CONSTRUCTION.
[Illustration: FIG. 88.-Two arrangements for tube holes in multi tubular boiler.]
The centres for the water-tubes, G and H, should be laid out, in accordance with Fig. 88, on the tops of B and C and the lower part of A, along lines scribed in the manner explained on p. 207. Tubes H must be bent to a template to get them all of the same shape and length, and all the tubes be prepared before any are put in place. If the tubes are set 7/8 inch apart, centre to centre, instead of 1-1/4 inches, the heating surface will be greatly increased and the furnace casing better protected.
Assembling.—When all necessary holes have been made and are of the correct size, begin by riveting and silver-soldering in the ends of the drums. Next fix the cross tubes, E, taking care that they and B and C form rectangles. Then slip the F, G, and H tubes half an inch into the main drum, and support A, by means of strips passed between the G and H tubes, in its correct position relatively to B and C. The E tubes can now be pushed into B and C and silver-soldered. The supports may then be removed, and the a and H tubes be got into position and secured. Drum D then demands attention. The connecting tubes, KK, should be silver-soldered in, as the boiler, if properly made, can be worked at pressures up to 100 lbs. per square inch.
The casing is of 1/20-inch sheet iron, and in five parts. The back end must be holed to allow A, B, and C to project 1 inch, and have a furnace-door opening, and an airway at the bottom, 5 inches wide and 1 inch deep, cut in it. The airway may be provided with a flap, to assist in damping down the fire if too much steam is being raised. In the front end make an inspection opening to facilitate cleaning the tubes and removing cinders, etc.
The side plates, m m, are bent as shown in Fig. 86, and bolted to a semicircular top plate, n, bent to a radius of 6 inches. A slot, 1-1/2 inches wide and 11-1/2 inches long, must be cut in the top, n, to allow it to be passed over drum D; and there must also be a 3 or 3-1/2 inch hole for the chimney. A plate, p, covers in D. A little plate, o, is slipped over the slot in n, and asbestos is packed in all round D. The interior of the end, side, and the top plates should be lined with sheet asbestos held on by large tin washers and screw bolts. To protect the asbestos, movable iron sheets may be interposed on the furnace side. These are replaced easily if burnt away. The pieces m m are bent out at the bottom, and screwed down to a base-plate extending the whole length of the boiler.
The fire-bars fill the rectangle formed by the tubes B, El, and E2. A plate extends from the top of E2 to the front plate of the casing, to prevent the furnace draught being “short circuited.”
Boiler Fittings.
[Illustration: FIG. 89.-Safety valve.]
Safety Valves.—The best all-round type is that shown in Fig. 89. There is no danger of the setting being accidentally altered, as is very possible with a lever and sliding weight. The valve should be set by the steam gauge. Screw it down, and raise steam to the point at which you wish the safety valve to act, and then slacken off the regulating nuts until steam issues freely. The lock nuts under the cross-bar should then be tightened up. In the case of a boiler with a large heating surface, which makes steam quickly, it is important that the safety-valve should be large enough to master the steam. If the valve is too small, the pressure may rise to a dangerous height, even with the steam coming out as fast as the valve can pass it.
[Illustration: FIG. 90.-Steam gauge and siphon.]
Steam Gauges.—The steam gauge should register pressures considerably higher than that to be used, so that there may be no danger of the boiler being forced unwittingly beyond the limit registered. A siphon piece should be interposed between boiler and gauge (Fig. 90), to protect the latter from the direct action of the steam. Water condenses in the siphon, and does not become very hot.
[Illustration: FIG. 91.-Water gauge.]
Water Gauges should have three taps (Fig. 91), two between glass and boiler, to cut off the water if the glass should burst, and one for blowing off through. Very small gauges are a mistake, as the water jumps about in a small tube. When fitting a gauge, put packings between the bushes and the glass-holders, substitute a piece of metal rod for the glass tube, and pack the rod tightly. If the bushes are now sweated into the boiler end while thus directed, the gauge must be in line for the glass. This method is advisable in all cases, and is necessary if the boiler end is not perfectly flat.
Pumps.—Where a pump is used, the supply should enter the boiler below low-water level through a non-return valve fitted with a tap, so that water can be prevented from blowing back through the pump. As regards the construction of pumps, the reader is referred to p. 164 and to Chapter XXII.
Filling Caps.—The filling cap should be large enough to take the nozzle of a good-sized funnel with some room to spare. Beat the nozzle out of shape, to give room for the escape of the air displaced by the water.
The best form of filling cap has a self-seating ground plug, which, if properly made, is steam-tight without any packing. If needed, asbestos packing can easily be inserted between plug and cap.
Mud-holes.—All but the smallest boilers should have a mud-hole and plug in the bottom at a point not directly exposed to the furnace. In Fig. 82 it is situated at the bottom of the barrel. In Figs. 86 and 87 there should be a mud-hole in one end of each of the three drums, A, B, and C. The plug may be bored at the centre for a blow-off cock, through which the boiler should be emptied after use, while steam is up, and after the fire has been “drawn.” Emptying in this way is much quicker than when there is no pressure, and it assists to keep the boiler free from sediment.
[Illustration: FIG. 92.—Steam cock.]
Steam Cocks.-The screw-down type (Fig. 92) is very preferable to the “plug” type, which is apt to leak and stick.
Testing Boilers.—The tightness of the joints of a boiler is best tested in the first instance by means of compressed air. Solder on an all-metal cycle valve, “inflate” the boiler to a considerable pressure, and submerge it in a tub of water. The slightest leak will be betrayed by a string of bubbles coming directly from the point of leakage. Mark any leaks by plain scratches, solder them up, and test again.
[Illustration: FIG. 94.—Benzoline lamp for model central-flue boiler.]
The boiler should then be quite filled with cold water, and heated gradually until the pressure gauge has risen to over the working pressure. There is no risk of an explosion, as the volume of the water is increased but slightly.
The third test is the most important and most risky of all-namely, that conducted under steam to a pressure well above the working pressure.
In order to carry out the test without risk, one needs to be able to watch the steam-gauge from a considerable distance, and to have the fire under control. My own method is to set the boiler out in the open, screw down the safety-valve so that it cannot lift, and raise steam with the help of a blow-lamp, to which a string is attached wherewith to pull it backwards along a board. If the boiler is to be worked at 50 lbs., I watch the steam gauge through a telescope until 100 lbs. is recorded, then draw the lamp away. After passing the test, the boiler, when pressure has fallen, say, 20 lbs., may safely be inspected at close quarters for leaks.
This test is the only quite satisfactory one, as it includes the influence of high temperature, which has effects on the metal not shown by “cold” tests, such as the hydraulic.
Do not increase your working pressure without first re-testing the boiler to double the new pressure to be used.
Fuels.—For very small stationary boilers the methylated spirit lamp is best suited, as it is smell-less, and safe if the reservoir be kept well apart from the burner and the supply is controllable by a tap or valve. (See Fig. 104.)
[Illustration: FIG. 95.-Paraffin burner for vertical boiler.]
For medium-sized model boilers, and for small launch boilers, benzoline or petrol blow-lamps and paraffin stoves have become very popular, as they do away with stoking, and the amount of heat is easily regulated by governing the fuel supply. Fig. 94 is a sketch of a blow-lamp suitable for the horizontal boiler shown on pp. 204, and 206, while Fig. 95 shows a convenient form of paraffin stove with silent “Primus” burner, which may be used for a horizontal with considerable furnace space or for vertical boilers. In the case of all these liquid fuel consumers, the amount of heat developed can be increased by augmenting the number of burners. Where a gas supply is available its use is to be recommended for small stationary boilers.
Solid Fuels.—The chief disadvantages attaching to these are smoke and fumes; but as a solid fuel gives better results than liquid in a large furnace, it is preferred under certain conditions, one of them being that steam is not raised in a living room. Charcoal, coke, anthracite coal, and ordinary coal partly burned are the fuels to use, the fire being started with a liberal supply of embers from an open fire. Every solid-fuel boiler should have a steam-blower in the chimney for drawing up the fire; and if a really fierce blaze is aimed at, the exhaust from the engine should be utilized for the same purpose.
XIX.
QUICK BOILING KETTLES.
[Transcriber’s note: Do not use lead solder on articles associated with human or animal consumption.]
The principles of increasing the area of heating surface in model boilers may be applied very practically to the common kettle. The quick-boiling kettle is useful for camping out, for heating the morning tea water of the very early riser, and for the study “brew,” which sometimes has to be made in a hurry; and, on occasion, it will be so welcome in the kitchen as to constitute a very useful present to the mistress of the house.
As the putting in of the tubes entails some trouble, it is worth while to select a good kettle for treatment. Get one that is made of thick tinned sheet iron (cast-iron articles are unsuitable), or even of copper, if you are intent on making a handsome gift which will last indefinitely. The broad shallow kettle is best suited for tubing, as it naturally has a fair heating surface, and its bottom area gives room for inserting plenty of tubes. Also, the tubes can be of good length. Let us, therefore, assume that the kettle will be of at least 8 inches diameter.
In Figs. 96 (a) and 96 (b) are shown two forms of fire-tube kettles (a and b) and two of water-tube (c and d). For use over a spirit or Swedish petroleum stove the first two types are most convenient; the third will work well on a stove or an open fire; and the last proves very efficient on an open fire. One may take it that, as a general rule, areas of heating surface being equal, the water-tube kettle will boil more quickly than the fire-tube.
Fire-tube Kettles.
The tubing of Figs. 96 (a) and 96 (b) presents a little difficulty in each case. The straight tube is the more difficult to insert, owing to the elliptical shape of the ends; whereas the bent tube requires only circular holes, but must be shaped on a template.
The tubing used for (a) should have at least 5/8-inch internal diameter, for (b) 1/2 inch, and be of thin copper. Hot gases will not pass willingly through tubes much smaller than this, in the absence of induced or forced draught.
For convenience in fitting, the tubes should run at an angle of 45 degrees to the bottom and side of the kettle, as this gives the same bevel at each end. Find the centre of the bottom, and through it scratch plainly four diameters 45 degrees apart. From their ends draw perpendiculars up the side of the kettle.
[Illustration: FIG. 96 (a).]
Now draw on a piece of paper a section of the kettle, and from what is selected as a convenient water-level run a line obliquely, at an angle of 45 degrees, from the side to the bottom. Measuring off from this diagram, you can establish the points in the side and bottom at which the upper and longer side of the tubes should emerge. Mark these off.
Next bevel off a piece of tubing to an angle of 45 degrees, cutting off roughly in the first instance and finishing up carefully with a file till the angle is exact. Solder to the end a piece of tin, and cut and file this to the precise shape of the elliptical end. Detach by heating, scribe a line along its longest axis, and attach it by a small countersunk screw to the end of a convenient handle.
Place this template in turn on each of the eight radii, its long axis in line with it, being careful that the plate is brought up to the marks mentioned above, and is on the bottom corner side of it. Scratch round plainly with a fine steel point.
To remove the metal for a tube hole, it is necessary to drill a succession of almost contiguous holes as near the scratch as possible without actually cutting it. When the ring is completed, join the holes with a cold chisel held obliquely. Then file carefully with a round file, just not cutting the scratch. As the side of the hole nearest to the bottom corner should run obliquely to enable the tube to pass, work this out with the file held at an angle.
As soon as a pair of holes (one in the bottom, the other in the side) have been made, true up the side hole until a piece of tubing will run through it at the correct angle. Then bevel off the end to 45 degrees and pass the tube through again, bringing the bevel up against the bottom hole from the inside. If it is a trifle difficult to pass, bevel off the edge slightly on the inside to make a fairly easy driving fit. (Take care not to bulge the bottom of the kettle.) Mark off the tube beyond the side hole, allowing an eighth of an inch extra. Cut at the mark, and number tube and hole, so that they may be paired correctly later on.
When all the tubes are fitted, “tin” the ends with a wash of solder before returning them to their holes. If there is a gap at any point wide enough to let the solder run through, either beat out the tube from the inside into contact, or, if this is impracticable, place a bit of brass wire in the gap. Use powdered resin by preference as flux for an iron kettle, as it does not cause the rusting produced by spirit of salt. If the latter is used, wipe over the solder with a strong ammonia or soda solution, in order to neutralize the acid.
As the hot gases may tend to escape too quickly through large tubes, it is well to insert in the upper end of each a small “stop,” x—a circle of tin with an arc cut away on the bottom side. To encourage the gases to pass up the tubes instead of along the bottom, a ring of metal, y, may be soldered beyond the bottom holes, if an oil or spirit stove is to be used. This ring should have notches cut along the kettle edge, so as not to throttle the flame too much.
[Illustration: FIG. 96—(b), (c), and (d).]
As the tubes for these require bending to shape in each case, the three types may be grouped together. The tubes of c and d, which require bending to somewhat sharp curves, may be of 3/8-inch internal diameter. In the last two cases the direction of the water travel is shown. The up-flow end, which projects farther through the bottom than the down-flow, is nearer the centre, where, if a gas stove is used, the heat is more intense than at the circumference of the bottom. (Note.-If type c is for use on a three-support stove, increase the number of tubes to 9, equally spaced, 40 degrees apart, so that the kettle may be adjusted easily.)
The copper tubing should be annealed or softened by heating to a dull red and plunging in cold water. Cut a wooden template of the exact outline of the inside line of the shape that the tube is to assume, and secure this firmly to a board. Fill the tube with melted resin, to prevent, as much as possible, “buckling” or flattening on the curves. The tube must be kept up to the template by a stop of hard wood, at the end at which bending commences. Don’t cut the tube into lengths before bending, as short pieces are more difficult to handle. When a piece sufficient for a tube has been bent, cut it oft, and remove the resin by heating.
The fitting of the tubes is an easy matter, as the holes are circular. Pair off a tube with its holes and number it. A fluted reamer will be found invaluable for enlarging them to the correct size. Tin all tubes at points where they are to be attached to the kettle.
In Fig. 96 (c) and (d) care should be taken to make all the tubes project the same distance, so that the kettle may be level when resting on them.
XX.
A HOT-AIR ENGINE.
The pretty little toy about to be described is interesting as a practical application to power-producing purposes of the force exerted by expanding air. It is easy to make, and, for mere demonstration purposes, has an advantage over a steam-engine of the same size in that it can be set working in less than a minute, and will continue to act as long as a small spirit flame is kept burning beneath it; it cannot explode; and its construction is a simpler matter than the building of a steam-engine.
[Illustration: FIG. 97.—Vertical section of hot-air engine.]
Principles of the Hot-air Engine.—Fig. 97 gives a sectional view of the engine. The place of what would be the boiler in a steam-engine of similar shape is taken by an air chamber immediately above the lamp, and above that is a chamber through which cold water circulates. In what we will call the heating chamber a large piston, known as the displacer, is moved up and down by a rod D and a connecting rod CR1. This piston does not touch the sides of the chamber, so that the bulk of the air is pushed past it from one end of the chamber to the other as the piston moves. When the displacer is in the position shown—at the top of its stroke—the air is heated by contact with the hot plate C, and expands, forcing up the piston of the power cylinder, seen on the left of the engine. (The power crank and the displacer crank are, it should be mentioned, set at right angles to one another.) During the second half of the power stroke the displacer is moved downwards, causing some of the air to pass round it into contact with the cold plate D. It immediately contracts, and reduces the pressure on the power piston by the time that the piston has finished its stroke. When the power piston has reached the middle of its downward stroke, the displacer is at its lowest position, but is halfway up again when the power piston is quite down. The air is once again displaced downwards, and the cycle begins anew. The motive power is, therefore, provided by the alternate heating and cooling of the same air.
Construction.—The barrel and supports were made out of a single piece of thin brass tubing, 2-7/16 inch internal diameter and 5-5/8 inch long. The heating end was filed up true, the other cut and filed to the shape indicated in Fig. 98 by dotted lines. The marking out was accomplished with the help of a strip of paper exactly as wide as the length of the tube, and as long as the tube’s circumference. This strip had a line ruled parallel to one of its longer edges, and 2-1/2 inches from it, and was then folded twice, parallel to a shorter edge. A design like the shaded part of Fig. 98 was drawn on an end fold, and all the four folds cut through along this line with a pair of scissors. When opened out, the paper appeared as in Fig. 98.
[Illustration: FIG. 98.]
We now—to pass into the present tense—wrap this pattern round the tube and scratch along its edges. The metal is removed from the two hollows by cutting out roughly with a hack saw and finishing up to the lines with a file.
The next things to take in hand are the displacer rod D and the guide tube in which it works. These must make so good a fit that when slightly lubricated they shall prevent the passage of air between them and yet set up very little friction. If you cannot find a piece of steel rod and brass tubing which fit close enough naturally, the only alternative is to rub down a rod, slightly too big to start with, until it will just move freely in the tube. This is a somewhat tedious business, but emery cloth will do it. The rod should be 3-3/8 inches, the tube 2-1/8 inches, long. I used rod 3/16 inch in diameter; but a smaller rod would do equally well.
[Illustration: FIG. 99.]
The two plates, A and B, are next prepared by filing or turning down thin brass[1] discs to a tight fit. (Note.—For turning down, the disc should be soldered centrally to a piece of accurately square brass rod, which can be gripped in a chuck. I used a specially-made holder like that shown in Fig. 99 for this purpose.)
[Footnote 1: Thin iron plate has the disadvantage of soon corroding.]
When a good fit has been obtained, solder the two discs together so that they coincide exactly, and bore a central hole to fit the guide tube tightly. Before separating the plates make matching marks, so that the same parts may lie in the same direction when they are put in position. This will ensure the guide tube being parallel to the barrel.
The power cylinder is a piece of brass tubing 2 inches long and of 7/8-inch internal diameter. The piston is of 7/8-inch tubing, fitting the cylinder easily, and thick enough to allow a shallow packing recess to be turned in the outside. Brass washers turned or filed to size form the ends of cylinder and piston. The connecting rod CR2 is a piece of strip brass, 3-3/16 inches long, between centres of holes. This had better be cut off a bit long in the first instance, and be fitted to the little stirrup which attaches its lower end to the piston. The drilling of the crank pinhole should be deferred till the cylinder and crank are in position.
[Illustration: FIG. 100.—Exterior view of hot air engine.]
Putting in the Water-chamber Discs.—Clean the inside of the barrel thoroughly with sandpaper; also discs A and B round the edges and the central holes. Disc A is forced in from the crank end a little further down than it is to be finally, and then driven up from below until at all points its lower side is exactly three inches from the bottom edge of the barrel. Disc B is then forced up 1-1/2 inches from the bottom end. The guide tube— which should have been cleaned—having been driven into place, solder is run all round the joints. If the barrel is heated over a spirit lamp, this operation is performed very quickly. (“Tinol” soldering paste is recommended.) Before soldering in B, drill a small hole in the barrel between A and B to allow the air to escape.
Attaching the Cylinder.—Scratch a bold line through the centre of one of the crank holes to the bottom of the barrel, to act as guide. Drill a 5/32-inch hole in the barrel on this line just below plate B, and a similar hole in the bottom of the cylinder. (The cylinder end should be put in position temporarily while this is done to prevent distortion.) Flatten down the cylinder slightly on the line of the hole, so that it may lie snugly against the barrel, and clean the outside of the barrel. Lay the cylinder against the barrel with the holes opposite one another, and push a short piece of wood through to exclude solder from the holes and keep the holes in line. Half a dozen turns of fine wire strained tightly round cylinder and barrel will hold the cylinder in place while soldering is done with a bit or lamp. The end of the cylinder should then be made fast.
The Displacer.—This is a circular block of wood—well dried before turning—5/8 inch thick and 3/32 inch less in diameter than the inside of the barrel. The rod hole in it should be bored as truly central as possible. A hole is drilled edgeways through the block and through the rod to take a pin to hold the two together. To prevent it splitting with the heat, make a couple of grooves in the sides to accommodate a few turns of fine copper wire, the twisted ends of which should be beaten down flush with the outside of the block. The bottom of the block is protected by a disc of asbestos card held up to the wood by a disc of tin nailed on.
The Crank Shaft and Crank.—The central crank of the crank shaft—that for the displacer—has a “throw” of 1/4 inch, as the full travel of the displacer is 1/2 inch. If the bending of a rod to the proper shape is beyond the reader’s capacity, he may build up a crank in the manner shown in Fig. 101. Holes for the shaft are bored near the tops of the supports, and the shaft is put in place. After this has been done, smoke the shaft in a candle flame and solder two small bits of tubing, or bored pieces of brass, to the outside of the supports to increase the length of the bearing. The power-crank boss is a 1-1/2-inch brass disc. This crank has a “throw” of 1/2 inch.
[Illustration: FIG. 101.-Details of built-up crank.]
Connecting Rods.—Put a piece of card 1/16 inch thick in the bottom of the cylinder and push the piston home. Turn the power crank down and mark off the centre of the hole for the crank pin in the connecting rod CR2. Solder a piece of strip brass on each side of the rod at this point; measure again, and drill.
The top of the displacer rod D is now filed flat on two sides and drilled. Slip a ring 1/16 inch thick over the rod and push the rod upwards through the guide tube till the displacer can go no farther. Turn the displacer crank up and measure from the centre of the hole in the rod to the centre of the crank. The top of the connecting rod should be filed to fit the under side of the crank, against which it should be held by a little horseshoe-shaped strap pinned on. (Fig. 102). (Be sure to remove the ring after it has served its purpose.)
The Water Circulation.—The water chamber is connected by two rubber tubes with an external tank. In Fig. 97 the cooling water tank is shown, for illustrative purposes, on the fly-wheel side of the engine, but can be placed more conveniently behind the engine, as it were. Two short nozzles, E1 and E2, of 1/4-inch tube are soldered into the water chamber near the top and bottom for the rubber pipes to be slipped over, and two more on the water tank. For the tank one may select a discarded 1 lb. carbide tin. Cut off the top and solder on a ring of brass wire; make all the joints water-tight with solder, and give the tin a couple of coatings of paint inside and outside.
[Illustration: FIG. 102.]
Closing the Hot-air Chamber.—When all the parts except the lamp chamber have been prepared, assemble them to make sure that everything is in order. The lower end of the hot-air chamber has then to be made air-tight. Soldering is obviously useless here, as the heat of the lamp would soon cause the solder to run, and it is impossible to make a brazed joint without unsoldering the joints in the upper parts of the engine. I was a bit puzzled over the problem, and solved it by means of the lower part of an old tooth-powder box stamped out of a single piece of tin. This made a tight fit on the outside of the barrel, and as it was nearly an inch deep, I expected that if it were driven home on the barrel and soldered to it the joint would be too near the water chamber to be affected by the lamp. This has proved to be the case, even when the water is nearly at boiling point. If a very close-fitting box is not procurable, the space between box and barrel must be filled in with a strip of tin cut off to the correct length.
The Lamp Chamber.—Cut out a strip of tin 4 inches wide and 1 inch longer than the circumference of the lower end of the hot-air chamber. Scratch a line 1/2 inch from one of the sides, a line 3/4 inch from the other, and a line 1/2 inch from each of the ends.
A lamp hole is cut in the centre, and ventilation holes 1 inch apart, as shown in Fig. 103. If the latter holes are made square or triangular (base uppermost), and the metal is cut with a cold chisel so as to leave the side nearest the edge unsevered, the parts may be turned up to form supports for the barrel.
[Illustration: FIG. 103.—Plate for lamp chamber cut out ready for bending.]
The slit lower side of the plate is splayed out into a series of “feet,” by three or more of which, the chamber is secured to the base. Bend the plate round the barrel and put the two screws and bolts which hold the ends in place, and tighten them until the barrel is gripped firmly. Screw the engine to its base, fit on the rubber water connections, and fasten down the tank by a screw through the centre of the bottom. The screw should pass through a brass washer, between which and the tank should be interposed a rubber washer to make a water-tight joint.
The Lamp.—The lamp shown in Fig. 104 was made out of a truncated brass elbow, a piece of 5/16-inch brass tube, and a round tin box holding about 1/3-pint of methylated spirit. A tap interposed between the reservoir and burner assists regulation of the flame, and prevents leakage when the lamp is not in use.
Running the Engine.—The power and displacer cranks must be set exactly at right angles to one another, and the first be secured by soldering or otherwise to the crank shaft. The fly wheel will revolve in that direction in which the displacer crank is 90 degrees ahead of the other.
[Illustration: FIG. l04.-Spirit lamp for hot-air engine, with regulating tap.]
The packing of the piston should be sufficiently tight to prevent leakage of air, but not to cause undue friction. When the packing has settled into place, an occasional drop of oil in the cylinder and guide tube will assist to make the piston and slide air-tight.
The engine begins to work a quarter of a minute or so after the lamp is lit, and increases its speed up to a certain point, say 300 revolutions per minute. When the water becomes very hot it may be changed. The power might be applied, through demultiplying gear, to a small pump drawing water from the bottom of the tank and forcing it through the water chamber and a bent-over stand pipe into the tank again. This will help to keep the water cool, and will add to the interest of the exhibit by showing “work being done.”
XXI.
A WATER MOTOR.
FIG. 105 is a perspective view of a simple water motor which costs little to make, and can be constructed by anybody able to use carpenter’s tools and a soldering iron. It will serve to drive a very small dynamo, or do other work for which power on a small scale is required. A water supply giving a pressure of 40 lbs. upwards per square inch must be available.
We begin operations by fashioning the case, which consists of three main parts, the centre and two sides, held together by brass screws. For the centre, select a piece of oak 1 inch thick. Mark off a square, 7 inches on the side; find the centre of this, and describe a circle 5 inches in diameter. A bulge is given to the circle towards one corner of the square, at which the waste-pipe will be situated.
Cut out along the line with a keyhole saw. Then saw out the square of wood. A 5/8-inch hole is now bored edgeways through the wood into the “bulge” for the escape, and in what will be the top edge is drilled a 1/4-inch hole to allow air to enter.
[Illustration: FIG. l05.—Simple water turbine.]
Cut out the sides, and screw them on to the centre at the four corners, taking care that the grain runs the same way in all three pieces, so that they may all expand or contract in the same direction. Plane off the edges of the sides flush with the centre.
The parts should now be separated, after being marked so that they can be reassembled correctly, and laid for a quarter of an hour in a pan of melted paraffin wax, or, failing this, of vaseline, until the wood is thoroughly impregnated. Reassemble the parts, and put in the rest of the holding screws, which should have their heads countersunk flush with the wood.
[Illustration: FIG. 106.—Water turbine, with pulley side of casing removed.]
For the shaft select a piece of steel rod 5/32 inch in diameter, and 3 or 4 inches long; for the bearings use two pieces, 3/4 inch long each, of close-fitting brass tube. Now take a drill, very slightly smaller in diameter than the bearings, and run holes right through the centres of, and square to, the sides. Both holes should be drilled at one operation, so that they may be in line.
With a wooden mallet drive the bearings, which should be tapered slightly at the entering end, through the sides. Push the shaft through them. If it refuses to pass, or, if passed, turns very unwillingly, the bearings must be out of line; in which case the following operation will put things right. Remove the bearing on the pulley side, and enlarge the hole slightly. Then bore a hole in the centre of a metal disc, 1 inch in diameter, to fit the bearing; and drill three holes for screws to hold the disc against the case. Rub disc and bearing bright all over.
Replace the bearing in its hole, slip the disc over it, and push the shaft through both bearings. Move the disc about until the shaft turns easily, mark the screw holes, and insert the screws. Finally, solder the bearing to the disc while the shaft is still in place.
The wheel is a flat brass disc 4 inches in diameter. Polish this, and scratch on one side twelve equally spaced radii. At the end of each radius a small cup, made by bending a piece of strip brass 1/4 inch wide and 1/2 inch long into an arc of a circle, is soldered with its extremities on the scratch. A little “Tinol” soldering lamp (price 1s. 6d.) comes in very handy here.
To fix the wheel of the shaft requires the use of a third small piece of tubing, which should be turned off quite square at both ends. Slip this and the wheel on the shaft, and make a good, firm, soldered joint. Note.— Consult Fig. 107 for a general idea of the position of the wheel, which must be kept just clear of the case by the near bearing.
[Illustration: FIG. 107.—Plan of water turbine, showing arrangement of nozzle.]
The nozzle should be a straight, tapered tube of some kind—the nose of a large oil can will serve the purpose. The exit must be small enough to allow the water to leave it at high velocity; if too large, the efficiency of the wheel will be diminished. To the rear end of the nozzle should be soldered a piece of brass tubing, which will make a tight fit with the hose pipe leading from the water supply. A few small brass rings soldered round this piece will prevent the hose blowing off if well wired on the outside.
Now comes the boring of the hole for the nozzle. Fig. 106 shows the line it should take horizontally, so that the water shall strike the uppermost bucket just below the centre; while Fig. 107 indicates the obliquity needed to make the stream miss the intervening bucket. A tapered broach should be used to enlarge the hole gradually till the nozzle projects sufficiently. If the line is not quite correct, the tip should be bent carefully in the direction required. One must avoid distorting the orifice, which should be perfectly circular; clean it out with a small twist drill of the proper size.
A brass elbow, which may be purchased for a few pence, should be driven into the waste hole, and a small shield be nailed under the air hole. A couple of screwed-on cross pieces are required to steady the motor sideways and raise the elbow clear of the ground.
The motor may be geared direct to a very small dynamo, if the latter is designed to run at high speeds. If a geared-down drive is needed, a small pulley—such as is used for blinds, and may be bought for a penny—should be attached to the shaft, and a bootlace be employed as belt. Avoid overloading the wheel, for if it is unable to run at a high speed it will prove inefficient.
[Illustration: FIG. l08.-Water motor working a photographic dish-rocker.]
Lubrication.—The water will keep the bearings cool, but the bearings should be well lubricated. The most convenient method of effecting this is to bore holes in the bearings, and from them run small pipes to an oil reservoir on the top of the case (as in Fig. 70), where they are fed on the siphon principle through strands of worsted.
Alternative Construction.—If an all-metal case is preferred, the reader might utilize the description given of a steam turbine on pp. 170-178. The details there given will apply to water as well as steam, the one exception being that a nozzle of the kind described above must be substituted for the steam pipe and small ports.
XXII.
MODEL PUMPS.
Every steam boiler which has to run for long periods and evaporate considerable quantities of water should be in connection with a pump capable of forcing water in against the highest pressure used. On a previous page (p. 158) we have described a force pump driven directly off the crank shaft of an engine. As the action of this is dependent on the running of the engine, it is advisable, in cases where the boiler may have to work an engine not provided with a pump of its own, to install an independent auxiliary pump operated by hand or by steam, and of considerable capacity, so that in an emergency water may be supplied quickly.
[Illustration: FIG. l09.-Vertical section of force pump.]
Making a Hand pump.—Fig. 109 shows the details of a hand pump which is easy to make. The barrel is a length of brass tubing; the plunger a piece of brass or preferably gun-metal rod, which fits the tube closely, but works easily in it. The gland at the top of the barrel, E, is composed of a piece, D, of the same tubing as the barrel, sliding in a collar, C, soldered to E. The bottom of D and top of E are bevelled to force the packing against the plunger. The plates A and B, soldered to D and C respectively, are drawn together by three or more screws. A brass door-knob makes a convenient top for the plunger. When the knob touches A, the bottom of the plunger must not come lower than the top of the delivery pipe, lest the water flow should be impeded and the valve, V, injured. Round off the end of the plunger, so that it may be replaced easily and without disarranging the packing if pulled out of the pump.
The valves are gun-metal balls, for which seats have been prepared by hammering in steel cycle balls of the same size. Be careful to select balls considerably larger than the bore of the pipes on which they rest, to avoid all possibility of jamming. An eighth of an inch or so above the ball, cross wires should be soldered in to prevent the ball rising too far from its seat.
[Illustration: FIG. 110.]
A convenient mounting for a hand pump is shown in Fig. 110. The plate, F, of the pump is screwed to a wooden base resting on a framework of bent sheet zinc, which is attached to the bottom of a zinc water tray. The delivery pipe, G, will be protected against undue strains if secured by a strap to the side of the wooden base.
The same pump is easily adapted to be worked by a lever, which makes the work of pumping easier. Fig. 111 gives details of the top of the plunger and the links, B. A slot must be cut in the plunger for the lever, A, to pass through, and the sides bored for a pivot pin. The links are straddled (see sketch of end view) to prevent the back end of the lever wobbling from side to side.
[Illustration: FIG. 111.—Details of lever for force pump.]
A Steam Pump.—The pump illustrated in Fig. 112 belongs to what is probably the simplest self-contained type, as no fly wheel, crank, or eccentric is needed for operating the valve.
The steam cylinder and the pump are set in line with one another (in the case shown, horizontally), and half as far apart again as the stroke of the cylinder. The plunger is either a continuation of the piston rod, or attached to it.
[Illustration: FIG. 112—View of steam pump, showing details.]
An arm, S, fixed at right angles to the piston rod, has a forked end which moves along the rod. This rod is connected with the slide valve through the rocking arm, R1 and the rod, R2. On it are two adjustable stops, T1 T2, which S strikes alternately towards the end of a stroke, causing the valve to shift over and expose the other side of the piston to steam pressure. The absence of the momentum of a fly wheel makes it necessary for the thrust exerted by the piston to be considerably greater than the back pressure of the water, so that the moving parts may work with a velocity sufficient to open the valve. If the speed falls below a certain limit, the valve opens only part way, the speed falls, and at the end of the next stroke the valve is not shifted at all.
The diameter of the plunger must be decided by the pressure against which it will have to work. For boiler feeding it should not exceed one-third that of the piston; and in such case the piston rod and plunger may well be one.
A piston valve, being moved more easily than a box valve, is better suited for a pump of this kind, as friction should be reduced as much as possible.