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RELATED U.S. APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     REFERENCE TO MICROFICHE APPENDIX 
     Not applicable. 
     FIELD OF THE INVENTION 
     The present invention relates to a duct coupler, and more particularly to a coupler for providing a water-tight joint between adjacent sections of duct used to provide a channel for multi-strand post-tensioning of concrete structures. 
     BACKGROUND OF THE INVENTION 
     For many years, the design of concrete structures imitated the typical steel design of column, girder and beam. With technological advances in structural concrete, however, its own form began to evolve. Concrete has the advantages of lower cost than steel, of not requiring fireproofing, and of its plasticity, a quality that lends itself to free flowing or boldly massive architectural concepts. On the other hand, structural concrete, though quite capable of carrying almost any compressive load, is weak in carrying significant tensile loads. It becomes necessary, therefore, to add steel bars, called reinforcements, to concrete, thus allowing the concrete to carry the compressive forces and the steel to carry the tensile forces. 
     Structures of reinforced concrete may be constructed with load-bearing walls, but this method does not use the full potentialities of the concrete. The skeleton frame, in which the floors and roofs rest directly on exterior and interior reinforced-concrete columns, has proven to be most economic and popular. Reinforced-concrete framing is seemingly a quite simple form of construction. First, wood or steel forms are constructed in the sizes, positions, and shapes called for by engineering and design requirements. The steel reinforcing is then placed and held in position by wires at its intersections. Devices known as chairs and spacers are used to keep the reinforcing bars apart and raised off the formwork. The size and number of the steel bars depends completely upon the imposed loads and the need to transfer these loads evenly throughout the building and down to the foundation. After the reinforcing is set in place, the concrete, a mixture of water, cement, sand, and stone or aggregate, of proportions calculated to produce the required strength, is placed, care being taken to prevent voids or honeycombs. 
     One of the simplest designs in concrete frames is the beam-and-slab. This system follows ordinary steel design that uses concrete beams that are cast integrally with the floor slabs. The beam-and-slab system is often used in apartment buildings and other structures where the beams are not visually objectionable and can be hidden. The reinforcement is simple and the forms for casting can be utilized over and over for the same shape. The system, therefore, produces an economically viable structure. With the development of flat-slab construction, exposed beams can be eliminated. In this system, reinforcing bars are projected at right angles and in two directions from every column supporting flat slabs spanning twelve or fifteen feet in both directions. 
     Reinforced concrete reaches its highest potentialities when it is used in pre-stressed or post-tensioned members. Spans as great as one hundred feet can be attained in members as deep as three feet for roof loads. The basic principle is simple. In pre-stressing, reinforcing rods of high tensile strength wires are stretched to a certain determined limit and then high-strength concrete is placed around them. When the concrete has set, it holds the steel in a tight grip, preventing slippage or sagging. Post-tensioning follows the same principle, but the reinforcing tendon, usually a steel cable, is held loosely in place while the concrete is placed around it. The reinforcing tendon is then stretched by hydraulic jacks and securely anchored into place. Pre-stressing is done with individual members in the shop and post-tensioning as part of the structure on the site. 
     In a typical tendon tensioning anchor assembly used in such post-tensioning operations, there are provided anchors for anchoring the ends of the cables suspended therebetween. In the course of tensioning the cable in a concrete structure, a hydraulic jack or the like is releasably attached to one of the exposed ends of each cable for applying a predetermined amount of tension to the tendon, which extends through the anchor. When the desired amount of tension is applied to the cable, wedges, threaded nuts, or the like, are used to capture the cable at the anchor plate and, as the jack is removed from the tendon, to prevent its relaxation and hold it in its stressed condition. 
     Multi-strand tensioning is used when forming especially long post-tensioned concrete structures, or those which must carry especially heavy loads, such as elongated concrete beams for buildings, bridges, highway overpasses, etc. Multiple axially aligned strands of cable are used in order to achieve the required compressive forces for offsetting the anticipated loads. Special multi-strand anchors are utilized, with ports for the desired number of tensioning cables. Individual cables are then strung between the anchors, tensioned and locked as described above for the conventional monofilament post-tensioning system. 
     As with monofilament installations, it is highly desirable to protect the tensioned steel cables from corrosive elements, such as de-icing chemicals, sea water, brackish water, and even rain water which could enter through cracks or pores in the concrete and eventually cause corrosion and loss of tension of the cables. In multi-strand applications, the cables typically are protected against exposure to corrosive elements by surrounding them with a metal duct or, more recently, with a flexible duct made of an impermeable material, such as plastic. The protective duct extends between the anchors and in surrounding relationship to the bundle of tensioning cables. Flexible duct, which typically is provided in 20 to 40 foot sections, is sealed at each end to an anchor and between adjacent sections of duct to provide a water-tight channel. Grout then may be pumped into the interior of the duct in surrounding relationship to the cables to provide further protection. 
     Several approaches have been tried to solve the problem of quickly, inexpensively and securely sealing the joints between adjacent sections of duct used in multi-strand post-tensioned applications. However, all prior art devices have utilized a plurality of arcuate sections which must be assembled at a joint around the ends of adjacent duct sections. Wedges, compression bolts or the like then are used to compress the joined sections into sealing engagement with the duct and with each other. Such prior art devices have been cumbersome to use and have proved somewhat unreliable in their ability to exclude moisture or other corrosive elements from the interior of the ducts. 
     Several patents have issued relating to duct couplers. For example, U.S. Pat. No. 5,320,319, issued on Jun. 14, 1994 to K. Luthi, describes a coupling element which is fitted with chamfered flanges. The sheaths of the coupler have protrusions which are inserted into the coupling element with a tubular element which forms the end of the sheaths. A sealing ring is inserted between each of the flanges and protrusions of the sheaths. The flanges and the protrusions are held together by sloping surfaces and by a groove worked within each socket. Also, U.S. Pat. No. 5,474,335, issued on Dec. 12, 1995 to the present inventor, describes a duct coupler for joining and sealing between adjacent sections of the duct. The coupler includes a body, flexible cantilevered sections on the end of the body adapted to pass over annular protrusions on the duct and locking rings for locking the cantilevered flexible sections into position, so as to lock the coupler onto the duct. 
     U.S. Pat. No. 5,775,849, issued on Jul. 7, 1998 to the present inventor, describes a coupler as used for ducts in post-tension anchorage systems. This duct system includes a first duct having a plurality of corrugations extending radially outwardly therefrom, a second duct having a plurality of corrugations extending radially outwardly therefrom, and a tubular body threadedly receiving the first duct at one end and threadedly receiving the second duct at the opposite end. The tubular body has a first threaded section formed on an inner wall of the tubular body adjacent one end of the tubular body and a second threaded section formed on the inner wall of the tubular body adjacent an opposite end of the tubular body. The threaded sections are formed of a harder polymeric material than the polymeric material of the first and second ducts. The tubular body has an outer diameter which is less than the diameter of the ducts at the corrugations. The first and second threaded sections have a maximum inner diameter which is less than the outer diameter of the ducts at the end of the ducts. First and second elastomeric seals are affixed to opposite end of the tubular body and juxtaposed against a surface of a corrugation of the first and second ducts. 
     U.S. Pat. No. 5,954,373, issued on Sep. 21, 1999 to the present inventor, describes a different type of duct coupler apparatus. The duct coupler apparatus of this patent includes a tubular body with an interior passageway between a first open end and a second open end. A shoulder is formed within the tubular body between the open ends. A seal is connected to the shoulder so as to form a liquid-tight seal with a duct received within one of the open ends. A compression device is hingedly connected to the tubular body for urging the duct into compressive contact with the seal. The compression device has a portion extending exterior of the tubular body. The compression device includes an arm with an end hingedly connected to the tubular body and having an abutment surface adjacent the end. The arm is movable between a first position extending outwardly of an exterior of the tubular body and a second position aligned with an exterior surface of the tubular body. A latching member is connected to an opposite end of the arm and serves to affix the arm in the second position. The abutment surface of the arm serves to push a corrugation of the duct against the seal and against the shoulder so as to form a liquid-tight seal between the duct and the interior of the coupler. 
     U.S. Pat. No. 6,764,105, issued on Jul. 20, 2004 to the present inventor, describes a duct coupler apparatus for use with precast concrete segmental construction. This coupler has a first duct, a first coupler member extending over and around an exterior surface of the first duct and having a seat opening adjacent an end of the first duct, a second duct, a second coupler member extending over and around an exterior surface of the second duct and a seat opening adjacent to an end of the second duct, and gasket received in the seats of the first and second coupler members. An external seal is affixed to an opposite end of the first coupler member and affixed to an exterior surface of the first duct. The seats of the first and second coupler members have slots facing one another. The gasket is received within these slots. 
     U.S. Pat. No. 6,752,435, issued on Jun. 22, 2004 to the present inventor, describes a symmetrical coupler apparatus for use with precast concrete segmental construction. This coupler member has a first duct, a first coupler member extending over and around an exterior surface of the first duct and an end opening adjacent an end of the first duct, a second duct, a second coupler member extending over and around an exterior surface of the second duct and an end opening adjacent to an end of the second duct, and a gasket received in the ends of the first and second coupler members. The gasket serves to prevent liquid from passing between the ends of the coupler members into an interior of either of the first and second ducts. An external seal is affixed to an opposite end of the first coupler member and affixed to an exterior surface of the first duct. An internal seal is interposed in generally liquid-tight relationship between an interior surface of the second coupler member and an exterior surface of the second duct. 
     U.S. Pat. No. 6,834,890, issued on Dec. 28, 2004 to the present inventor, teaches a coupler apparatus for use with a tendon-receiving duct in a segmental precast concrete structure. This coupler apparatus includes a coupler body having an interior passageway for receiving the duct therein. The coupler body has a generally U-shaped channel formed at one end thereof. The coupler element has a connector element formed on interior thereof adjacent one end of the coupler body so as to allow the coupler element to receive a variety of implements for the formation of the precast concrete segment. 
     U.S. Pat. No. 6,874,821, issued on Apr. 5, 2005 to the present inventor, describes a coupler apparatus for use with angled post-tension cables in precast concrete segmental construction. This coupler apparatus has a first duct, a first coupler member extending over and around the first duct, a second duct, a second coupler member extending over and around the second duct and a gasket received at the ends of the first and second coupler members so as to prevent liquid from passing between the coupler members into an interior of either of the ducts. The ducts extend at a non-transverse acute angle with respect to the ends of the coupler members. Heat shrink seals are affixed to the opposite ends of the coupler member so as to secure the coupler members to the ducts in liquid-tight relationship. The ends of the coupler member have generally V-shaped grooves facing each other. The gasket is received in compressive relationship within the V-shaped grooves. 
     U.S. Pat. No. 7,273,238, issued on Sep. 25, 2007 to the present inventor, teaches a duct coupler apparatus with compressible seals. This apparatus is used for joining the ends of a pair of ribbed ducts together. The apparatus has a collar with an interior suitable for receiving the ends of the pair of ducts therein. A first coupler element is translatably secured adjacent a first end of the collar. A compressible seal is disposed between a surface of the first coupler element and the first end of the collar. A second coupler element is secured adjacent a second end of the collar. A second seal is disposed between a surface of the second coupler element and the second end of the collar. The coupler elements are translatable so as to compress the seal such that a surface of the seal will bear against a respective rib of the pair of ducts. 
     U.S. Pat. No. 7,267,375, issued on Sep. 11, 2007 to the present inventor, describes a duct coupler apparatus. This apparatus is for joining ends of a pair of ducts together in end-to-end relationship. The apparatus has a collar with a first end portion and a second end portion. A first coupler element is translatably secured to an exterior of the collar for moving the first end portion between first and second positions. A second coupler element is translatably secured to the exterior of the collar so as to move the second end portion between first and second positions. The end portions have a plurality of fingers that are movable so as to be free of the surfaces of the duct when in the first position and which contact a rib of the duct when in the second position. The collar and the coupler elements form a liquid-tight seal over the respective ends of the pair of ducts. 
       FIGS. 1-3  herein describe the prior art coupler apparatus similar to that disclosed in U.S. Pat. No. 7,267,375. Referring to  FIG. 1 , there is shown the coupler apparatus  10  in of the prior art. The coupler apparatus  10  includes a collar  12 , a first coupler element  14  and a second coupler element  16 . A first duct  18  is received within the interior of the collar  12  and within the interior of the first coupler element  14 . A second duct  20  is received within the collar  12  and within the interior of the second coupler element  16 . The collar  12  has an interior suitable for receiving the ducts  18  and  20  in end-to-end relationship and in generally longitudinal alignment. The collar  12  has first end portion  22  at one end thereof and a second end portion  24  at an opposite end thereof. Each of the end portions  22  and  24  are movable between a first position (illustrated by end portion  24 ) spaced away from the interior of the collar  12  and a second position (illustrated by end portion  22 ) which extends toward the interior of the collar  12 . The first coupler element  14  is translatably secured to the exterior of the collar  12 . The first coupler element  14  is translatable so as to move the first end portion  22  between the first and second positions. The second coupler element  16  is also translatably secured to the exterior of the collar  12 . The second coupler element  16  is translatable so as to move the second end portion  24  between the first and second positions. 
     As can be seen in  FIG. 1 , the first duct  18  has a plurality of ribs  26  formed thereon. Longitudinal channels  28  extend between the ribs  26  and allow liquid and grout therein to communicate between the ribs  26 . Longitudinal channels  28  have an outer edge which is flush with the outer diameter of the respective ribs  26 . The first duct  18  has an outer wall which extends between the ribs  26  and defines the interior of the duct  18 . The second duct  20  similarly has a plurality of ribs  32 , longitudinal channels  34  and wall  36 . The first duct  18  is identical to the second duct  20 . In normal use, the ducts  18  and  20  will receive tendons therein and allow a grout material to fill the interior thereof. The respective channels  28  and  34  allow grout to fill the interior of the respective ducts  18  and  20  and to flow into the ribs  26  and  32 , respectively. 
     As can be seen, the first end portion  22  has a plurality of finger elements  38 ,  40 ,  42 ,  44  and  46  extending outwardly therefrom. In  FIG. 1 , for the purposes of illustration, the finger element  38  is illustrated in its second position which serves to lock the first duct  18  in its proper position. The finger element  22  has a lower surface  48  which will reside in surface-to-surface relationship with the wall  30  of duct  18 . An extension element  50  extends outwardly as a tip from the finger element  38  so as to reside over the outer surface of the rib  26 . An inclined surface extends between the tip  50  and the surface  48  so as to reside against the slanted surface of the rib  26 . The remaining finger elements  40 ,  42 ,  44  and  46  are illustrated in the first position extending away from the surface of the duct. In normal use, the finger elements  38 ,  40 ,  42 ,  44  and  46  will move cooperatively relative to the translation of the first coupler element  14  on the collar  12 . 
     The collar  12  has a plurality of finger elements  52 ,  54 ,  58 , and  60  extending outwardly from an opposite end thereof of finger elements  22 . Each of the finger elements  52 ,  54 ,  58 , and  60  is illustrated in the first position spaced away from the exterior surface of the duct  20 . The coupler element  16  is translatable relative to the collar  12  so as to move the finger elements  52 ,  54 ,  58 , and  60  to the second position. 
     In  FIG. 1 , it can be seen that there is an indented portion  62  formed in the collar  12  generally between the ends of the ducts  18  and  20 . The indented surface  62  will have an interior surface aligned with interior surface of the respective ducts  18  and  20 . 
     The collar  14  is translatable about one end of the collar  12 . The translating motion in the preferred embodiment of the present invention is established by a threaded relationship between the exterior surface of the collar  12  and the interior surface of the coupler  14 . In other embodiments of the present invention, the coupler element  14  is translatable by slidable or ratcheting motion. Suitable hinging mechanisms or other cantilever or lever actions can be incorporated within the apparatus  10  so as to facilitate proper translatable motion of the coupler elements  14  and  16  on the collar  12 . Coupler element  16  will have a configuration similar to that of coupler element  14  and will translate in the same manner as coupler element  14 . Each of the coupler elements  14  and  16  has a plurality of ribs  64  formed on an exterior surface thereof. Each of the plurality of ribs  64  extends longitudinally for at least a portion of the length of the respective coupler elements  14  and  16 . The plurality of ribs are radially spaced from each other around the diameter of the respective coupler elements  14  and  16 . Ribs  64  facilitate the ability of a worker to grasp the exterior surface of the coupler elements  14  and  16  and to provide the necessary translatable motion with respect to the movement of the coupler elements  14  and  16  onto the respective end portions  22  and  24 . 
       FIG. 2  illustrates the collar  12  as having the end portions  22  and  24  in the first position away from the respective ducts  18  and  20 . In  FIG. 2 , the collar  12  is illustrated as having the indented portion  62  formed between the respective ends  66  and  68  of ducts  18  and  20 . The inward surface of the indented portion  62  is in coplanar alignment with the inner surface  70  of duct  18  and inner surface  72  of duct  20 . The collar  62  has an annular seal  74  extending around the interior of the collar  12 . A second annular seal  76  is also affixed to the collar  12  and extends around the interior of the collar  12 . The annular seals  74  and  76  can be formed of a suitable elastomeric material such that the seal  74  establishes a liquid-tight relationship with the rib  26  of duct  18 . The annular seal  76  will establish a liquid-tight seal with the rib  32  of duct  20 . It can be seen that the collar  12  has an inner surface which will generally abut the tops of the respective shoulders  26  and  32  of the ducts  18  and  20 . As such, the ducts  18  and  20  can be easily installed within the interior of the collar  12  by slidably inserting the ends  66  and  68  of ducts  18  and  20  into opposite ends of the collar  62 . 
     In  FIG. 2 , it can be seen that the collar  12  has a threaded exterior surface  78 . The collar  12  also has another threaded exterior surface  80  formed thereon. The end portion  22  is integrally formed with the collar  12  at one end of the collar  12 . The second end position  24  is also integrally formed with the collar  12  at the opposite end of the collar  12 . The threaded portions  78  and  80  are respectively interposed between the indented portion  62  and the end portions  22  and  24 . The end portion  22  has a shoulder  82  formed thereon. The end portion  24  also has a shoulder  84  formed thereon. Underlying surface  48  extends from shoulder  82  outwardly therefrom. Another underlying surface  86  is formed on the end portion  24  and extends outwardly from the shoulder  84 . End surfaces  48  and  86  will extend generally upwardly at an acute angle with respect to a longitudinal axis of the collar  12 . In  FIG. 2 , the first position of the end portions  22  and  24  is particularly illustrated. As such, the shoulders  82  and  84 , along with the surfaces  48  and  86 , will be generally spaced away from the respective ducts  18  and  20  so as to allow for the free insertion of the ends  66  and  68  of ducts  18  and  20  into the collar  12 . 
     The first coupler element  14  is illustrated as having interior threads  88  engaged with the exterior threads  78  of the collar  12 . The first coupler element  14  has an abutment end  90  extending into contact with a surface of the end portion. Similarly, the second coupler element  16  has an interior threaded section  92  threadedly engaged with the exterior threads  80  of the collar  12 . An abutment end  94  is formed on the coupler element  16  so as to reside against the surface of the end portion  24 . 
       FIG. 3  illustrates how the coupler elements  14  and  16  translate so as to move the end portions  22  and  24  into their second or locking positions. In normal use, the coupler elements  14  and  16  will be rotated so that the interior threads  88  will translate along the exterior threads  78  at one end of the collar  12 . The second coupler element  16  will similarly have its interior threads  92  rotate with respect to the exterior threads  80 . This causes the abutment end  90  of coupler element  14  to urge against the surface of the end portion  22  and to move the end portion  22  downwardly. As a result, the shoulder  82  will reside in contact (illustrated in broken line fashion) against a surface of rib  26 . The second coupler element  16  will work in a similar manner so that the shoulder  84  will reside in contact against a surface of the rib  32 . In this locked position, it will be impossible to pull the first duct  18  away from the second duct  20 . A secure seal is formed between the interior surfaces of the collar  12  and the exterior surfaces of the ducts  18  and  20 . The annular seal  74  and  76  will further provide a strong liquid-tight seal against the outer surfaces of the respective ducts  18  and  20 . 
     It has been found with the prior art coupler apparatus illustrated in  FIGS. 1-3  that it is often somewhat complicated to properly install the apparatus. In certain circumstances, the installation can be somewhat time consuming. As such, it has been found that there is a need to provide a coupler apparatus for ducts which allows workman at the construction site to easily connect the ends of the ducts through a use of a coupler. The coupler should be of a type that is suitable for effectively engaging the ends of the ducts in a liquid-tight manner. The coupler apparatus should have a minimum number of moving parts so as to effectively create the necessary seal while, at the same time, avoids complexities in the actual manufacturing injection molding of such a coupler apparatus. 
     It is an object of the present invention to provide a duct coupling system that allows the ends of tendon-receiving ducts to be joined in a proper end-to-end relationship. 
     It is another object the present invention to provide a duct coupling system that effectively establishes a liquid-tight seal between the respective coupled ducts. 
     It is another object of the present invention to provide a duct coupling system which allows the coupler to be formed through an injection molding process. 
     It is still another object of the present invention to provide a duct coupling system which allows the ducts to be effectively coupled in a minimal amount of time with a minimum complexity. 
     These and other objects and advantages of the present invention will become apparent from a reading of the attached specification and appended claims. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a duct coupling system that comprises a first duct, a second duct, and a coupler that joins the first duct to the second duct. The first duct has an interior passageway at a first end. The first end has threads thereon. The interior passageway opens at the first end of the first duct. The second duct also has an interior passageway at a first end. The interior passageway of the second duct opens at the first end of the second duct. The first end of the second duct has threads thereon. The coupler is a generally tubular body with a first end and a second end and an interior passageway extending therebetween. The first end of the tubular body is threadedly engaged with the threads of the first end of the first duct. The second end of the coupler is threadedly engaged with the threads at the first end of the second duct. 
     The interior passageway of the first duct and the interior passageway of the second duct and the interior passageway of the coupler are axially aligned. 
     The thread of the duct has a unique configuration. The thread of the first end of the first duct has a narrow width portion and a wide width portion. This thread extends radially outwardly of the first duct for a lesser distance at the narrow width portion than a distance at the wide width portion. The narrow width portion of this thread is spaced from the narrow width portion of an adjacent thread. The wide width portion is offset by approximately 90° from the narrow width portion. 
     Each of the first and second ducts has a ridge extending circumferentially therearound adjacent the first end thereof. The coupler has a first lip extending longitudinally outwardly therefrom so as to overlie the ridge of the first duct. The coupler having a second lip extending longitudinally outwardly therefrom so as to overlie the ridge of the second duct. The body of the coupler has a first ring seal juxtaposed between an inner surface of the body and a surface of the ridge of the first duct. The body of the coupler also has a second ring seal juxtaposed between another inner surface of the body and surface of the ridge of the second duct. The body of the coupler has a radially indented area extending circumferentially therearound between the first and second ends of the coupler. This radially indented area is positioned between the first end of the first duct and the first end of the second duct. The first end of the body of the coupler has a square threads extending inwardly therefrom and engaged with the threads at the first end of the first duct. The second end of the body of the coupler has square threads extending inwardly therefrom and engaged with the threads at the first end of the second duct. The thread at the first end of the first duct has an end and a portion circumferentially spaced from this end of the thread. The end of the thread extends radially outwardly of the first duct for a lesser distance than a distance that the portion extends outwardly of the first duct. The first end of the body of the coupler is slidable over the ends of the thread. The coupler is rotatable relative to the first duct so that the threads at the first end of the body of the coupler engaged with the portion of the threads at the first end of the first duct. 
     In the present invention, each of the first duct and the second duct and the coupler are integrally formed of a polymeric material. A plurality of tendons extend through the interior passageways of the first duct, the second duct, and the coupler. 
     The present invention is also a duct coupler that includes a generally tubular body having a first end and a second end with interior passageway extending therebetween. The first end and the second end are interiorly threaded. The body is formed of a polymeric material. The threads at the first end and the threads at the second end of the body are square threads. The body has an outer surface with a first lip extending longitudinally outwardly at the first end and a second lip extending outwardly at the second end. A first ring seal is affixed against an inner surface of the body adjacent the first lip. A second ring seal affixed against an inner surface of the body adjacent the second lip. The body has a radially indented area around a circumference thereof in an area between said first and second ends. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a side elevational view of a prior art duct coupler. 
         FIG. 2  is a cross-sectional view showing the end portions of the collar of the coupler apparatus of the prior art in a first position. 
         FIG. 3  is a cross-sectional view showing the end portions of the collar of the coupler of the prior art in the second locked position. 
         FIG. 4  is a cross-sectional view showing the duct coupling system in accordance with the preferred embodiment of the present invention. 
         FIG. 5  is a cross-sectional view showing the duct coupling system of the present invention with the coupler rotated 90° with respect to the duct. 
         FIG. 6  is a upper perspective view of an end of either the first duct and a second duct as used with the coupler apparatus of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 4 , there is a shown the duct coupling system  100  in accordance the preferred embodiment of the present invention. The duct coupling system  100  includes a first duct  102 , a second duct  104  and a coupler  106 . Coupler  106  joins the ducts  102  and  104  in a liquid-tight relationship. 
     The first duct has an interior passageway  108  and a first end  110 . The interior passageway  108  opens at the first end  110 . The first end  110  has threads  112  formed thereon. The second duct  104  has an interior passageway  114  and a first end  116 . The interior passageway  114  of the second duct  104  opens at the first end  116 . The first end  116  of the second duct  104  has threads  118  formed thereon. 
     The coupler  106  has a generally tubular body  120 . The tubular body  120  has a first end  122  and a second end  124 . The coupler  106  also has an interior passageway  125  extending between the first end  122  an the second end  124 . It can be seen that the first end  122  of the body  120  is threadedly engaged with the threads  112  at the first end  110  of the first duct  102 . The second end  124  of the coupler  106  is threadedly engaged with the threads  118  at the first end  116  of the second duct  104 . 
     The interior passageway  108  of the first duct  102  and the interior passageway  114  of the second duct  104  and the interior passageway  125  of the coupler  106  are longitudinally axially aligned. 
     In  FIG. 4 , it can be seen that the first duct  102  has a ridge  130  extending circumferentially therearound adjacent to the first end  110  thereof. The coupler  106  has a first lip  132  extending longitudinally outwardly therefrom so as to overlie the ridge  130  of the first duct  102 . The coupler  106  also has a second lip  134  extending longitudinally outwardly therefrom so as to overlie the ridge  136  of the second duct  104 . It can be seen that there is a first elastomeric ring seal  138  juxtaposed between an inner surface of the body  120  of the coupler  106  and a surface of the ridge  130  of the first duct  102 . A second elastomeric ring seal  140  is juxtaposed between another inner surface of the body  120  of the coupler  106  and a surface of the ridge  136  of the second duct  104 . 
     The coupler  106  has a radially indented area  142  extending circumferentially therearound between the first end  122  and the second end  124  of the coupler  106 . This radially indented area  142  is positioned between the first end  110  of the first duct  102  and the first end  116  of the second duct  104 . 
     In  FIG. 4 , it can be seen that the first end  122  of the body  120  has square threads  150  extending inwardly therefrom and engaged with the threads  112  at the first end  110  of the first duct  102 . The second end  124  of the body  120  of coupler  106  also has square threads  152  that are engaged with the threads  118  at the first end  116  of the second duct  104 . 
       FIG. 5  shows another form of the present invention in which the coupler  106  is illustrated as being located in another position with respect to the threads of the respective ducts  200  and  202 . It can be seen in  FIG. 5  that the first elastomeric ring seal  204  that juxtaposed against an inner surface of the lip  206  at the end of coupler  106  and an outer surface of the first duct  200 . There is also another elastomeric ring seal  208  that is juxtaposed in liquid-tight relationship between the end  210  of the coupler  106  and an outer surface of the duct  202 . In this embodiment, the lips at the end of the coupler  106  do not overlie the ridge  220  of the first duct  200  and the ridge  222  of the second duct  202 . 
       FIG. 6  shows the ends of the respective ducts. In particular, duct  104  is particularly illustrated in  FIG. 6 . The unique configuration of the threads  118  adjacent to the end  116  of the duct  104  are particularly illustrated. The duct  104  is illustrated as having interior passageway  114  opening at the end  116  and extending therethrough. The duct  104  is also illustrated as having the ridge  136  extending circumferentially therearound and forming a raised surface with respect the to threads  118 . 
     In particular, in  FIG. 6 , it can be seen that there is illustrated a single thread  300 . The threads  300  includes a narrow width portion  302  and a wide width portion  304 . The narrow width portion  302  is of set by approximately 90° from the wide width portion  304 . In the preferred embodiment of the present invention, the narrow width portion  302  extends outwardly of the surface  306  of the duct  104  for a distance less than the distance that the wide width portion  304  extends outwardly from surface  306 . The narrow width portion  302  is in spaced relationship to another narrow width portion  310  of an adjacent thread. The narrow width portion  310  of the adjacent thread  312  extends pass the end of the thread  300 . The alignment of the various narrow width portions of the various thread  300  will serve to allow the teeth of the duct coupler to be slidably positioned thereover. To install the duct, it is only necessary to push the threaded portion of the coupler over the narrow width portion so that the end of the coupler abuts the ridge  136 . The coupler can then be rotated upwardly or downwardly such that the threads become wedged between the wide width portion  304  of the various threads  300 . As such, a 90° rotation of the coupler in one direction or another will cause the coupler to be installed effectively in a liquid-tight sealing manner. As such, installation in the coupler can be accomplished in a very efficient and effective manner. If it is desired to remove the coupler for any reason, it is only necessary to rotate the coupler backward by 90° so that the threads can slide over the narrow width portions of the duct  104 . A similar operation can be used so as to install the coupler over the respective threads of the first duct  102 . 
     In the present invention, the coupler is able to establish a liquid-tight seal in a fast and efficient manner. Additionally, the coupler can be formed through an injection molding process. It is only necessary to form the threads on the inner surface of the coupler. The lips of the coupler will extend outwardly so as to effectively center the coupler with respect to ridges formed on the ducts. As such, a proper alignment of the couplers with the duct is effectively achieved. The liquid-tight sealing relationship is established by virtue of the rotation of the coupler with respect to the ducts. 
     The foregoing description of the invention is illustrative and explanatory thereof. Various changes in the details of the illustrated construction may be made within the scope of the appended claims without departing from the true spirit of the invention. The present invention should only be limited by the following claims and their legal equivalents.

Summary:
A duct coupling system has a first duct with and end having a threads thereon, a second duct having an end with threads thereon, and a coupler having a first end threadedly engaged with the threads of the first duct and a second end threadedly engaged with the threads of the second duct. The ducts and the coupler are each integrally formed of a polymeric material. A plurality of tendons extend through the interior passageways of the ducts of the coupler.