THE VALVE-GEAR.
QUESTION 182. _What is meant by the valve-gear of a locomotive?_
_Answer._ By the valve-gear is meant the arrangement of eccentrics,
rods, links, rockers, etc., by which the valves are moved and their
motion regulated.
QUESTION 183. _What is required of the valve-gear in working a
locomotive?_
_Answer._ It must be so arranged that the locomotive can be run
either backward or forward, and so that the motion of the wheels can
be reversed quickly and with certainty. It should enable the runner
to employ the greatest power of the engine by admitting steam into
the cylinders during the whole or nearly the whole of the stroke of
the pistons, or when less power is required, to use the steam more
economically by working it expansively, which latter is accomplished
with the present appliances by changing the travel of the valve.
QUESTION 184. _How is the valve-gear constructed so as to run the
engine either backward or forward?_
_Answer._ As already explained, in answer to question 76, two
eccentrics are provided for each cylinder. These are set so that one of
each pair will run the locomotive in one direction, and the other two
the reverse way.
QUESTION 185. _How must the eccentrics for each cylinder be set in
order that the one may run the engine forward and the other backward?_
_Answer._ This can be best explained by reference to fig. 100, in
which the piston, _P_, is represented at the beginning of the backward
stroke, and the valve _V_ has the requisite lead and is just about to
open the front steam-port. It is obvious that, in order to complete the
backward stroke of the piston, the front port must be opened to admit
steam into the front end of the cylinder, and therefore the valve must
be moved in the direction indicated by the dart _a_. To do this, the
upper arm of the rocker _r_ must move in the same direction, and the
lower arm must be moved the reverse way, as indicated by the dart _e_.
If the crank is intended to move in the direction indicated by the dart
_N_, then the centre of the eccentric must be above the centre of the
shaft or axle, in order to move the rocker in the direction indicated
by the dart _e_. Supposing, however, it was intended to move the crank
the reverse direction, as shown by the dart _N_ in fig. 101; it is
evident in that case that the valve must be moved in the same direction
as before, in order to open the front steam-port and thus admit steam
to force the piston back. But if the crank turns in the direction shown
by the dart _N_, fig. 101, then the centre of the eccentric must be
placed _below_ the centre of the axle in order to move the lower rocker
arm in the direction of the dart _e_ and the valve in that indicated by
_a_. It will thus be seen that the centres of the eccentric for running
forward and that of the one for running backward must be placed, the
one above and the other below the centre of the axle at the beginning
of the stroke of the piston, as shown in figure 101.
[Illustration:
_Fig. 100._
_Fig. 101._
_Fig. 102._
Scale ³⁄₈ in. = 1 foot.]
QUESTION 186. _Why is it that the centres of the eccentrics are not
placed opposite to each other on the axle?_
_Answer._ Because before the beginning of the stroke of the piston it
is necessary to move the valve from its middle position a distance
equal to the lap before the steam-port begins to open. If we have a
valve like that shown in fig. 10--that is, without any lap--the centres
of the eccentrics could be placed at right angles, or, as mechanics
say, “square” with the crank, as was shown in fig. 11, and exactly
opposite to each other, because such a valve begins to take steam
as soon as it moves from the middle of the valve-face. If, however,
we have a valve like that shown in fig. 27, it is plain that before
it will admit or _take_ steam, as it is called, in either of the
steam-ports, it must be moved from the centre of the valve-face, or its
_middle position_, a distance equal to the lap, L. For this reason,
therefore, the eccentric, instead of being placed at _half-throw_,[49]
as it is called, must be so far ahead of the middle position as to
have moved the valve a distance equal to the lap, and if any lead is
given to the valve, equal to the lap and lead together. In figs. 100
and 101, _f g_ is a vertical line at right angles to the crank at the
beginning of the stroke. It will be seen that the centre of each of the
eccentrics is set far enough ahead of this line to give the valve the
required lead. When the piston reaches the back end of the cylinder,
the two eccentrics will occupy the position shown in fig. 102, in which
position the lower one would move the valve so as to turn the crank
in the direction of the dart _N_, and the upper one in the reverse
direction. It will be seen that in this position both of the eccentrics
are again ahead of half-throw, when the piston is at that end of its
stroke.
[49] This would be at right angles to the crank when the piston is at
the end of the stroke.
[Illustration: _Fig. 103._
Scale ³⁄₄ in. = 1 foot.]
QUESTION 187. _How is the motion of either eccentric communicated to
the valve?_
_Answer._ The ends of each pair of eccentric-rods are connected
together by a link, _a b_, fig. 103. This link has a curved groove or
slot, _a b_, in it, in which a block, _B_, fits accurately, so that it
can slide freely from one end to the other. This block is attached to
the lower rocker-arm by a pin, _c_, which works freely in the block.
The two eccentric-rods _C_ and _D_ are attached to the ends of the
link at _e_ and _f_ by pins and knuckle-joints. It is apparent that if
the link is down, or in the position shown in fig. 103 and also on a
smaller scale in fig. 104, the motion of the upper eccentric-rod, which
is usually used for the forward motion, will be imparted to the rocker,
and thus to the valve, and when the link is in the position shown
in fig. 105, that the valve will be moved by the lower or backward
eccentric-rod _B_. In order to reverse the engine, it is then only
necessary to provide the means of raising and lowering the links. This
is done by a shaft, _A_, fig. 103, called a _lifting-shaft_, which has
two horizontal arms, _E_,[50] one for each link, and a vertical arm,
_F_. The links are suspended from the ends of the horizontal arms by
rods or bars, _g h_, called _link-hangers_, which are connected to
the links and to the arms above by pins, which enable the hangers to
vibrate freely. The lower pin is attached to a plate, _L d_, called
a _link-saddle_, which is bolted to the link. The vertical arm of the
lifting-shaft is connected by a rod, _G G_, called the _reverse-rod_,
to a lever _O_, _O_, Plate II. in the cab called a _reverse-lever_, the
construction of which will be explained hereafter. This lever is worked
by the locomotive runner, and by moving the upper end of it forward,
the link will be lowered, and the rocker and valve will be moved by the
forward eccentric; and if the reverse-lever is moved back, the link
will be raised, and the backward eccentric will move the valve. When
this is done, the valve-gear is said to be thrown into the _forward_ or
_backward motion_, or _forward_ or _back gear_.
[50] Only one of these is shown in the engraving.
[Illustration:
_Fig. 104._
_Fig. 105._
Scale, ³⁄₈ inch = 1 foot.]
[Illustration:
_Fig. 106._
_Fig. 107._
Scale, ³⁄₈ inch = 1 foot.]
QUESTION 188. _How is the travel of the valve changed by the motion of
the link?_
_Answer._ By either raising or lowering the link, so that the
link-block and rocker-pin will be some distance above or below
the eccentric-rods. Thus in fig. 104, the motion of the upper
eccentric-rod, and in fig. 105 that of the lower or _back_
eccentric-rod is communicated to the rocker-pin and the valve.
If, however, the link should be raised so that the link-block and
rocker-pin are somewhat below the upper or forward eccentric-rod, as
shown in fig. 106, then the motion imparted to the rocker and valve
will partake somewhat of that of the upper and also of the lower
eccentric-rod. So long as the rocker-pin is above the centre of the
link, the motion of the valve will partake most of that of the upper
or forward rod, and the engine will then run forward, but when the
rocker-pin is below the centre of the link, its motion will be
influenced more by the back eccentric-rod, and the engine will then run
backward.
The motion of the link, which is somewhat complex and difficult to
understand clearly, will perhaps be understood better if we represent
it in a number of successive positions of the whole stroke of the
piston, as was done to show the motion of the eccentric in figs. 11 to
24. We will therefore suppose that the link is in what is called _full
gear forward_, as shown in figs. 103 and 104. In fig. 108 the link is
in the position it would occupy at the beginning of the stroke of the
piston; in fig. 109 it is in that which it will be in when the piston
has moved four inches; in fig. 110, when it has moved eight inches; in
fig. 111, twelve; and in figs. 112, 113 and 114, sixteen, twenty and
twenty-four inches. Figs. 114 to 119 represent the successive positions
of the link during the return stroke. In order to show the different
positions of the link we have represented on a larger scale, in fig.
120, the successive positions of the centre line of the link, which
will indicate the motion imparted by it to the rocker. In order to
designate each of these positions, the centre lines in fig. 120 are
numbered + and -0, 4, 8, etc., etc., to correspond with similar numbers
in figs. 108 to 119.
[Illustration:
_Fig. 108._
_Fig. 109._
_Fig. 110._
_Fig. 111._
_Fig. 112._
_Fig. 113._]
[Illustration:
_Fig. 119._
_Fig. 118._
_Fig. 117._
_Fig. 116._
_Fig. 115._
_Fig. 114._
Scale ³⁄₈ in. = 1 foot.]
[Illustration:
_Fig. 120._
Scale ³⁄₁₆ in. = 1 inch.]
[Illustration:
_Fig. 121._
Scale ³⁄₁₆ in. = 1 inch.]
[Illustration:
_Fig. 122._
Scale ³⁄₁₆ in. = 1 inch.]
Thus the line _-0 -0_ represents the position of the centre of the
link which it occupies at the beginning of the stroke as shown in fig.
108. The line -4 -4, that represented by fig. 109, when the piston has
moved 4 in. The lines -8 -8, -12 -12, -16 -16, -20 -20, 24 24, +4 +4,
etc., the successive positions of the centre of the link represented
in figs. 108 to 119. The dotted lines _h a_ and _h b_ represent the
two extreme positions into which the rocker-arm would be moved by
the action of the link. It will be seen that when the link is in the
position shown, it imparts the full stroke of the eccentrics to the
rocker-pin and consequently to the valve. We will now suppose that the
link is raised up as shown in fig. 106, so that the position of the
rocker-pin is just half-way between the end of the eccentric-rod and
the centre of the link. This position is called _half-gear_. In fig.
121 the different positions of the centre line of the link and of the
rocker have been laid out for half-gear in the same way as was done
for full-gear before. From this it will be seen that the travel, _a
b_, imparted to the rocker-pin and valve by the link when it is in the
position shown, instead of being 5 in. is only 3¹⁄₂ in. In fig. 107 the
link is raised up, so that the rocker-pin is in the centre of it or
midway between the eccentrics. This position is called _mid-gear_. The
successive positions of the centre line of the link in this position
have been laid down in fig. 122 in the same way as was done for full
and half-gear. The movement of the rocker, it will be seen, is, for
mid-gear, only 2¹⁄₂ in. These diagrams show that when the rocker-pin is
opposite the eccentric-rod, the valve receives the full throw of the
eccentric, and that the motion imparted by the eccentric diminishes
as the rocker-pin approaches the centre of the link, so that, with
eccentrics having 5 in. throw and a valve with ⁷⁄₈ lap and ¹⁄₈ in.
lead, we can increase or diminish the travel of the valve from 2¹⁄₂
to 5 in. by simply raising or lowering the link, which is done by the
reverse-lever.
QUESTION 189. _What is the effect of this variation of travel on the
working of the valve and the admission and release of steam to and from
the cylinder?_
_Answer._ It is almost precisely the same as that which is effected
by increasing or diminishing the throw of the eccentric, which was
explained in the answer to Question 52. In order to show this effect
more clearly, we have represented by motion-curves,[51] fig. 123,
the movement imparted to the valve by the link when it is in full,
half and mid-gear, as illustrated in the preceding figures. The curve
for full-gear is engraved in full heavy lines; that for half-gear in
lighter lines, and for mid-gear in dotted lines. From these curves it
will be seen that when the valve is worked in full-gear the steam-port
is opened wide at 2 in. of the stroke and steam cut off at 21 inches.
When the valve is worked in half-gear the port is not at any time
opened wide and steam is then cut off at 17¹⁄₂ in. of the stroke, and
when worked in mid-gear the greatest opening of the steam-port is no
greater than the lead and the cut-off occurs at 4 inches of the stroke.
[51] The nature of these curves was explained in answer to Question
44.
It is of course possible to work the link in any intermediate position
between those which we have represented. Usually the reverse-lever is
arranged so that the steam will be cut off at 6, 8, 10, 12, 15, 18, and
20 inches of the stroke.
QUESTION 190. _What is the greatest and the least admission of steam
possible with the ordinary link motion?_
_Answer._ With 24 in. stroke of piston and 5 in. travel and ⁷⁄₈ in.
lap, steam can be admitted as shown by the motion-curves during 21
in. or 87¹⁄₂ per cent. of the stroke, and can be cut off at about 4
in. or 16²⁄₃ per cent. It will be seen, however, that in mid-gear the
motion-curve becomes a straight line, and that the _pre-admission_
of steam, that is the admission of steam before the piston reaches
the end of the stroke, is equal to that admitted after, so that it
is impossible to work the locomotive with the link in that position.
Practically it is found that no useful work can be done with a link
if the steam is cut off at less than six inches, or one-fourth of the
stroke. Even then the opening of the steam-ports is so small that the
steam which enters the cylinders is very much wire-drawn.
QUESTION 191. _How are the curves drawn which represent the motion of
the valve?_
_Answer._ These motion-curves as produced by the link-motion are very
difficult to draw, as the motion of the link is extremely complicated.
It is doubtful, therefore, whether those who have no knowledge of
mechanical drawing will be able to understand the following description
of the method of doing it, which we will try to make as clear as
possible.
[Illustration:
_Fig. 123._
Scale ³⁄₁₆ in. = 1 inch.]
[Illustration:
_Fig. 124._
Scale ³⁄₄ in. = 1 foot.]
In the first place, the centre _S_, fig. 124, of the axle, _A_, of
the rocker, and _B_ of the lifting-shaft, must be laid down in their
proper positions. If, now, the valve has ⁷⁄₈ in. lap and ¹⁄₈ lead, the
lower rocker-pin must be one inch ahead of its middle position when
the piston is at the front end of the cylinder, and at the beginning
of the backward stroke. We will, therefore, mark the centre, _a_, of
the rocker-pin in this position. If from the centre of the axle a
circle, _c d e_, be drawn whose diameter is equal to the throw of the
eccentrics, this circle will represent the path in which the centres of
the eccentrics will revolve. If, now, the distance from the centre of
the axle to the centre of the lower rocker-pin, _a_, when the latter
is in its middle position, be taken for a radius,[52] and from the
position of the rocker-pin at the beginning of the stroke as a centre,
the circle representing the path of the eccentrics be intersected at
two points, _c_ and _d_, the points of intersection will represent
the positions of the centres of the forward and backward eccentrics.
Having determined these positions, draw arcs of circles, _f_ and _g_,
from these centres with a radius equal to the distance from the centres
of the eccentrics to the centres of the pins which connect the rods
to the link. It is evident that at the beginning of the stroke the
centres of the pins in the link must each be in one of these arcs.
But the link is suspended by the hanger, _i h_, which oscillates from
the end, _i_, of the lifting-arm, which for any one point of cut-off
is stationary; and therefore the point of suspension of the link must
always be in the arc, _j k_, described from the centre of the pin, _i_,
in the lifting-arm, with a radius equal to the length of the hanger.
There are, therefore, three points in the link, each of which must be
in one of the arcs which have been drawn, and which will determine
the position of the link. This can be done easiest by drawing the
link, _L_, fig. 125, on a stiff piece of paper, _m n_, and cutting off
the back, _p q_, of it through the centres of the pins, _s_ and _t_,
and also cutting out a triangular piece, _u_, the apex of which will
correspond with the centre of the point of suspension, _o_. By placing
this piece of paper on the drawing it can be moved, so that the three
centre points, _s_, _t_ and _o_, will respectively conform with the
arcs, _f g_ and _j k_, fig. 124. In this position the piece of paper
will then be in the position of the link for the point of the stroke
represented. By marking the centres of the link-pins on the arcs _f_
and _g_, and from them as centres, with the length of the rods used to
draw the arcs, two other arcs, _v_, _w_, be drawn intersecting each
other, the point _d_, where they intersect will be the centre from
which the centre line of the link can be drawn with a radius, _l d_,
equal to the distance from the centre of the eccentrics to the centre
of the link. This will give the first position _-o -o_, of the centre
line of the link. As the rocker-pin must always be in the centre of
the link, it is obvious that the point at which the centre-line of the
link intersects the arc in which the rocker-pin oscillates must be the
position of the centre of the rocker-pin. With this determined the
position of the valve can easily be located.
[52] This is usually the radius of the link but in some cases
either a longer or shorter radius is taken to draw the link. In the
following explanation it is assumed that the link is drawn with this
radius, or from the centre of the axle. Of course if a greater or
lesser radius is used, due allowance must be made therefor.
[Illustration:
_Fig. 125._
Scale, ³⁄₄ in. = 1 foot.]
In order to represent the link at any other point of the stroke, say
after the piston has moved four inches, the position of the crank
must first be laid down. To do this, allowance must be made for the
irregularity due to the angularity of the connecting-rod, which was
explained in answer to Question 54. From the centre of the axle, a
circle, _C D_, whose diameter is equal to the stroke of the piston,
is first drawn, which will represent the path of the crank-pin. A
horizontal centre line, _E F_, should also be drawn through the centre
of the axle and the centre of the cylinder. The intersection _o_ of
this line with the path of the crank-pin will be the position of the
latter at the beginning of the stroke. If from this point a distance,
_o o_, be laid off on the centre line equal to the length of the
connecting-rod,[53] it will give the position of the wrist-pin at the
beginning of the stroke, so that from this its successive positions
for each inch of the stroke can be laid off. From its position after
the piston has made say four inches of the stroke as a centre, and the
length of the connecting-rod as a radius, if the path of the crank-pin
be intersected at -4, the point of intersection will represent the
position of the crank-pin at four inches of the stroke. The distance
from _o_ to -4 is equal to 44 degrees of the whole circle. The
eccentrics, being attached to the axle, of course move the same number
of degrees that the crank does, and therefore, in order to determine
their position when the crank has moved any distance, it is only
necessary to move them as many degrees as the crank has. This can
be done very easily by extending the radii of the eccentrics, when
they are in the first position, until they intersect the path of the
crank-pin at _c′_ and _d′_. By stepping off from the latter points of
intersection a distance _c′ c‴_ and _d′ d‴_, equal to _o_ -4, which the
crank has moved, and then drawing other radii from the two points _c‴_,
_d‴_, their intersection, _c″ d″_, with the path of the eccentrics
will represent the position of the centres of the eccentrics when the
crank is at -4. Having determined the position of the eccentrics,
the link can be laid down as before, that is, from _c″_ and _d″_ as
centres and with the length of the eccentric-rods as a radius arcs,
_f″_ and _g″_, are drawn. Then with the paper template the positions of
the centres of the link-pins in these arcs are determined and marked,
and from them with the length of the eccentric rods as a radius, two
intersecting arcs, _v″_, _w″_, are drawn, whose intersection gives
the centre of the link from which its centre line, -4 -4, is drawn.
This will give the position of the rocker-pin for another point of
the stroke. In a similar manner its position can be determined for
any number of points of the stroke, from which the position of the
valve can easily be determined and laid down on the diagram for the
motion-curve as was described in the answer to Question 44. Of course
the valve will be moved from its middle position the same distance
that the rocker-pin is,[54] only in an opposite direction. In order to
lay down the position of the valve on the diagram for motion-curves, it
is, therefore, only necessary to draw it in the same relative position
as that of the rocker-pin which is given by the point of intersection
of the center line of the link with the path in which the rocker-pin
oscillates. To construct the motion-curves it is necessary to determine
the positions of the valve for different points of the stroke and
mark them on the horizontal lines which represent the respective
positions of the piston. Curves are then drawn through these points,
either by hand or by constructing templates. The more points there are
determined, the more accurate will be the curves. It is, therefore,
best to lay down the position of the valve for each inch of the stroke
of the piston. They should also be drawn full size, which of course was
impossible for the illustrations which are given herewith.
[53] In order to get the engraving within the required limits, the
diagram is drawn with a connecting-rod only 5¹⁄₂ instead of 7 feet.
The latter is the length used in previous illustrations.
[54] This will be the case when the two arms of the rocker are of the
same length, as they usually are. Sometimes, though rarely, they are
of different lengths.
QUESTION 192. _Is there any other method of drawing these motion
curves?_
_Answer._ Yes: models which show the working of the valve-gear have
been constructed with a pencil, to which the reciprocating motion of
the valve is imparted, and which traces a curve on a surface having
the same motion as the piston. This method has been employed by the
writer in an instrument which he has applied to the locomotive itself.
The principle upon which it works will be understood by supposing
that the steam and exhaust-ports as represented in the diagram for
motion-curves, fig. 123, be drawn on a board, _A B C D_, fig. 126, but
instead of standing vertical, as in fig. 123, they are represented in a
horizontal position, and the board on which they are drawn is fastened
to the cross-head, _L_, so that the former will move backward and
forward simultaneously with the latter and the piston. A small shaft,
_F_, is attached to suitable supports, _j_, which are fastened to the
guides. This shaft has two arms, _G_ and _E_, one vertical and the
other horizontal and of the same length. The upper end of the vertical
one, _G_, is then attached to the valve-stem or rocker-arm by a short
connecting-rod, _H_, or other suitable means, so that the movement of
the valve-stem will be imparted to the arm and shaft. Of course the end
of the horizontal shaft then has exactly the same motion vertically
that the valve-stem and valve have horizontally, with the very trifling
inaccuracy due to the fact that the movement of the one is in a
straight line, whereas the other is in the arc of a circle.
Now if a pencil, _P_, is attached to the end of the horizontal arm,
_E_, and is set so that its point indicates the exact position of the
steam edge, _h_, of the valve, as shown in fig. 123, it is obvious that
when the piston and board have moved four inches, the pencil will have
moved downward and have drawn the portion of the motion-curve from _h_
to _i_; and when the piston has moved eight inches the curve will be
drawn to _j_, and at 12, 16, 20 and 24 inches of the stroke the curve
will be drawn to _k_, _l_, _m_ and _n_. During the return stroke a
corresponding curve, _n o h_, will, of course, be drawn. With such an
instrument curves can be drawn for any position of the link, and they
will show the exact movement of the valve during the whole stroke,
and will indicate all the defects resulting from bad proportions or
construction, lost motion in the parts, or other causes of error or
irregularity.
[Illustration: Fig. 126.--Scale ³⁄₄ in. = 1 foot.]
In using this instrument, however, it is impracticable to attach
a board to the inside of the cross-head, and it must therefore be
fastened to the outside. The horizontal arm _E_ should be made of thin
steel, so as to form a spring. The end has a small _boss_[55] with a
hole in it ³⁄₁₆ of an inch in diameter. This hole has a screw thread
cut in it, into which an ordinary hard drawing pencil is screwed. The
spring is so arranged that the pencil will not be in contact with the
board unless it be pressed against it. The locomotive is then placed
on a smooth piece of track with steam on and run very slowly, so that
a person walking alongside can press the pencil against the surface
of the board, which should be covered with drawing paper. By watching
the cross-head when it reaches the end of the stroke, the pencil can
then be pressed against the paper and kept in contact through the whole
stroke and instantly released when the motion-curve is completed. The
link can then be placed in another position, and thus any number of
curves can be drawn, which will furnish the most accurate means of
analyzing the motion of the valve.
[55] The term “boss” is used to imply an enlargement or increased
thickness of any part.
In practice it is best not to draw the lines which represent the edges
of the ports, until after the curves are drawn and the paper removed
from the board. A centre line must, however, be drawn on the engine
from which to lay off the ports. This can be done by placing the valve
in its middle position, and then fastening the shaft _F_ in that
position with a nut which should be provided for that purpose on the
end of the shaft. After it is fastened in this position, detach the
connecting-rod _H_, and with one stroke of the piston a centre line can
be drawn with the pencil _P_. From this centre line the edges of the
ports can easily be laid off and drawn on the paper after it is taken
off the engine.
QUESTION 193. _Can the position of each edge of the valve, with any
given amount of travel, be shown in its relation to the ports by one
motion-curve, or is it necessary to draw such curves for each edge of
the valve, as shown in fig. 28?_
_Answer._ One motion-curve is sufficient to represent the position of
any part of the valve during the entire stroke. This will be apparent
if it is remembered that each motion-curve is exactly like the others,
as shown in fig. 28, the only difference being that the ports occupy
different positions in relation to the curves. It is, therefore, only
necessary to draw lines to represent the relative positions of the
ports to the other curves to show the entire motion of the valve by
one curve. To illustrate this it will be assumed that a motion-curve,
_h i j k l m n o p_, and a centre line, _a b_, fig. 127, have been
drawn with the instrument described in the answer to the previous
question. The centre line _a b_, which will be equal in length to the
stroke of the piston, should then be divided into inches, and lines
_ff_, 23 1, 22 2, etc., should be drawn through the points of division
and at right angles to _a b_. If, now, we want to show the movement
of the front steam edge of the valve in relation to the corresponding
steam port, a line, _t_, should be drawn perpendicular to _f f_, to
represent that edge of the valve at the beginning of the stroke. As it
is impossible to determine accurately the position of this steam edge
at the beginning of the stroke from the motion-curve, which is then
_tangent_[56] to the line _f f_, we must lay it off from the centre
line, _a b_. This can readily be done if we remember that if a valve
has ⁷⁄₈ in. lap when it is in the middle position, as shown in fig. 27,
and ¹⁄₁₆ in. lead at the beginning of the stroke, it must have moved
¹⁵⁄₁₆ in. from the middle position at the beginning of the stroke as
shown in fig. 28. The line _t_ must therefore be drawn ¹⁵⁄₁₆ in. from
_a b_ to represent its proper position in relation to the motion-curve,
and as it has ¹⁄₁₆ in. lead, the steam edge, _h h′_, of the steam-port
must be drawn at that distance from _t_. Another line, _m m′_, can then
be drawn to represent the width of the front steam-port, _c c′_. From
these lines the movement of the valve in relation to the front port, _c
c′_, and the admission of steam are shown as clearly as in fig. 28.
[56] A curve is said to be _tangent_ to another carve or to a
straight line when the two just touch, but do not intersect or cross
each other.
If now we want to represent the motion and relative position of the
back steam edge of the valve in relation to its port, it is only
necessary to assume that the line _t_ represents that edge, and that
the curve _h i j k l m n o h_ represents its motion, and to draw the
back steam-port in its proper relation to it. When the valve is in its
middle position, as shown in fig. 27, the outside edge of the port _h_
is ⁷⁄₈ in., or a distance equal to the lap, from the steam edge _q_ of
the valve. As the center line _a b_, fig. 127, represents the middle
position of the edge of the valve, it is only necessary to draw a line,
_n n′_, ⁷⁄₈ in., or the same distance from the centre line _a b_ that
the outer edge of the port _d_ is from _q_ in fig. 28, to represent
this edge of the port in fig. 127, and another _b b′_, at a distance
from the former equal to the width of the port, to represent its inner
edge. A line, _q_, below the line 0 24, will represent the edge of the
valve at the beginning of the forward stroke. The curves in relation to
the port _d_ will then show the motion of the valve in relation to this
port, in the same way that the dotted curve _d f_ does in fig. 28.
[Illustration:
Fig. 127.
Scale ³⁄₁₆ inch = 1 inch.]
If it is desired to represent the motion of the exhaust edge _h′_, fig.
28, of the valve, it is only necessary to imagine that the line _t_,
fig. 127, represents that edge, and then draw in the port _d_ in the
same relation to it that it bears to the edge _h′_, in fig. 28. This
has been done in dotted lines, _c c′_ and _e e′_, in fig. 127.
If the reader will cut a paper section of a valve like that shown in
fig. 27 and place the different edges, _h_, _i_, _h′_ and _q_, so that
they will successively correspond with the line _t_ in fig. 127, the
diagram will perhaps be more clear. If, for example, the paper section
be placed to the right of the line _t_, so that the edge _h_ will
correspond with _t_, then it will be seen that the port _c_ occupies
the same relation to it that it does in fig. 28. If the valve be placed
to the left, so that the edge _q_ corresponds with _t_, then the port
_d_ will be in the same relation to it that it has in fig. 28. If the
edges _i_ and _h′_ be made to correspond with _t_, then the ports drawn
in dotted lines in fig. 127 will represent the ports _c_ and _d_ in
fig. 28.
The position of the ports in relation to the centre line of the
motion-curve can be determined, if it is kept in mind that the centre
line _a b_, fig. 127, represents the position of the different edges
of the valve when the latter is in the middle of the valve-face as
shown in fig. 27, and that the ports must be on the same side, and the
same distance from the centre line that they are from the edge of the
valve whose motion is represented. Thus if the movement of the steam
edge _h_ in relation to its port, _c_, was represented, the edge of
the latter must be drawn on the motion diagram the same distance from
the centre line that it is from _h_ when the valve is in its middle
position as shown in fig. 27. This distance is of course just equal
to the lap of the valve. If the motion of the exhaust edge _h′_ was
represented in relation to the steam port _d_, then the inside edge
of the latter would be drawn the same distance from the centre line
_a b_ in the diagram that the inner edge of the port is from the edge
_h′_ of the valve, which is equal to the inside lap. The exhaust
port could also be drawn in the same way, but it would be liable to
confuse a diagram made to so small a scale as that which has been
employed for the accompanying illustrations, and it has therefore
been omitted. Diagrams of this kind which are made full size will,
of course, show the movement of the valve more distinctly than is
possible in the space occupied by the illustrations herewith. When
they are made of full size, the lines indicating the ports should be
drawn of different colors, so as to distinguish them from each other
easily. Such diagrams will show the position of the valve in relation
to the ports, and indicate the distribution of the steam during the
whole stroke. It is only necessary to refer the curve to the proper
line to determine the position of the valve in relation to either of
the ports for either the admission or release of the steam. If, for
example, we want to observe how the admission of steam is governed by
the valve, by referring to fig. 127 we see that at the beginning of
the backward stroke the valve has ¹⁄₁₆ inch lead; that at 1³⁄₄ inches
of the stroke the port _c_ is wide open, as shown by the intersection
of the motion-curve with the line _m m′_; that the valve has received
its maximum backward travel at 9 inches of the stroke, and begins to
close the port at 15¹⁄₂ inches, and completely closes it at 21 inches
of the stroke. By referring the motion-curve to the lines _n n′_ and _b
b′_, we see that the valve as shown by the line _q_ at _n′_ again has
¹⁄₁₆ inch lead at the beginning of the forward stroke; that the steam
port is wide open at 1³⁄₈ inches of the stroke; begins to close at
16¹⁄₄ inches, and is completely closed at 21 inches. By referring the
curve to the lines _e e′_ and _c c′_ we see that the front port begins
to open to the exhaust before the piston has completed its forward
stroke and when it has nearly an inch to move, that it is wide open
almost immediately after the piston begins its stroke, does not begin
to close until the piston has moved 19¹⁄₂ inches of its stroke, and is
completely closed at 23 inches of the stroke. By referring the curve to
the lines _d d′_ and _g g′_, almost the same phenomena will be observed
for the forward stroke. In fact from such a diagram the whole motion
of the valve can be studied and analyzed with the greatest accuracy;
and, as has already been shown, the motion imparted to a slide valve
by a link is of so complicated a nature that it is almost or quite
impossible to observe its exact nature without such diagrams.
QUESTION 194. _Can a motion diagram be constructed to represent the
motion of the valve with different amounts of travel?_
_Answer._ Yes; it is only necessary to construct motion-curves for
the same diagram for each distance traveled, and they will show the
movement of the valve for the given amount of travel represented by the
curves. This has been done in fig. 128, which is a reduced copy of a
series of motion-curves taken from a locomotive. From this diagram the
movement of a slide-valve worked by the link-motion can be seen from
the highest to the lowest practicable point of cut-off. For convenience
of reference the curves have been numbered.
[Illustration:
_Fig. 128._
Scale ³⁄₁₆ inch = 1 inch.]
The smallest travel of the valve represented by curve No. 1 is a little
less than 2¹⁄₂ in., and the ports are then opened only about ⁵⁄₁₆
in., and the steam is cut off at 8 in. on the backward and 6³⁄₄ in.
on the forward stroke. The exhaust is opened or the steam is released
during the backward stroke at 17 in., and during the forward stroke at
16⁵⁄₈. When the valve works with its greatest travel, as represented by
curve 8, it travels 5 in., and opens the steam port wide at 3 in. of
the backward stroke and 2¹⁄₄ in. of the forward stroke. The steam is
cut off at 20³⁄₄ and 20¹⁄₂ in., and its release takes place at 23¹⁄₈
in. of each stroke. The following table gives the greatest width of
opening, the point of cut-off, the point of release, and the lead for
each motion-curve on the diagram. This table has been made up from the
motion-curves drawn with the instrument described in answer to Question
191, on a locomotive which had been running about eighteen months and
whose valve-gear consequently was considerably worn, as s indicated
by the flatness of the motion-curves on each side at the point when the
motion of the valve was reversed. This flatness was caused by the lost
motion in the valve-gear, the pencil remaining for a time stationary
when the motion was reversed and while the parts were moving from their
bearings on one side to those on the other. The curves and the table
therefore show the operation not of a theoretically perfect valve-gear,
but are examples of actual practice, with such imperfections as are
incidental to ordinary locomotives. It will be seen that the instrument
shows not only what the valve-gear should, but what it actually does
do, and delineates all its imperfections.
======+======+===============+===============+===============+=====
| | Width of | | |
| | opening | Point of | Point of |
| | of steam-port.| cut-off. | release. |
No. |Travel+-------+-------+-------+-------+-------+-------+
of | of | Backwd|Forw’rd| Backwd|Forw’rd|Backwd |Forw’rd|
curve.|valve.|stroke.|stroke.|stroke.|stroke.|stroke.|stroke.|Lead.
------+------+-------+-------+-------+-------+-------+-------+-----
| in. | in. | in. | in. | in. | in. | in. | in.
1 | 2¹⁄₂ | ¹¹⁄₃₂| ⁵⁄₁₆ | 8 | 6³⁄₄ |17 |16⁵⁄₈ |⁹⁄₃₂
2 | 2⁵⁄₈ | ⁷⁄₁₆ | ¹³⁄₃₂ | 9¹⁄₂ | 9¹⁄₂ |18⁵⁄₈ |18⁵⁄₁₆ |¹⁄₄
3 | 2⁷⁄₈ | ⁹⁄₁₆ | ¹⁄₂ | 12 | 11³⁄₄ |19³⁄₄ |19⁹⁄₁₆ |⁷⁄₃₂
4 | 3¹⁄₈ | ¹¹⁄₁₆| ⁴¹⁄₆₄ | 14 | 14 |20¹¹⁄₁₆|20⁹⁄₁₆ |³⁄₁₆
5 | 3¹⁄₂ | ⁷⁄₈ | ²⁷⁄₃₂ | 16¹⁄₂ | 16¹⁄₂ |21¹¹⁄₁₆|21¹⁄₂ |⁵⁄₃₂
6 | 4 | 1¹⁄₈ |1³⁄₃₂ | 18¹⁄₄ | 18¹⁄₈ |22³⁄₈ |22¹⁄₄ |¹⁄₈
7 | 4¹⁄₂ | 1¹⁄₄ |1¹⁄₄ | 19⁵⁄₈ | 19¹⁄₂ |22¹³⁄₁₆|22³⁄₄ |³⁄₃₂
8 | 5 | 1¹⁄₄ |1¹⁄₄ | 20⁵⁄₈ | 20¹⁄₂ |23¹⁄₈ |23¹⁄₈ |¹⁄₁₆
-=====+======+=======+=======+=======+=======+=======+=======+=====
QUESTION 195. _What are the chief dimensions of the valve-gear
represented in fig. 128?_
_Answer._ The throw of eccentrics was 5 in., the steam-ports were 1¹⁄₄
in., and the exhaust-port 2³⁄₄ in. wide, the valve had ⁷⁄₈ in. outside
and ¹⁄₁₆ inside lap and ¹⁄₁₆ in. lead at full stroke.
QUESTION 196. _What relation is there between the distance which the
ports are opened by the valve, and its travel when worked by a link?_
_Answer._ As explained in the answer to Question 52, the width which
the steam-ports are opened by the valve for the admission of steam
diminishes with the travel of the valve. This is shown very clearly by
the motion-curves, and also in the above table, from both of which it
will be seen that when the valve travels only 2¹⁄₂ in. the steam-ports
are opened only ¹¹⁄₃₂ in. for the back stroke and ⁵⁄₁₆ for the front.
With 2⁵⁄₈ travel the opening is ⁷⁄₁₆ and ¹³⁄₃₂ in. With 4 in. travel
the port is opened 1¹⁄₈ and 1³⁄₃₂ in. and with 4¹⁄₄ in. travel they
would be opened wide. With 4¹⁄₂ and 5 in. travel, as will be seen from
the motion diagram, the ports are not only opened wide, but the valve
throws “over” them, or travels beyond their inner edges.
QUESTION 197. _How is the point of cut-off affected by the link?_
_Answer._ Changing the travel of a valve with a link has a very similar
effect to that produced by eccentrics of different throw--that is, the
period of admission is increased with the throw of the eccentric and
that for expansion lessened. This is shown clearly in both the motion
diagram and the table. With the first curve and a travel of 2¹⁄₂ in.
the steam is cut off at 8 in. for the backward stroke and 6³⁄₄ in. for
the front, and with 5 in. travel steam is admitted during 20⁵⁄₈ in. of
the backward and 20¹⁄₂ in. of the forward stroke.
QUESTION 198. _How is the point of release or exhaust of the steam
affected by the link?_
_Answer._ As the travel increases, it is delayed until later in the
stroke. Thus, with 2¹⁄₂ in. travel the steam is exhausted or released
from the cylinder during the backward stroke when the piston has moved
17 in., and on the return stroke at 16⁵⁄₈ in., whereas, with 5 in.
travel of the valve, the release is delayed until 23¹⁄₈ in. of the
stroke. An examination of the diagram and table will show very clearly
the relation of the point of release to the travel.
QUESTION 199. _How is the lead affected by the ordinary link motion?_
_Answer._ It is increased as the travel is diminished, as is shown in
the table, and also by the inclination of the curves at the top and
bottom of the diagram.
[Illustration:
_Fig. 129._
_Fig. 130._
_Fig. 131._
Scale ³⁄₄ in. = 1 foot.]
QUESTION 200. _What is the cause of this change of the amount of lead?_
_Answer._ This can be best explained by reference to fig. 129, which
represents a link with very short eccentric rods. If now the centre
from which the link was drawn was in the centre of the axle _S_, and
the eccentric straps embraced the axle instead of the eccentrics, their
ends _c_ and _d_ would each describe the same arc, _a b_, parallel
with the centre line, _x y_, of the link, and the latter could then
obviously be raised and lowered without moving the rocker-pin at all.
But the eccentric straps being attached to the eccentrics, as shown by
the dotted lines, when the rods are raised or lowered they describe
arcs, _c e_ and _g h_, from the centres _s_ and _t_ of the eccentrics,
and not from the centre of the axle. When the link is raised then, the
end of the upper rod obviously moves in the arc _c e_, and the top
of the link is moved from the axle, as shown in fig. 130, a distance
equal to the interval between the arc, _a b_, drawn from the centre
of the axle, and _e f_, which the rod describes from the centre
of its eccentric. When the link is lowered from back to mid-gear, a
similar action takes place, as the end, _d_, fig. 130, of the lower
rod describes an arc, _f g_, so that the whole link is thrown from the
axle a distance equal to the space between the arcs described from the
centre of the axle and the centres of the eccentrics. When the position
of the eccentrics is reversed, as shown in fig. 131, the link is moved
towards the axle, thus causing an increase of lead on the opposite
side of the valve. We have employed for our illustrations very short
eccentric rods, in order to make this action apparent by exaggerating
it. It is obvious from the engravings that the difference in the lead
is increased as the eccentric rods are shortened, and also as the
distance between the points of connection of the rods with the link
is increased. It will also be plain that increasing the throw of the
eccentrics, that is, increasing the distance of the centres _s_, _s_,
of the eccentrics from the centre _S_ of the axle will also increase
the variation in the lead in full and mid-gear.
QUESTION 201. _What is meant by the distribution of steam in the
cylinder?_
_Answer._ It means the admission and exhaust of steam to and from the
cylinder in relation to the stroke of the piston or the revolution of
the crank.
QUESTION 202. _What are the principal periods or elements of the
distribution of steam by the slide-valve and link motion?_
_Answer._ They are:
1. The _pre-admission_ or lead, that is, the admission of steam into
the cylinders in front of the piston before it has completed its
stroke.
2. The _admission_ of steam after the piston has commenced its stroke.
3. The _expansion_ of steam in the cylinder.
4. The _pre-release_, or exhaust of steam before the piston has
completed its stroke.
5. The _release_, or exhaust during the return stroke of the piston.
6. The _compression_ of steam, or closing the exhaust before the piston
has completed its return stroke.
QUESTION 203. _What is meant by the clearance of the piston?_
_Answer._ It is the space between the piston and the cylinder-head
when the former is at the end of the stroke. If the piston touched the
cylinder-head at the end of each stroke, it would cause a concussion
or “thump” which would injure these parts. Owing to the impossibility
of constructing machinery with absolute accuracy, it is therefore
necessary to leave a space, usually from ¹⁄₂ to ¹⁄₄ in. wide, between
the piston and the cylinder-heads, so as to be certain that they will
not strike each other should there be any slight inaccuracies in the
length of the piston-rods, connecting-rods, frames or other parts.
QUESTION 204. _Why is it desirable to open the steam-port and admit
steam at the end of the cylinder towards which the piston is moving
BEFORE the latter has completed its stroke?_
_Answer._ Because it is essential, in order to insure a good action of
the steam, that the maximum cylinder pressure should be attained at the
very commencement of the stroke. If the steam-port was not opened until
after the piston had commenced its stroke, some appreciable time would
be consumed in filling the clearance space and the _steam-way_ with
steam.[57] It is also found, especially if an engine is working at a
high speed, that a slide-valve worked by the ordinary link-motion will
not open the steam-port rapidly enough to enable steam of the maximum
boiler pressure to fill the space after the receding piston, unless the
valve begins to open the port _before_ the piston reaches the end of
its stroke.
[57] The _steam-ways_ are the passages which lead from the
steam-chest to the cylinder, and are sometimes called steam-ports,
but the term steam-ways is used to distinguish the passages from
their openings in the valve-seat, which latter are more properly
called steam-ports.
Another advantage resulting from the pre-admission of steam consists in
the smooth working of the engine at high speeds, a circumstance which
reduces greatly the wear and tear of the working gear. As the piston
approaches the end of its stroke, the pre-admitted steam forms a kind
of elastic cushion, which is well calculated to absorb the momentum
of the reciprocating parts at that instant. The pressure due to the
momentum of these parts will, of course, depend upon their weight and
the speed of working, increasing directly as the square of the speed.
It follows from this that the lead should increase with the speed, and
that it should be greatest at high speeds. As has been shown before,
this condition is fully accomplished by the ordinary shifting-link
motion.
QUESTION 205. _Upon what does the admission of steam into the cylinder
depend?_
_Answer._ It depends in the first place upon the opening of the
throttle-valve, and the size of the pipes and passages through which it
is conveyed from the boiler to the cylinder. In the second place, it
depends upon the time and amount of opening of the steam-port by the
valve.
QUESTION 206. _What should be the pressure of the steam in the cylinder
during admission?_
_Answer._ In order that the steam may be used to most advantage, it
should be admitted and maintained in the cylinder at full boiler
pressure during the whole period of admission. If the opening of
either the throttle-valve or the steam-ports is not sufficient to
allow the steam to flow into the cylinder at full boiler pressure, the
steam is said to be wire-drawn, and much of the advantage of using it
expansively as has already been explained in answer to Question 59, is
then lost.
QUESTION 207. _Why is it difficult to admit and maintain steam at the
full boiler pressure in the cylinder during admission?_
_Answer._ Because it is necessary to reduce the travel of the
slide-valve in order to cut off the steam “_short_,” or soon after the
beginning of the stroke of the piston. When the travel is reduced, the
valve opens the port only a small distance, so that the area of the
opening is not then sufficient to allow the steam to flow into the
cylinder with sufficient rapidity to fill it at full boiler pressure,
especially if the engine is working at a high speed. Thus, by referring
to the table given on page 216 and to the motion curves in fig. 128, it
will be seen that when the steam is cut off at from ¹⁄₄ to ¹⁄₂ stroke,
the port is opened for the admission of steam only from ¹⁄₄ to ¹⁄₂ inch
wide. From the curves it will also be seen that the valve then acquires
its maximum travel and the steam-port its greatest width of opening
very soon after the piston begins its stroke; after which the port is
gradually closed, so that before the steam is entirely cut off the
opening is so much reduced in area that the steam cannot flow through
it rapidly enough to maintain the steam at full boiler pressure in the
cylinder when the engine is working at high speeds.
QUESTION 208. _What means are used to overcome this difficulty and
thus admit steam at full boiler pressure when the valve is cutting off
short?_
_Answer._ In the first place, the steam-ports are made from ten to
twelve times as long as they are wide, so that a narrow opening will
have a comparatively large area. In the second place, by giving the
valve lead, not only are the clearance space and the steam-way filled
with steam when the piston begins its stroke, but the port is then
open a distance equal to the lead. With the ordinary link motion, as
has already been shown, this lead increases as the travel and period
of admission diminish, so that the smaller the total distance that
the port is opened, the greater is its opening at the beginning of
the stroke. As the steam is usually cut off short when locomotives
run at high speeds, it will be seen that the increased lead which is
imparted to the valve by the shifting link is an advantage rather than
a disadvantage. But while it is often possible in this way to secure
a pressure of steam in the cylinder at the beginning of the stroke
equal or nearly so to that in the boiler, yet it is almost impossible
to maintain this pressure during the whole period of admission, when
the steam is cut off short and the engine working at a high speed. To
obviate this evil what is called the Allen valve was designed, which
is represented in fig. 132. This valve has a channel or supplementary
port, _a a_, which passes over the exhaust cavity, and has two
openings, _b_, _b′_, in the valve-face. When the valve begins to admit
or “_take_” steam at _c_, as shown in fig. 133, it will be seen that it
also uncovers the opening _b′_ at _e_ and admits steam at _b′_, which
passes through the channel _b′ a a b_ and enters the steam-port _c_
at _b_, and in this way there is a double opening for the admission
of steam. The opening _b_ of the supplementary port is closed as the
valve advances, but when this takes place the steam-port is uncovered
far enough to admit all the steam that is required. This form of valve
is very efficient when the travel and point of cut-off are very short.
It then gives just twice as much opening as the ordinary valve for the
admission of steam. This improved valve has been much used in Europe;
but, although it is an American invention, has not received the
attention in this country which its merit deserves.
[Illustration:
_Fig. 132._
_Fig. 133._
Scale ³⁄₁₆ in. = 1 inch.]
QUESTION 209. _What is meant by the pre-release of steam?_
_Answer._ It is the release of the steam before the piston has
completed its stroke. If it is confined until the piston has reached
the end of the cylinder, there will not be time nor will it be
possible, with a slide valve and link-motion, to secure a sufficiently
large opening of the port to permit the steam to escape from the
cylinder before the piston begins its return stroke. If there were no
pre-release, there would therefore be more or less back pressure on the
piston.
QUESTION 210. _Upon what does the amount of pre-release depend?_
_Answer._ First, as has already been explained in answer to Question
51, on the amount of inside lap; and second, on the outside lap of
the valve and lead of the eccentrics; and third, on the travel of
the valve. The less the inside lap, the greater the outside lap and
consequent lead of the eccentrics; and the shorter the travel of the
valve, the earlier will be the release. The proper amount of this
pre-release depends upon the velocity of the piston and the quantity
of steam to be discharged or the degree of expansion. From the
motion-curves in fig. 128 it will be seen that it is a marked feature
of the shifting-link motion that the pre-release occurs earlier in the
stroke as the link approaches mid-gear, or as the travel of the valve
diminishes. As the link is usually worked near that position when the
engine is run at a high speed, it will be seen that in this respect
again the link-motion is well adapted for working the slide-valves of
locomotives.
QUESTION 211. _What governs the period of release?_
_Answer._ The release like pre-release is dependent upon the amount of
inside lap, the outside lap and consequent lead of the eccentrics, and
the travel of the valve.
The addition of inside lap has the effect of closing the port earlier
than it would be closed without, and thus shortening the period of
release and also of reducing the area of the opening of the port. This
will be apparent by referring to fig. 128, in which the valve had ¹⁄₁₆
in. lead. The dotted lines which represent the edges of the ports in
relation to the exhaust edges of the valve are therefore drawn ¹⁄₁₆ in.
from the centre line _a b_. If, however, there had been no inside lap,
then the edges of the ports would have conformed to the line _a b_. It
will be observed that the first curve crosses the dotted line _g g′_
at 15¹⁄₂ in. of the forward stroke, which is the point at which the
port is closed to the exhaust, or where the period of release ends and
compression begins. If there had been no lap and the line _g g′_ had
therefore occupied the same position as _a b_, then the motion-curve
would not have crossed it until the piston had reached 16 in. of its
stroke, thus showing that the period of release had been lengthened
and compression delayed. As the width of the opening of the port is
represented by the distance of the motion-curve from the right hand
side of the line _g g′_, which represents the edge of the port, it is
obvious that if there had been no lap, so that the position of the line
representing the edge of the port had occupied the position of _a b_,
then the space between it and the motion-curve would have been greater,
thus showing that the port would have been opened wider if there had
been no inside lap. The width of the opening of the port to the exhaust
is in fact always diminished by an amount equal to the inside lap.
With the same travel, increase of outside lap and lead shortens the
period of release, but has no effect on the width of the opening of the
port to the exhaust.
Increase of travel, with the same outside lap, lengthens the period of
release and also increases the width of the opening of the port to the
exhaust.
QUESTION 212. _What governs the period of compression?_
_Answer._ As compression begins when release ends, or when the port is
closed to the exhaust, it is controlled by exactly the same causes, and
as the two events occur simultaneously, of course whatever shortens the
period of release lengthens that of compression.
QUESTION 213. _What effect do the clearance spaces and steam-ways have
upon the compression of the confined steam?_
_Answer._ By referring to the motion-curves in fig. 128, it will be
seen that the steam-port is closed by the exhaust edge of the valve,
or compression begins some time before the piston reaches the end of
the stroke. The result is that the remaining portion of the cylinder,
through which the piston must move _after_ the port is closed to the
exhaust, is filled with steam of atmospheric pressure, or possibly
a little above that pressure. As this is confined in the cylinder,
it is compressed by the advance of the piston. If there was no room
between it and the cylinder at the end of the stroke, then either the
cylinder would be burst or the valve would lift so as to allow the
compressed steam to flow back into the steam-chest. The clearance and
the steam-passages, however, afford considerable room, into which
the confined steam can be compressed without danger of bursting the
cylinder, or of raising the slide-valve when there is steam in the
steam-chest. As the clearance spaces and steam-ways must be filled
with high-pressure steam at the beginning of each stroke, it must be
obtained either by taking a supply of “_live_”[58] steam from the
boiler, or by compressing into the clearance spaces the low-pressure
steam that still remained in the cylinder when the port was closed to
the exhaust. By the latter process, a certain quantity of steam is
saved at the expense of increased back pressure. It should be borne in
mind also that the total heat of the compressed steam increases with
its pressure, and as this latter approaches that in the boiler, the
temperature of the former must have been raised from that due to about
atmospheric pressure to nearer the temperature of that in the boiler.
These changes of temperature which the steam undergoes will affect
the surface of the metal with which the steam is in contact during
the period of compression; it follows from this, that the ends of the
cylinder principally comprising the clearance spaces must acquire a
higher temperature than those parts where expansion only takes place.
This is an important consideration, since the fresh steam from the
boiler comes first in contact with these spaces, and by touching
surfaces which have thus previously been heated, as it were, by the
high temperature of the compressed steam, less heat will be abstracted
from the fresh steam, and therefore a less amount of water will be
deposited in the cylinder.[59]
[58] The term “live” steam means steam taken direct from the boiler
and which has not been used in the cylinder or to do any work.
[59] Bauschinger’s Indicator Experiments on Locomotives, published in
Vol. III. of the RAILROAD GAZETTE.
It will thus be seen that the effect of compression is to fill
the clearance spaces and steam-ways with compressed steam before
pre-admission begins. As already stated, this is done at the expense
of back pressure in the cylinder. It must be remembered that all
the energy, excepting that part which is wasted by loss of heat,
friction, etc., which is consumed in compressing the confined steam,
is again given out to the piston by expansion. The confined steam
also acts as an elastic cushion to receive the piston, just as the
steam which is admitted before the end of the stroke would if there
were no compression. Compression, therefore, has the effect of saving
the quantity of live steam which it would otherwise be necessary to
admit before the end of the stroke to fill the clearance spaces and
steam-ways and also to “cushion” the piston. As already stated, the
momentum of the piston and other parts depends upon their weight
and the speed at which they are working, increasing directly as the
square of the speed, from which it follows that the compression should
increase rapidly with the speed, and should be the greatest at high
speeds. As the ports are prematurely closed to the exhaust with the
shifting-link motion, and as the lead increases rapidly as the link
approaches mid-gear, and the amount of compression is at the same time
correspondingly augmented, it will be seen that the shifting-link
motion fulfills these conditions very perfectly.
The pressure to which the confined steam will rise depends of course
upon the amount of the period of compression, and also on the size
of the clearance spaces. As it is possible to have such an amount of
compression that it will exceed the boiler pressure, and thus raise
the valve from its seat and be forced back into the steam-chest, some
care must be exercised to proportion the one to the other, so that the
degree of the confined steam may not be excessive.
QUESTION 214. _How can the effect of the distribution of the steam upon
its action in the cylinder be determined by experiment?_
_Answer._ As already explained in answer to Question 55, this can be
done by an instrument called a steam indicator.
[Illustration: Fig. 134.]
[Illustration: Fig. 135.
Scale 3 in. = 1 foot.]
QUESTION 215. _What is the construction of this instrument?_
_Answer._ The indicator now ordinarily used is the Richards indicator,
the outside of which is represented in fig. 134 and a section in fig.
135. It consists of a cylinder, _B_, into which a piston, _C_, is
accurately fitted, but so that it will move freely in the cylinder.
The piston rod is surrounded with a spiral spring, _D_, the lower
end of which is attached to the top of the piston, and the upper end
to the cylinder cover. When steam is introduced below the piston it
pushes it up in the cylinder and the spring is compressed. If there
should be a vacuum below the piston, the air above it will press the
piston downward and extend the spring. This latter occurs only when
the indicator is used on condensing engines. Of course the distance
which the piston is forced up by the steam pressure below it depends
upon the amount of pressure and also on the tension of the spring; and
therefore by attaching a pencil to the piston-rod so that it can mark
on a moving card in front of it, a diagram will be drawn which would
indicate the steam pressure, as was explained in answer to Question 55.
But there are some practical difficulties in the way of doing this.
It is found that if the pencil is attached directly to the piston-rod
of the indicator, the distance through which they must move, in order
to make the scale of the diagram sufficiently large to be clear, is
so great that the momentum of the parts carries them further than the
pressure of the steam alone would move them. The distance through
which the pistons or instruments move, moreover, makes it impossible
that the changes of pressure should be indicated simultaneously with
the position of the piston; the latter must travel while the action
is taking place, and thus the diagram shows changes of pressure later
or more gradually than they occur.[60] To overcome these and other
difficulties, the piston-rod of the indicator which we have illustrated
is attached at _h_ to the short arm of a lever, _F G_, and to the end
of the long arm a piece, _F I_, is attached, which carries a pencil,
_J_. By this means the piston has only one-fourth of the motion that
it imparts to the pencil, so that the momentum of the moving parts is
comparatively slight. If the pencil was attached directly to the end of
the lever, it is obvious that it would move in the arc of a circle, and
that this would be a source of error in the diagram. To avoid this the
pencil is attached to what is called a “parallel motion.” This consists
of a coupling-rod, _F I_, which connects the ends of two levers, _F
G_ and _I H_. The centre of the rod _F I_, to which the pencil is
attached, will with this arrangement move in a straight line. The
levers and all the parts are of course all made as light as possible,
so that their weight will have little effect on the motion of the
indicator piston.
[60] Richards’ Steam Indicator, by Charles T. Porter.
The paper or card on which the diagram is drawn is wrapped around a
brass cylinder, _A A_. This cylinder is made to revolve part of the
way around by a strong twine, _a b_, which is wrapped around a pulley,
_b_, at the bottom of the cylinder. The twine is attached to a lever,
similar to that shown in fig. 30, which receives a reciprocating
motion from the piston of the engine. The twine can of course move
the cylinder only in one direction, and therefore a coiled spring
similar to a clock spring is placed inside of the cylinder to draw it
back when the twine is relaxed. In this way the paper cylinder or drum
receives a part of a revolution at each stroke of the piston, and moves
simultaneously with it. This drum is used instead of a flat card, on
account of the practical difficulties of employing the latter. The
motion of the paper on this drum will, however, be exactly the same in
relation to the pencil as the motion of a flat card would be.
[Illustration: Fig. 136a.]
The method of attaching an indicator to a locomotive is represented in
fig. 136a. It will be seen from this that it is placed over the center
of the steam chest and connected to each end of the cylinder with
³⁄₄-inch pipes. A globe valve was in the case represented placed on
each side of the indicator, so that it could be put into communication
with either end of the cylinder, or could be completely shut off from
both. A better plan, however, is to have a three-way cock at the point
where the horizontal pipe connects with the vertical one leading to the
indicator, as the passages in a three-way cock are more direct than
those in globe valves. The arrangement of the levers for giving motion
to the indicator drum, and of the seat, which is very requisite for the
experimenter, will be readily understood from the engraving without
further explanation. It is thought by some engineers that the indicator
should be applied as near to each end of the cylinder as possible. It
is believed, however, that if the pipes, cocks, and their connections
are made large enough so as not to impede the motion of the steam, no
appreciable error will arise from the method illustrated in fig. 136a.
QUESTION 216. _What should be the form of an indicator diagram, if
the steam is distributed by a link motion so as to produce the best
practicable action in the cylinders?_
_Answer._ It should approximate to that shown in fig. 136b. In this
diagram the vertical lines represent inches of the stroke, and the
scale on the left the steam pressure in pounds per square inch. The
atmospheric and vacuum lines are also indicated, as already explained
in answer to Question 55. The points at which the different periods
of the distribution begin are indicated by the letters _a_, _b_, _c_,
_d_, _e_ and _f_. These are in the order in which they occur: _a_,
pre-admission; _b_, admission; _c_, expansion; _d_, pre-release; _e_,
release; and _f_, compression. The lines forming the outline of the
diagram will be designated for convenience of description as follows:
The line from _a_ to _b_, the _admission line_.
The line from _b_ to _c_, the _steam line_.
The line from _c_ to _d_, the _line or curve of expansion_.
The line from _d_ to _e_, the _exhaust line_.
The line from _e_ to _f_, the _line of back pressure_.
The line from _f_ to _a_, the _line or curve of compression_.
[Illustration:
_Fig. 136b._]
The diagram represents a distribution of steam produced by a valve
having ⁷⁄₈ in. outside and ¹⁄₁₆ inside lap, and operated by the link
motion represented in fig. 103. The eccentrics have 5 in. throw, and
the steam-ports are 1¹⁄₄ and the exhaust 2³⁄₄ in. wide. The valve as
shown by the diagram is cutting off at 8 in., or one-third of the
stroke. Pre-admission begins when the piston still has ¹⁄₂ in. to move
before reaching the end of its stroke. Admission of course begins with
the stroke, expansion at 8 in., pre-release at 18¹⁄₂ in., release
at the end of the stroke, and compression at 17¹⁄₂ in. of the return
stroke. The valve is supposed to be set without any lead, or “_line
and line_,”[61] as it is called at full stroke. When the steam is cut
off at 8 in. of the stroke, the valve has 2⁵⁄₈ in. travel and ³⁄₁₆ in.
lead. The steam pressure in the boiler is supposed to be 100 pounds
above the atmosphere. Of course, when the valve cuts off at different
points of the stroke, the periods of distribution will be somewhat
changed; but from the above diagram the principal features of a good
distribution can be explained.
[61] That is, the steam edges of the valve correspond with the steam
edges of the port at the beginning of the stroke.
These are: First, that the steam pressure should rise rapidly during
the period of pre-admission, so that there will be full boiler pressure
in the cylinder at the beginning of the stroke. When this occurs, the
pre-admission line will rise from _a_ to _b_, to such a point at _b_
which will indicate full boiler pressure in the cylinder. The same
pressure should then be maintained in the cylinder during the whole
period of admission, and the admission line from _b_ to _c_ should
therefore be a straight horizontal line, as shown in fig. 136b. When
expansion begins, the pressure will fall, as was explained in answer
to Question 55. The expansion line should approximate a hyperbolic
curve, but if there is much loss of heat by radiation or other causes,
the diagram will fall considerably below the theoretical curve. With
cylinders well protected and with dry steam the expansion line will
fall slightly below a hyperbolic curve at the beginning of the period
of expansion, and rise above it during the latter part of the same
period. The reason of this is that the cylinder is heated by the
admission of live steam of comparatively high pressure and temperature,
so that, when the pressure becomes reduced by expansion, a part of
the water which is condensed in the cylinder will be re-evaporated by
the heat in the latter. From the point of the pre-release, _d_, to
the end of the stroke, _e_, the exhaust line should fall rapidly, so
that there will be no pressure behind the piston during its return
stroke. To explain the theoretical form of the exhaust line would
lead us into a very abstruse discussion, which would be out of place
here. It will be sufficient for our purpose to call attention to the
fact that the pre-release should allow all the steam in the cylinder
to escape before the piston reaches the end of the stroke, so that
the back pressure during the return stroke may be as low as possible.
It is, however, only at comparatively slow speeds that the steam in
locomotive cylinders escapes during the period of pre-release, so that
the back pressure is reduced to that of the atmosphere. It is necessary
in locomotives, as has already been explained, to contract the area
of the blast orifices or exhaust nozzles, in order to stimulate the
draft through the fire, so that the steam cannot escape with sufficient
rapidity to reduce the back pressure to that of the atmosphere if the
engine is running fast. Of course every pound of back pressure on
the piston is so much loss of energy, and a reduction of the amount
of work done by the engine; but it is a sacrifice which must be made
in order to be able to generate the requisite quantity of steam. In
studying the distribution of steam, however, every effort should be
made to reduce the back pressure as much as is practicable, and yet
maintain a sufficient supply of steam, and therefore the line of back
pressure should conform as closely as possible to the atmospheric
line. The compression line should be a hyperbolic curve, beginning
with the period of compression. In calculating both the compression
and expansion, allowance must be made for the clearance space and
steam-way. In a cylinder like that illustrated in fig. 92, their
contents would be about equal to that of two inches of the cylinder.
Therefore, when steam is cut off at 8 in. of the stroke, instead of
having a quantity of steam which will fill a cylinder 16 in. diameter
and 8 in. long, we have as much as would fill a cylinder of that
diameter and 10 in. long. The same thing is true of the compression.
This must occur in the above example when the piston has 6¹⁄₂ in. more
to move before completing its stroke. There is therefore a quantity
of steam in front of it sufficient to fill a cylinder 8¹⁄₂ in. in
diameter. This steam is of course compressed by the advance of the
piston, and if its pressure when compression begins is the same as
that of the atmosphere, then it will be 0.9 lbs. above it when the
piston has only 6 in. to move and 3.2, 6.2, 10.5, 16.9, and 27.5
lbs. effective pressure when the piston has 5, 4, 3, 2 and 1 inches
to move respectively, and when pre-admission begins, the pressure
will have risen to 48.7 lbs. If the back pressure is above that of
the atmosphere, of course the compression will be correspondingly
increased. It will also be seen that, without any or with very little
clearance space, the compression would at the end of each stroke rise
above the boiler pressure. It being a peculiarity of the ordinary
shifting-link motion that as the period of admission is reduced that of
compression is lengthened, the latter becomes very excessive when the
steam is cut off at less than one-third or one-fourth of the stroke.
QUESTION 217. _In what respect would a diagram made by an indicator
differ from the theoretical form represented in fig. 136b?_
_Answer._ It would be drawn with less exactness; that is, the corners
instead of being sharply defined, as in fig. 136b, would be more or
less rounded, as in fig. 137, and the curves and straight lines would
vary somewhat from the exact mathematical form indicated in fig. 136b.
The higher the speed at which the engine is working when the diagrams
are taken, the greater will be the variation from the theoretical form.
[Illustration:
_Fig. 137._]
QUESTION 218. _If the amount of pre-admission is insufficient, how will
it be shown in the indicator diagram?_
_Answer._ The effect of too little pre-admission is to lower the
pressure of the steam at the beginning of the stroke, and at high
speeds there will not be time enough nor sufficient opening of the
steam-port to supply the deficiency after the stroke has commenced.
The corner of the diagram at _b_ will then be very much rounded, as
shown in fig. 138. This is apt to be the case when steam is admitted
during a considerable part of the stroke, as a shifting-link motion
then gives less lead than when it is worked nearer mid-gear. If the
steam is cut off short, then the pressure in the cylinder during
admission is very much below boiler pressure, and is apt to fall
rapidly after the commencement of the stroke, as shown in fig. 138.
[Illustration:
_Fig. 138._]
QUESTION 219. _If the opening of the steam-ports during admission is
too small, what will be the form of the diagram?_
_Answer._ The effect will be very much the same as that produced by too
little pre-admission or lead; that is, the pressure in the cylinder
will be much lower than in the boiler and will fall rapidly during the
periods of admission, as shown in fig. 138.
QUESTION 220. _What defects will be indicated by the expansion curve of
indicator diagrams?_
_Answer._ If the cylinders are not well protected, and there is much
loss of heat from radiation, there will be a rapid fall of pressure
during the period of expansion, which will be shown by the expansion
curve falling below the theoretical curve shown in fig. 136b. If, on
the contrary, the indicator curve is much above the theoretical curve,
it may be caused by a leak in the valve. As steam is quite as likely
to leak from the steam-port into the exhaust as from the steam-chest
into the steam-port, a valve which is not tight may produce just the
contrary effect upon the indicator diagram. As it is usually quite easy
to detect a leak in the valve by other means, the use of the indicator
for this purpose is unnecessary. Attention is called to it, however,
to show the impossibility of getting results of any value with the
indicator if the valves are not steam-tight.
QUESTION 221. _What should be observed regarding the exhaust line of
the indicator diagram?_
_Answer._ The most important point to be observed is, whether the
pressure at the end of the stroke is reduced as low as possible, as at
high speeds it is usually much more difficult to exhaust the steam from
than to admit it into the cylinder. As already stated, the blast in the
chimney makes it almost impossible to exhaust the steam to atmospheric
pressure when the locomotive is running fast. If the steam is released
too late in the stroke, as already explained, there will not be time
enough nor sufficient opening of the port to allow the confined steam
to escape from the cylinder before the end of the stroke, and this
will be indicated on the diagram by the space between the line of back
pressure and the atmospheric line during the commencement of the return
stroke, as shown in fig. 138.
QUESTION 222. _What should be observed regarding the line of back
pressure?_
_Answer._ The most important point is, that it should approximate as
closely as possible to the atmospheric line, as all the back pressure
not only diminishes the efficiency of the engine, but is a total
loss of energy. Too much inside lap will increase the amount of back
pressure, but generally it is more influenced by the area of the
blast orifices than by any other cause. Every effort should be made,
therefore, to have them as large as possible and yet have the boiler
make as much steam as is needed.
When only one blast orifice is used for both cylinders, it often
happens that when the steam is exhausted from the one cylinder it
“blows” over into the other, and thus produces an additional amount of
back pressure. This is shown by a rise or “hump” in the line of back
pressure, as indicated in fig. 138.
QUESTION 223. _Can the amount of compression which is needed be
determined by calculation?_
_Answer._ Yes; but it involves more abstruse principles of mathematics
than it is thought best to introduce here. Some of the reasons can,
however, be given, which will make the subject clearer, and enable the
reader, if he has sufficient knowledge of mathematics, to investigate
the subject still further.
[Illustration:
_Fig. 140._]
In the first place it is a well-known fact that the motion of a piston
in the cylinder of a steam engine is not a uniform one, but increases
in speed from the beginning of the stroke to the middle, and diminishes
in speed from the middle to the opposite end. The cause of this is that
the crank revolves at a uniform speed during the entire revolution,
but the piston moves much less at the beginning of the stroke, with a
given amount of revolution of the crank, than it does at the middle.
This is shown in fig. 140, in which _A_ is a cylinder and _B_ the
piston and _a b c d_ the path of the crank. Now while the crank moves
from _a_ to 1, or ¹⁄₁₂ of a revolution, the piston has moved 1³⁄₈
in., or a distance equal to that from _a_ to 1′ or to the base of a
perpendicular drawn from 1 to the centre line _a c_. While the crank
moves from 1 to 2, or through the second twelfth of a revolution, the
piston has moved from 1′ to 2′, or 4³⁄₈ in., or 2³⁄₄ in. further than
during the first twelfth of the crank’s revolution. During the third
twelfth of the revolution the piston moves from 2′ to 3′, or 6 in.,
thus showing that it continues to increase in the distance moved during
each period of the revolution of the crank until the latter has made
a quarter revolution. The speed of the piston then begins to diminish
until it reaches the end of the stroke. It is slightly affected by the
angularity of the connecting-rod, as already explained, but for the
present this is disregarded. It is obvious now that if the momentum,
or actual energy stored up in the piston and other reciprocating parts
after they have passed the middle of the stroke, added to the pressure
behind the piston, is greater than the resistance offered by the crank,
the motion of the latter will then be accelerated and thus conveyed
to the moving engine and train. If, however, there is any momentum in
the piston when it reaches the end of the stroke, evidently it can
exert no power to cause the crank to revolve, but must be expended by
producing a pressure on the crank-pin and thus on the axle-boxes. Not
only will such a pressure not cause the crank to revolve, but it
will be more difficult to turn the crank with such a pressure against
it than it would be without. The momentum of the piston and other
reciprocating parts at the dead point therefore creates a resistance
to the movement of the crank instead of helping to turn it. It will
also be observed that after the crank has moved slightly from the dead
point, any pressure on the piston will exert very little force which
will tend to turn the crank. In fact the nearer the piston is to the
end of the stroke the greater is the proportion which the friction
of the crank-pin and axle bears to the useful effect of the strain
in causing the crank to turn. Calculation shows that for about three
degrees on either side of the dead points the effect of pressure on the
crank-pin is actually to retard the engine. If now the piston reaches
the end of the stroke with a certain amount of unexpended momentum
stored up in it, if this energy is expended by producing pressure on
the crank, then it will not only be a waste of energy but a double
waste by retarding the motion of the crank. If, however, this energy
can be absorbed by compressing steam which will fill the clearance
spaces, it will not only prevent the retarding effect referred to,
but the energy in the piston and other parts will be converted into
steam pressure, which will be given out in useful work during the next
stroke. It would, of course, be impossible to arrest the motion of the
piston instantly, and therefore its momentum is gradually absorbed from
the time compression begins until it reaches the end of the stroke. As
the energy of a moving body is equal to its weight multiplied by the
square of its speed, it is obvious that to overcome this a different
amount of compression would be required for each speed, and also that
it must be adjusted to the weight of the moving parts. Such exact
adaptation is not practicable on locomotives, nor does the link motion
enable us to alter the amount of compression with so much exactness:
but the explanation shows the value of increasing the amount of
compression with the speed, which fortunately the peculiarities of the
shifting-link motion enable us to do without difficulty.
[Illustration:
_Fig. 139._]
QUESTION 224. _What cause produces the form of diagram represented by
Fig. 139?_
_Answer._ It is produced by excessive compression, which causes
the pressure in the cylinder to rise above boiler pressure before
pre-admission begins. As soon as the port is opened, part of the steam
in the cylinder flows back into the steam-chest, and thus the pressure
is reduced, as shown by the diagram.
QUESTION 225. _How can we determine whether the steam is distributed in
the cylinders to the best advantage, and how can we discover the fault,
if there is one, in the link motion?_
_Answer._ The indicator will show the action of the steam in the
cylinder, and motion-curves drawn with the instrument described in
answer to Question 192 will show the exact movement of the valve. By
comparing the indicator diagram with the motion-curves, the one will
show the defects in the other.[62]
[62] See description of Richards’ Improved Steam Engine Indicator,
with directions for its use, by Charles T. Porter, London.
QUESTION 226. _To what extent can the movement of the valve be modified
by alterations in the proportions of the link motion?_
_Answer._ The motion of the valve is susceptible of an almost infinite
number of changes, by different variations and combinations of
proportions of the working parts of the link motion. These changes
are, however, limited by the general laws which govern the motion of
eccentrics, and therefore cannot influence the motion of the valve
beyond certain limits. Hardly any variation can be made either in
the proportions or arrangement of the working parts which will not
have some influence upon the movement of the valve. Aside from the
proportions of the valve itself, which have already been discussed,
the throw of the eccentrics, the length of the rods and of the link,
the point of connection of the rods with the link, the point of
suspension, the position of the lifting shaft, the length of the arms,
the length and position of the rocker arms will each of them effect
the distribution of steam. The number of combinations of all these
different proportions is of course almost infinite, and therefore any
full discussion of them will be impossible here.
QUESTION 227. _What are the most important points which require
attention in designing a link motion?_
_Answer._ It should be proportioned so that--
First, the lead and the period of admission should be the same for each
end of the cylinder, for each point of cut-off, and, if possible, in
back as well as forward gear.
Second, the width of opening for both admission and exhaust should be
as large as possible when steam is cut off short.
Third, the exhaust or pre-release should occur early enough and be
maintained long enough to reduce back-pressure as low as possible.
QUESTION 228. _How can the lead and period of admission be equalized?_
_Answer._ It is impossible to make the periods of admission absolutely
alike for every point of cut-off in both fore and back gear. It is
therefore customary to disregard the back gear, as engines are worked
but little with the link in that position. Even for forward gear the
periods of admission cannot be made exactly alike for each end of the
cylinder and for each point of cut-off, and therefore it is usual to
make the periods of admission alike for half-gear forward, in which
position the link is worked most.
The periods of admission for the front and back ends of the cylinder
can be changed most in relation to each other by altering the position
of the point of suspension on the link. This can be done either by
moving this point up or down, or horizontally. Usually links are
suspended from a point halfway between the points of connection of the
eccentric-rods and from ¹⁄₄ to ³⁄₄ in. back of the centre line of the
slot in the link. A somewhat better distribution can be secured by
suspending it about 3 in. above the centre, but the suspending link
must then be made so short that it is subjected to very great strains
by the motion of the link, and this evil is usually considered much
greater than the advantage which is gained thereby in the more equal
distribution. The point at which the upper end of the suspension link
is hung also influences the relative amount of admission front and
back. This point, of course, varies as the end of the lifting arm is
raised or lowered. In designing valve gear it is usually tested by a
full-sized model, which will show the exact motion of all the parts.
The best position for the lifting shaft and the length of its arm can
be determined perhaps most satisfactorily by placing the link in full
gear forward, then moving the point of suspension of the upper end of
the link-hanger horizontally so that the front and back admission will
be alike, and then marking this position. The same process should then
be repeated for half gear and for the shortest point of cut-off. If the
position of the lifting shaft and the length of its arm are then so
arranged that the end of the latter will move through the three points
which have been thus determined, the admission will be very nearly
equal for each end of the cylinder. Usually, however, it is impossible
to arrange the shaft and arm so that they will conform exactly to these
conditions, and therefore an approximation is made which will come
as near as possible to what is required. It may be stated, however,
that the lifting shaft should be kept as low as possible, so as not to
interfere with the eccentric-rods. In some cases the shaft has been
suspended from the boiler, so that the outside eccentric-rod would work
past or over the end of the lifting shaft, thus allowing the latter to
be located lower than would otherwise be possible.
QUESTION 229. _Which parts of the link-motion have the greatest
influence on the distribution of steam?_
_Answer._ The lap of the valve and the throw of the eccentrics. The
effect of any change of these upon the distribution is very similar to
that produced if a single eccentric is used, which was explained in the
answers to Questions 49, 50 and 52.
[Illustration:
_Fig. 141._]
QUESTION 230. _What is the effect upon the admission of increasing the
throw of the eccentrics with the same lap?_
_Answer._ As already explained, the effect is to increase the period of
admission, or in other words to cut off later in the stroke, and also
to increase the width of the opening of the steam-port or the distance
which the valve throws over the port. This has an important influence
upon the admission, when the link-motion is used.
QUESTION 231. _What is meant by the angular advance of the eccentrics?_
_Answer._ It is the angle which a line, _e f_, fig. 141, drawn through
the centre of the axle and the centre of the eccentric makes with a
vertical line _a b_, when the crank is on one of the dead-points or
centres. Thus in fig. 141 the crank _A_ is represented on the front
centre. In order to give the valve the necessary lead the eccentric
must be moved ahead of the vertical line _a b_. The angle _c_ which the
line _e f_ (drawn through the centre of _g_ of the eccentric and _f_ of
the axle) makes with the vertical line is called the _angular advance_.
QUESTION 232. _What is meant by linear advance?_
_Answer._ By linear advance is meant the distance which the valve has
moved from its middle position at the beginning of the stroke of the
piston. This, when the two rocker arms are the same length, is the same
as the distance of the centre of the eccentric _g_ from the vertical
line _a b_, fig. 141.
QUESTION 233. _Why does the cut-off occur earlier with an eccentric
having a short throw than with one which gives more travel to the
valve?_
_Answer._ Because it is necessary to give the eccentric with the short
throw more angular advance in order to give the valve the required
lead. This is illustrated in fig. 142, in which a section of a valve,
_V_, and ports _c_, _g_, and _d_, are represented. In order to simplify
the diagram as much as possible the rocker is left out and the valve
is supposed to be moved by the rod _R_ directly from the centre _a_ of
the eccentric.[63] The effect of the angularity of the connecting rod
and eccentric rod is also neglected. The circle _a b e f_ represents
the path of the centre of an eccentric having 5 in. throw, and _h i
j_ the path of one having 3¹⁄₂ in. throw. In order to give the valve
the required lead, which is supposed to be just line-and-line at the
beginning of the stroke, the linear advance of the valve must be equal
to the lap, or ⁷⁄₈ in. If therefore we draw a line, _p a_, parallel to
the vertical centre line, _e k_, and ⁷⁄₈ in. from it, the intersection
of _p a_ at _a_ and _h_ with the paths of the eccentric will be the
centres of the eccentrics. If through these centres and the centre of
the circle, lines, _o a_ and _o p_, be drawn, the angles which they
make with the vertical _e k_ will be the angular advance. It will be
seen from these lines that in order to give the valve the required
lead it is necessary to give the eccentric with the small travel more
angular advance than is necessary for the one with the larger throw.
It is obvious, too, that when the centre of the larger eccentric has
reached the point _b_ the valve will have received its greatest travel,
and that when it reaches _p_ the steam-port _c_ will again be closed or
the steam cut off. If the small eccentric is employed, the valve will
then have its maximum travel when the centre _h_ reaches _s_, and the
port will be closed when it reaches _i_. By drawing lines, _o p_ and _o
n_, through _i_ and _p_, it will be seen that from the beginning of the
stroke until the steam is cut off, if the large eccentric is employed,
it, and consequently the shaft and crank, must move over an angle
measured by the arc _q t p_. If the small eccentric is used, it and the
crank must move through an angle measured by the arc _u t n_. In other
words, the crank must turn a considerably greater distance before steam
is cut off with an eccentric having a large than with one having a
small throw.
[63] It will be seen that this causes the position of the centre of
the eccentric to be reversed.
[Illustration:
_Fig. 142._
_Fig. 143._
Scale ³⁄₁₆ in. = 1 inch.]
It is also quite obvious from fig. 142 why the port is opened a shorter
distance with a small than with a large eccentric. The distances _o
s_ and _o b_ are equal to half the throws of the eccentrics, or 1³⁄₄
and 2¹⁄₂ in. The linear advance _o r_ is in both cases ⁷⁄₈ in., and
therefore after the port begins to open the valve will be moved by the
small eccentric a distance which is equal to 1³⁄₄ - ⁷⁄₈ = ⁷⁄₈ in., and
by the large one 2¹⁄₂ - ⁷⁄₈ = 1⁵⁄₈ in.
[Illustration:
_Fig. 144._
Scale ³⁄₁₆ in. = 1 inch.]
QUESTION 234. _What is the effect on the admission of giving an
eccentric with a small throw the same angular advance as one with a
large throw, and then reducing the lap of the valve so that the lead
will be the same in both cases?_
_Answer._ The admission and the cut-off will then occur at the same
points of the stroke, but the ports will not be opened so wide. This is
illustrated in fig. 143, in which the paths of two eccentrics having
the same throw as those in fig. 142 are represented. The centre, _a_,
of the larger eccentric is represented in the same position in fig.
143 as in fig. 142. If a line is drawn from the centre of the larger
eccentric to that of the axle, and if the centre, _h_, of the smaller
eccentric is located on the intersection of this line with the circle
representing its path, then the smaller eccentric will have the same
angular advance, but the linear advance measured by the distance _o t_
will be only ⁵⁄₈ in. If the valve have the same lap as in fig. 142,
its steam edges at the beginning of the stroke, if the small eccentric
is employed, will occupy the position represented by the dotted lines
_A_ and _B_. If these edges are cut off, as shown by the full lines
and shading, then the valve will have the same lead as in fig. 142.
It is obvious, too, that if the smaller eccentric has the same
angular advance it will reach the point _v_, at which, with the reduced
lap, the steam will be cut off, at the same time that the centre,
_a_, of the large eccentric will reach _p_, at which point it cuts
off the steam with the valve having the large lap. There is, however,
this difference in the distribution, that in the one case the valve
opens the port a distance equal to _t s_, and in the other a distance
equal to _r b_. As _o t_ is equal to the linear advance of the small
eccentric, or ⁵⁄₈ in., and _o s_ to half the throw of the eccentric,
or 1³⁄₄, _t s_ is equal to 1³⁄₄ - ⁵⁄₈ = 1¹⁄₈ in. The distance _r b_,
as shown above, is equal to 2¹⁄₂ - ⁷⁄₈ = 1⁵⁄₈ in., so that the effect
produced upon the admission of using an eccentric with a small throw
and corresponding amount of lap is, that the ports are not opened so
wide as with an eccentric having a larger throw.
[Illustration:
_Fig. 145._
Scale ³⁄₁₆ in. = 1 foot.]
QUESTION 235. _How do eccentrics with a short throw, and valves with a
corresponding amount of lap, affect the admission with a link motion as
compared with eccentrics having a larger amount of throw and greater
lap of valve?_
_Answer._ The chief difference is that the ports are not opened so
wide for the same period of admission. Thus in fig. 144 is a series of
motion-curves drawn with a model of a link motion like that illustrated
in fig. 103. The eccentrics had 5 in. throw, and the valve ⁷⁄₈ in. lap
outside and ¹⁄₁₆ in. inside. Fig. 145 represents a series of curves,
drawn with the same arrangement of valve-gear, excepting that the
eccentrics had 3¹⁄₂ in. throw and the valve ¹⁄₂ in. lap. In both cases
the curves represent the motion of the valve when cutting off at the
same point of the stroke. The following table will show the relative
amount of opening of the port.
========+============================
| Width of Opening
| of Steam-port.
+------------+---------------
Point of| Eccentric | Eccentric
Cut-Off.|5 in. throw.|3¹⁄₂ in. throw.
--------+------------+---------------
6 in. | ⁷⁄₃₂ in. | ⁵⁄₃₂ in.
8 „ | ⁹⁄₃₂ „ | ³⁄₁₆ „
10 „ | ¹¹⁄₃₂ „ | ⁷⁄₃₂ „
12 „ | ⁷⁄₁₆ „ | ⁹⁄₃₂ „
15 „ | ⁵⁄₈ „ | ³⁄₈ „
18 „ | ³¹⁄₃₂ „ | ¹¹⁄₁₆ „
21 „ | 1¹⁄₄[64] „ | 1¹⁄₃₂ „
--------+------------+---------------
[64] The valve throws over 1³⁄₄ in. at this point.
It will be seen from this that the eccentric with 5 in. throw gives a
greater width of opening for every point of cut-off than the one with
3¹⁄₂ in. throw. For the higher admissions this is not important, but
when steam is cut off short it will be observed that the width of the
opening is very small. At high speeds the small opening is a great
disadvantage.
QUESTION 236. _Has it been determined what amount of opening is
required for given speeds of the piston?_
_Answer._ Not with any degree of accuracy. It is customary to make
the area of the ports about one-tenth that of the piston. It is
certain, however, that with steam-ports of this proportion an opening
considerably less than their whole area is sufficient to maintain steam
at boiler pressure in the cylinders. One of the defects of the link
motion is that the opening of the port is very small when the steam is
cut off short. It is best, therefore, to secure the largest practicable
opening of the ports for the lower points of cut-off.
QUESTION 237. _What are the proportions of the valves and eccentrics
used in the ordinary practice in this country?_
_Answer._ The following report made by a committee of the Master
Mechanics’ Association will show the proportions used on thirty-five
different railroads, and is a fair indication of the common practice.
===================================================+=======+======+
TABLE | | |
SHOWING THE AMOUNT OF LAP, LEAD AND TRAVEL OF THE | | |
VALVES OF LOCOMOTIVES USED ON 35 OF THE RAILROADS |Outside|Inside|
IN THE UNITED STATES AND CANADA. | Lap | Lap |
---------------------------------------------------+-------+------+
| in. | in. |
For locomotives running express pass. trains 25 use| ⁷⁄₈ | ¹⁄₈ |
„ „ „ „ „ „ 6 „ | ³⁄₄ | ¹⁄₁₆ |
„ „ „ „ „ „ 4 „ | 1¹⁄₄ | ¹⁄₄ |
„ „ „ accom. „ „ 20 „ | ³⁄₄ | ³⁄₈ |
„ „ „ „ „ „ 10 „ | ⁷⁄₈ | ¹⁄₁₆ |
„ „ „ „ „ „ 5 „ | ⁵⁄₈ | ³⁄₁₆ |
„ „ „ heavy freight „ 19 „ | ³⁄₄ | ¹⁄₁₆ |
„ „ „ „ „ „ 11 „ | ⁵⁄₈ | ¹⁄₈ |
„ „ „ „ „ „ 5 „ | ¹⁄₂ | ³⁄₁₆ |
===================================================+=======+======+
===================================================+======+====
TABLE | |Lead
SHOWING THE AMOUNT OF LAP, LEAD AND TRAVEL OF THE |Travel| in
VALVES OF LOCOMOTIVES USED ON 35 OF THE RAILROADS | of |full
IN THE UNITED STATES AND CANADA. | Valve|Gear
---------------------------------------------------+------+----
| in. | in.
For locomotives running express pass. trains 25 use| 5 |¹⁄₁₀
„ „ „ „ „ „ 6 „ | 4³⁄₄ |¹⁄₈
„ „ „ „ „ „ 4 „ | 5 |¹⁄₈
„ „ „ accom. „ „ 20 „ | 5 |¹⁄₁₀
„ „ „ „ „ „ 10 „ | 5¹⁄₂ |¹⁄₁₆
„ „ „ „ „ „ 5 „ | 4¹⁄₂ |¹⁄₈
„ „ „ heavy freight „ 19 „ | 5 |¹⁄₁₀
„ „ „ „ „ „ 11 „ | 4¹⁄₂ |¹⁄₁₆
„ „ „ „ „ „ 5 „ | 4³⁄₄ |¹⁄₁₀
===================================================+======+====
QUESTION 238. _What should be the width of the bridge between the steam
and exhaust ports?_
_Answer._ It is usually made about the same thickness as the sides of
the cylinder, in order to secure a good casting; but sometimes it is
necessary to make it wider, in order to prevent steam from escaping
from the steam-chest into the exhaust, which is apt to be the case if a
valve has little lap and a long travel.
QUESTION 239. _What determines the width of the exhaust-port?_
_Answer._ The throw of the valve. This will be clear if we refer to
fig. 146, which represents a valve with a travel of 5¹⁄₂ in. It will
be seen that when it is in the extreme position in which it is shown
the width _A_ of the opening of the exhaust-port is very small. If
this opening is contracted too much it will of course interfere with
the free escape of the exhaust steam. It is therefore best to make the
exhaust-port so wide that with the greatest travel of the valve the
width of its opening will be either quite or very nearly equal to the
width of the steam-port.
[Illustration:
_Fig. 146._
Scale ³⁄₁₆ in. = 1 foot.]
[Illustration: Fig. 148.
Fig. 147.
Scale ³⁄₄ in. = 1 foot.]
QUESTION 240. _Where is the reverse lever located and how is it
constructed?_
_Answer._ It is located on the _foot-board_[65] _K′ K′_, as shown in
plate II. It consists of a lever _O_, _O_, with the fulcrum at the
lower end. The _reverse-rod e k_, which connects the lever with the
vertical arm _k_ of the lifting-shaft, is attached above the fulcrum of
the reverse lever. Figs. 147 and 148 represent side and end views of
the lever on an enlarged scale and with some of the details attached
which are omitted on plate II. _C_, _C_ are two curved bars, which in
this country are usually called _quadrants_, but in England are called
(and more properly) _sectors_. These are placed on each side of the
reverse-lever and are fastened to some portion of the engine. They have
notches, _n_, _n_, _n_, cut in them to receive the _latch L_, which
slides in a clamp, _H_, and holds the reverse-lever in the notches in
which it is placed. This latch is operated by a _trigger_, _D_, which
is grasped by the locomotive runner when he takes hold of the handle
_A_ of the reverse-lever. The trigger works on a pin, _E_, as a fulcrum
and is attached to the latch by a rod, _r r_. When the trigger is
pressed up against the handle, the latch is raised out of the notches
by the rod _r r_, and is pressed into them again by the spring _s_
when the trigger is released. _F_ is a set-screw which presses against
a gib, _G_, and is intended to keep the latch tight and prevent the
reverse-lever from shaking.
[65] The _foot-board_ _K′ K′_, plates 2 and 3, is a platform for the
locomotive runner and fireman to stand on and is located at the back
end of the engine.
QUESTION 241. _How are the notches in the sector arranged?_
_Answer._ They are usually arranged so that the steam will be cut off
at some full number of inches of the stroke when the reverse-lever is
in each one of the notches. They are therefore located so that the
steam will be cut off at 6, 9, 12, 15, 18 and 21 inches, or at 6, 8,
10, 12, 15, 18 and 21 inches of the stroke. A notch is also placed so
as to hold the link in mid-gear. In some cases as many notches as there
is room for are put into the sectors. The latter seems to be much the
best plan, as it gives more gradations in which the valve-gear can be
worked, and it is a matter of no consequence whatever in the working of
an engine whether the steam is cut off at some full or some fractional
number of inches of the stroke. By referring to fig. 144 it will be
seen how very great the difference of the distribution of steam must
be, as indicated by the 5th, 6th and 7th motion-curves.
QUESTION 242. _How long should the reverse-lever be?_
_Answer._ The lever should be sufficiently long so that in throwing the
link from full gear forward to full gear backward the handle _A_ will
move _not less_ than four times the distance that the link is moved. It
is much better to give the end of the handle _A_ five or even six times
the motion of the link, as there will then be a much easier action in
reversing the engine. This will also make it possible to use longer
sectors, and give room for more notches.
QUESTION 243. _What provision is made in the reversing gear for
overcoming or neutralizing the weight of the link and other parts of
the valve-gear?_
_Answer._ Their weight is counterbalanced by the pressure of a spring
of some kind. In fig. 103 the two volute springs enclosed in a case,
_H_, are used for this purpose. These are compressed by the rod _m_,
which is attached to a short arm _l_, on the reverse shaft _A_, when
the link is lowered, and consequently the tension of the spring resists
the weight of the link when the latter is down or in forward gear.
Different kinds of springs are used for this purpose and sometimes are
attached to the reverse-lever instead of to the lifting-shaft.
QUESTION 244. _What is meant by “setting” a slide-valve?_
_Answer._ It is to fasten the eccentrics in the right position on the
axle and to adjust the length of the eccentric-rods and valve-stem so
that the valves will give the required distribution of steam.
QUESTION 245. _How are the valves of a locomotive set?_
_Answer._ After the wheels, axles, main connecting-rods and valve-gear
are connected together, put the rocker-arm in its middle position, and
lengthen or shorten the valve-stem so that the valve will be in the
centre of the valve-face. Then place the crank on the forward centre
and the full part of the forward motion eccentric above and that of the
backward motion eccentric below the axle, and fasten them to the axle
temporarily by tightening up the set-screws. Then throw the link down
until the block comes opposite to the end of the eccentric-rod, and
turn the wheels,[66] and at the same time, observe whether the travel
of the valve is equal to the throw of the eccentric and also whether it
travels equally on each side of the centre of the valve-face. If its
travel is greater than the throw of the eccentric, raise the link up;
if less, lower it down until the two are just equal, and then mark the
position for the notches on the sections or quadrants to receive the
latch of the reverse-lever. If the valve does not travel equally on
each side of the centre of the valve-face, either lengthen or shorten
the eccentric-rod, as may be necessary. Repeat this operation for the
backward motion, by raising the link up until the block is opposite
the end of the lower eccentric-rod. After having done this, go over
the whole process again to see whether it is all correct. Now with
the crank on the forward centre, and the link in full gear forward,
loosen the set-screws in the forward eccentric, and move it around the
axle so that the valve will have the required lead and then fasten it
again. Now raise the link up into full back gear, and set the backward
eccentric in the same way. Then turn the wheels so as to bring the
crank on the back centre, and observe whether the lead is correct
for the back end of the cylinder. If it is not, lengthen or shorten
the valve-stem or eccentric-rod so as to make the lead alike at both
ends, and if it is then too much or too little, it can be increased or
diminished by moving the eccentrics on the axle.
[66] This can be done by moving the engine on the track, or by
raising it off its wheels, so that the latter can be turned without
moving the former. In some shops a pair of rollers is put in the
track so that by placing the driving wheels on them they can be
turned without any difficulty.
Great care must be taken in setting valves to be sure that the cranks
are exactly on the centres or dead points, and it is impossible to
set them in that position with sufficient accuracy from the motion of
the piston or cross-head, and therefore the centres of the crank-pins
should always be set so as to conform to a line drawn through the
centre of the cylinder and the axle. When the cylinders are horizontal,
it is of course only necessary to place the cranks on a horizontal line
drawn through the centre of the axle.
When the valves are set it should also be noticed whether the
axle-boxes (whose construction will be explained hereafter) are in
the centre of the jaws, and if not they should be moved to the centre
by driving wooden wedges between them and the frames, either above
or below, as may be required. The position of the boxes has a very
material influence on the valve-gear.
If it is intended to lay off the notches on the sectors so as to cut
off steam at certain definite points of the stroke, these points should
be laid off in the guides from the motion of the cross-head. The latter
being placed in any of the required positions at which steam is to be
cut off, the reverse-lever should then be moved so that the link will
just close the admission port. The lever can then be clamped to the
sectors, and the wheels turned so as to show whether its position is
correct for each end of the stroke. As before stated it is impossible
to get the ordinary link-motion to cut off at exactly the same points
at both ends of the cylinder, but a very close approximation can be
made by proportioning the different parts properly. As has already been
stated, it is believed to be a much better plan to put as many notches
in the sectors as possible, than to locate them for certain definite
points of the stroke.
In setting the valves of locomotives, care must be taken to turn the
wheels _forward_ for the _forward motion_ and _backward_ for the
_backward motion_.
After the valves are set the position of the eccentrics on the shaft
should be marked, so that in case they become loose on the road they
can easily be set again. It is usual, too, to mark the position of
the valves with centre-punch marks on the valve-stem and on the
stuffing-box of the steam-chest, so that with a gauge made for the
purpose the position of the valve can be determined without taking off
the steam-chest cover.
In some cases the eccentrics are keyed on, which is done after
their position is determined by setting the valves. The ends of the
set-screws which are used to fasten the eccentrics should be cup-shaped
and case-hardened, so as to hold as securely as possible to the axle
when they are screwed down.