The Young Mechanic / Containing directions for the use of all kinds of tools, and for the construction of steam engines and mechanical models, including the art of turning in wood and metal

by James Lukin

ENGINES WITHOUT SLIDE-VALVES EASY TO MAKE.

Having been led on from the atmospheric engine to that of Watt’s, and to slide-valve engines generally, I am now going backward a little to a class easier to make, because they have no slide-valves, nor even four-way cocks; and then I shall have done with engines. But I dare say some of my readers will wonder why I have said so little about condensers and condensing engines. I am sure they will wonder at it if they understood what I explained of the advantage of a vacuum under the piston; so that 15 lbs. pressure upon the piston means 15 lbs. of useful work, instead of 30 lbs. being required for that purpose. But condensing engines are utterly beyond a boy’s power. They require not only a vessel into which the steam is injected at each stroke, but there must be a pump to raise and inject cold water to condense the steam, and a pump to extract from the vessel again this water, after it has been used, and a cistern, and cold and hot wells; and all this is difficult to make so as to act; and I am sure no boy cares for a steam engine that will not work. Moreover, I have given you difficult work as it is—work that many of my readers will no doubt be afraid to try—yet I did it on purpose; because if small boys are unequal to some of it, their big brothers are not, or ought not to be; and mechanical boys must look at difficulties as a trained hunter looks at a hedge—viz., with a strong desire to go over it, or through it, or any how and some how to get to the other side of it. Indeed, you must ride your mechanical hobby very boldly and with great pluck, or you won’t half enjoy the ride. However, I am quite aware that I have led you into several difficulties, and therefore now I propose to set before you some easy work as a kind of holiday task which will send you with fresh vigour to what is not so easy.

The engines without slide-valves have also no eccentrics and no connecting-rods. There is just a boiler, a cylinder, piston, piston-rod, and crank, and you have the sum total, save and except the fly-wheel. These are direct-action engines, the cylinders of which oscillate like a pendulum, and the piston-rod itself is connected to the crank, doing away with the necessity for guides.

Fig. 69, A, shows one of these engines, and you see that the cylinder leans to the left when the crank is turned to that side; and if you turn the wheel to the right, the crank will presently cause it to lean the other way; and thus, as it turns on a pin, or “trunnion,” as it is called, it keeps on swinging from side to side as the wheel goes round.

Now, when it is in its first position, the piston is at the bottom of the cylinder, and it then needs to have the steam admitted below it to drive up the piston; but when this has passed its highest position, and the cylinder is turned a little to the right, the piston must be allowed to descend, and, therefore, we must let out the steam below it. We ought, at the same time, to admit steam above the piston to force it down; but, in the simplest models, which are called single-action engines, this is not done. The fly-wheel, having been set in motion, keeps on revolving, and, by its impetus, sends down the piston quite powerfully enough to overcome the slight resistance which is offered by the friction of the parts.

Now, you can, I daresay, easily understand that it is possible to make this to-and-fro motion of the oscillating cylinder open first a steam-port to allow steam to raise the piston, and then an exhaust-port to let it blow off into the air. This is exactly what is done in practice, and it is managed in the following manner:—

Fig. 69.

B, of Fig. 69, shows the bottom of the cylinder, which is a solid piece of brass filed quite flat on one side, and turned out to receive the end of the brass tube, which, generally speaking, is screwed into it to form the cylinder, this being the easiest way to make it. In the middle of the upper part of the flat side you see a white steam-port, and below it a round white spot, which is the position of the pin, or trunnion, on which it oscillates. Fig. 69, C, is a similar piece of brass, which is fixed to the top of the boiler. In this, on the left of the upper part, is also a port, which is connected with the boiler by a hole drilled below it to admit steam. On the right is also a port, which is merely cut like a notch, or it may go a little way into the boss, and then be met by a hole drilled to meet it, so as to form the escape or exhaust port. Between and below these is the hole for the trunnion.

Now, you can, I think, see that if the cylinder stands upright against this block, as it does when the crank is vertical (or upright) and on its dead points, the port at the bottom of the cylinder would fall between the two on this block of brass, and, as they are both flat and fit closely, no steam from the boiler can enter the cylinder. Nor do we want it to do so, because, if the crank is on a dead point, no amount of steam can make the piston rise so as to move it. But now, if we move the cylinder to the left, which we can do by turning the wheel, we shall presently get the crank at right angles to its former position, and, also, we shall bring the steam-ports in the cylinder and block together, so that steam will enter below the piston. But, practically to get as long a stroke as possible, steam is not allowed to enter fully until the crank is further on than in a horizontal position, that is, approaching its lower dead point; and this is the position in which to put it to start the engine. By altering the shape or the position of the port a little, we can so arrange matters as to let steam enter at any required moment.

Steam having entered, the piston will rise rapidly, forcing up the piston, and presently, by the consequent revolution of the fly-wheel, the cylinder will be found leaning to the left, and at this moment the piston must evidently begin to descend. At this very time the steam-ports will have ceased to correspond, but the port in the cylinder will come opposite the exhaust-port in the brass block, and this port is made of such size and shape that the two shall continue to be together all the time the piston is descending; but, the moment it has reached the end of its downward stroke, they cease to correspond in position, and the steam-port begins again to admit a fresh supply of steam.

The pillar attached to the brass boss has nothing to do with it, but is one of the supports of the axle of the fly-wheel, as you will understand by inspection of A of this same drawing.

Such is the single-action model engine, of no power, but a very interesting toy and real steam engine.

The double-action engine is very superior to the foregoing, which, I may remark, has no stuffing-box, and of which the piston is never packed. I may also add, that the crank is formed generally by merely bending the wire that forms the axle of the wheel, and putting the bent end through the hole of a little boss or knob of brass, screwed to the end of the piston-rod. Here you have no boring of cylinders to accomplish, but the cylinder cover, piston, and wheel (often of lead or tin) require the lathe to make them neatly. Many an engine, however, has been made without a lathe, and I have seen one with a bit of gun-barrel for a cylinder, and a four-way cock of very rough construction, that was used to turn a coffee-mill, and did its work very well too.

But I must go at once to the double-action oscillating cylinder, in which, although a similar mode of admitting steam is used, it is arranged to admit it alternately above and below the piston, the exhaust also acting in a similar manner.

After the explanation I have given you, however, of the single-action engine, you will, some of you, I think, jump at a conclusion almost directly, and perhaps be able to plan for yourselves a very easy arrangement to accomplish the desired end. All boys, however, are not “wax to receive, and adamant to retain” an impression; for I have known some who need an idea to be driven into their brains with a good deal of hard hammering. Stupid?—No. Dull?—No, only slow in getting hold, and none the worse for that generally, if the master will but have a little patience; for when they do get hold, they are very like bulldogs, they won’t let go in a hurry, but store up in most retentive minds what they learned with such deliberation.