Chapter XV: (the most elaborate mechanism included in the book) with (3)

It is advisable to lay out a set of points, together with motor and signals, on a separate board.

Preservation of Track.—All the wooden parts of an outdoor track should be well creosoted before use.

The Electric Locomotive.

An elevation and a plan of this are given in Fig. 47. The two pairs of wheels are set close together, so that they may pass easily round curves.

The Motor.—A motor of ordinary type, with electro field magnets, is unsuitable for traction, as it cannot be reversed by changing the direction of the current, unless a special and rather expensive type of automatic switch be used. While a motor of this kind is, in conjunction with such a switch, the most efficient, the motor with permanent field magnets is preferable as regards cost and ease of fixing. It can be reversed through the rails. The armature or revolving part must be tripolar to be self-starting in all positions.

A motor of sufficient power can be bought for half a crown or less—in any case more cheaply than it can be made by the average amateur.

The motor used for the locomotive illustrated was taken to pieces, and the magnet M screwed to a strip of wood 1-5/8 inches wide; and for the original armature bearings were substituted a couple of pieces of brass strip, HH, screwed to two wooden supports, SS, on the base, E (Fig. 47, a). It was found necessary to push the armature along the spindle close to the commutator piece, C, and to shorten the spindle at the armature end and turn it down to the size of the original bearing, in order to bring the motor within the space between the wheels.

The place of the small pulley was taken by an 8-toothed pinion wheel, engaging with a pinion soldered to the near driving wheel, the diameter of which it exceeded by about 3/16 inch. The pair, originally parts of an old clock purchased for a few pence, gave a gearing-down of about 9 times.

The position of the driven wheels relatively to the armature must be found experimentally. There is plenty of scope for adjustment, as the wheels can be shifted in either direction longitudinally, while the distance between wheel and armature centres may be further modified in the length of the bearings, BE. These last are pieces of brass strip turned up at the ends, and bored for axles, and screwed to the under side of the base. To prevent the axles sliding sideways and the wheels rubbing the frame, solder small collars to them in contact with the inner side of the bearings.

The Frame.—Having got the motor wheels adjusted, shorten E so that it projects 2 inches beyond the centres of the axles at each end. Two cross bars, GG, 3-1/2 inches long, are then glued to the under side of E, projecting 1/8 inch. To these are glued two 3/8-inch strips, FF, of the same length as E. A buffer beam, K, is screwed to G. A removable cover, abedfg, is made out of cigar-box wood or tin. The ends rest on GG; the sides on FF. Doors and windows are cut out, and handrails, etc., added to make the locomotive suggest the real thing—except for the proportionate size and arrangement of the wheels.

Electrical Connections.—The current collector, CR, should be well turned up at the end, so as not to catch on the centre rail joints, and not press hard enough on the rail to cause noticeable resistance. The fixed end of CR is connected through T2 with one brush, B, and both wheel bearings with T1.

Electrical Fittings.—The best source of power to use is dry cells giving 1-1/2 to 2 volts each. These can be bought at 1s. apiece in fairly large sizes. Four or five connected in series will work quite a long line if the contacts are in good condition.

A reversing switch is needed to alter the direction of the current flow. The construction of one is an exceedingly simple matter. Fig. 48 gives a plan of switch and connection, from which the principle of the apparatus will be gathered. The two links, LL, are thin springy brass strips slightly curved, and at the rear end pivoted on the binding posts T1 T2. Underneath the other ends solder the heads of a couple of brass nails. The links are held parallel to one another by a wooden yoke, from the centre of which projects a handle. The three contacts C1 C2 C3 must be the same distance apart as the centres of the link heads, and so situated as to lie on the arcs of circles described by the links. The binding post T3 is connected with the two outside contacts—which may be flat-headed brass nails driven in almost flush with the top of the wooden base—by wires lying in grooves under the base, and T4 with the central contact. As shown, the switch is in the neutral position and the circuit broken.

Multiple Battery Switch.—To control the speed of the train and economize current a multiple battery switch is useful. Fig. 49 explains how to make and connect up such a switch. The contacts, C1 to C5, lie in the path of the switch lever, and are connected through binding posts T1 to T6 with one terminal of their respective cells. The cells are coupled up in series to one another, and one terminal of the series with binding posts T0 and T6. By moving the lever, any number of the cells can be put in circuit with T7. The button under the head of the lever should not be wide enough to bridge the space between any two contacts. Change the order of the cells occasionally to equalize the exhaustion.

Resistance.—With accumulators, a “resistance” should be included in the circuit to regulate the flow of current. The resistance shown in Fig. 50 consists of a spiral of fine German silver wire lying in the grooved circumference of a wood disc. One of the binding posts is in connection with the regulating lever pivot, the other with one end of the coil. By moving the lever along the coil the amount of German silver wire, which offers resistance to the current, is altered. When starting the motor use as little current as possible, and open the resistance as it gets up speed, choking down again when the necessary speed is attained.

General.—All the three fittings described should for convenience be mounted on the same board, which itself may form the cover of the box holding the dry cells or accumulators.

SOME SUGGESTIONS.

Instead of dry cells or accumulators a small foot or hand operated dynamo generating direct, not alternating current, might be used. Its life is indefinitely long, whereas dry cells become exhausted with use, and accumulators need recharging from time to time. On occasion such a dynamo might prove very convenient.

Anyone who possesses a fair-sized stationary engine and boiler might increase the realism of the outdoor track by setting up a generating station, which will give a good deal of extra fun.

XIV. A SIMPLE RECIPROCATING ENGINE.

Figs. 51 and 52 illustrate a very simple form of fixed-cylinder engine controlled by a slide valve.

An open-ended “trunk” piston, similar in principle to that used in gas engines, is employed; and the valve is of the piston type, which is less complicated than the box form of valve, though less easily made steam-tight in small sizes. The engine is single-acting, making only one power stroke per revolution.

The cylinder is a piece of brass tubing; the piston another piece of tubing, fitting the first telescopically. Provided that the fit is true enough to prevent the escape of steam, while not so close as to set up excessive friction, a packing behind the piston is not needed; but should serious leakage be anticipated, a packing of thick felt or cloth, held up by a washer and nuts on the gudgeon G, will make things secure. Similarly for the built-up piston valve P may be substituted a piece of close-fitting brass rod with diameter reduced, except at the ends, by filing or turning, to allow the passage of steam.

CONSTRUCTION.

The bed is made of wood, preferably oak, into the parts of which linseed oil is well rubbed before they are screwed together, to prevent the entry of water. A longitudinal groove is sawn in the top of the bed, as indicated by the dotted line in Fig. 51, to give room for the connecting rod in its lowest position, and a cross groove is scooped in line with the crank shaft to accommodate the lower part of the crank disc and the big end of the rod. (If the wing W under the cylinder is screwed to the side of the bed, instead of passing through it, as shown, a slight cutting away of the edge will give the necessary clearance in both cases. )

The cylinder and valve tube A should be flattened by filing and rubbing on emery cloth, so that they may bed snugly against one another and give a good holding surface for the solder. A steam port, S P, should next be bored in each, and the “burr” of the edges cleaned off carefully so as not to obstruct valve or piston in the slightest degree. “Tin” the contact surfaces thinly, and after laying valve tube and cylinder in line, with the portholes corresponding exactly, bind them tightly together with a turn or two of wire, or hold them lightly in a vice, while the solder is made to run again with the aid of a spirit lamp. If it seems necessary, run a little extra solder along the joint, both sides, and at the ends.

The valve, if built up, consists of a central rod, threaded at the rear end, four washers which fit the tube, and a central spacing-piece. The forward washer is soldered to the rod. Behind this is placed a felt packing. Then come in order the central spacing-piece, with a washer soldered to each end, a second packing, and a fourth washer. The series is completed by an adjusting nut to squeeze the packings, and a lock nut to prevent slipping. The back end of the valve must be wide enough to just more than cover the steam port. If the felt proves difficult to procure or fit, one may use a ring or two of brass tubing, with an external packing of asbestos cord.

The cylinder wing W should have the top edge turned over for an eighth of an inch or so to give a good bearing against the cylinder, and be held in position by a wire while the soldering is done. It is important that the line of the wing should be at right angles to a line passing through the centres of the valve tube and cylinder.

Shaft Bearings.—Take a piece of strip brass half an inch or so wide and 3-1/2 inches long. Bore four holes for screws, and scratch cross lines an inch from each extremity. Turn up the ends at these lines at right angles to the central part, stand the piece on some flat surface, and on the outer faces of the uprights scratch two cross lines at the height of the centre of the cylinder above the bed. Mark the central points of these lines.

Next select a piece of brass tubing which fits the rod chosen for the crank shaft, and bore in the bearing standards two holes to fit this tubing. Slip the tubing through the standards and solder it to them. The ends and central parts of the tubing must now be so cut away as to leave two bearings, BB—that at the fly-wheel end projecting far enough to allow the fly wheel, when brought up against it, to just clear the bed; that at the crank end being of the proper length to allow the eccentric to be in line with the valve rod, and the crank disc to occupy its proper position relatively to the central line of the cylinder. Finish off the standards by filing the tops concentrically with the bearings.

The eccentric may be built up from a metal disc about 3/4 inch diameter and two slightly larger discs soldered concentrically to the sides. The width of the middle disc should be the same as that of the eccentric rod. A careful filer could make a passable eccentric by sinking a square or semicircular groove in the edge of a wide disc. The centre of the eccentric must be found carefully, and a point marked at a distance from it equal to half the travel of the valve. To ascertain this, pull the valve forward until the steam port is fully exposed, insert a bar at the rear end of the valve tube, and mark it. Then push the valve back until a wire pushed through the port from the cylinder side shows that the port is again fully exposed. Insert and mark the bar again. The distance between the marks gives you the “travel” required.

Order of Assembly.—The following list of operations in their order may assist the beginner:

Make the bed.

Cut out cylinder barrel, piston, and valve tube.

Bevel off the ends of the last inside to allow the valve to enter easily.

Make the valve.

Bore the steam ports, and solder valve tube and cylinder together.

Solder holding-down wing, W, to cylinder.

Finish off the piston.

Solder the bearings in their standards.

Prepare shaft, crank disc, crank pin, and piston rod.

Fix the cylinder to the bed, in which a slot must be cut for the wing and holding-down bolt.

Attach the piston rod to the piston, and insert piston in cylinder.

Bore hole for shaft in centre of crank disc, and another, 9/16 inch away (centre to centre), for crank pin.

Solder in crank pin squarely to disc.

Pass shaft through bearings and slip on the crank disc.

Pass front end of piston rod over the crank pin.

Lay bearing standard on bed squarely to the centre line of the cylinder, turn crank fully back, and move the standard about till the back end of the piston clears the back end of the cylinder by about 1/32 inch.

Get standard quite square, and adjust sideways till connecting rod is in line with axis of cylinder.

Mark off and screw down the standard.

Make the eccentric, eccentric rod, and strap. Slip eccentric on shaft.

Put valve in position and draw it forward till the port is exposed.

Turn the eccentric forward, and mark the rod opposite centre of valve pin.

Bore hole for pin, and insert pin.

Hold the crank shaft firmly, and revolve eccentric till the port just begins to open on its forward stroke. Rotate crank disc on shaft till the crank pin is full forward.

Solder eccentric and disc to shaft.

Solder steam pipe to cylinder, and a brass disc to the rear end of the cylinder.

Fit a fly wheel of metal or wood. This must be fairly heavy, as it has to overcome all friction during the return or exhaust stroke.

Action of Engine.—During the forward motion of the piston the valve is pushed back by the eccentric until the steam port is fully opened, and is then drawn forward, covering the port. At the end of the power stroke the port has begun to open to the air, to allow the steam to escape throughout the exhaust stroke, in the course of which the valve is pushed back until, just at the end of the stroke, the steam port begins to open again.

Notes.— (l.) The connecting rod may be made shorter than shown in Figs. 51 and 52; but in that case the piston also must be shortened to allow for the greater obliquity of the rod at half-stroke.

(2.) If two opposed cylinders are made to operate the one crank, a double-acting engine is obtained. Both valves may be operated by a single eccentric, the connecting rod of one being pivoted to a small lug projecting from the eccentric strap. If three cylinders are set 120 degrees apart round the crank shaft, a continuous turning effect is given. This type will be found useful for running small dynamos.

(3.) If it is desired to use the exhaust steam to promote a draught in the boiler furnace, it should be led away by a small pipe from the rear end of the valve tube.

XV. A HORIZONTAL SLIDE-VALVE ENGINE.

The reader who has succeeded in putting together the simple engine described in the preceding chapter may wish to try his hand on something more ambitious in the same line. The engine illustrated in Figs. 53 to 66 will give sufficient scope for energy and handiness with drill and soldering iron. The writer made an engine of the same kind, differing only from that shown in the design of the crosshead guides, without the assistance of a lathe, except for turning the piston and fly wheel—the last bought in the rough. Files, drills, taps, a hack saw, and a soldering iron did all the rest of the work.

Solder plays so important a part in the assembling of the many pieces of the engine that, if the machine fell into the fire, a rapid disintegration would follow. But in actual use the engine has proved very satisfactory; and if not such as the highly-skilled model-maker with a well-equipped workshop at his command would prefer to expend his time on, it will afford a useful lesson in the use of the simpler tools. Under 50 lbs. of steam it develops sufficient power to run a small electric-lighting installation, or to do other useful work on a moderate scale.

The principal dimensions of the engine are as follows:

Bedplate (sheet zinc), 13-1/2 inches long; 4-1/2 inches wide; 1/8 inch thick.

Support of bedplate (1/20 inch zinc), 3 inches high from wooden base to underside of bedplate.

Cylinder (mandrel-drawn brass tubing), 1-1/2 inches internal diameter; 2-13/16 inches long over all.

Piston, 1-1/2 inches diameter; 1/2 inch long.

Stroke of piston, 2-1/4 inches.

Connecting rod, 5 inches long between centres; 5/16 inch diameter.

Piston rod, 5-1/8 inches long; 1/4 inch diameter.

Valve rod, 4-1/8 inches long; 3/16 inch diameter.

Crank shaft, 5 inches long; 1/2 inch diameter.

Centre line of piston rod, 1-1/4 inches laterally from near edge of bed; 1-5/8 inches from valve-rod centre line; 1-5/8 inches vertically above bed.

Centre line of crank shaft, 10-3/8 inches from cross centre line of cylinder.

Bearings, 1 inch long.

Eccentric, 9/32-inch throw.

Fly wheel, diameter, 7-1/2 inches; width, 1 inch; weight, 6 lbs.

Pump, 3/8-inch bore; 3/8-inch stroke; plunger, 2 inches long.

Other dimensions will be gathered from the various diagrams of details.

The reader will, of course, suit his own fancy in following these dimensions, or in working to them on a reduced scale, or in modifying details where he considers he can effect his object in a simpler manner.

The diagrams are sufficiently explicit to render it unnecessary to describe the making of the engine from start to finish, so remarks will be limited to those points which require most careful construction and adjustment.

The Bedplate.—This should be accurately squared and mounted on its four arch-like supports. (For dimensions, consult Fig. 55.) Half an inch is allowed top and bottom for the turnovers by which the supports are screwed to the bedplate and base. The ends of the longer supports are turned back so as to lie in front of the end supports, to which they may be attached by screws or solder, after all four parts have been screwed to the bed. Care must be taken that the parts all have the same height. Drill all holes in the turnovers before bending. Use 1/8-inch screws. Turn the bed bottom upwards, and stand the four supports, temporarily assembled, on it upside down and in their correct positions, and mark off for the 3/32-inch holes to be drilled in the bed. A hole 3/4 inch in diameter should be cut in the bedplate for the exhaust pipe, round a centre 2 inches from the end and 1-5/8 inches from the edge on the fly-wheel side, and two more holes for the pump.

Making the Cylinder Slide and Valve.—The cylinder barrel must be perfectly cylindrical and free from any dents. Mandrel-drawn brass tubing, 1/16-inch thick, may be selected. If you cannot get this turned off at the ends in a lathe, mark the lines round it for working to with the aid of a perfectly straight edged strip of paper, 2-13/16 inches wide, rolled twice round the tube. The coils must lie exactly under one another. Make plain scratches at each end of the paper with a sharp steel point. Cut off at a distance of 1/16-inch from the lines, and work up to the lines with a file, finishing by rubbing the ends on a piece of emery cloth resting on a hard, true surface.

A square-cornered notch 1/8 inch deep and 7/8 inch wide must now be cut in each end of the barrel, the two notches being exactly in line with one another. These are to admit steam from the steam ways into the cylinder.

Cylinder Standards.-Use 5/64 or 3/32 inch brass plate for these. Two pieces of the dimensions shown in Fig. 56 are needed. Scratch a line exactly down the middle of each, and a cross line 1/2 inch from one end. The other end should be marked, cut, and filed to a semicircle. Drill three 3/16-inch holes in the turnover for the holding-down screws. The two standards should now be soldered temporarily together at the round ends and trued up to match each other exactly. Place them in the vice with the bending lines exactly level with the jaws, split the turnovers apart, and hammer them over at right angles to the main parts. Whether this has been done correctly may be tested by placing the standards on a flat surface. Take the standards apart, and scratch a cross line on each 1-5/8 inch from the lower surface of the foot on the side away from the foot. Make a punch mark where the line crosses the vertical line previously drawn, and with this as centre describe a circle of the diameter of the outside of the barrel. Cut out the inside and file carefully up to the circle, stopping when the barrel makes a tight fit. On the inside of the hole file a nick 1/8 inch deep, as shown in Fig. 56. Remember that this nick must be on the left of one standard and on the right of the other, so that they shall pair off properly.

Standards and barrel must now be cleaned for soldering. Screw one standard down to a wood base; slip one end of the barrel into it; pass the other standard over the other end of the barrel, and adjust everything so that the barrel ends are flush with the, outer surfaces of the standard, and the nicks of the barrel in line with the standard nicks. Then screw the other standard to the base. Solder must be run well into the joints, as these will have to stand all the longitudinal working strain.

The next step is the fitting of the cylinder covers. If you can obtain two stout brass discs 2-1/8 inches in diameter, some trouble will be saved; otherwise you must cut them out of 3/32-inch plate. The centre of each should be marked, and four lines 45 degrees apart be scratched through it from side to side. A circle of 15/16-inch radius is now drawn to cut the lines, and punch marks are made at the eight points of intersection. Solder the covers lightly to the foot side of their standards, marked sides outwards, and drill 1/8-inch holes through cover and standard at the punch marks. Make matching marks on the edges. Unsolder the covers, enlarge the holes in them to take 5/32-inch screws; and tap the holes in the standards. This method will ensure the holes being in line, besides avoiding the trouble of marking off the standards separately.

Bore a 1/4-inch hole in the centre of one cover—be sure that it is the right one—for the piston rod.

You can now proceed to the making of the piston-rod gland (Fig. 54, G1). Fig. 57 shows how this is built up of pieces of tubing and brass lugs for the screws. If possible, get the tubular parts trued in a lathe.

Before the gland is soldered to the cover, the cover should be put in place, the piston rod attached to the piston, and the parts of the gland assembled. Push the piston rod through the cover until the piston is hard up against the back of the cover. Slip the gland over the rod, turn it so that the screws are parallel to the foot of the standard, and make the solder joint. This is the best way of getting the gland exactly concentric with the cylinder so that the piston rod shall move without undue friction. But you must be careful not to unsolder the cylinder from its standard or the parts of the gland. Blacken the piston rod in a candle flame to prevent solder adhering.

Steam Chest.—The walls of the steam chest are best made in one piece out of 1/2-inch brass by cutting out to the dimension given in Fig. 58. A sharp fret saw will remove the inside rectangle. Get both inside and outside surfaces as square as possible in all directions, and rub down the two contact faces on emery cloth supported by an old looking-glass.

Two perfectly flat plates of 1/8-inch brass are cut to the size given in Fig. 59, or a little longer both ways, to allow for working down to the same area as the wall-piece. This operation should be carried out after soldering the three pieces together. File and rub the sides until no projections are visible. Then drill twelve 3/32-inch holes right through the three parts. After separating them, the holes in the walls and what will be the cover must be enlarged to an easy fit for 1/8-inch bolts, and the valve plate tapped.

Now drill 3/16-inch holes centrally through the ends of the walls for the valve rod. If the first hole is drilled accurately, the second hole should be made without removing the drill, as this will ensure the two holes being in line. If, however, luck is against you, enlarge the holes and get the rod into its correct position by screwing and soldering small drilled plates to the outside of the chest. Also drill and tap a hole for the lubricator. The attachment of the gland (Fig. 54, G2) is similar to that of the cylinder gland, and therefore need not be detailed.

The Valve Plate (Fig. 59).—Three ports must be cut in this—a central one, 7/8 by 3/32 inch, for the exhaust; and two inlets, 7/8 by 3/32 inch, 1/8 inch away from the exhaust. These are easily opened out if a series of holes be drilled along their axes.

The Steam Ways.—The formation of the steam ways between valve plate and cylinder is the most ticklish bit of work to be done on the engine as it entails the making of a number of solder joints close together.

We begin by cutting out of 1/20-inch sheet brass a piece shaped as in Fig. 60. Parallel to the long edges, and 3/8 inch away, scribe bending lines. Join these by lines 5/8 inch from the short edges, and join these again by lines 1/4 inch from the bending lines. Cuts must now be made along the lines shown double in Fig. 60. Bend parts CC down and parts BB upwards, so that they are at right angles to parts AA. The positions of these parts, when the piece is applied to the cylinder, are shown in Fig. 62.

One must now make the bridge pieces (Fig. 61, a, a) to separate the inlet passages from the exhaust. Their width is the distance between the bent-down pieces CC of Fig. 60, and their bottom edges are shaped to the curvature of the cylinder barrel. Finally, make the pieces bb (Fig. 61), which form part of the top of the steam ways.

In the assembling of these parts a blowpipe spirit lamp or a little “Tinol” soldering lamp will prove very helpful.

The following order should be observed:

(1.) Solder the piece shown in Fig. 60 to the cylinder barrel by the long edges, and to the cylinder supports at the ends. This piece must, of course, cover the steam ports in the cylinder.

(2.) Put pieces aa (Fig. 61) in position, with their tops quite flush with the tops of BB (Fig. 62), and solder them to the cylinder barrel and sides of the steam-way piece.

(3.) Solder the valve plate centrally to BB, and to the tops of aa, which must lie between the central and outside ports. Take great care to make steam-tight joints here, and to have the plate parallel to the standards in one direction and to the cylinder in the other.

(4.) Solder in pieces bb. These should be a tight fit, as it is difficult to hold them in place while soldering is done.

(5.) Bore a 5/16-inch hole in the lower side of the central division and solder on the exhaust pipe.

Slide Valve.—The contact part of this is cut out of flat sheet brass (Fig. 63), and to one side is soldered a cap made by turning down the edges of a cross with very short arms. The little lugs aa are soldered to this, and slotted with a jeweller’s file to engage with notches cut in the valve rod (see Figs. 58 and 62).

The Crank and Crank Shaft.—The next thing to take in hand is the fixing of the crank shaft. This is a piece of 3/8 or 1/2 inch steel rod 5 inches long.

The bearings for this may be pieces of brass tubing, fitting the rod fairly tight. By making them of good length—1 inch—the wear is reduced to almost nothing if the lubricating can is used as often as it should be.

Each bearing is shown with two standards. The doubling increases rigidity, and enables an oil cup to be fixed centrally.

The shape of the standards will be gathered from Fig. 53, their outline being dotted in behind the crank.

Cut out and bend the standards—after drilling the holes for the foot screws—before measuring off for the centres of the holes; in fact, follow the course laid down with regard to the cylinder standards.

Make a bold scratch across the bedplate to show where the centre line of the shaft should be, and another along the bed for the piston-rod centre line. (Position given on p. 138.)

Bore holes in the bearings for the oil cups, which may be merely forced in after the engine is complete.

The crank boss may be made out of a brass disc 2-3/4 inches diameter and 3/16 inch thick, from which two curved pieces are cut to reduce the crank to the shape shown in Fig. 53. The heavier portion, on the side of the shaft away from the crank pin, helps to counterbalance the weight of the connecting and piston rods. In Fig. 54 (plan of engine) you will see that extra weight in this part has been obtained by fixing a piece of suitably curved metal to the back of the boss.

The mounting of the crank boss on the shaft and the insertion of the crank pin into the boss might well be entrusted to an expert mechanic, as absolute “squareness” is essential for satisfactory working. Screw-thread attachments should be used, and the crankshaft should project sufficiently to allow room for a flat lock nut. The crank pin will be rendered immovable by a small lock screw penetrating the boss edgeways and engaging with a nick in the pin.

Fixing the Standards and Bearings.—Place the two bearings in their standards and slip the crank shaft through them. Place standards on the bed, with their centre lines on the crank-shaft centre line. The face of the crank should be about 3/8 inch away from the piston rod centre line. Bring the nearer bearing up against the back of the disc, and arrange the standards equidistantly from the ends of the bearing. The other bearing should overlap the edge of the bed by about 1/8 inch. Get all standards square to the edge of the bed, and mark off the positions of screw holes in bed. Remove the standards, drill and tap the bed-plate holes, and replace parts as before, taking care that the lubricating holes in the bearings point vertically upwards. Then solder bearings to standards.

If any difficulty is experienced in getting all four standards to bed properly, make the bearing holes in the two inner ones a rather easy fit. The presence of the crank-shaft will assure the bearings being in line when the soldering is completed.

The standards and bed should have matching marks made on them.

The Eccentric.—This can be formed by soldering two thin brass discs 1-15/16-inch diameter concentrically to the sides of a disc of 1-15/16-inch diameter and 5/16 inch thick. The centre of the shaft hole must be exactly 9/32 inch from the centre of the eccentric to give the proper valve-travel. Drill and tap the eccentric edgeways for a lock screw.

A piece to which the eccentric strap, eccentric rod, and pump rod are attached is cut out of 5/16-inch brass. Its shape is indicated in Fig. 53. The side next the eccentric must be shaped as accurately as possible to the radius of the eccentric. The strap, of strip brass, is fastened to the piece by four screws, the eccentric rod by two screws.

Crosshead and Guides.—The crosshead (Figs. 53 and 54) is built up by soldering together a flat foot of steel, a brass upright, and a tubular top fitting the piston rod. The guides, which consist of a bed, covers, and distance-pieces united by screws (Fig. 64), have to withstand a lot of wear, and should preferably be of steel. The importance of having them quite flat and straight is, of course, obvious.

The last 1-3/8 inches of the piston rod has a screw thread cut on it to engage with a threaded hole in the fork (cut out of thick brass plate), to which the rear end of the connecting rod is pinned, and to take the lock nut which presses the crosshead against this fork.

Assuming that all the parts mentioned have been prepared, the cylinder should be arranged in its proper place on the bed, the piston rod centrally over its centre line. Mark and drill the screw holes in the bed.

The Valve Gear.—We may now attend to the valve gear. A fork must be made for the end of the valve rod, and soldered to it with its slot at right angles to the slots which engage with the valve lugs. Slip the rod into the steam chest, put the valve on the rod, and attach the chest (without the cover) to the valve plate by a bolt at each corner. Pull the valve forward till the rear port is just uncovered, and turn the eccentric full forward. You will now be able to measure off exactly the distance between the centres of the valve-rod fork pin and the rear screw of the eccentric. The valve connecting rod (Fig. 53, VCR) should now be made and placed in position. If the two forward holes are filed somewhat slot-shaped, any necessary adjustment of the valve is made easier. If the adjustment of VCR and the throw of the eccentric are correct, the valve will just expose both end ports alternately when the crank is revolved. If one port is more exposed than the other, adjust by means of the eccentric screws till a balance is obtained. Should the ports still not be fully uncovered, the throw of the eccentric is too small, and you must either make a new eccentric or reduce the width of the valve. (The second course has the disadvantage of reducing the expansive working of the steam.) Excess movement, on the other hand, implies too great an eccentric throw.

Setting the Eccentric.—Turn the crank full forward, so that a line through the crank pin and shaft centres is parallel to the bed. Holding it in this position, revolve the eccentric (the screw of which should be slackened off sufficiently to allow the eccentric to move stiffly) round the shaft in a clockwise direction, until it is in that position below the shaft at which the front steam port just begins to show. Then tighten up the eccentric lock screw.[1]

[Footnote 1: The reader is referred to an excellent little treatise, entitled “The Slide Valve” (Messrs. Percival Marshall and Co., 26 Poppin’s Court, Fleet Street, E.C. Price 6d.), for a full explanation of the scientific principles of the slide valve.]

The Connecting Rod.—The length of this from centre to centre of the pins on which it works should be established as follows:—Slip over the piston rod a disc of card 1/32 inch thick. Then pass the rod through the gland and assemble the crosshead and fork on its end, and assemble the guides round the crosshead foot. Turn the crank pin full forward, pull the piston rod out as far as it will come, measure the distance between pin centres very carefully, and transfer it to a piece of paper.

The rod consists of a straight central bar and two rectangular halved ends. The ends should be cut out of brass and carefully squared. Through their exact centres drill 1/8-inch holes, and cut the pieces squarely in two across these holes. The sawed faces should be filed down to a good fit and soldered together. Now drill holes of the size of the pins, using what remains of the holes first made to guide the drill. The bolt holes are drilled next, and finally the holes for lubrication and those to take the rods. Then lay the two ends down on the piece of paper, so that their pinholes are centred on the centre marks, and the holes for the rod are turned towards one another. Cut off a piece of steel rod of the proper length and unsolder the ends. The rod pieces must then be assembled on the rod, and with it be centred on the paper and held in position while the parts are soldered together.

OTHER DETAILS.

Adjusting the Guides.—Put the connecting rod in place on its pins, and revolve the crank until the guides have taken up that position which allows the crosshead to move freely. Then mark off the holes for the guide holding-down screws, and drill and tap them.

Packings.—The glands and piston should be packed with asbestos string. Don’t be afraid of packing too tightly, as the tendency is for packing to get slacker in use. The rear end of the cylinder should be bevelled off slightly inside, to allow the packed piston to enter easily.

Joints.—The cylinder head and valve chest joints should be made with stout brown paper soaked in oil or smeared with red lead. All screw holes should be cut cleanly through the paper, and give plenty of room for the screws.

When making a joint, tighten up the screws in rotation, a little at a time so as not to put undue strain on any screw. Wait an hour or two, and go round with the screw-driver again.

Lubrication.—When the engine is first put under steam, lubrication should be very liberal, to assure the parts “settling down” without undue wear.

The Pump.—Fig. 65 shows in section the pump, which will be found a useful addition to the engine. (For other details, see Figs. 53 and 54.) Its stroke is only that of the eccentric, and as the water passages and valves are of good size, it will work efficiently at high speed. The method of making it will be obvious from the diagrams, and space will therefore not be devoted to a detailed description. The valve balls should, of course, be of gun-metal or brass, and the seatings must be prepared for them by hammering in a steel ball of the same size.

In practice it is advisable to keep the pump always working, and to regulate the delivery to the boiler by means of a by-pass tap on the feed pipe, through which all or some of the water may be returned direct to the tank.

The tank, which should be of zinc, may conveniently be placed under the engine. If the exhaust steam pipe be made to traverse the tank along or near the bottom, a good deal of what would otherwise be wasted heat will be saved by warming the feed water.

Making a Governor.

It is a great advantage to have the engine automatically governed, so that it may run at a fairly constant speed under varying loads and boiler pressures. In the absence of a governor one has to be constantly working the throttle; with one fitted, the throttle can be opened up full at the start, and the automatic control relied upon to prevent the engine knocking itself to pieces.

The vertical centrifugal apparatus shown in Fig. 66 was made by the writer, and acted very well. The only objection to it is its displacement of the pump from the bed. But a little ingenuity will enable the pump to be driven off the fly wheel end of the crank shaft, or, if the shaft is cut off pretty flush with the pulley, off a pin in the face of the pulley.

Turning to Fig. 66, A is a steel spindle fixed in a base, L, screwed to the bed. B is a brass tube fitting A closely, and resting at the bottom on a 1/4-inch piece of similar tubing pinned to A.

A wooden pulley jammed on B transmits the drive from a belt which passes at its other end round a similar, but slightly larger, pulley on the crank shaft. This pulley is accommodated by moving the eccentric slightly nearer the crank and shortening the fly-wheel side bearing a little.

The piece G, fixed to B by a lock screw, has two slots cut in it to take the upper ends of the weight links DD; and C, which slides up and down B, is similarly slotted for the links EE. Each of the last is made of two similarly shaped plates of thin brass, soldered together for half their length, but separated 3/32 inch at the top to embrace the projections of D. To prevent C revolving relatively to B, a notch is filed in one side of the central hole, to engage with a piece of brass wire soldered on B (shown solid black in the diagram). A spiral steel spring, indicated in section by a number of black dots, presses at the top against the adjustable collar F, and at the bottom against C.

The two weights WW are pieces of brass bar slotted for driving on to DD, which taper gently towards the outer edge.

When the pulley revolves, centrifugal force makes WW fly outwards against the pressure of the spring, and the links EE raise C, which in turn lifts the end of lever M. A single link, N, transmits the motion from a pin on M to the double bell-crank lever O (see Fig. 66) pivoted on a standard, P, attached to the bedplate. The slotted upper ends of P engage with pins on an adjustable block, R, which moves the governing valve V (solid black), working in the tube S through a gland. The higher M is raised the farther back is V moved, and its annular port is gradually pushed more out of line with two ports in the side of the valve tube, thus reducing the flow of steam from the supply pipe to the cylinder connection on the other side of the tube. This connection, by-the-bye, acts as fulcrum for lever M, which is made in two parts, held together by screws, to render detachment easy.

The closer the fit that V makes with S the more effective will the governing be. The gland at the end of S was taken from an old cylinder cover.

Regulation of the speed may be effected either

(1) by driving the governor faster or slower relatively to the speed of the crank shaft;

(2) by altering the position of W on D;

(3) by altering the compression of the spring by shifting F;

(4) by a combination of two or more of the above.

Generally speaking, (3) is to be preferred, as the simplest.

The belt may be made out of a bootlace or fairly stout circular elastic. In either case the ends should be chamfered off to form a smooth joint, which may be wrapped externally with thread.

FINAL HINTS.

All parts which have to be fitted together should have matching marks made on them with the punch. To take the parts of the valve chest as an example. As we have seen, these should be soldered together, finished off outside, and drilled. Before separating them make, say, two punch marks on what will be the upper edge of the valve plate near the end, and two similar marks on the chest as near the first as they can conveniently be. In like manner mark the chest cover and an adjacent part of the chest with three marks. It is utterly impossible to reassemble the parts incorrectly after separation if the marks are matched. Marking is of greatest importance where one piece is held up to another by a number of screws. If it is omitted in such a case, you may have a lot of trouble in matching the holes afterwards.

Jacket the cylinder with wood or asbestos, covered in neatly with sheet brass, to minimize condensation. If the steam ways, valve chest, and steam pipe also are jacketed, an increase in efficiency will be gained, though perhaps somewhat at the expense of appearance.

Boiler.—The boiler described on pp. 211-216, or a vertical multitubular boiler with about 800 sq. inches of heating surface will drive this engine satisfactorily.

XVI. MODEL STEAM TURBINES.

Steam turbines have come very much to the fore during recent years, especially for marine propulsion. In principle they are far simpler than cylinder engines, steam being merely directed at a suitable angle on to specially shaped vanes attached to a revolving drum and shaft. In the Parsons type of turbine the steam expands as it passes through successive rings of blades, the diameter of which rings, as well as the length and number of the blades, increases towards the exhaust end of the casing, so that the increasing velocity of the expanding steam may be taken full advantage of. The De Laval turbine includes but a single ring of vanes, against which the steam issues through nozzles so shaped as to allow the steam to expand somewhat and its molecules to be moving at enormous velocity before reaching the vanes. A De Laval wheel revolves at terrific speeds, the limit being tens of thousands of turns per minute for the smallest engines. The greatest efficiency is obtained, theoretically, when the vane velocity is half that of the steam, the latter, after passing round the curved inside surfaces of the vanes, being robbed of all its energy and speed. (For a fuller description of the steam turbine, see How It Works, Chap. III., pp.74-86.)

The turbines to be described work on the De Laval principle, which has been selected as the easier for the beginner to follow.

A Very Simple Turbine.

We will begin with a very simple contrivance, shown in Fig. 67. As a “power plant” it is confessedly useless, but the making of it affords amusement and instruction. For the boiler select a circular tin with a jointless stamped lid, not less than 4 inches in diameter, so as to give plenty of heating surface, and at least 2-1/2 inches deep, to ensure a good steam space and moderately dry steam. A shallow boiler may “prime” badly, if reasonably full, and fling out a lot of water with the steam.

Clean the metal round the joints, and punch a small hole in the lid, half an inch from the edge, to give egress to the heated air during the operation of soldering up the point or joints, which must be rendered absolutely water-tight.

For the turbine wheel take a piece of thin sheet iron or brass; flatten it out, and make a slight dent in it an inch from the two nearest edges. With this dent as centre are scribed two circles, of 3/4 and 1/2 inch radius respectively. Then scratch a series of radial marks between the circles, a fifth of an inch apart. Cut out along the outer circle, and with your shears follow the radial lines to the inner circle. The edge is thus separated into vanes (Fig. 68), the ends of which must then be twisted round through half a right angle, with the aid of a pair of narrow-nosed pliers, care being taken to turn them all in the same direction.

A spindle is made out of a large pin, beheaded, the rough end of which must be ground or filed to a sharp point. Next, just break through the metal of the disc at the centre with a sharpened wire nail, and push the spindle through till it projects a quarter of an inch or so. Soldering the disc to the spindle is most easily effected with a blowpipe or small blow-lamp.

The Boiler.—In the centre of the boiler make a dent, to act as bottom bearing for the spindle. From this centre describe a circle of 5/8-inch radius. On this circle must be made the steam port or ports. Two ports, at opposite ends of a diameter, give better results than a single port, as equalizing the pressure on the vanes, so that the spindle is relieved of bending strains. Their combined area must not, however, exceed that of the single port, if one only be used. It is important to keep in mind that for a turbine of this kind velocity of steam is everything, and that nothing is gained by increasing the number or size of ports if it causes a fall in the boiler pressure.

The holes are best made with a tiny Morse twist drill. As the metal is thin, drill squarely, so that the steam shall emerge vertically.

For the upper bearing bend a piece of tin into the shape shown in Fig. 67. The vertical parts should be as nearly as possible of the same length as the spindle. In the centre of the underside of the standard make a deep dent, supporting the metal on hard wood or lead, so that it shall not be pierced. If this accident occurs the piece is useless.

Place the wheel in position, the longer part of the spindle upwards, and move the standard about until the spindle is vertical in all directions. Scratch round the feet of the standard to mark their exact position, and solder the standard to the boiler. The top of the standard must now be bent slightly upwards or downwards until the spindle is held securely without being pinched.

A 3/16-inch brass nut and screw, the first soldered to the boiler round a hole of the same size as its internal diameter, make a convenient “filler;” but a plain hole plugged with a tapered piece of wood, such as the end of a penholder, will serve.

Half fill the boiler by immersion in hot water, the large hole being kept lowermost, and one of the steam vents above water to allow the air to escape.

A spirit lamp supplies the necessary heat. Or the boiler may be held in a wire cradle over the fire, near enough to make the wheel hum. Be careful not to over-drive the boiler. As a wooden plug will probably be driven out before the pressure can become dangerous, this is a point in favour of using one. Corrosion of the boiler will be lessened if the boiler is kept quite full of water when not in use.

A Practical Steam Turbine.

The next step takes us to the construction of a small turbine capable of doing some useful work. It is shown in cross section and elevation in Fig. 69.

The rotor in this instance is enclosed in a case made up of two stout brass discs, D and E, and a 3/4-inch length of brass tubing. The plates should be 1/2-inch larger in diameter than the ring, if the bolts are to go outside. The stouter the parts, within reason, the better. Thick discs are not so liable to cockle as thin ones, and a stout ring will make it possible to get steam-tight joints with brown-paper packing.

The wheel is a disc of brass, say, 1/25 inch thick and 4 inches in diameter; the spindle is 3/16 inch, of silver steel rod; the bearings, brass tubing, making a close fit on the rod.

If you cannot get the ring ends turned up true in a lathe—a matter of but a few minutes’ work—rub them down on a piece of emery cloth supported on a true surface, such as a piece of thick glass.