How to Make a Horizontal Slide Valve Engine - Part 2
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).
FIG. 63.- Parts of slide valve.
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 pistonrod 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.
FIG. 64. - Cross section of crosshead and guide.
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.
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.
[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.]
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
FIG. 65.- Vertical section of force pump driven by engine.
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.
FIG. 66. -- Elevation of governor for horizontal engine. Above is plan of valve and rod gear.
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, bythe- 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.
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.