You are an expert at summarizing long articles. Proceed to summarize the following text:

You are an expert at summarizing long articles. Proceed to summarize the following text: 
[0001]     A barrel, or butt hinge is a hinge composed of interlocking plates attached to abutting surfaces of fixed and moveable elements, the fixed element having an opening which can be covered by the moveable element, such as a door and a door jam. The interlocking plates have curved leaves that wrap around a common pin. The barrel hinge is one of the most common types of door attachments employed in modern building construction.  
       BACKGROUND AND PRIOR ART  
       [0002]     Vertical doors are generally installed with two or more independent hinge assemblies. The hinge assemblies perform two functions; bearing weight and providing an axis of rotation to enable opening and closing the door. Ideally, this axis of rotation parallels the direction of the gravitational force acting on the mass of the door. However, the center of mass of the door is physically offset from the axis of rotation. In addition, the hinge assemblies may be slightly misaligned at initial installation or become so over time due to a shifting door jam. The net effect of even slight misalignments or unbalanced tension between hinge assemblies often results in unintended and undesirable door movement.  
         [0003]     U.S. Pat. No. 6,574,836 B1 addresses this problem by providing an adjustable lateral force to the exterior knuckles of a barrel type door hinge. While the &#39;836 patent indeed provides an external force resistant to unintended door movement, it requires the installation of an external visible device. The &#39;836 patent operates by applying resistance to the outside surface of adjoining knuckles on separate leaves. The resistance is applied by means of a series of “brake pads”, the placement of which is dependent on the particular geometry of a given hinge model. Moreover, the &#39;836 device will move slightly with each opening and closing of the door, requiring routine maintenance and potentially interfering with its range of motion.  
         [0004]     It is an objective of this invention to provide a simple, inexpensive internal friction restraint for a hinge that will function with a hinge of any length, width or leaf configuration.  
         [0005]     It is an objective of this invention to provide a simple, inexpensive internal friction restraint for a hinge that can be installed with a minimum of effort and require only basic tools.  
         [0006]     It is an objective of this invention to provide a simple, inexpensive internal friction restraint for a hinge that is easily manufactured.  
         [0007]     It is an objective of this invention to provide a simple, inexpensive internal friction restraint for a hinge that is visibly innocuous and requires minimal change to existing hardware.  
         [0008]     The features of the invention believed to be novel are set forth with particularity in the appended claims. However the invention itself, both as to organization and method of operation, together with further objects and advantages thereof may be best understood by reference to the following description taken in conjunction with the accompanying drawings.  
       SUMMARY  
       [0009]     An internal friction system for a typical hinge is described wherein friction is created at the contacting surface between the hinge pin and the leaf knuckle. Either or both of the inside surface of the leaf knuckle or the outside surface of the hinge pin shaft can be covered with a motion resistive material. The system can be either adjustable or non-adjustable. The system can further be either removeable or non-removeable.  
       DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0010]     The definitions below serve to provide a clear and consistent understanding of the specification and claims, including the scope given to such terms.  
         [0011]     Hinge Pin—Rod running the length of the hinge, holding the leaves together.  
         [0012]     Leaf—the portion of a hinge extending laterally from the knuckle  
         [0013]     Knuckle—the hollow circular part at the joint of a hinge through which a pin is passed.  
         [0014]     Motion resistive material—substance amenable to creating friction upon contact with another surface. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]      FIG. 1  is an illustration of a typical hinge assembly.  
         [0016]      FIG. 2  is an illustration of a typical hinge pin.  
         [0017]      FIG. 3  illustrates the basic features of the expandable hinge pin embodiment of the present disclosure.  
         [0018]      FIG. 4  illustrates the non-expandable hinge pin embodiment of the present disclosure.  
       IDENTIFICATION OF NUMBERS USED IN THE DRAWINGS  
       [0019]      100 —typical hinge assembly  
         [0020]      110 —leaves of hinge  
         [0021]      120 —knuckles  
         [0022]      130 —cylindrical void of knuckle through which pin is inserted  
         [0023]      140 —standard hinge pin  
         [0024]      200 —head of standard hinge pin  
         [0025]      210 —hinge pin shaft  
         [0026]      300 —adjustable hinge pin of the present invention  
         [0027]      310 —hinge pin head  
         [0028]      320 —hinge pin end assembly  
         [0029]      330 —hinge pin core  
         [0030]      340 —expandable sleeve comprised of motion resistive material surrounding hinge pin core  
         [0031]      400 —non-adjustable hinge pin of the present invention  
         [0032]      410 —hinge pin head  
         [0033]      430 —hinge pin core  
         [0034]      440 —motion resistive material surrounding hinge pin core 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0035]     A typical door hinge ( 100 ) is shown in  FIG. 1 . It has two leaves ( 110 ), one of which is attached to a moveable structure such as a door and the other attached to a fixed structure such as door jam, but may also be used when both leaves are mounted on a movable structure, such as on multi-panel shutters.  
         [0036]     The leaves have two or more interlocking knuckles ( 120 ) with a cylindrical void ( 130 ) in common alignment with each. A hinge pin shaft ( 210 ) fits snugly within the cylindrical void of each knuckle ( 130 ) so as to connect the two leaves of the hinge together while allowing rotational movement about the hinge pin axis. The pin commonly has a head ( 200 ) to prevent the pin from sliding through the knuckle ( 130 ),  
         [0037]     Two general embodiments are described in this disclosure. A removable hinge pin with an optionally expandable radius is described as well as a non-removable pin included as part of a pre-built hinge assembly. The non-removable pin may be inserted as a replacement pin in a pre-installed hinge.  
         [0038]     In the first embodiment, the standard hinge pin ( 140 ) is replaced by a pin with an expandable radius ( 300 ) in order to create an adjustable friction coupling between the leaves ( 110 ) of the hinge. The expandable pin may be used on one or more the hinges of the door, depending on the restraining force desired. The weight of the door may dictate that the expandable hinge pin be part of a supplemental hinge as a non weight-bearing assembly.  
         [0039]     The expandable pin ( 300 ), shown in  FIG. 3 , consists of a shaft ( 330 ), an expandable sleeve ( 340 ), and constraining members ( 310 ) and ( 320 ) on the ends of the shaft. The constraining members act to snugly contain the expandable sleeve ( 340 ) by applying pressure on the ends. A simple machine screw, a lock nut and Tygon® tubing, for example, can perform the basic functions of the expandable sleeve and constraining members.  
         [0040]     One or both of the constraining members ( 310 ) and ( 320 ) are optionally movable along the length of the shaft ( 330 ). When one or both are moved in such a way as to shorten the distance between them, the expandable sleeve is squeezed at both ends by the constraining members and becomes distorted in the radial direction. Effectively, the sleeve bows out in the mid-shaft region, thereby increasing its radius and providing an increased amount of friction between the outer surface of the sleeve ( 340 ) and the inner wall defining the cylindrical void ( 130 ) running through each knuckle of its leaf.  
         [0041]     The expandable sleeve may be of a variety of composite elastic materials such as rubber, silicon rubber, or polyurethane. For the pin to be removable, the material must have an elastic hysteresis such that it will not cold flow or take a “set” under pressure. When the compression force is relieved, the sleeve must be able to return to its approximate original diameter in order to facilitate removal.  
         [0042]     A second embodiment of the friction hinge pin is appropriate for a pre-built assembly. Such a configuration would include a set of leaves with the standard hinge pin replaced by a non-removable pin made of a motion resistive material such as natural rubber, silicon rubber, or a coated metal pin as shown in  FIG. 4 , capable of creating friction upon contact with the inner surface of the knuckles. Such an embodiment allows the use of a hinge pin having the standard shape and size. The leaves of such an assembly could be made much thinner and smaller than standard leaves so as to render mortising unnecessary if installed at a non-weight bearing location. A coated pin may also be field insertable as a replacement for a standard metal pin.  
         [0043]     A variation of the second embodiment, also appropriate for a pre-built assembly, involves using a standard hinge pin in combination with a set of leaves, wherein the inside surface of the leaf knuckles are coated with a motion resistive material.

Summary:
Means to create a friction restraint between the interlocking leaves of a hinge. The outer surface of the hinge pin shaft is in contact with the inner surface of the interlocking elements. Friction is generated between the contacting surfaces through a motion resistive material comprising the outside surface of the hinge pin shaft, the inner surface of the knuckle, or both.