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FIELD OF THE INVENTION 
   The invention relates to the field of closures; specifically hinge closures. In particular the invention relates to safety structures that incorporate hinges that operate primarily under the influence of gravity to secure closure of safety fences and gates. 
   BACKGROUND OF THE INVENTION 
   The present invention relates to safety barriers, and in particular relates to safety fences and gates in which opened gates return automatically to a closed position. One particular use for hinges along this line is pedestrian traffic control in industrial work areas. For example, Federal regulatory authorities (e.g., OSHA and EPA) require extensive systems to control the path and flow of workers in industrial plants. Hinged gates and doors are often used to restrict movement in areas deemed dangerous or to seal off areas containing harmful materials. Typically these regulations are implemented by installing extensive railing systems painted in fluorescent colors, usually bright yellow. 
   A common feature of these systems are self-closing gates and doors. Currently, spring loaded gates which automatically close via the tension in the spring are most commonly used. Other types of gates that are known and could be used are gravity gates that close automatically via the action of gravity. 
   Gravity gates typically employ a cylindrical hinge consisting of at least two parts: a lower portion and an upper portion that rotates about an oblique junction upon the application of a rotational force. As the upper portion rotates, the two portions separate due to the oblique junction. The upper portion “rises” thereby storing potential energy which will cause the upper portion to “fall” or rotate back to a neutral position when the rotational force is terminated. Examples of such a gate are shown in U.S. Pat. No. 4,631,777 to Takimoto, U.S. Pat. No. 3,733,650 to Douglas and U.S. Pat. No. 4,991,259 to Finkelstein et al. 
   One problem associated with known gravity gates is common to all devices that employ moving parts: friction. In many instances the rotating portions of the hinges are in direct contact with one another which causes friction. If the portions are made of metal, as they often are, the friction could lead to premature failure of the hinge absent some form of external lubrication. External lubrication, most often in the form of grease, is messy and transitory thereby leading to frequent maintenance. 
   More recent designs of gravity gates incorporate polymers to reduce the weight of the hinge and friction. The Douglas, Takimoto and Finkelstein patents cited above discuss implementing polymers in the design of gravity gates. These patents discuss hinges that use polymer cams to translate rotational energy to potential energy. Although polymer cams reduce friction, polymer cams are far more susceptible to torsional failures than metallic cams. Furthermore, the devices of these patents utilize multiple polymeric parts which increases the likelihood of torsional failure. When these weaknesses are combined with the difficulties relating to machining and molding such intricate polymer parts, the impracticality of these hinges is readily apparent. 
   Another weakness of known hinge and gate designs is the free conduction of electricity. Many hinges and gates employ metal on metal contact which leads to the conduction of electricity. Such conduction can be fatal. For example, a hot wire falling on a metal railing could electrocute someone passing through a swing gate attached to the railing. 
   OBJECT AND SUMMARY OF THE INVENTION 
   Therefore, an object of the present invention is to provide a hinge that automatically closes upon the application of gravity. 
   A further object of the invention is to provide a gravity hinge that is efficiently designed and easy to maintain. 
   A still further object of the invention is to provide a gravity hinge that eliminates the need for periodic lubrication of the hinge joint. 
   A still further object of the invention is to provide a gravity hinge, gate and fencing system that reduces or eliminates electrical conduction between the fence portion and the gate portion of the system. 
   The gravity hinge according to the invention meets these and other objects. The gravity hinge comprises an upper cylindrical knuckle having a first terminating surface and an opposing second terminating surface. The second terminating surface is oblique to the axis of the upper knuckle. The gravity hinge also comprises a lower cylindrical knuckle having a first terminating surface oblique to the axis of the lower knuckle. Preferably the oblique angle of the lower knuckle first terminating surface is approximately the same as the second surface of the upper knuckle. The lower knuckle also has an opposing second terminating surface. 
   A spindle, which is received by at least one of the knuckles, establishes rotating communication between the upper and lower knuckles. The upper and lower knuckles are situated such that the second terminating surface of the upper knuckle is opposed to the first terminating surface of the lower knuckle. 
   A bushing surrounds the spindle and separates the upper and lower knuckles. The bushing has a lower coefficient of friction with respect to the respective oblique surfaces of the upper and lower knuckles than the respective surfaces have for each other. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view of one embodiment of the hinge according to the invention. 
       FIG. 2  is an exploded view of one embodiment of the hinge according to the invention. 
       FIG. 3  is a partial cross-section of one embodiment of the hinge according to the invention. 
       FIG. 4  is an illustration of the action of a rotational force on a hinge according to the invention. 
       FIG. 5  is an exploded view of one embodiment of the hinge according to the invention. 
       FIG. 6  is an exploded view of one embodiment of the hinge according to the invention. 
   

   DETAILED DESCRIPTION 
   The invention provides a gravity hinge for use with a gate, door or other hinged closure. As used herein, the term “hinge” has its usual definition; e.g., “a jointed or flexible device on which a door, lid or other swinging part turns.” Merriam-Websters&#39; Collegiate Dictionary (online edition, cited as of Jan. 9, 2001). Referring now to  FIG. 1 , there is shown a hinge  10  in accordance with the invention. For ease of discussion and presentation to the reader, the hinge  10  is shown in conjunction with a gate or door  12  and support post  14  such as would be commonly found in the practice of the invention. This particular setting, however, should not be interpreted as limiting in any way the scope of the invention. 
   Referring now to  FIG. 2 , in a preferred embodiment the hinge  10  according to the invention comprises an upper tubular cylindrical knuckle  16 . As used herein relational terms such as upper and lower are used for explanatory purposes and as an aid to the reader&#39;s interpretation of the drawings. Such terms should not be read or interpreted as any type of limitation on the scope of the invention. The upper tubular cylindrical knuckle  16  is defined by a first terminating surface  18  that is preferably perpendicular to the axis of the upper tubular cylindrical knuckle  16 . The upper tubular cylindrical knuckle  16  is also defined by a second terminating surface  20  that is separated from and opposed to the first terminating surface  18 . The second terminating surface  20  is oblique to the axis of the upper tubular cylindrical knuckle  16 . As shown in  FIG. 2 , the first and second terminating surfaces  18 ,  20  give the upper knuckle  16  the general appearance of a truncated right circular cylinder. Preferably, a flange  22  is attached to the outer surface of the upper tubular cylindrical knuckle  16  to secure the knuckle to the gate, door or closure used in conjunction with the hinge  10 . The flange  22  may be manufactured integral with the upper tubular cylindrical knuckle  16  or manufactured separate from the upper tubular cylindrical knuckle  16 . 
   The upper tubular cylindrical knuckle  16  is preferably made of metal but may be made of any suitable material (i.e., ceramic, polymers) provided the material possesses the requisite physical properties required for the particular use. For example, the hinge may be made electrically or thermally insulating by choosing an insulating ceramic or polymer. Suitable polymers include but are not limited to neat or “unfilled” polytetrafluoroethyelene (PTFE), polyetheretherketone (PEEK) and ultra-high molecular weight polyethylene (UHMW). Similarly, the hinge may be made electrically or thermally conductive by choosing an appropriate metal or modified polymer such as “filled” PTFE, PEEK or UHMW. The term polymer as used herein includes, but is not limited to, polymers and composites comprising polymers including fiberglass. 
   Theoretically, the angle of the oblique second terminating surface  20  may be any angle between 0° and 90°. As a practical matter, however, angles between about 30° and 50° are preferred, with angles of about 45° being most preferred. 
   The hinge  10  also comprises a lower cylindrical knuckle  24 . The lower cylindrical knuckle  24  has a first terminating surface  26  that is oblique to the axis of the lower knuckle  24 . Preferably the angle of the oblique first terminating surface  26  is approximately equal to the angle of the second terminating surface  20  of the upper tubular cylindrical knuckle  16 . The lower cylindrical knuckle  24  also has a second terminating surface  28  separate from and opposing the first terminating surface  26 . Preferably, the second terminating surface  28  is perpendicular to the first terminating surface  26 , thus forming a structure resembling a truncated right circular cylinder. Just as with the upper tubular cylindrical knuckle  16 , the lower cylindrical knuckle  24  may be made of metal or any other suitable manufacturing material (i.e., ceramic or polymer). 
   A spindle  30 , which is received by at least one of the two knuckles, establishes rotating communication between the upper and lower knuckles. Stated alternatively, the spindle  30  rotatably engages the upper and lower knuckles  16 ,  24  such that the oblique terminating surfaces  20 ,  26  are proximate to each other. 
   In the embodiment shown in  FIG. 2 , the spindle  30  extends from the first terminating surface  26  of the lower cylindrical knuckle  24 . The spindle  30  should have a diameter that is smaller than the diameter of the lower cylindrical knuckle  24  thereby creating an oblique ledge  32  corresponding to the outer perimeter portion of the oblique terminating surface  26 . The spindle  30  may be integral to the lower cylindrical knuckle  24  as shown in  FIG. 2 . For example, the spindle  30  could be a machined extension of the lower knuckle  24  or physically attached to the lower knuckle  24  (e.g., welded). Alternatively, the spindle  30  may be separate from the lower cylindrical knuckle  24 . In this latter embodiment, the lower cylindrical knuckle  24  must possess a recess for receiving a portion of the spindle  30 . Such a recess is represented by the dotted lines  36  in the lower knuckle  24  shown in  FIG. 2 . 
   In preferred embodiments, the spindle  30  and the lower cylindrical knuckle  24  are integral and the length of the spindle  30  extending from the lower knuckle  24  is greater than the maximum length of the upper cylindrical knuckle  16 . Just as with the knuckles, the spindle  30  may be made of metal or any other suitable manufacturing material (i.e., ceramic or polymer). 
   A flange  22  attached to the outer surface of the lower cylindrical knuckle  24  secures the knuckle to the post or static structure utilized in conjunction with the hinge  10 . The flange  22  may be manufactured integral with the lower cylindrical knuckle  24  or manufactured separate from the knuckle. 
   As shown in  FIGS. 2 and 3 , one embodiment of a gravity hinge  10  according to the invention is formed when the upper tubular cylindrical knuckle  16  receives in a close mating relationship the spindle  30  extending from the lower knuckle  24 . The upper knuckle second terminating surface  20  thus comes into close contact with the lower knuckle first terminating surface  26 , specifically the oblique ledge  32  of the lower knuckle first terminating surface  26 . If the knuckles are made from a non-polymeric substance, such as metal, significant friction will typically develop at the interface between the knuckles. Thus, a complete gravity hinge  10  according to the invention also comprises a self-lubricating friction reducer  34  that separates the upper knuckle second terminating surface  20  and the lower knuckle first terminating surface  26 . In a preferred embodiment, the self-lubrication friction reducer  34  is a bushing possessing an opening  36  that allows the bushing to slide down and surround the base of the spindle  30  and rest upon the oblique ledge  32  of the lower cylindrical knuckle  24 . 
   The self-lubricating friction reducer  34  is formed of a material possessing a coefficient of friction with respect to the oblique surfaces of the upper and lower knuckles ( 20 ,  32 ) that is lower than the coefficient of friction between the two oblique surfaces ( 20 ,  32 ). Accordingly, the self-lubricating friction reducer  34  is not limited to any particular material. For example, in certain circumstances, it could be formed of metal (i.e., brass) or ceramic provided its coefficient of friction with respect to the oblique surfaces is lower than the coefficient of friction between the two oblique surfaces. As with the knuckles, the exact material of construction for the bushing will depend on individual circumstances. Suitable materials include but are not limited to those discussed in relation to the knuckles. In a preferred embodiment, the self-lubricating friction reducer is made from a polymer. 
   Preferably, the bushing  34  incorporates the same oblique angle as the oblique ledge  34 . In other words, the bushing  34  is a uniform angled slice from a hollow right circular cylinder. Thus, bushing  34  separates the oblique surfaces ( 20 ,  32 ) while maintaining the angled relationship of the surfaces. The bushing  34  may be made of any suitable polymer that has an appropriate coefficient of friction and that is otherwise compatible with the structure and function of the hinge, gate and fence. Polyethylene, polyester, polypropylene, PTFE and PEEK are representative, and the exact polymer choice can be made by those of skill in this art and without undue experimentation based on factors such as cost, weight, ease of manufacture and industrial purpose. Alternatively, the bushing may be formed of a core material (e.g., metal) that is coated with a polymer. Furthermore, the efficient design of the hinge  10  (shown in  FIG. 2 ) provides great versatility in the choice of polymer. If for some reason the bushing must be changed (for example to a more solvent-resistant bushing), all one must do is lift off the gate portion of the hinge  10  (i.e., the upper knuckles  16 ) and slide on a different bushing. Known designs of polymer containing gravity gates do not provide this flexibility. 
     FIGS. 1 ,  3  and  4  illustrate the operation of one embodiment the hinge  10  according to the invention. In the absence of any rotational force (i.e., at rest) the hinge  10  appears as shown in  FIGS. 1 and 3 . The oblique surfaces of the knuckles are proximate and parallel to one another and are separated by the bushing  34 . Note the relative positions of the screws or bolts  35  that attach the hinge knuckles to the gate  12  and support post  14 . 
     FIG. 4  illustrates the relative positioning of the knuckles ( 16 ,  24 ) upon the application of a rotational force, such as that applied by a person traversing the gate  12 . The oblique angle of the junction of the knuckles ( 16 ,  24 ) and bushing  34  causes the upper tubular cylindrical knuckle  16  to rotate upwards upon the application of the rotational force thereby storing potential energy in the hinge  10 . Note the relative positions of the screws or bolts  35  in  FIG. 4 . Upon the release of the rotational force, gravity (and the low friction bushing  34 ) causes the upper knuckle  16  to “fall” and rotate back to its starting or resting position shown in  FIGS. 1 and 3 . 
   Another embodiment of the invention is shown in  FIG. 5 . In this embodiment, a lower knuckle receives a spindle extending from an upper knuckle thereby rotatably engaging the upper knuckle with the lower knuckle. All elements of this embodiment may be made from the materials discussed in conjunction with previous embodiments. 
   More specifically and in reference to  FIG. 5 , this embodiment of the invention comprises an upper cylindrical knuckle  50  possessing a first terminating surface  52  and a second terminating surface  54  oblique to the axis of the upper knuckle  50 . 
   A spindle  30  extends from the second terminating surface  54  of the upper knuckle  50 . As with previous embodiments, the spindle  30  may be integral to the upper knuckle  50  or separate from the upper knuckle  50 . In the latter embodiment, the upper knuckle  50  is preferably tubular and the spindle  30  possesses a cap or nut  56  that is larger than the diameter of the knuckle&#39;s tubular opening. The spindle  30  traverses the length of the upper knuckle  50  and the cap  56  prevents the spindle  30  from passing through tubular knuckle  50 . The spindle  30  engages with a lower knuckle  58  which is also tubular as shown in  FIG. 5 . The lower knuckle  58  has a first oblique surface  60  and an opposing second surface  62  similar to the lower knuckles of the previous embodiments. Preferably, the spindle  30  extends to a point beyond the opposing second surface  62  thereby allowing the spindle  30  to remain engaged with the lower knuckle  58  when the spindle  30  rises with the upper knuckle  50  upon the application of a rotational force such as that applied by a person traversing a gate incorporating the hinge. 
   A self-lubricating friction reducer  34  similar to that described in previous embodiments separates the upper and lower knuckles  50 ,  58 . 
   A further embodiment of the invention is shown in  FIG. 6 . This embodiment is similar to previous embodiments in that it comprises an upper knuckle  80  and a lower knuckle  82 . Each knuckle has an oblique surface ( 91 ,  92 ) that functions similarly to the oblique surfaces of previously described knuckles. A spindle  30  rotatably engages the two knuckles. A self-lubricating friction reducer  86  separates the knuckles from each other and at least one of the knuckles from the spindle  30 .  FIG. 6  incorporates the basic knuckle design shown in  FIG. 5  for ease of explanation. It should be understood, however, that the principles of this embodiment may work in conjunction with any of the previous embodiments. 
   As shown in  FIG. 6 , the self-lubricating friction reducer  86  comprises an oblique upper portion (or bushing)  88  and a sleeve  84  that is preferably integral with the oblique upper portion  88 . The cylindrical sleeve  84  preferably possess an outer diameter d′ that is less than the outer diameter of the oblique upper portion (or bushing)  88  thereby creating a ledge  90  that may rest on the oblique surface  92  of the lower knuckle  82 . 
   One knuckle should possess an opening of a size sufficient to receive both the spindle  30  and the tubular sleeve  84  of the self-lubricating friction reducer  86 . In the embodiment shown in  FIG. 6 , the lower knuckle  82  is tubular as in previous embodiments and possesses an opening  89  having a diameter such that the knuckle  82  may receive a spindle  30  having a diameter (d) and a cylindrical sleeve  84  having an outer diameter of (d′). 
   Those skilled in the art will readily recognize that the self-lubricating friction reducer  86  may be oriented such that the sleeve  84  is received by the upper knuckle  80  in which case the upper knuckle  80  should possess an opening having a diameter sufficient to receive a spindle  30  and the sleeve  84  of the self-lubricating friction reducer  86 . Alternatively, the self-lubricating friction reducer  86  could have sleeves  84  extending from both sides of the oblique portion (or bushing)  88 . In this instance both knuckles should possess a suitable opening  89  to receive the sleeve  84 . 
   In a further preferred embodiment the sleeve discussed above may be separate from the bushing. Referring again to  FIG. 6 , the upper knuckle may possess an opening sufficient to receive a separate self-lubricating sleeve that separates the spindle from the upper knuckle. Such a sleeve  93  is shown in each embodiment of the upper knuckle in  FIGS. 2 ,  3 ,  5  and  6 . In yet another alternative, the lower knuckle may be designed to receive a separate sleeve such as that identified by the numeral  93 . 
   As with previous embodiments, the knuckles, self-lubricating friction reducer and sleeves may be made of any suitable material provided the material possesses the requisite structural, chemical and electrical properties. Self-lubricating friction reducers (including the sleeves) made of non-conducting polymers are particularly well suited for applications in which insulating a portion of the overall hinge, gate or fence is desired. 
   An additional embodiment of the invention is shown in  FIG. 1 . The embodiment comprises a gravity gate  11  formed of the gravity hinges discussed above. This embodiment incorporates the embodiment of the hinge shown in  FIG. 2  but may incorporate any of the other embodiments of the hinge as well. As shown in  FIG. 1 , the gravity hinge  10  joins a frame member  12  and a static structure  14 . The frame member  12  may be a gate or door or other such suitable closure. The static structure  14  maybe a post or wall or any other structure forming part of an opening that is regulated by a gate or door. In preferred embodiments, the gravity gate  11  is used in conjunction with a fence to control the flow of traffic through a secured area. The frame member  12 , static structure  14 , and fence may be made of metal, wood, polymer or any other suitable material. 
   The invention has been described in detail, with reference to certain preferred embodiments, in order to enable the reader to practice the invention without undue experimentation. However, a person having ordinary skill in the art will readily recognize that many of the components and parameters may be varied or modified to a certain extent without departing from the scope and spirit of the invention. Furthermore, titles, headings, or the like are provided to enhance the reader&#39;s comprehension of this document, and should not be read as limiting the scope of the present invention. Accordingly, only the following claims and reasonable extensions and equivalents define the intellectual property rights to the invention thereof.

Summary:
A gravity hinge is disclosed that is simple to manufacture and avoids limitations found in known gravity hinges. The hinge includes two cylindrical knuckles joined by a spindle and separated by a polymer bushing. The junction of the knuckles and the bushing is at an angle oblique to the vertical axis of the knuckles and spindle. The oblique angle causes the upper knuckle to rotate upward upon the application of a rotational force thereby storing potential energy. Upon the release of the rotational force the upper knuckle falls or (rotates) back into place thereby returning the hinge to its natural, closed position.