CHAPTER I.
THE SIMPLE SLIDE.
The slide valve has always been regarded by the writer as the _pons asinorum_ of the student of the steam-engine. His own early attempts at crossing that bridge were greatly facilitated by the simple little device to be presently submitted to the reader; with the aid of this expedient the student may easily obtain clear notions upon all the functions and operations of the slide valve, at a nominal cost, and with but a small expenditure of time and trouble.
It is necessary that at the outset the nature of the construction above alluded to should be clearly appreciated, for the reason that many of the explanations given in the succeeding pages are referred thereto.
In passing, it may be remarked that much of the difficulty which the ordinary student experiences in the study of valves and valve gear arises because he approaches the subject with a false impression that it is of necessity a difficult one; this idea is generated on the one hand in the atmosphere of mystery which in the shop is made to surround the matter, and on the other hand by the somewhat formidable aspect of most of the geometrical treatises on the subject. If the beginner can bring himself to believe that the slide valve is nothing more complex than a sliding shutter with a cavity in its face, travelling backwards and forwards over three ports, the outermost of which are alternately opened to steam and placed in communication with the central exhaust-port, he will have made a satisfactory commencement.
Take a piece of stout white cardboard and upon it set out an enlarged copy of fig. 1.[1]
[Illustration: FIG. 1.—Explanatory Diagram.]
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Footnote 1:
See also fig. 41.
The upper portion of this diagram represents a section taken at right angles to the port-face through a set of steam and exhaust-ports as ordinarily arranged; the graduated ring beneath the section will receive consideration later.
Postponing for the moment the further investigation of fig. 1, reference should here be made to figs. 2, 3, and 4.
Fig. 2 shows, in perspective, a cylinder and valve-chest with parts removed to make clear the manner in which the two steam-ports (S_{1}S_{2}) at the upper ends of the two outer passages (P_{1}P_{2}) afford a communication between the valve-chest (VC) and the opposite ends (C_{1}C_{2}) of the cylinder (C), and to show the passage from the central exhaust-port (EP) to the outlet (O) at which the exhaust steam is discharged. The relationship between this view and the upper part of fig. 1 will be obvious upon a comparison of the two.
[Illustration: FIG. 2.—Perspective Section of Steam-Engine Cylinder.]
A typical slide valve is shown in section, in its place above the ports in fig. 2, and separately in figs. 3 and 4. From these latter, the large ratio of the width to the length of the valve is apparent, and it will be seen from fig. 2 that the ports are correspondingly made wide and short. This arrangement enables a small linear movement of the valve to effect the uncovering of a large area of port, and to secure free passage for the steam with but a small amount of valve-travel, and so to reduce to a minimum the work unavoidably wasted in overcoming friction between the port-face and the valve heavily loaded by the pressure of steam upon its back.
[Illustration: FIGS. 3 and 4.—Separate Views of the Slide Valve.]
Let us return now to fig. 1. Draw, near the edge of a strip of moderately stiff paper, the section of the elementary form of slide valve illustrated by fig. 5, taking its dimensions from the ports of fig. 1, so that the two views agree in the manner indicated. By sliding this diagram backwards and forwards across the ports of fig. 1, the following explanation of the action of this, the simplest form of slide valve, will be readily appreciated.
[Illustration: FIG. 5.—Section of Slide Valve.]
This valve has first to open one port to steam, and when the steam thus admitted has forced the piston towards the opposite end of the cylinder, to put the same port in communication with the exhaust, while opening the other port to the steam which effects the return movement of the piston. The duration of the admission of steam is the same as that of the exhaust, the admission, owing to the dimensions of the valve in relation to the ports, necessarily taking place on one side of the piston simultaneously with the occurrence of exhaust from the other side (see fig. 6). The travel of this rudimentary valve equals twice the amount to which the port is opened to steam. It happens to open each port fully in this case (although this is rarely the case with modern slide valves), so that the “travel” equals twice the width of steam-port: _twice_, because (starting with the valve in its central position, as shown in fig. 5) it has to move to the right, say, by an amount equal to the _width_ of _the left-hand_ port, in order to open that port fully to steam, and then it moves back again and travels to the other side of the central position by an equal amount, in order to open the equally dimensioned right-hand port to steam. In fig. 6 the valve is shown at the two extremes of its travel, in full lines at one end, and in dotted lines at the other end.
[Illustration: FIG. 6.—Section showing Position of Slide Valve in relation to Steam and Exhaust-Ports.]
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