User Tools

Site Tools


h:hydraulic-press

Differences

This shows you the differences between two versions of the page.

Link to this comparison view

h:hydraulic-press [2018/03/30 02:06] (current)
Line 1: Line 1:
 +<< [[contents:​index| Dictionary Index]] << [[contents:​h|Definitions under H]]
  
 +----
 +
 +====== Hydraulic Press ======
 +
 +This press was invented and introduced to the public by Mr. Joseph [[b:​bramahs-press|Bramah]],​ of Piccadilly, Engineer. Mr. W. Nicholson, in his Journal of Natural Philosophy, Chemistry, and the Arts, vol. 1. April, 1797, gave an account of it, with engravings, from which the following observations are extracted, which will give a brief description of this powerful machine, that is superseding the common book press with a screw in all extensive establishments. ​
 +
 +- "Its action is as follows: when the lever or pump handle is raised, it brings up the piston, which would leave a vacuum beneath if the pressure of the atmosphere did not force the water in through a side valve. The lever is then to be pressed down, which causes the side valve to shut, and forces the water through a valve at the bottom, whence it passes through a pipe into the cavity of the great [[c:​cylindrical-printing|cylinder]],​ and raises the piston or pressing rammer. A repetition of the same process forces more water in, and the pressure may in this manner be carried to a great extent. ​
 +
 +" There is no difficulty in computing the force of this instrument. If the diameter of the pump barrel be one quarter of an inch, and that of the cylinder one inch, that is to say, four quarters of an inch; one pound lodged upon the piston rod of the pump will be in equilibrio with sixteen pounds lodged upon the table of the press; the weights of the parts of the engine attached to, and moving with each piston, being respectively included. And if the length of the pump lever be fifteen inches, and the distance between the centres of motion and of action be two inches, one pound at the end of the lever will gain an advantage of 7½ times when compared with that at the piston rod. Instead, therefore, of sixteen pounds upon the table being equal in effect to counterpoise this last action, there will be required upwards of 120 pounds. But a man in this action of pumping by a downward pressure, can without difficulty apply his whole weight, and with great ease one third or one fourth of his weight, suppose 50 pounds. In this case the pressure will be equivalent to fifty times 120 pounds, or 6000 pounds, that is to say, nearly three tons. 
 +
 +" To compare this engine with a screw, in theory, we must enquire what fineness of thread and length of lever would afford a purchase of 120 to one. Let us suppose the thread of a screw, substituted in the place of the cylinder, to be one tenth of an inch thick; the distance from the top of one. thread to the top of the next will in this case be one fifth of an inch. This is the space through which the weight must rise in one revolution. The power must therefore move through 120 times that space, namely twenty-five inches; but a lever, or radius four inches long will describe a circle somewhat larger than this, and consequently such an engine would in theory be equal in power to the hydraulic engine we have been contemplating. ​
 +
 +" But when the subject is viewed practically,​ the difference between the two engines appears to be very remarkable. All practical men know how very large a part of the force operating by means of engines is employed in overcoming frictions. Every one is aware of the extreme friction between solids, and the very slight friction which takes place between the parts of fluids. This is seen in the common expedient of oiling the pivots of wheels, and in the very gradual decay of motion in fluid bodies; while solids moving on each other stop at once, as soon as the force is diminished to a certain degree. The screw is an organ peculiarly liable to friction, and this friction is always much greater than the whole of the reacting force; for there are few instances where a screw will return from extreme pressure, when the agency upon the lever is withdrawn. It is also to be considered, that the whole force of the weight or resistance acts directly upon the face of the screw, at which the motion is required to take place. It has not been appreciated in what degree this resistance or friction increases with the weight. In lighter actions the simple ratio has been inferred; but under more severe pressures the two metallic faces extrude the greater part of the half-fluid matter between them, and appear, by the magnitude of their resistance, to be attached to each other by a process of the nature of cohesive attraction. For these and other reasons, it appears nearly impracticable to form any comparison between two engines so different in principle, but such as shall be deduced from immediate experiment of their effects. I am not in possession of numerical data to indicate the actual power of screw-engines or presses; which are perhaps the less necessary, because those who are the most interested in the success of an improvement like the present, are for the most part able to come at these without difficulty. ​
 +
 +" In an engine of this kind, the diameter of the great piston was four inches, and of the smaller three-eighths of an inch; and the advantage given by the lever or handle was twelve to one. Above the piston of the great cylinder was applied a long lever, at one end of which was an axis, and at the other end a large scale to hold weights: it contained twenty hundred weight. The distance between the axis of motion of this lever and the part where it acted on the piston was six inches; and the distance from the same axis to the extremity where the scale was hung was 126 inches. Every hundred weight in the scale consequently pressed upon the piston with a force equal to twenty-one hundred weight; whence the whole pressure was twenty-one tons. It was easy to work the lever briskly with one hand, and each stroke raised the scale near one-third of an inch. Forty-seven pounds hung at the end of the lever, carried it down with a moderate swiftness of working; but a weight of only forty-three pounds remained in equilibrio, and did not descend. Now, as the true weight in theory was thirty-two pounds, it follows that less than one-third of the actual power was employed to give velocity and overcome all friction. ​
 +
 +" It may be remarked, that the principal frictions in these machines must be at the circumference of the pistons, and that these do not increase in the simple, but in less than the subduplicate,​ ratio of the power. For if the diameter of the great cylinder were double, every thing else remaining unchanged, the surface of its piston, and consequently the power, would be quadrupled. But the friction would be only doubled, and that merely at the leathering of the greater piston. ​
 +
 +" As the pressure in the experiment last mentioned amounted to 47040 pounds upon the great piston of four inches in diameter, or sixteen circular inches surface, it amounted to 2940 pounds upon each round inch. But the medium pressure of the atmosphere on a round inch is near twelve pounds, consequently the action was equal to 245 atmospheres:​ and as each of these corresponds with a column of 34 feet of fresh water at a medium, the water in the cylinder was pressed in the same manner as if the whole column had been 8330 feet, or 1⅔ mile, long. 
 +
 +"Large presses of this construction are made with two pumps of 1¼ inch bore, and a cylinder of seven inches. These have been used in pressing hay and cotton for package; and, as I am informed, are effective in producing a greater condensation on the material with a much less application of moving power and consumption of time." ​
 +
 +The following description and figures are taken from Dr. Ure's [[http://​archive.org/​details/​dictionaryofarts00urea|Dictionary of Arts, Manufactures and Mines]], 8vo. 1839. 
 +
 +
 +
 +<WRAP right centeralign>​{{:​h:​hydraulic-press.png?​direct|Hydraulic Press}}\\
 +Fig. 1: Hydraulic Press <wrap magnify>​[[http://​www.oldbookillustrations.com/​img/​hydraulic-press-large.png|{{:​wiki:​magnify.png|Enlarge}}]]</​wrap></​WRAP>​
 +
 +" The framing consists of two stout cast-iron plates //a, b//, which are strengthened by projecting ribs, not seen in the section,// fig//. 1. The top or crown plate //b//, and the base plate //a//, are bound most firmly together by four cylinders of the best wrought iron, //c//, //c//, which pass up through holes near the ends of the said plates, and are fast wedged in them. The flat pieces //e, e//, are screwed to the ends of the crown and base plates, so as to bind the columns laterally, //f// is the hollow cylinder of the press, which, as well as the ram //g//, is made of cast iron. The upper part of the cavity of the cylinder is cast narrow, but is truly and smoothly rounded at the boring-mill,​ so as to fit pretty closely round a well-turned ram or piston; the under part of it is left somewhat wider in the casting. A stout cup of leather, perforated in the middle, is put upon the ram, and serves as a valve to render the neck of the cylinder perfectly water-tight,​ by filling up the space between it and the ram; and since the mouth of the cup is turned downwards, the greater the pressure of water upwards, the more forcibly are the edges of the leather valve pressed against the inside of the cylinder, and the tighter does the joint become. This was Bramah'​s beautiful invention. ​
 +
 +" Upon the top of the ram, the press-plate or table //h//, strengthened with projecting ridges, rests, which is commonly called the follower, because it follows the ram closely in its descent. This plate has a half-round hole at each of its four corners, corresponding to the shape of the four iron columns along which it glides in its up-and-down motions of compression and relaxation. <WRAP leftimg centeralign>​{{:​h:​pump-hydraulic-press.png?​direct|Pump of a Hydraulic Press}}\\
 +Fig. 2: Pump of a Hydraulic Press <wrap magnify>​[[http://​www.oldbookillustrations.com/​img/​pump-hydraulic-press-large.png|{{:​wiki:​magnify.png|Enlarge}}]]</​wrap></​WRAP>​
 +
 +" //k, k, figs. 1.// and //2//., is the framing of a force pump with a narrow barrel; //i// is the well for containing water to supply the pump. To spare room in the engraving, the pump is set close to the press, but it may be removed to any convenient distance by lengthening the water-pipe //u//, which connects the discharge of the force pump with the inside of the cylinder of the press. //Fig. 3//. is a section of the pump and its valves. The pump// m//, is of bronze; the suction-pipe //n//, has a conical valve with a long tail; the solid piston or plunger //p//, is smaller than the barrel in which it plays, and passes at its top through a stuffing-box //q//; //r// is the pressure-valve,​ //s// is the safety-valve,​ which, in //fig. 2//., is seen to be loaded with a weighted lever; //t// is the discharge-valve,​ for letting the water escape, from the cylinder beneath the ram, back into the well. See the winding passages in //fig. 4.// //u// is the tube which conveys the water from the pump into the press-cylinder. In //fig. 2.// two centres of motion for the pump-lever are shown. By shifting the bolt into the centre nearest the pump-rod, the mechanical advantage of the workman may be doubled. Two pumps are generally mounted in one frame for one hydraulic press; the larger to give a rapid motion to the ram at the beginning, when the resistance is small; the smaller to give a slower but more powerful impulsion, when the resistance is much increased. A pressure of //500// tons may be obtained from a well-made hydraulic press with a ten-inch ram, and a two and a one inch set of pumps." ​
 +
 +In a Report addressed to the Commissioners of Her Majesty'​s Woods, Forests, &c., in July 1839, as the result of an inquiry with reference to the selection of stone for building the new Houses of Parliament, it is stated the experiments relating to the cohesive strength of the stones, or their resistance to pressure, were made at the manufactory of Messrs. Bramah and Robinson, with a six-inch hydraulic press, the pump of which was one inch in diameter. According to trials previously made by Messrs. Bramah and Robinson, one pound weight at the end of the pump lever produced a pressure on the face of the cube [two inches square] equal to 2.53 cwt., or to 71.06 lbs. on the square inch; from this datum it may be estimated how immense the pressure is that can be obtained by this press, when the strength of a man is exerted at the pump. I have used the common book press with an iron screw to press printed paper, and I have also used a Hydraulic press of an estimated power of eighty tons: besides the greater expedition in pumping this press up than screwing the other down, I can state from my own observation,​ that the hydraulic press produced as great an effect upon the paper in three hours as the screw press did in a night, or at least fourteen hours. This may show the great superiority of this press over that which has been, in general use in printing offices. The hydraulic press has fully accomplished in practice all that was expected from it, and has established for itself a high character, which it richly deserves. ​
 +
 +
 +
 +
 +
 +
 +
 +[<6>]