{PG,1 A self-locking hinge for use as a pivotal connector between two pivoted members and adapted to lock the members in a fixed angular relation. The hinge comprises a pair of circular plates that are rigidly affixed to opposite faces of one member and rotatably connected to the other member. The circular plates are used to laterally support both of the members and to guide the other member as it is rotated from a folded position to a fully extended position. The hinge is locked in the fully extended position by means of a locking ring that is slidably engaged with the other member. When such other member is fully extended, the ring engages corresponding locking indentations at the periphery of the circular plates and thereby blocks the movement of the other member in one angular direction. The movement of the other member is blocked in the opposite angular direction due to the abutting of the hinge ends of the members. The members may be returned to the folded position by releasing the locking ring from its engagement with the indentations in the circular plates. The operation of the self-locking hinge of the invention is particularly described with respect to a door security apparatus adopted to engage a door and a floor to resist the opening of the door. A further embodiment uses the self-locking hinge of the invention to provide a means to convert a stepladder to a straight ladder of increased height.
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