Chapter IX: Model Steam-Engines, and How to Make Them

BY PAUL N. HASLUCK, Author of _Lathe-work_, &c.

I.--PRINCIPLES OF THE STEAM-ENGINE.

This chapter is intended to fully describe the constructive details of miniature steam-engines. It is proposed to first give an idea of the general principles which govern steam-engines, and to explain the various characteristics and methods of constructing different types of engines. The boiler and its several fittings and attachments will be duly described, and then minute directions given for constructing engines with oscillating and slide-valve cylinders. Illustrations of both vertical and horizontal engines will be given, and also sketches in all cases where they will serve to explain more fully the meaning of the text.

This is a brief outline of the scope of the present chapter. Those readers who have acquired only slight manual dexterity in the use of tools will find little difficulty in making the engines illustrated, if the instructions given are carefully followed. In each case the minute details of the various processes incidental to our engineering work will be carefully described, so that those unacquainted with the mechanical arts will be able to comprehend the method of procedure.

Model engines, in every stage of manufacture, from the rough castings direct from the foundry to the complete, highly-finished working model, may now be purchased in nearly every town of importance throughout Great Britain. Though this trade is of but recent growth, its continual extension proves that model engines are objects of interest to a large number of the rising generation, and hence it is felt that information as to their manufacture will prove acceptable to very many readers.

It will be advisable to gain an insight into the principles which govern the action of a steam-engine, and to learn some of the technical peculiarities, before proceeding to attempt its manufacture. There are numerous text-books on the steam-engine, which may be studied with advantage, and which show the theoretical principles.

The modern engine, which now claims our attention, is the result of numerous successive improvements. The application of steam as a motive power was probably originally made by Hero, who, 150 B.C., constructed, or at least described, an Æolipile. This was a hollow sphere with hollow bent arms attached; when water placed inside the sphere was heated, and steam generated, it issued from the arms, and reacting on the air caused the sphere to rotate. A model of this, the primogenitor of the modern steam-engine, can be bought at many opticians’ shops for about one shilling.

The commencement of the eighteenth century began the first steps towards the development of the modern form of engine. Savery and Newcomen made improvements, which were perfected by James Watt, who was born at Glasgow in 1737. Amongst other valuable improvements he first contrived to convert the reciprocating motion into a rotary one by means of the crank. In the year 1800 Watt retired from business, leaving the steam-engine in much the same condition as we find it now. The application of steam-power for locomotion on both land and water followed, and now stationary, locomotive, and marine engines, driven by steam, are distributed all over the civilised world.

The varieties of model engines are in many cases indicated by their names. Stationary engines are intended to be fixed, as those used for driving machinery. Locomotives are those which are intended to travel by steam, and are self-moving. Marine engines are those used to propel ships. Of these three classes we shall deal only with the first and third in the present chapter. Locomotives are much more complicated in their construction, and consequently are more difficult to make.

Horizontal engines are those having the cylinder lying with its axis in a horizontal position. Vertical engines have the cylinder upright; sometimes they are designated by the latter adjective. Beam engines have an oscillating beam; one end is connected to the piston and the other to a rod which drives the crank. Cylinders are single-acting when the steam is admitted only at one end, and consequently with these the crank is only propelled during half of its rotation. Double-acting cylinders are provided with valves which admit the steam at each end of the cylinder alternately. Oscillating cylinders are fitted to oscillate with the motion of the crank, and the steam-valves are usually contrived to act by this oscillating motion. Slide-valve cylinders have a sliding valve, worked by a rod connected to an eccentric on the crank shaft, which opens the steam ports to alternately admit live steam and exhaust at both ends of the cylinder. Slide-valve cylinders are invariably double-acting.

Boilers, which are the vessels in which water is converted into steam, are usually described by their shape and position. They may be cylindrical, spherical, etc., and horizontal or vertical. The construction also forms a distinguishing characteristic. Tubes are usually inserted in the boiler to convey the heat from the fire. These tubes--which are more properly called flues, especially in large boilers--vary in number from one of large gauge to scores of small ones, thus naming the respective boilers single-flue or multiflue. It may be advisable to mention here that _tubular_ boilers are those in which the water circulates in the tubes, and the fire impinges on the outer surface. When the fire operates inside the tube it is called a _flue_. A tube carries water; a flue carries flame and the volatile products of combustion.

Boilers, or steam generators, that are used to contain the water which, when converted into steam, drives the engine, require to be sufficiently strong to withstand an internal or bursting pressure. This pressure is very great in high-pressure engines, but in models it is generally very low, and seldom exceeds twenty pounds to the square inch. The evaporating capacity of the boiler is according to the requirements of the engine it has to supply. The resistance of the piston to the steam shows the pressure at which it should be supplied. Boilers are generally tested, by means of a hydraulic pump, to stand a pressure at least double that at which it is intended to use them. It is unsafe to generate steam in any vessel that has not been properly tested. This fact cannot be too strongly impressed upon the mind of the reader.

Suppose a double-action cylinder, 1-inch bore and 2-inch stroke, is to make one hundred revolutions of the crank per minute, let us see how much steam will be wanted to drive it. The area of the piston is ·785 inch, and each revolution of the crank will require the cylinder to be filled twice--that is, one stroke in each direction. This will take a column of steam ·785 inch in diameter and 4 inches long for each revolution, or 314 cubic inches of steam per minute. If the speed is greater, the quantity of steam must be increased proportionately; and when running at the rate of one thousand revolutions per minute--a speed often attained--3,140 cubic inches of steam will be wanted to supply the cylinder. That is at the rate of about 100 cubic feet per hour.

The pressure of the steam has not yet been taken into account, but it obviously forms a most important factor in the calculation. Water in an open vessel boils at a temperature of 212° Fahr. Provided that the vessel allows the steam to escape freely, all the heat that can be applied will only generate steam at the same pressure, though it will escape faster. As the bubbles of steam ascend to the surface they escape, having only the pressure of the atmosphere to overcome. When water is confined in a closed vessel, like the boiler of a steam-engine, the temperature may be raised to considerably above the usual boiling-point. The heat is always proportionate to the pressure, and steam at a pressure of 120 lb. per square inch is equivalent to the heat represented by 345° Fahr.

A correct knowledge of the fact that pressure depends on temperature cannot be urged too strongly on the mind of the model engineer. In many model boilers it is quite impossible to raise the heat sufficiently to produce an adequate pressure. Boiling water at 212° Fahr. does not produce any available pressure of steam, it merely counterbalances the weight of the atmosphere, which is 15 lb. to the square inch. By increasing the heat, which can only be done in a closed vessel, available pressure is obtained. Thus 228° = 5 lb., 241° = 10 lb., 251° = 15 lb., and so on. The steam, and the water from which it is generated, and with which it remains in contact, have both the same temperature.

A cubic foot of water weighs 62·5 lb., and it will produce 882 cubic feet of steam, at a pressure of 15 lb. to the square inch above the normal atmospheric pressure; this is equal to a temperature of 251° Fahr. If the pressure is raised to 150 lb., which requires a temperature of 371°, only 187 cubic feet of steam will be produced. Steam is elastic, and hence the more it is compressed the greater will be its force. If one cubic inch of steam, at a pressure of 30 lb., is admitted into a cylinder, and the supply cut off when half filled, the steam will expand till it has filled the cavity, and in increasing its bulk twofold its force will diminish in inverse ratio. The pressure will therefore diminish to 15 lb. to the square inch. The expansive force of steam is always at work on the piston of the engine, and it varies in accordance with the arrangement of the valves.

Let us now trace the effect of the steam when admitted to the cylinder. When the governor valve is opened the steam flows along the pipe to the slide valve chest, and if one of the ports are open it reaches the cylinder. In traversing the pipes which conduct it to the cylinder the steam is cooled considerably and its force diminished. In course of time the parts become heated to a certain extent, and then the loss of power is less. When the steam enters the cylinder it at once exerts a certain force on the piston. This has the effect of turning the crank shaft, and in due course the slide valve closes the steam inlet. Now the steam within the cylinder acts expansively, and continues to drive the crank shaft to the end of the stroke. Then the exhaust port is opened, and allows the spent or dead steam to escape. At the same time the inlet at the other end is opened and the live steam rushes in and exerts its full pressure on the piston, causing it to travel in the opposite direction. The opening and shutting of the steam ports is effected by an eccentric on the crank shaft. In treating of the construction of these parts, the relative sizes will be given and the correct motion explained.

II.--A SIMPLE TOY ENGINE.

The most simple form of toy engine is that illustrated herewith. It consists of a tin boiler, a single-action oscillating cylinder, and a fly-wheel. These parts are sold ready for putting together at a very low price, and a complete engine may be bought for a couple of shillings, though one of ‘superior make’ at twice that sum is by far a preferable investment.

The drawing represents 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 1-3/4 in. 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, and 1/8 in. 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 plug A (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 in Fig. 2 being perhaps as good as any. The lower part is circular, 1/2 in. in diameter, and it has a flat face on one side, against which the valve face of the cylinder works. Fig. 3 shows this. The centre of the pillar is bored up in the middle of the screwed part to meet _one_ of the holes _a 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 that _a_ is bored into the central hole, and _b_ is bored through the pillar; then, when the pillar is screwed on to the boiler, and steam is generated, it issues from the port hole _a_.

The upper end of the pillar is bored through at right angles to the flat at the bottom (see Fig. 2). Through the top a piece of brass tubing about 5/8 in. 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.

The cylinder itself is shown at Fig. 3, and also in Figs. 1 and 2. The piston, piston-rod, and bearing which fits the crank-pin are shown in Fig. 4. It will be evident that the dimensions of this engine are microscopic. The bore of the cylinder is 5/16 in., 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 in Fig. 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 the bore 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 to Fig. 3 will show the working of the oscillating valve. The face of the pillar is shown on the right. On this, _a_ is the hole from which the live steam issues, and _b_ is the exhaust hole. These holes are technically called ports. The hole _c_ is bored through the pillar, and takes the trunnion, or pin on which the cylinder oscillates. Fig. 2 shows 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 to Fig. 3, the holes in the cylinder on the left are:--_c_, into which the trunnion is screwed; and _d_, the steam-way.

When the cylinder is in the position shown in Figs. 1 and 2, the port-hole _d_ (Fig. 3) is over the solid metal between the holes _a_ and _b_. On turning the fly-wheel the crank draws the piston out very slightly and inclines the cylinder sideways, bringing the port _d_ over _a_. 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 hole _d_ is mid-way between _a_ and _b_. The momentum of the fly-wheel carrying the crank round brings the hole _d_ opposite _b_, 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 side _d_ comes over _a_, 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 a [V]-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 about 1/16 in. 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.

The body of the valve is A, shown in section. B is the valve itself, fitted to a rod, D; it rests on the conical seat of A, and is pressed down by the spiral spring within the barrel of A. The body is screwed into the boiler by the thread at the bottom, and the steam coming up the hole C presses on the under side of the valve B. 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 holes F F. The cover E is screwed on the body part and confines the spring; it has a hole through its centre to allow the valve-rod D to 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.

III.--SMALL MODEL ENGINES.

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 been done 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, and 8/10 × 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 1-3/8-inch bore and the same length of stroke, viz., 2 inches, would give 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 is 4/12 × 100 = 33-1/3 (feet). Multiply these together, 16 × 33-1/3 = 533-1/3, and divide 533/33000 = ·016. That is, the engine is 16/1000 of a horse-power, or capable of raising 528 pounds 1 foot high in one minute. That is supposing all the power was available for _duty_. 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.

The _power_ of an engine is the nominal, and the _duty_ is 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. _Duty_ is 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.

IV.--THE HORIZONTAL ENGINE.

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.

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. 1 shows a plan view, and Fig. 2 an elevation of the complete engine. In both drawings the lettering is the same. The bed-plate A A is 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 1-1/2 inches in the bore and upwards 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 in Figs. 1 and 2 at B; at C (in Fig. 1 only) is the steam-chest containing the slide-valve. D is the fly-wheel fixed on the shaft E, which has at its opposite end the crank F. The piston-rod is shown passing through a guide G fixed to the bed by two screws. The connecting-rod from the piston to the crank-pin is marked H. The eccentric is shown at I, and the rod from it to the steam-chest is the eccentric-rod. J J, Fig. 2 only, 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 in Figs. 1 and 2. The dimensions are suited to the size known as ‘3/4-inch bore and 1-1/2-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.

A section of the cylinder is shown in Fig. 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 1-3/8 inches diameter across the flanges. The bore is 3/4 inch and 1-5/8 inches (full) long. The face of the cylinder where the valve works is level with the diameter of the flanges. This face is shown at Fig. 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 to Fig. 3, the steam-ways are shown at A A. These are drilled from the ends to meet the inlet ports B and C, which are closed by the slide-valve (see Fig. 5). The exhaust way is at D, and the port-hole communicating with it needs no special mention. The steam inlet is E; the threaded exterior is for attaching the steam-pipe from the boiler. The glands and stuffing-boxes, for keeping the piston and valve-rod steam-tight, are shown in section. G G are the glands screwed into the castings; the parts bored out to receive the packings are marked H H. 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 to Fig. 3 the passage of the steam may be traced. It enters at E, filling the steam chest, and as the valve is shown it could find no outlet. The valve on being moved would uncover one port, say B, and allow the steam to enter by the steam-way A, 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 opening B. 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 from B to D, thus allowing the steam in the cylinder to escape. The port-hole C would 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 into B and C alternately, and thus the reciprocating motion of the piston is maintained.

The slide-valve is shown at Fig. 5, where A is a view of the face. The centre is hollowed out, as shown at B of the section, to allow the steam to pass into the exhaust. The back is shown at C; 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 of A, 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, marked E in Fig. 1, is shown alone full size at Fig. 6. This is a rod of round steel 1/4 inch in diameter, the total length is 3-1/8 inches. At the right-hand end it is reduced in size a length of 7/8 inch, 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 at Fig. 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 at Fig. 7. A is a side view, B an edge view, and C a 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 nearly 1/4 inch. The height must be precisely that which will bring the centre of the crank-shaft level with the centre of the piston-rod.

Fig. 8 is 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 about 3/16; 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, is 5/8 inch. This gives 1-1/4-inch stroke; there is plenty of space in the cylinder for another 3/16 inch, and possibly the nominal stroke, 1-1/2 inches, could be managed by using a thin piston-head. The crank-pin is shown at the top of 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.

Fig. 10 shows the crank-pin and three views of the head of the connecting-rod. The crank-pin is steel, 3/16 inch diameter, turned down to 1/8 inch 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 at Fig. 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.

Fig. 12 shows 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 is 1/16 inch, which gives a travel of 1/8 inch 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 of Fig. 12 is 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 at Figs. 1 and 2 will show the relative position of each.

V.--THE OSCILLATING ENGINE.

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.

The illustration shows 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. (See page 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. The form shown possesses 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. 1 gives a side elevation, and Fig. 2 an end view of the same engine. The cylinder is 1 in. bore and 1 in. stroke. The length without covers is 1-1/2 in., that allows 1/4 in. for thickness of piston, 1/16 in. 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 1-1/2 in., 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 steel 1/8 in. 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 head was cut across horizontally at the diameter of the crank-pin, and the cap secured by two screws. The crank-pin is marked E. 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 disc F is fixed on the crank-shaft either by screwing, by a transverse pin, or by a key.

The crank-shaft is 1/4 in. 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 bearing H through which the trunnion passes.

The fly-wheel is marked A in Fig. 1; it is cast-iron, 2-1/2 inches in diameter and 3/4 in. wide on the rim. The rim should be half an inch thick at least, and the boss in 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.

The valve-face of the standard, B, Fig. 1. must be made perfectly flat and at right angles to the boring for the crank-shaft C. Fig. 3 shows the face of this standard as it would be seen in Fig. 2 if 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.

Through the centre of the valve-face a hole is bored to receive the cylinder trunnion. This trunnion, Fig. 4, is a steel pin screwed into the valve-face of the cylinder (see Fig. 3, section of cylinder showing valve-face and steam-ways). The outer end is threaded for a nut, D; this has a washer beneath it, and keeps the cylinder close against the standard, with the faces of the valves held together steam-tight, yet so that the cylinder may oscillate freely. A spiral spring beneath the nut is sometimes used, but in good work the adjustment is made with a pair of lock-nuts. The hole through the standard must be perfectly at right angles to the face, and the trunnion in the cylinder must also be perpendicular to the valve-face, or the two faces cannot come together steam-tight.

The stuffing-box, 1, of the piston is made with a gland drawn down on the packing by two screws. This arrangement is shown in section at Fig. 5. The method of fitting the gland, whether by screwing direct into the boss of the cylinder cover, as shown at the right in Fig. 5, or by screws tapped through the flange, as in the left-hand illustration, is quite optional. By referring to Fig. 5, the construction of the two forms of stuffing-boxes will be understood. The gland belonging to each is shown separate immediately above the sections. The same lettering is used in both. A is the cylinder cover, with the projecting boss into which the gland B is fitted. The space for the stuffing or packing is marked C. This is filled with lamp cotton, and when the gland is screwed down the cotton is compressed, so that it makes a steam-tight fitting for the piston-rod to slide in. The hole for the piston-rod is shown through the centres of both sections. As before explained, the gland on the left is secured by two screws shown in the section; it is fitted into a plain cylindrical hole. The other gland is threaded to screw direct into the cylinder cover, which is tapped to receive it. The first method is the one always employed in large engines. The screwed gland has a milled edge, so that it may be turned with the thumb and finger.

The action of the valves in a double-action oscillating cylinder will be best explained by reference to Fig. 3. This shows the face of the standard and the section of the cylinder. There is a flat face to the cylinder, usually circular; this has the two steam-ways _a_, _a_, bored in it. These holes meet others, drilled from the ends of the cylinder, parallel with its bore, and conduct the steam to the ends of the cylinder through the passages at _b_, _b_. On the face of the standard are two holes _a_ and _b_ drilled from the back, one to receive the steam from the boiler, the other to take the exhaust pipe. These holes are not bored through, but communicate with the circular groove _c_, _d_. This groove is stopped at _e_ and _f_. The cylinder is placed against the standard and held close to it, as shown in Fig. 1, by means of the trunnion illustrated by Fig. 4.

When the cylinder is vertical, as shown in Fig. 2, the port-holes are opposite the solid part of the face _e_ and _f_. Suppose live steam issues from _a_ and fills _c_, directly the cylinder is moved on one side and one of the port-holes comes over the groove _e_, the steam enters the cylinder and, pressing against the piston, compels the crank to revolve. By the same motion the other port is uncovered into _d_, and the dead steam escapes. When the cylinder again reaches a vertical position the steam-ports are again closed, but the momentum of the fly-wheel carries it over the dead centre, and then the positions of the ports are reversed. The one formerly over the exhaust now opens to the live steam and _vice versâ_. Thus the steam is admitted alternately at both sides of the piston, and so the engine continues to work.

VI.--MODEL BOILERS AND THEIR CONSTRUCTION.

A few words on model boilers and their construction will now be advisable. They have been mentioned several times incidentally in the course of these sections, but, with the exception of the small tin boiler for the oscillating engine first described, particulars of their construction have been omitted. It is not an easy task making a steam boiler, and in most cases it will be found cheapest in the end to purchase ready made.

The materials most generally used are brass and copper; sometimes iron, or what amounts to the same thing, tin-plate, is employed. Brass or copper, from the ease with which they can be manipulated, are the best for a beginner to work on.

Brass can be bought in the form of tube of sufficient size for small models, and strong enough to stand the steam pressure. The edges of the bought tube are brazed together, and thus the joint is made nearly as strong as the other part. The tube is afterwards drawn, and, except from a slight discoloration, the joint is not noticeable.

Brass tube, from two inches to six inches in diameter, cut in lengths suited for boilers, is sold by most of the model engineers. The price of the tube ranges from about 2_s._ per foot for the small to about 10_s._ per foot for the large size; the short length necessary for a boiler being charged at about the same proportion. This is merely for the tubular body part of the boiler, and it may be placed vertically or horizontally as required.

The ends or flanges which have to be fitted on are extra pieces. Sometimes a plain disc of metal is fixed by soldering with pewter; but this plan should be strenuously avoided. The ends should at least be brazed on. It is best also to use discs with a rim round them to fit over the boiler tube. This gives a much stronger hold than is possible with a plain disc of sheet metal.

Castings used for the boiler ends must be quite free from any flaws, or the weak part will be apt to give way under the steam pressure. It is often advisable to use castings, which may be made of a shape exactly suited to certain requirements. An inverted cup-shaped casting for the lower end of a vertical boiler gives a good heating surface. A flue for the chimney must be put in it, and this goes up to the top end of the boiler, which may appropriately be dome-shaped.

The flue and both ends of the boiler should be brazed in their places, not soft-soldered. Some prefer to use silver solder for such purposes, and this is an excellent material. When the joints are made to fit properly, as they should do before soldering, only very little solder is required to unite the parts. Borax is used as the flux, both for the alloy employed in brazing and for silver solder. The heat required to flow these properly may be got from an ordinary gas jet, with the burner or nipple removed, using a common blow-pipe to urge the flame.

A horizontal boiler is frequently only a plain tube, with the ends soldered in, and supported on legs to raise it sufficiently to allow a lamp to be put underneath. The heat applied in this manner does not take effect as it should. The flame is deflected from the surface of the boiler, and, moreover, any breath of wind stirring will blow the flame aside. A plain saddle-shaped boiler is much better; in this form the heating surface is large, and the heat from the furnace is applied to it direct, and cannot well be deflected.

Flues or tubes are very desirable in any form of boiler, and one or the other should be used. The plain straight chimney put through the boiler is the most simple form of flue. If this is of spiral form, like a corkscrew, the effect is infinitely increased, because the heat, instead of ascending straight up through the vertical tube, is met at every turn with a fresh surface of metal. In winding its way through a spiral tube, the heat is absorbed in a way quite unattainable when a straight tube is used. Several small tubes are of course better than one large one of the same area. By increasing the number of flues the cost of making a boiler is also increased, and it is to save expense that large flues are used.

Boilers for locomotives, which are required to make steam very fast, have an immense number of tubes running through them. The space between the tubes, which is occupied by the water, is often very small, and in fact the tubes are put as closely together as possible. As the heat rushes through them it is absorbed by the water in contact with the tube, and turns it into steam. The greater the heating surface the more readily is the steam generated.

Tubes are often put across the fire-grate; they are then called cross-tubes. Two, placed one above the other and crossing each other, will give a large amount of heating surface. By adding this simple contrivance to a vertical boiler with a straight flue it may be made to give off much more steam. One or two cross-tubes generally suffice to convert a useless boiler, that is, one that will not generate enough steam, into an effective one.

The fuel used to heat small boilers is generally spirits of wine. This is put in a suitable receptacle and burnt through a cotton wick. Several wicks are used in large boilers, and they are placed to heat the largest surface available. Spirit lamps are a source of danger if proper precautions are not taken. Unless there is a free outlet for the air within the lamp, it will be expanded by the heat and cause the spirit to rise too quickly in the wick. Sometimes it will overflow, and then it burns wherever it may be. Care must therefore be exercised in using spirit fuel. In model boats it occasionally happens that the spirit overflows, and the boat is all ablaze. An iron tea-tray, or some such utensil, should be used to stand the boiler on when the furnace is to be lighted.

Charcoal is a better fuel, when there is sufficient space in the fire-box to contain a supply. The waste steam from the cylinder must always be conveyed to the chimney and escape up it to make a draft through the fire. Without this it cannot be made to burn sufficiently fierce for the purpose. A charcoal fire will act very well, with a little attention, and except for the smallest engines it is always preferable to methylated spirit.

As it is not possible to give any adequate instructions on boiler-making in the limited space at my disposal, the above hints are chiefly intended for the guidance of purchasers.

A safety valve should always be fitted to a steam boiler. One of the spring valves has been illustrated in the chapter treating of the small oscillating engine. The lever safety valve is more certain in its action, especially in model work, and is better adapted for stationary purposes. A weighted lever is of no use to a locomotive or marine boiler, as the motion of travelling would disarrange the gear. The safety valve of every engine should be tested frequently, to make sure that it does not stick in its place and that all works perfectly free.

A glass gauge, by which the height of water may be seen at a glance, is frequently attached to boilers having any pretension to high-class workmanship. There is a good deal of work in a properly made gauge, and the cost is correspondingly high. Two or three stop-cocks are required in a gauge, and these involve good workmanship, or they will not stand the pressure. Leaky taps are a sign of inferior work.

Gauge cocks are sometimes used instead of the water gauge just mentioned. These are plain taps with straight noses. Two are wanted on a boiler; they are screwed in, one at high-water and the other at low-water level. By turning on these taps it is easy to see whether the water is within these limits; but the precise height cannot be ascertained. The gauge-glass is therefore much preferable.

Whistles are fitted to boilers only as ornaments. They are quite useless as signals, except such as can be given by word of mouth, are not required in working model engines. These attachments are made to sound by allowing the steam to act as the breath does in common whistles.

Force-pumps are used to force water into the boiler to make up for that converted into steam, and conveyed through the cylinder. These pumps are actuated by an eccentric on the crank-shaft, and, at every revolution of the crank, throw a small quantity of water into the boiler. When we consider how much water is evaporated to make the quantity of steam used for each revolution of the cylinder, we may arrive at an idea of the work required of a force pump. Practically the water to be injected at each stroke is too small to be dealt with, unless a large cylinder has to be supplied. The only way to work a force pump for a model satisfactorily is by gearing, so that a stroke of the plunger is performed about once to each hundred revolutions of the crank.

A better plan for feeding small boilers is by hand. The force pump is attached to the boiler in the usual way, but not connected to the engine. The plunger is worked by a hand lever, and when it is seen that water is wanted in the boiler, a few strokes of the lever will suffice.

Governors are used to control the speed of the engine. Without any such contrivance the engine runs at a speed corresponding to the work it has to do. The heavier the load the slower the speed, and immediately that the load is decreased the speed increases. A governor consists of a pair of balls, which are attached to arms pivoted to an axis revolved by the engine. The faster the speed the greater is the centrifugal force of the balls, and by connecting these with a valve, called a throttle valve, in the steam pipe, the supply of steam is reduced as the speed increases. By this means a uniform rate of speed is attained, irrespective of the steam pressure or the duty demanded of the engine.