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 the _outside lap_ of the valve, and the amount which it projects on the inside is called the _inside lap_. When the term lap is used without any qualification, outside lap is to be understood. In fig. 58 it will be seen 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.

[Ill.u.s.tration: FIG. 58.]

When the piston is at the beginning of its stroke the steam port is generally open by a small amount called the _lead_ of 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 bra.s.s, bronze, or cast iron.

EXERCISE 59: _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 bra.s.s, and the valve spindle and nuts of wrought iron.

EXERCISE 60: _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 the _cylinder 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.

_c.o.c.ks._--A c.o.c.k 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 pa.s.s, or it may be turned round so that the solid part of the plug lies across the hole in the pipe, and thus prevent the fluid from pa.s.sing. As the student will be quite familiar with the common water c.o.c.k or tap such as is used in dwelling-houses we need not ill.u.s.trate it here.

[Ill.u.s.tration: FIG. 59.]

Fig. 59 shows a c.o.c.k 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 c.o.c.k is taken from the bore of the pipe in which it is placed; thus fig. 59 shows a 2-1/4-inch c.o.c.k.

EXERCISE 61: 2-1/4-_inch Steam or Water c.o.c.k._--First draw the views of this c.o.c.k 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 pa.s.s 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 bra.s.s.

XVII. MATERIALS USED IN MACHINE CONSTRUCTION.

_Cast Iron._--The essential const.i.tuents 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.

In _white cast iron_ the whole of the carbon is in chemical combination with the iron, while in _grey cast iron_ the carbon is princ.i.p.ally 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 produces _mottled 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 about 1/8 of 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 called _foundry irons_ or _foundry pigs_, while the white varieties are called _forge irons_ or _forge 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 making _chilled 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 from 1/8 to 1/2 an 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 hemat.i.te (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 pa.s.ses off as gas. In other words the carbon is burned out. The iron is removed from the puddling furnace in soft spongy ma.s.ses called _blooms_, which are subjected to a process of squeezing or hammering called _shingling_.

These shingled blooms still contain enough heat to enable them to be rolled into rough _puddled 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 called _merchant 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 and rolling or hammering, but beyond that a repet.i.tion 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 of _annealing_, 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 called _forging_. 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 called _welding_, 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 the _cementation_ process, 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 form _blister 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 of _shear steel_.

To form _cast steel_ the bars of blister steel are broken into pieces and melted into crucibles.

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

_Bessemer steel_ is 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 quant.i.ty of '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 of _annealing_, which consists in heating the hardened steel to redness, and then allowing it to cool slowly. Hardened steel is _tempered_, 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._--_Bra.s.s_ contains two parts by weight of copper to one of zinc. _Muntz metal_ consists of three parts of copper to two of zinc.

Alloys consisting of copper and tin are called _bronze_ or _gun-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 bronze_ contains copper and tin with a little phosphorus; it has this advantage 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. _Beech_ and _hornbeam_ are used for the cogs of mortise wheels. _Yellow pine_ is 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-vitae_ is a very hard dark-coloured wood, and remarkable for its high specific gravity, being 1-1/3 times the weight of the same volume of water. This wood is much used for bearings of machines which are under water.

XVIII. MISCELLANEOUS EXERCISES.