The present invention relates in general to paper binders and in particular, to a new and useful binding mechanism for binding perforated papers having at least two elastically flexible tangs which are arranged on the top of a flat support strip of plastic in one piece and which are insertable in passages of the support strip for the formation of loop-like ring arcs of different sizes, which passages are arranged directly below the tangs and extending crosswise to the length of the support strip. 
In a known binding device of the above-mentioned kind German GM No.  14 36 185, the elastically flexible tangs having a flat, ribbon-like cross-section are integrally formed in one piece with a plastic plate which together with three other plates of equal size forms the base part of the binding device.  The tangs are arranged to protrude laterally at a longitudinal edge of the plate and insertable by their free ends into passages for the formation of ring arcs, the passages being arranged below the plate and machined into the plates located therebelow.  So that the formed ring arcs cannot open by themselves, the tangs are provided with lock cams which can be introduced into bayonet grooves of a lock plate.  These grooves are substantially perpendicular to the insertion direction of the passages.  For taking out any sheets from the stack of bound sheets, or for adding new sheets, the ring arcs must be opened completely, that is, the tang ends inserted in the passages must be pulled out of the passages completely.  This means that the portion of the stack which happens to be on top of the sheet that is to be removed, or on top of the place where a new sheet is to be inserted, must be taken off the tangs completely and must then be threaded on again.  This complicates the handling of such binders.  In addition, due to the locking by means of the lock cams at the individual tangs and because of the bayonet grooves, the adaptation of the size of the ring arcs formed to the thickness of the stack of sheets can, at best, take place in large gradations. 
In the binder of British Pat.  No.  968,650, two separate base plates are fastened side by side on the inside of the binder cover.  They may be provided with integrally formed or separate tangs.  The tangs of one base plate are inserted into crosswise extending openings of the other base plate to form loops.  In this way it is indeed possible to open the rings formed by pairs of tangs with the stack divided.  But rethreading is cumbersome because the passages of the opposite base plate are concealed by the stack portions lying thereon and because the tangs must be bent at right angles for insertion in the passages. 
In another known binding mechanism German GM No.  75 27 298, elastically bending tangs are provided which can be fastened by one end at a paper carrier, while the other ends are detachably connectable with extension pieces which are arranged laterally in one piece at a so-called cover strip by which the papers are held together as in a stack.  For connecting the tangs with the extension pieces of the cover strip, the free ends of the tangs are tubular and the extension pieces are provided at their ends with conical pins which can be fitted into the tubular ends of the tangs.  It may be provided also that the tangs are made in the form of an elastic plastic hose or a flexible, wound metal spring.  The cover strip has the form of a flat band with laterally open conical slits which can receive the tangs and clamp them.  The extension pieces consist of non-flexible cylindrical pins.  For the papers to be threaded on the tangs, the tangs must be separated from the extension pieces, and also the support strip must be removed from the tangs.  The length of the extension pieces is chosen so that it is approximately as great as or somewhat greater than the thickness of the maximum amount of papers to be bound, in order that they can receive a bound papers stack normally threaded on the tangs.  For transferring the paper stack from the tangs to the extension pieces, the tang ends are connected with the extension pieces and the cover strip is detached from the tangs.  In any event, the extension pieces are substantially shorter than the tangs themselves, whose length must be greater by a multiple, than the maximum stack height of the papers to be bound, because otherwise bends that might cause breakage of the tangs are inevitable if, with the cover strip clamped on the stack of the tangs, the ends of the tangs are to be connected with the extension pieces extending downwardly parallel to the surface of the stack or even with the back of the stack, in order that their end sections exceeding the stack height, will not hang over freely.  When the threaded paper stack is transferred wholly or partially onto the extension pieces and the tangs must be separated from the extension pieces for the purpose of placing in or taking out one or more sheets, the cover strip with its extension pieces no longer has any connection with the paper carrier.  As a rule, the cover strip then stands on its narrow lengthwise edge opposite the extension pieces, so that it may easily tip over and consequently some sheets may become detached from the stack portion threaded on the extension pieces, or it may not readily be possible to prevent this. 
When, to secure the threaded paper stack, the cover strip is clamped fast to the tangs and the free ends of the tangs are connected with the extension pieces, these free ends of the tangs form loops of a size depending on the stack thickness which lie completely over the stack plane.  If these loops are not to be in the way, for example when closing a file in which this binding mechanism is employed, it must be possible to tip them over to the side, for which reason the tangs must have a high flexibility but be little resilient, because a file cover resting on the tipped tang loops would be flapped open by the loops standing up again.  High flexibility however, and low resilience or low stiffness of the tangs means at the same time, instability of the tangs, so that, in particular in the case of hanging files, the tangs tend to assume an oblique position, making it difficult to turn the pages or the entire stack, as they cannot firmly guide the sheets to be turned in their perforations. 
To avoid that the cover strip, when it is clamped on the tangs, will easily slide along them, the tangs are provided with a sawtooth or sinuous surface.  An uneven surface of the tangs, however, means reduced ease in threading as well as removal or turning over of the individual sheets on the tangs.  There is also a great danger that the holes of the paper will tear. 
In another threading device German GM Nos.  76 30 776 and 69 14 733, which is intended for quick binders, letter files and the like, two rigid threading pins and two movable bows are arranged on an attachment plate, the ends of the pins and bows facing each other being adapted to be detachably connected together by means of matched detent and counter-detent elements.  The bows consist of elastic material, and they are made in teh form determined by the desired open position, so that they can be connected with the rigid threading pins by deformation overcoming their inherent elasticity.  The bows may be made as separate parts and provided at both ends with detent/counter-detent elements, and in addition to the threading pins rigid connecting pins may be arranged, at whose ends the bows are lockable.  With this threading device the space requirement is constant independently of the thickness of the bound paper stack, as the length of the threading pins and of the elastic bows is invariable.  When the bow ends are locked with the threading pins and with the attachment plate or the connecting pins thereof, the bow height is always the same.