Lith′o-graph′ic press.
The manner of applying pressure to a lithographic stone for the purposes of printing or transferring differs essentially from that employed for analogous work in the type and copperplate hand-presses.
In the latter two, up and down pressure of the platen, or the progressive rolling pressure of the cylinder, is employed; in the fermer, a scraper is used.
In power-press printing, however, it will be seen that these distinctions do not hold good.
The lithographic hand-press consists of a frame provided with rollers
b b b, on which the carriage
a runs to and fro. The scraper
c consists of a flat piece of boxwood, beveled from both sides to about a quarter-inch edge, and firmly clamped in the sliding iron piece above.
The hight of the scraper above the carriage can be adjusted by the screw
d to suit the varying thickness of the stones.
The manner of printing is as follows: The stone
i, with the design upon it, is placed face up upon the carriage; it is damped and inked, and the sheet of paper which is to receive the impression is laid thereon.
The tympan
h of leather, stretched in an
iron frame, is then lowered upon it, as in the figure, and the carriage pushed under the scraper.
By depressing the lever
e to the position indicated by the dotted lines, the toggles at each end of the shaft
f raise the bearings of the winch
g, upon the axle of which a strong iron roller is made fast.
This roller, or cylinder, is the full width of the press, and the depression of the lever brings it into contact with the under side of the carriage, lifting the latter at the same time off the rollers
b b b, and pressing the forward end of the stone against the edge of the scraper.
By revolving the winch-handle, the stone is slowly dragged under this stationary edge, which applies its pressure progressively through the leather of the tympan, to a narrow strip of the work, the result being equivalent to a very much greater crushing force applied over a larger surface.
When the whole of the design has passed under the scraper, the lever
c at the side is restored to its upright position, the carriage falls upon the rollers
b b b, it is brought back by hand, the tympan lifted, and the printed sheet removed.
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Lithographic hand-press. |
This, with unimportant modifications, is the form given to the lithographic hand-press in
England and
America.
In
Germany,
Belgium, and
France, presses of this construction are rarely seen.
In
Prussia, for instance, a press is used, occupying nearly double the space, but admirably constructed and well adapted for the execution of first-class work.
In this machine, the scraper and its holder swing from the side across the carriage, the holder or
head is then locked, and the pressure applied by a compound lever with the foot.
The carriage is then drawn through by a broad girth or belt, one end of which is made fast to it and the other wound about an axle of small diameter at the extreme end of the frame, rotated by a
star similar to that of a copperplatepress.
The result is an exceedingly steady and uniform motion, but the loss of time as compared with the
English press is considerable.
In the construction of lithographic power-presses, the scraper principle has been tried without success; power has long been and is still applied to the ordinary hand-presses, to relieve the workmen from the labor of pulling the carriage through; but for all automatic presses driven by steam, a cylinder of large diameter is now employed to apply the pressure.
Since 1865, many press-builders in
Europe and
America have turned their attention actively to this subject, and produced serviceable machines.
One of the most satisfactory presses of the kind, though not the earliest, is that now well known as Hoe's Steam Lithographic Machine.
It is a stopcylinder press, or one in which the cylinder comes to rest pending the adjustment of the sheet.
The paper is fed to grippers on the cylinder from the inclined table above.
The traveling-bed, on which the stone rests, is drawn under the cylinder by a crank and connecting-rod from the end of the frame below, and the cylinder, after being thrown into gear, is rotated at the same time (carrying the sheet with it) by a rack attached to the side of the bed. At the end of the stroke the cylinder goes out of gear, and remains stationary and locked during the return of the bed and stone; the latter passing under a cut-away part of the cylinder, so as not to come in contact with it. In place of a tympan, the cylinder is covered with a thin rubber blanket.
The inking of the stone is effected by parallel rollers (from three to six) in front of the cylinder, upon which are heavy riding rollers of iron; the latter being made to vibrate laterally to aid the distribution of the ink. These inking-rollers are covered with leather, like the ordinary hand-rollers for lithographic printing; they receive their ink from a table which travels with the bed, and are driven by a rack or friction-pieces on the sides of the bed. The ink is fed to said table from a fountain at the end of
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the press, and distributed by a number of obliquelying rollers, also covered with leather.
The automatic damping arrangement is at the back of the cylinder.
It consists of a shallow trough containing water, partially immersed in which a cylinder of wood is made slowly to revolve.
An absorbent roller is held in contact with the wet surface of this cylinder for a longer or shorter time, according to the amount of water required upon the stone, after which it carries its increase of moisture over to a heavy riding-roller, which again gives it up to two damping-rollers covered with linen, which traverse the stone as it passes beneath them, just before it meets the inking-rollers at the other side of the cylinder; the feed of water admits of adjustment as to quantity, while the press is in motion.
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Lithographic printing-machine. |
The pressure in this press is adjusted by means of butting-screws, which lift or lower the bed in the traveling-carriage; these screws are turned by a key from above.
When the sheet is printed it is conveyed by an intermediate cylinder provided with grippers to the fly at the end of the press, and there deposited, face up, on the pile of printed work.
This press, though by no means identical with
European machines of the same class, may be regarded as furnishing an illustration of the essential features of them all.
The introduction of the lithographic power-press has totally remodeled the lithographic trade throughout the world within the short period of four years (from 1868-72), increasing the possible production about tenfold.
It has lowered the cost of, and, in fact, rendered possible, large editions from stone, which, in former times, found their way to the typepress with very inferior results.
By this change the general public have profited largely.
