental volume phase transition region of the gel and varies depending on the type of gel material. By way of example, a particular gel may contract as the temperature of the gel or a fluid in contact with the gel increases. Similarly, the gel may expand as the temperature of the gel or fluid in contact with the gel decreases.  
         [0047]     In some embodiments, a gel material is selected for use in the system  100  based on the gel&#39;s ability to undergo a “discontinuous” volume change. Discontinuous volume changes involve a reversible transition by the gel material from the expanded to the contracted (or collapsed) state and back again. In some embodiments, the volume change is a substantial volume change in response to a relatively small change in the environmental condition. In some embodiments, temperature changes of less than about 0.1° C. result in a discontinuous volume change. Such gel materials may be referred to as “phase-transition gels.” The environmental condition can be referred to as the “phase transition temperature” because the gel undergoes volumetric change at or about the phase transition temperature. In some embodiments, as the temperature drops to below the phase transition temperature, the gel material expands and urges the lips  126   a  and  126   b  into contact with the shaft  102 . In some embodiments, absence of a fluid or as the temperature rises above the phase transition temperature causes the gel material to contract and permits the lips  126   a  and  126   b  to deform away from the shaft  102 .  
         [0048]     In some embodiments, the gel material (e.g., pellets or packets of gel material) is embedded into a foam material. In some embodiments, the foam material is an open cell foam material. In some embodiments, the open cell foam material is a soft open cell foam material. Examples of suitable open-cell foam materials include urethane foam or low-density polyurethane foam, for example, sold by Rynel, Inc. of Boothbay, Me.  
         [0049]     In some embodiments, less gel material is used when a foam material is used because the foam material occupies space within the pockets  128   a  and  128   b . The foam material is used as a containment vehicle for the gel material (e.g., individual gel particles or polymer strands) and can prevent migration of the gel material. In some embodiments, the gel material within the foam expands when exposed to a fluid. The expansion of the gel causes the foam to expand. The expanding foam reacts against the pockets  128   a  and  128   b  of the seals  120   a  and  120   b  to deform the seals  120   a  and  120   b  and urge the lips  126   a  and  126   b  into contact with shaft  102 .  
         [0050]     In some embodiments, the gel material is disposed within a liquid permeable sleeve or tube (not shown). The sleeve can be placed within the pockets  128   a  and  128   b  of the seals  120   a  and  120   b . In some embodiments, the sleeve is made from a mesh material that permits water to permeate in and out of the sleeve but prevents the gel material from permeating out of the sleeve. A sleeve containing gel material can be made and used in seals having a variety of geometries because the sleeve and gel material can assume the shape of the seal or pockets of the seal. In some embodiments, a greater amount of gel material can be contained in a sleeve than could otherwise be embedded in a foam material.  
         [0051]      FIG. 2  is an exploded perspective view of a system  200  that reduces a flow of a fluid between a shaft  102  and a housing. The system  200  includes two housing portions  202   a  and  202   b  that cooperate to form a single housing (collectively  202 ) disposed circumferentially about the shaft  102 . The shaft  102  defines a longitudinal axis A. Each of the housing portions  202   a  and  202   b  define a plurality of through-holes  108  for securing the housing portions  202   a  and  202   b  to a surface or a bulkhead (not shown) through which the shaft  102  passes. Each of the housing portions  202   a  and  202   b  are a one-half ring-shaped structure each circumscribing approximately one-half of the shaft  102 . Other configurations for the housing portions are possible (e.g., one-quarter-circles or one-third circles) for disposing the housing  202  about the shaft  102 .  
         [0052]     The system  200  includes two alignment rings  206   a  and  206   b  that cooperate to form a single alignment ring disposed within the housing  202  and circumferentially about the shaft  102 . The alignment rings  206   a  and  206   b  are secured with respect to each other by connections  208   a  and  208   b . The connection  208   a  features two connection plates  210   a  and  210   b . An identical set of connection plates are used for the connection  208   b  (hidden in perspective). A plurality of connectors  212  (e.g., bolts, screws, rivets, or fasteners) pass through the first connection plate  210   a , a portion of either or both of the alignment rings  206   a  and  206   b , and the second connection plate  210   b , and are axially secured to form a tight fit among the components. In some embodiments, the alignment rings  206   a  and  206   b  are fitted tightly together to form the alignment ring (e.g., with a friction fit).  
         [0053]     Each of the alignment rings  206   a  and  206   b  and the connection plates  210   a  define annular grooves  117  (e.g., the annular grooves  117  of  FIG. 1 ) for accommodating o-rings  118 . Similarly, each of the alignment rings  206   a  and  206   b  and the connection plates  210   b  define annular grooves  117  (hidden in perspective) for accommodating o-rings  118  (hidden in perspective) on the opposite face along the axis A of the alignment rings  206   a  and  206   b  and connection plates  210   b . The o-rings  118  are circumferentially continuous about the shaft  102  within the annular groove  117 . In some embodiments, the o-rings  118  are not circumferentially continuous (e.g., they are split  0 -rings) to enable positioning about the shaft  102  without sliding the o-rings  118  over the length of the shaft  102  (e.g., to a location mid-shaft).  
         [0054]     The system  200  also includes a ring  122  and seals  120   a  and seal  120   b  disposed circumferentially about the shaft  102 . For clarity of illustration purposes, the seal  120   a  is not shown. The ring  122  and the seal  120   b  are circumferentially continuous about the shaft  102 . In some embodiments, neither the alignment ring  122  nor the seal  120   b  is circumferentially continuous to enable positioning about the shaft  102  mid-shaft similar to the split o-ring.  
         [0055]      FIG. 3  is a cross-sectional perspective view of a lip seal  300  including a pocket  308  designed to contain a gel material that embodies the invention. The seal  300  is illustrated as a half circle for disposing about a shaft (not shown) along the longitudinal axis A (e.g., the longitudinal axis A of the shaft  102  of  FIGS. 1 and 2 . In some embodiments, the seal  300  is formed of an elastomer or a rubber material. In some embodiments, the seal  300  is a unitary structure and forms a substantially continuous ring for disposing about the shaft. In some embodiments, the seal  300  may be formed of one or more components that cooperate to form a substantially continuous ring.  
         [0056]     The seal  300  includes a first surface  304  for disposing adjacent the shaft and a second surface  306  for reacting against an external component (e.g., the alignment ring  116 , the connection plate  110 , the housing  104 , or a combination of these components of  FIG. 1 ). The seal  300  defines a pocket  308  designed to house or contain a gel material (not shown) that expands in the presence of a fluid. When a fluid is present, the gel material expands and reacts against the interior of the pocket  308 .  
         [0057]     In some embodiments, the seal  300  is formed of a material sufficiently rigid to prevent the first surface  304  from engaging the shaft in the absence of a fluid. The material is sufficiently pliable to permit expansion of the gel material to deform the seal  300  in the presence of a fluid such that the first surface  304  engages the shaft. For example, the seal may be formed of an elastomer material or a rubber material (e.g., natural or synthetic rubber). Because the second surface  306  reacts against the relatively fixed external component (e.g., the housing  202  or alignment rings  206   a  and  206   b  of  FIG. 2 ), the first surface  304  expands substantially radially toward the shaft that is aligned with the axis A.  
         [0058]     In one embodiment, tests have shown that in the absence of fluid, the distance  310   a  is about 12.7 mm, and the distance  310   b  is about 14.6 mm, providing a clearance or gap relative to the shaft of about 1 mm from the seal. When fluid was present, the gel material in the pocket  308  deformed the seal  300 . The distance  310   a  remained approximately the same (i.e., about 12.7 mm). The distance  310   b  expanded to about 16.5 mm, sufficient to overcome the gap of about 1 mm clearance between the seal  300  and the shaft. In this embodiment, the expansion of the distance  310   b  exceeded about 16.5 mm when an external component (e.g., the housing  202  or the alignment rings  206   a  and  206   b  of  FIG. 2 ) was in contact with the second surface  306 . The greater expansion provides a stronger (e.g., reinforced) and more robust sealing interface between the seal  300  and the shaft.  
         [0059]      FIG. 4  is a cross-sectional view of a system  400  that embodies the invention. The system  400  includes a housing  402  disposed relative to an alignment ring  404 . The housing  402  is coupled to an extensive portion  406  having one or more through-holes  408  for securing the system  400  to a wall or surface (e.g., bulkhead) through which a shaft  410  passes along a longitudinal axis A. The housing  402  defines an interior surface  412 . The interior surface  412  can create a seal with respect to the alignment ring  404  by reacting against one or more o-rings  414  disposed with respect to the alignment ring  404 . Annular grooves  416  within the alignment ring  404  accommodate the o-rings  414 .  
         [0060]     The system  400  includes a seal  418  disposed relative to a cavity  420  in a portion  422  of the alignment ring  404  configured to be disposed adjacent the shaft  410 . The seal  418  defines a pocket  424  and an opening  426  to permit a fluid to interact with a gel material (not shown) disposed in the pocket  424 . In some embodiments, the seal  418  is referred to as a “back-to-back” seal because when the gel material expands in the presence of a fluid, the gel material reacts against opposing faces  428  of the pocket  424  to force the bottom portion  430  (e.g., two oppositely-oriented axial lips  432   a  and  432   b ) of the seal  418  radially toward the shaft  410 . The system  400  also includes two rings  434  disposed between the shaft  410  and the system  400 . In general, each of the rings  434  is similar to the ring  122  discussed above with respect to  FIG. 1 .  
         [0061]     In some embodiments, the seal  418  is positioned or secured with respect to the alignment ring  404  (e.g., the cavity  420 ), for example, by a friction fit. In some embodiments, the seal  418  is coupled to the alignment ring  404 , for example, by bonding the seal  418  to the envelope  420  or by forming the seal  418  from the same material as the alignment ring  404 . As fluid from a first compartment I migrates through a gap  436  or radial distance between the alignment ring  404  and the shaft  410  toward the second compartment II, the fluid enters the opening  426  of the seal  418 . Fluid entering the opening  426  interacts with, contacts, or wets the gel material disposed inside the pocket  424 . The gel material expands and forces the lips  432   a  and  432   b  into contact with the shaft  410  to create a fluid-tight seal with respect to the shaft  410 . Expansion of the gel material within the pocket forces the seal  418  to engage the shaft  410 . The system  400  resists premature wear on the seal  418  associated with frictional forces because the seal  418  does not contact or minimally contacts the shaft  410  in the absence of a fluid. In general, the gel material can include one or more of the features described regarding the embodiments discussed herein.  
         [0062]      FIG. 5  is a cross-sectional perspective view of a back-to-back lip seal  500  for use in the system of  FIG. 4  (as, for example, the seal  418  of  FIG. 418 ). In some embodiments, the seal  500  is formed of an elastomer or a rubber material. In some embodiments, the seal  500  is a unitary structure and forms a substantially continuous ring for disposing about the shaft (not shown) defining the longitudinal axis A. In some embodiments, the seal  500  may be formed of one or more components that cooperate to form a substantially continuous ring.  
         [0063]     The seal  500  includes a first portion  504  for disposing adjacent the shaft and a second surface  506  for reacting against an external component (e.g., the alignment ring  404 , the housing  402 , or both of these components of  FIG. 4 ). The first portion  504  includes two oppositely-facing lips  508   a  and  508   b  with an opening  426  disposed between the lips  508   a  and  508   b . The opening  426  permits a fluid that flows along the first portion  504  to contact or wet a gel material (not shown) disposed within a pocket  424  of the seal  500 . The gel material expands in the presence of a fluid and reacts against an interior of the pocket  424 . As the gel material expands, one or both of the lips  508   a  and  508   b  are deformed and moved toward the shaft to establish contact with the shaft to create a fluid-sealing interface. Contact between the lips  508   a  and  508   b  and the shaft reduces a flow of the fluid between the shaft and the seal  500 . The gel material continues to provide expansion forces to reinforce or strengthen the seal  500  with respect to the shaft as the pressure differential increases between compartments of a structure (e.g., between compartment I and compartment II of  FIG. 4 ).  
         [0064]     In some embodiments, the seal  500  is formed of a material sufficiently rigid to prevent the first portion  504  or the lips  508   a  and  508   b  from engaging the shaft in the absence of a fluid. The seal  500  is formed of a material sufficiently pliable to permit deformation of the lips  508   a  and  508   b  by expansion of the gel material in the presence of a fluid to move the lips  508   a  and  508   b  towards the shaft. Because the second surface  506  reacts against the relatively fixed external component, the expansion is substantially toward the shaft. Although the seals  300  and  500  of  FIGS. 3 and 5  respectively are illustrated as semi-circles, seals having other geometries (e.g., rectilinear geometries or other curvilinear geometries or combinations thereof) are within the scope of invention.  
         [0065]      FIG. 6  is a graphical representation of leak rate data for a system that embodies the invention. The data was obtained from observation of the performance of a prototype system. The prototype system was designed as an approximately ⅛ th -scale model of a system that could be used on a propeller shaft of a DDG-type destroyer. The system incorporated the elements of  FIG. 1 . For example, the diameter of the prototype shaft was about 63.5 mm. The diameter of the full-scale shaft on a DDG-type destroyer propeller is about 508 mm. The maximum revolutions per minute (RPM) for the full-scale shaft permitted by shipbuilder regulations is about 168 RPM. The ⅛ th -scale prototype was tested at about 1,344 RPM to account for the difference in shaft diameter. The test setup included a first compartment in an ambient pressure environment and a second compartment in a pressurized environment (e.g., in the presence of water). The pressure drop between the first compartment and the second compartment caused the water to seek to migrate towards the lower-pressure first compartment. The leak rate of water entering the first compartment was measured during the experiment.  
         [0066]     The graph  600  includes a first curve  602  of the leak rate obtained with the prototype system. The first curve  602  corresponds to the leak rate values in U.S. pints/hour of the vertical axis  604  against time (minutes) on the horizontal axis  606 . The graph  600  also includes a second curve  608  mapping the leak rate in U.S. pints/hour of the vertical axis  610  against time (minutes) on the horizontal axis  606 . The curve  608  represents the predicted performance of a system designed for use in the full-scale system for the DDG-type destroyer illustrating expected data based on the first plot  602 . The data represented by the second curve  608  is a linear extrapolation of the data represented by the first curve  602  (scaled by a factor of 8 to compensate for the ⅛ th -scale of the prototype).  
         [0067]     The data associated with plot  602  reflects a seal (e.g., the seals  120   a  and  120   b ) having hardness of about Shore durometer of A60. Seals of other hardness were also successfully tested. Seals having hardness in a range from about a Shore durometer of A20 to about a Shore durometer of A60 also have been used. In some embodiments, the hardness value chosen for a particular seal depends on usage parameters associated with the seal, for example, the amount of time that the seal will be active. During testing, the gel material disposed within the seal (e.g., within a pocket of the seal) responded to the presence of a fluid, in this case water, within a few seconds of exposure to the fluid. The first curve  602  illustrates that the leak rate from the first compartment I to the second compartment II decreases to about 1.5 U.S. pints/hour (about 0.197 ml/s) after about 10 minutes. Further, the leak rate is less than 0.1 U.S. pints/hour (about 0.0131 ml/s) after about one hour.  
         [0068]     The second curve  608  illustrates that the leak rate is expected to be about 9 U.S. pints/hour (about 1.183 ml/s) after about 10 minutes and less than about 0.5 U.S. pints/hour (about 0.065 ml/s) after one hour. Additionally, the leak rate observed under exemplary pressure differentials of about 7.5 psid (pounds per square inch difference) (about 5,273 kg/m 2 ) and 15.0 psid (about 10,550 kg/m 2 ) was lower than that of the first curve  602  or the second curve  608  because increases in differential pressure increase the sealing ability of the seal. In some embodiments, marginally higher leak rates were obtained when the ⅛ th -scale shaft was spinning at about 720 and 1440 RPM (corresponding to about 90 and 180 RPM for the full-scale shaft).  
         [0069]      FIG. 7  is a cross-sectional perspective view of a seal  700  that embodies the invention. The seal  700  includes a lip portion  702  and an upper portion  704 . A pocket  706  is defined by the lip portion  702  and the upper portion  704 . In some embodiments, a gel material (not shown) is disposed within the pocket  706 . The seal  700  includes two connector portions  708   a  and  708   b  (generally  708 ) that extend from the lip portion  702  toward the upper portion  704  along the Y-axis. The upper portion  704  defines corresponding surfaces  710   a  and  710   b  (generally  710 ) configured for mating with the connector portions  708   a  and  708   b . In some embodiments, the seal  700  is made from an elastomer material or a rubber material (e.g., natural or synthetic rubber).  
         [0070]     In some embodiments, the connector portions  708   a  and  708   b  prevent the lip portion  702  from moving away from the upper portion  704  (e.g., along the Y-axis) under, for example, the influence of gravity or from the weight of the gel material on the lip portion  702 . In the presence of a fluid, the gel material within the pocket  708  expands and urges the lip portion  702  away from the upper portion  704 . The connector portions  708   a  and  708   b  react against the corresponding surfaces  710   a  and  710   b  of the upper portion  704  to resist movement of the lip portion  702 . The gel material provides an expansion force sufficient to overcome this resistance and dislodge the connector portions  708   a  and  708   b  from the corresponding surfaces  710   a  and  710   b . After the connector portions  708   a  and  708   b  have been dislodged, the gel material can move the lip portion  702  away from the upper portion  704 . Alternative geometries, quantities, and configurations of the connector portions  708  and the corresponding surfaces  710  are contemplated in alternative embodiments of the invention.  
         [0071]     While the invention has been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.