Part 16 (1/2)
[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.