Abstract:
A pressure vessel is provided for housing electronic components in an underwater environment and permitting connection of the components to signal transmission elements of a signal cable. The pressure vessel comprises a hollow steel shell defining an interior chamber adapted to house the electronic components. A layer of thermal-sprayed aluminum covers the shell. The shell has an opening adapted to pass the signal transmission elements into the interior chamber. A seal adapted to sealingly surround the transmission elements is disposed in the opening in the shell so that an outer peripheral surface of the seal contacts an inner peripheral surface of the opening in the shell for preventing moisture penetration into the interior chamber. The seal may be formed of epoxy and the outer peripheral surface have a plurality of compressible o-rings disposed in axially-spaced circumferential grooves for contacting the inner peripheral surface of the opening in the shell.

Description:
BACKGROUND 
     This invention relates generally to a pressure vessel assembly, and more particularly to a pressure vessel assembly for housing electronic components in an underwater environment. 
     Underwater communication systems provide signal transmission between, or to, land-based positions. A typical system generally comprises a cable for signal transmission and one or more housings containing electrical components which are spaced along the cable between the land-based positions. The signal cable has a core which may include electrical conductors, fiber optic cable, or other signal transmission elements surrounded by a protective jacket. Metal strength members are disposed about the core of the cable between the cable core and the outer surface of the jacket. The strength members bear the tensile and compressive loads placed on the cable while in operation. The cable may carry various signals, including low voltage signals such as information and data signals, higher voltage signals for providing electrical power, or other types of signals. 
     The housings for the electronic components are referred to as pressure vessels. A pressure vessel is typically a cylindrical tube with open ends capped by circular bulkheads. The signal cable is terminated adjacent to each of the bulkheads in a termination assembly. The termination assemblies are secured to the bulkheads and provide mechanical continuity between the cable ends and the pressure vessel while relieving the stress on the signal transmission elements of the cable. The signal transmission elements pass into the pressure vessel through seals in the bulkheads for connection to the electronic components in the pressure vessel. 
     The pressure vessel assembly must protect the electronic components, signal cable and their connections from exposure to water at depths of up to 20,000 feet and pressures of up to 10,000 pounds per square inch for periods of up to 25 years. This harsh environment contributes to problems related to the performance reliability and product life of pressure vessel assemblies. 
     Conventional solutions to the problems of designing reliable and durable pressure vessel assemblies are plagued by high cost. The present practice is to use pressure vessels formed from beryllium-copper or titanium with polyethylene or gland cable seals and polyethylene-overmolded cable termination assemblies. Beryllium-copper or titanium is used for the pressure vessel because of their excellent resistance to corrosion in underwater applications. However, this material is very expensive, and machining is difficult. The polyehtylene or gland cable seals are expensive, consist of numerous parts, and are difficult and time-consuming to install. In addition to the seals, a water block of some sort must be used to prevent water ingress in the event of a cable cut. 
     The termination assemblies are overmolded with polyethylene to prevent water from accessing the internal portions of the signal cable and pressure vessel. In the overmolding process, high density polyethylene is molded around the cable termination assembly thereby sealing the areas between the outer surface of the cable jacket and the termination assembly. In some cases, portions of the cable and the pressure vessel are also overmolded with polyethylene. However, polyethylene overmolding is not cost effective in most applications because the required molding equipment is expensive and the process time consuming thereby restricting production rates. Moreover, the overmolding is a difficult process, requiring a high operator skill level and has yield and reliability problems. 
     For the foregoing reasons, there is a need for a reliable, long life, low cost pressure vessel assembly for housing electrical components in underwater communication systems. The new pressure vessel assembly must be capable of withstanding deep underwater pressures for many years. Seals for the passage of the signal cable transmission elements into the pressure vessel should be easy to install and effectively prevent moisture penetration. The termination assembly sealing process should be fast and simple to perform, requiring minimal operator skill level. Moreover, the components for sealing the cable termination assembly should be adaptable to seal various cable termination assembly types. 
     SUMMARY 
     Therefore, it is an object of the present invention to provide a low cost underwater pressure vessel assembly which is readily and economically produced for use in underwater communications systems. 
     A further object of the present invention is to provide moisture protection to the electronic components in the pressure vessel. A related object is to provide a simple, effective seal for the signal transmission elements passage into the pressure vessel. 
     A still further object of the present invention is to provide a seal and method for readily and simply sealing a cable termination assembly for use in connecting signal cable to the pressure vessel. The termination assembly seal and method should allow for adaptability to various cable types. 
     Another object of the present invention is to provide a pressure vessel assembly which is reliable and durable enough for an extended useful life submerged in the underwater environment. 
     According to the present invention, a pressure vessel is provided for housing electronic components in an underwater environment and permitting connection of the components to signal transmission elements of a signal cable. The pressure vessel comprises a hollow steel shell defining an interior chamber adapted to house the electronic components. A layer of thermal-sprayed aluminum covers the shell. The shell has an opening adapted to pass the signal transmission elements into the interior chamber. A seal adapted to sealingly surround the transmission elements is disposed in the opening in the shell so that an outer peripheral surface of the seal contacts an inner peripheral surface of the opening in the shell for preventing moisture penetration into the interior chamber. The seal may be formed of epoxy and the outer peripheral surface have a plurality of compressible o-rings disposed in axially-spaced circumferential grooves for contacting the inner peripheral surface of the opening in the shell. 
     Also according to the present invention, an underwater telecommunication system is provided comprising electronic components and a hollow steel shell for housing the electronic components. The shell is covered by a layer of thermal-sprayed aluminum. A signal cable is mechanically connected to the shell in watertight relation. The signal cable includes at least one transmission element and the shell has an opening for passing the transmission element into the interior chamber for connecting the transmission element to the electronic components for signal transmission. A seal surrounding the transmission element is disposed in the opening in the shell for preventing moisture penetration into the shell. 
     According to another aspect of the present invention, a cable seal is provided for sealing the passage of a cable into a cable-receiving structure. The seal comprises a first plurality of o-rings disposed along a length of the cable adjacent the cable-receiving structure and a first length of heat-shrinkable tube which fits over the cable and o-rings for compressive engagement with the cable and o-rings when the first tube is heated to prevent moisture penetration between an inner surface of the first tube and the outer surface of the cable. A second plurality of o-rings is disposed over the first heat-shrinkable tube adjacent the first plurality of o-rings. A second length of heat-shrinkable tube fits over the second plurality of o-rings, the first heat-shrinkable tube, the first plurality of o-rings, the cable and a portion of the cable-receiving structure for compressive engagement with the second plurality of o-rings, the first heat-shrinkable tube, the first plurality of o-rings, the cable and the portion of the cable-receiving structure when heated to prevent moisture penetration between an inner surface of the second tube and an outer surface of the cable-receiving structure and between the inner surface of the second tube and the outer surface of the cable. 
     According to a still further aspect of the present invention, a sealed cable end assembly comprises a structure having a through passage for receiving a cable end and means for preventing relative axial movement of the structure and cable. A first plurality of o-rings is disposed along a length of the cable adjacent the cable-receiving structure. A first length of heat-shrinkable tube is positioned around the cable and o-rings for compressive engagement with the cable and o-rings when the first tube is heated to prevent moisture penetration between an inner surface of the first tube and the outer surface of the cable. A second plurality of o-rings is disposed over the first heat-shrinkable tube, the second plurality of o-rings positioned adjacent the first plurality of o-rings. A second length of heat-shrinkable tube fits over the second plurality of o-rings, the first heat-shrinkable tube, the first plurality of o-rings, the cable and a portion of the cable-receiving structure for compressive engagement with the second plurality of o-rings, the first heat-shrinkable tube, the first plurality of o-rings, the cable and the portion of the cable-receiving structure when heated to prevent moisture penetration between an inner surface of the second tube and an outer surface of the cable-receiving structure and between the inner surface of the second tube and the outer surface of the cable. 
     Also according to the present invention, a method is provided for sealing a cable termination including a cable end positioned in an opening of a cable-receiving structure for preventing relative axial movement of the cable end and structure. The sealing method comprises disposing a first plurality of o-rings along a length of the cable adjacent the cable-receiving structure and positioning a first length of heat-shrinkable tube around the cable and o-rings. The first heat-shrinkable tube is heated causing the tube to compressively engage the cable and o-rings to prevent moisture penetration between an inner surface of the first tube and the outer surface of the cable. A second plurality of o-rings is disposed around the first heat-shrinkable tube adjacent the first plurality of o-rings and a second length of heat-shrinkable tube is positioned around the second plurality of o-rings, the first heat-shrinkable tube, the first plurality of o-rings, the cable and a portion of the cable-receiving structure. The second heat-shrinkable tube is then heated causing the tube to compressively engage the second plurality of o-rings, the first heat-shrinkable tube, the first plurality of o-rings, the cable and the portion of the cable-receiving structure to prevent moisture penetration between an inner surface of the second tube and an outer surface of the portion of the structure and between a portion of the inner surface of the second tube and the outer surface of the cable. 
     The pressure vessel assembly of the present invention provides a low cost, structurally robust, reliable, and durable device for use in underwater communication systems. The thermal-sprayed aluminum coating of the steel pressure vessel provides a housing for the electronic components of the system which is resistant to corrosion, particularly galvanic corrosion in seawater. The two-layers of heat-shrinkable tubes over sets of o-rings function as multiple redundant seals for the cable termination assembly. The components of the pressure vessel assembly of the present invention are low cost and require minimal assembly expertise and time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference should now be had to the embodiments shown in the accompanying drawings and described below. In the drawings: 
     FIG. 1 is a perspective view of a pressure vessel assembly according to the present invention showing a portion of the cables extending from each end; 
     FIG. 2 is an exploded view of an electronics pressure vessel assembly as shown in FIG. 1; 
     FIG. 3 is a close-up exploded view of a bulkhead assembly for use with the electronics pressure vessel assembly as shown in FIG. 2; 
     FIG. 4 is a close up view of a cable seal for use in the present invention; 
     FIG. 5 is a partial cross-section of an electronics pressure vessel assembly taken along line  5 — 5  of FIG. 2; 
     FIG. 6 is a perspective view of a first plurality of o-rings in place along a cable leading into a cable termination assembly; 
     FIG. 7 shows a first heat-shrink tube in place around the first set of o-rings shown in FIG. 6; 
     FIG. 8 shows a second set of o-rings in place around the shrunken first heat shrink tube around a cable leading into a termination assembly as shown in FIG. 7; 
     FIG.  9 . shows a second heat-shrink tube around the second set of o-rings shown in FIG. 8; 
     FIG. 10 shows the shrunken second heat-shrink tube around a cable leading into a termination assembly as shown in FIG. 9; 
     FIG. 11 shows a third heat-shrink tube positioned around a cable termination housing as shown in FIG. 10; 
     FIG. 12 shows the shrunken third heat-shrink tube around the cable termination housing as shown in FIG. 11; 
     FIG. 13 is a side elevational view partially cut-away of the sealed cable termination assembly shown in FIG. 12; and 
     FIG. 14 is side elevational view partially cut-away of another embodiment of a sealed cable and cable termination assembly. 
    
    
     DESCRIPTION 
     Certain terminology is used herein for convenience only and is not to be taken as a limitation on the invention. For example, words such as “upper”, “lower”, “left”, “right”, “horizontal”, “vertical”, “upward”, “downward”, “clockwise” and “counter-clockwise” merely describe the configuration shown in the figures. It is understood that the components may be oriented in any direction in the terminology, therefore, it should be understood as encompassing such variations unless specified otherwise. 
     Referring now to the drawings wherein like reference numerals designate corresponding or similar elements throughout the several views, there is shown in FIG. 1 a pressure vessel assembly according to the present invention, generally designated at  20 , for use in an underwater communication system. The pressure vessel assembly includes a pressure vessel  22 , a signal cable  24 , portions of which are shown extending from each end of the pressure vessel assembly  20 , and a bend strain relief  26  surrounding the signal cable  24  at each end of the pressure vessel  22 . 
     Other components of the pressure vessel assembly  20 , are seen in the exploded view of FIG. 2, including bulkheads  28 ,  30  for sealing the ends  32 ,  34  of the pressure vessel  22  in cooperation with a lock ring  36 , and termination assemblies  38  at each end of the signal cable  24 . 
     The pressure vessel  22  is a hollow, cylindrical shell formed from carbon or stainless steel for housing a card cage assembly  40  (shown in phantom in FIG. 2) supporting card trays carying electronic components of the communications system. The pressure vessel  22  has an inner surface defining a cylindrical bore having annular seats  42  on each end. 
     The outer surface pressure vessel  22  is coated with a layer of aluminum  44   42  (FIG.  5 ). Preferably, the aluminum layer  44  is applied using a thermal spraying process. Thermal spraying is a process of depositing on substrate materials molten or semi-molten materials which solidify and bond to the substrate. The process is also called metallizing and flame or metal spraying. The spray materials may be in the form of wire, rod, or powder. As the materials pass through the spray unit, they are heated to a molten or semi-molten state and then projected onto the substrate. A coating of thermal-sprayed aluminum (TSA) has been shown to provide a 20-year life to steel structures in sea water splash zones. The TSA coating improves corrosion resistance by providing a “sacrificial” aluminum anode over components susceptible to corrosion in seawater. Thus, in the present invention, the TSA layer  44  will galvanically protect the carbon steel base material of the pressure vessel  22  thereby slowing the corrosion of the pressure vessel to a negligible level. Preferably, a layer of silicone aluminum sealer  46  is applied over the TSA layer  44  to further increase the life of the pressure vessel. 
     The bulkheads  28 ,  30  (FIG. 2) form the ends of the pressure vessel  22  and are preferably machined from the same grade of carbon steel as the pressure vessel. The bulkheads  28 ,  30  are generally circular in cross section with an outwardly extending central cylindrical coaxial portion  48 . Each bulkhead  28 ,  30  is secured to the card cage assembly  40 . 
     One of the bulkheads  28  has a cross section which is slightly less than the diameter of the inner cylindrical bore of the pressure vessel  22  for allowing insertion of the assembled bulkheads  28 ,  30  and card cage assembly  40  through one end of the pressure vessel. This smaller bulkhead  28  is externally threaded  50  for receiving the lock ring  36  which has an inner thread  52  for engaging the external threads  50  on the bulkhead  28  so that the inner surface of the lock ring contacts the seat (not shown) inside the end of the pressure vessel  22 . After the bulkheads  28 ,  30  and the card cage assembly  40  are inserted into the pressure vessel  22 , the bulkheads are welded to the pressure vessel to form an enclosed pressure vessel. 
     A close-up view of the larger bulkhead  30  is shown in FIG.  3 . The outward extension  48  includes three portions of varying diameter, including a large externally threaded portion  54 , a small intermediate portion  56  having external circumferential grooves, and a distal housing  58  having circumferentially-spaced threaded openings  60 . The bulkheads  28 ,  30  electrical power lines  66 , fiber optic cable  68  are provided with axial openings  62  which allow them to pass signal transmission elements  64  in the signal cable  24 , such as telecommunication lines  70 , and the like, into the pressure vessel  22 . 
     According to the present invention, the openings  62  into the pressure vessel  22  are protected against moisture penetration using seals  72 . As seen in FIG. 4, the seal comprises a cylindrical body  74  formed to correspond to the size of the outer end of the bulkhead openings  62 . The seal  72  is formed from a thermal setting polymeric material such as an epoxy resin or cross-linked polymer, for example, a cross-linked elastic polymer. The seals  72  are formed by casting the transmission elements  64  in an epoxy base that hardens to provide a strong, non-porous moisture seal around the transmission elements  64 . Short lengths of transmission elements  64  extend out from the ends of the seal  72  for connection to components of system. The formed epoxy seal  72  has three, spaced annular grooves  76  for holding two radial o-rings  78  and one face-seal o-ring  80 , respectively. Back-up rings  82  fit in two of the radial seal grooves  76  of the epoxy seal to prevent extrusion of the radial o-rings  78  at high pressure and also compensate for loose tolerance in the bulkhead openings  62 . 
     Referring to FIG. 5, the epoxy seals  72  are slid into the outer end of the bulkhead openings  62  for introducing the transmission elements  64  through the openings and into the pressure vessel  22  where the transmission elements may be connected to the electrical components. Insertion depth of the seals  72  into the bulkhead  30  is limited by the depth of a larger diameter outer bore of the openings  62 . The seals  72  are positively secured within the bores by retaining c-clips  84 . so that the face seal o-ring  80  engages the bottom of the bore. The radial o-rings  78  seal the periphery of the epoxy seal  72  against the inner surface of the bore to provide a fluid tight seal that prevents any fluid from reaching the interior of the pressure vessel  22 . The transmission element  64  ends from the outer end of the seal  72  extends externally of the bulkhead  30 . A fiber splice tray  86  (FIG. 3) is attached in bulkhead housing  58  using a c-clip  88  when necessary for splicing fiber optic cable. 
     A third axial opening  90  in the bulkhead is provided for vacuum evacuation of the pressure vessel  22  and for filling the evacuated vessel with nitrogen, as is known in the art 
     Referring now to FIG. 2, cable termination may be accomplished using standard cable termination assemblies  38 , which are generally of the cone-in-socket or crimp type. In both types, the termination assembly housing  92  is mechanically fastened to the signal cable  24  end using conventional fastening means for securing the metal strength members in the signal cable  64  to the housing  92 . As described above, the termination assembly  38  functions to pass the signal transmission elements  64  while assuming the mechanical stress on the signal cable  24 . The termination assembly housing  92  holding the terminal end of the signal cable  24  is designed to prevent the entry of water into the internal portions of the signal cable. A fiber splice tray  94  is provided when necessary for splicing of fiber optic cable. 
     A carbon steel connector sleeve  96  is provided for connecting the pressure vessel  22  and termination assembly  38 . The ends of the sleeve  96  fit tightly over the termination assembly housing  92  and bulkhead housing  58 . The ends of sleeve  96  have circumferentially-spaced holes  98  in the periphery which align with corresponding holes  100  in the termination assembly housing  92  and the holes in the bulkhead housing. The holes in the sleeve  96  receive screws or other fasteners (not shown) for securing the bulkhead housing  58  and the termination assembly  38 . When connected, the sleeve  96  houses the optical fiber splice trays assemblies  86 ,  94  and defines an area where other transmission elements  64  are spliced or branched. The sleeve  96  thus becomes a load bearing member of the pressure vessel assembly  22  to prevent mechanical stress from being applied to the transmission elements  64 . Moreover, the signal cable  24  is secured relative to the pressure vessel  22  such that the application of force to pull the cable from the cable termination assembly  38  will not be transmitted to connections between the signal cable transmission elements  64  and electronic components in the pressure vessel  22 . 
     The internal area defined by the cable termination assembly  38 , bulkhead housing  58  and sleeve  96  not otherwise occupied is substantially filled with a water blocking compound, such as a polybutene compound, to substantially prevent the entry of moisture into the area and the vessel  22 . This is accomplished by providing passages  100  (FIG. 5) at the bottom of the screw holes  60  in the bulkhead housing  58  which open into said area. During assembly, two opposed screw holes  60  are left open and polybutene is added to the area through one hole until the polybutene exits the opposed hole indicating the area is filled. 
     According to the present invention, a series of heat shrink tubes  102 ,  104 ,  106  are used to seal the termination assembly  38 . Heat-shrink tubes are known and include, for example, polyolefin polymeric materials with a low shrinking temperature such as the polyolefin marketed under the trade name “Sigmaform” by the Raychem which shrinks completely at a low temperature of about 250 degrees Fahrenheit. 
     After the termination assembly  38  is connected through the sleeve  96  to the bulkhead housing  58 , a length of cable adjacent the termination assembly housing  92  (FIG. 6) and a portion of the housing  92  are lightly abraded. The abraded surfaces are then cleaned with industrial grade alcohol. A first set of three o-rings  108  is installed along the cable  24  adjacent the housing  92 . The first heat-shrink tube  102  is positioned over the o-rings  108  and cable  24  (FIG.  7 ). The tube  102  is selected to have a slightly greater inside diameter than the outside diameter of the o-rings  108  so that the tube slips easily over the o-rings. One end of the tube  102  seats against the termination assembly housing  92 . Preferably, the inside of the tube  102  is coated with an adhesive sealing material which, along with the roughened cable surface, promotes adherence and sealing by the tubing  102 . A suitable adhesive is available under the designation “S-1030″ ” from the Raychem company. A heat gun (not shown) is used to shrink the diameter of the tube  102  (FIG.  8 ). The shrinking of the tube  102  causes a portion of an inner surface of the heat-shrinkable tube to come into compressive engagement with the outer jacket of the cable  24  and the o-rings  108  and causes the adhesive to flow providing an excellent water blocking seal even in the presence of high exterior water pressure. The compressive engagement of the tube  102  with the jacket and o-rings  108  also provides a series of redundant seals between a portion of an inner surface of the heat-shrink tube and the signal cable  24  jacket. 
     After cooling, the surface of the first heat shrink tubing  102  is abraded to form a rough surface and cleaned with alcohol. As seen in FIG. 8, a second set of three o-rings  110  is installed over the first heat shrink tubing  102  just behind the first three, already-installed o-rings  108 . A second, larger heat-shrinkable tube  104  is positioned over the o-rings  110  and cable  24  over a portion of the termination assembly housing  92  (FIG.  9 ). The preferred length of the second heat-shrink tube  104  is sufficient to cover the first tube  102 . The second tube  104  is heated and shrunk down over the cable  24 , and o-rings  110  and housing  92  (FIG.  10 ). Preferably, the termination assembly housing  92  includes a peripheral flange which is engaged by the shrunken tube  104  for sealing the end of the tube  104  around the housing  92 . 
     As seen in FIG. 11, a third heat-shrinkable tube  106  is then placed over the sleeve  68  and portions of the bulkhead housing  58  and termination assembly housing  92  and shrunk into place (FIG.  12 ). The shrunken tube  106  engages the series of grooves provided around the intermediate portion  56  of the axial extension of the bulkhead  30  for sealing the corresponding end of the tube  106 . The third heat-shrink tube seal works to keep moisture from the area in the sleeve  96  and to retain the polybutene in the sleeve  96 . 
     The rubber bend strain  26  relief is attached to each bulkhead  28 ,  30  to protect the signal cable  24  from being bent at a radius that could damage the cable. The bend strain relief  26  (FIG. 2) has a profiled passage extending therethrough for passing the cable  24 . The profiled passage has a forward bore  112  which is sized to accept the termination assembly  38 . The end of the bend strain relief  26  holds a metal ring  114  which is internally threaded to cooperate with the external threads of the larger portion  54  of the bulkhead extension  48  to close and seal the end of the pressure vessel assembly  20   
     Moisture ingress barriers to the electronics within the pressure vessel  22  are provided by the heat-shrinkable tubing  102 ,  104 ,  106 , o-rings  108 ,  110 , the water-excluding polybutene compound, and the epoxy seals  72 . Thus, the electrical components are enclosed inside a watertight pressure vessel assembly  20  with all points of possible moisture entry sealed at several levels by mechanical and fluid seals. 
     The pressure vessel assembly, due to the aluminum coating, of the present invention is particularly effective in shallow water, or “splash zones”, where the oxygen content of seawater is high. However, since deep, oxygen-poor seawater corrodes steel only at a rate of 0.004″ per year, a vessel made according to the present invention is adequate for long life applications beyond 20 years. 
     The pressure vessel and cable termination assembly seals of the present invention thus provide a reliable, durable pressure vessel assembly for housing electronic components in the underwater environment. Moreover, the materials and production methods used result in inexpensive pressure vessels. Specifically, reduced product costs are realized by replacing the beryllium copper material used for conventional pressure vessels and bulkheads with carbon steel, thereby reducing costs on the order of about ten to one. Also, there are no safety hazards in working with the carbon steel, as opposed to beryllium copper. Epoxy cable seals described herein are about one fourth the cost of Bridgman-type seals and installation is more reliable. The use of heat-shrinkable tubing to seal the termination assemblies eliminates the need for costly, specialized capital equipment for polyethylene molding and reduces manufacturing time. Generally, manufacturing and assembly of the pressure vessel assembly of the present invention are less costly, less complex, require lower skill levels, and are less time-consuming than present pressure vessels. The manufacturing process of the pressure vessel assembly removes process dependent characteristics inherent with present pressure vessels by providing a more robust, repeatable product. 
     Although the present invention has been shown and described in considerable detail with respect to only a few exemplary embodiments thereof, it should be understood by those skilled in the art that we do not intend to limit the invention to the embodiments since various modifications, omissions and additions may be made to the disclosed embodiments without materially departing from the novel teachings and advantages of the invention, particularly in light of the foregoing teachings. For example, the cable termination assembly seal and method may be used to seal the passage of any cable into a cable-receiving structure. Accordingly, we intend to cover all such modifications, omission, additions and equivalents as may be included within the spirit and scope of the invention as defined by the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures.