Mechanics: The Science of Machinery

by A. Russell Bond

FIG. 39.—PRINCIPLE OF THE HYDRAULIC PRESS

Here, then, we have a convenient means of multiplying force or effort and it is a means that is used very largely in certain classes of machinery. Figure 39 is a diagrammatic representation of a hydraulic press. It consists of a cylinder A in which is fitted a ram B. An L-shaped tube C connects with the cylinder and is fitted with a plunger D. The cylinder and tube are filled with water and then when the plunger is depressed the ram B has to rise, If the area of the plunger is one square inch and that of the ram thirty square inches, a 100 pounds pressure on the plunger will exert 3,000 pounds of lift on the ram.

HYDRAULIC LEVERAGE

However, we must remember that in mechanics, as in all walks of life, we cannot get “something for nothing.” If we multiply the pressure or force, we must pay for it in some way, otherwise we should be getting more work out of the press than we put in it, which is what the perpetual motion crank is ever trying to do. As the cross-sectional area of the plunger D is only 1/30th of that of the ram, the plunger must descend thirty inches to raise the ram one inch. We need not consider the difference in the head of water because it would not amount to more than a few ounces at most, nor need we consider frictional losses. The case is parallel to that of the lever. In fact, we may consider the hydraulic press as a fluid lever with the water in tube C as the effort arm and that in cylinder A as the weight arm. The two arms are here so proportioned that the power arm must move thirty times as far as the weight arm. The work put into the press is exactly balanced by that we get out of it. An effort of 100 pounds exerted through a distance of thirty inches is exactly balanced by the moving of 3,000 pounds through a distance of one inch.

It is a decided disadvantage to have to move the plunger so far and in actual commercial practice hand-operated hydraulic presses are not worked in that way. A pump is used to force water into the cylinder so that a great many short strokes may be taken in place of one long one, and the pump handle provides an added leverage, enabling a man with little effort to exert an enormous lift. The water enters the ram cylinder through a valve, and the pressure is maintained on the ram until relieved by the opening of an outlet port.

MAUDSLEY’S LEATHER COLLAR

All this seems very simple and one would suppose that the inventor of the hydraulic press must have been exceptionally free from the troubles and trials that beset most inventors. However, there is a vast difference between a laboratory apparatus and a commercial machine. When, towards the close of the eighteenth century, Joseph Bramah, the eminent British tool builder, invented the hydraulic press, he experienced all sorts of difficulty in holding the water in the ram cylinder. Of course, the ram has to slide freely into and out of the cylinder, but how could he prevent the water from leaking out past the ram? He resorted to all the plumbing expedients of the day. He used a stuffing box and gland, but when this was packed tight enough to hold the water in, it gripped the ram so tightly that the latter would not move down into the cylinder on the return stroke. Bramah had in his employ a very clever young mechanic named Henry Maudsley, who later became famous as an inventor and designer of machine tools. We read of him in Chapter III. Maudsley attacked the baffling problem of the hydraulic press and provided a solution that survives to this day. In place of the stuffing box which is a means of jamming a mass of cotton waste about the collar, he provided a cupped leather collar. When the pressure was applied it expanded the collar and made it bear tightly against the ram, but on relieving the hydraulic pressure the pressure of the cup leather was also reduced automatically.

There are many machines analogous to the hydraulic press in principle. They do not use water in every case for the fluid lever. Where the fluid is used over and over again oil is frequently employed. The compactness of this form of lever makes it most useful wherever an operation calls for the overcoming of a very heavy load or resistance through a relatively short distance. For instance, there are machines for bending pipe, for curving railroad rails, for punching holes in metal, for pulling wheels off their shafts, for jacking up heavy weights, for baling cotton, paper, and other materials, all of which operate on the same principle as the hydraulic press. Water pressure is supplied sometimes by a hand pump, sometimes by a power-driven pump, and sometimes it is taken from a reservoir in which compressed air imparts the requisite pressure to the water.