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FIELD OF THE INVENTION 
     This invention relates generally to hinges for products such as, but not limited to, folding furniture, and, more particularly, to a self-locking and releasing dual-axis hinge that is suitable for use in folding furniture as well as with other products requiring a hinged joint that can lock and release without use of a separate locking device. 
     BACKGROUND 
     A typical hinge connects two objects, allowing a limited range of angular rotation about a fixed axis between the objects. Typically, a hinge does not lock in a position. Thus, the objects may be freely moved from one angular position to another angular position. 
     Various locking hinges have been devised. For example, hinges for folding ladders include a ratcheting mechanism to lock the hinge in a position. A releasable spring actuated pawl engages a toothed cog. The engaged pawl prevents rotation. To release the pawl a lever must be raised or pressed while pressure is relieved. Relieving pressure typically entails manipulating the hinged objects so that the pawl is between adjacent teeth, without considerable stress against either tooth. While such a mechanism is useful for a ladder that will be opened and closed by skilled adults, it is not well suited for a device that may be used by children. 
     Other locking mechanisms including latches, bolts and stops that may be positioned to prevent angular rotation. Such mechanisms are useful, but are separate from the hinge. 
     What is needed is a hinge that can be used in a wide variety of applications, including children&#39;s furniture. The hinge should lock into an open position. When locked, the hinge should resist closing during normal use of the object (e.g., furniture). The hinge should unlock by pivoting one particular side. Thus, by folding the object (e.g., furniture) in the correct sequence, unlocking is achieved. 
     The invention is directed to overcoming one or more of the problems and solving one or more of the needs as set forth above. 
     SUMMARY OF THE INVENTION 
     To solve one or more of the problems set forth above, in an exemplary implementation of the invention, a self-locking and releasing dual-axis hinge that is suitable for use in folding furniture is provided. The hinge automatically locks when unfolded, and automatically unlocks when folded in a determined sequence. The sequence for folding entails rotating about one axis of rotation to release a locking element and then rotating about the other axis of rotation. The exemplary hinge includes a first locking plate coupled to a first object (e.g., a portion of furniture to be hingedly pivoted). The first locking plate includes a first base having a first plate aperture and a first plate slot. The first plate aperture is centered at a first axis of angular rotation. The first plate slot extends from a locked slot position to an unlocked slot position. The unlocked slot position is closer to the first axis of angular rotation than the locked slot position. 
     A first unlocking plate is coupled to a second object (e.g., another portion of furniture to be hingedly pivoted). The first unlocking plate includes a second base having a second plate aperture and a periphery with a peripheral recess. The second plate aperture is centered at a second axis of angular rotation, and the peripheral recess defines a retaining position. An L-shaped hook-like portion may extend outwardly from the recess, to limit rotation therefrom. The periphery of the first unlocking plate includes a protruding stop spaced apart from the peripheral recess. 
     A link includes an elongated body, a first link aperture concentric with the first axis of angular rotation, a second link aperture concentric with the second axis of angular rotation, and a locking element slot extending between the first link aperture and the second link aperture. 
     A locking element has an elongated body with a first end and an opposite second end, and a first pin extending from the elongated body near the first end of the elongated body, and a second pin extending from the elongated body adjacent to the second end of the elongated body. A portion of the first pin is within the plate slot of the first locking plate. The second pin abuts the periphery of the first unlocking plate. The locking element is movably disposed within the locking element slot of the link. 
     The dual axis self-locking and unlocking hinge according to claim  2 , further includes an L-shaped hook extending outwardly from the periphery of the first unlocking plate at the peripheral recess. 
     The dual axis self-locking and unlocking hinge according to claim  1 , further includes a first shaft extending through the first plate aperture and the first link aperture. 
     A first shaft of a first barrel may extend through the first plate aperture and the first link aperture. Likewise, a second shaft of a second barrel may extend through the second plate aperture and the second link aperture. These shafts define axes of angular rotation of the hinge. 
     In one exemplary embodiment, there are a pair of locking plates and a pair of unlocking plates, with each of the locking plates being identical, and each of the unlocking plates being identical. The link is disposed between the pairs. Each plate of a pair is coupled to the other plate of the pair with a pin that extend through an arc-shaped slot in the link. The arc-shaped slot allows rotation of the link relative to the plate. 
     In an exemplary embodiment, a housing (e.g., pod-shaped housings) contains each pair of locking plates, and another housing contains each pair of unlocking plates. The plates are coupled to their respective housings. The housings provide an interface for attaching the first and second objects to be hingedly coupled. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other aspects, objects, features and advantages of the invention will become better understood with reference to the following description, appended claims, and accompanying drawings, where: 
         FIG. 1  is a perspective view that conceptually illustrates a foldable furniture item, namely a child&#39;s bed, equipped with an exemplary self-locking and releasing dual-axis hinge according to principles of the invention; and 
         FIG. 2  is a side view that conceptually illustrates a foldable furniture item, namely a child&#39;s bed, equipped with an exemplary self-locking and releasing dual-axis hinge according to principles of the invention; and 
         FIG. 3  is a perspective view that conceptually illustrates an exemplary self-locking and releasing dual-axis hinge assembly in a folded configuration according to principles of the invention; and 
         FIG. 4  is a profile view that conceptually illustrates an exemplary self-locking and releasing dual-axis hinge assembly in a folded configuration according to principles of the invention; and 
         FIG. 5  is a perspective view that conceptually illustrates an exemplary self-locking and releasing dual-axis hinge assembly in an unfolded configuration according to principles of the invention; and 
         FIG. 6  is a perspective view that conceptually illustrates components of an exemplary self-locking and releasing dual-axis hinge assembly in a unfolded configuration according to principles of the invention; and 
         FIG. 7  is a side view that conceptually illustrates components of an exemplary self-locking and releasing dual-axis hinge assembly in a unfolded configuration according to principles of the invention; and 
         FIG. 8  is a side view that conceptually illustrates components of an exemplary self-locking and releasing dual-axis hinge assembly in a folded configuration according to principles of the invention; and 
         FIG. 9  is a perspective view that conceptually illustrates a slotted link for an exemplary self-locking and releasing dual-axis hinge assembly according to principles of the invention; and 
         FIG. 10  is a perspective view that conceptually illustrates a barrel for an exemplary self-locking and releasing dual-axis hinge assembly according to principles of the invention; and 
         FIG. 11  is a perspective view that conceptually illustrates a lock for the slotted link for an exemplary self-locking and releasing dual-axis hinge assembly according to principles of the invention; and 
         FIG. 12  is a perspective view that conceptually illustrates a locking mechanism for an exemplary self-locking and releasing dual-axis hinge assembly in a folded configuration according to principles of the invention; and 
         FIG. 13  is a side view that conceptually illustrates a locking mechanism for an exemplary self-locking and releasing dual-axis hinge assembly in a folded configuration according to principles of the invention; and 
         FIG. 14  is a side view that conceptually illustrates a locking mechanism for an exemplary self-locking and releasing dual-axis hinge assembly in an unfolded configuration according to principles of the invention. 
     
    
    
     Those skilled in the art will appreciate that the figures are not intended to be drawn to any particular scale; nor are the figures intended to illustrate every embodiment of the invention. The invention is not limited to the exemplary embodiments depicted in the figures or the specific components, configurations, shapes, relative sizes, ornamental aspects or proportions as shown in the figures. 
     DETAILED DESCRIPTION 
     Referring now to  FIGS. 1 and 2 , perspective and side views that conceptually illustrates a foldable furniture item  100 , namely a portable child&#39;s bed, equipped with an exemplary self-locking and releasing dual-axis hinge assembly  130 , according to principles of the invention. A hinge according to the invention may be used with furniture and items other than a portable child&#39;s bed  100 . The child&#39;s bed  100  is used for illustrative purposes. 
     The exemplary bed  100  provides a framework onto which a fabric and cushion may be supported. In the exemplary embodiment, the bed  100  is comprised of three sections  105 ,  110 , and  120 . The two outer sections  105 ,  110  are pivotally attached to the intermediate section  120  with a plurality of hinge assemblies  125 ,  130  disposed therebetween. One pair of hinge assemblies  125  is shown in an unfolded configuration while another pair  130  is shown in a folded configuration. When in the unfolded configuration, the hinge assembly  125  may be unlocked only by folding the object (e.g., furniture) in the correct sequence, as described more fully below. Joints  115  connect or couple framework elements such as rigid tubular elements. 
     With reference to  FIGS. 3 and 4 , perspective and profile views that conceptually illustrates an exemplary self-locking and releasing dual-axis hinge assembly  130  in a folded configuration according to principles of the invention is provided. The hinge assembly includes two axes of angular rotation. The axes are defined by barrels  220 ,  230 . The axes are coplanar, e.g., aligned in the same plane, e.g., the vertical plane. The coplanar axes are spaced apart. The barrels  220 ,  230  are parallel, i.e., the longitudinal axes of the barrels  220 ,  230  are parallel. Additional barrels  225 ,  235  are provided for coupling tubing or other structural members to the hinge assembly  130 . 
     A locking mechanism (described below) is contained in a housing  200  of the hinge assembly  130 . The housing  200  includes first and second mating leg portions  205 ,  210 . The leg portions  205 ,  210 , are not essential to the hinge assembly  130 . Rather, they are used to support the bed  100 . A socket  215  is provided on one leg portion  210 . The socket  215  receives the end of a support (e.g., a rigid tube) to form the framework of the furniture  100 . 
     The housing  200  also includes pods  240 ,  243 . Each pod  240 ,  243  is comprised of mating first and second portions (e.g., portion  245  mating with portion  250  to form pod  240 , and portion  265  mating with portion  270  to form pod  243 ). The pods house portions of the hinge and provide sockets  242 ,  272  for receiving the end of a support (e.g., a rigid tube) to form the framework of the furniture  100 . Barrels  225 ,  235  are provided for coupling ends of the tubing or other structural members within the sockets  242 ,  272 . 
     A link  255  and lock  260  connect the pods  240 ,  243 . The link  255  and lock  260 , which are described in more detail below, extend from one pivoting axis to another. The axes are defined by barrels  220 ,  230 . 
     In  FIGS. 3 and 4 , the hinge assembly  130  is shown in a fully folded position. In a fully folded position, the sockets  242 ,  272  of the pods  240 ,  243  face in about the same direction.  FIGS. 1 and 2  show hinge assembly  130  in a folded configuration. The space between the axes of angular rotation defined by the link  255  allows space for overlapping portions  110 ,  120 . The angle between the pods  240 ,  243  when fully folded is acute, possibly 0°, depending upon the configuration and supported framework. 
     In  FIG. 5 , the hinge assembly  130  is shown in a fully open position. In a fully open position, the sockets  242 ,  272  of the pods face in about opposite directions, and the angle between the pods  240 ,  243  is about 180°.  FIGS. 1 and 2  show hinge assembly  125  in an open configuration. 
     In  FIGS. 6 and 7  perspective and side views conceptually illustrate components of an exemplary self-locking and releasing dual-axis hinge assembly in an unfolded configuration. Each end of the link  255  is sandwiched between a pair of leg plates  300 ,  400  (at one end) and  305 ,  405  at the opposite end. Leg plates  400  and  405  are better seen in  FIG. 12 . The leg plates  300  and  400 ,  305  and  405  are held in fixed relation to the containing pod or housing element. The leg plates may be contained in a compartment in the corresponding pod or housing element. Rotation of the pod formed by elements  245 ,  250  causes rotation of the contained pair of leg plates  305 ,  405  about barrel  230 . Link  255  may rotate about barrel  220  and, therefore, relative to leg plates  300 ,  400 , contained in the pod housing elements  265 ,  270  of the leg  205 . A locking element  260  fits in a slot in the link  255 . Each end of the locking element  260  includes a pair of protruding guide pins  262  (and one on the opposite side) and  264  (and one on the opposite side). In the open configuration, pin  262  sits in cove  308  of the leg plate  305 . The cove  308  is a hook shaped recess that includes a concave recess in the periphery  306  of the leg plate  305  that limits the range of rotation in one direction, while protruding stop  307  limits the range of rotation in the opposite direction. In the open configuration, the pin  262  sits in the recessed cove  308 . Because the pin  262  is in the recessed cove  308 , rotation about barrel  230  is prevented until the pin  262  is dislodged from the recess. Dislodgement occurs when locking element  260  moves away from the recessed cove  308 . Such movement occurs when link  255  is pivoted about barrel  220 . During such pivoting, pin  264  of locking element  260  travels in a curved slot  302 . The curved slot  302  moves closer to the central axis of the barrel  220  as the pin  264  moves from the open configuration. In other words, the distance between the slot  302  and the central axis of the barrel  220  varies, with said distance being greatest at the open position of the slot, which is where the pin  264  resides in  FIGS. 6 and 7 , when the hinge assembly is in an open configuration. The distance is least at the folded position of the slot, which is opposite the open position of the slot  302 , i.e., at the opposite end of the curved slot  302 . The pin  264  is shown in the folded position of the slot  302  in  FIG. 8 . This configuration of the curved slot  302  causes the locking element  260  to move linearly, albeit slightly, within the slot  256  of the link  255 , as shown in  FIG. 9 . Such movement is possible because the length of the locking element  260  is less than the length of the slot  256  of the link  255 . Such movement dislodges the pin  262  from the recessed cove  308 . When dislodged from the recessed cove  308 , the pin  262  may move away from the recessed cove  308 , along or adjacent to the periphery  306  of the leg plate  305 , until the pin  262  reaches the stop  307 . The stop  307  is a protrusion that prevents further rotation. Thus, the pin  262  may move between stop  307  and the cove  308 , when the pin  262  is dislodged. When the pin  262  resides in the recessed cove  308 , the recessed cove  308  prevents appreciable movement of the pin  262  until the pin  262  is dislodged (i.e., withdrawn) from the cove  308 . When the pin  262  resides in the recessed cove  308 , rotation of leg plate  305  relative to the barrel  230  is prevented by the recessed cove  308 . 
       FIG. 8  is a side view that conceptually illustrates components of an exemplary self-locking and releasing dual-axis hinge assembly in a folded configuration according to principles of the invention. Pin  262  is against the stop  307  in the folded configuration. Pin  264  is at the end of the curved slot  302  that is closest to the central axis of barrel  220 . The opening of socket  242  faces in about the same direction as the opening of socket  272 . 
       FIG. 9  is a perspective view that conceptually illustrates a slotted link for an exemplary self-locking and releasing dual-axis hinge assembly according to principles of the invention. The link  255  includes an elongated slot  256 , which in a preferred embodiment is approximately central. The slot includes a first end  257  and an opposite second end  258 . The length of the slot  256 , measured from the first end  257  to the second end  258 , exceeds the length of the locking element  260 . The locking element  260  fits within the slot  256  and may move from side to side between the ends  257 ,  258  of the slot  256 . The ends  257 ,  258  of the slot  256  are circular and sized to receive the shaft  221  of each barrel, such as barrel  220  shown in  FIG. 10 . A curved slot  253 ,  254  is provided between the periphery and elongated slot  256  of the link. The curved slots  253 ,  254  receive pins  222 ,  232 , which extend from one leg plate  300  to its mating leg plate  400 , and from the other leg plate  305  to its mating leg plate  405 . Pin  222  extends from leg plate  300 , through the curved slot  254 , to leg plate  400 . Pin  232  extends from leg plate  305 , through the curved slot  253 , to leg plate  405 . The pins  222 ,  232  are narrower than the width of the curved slots  253 ,  254 . Thus, the pins  222 ,  232  may move, relative to the link  255 , within the slots  253 ,  254 . 
       FIG. 10  is a perspective view that conceptually illustrates a barrel for an exemplary self-locking and releasing dual-axis hinge assembly according to principles of the invention. The barrel  220  includes opposite ends  223 A,  223 B and a cylindrical shaft  221  between the ends  223 A,  223 B. One or both ends  223 A,  223 B may be threadedly fastened to the shaft  221 , press fit onto the shaft or otherwise attached or formed thereon. Each barrel  220 ,  225 ,  230 ,  235  may be closely similar or identical in shape, size and construction. 
       FIG. 11  is a perspective view that conceptually illustrates a lock  260  (i.e., locking element) for the slotted link  255  for an exemplary self-locking and releasing dual-axis hinge assembly according to principles of the invention. The locking element  260  fits within the slot  256  and may move from side to side between the ends  257 ,  258  of the slot  256 . Each end of the locking element  260  includes a pair of protruding guide pins  262  (and one  261  on the opposite side) and  264  (and one  263  on the opposite side). In the open configuration, pins  261 ,  262  sit in recessed coves  308  of the leg plates  305 ,  405 . When the pins  261 ,  262  are in the recessed coves  308  of their respective leg plates  305 ,  405 , rotation about barrel  230  is prevented until the pins  261 ,  262  are dislodged from their recessed coves  308 . Dislodgement occurs when locking element  260  moves away from the recessed cove  308 . Such movement occurs when link  255  is pivoted about barrel  220 . During such pivoting, pins  263 ,  264  of locking element  260  travels in a curved slot  302  in their respective leg plates  300 ,  400 . The curved slots  302  move closer to the central axis of the barrel  220  as the pins  263 ,  264  move from the open configuration. This configuration of the curved slots  302  causes the locking element  260  to move linearly, albeit slightly, within the slot  256  of the link  255 , as shown in  FIGS. 13 and 14 . Such movement is possible because the length of the locking element  260  is less than the length of the slot  256  of the link  255 . Such movement dislodges the pins  261 ,  262  from the recessed coves  308 . When dislodged from the recessed coves  308 , the pins  261 ,  262  may move away from the recessed coves  308 , along or adjacent to the periphery  306  of their leg plates  305 ,  405 , until the pins  261 ,  262  reach stops  307 . The stop  307  in each leg plate  305 ,  405  is a protrusion that prevents further rotation. Thus, the pins  261 ,  262  may move between stop  307  and the cove  308 , when the pins  261 ,  262  are dislodged. When the pins  261 ,  262  reside in the recessed cove  308 , the recessed cove  308  prevents appreciable movement of the pins  261 ,  262  until the pins  261 ,  262  are dislodged (i.e., withdrawn) from the cove  308 . When the pins  261 ,  262  reside in the recessed cove  308 , rotation of leg plates  305 ,  405  relative to the barrel  230  is prevented by the recessed cove  308 . 
       FIG. 12  is a perspective view that conceptually illustrates a locking mechanism for an exemplary self-locking and releasing dual-axis hinge assembly in a folded configuration according to principles of the invention. In this view, the mating (i.e., paired) leg plates are shown. The paired leg plates are identical to each other. Thus, leg plate  405  is identical to leg plate  305 . Leg plate  400  is identical to leg plate  300 . Leg plates  300 ,  400  are unlocking plates because they move pins  263 ,  264 , to move the lock  260 , to dislodge pins  261 ,  262  from the recessed cove  308 . Leg plates  305 ,  405  are locking plates because they have the cove  308  into which the pins  261 ,  262  are retained until dislodged. 
       FIG. 13  is a side view that conceptually illustrates a locking mechanism for an exemplary self-locking and releasing dual-axis hinge assembly in a folded configuration according to principles of the invention.  FIG. 14  is a side view that conceptually illustrates a locking mechanism for an exemplary self-locking and releasing dual-axis hinge assembly in an unfolded configuration according to principles of the invention. In the unfolded (i.e., locked) configuration, the unfolded leg operably coupled to leg plates  305 ,  405  will not pivot, unless link  255  is pivoted around barrel  220 . If link  255  is pivoted around barrel  220 , then the hinge becomes unlocked and the leg operably coupled to leg plates  305 ,  405  is free to pivot within the range of angular motion provided by the hinge. 
     A hinge according to the principles of the invention may be used in a variety of applications, including in folding furniture where downward forces are applied to the hinge. Downward pressure exerted on the hinge keeps the hinge in locked configuration. Thus, unintended folding due to downward pressure is avoided. Additionally, the dual axes allow the hinge to accommodate structural members (e.g., rigid tubes) of thickness comparable to the link length (e.g., half of the link length) and still achieve a folded pattern with substantially parallel overlapping layers. Furthermore, complex and cumbersome additional locking mechanisms that must be actuated by a user to unlock the hinge are avoided. For a user to unlock the hinge, he or she must first rotate the correct portion of the hinge. As such rotation will be performed to fold the hinged object in any event, this does not represent an additional step. 
     While an exemplary embodiment of the invention has been described, it should be apparent that modifications and variations thereto are possible, all of which fall within the true spirit and scope of the invention. With respect to the above description then, it is to be realized that the optimum relationships for the components and steps of the invention, including variations in order, form, content, function and manner of operation, are deemed readily apparent and obvious to one skilled in the art, and all equivalent relationships to those illustrated in the drawings and described in the specification are intended to be encompassed by the present invention. The above description and drawings are illustrative of modifications that can be made without departing from the present invention, the scope of which is to be limited only by the following claims. Therefore, the foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly, all suitable modifications and equivalents are intended to fall within the scope of the invention as claimed.

Summary:
A self-locking and releasing dual-axis hinge automatically locks when unfolded, and automatically unlocks when folded in a determined sequence. The sequence for folding entails rotating about one axis of rotation to release a movable locking element and then rotating about the other axis of rotation. Such unlocking is automatic. The hinge is well suited for a wide range of applications including folding furniture.