Slide valve gears. An explanation of the action and construction of plain and cut-off slide valves

Front matter

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Preface

THIS work has been prepared to meet what the author considers a real want. It has been written with the aim of making it intelligible to any one who might be willing to make a serious effort to understand it. High authority exists for a mathematical treatment of the subject, but with this the author has no sympathy. Designing a valve gear is essentially a drawing board process, and a mathematical treatment of it is simply an uncalled for use of heavy artillery. The graphical treatment is therefore adopted throughout.

Acknowledgment is due to Mr. Hugo Bilgram for his courtesy in kindly permitting the use of his valve diagram. The author has all due respect for the Zeuner diagram, but that respect is not incompatible with the conviction that Mr. Bilgram's method is a marked improvement upon it. Valve diagrams are used for two purposes — to analyze existing valve motions and to design new ones. The Zeuner diagram fulfils the first purpose perfectly, but is unsatisfactory when applied to the second. The leading data that are given in designing a valve motion are the point of cut-off, the port opening, and the lead of the valve (not the lead angle of the crank, as is often conveniently assumed). It is the radi-

IV PREFACE.

cal defect of the Zeuner diagram that none of these dimensions can be laid off from known points. The lead must be laid off from an unknown point of the centre line, and the port opening from an unknown point on an unknown line. Finally, through these unknown points and the centre of the shaft the valve circle is to be drawn from an unknown centre and with an unknown radius. Under these circumstances the result sought is found only through blind trial. With Mr. Bilgram's method all this is changed. The lead is laid off from a fixed line, the port opening from a fixed point, and the cut-off position of the crank is located. The lap circle is then drawn tangent to these lines, and the problem is solved. Moreover, the awkward conception of the backward rotation of the crank is obviated. Finally, these marked advantages are not accompanied by any compensating disadvantages whatever.

Acknowledgment is also due to the American Machinist for the use of a number of engravings originally prepared to illustrate some of the author's articles in that paper.

The irregularities due to the connecting rod introduce peculiar difficulties into the study of the first principles of the slide valve, which difficulties were first overcome by the happy expedient of using the slotted cross-head instead of the connecting rod in the preliminary study. For this, together with many other original and highly valuable contributions to the subject, we are indebted to Mr. W. S. Auchincloss, who first published them in his well-known and standard work entitled Link and Valve Motions, to which those who wish to prosecute their studies beyond the scope of this work are referred.

PREFACE. V

The author has gone more fully than is customary into the methods of equalizing the various events of the stroke. The sections relating to these methods will be found more difficult to follow than the others, while at the same time they form no necessary part of a general treatment of the subject. Those who begin their studies of valve motions with this book, may find these chapters too difficult for the first reading. They have, therefore, been marked with a star (*)in the Table of Contents and in the body of the book, in order that they may be omitted, if desired, in the first reading; and it should be understood that the chapters not so marked form of themselves a complete connected treatise, of a more elementary character than the book as a whole.

Philadelphia, Oct. 19, 1889.

Table Of Contents

The chapters with an asterisk (*) prefixed may be omitted in the first reading without breaking the continuity of the subject.

PART I. The Slide Valve with Fixed Eccentric

PAGE

The Plain Slide Valve, 3

The Eccentric, .......... 4

The Scotch Yoke or Slotted Cross-head, ..... 5

The Primitive Engine, 7

Defects of the Primitive Engine, . . . . . , 13

Lap, 15

Angular Advance, ......... 18

Lead, 21

Exhaust Lap, 25

Backward Rotation, ......... 26

Laying out the Slide Valve, ....... 38

* Velocity of the Valve, ... .... 39

Limitations of the Plain Slide Valve, ...... 40

The Areas of the Ports and Pipes, . . . . . 42

* The Angular Vibration of the Connecting Rod, ... 45

" Eccentric Rod, .... 49

* Equalized Exhaust, ........ 50 vii

Vlll TABLE OF CONTENTS.

PART II. The Slide Valve with Shifting and Swinging Eccentric

PAGE

* Equal Lead and Constant Lead, . . . . . . 77

The Shifting Eccentric, 78

The Swinging Eccentric, ........ 80

* The Angularity of the Eccentric Rod, . . . . . 85

* Equalized Lead and Cut-off, . . . . . . . 95

PART III. The Slide Valve with Independent Cut-off

Introductory Remarks, ........ 101

The Buckeye Valve Gear, . . . . . . . .116

The Straight Line Independent Cut-off Valve Gear, . . .120 The Bilgram Valve Gear, . . , . , . , .122

Part I. THE SLIDE VALVE WITH FIXED ECCENTRIC

The Slide Valve with Fixed Eccentric.

THE PLAIN SLIDE VALVE.

Fig. I is a sectional view of a plain slide valve and its seat, the valve being shown in its central position, with the ports completely covered by it. The distance

Fig.1

a, by which the valve extends beyond the steam edge of the port, is called the outside lap, steam lap, or more usually, simply lap of the valve. The distance &, by which it extends beyond the exhaust edge of the port, is called the inside lap or exhaust lap.* The exhaust

* As will be more fully shown later on, valves are sometimes so made that the steam is admitted by the inside and exhausted by the outside edges. Hence the terms inside and outside lap are somewhat ambiguous. The terms steam lap and exhaust lap avoid this ambiguity, and are to be preferred, 3 lap is always much smaller than the steam lap. It is frequently absent, and frequently the exhaust edge of the valve does not reach the exhaust edge of the port, being made as shown by the dotted lines. In that case the distance c is usually called inside clearance, though a better name is negative inside or exhaust lap. It is sometimes called inside lead or exhaust lead ; but these terms should not be applied here, as they have properly another definite meaning, which will be explained farther on. The measurement for both steam and exhaust lap is made for one end of the valve only. Thus if a valve is said to have f inch lap, the meaning is that it has that much at each end.

THE ECCENTRIC.

The slide valve is usually driven by means of an eccentric on the crank shaft, and it becomes necessary at the outset to obtain a clear conception of the motion which the eccentric gives. In brief, the eccentric is a short crank with a large crank pin. It is obvious that the motion of a cross-head would not be changed by increasing the size of the crank pin. If a crank pin were enlarged until the crank shaft came within the circumference of the pin, the result would be an eccentric. The arm of a crank is the distance from the centre of the shaft to the centre of the crank pin, and similarly the " throw'' of an eccentric is the distance from the centre of the shaft to the centre of the eccentric disc. Usually the centre of the disc is within the circumference of the shaft ; but this does not alter

THE SCOTCH YOKE OR SLOTTED CROSS-HEAD. 5

the nature of the device, which remains simply a short crank with a large crank pin.

THE SCOTCH YOKE OR SLOTTED CROSS-HEAD.

The usual method of connecting the cross-head to the crank pin by means of a connecting rod introduces certain distortions and irregularities into the relative motions of the piston and crank. These will be more fully explained farther on, but it is desirable in the first instance to avoid the necessity for considering them, as they greatly complicate the subject. This is accomplished by considering in the first instance an engine having the piston and crank connected by means of the device called the Scotch yoke or slotted cross-head, since that connection is without the distortions mentioned.* As has been explained, the eccentric is essentially a crank ; and it follows that the distortions which are introduced by the connecting rod into the motions of the piston and crank, are also introduced by the eccentric rod into the motions of the valve and eccentric. The reasons which lead to the adoption of the slotted cross-head in place of the connecting rod also require its use in place of the eccentric rod. An engine fitted with slotted cross-heads is illustrated in Figs. 2-1 1. The slotted cross-head will be recognized at once, and is too familiar a device to need further description.

*The slotted cross-head is employed here with the permission of Mr. W. S. Auchincloss, to whom the thanks of the author are due.

SLIDE VALVE GEARS.

THE PRIMITIVE ENGINE. J

THE PRIMITIVE ENGINE.

When illustrating the action of the valve of a steam engine, it is essential for clearness that the valve be shown on the top of the cylinder. A valve so located in an actual engine would require the use of a rock shaft to communicate the motion of the eccentric rod to the valve rod — a construction which finds use in American locomotives. This rock shaft complicates the action of the parts, and it is desirable in this preliminary work to avoid it. To accomplish this the unmechanical arrangement of Figs. 2-1 1 is adopted.

Figs. 2-6 represent the primitive engine with the parts in a number of successive positions. The valve has no lap on either steam or exhaust side, and the eccentric is set at right angles to the crank, and in advance of it in the direction of the rotation. The eccentric, being in fact a crank, is represented as such, and the valve is driven from it by a slotted cross-head, which is secured to the valve stem by the bracket shown. In Figs. 3-6 the slotted cross-heads are represented by their centre lines only, for greater clearness and simplicity.

Referring to Fig. 2, the crank is on the "centre," and the parts are ready to begin movement, the direction of rotation being as shown by the arrow. In Fig. 3 the crank shaft has turned through an angle of forty five degrees, carrying the parts to the positions shown. Considering Figs. 2 and 3, it is clear that the first movement of the crank shaft carried the valve to the right, and thereby opened port x to steam and y to exhaust. Opening port x admitted steam behind the piston to

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THE PRIMITIVE ENGINE.

THE PRIMITIVE ENGINE. II

drive it forward, and opening y enabled the steam which previously filled the space in front of the piston to escape to the cavity z, which communicates through the exhaust pipe with the atmosphere or condenser, as the case may be. In Fig. 4, the crank shaft has turned through an additional angle of forty five degrees, bringing it at right angles to its initial position. The piston is now at the centre of its travel, and the valve at its extreme right hand position. As the rotation continues, the piston continues to advance ; but the valve reverses its motion, and gradually closes its ports, until when the crank completes a half revolution, as shown in Fig. 5, the valve reaches its middle position at which it stood in Fig. 2, with all ports closed. Continuing the motion, the valve is carried to the left, opening port y to steam and x to exhaust, as shown in Fig. 6, and the piston is driven back to its original position ; and this sequence of operations will obviously continue indefinitely.

With the eccentric located as in the figures, the direction of rotation must be as described. This will be apparent if, starting with Fig. 2, rotation in the opposite direction be imagined. The effect of this would be to open port x to exhaust and y to steam, thereby effectually stopping the rotation in the direction imagined. To effect this reverse rotation the eccentric must be located diametrically opposite to the position shown in the figures.* The student should satisfy himself of

* This is true with the primitive form of valve only. With valves having lap, as actually used, the eccentric position for reverse rotation is not diametrically opposite from the position required for forward rotation. This subject will be referred to again.

SLIDE VALVE GEARS.

DEFECTS OF THE PRIMITIVE ENGINE. 1 3

the correctness of this fact by supposing the eccentric so located, and then following the motion through a revolution.

Throughout this book, whether shown or not, it will be understood that the cylinder is located as in the figures already explained, i.e., to the left of the shaft ; and unless otherwise specified, that the direction of rotation is the same as in these figures, i.e., " over."

DEFECTS OF THE PRIMITIVE ENGINE.

With the construction of Figs. 2-6 the opening and closing of the ports are coincident with the passing of the centre by the crank. Economy of steam and successful running require that the following changes be made in this distribution of the steam :

I. The opening of the steam port or "admission" should occur slightly before the crank reaches the centre.* In a general sense this is called giving the valve steam lead or simply lead. In a more strict sense, that term means the width of opening in fractions of an inch which the valve has given to the steam port at the instant the crank passes the centre.

II. The closing of the steam port or " cut-off " should occur a good deal before the crank passes the centre.

III. The opening of the exhaust port or " release,"

* Of late years a difference in practice has arisen in this respect. Some makers now set their valves to open the port just as the crank passes the centre, and in some cases the admission is delayed until after that event. The statement in the text, however, represents general practice. This subject will be referred to again farther on.

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LAP. 15

and its closing or " compression," should occur earlier than the opening of the steam port, but not so early as its closing. As the width of port opening to steam as the crank passes. the centre is called steam lead, so the width of opening to the exhaust at the same instant is called exhaust lead or inside lead (compare page 4).

These changes in the steam distribution are brought about by two changes in the valve gear . (1.) The valve is given lap, and (II.) The eccentric is advanced on the shaft ahead of the position given.

LAP.

Fig. 7 is a reproduction of Fig. 2, with the addition of outside or steam lap to the valve. There is no change in the exhaust side of the valve nor in the angular position of the eccentric, and it is obvious that the ports will be opened and closed to the exhaust as the crank passes the centre exactly as before ; but the port x will not be opened to steam until the valve has been carried to the right an amount equal to its lap.

This will happen as shown in Fig. 8, when the shaft has turned through an angle def, such that df, or what is the same thing, eg, is equal to the lap. This angle def is called the lap angle. The valve opens port x to steam when the edge of the valve passes the edge of the port going to the right, and it closes it when the edge of the valve passes the edge of the port going to the left. The position of the valve is the same at closing as at opening, the only difference between the two acts being in the direction of the valve's motion ; coni6

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LAP, sequently the port must close with the eccentric at h, vertically below/, as shown in Fig. 9. From this, two important and fundamental facts can be learned :

I. During the time that steam was being admitted the eccentric (and with it the shaft and crank) turned through the angle fch. Now feh is equal to a semicircle less def and less hei, that is, a semicircle less twice the lap angle, and this is the first result of the addition of lap to the steam side of the valve. During the admission of steam the crank turns through an angle equal to a semicircle less twice the lap angle.

II. In the case of the primitive valve of Figs. 2-6, the port began to open with the eccentric in the position of Fig. 2. It attained its greatest opening in the position of Fig. 4, and the maximum width of port opening was equal to the throw of the eccentric. With the valve of Figs. 7-9, however, the port does not begin to open until the eccentric reaches the point f, and the remaining travel gk only is available as port opening. This width of opening, gk, is equal to ek less eg, that is, to the throw of the eccentric less the lap of the valve ; and it follows at once that if a valve with lap is to give the same port opening as another without lap, the eccentric throw of the former must exceed that of the latter, and the greater the lap the greater must be the throw.

ANGULAR ADVANCE.

The last section has shown how, by the addition of lap, the period of port opening may be shortened as desired. While, however, the method of regulating

ANGULAR ADVANCE. 1 9

the length of period of port opening was pointed out, nothing was said about properly timing the opening or closing of a valve having lap with reference to the position of the piston, and in point of fact in Fig. 8 the port opening to steam occurred long after the crank had passed the centre. The correct timing of the events of the stroke, and especially of the admission of steam, is obtained by advancing the eccentric around the shaft from the position thus far shown. In order to give admission to the steam at the instant the crank passes the centre, it is necessary to first locate the crank on the centre, then to advance the eccentric around the shaft such an amount as to draw the valve to the right a distance equal to the steam lap, and finally to secure the eccentric in that position. Such a setting of the eccentric is shown in Fig. 10, in which the eccentric has been turned forward until the distance df ox its equal, eg, is equal to the lap. Such advance of the eccentric on the shaft is called the angular advance or the advance angle of the eccentric ; and if the admission is to occur with the crank on the centre, as in this instance, the advance angle of Fig. 10 is equal to the lap angle of Fig. 8. Having secured a proper admission to the steam, it is proper to inquire next into the effect which this change in the angular position of the eccentric has had on the other events of the stroke. It is sufricientlv obvious that turning- the eccentric forward a given angle would simply cause each event to occur that much earlier in the rotation of the crank. After steam lap had been added, and before the advance of the eccentric, the cut-off occurred with the crank lacking one lap angle of having reached

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LEAD. 21

the centre (see Fig. 9). Advancing the eccentric one lap angle will cause cut-off to occur one lap angle earlier still, or with the crank lacking two lap angles of having reached the centre. Before the advance of the eccentric, the opening and closing of the ports to the exhaust occurred as the crank passed the centre (see Figs. 5 and 7). Advancing the eccentric one lap angle swill therefore cause both release and compresssion to occur with the crank lacking one lap angle of having reached the centre.

Summarizing then, the valve has had steam lap added to it, and the eccentric has been advanced by an angle equal to the lap angle, and the resulting steam distribution is as follows : Admission occurs as -the crank passes the centre ; cut-off occurs two lap angles before the centre ; and release and compression occur one lap angle before the centre.

Throughout this book the advance angle will be designated on the diagrams by the letter $ (delta).

LEAD.

It was explained on page 13 that the admission of steam should take place slightly before the crank reached the centre, and that such early admission was called lead. In the last section, for the sake of simplicity, the admission was supposed to occur as the crank passed the centre. In other words, the lead was made zero. It now becomes in order to examine the method for the introduction of lead, and the changes in the other events of the stroke which follow. In Fisr.

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LEAD. 33

10 the eccentric was advanced to cause admission to occur on the centre. If it is proposed to give the valve lead, the eccentric must be advanced still further, so as to draw the valve to the right an additional amount equal to the lead desired. In Fig. 11 this additional advance has been made, the eccentric having been moved from /of Fig. 10 (reproduced in Fig. 11) to /. The angle fel is called the lead angle, and it follows that in all cases the angular advance is equal to the lap angle plus the lead angle. If the lead is zero, then, as before found, the angular advance is equal to the lap angle. If in Fig. 1 1 the crank shaft be turned backward, the valve will close the port when the eccentric reaches point f and the crank stands at m, the angle nem being equal to the angle/*?/. In other words, the lead angle is equal to the angular distance which the crank lacks of having reached the centre when admission occurs.

It was found on page 19 that advancing the eccentric on the shaft a given angle advanced all the events of the stroke correspondingly, and the resulting distribution of steam with no lead was summarized on page 21. If now the valve have lead so that the angular advance be greater than the lap angle, the steam distribution given on page 21 is changed as follows:

Admission occurs with the crank lacking the lead angle of having reached the centre.

Cut-off occurs with the crank lacking two lap angles and one lead angle of having reached the centre.

Release and compression occur with the crank lack ing one lap angle and one lead angle of having reached the centre.

An application of the above principles is all that is

SLIDE VALVE GEARS.

necessary for the analysis of the steam side of any existing plain slide valve, as will be seen from the following example :

An eccentric has a throw of if " , the valve has onee m

Fig. 12 inch steam lap, no exhaust lap, and the eccentric is set to give^Jg-" lead. Required the greatest port opening, and the crank positions for admission, cut-off, release, and compression.

In Fig. 12 strike the circle with a radius equal to the

EXHAUST LAP. 25

throw of the eccentric, and lay off Oa equal to the lap of the valve and a b equal to the lead. Erect perpendiculars from points a, b, giving points c, d. Now eOc is the lap angle, cOd is the lead angle, and eOd is the angular advance. Lay off fg equal to cd, giving Og the crank position for admission. Make hi equal to twice ec plus cd, giving the crank position Oi for cut-off. Make kh equal to ec plus cd, giving Ok the crank position for release and compression. Finally, ah is the greatest steam port opening.

The student should familiarize himself with the principles thus far treated, by the solution of a variety of problems similar to the following :

Problem I. Eccentric throw if, lap f, lead ^ Required crank positions for lead, cut-off, compression and release, and port opening to steam.

Problem II. Eccentric throw if, lap -J-J, lead 3%-. Required as before.

EXHAUST LAP.

The addition of lap to the inside of the valve has the same effect upon the opening and closing of the port to the exhaust that steam lap has upon opening and closing it to steam, i.e., it delays the opening and hastens the closing, and it delays the opening and hastens the closing by an angle of rotation of the crank equal to the exhaust lap angle — that angle being found exactly as was the steam lap angle in Figs. 8 and 12. Returning to Fig. 12, if the valve were to have an ad dition of -§• inch inside lap, its effect upon the release and compression could be found as follows : Make 01 equal the inside lap, and by the line Im find the exhaust lap angle eOm. From k lay off em both upward and downward. Draw the radii On and Op, and the results are On for the crank position at release, and Op for compression. It is unnecessary to go in detail through the results following the introduction of negative inside lap. It will be seen at once that its results are the direct reverse of the preceding, i.e., to hasten the opening and delay the closing of the ports. If in the illustration preceding, the valve had been given -J negative inside lap, the crank positions found for release and compression would have changed places.

It was shown on page 18 that the steam port opening given by a valve having steam lap is equal to the throw of the eccentric less the steam lap, and it follows by the same course of reasoning that the exhaust opening is equal to the throw less the exhaust lap. Since the exhaust lap is always less than the steam lap, it follows that the port opening to exhaust is always greater than to steam.

BACKWARD ROTATION.

It was explained on page 1 1 that with a primitive valve the eccentric location for rotation in the reverse direction would be diametrically opposite that shown in Figs. 2-6. For a valve having lap, the position for reverse rotation is found by laying off the advance angle in the direction of the proposed rotation from the position for a primitive valve. The effect of a rock

BACKWARD ROTATION. 27

shaft in the valve motion (for an example of which see any American locomotive) is to reverse the motion of the valve as compared with the eccentric, and hence to require a location of the eccentric which will provide for this reversal. The position of the eccentric for either direction of rotation, and with or without a rocker, may be located from the following facts :

I. Without a rocker the eccentric for a primitive valve is 90 in advance of the crank in the direction of the rotation. With a rocker the eccentric is behind the crank.

II. The advance angle is laid off in all cases in the direction of the rotation from the position for a primitive valve.

One qualification should be added to the above, as follows : In all the cases thus far shown, the location of the eccentric for the primitive valve is as stated at right angles to the crank. In certain cases, owing to the character of the connections between the eccentric and valve, this is not true. For an example see Figs. 58 and 59. In cases of this kind the location of the eccentric can be found as follows : Carry the centre of the eccentric strap to the centre of the crank shaft. Through the centre of the shaft draw a line perpendicular to the location of the eccentric rod thus found. This perpendicular gives the location of the eccentric for the primitive valve, and the angular advance is to be laid off from it in the direction of the rotation.

THE BILGRAM DIAGRAM.

Any existing slide valve can be analyzed by the methods that have been followed in explaining the action of the valve, and new valves could be designed by a tentative application of the same methods. Such a plan of procedure, however, would be exceedingly tedious, and much ingenuity has been expended in devising briefer and better methods. Of these, by far the best is the diagram devised by Mr. Hugo Bilgram, and explained below. The chief office of such a diagram is to show briefly and accurately the position of the valve for any and every position of the crank.

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Fig. 13

The demonstration of the Bilgram diagram depends upon the following theorem of geometry : In Fig. 13 let ABC and abc be two triangles, such that any two of their angles, as those at A y C and a, c, and any one side, as BC and be, are respectively equal. Then this theorem asserts that all of the other parts of the triangles are equal, i.e., angle B to b, side AC to ac, and side AB to ab.