Many devices are known in the art, which form a resilient hinge suitable for allowing a lens-carrying frame to be connected to the arms of spectacles.
Said devices allow, in operation, the arms to be bent towards a resting position, in which they are positioned and resiliently held substantially parallel to the frame; furthermore, they allow a straddling movement of each arm beyond the normal opening position, said straddling movement undergoing a resilient restoring force which urges the arm towards the normal opening position.
Devices of this kind are known, for instance, from document IT-A-588.580, in which a screw forming the rotation pivot of the hinge is encircled by a tubular spring which exerts the restoring function; furthermore, document IT-A-612.322 discloses a device in which the rotation pivot is constituted by a small prismatic block of the arm, and the resilient restoring movement is carried out by means of a leaf spring acting against said small prismatic block; Italian utility model IT-U-134.731 describes a spectacles hinge in which an element consituting the pivot, assembled on the frame, is connected to an element having a cam surface carried by the arm, and in which the frame is provided with a dead hole inside of which acts, under the action exerted by a compression spring, a cam follower formed by a metallic cylinder having a rounded tip which cooperates with the cam surface. This last form of embodiment, according to which the cam follower may have various shapes, for instance the shape of a sphere made of a metallic or self-lubricating material, is commonly used in the most of modern resilient hinges for spectacles.
The devices known in the art provide for a plurality of drawbacks and disadvantages, since they comprise a plurality of elements suitable for assuring the functionality of the hinge, such as screws, which need corresponding female threads in the connecting elements, spiral springs, cam followers and so on; the presence of these elements raises, thus, the costs and the working time necessary for producin the hinges, taking also account of the required working precision.