THE
SLIDE VALVE,
SIMPLY EXPLAINED.
BY W. J. TENNANT, ASSO. M.I.M.E
REVISED AND MUCH ENLARGED
BY J. H. KINEALY, D. E.,
_Professor of Mechanical Engineering in Washington University;
M. Am. Soc. Mech. Engrs. Author of “Steam Engines and
Boilers;” “Low Pressure Steam Heating Charts.”_
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FULLY ILLUSTRATED WITH ORIGINAL DRAWINGS AND DIAGRAMS.
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SPON & CHAMBERLAIN,
12 CORTLANDT STREET, NEW YORK.
1899.
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Entered according to Act of Congress in the year 1899,
BY SPON & CHAMBERLAIN,
in the Office of the Librarian of Congress, Washington, D.C.
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THE BURR PRINTING HOUSE, NEW YORK, U. S. A.
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PREFACE.
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.
W. J. T.
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CONTENTS.
PAGE
INTRODUCTION TO AMERICAN EDITION, 1
CHAPTER I.
THE SIMPLE SLIDE, 5
CHAPTER II.
THE ECCENTRIC A CRANK. SPECIAL MODEL TO
GIVE QUANTITATIVE RESULTS, 13
CHAPTER III.
ADVANCE OF THE ECCENTRIC, 20
CHAPTER IV.
DEAD CENTRE. ORDER OF CRANKS. CUSHIONING
AND LEAD, 24
CHAPTER V.
EXPANSION—INSIDE AND OUTSIDE LAP AND
LEAD; ADVANCE AFFECTED THEREBY.
COMPRESSION, 31
CHAPTER VI.
DOUBLE-PORTED, MULTIPLE ADMISSION AND
PISTON VALVES, 43
CHAPTER VII.
THE EFFECT OF ALTERATIONS TO VALVE AND
ECCENTRIC, 57
CHAPTER VIII.
NOTE ON LINK MOTIONS, 66
CHAPTER IX.
NOTE ON VERY EARLY CUT-OFF, AND ON
REVERSING GEARS IN GENERAL, 73
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INTRODUCTION TO AMERICAN EDITION.
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.
J. H. KINEALY, D.E.,
Prof. of Mech. Eng., Washington Univ.,
St. Louis, Mo.
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THE SLIDE VALVE.
CHAPTER I.
THE SIMPLE SLIDE.
The slide valve has always been regarded by the writer as the _pons
asinorum_ of the student of the steam-engine. His own early attempts at
crossing that bridge were greatly facilitated by the simple little
device to be presently submitted to the reader; with the aid of this
expedient the student may easily obtain clear notions upon all the
functions and operations of the slide valve, at a nominal cost, and with
but a small expenditure of time and trouble.
It is necessary that at the outset the nature of the construction above
alluded to should be clearly appreciated, for the reason that many of
the explanations given in the succeeding pages are referred thereto.
In passing, it may be remarked that much of the difficulty which the
ordinary student experiences in the study of valves and valve gear
arises because he approaches the subject with a false impression that it
is of necessity a difficult one; this idea is generated on the one hand
in the atmosphere of mystery which in the shop is made to surround the
matter, and on the other hand by the somewhat formidable aspect of most
of the geometrical treatises on the subject. If the beginner can bring
himself to believe that the slide valve is nothing more complex than a
sliding shutter with a cavity in its face, travelling backwards and
forwards over three ports, the outermost of which are alternately opened
to steam and placed in communication with the central exhaust-port, he
will have made a satisfactory commencement.
Take a piece of stout white cardboard and upon it set out an enlarged
copy of fig. 1.[1]
[Illustration: FIG. 1.—Explanatory Diagram.]
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Footnote 1:
See also fig. 41.
The upper portion of this diagram represents a section taken at right
angles to the port-face through a set of steam and exhaust-ports as
ordinarily arranged; the graduated ring beneath the section will receive
consideration later.
Postponing for the moment the further investigation of fig. 1, reference
should here be made to figs. 2, 3, and 4.
Fig. 2 shows, in perspective, a cylinder and valve-chest with parts
removed to make clear the manner in which the two steam-ports
(S_{1}S_{2}) at the upper ends of the two outer passages (P_{1}P_{2})
afford a communication between the valve-chest (VC) and the opposite
ends (C_{1}C_{2}) of the cylinder (C), and to show the passage from the
central exhaust-port (EP) to the outlet (O) at which the exhaust steam
is discharged. The relationship between this view and the upper part of
fig. 1 will be obvious upon a comparison of the two.
[Illustration: FIG. 2.—Perspective Section of Steam-Engine Cylinder.]
A typical slide valve is shown in section, in its place above the ports
in fig. 2, and separately in figs. 3 and 4. From these latter, the large
ratio of the width to the length of the valve is apparent, and it will
be seen from fig. 2 that the ports are correspondingly made wide and
short. This arrangement enables a small linear movement of the valve to
effect the uncovering of a large area of port, and to secure free
passage for the steam with but a small amount of valve-travel, and so to
reduce to a minimum the work unavoidably wasted in overcoming friction
between the port-face and the valve heavily loaded by the pressure of
steam upon its back.
[Illustration: FIGS. 3 and 4.—Separate Views of the Slide Valve.]
Let us return now to fig. 1. Draw, near the edge of a strip of
moderately stiff paper, the section of the elementary form of slide
valve illustrated by fig. 5, taking its dimensions from the ports of
fig. 1, so that the two views agree in the manner indicated. By sliding
this diagram backwards and forwards across the ports of fig. 1, the
following explanation of the action of this, the simplest form of slide
valve, will be readily appreciated.
[Illustration: FIG. 5.—Section of Slide Valve.]
This valve has first to open one port to steam, and when the steam thus
admitted has forced the piston towards the opposite end of the cylinder,
to put the same port in communication with the exhaust, while opening
the other port to the steam which effects the return movement of the
piston. The duration of the admission of steam is the same as that of
the exhaust, the admission, owing to the dimensions of the valve in
relation to the ports, necessarily taking place on one side of the
piston simultaneously with the occurrence of exhaust from the other side
(see fig. 6). The travel of this rudimentary valve equals twice the
amount to which the port is opened to steam. It happens to open each
port fully in this case (although this is rarely the case with modern
slide valves), so that the “travel” equals twice the width of
steam-port: _twice_, because (starting with the valve in its central
position, as shown in fig. 5) it has to move to the right, say, by an
amount equal to the _width_ of _the left-hand_ port, in order to open
that port fully to steam, and then it moves back again and travels to
the other side of the central position by an equal amount, in order to
open the equally dimensioned right-hand port to steam. In fig. 6 the
valve is shown at the two extremes of its travel, in full lines at one
end, and in dotted lines at the other end.
[Illustration: FIG. 6.—Section showing Position of Slide Valve in
relation to Steam and Exhaust-Ports.]
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