LITMIR.BIZ

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centre of the shaft to the centre of the crank pin is called the crank's throw, which is half of the piston's stroke. An engine of this type is called double-acting, as the piston is pushed alternately backwards and forwards by the steam. When piston rod, connecting rod, and crank lie in a straight line—that is, when the piston is fully out, or fully in—the crank is said to be at a "dead point;" for, were the crank turned to such a position, the admission of steam would not produce motion, since the thrust or pull would be entirely absorbed by the bearings.

      

       Fig. 18.—Sectional plan of a horizontal engine.

      

      DOUBLE-CYLINDER ENGINES.

       Fig. 19.

       Fig. 20.

      Locomotive, marine, and all other engines which must be started in any position have at least two cylinders, and as many cranks set at an angle to one another. Fig. 19 demonstrates that when one crank, C₁, of a double-cylinder engine is at a "dead point," the other, C₂, has reached a position at which the piston exerts the maximum of turning power. In Fig. 20 each crank is at 45° with the horizontal, and both pistons are able to do work. The power of one piston is constantly increasing while that of the other is decreasing. If single-action cylinders are used, at least three of these are needed to produce a perpetual turning movement, independently of a fly-wheel.

      THE FUNCTION OF THE FLY-WHEEL.

      A fly-wheel acts as a reservoir of energy, to carry the crank of a single-cylinder engine past the "dead points." It is useful in all reciprocating engines to produce steady running, as a heavy wheel acts as a drag on the effects of a sudden increase or decrease of steam pressure. In a pump, mangold-slicer, cake-crusher, or chaff-cutter, the fly-wheel helps the operator to pass his dead points—that is, those parts of the circle described by the handle in which he can do little work.

      THE CYLINDER.

       Fig. 21.—Diagrammatic section of a cylinder and its slide-valve.

      The cylinders of an engine take the place of the muscular system of the human body. In Fig. 21 we have a cylinder and its slide-valve shown in section. First of all, look at P, the piston. Round it are white grooves, R R, in which rings are fitted to prevent the passage of steam past the piston. The rings are cut through at one point in their circumference, and slightly opened, so that when in position they press all round against the walls of the cylinder. After a little use they "settle down to their work"—that is, wear to a true fit in the cylinder. Each end of the cylinder is closed by a cover, one of which has a boss cast on it, pierced by a hole for the piston rod to work through. To prevent the escape of steam the boss is hollowed out true to accommodate a gland, G¹, which is threaded on the rod and screwed up against the boss; the internal space between them being filled with packing. Steam from the boiler enters the steam-chest, and would have access to both sides of the piston simultaneously through the steam-ways, W W, were it not for the

      SLIDE-VALVE,

      a hollow box open at the bottom, and long enough for its edges to cover both steam-ways at once. Between W W is E, the passage for the exhaust steam to escape by. The edges of the slide-valve are perfectly flat, as is the face over which the valve moves, so that no steam may pass under the edges. In our illustration the piston has just begun to move towards the right. Steam enters by the left steam-way, which the valve is just commencing to uncover. As the piston moves, the valve moves in the same direction until the port is fully uncovered, when it begins to move back again; and just before the piston has finished its stroke the steam-way on the right begins to open. The steam-way on the left is now in communication with the exhaust port E, so that the steam that has done its duty is released and pressed from the cylinder by the piston. Reciprocation is this backward and forward motion of the piston: hence the term "reciprocating" engines. The linear motion of the piston rod is converted into rotatory motion by the connecting rod and crank.

       Fig. 22.—Perspective section of cylinder.

      The use of a crank appears to be so obvious a method of producing this conversion that it is interesting to learn that, when James Watt produced his "rotative engine" in 1780 he was unable to use the crank because it had already been patented by one Matthew Wasborough. Watt was not easily daunted, however, and within a twelvemonth had himself patented five other devices for obtaining rotatory motion from a piston rod. Before passing on, it may be mentioned that Watt was the father of the modern—that is, the high-pressure—steam-engine; and that, owing to the imperfection of the existing machinery, the difficulties he had to overcome were enormous. On one occasion he congratulated himself because one of his steam-cylinders was only three-eighths of an inch out of truth in the bore. Nowadays a good firm would reject a cylinder 1⁄500 of an inch out of truth; and in small petrol-engines 1⁄5000 of an inch is sometimes the greatest "limit of error" allowed.

       Fig. 23.—The eccentric and its rod.

      THE ECCENTRIC

      is used to move the slide-valve to and fro over the steam ports (Fig. 23). It consists of three main parts—the sheave, or circular plate S, mounted on the crank shaft; and the two straps which encircle it, and in which it revolves. To one strap is bolted the "big end" of the eccentric rod, which engages at its other end with the valve rod. The straps are semicircular and held together by strong bolts, B B, passing through lugs, or thickenings at the ends of the semicircles. The sheave has a deep groove all round the edges, in which the straps ride. The "eccentricity" or "throw" of an eccentric is the distance between C², the centre of the shaft, and C¹, the centre of the sheave. The throw must equal half of the distance which the slide-valve has to travel over the steam ports. A tapering steel wedge or key, K, sunk half in the eccentric and half in a slot in the shaft, holds the eccentric steady and prevents it slipping. Some eccentric sheaves are made in two parts, bolted together, so that they may be removed easily without dismounting the shaft.

      The eccentric is in principle nothing more than a crank pin so exaggerated as to be larger than the shaft of the crank. Its convenience lies in the fact that it may be mounted at any point on a shaft, whereas a crank can be situated at an end only, if it is not actually a V-shaped bend in the shaft itself—in which case its position is of course permanent.

      SETTING OF THE SLIDE-VALVE AND ECCENTRIC.

The subject of valve-setting is so extensive that a full exposition might weary the reader, even if space permitted its inclusion. But inasmuch as the effectiveness of a reciprocating engine depends largely on the nature and arrangement of the valves, we will glance at some of the more elementary principles.

       Fig. 24.

       Fig. 25.

      In Fig. 24 we see in section the slide-valve, the ports of the cylinder, and part of the piston. To the right are two lines at right angles—the thicker, C, representing the position of the crank; the thinner, E, that of the eccentric. (The position of an eccentric is denoted diagrammatically by a line drawn from the centre of the crank shaft through the centre of the sheave.) The edges of the valve are in this case only broad enough to just cover the ports—that is, they have no lap. The piston is about to commence its stroke towards the left; and the eccentric, which is set at an angle of 90° in advance of the crank, is about to begin opening the left-hand port.

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