CHAPTER XVI.

SLIDE-VALVES AND FITTINGS.

Slide-jacket. The steam, after passing the regulating valve, enters the slide-jacket or casing, which is simply a rectangular or cylindrical box bolted to the cylinder, in which the slide-valve works. This slide casing is either cast in one with the cylinder, or bolted to it. Slide-valve. The distribution of steam in each of the steam cylinders of an engine, involving the processes of admission, expansion, and finally exhaust into the receiver or the condenser as the case may be,

CYLINDER

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pipes of the succeeding cylinder is now effected by the agency of a single valve, called the slidevalve.' The slide-valve is one of the most important parts of the engine, and on the skill and care exercised in its design and fitting, the satisfactory working of the machinery will greatly depend.

A section of an ordinary single-ported slide-valve is shown in Fig. 131.

In the cylinder face there are three passages, called 'ports,' marked respectively A, B, and c. A and B are the steam passages or ports, one leading to each end of the cylinder, and c is the exhaust port leading to the condenser. s is the slide-valve, which is rectangular in plan, and the hollow space, D, in its centre is called the exhaust cavity of the valve. The valve is shown in its central position, and it will be seen that it not only closes the steam ports, but overlaps the edges for some distance on the steam side. The object of this will be explained later.

FIG. 131.

The slide-valve has a flat face, and it works steam tight on the corresponding flat face of the cylinder. The casing around it is supplied with steam, while the exhaust cavity is connected either to the condenser, the reservoir, or the atmosphere, depending on the type of engine.

Action of the slide-valve.-We will examine first the motion of such a valve and the distribution of steam in the cylinder during one revolution of the engine, noting the movements of the piston at the same time.

We will assume the piston to be just commencing its stroke, it being always arranged that the slide-valve shall then have uncovered the steam port by a certain distance called the 'lead,' this condition being represented in Fig. 132. Steam enters through the port A and pushes the piston in the direction of the arrow, the valve moving also in the same direction. The other port, B, is open on the inside edge, and exhausts the steam on the side E of the piston to the exhaust pipe shown in the exhaust cavity c. When the piston and valve have travelled a certain distance in the direction indicated, the valve reaches the end of its travel and is for an instant at rest, the piston, however, continuing to move in the same direction as shown in Fig. 133. The exhaust port, B, is now wide open, but the valve and

ports are generally arranged so that the ports are never wide open to the steam, so that A is not wide open. The reason for this will be explained later.

This

The valve now commences its return stroke, the piston and valve travelling in opposite directions, the next important phase being shown in Fig. 134, when the admission of steam to the cylinder is stopped by the steam edge of the valve closing the port a. is called the instant of cut-off,' and the remainder of the piston's motion in the direction of the arrow is caused by the expansion of the steam previously admitted. It will be noticed that the port B is still open to exhaust. The piston and valve now proceed still further in opposite directions until the piston has travelled nearly the whole of its stroke and the valve reaches the middle of its travel, as

in Fig. 135. In this position the two inner or exhaust edges coincide with the inner edges of the port.

Two important operations now occur. On the side E of the piston the steam or vapour which had previously been passing out through the port в into the exhaust pipe is now confined by the closing of the port, and as the piston proceeds further in the same direction the steam still remaining in the end E of the cylinder is compressed, and its pressure will gradually increase as the piston gets nearer the end

of its stroke. This is called the instant of 'compression.' Its effect is to provide an elastic cushion of steam to absorb the momentum of the piston and parts attached, and bring them to rest gently before the opposite stroke is commenced, thus avoiding shocks, and assisting the entering steam to start the piston on its return stroke. It has also an important effect as regards efficiency of the steam, as by its means the clearance spaces are filled with compressed steam, and a smaller quantity of steam from the steam pipe is thus required for each stroke. See Chapter XXVI.

On the other side, F, of the piston another important operation also occurs, for the valve is still travelling in the direction of the arrow, and the inner edge of the valve now commences to open the port A to the exhaust pipe, and the steam which has previously been driving the piston forward by its expansive force now rushes off to the condenser, and the pressure on the side F is suddenly reduced. This is called the instant of 'release.' It will be noted that with the valve as shown, having both its exhaust edges exactly corresponding to the exhaust edges of the cylinder ports at the same time, the operations of 'exhaust' on one side of the piston and 'compression' on the other occur at the same instant. If, as is often the case, these edges do not correspond exactly, the exhaust and compression will occur at different instants.

As the piston travels still further towards the end of its stroke, the valve proceeds in the direction of the arrow, and rapidly uncovers the port a to the exhaust pipe, and also the compression on the side E proceeds till just before the piston reaches the end of its stroke, when the steam edge of the valve reaches the edge of the port B and commences to admit steam. This is termed the instant of admission' (Fig. 136). The pressure on the side E then rises to the full steam pressure, and the small remaining part of the piston's stroke is completed against this steam pressure, which continues the action of the compressed steam in bringing the piston gradually to rest prior to the commencement of the return stroke.

When the stroke of the piston is completed, as in Fig. 137, the valve is again open to steam by an amount equal to the 'lead,' and the operations described above are repeated on the opposite side of the piston, while the latter performs the return stroke, until the piston and valve are again in the same position as in Fig. 132. The steam is thus

admitted to, and exhausted from, the opposite ends of the cylinder, and a motion of the piston to and fro in the cylinder caused, and this reciprocating motion of the piston is communicated to the crank-shaft of the engine by the mechanism described in Chapter XXI, the shaft being thus continuously rotated so long as steam is supplied to the cylinder. The effect of arranging the slide-valve with 'lead' is to considerably increase the opening to steam when the piston is commencing its stroke, so as to assist in avoiding any considerable fall of pressure due to contraction of the steam inlet at this period. It also, as explained above, allows steam to be admitted just prior to the completion of the stroke, and thus helps in bringing the piston gradually to rest and avoiding shocks at the end of the stroke.

It should be clearly noticed that the points of admission and cut-off are determined by the steam edge of the valve, and those of release and compression, by the exhaust edge.

In considering the motion of the slide-valve the student will find it a very instructive exercise to draw to scale a section of the cylinder ports as shown in the diagrams, and make a cardboard model of the section of the slide-valve, so that it may be worked over the cylinder ports as desired.

FIG. 138.

Motion of the slide valve, eccentric, and eccentric rod.-The motion of the slide-valve to and fro, causing the reciprocating motion of the piston, is generally produced by means of an eccentric and rod, sketches of a small example of which are given in Fig. 138. A circular castiron sheave, E, has bored in it, eccentrically with its own circumference, a hole of the same diameter as the crank-shaft. This eccentric sheave is keyed firmly on the shaft, so as to revolve with it. The centre of the eccentric is indicated at D, while A is the centre of the shaft, which remains fixed, and about which centre the shaft, carrying with it the eccentric, rotates. On the circumference of the eccentric there works a ring, s, called the eccentric strap, to which the eccentric rod, R, is attached. The end, B, of the eccentric rod is connected by a joint to the slide-valve rod. In large marine engines this connection is not made direct, but through the agency of a 'link,' as explained in Chapter XVII.

When the shaft revolves, carrying the sheave with it, as the end B of the eccentric rod is prevented from moving except along the line AF, the sheave must slide in the strap and sway the latter to and fro, thus producing a reciprocating motion in the end B, of the eccentric rod, and consequently in the slide-valve itself, to which it is connected.

The extent of the travel of the end B of the rod along the line AF is evidently equal to twice A D; i.e. twice the distance between the centres of the crank-shaft and eccentric sheave. This distance A D is called the 'eccentric radius' or 'eccentric arm,' or 'throw of eccentric.

Eccentric and rod equivalent to crank and connecting rod.This motion is evidently the same as that outlined in Fig. 139, where the eccentric sheave and shaft are replaced by a solid plate revolving about the point c. The eccentric rod E D is evidently always normal to