The Library of Work and Play: Mechanics, Indoors and Out

by Fred. T. Hodgson

Fig. 31. Steam cylinder and piston

Steam, as everybody knows, is generated by heat being applied to a closed metal kettle or boiler containing water. This boiler must be strong and properly arranged so as to admit more water—which is usually injected with a force pump—and it must have an outlet for the release of the steam to the cylinder of the engine. Generally, there is a small dome on the top of the boiler, called the "steam dome," and to this the steam outflow pipe is attached. The actual use of this dome is to hold a volume of steam that will remain unmixed with water, as it is placed considerably above water level. On the top of the dome there is an automatic arrangement called a "safety valve," so that when there is too much pressure of steam in the boiler, it will open and allow the over-pressure to escape, and thus prevent the boiler from exploding or being over strained. This valve is controlled by a simple device, somewhat similar to a steelyard. A movable weight is arranged to slide on a long arm which is loosely fixed to the valve flange by a bolt and nut, and extends some distance past the seat of the valve. The arm or lever has an iron pin attached to it directly over the valve seat, which holds down the valve and keeps the steam from escaping. The movable weight on the arm is adjusted so as to regulate the pressure on the valve. When there is too great a pressure, the valve forces up the lever, and at the same time opens a passage for the extra pressure of steam to escape. There are several other contrivances for relieving the boiler of over stress, but the one described, or rather the principle on which it is built, is most in use on this country. There are many kinds of boilers, or steam generators, but the best, and very likely the strongest, are those employed on our first-class railway locomotives. These are frequently under a pressure of 200 or more pounds to the square inch, which seems an enormous load for a hollow shell to carry, yet, so near perfection are they, we rarely hear of a locomotive boiler explosion. As there are many kinds of boilers, so also are there many kinds of steam engines, but all of these latter, with very few exceptions, have a cylinder and piston for converting the force of the steam into useful and effective motion. The manner of using this force and keeping it under proper control is somewhat complex and difficult to describe briefly, without elaborate diagrams, but Mr. Gregg explained, in his own way, how the great force was converted into motion. On the blackboard he drew a rough diagram of a cylinder and valve or steam-chest, with piston and slide-valve, about as shown in [Fig. 31], which gives a longitudinal section of the whole arrangement. Here we see near each end, the opening of a double conduit aa, made in the thickness of the side; these are the openings by which the steam comes alternately to work on one end, then on the other, of the piston. These are called the steam-ports. These two open outward on a well-polished surface, and between the two a third opening, E, is seen, which serves to let the steam escape when it has done its work, and is called for that reason the exhaust port. C is the pipe by which the steam gains access to the open air or to the condenser, where it parts with its elastic force.

Here is shown by what contrivance the distribution is effected, consisting, as it does, of two partial operations; the admission of the steam and its escape, which must be repeated twice to obtain a complete phase of the to-and-fro movement of the slide-valve. There are various methods employed according to different engines—but the first described is the one represented by the illustration.

In the valve chest, BB, is seen a prismatic box, open on one side, called the slide-valve, and this is applied by its open face to the well-polished plane on which, as mentioned before, the three ports open. The space BB, is called the valve or steam chest. The steam coming from the boiler by the pipe C spreads out freely in it, but the inside of the slide-valve, on the contrary, is always closed to the entering steam, though constantly in communication with the escape pipe and also with first one then the other of the entrances to the cylinder. Lastly, the movement of the slide-valve is produced by the engine itself, aided by a rod and an eccentric fixed to the shaft of the fly-wheel.

Fig. 32. Steam valves—different positions

By following the successive and alternating motions of the slide-valve, as represented in [Fig. 32], we can easily comprehend the different phases of the distribution of the steam.

This is the machinery for the distribution of steam generally. There are other engines, such as rotary and oscillating, that are supplied by other contrivances, but most of these have fallen, or are fast falling into disuse, as they are not so satisfactory as the ordinary slide-valve. It will be seen upon examination of the sketch, shown in [Fig. 32], how the steam enters and leaves the cylinder and the position of the piston under the various positions of the valves. The arrows show the direction of the slide, also the direction of the piston and its position when the slide covers the ports X, or leaves them open, or partly so. The ports for egress or ingress are shown at X, the slide-valve at V, and the cylinder at C. When the piston is near one end of the cylinder, the steam is admitted and forces the piston in the opposite direction, while the valve is so arranged that when the piston starts in that other direction, it begins to open the port at the other end of the cylinder through which the exhausted steam escapes. This makes the noise Jessie asked her father about. There are some engines so devised that the exhaust is made to assist in driving another engine.

Of course, there are many kinds of steam engines, but all are run on the same principle, or nearly so. As you know, steam is generated in boilers by fire being applied to the outside and the water made hot enough to raise steam. A steam engine is said to be externally heated, while gas, oil, and other similar engines are internally heated, because instead of the steam driving the piston, the gas, oil, or other explosive matter is admitted into the clearance or space between the piston and the end of the cylinder, where it is exploded by an electric spark from a battery provided for the purpose, and this is called the "ignition." The explosion causes the gas and air in the cylinder to expand, bringing a great pressure on the piston, forcing it to move toward the other end of the cylinder, and making the whole machine move. One great advantage of employing a gas engine is that no boiler is required, a very important matter, as boilers take up a great deal of space. The coal or wood necessary to keep up steam also takes space that could be used for other purposes, all of which make the use of steam objectionable when it is possible to employ suitable gas engines. Besides, the make-up of a steam engine is of such a character that it is very expensive, while the first cost of gas engines is much lower.