Au equally good representative of the parallel type is Whitworth's large bar-iron shear (fig. 4, pl. 30). A is the stationary lower blade, secured to a projection of the frame, and B the movable vertical blade, screwed to a carriage, which can be shifted in the prismatic guide a. The carriage is pushed up and down by the rod b from an eccentric on the front end of a horizontal shaft placed in the frame at c. The hack end of this shaft carries a large spur-wheel (d) meshing into a smaller one (e) upon the fly-wheel shaft of a special engine (k). This fly-wheel is designated by f and the crank by g; h represents the slide-rod, i the piston rod, k the steam-cylinder, I the sliding eccentric, m the eccentric-rod, n the valve-chest, and o the steam-admission steam-valve, which can be opened and shut by the hand-wheel p. These shears are intended for use in rolling-mills, for cutting rolled puddle-bars into pieces for " piling" or reheating. On account of the varying resistance in cutting such bars, and to prevent disturbance in the working of other machines, it is advan tageous to use a separate motor for working the shear.
The large shears for sheet iron shown on Plate 31 (Jigs. 4, 5) are dis tinguished by the great length of the blades, by the use of which the edges of large plates can be trimmed at a single cut. For machines of this class a separate motor is usually provided. A special arrangement, shown in Figure 4, serves for quickly stopping the movable blade without stopping the engine and waiting for the heavy rotating masses to come to a standstill.
Figures i and 2 (pl. 32) show two frequently-used combinations of parallel shears and punching-machines, one transportable, the other sta tionary. In Figure 2 the shear and the punch are placed one over the other and in Figure r opposite each other, while both are belt-driven. In Figure r the power applied to the live pulley is transmitted through the pinion a' to a spur-wheel (c), upon whose shaft is a second pinion (b) , which in turn drives the large spur-wheel a. The shaft of this wheel drives the punch D and the shear-blade 13; the fly-wheel f is to aid the belt in overcoming the resistance of the material to be punched or sheared. The frame is a hollow casting. Figure 2 (Pi. 32), which is a German portable punching- and shearing-machine, may he operated by belt- or hand-power. The same sliding piece carries at its lower end the punch and at its upper end the movable shear-blade. This machine has a truss frame.
Hydraulic the hydraulic-machine (IL 3o, fig. 3) the cut ting-stroke of the punch is effected by a small force-pump (concealed in the framework and controlled by the upper hand-lever), conveying water or oil from a reservoir into a cylinder with its piston forming one piece with the punch. The up-stroke is effected, with the assistance of the lower hand-lever, after communication has been established between the cylinder and the reservoir. This arrangement is practically a combina
tion hydraulic-press and punch. Shears may be classed as well under " Presses " as under any other head, as they operate by simple pressure.
Presses are machines that do their work by pressure, which, whether exerted gradually or instantaneously, has the same effect as regards the energy applied, although sometimes, on account of the inertia of the material, there is an unequal distribution of that energy. The blacksmith's hammer as it shapes the iron on the anvil (N. 32, fig. 3) is a primitive form of press; but if a pair of upright guides were attached to the anvil, restricting the hammer to a vertical motion, it would approach more nearly to the machine generally known as a press, and, in fact, would represent a class called " drop-presses " (fig. 4). While there are numer ous styles of presses adapted for punching, shearing, and forming sheet or bar-metal and other materials, they all contain the parts represented by the anvil or "bed," the hammer or "ram," and the guides or "slide bearings." The bed and the slide-bearings are always at right angles to each other, and usually form part of the same casting, called the "frame." a drop-press (fig. 4) the rain, after being lifted to a certain height, is allowed to descend by gravity, the amount of work pro duced depending on the height of the fall and the weight of the rain. In doing work which requires sudden pressure, such as is given by a sledge hammer, the drop-press or its equivalent is indispensable, but in most cases gradual pressure is required.
a " power-press " this gradual pressure is usually obtained by using a fly-wheel connected with the ram by an interven ing shaft, crank, and pitman (fig. 5), and in a foot-press it is got by the intervention of a series of levers and their connections between the foot and the ram (fig. 7). In most power-presses the fly-wheel revolves loosely on the shaft, to which motion is given—only when connected with the wheel—by a " clutch," which device is generally a pin that slides in a projection on the shaft and is capable of being moved out by a spring until it engages with a notch in the hub of the wheel. Most clutches are "tripped," or caused to engage with the revolving wheel, by a treadle and a rod connected to a wedge or other device which by the action of the press itself has previously locked the clutch-pin out of gear with the wheel. The depression of the treadle unlocks this device and allows the pin to re-enter the wheel for another stroke. Press-clutches are usually thus made auto matic—that is, they are so constructed as to throw the fly-wheel " out of gear" with the shaft at the completion of one revolution of the wheel, and the consequent completion of a stroke of the press. The clutch being operated by a treadle, the operator's hands are left free to handle the work.