PART IV.

THE SLIDE-VALVE.

QUESTION 41. _What are the essential conditions which a slide-valve must fulfill in governing the admission and exhaust of steam to and from the cylinder of an ordinary engine?_

_Answer._ 1. It must admit steam to one end only of the cylinders at one time. 2. It must allow the steam to escape from one end at least as soon as it is admitted to the other end; and 3. it must cover the steam-ports so as not to permit the steam to escape from the steam-chest into the exhaust-port.

[Illustration:

_Fig. 10._

Scale ³⁄₁₆ in. = 1 inch.]

QUESTION 42. _What was the first form of slide-valve used?_

_Answer._ That represented in fig. 10. The smallest movement of this valve either way opens one of the steam-ports for the admission of steam and puts the other in communication with the exhaust-port. By cutting a piece of ordinary writing paper to the form of the section of the valve, and moving it on the line _f f_, the action of the valve will be clearly shown.

[Illustration: _Plate I._

AMERICAN LOCOMOTIVE. By The Grant Locomotive Works, Paterson, New Jersey. _Scale, ³⁄₈ in. = 1 foot._]

QUESTION 43. _How was the admission and escape of the steam effected by this valve?_

_Answer._ In order to explain this clearly, a series of diagrams will be necessary. Before referring to them, however, it should be explained first that the motion of an eccentric is exactly the same as that of a small crank. It is in fact a crank with a crank-pin whose diameter is very much enlarged. In the diagrams, figs. 11 to 25, the eccentrics will therefore be represented as small cranks, and most of the other parts by their centre-lines and points only, so as to make the diagrams as simple as possible. The dimensions selected for these illustrations are for the cylinder 16 in. diameter and 24 in. stroke, and a connecting-rod 7 ft. long. The steam-ports are 1¹⁄₄ in., the exhaust-port 2¹⁄₂ in., and the metal or bars between them, which are called _bridges_, are 1¹⁄₈ in. wide. The eccentric produces a lateral movement of 3 in., which is called its _throw_. In fig. 11 the piston is at the beginning of the backward stroke. The valve is then in the centre of the valve-face, and the eccentric is consequently at half-throw. The slightest movement of the crank in the direction of the dart _N_ will move the eccentric enough to open the front steam-port to the steam and the back one to the exhaust. In fig. 12 the piston is represented as having moved 4 in. of its stroke; the valve has then partly opened the front steam-port, and the other one is open to the exhaust. In fig. 13 the piston has moved 8 in. of its stroke, and the ports are now wide open, the front one to the steam and the back one to the exhaust. In fig. 14 the piston has moved 12 in., or is at half-stroke, and the valve has then moved as far as it will in that direction. In fig. 15 the piston has moved 16 in. and the valve has begun to return. In fig. 16 the piston has moved 20 in., and the valve has nearly closed the front port to the steam and the other to the exhaust. In fig. 17 the forward stroke is completed, and both ports are closed by the valve. Figs. 18, 19, 20, 21, 22 and 23 represent the piston and the valve on the return stroke in the positions corresponding with those described for the backward stroke.

[Illustration:

_Fig. 11._

_Fig. 12._

_Fig. 13._

_Fig. 14._

_Fig. 15._

_Fig. 16._

_Fig. 17._

Scale ¹⁄₄ in. = 1 foot.

_Fig. 24._

_Fig. 23._

_Fig. 22._

_Fig. 21._

_Fig. 20._

_Fig. 19._

_Fig. 18._

Scale ¹⁄₄ in. = 1 foot.]

QUESTION 44. _Is there any other method by which the motion of a valve can be represented by a drawing?_

_Answer._ Yes, by what are called _motion-curves_. It is, however, difficult to explain these clearly, and as they are purely imaginary, it is difficult to understand their nature and purpose. Close attention will therefore be required to the following description:

We will suppose, in the first place, that the line _f f_, fig. 25, represents the valve-face, _c_ and _d_ the steam and _g_ the exhaust-port, drawn to a larger scale than in the preceding figures. We will now draw the valve _H_ in the position represented in fig. 11, where the piston is at the beginning of the stroke. In order to show the valve in the position represented in fig. 12, where the piston has moved 4 in. of the stroke, we will draw a line 4-20, four inches below and parallel to _f f_, and extend the lines, representing the edges of the ports _c_, _d_ and _g_, downward. On the horizontal line 4-20 we will now draw the edges _i_, _r_, _i′_, _r_, of the valve in the same position in relation to the port _c_ that it has in fig. 12. We will then draw another horizontal line, 8-16, eight inches below _f f_, and parallel to it, and on this represent the valve in the position shown in fig. 13. In the same way we will draw lines 12, 16, 20 and 24 in. below _f f_, and draw the valve on each one respectively in the positions shown in figs. 14, 15, 16 and 17. The distance between the lower line 24-0, and _f f_, will then represent the stroke of the piston, or 24 in. If now we begin from the edge _h_ of the valve on the line _f f_, and draw a curve, _h i j k l m n_, through the same edge of the valve, represented on each of the parallel lines below, the curve will indicate the position of the valve in relation to the steam-port _c_ at each point of the stroke. To illustrate this, suppose we draw lines 1-23, 2-22, and 3-21, one inch apart and parallel to _f f_, and between it and 4-20. They will then represent the position of the piston after it has moved 1, 2 and 3 in. from the beginning of the stroke, and where they intersect the curved line will be the position of the edge of the valve when the piston has moved 1, 2 and 3 in. of the stroke. The curved line will in fact represent the position of the valve at any point of the stroke between these lines. Other horizontal lines, 5-19, 6-18, etc., can be drawn to represent every inch of the rest of the stroke. The curve line _h i j k l m n_, or _motion-curve_ as it is called, will then show the exact position of the edge of the valve and of the width of the opening of the steam-port during the whole stroke. From it we see that the valve opens the port _c_ for the admission of steam simultaneously with the movement of the piston, and when the latter has made one inch of its stroke the port _c_ is half open. At 4¹⁄₂ inches of the stroke the port is wide open,[12] and at 19¹⁄₂ inches it begins to be closed, but is not completely closed until the end of the stroke.

[12] By cutting a paper section of the valve and placing it on the diagram in each position named, it will probably help the reader to understand the movement of the valve more and the nature of the motion-curves.

[Illustration:

_Fig. 25._

Scale ³⁄₁₆ in. = 1 inch.]

Similar motion-curves, such as _h′ i′ j′ k′ l′ m′ n′_, (represented in fig. 25) can be drawn to represent the position of the other edges of the valve, and also for the return stroke. The latter are shown in dotted lines. If we follow the curve _h′ i′ j′ k′ l′ m′ n′_, which represents the position of the edge of the valve _h′_ which governs the exhaust from the back end of the cylinder, we see that the port _d_ is opened and closed to the exhaust simultaneously with the opening and closing of the port _c_ for the admission of steam to the front end of the cylinder, and that they both remain open until the completion of the stroke.

The width that the ports are opened by the valve is thus ascertainable from these diagrams, for any point of the stroke, and in fact can be seen at a glance. By the aid of such motion-curves, the movement of slide-valves can therefore be analyzed more perfectly than is possible without them.

QUESTION 45. _What were some of the disadvantages of valves, like that shown in fig. 10, and which are shown by the motion-curves in fig. 25._

_Answer._ The free admission of the steam until the completion of the stroke by the piston was hurtful to the machinery, as it co-operated with the momentum of the piston and its connections in producing undue strains in the working parts. The steam then escaped from the cylinder without expansion, so that much of its useful energy was lost. The steam was not allowed to escape from one end of the cylinder until it was admitted at the opposite end, and as the process of exhausting it occupies some time, there was always more or less back pressure until all the exhaust steam was expelled from the cylinder. In practice, the imperfections of the valve-gear frequently delayed the opening of the ports, both for admitting and exhausting steam, until after the commencement of the stroke of the piston.[13]

[13] D. K. Clark’s “Railway Machinery.”

QUESTION 46. _How may some of these evils be overcome?_

_Answer._ By moving the eccentric forward on the axle so that the motion of the valve is advanced to the same extent, and the admission and exhaust of the steam will occur a little before the completion of the stroke of the piston. In this way the steam is admitted into the cylinder so as to act as a cushion to receive the momentum of the piston, and some time is given to the exhaust steam to escape, before the return stroke.

QUESTION 47. _What is meant by lead?_

_Answer._ By _lead_ is meant the width of the opening of the steam-ports at the beginning of the stroke of the piston. On the steam side of the valve it is called _outside lead_; on the exhaust side _inside lead_. In fig. 26 the opening _h_ of the steam-port is the outside lead and _h′_ the inside lead.

[Illustration:

_Fig. 26._

Scale ³⁄₁₆ in. = 1 inch.]

QUESTION 48. _What is meant by the travel of a valve?_

_Answer._ By the _travel_ we mean the motion of the valve back and forth, or in other words its stroke. If the arms of the rocker are of the same length, the travel of the valve is equal to the throw of the eccentric. For the preceding illustrations we have selected an eccentric with three inches throw, which is the travel of the valve.

[Illustration:

_Fig. 27._

Scale ³⁄₁₆ in. = 1 inch.]

QUESTION 49. _How is the steam made to work expansively with a slide-valve?_

_Answer._ By giving the valve what is called _lap_. That is, by allowing the edges of the valve when it is in the center of the valve-seat to overlap the edges of the steam-ports, as shown in fig. 27. Where this overlap, _L L_, is on the outside of the valve, it is called _outside lap_; when on the inside, _l l_, _inside lap_. When a valve has lap, those portions of the _face_[14] _h_, _i_, and _h′_, _q_, which cover the steam-ports, being wider than the ports, therefore occupy some time in moving over them, during which time the steam is enclosed in the end of the cylinder, as there is then no communication either with the steam-chest or the exhaust-port. This action is shown very clearly by the motion-curves in fig. 26. The valve in this case has ¹⁄₄ inch lead. At 4¹⁄₂ inches of the stroke of the piston the valve has moved as far as it will go in that direction, and the steam-port has its maximum width of opening. From that point the valve will begin to close the steam-port, and at 14¹⁄₂ inches of the stroke the port will be entirely covered, and the steam therefore be _cut off_. The port will remain closed until the piston has moved 21³⁄₄ inches, when it will be observed from the motion-curve _r s t u v w x_, that the port _c_ is opened to the exhaust and the steam escapes, or, as it is technically called, the _release_ takes place. From the time the steam is cut off to the time it is released, it works _expansively_ in the cylinder.

[14] The valve-face is the surface of the valve in contact with the valve-seat.

QUESTION 50. _What relation is there between the amount of lap_[15] _and the degree of expansion?_

[15] In speaking simply of _lap_, outside lap is always meant.

_Answer._ The greater the lap with any given travel, the shorter will be the period of admission of steam, and, consequently, the more time and space for expansion.

QUESTION 51. _What is the effect of inside lap?_

_Answer._ It delays the release of the steam. Thus in fig. 26 the valve has ¹⁄₈ in. inside lap. The motion-curve _r s t u v w x_ shows that the release takes place during the back stroke at 21³⁄₄ in. If now there was no inside lap, the dotted line _y_, _x_ would represent the exhaust edges of the valve, and the release would then occur somewhat earlier, or at 21 in. For this reason no inside lap is usually given to valves for engines which run at high rates of speed, as it allows too little time for the steam to escape. In fact, in some cases, what is called _inside clearance_ is given to the valve; that is, the valve as shown in fig. 29, when it is in the middle of the valve-face, does not entirely cover the steam-ports. The effect of this is just the reverse of that produced by inside lap; that is, it causes the release to occur earlier in the stroke.

[Illustration:

_Fig. 29._

Scale ³⁄₁₆ in. = 1 inch.]

QUESTION 52. _With the same outside lap, what is the effect of changing the travel of the valve?_

_Answer._ By increasing the travel, the period of admission is increased and that for expansion lessened; and by reducing it, the admission is lessened, and the degree of expansion is increased. This is shown by the motion-curves in fig. 28, in which the same valve and ports are represented as are shown in fig. 26, but the valve has a travel of 5 instead of 3 inches. The valve also has the same lead. By following the motion-curve _h i j k l m n_, it will be seen that the steam is thus admitted up to 20¹⁄₂ inches of the stroke of the piston, and the period of expansion, as compared with that in fig. 26, is correspondingly lessened. It will also be seen by comparing fig. 26 with fig. 28 that with the short travel of the valve the ports are not opened so wide as they are when the travel is increased. This evil is practically obviated, however, by making the ports so long that with a comparatively small opening they will still have area sufficient to admit enough steam to fill the cylinders, and it is known that an opening less than the whole area of the steam-ports is sufficient to facilitate the passage of steam into the cylinder.

[Illustration:

_Fig. 28._

Scale ³⁄₁₆ in. = 1 inch.]

QUESTION 53. _How is the exhaust affected by lap and lead?_

_Answer._ The steam is released earlier in the stroke in proportion as the amount of outside lap and lead is increased, but the steam-port is also closed to the exhaust, or _compression_, as it is called, begins earlier with lap and lead than without. Thus, in fig. 25, it will be seen that at the beginning of the stroke both ports are entirely closed; in fig. 26, however, in which the valve has both lap and lead, the port _d_ is nearly wide open at the beginning of the stroke, and by following the motion-curve _r s t u v w x_, which represents the position of the exhaust edge of the valve, it will be seen that the steam was released from the port _c_ before the piston had completed its stroke, or when it had still nearly 3¹⁄₂ inches to move. In fig. 25 the port _c_ is not opened to the exhaust until the commencement of the stroke, but it remains open to its completion, whereas in fig. 26 it is closed, or compression begins, at 18 inches of the return stroke, as shown by the dotted motion-curve.

QUESTION 54. _How does the action of the connecting-rod influence the motion of the valve in relation to the piston?_

_Answer._ By delaying the movement of the crank in the backward stroke of the piston, and accelerating it in the forward stroke. This will be best explained by reference to fig. 14, in which the piston is represented in the center of the cylinder, or the middle of the backward stroke. If now we take a pair of dividers set to a length equal to that of the connecting-rod, and from the center, _f_, describe an arc of a circle, _a b_, from the center of the shaft, and through the lower half of the circle which represents the path of the crank-pin, we will find that the point of intersection, _a_, falls short of the vertical line, _c d_, and that the crank-pin has not made quite one-quarter of a revolution while the piston was moving through the first half of the backward stroke. By referring to fig. 21, in which the piston is again in the middle of its stroke, but is moving forward, and by describing another arc of a circle, _b a_, from the center of the shaft and intersecting the path of the crank-pin, it will be seen that the latter has moved _more_ than a quarter revolution, while the piston has made the first half of the forward stroke. Owing to this _angularity_, as it is called, of the connecting-rod, the crank-pin is behind the piston during its backward stroke and ahead of it during the forward stroke. As the valve is moved by the eccentric, and it in turn by the shaft and crank, any irregularities of the latter are of course communicated to the valve. We therefore find, by referring to fig. 26, that the point of cut-off occurs during the backward stroke at 14¹⁄₂ inches, and during the forward stroke at 12 inches. A similar inequality is observable in the points of release for the front and back strokes. It is not, however, a matter of very great practical importance with stationary engines which run at comparatively slow speeds; but if it is thought desirable, the period of admission and the point of release for both strokes can be equalized, either by giving the valve more lead or lap at one end than the other, or by making the one steam-port wider than the other. The mechanism employed for moving locomotive slide-valves, however, furnishes us with the means of modifying their motion in relation to that of the piston, and of thus equalizing the periods of admission and release for the front and back strokes. The methods of doing this will be more fully explained hereafter.