PART X.
THE CYLINDERS, PISTONS, GUIDE-RODS AND CONNECTING-RODS.
QUESTION 159. _How are the steam cylinders constructed?_
_Answer._ They are made of hard cast iron, and have the steam and exhaust ports and valve-seats cast with them. The harder the iron the better will the cylinders withstand the wear of the pistons and valves, but they must at the same time be made soft enough, so that after they are cast the inside can be bored out perfectly cylindrical, the ends turned off, the bolt-holes drilled, and the valve-seats planed smooth.
Fig. 92 represents a longitudinal section through the centre of the cylinder and steam-chest. Fig. 94 is a plan of the same parts with the cover of the steam-chest and the valve removed. The left-hand side of fig. 95 shows a transverse section through the centre _c d_, fig. 92, of the cylinders, and the right side is a section through the steam-pipe _G h_, fig. 94. The same letters indicate like parts in the three different views.
The cylinders of locomotives in this country are now universally placed on the outside of the wheels, as has already been described. In order to fasten them securely together and to the boiler, they are attached to what is called a _bed-plate_ or _bed-casting_, _D D_, figs. 94 and 95, which is placed between them. Some builders make this bed-casting in a separate piece, and the cylinders are then bolted to it on the outside, about at the dotted lines, _l_, _m_, fig. 95. Others cast one-half of it with each cylinder, as shown in our engravings, and then bolt them together at the line _i_, _j_, which is the centre of the engine. The bed-casting is also bolted to the smoke-box by the flanges _E_, _E_. The cylinders are bolted to the frame _F_ with bolts, _m_ and _k_, fig. 95.
After the cylinders are bored out, and the ends turned off, _heads_, _A_ and _B_, figs. 92 and 94, are fitted with steam-tight joints to each end. These heads are fastened with bolts and nuts, _a_, _a_, _a_, to flanges, _C_, _C_.
QUESTION 160. _How is the steam conducted to and from the cylinders?_
_Answer._ Two pipes or passages are cast in each cylinder, the one, _G G′_, fig. 95, for admitting steam into the steam-chest, and the other, _H H′_, for exhausting it from the cylinders. The one _G G′_ is called the _steam-passage_, and the other, _H H′_, the _exhaust-passage_. The steam-passage terminates at one end with a round opening, _G_, figs. 94 and 95, to which the steam-pipe _o_, figs. 40 and 95, is attached inside of the smoke-box. At the other end it divides into two branches, _G′_, _G′_, fig. 94, each of which terminates in an opening, _g_, _g′_, inside of the steam-chest. The steam is thus delivered at both ends of the chest, and can pass freely into each of the steam-ports. By making the cylinders in this way, they are exactly alike for each side of the engine, or, to use a shop phrase, there are “_no rights and lefts_,” so that a cylinder casting can be used for either side of the engine. This method of making cylinders has been adopted by a number of the principal builders in this country, but is by no means universal.
[Illustration: Fig. 92. Scale ³⁄₄ in. = 1 foot.]
[Illustration: _Fig. 93._]
[Illustration: Fig. 94. Scale ³⁄₄ in. = 1 foot.]
[Illustration: Fig. 95. Scale ³⁄₄ in. = 1 foot.]
QUESTION 161. _How is the steam-chest constructed?_
_Answer_. It usually consists of two castings, one of which, _J_, figs. 92 and 94, is a square cast-iron box made open at the top and bottom. This rests on the top of the cylinder casting and is joined to the latter with a steam-tight joint. On top of it is a cast-iron cover, _K K_. The steam-chest and cover are held down by bolts, _p_, _p_, which are screwed into the cylinder casting and have nuts on top.
QUESTION 162. _How are the slide-valves made to work steam-tight on the valve-seats?_
_Answer_. They are first planed off smooth, and then filed and scraped until the two touch each other over the whole of their surfaces in contact. The valve-stem _v_, fig. 92, works steam-tight through a stuffing-box on the steam-chest.
QUESTION 163. _How are the valves and pistons oiled?_
_Answer_. The oil is usually introduced into the steam-chest through a cock, _c_, fig. 92, called an _oil-cock_. From this cock it flows down upon the valve and is conducted by suitable holes and channels to the valve-face and from there through the steam-ports to the cylinder and piston. Sometimes, for greater convenience, oil-cocks _c_, _c_, fig. 71, are placed inside of the cab and communicate by pipes with the steam-chests.
The valves are oiled by pouring oil or melted tallow into the oil-cocks when the steam is shut off from the steam-chests and cylinders. When the pistons are working in the cylinders without steam, they create a partial vacuum, so that if oil is then poured into the oil-cocks it will be sucked into the steam-chests, or, in other words, it will be forced in by the pressure of the air above it. _Q_, fig. 71, is a shelf attached to the boiler to receive an oil-can filled with oil or tallow, which is thus melted or kept in a fluid condition by the heat of the boiler.
QUESTION 164. _How are the cylinders and steam-chests protected so as to prevent, as far as possible, the heat in the steam from being lost?_
_Answer._ The sides of the cylinders are covered with wood, _w_, _w_, _w_, fig. 95, called the _cylinder lagging_, and the wood is covered outside with Russia iron or brass, which is called the _cylinder-casing_. The ends of the cylinders have light metal covers, called _cylinder-head covers_, made of cast iron, brass or sheet metal. The steam-chest is covered in a similar way so as to be surrounded either with a covering of wood or of confined air. Sometimes coarse felt is used for the purpose. The covering, excepting the cylinder lagging, is not shown in the engravings.
QUESTION 165. _For what purpose are the cocks C, C, figs. 92 and 95, at each end of the cylinder, used?_
_Answer._ They are used to exhaust the water which collects in the cylinders. When the engine is not working the cylinders and steam-pipes are all cooled off, so that when steam is first introduced into them a great deal of it is condensed until they become warmed. Water is also frequently carried over from the boiler with the steam. When this occurs the boiler is said to _prime_, or to “_work water_.” This water and that produced by the condensation of steam collect in the bottom of the cylinder and will not escape through the exhaust-pipes until the piston moves up so near to the end of the cylinder that the water will fill the whole space between it and the cylinder-head. As has already been stated, it will then escape so slowly that the momentum of the piston and other machinery is liable to “knock out” the cylinder-heads or even break the cylinder itself. The cocks _C_, _C_, called _cylinder-cocks_, are therefore placed in the under side of the cylinder, so that when they are open if there is any water in the cylinder it will escape through the cocks. They are therefore always opened when the engine is starting, or at any other time when there is any indication that there is water in the cylinders.
QUESTION 166. _How are these cocks opened and closed?_
_Answer._ A shaft, _R R_, figs. 92 and 94, which extends across the frames, has an arm, _R S_, fig. 92, at each end. These arms are connected by rods, _S T_, with the handles of the cylinder-cocks. The shaft also has a vertical arm, _R U_, the upper end of which is connected by a rod with the cab. At the end of the rod is a suitable handle, _f_, fig. 71, by which the cocks can be either opened or closed at pleasure by the locomotive runner.
QUESTION 167. _How is the piston-rod fastened to the piston?_
_Answer._ It fits into a straight or tapered hole in the piston-head, in which it is fastened either with a key, _k k_, as shown in figs. 96 and 97,[44] or by a nut on the front side of the piston.
[44] Fig. 96 is an end view of the piston with the follower-plate removed. Fig. 97 is a section through the centre.
[Illustration: Fig. 96.]
[Illustration: Fig. 97.
Scale 1¹⁄₂ in. = 1 foot.]
QUESTION 168. _How is the piston constructed?_
_Answer._ It is made of two cast-iron pieces, _B_ and _C_, fig. 97, the one, _B_, called the _piston-head_ or _spider_, to which the piston-rod _D_ is attached. The other part, _C_, called the _follower-plate_, is bolted to the piston-head by the bolts _c_, _c_, called _follower-bolts_. The piston-head has lugs or projections, _d_, _d_, _d_, fig. 96, cast on the inside, to which the follower plate is bolted. Hollow spaces are thus left between these lugs.
QUESTION 169. _How is the piston made to work steam-tight in the cylinder?_
_Answer._ By means of two rings, _A_, _A_, figs. 96 and 97, called _packing-rings_. These rings are turned of the same size or a little larger in diameter than the cylinder. They are then cut open at one point in their circumference so that they can be pressed apart or expanded by the springs _a_, _a_, called _packing springs_, on the inside of the rings. These springs are pressed out by the nuts and bolts _b_, _b_, called _packing-bolts_ and _packing-nuts_, so that when the rings wear they can be expanded so as to fill the cylinder completely. The place where the one ring is cut is placed opposite that of the opening in the other ring, or they are made to _break joints_, as it is called. This is done to prevent the steam which leaks through the opening where the one ring is cut from passing through to the other side of the piston. These rings are usually made of brass and have grooves, _c_, _c_, fig. 97, turned in them, which are filled with what is called Babbitt’s metal. This metal is used because it is less liable to scratch the cylinders than brass alone. Another ring, _l l_, made of cast iron and as wide as the two brass rings, is placed inside of the latter and is intended to furnish a bearing for the springs, and thus distribute the pressure of the springs equally on the packing rings. This iron ring is also cut open at one point.
QUESTION 170. _How is the piston-rod made to work steam-tight through the cylinder-head?_
_Answer._ By what is called a _stuffing-box_. This consists of a cylindrical chamber, _r r_, fig. 92, which is made about 1¹⁄₂ inches larger in diameter than the piston-rod. This leaves a space ³⁄₄ of an inch wide all around the rod. This space is filled with hemp or some other fibrous material, called _packing_, saturated with oil or melted tallow. This packing is compressed by a hollow cylinder, _s s_, called a _gland_, the inside of which fits the piston-rod and the outside the stuffing-box. This gland is forced into the stuffing-box by nuts, _t_, _t_, which are screwed down on a flange, _u_, attached to the gland. The packing is thus compressed in the stuffing-box and forced against the piston-rod, which is made smooth and perfectly round and straight, and against the side of the stuffing-box, so that no steam can escape around the piston-rod. A brass ring or “_bushing_” is often put into the cylinder-head and in the gland where it touches the piston-rod,[45] because brass will bear the friction of the rod better than cast iron, and when it is worn out it can be removed and a new one substituted in its place.
[45] Locomotive piston-rods are now usually made of steel.
QUESTION 171. _Why is the end of the piston-rod made to work in guides?_
_Answer._ Because it must move in a straight line if it and the piston work steam-tight in the cylinder. By referring back to fig. 2, it is obvious that if a pressure be exerted against the piston _B_ and communicated to the crank-pin _N_ by the connecting-rod _E_, the latter, excepting at the dead-points, will exert a pressure either upward or downward, according to the direction the piston is moving. This pressure would bend the piston-rod if no provision were made to prevent it. For this reason, therefore, the end of the piston-rod is attached to what is called a _cross-head_, _L_, figs. 92 and 94, which works in guides, _M_, _M_. The cross-head is made of cast iron and has slides, _N_, _N_, figs. 93 and 94, one on each side, each of which works between a pair of guide-bars or rods, _M_, _M_, shown in section in fig. 93.[46] These guide-rods, or _guides_ as they are called, are planed and finished with great accuracy so as to be straight and smooth, and are attached to the cylinder-head at one end, and to a support, _O_, called the _guide-yoke_, which is fastened to the frame at _F_, fig. 94, and also usually attached to the boiler. The guides are set with great care, so as to be exactly parallel with the axis or centre line of the cylinder, so that the cross-head will slide in exactly the same path that the piston-rod will if it moves in a straight line. If then the piston-rod and the connecting-rod are attached to the cross-head, all the strain produced by the obliquity of the connecting-rod will be borne by the guides, thus relieving the piston-rod, and making it certain that it will move in a straight line.
[46] Fig. 93 is a transverse section through the guides at _n_, fig. 92.
QUESTION 172. _How are the piston and connecting-rods attached to the cross-head?_
_Answer._ The end of the piston-rod fits into a tapered hole in the cross-head and is held by a key, _w_, figs. 92 and 94. The connecting-rod is attached to a pin, _Q_, called a _wrist-pin_, which is cast with the cross-head.
QUESTION 173. _How is the wear of the slides lessened and compensated?_
_Answer._ Sometimes they are made with brass wearing pieces called _gibs_, shown at _N_, _N_, fig. 93, which are placed between the slides and the guides. These gibs can either be removed and new ones substituted when they become very much worn, or by inserting thin pieces of metal, called liners, between them and the cross-head, they will be spread apart so as to fill the space between the slides. The slides are now, however, oftener made without gibs, and have recesses either cast or drilled in them, which are filled with either Babbitt’s metal or glass bearings, which latter are said to wear very well. The guides are bolted at each end to blocks, _x_, _x_, called _guide-blocks_, which can be planed off so as to bring the guides nearer together when they and the slides are worn. Sometimes liners are placed between the blocks and the guides, which can be removed when it is necessary to bring the guides nearer together.
QUESTION 174. _Are the top and bottom guides worn alike?_
_Answer._ No: the top guide in ordinary engines is worn the most, because the pressure of the slides is always on the top guide when the engine is running forward, and on the bottom guide when it is running backward. This will be understood by referring back to the series of figures from 11 to 24. It will be noticed that in the backward stroke of the piston, represented by figs. 11 to 17, the strain on the connecting-rod tends to _push_ the cross-head upward, and in the forward part of the stroke, figs. 18 to 24, the connecting rod _pulls_ the cross-head in the same direction. If the crank turned the opposite way, this action would be reversed and the cross-head would then be alternately pushed and pulled downward, and the bottom guides would then be worn the most. As nearly all locomotives run forward more than backward, the top guides are usually worn the most.
QUESTION 175. _How are the slides oiled?_
_Answer._ Oil cups, _n_, _o_, figs. 92, 93 and 94, are placed about the middle of the top guide. These cups usually have a reservoir to hold a supply of oil, and are so constructed that it will be gradually fed on the slides, which are thus constantly and regularly lubricated.
QUESTION 176. _How are the pumps worked from the piston-rods?_
_Answer._ The pump-plunger is attached to a projection, _W_, figs. 92 and 93, called the _pump-lug_, cast on one of the slides of the cross-head. The plunger thus receives a reciprocating motion from the piston.
QUESTION 177. _How are the connecting-rods made?_
_Answer._ They are made of flat bars of wrought iron. The rods which connect the cross-heads with the driving-wheels are called _main connecting-rods_, and the rods which connect or couple the driving-wheels together are called _coupling-rods_.[47] Fig. 98 represents a side view and a plan of a main connecting-rod. In the side view the end _B_, and in the plan both ends of the rod are shown in section. It is attached to the wrist-pin at _A_ and to the crank-pin at _B_. Fig. 99 represents similar views of a coupling-rod. To save room in the engraving each of these rods is represented with a part of the middle broken away. The main rods are usually made wider at _G_, next the crank-pin, than at the other end, as it has been found that they are most liable to break at that end. The coupling-rods are now made either straight or somewhat wider in the centre.
[47] They are also often called _side_ or _parallel-rods_, but the term _coupling-rods_ is considered the best.
[Illustration: _Fig. 98._]
[Illustration: _Fig. 99._
Scale ³⁄₄ in. = 1 foot.]
QUESTION 178. _How are these rods prevented from getting loose on the pins from the wear of the latter in the inside of the holes of the rods?_
_Answer._ The ends of the rods are provided with what are called _brass-bearings_,[48] or “_brasses_,” _c_, _d_, and _e_, _f_. These brasses are made in pairs, so as to embrace the pins, from each side. They are held by ~⊃~-shaped clamps, _s s s_, called _straps_, which are bolted to the rods. When the brass-bearings become worn, they are taken out of the straps, and a portion of their surfaces of contact with each other is filed away, thus allowing them to come nearer together, and thereby reducing the size of the hole which receives the pin or journal. In order to prevent their being loose in the straps, tapered or wedge-shaped _keys_, _k_, _k_, which bear against the brasses, are fitted in the straps and rods. By driving down these keys the brass bearings are forced together, thus reducing the size of the hole for the journal, and making the rods fit tightly on the pins. A hard steel plate, shown by dark shading in the engraving, is sometimes interposed between the keys and the brasses to prevent the key from indenting the surface of the soft brass. As the keys are very liable to get loose and fall out, they are held either by screws and nuts, _x_, _x_, as shown in the engraving, or by _set-screws_ on the side of the rods. The whole arrangement of straps, keys and brasses is called a _stub-end_.
[48] The portion of a shaft pin or spindle subjected to friction is called a _journal_, and the surface which presses or “bears” against it is called a _bearing_.
QUESTION 179. _How are the journals of the crank-pins oiled?_
_Answer._ By oil-cups attached to the straps, above the journals, similar to the cups used on the _guide-rods_, but which are not shown in the engravings of the connecting-rods. Sometimes _oil-cellars_, as they are called, are attached to the under side of the straps. These are metal boxes, which are filled with oil, which is agitated violently by the rapid motion of the rods, and is thus applied to the journals through holes drilled in the straps. In order to confine the oil and prevent its leaking out around the journals of the coupling-rods, the brasses at _n_, _n_, fig. 99, are usually made so as to enclose the outside end of the crank-pin, which thus not only keeps the oil in, but excludes the dust. The brasses are usually lined with Babbitt’s or some other kind of soft metal, which is thought to be less liable to heat from the friction of the journals.
QUESTION 180. _Are the coupling-rods always made with stub-ends?_
_Answer._ No; their ends are sometimes made in one piece--that is, without straps or keys. The holes which receive the crank-pins then have brass rings or _bushings_, as they are called, which fit tightly and are driven into the holes, and form the bearings on the pins. When these rings become worn they are driven out and new ones put in.
QUESTION 181. _What is meant by the term lost motion?_
_Answer._ It is used to designate the wear of machinery, which causes a loss of motion in some of the parts. Thus if the bearings of the main connecting-rods are worn, the piston must move a distance equal to the wear at each end of the stroke before it moves the crank-pin. Lost motion might therefore be called the looseness of the parts. When we speak of _taking up_ the lost motion, we mean making parts which were loose fit tightly.