When it is required to unscrew the press, the hooked end of the lever l is placed in contact with the circle of teeth on the upper surface of the ratchet-wheel; the lever being then pulled the reverse way, the screw is raised, and the pressure taken off.
A very substantial and practical standing press has been made by Mr. J. L. Pouchee, type-founder, of Holborn: it is particularly designed for the use of bookbinders, stationers, and printers; the arrangement dispensing with the long inconvenient lever,as in Mr. Dunn's. It is represented in the following perspective outline. There is little in its structure that varies essentially from other presses of the kind; the head, bed, cheeks, screw, and nut, may be regarded as the same. The chief novelty consists in employing, in addition to these parts, a toothed wheel b, fixed on the axis of the screw, and operating upon it by the small pinion c turned by the lever d, which fits on the square end of the axis of c, whereon it is shifted at every fresh pull. The power of the press, when brought down to the work, may thus be increased in proportion to the difference of the diameters between the large wheel and the little pinion; the slow operation of the press at this time is of little consequence. This press stands in but little room, considering its mechanical efficacy, and it is manufactured at a low price.
The foregoing are sufficient examples of the construction of screw presses; we shall therefore proceed to give a description of a most ingenious, cheap, and effective press, in which all the other mechanical powers are brought into operation; viz. the wheel and axlelever, wedge, inclined plane, and pulley. It is one of the inventions of Mr. Ewings, a talented member of the London Mechanics' Institution, who obtained for it Dr. Fellowes's annual prize of ten pounds. This press, which is applicable to the packing of goods, pressing of juice from fruits, oil from seeds, or other purposes to which the screw-press is usually applied, consists of a frame-work, and two or more blocks or beams, between which the articles to be subjected to pressure are to be placed; and these vary in form, size, and material, according to the purposes for which they are intended. Mr. Ewings does not claim any novelty in the construction of these parts, but only in his method of producing the pressure, which is effected by bringing together the pieces that act on the articles to be pressed by wedges; these are forced in by levers (in the manner represented by the following figures 1 and 2), in both of which the same letters of reference represent similar parts, a is the base of the press, furnished at each end with ratchet notches b b, which constitute the fulcrums of the levers h h; c is the top of the press, supported by the frames d d; and ee are the pieces acting on the goods, either downwards, upwards, or both, according as the pressure may be required: in Fig. 1 it is represented acting upwards; and in Fig. 2 it is represented acting both ways: f f f f are friction rollers, between which the wedges g g are projected.
A cord is fixed to a hook on the end of one of the levers, and passing over a pulley k, on the end of the other, is attached to a small drum l, which is furnished with a ratchet-wheel and pall, and is turned by a winch "We have seen other modifications of Mr. Ewings's press, but we have selected the above as best calculated to show the principle of its action. Amongst the advantages of this press have been noticed the simplicity and economy of its construction, as it may be made by almost any person accustomed to handle carpenters' or smiths' tools, of very cheap materials; and alsa the facility with which its power may be varied: it may be diminished or increased to any extent, simply by changing the form of the wedges; but the drawings exhibit its various applications and mode of operation so obviously as to preclude the necessity of further remarks.
We shall now proceed to the consideration of those presses wherein the power applied is communicated through the medium of an incompressible fluid.
The hydrostatic, or water press, as it is sometimes called, was first brought into a practical form by the late Mr. Joseph Bramah, and was patented by him in 1796. Since this period it has undergone many new improvements in the constructive department, which, although they have not sensibly added to its mechanical energy, have materially added to its convenience, by rendering its operation more easy and certain. The following diagram is explanatory of the principle upon which it acts, a represents the foundation plate of the machine, and b the head-plate, connected together by four strong standards cc; the latter should be of wrought iron; and the whole of the utmost strength and solidity, to lesist the entire force of the press, which is exerted upon the goods placed between the follower d and the head of the press. The piston or ram e, (which supports the follower and goods) moves up and down in a very massive hollow cylinder f f, bored very accurately at its upper part to fit the ram, and at its lower end somewhat widened, as shown by the dotted lines, to admit a small quantity of water, which is forced into it by a small force-pump g, along the pipe h.
Just above that part of the cylinder where the water discharges itself in a minute crevice, an annular cavity is formed around the cylinder, wherein is fitted a folding collar of leather, which presents a thin edge both to the ram and to the cylinder, to render the junction between them water-tight, which it does most effectually by the action of the pressure itself. The top of the cylinder where the ram emerges from it, is provided with a stuffing box, well packed, and secured by a covering plate. Now if we suppose the area of the valve by which the water is admitted into the cylinder to be one-eighth of an inch (as usual), and the power applied thereto by the lever of the pump to be a ton, and the area of the section of the ram to be 64 inches, we have 64 X 64 = 4096 tons applied to the goods in this press, according to the known laws of the pressure of fluids, as explained under the article Hydrostatics The power mentioned is unnecessarily great for the general purposes of a press.- but it it obvious that it may be reduced to any required extent by altering the proportions of the lever, the valve of transmission, and the ram; and it is equally obvious that the power may, by other modifications, be increased to an indefinite extent.
It should, however, be noticed, that in the hydrostatic press of Bramah, in common use, the same time is occupied in pumping against a small, as against a great resistance: in almost all cases the operation is commenced when the resistance is at a minimum: during the process the increase is gradual, and at the termination the resistance is at a maximum. As a remedy for this practical inconvenience, hydrostatic presses are generally made with two levers of different powers, with the view of changing the power at some time during the process. Notwithstanding this provision, however, the time and trouble attending the change, renders its assumed advantages a doubtful question of economy, and it is, consequently, rarely resorted to in practice.
To obviate these objections, Mr. James Murdoch has proposed a self-regulating hydrostatic press, in which the change of power proceeds in the same ratio as the resistance, without any care or interference on the part of the operator. The ingenious arrangement proposed will be comprehended by the annexed drawings, marked Fig. 1 and Fig. 2, together with the following description:a represents the vacuum chamber, being similar to the exhausted receiver of an air-pump; b a double-barrelled air-pump; c a four-way cock, connecting a with d and e, the two force pumps, and its lower end communicating with the atmosphere; d is a section of one of the force pumps; fis the plunger, working through the stuffing-box m, and having a solid piston h keyed on to it, which works air-tight in the enlarged part of d; g is a valve opening upwards; n the exit pipe, leading to the press, which is not here shown, it being of the usual construction; o is a wheel, over which passes a chain connecting the two plungers; it is fixed square on the axle t, as is the wheel p, which serves to turn the cock by means of a cord passing round it and r, which is a pulley playing loose on v, and having a projecting shoulder on its lower part; s is a key fixed square on w, having shoulders at its lower end, and a weight at its upper end; v is a rod attached to the plug of the cock c.
The action is as follows: suppose the pistons in the situation shown in Fig. 1. the enlarged chamber of the forcing pump d is now open to the vacuum chamber a, and the chamber of c is open to the atmosphere s; the lower barrel of d is full of water. Upon rarefying the air in the vacuum chamber a, by means of the air-pump b, the air in the chamber d likewise becomes rarefied, and the piston h will descend as soon as the pressure on it exceeds the pressure on the plungerf, and a portion of water is thus forced into the press by the pipe n. By the descent of the piston h, the wheel o revolves, and brings up the piston of the chamber e; the smaller wheel p is carried round at the same time, and turns the cock r, the shoulder on which, taking the shoulder on s, carries it round a little past the vertical line, when it (s) falls into the position of the dotted line, and opens e to the vacuum chamber, and d to the atmosphere. The air under the piston of e now becomes rarefied, and it descends in like manner as the other.
The larger the vessel a is made in proportion to the chambers d and e, the better will the press accommodate itself to the changes of resistance.
Printing presses are described under the article Printing: see also Oil, Copying-machine, etc.