Fig. 153.—Cowper’s Double Cylinder Machine.
Fig. [152] is a similar diagram, to show the action of the double or perfecting printing machine, in which the sheets are printed on both sides. It will be observed that the general arrangement of impression cylinder, rollers, &c., is represented in duplicate, but reversed in direction. There are also two cylinders, B B, the purpose of which, as may be gathered from an inspection of the diagram, is to reverse the sheets of paper, so that after one side has been printed under the cylinder, E´, the blank surface may be turned downward, ready to receive the impression from the form, A B. Fig. [153] gives a view of the Cowper and Applegath double machine, as actually constructed. The man standing up is called the feeder or layer-on. He pushes the sheets forward, one by one, towards the tapes, which carry them down the farther side of the more distant cylinder, under which they pass, receiving the impression; and so on in the manner already indicated in the diagram, Fig. [152], until finally they reach a point where, released by the separation of the two sets of tapes, they are received by the taker-off (the boy who is represented seated on the stool), and are placed by him on a table. The bed or table which carries the form moves alternately right and left, impelled by a pinion acting in a rack beneath it, in such a manner that the direction of the table’s motion is changed at the proper moment, while the driving pulley continues to revolve always in the same direction. The movements of the table and of the cylinders are performed in exact harmony with each other, for these pieces are so connected by trains of wheels and rack-work that the sheets of paper may always receive the impression in the proper position as regards the margins, and therefore, when the sheets are printed on both sides, the impressions will be exactly opposite to each other. This gives what is technically called “true register,” and as this cannot be secured unless the paper travels over both cylinders at precisely the same rate, these are finished with great care by turning their surfaces in a lathe to exactly the same diameter. The action of the machine will not be fully understood without a glance at the arrangement of the endless tapes which carry the paper on its journey. The course of these may be followed in Fig. [154], and a simple inspection of the diagram will render a tedious description unnecessary.
Fig. 154.—Tapes of Cowper’s Machine.
In Fig. [155] we have a representation of a steam-power printing machine, such as is now very largely used for the ordinary printing of books, newspapers of moderate circulation, hand-bills, &c., and in all the ordinary work of the printing press. In this the table on which the form is placed has a reciprocating motion, but the large cylinder moves continuously always in the same direction. The feeder, or layer-on, places the sheet of paper against certain stops, and at the right moment the sheet is nipped by small steel fingers, and carried forwards to the cylinder, which brings it into contact with the inked type. This is done with much accuracy of register, for the impression cylinders gear in such a manner with the rest of the parts that their revolutions are synchronous. This is a perfecting machine, for the paper, after having received the impression on one side, is carried by tapes round the other cylinder, where it receives the impression on the other side, “set-off sheets” being passed through the press at the same time. The axles of the impression cylinders are mounted at the ends of short rocking beams, by small oscillations of which the cylinders are alternately brought down upon, or lifted off, the form passing below them. A machine of this kind can print 900 impressions per hour, even of good bookwork, and for newspaper or other printing, where less accuracy and finish are required, it may be driven at such a rate as to produce 1,400 perfected impressions per hour.
Fig. 155.—Messrs. Hopkinson and Cope’s Perfecting Machine.
The machines used for lithographic printing by steam power are almost identical in their general arrangement with that just described, which may be taken as a representative specimen of the modern printing machine.
To such machines as those already described the world is indebted for cheap books, cheap newspapers, and cheap literature in general. But when, with railways and telegraphs, came the desire for the very latest intelligence, the necessities of the newspaper press, as regards rapidity of printing, soon required a greater speed than could possibly be attained by any of the flat form presses; for in these the table, with the forms placed upon it, is unavoidably of a considerable weight, and this heavy mass has to be set in motion, stopped, moved in the opposite direction, and again stopped during the printing of each sheet. The shocks and strains which the machine receives in these alternate reversals of the direction of the movement impose a limit beyond which the speed cannot be advantageously increased. When Mr. Applegath was again applied to by the proprietors of the “Times” to produce a machine capable of working off a still larger number of impressions, he decided upon abandoning the plan of reciprocating movement, and substituting a continuous rotary movement of the type form. And he successfully overcame the difficulties of attaching ordinary type to a cylindrical surface. The idea of placing the type on a rotating cylinder is due to Nicholson, who long ago proposed to give the types a wedge shape, so that the pieces of metal would, like the stones of an arch, exactly fit round the cylindrical surface. The wedge-shaped types were, however, so liable to be thrown from their places by the centrifugal force, that Nicholson proposed also certain mechanical methods of locking the types together after they had been placed on the circumference of the drum. The plan he suggested for this purpose involved, however, such an expenditure of time and trouble that his idea was never carried into practice. Mr. Applegath used type of the ordinary kind, which was set up on flat surfaces, forming the sides of a prism corresponding to the circumference of his revolving type cylinder, which was very large and placed vertically. The flat surfaces which received the type were the width of the columns of the newspaper, and the type forms were firmly locked up by screwing down wedge-shaped rules between the columns at the angles of the polygon. These form the “column rules,” which make the upright lines between the columns of the page, and by their shape they served to securely fix the type in its place. The diameter of the cylinder to which the form was thus attached was 5 ft. 6 in., but the type occupied only a portion of its circumference, the remainder serving as an inking table. Round the great cylinder eight impression rollers were placed, and to each impression roller was a set of inking rollers. At each turn, therefore, of the great cylinder eight sheets received the impression. These cylinders were, as already stated, placed vertically, and, as it was necessary to supply the sheets from horizontal tables, an ingenious arrangement of tapes and rollers was contrived, by which each sheet was first carried down from the table into a vertical position, with its plane directed towards the impression roller, in which position it was stopped for an instant, then moved horizontally forwards round the impression cylinder, and was finally brought out, suspended vertically, ready for a taker-off to place on his pile. This machine gave excellent results as to speed and regularity. From 10,000 to 12,000 impressions could be worked off in an hour, and the advantage was claimed for it of keeping the type much cleaner, by reason of its vertical position. The power of this machine may be judged of from one actual instance. It is stated that of copies of the “Times” in which the death of the Duke of Wellington was announced, 14th November, 1852, no less than 70,000 were printed in one day, and the machines were not once stopped, either to wash the rollers or to brush the forms. It may be mentioned, in order to give a better idea of the magnitude of the operation of printing this one newspaper, that one average day’s copies weigh about ten tons, and that the paper for the week’s consumption fills a train of twenty waggons.