XV. STUFFING-BOXES.

Exercise 48:Locomotive Piston.—A part elevation and part section of a locomotive piston, for a cylinder having a bore 18 inches in diameter, is shown in fig. 48. Draw this, and also a view looking on the nut in the direction of the axis of the piston rod. Scale 6 inches to a foot.Note.—The reason why the part of the piston rod within the piston has such a quick taper is that the piston has to be taken off the rod while it is in the cylinder. The cross-head being forged on the end of the piston rod prevents the piston and piston rod being withdrawn together.

Note.—The reason why the part of the piston rod within the piston has such a quick taper is that the piston has to be taken off the rod while it is in the cylinder. The cross-head being forged on the end of the piston rod prevents the piston and piston rod being withdrawn together.

Large Pistons.—Pistons of large diameter are generally provided with two cast-iron packing rings placed within the same groove. These rings are pressed outwards against the cylinder, and also against the sides of the groove by one or more springs. One form of this packing (Lancaster's) is shown in fig. 49. Here one spring only is used, and it is first made a straight spiral spring, and then bent round and its ends united. The action of the spring will be clearly understood from the illustration. For the purpose of admitting the packing rings the piston is divided into two parts, one the piston proper, and the other thejunk ring. In fig. 49, A is the junk ring, which is secured to the piston by means of bolts as shown.

Exercise 49:Marine Engine Piston.—The piston illustrated by fig. 49 is for the high-pressure cylinder of a marine engine. The piston, junk ring, and packing rings are of cast iron. The piston rod and nut are of wrought iron, so also are the junk ring bolts. The nuts for the latter are of brass. The spiral spring is made from steel wire3⁄8inch diameter. An enlarged section of one of the packing rings is shown at (a). A front elevation of the locking arrangement for the piston rod nut is shown at (b). A sectional plan of one of the nuts for the junk ring bolts is shown at (c).First draw the vertical section of this piston, next draw a plan, one-third of which is to show the piston complete, one-third to show the junk ring removed, and the remaining third to be a horizontal section through between the packing rings. The details (a) and (c) need not be drawn separately. Scale 3 inches to a foot.

First draw the vertical section of this piston, next draw a plan, one-third of which is to show the piston complete, one-third to show the junk ring removed, and the remaining third to be a horizontal section through between the packing rings. The details (a) and (c) need not be drawn separately. Scale 3 inches to a foot.

Fig. 49.Fig. 49.

Proportions of Marine Engine Pistons.—Mr. Seaton, in his 'Manual of Marine Engineering,' gives the following rules for designing marine engine pistons:—

D=diameter of piston in inches.p=effective pressure in lbs. per square inch.x=D—50× √p+1.

Thickness offront of piston near boss0·2 ×x.”” ” rim0·17 ×x.”back of piston0·18 ×x.”boss around rod0·3 ×x.”flange inside packing ring0·23 ×x.”” at edge0·25 ×x.”junk ring at edge0·23 ×x.”” inside packing ring.0·21 ×x.”” at bolt-holes0·35 ×x.”metal around piston edge0·25 ×x.Breadth of packing ring0·63 ×x.Depth of piston at centre1·4 ×x.Lap of junk ring on piston0·45 ×x.Space between piston body and packing ring0·3 ×x.Diameter of junk-ring bolts0·1 ×x+ ·25 inch.Pitch of junk-ring bolts10 diameters.Number of webs in pistonD + 20———.12Thickness”0·18 ×x.

Exercise 50:Design for Marine Engine Piston.—Calculate by Seaton's rules the dimensions for a marine engine piston 40 inches in diameter, and subjected to an effective pressure of 36 lbs. per square inch. Then make the necessary working drawings for this piston to a scale of, say, 3 inches to a foot.Note.—Take the dimensions got by calculation to the nearest 1-16th of an inch.

Note.—Take the dimensions got by calculation to the nearest 1-16th of an inch.

Fig. 50.Fig. 50.

In fig. 50 is shown a gland and stuffing-box for the piston rod of a vertical engine. A B is the piston rod, C D a portion of the cylinder cover, and E F thestuffing-box. Fitting into the bottom of the stuffing-box is a brass bush H. The space K around the rod A B is filled withpacking, of which there isa variety of kinds, the simplest being greased hempen rope. The packing is compressed by screwing down the cast-iron gland L M, which is lined with a brass bush N. In this case the gland is screwed down by means of three stud-bolts P, which are screwed into a flange cast on the stuffing-box. Surrounding the rod on the top of the gland there is a recess R for holding the lubricant.

Figs. 51, 52.Fig. 51.Fig. 52.

Fig. 51.Fig. 52.

The object of the gland and stuffing-box is to allow the piston rod to move backwards and forwards freely without any leakage of steam.

Fig. 51 shows a gland and stuffing-box for a horizontal rod. The essential difference between this example and the last is in the mode of lubrication. The gland flange has cast within it an oil-box which is covered by a lid; this lid is kept shut or open by the action of a small spring as shown. A piece of cotton wick (not shown in the figure) has one end trailing in the oil in the oil-box, while the other is carried over and passed down the hole A B. The wick acts as a siphon, and drops the oil gradually on to the rod. In this example only two bolts are used for screwing in the gland; and the flanges of the gland and stuffing-box are not circular, but oval-shaped.

In the case of small rods the gland is made entirely of brass, and no liner is then necessary. Fig. 52 shows a form of gland and stuffing-box sometimes used for small rods. The stuffing-box is screwed externally, and carries a nut A B which moves the gland.

Exercise 51:Gland and Stuffing-box for a Vertical Rod.—Draw the views shown in fig. 50 to the dimensions given. Scale 6 inches to a foot.Exercise 52:Gland and Stuffing-box for a Horizontal Rod.—Fig. 51 shows a plan, half in section, and an elevation half of which is a section through the gland flange. Draw these to a scale of 6 inches to a foot, using the dimensions marked in the figure.Exercise 53:Screwed Gland and Stuffing-box.—Draw, full size, the views shown in fig. 52 to the given dimensions.

Exercise 51:Gland and Stuffing-box for a Vertical Rod.—Draw the views shown in fig. 50 to the dimensions given. Scale 6 inches to a foot.

Exercise 52:Gland and Stuffing-box for a Horizontal Rod.—Fig. 51 shows a plan, half in section, and an elevation half of which is a section through the gland flange. Draw these to a scale of 6 inches to a foot, using the dimensions marked in the figure.

Exercise 53:Screwed Gland and Stuffing-box.—Draw, full size, the views shown in fig. 52 to the given dimensions.

A more elaborate form of gland and stuffing-box is shown in fig. 53. This is for a large marine engine with inverted cylinders, such as is used on board large ocean steamers. The stuffing-box is cast separate from the cylinder cover to which it is afterwards bolted. The lubricant is first introduced to the oil-boxes marked A, from which it passes to the recess B, where it comes in contact with the piston rod. To prevent the lubricant from being wasted by running down the rod, the main gland is provided with a shallow gland and stuffing-box which is filled with soft cotton packing, which soaks up the lubricant.

Fig. 53.Fig. 53.

The main gland is screwed up by means of six bolts, and to prevent the gland from locking itself in the stuffing-box, itis necessary that the nuts should be turned together. This is done in a simple and ingenious manner. One-half of each nut is provided with teeth, and these gear with a toothed wheel which has a rim only; this rim is held up by a ring C. When one nut is turned, all the rest follow in the same direction.

Exercise 54:Gland and Stuffing-box for Piston Rod of Large Inverted Cylinder Engine.—The lower view in fig. 53 is a half plan looking upwards, and a half section of the gland looking downwards. The upper view is a vertical section. Complete all these views and add an elevation. Scale 3 inches to a foot.Note.—The large nuts, the wheel, the supporting ring, and small gland are made of brass.

Note.—The large nuts, the wheel, the supporting ring, and small gland are made of brass.

Dimensions of Stuffing-boxes and Glands.

d= diameter of rod.t1= thickness of stuffing-box flange.d1= diameter of box (inside).t2= thickness of gland flange.l= length of stuffing-box bush.t3= thickness of bushes in box and gland.l1= length of packing space.d2= diameter of gland bolts.l2= length of gland.n= number of bolts.t= thickness of metal in stuffing-box.

dd1ll1l2tt1t2t3d2n11¾3⁄421½7⁄161⁄2t2=t3⁄167⁄1621½2½1¼25⁄829⁄1611⁄16when gland1⁄45⁄8223½1¾3¼2½11⁄167⁄8flange is5⁄163⁄422½41⁄82¼37⁄827⁄813⁄1611⁄16made of5⁄167⁄8234¾2¾4½3¼15⁄161¼cast iron3⁄8123½5¼351⁄835⁄8113⁄8andt2=t13⁄812457⁄83¼5¾4113⁄8when gland7⁄16124½63⁄83½63⁄843⁄811⁄1619⁄16flange is7⁄167⁄84573¾745⁄811⁄1619⁄16made of7⁄1614684¼8¼511⁄8111⁄16brass.1⁄21¼4

The proportions of glands and stuffing-boxes vary considerably but the above table represents average practice.

Exercise 55:—Make the necessary working drawings for a gland and stuffing-box for a locomotive engine piston rod 2½ inches in diameter, to the dimensions given in the table.

Professor Unwin divides valves, according to their construction into three classes as follows:—(1) flap valves, which bond or turn upon a hinge; (2) lift valves, which rise perpendicularly to the seat; (3) sliding valves, which move parallel to the seat.

Examples of flap valves are shown in figs. 54 and 55; two forms of lift valves are shown in figs. 56 and 57, and in figs. 58 and 59 are shown two forms of slide valve. The slide valve shown in fig. 58 moves in a straight line, while that shown in fig. 59 (called a cock) moves in circle.

India-rubber Valves.—In india-rubber valves there is a grating covered by a piece of india-rubber, which may be rectangular, but is generally circular, and which is held down along one edge if rectangular, or at the centre if circular. Water or other fluid can pass freely upwards through the grating, but when it attempts to return the elasticity of the india-rubber, and the pressure of the water upon it, cause it to lie close on the grating, and thus prevent the return of the water. The india-rubber is prevented from rising too high by a perforated guard. In fig. 54 is shown an example of an india-rubber disc valve. A is the grating, B the india-rubber, C the guard secured to the grating or seat by the stud D and nut E. The grating is held in position by bolts and nuts F. The grating and guard are generally of brass.

India-rubber disc valves are also shown on the air-pump bucket, fig. 47.

Exercise56:India-rubber Disc Valve.—Fig. 54 shows a vertical section and a plan of an india-rubber disc valve. In the plan one-half of the guard and india-rubber are supposed to be removed so as to show the grating or seat. Draw these views, and also an elevation. A detail drawing of the central stud is shown in fig. 16, page 18. In fig. 54 the elevation of the guard is drawn as it is usually drawn in practice, but if the student has a sufficient knowledge of descriptive geometry he should draw the elevation completely showing the perforations. Scale 6 inches to a foot.

Fig. 54.Fig. 54.Fig. 55.Fig. 55.

Fig. 54.Fig. 54.

Fig. 55.Fig. 55.

Kinghorn's Metallic Valve.—The action of this valve is the same as that of an india-rubber valve, but a thin sheet of metal (phosphor bronze) takes the place of the india-rubber.

This valve is now largely used in the pumps of marine engines, and is shown in fig. 55 as applied to an air-pump bucket. Three valves like the one shown are arranged round the bucket.

Exercise57:Kinghorn's Metallic Valve.—Fig. 55 shows an elevation and plan of one form of this valve. In the plan one-half of the guard and metal sheet are supposed to be removed, so as to show the grating, which in this case is part of an air-pump bucket. Draw the views shown, and also a vertical section of the guard through the centres of the bolts. All the parts are of brass except the valve proper, which is of phosphor bronze. Scale 6 inches to a foot.

Conical Disc Valves.—A very common form of valve is that shown in figs. 56 and 57. This form of valve consists of a disc, the edge of which (called the face) is conical. The conical edge of this disc fits accurately on a corresponding seat. The angle which the valve face makes with its axis is generally 45°. If the disc is raised, either by the action of the fluid as in the india-rubber valve, or by other means, an opening is formed around the disc through which the fluid can pass. The valve is guided in rising and falling either by three feathers underneath it, as in fig. 56, or by a central spindle which moves freely through a hole in the centre of a bridge which stretches across the seat, as in fig. 57. The lift of the valve is limited by a stop above it, which forms part of the casing containing the valve. The lift should in no case exceed one-fourth of the diameter of the valve, and it is generally much less than this. The guiding feathers (fig. 56) are notched immediately under the disc for the purpose of making available the full circumferential opening of the valve for the passage of the fluid. These notches also prevent the feathers from interfering with the turning or scraping of the valve face.

Conical disc valves and their seats are nearly always made of brass.

Exercise58:Conical Disc Valves.—Draw, half size, the plans and elevations shown in figs. 56 and 57. In fig. 57 the valve is shown open in the elevation, and in the plan it is removed altogether in order to show the seat with its guide bridge.

Fig. 56.Fig. 56.Fig. 57.Fig. 57.

Fig. 56.Fig. 56.

Fig. 57.Fig. 57.

Simple Slide Valve.—The form of valve shown in fig. 58, often called thelocomotive slide valve, is very largely used in all classes of steam-engines for distributing the steam in the steam cylinders. The valve is shown separately at (d), (e), and (f), while at (a), (b), and (c) is shown its connection with the steam cylinder.

It will be observed that the valve itself is in the shape of a box with one side open, the edges of the open side being flanged. When the valve is in its middle position, as shown at (a), two of these flanged edges completely cover two rectangular openings S1and S2, calledsteam ports, while the hollow part of the valve is opposite to a third port E, called theexhaust port. As shown at (a) the piston P would be movingupwards and the valve downwards. By the time the piston has reached the top of its stroke the valve will have moved so far down as to partly uncover the steam port S1, and admit steam from the valve casing C through S1and the passage P1to the top of the piston. The pressure of this steam on the top of the piston will force the latter down. While the above action has been going on, the port S2will have become uncovered, and the hollow part of the valve will be opposite both the steam port S2and the exhaust port E, so that the steam from the under side of the piston, and which forced the piston up, can now escape by the passage P2, the steam port S2, and the exhaust port E to the exhaust outlet O, and thence into the atmosphere, if it is a non-condensing engine, or into the condenser if it is a condensing engine, or into another cylinder if it is a compound engine. After the piston has performed, a certain part of its downward stroke, the valve, which has been moving downwards, will commence to move upwards, and when it has reached a certain point it will cover the port S1, and shut off the supply of steam to the top of the piston. It is generally arranged that the steam shall be cut off before the piston reaches the end of the stroke. When the piston reaches the bottom of its stroke the valve has moved far enough up to uncover the port S2and admit steam to the bottom of the piston, and to uncover the port S1and allow the steam to escape from the top of the piston through the passage P1, the port S1, the port E, and outlet O. In this way the piston is moved up and down in the cylinder.

The valve is attached to a valve spindle S by nuts as shown, the hole in the valve through which the spindle passes being oval-shaped to permit of the valve adjusting itself so as to always press on its seat.

When the valve is in its middle position it generally more than covers the steam ports. The amount which the valve projects over the steam port on the outside, the valve being in its middle position, is called theoutside lapof the valve, and the amount which it projects on the inside is called theinside lap. When the term lap is used without any qualification, outside lap is to be understood. In fig. 58 it will beseen that the valve has no inside lap, and that the outside lap is three-eighths of an inch. The inside lap is generally small compared with the outside lap.

Fig. 58.Fig. 58.

When the piston is at the beginning of its stroke the steam port is generally open by a small amount called theleadof the valve.

The reciprocating motion of the slide valve is nearly always derived from an eccentric fixed on the crank-shaft of the engine. Slide valves are generally made of brass, bronze, or cast iron.

Exercise59:Simple Slide Valve.—At (d), fig. 58, is shown a sectional elevation of a simple slide valve for a steam-engine, the section being taken through the centre line of the valve spindle, while at (e) is shown a cross section and elevation, and at (f) a plan of the same. Draw all these views full size, and also a sectional elevation at A B. The valve is made of brass, and the valve spindle and nuts of wrought iron.Exercise60:Slide Valve Casing, &c., for Steam-engine.—Draw, half size, the views shown at (a), (b), and (c), fig. 58; also a sectional plan at L M. (b) is an elevation of the valve casing with the cover and the valve removed. (a) is a sectional elevation, the section being taken through the axes of the steam cylinder and valve spindle. (c) is a sectional plan, the section being a horizontal one through the centre of the exhaust port. The inlet and outlet for the steam are clearly shown in the sectional plan: in the sectional elevation their positions are shown by dotted circles.The stroke of the piston is in this case 12 inches, so that from the dimensions given at (a) it must come within a quarter of an inch of each end of the cylinder; this is called thecylinder clearance.The piston has three Ramsbottom rings, a quarter of an inch wide and a quarter of an inch apart.The steam cylinder and valve casing are made of cast iron.

Exercise60:Slide Valve Casing, &c., for Steam-engine.—Draw, half size, the views shown at (a), (b), and (c), fig. 58; also a sectional plan at L M. (b) is an elevation of the valve casing with the cover and the valve removed. (a) is a sectional elevation, the section being taken through the axes of the steam cylinder and valve spindle. (c) is a sectional plan, the section being a horizontal one through the centre of the exhaust port. The inlet and outlet for the steam are clearly shown in the sectional plan: in the sectional elevation their positions are shown by dotted circles.

The stroke of the piston is in this case 12 inches, so that from the dimensions given at (a) it must come within a quarter of an inch of each end of the cylinder; this is called thecylinder clearance.

The piston has three Ramsbottom rings, a quarter of an inch wide and a quarter of an inch apart.

The steam cylinder and valve casing are made of cast iron.

Cocks.—A cock consists of a slightly conical plug which fits into a corresponding casing cast on a pipe. Through the plug is a hole which may be made by turning the plug to form a continuation of the hole in the pipe, and thus allow the fluid to pass, or it may be turned round so that the solid part of the plug lies across the hole in the pipe, and thus prevent thefluid from passing. As the student will be quite familiar with the common water cock or tap such as is used in dwelling-houses we need not illustrate it here.

Fig. 59.Fig. 59.

Fig. 59 shows a cock of considerable size, which may be used for water or steam under high pressure. The plug in this example is hollow, and is prevented from coming out by a cover which is secured to the casing by four stud bolts. An annular ridge of rectangular section projecting from the under side of the cover, and fitting into a corresponding recess on the top of the casing, serves to ensure that the cover and plug are concentric, and prevents leakage. Leakage at the neck of the plug is prevented by a gland and stuffing-box. The top end of the plug is made square to receive a handle for turning it. The size of a cock is taken from the bore of the pipe in which it is placed; thus fig. 59 shows a 2¼-inch cock.

Exercise61: 2¼-inch Steam or Water Cock.—First draw the views of this cock shown in fig. 59, then draw a half end elevation and half cross section through the centre of the plug. Scale 6 inches to a foot.Instead of drawing the parts of the pipe on the two sides of the plug in the same straight line as in fig. 59, one may be shown proceeding from the bottom of the casing, so that the fluid will have to pass through the bottom of the plug and through one side. This is a common arrangement.All the parts of the valve and casing in this example are made of brass.

Instead of drawing the parts of the pipe on the two sides of the plug in the same straight line as in fig. 59, one may be shown proceeding from the bottom of the casing, so that the fluid will have to pass through the bottom of the plug and through one side. This is a common arrangement.

All the parts of the valve and casing in this example are made of brass.

Cast Iron.—The essential constituents of cast iron are iron and carbon, the latter forming from 2 to 5 per cent. of the total weight. Cast iron, however, usually contains varying small amounts of silicon, sulphur, phosphorus, and manganese.

In cast iron the carbon may exist partly in the free state and partly in chemical combination with the iron.

Inwhite cast ironthe whole of the carbon is in chemical combination with the iron, while ingrey cast ironthe carbon is principally in the free state, that is, simply mixed mechanically with the iron. It is the free carbon which gives the grey iron its dark appearance. A mixture of the white and grey varieties of cast iron when melted producesmottled cast iron. The greater the amount of carbon chemically combined with the iron, the whiter, harder, and more brittle does it become.

The white cast iron is stronger than the grey, but being more brittle it is not so suitable for resisting suddenly applied loads. White iron melts at a lower temperature than grey iron, but after melting it does not flow so well, or is not so liquid as the grey iron. White iron contracts while grey iron expands on solidifying. The grey iron, therefore, makes finer castings than the white. Castings after solidifying contract in cooling about1⁄8of an inch per foot. Castings possessing various degrees of strength and hardness are produced by melting mixtures of various proportions of white and grey cast irons. White cast iron has a higher specific gravity than grey cast iron.

Cast iron gives little or no warning before breaking. The thickness of the metal throughout a casting in cast iron should be as uniform as possible, so that it may cool and therefore contract uniformly throughout; otherwise some parts may be in a state of initial strain after the casting has cooled, and will therefore be easier to fracture. Re-entrant angles should be avoided; such should be rounded out with fillets.

The presence of phosphorus in cast iron makes it more fusible, and also more brittle. The presence of sulphur diminishes the strength considerably.

The grey varieties of cast iron are calledfoundry ironsorfoundry pigs, while the white varieties are calledforge ironsorforge pigs, from the fact that they are used for conversion into wrought iron.

Amongst iron manufacturers the different varieties of cast iron are designated by the numbers 1, 2, 3, &c., the lowest number being applied to the greyest variety.

Chilled Castings.—When grey cast iron is melted a portion of the free carbon combines chemically with the iron; this, however, separates out again if the iron is allowed to cool slowly; but if it is suddenly cooled a greater amount of the carbon remains in chemical combination, and a whiter and harder iron is produced. Advantage is taken of this in makingchilled castings. In this process the whole or a part of the mould is lined with cast iron, which, being a comparatively good conductor of heat, chills a portion of the melted metal next to it, changing it into a hard white iron to a depth varying from1⁄8to1⁄2an inch. To protect the cast-iron lining of the mould from the molten metal it is painted with loam.

Malleable Cast Iron.—This is prepared by imbedding a casting in powdered red hematite (an oxide of iron), and keeping it at a bright red heat for a length of time varying from several hours to several days according to the size of the casting. By this process a portion of the carbon in the casting is removed, and the strength and toughness of the latter become more like the strength and toughness of wrought or malleable iron.

Wrought or Malleable Iron.—This is nearly pure iron, and is made from cast iron by the puddling process, which consists chiefly of raising the cast iron to a high temperature in a reverberatory furnace in the presence of air, which unites with the carbon and passes off as gas. In other words the carbon is burned out. The iron is removed from the puddling furnace in soft spongy masses calledblooms, which are subjected to a process of squeezing or hammering calledshingling. These shingled blooms still contain enough heat to enable them to be rolled into roughpuddled bars. These puddled bars are of very inferior quality, having less than half the strength of good wrought iron. The puddled bars are cut into pieces which are piled together, reheated, and again rolled into bars, which are calledmerchant bars. This process of piling, reheating, and re-rolling may be repeated several times, depending on the quality of iron required. Up to a certain point the quality of the iron is improved by reheating androlling or hammering, but beyond that a repetition of the process diminishes the strength of the iron.

The process of piling and rolling gives wrought iron a fibrous structure. When subjected to vibrations for a long time, the structure becomes crystalline and the iron brittle. The crystalline structure induced in this way may be removed by the process ofannealing, which consists in heating the iron in a furnace, and then allowing it to cool slowly.

Forging and Welding.—The process of pressing or hammering wrought iron when at a red or white heat into any desired shape is calledforging. If at a white heat two pieces of wrought iron be brought together, their surfaces being clean, they may be pressed or hammered together, so as to form one piece. This is calledwelding, and is a very valuable property of wrought iron.

Steel.—This is a compound of iron with a small per-centage of carbon, and is made either by adding carbon to wrought iron, or by removing some of the carbon from cast iron.

In thecementationprocess, bars of wrought iron are imbedded in powdered charcoal in a fireclay trough, and kept at a high temperature in a furnace for several days. The iron combines with a portion of the carbon to formblister steel, so named because of the blisters which are found on the surface of the bars when they are removed from the furnace.

The bars of blister steel are broken into pieces about 18 inches long, and tied together in bundles by strong steel wire. These bundles are raised to a welding heat in a furnace, and then hammered or rolled into bars ofshear steel.

To formcast steelthe bars of blister steel are broken into pieces and melted into crucibles.

In theSiemens-Martinprocess for making steel, cast and wrought iron are melted together on the hearth of a regenerative gas-furnace.

Bessemer steelis made by pouring melted cast iron into a vessel called a converter, through which a blast of air is then urged. By this means the carbon is burned out, and comparatively pure iron remains. To this is added a certain quantityof 'spiegeleisen,' which is a compound of iron, carbon, and manganese.

Hardening and Tempering of Steel.—Steel, if heated to redness and cooled suddenly, as by immersion in water, is hardened. The degree of hardness produced varies with the rate of cooling; the more rapidly the heated steel is cooled, the harder does it become. Hardened steel is softened by the process ofannealing, which consists in heating the hardened steel to redness, and then allowing it to cool slowly. Hardened steel istempered, or has its degree of hardness lowered, by being heated to a temperature considerably below that of a red heat, and then cooling suddenly. The higher the temperature the hardened steel is raised to, the lower does its 'temper' become.

Case-hardening.—This is the name given to the process by which the surfaces of articles made of wrought iron are converted into steel, and consists in heating the articles in contact with substances rich in carbon, such as bone-dust, horn shavings, or yellow prussiate of potash. This process is generally applied to the articles after they are completely finished by the machine tools or by hand. The coating of steel produced on the article by this process is hardened by cooling the article suddenly in water.

Copper.—This metal has a reddish brown colour, and when pure is very malleable and ductile, either when cold or hot, so that it may be rolled or hammered into thin plates, or drawn into wire. Slight traces of impurities cause brittleness, although from 2 to 4 per cent. of phosphorus increases its tenacity and fluidity. Copper is a good conductor of heat and of electricity. Copper is largely used for making alloys.

Alloys.—Brasscontains two parts by weight of copper to one of zinc.Muntz metalconsists of three parts of copper to two of zinc. Alloys consisting of copper and tin are calledbronzeorgun-metal. Bronze is harder the greater the proportion of tin which it contains; five parts of copper to one of tin produce a very hard bronze, and ten of copper to one of tin is the composition of a soft bronze.Phosphor bronzecontains copper and tin with a little phosphorus; it has thisadvantage over ordinary bronze, that it may be remelted without deteriorating in quality. This alloy also has the advantage that it may be made to possess great strength accompanied with hardness, or less strength with a high degree of toughness.

Wood.—In the early days of machines wood was largely used in their construction, but it is now used to a very limited extent in that direction.Beechandhornbeamare used for the cogs of mortise wheels.Yellow pineis much used by pattern-makers.Box, a heavy, hard, yellow-coloured wood, is used for the sheaves of pulley blocks, and sometimes for bearings in machines.Lignum-vitæis a very hard dark-coloured wood, and remarkable for its high specific gravity, being 11⁄3times the weight of the same volume of water. This wood is much used for bearings of machines which are under water.

The illustrations in this chapter are in most cases not drawn to scale; they are also in some parts incomplete, and in others some of the lines are purposely drawn wrong. The student must keep to the dimensions marked on the drawings, and where no sizes are given he must use his own judgment in proportioning the parts. All errors must be corrected, and any details required, but not shown completely in the illustrations, must be filled in.

Exercise 62:Single Riveted Butt Joint with Tee-iron Cover Strap.—Two views, one a side elevation and the other a sectional elevation, of a riveted joint are shown in fig. 60. Draw these views, and also a plan projected from one of them. Show the rivets completely in all the views. Scale 4 inches to a foot.

Fig. 60.Fig. 60.

Fig. 61.Fig. 61.

Exercise 63:Girder Stay for Steam Boiler.—The flat crown of the fire-box of locomotive and marine boilers is generally supported or stayed by means of girder stays, an example of which is shown in fig. 61. A B is the side elevation of a portion of one of these girders. Each girder is supported at its ends by the plates forming the vertical sides of the fire-box. The flat crown is bolted to the girders as shown. Observe that the girders are in contact with the crown only in the neighbourhood of the bolts. Consider carefully this part of the design, and then answer the followingquestions: (1) What objections are there to supporting the girders at the ends only without the contact pieces at the bolts? (2) What objections are there to having the girders in contact with the crown plate of the fire-box throughout their whole length?Draw the views shown in fig. 61, and from the right-hand one project a plan. Scale 4 inches to a foot.

Draw the views shown in fig. 61, and from the right-hand one project a plan. Scale 4 inches to a foot.

Fig. 62.Fig. 62.

Exercise 64:End of Bar Stay for Steam Boiler.—On page 12 one form of stay for supporting the flat end of a steam boiler is described. Another form of stay for the same purpose is shown in fig. 62. A B is a portion of the end of a steam boiler. C D is one end of a bar which extends from one end of the boiler to the other. The ends of this bar are screwed, and when the bar is of wrought iron the screwed parts are generally larger in diameter than the rest of the bar. When made of steel the bar is generally of uniform diameter throughout. In the case of wrought-iron bar stays the enlarged ends are welded on to the smaller parts. Welding is not so reliable with steel as with wrought iron. Write out answers to the following questions: (1) What is the advantage of having the screwed part of the bar larger in diameter than the rest? (2) Why are steel bar stays not generally enlarged at their screwed ends?Draw the views shown in fig. 62, and project from one of them a third view. Scale 4 inches to a foot.Exercise 65:Knuckle Joint.—Draw the plan and elevation of this joint shown in fig. 63, and also draw an end elevation looking in the direction of the arrow. The parts at A and B are octagonal in cross section. Scale 4 inches to a foot.

Draw the views shown in fig. 62, and project from one of them a third view. Scale 4 inches to a foot.

Exercise 65:Knuckle Joint.—Draw the plan and elevation of this joint shown in fig. 63, and also draw an end elevation looking in the direction of the arrow. The parts at A and B are octagonal in cross section. Scale 4 inches to a foot.

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