Question 155. How is the steam admitted to and the supply regulated or shut off from the cylinders?

Answer. By a valve, h, fig. 41, called a throttle-valve, which is usually placed at the end of the pipe I, near the top of the dome. Throttle-valves are sometimes placed in the smoke-box at the front end, n, fig. 41, of the dry-pipe. Until within a few years they consisted of plain slide-valves which covered openings similar in form to the steam-ports, but smaller in size. The pressure on such valves is of course greatest when there is no steam underneath, which is the case when the valves are closed. It is then very difficult to open them, and as it is important that the supply of steam admitted to the cylinders when the locomotive is started should be easily regulated, such valves are objectionable, and therefore the form has been introduced which is illustrated in fig. 41, and also on a larger scale in fig. 85, which represents a longitudinal section of the throttle-pipe and valve. This is what is called a double-poppet valve, and consists of two circular discs, a and b, which cover two corresponding openings in the end of the pipe I. When these discs are raised up, as shown in fig. 85, steam flows in around their edges, as represented by the darts. It will be observed that the steam pressure in the boiler comes on top of the disc a and against the under side of b. The pressure on the one thus neutralizes or balances that on the other. If the two discs were of the same size, the pressure of the one would be exactly the same as on the other; but as they are joined together and are made to fit steam-tight on their seats by what are called countersunk joints, their diameters must be somewhat larger than the openings they cover. The only practicable way, therefore, by which the lower disc b can be introduced into the end h of the pipe I so as to cover the lower opening is through the upper opening a. For this reason the lower disc must be made smaller than the upper one, and therefore the pressure on the upper one, being in proportion to its size, has a constant tendency to close the valve. As it is of the greatest importance that a throttle-valve should remain closed after steam is shut off, and never be opened at any time accidentally, the arrangement described accomplishes just what is needed—that is, makes the valve work comparatively easily, and at the same time keeps it closed after the steam has been shut off.

Question 156. How is the valve opened and closed?

Answer. By a lever, O′ O, called a throttle-lever, figs. 85, 86[41] and 71. This lever is connected by a rod, d, called the throttle-stem, with a bell crank, i, the other arm of which works the rod c, to which the throttle-valve is attached. The rod d works through a steam-tight stuffing-box, f, in the back end of the boiler. The end of the throttle-lever is attached to two links, g, fig. 86, which are fastened by a pin to the stud h. These links have a slight vibratory motion, which enables the pin k, by which the lever O′ O, is fastened to the rod d, to move in a straight line, which is necessary in order that the rod d may work steam-tight in the stuffing-box, f. The throttle-lever has a latch, l, which gears into a curved rack, n, so as to hold the lever and valve in any required position. This latch is operated by a trigger, m. Various other devices are used to fasten the throttle-lever and thus hold it in any position required.

Question 157. How are the steam pipes constructed?

Answer. The steam, after it is admitted by the throttle-valve, as was explained in answer to Question 155, passes into the throttle-pipe I and the dry-pipe m m, fig. 41. At the front end of the dry-pipe a pipe, n, figs. 40 and 41, which divides into two branches like the top of the letter T and is therefore called a T-pipe, is attached. The steam-pipes o, o, fig. 40, are connected to each of the two branches of the T-pipe at one end and to the cylinders at the other.[42]

These pipes, being in the smoke-box, are exposed to great changes of temperature, and are therefore subjected to expansion by heat and contraction by cold. The joints are therefore constantly subject to disturbance by the contraction and expansion of the pipes and so are difficult to keep tight. It is also practically impossible to construct the boiler, the cylinders and the pipes with perfect accuracy, and therefore a small amount of adjustability and flexibility is necessary in the joints of the pipes. If, for example, the opening x in the cylinder, fig. 40, were either too near or too far from the cylinder of the engine, it would be necessary to move the end of the pipe o either to the right or to the left in order to connect it with x. If the joint of the upper end of the steam-pipe were attached to the T-pipe with a flat joint like that shown at a b, fig. 87, it would be impossible to move the lower end of the steam-pipe either to the right or to the left without disturbing the joint and causing it to leak. For this reason these pipes are connected with what are called ball joints, fig. 88, that is, the end a b of one of the pipes is turned into the form of a part of a sphere,[43] and the other end into a corresponding concave form. It is known that a sphere will fit into a corresponding socket in any position; for example, an acorn in its cup or the bones at the hip or shoulder joints. If, therefore, the pipes are joined with such spherical or ball-joints as they are called, the lower end can be moved sideways several inches either way, and the joint will still be steam-tight if it is then firmly bolted together. Even after it is bolted together it will have so much flexibility that the expansion and contraction of the pipes will not cause it to leak.

There is, however, still another difficulty. Although the lower end of the pipe, o o, fig. 40, can, with a ball-joint above, be moved in any direction horizontally, yet if the pipe is too long or too short it is obvious such a joint will not permit it to be moved up or down. A joint with a flat surface, like that shown in fig. 87, would, however, permit such motion in the pipe without leaking. If, for example, the steam-pipe were ¹⁄₈ of an inch too short, it might be drawn down that distance, and if the upper joint were then screwed up it would still be steam-tight. In order, then, to get both vertical and lateral flexibility in the joints of the steam-pipes, a ring, a b, fig. 89, is interposed between the pipes. One side of this ring is spherical and the other flat, so that the pipes can move either around the spherical part or slip up or down or sideways on the flat surface of the ring. In this way the pipes are flexible and adjustable in every direction, and for all kinds of motion caused by expansion, or which may be needed when the parts are put together. Sometimes the joints at one end only of the steam-pipes are made in this way, and the other is connected with a simple ball-joint.

In designing these joints their form should be drawn with a radius, c d, fig. 88, from one centre, c, so that the surface of the joint will form a part of a sphere. If they are drawn from two centres, as is sometimes done, it is obvious that the surface of the joint will not be a part of a sphere, and therefore will not have the requisite flexibility. The surfaces of the joints are carefully turned to the proper form, and then made steam-tight by scraping or grinding them with emery and oil, and the pipes are then fastened together with bolts, g, g, fig. 89, and flanges, f, f, cast on the pipes.

Question 158. How are the exhaust pipes constructed?

Answer. They are made of cast iron. When two nozzles are used they are generally cast together, as shown in fig. 90. When only one is used, the form of the pipes resembles somewhat that of an inverted letter ⅄, as shown in fig. 91, so as to cover the two openings which connect with the cylinders. The tops of these pipes have rings or bushings, a a, fitted into them, which are held by set screws, b, so that they can easily be removed and others with larger or smaller openings be substituted. If the openings in the exhaust-nozzles are small, the steam must be discharged at a higher rate of speed, in order to exhaust all that is in the cylinders, than if the blast orifices are larger. Therefore, if the latter are reduced in size, the draft becomes more violent, but at the same time the back-pressure in the cylinder (which will be explained hereafter) is increased. It therefore becomes necessary to adjust the size of the blast orifices with the greatest care, so as to have them just small enough to produce the required draft and yet leave them as large as possible, so as to reduce the back-pressure. For these reasons what are called variable exhausts are sometimes used. In these the blast orifice can be increased or diminished at pleasure, and thus regulated to suit the conditions under which an engine is working. A great variety of such devices has been used, but now nearly all have been abandoned for the simpler arrangement described, which is not variable when the engine is working.