An important mechanism was intrinsic to the kind of printing press that was used by Johannes Gutenberg in the mid-fifteenth century, a modification that prevented the turning of the weight pushing the paper down onto the inked surface from causing any turn at the point of contact, and thus causing the paper to rotate as it was being pressed down.
Gutenberg’s press was based on the kind of press used to squeeze water out of cheese, or juice out of fruit. Continued rotation at the point of contact and increasing pressure did not matter. But if this mechanism unmodified was used to press paper down onto inked type, there would have been a slight rotation of the paper, causing the ink to slur. We do not know how Gutenberg avoided this, but the great Bible of 1454-5 and later printing of the same period shows that the issue was resolved. But images of printing presses from 1499 onwards give us no indication of the specific mechanics, which must have involved the weight of the platen coming to rest on top of the paper before the pressure of the screw was applied. Later images show the platen hanging by cords from the descending screw spindle, but not rotating, which must have involved a device hanging from the descending and rotating spindle, a device which did not itself rotate.
Variously described as a ‘hose’, a ‘box’, with a ‘garter’, this device from which the platen hung rose and fell as the spindle rose and fell. It was in some way suspended from the spindle without being attached; and it rose and fell between guides which prevented it from rotating. As the press was derived from other earlier mechanical implements, we may ask if existing mechanical apparatus offered a model. The Archimedean screw? The water pump?
Close observation of the images of the press indicate that the spindle was inverted from its usual position in the fruit press – instead of the turning handle being above the screw-spindle, it was now below it. This allowed the downwards extension of the shaft with no screw carving to interfere with a hanging mechanism. Gutenberg’s inventiveness included seeing the effects of inversion: just as he inverted the printing bed, turning upside-down the process of printing cloth, which printed by pressing the inked block onto cloth, into the process of pressing the paper down onto the inked type, he inverted the fruit-press screw-spindle, which allowed space for a non-rotating rising and falling hanger for the platen.
Two square pegs running horizontally through the shaft and protruding on either side could have supported a ring, which could have supported a collar, round on its inner edge and square on its outer edge, which created the top of an open-bottom box, to whose bottom corners were connected the cords from which the platen hung. Generous greasing between the ring and the collar would have ensured easy rotation of the ring, while the box rose and fell within a square space in a bar running between the uprights of the press, ensuring the box could not turn. The inverted screw-spindle could easily be extended downwards to meet the gentle socket in the centre of the platen and apply the necessary pressure. Further guide-bars at the sides of the platen, and lying just outside the uprights of the press could have ensured no rotation of the platen.