Abstract:
According to a preferred embodiment, a mechanical face seal assembly comprises four generally ring-shaped members and plural spring devices. The first ring-shaped member is directly fastened (e.g., bolted) to the flanged structural section located at the end of the shaft sleeve. The first and second ring-shaped members are coaxially aligned and rotatively communicative at respective radial surfaces. The first ring-shaped member is made of a metal (or ceramic) matrix composite material. The second ring-shaped member is made of a polymer matrix composite material. The second and third ring-shaped members are mated via radial openings (in the second ring-shaped member) and corresponding radial projections (in the third ring-shaped member). The third ring-shaped member has axial projections and is thereby directly fastened (e.g., bolted) to the fourth ring-shaped member, which clampingly secures the third ring-shaped member with respect to the shaft. The third ring-shaped member contains the spring devices so that they push against the second ring-shaped member, which is consequently biased against the first ring-shaped member. Since each ring-shaped member is a “split” structure having two joinable semicircular halves, the entire sealing system can be installed without disassembling the shafting mechanism. The matrix composite compositions confer structural qualities on the first and second ring-shaped members, and further afford tribological “self-healing” properties of the wear junction therebetween. The resultant benefits include fewer parts, greater compactness, longer service life, and less frequent maintenance and repair.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to methods and apparatuses for sealing machinery, more particularly to mechanical seal systems for rotary shafting mechanisms such as used in propulsors, pumps and compressors. 
     Current mechanical seal systems for the main propulsion shaft apparatus of marine vessels typically comprise either circumferential-type seals or face seals. 
     A known kind of circumferential seal includes packing which seals by being hydrostatically compressed from the axial clamping force of bolts acting on a stuffing box and packing gland arrangement. Other conventional methodologies of circumferential sealing include lip seals (or variations thereof) which make use of a spring to radially load an elastomer around the rotating shaft. Circumferential seals are inexpensive, but the failure mode involves leakage and shaft wear, with concomitant repairs entailing a new shaft sleeve or new shaft. 
     Face seals, on the other hand, have replaceable wearing elements which wear on their axial faces (i.e., perpendicular to the radial direction). These seals are easy to maintain once installed, since there is usually no or minimal leakage and there are no adjustments required. Generally, the failure mode for a face seal is leakage over time, which leads to replacement or seal face refurbishment. 
     A notable type of failure mechanism for propulsion shaft seals involves dirt and contamination which will wear at the sealing interface. An ideal seal behavior is one that exhibits recovery from leakage attributable to the ingestion of wear debris. The present invention has basis in the recognition that the design of the seal, including the design of the materials, plays an important role in promoting seal recovery. 
     Currently, the U.S. Navy almost exclusively uses face seals on its large surface ship combatants and submarines; its small combatants and commercial boats, however, most often implement circumferential seals. As compared with a circumferential seal, it is more desirable to use a face seal in smaller water craft (e.g., smaller naval combatants) because of the relative lack of attention required as well as the minimization of bilge leakage. In terms of commercial availability, there is an apparent dearth of completely split face seal designs such as would be suitable for the U.S. Navy&#39;s smaller marine vessels. Although small unsplit face seal designs are commercially available, these do not appear to be rugged enough for the naval marine environment. Moreover, the U.S. Navy lacks knowledge and experience with the design and materials used in the commercially available “off-the-shelf” products. The present invention appreciates that the design and materials for a small split face seal can be improved or optimized to suit U.S. Navy and other applications. 
     Incorporated herein by reference are the following pertinent United States patents: Reagan U.S. Pat. No. 5,820,129 issued Nov. 13, 1998; Clark et al. U.S. Pat. No. 5,725,220 issued Mar. 10, 1998; Duffee et al. U.S. Pat. No. 5,716,054 issued Feb. 10, 1998; Clark et al. U.S. Pat. No. 5,711,532 issued Jan. 27, 1998; Bessette et al. U.S. Pat. No. 5,662,340 issued Sep. 2, 1997; Reagan U.S. Pat. No. 5,615,893 issued Apr. 1, 1997; Azibert U.S. Pat. No. 5,571,268 issued Nov. 5, 1996; Borino et al. U.S. Pat. No. 5,529,315 issued Jun. 25, 1996; Radosav et al. U.S. Pat. No. 5,490,682 issued Feb. 13, 1996; Bowers U.S. Pat. No. 5,409,241 issued Apr. 25, 1995; Carmody U.S. Pat. No. 5,354,070 issued Oct. 11, 1994; Glynn et al. U.S. Pat. No. 5,292,138 issued Mar. 8, 1994; Pecht et al. U.S. Pat. No. 5,217,233 issued Jun. 8, 1993; Radosav et al. U.S. Pat. No. 5,199,720 issued Apr. 6, 1993; McOnie U.S. Pat. No. 5,192,085 issued Mar. 9, 1993; Radosav U.S. Pat. No. 5,114,163 issued May 19, 1992; Nagai et al. U.S. Pat. No. 5,067,733 issued Nov. 26, 1991; Mullaney U.S. Pat. No. 5,020,809 issued Jun. 4, 1991; Lowe et al. U.S. Pat. No. 4,795,169 issued Jan. 3, 1989; Azibert U.S. Pat. No. 4,576,384 issued Mar. 18, 1986; Copes U.S. Pat. No. 4,410,188 issued Oct. 18, 1983; Wilkinson U.S. Pat. No. 4,239,240 issued Dec. 16, 1980; Adams U.S. Pat. No. 3,773,337 issued Nov. 20, 1973. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the present invention to provide a reliable face sealing system for a rotary shafting mechanism, such as a ship&#39;s main propulsion shafting mechanism. 
     It is another object of the present invention to provide such a sealing system which will remain leak-free for a significant period of time, such as throughout the life cycle of a ship. 
     It is a further object of the present invention to provide such a sealing system which is easily maintainable. 
     Another object of the present invention is to provide such a sealing system which may be used in certain applications, such as in association with small naval vessels, wherein circumferential sealing systems are customarily used. 
     In accordance with the present invention, a mechanical face seal combination is suitable for use in connection with machinery of the type wherein an axially rotative shaft passes through an immobile housing. The inventive combination comprises a first annulus, a second annulus and support apparatus. The first annulus has a first annular face which is at least substantially planar. The second annulus has a second annular face which is at least substantially planar. The support apparatus includes biasing means. The first annulus, the second annulus and the support apparatus are adaptable to arrangement wherein: the first annulus is attached to the housing; the support apparatus is attached to the shaft; the first annular face and the second annular face coaxially abut; and, the support apparatus holds the second annulus in position with respect to the shaft whereby the biasing means contactingly presses the second annulus so as to approximately axially bias the second annular face against the first annular face. 
     Notable among the features of the present invention is the robust, integral construction of the rotational structure inclusive of the face (surface) designed for dynamic abutting contact with the face (surface) of the non-rotational structure. This rotational structure, viz., the second annulus, is able to directly bear, in the absence of any intermediary structure, the loading imposed by the support apparatus which holds the second annulus in place with respect to the rotational shaft. Similarly featured is the robust, integral construction of the non-rotational structure inclusive of the face (surface) designed for dynamic abutting contact with the face (surface) of the rotational structure. This rotational structure, viz., the first annulus, is able to be directly coupled with the housing—e.g., directly fastened to the flanged (radially annularly projecting) end portion of a ship&#39;s stern tube. 
     A resultant advantage of the superior material strength and unitary construction of both the first annulus and the second annulus is the reduction of parts needed for the overall seal configuration, with the concomitant advantage of a simplified design. In particular, obviated is the functional or structural need for additional or auxiliary structure—whether it be of an intermediary, intervening, interpositional, dampening, buffering, holding and/or containing nature—used in association with: (i) the first (stationary) annulus in relation to the stationary housing; and, (ii) the second (rotational) annulus in relation to the support apparatus which holds the second annulus in place with respect to the rotational shaft. 
     According to many inventive embodiments, the second (rotational) annulus includes (e.g., is at least substantially made of a polymeric matrix composite characterized by sufficient structural integrity (e.g., in terms of strength, rigidity and elastic deformation) for enduring the holding by the support apparatus (under both rotating and non-rotating conditions of the shaft), including enduring the pressing by the biasing means. According to typical inventive practice, the second annulus is provided with openings (e.g., slots) and the support apparatus is provided with projections (e.g., cogs); the holding operation of the second annulus by the support apparatus includes the interlocking or mating of the openings and projections; at the same time, spring devices (or other spring-loading means) pushing off the support apparatus are exerting an axial force against the second annulus. The robust material composition of the second apparatus enables it to resist the stresses and strains associated with such modes of containment by the support apparatus. 
     Another feature of note according to many inventive embodiments is a complementarity of the respective material compositions of the rotational structure and the non-rotational structure, thus giving rise to the complementarity of their respective wear surfaces. According to many inventive embodiments, the first (stationary) annulus includes (e.g., at least substantially consists of) a metal matrix composite (MMC) or a ceramic matrix composite (CMC), and the second (rotational) annulus includes (e.g., at least substantially consists of) a polymer matrix composite (PMC). It is believed that the present invention&#39;s matrix-composite-on-matrix-composite material wear combination will imbue the wear interface, on a continual basis, with “self-healing” (“leakage-restorative”) and contamination-resistant attributes. 
     The present invention thus features a single stationary wearing piece and a single rotational wearing piece. These inventive features are predicated on the inventive observation that a wearing face structure made of an appropriate matrix composite material will be accorded both structural and tribological properties. According to preferred practice of this invention, both the stationary wearing piece and the rotational wearing piece will be made of a matrix composite material; that is, the stationary wearing piece will be made of MMC or CMC, and the rotational wearing piece will made of PMC. 
     Therefore, according to this invention, both the stationary wearing piece and the rotational wearing piece will be endowed with “structural” attributes, especially in terms of load-bearing capabilities. Furthermore, the PMC material of the rotational wearing piece, in interacting with the MMC or CMC material of the stationary wearing piece, will provide “lubricity” to the wear interface. In other words, the rotational wearing piece&#39;s PMC material will afford a transfer of polymeric material so as to develop a “transfer film” of low shear strength, thus allowing for a low coefficient of friction against a stationary wearing piece made of a wear-resistent matrix composite material such as MMC (e.g., a bronze ceramic matrix, which is essentially a ceramic-filled bronze) or a CMC. 
     According to conventional seal technology, the seal faces must maintain their original flat surfaces in order for them to to actually seal. If either or both wear surfaces become “out of flat” or worn, then the sealing capabilities will degrade. The traditional approach is to utilize hard and brittle materials such as silicon carbide or carbon, which will tend to maintain their original flat surface condition for some period of time; however, once there is a wear spot, the seal must be removed and replaced. The conventional requirement of a high degree of flatness in a hard brittle material can be costly and time-consuming. 
     By contrast, according to this invention, the utilization of a combination of two kinds of matrix composite materials within a certain sealing geometry will promote leakage restoration, and therefore will not require brittle materials and precise surface conditions. According to the present invention, the flatness tolerance is relaxed due to the implementation of the selected engineered matrix composite materials. The combination of matrix composite materials can be “lapped” in place via the natural machining action of the seal and the concomitant rubbing of the wearing surfaces. The material design is hence also contamination-resistant, since the contaminants will be subsumed in the ongoing self-healing process of the wear interface. 
     The present invention thus provides a lower cost seal system requiring significantly less “down” time for repair or maintenance, which conventionally involves disassembly. Another benefit of using metal matrix composite and polymeric matrix composite materials is that they are both corrosion-resistant. An additional benefit of the inventive seal system, arising from its “self-healing quality,” is the minimization of installation space around the seal, which would traditionally be needed for maintenance and repair; the present invention thus allows for the insertion of additional machinery in spaces that would conventionally be reserved to make room for personnel. 
     The inventive use of wear-resistant and self-lubricating matrix composite materials will thus provide long wearing surfaces. The characteristics of the matrix composite materials can be optimized or tailored according to the particular inventive application. The selected matrix composite materials can be specially developed, custom engineered or purchased commercially (e.g., “off the shelf”). The matrix composite materials can be selected, according to this invention, on the basis of a general model wherein a stationary face piece made of a metal matrix material or ceramic matrix material is in rubbing contact with a rotative face piece made of a polymeric matrix material, and wherein the metal or ceramic matrix material affords wear resistance while the polymeric matrix material affords lubricity via transfer film formation. Generally, each of these kinds of matrix composite materials (viz., MMC, CMC and PMC) are castable and/or moldable materials which allow for near net shape and/or final shape, thus requiring a minimal amount of processing. 
     Another feature of many inventive embodiments is a completely split seal design. In other words, the entire inventive split seal assembly can be applied in place to the machinery, without any need for disassembly of the shaft line. According to many inventive embodiments, there are four main generally annular structures, viz.: (i) the stationary face structure (e.g., stationary seal face); (ii) the rotative face structure (e.g., floating seal ring); (iii) the coupling structure (e.g., drive ring), which serves to hold the rotative face structure in position with respect to the shaft); and, (iv) the clamp structure (clamp ring), which serves to clamp the coupling structure with respect to the shaft. The terms “annular” and “ring-shaped,” as used herein, refer to a shape which generally, substantially, essentially or approximately describes that of a circular ring. All four ring-shaped structure basically describe an overall annular shape when -viewed elevationally in the axial direction; further, each ring-shaped structure has its own cross-sectional geometry. All of these ring-shaped structures lend themselves to having a “split” construction wherein two semi-annular halves can be united (into one annular piece) and disunited (into two semi-annular pieces). 
     According to this invention, there is no need for any complex locking mechanism (e.g., such as would have to account for seawater), or for any difficult machining process such as would be applied to the shaft sleeve or shaft. In this regard, the present invention provides for uncomplicated engagement of the stationary face ring with the housing, of the rotational face ring with its support ring, and of the face ring&#39;s support ring with its clamp ring—all requiring no real structural modification (other than the machining of holes). This is especially beneficial for applications wherein the shaft, shaft sleeve and or other parts are made of composite materials. 
     Inventive practice can also admit of configurational adjustment of the inventive seal system under damaged conditions. Parts of an inventive face seal system can be removed and rearranged, with packing introduced, thereby basically “salvaging” the original component; for continous use, through minimal effort. 
    
    
     Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the present invention may be clearly understood, it will now be described, by way of example, with reference to the accompanying drawings, wherein like numbers indicate the same or similar components, and wherein: 
     FIG. 1 is a diagrammatic cutaway side elevation view of a ship hull form illustrating, in association with a propulsion unit, an embodiment of a seal assembly in accordance with the present invention. 
     FIG. 2 is a detail view, as indicated by encirclement A in FIG. 1, of the seal assembly and propulsion unit shown in FIG. 1 
     FIG. 3 is a detail view, as indicated by encirclement B in FIG. 2, of the abutment of the seal face and seal ring shown in FIG.  2 . 
     FIG. 4 is a diagrammatic end elevation view looking aft, as indicated by C—C in FIG. 2, of the seal assembly and propulsion unit shown in FIG.  2 . 
     FIG. 5, FIG. 5 a , FIG. 5 b  and FIG. 5 c  are views, similar to that shown in FIG. 3, of embodiments wherein a seal face and seal ring obliquely abut. 
     FIG. 6 is a diagrammatic elevation view, similar to the view shown in FIG. 2, of another embodiment of a seal assembly in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, propulsion shaft  10  transmits power developed from propulsion plant  20 , located within ship  30 . Ship  30  operates in the seawater  60  environment. The seawater  60  surface rises approximately to waterline  65  on the hull of ship  30 . A shaft sleeve, stem tube  40 , contains the rotating main propulsion shaft  10 . Located at the inboard end is a seal assembly  50  which prevents the ingress of seawater  60 . 
     With reference to FIG. 2, FIG.  3  and FIG. 4, shaft  10  has rotational axis a. Seal assembly  50  is bolted via machined through-holes  70 , using fasteners  71 , to a mating (mounting) flange  90 , located at the stem tube  40  inboard end. More specifically, stationary seal face  110  is bolted to flange  90 . Flange  90  has an annular flange surface  91  which fixedly abuts (with gasket  100  interposed) the annular outboard end surface  112  of stationary seal face  110 . A flexible gasket  100  provides a static seal between the seal assembly  50  and the mating flange  90 . Inlet  15  in stem tube  40  provides a cooling water connection. Accordingly, adjacently attached to mating flange  90  is seal assembly  50 , which comprises the following main components, described hereinbelow and generally considered in order from left to right as represented in FIG.  2 : stationary seal face  110 ; rotatable seal ring  120 ; o-ring seals  150  and  151 ; drive ring  170 ; clamp  180 . 
     A split stationary seal element referred to herein as a “seal face,” seal face  110 , is made of a metal matrix composite (MMC) material or a ceramic matrix composite (CMC) material. As shown in FIG.  2  and FIG. 3 (and also FIG.  5 ), seal face  110  has a radial cross-section which describes an “L-shape.” seal face  110  includes two halves which are assembled around shaft  10 . The two half-portions of seal face  110  are joined using fasteners  141  which are inserted through machined through-holes  140 . Seal face  110  is then fastened, using fasteners  71 , to mating flange  90 . 
     A split rotatable seal element referred to herein as a “seal ring,” seal ring  120 , is made of a polymer matrix composite (PMC) material. As shown in FIG.  2  and FIG. 3 (and also FIG.  5 ), seal ring  120  has a radial cross-section which describes a “sideways U-shape” (or, equivalently, a “C-shape”). Similar to seal face  110 , seal ring  120  is a split assembly comprising two halves which are held together by fasteners  143  inserted through machined through-holes  142 . 
     As particularly well shown in FIG.  3  and FIG. 5, the two wear elements, viz., stationary seal face  110  and rotatable seal ring  120 , have corresponding annular planar surface portions (viz., wear surface  111  and wear surface  121 , respectively) which abut each other. Wear surface  111  (which seal face  110  includes) and wear surface  121  (which seal ring  120  includes) are the adjoining annular wear surfaces (“mating faces”) in relation to each other. 
     “Floating” seal ring  120  is. loaded against seal face  110  via the force f from plural springs  130  and from seawater  60  pressure acting against the seal ring  120  area. Seal ring  120 , having a radially cross-sectional sideways U- shape, is propitiously configured to both hold and protect springs  130 . Preferably, springs  130  are approximately equally distributed around the circumference defined by the interface between the annular alcove surface  122  of seal ring  120  and the annular outboard end surface  171  of drive ring  170 . 
     Still referring to FIG.  2  through FIG. 4, seal ring  120  requires a secondary seals, e.g., static O-ring seal  150 . O-ring seal  150  circumferentially seals between seal ring  120  and drive ring  170 . Shaft  10  also requires a secondary seal, e.g., static O-ring seal  151 . O-ring seal  151  circumferentially seals between shaft  10  and drive ring  170 . 
     Drive ring assembly  160  includes drive ring  170 , split shaft clamp  180  and fasteners  190 . Drive ring assembly  160  (in particular, drive ring  170 ) is fastened around shaft  10  in a split fashion. Drive ring  170  is characterized by a radial cross-section which describes an “inverted L-shape.” Like seal face  110  and seal ring  120 , drive ring  170  is characterized by divisibility (“splittability”) into two halves or semi-sections; installation is accomplished by appropriately disposing the semi-sections in relation to shaft  10  and then uniting the semi-sections to form a ring-shaped unit which encircles shaft  10 . Many embodiments thus advantageously afford “splittability” (and hence ease of installation and repair) of the three principle annular components of the present invention&#39;s mechanical split seal assembly, viz., seal face  110 , seal ring  120  and drive ring  170 . 
     Ears  175 , protruding axially from drive ring  170 , attach to the split shaft clamp ring  180  which is held in place with fasteners  190  through aligned holes  191 . Again, clamp ring  180  is characterized by divisibility into two half-annular portions, similarly as are seal face  110 , seal ring  120  and drive ring  170 . Installation of clamp ring  180  is similarly accomplished by appropriately placing the half-annular portions with respect to shaft  10  and then joining the semi-sections to form a ring-shaped clamp ring  180  unit which encircles shaft  10 . Each half-annular portion of clamp ring  180  has, at opposite ends, two apertured axial protrusions  181 ; thus, when the half-annular portions are joined to become clam ring  180 , at diametrically opposed locations each ear  175  is sandwiched and fastened (e.g., bolted) to and between two protrusions  181 . 
     Drive ring assembly  160  contains the static seals  150  and  151  along seal ring  120  and shaft  10 , respectively, and transmits the rotational torques from friction between seal face  110  and seal ring  120 . At least two axial projections  172  (which resemble tangs, cogs, etc.), emanating from the outside diameter of drive ring  170 , engage (e.g., mesh or mate with) corresponding slots  173  of seal ring  120 . 
     As shown in FIG.  2  and FIG. 3, seal face  110  wear surface  111  and seal ring  120  wear surface  121  are coaxially and contiguously situated so as to describe a planar interface or junction  213  which is disposed at a ninety degree angle α in relation to the axis a of shaft  10 . However, according to the present invention, junction  213  need not be perpendicular to shaft  10  axis a. Referring to FIG.  5  through FIG. 5 c , junction  213  variously is non-perpendicularly inclined at an angle α with respect to axis a. As shown in these figures, the plane defined by junction  213  is inclined at an oblique angle α of at least about forty-five degrees. 
     In accordance with the present invention, seal face  110  is preferably made of wear resistant material such as a ceramic matrix composite (CMC) material or a metal matrix composite (MMC) material. Further, in accordance with the present invention, seal ring  120  is preferably made of a self-lubricating polymer matrix composite (PMC) material. An MMC seal face  110  composition may be preferable to a CMC seal face  110  composition for many inventive embodiments, because MMC material has some superior properties vis-a-vis&#39; CMC material. For instance, ceramic matrix composites have been known to be beset with problems such as matrix brittleness; see, e.g., below mentioned book John W. Weeton, Dean M. Peters and Karyn L. Thomas,  Engineers&#39; Guide to Composite Materials , American Society for Metals, Metals Park, Ohio, 1987, pp 1-2 to 1-4. An example of a preferred MMC material for many inventive embodiments is bronze ceramic matrix composite (i.e., ceramic-filled bronze matrix material). 
     According to some inventive embodiments, stationary seal face  110  is at least substantially composed of a kind of matrix composite material other than an MMC or a CMC. As an alternative to MMC or CMC, the following matrix composites may also be suitable for inventive practice of the stationary wear structure (such as seal face  110 ), for instance in terms of affording requisite wear-reisistance: glass matrix composites; carbon matrix composites; and, graphite matrix composites. 
     Hence, according to this invention, the materials used in the wearing elements (viz., seal face  110  and seal ring  120 ) ideally are non-corroding and are integral with the structure so as to minimize components. Thus, in inventive practice, the wearing elements will be at least substantially composed of near net shape materials (e.g., fiber-reinforced matrix materials) which are castable, machineable and/or moldable. The respective matrix composite compositions of the two wearing elements are features which give rise to significant advantages in comparison to conventional approaches to mechanical sealing. 
     Among the several advantages of the present invention&#39;s double matrix composite (MMC-contacting-PMC or CMC-contacting-PMC) wear combination, such wear combination will generally promote leakage restoration, thereby affording a “self-healing” quality which will obviate the need for great geometrical or configurational precision of the wearing elements and their interrelationship. Leakage will be tolerated to the point of diminshment as the seal wears in. The self-healing attribute will permit less precision since leakage will be tolerated to the point of diminishment as the seal wears in. Since this wear combination will advance leakage restoration, it will reduce the need for disassembly due to wear or leakage. 
     Moreover, such double matrix composite wear combination will generally allow for a reduced number of parts or a simplified design, such as exemplified by the inventive embodiment shown in FIG.  1  through FIG.  4 . In particular, the wearing elements will have structural integrity because the matrix composite materials will be structural materials. Furthermore, these matrix composite wearing elements will lend themselves to easy manufacturing techniques. A PMC part might be cast, extruded or machined with an alignment (locational) fit and therefore might not require, in the context of the mechanical seal system, a centering or holder device; what might be required is a band clamp to hold the pieces together. 
     A composite is a combination of two or more materials which differ at the macroscopic level, each different material being a constituent of the composite. A matrix composite comprises (i) a filler or reinforcing agent (e.g., fibers, particles or fillers) and (ii) a matrix binder (e.g., a resin). The matrix is the principal phase or aggregate in which the filler or reinforcing agent is embedded or surrounded. Generally, the matrix serves two functions, viz., (i) it holds the reinforcement phase in place, and (ii) under an applied force, it deforms and distributes the stress to the reinforcement constituents. 
     Examples of metals (metal elements, or alloys of two or more metal elements) conventionally used as matrices in metal matrix composites are aluminum, titanium; bronze and magnesium. A broad range of fillers or reinforcing agents (e.g, fibers) can be used with lower-melting point matrices. For instance, most metals, ceramics and compounds can be used as fillers or reinforcing agents in an aluminum or magnesium matrix. The choice of fillers or reinforcing agents becomes increasingly limited as the melting point of the metal matrix material increases. 
     Ceramic compounds are formed by the combination of one or more metallic elements with one or more nonmetallic elements. Examples of ceramic materials include aluminum oxide, magnesium oxide and silica. 
     There are two main types of polymers, viz., thermoplastics and thermosets. Examples of thermoplastics which can be used as matrix resins include polycarbonate, polyethylene, polystyrene, polypropylene, polyamide, fluoropolymer, thermoplastic polyester, nylon, vinyl, acetal, polycarbonate, polyphenylene oxide, polyetheretherketone (PEEK), polyphenylene sulfide (PPS), polyetherketone ketone (PEKK) and polyetherketone (PEK). Examples of thermosets which can be used as matrix resins include epoxy, polyester, vinyl ester, phenolic, polyimide and bismaleide. 
     Conventional types of fillers and reinforcing agents (e.g., reinforcing fibers) used in the fabrication of matrix composites include glass, cotton, aramid, carbon, graphite, polyethylene, boron, steel, polyamide, alumina, silicon carbide and aluminaboria-silica. 
     The following references, incorporated herein by reference, are instructive generally regarding matrix composites and particularly regarding metal matrix composites, polymer matrix composites and ceramic matrix composites: John W. Weeton, Dean M. Peters and Karyn L. Thomas,  Engineers&#39; Guide to Composite Materials , American Society for Metals, Metals Park, Ohio, 1987 (See, esp., Section 1, entitled “Introduction to Composite Materials”); George Lubin,  Handbook of Composites , Van Nostrand Reinhold Company, New York, 1982 (See, esp., Chapter 1, entitled “An Overview of Composites”); Roy L. Harrington, Editor,  Marine Engineering , the Society of Naval Architects and Marine Engineers, Jersey City, N.J., 1992 (See, esp., Chapter XXII, entitled “Construction Materials,” Section  5 , entitled “Composite Materials”). 
     The following United States patents, incorporated herein by reference, are exemplary of various composite matrix materials which may be suitable for inventive practice of the wearing elements: Cohen et al. U.S. Pat. No. 6,000,851 issued Dec. 14, 1999; Cohen U.S. Pat. No. 5,389,411 issued Feb. 14, 1995; Divecha et al U.S. Pat. No. 5,337,803 issued Aug. 10, 1994 Karmarkar et al. U.S. Pat. No. 5,025,849 issued Jun. 25, 1991. Cohen et al. &#39;851 disclose (column 2) “a spin castable multiphase bearing material, such as a metal matrix composite, ceramic matrix composite and/or a polymer matrix composite to minimize wear over the life cycle of the motor or apparatus with which the bearing assembly is associated.” Karmarkar et al. disclose a process involving spin (centrifugal) casting into symmetrical shapes of fiber-reinforced metal matrix material. 
     The present invention can alternatively be embodied in a reconfigurable “packing gland” arrangement—a circumferentially sealing configuration which may be appropriate as an inventive modification in response to a damage condition such as worn seal faces or failed spring operation. Under circumstances wherein an inventive face sealing system such as seal assembly  50  shown in FIG.  1  through FIG. 4 is in need of repair, it can be inventively converted to what would essentially represent an inventive circumferential sealing system such as seal assembly  50   a  shown in FIG.  6 . The inventive arrangement shown in FIG. 6 can also be practiced “from scratch” rather than adaptively. 
     With reference to FIG. 6, seal assembly  50   a  operates on packing gland  200 . A notable feature of seal assembly  50   a  is that the following parts/components are absent or removed, viz., seal ring  120 , both secondary seals (o-rings)  150  and  151 , and clamp ring  180 . Thus, the drive ring assembly  160   a  shown in FIG. 6 represents a sort of “stuffing box” drive ring assembly, wherein loading pressure is applied via the packing gland bolts  81  which are inserted through through-holes  70  (which are machined in flange  90  and seal face  110 ) and through-holes  80  (which are machined in drive ring  170   a ). 
     Many small marine craft have shaft tubes  40  without a mounting (mating) flange  90 . A flexible hose with clamps is often used to hold a floating packing gland such as packing gland  200 . In order to provide a mounting surface, a flange must be provided, or the seal face  110  must be connected to the stern tube  40  using the existing hose and clamps. 
     There are other possible inventive configurational combinations of the various components which will allow for placement of the seal assembly with respect to the propulsion shaft. For instance, with regard to the packing gland arrangement shown in FIG. 6, at a propitious time the location of the stuffing box relative to the packing gland can be reversed, thus possibly permitting the reutilization or extended utilization of existing components. 
     There are also alternative fastening methodologies according to this invention. For instance, clamp bands can be used in place of fasteners. As another example, a hydraulic shaft locking device can be used in place of the shaft clamp band; this may eliminate tampering and provide a safe alternative to bolting. 
     Alternative applications of the present invention include marine propulsors, pumps and compressors. In fact, the present invention admits of application as a shaft seal device in any similar mechanical context which requires a split design, long wearing materials and non-wearing secondary sealing elements on the shaft. 
     Typical inventive embodiments feature plural ring-shaped objects which are each characterized by a two-piece construction, wherein two semicircular halves are attachable and detachable with respect to each other. Each ring-shaped object thereby affords the capability of being assembled onto and around a cylindrical object (such as a shaft) and disassembled therefrom, and of accomplishing same without significantly disturbing the cylindrical object. As exemplified in FIG.  2  and FIG. 4, inventive seal assembly  50  includes four dichotomous (“split”) ringshaped objects (stationary seal face  110 , rotatable seal ring  120 , drive ring  170  and clamp ring  180 ) which, when appropriately installed, are closely and sequentially disposed in coaxial alignment along shaft  10 . Inventive seal assembly  50  thus admits of ease and totality of installation, absent disruption or displacement of shaft  10  or other parts of the shafting mechanism. 
     Other embodiments of this invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. Various omissions, modifications and changes to the principles described may be made by one skilled in the art without departing from the true scope and spirit of the invention which is indicated by the following claims.