Abstract:
A rotating union for transferring a material stream, and especially a bearing damaging material stream, includes, a generally cylindrical body and an annular sleeve that is annularly spaced from the body and rotatably coupled thereto via a pair of axially-spaced bearings. The body and the sleeve are formed with ports and interconnecting passages for carrying the material stream and these ports and passages communicate with each other via a material transfer interface that is sealed by at least one dynamic seal assembly. Finely lapped material is used to improve the quality of the dynamic seal faces, and the seal faces are uniformly spring loaded. This, along with the axial spacing of the bearings, helps to minimize seal face misalignment and consequent leakage. In the event that material leakage does occur, the rotating union further includes a leakage collection area filled with a barrier fluid. The leakage collection area may encompass the bearings or it may be separate therefrom. A barrier fluid circulation system removes barrier fluid from the leakage collection area along with any material that leaks past the dynamic seals. The barrier fluid circulation system may be passive or active.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     This invention relates generally to rotating unions for transferring material streams between stationary and rotating material-carrying equipment. More specifically, the invention concerns rotating unions for handling bearing damaging fluids and semi-fluids, gaseous fluids having poor heat dissipation characteristics, environmentally sensitive compositions, and other materials wherein internal material leakage is an important concern. 
     A rotating union is used to transfer a material stream between a material provider and a material recipient that must rotate relative to each other. Typically, the material provider, such as an inlet conduit leading from a material source, is stationary while the material recipient, such as an outlet conduit leading to fluid processing equipment, rotates relative to the material provider. In other implementations, the material provider rotates relative to a stationary material recipient. 
     In either case, material to be transferred is directed through a material stream passage formed in a first rotating union member, across a dynamically sealed material transfer interface, to a material stream passage formed in a second rotating union member that is rotatably coupled to the first rotating union member via a bearing system. Dynamic sealing is provided by a dynamic seal assembly that mounts to the first and second rotating union members. The seal assembly has dynamic seal faces that are spring-biased into mutual rotational engagement to contain the transferred material as it passes between the first and second rotating union members. 
     Some rotating union applications call for the transfer of bearing damaging (e.g., abrasive) fluids and semi-fluids, gaseous fluids having poor heat dissipation characteristics, environmentally sensitive compositions, and other materials into or out of processing equipment. Such materials include, by way of example only, paint (having a ratio of 70% or more solids to 30% or less solvents), glue (hot and cold), rubber, and any of a variety of plastics or other polymers, such as sealants. 
     In certain rotating unions of the prior art, the handling of such materials has been problematic. For example, when bearing damaging material has been transferred, it has tended to leak past the dynamic seal and contact the bearings, ultimately degrading or damaging them. This condition is believed to be attributable to a variety of factors, including (1) misalignment of the dynamic seal faces causing them to intermittently separate during rotation, (2) unequal spring loading on the dynamic seal faces so as to aggravate the effects of seal misalignment, and (3) the selection of dynamic seal face materials that lack sufficient hardness. In addition, none of the rotating unions of the prior art, as far as known, have sought to affirmatively remove heat when transferring gaseous fluids having poor heat dissipation characteristics. There are also no known rotating unions that remove leakage material from the vicinity of the bearings once it does leak past the dynamic seal. One prior art approach has utilized drainage holes to remove leakage material; however, this is a passive approach to material removal, and does not affirmatively work to prevent the material from contacting the bearings. 
     It is with overcoming the foregoing deficiencies of the prior art that the present invention is concerned. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved rotating union for use with bearing damaging fluids and semi-fluids, gaseous fluids having poor heat dissipation characteristics, environmentally sensitive compositions, and other materials, that greatly reduces the likelihood that material will leak past the dynamic seal faces and into the bearing area of the rotating union. A further object of the invention is to provide an improved rotating union that employs a system for affirmatively removing material that does leak past the dynamic seal faces and into the vicinity of the bearings. 
     To that end, a rotating union is provided that comprises two separate circulating systems. One circulating system is a primary system for transferring a material stream through the union from a material provider to a material recipient arranged for mutual relative rotation. The other circulating system is a secondary system for circulating a barrier fluid past the dynamic seal(s) of the union and carrying leakage material from the primary system away from the dynamic seal(s). 
     In a first preferred embodiment, the rotating union of the invention includes a generally cylindrical body and a generally annular sleeve that is radially spaced from the body and rotatably coupled thereto via a pair of axially-spaced bearings. The body and the sleeve are formed with ports and interconnecting passages for transferring a material stream therethrough and these ports and passages communicate with each other via a material transfer interface that is dynamically sealed by a dynamic seal assembly. A very finely lapped (preferably ceramic) material is used for the seal faces and the seal faces are finished square to the axial centerline of the rotating union to provide correct, tight, lapped sealing surfaces. The wide spacing of the bearings and uniform spring loading of the dynamic seal faces minimizes seal face misalignment, thus further minimizing the possibility of material leakage across the dynamic seal assembly. In the event that material leakage does occur, the rotating union further includes a leakage collection area on the opposite side of the dynamic seal assembly. The leakage collection area is charged with a barrier fluid and is sealed so that the barrier fluid cannot contact the bearings. A passive barrier fluid circulating system is provided for removing barrier fluid from the leakage collection area along with material that leaks past the dynamic seal faces. The barrier fluid circulating system includes a barrier fluid reservoir that surrounds the sleeve and passages through which barrier fluid is circulated from the leakage collection area under the rotating action of the body. 
     In a second preferred embodiment, the rotating union of the invention includes a generally cylindrical body and a generally annular sleeve that is radially spaced from the body and rotatably coupled thereto via a pair of axially-spaced bearings. As with the first preferred embodiment, the body and the sleeve are formed with ports and interconnecting passages for transferring a material stream therethrough and these ports and passages communicate with each other via a material transfer interface that is dynamically sealed. Again, the seal faces are made from finely lapped (preferably ceramic) material and they are uniformly spring loaded to minimize seal misalignment. Unlike the first preferred embodiment, a pair of dynamic seal assemblies are provided, and they are positioned between the bearings such that seal rocking becomes nearly impossible. The second preferred embodiment also includes a pair of leakage collection areas located between the dynamic seal assemblies and the bearings. To prevent bearing damaging material from contacting the bearings, the leakage collection areas are charged with a barrier fluid and an active barrier fluid circulating system is provided for pumping barrier fluid through the bearings themselves and toward the dynamic seal assemblies. The barrier fluid is then removed from the leakage collection areas along with bearing damaging material that leaks past the dynamic seal faces. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The various aspects of the present invention will be more fully understood when the following portions of the specification are read in conjunction with the accompanying drawing wherein: 
     FIG. 1 is a bottom view of a rotating union constructed in accordance with a first preferred embodiment of the present invention; 
     FIG. 2 is a side elevational view of the rotating union of FIG. 1 with a section removed along line  2 — 2  in FIG. 1; 
     FIG. 3 is an end view of a rotating union constructed in accordance with a second preferred embodiment the present invention; 
     FIG. 4 is a cross-sectional view of the rotating union of FIG. 3 taken along line  4 — 4  in FIG. 3 to show a primary circulation system for circulating fresh material through the union from a material provider to a material recipient, and for recycling unused material from the material recipient back to the material provider; 
     FIG. 5 is a cross-sectional view of the rotating union of FIG. 3 taken along line  5 — 5  in FIG. 3 to show a secondary circulation system for circulating a barrier fluid past the dynamic seals of the union to pick up any material that leaks from the primary circulation system; 
     FIG. 6 is an end view of a dynamic chair seal member used in the rotating union of FIG. 3 showing a plurality of spring-receiving holes therein; 
     FIG. 7 is an enlargement of inset view  7 — 7  in FIG. 4 showing a pin arrangement for connecting the dynamic chair seal member to a sleeve member of the rotating union of FIG. 3 to prevent relative rotation between the chair seal member and the sleeve; 
     FIG. 8 is an enlarged cross-sectional view taken along line  8 — 8  in FIG. 7 showing a clip for securing a body ring seal member against rotation relative to a body member of the rotating union; and 
     FIG. 9 is a diagrammatic view showing a barrier fluid recirculation and filtering or centrifuge system in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawing, wherein like reference numerals designate like elements in all of the several views, FIG. 1 illustrates a bottom view of a rotating union  2  for transferring a material stream from a material provider (not shown) to a material recipient (not shown) arranged for mutual relative rotation. FIG. 2 illustrates a side elevation view of the rotating union  2  with a section removed to show interior components. Unless otherwise indicated, all of the structural members of the rotating union  2  described hereinafter are made of a suitable metal, such as aluminum, steel or stainless steel, depending on the application. The rotating union  2  includes a generally cylindrical body  4  and a generally annular sleeve  6  arranged to surround the body  4  in radially spaced relationship therewith over a portion of the body&#39;s axial length. In particular, the sleeve  6  covers the upper end of the body  4  while leaving the lower end exposed. A cap ring  8  is mounted to the bottom of the sleeve  6 , and includes outside and inside walls respectively shown by reference numerals  10  and  12 . The cap ring  8  is attached to the sleeve  6  by plural bolts (not shown) that are received in respective countersunk holes  14  formed in the cap ring and mating holes  15  formed in the sleeve. 
     The lower end of the body  4  is threaded as shown by reference numeral  16  and adapted for attachment to a material recipient (not shown). The threaded portion  16  includes a threaded outside wall  18  and a smooth inside wall  20 . It will be seen in FIG. 2 that the inside wall  20  continues throughout the length of the body  4 , thus defining an interior bore of constant diameter. A circulation tube or pipe  22  extends coaxially within this bore, in spaced relationship with the inside wall  20 . The pipe  22  connects at its upper end to the sleeve  6 , as shown at reference numeral  23 . As described in more detail below, the interior of the pipe  22  provides a circulation return passage  24  for a material stream returning from the material recipient. The annular space formed between the pipe  22  and the inside wall  20  of the body  4  provides a supply passage  26  for delivering a material stream to the material recipient. 
     A barrier fluid cup  28  is seated on a shoulder  29  that is formed on an upper portion of the sleeve  6 . The cup  29  surrounds the sleeve  6  in coaxial relationship therewith. It has an upwardly extending annular side wall  30  and a flat bottom wall  31 , such that the cup  29  forms a pond or reservoir for holding a barrier fluid, as described in more detail below. In this configuration, the rotating union  2  is best suited for an operational setup wherein the sleeve  6  remains stationary, while the body  4  rotates relative thereto. However, the body  4  could remain stationary while the sleeve  6  rotates, if there was reason to do so. 
     With particular reference now to FIG. 2, the body  4  extends upwardly from the threaded portion  16  to a material transfer interface  32  located at the top of the supply passage  26  and representing the area where material is transferred from the sleeve  6  to the body  4 , as described in more detail below. As previously indicated, the interior of the body  4  has a smooth wall  20  providing a constant diameter bore. The outside of the body  4  has several outer wall sections of different diameter, beginning with the threaded outside wall  18  of the threaded portion  16 . These outer wall sections include a pair of upper and lower bearing seats  34  that mount respective first and second bearings  36  and  38 . The bearings  36  and  38  are conventional in nature and provide a rotational interconnection or coupling between the body  4  and the sleeve  6 . The bearings  36  and  38  are mounted so as to be relatively widely spaced from each other. This wide spacing helps minimize dynamic seal misalignment by maintaining the body  4  and the sleeve  6  in relatively non-rocking rotational alignment. 
     A generally annular leakage collection area  40  is provided between the uppermost portion of the body  4  and a medial portion of the sleeve  6 , above the second bearing  38 . The leakage collection area  40  is in fluid communication with the fluid reservoir formed by the cup  28  via a pair of radially extending ports  42  (only one is shown). The ports  42  are preferably spaced 180 degrees from each other. A suitable viscous, cooling, lubricating barrier fluid, such as mineral oil, is carried in the reservoir formed by the cup  28 . The barrier fluid is free to flow between the cup  28  and the leakage collection area  40  via the ports  42 . At the bottom of the leakage collection area  40  is a U-shaped cup seal  44 . The cup seal  44  is of conventional design, and made from a suitable material such as PTFE (Polytetraflouroethylene). An optional impeller  46  is mounted above the cup seal  44 . It is preferably press-fit onto the body  4 , and may be further supported by a spring clip  48 . The purpose of the impeller  46  is to pump material that enters the leakage collection area  40  (and becomes suspended in the barrier fluid) through one of the ports  42  (the other port  42  being a return port, as described below) and into the reservoir provided by the cup  28 . Once in the cup  28 , the material (still carried in suspension by the barrier fluid) will gravitate to the bottom of the cup  28  where it can be safely removed. In this way, the material is prevented from coagulating and is carried away from the area where the bearings  36  and  38  are located. 
     A first material stream supply port  50  is formed in the sleeve  6  and receives material from a material provider (not shown) that will typically remain stationary during material transfer operations. An inlet conduit leading from a material source reservoir is one example of a material provider that could be connected to the supply port  50 . A second material stream return port  52  is formed in the sleeve  6  and is adapted for connection to a material return, which could be a return conduit that leads back to a material source reservoir, thus returning material back to the material source. This allows the material to be circulated for remixing, which is useful for slurries and other mixtures containing particulates that need to remain in suspension in a fluid medium. 
     The material that enters the rotating union  2  through the supply port  50  is carried into an annular receiving area  54  formed between the sleeve  6  and the pipe  22 . From there, the material travels through an annular passage  56  formed between the pipe  22  and an interior wall of an annular dynamic chair seal element  58 . At this point, the material enters the area of the material transfer interface  32 , which may be thought of as including the lower part of the annular passage  56  and the upper part of the supply passage  26 . The material then traverses the supply passage  26  and exits the rotating union  2  through the bottom of the body  4 . Material returning from the material recipient enters the rotating union  2  via the return passage  24  and is carried through the pipe  22  to a material return area  60  in fluid communication with the return port  52 . 
     The dynamic chair seal element  58  forms a dynamic seal with the upper end  36  of the body  4  to retain material in the material transfer interface  32  as the material transfers from the sleeve  6  to the body  4 . A pair of very finely lapped ceramic inserts  62  and  64  are respectively formed in the chair seal element  58  and the upper end of the body  4 ,and provide the mating dynamic seal faces. These surfaces are preferably formed by respectively making a slight undercut in the chair seal member  58  and the upper end of the body  4  and applying (as by bonding or spraying) a ceramic coating to fill each undercut. The ceramic material is then lapped to form an extremely smooth, hard surface. Lapping is preferably performed using a silicon carbide lapping tool to less than about 1 micron roughness. Alternatively, ceramic-on-ceramic lapping could be performed. Thus configured, the dynamic seal faces provide a superior dynamic seal that can withstand many hours of operation in an abrasive fluid environment without significant leakage. In a modified configuration, the chair seal element  58  could be manufactured as a one-piece ceramic casting which is then finish ground and lapped in the manner described above. 
     One or more washer-type springs  66  are used to exert a sealing force on the chair seal element  58 . Advantageously, the spring  66  exerts a uniform spring force on the chair seal element  58  due to its multiple contact points around 360 degrees of diameter therewith, thus minimizing eccentric loading of the chair seal element  58  and the attendant leakage of material across the dynamic seal faces. Alternatively, multiple coil springs (not shown) could be located around the periphery of the chair seal element  58  to provide an equally uniform spring force. The wide axial spacing of the bearings  36  and  38  likewise minimizes dynamic seal rocking and attendant seal misalignment. A pair of locking pins  68  located 180 degrees apart (only one is shown) are used to stabilize the chair seal element  58  against rotation. A pair of conventional static seals  70  made from rubber or similar flexible material are seated in grooves on the sleeve  6  to engage the chair seal element  58 , thus sealing against leakage of the barrier fluid from out of the upper end of the leakage collection area  40 . 
     During operation of the rotating union  2 , the barrier fluid is held in the reservoir defined by the cup  28 . A cover (not shown) is secured over the top of the cup wall  30 . In some cases, the reservoir can be&#39;sealed (enclosed) and the barrier fluid can be pressurized, particularly where the barrier fluid is water, under a pressure. This pressure may even be raised to the point where barrier fluid is forced across the dynamic seal faces and into the material transfer interface  32 , thus cooling and lubricating the seal faces, and assisting the dynamic seal in sealing the material inside the material transfer interface  32 . 
     The barrier fluid is passively circulated by rotary motion of the body  4  relative to the sleeve  6  during operation, which creates a pumping action (assisted by the impeller  46 , if present) from the leakage collection area  40  to the cup  28 . Note that the impeller  46  is radially aligned with the centerline of the port  42  illustrated in FIG.  2 . This allows free movement of the impelled material from the leakage collection area  40  to the cup  28 . Although not shown, the other port  42  is positioned higher on the sleeve  6  than the illustrated port  42 , such that it is above the impeller  46 . This higher port  42  serves as a return passage for barrier fluid returning to the leakage collection area, thus facilitating a circulating flow of barrier fluid. 
     Advantageously, in the event that any material leaks out of the material transfer interface  32  past the dynamic seal faces, it will enter the leakage collection area  40  and become suspended in the barrier fluid, which is selected according to the type of material being carried through the rotating union  2 . As this suspended material circulates into the cup  28 , it will tend to gravitate to the bottom of the cup  28  while clean barrier fluid enters the leakage collection area  40  from the upper portion of the cup  28  through the upper port  42 . In the event that the barrier fluid becomes contaminated with too much material, the cup  28  can be drained via a drain plug  72  and refilled with clean barrier fluid. 
     To maintain the bearings  36  and  38  in serviceable condition, a grease fitting inlet  74  is provided in the side of the sleeve  6 . To prevent leakage of barrier fluid from the cup  28 , a static seal  76  engages the portion of the cup&#39;s bottom wall  31  through which the sleeve  6  extends. To help retain the cup  28  in position, a snap ring  78  is mounted on the sleeve  6  to engage the bottom wall  31 . If necessary, a torque arm (not shown) may be bolted to stabilize the body  6  using a countersunk bolt hole  80  formed in the cap ring  8 . A static seal  82  of conventional design is placed between the cap ring&#39;s inside wall  12  and the body  4  to seal the bearings  36  and  38  against dust and dirt. 
     As previously indicated, the rotating union  2  may carry many different types of bearing damaging material, or it may carry other material that is not necessarily bearing damaging, but which should not leak into the outside environment in any event. The barrier fluid is indicated by way of example above as being mineral oil. This type of barrier fluid can be used when the rotating union  2  carries organic solvent-based materials. For water-based materials, a water-based barrier fluid can be used, although it may need to be treated, as with a soap solution or the like, to reduce shear-sensitivity. For food processing applications, mineral oil is preferred as the barrier fluid because it complies with FDA requirements. For non-consumable material applications, other fluids such as lubricating oils, hydraulic oils, water-based fluids, water, and even gasses, are all feasible barrier fluids that could be used with the present invention. 
     Having now disclosed a first preferred embodiment of the invention, a second preferred embodiment will next be described with initial reference being made to FIG.  3 . FIG. 3 is an end view showing a rotating union  102  for transferring a material stream from a material provider (not shown) to a material recipient (not shown) arranged for mutual relative rotation. Unless otherwise indicated, all of the structural members of the rotating union  102  described hereinafter are made of a suitable metal, such as aluminum, steel or stainless steel, depending on the application. The rotating union  102  includes a generally cylindrical body  104  and a generally annular sleeve  106  arranged to surround the body  104  in radially spaced relationship over a portion of the body&#39;s axial length, and to cover one end of the body while leaving the other end, i.e., the end which is shown in FIG. 3, exposed. Four axially-oriented ports  108 ,  110 ,  112  and  114  (axial ports) are formed at the exposed end of the body  102  and serve several functions as described in more detail hereinafter. 
     Turning now to FIG. 4, the body  104  is seen as having a first covered base end  116  and a second exposed face end  118  in which the axial ports  108 ,  110 ,  112  and  114  are formed. To help facilitate assembly of the rotating union  102 , the sleeve  106  is preferably implemented as a multi-part assembly. FIG. 4 illustrates a preferred embodiment wherein the sleeve  106  is configured as a three-part assembly comprising a central sleeve member  120  and two end caps  122  and  124 . The end cap  122  defines a first closed end  126  of the sleeve  106  that is proximate to, and covers, the base end  116  of the body  104 . The end cap  124  defines a second open end  127  of the sleeve  6  that is proximate, but not necessarily adjacent, the face end  118  of the body  104 . It will be appreciated that the multi-part sleeve configuration of FIG. 4 greatly simplifies the construction of the rotating union  102 . 
     The end caps  122  and  124  are attached to the central sleeve member  120  using a plurality of bolts (not shown). These bolts are received through countersunk bolt holes  128  and  129  that are formed in the end caps  122  and  124 , respectively. The bolts are then secured in threaded holes  130  and  132  that are formed in the central sleeve member  120  in alignment with the bolt holes  126  and  127 , respectively. As can be seen in FIG. 3, the bolt holes  129  (and the same holds true for bolt holes  128 ) are arranged in a spaced pattern around the circumferential face of the end cap open end  127 . In the embodiment of FIG. 3, there are  6  bolt holes  129  (and  128 ) spaced 30 degrees apart. Other bolt patterns could also be used. 
     A generally annular gap is maintained between the body  104  and the sleeve  120 . This gap is covered at the open end  127  of the sleeve  106  by an annular lip seal  140  of conventional design, made from rubber or the like. First and second bearings  150  and  152  are disposed in the aforementioned gap and provide a rotational interconnection or coupling between the body  104  and the sleeve  106 . The bearings  150  and  152  are mounted near the respective ends  126  and  127  of the sleeve  106  so as to be widely axially spaced from each other. As described in more detail below, this wide spacing helps minimize dynamic seal misalignment by maintaining the body  4  and the sleeve  106  in concentric non-rocking rotational alignment. Snap rings  154  and  156  secure one side of each bearing  150  and  152  against axial movement, respectively. More specifically, the snap rings  154  and  156  engage the sides of the inner bearing face of each bearing  150  and  152 . The snap rings  154  and  156  are seated in respective annular grooves  158  and  160  formed in the radial outer surface of the body  104 . The opposite sides of the bearings  150  and  152 , and more specifically, the sides of the outer bearing races, abut against annular shoulders  162  and  164  formed on the end caps  122  and  124  of the sleeve  106 , respectively. 
     A first material stream passage  170 , and an optional second material stream passage  172  (body passages), are formed in the body  104  to carry a material stream through the body  104  for ultimate delivery to the sleeve  106 . The body passage  170  extends axially in the body  104  between the axial port  108  and a first radially-oriented port  174  (radial port) located at the radial exterior surface of the body  104 . The optional body passage  172  extends axially in the body  104  between the axial port  110  and a second radially-oriented port  176  (radial port) located at the radial exterior surface of the body  104 . As can be seen more clearly in FIG. 3., the radial ports  174  and  176  are oriented 180 degrees apart from each other when viewing the rotating union  102  from the face end  118  of the body  104 . 
     The axial port  108  serves as a material stream inlet port to the rotating union  102  and is adapted for connection to a material provider (not shown) that typically, but not always, remains stationary during material transfer operations. An inlet conduit leading from a material source reservoir is one example of a material provider that could be connected to the axial port  108 . The axial port  108  acts in combination with the body passage  170  and the radial port  174  to provide a material stream delivery path for delivering a material stream for transfer to the sleeve  106 . The axial port  110  is adapted for connection to a material return, which could be a return conduit that leads back to a material source reservoir. The axial port  110  acts in combination with the body passage  172  and the second radial body port  176  to provide a material stream return path for returning portions of the material stream that are not transferred to the sleeve  106 , or which return from a material recipient (see below), back to the material provider. 
     A radially-oriented material stream port  180  (radial port) is formed in the sleeve  106 . The radial port  180  is spaced from the radial port  174  on the body  104  in an axially offset relationship, and is adapted to receive material that is provided by the radial port  174 . The radial port  180  serves as a material stream outlet port from the rotating union  102 , and is adapted to be connected to a material recipient that typically, but not always, rotates relative to a stationary material provider. It will be appreciated that an axial material stream outlet port could be provided as an alternative to the radial port  180 . 
     An annular material transfer interface  190  is provided to transfer a material stream from the radial port  174  of the body  104  to the radial port  180  of the sleeve  106 . The material transfer interface  190  thus extends between the radial ports  174  and  182  so as to facilitate material transfer therethrough in a radial, and if desired, axial direction. The material transfer interface  190  is defined in part by an annular slot  191  formed in the radial outer surface of the body  104 , and is bounded by a pair of axially-spaced dynamic seal assemblies  192  and  194  (primary seal assemblies) that are centrally located between the bearings  150  and  152 , and adjacent to the ends of the slot  191 . 
     The primary seal assemblies  192  and  194  each include a generally annular chair seal member, respectively labelled by reference numerals  196  and  198  in FIG.  4 . The chair seal members  196  and  198  are mounted in axially slidable, rotationally fixed engagement with the sleeve  106 . The non-rotational connection between each chair seal member  196 / 198  and the sleeve  106  is provided by a pair of axially oriented anti-rotation pins  200  that are press fit into holes  202  formed in the sleeve  106 , and which extend into notches  204  formed in the chair members  196  and  198  (see FIGS.  6  and  7 ). The axial slidable coupling between each chair seal member  196 / 198  and the sleeve  106  is provided by a circumferential arrangement of springs  206  (see FIG. 5) that axially bias the chair seal members  196  and  198  toward the respective ends  116  and  118  of the body  104 . One end of each spring  206  is received in one of a plurality of holes  208  (see FIG. 6) formed in a circumferential pattern in the chair seal members  196  and  198 . The other end of each spring  206  engages the side of a central shoulder flange  210  (see FIG. 5) disposed on the inner radial surface of the sleeve  106  at a central location between the closed and open sleeve ends  126  and  127 . 
     The primary seal assemblies  192  and  194  each further include a generally annular body ring seal member  220  and  222 , respectively. As shown more clearly in FIGS. 7 and 8, each body ring seal member  220  and  222  is mounted in fixed axial and rotational engagement with the body  104  using a snap ring  224  that seats in a circumferential slot  226  (see FIG. 8) formed in the radial outer surface of the body  104 , and a pair of U-shaped clips (U-clips)  228  (only one is shown). The U-clips  228  are spaced 180 degrees apart when viewing the rotating union  102  from the face end  118  of the body  104 , as in FIG.  3 . Each U-clip  228  has an axially oriented base  230  that is trapped underneath the snap ring  224  in a corresponding well  232  formed in the body  104  as an enlargement of the slot  226  that seats the snap ring  224 . The U-clips  228  further include radially oriented legs, one of which fits into a corresponding notch  236  formed in one side of each of the body ring seal members  220  and  222 , namely, the side that does not engage a chair seal member  196 / 198 . 
     Where the chair seal members  196 / 198  and the body ring seal members  220 / 222  engage each other, i.e., to form the dynamic sealing surfaces, the seal members  196 / 98  and  220 / 222  each have very finely lapped ceramic seal faces  240  and  242 , respectively, as shown in FIG.  7 . As in the first preferred embodiment, these surfaces are preferably formed by making a slight undercut in one side of the seal members  196 / 198  and  220 / 222 , and applying a ceramic coating to fill the undercut. The ceramic material is then lapped in the manner described relative to the first preferred embodiment to form an extremely smooth surface that is preferably less than about 1 micron roughness. 
     Further assisting the effectiveness of the dynamic seal are the plural springs  206 , which provide 360 degrees of uniformly distributed biasing force on the chair seal members  196  and  198 , to urge them into sealing engagement with the body ring seal members  220  and  222 . The chair seal members  196  and  198  are also able to maintain precise alignment as they engage the body ring seal members  220  and  222  because a slight gap, e.g., about 0.005 inches, is maintained to provide a clearance fit between an outer radial surface of each chair seal members  196  and  198 , and a mating surface of the sleeve  106  that each chair seal members  196  and  198  slidably engages. In particular, as shown in FIG. 7, this slidable engagement occurs between a chair seal member surface  250  and a corresponding sleeve surface  252 . The surface  252  represents part of the central shoulder flange  210 . 
     The small gap between the slidably engaging surfaces of the chair seal members  196  and  198  and the sleeve  106  permit the chair seal members  196  and  198  to float slightly. This allows the chair seal members  196  and  198  to adjust to any eccentricities in the alignment of the body ring seal members  220  and  222  that arise due to twisting of the U-clips  228  as they rotatably couple the body ring seal members  220  and  222  to the body  104 . If the body ring seal members  220  and  222  become eccentrically aligned, the chair seal members  196  and  198  will self-align to the body ring seal member eccentricity. The seal faces  240  and  242  will thus remain in mutual parallel contact with each other. A pair of conventional static O-ring seals  258  and  260 , made from rubber or the like, are further provided to minimize material leakage past the primary seal assemblies  192  and  194 . 
     Turning now to FIG. 5, a barrier fluid system is provided to further limit the damaging effect on the bearings of material leakage past the primary seal assemblies  192  and  194 . This is done by affirmatively working to prevent material that does leak past the dynamic seal assemblies  192  and  194  from reaching the bearings  150  and  152 . To that end, a pair of axial barrier fluid passages  270  and  272  are formed in the body  104 . The axial passage  270  is a barrier fluid supply passage that extends between the axial port  112  formed at the face end  118  of the body  104 , which acts as a barrier fluid inlet port, and a pair of barrier fluid injection ports  274  and  276  (injection ports). It will be seen that, unlike the various other ports of the rotating union  102 , the injection port  274  is formed by the disk-shaped gap that exists between the base end  116  of the body  104  and the central inside surface of the end cap  122  of the sleeve  106 . Collectively, the axial port  112 , the axial passage  270  and the injection ports  274  and  276  provide a barrier fluid injection path within the rotating union  102 . More specifically, barrier fluid introduced to the axial port  112  is fed through the axial passage  270  to the injection ports  274  and  276 . The injection ports  274  and  276  are positioned to communicate with a pair of leakage collection areas  277  located between the bearings  150  and  152  and the primary seal assemblies  192  and  194 . In particular, barrier fluid is injected through the bearings  150  and  152  and into the leakage collection areas  277 , where it can collect any material that might leak past the primary seal assemblies from the material transfer interface  190 . As stated relative to the first preferred embodiment, the barrier fluid used in the rotating union  102  can be made from any of a variety of materials. 
     The axial passage  272  is a barrier fluid return passage that extends between the axial port  114  formed at the face end  118  of the body  104 , which acts as a barrier fluid outlet port, and a pair of radially-oriented barrier fluid return ports  278  and  279  (return ports). Collectively, these components provide a barrier fluid return path within the rotating union  102 . More specifically, each of the return ports  278  and  279  communicates with the leakage collection areas  277 . The return port  278  receives barrier fluid that passes through the bearing  150  and enters the left-hand leakage collection area  277  between the bearing  150  and the primary seal assembly  192 . The return port  279  receives barrier fluid that passes through the beating  152  and enters the right-hand leakage collection area  277  between the bearing  152  and the primary seal assembly  194 . The axial passage  272  carries barrier fluid removed from the leakage collection areas  277  to the axial port  114 . 
     As can be seen in FIG. 5, the barrier fluid injection ports  274  and  276  inject barrier fluid so that it flows axially through the bearings  150  and  152 . An alternative to this approach would be to inject the barrier fluid through holes in the outer bearing races such that the barrier fluid enters the bearings radially and exits axially. In either case, the flow of barrier fluid through the bearings works to isolate the bearings  150  and  152  from contact with any material that leaks past the dynamic seal assemblies  192  and  194 . The barrier fluid works to flush out such material and remove it from the leakage collection areas  277  before it contacts the bearings  150  and  152 . 
     Turning now to FIG. 9, an active (positive) barrier fluid recirculation system  280  is connected to the axial ports  112  and  114 . The recirculation system  280  recovers the barrier fluid after it has circulated through the rotating union  102 , including the axial passage  270 , the injection ports  274  and  276 , the bearings  150  and  152 , the leakage collection areas  277 , the return ports  278  and  279 , and the axial passage  272 . The barrier fluid received from the axial port  114  is first passed through a filter or centrifuge  282  to remove any material that may have leaked past the dynamic seal assemblies  192  and  194  and been picked up by the barrier fluid in the leakage collection areas  277 . The filtered barrier fluid then passes to a barrier fluid collection reservoir  284 . A pump  286  sends barrier fluid from the reservoir  284  back to the axial port  112 . Because it has been found that the barrier fluid need not necessarily circulate through the rotating union  102  on a continuous basis to be effective, a timer  288  is provided to activate the pump  286  at selected time intervals, following which the pump  286  is deactivated. 
     Accordingly, an improved rotary union for abrasive, bearing damaging, or other material has been shown and described. Although various embodiments have been disclosed, it should be apparent that many variations and alternative embodiments would be apparent to those skilled in the art in view of the teachings herein. For example, the second preferred embodiment of the invention could be modified by adding a barrier fluid injection port that injects barrier fluid into the material transfer interface  190 . This barrier fluid could be placed under a slightly positive differential pressure relative to the remainder of the barrier fluid system line pressure. The barrier fluid injected into the material transfer interface would tend to flow slowly across the primary seal assemblies  192  and  194  if leaks were present. This would clean and cool the dynamic seal faces  240  and  242  so as to reduce wear. It is understood, therefore, that the invention is not to be in any way limited except in accordance with the spirit of the appended claims and their equivalents.