Things To Make Part 16

[Ill.u.s.tration: 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 bra.s.s, to the outside of the supports to increase the length of the bearing. The power-crank boss is a 1-1/2-inch bra.s.s disc. This crank has a "throw" of 1/2 inch.

[Ill.u.s.tration: 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 bra.s.s 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 ill.u.s.trative 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 bra.s.s wire; make all the joints water-tight with solder, and give the tin a couple of coatings of paint inside and outside.

[Ill.u.s.tration: FIG. 102.]

Closing the Hot-air Chamber.--When all the parts except the lamp chamber have been prepared, a.s.semble 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 s.p.a.ce 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 circ.u.mference 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.

[Ill.u.s.tration: 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 pa.s.s through a bra.s.s 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 bra.s.s elbow, a piece of 5/16-inch bra.s.s tube, and a round tin box holding about 1/3-pint of methylated spirit. A tap interposed between the reservoir and burner a.s.sists 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.

[Ill.u.s.tration: 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 a.s.sist 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 fas.h.i.+oning the case, which consists of three main parts, the centre and two sides, held together by bra.s.s 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.

[Ill.u.s.tration: 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 rea.s.sembled 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. Rea.s.semble the parts, and put in the rest of the holding screws, which should have their heads countersunk flush with the wood.

[Ill.u.s.tration: 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 bra.s.s 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 pa.s.s, or, if pa.s.sed, 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 bra.s.s disc 4 inches in diameter. Polish this, and scratch on one side twelve equally s.p.a.ced radii. At the end of each radius a small cup, made by bending a piece of strip bra.s.s 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.

[Ill.u.s.tration: 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 bra.s.s tubing, which will make a tight fit with the hose pipe leading from the water supply. A few small bra.s.s 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 bra.s.s elbow, which may be purchased for a few pence, should be driven into the waste hole, and a small s.h.i.+eld 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.

[Ill.u.s.tration: 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 subst.i.tuted for the steam pipe and small ports.

XXII. MODEL PUMPS.

Every steam boiler which has to run for long periods and evaporate considerable quant.i.ties 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.

[Ill.u.s.tration: FIG. l09.-Vertical section of force pump.]