THE BURR PRINTING HOUSE, NEW YORK, U. S. A.

This work is based upon notes and diagrams which were prepared by the writer with no more ambitious object, originally, than to help his railway students toward the obtainment of clear general notions upon the important subject of the slide valve.

For practical demonstration of the functions and operation of the slide valve, a large sectional model engine, with provision for variation of the operation of its parts, is very valuable; but it seems to the writer to be desirable that each student should have in his own possession a model to cogitate upon, and that his model should give him, graphically, results which should be not merely qualitative, but also, in some degree, quantitative, to enable him to institute comparisons between the actions of different valves operated under varying conditions. With this end in view, the writer conceived the idea of using on a base-board a rotary disc to represent a crank-shaft, together with the idea of obtaining concentric circular diagrams of results (see fig. 21, for instance) by using a crank-arm marked on the disc as an index-finger, and recording on the base-board the beginnings and ends of the arcs swept through by the crank in the various distribution-periods. Following upon this, it was thought that the eccentric and its rod could be eliminated by bringing the disc under the valve and fixing upon the valve a rigid slotted arm to extend down across the face of the disc and engage a pin on the latter, so that rotation of the disc would cause reciprocation of the slotted arm and the valve. This promised, however, to be unmechanical and expensive, and was discarded in favor of the model described in Chapter I., which comprises the simple method of correlating (as described in Chapter II.) the movements of valve and crank-shaft by scales adjacent to each, so that the reading given by the valve on the valve-scale at any position of the valve would show what position on the eccentric-scale the eccentric ought correspondingly to occupy. The fact that this involved a step-by-step adjustment of the valve and shaft alternately was found in practice to be advantageous rather than the reverse.

As to the diagrams and sketches, the writer has sought, in some of them, to clear up graphically some of the incidental minor difficulties related to the subject; see, for instance, figs. 12 and 13, illustrating “Order of Cranks,” fig. 22 on “Width of Port”; also compare figs. 23, 24, and 25, “Double-ported Valve,” and see fig. 37, which last is in effect a graphical summary of results from nearly all the different examples which precede it.

Fig. 36, which concerns the reversal of an engine in motion under steam, will probably have a special interest to students of the locomotive.

There is no more important part of the steam-engine than the valve—the part which determines when and for how long the steam shall be admitted to the cylinder, how long it shall stay there and how long it shall be allowed for leaving. And the principles which should govern the construction of the valve of an engine in order that the steam shall be properly admitted and released are the same for American, English, French and German engines. The principles of their construction and action are the same, but the details vary. The action of every steam-engine depends upon its valve, and hence it is important that every engineer of whatever country should be thoroughly conversant with the principles of the construction of valves of different types; should know the meaning of “lap,” “lead,” “advance,” etc.; and should know how a change of one of these will affect the others and the admission and release of the steam. One of the best ways to learn all about valves and valve motions is to study them with the aid of sectional models, made so that the inside as well as the outside of the parts may be seen, but such models are difficult to obtain except at considerable cost. Diagrams, such as are so freely used in this work, rank next to models in utility and as a means of enabling one to thoroughly understand the workings of the different parts of machines. The use of diagrams requires no knowledge of mathematics, and enables explanations to be presented in a clear, concise manner. From an educational point of view, diagrams possess an advantage which models do not, as they make one think more. Their use gets one in the habit of forming mental pictures of the parts of the machines discussed, and thus enables one to more readily and quickly think out the effect of a change in one part on other parts. And this ability to picture in the mind the various relations of the parts to one another is absolutely necessary in order that these relations may be thoroughly understood. The man who is accustomed to work out his problems from diagrams and drawings, reasons from cause to effect and from effect to cause; while the man who must have a model to work with, works on the “cut-and-try” method.

This little book is confined strictly to an explanation of the principles which underlie the action of the different types of slide valves. The plain, simple D-valve, as it is called in this country, without lap or lead, is first taken up and discussed; and gradually lap and lead are introduced, and the effect of each upon the admission, the cut-off, the release, and the compression is fully worked out and shown. Then other types of slide valves are taken up and discussed. The advance is made step by step, from the most simple form of slide valve to the more complicated forms used for attaining certain results not so easily attained by the use of the simple forms.

The form of multi-ported valve shown in fig. 25 is so much like the Giddings valve used on the Russell and some other single-valve engines, that any one reading the explanation given here will have no trouble in understanding the Giddings valve (shown in fig. 26).

The multiple admission valve is used to such an extent in America that it is but proper that a few words should be said of it, and that the most common form of multiple admission valves should be discussed. And hence the Straight Line and the Woodbury valves are shown in figs. 27 and 28.

There are so many engines in this country in the front rank of automatic, high-speed engines which use piston-valves, that a treatise on slide valves would be incomplete without some mention of this type of slide valve. The form of piston-valve shown in fig. 30, in which the steam is admitted at the middle and exhausted at the ends, is well known to all engineers who have used an Ide or an Ideal engine. The valve used on these engines is shown in fig. 31.

Many American engineers prefer engines on which the cut-off may be changed without in any way affecting the lead, release, or compression, and, therefore, the description of the Meyer valve will be read with interest. Many will recognize at once in the valve used on the Watertown engine the Meyer valve in almost its original form, and in the valve of the Buckeye engine (shown in section in fig. 39), the Meyer valve in a modified form.

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]

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 (S₁S₂) at the upper ends of the two outer passages (P₁P₂) afford a communication between the valve-chest (VC) and the opposite ends (C₁C₂) 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.

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.

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.

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.

The necessary travel of the valve is given to it by means of an eccentric, which is keyed upon or formed as part of the crank-shaft. The eccentric is, in effect, a crank, whose throw equals the amount of eccentricity of the sheave. This may not be obvious; let us investigate a little. Suppose that we have a big crank-shaft, and want to put a little crank in the middle of it (for this is what happens in the case of a steam-engine, the travel of the valve being such that a crank of the usual form would be disproportionate to the shaft of which it formed part—like fig. 7, perhaps). Now, that such a shaft would be extremely weak at the pin goes without saying. Imagine, that to lessen the weakness the crank-pin of this little crank is made of greater diameter, as in fig. 8, or even more so, as in fig. 9; still we get the same result, which is that the throw of the crank remains the same, and equals the distance of the centre of the crank-pin from the centre of the crank-shaft—i.e., the amount that this crank-pin is “ex-centric,” out of centre, and the eccentric sheave being simply an exaggerated crank-pin we get back to our original statement that it is virtually a crank whose throw equals the half-travel of the valve, which in turn equals the amount of eccentricity of the sheave.

Being fixed to the crank-shaft and moving with it, always in a fixed relationship to the main crank, the eccentric operates the valve in the necessary accordance with the movement of the crank and piston, as will presently be explained.

Returning now to fig. 5, it must be understood that a valve of the type exemplified therein must be in the middle of its travel whenever the piston is at either end of its stroke. The reason for this must be clearly appreciated for the sake of what follows hereafter; it may be arrived at very easily with the aid of fig. 1. Put a disc of card in the circle provided in that figure, and let it turn about a drawing-pin stuck through its centre; this disc is the equivalent of a crank-shaft. A crank-arm (C) may be permanently marked upon it in ink, as in fig. 10; another arm, which may be marked upon it in pencil, will serve to represent a “valve-crank,” the equivalent of the eccentric by which the valve is to be operated.

Actual connection between that valve-crank and the valve being dispensed with, some means for making the movement of the valve correspond accurately with that of its crank must be provided. This provision is made by the numbered scales^[2] in fig. 1; the circular scale within which the card disc revolves is numbered in correspondence with scales on the ports, and to these latter an arrow on the strip of paper which carries the valve is adjusted (see also fig. 10). To find the position in which to mark the arrow on the valve, put the latter in the centre of its travel and mark the arrow-head just above the “0” on the No. 1 scale. When the disc is turned, the end of the eccentric-arm or valve-crank travels with it around the circular scale. When the eccentric-arm is moved step by step around the circle to the various numbered graduations, the valve must be moved step by step to bring its arrow-head to the corresponding numbers on the horizontal scale on the ports. Thus the eccentric and valve, although not moved simultaneously, as they would be if a rod connected them, are kept in accurate relationship whilst moved independently. If the reader will now read the preface, which he has probably skipped, he will the more readily appreciate the principle of this arrangement.

Further, taking the main crank-arm (C) as an index-finger, the position of its end at the points of cut-off, release, etc., with different valves and eccentrics, may be marked in pencil on the circular scale, or better, on a piece of tracing-paper interposed between the disc and that scale; by this means the student will obtain circular diagrams enabling him to compare the results due to different settings and proportions of gear, and he will also discover in what manner those results are affected by individual elements of the mechanism in any given example.

Assume a horizontal engine, with the main crank moved into a horizontal position so that the piston would be at, say, the left-hand end of its stroke. Assume also that the engine is at rest, and put the valve in the centre of its travel (as in fig. 5), when its faces exactly cover the steam-ports and its cavity covers the bridges and the exhaust-port; the first movement of the valve is wanted to be to the right, in order to admit steam to the left-hand end of the cylinder to move the piston also to the right, no matter whether the crank-shaft is to have negative or positive rotation. Now, a valve of this pattern evidently will not admit steam to the cylinder to start the movement of the piston (supposing that the engine has not yet been started), which will therefore have to be helped by some external agency; admitting this as a point which shall be touched again presently, place the crank next, so that the piston would be at the opposite end of its stroke; the valve, which had to move to the right as explained, must have come back into the position whence it started, and be in readiness to move equally to the left, afterwards returning once more to the central position by the time that the piston gets back to the left-hand end of the cylinder. Hence we see that, whenever the piston is at either end of its stroke, this valve must be in the middle of its travel.

If this be granted, we may proceed further:—Having the piston at the right-hand end of its stroke, for instance, draw a line on the disc at right angles with the crank (see fig. 10); this will represent two “eccentric-arms” of the necessary throw if each arm equals in length the half-travel of the valve; either of them at present suits the position of the valve, which is in the middle of its travel. Choose, now, the direction in which your crank-shaft shall rotate; let us suppose, for example, that it shall have “right-handed” rotation, and place an arrow on the disc to indicate the direction of rotation. Under these conditions the upper eccentric will follow the crank round, operating the valve to close the right-hand port and open the left-hand one, admitting steam to stop the engine. Cross this upper arm out, then, for manifestly we have to use the lower eccentric, which, going ahead of the crank, will cause the valve to open the right-hand port when required. But now conversely, suppose that the engine is to be run in the opposite direction; the upper eccentric (represented by the crossed-out line) now goes ahead of the crank, and gives the valve motion in the right direction, consequently the lower one must now go out of use, for under the altered conditions of working it tends to move the valve in the wrong direction. The deduction from this reasoning is, that in whichever direction an engine runs, the eccentric used for the time being must be set in advance of the crank, and that advance must be at least 90°.

If it were less than 90° the wrong port would be opened—the port at the opposite end of the cylinder to that at which the piston might happen to be. This effect may be shown by purposely giving the eccentric less advance.

The amount which the angle between the eccentric and the crank exceeds 90° is the “angle of advance”; and the distance which the valve is moved from midposition when the piston is at the end of the stroke is the “linear advance.”

Because of the necessity for different settings of the eccentric for different directions of rotation of the crank, engines required to be reversible usually have either:

(1) Means for shifting the position of a single eccentric upon the shaft, that it may always be placed so as to lead the crank. Or

(2) Two eccentrics fixed upon the crank-shaft so that one of them is always ahead of the crank in whichever direction it rotates, mechanism being arranged in connection with the eccentrics, so that the valve can be driven by the leading eccentric, or operated by the conjoint action of both eccentrics. Or

(3) Radial or other special valve gears.

The simple form of slide valve shown in fig. 10 does not permit of the economical use of steam, inasmuch as it allows steam at full pressure to follow the piston for the whole of the stroke, and does not admit of the use of its expansive properties, for the simple reason that, at the instant the admission of steam ceases, the exhaust of the same body of steam must immediately commence, as may be clearly seen from the circular diagram.

It is desirable at this point to clear the ground by touching for a moment upon one or two elementary matters which, if not now explained, might cause confusion subsequently.

Dead Centres.—A crank is said to be “on the centre” or “on the dead centre” when the connecting-rod and crank are in line, and this occurs twice in every revolution, as shown in fig. 11.

Right-hand Crank to Lead.—The phrases “right-hand crank to lead,” or “left-hand crank to lead,” are sometimes used, and are confusing to beginners; the “leading” crank is that one which leads when the engine is running ahead. In most two-crank engines one of the cranks will be a quarter of a revolution (or less than half a revolution, where the cranks are not set at 90 degs.) in advance of its neighbour, and will, therefore, lead it in the direction in which it should go. Of course, that neighbour might be said to lead the other by being three-quarters of a revolution in advance (or more than half a revolution, where the cranks are not set at 90 degs.), and this is where a little confusion sometimes arises; the leading crank must always be taken as that one which is less than half a revolution in advance of its fellow when the engine is running ahead, and in locomotives and similar engines is “left-hand” or “right-hand,” according as it lies to the left or right of a spectator looking from behind the crank-shaft towards the cylinders. (See fig. 12, which shows two arrangements of the driving cranks of an English locomotive. The cylinders are supposed to be to the right of the wheels.)

The accompanying diagram (fig. 13) will perhaps serve to make clear without further explanation the meaning of the expression, used with reference to three-crank marine engines, of “order of cranks, high, intermediate, low,” or “order of cranks, high, low, intermediate,” as the case may be.

Let us revert here to one of the points previously noted, which was that the simple valve, when we had it in connection with an eccentric set 90 degs. in advance of the crank, would not admit steam to the cylinder at the exact commencement of the stroke, i.e., when the crank is on either of the “dead centres.”

Keeping this fact in mind, we will consider the matters of

Cushioning and Lead.—It has long been found desirable (especially in quick-running engines) that the motion of the returning piston should be opposed as it arrives at either end of its stroke, so that the moving weights of piston, piston-rod, and connecting-rod may be “cushioned,” to prevent injurious stresses and to conduce to easy running. To use a homely illustration, one might say that just in the same way is it desirable, when striking out from the shoulder, to have one’s adversary within range, that he may provide the necessary “cushioning,” and so prevent stresses in one’s arm and possible dislocations.

This “cushioning” is provided in part by checking the exit of the exhaust steam, as will be shown later, and in part by setting the eccentric a little more than 90 degs. (fig. 14) in advance of the crank, so that the valve commences to open the port to admit steam in front of the returning piston before it arrives at the end of its stroke, continuing to open the port as the piston comes to a rest, and is started on the return stroke.

The amount to which the port is found to be open when the piston is at the end of its stroke is called the “lead” of the valve, and it would be expressed as “one-eighth of an inch lead,” or “three-sixteenths of an inch lead.” etc., as the case might be. To set this valve for lead, keep the piston at the end of its stroke, and set the eccentric back until the valve is opened to the desired lead; then the eccentric would be fixed in its newly-found position, as shown in fig. 14. The radius of the “eccentric-arm,” which is the lighter of the two radial lines on the disc in fig. 14, is equal in length to one-half of the travel of the valve, and therefore does not reach the edge of the disc, at which, however, there is a short guiding-mark in the line of the eccentric-arm “produced.” By using this guiding-mark and moving the arrow-head of the valve over the scale on the ports, in correspondence with the movement of the guiding-mark on the edge of the disc within the circular scale, the valve, the main crank (C), and the eccentric can be operated in concord, very much as if they were mechanically connected.

The explanation just given of the meaning of lead, and of its effect, should be borne in mind, as we shall presently have to consider it in conjunction with other matters, for, as has been said, there is something more than “lead” employed in the production of cushioning.

Expansion.—In order that the expansive properties of steam may be utilised, it must be cut off before the piston arrives at the end of its stroke, and on being cut off must not be allowed to commence exhausting immediately, but must be confined in the cylinder, in order that by expanding it may force the piston further before it. The old valve, as we have seen it, did not allow this, and to effect it the faces of the valve are lengthened, so that after the steam is cut off by the edge (a) of the valve (fig. 15) the steam is confined, and expands in the cylinder until the edge (b) arrives at c, when exhaust immediately commences. Now put the piston at the end of its stroke, and put the new valve in a central position (fig. 16). The distance that the valve, when in this central position, overlaps the outer edge of the steam-port, is called the “outside or steam lap,” or more commonly the “lap” of the valve, that is, the distance d a (fig. 16).