The slide-valve and its functions, with special reference to modern practice in the United States

Preface

The Slide- Valve has been called the heart of the steam engine, and the simile is not badly chosen ; for the valve is a distinct and vital part of the engine, controHing and regulating the circulation of its life-fluid in a manner not entirely unlike that of a living heart. It is the office of the slide-valve to direct the motion and action of this subtile and expensive fluid to best advantage and without waste, so as to make the engine an effective and economical motor.

As the valve must be designed so as to effect an economical steam distribution in the cylinder, it has attained a peculiar significance in scientific steam engine construction; but durability and permanency of form are requirements not less imperative, and they involve constructive problems of a different order. In recognition of this fact an attempt has been made in this work to treat the subject with due regard to the various requirements of modern practice. The fundamental principles are fully explained, and are illustrated by new graphical methods, and a number of special valve constructions are described and analyzed, in order to exhibit in a comprehensive manner how the exacting conditions of higher steam pressure and higher speed have been met by modern engine-builders.

An endeavor has been made to present the subject-matter of this book in a condensed form, as being best adapted to the requirements of practical men, and the author has in this respect followed the suggestions of an extensive personal experience.

IV PREFA CE.

The information is presented in more or less explanatory form, which is necessary in order to preserve its scope and general character by the absence of a multiplicity of details ; but lengthy explanations are studiously avoided, for it is the author's opinion that practical knowledge — which is the more complete knowledge — cannot be imparted by the use of many words. Nothing is fully comprehended before the learner can follow the thoughts of the teacher, but he must be allowed to learn this in his own way, or by his own efforts, if the acquired knowledge 'is to be of any actual use. All the book really can accomplish is to start the reader thinking in the right direction, which often may be done by a few carefully selected words.

Both verbal and graphical demonstrations are used, that one form may supplement the other. The verbal treatment is the broader, but the graphical representation is indispensable as an illustrative and explanatory supplement, and as far as the valve motion is concerned, it is the only method practiced by those who build engines, or make the drawings from which they are built. Further, the graphical representation has the advantage that it presents a number of associated facts in one frame, as it were ; and it may, therefore, eventually lead to those broad conceptions which are of so great practical utility, and which the best verbal exposition sometimes may fail to disclose.

The valve-diagrams presented in this book have been used for the; last ten years, on many different occasions, and they have given more general satisfaction than others which are better known ; and the reader will doubtless share this opinion, if the methods here used are accorded a fair trial.

Comparatively few letters of reference are used in the text, and it is believed that this will make it easier to follow the demonstrations.

J. BEGTRUP.

Chapter I. The Common Slide-valve

Page

Introductory Remarks i

The Common D-Valve . . ; 2

Simplest Form of Slide-Valve 4

The Valve-Motion in Relation to the Piston-Motion .... 9

Valves Without Lap ii

The Primitive Valve-Diagram 12

Sweet's Valve-Diagram 14

Variable Valve Travel 15

Limitations of the Combination Valvk 16

Steam-Ports 18

Port-Opening 19

Valve Dimensions , . . . 20

Valve Diagram for Shifting-Eccentric Engines 21

Variation in Port-Opening 24

Notes About Lead 25

Compression 27

The Motion of Locomotive Valves ....■%.,. 28

Multiporting 31

Setting the Engine on Dead Centers 32

CHAPTER n.

improved slide-valves.

The Double-Ported Marine Slide-Valve 34

The Allen Locomotive Valve 35

The Straight-Line Balanced Valve 36

Variations of the Straight-Line Valve 38

The McEvven Valve 39

The Ball Telescopic Valve 41

Balancing a Common D-Valve 42

The Richardson Balanced Valve 44

v

VI TABLE OF CONTENTS.

Page

The Thomas Balance 44

Piston-Valves 46

Valves of the " Ideal Engine " 47

The Westinghouse Standard Valve 49

Valves on Compound Engines 50

The Westinghouse Compound Valve 53

The Vauclain Valve 54

The Allfree Valve-Gear 57

Chapter III

foir-valve systems.

Introductory Remarks 61

Corliss Valves 62

JLlMITATIONS of THE CoRLISS GeAR 65

The Single-Eccentric Valve Dl\gram 67

Two Eccentrics 69

Corliss Valve Dimensions 71

Directions for Setting the Valve-Gear 72

Valves of the Porter-Allen Engine 72

Gridiron Valves 75

The Hill Valves 76

The Wheelock Valves 78

CHAPTER IV. independent cut-off.

Introductory Remarks 80

The Cut-Off Valve on a Stationary Valve-Seat 81

The Cut-Off Valve on the Back of the Main Valve .... 83

Fundamental Principles 83

The Meyer Cut-Off 85

The Meyer Cut-Off Diagram 86

Limitations of the Meyer Cut-Off 89

The Rider Cut-Off Valve 89

Cut-Off Varied by Rotating Eccentric on Shaft 90

Gridiron Valves With Independent Cut-Off 96

Begtrup's Eccentric 97

Cut-Off Valve With Constant Travel on Main Valve .... 99

The Buckeye Valve-Gear 102

Piston-Valves With Independent Cut-Off 105

Cut-Off Valve Worked by Means of a Link 106

Independent Cut-Off on Four- Valve Engines 108

TABLE OF CONTENTS. vii

Page

The McIntosh & Seymour Valves io8

Valves of the Russell Engines lio

The Buckeye Vibrating Cut-Off iii

CHAPTER V. THE slide-valve ON PUMPS

Blake 117

Dean Brothers 119

Knowles 121

Davidson 121

Cameron 123

Worthington •. . . . 124

Chapter VI

angularity of connecting-rod and eccentric-rod.

The Angular Motion of the Connecting-Rod 128

The Angular Motion of the Eccentric-Rod 132

Steam- and Exhaust-Laps 134

Equalizing both Lead and Cut-Off 134

The Inclined Rocker for Shifting-Eccentric Engines .... 136

Equalizing the Lead on Shifting-Eccentric Emgines . . . . 137

Unequal Laps on Variable Cut-Off Engines 139

THE SLIDE-VALVE AND ITS FUNCTIONS.

Chapter I. The Common Slide-valve

INTRODUCTORY REMARKS.

The term << slide-valve ** is applied to flat-faced valves having a continuous reciprocating sliding motion, whereby steam is alternately admitted to and exhausted from a cylinder ; and this term will also apply to similar parts of gas, air, and water engines and pumps. The steam engine slide-valve and some typical pump valves will be considered here.

The sliding surfaces may be cylindrical, and this does not affect the principles of the valve motion ; but in such case different names are used, as piston-valve, Corliss-valve, semi-rotary or oscillating valve, etc. These valves are only modifications of the original slide-valve, and they will be included as such here.

Slide-valves, of whatever design, can easily be referred to and compared with the common D-valve, their functional properties, in all cases, being closely related. But the D-valve — so called because in section slightly similar to the letter D — is not the simplest slide-valve, per se ; and in order to fully understand the principles and master the analysis of the subject, it is advisable to commence with a careful study of the simplest elementary form.

By thus taking up the elements of the subject, or going to the bottom of it, as it were, the consequent study of more or less elaborate forms and combinations will be greatly facilitated ; and such a course may lead to broad and independent views, so essential to successful and progressive engineering.

The diagrams presented in this chapter are not supposed to represent actual practice. They are only here to illustrate and explain the text, and to give graphical directions for laying out the essential parts of valve and gear, and no attempt is made to show details. Some of the sketches are purposely out of proportion, for otherwise almost unlimited space would be required, or else clearness and distinctness of the principal parts would be sacrificed ; and it can hardly be considered a serious defect, for some imagination is a prerequisite in constructive engineering, for which there is no substitute.

THE COMMON D-VALVE.

Fig. I shows the valve in its central position and part of the cylinder. By moving the valve to the right steam is

exhausted from the right hand side of the cylinder and admitted to the left hand side ; and by moving it to the left steam is exhausted from the left hand side, and admitted to the right hand side. Steam is admitted past the outer edges ^^* '• of the valve, and the exhaust

escapes past the inner edges into the exhaust-cavity, and from thence to the exhaust-pipe (not shown).

The flat surface on which the valve slides is called the valve face of the cylinder or the valve-seat, and the corresponding surface of the valve is the face of the valve. The orifices of steam -passages, covered by the face of the valve, are the steam-ports, and the central opening in the cylinder casting is called the exhaust-port.

The conventional use of this term is somewhat unfortunate, for, as the exhaust from the cylinder passes out through the same ports through which steam is admitted, these ports are also entitled to be named exhaust-ports. True, the terms steam and exhaust signify here only different conditions of the steam, but the name '' exhaust-port " for the middle opening conveys unconsciously the impression that the two exterior ports are for steam only, and the central port is for the exhaust. Besides, the name port is too dignified in this connection, where it only means a hole or opening through which the exhaust escapes after it has left the steam-port, and no particular importance attaches to this hole ; it can be of any shape and size, provided it is big enough ; in fact, it hardly needs a special name.

The steam-ports, on the other hand, are essential and characteristic components of the valve-face, for in conjunction with the valve, they regulate the steam distribution in the cylinder, and their location and dimensions require careful consideration. Engines having separate exhaust-valves have also separate exhaust-ports, but these bear no relation to the "exhaust-port," so called, of the common D-valve. The force of this argument will become apparent later on.

That part of the face of the valve which overlaps the outside edge of the steam-port, when the valve. Fig. i, is in its central position, is called steam-lap, and the over-lapping of the inside edges is called exhaust-lap. The terms " inside " and "outside " lap are often used, but are not to be recommended, for many valves have steam-lap on the inside and exhaust-lap on the outside. Valves are often constructed without exhaust -lap, and occasionally the exhaust-edge of the valve does not reach to the port-edge, in which case the face of the valve does not entirely cover the ports in its central position, and the uncovered part is then referred to as negative exhaust-lap or exhaust clearance. If not otherwise specified, lap means steam-lap. The two partitions which separate the steam-ports from the exhaust-opening are named bridges.

It is proper to study the slide-valve as it relates to the action of steam on one side of the piston only ; for in a double-acting cylinder, being, as it were, a combination of two single-acting cylinders, the action on one side of the piston is practically an exact repetition of that on the other side ; and the D-valve can be divided into two equal halves, each of which will take care of the steam on one side of the piston ; and in order to place the back pressure in its proper relation to the forward stroke, it is only necessary to consider the exhaustaction in reversed order.

The duty of the shde-v^alv£, broadly speaking, is to admit steam during the forward stroke and exhaust it during the return stroke, and which includes expansion and compression.

The majority of engines are double-acting ; that is, steam is admitted alternately on both sides of the piston, and the common slide-valve is charged with the fourfold duty of admitting and releasing steam at both ends of the cylinder ; and we may conceive this being attained by a unification of four valves in one body. The regularly alternating periods of admission and exhaust, and the symmetrical arrangement of steam-ports admit of a single valve being used.

SIMPLEST FORM OF SLIDE-VALVE.

Fig. 2 represents a slide-valve, whose only duty is to open and close for admission of steam to one side of the piston. When it is moved to the right, a sufficient distance, it uncovers part of the port and admits steam to the. cylinder, and by its retrograde motion the port is closed again. It controls the admission of steam only, and there will have to be another valve to open and close for the exhaust ; and in case of a double-acting engine two such pairs, or four valves in all, would be needed.

THE COMMON SLIDE-VALVE.

The reciprocating motion of slide-valves is generally produced by eccentrics attached to the crank shaft of the engine, and an eccentric - — being nothing but an enlarged crank-pin —

Fig. 2.

^HC^o

Fig. 3.

Fig. 4.

?.io

Fig. 5.

is in the annexed diagrams represented by a small crank, or arm, E, and which communicates motion to the valve through a rod, as indicated. The diameter of the circle described by the center of the eccentric is called the throw of the eccentric, and it represents here the full stjoke or travel of the valve.

According to this definition the throw of an eccentric is twice its eccentricity or radius ; but, as the throw of a crank means the distance from center of shaft to center of crank-pin, it ought to mean the same when speaking of an eccentric. However, rhetoric is not cultivated in the shop or engine room, and when they talk about the throw of an eccentric it is well understood what it means.

For convenient representation in a limited space, the distance between eccentric and valve is made short in the diagram, which makes the angularity of the eccentric-rod appear abnormal. In ordinary stationary engines this angularity is always very slight, and its effect on the valve motion may be ignored ; and for this reason we may assume the eccentric-rod parallel with its medial-line of action, as also would be the case if we assume an eccentric-rod of infinite length.*

Fig. 3 shows the valve in the position where it commences to open the steam-port. Edge A of the valve and edge B of the port are now '■'■ line and line," and by continued turning of the eccentric in the direction shown by the arrow, these edges will separate, leaving an increasing opening for the passage of steam, till the eccentric has reached the extreme of its throw, when by further turning it will push the valve back towards edge B ; and when the eccentric is in position 4, vertically opposite position i, the port is closed again.

It should be observed that we are here only considering one edge of the valve and one edge of the steam-port ; other relations of port and valve being for the present quite immaterial. These edges come together twice during each revolution of the eccentric, when it is at the vertically opposite points i and 4,

* The actual effect produced by the angular motion of the eccentric-rod is explained in the last chapter of the book.

and the valve, therefore, opens and closes the port once during each revolution.

If arc 1-4 represents the period during which the port is open, arc 4-1 will represent the period during which it is closed. The greatest port opening is equal to the distance G-3, and for any position of the eccentric, the corresponding port opening can be measured horizontally on the shaded segment. When the eccentric is at << half throw,*' at point 10, the valve is in the position shown in Fig. 2, and the amount it overreaches the port in this position is the << lap,** and which is equal to the distance L. Thus the length of the period during which steam is admitted depends on the amount of lap, for the valve must move from its middle position a distance equal to the lap before it uncovers the steam-port, and it is evident, that the longer the period during which the port is closed the shorter must be the period during which it is open. A correct idea of this relationship can be had by an inspection of the figure, which clearly shows how an increase or diminution of distance L affects the period of steam admission.

Comparing the valve here described with that shown in Fig. I it will be seen, that by cutting off one-half of the latter and filling up the exhaust-cavity, we have a valve very similar to that shown in Fig. 2, and operating in exactly the same manner. It will also readily be seen that a valve like that shown in Fig. 2, working in a separate chamber, could be made to open and close an exhaust-port in much the same manner as the steam-valve controls the admission of steam or as one of the inner edges of the D-valve, Fig. i, controls the exhaust, and that the period of exhaust for each turn of the eccentric would depend on an exhaust-lap corresponding to the steam -lap in Fig 2 or the exhaust-lap in Fig i.

Fig. 5 shows an exhaust-valve and its eccentric at half throw ; the main difference between this valve and the steam-valve. Fig. 2, being that in Fig. 5 the valve is turned end for end and has a smaller lap, which makes the period of exhaust longer than the period of admission. It would also be necessary to provide means for holding this valve to its seat, which is not shown.

The period of exhaust is represented by the arc of the shaded segment. Fig. 5. If the exhaust eccentric is fixed on the shaft in the same position as the steam eccentric, the period of exhaust will, in a manner, be diametrically opposite the period of admission ; that is, if we imagine the period of revolution represented by a circle. Under such conditions, it is evident that all four valves for a double acting engine might be driven by one eccentric ; and by uniting the four valves in one body, and by providing an exhaust cavity to keep the exhaust separate from the live steam, we arrive at the construction shown in Fig. i.

It should be noted again, that here the exhaust passes through a port which also serves for admission of steam, and that steam passes over the outer edge of this port on its way to the cylinder, and is exhausted from the cylinder over the opposite edge, and that the steam- and exhaust-lap, therefore, extend in opposite directions, as on the separate valves Figs. 2. and 5. Each operation of this valve, therefore, depends on the relative position of two individual edges, just as does each operation of the simple valves referred to ; and thus is established the proposition that such simple valves are, in a sense, integral parts of the D-valve, or, in other words, the D-valve is equivalent to four simple valves driven by one eccentric.

Let the circle in Fig. 6, p. 12, represent the path of the center of the eccentric, and arcs 1-4 and 6-9 represent steam and exhaust periods respectively, during one revolution of the eccentric, for the left half of the D-valve, and the left side of the piston. It is here observable that the greatest steam and exhaust openings occur wjien the valve is at the two extremes of its travel, that is, at diametrically opposite points of the period of revolution; that any opening of the port can be measured horizontally on the segments ; that if there be no exhaust lap, the exhaust period will cover one-half revolution ; and that lap plus port-opening equals half travel of valve.

THE VALVE-MOTION IN RELATION TO THE PISTON-MOTION.

The main object of the discussion so far has been to establish a clear conception of the simple relationship which exists between laps and periods of admission and exhaust. The next subject for consideration is the relation of these periods to the motion of the piston in the cylinder, which includes determination of the position of the eccentric relative to the crank. It has been shown how these periods may be lengthened or shortened by changing the laps of the valve ; and any one of them can evidently be made to coincide with any period of the piston-stroke by fixing the eccentric in a suitable position on the crank-shaft.

First : Where should the valve be when the piston is at the end of the cylinder and commencing its forward stroke .'' As the port must open at or near the beginning of the piston-stroke, when the crank is on its first << dead center/' and as the period of admission occurs during the opposite half-revolution of the eccentric, and must be considerably less than one-half period of revolution, it follows that the eccentric must be past the middle of its throw, or more than one-quarter of a revolution ahead of the crank. Let C, Fig. 3, represent the crank when the valve is at the point of admitting steam, at i, then considering that eccentric and crank are moving together with the crank shaft, it is clear that when the crank has arrived at the end of its return stroke, at C, Fig. 4, the eccentric has advanced the same angular distance, from position i to 2, and there will be a small opening between the valve and the port-edge, as shown ; and during this short interval steam enters the cylinder in advance of the piston. This initial port opening is called lead or sometimes steam-lead, in contradistinction from exhaust-lead, which means the opening for exhaust at commencement of the return stroke. Steam-lead is usually supposed beneficial, and the desirability of exhaust-lead is unquestionable. Lead is increased by turning the eccentric forward on the shaft, and is diminished by turning it backward.

Sometimes, under certain conditions, the port opens after the forward stroke has commenced, and in such case the expression << negative-lead'* may be used, to indicate that the valve overlaps the steam -port to a certain extent at the commencement of the stroke.

Let the position of the eccentric at the beginning of the stroke be at 2, Pig. 4 ; then period of admission from the time the piston starts on its forward stroke is represented by arc 2-4, which obviously always should be considerably less than the period of one-half revolution in order to provide for expansion of the steam. The angle 10-2 has been named the angle of advance, but, thus should be named the angle between the crank arm and eccentric arm ; for it represents how much the eccentric actually is in advance of the crank. Any other designation is confusing, and there is no valid reason for any other use of this term. If there ever was a good reason for it, it does not exist now. In this book, the <* angle of advance" means the angular advance of the eccentric relative to the crank, as represented by angle 8-2, Fig. 4 and 6.

When, by continued turning of the shaft, the eccentric reaches position 4, the admission of steam to the cylinder is cut off. Where will the piston then be } Mark off the angle of advance from point 4 in a reverse direction, and a radial line intersecting the crank-circle marks the location of the crank-pin, and its horizontal projection will indicate approximately the position of the piston in the cylinder when the admission of steam is cut off and expansion commences. The exact location

HIE COMMON SLIDE-VALVE. 11

of this point — the point of cut-off — is sometimes required, and the angularity of the connecting-rod must then be taken into account. (See last chapter.)

As steam always is admitted at a point very near the commencement of the stroke, the period of admission must be made shorter or longer according as an earlier or later "cut-off" is required. This can be effected by lengthening or shortening the steam-lap ; but as the point of admission is fixed near the commencement of the stroke, the period as a whole must occur earlier or later according to the location of the point of cut-off, and the angle of advance must be changed accordingly — which will become quite clear by an inspection of Fig. 4. Thus is established the important rule : Period of admission is shortened by increasing the lap and the angle of advance, and is lengthened by diminishing the lap and angle of advance. Also mark : that all periods of steam, distribution occur earlier or later in the stroke, according as the eccentric is turned ahead or back on the shaft.

VALVES WITHOUT LAP.

Valves have been used without steam- and exhaust-lap at a time when the gain by expansion was not fully recognized and compression of exhaust had no significance. When such valves have their eccentrics at right angles to the crank, the points of admission and release of the steam occur when the crank is on its " dead center " ; but the eccentric was presumably moved forward some to give the valve-lead. Such valves are not used now, and they may be classed with historical curiosities. In treatises on the steam-engine this lap-less construction is usually discussed at length as an introductory to a chapter on slide-valves, though the discussion of this special and obsolete construction seems hardly profitable, and it is not conducive to a broad, practical conception of the general subject.

THE PRIMITIVE VALVE-DIAGRAM.

Fig. 6 is a combination of diagrams 4 and 5, and it represents a simple D-valve diagram. The crank is supposed to be at its dead center, or at the extremity of its throw, at the commence-

Fig. 6.

ment of the forward stroke, and significant positions of the valve during one revolution of the crank are indicated diagram mat ically to the left. The various positions of the eccentric are marked on the eccentric-circle in the following order.

1. Point of admission, just before end of return stroke.

2. Lead at commencement of forward stroke.

3. Extreme throw of valve, port "vvide open.

4. Steam is cut off, expansion commences.

5. Valve in central position.

6. Point of release.

7. Exhaust-lead, at commencement of return stroke. ■ 8. Extreme throw of valve, exhaust wide open.

9. Exhaust-closure ; compression commences. 10. Valve in central position.

THE COMMON SLIDE-VALVE. IS

This diagram exhibits in a natural manner all that is required for a full understanding of the functions and principles of construction of a common slide-valve, and from it the position of the valve relatively to the ports, at any point of the stroke, can readily be ascertained by marking off the angle of advance on the eccentric-circle. Note that in this diagram the eccentric-arm is supposed to be moved in unison with the crank-arm, as it does move in reality.

The simple and direct relationship between eccentric and valve motion, as here assumed, is often changed by the intervention of a special valve-gear ; but the resolution of such cases presents no special difficulty. Usually the valve-motion is assumed, and the throw and position of the eccentric are then deduced from the design of the valve gear.

It should be particularly noted that the valve-end of the eccentric-rod must be in the same position when the valve opens and closes the port, and that, therefore, each pair of points, 1-4, and 6-9, which mark the beginning and end of the admission and the exhaust period respectively, must be equally distant from a certain point which marks the location of the eccentric-rod pin when the valve and port edges come together, and that, therefore, if the valve-end of the eccentric-rod travels in a straight line, which extended passes through the center of the crank-shaft, chords 1-4 and 6-9 must be parallel. A slight deviation from this ' direction, caused by the swing of an ordinary rocker arm, is of so little consequence that it may be entirely ignored.*

The foregoing discussion presents the fundamental- principles which must govern the construction of slide-valves of any description, and it will serve as an introduction to the study of other forms of valves.

* The general effect of a rocker motion in other directions than the center of the shaft is discussed in the last chapter of the book.

THE SLIDE-VALVE AND ITS FUNCTIONS.

SWEET S VALVE-DIAGRAM.

In Fig. 6 the various positions of the valve relative to the port during a complete revolution of the eccentric are indicated by the eccentric-arm, as a pointer ; but if the circular figure be turned backwards through an angle equal to the angular advance of the eccentric, without disturbing the crank, all the points in the circumference will be placed in the same position relative tO'

the crank-arm as they now occupy relative to the eccentric-arm ; and if then the crank-arm, or its center line, be moved into any new position, the corfesponding position of the valve will appear directly from the marks in the circumference of the circle. A valve-diagram thus modified is shown in Fig. 7. Note that valve measurements and port-openings are taken at right angles to the oblique lines, and that these lines must be parallel. For convenience, the eccentric-circle is drawn full size, and the crank-circle is reduced, so as to make both circles appear of same size.

From this circular diagram a theoretical indicator diagram can readily be constructed, as shown, which adds greatly to its usefulness ; for to many engineers the indicator diagram exhibits and explains, at a single glance, all the peculiarities of the steam distribution in the cylinder, and it sometimes contains a whole story in graphic language.