Paper-picking type Document Feeders

    A mechanical plate feeds the paper

A very few document feeders employ the paper-picking principle. These machines use left-over technology of the punched-card era, which it must have seemed a waste not to have adapted to more modern needs, but which turned out to be inadequate in several respects.

The principle is quite simple. A flat plate holds the paper level, under slight pressure. At the edge of the plate is a narrow lip, which, when the plate is moved forward over the paper stack, engages with the edge of the top sheet of paper and pushes it over into the machine. Obviously the lip is of a very similar size to the thickness of the paper, and obviously it cannot push too hard or the paper will wrinkle or jam; likewise it cannot hold the paper too strongly for the same reasons. The plate must be metal or of some electrically conductive substance, because static electricity is even less acceptable here than in roller-type systems.

Design problems will occur to the reader. The mechanism will only work on the top of the paper stack - otherwise the weight of the stack would prevent the piece of paper on the bottom from moving at all - unless there were rollers involved, in which case a picking mechanism wouldn't be needed! There must be a way of lifting the stack so that the top piece of paper is in the correct alignment with the picking lip: not a problem per se but great precision is needed. Evidently only a limited range of paper thicknesses will work with such a mechanism. And the moving plate can only propel the paper so far, or else the mechanism will be too wide to be practical: perhaps one quarter of the paper width is the maximum.

As noted, these systems originate in the days of the 80-column card. The paper stock used in the punched card was very robust and strong, and thicker than most paper; the card paths in readers and punches routinely banged the cards head-on into some sort of barrier, to stack them properly, or changed their shape by sending them around 90-degree corners; in spite of this the sustained speed of the readers (punches were slower) was 2,000 per minute or more by the end of the punched-card era. And because of this robustness, the cards could be picked off the bottom of a stack, which simplified the mechanism very greatly. But paper will not put up with this sort of treatment.

Still, some vestige of the technology is in use: a very slight picking push can be useful in the right applications, and belts (as used in card machines) may be employed instead of rollers if the paper path is very long.

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