The Building of a Book / A Series of Practical Articles Written by Experts in the Various Departments of Book Making and Distributing

by Unknown

The impression cylinder is located at a distance from the front of the press of about two-thirds of the entire length of the press. The circumference of the cylinder is equal to the distance that the type-bed travels in one direction. When the type-bed moves from the front to the rear, the cylinder rotates in unison with it, and thus the cylinder makes one revolution. While the bed returns the cylinder does not move.

Near the rear of the press is a large wooden board extending across the press and lying in a slightly inclined position with its lower edge almost directly above the centre of the impression cylinder. This is the "feed board" upon which the sheets of paper lie before they are printed. The impression cylinder has a set of grippers, and when the cylinder is at rest, these grippers are close to the edge of the feed board and stand open to receive the edge of the sheet of paper. Extending a little over the front of the feed board are two gauges against which the front edge of the sheet of paper is placed, while one side edge of the sheet is placed against a gauge at the side of the feed board. Just an instant before the cylinder commences to rotate, the grippers seize the front edge of the sheet, and the gauges lift out of the way. The cylinder then carries the sheet around, meets the moving inked form, and makes the impression. Before the cylinder completes its revolution, the grippers open and release the sheet, and at the same instant another set of grippers on an adjoining cylinder, called the "delivery cylinder," seize the sheet. From this delivery cylinder the sheet runs down over a set of strings, and is lifted off the strings by a sort of fan, or "sheet flier," and deposited on a table at the rear of the press. This method of delivering the sheets is known as the cylinder or rear delivery. This press may also be fitted for "front delivery." By this method the sheet of paper after being printed is carried around on the impression cylinder until the front edge comes again to the feeding point. Just as the impression cylinder comes to a stop, a set of grippers seize the front edge of the printed sheet, draw it over and away from the impression cylinder, and deposit it, with the printed side up, upon a table near the front of the press and above the ink-fountain and distributing rollers.

The average speed of one of these presses is from one thousand to fifteen hundred impressions an hour, depending upon the desired quality of the work.

Notwithstanding the excellent qualities of the stop-cylinder press, commercial necessities often demand a sacrifice of quality to speed, and this has brought the two-revolution press into very general use. As the name implies, the cylinder makes two revolutions, one to print the sheet, and the other, an idle one, to allow the bed to return. While the bed is returning, the impression cylinder is lifted to clear the type-form. As the cylinder rotates continually at a uniform speed, the type-bed must also travel at a constant speed. The reversal of the movements of the bed must, therefore, take place in a short space of time.

The study of inventors has been concentrated upon this subject more than upon any other connected with flat-bed presses, and hundreds of patents for "bed motions" have been taken out. Considering the fact that in the larger presses the weight of the bed and form is about one and a half tons and that this weight moving at a speed of about six feet in a second must be brought to a full stop and put into motion again in the opposite direction at full speed in about one-quarter of a second, it is obvious that the problem was not an easy one, especially when the reversal of the bed must be accomplished without a jar or vibration. The mechanism employed has always been a driving gear and one or two toothed racks. In Koenig's original movement, the driving gear on the end of a rising and falling shaft ran on top of a rack attached to the bottom of the bed in order to drive the bed in one direction, and then descending around the end of the rack ran in the bottom to the same rack to drive the bed in the other direction and ascending at the other end to repeat the movement. This, as already stated, has proven a very efficient mechanism and is employed, with improvements, by some of the press manufacturers of the present time.

In a pamphlet entitled "A Short History of the Printing Press" (New York, 1902), by Robert Hoe, the writer describes a method of reversing the bed. Although somewhat technical, it seems desirable to quote him as follows: "As early as 1847, Hoe & Co. patented an entirely new bed-driving mechanism. To a hanger fixed on the lower side of the bed were attached two racks facing each other, but not in the same vertical plane, and separated by a distance equal to the diameter of the driving wheel, which was on a horizontal shaft and movable sideways so as to engage in either one or other of the racks. By this means, a uniform movement was obtained in each direction. The reversal of the bed was accomplished by a roller at either end of the bed entering a recess in a disc on the driving shaft, which in a half-revolution brought the bed to a stop and started it in the opposite direction. This involved a new principle; a crank action operating directly upon the bed from a shaft having a fixed centre, and within recent years modifications of this patent have been successfully employed to drive the type-bed at a high velocity and reverse it without a shock or vibration."

This invention appears to have been the forerunner of the more recent improvements in bed motions. A notable one is that employed in the Miehle presses, which have gained much celebrity, run at a high rate of speed, and are used in many printing-offices in this and other countries. The reversal of the bed movement is accomplished by a so-called "true crank" movement and with an absence of jar and vibration never before obtained in any other than the stop-cylinder presses.

At the present time, the latest development in printing presses is Hoe & Co.'s new two-revolution press, in which, also, the reversal of the bed is accomplished by the true crank movement, but with an improvement which brings it to an easy stop and returns it without the least vibration.

On all two-revolution presses there are employed, to assist in the reversal of the bed, air-chambers or cylinders, without which the reversing mechanisms could not withstand the enormous strain to which they are subjected. These are iron cylinders, closed at one end, approximately six inches in diameter and eighteen inches long, and varying in size according to the size of the press. Some presses have two and others four of these cylinders, one or two at each end. The open ends of the cylinders are toward the bed, and attached to the bed are two or four pistons which enter the air-chambers as the bed nears the end of its stroke. The compression of the air in the cylinders makes a cushion and checks the momentum of the moving bed. The pistons can be adjusted to regulate the air compression to suit the velocity of the bed and the weight of the form, which vary in different kinds of work.

The delivery of the printed sheets is performed either by a delivery cylinder or by a front delivery with the printed side of the paper uppermost as already described for the stop-cylinder presses. Grippers are not used in the front delivery carriage, as the sheet is discharged from the cylinder by its continuous rotation.