LECTURE XX CONTENTS.-Bramah's Hydraulic Press-Bramah's Leather Collar Pack ing-Examples I. II.-Large Hydraulic Press for Flanging Boiler Plates The Hydraulic Jack-Weem's Compound Screw and Hydraulic Jack-Example_III.-The Hydraulic Bear or Portable Punching Machine-The Hydraulic Accumulator-Example IV.-Questions. Bramah's Hydraulic Press. This useful machine was invented by Pascal, but he could not make the moving parts watertight. Bramah, about the year 1796, discovered a means by which this difficulty was effectually overcome; and thus the instrument has been handed down to us under his name. As may be seen from the following figure, it consists of a single-acting force pump in connection with a strong cylinder containing a plunger or ram, which is forced outwards from the cylinder through a tight collar by the pressure of the water delivered into the cylinder from the force pump. From what was said in Lecture XIX. about force pumps, we need not particularise about this part of the machine, except to say that the suction and delivery valve boxes can be disconnected from the pump, and the valve cover-checks removed at any time for the purpose of examining the parts, or of regrinding the valves into their seats. The plunger extends through a stuffingbox and gland filled with hemp packing, and is guided by a centrally bored bracket bolted to the top flange of the pump. The lever fits through a slot in this guide-bar, whereby it has an easy free motion, when communicating the force applied through it to the pump plunger. The relief-valve RV has a loaded lever, adjusted like the lever safety valve in Lecture IV., so as to rise and let the water escape when the pressure exceeds a certain amount. It may also be used for taking the pressure of the object under compression, or for lowering the ram R by simply lifting the little lever and pressing down the table T, when the water flows easily from the cylinder, and out of DP by the relief valve. The delivery pipe DP is made of solid drawn brass, and the ram cylinder is carefully rounded at the bottom end, instead of being flat, in order that it may be naturally of the strongest shape.* * In the case of large cylinders for very great pressures, the lower or The guide pillars are securely bolted to the base B by nuts and iron washers, not shown. The cup leather packing CL deserves special attention, because it formed the chief improvement by VERTICAL SECTION OF A BRAMAH HYDRAULIC PRESS, Made in the Engineering Workshop of The Glasgow Technical College. inner end of the cylinder should be carefully rounded off, both inside and outside. For, if left square, or nearly square, the crystals formed in the casting of the metal naturally arrange themselves whilst cooling in such a manner as to leave an initial stress, and consequent weakness, inviting fracture along the lines joining the inside to the outside corners of the cylinder end. The severe shocks and stresses to which this weak line of division is subjected during the working of the press would sooner or later force out the end of the cylinder, in the shape of the frustrum of a cone, unless the cylinder had been made unnecessarily thick and strong at the bottom end. Bramah on Pascal's press. It consists of a leather collar of n section, placed into a cavity turned out of the neck of the cylinder, and kept there by the gland of the cylinder cover. The following figure shows an enlarged section of Bramah's packing suitable for a huge press, where the desired shape of the leather collar LC is maintained by an internal brass ring, BR, and an outside metal guard ring GR, resting on a bedding of hemp H. It will be observed at once, from an inspection of this figure, that the water which leaks past the easy fit between the plunger or ram R, and the cylinder C, presses one of the sharp edges of the leather collar against the ram, and the other edge against the side of the bored cavity in the neck of the cylinder, with a force directly proportional to the pressure of the water in the cylinder. By this simple automatic action, the greater the pressure in the cylinder the tighter does the leather collar grip the ram and bear on the cylinder's neck. Bramah's Leather Collar Packing.-This collar is made from a flat piece of new strong well-tanned leather, thoroughly soaked in water, and forced into a metal mould of the requisite size and shape until it has assumed the form of a U collar. The central or disc portion of the leather is then cut out, and the circular edges are trimmed up sharp in the bevelled manner shown by the above figure. P Formula for the Pressure on the Ram of a Bramah Press. -Referring again to the first figure in this Lecture, it will be found that by taking moments about the fulcrum at F, we obtain the pressure or reaction Q on the plunger of the force pump Therefore, neglecting weight of lever and friction, we get— Further, by Pascal's law for the transmission of pressure by liquids, enunciated in Lecture XVII., we know that the statical pressure Q is transmitted with undiminished force to every corresponding area of the cross section of the ram. Or, Q: W: area of plunger : area of ram. .. W x area of plunger =Qx area of ram. W × πr2=Q × πR2 Where r-radius of plunger, and R=radius of ram, both in the same unic. Substituting the previous value for Q, and dividing each side of the equation by π, we get PX AF Since the radius of a circle is directly proportional to its diameter, we may write the formula thus, where D is the diameter of the ram and d the diameter of the plunger, both in the same unit PX AF D3 EXAMPLE I.-In a small Bramah press, P= 50 lbs., AF = 20 in., BF=2 in., area of plunger = 1 sq. in., whilst area of ram = in. Find W, neglecting friction and weight of lever. ANSWER. By the above formula— = 14 sq. EXAMPLE II.-In Bramah's original press at South Kensington the plunger is 3" in diameter, and it acts at a distance of 6 inches from the fulcrum, which is at one end of a lever 10 feet 3 inches long, carrying a loaded scale-pan at the other end. What should be the pressure of the water in the press in order to lift a weight of 3 cwt. in the scale-pan, neglecting the weight of the lever? Make a diagram of the arrangement. (S. and A. Exam. 1892.) = 4 ANSWER.-Here d= 3 in., consequently the area of the plunger =-d3='7854 × 3′′ × 3′′ = 7 sq. in.; BF=6′′; AF=10′ 3′′=123′′; P=3 cwt.3 × 112 = 336 lbs.; and we have to find the pressure per sq. in. on the ram that will balance P, acting with the stated advantage, since the area of the ram is not given. By the above formula Large Hydraulic Press for Flanging Boiler Plates, &c.— As an example of the practical application of the Bramah press to modern boiler-making, the accompanying illustration shows the form which it takes when worked by a high-pressure water supply derived from a central accumulator, which may at the same time be used to work cranes, punching, riveting, and other similar machine tools, in the same works. The operation of flanging, say the end tube-plates of the cylindrical barrel of a locomotive boiler, is carried out in the following manner :-The ram R is lowered to near the bottom of the hydraulic cylinder HC, thus leaving room to place the boiler plate (which has been heated all round the outside edge) on the movable table T, High-pressure water is then admitted from the central accumulator to the auxiliary cylinders AC, thus forcing the side rams SR, SR, with their table T,, and the plate P, vertically upwards, until the upper surface of the plate bears hard against the bearers B, B, or internal part of the dies. Water from the same source is now admitted into the hydraulic cylinder HC, which forces up the ram R, with its table T1, supporting columns SC, SC, and the external part of the dies D, D, until the latter has quietly and smoothly bent the hot edge of the plate round the curved corner of the internal bearer B, B. The ram R is now lowered, carrying with it the table T, and dies D, by letting out water from HC, and then the table T,, with the flanged plate, are lowered by letting out water from AC. The plate is removed from its table, allowed to cool, placed in position in the barrel of the boiler, marked off for the rivet holes, drilled and riveted in the usual manner. The student will now understand what a useful and powerful servant a hydraulic press is to the engineer in the hands of a skilful workman, for it can be made to do work in the manner indicated above in far less time, and with far greater certainty of uniformity and exactitude, than the boiler-smith could turn out, with any number of hammermen to help him. It is fast replacing, the steam-hammer for cressing work, and the steam or belt-driven punching and riveting |