steam engineFig. 1.steam engineFig. 2.
steam engineFig. 1.
Fig. 1.
steam engineFig. 2.
Fig. 2.
Thedrawingrepresents the most simple way of constructing a steam-engine, and, if the workmanship is fairly good, a working model will be produced. First is the boiler; a tin box 13⁄4in. deep and 2 in. in diameter, will serve for this. The joint at the side should be made by folding the edges of metal over each other, and then soldering. The top and bottom are both soldered on their respective places, steam-tight, of course. The top of the boiler must be provided with small bosses of metal, soldered on the inner side, into which the pillar (Fig. 3) and the safety-valve (Fig. 5) are screwed.
The tin plate is not sufficiently thick to afford a hold for the thread on the pillar and valve. A disc of brass, say the size of a sixpence, and1⁄8in. thick, is soldered on the under side of the lid, and the holes, which are tapped to receive the pillar and valve, are bored and threaded before the lid is fixed. By this means a strong hold is secured for the fittings. The screw plugA(Fig. 1) is similarly provided for. When each piece is screwed into its place a little hemp or cotton, placed between the shoulder of the ‘fitting’ and the surface of the tin plate, will assist to ensure a steam-tight joint.
The standard or pillar is brass, about 2 in. long from end to end. Any form may be given to it, according to fancy, the one shown inFig. 2being perhaps as good as any. The lower part is circular,1⁄2in. in diameter, and it has a flat face on one side, against which the valve face of the cylinder works.Fig. 3shows this. The centre of the pillar is bored up in the middle of the screwed part to meetoneof the holesa b, it is immaterial which. The other hole is bored right through the pillar to the opposite side, and forms the exhaust port, the one communicating with the central hole in the pillar being the steam port. For the sake of distinction we will suppose thatais bored into the central hole, andbis bored through the pillar; then, when the pillar is screwed on to the boiler, and steam is generated, it issues from the port holea.
The upper end of the pillar is bored through at right angles to the flat at the bottom (seeFig. 2). Through the top a piece of brass tubing about5⁄8in. long is fixed, generally by soldering; this is the bearing for the crank-shaft. The crank-shaft itself is a piece of steel wire bent to the form required. The fly-wheel is fixed to one end, and prevents the shaft coming out of the bearing, the bend of the arm serving the same purpose at the other end.
steam engineFig. 3.steam engineFig. 4.
steam engineFig. 3.
Fig. 3.
steam engineFig. 4.
Fig. 4.
The cylinder itself is shown atFig. 3, and also inFigs. 1and2. The piston, piston-rod, and bearing which fits the crank-pin are shown inFig. 4. It will be evident that the dimensions of this engine are microscopic. The bore of the cylinder is5⁄16in., and the barrel itself is often made of triblet-drawn brass tube. The enlarged part at the bottom is a casting with a flat face, as shown inFig. 3, on one side. Some makers use a casting for the entire cylinder, but the tube is perhaps the cheaper method of making. A piece of good tube is sufficiently accurate in thebore for use as bought, so that the trouble of boring the cylinder is dispensed with. The base, for the tube to fit in, is bored to the external diameter, and the tube fixed with solder. The lid or cover is fixed only by being snapped on. Its object is only to guide the piston-rod.
A reference toFig. 3will show the working of the oscillating valve. The face of the pillar is shown on the right. On this,ais the hole from which the live steam issues, andbis the exhaust hole. These holes are technically called ports. The holecis bored through the pillar, and takes the trunnion, or pin on which the cylinder oscillates.Fig. 2shows this trunnion-pin prolonged and having a nut on the end. A spiral spring around the trunnion, between the nut and the pillar, keeps the valve face in close contact with the pillar face. Again, turning toFig. 3, the holes in the cylinder on the left are:—c, into which the trunnion is screwed; andd, the steam-way.
When the cylinder is in the position shown inFigs. 1and2, the port-holed(Fig. 3) is over the solid metal between the holesaandb. On turning the fly-wheel the crank draws the piston out very slightly and inclines the cylinder sideways, bringing the portdovera. The live steam from the boiler at once enters and forces the piston upwards, and on the crank reaching the highest point the cylinder is again vertical and the holedis mid-way betweenaandb. The momentum of the fly-wheel carrying the crank round brings the holedoppositeb, and allows the steam to escape. There is no force to keep the engine going during this part of the time except the momentum of the fly-wheel. When the cylinder again inclines to the opposite sidedcomes overa, and force is again applied under the piston. This will keep the engine going.
The single-action oscillating cylinder, being supplied with steam at one end only, exerts power only during half the revolution of the crank. The return stroke is dependent entirely on the momentum of the fly-wheel, which also has to drive the steam out of the cylinder. Steam only acts in the lower part of the cylinder, and as there is no power tending to force off the cover, it may be simply snapped on like the lid of a pill-box. The piston,Fig. 4, has for its head a disc of brass, with aV-shaped groove in its edge. This is packed with hemp or lamp cotton, to make it fit the cylinder steam tight. The rod is a steel wire about1⁄16in. diameter; it is fixed in the piston by riveting, to save the trouble of screwing. The end of the rod has a small piece of brass fixed on to it which fits on the crank-pin.
The crank itself is all of one piece; a straight length forms the shaft; it is bent at right angles to form the throw, and a piece bent from this, parallel to the shaft, forms the pin. This is the most simple way of making a crank, and when large quantities are made the wire is bent upon a template. A better type of crank is made by using a steel rod for the shaft, a brass arm riveted on to it, and a steel pin riveted into that. In the portion of this chapter devoted to the horizontal engine will be found a more complete description of such a crank.
The safety-valve,Fig. 5, is very important as a safeguard in working. Though they are sometimes omitted, yet safety-valves are essential for security. They allow steam to escape from the boiler when the pressure exceeds a certain amount, and thus the danger of an explosion is removed. The valve illustrated in section has a spiral spring to keep the valve itself on its seat. This is effective when the power of the spring has been definitely gauged, but when the valves are put together haphazard no dependence can be placed on the pressure at which the valve will blow off.
steam engineFig. 5.
Fig. 5.
The body of the valve isA, shown in section.Bis the valve itself, fitted to a rod,D; it rests on the conical seat ofA, and is pressed down by the spiral spring within the barrel ofA. The body is screwed into the boiler by the thread at the bottom, and the steam coming up the holeCpresses on the under side of the valveB. When the pressure of the steam is sufficient to overcome the pressure of the spiral spring the valve is lifted and the steam escapes through the holesF F. The coverEis screwed on the body part and confines the spring; it has a hole through its centre to allow the valve-rodDto pass. Especial attention should always be given to the safety-valve when heat is to be applied to a boiler. See that the valve is not fixed to its seat or in any way confined, or an explosion may follow the want of care.
The engine shown by the illustrations is usually mounted on a three-legged stand, which raises it about two or three inches. A wire stand may be made according to fancy, or perhaps some contrivance may be improvised to support the boiler at a convenient height for applying the heat under it.
A small lamp burning methylated spirits—that is, spirits of wine—will supply the requisite heat. It should have a clean and dry wick of lamp cotton; the size of the flame may be regulated, to an extent, by the amount of wick which is drawn out. The lamp must not be quite filled with spirit—about two-thirds full will be ample—and thus the spirit will not be liable to overflow.
When charging the boiler it is best to use boiling water from a kettle. This will save the time which would be lost in heating cold water with the spirit lamp. The water is poured in through the water-plug hole,A,Fig. 1. The boiler must only be filled to a little over half way. The plug is screwed in again and the lamp put under; steam will be generated in due course, and if the fly-wheel is turned in the right direction by hand for a few turns the engine will presently work of its own accord.
It is scarcely necessary to say that the engine above described is of the most simple kind, and every unnecessary detail is omitted. I will now proceed to describe engines of a more elaborate character.
Small model engines are composed mainly of brass castings and of steel which requires no special forging for the purpose. The screws or bolts used to unite the parts are usually purchased in a finished state. Makers of these employ machinery which acts almost automatically, and the screws are sold at a very cheap rate. Larger models require special forgings for the crank shaft, and the castings employed are of iron, which is considerably cheaper than brass.
The castings are made from patterns which are counterparts of the object required. These are imbedded in sand, and leave a matrix, into which molten metal is poured, producing, on solidifying, a facsimile of the pattern. The operation is always carried out in a foundry, where the necessary furnaces and moulding appliances are at hand. The founders charge for the rough castings by weight, and they cost merely a trifle over the value of the metal. It is, however, necessary to supply the requisite patterns before a founder can proceed to do his part of the work.
All vendors of castings have patterns from which their castings are moulded, and of course they charge, in addition to a profit on the cost of the metal, something for the use of the patterns. The patterns for a founder’s use require certain modifications, which it is unnecessary to explain in detail. Some are made in two or more parts, with pins to hold them together. Some have projections affixed to them; these make prints in the mould to receive cores, which form holes in the casting. Those patterns which enter deeply into the moulding sand are made tapering, to draw out easily. In all cases they must be made sufficiently large to allow for shrinkage in the metal. Ordinary iron castings shrink about one-eighth of an inch to the foot; brass about half as much again. Pattern-makers use a ‘contraction-rule’ to work by; this is made longer than the standard measurement, and patterns made according to it are the correct size to allow for shrinkage.
From what has just been said it will be readily understood that vendors of castings charge various prices for their goods. Nor in every case is the quality in accordance with the price, and it is difficult to give the exact prices that should be paid for good castings. Speaking generally, the price is regulated by the weight, and the rate per pound is decided by the seller. In the catalogues issued by various firms will be found the prices charged. As an example of the difference, I notice that a certain size of bolts made by one firm are retailed by shopkeepers at rates varying from 33 to 200 per cent. profit; the same rule probably holds good in all other items.
Those readers who are not possessed of a lathe will not have the means of finishing the cylinders and some other parts which have to be turned. These can, however, be bought in various stages of completion, and the beginner who has only a screwdriver may now purchase the component parts, and, having screwed his engine together, he may claim some merit for his share in the erecting department.
Sets of castings quite finished and ready to be screwed together are now sold. These are generally of the cheaper class, and, tacked on cards, may be seen in the windows of opticians. The prices for the complete engine, with boiler, lamp, and all other parts, range from about five shillings upwards. A few words on the better type of partially finished parts.
These castings are more expensive than those quite rough, but they afford an opportunity of displaying considerable skill and judgment in completing them.
Boring the cylinders is the operation most likely to baffle the tyro. This is done by vendors of castings for about two shillings and sixpence for cylinders 1-in. bore. This charge includes turning the flanges ready to receive the covers, and also boring the steam-ways and cutting the port-holes. When all this has beendone it will be necessary to use a lathe to turn the covers for the cylinder, and also for making the piston. The cylinder may be purchased complete with the covers screwed on and the slide-valve fitted. One an inch in the bore costs half a guinea. Every piece of an engine may be bought separately in a finished state, so that they only require putting together, and when the young engineer has not the requisite tools for doing the work his best plan will be to purchase the finished parts.
A glance at an engine will show that nearly every part of it has been fashioned on a lathe. This tool is indispensable for all kinds of engineering work, but as it is somewhat costly it frequently occurs that tyros are compelled to forego its ownership and get the necessary turning executed by a professional latheman. Those readers who are happily possessed of the king of tools—or the father of mechanism, as the lathe has been aptly dubbed—will have the advantage of being able themselves to execute the work throughout.
A few particulars of the different kinds of engines which a beginner may make, will assist him in deciding as to the form and size best suited to his requirements. An idea of the general forms and peculiarities of engines may be gleaned from what has been already said. It is a matter entirely at the choice of the maker whether he will build a vertical or a horizontal engine—whether it shall have oscillating or slide-valve cylinders, and whether it shall be of microscopic dimensions or a powerful model. All these points are for the consideration of the constructor, and some hints will be of service to, and assist him in arriving at a useful result—that is, the production of a working model.
The dimensions of the cylinder to an extent indicates the power; the pressure of steam must also be considered. The friction in models is very great in proportion to their size, and hence the very small ones are often barely able to generate sufficient power to keep them going. The bore of the cylinder governs the area of the piston, and this multiplied by the pressure of steam and the length of stroke gives the power of the engine.
Let us compare the power of two small cylinders, one half-inch in bore and one-inch in stroke, the other one-inch bore and two-inch stroke. We will suppose the pressure of steam to be the same in both cases, viz., ten pounds to the square inch. Speaking off-hand, many tyros would be apt to say that one cylinder was twice the size of the other, and, as a natural deduction, twice the power. Comparison will at once show the fallacy of the idea.
The area of the half-inch cylinder is nearly two-tenths of a square inch, that of the other nearly eight-tenths. Thus we see that the larger one has four times the area; also the length of stroke is twice as much. According to the rule given above we find the power thus:2⁄10× 10 × 1 = 2, and8⁄10× 10 × 2 = 16. So that the power of the larger cylinder is precisely eight times that of the small one. In every case it is necessary to allow a certain percentage of the power to overcome friction. The smaller the engine the greater will be the percentage lost in friction. These simple facts will at once show that size is a most important consideration. If the friction in the small engine was two pounds, the power would not drive it, whereas if it were that much in the large one, there would still be fourteen pounds of available power.
A cylinder 13⁄8-inch bore and the same length of stroke, viz., 2 inches, wouldgive exactly double the power of the 1-inch bore cylinder just mentioned. If the bore was increased to 2 inches, the power would be exactly four times that of the 1-inch bore, the length of stroke and pressure of steam remaining the same.
The velocity of the piston also forms a factor in calculating the power, which is increased in the same proportion as the velocity. It will be readily understood that when the pressure of the steam is constant, the speed of the engine will depend on the amount of work it has to do. It must also be remembered that the pressure of steam against the piston is by no means necessarily the same as it is in the boiler. In passing from the boiler to the cylinder the steam pressure is always reduced, and the greater the distance, and more exposed or tortuous the steam-pipe, the greater will be the loss of pressure.
Every one knows that the power of steam-engines is given in ‘horse-power.’ This was a term originated by James Watt, and it is now universally adopted. The mechanical equivalent is a lifting power that will raise 33,000 pounds 1 foot high in one minute. On this estimate the power of an engine is calculated. The rule is this: Multiply the pressure on the piston by velocity per minute and divide by 33,000. The velocity of the piston is twice the length of stroke in feet multiplied by the number of revolutions per minute.
Let us calculate the horse-power of the 1 × 2 inch cylinder, already dealt with at a pressure of 10 pounds, the speed being 100 revolutions per minute. By the previous calculation we found that the pressure was 16 pounds. The velocity is4⁄12× 100 = 331⁄3(feet). Multiply these together, 16 × 331⁄3= 5331⁄3, and divide533⁄33000= ·016. That is, the engine is16⁄1000of a horse-power, or capable of raising 528 pounds 1 foot high in one minute. That is supposing all the power was available forduty. In large engines about 20 per cent. is allowed for friction, and in the model we must allow at least 50 per cent. This at once reduces the calculated power to half.
Thepowerof an engine is the nominal, and thedutyis the actual work that it will perform. When the horse-power of an engine is spoken of it must be taken in a qualified sense. By urging the furnace greater effect may be obtained, and by keeping the furnace low an effect less than the nominal power is produced.Dutyis the term used to represent the amount of work absolutely done; it disregards the size of the engine, and simply inquires how much work is done by a given expenditure of fuel. True economy in working will add to the duty of an engine, whilst woful waste in no way affects the power.
In order to supply the requisite quantity of steam, boilers should evaporate at the rate of one cubic foot of water per hour per horse-power; that will produce 1700 cubic feet of free steam. The capacity of a boiler should be four or five times as much as the water it boils off per hour, and the steam space should be at least ten times as large as the consumption of steam at each stroke. The heating surface should be from fifteen to twenty square feet per horse-power. Many circumstances tend to modify these rules, but they maybe taken as fairly reliable.
Engines of the horizontal type are usually employed to furnish the power required to drive fixed machinery in factories. The construction is simple, and the form is adapted for fixing readily anywhere where a tolerably level foundation is to be found.
steam engineFig. 1.
Fig. 1.
steam engineFig. 2.
Fig. 2.
The several illustrations given herewith are drawn to scale, and they will show at a glance constructive details which could not well be explained in letterpress.Fig. 1shows a plan view, andFig. 2an elevation of the complete engine. In both drawings the lettering is the same. The bed-plateA Ais the foundation on which the parts of the engine are fixed. A piece of sheet brass is used for small models, but larger ones have cast-iron foundations. Cylinders 11⁄2inches in the bore andupwards are usually mounted on iron bed-plates, the saving in cost of metal being considerable when the castings are so large. Cast bed-plates have a moulded edge, which adds both to their strength and appearance. Sheet metal has to be mounted on columns sufficiently high to raise the fly-wheel above the ground-level.
The cylinder is shown inFigs. 1and2atB; atC(inFig. 1only) is the steam-chest containing the slide-valve.Dis the fly-wheel fixed on the shaftE, which has at its opposite end the crankF. The piston-rod is shown passing through a guideGfixed to the bed by two screws. The connecting-rod from the piston to the crank-pin is markedH. The eccentric is shown atI, and the rod from it to the steam-chest is the eccentric-rod.JJ,Fig. 2only, show two screws which fix the cylinder to the bed-plate. These references are sufficient to enable the inexperienced reader to identify the principal parts of the engine. By carefully studying the drawings the whole combination of the machine will be understood.
Each of the chief component parts which possess any intricacy of detail are shown on a much larger scale. The description of each one may be taken as generally applicable to engines of the type shown inFigs. 1and2. The dimensions are suited to the size known as ‘3⁄4-inch bore and 11⁄2-inch stroke.’ These measurements refer to the cylinder. It will not be difficult to modify any of the minor details to suit another size, whether it be larger or smaller.
steam engineFig. 3.
Fig. 3.
steam engineFig. 4.
Fig. 4.
steam engineFig. 5.
Fig. 5.
A section of the cylinder is shown inFig. 3; the piston and its rod are absent, to prevent confusion of the parts. The cylinder with the covers on is 2 inches long and 13⁄8inches diameter across the flanges. The bore is3⁄4inch and 15⁄8inches (full) long. The face of the cylinder where the valve works is level with the diameter of the flanges. This face is shown atFig. 4, where the size and position of each porthole may be seen. The rectangle represents the steam-chest itself, and the four small circles are the screw-holes in the valve-face for attaching the steam-chest.
Returning toFig. 3, the steam-ways are shown atA A. These are drilled from the ends to meet the inlet portsBandC, which are closed by the slide-valve (seeFig. 5). The exhaust way is atD, and the port-hole communicating with it needs no special mention. The steam inlet isE; the threaded exterior is for attaching thesteam-pipe from the boiler. The glands and stuffing-boxes, for keeping the piston and valve-rod steam-tight, are shown in section.GGare the glands screwed into the castings; the parts bored out to receive the packings are markedHH. It will not be necessary to make special reference to the body of the cylinder, the covers, &c., as the reader will have become acquainted with these in the previous chapters.
By reference toFig. 3the passage of the steam may be traced. It enters atE, filling the steam chest, and as the valve is shown it could find no outlet. The valve on being moved would uncover one port, sayB, and allow the steam to enter by the steam-wayA, through the slot filed in the edge. When in the cylinder it would force the piston towards the bottom, the action of the eccentric meanwhile pushing the valve along and further openingB. When the piston had made half its stroke the valve would commence to close again, and by the time the end was reached the valve would be again in the position shown. The momentum of the fly-wheel would carry round the eccentric, and with it the valve would move so as to open the way fromBtoD, thus allowing the steam in the cylinder to escape. The port-holeCwould also be opened to the live steam, which would then exert its pressure on the lower side of the piston. By the motion of the valve the steam is let intoBandCalternately, and thus the reciprocating motion of the piston is maintained.
steam engineFig. 6.
Fig. 6.
steam engineFig. 7.steam engineFig. 8.
steam engineFig. 7.
Fig. 7.
steam engineFig. 8.
Fig. 8.
The slide-valve is shown atFig. 5, whereAis a view of the face. The centre is hollowed out, as shown atBof the section, to allow the steam to pass into the exhaust. The back is shown atC; the saw-cut receives the valve-rod, which is thinned down to fit it. The face of the valve, that is, all the outer part ofA, is made perfectly flat, to fit steam-tight on the valve face of the cylinder. Contact is ensured by the pressure of the live steam in the steam-chest; this is always more than that of the exhaust.
The crank-shaft, markedEinFig. 1, is shown alone full size atFig. 6. This is a rod of round steel1⁄4inch in diameter, the total length is 31⁄8inches. At the right-hand end it is reduced in size a length of7⁄8inch, to receive the fly-wheel and the driving pulley. These are generally screwed on to a thread cut on the shaft, but wedging is a more workmanlike way of securing driving wheels and pulleys. The two journals are to rest in the bearings shown atFig. 7; the neck at the left-hand end is to receive the crank-arm. The collars on the shaft outside of each journal are of the widths shown. One of the bearings for the crank-shaft is shown atFig. 7.Ais a side view,Ban edge view, andCa view from the top; in this the dotted lines represent the screw-heads. These bearings are usually brass castings; they are fixed on the bed-plate by two screws each, and the cap is also held on by two other screws.Various designs may be obtained, but the one illustrated is as good as any. The thickness of the bearing is nearly1⁄4inch. The height must be precisely that which will bring the centre of the crank-shaft level with the centre of the piston-rod.
Fig. 8is the crank-arm, giving an end and side view. It should be made of steel and fixed on the shaft by keying, though more often it is screwed on. The thickness is shown about3⁄16; the shape may be according to fancy. The hole at the bottom is for the crank-pin, which is riveted in. The ‘throw’ of the crank is an important point, and it must never be so much that the piston touches the ends of the cylinder. In the present case the ‘throw,’ that is, the distance from the centre of the crank-shaft to the centre of the crank-pin, is5⁄8inch. This gives 11⁄4-inch stroke; there is plenty of space in the cylinder for another3⁄16inch, and possibly the nominal stroke, 11⁄2inches, could be managed by using a thin piston-head. The crank-pin is shown at the top ofFig. 10.
steam engineFig. 9.steam engineFig. 10.
steam engineFig. 9.
Fig. 9.
steam engineFig. 10.
Fig. 10.
The guide-block,Fig. 9, serves to guide the piston-rod, and steadies it against the influence of the crank. The shape is shown by the illustrations. The hole for the piston-rod is bored on a level with the axis of the cylinder and the centre of the crank-shaft. The block is secured to the bed-plate by two screws, holes for which are shown in the top view.
steam engineFig. 11.
Fig. 11.
Fig. 10shows the crank-pin and three views of the head of the connecting-rod. The crank-pin is steel,3⁄16inch diameter, turned down to1⁄8inch at the journal and at the neck, which is riveted into the arm (Fig. 8). The head of the rod is fitted with a cap, held by two screws, so that it may be placed over the crank-pin into the groove. The other end of the rod, which is forked, is shown atFig. 11. Here a section and elevation are given; the round piece, called the cross-head, which receives the two screws (see section), is bored to fit the piston-rod, and it is clamped to this by the points of the screws shown. The sides of the fork are bored to fit freely over the threads of the screws, so that it may oscillate with the motion of the crank. The position of the cross-head on the piston is determined when the engine is together; it is placed so that the piston slides midway between the ends of the cylinder.
steam engineFig. 12.
Fig. 12.
Fig. 12shows the eccentric and the eccentric strap. The first is a piece of brass; the large circle has a groove turned in it to receive the strap, and the boss is eccentric, as shown in the left-hand figure. The amount of eccentricity is1⁄16inch,which gives a travel of1⁄8inch to the slide-valve. These eccentrics are turned on a mandrel having double centres, one pair serving when turning the boss, and the other when turning the eccentric itself. A set-screw tapped through the boss serves to secure it on the crank-shaft.
The strap on the right ofFig. 12is cast in the form shown, the centre is bored to fit the groove in the eccentric, and the strap then cut in halves through the lugs. These lugs serve to take screws, which hold the strap together. The projecting piece on the right is to receive the eccentric rod, which is screwed into the strap at this point.
This completes the description of the various parts of a model horizontal engine. A glance atFigs. 1and2will show the relative position of each.
Herewith are drawings of an engine with an oscillating cylinder. This form of construction economises space and weight; it is also more simple than slide-valve cylinders. In all oscillating engines the cylinder is mounted on trunnions or gudgeons, so that it may swing to and fro through a small arc, and allow the piston-rod to follow the motion of the crank. No connecting-rod is required in this engine, the piston-rod being attached direct to the crank-pin.
Theillustrationshows an engine specially adapted for propelling a model boat. The entire machine is kept low down, which is generally necessary for small boats. The fly-wheel is shown much heavier than are those attached to toyshop engines, but it is not unnecessarily large. Experiments show that a weighty fly-wheel is required on an engine which has the constant drag of a screw propeller to overcome. This fact is ignored by some makers of engines, and I have known cases where a useless engine has been made effective by the substitution of a much heavier fly-wheel. (Seepage 132.)
The framework on which the cylinder is mounted, and which also generally serves to carry the bearings for the driving shaft, may be of almost any design. There is no set pattern for this purpose, and it rests with the designer to fashion his pattern according to fancy. Theform shownpossesses the essential characteristics. It is strong, yet light; there is a good base by which to secure the engine to the hull of the boat. Suitable provisions are made for the bearings of the crank-shaft, also for the valve-face and the cylinder trunnion. So long as these are provided for, the mere contour is of little importance.
Fig. 1gives a side elevation, andFig. 2an end view of the same engine. The cylinder is 1 in. bore and 1 in. stroke. The length without covers is 11⁄2in., that allows1⁄4in. for thickness of piston,1⁄16in. for each of the projections of the two covers, and the same distance left vacant at each end. The diameter of the cylinder across the flanges is 11⁄2in., and a semicircular rib is shown in the middle. Each cover is held on by six hexagon-headed bolts, placed equidistant round it, tapped into the flange. These bolts are not shown in the lower cover.
The piston-rod is shown out from the cylinder to its fullest extent. The rod is of round steel1⁄8in. diameter. The crank-pin head is of brass screwed on to the end of the rod. Though shown as a solid piece, it would be better if this headwas cut across horizontally at the diameter of the crank-pin, and the cap secured by two screws. The crank-pin is markedE. It is steel riveted into the disc which forms the crank. A crank-arm would do equally well, and the disc is shown simply as illustrating a different plan. The discFis fixed on the crank-shaft either by screwing, by a transverse pin, or by a key.
steam engineFig. 1.steam engineFig. 2.
steam engineFig. 1.
Fig. 1.
steam engineFig. 2.
Fig. 2.
The crank-shaft is1⁄4in. steel, and should be turned smooth and parallel to fit the hole in the standard. This hole should also be smooth and parallel, which it will be if properly bored with a suitable tool. A long bearing has no more friction than a short one, though a contrary opinion seems to be prevalent. A small hole for supplying the oil necessary for lubrication should be made near the middle of this bearing. The same remarks apply to the bearingHthrough which the trunnion passes.
The fly-wheel is markedAinFig. 1; it is cast-iron, 21⁄2inches in diameter and3⁄4in. wide on the rim. The rim should be half an inch thick at least, and the bossin the centre as wide as the rim. If bored fairly true, the casting need not be turned on its edge, though it will look better if bright. A small key should be used to fix the fly-wheel on the shaft. This latter, shown broken off in the drawing, projects slightly, and carries a small disc with two pins, which engage in a fork on the end of the propeller shaft and so drive it, and the screw is attached to its end.
steam engineFig. 3.
Fig. 3.
The valve-face of the standard,B,Fig. 1. must be made perfectly flat and at right angles to the boring for the crank-shaftC.Fig. 3shows the face of this standard as it would be seen inFig. 2if the cylinder was removed. It is convenient to turn the valve-face in the lathe, and at the same time cut the circular groove, which, after being stopped by plugging at both top and bottom, forms the supply and exhaust ports respectively. The face may be made flat by filing when a lathe is not available, and the groove cut by means of a circular cutter. This is an annular bit with teeth on its edge, which cut a channel but do not touch the inner part.