_THE VALVE-MOTION._
THE LOCOMOTIVE SLIDE-VALVE.
The nature of the service required of locomotive engines, especially
those employed on fast-train service, makes it necessary that the
steam-distribution gear shall be free from complication; and, for
convenience in working the engine, it is essential that means should
be provided for reversing the motion promptly, without endangering the
working-parts. The valve-gear should also be capable of regulating
the admission and exhaust of steam, so that the engine shall be
able to maintain a high rate of speed, or to exert a great tractive
force. These features are admirably combined in the valve-gear of the
ordinary locomotive. Designers of this form of engine have given great
consideration to the merit of simplicity. Numerous attempts have been
made to displace the common D slide-valve, but every move in that
direction has ended in failure.
INVENTION AND APPLICATION OF THE SLIDE-VALVE.
The slide-valve, in a crude form, was invented by Matthew Murray
of Leeds, England, towards the end of last century; and it was
subsequently improved by Watt to the D form. It received but little
application in England till the locomotive era. Oliver Evans of
Philadelphia appears to have perceived the advantages possessed by
the slide-valve, for he used it on engines he designed years before
locomotives came into service. The D slide-valve was better adapted for
high-speed engines than any thing tried during our early engineering
days, but it was on locomotives where it first properly demonstrated
its real value. The period of necessity brought the slide-valve into
prominence; and the galaxy of mechanical genius that heralded the
locomotive into successful operation recognized its most valuable
features, and it soon obtained exclusive possession of that form of
engine. Through good and evil report, and against many attempts to
displace it, the slide-valve has retained a monopoly of high-speed
reversible engines.
DESCRIPTION OF THE SLIDE-VALVE.
The slide-valve in common use is practically an oblong cast-iron box,
which rests and moves on the valve-seat. In the valve-seat, separated
by partitions called bridges, are three ports, those at the ends
being the openings of the passages for conveying steam to and from
the cylinders, while the middle port is in communication with the
blast-pipe, which conveys the exhausted steam to the atmosphere. On
the under side of the valve is a semicircular cavity, which spans the
exhaust-port and the bridges when the valve stands in its central
position. When the steam within the cylinder has performed its duty
of pushing the piston towards the end of the stroke, the valve cavity
moves over the steam-port, and allows the steam to pass into the
exhaust-port, thence into the exhaust-pipe. The cavity under the valve
thus acts as a door for the escape of the exhaust steam. This is a very
convenient and simple method of educting the steam; and the process
helps to balance the valve, since the rush of escaping steam striking
the under part of the valve tends to counteract the pressure that the
steam in the steam-chest continually exerts on the top of the valve.
PRIMITIVE SLIDE-VALVE.
[Illustration: FIG. 6.]
In its primitive form, the slide-valve was made merely long enough
to cover the steam-ports when placed in the central position, as
shown in Fig. 6. With a valve of this form, the slightest movement
had the effect of opening one end so that steam would be admitted
to the cylinder, while the other end opened the exhaust. By such an
arrangement, steam was necessarily admitted to the cylinder during the
whole length of the stroke; since closing at one end meant opening at
the other. There were several serious objections to this system. It was
very difficult to give the engine cushion enough to help the cranks
over the centers without pounding, and a small degree of lost motion
was sufficient to make the steam obstruct the piston during a portion
of the stroke. But the most serious drawback to the short valve was,
that it permitted no advantage to be taken of the expansive power of
steam. For several years after the advent of the locomotive, the boiler
pressure used seldom exceeded fifty pounds to the square inch. With
this tension of steam, there was little work to be got from expansion
with the conditions under which locomotives were worked; but, so soon
as higher pressures began to be introduced, the loss of heat entailed
by permitting the full-pressure steam to follow the piston to the end
of the stroke became too great to continue without an attempted remedy.
A very simple change served to remedy this defect, and to render the
slide-valve worthy of a prominent place among mechanical appliances for
saving power.
OUTSIDE LAP.
The change referred to, which so greatly enhanced the efficiency of
the slide-valve, consisted in lengthening the valve-face, so that,
when the valve stood in the center of the seat, the edges of the valve
extended a certain distance over the induction ports, as in Fig. 7.
This extension of the valve is called outside lap, or simply lap. The
effect of lap is to close the steam-port before the piston reaches the
end of the stroke, and the point at which the steam-port is closed is
known as the point of cut-off. When the steam is cut off, and confined
within the cylinder, it pushes the piston along by its expansive
energy, doing work with heat that would be lost were the cylinder left
in communication with the steam-chest till the end of the stroke.
[Illustration: FIG. 7.]
When a slide-valve is actuated by an eccentric connected directly
with the rocker-arm or valve-stem, the point of cut-off caused by the
extent of lap, remains the same till a change is made on the valve,
or on the throw of the eccentric, unless an independent cut-off valve
be employed. Locomotives having the old hook motion worked under this
disadvantage; because the hook could not vary the travel of the valve,
which is the method usually resorted to for producing a variable
cut-off. The link and other simple expansion gears perform their office
of varying the cut-off in this way.
SOME EFFECTS OF LAP.
In addition to cutting off admission of steam before the end of the
stroke, lap requires the valve to be set in such a way that it has also
the effect of leading to the exhaust-port being opened before the end
of the stroke. The point where the exhaust is opened is usually known
as the point of release. The change which causes release to happen
before the piston completes its stroke, leads to the closure of the
exhaust-port before the end of the return stroke is reached, which
imprisons the steam remaining in the cylinder, causing compression.
Where a valve has no inside lap, release and compression happen
simultaneously; that is, the port at one end of the cylinder is opened
to release the steam, and that at the other end is closed, letting the
piston compress any steam remaining in the cylinder into the space left
as piston clearance.
INSIDE LAP.
In some cases the inside edges of the valve cavity do not reach the
edges of the steam-ports when the valve is on the middle of the seat,
but lap over on the bridge a certain distance, as shown by the dotted
lines in Fig. 7. This is called inside lap, and its effect upon the
distribution of steam is to delay the release. By this means it
prolongs the period of expansion, and hastens compression on the return
stroke. Inside lap is an advantage only with slow-working engines. When
high speed is attempted with engines having much inside lap, the steam
does not have enough time to escape from the cylinders, and the back
pressure and compression become so great as to be very detrimental to
the working of the engine. As locomotive engineers have it, the engine
is “logy.”
THE EXTENT OF LAP USUALLY ADOPTED.
In locomotive practice, the extent of lap varies according to the
character of service the engine is intended to perform. With American
standard gauge engines, the lap varies from ½ inch to 1¼ inch. For
high-speed engines, the extent of lap ranges from ⅞ to 1¼. Freight
engines commonly get ⅝ to ¾ outside lap, and from 1/16 to ¼ inside lap.
With a given travel, the greater the lap the longer will the period for
expansion be.
FIRST APPLICATION OF LAP.
Lap was applied to the slide-valve in this country before its advantage
as an element of economy was understood in Europe. As early as 1829,
James of New York used lap on the valves of an engine used to run a
steam-carriage; and in 1832 Mr. Charles W. Copeland put a lap-valve on
a steamboat engine, and his father understood that its advantage was in
providing for expansion of the steam. Within a decade after our first
steam-operated railroad was opened, the lap-valve became a recognized
feature of the American locomotive; but the cause of the saving of
fuel, effected by its use, was not well comprehended. Many enlightened
engineers attributed the saving to the early opening of the exhaust,
brought about where outside lap was used, which they theorized reduced
back pressure on the piston; and in that way they accounted for the
enhanced economy resulting from the application of lap. It was not till
Colburn applied the indicator to the locomotive, that the true cause of
economy was demonstrated to be in the additional work taken from the
steam by using it expansively.
THE ALLEN VALVE.
[Illustration: FIG. 8.]
An improvement on the plain _D_ slide-valve has been effected in a
simple and ingenious manner in the Allen valve, which is receiving
considerable favor for high-speed locomotives. This valve is shown in
Fig. 8. The valve has a supplementary steam-passage, _A_, _A_, cast
above the exhaust cavity. The valve and seat are so arranged, that, so
soon as the outside edge of the valve begins to uncover the steam-port
at _B_, the supplementary passage begins receiving steam at _C_; and
this gives a double opening for the admission of steam to the port when
the travel is short. As the travel of the valve is always short when an
engine is running at high speed, the advantage of this double opening
is very great; for it has the effect of admitting the steam promptly
at the beginning of the stroke, and maintaining a full pressure on the
piston till the point of cut-off.
ADVANTAGES OF THE ALLEN VALVE.
With an ordinary valve cutting off at six inches, and having five
inches eccentric throw, the port opening seldom exceeds ⅜ inch. It
is a hard matter getting the full pressure of steam through such a
small opening in the instant given for admission. If an Allen valve
is used with that motion, the opening will be double, making ¾ inch,
which makes an important difference. The practical effect of a change
of this kind is that an engine will take a train along, cutting off
at six inches with the Allen valve, when, with the ordinary valve,
the links would have to be dropped to eight or nine inches. The valve
can be designed to work on any valve-seat, but the dimensions given
in Fig. 8 are those that have been found most satisfactory with our
large passenger engines. In designing an Allen valve for an old seat,
it is sometimes advisable to widen the steam-ports a quarter of an
inch or more, by chamfering off the outside edges that amount. Care
must be taken to prevent the valve from traveling so far as to put the
supplementary port over the exhaust-port, for that would allow live
steam to pass through. The proper dimensions can best be schemed out on
paper before making the required change on the seat.
In very carefully conducted experiments made on the Boston and Albany
Railroad, to compare the performance of the Allen valve with an engine
equipped with a common valve, it was found that the Allen valve
effected a fuel saving of seven per cent.
CASE WHERE THE ALLEN VALVE PROVED ITS VALUE.
On one of the leading railroads in this country, an engineer was
running a locomotive on a fast train where it was a hard matter making
the card-time. A few minutes could be saved by passing a water-station;
but this was done at serious risk, for the tender would nearly always
be empty by the time the next water-station was reached. The master
mechanic of the road determined to equip this engine with the Allen
valve: and, after the change was made, there was no risk in passing
the water-station; for there always was a good margin of water in the
tank when the next watering-place was reached. The engine seemed to
steam better, because the work was done with less steam; and there
was a decided saving of fuel. The change made the engine smarter, and
there seems to be no limit to the speed it can make. This valve can
be applied to any locomotive with trifling expense. When an engine is
designed specially for the Allen valve, the steam ports and bridges are
usually made a little wider than for the ordinary valve. The only real
difficulty in adopting the valve is getting the casting properly made,
so that the supplementary port will not be too rough for the passage of
steam, and the thin shell will be strong enough to stand the pressure.
INSIDE CLEARANCE.
For high-speed locomotives, where there is great necessity for getting
rid of the exhaust steam quickly, the valves are sometimes cut away
at the edges of the cavity, so that, when the valve is placed in the
middle of the seat, it does not entirely cover the inside of either of
the steam-ports. This is called inside clearance. In many instances
inside clearance has been adopted in an effort to rectify mistakes made
in designing the valve-motion, principally to overcome defects caused
by deficiency of valve-travel. The fastest locomotives throughout the
country do not require inside clearance, because their valve-motion
is so designed that it is not necessary. Inside clearance induces
premature release, and diminishes the period of expansion. Consequently
inside clearance wastes steam, and ought to be avoided.
LEAD.
There are certain advantages gained in the working of a locomotive,
by having the valves set so that the steam-port will be open a small
distance for admission of steam, when the piston is at the beginning of
the stroke. This opening is called lead. On the steam side of the valve
the opening is called steam-lead: on the exhaust side it is called
exhaust-lead. Lead is generally produced by advancing the eccentric on
the shaft, its effect being to accelerate every event of the valve’s
movement; viz., admission, cut-off, release, and compression. In
the most perfectly constructed engines, there soon comes to be lost
motion in the rod connections and in the boxes. The effect of this
lost motion is to delay the movement of the valves; and, unless they
are set with a lead opening, the stroke of the piston would in some
instances be commenced before steam got into the cylinder. It is also
found in practice, that this lost motion would cause a pounding at
each change in the direction of the piston’s travel, unless there is
the necessary cushion to bring the cranks smoothly over the centers.
Without cushion, the change of direction of the piston’s travel is
effected by a series of jerks that are hard on the working-parts. So
long as the lead opening at the beginning of the stroke is not advanced
enough to produce injurious counter pressure upon the piston, it
improves the working of the engine by causing a prompt opening for
steam admission at the beginning of the stroke. This is the time that
a full steam-pressure is wanted in the cylinder, if economical working
be a consideration. A judiciously arranged lead opening is therefore an
advantage; since it increases the port opening at the proper time for
admitting steam, tending to give nearly boiler pressure in the cylinder
at the beginning of the stroke. With the shifting link-motion, the
amount of lead opening increases as the links are hooked back towards
the center notch; the magnitude of the increase, in most cases, being
in direct proportion to the shortness of the eccentric-rods. A common
lead opening in full gear with the shifting link is 1/16 inch, which
often increases to ⅜ inch in the center notch. The tendency of wear
and lost motion is to neutralize the lead, so that, when a locomotive
motion gets worn, increasing the lead will generally improve the
working of the engine.
OPERATION OF THE STEAM IN THE CYLINDERS.
As the work performed by a steam-engine is in direct proportion to
the pressure exerted by the steam on the side of the piston which is
pulling or pushing on the crank-pin, it is important that the steam
should press only on one side of the piston at once. Hence, good
engines have the valves operated so that, by the time a stroke is
completed, the steam, which was pushing the piston, shall escape, and
not obstruct the piston during the return stroke, and so neutralize the
steam pressing upon the other side. When an engine is working properly,
the steam is admitted alternately to each side of the piston; and its
work is done against a pressure on the other side not much higher than
that of the atmosphere.
BACK PRESSURE IN THE CYLINDERS.
When, from any cause, the steam is not permitted to escape promptly and
freely from the cylinder at the end of the piston stroke, a pressure
higher than that of the atmosphere remains in the cylinder, obstructing
the piston during the return stroke, and causing what is known as back
pressure. There is seldom trouble for want of sufficient opening to
admit steam to the cylinders, for the pressure is so great that the
steam rushes in through a very limited space; but, when the steam has
expanded two or three times, its pressure is comparatively weak, and
needs a wide opening to get out in the short time allowed. This is one
reason why the exhaust-port is made larger than the admission-ports.
Nearly all engines with short ports suffer more or less from back
pressure, but the most fruitful cause of loss of power through this
source is the use of extremely contracted exhaust nozzles. Were it
not for the necessity of making a strong artificial draught in the
smoke-stack, so that an intense heat shall be created in the fire-box,
quite a saving of power, now lost by back pressure, would be effected
by having the exhaust opening as large as the exhaust-pipe. This not
being practicable with locomotives, engineers should endeavor to have
their nozzles as large as possible consistent with steam-making.
Engines with very limited eccentric throw will often cause back
pressure when hooked up, through the valve not opening the port wide
enough for free exhaust.
Locomotives suffering from excessive back pressure are nearly always
logy. The engine can not be urged into more than moderate speed under
any circumstances; and all work is done at the expense of lavish
waste of fuel, for a serious percentage of the steam-pressure on the
right side of the piston is lost by pressure on the wrong side. It is
like the useless labor a man has to do turning a grindstone with one
crank, while a boy is holding back on the other side. The weight of
obstruction done by the boy must be subtracted from the power exerted
by the man to find the net useful energy exerted in turning the
grindstone. In the same way, every pound of back pressure on a piston
takes away a pound of useful work done by the steam on the other side.
EFFECT OF TOO MUCH INSIDE LAP.
Engines that have much inside lap to the valves are likely to suffer
from back pressure when high speed is attempted. The inside lap delays
the release of the steam; and, where the piston’s velocity is high
the steam does not escape from the cylinder in time to prevent back
pressure.
RUNNING INTO A HILL.
Most of engineers are familiar with the tendency of some engines to
“run into a hill.” That is, so soon as a hill is struck, they suddenly
slow down till a certain speed is reached, when they will keep going.
This is generally produced by back pressure, its obstructing effect
being reduced when the engine is moving slow.
COMPRESSION.
The necessity which requires lap to be put on a slide-valve to produce
an early cut-off, in its turn causes compression, by the valve passing
over the steam-port, and closing it entirely for a limited period
towards the end of the return stroke. As the cylinder contains some
steam which did not pass out while the exhaust-port was open, this
is now squeezed into a diminishing space by the advancing piston. In
cases where too much steam was left in the cylinders through contracted
nozzles or other causes, or where, through mistaken designing of the
valve-motion, the port is closed during a protracted period, the
steam in the cylinder gets compressed above boiler tension, and loss
of useful effect is the result. Under proper limits, the closing of
the port before the end of the stroke, and the consequent compression
of the steam remaining in the cylinder, have a useful effect on the
working of the engine by providing an elastic cushion, which absorbs
the momentum of the piston and its connections, leading the crank
smoothly over the center. Where it can be so arranged, the amount of
compression desirable for any engine is the degree that, along with
the lead, will raise the pressure of the cylinder up to that of the
boiler at the beginning of the stroke. When this can be regulated, the
compression performs desirable service by cushioning the working-parts,
thereby preventing pounding, and by filling up the clearance space and
steam passages, by that means saving live steam. Compression probably
does some economical service by reheating the cylinder, which has
a tendency to get cooled down during the period of release, and by
re-evaporating the water, which forms by condensation of steam in the
cool cylinder.
Engines that are running fast require more cushioning than those that
run slow, or at moderate speeds. The link-motion, by its peculiarity
of hastening compression when the links are hooked up, tends to make
compression a useful service in fast running.
DEFINITION OF AN ECCENTRIC.
The reciprocating motion which causes the valves to open and close
the steam-ports at the proper periods, is, with most locomotives,
imparted from eccentrics fastened upon the driving-axle. An eccentric
is a circular plate, or disk, which is secured to the axle in such a
position that it will turn round on an axis which is not in the center
of the disk. The distance from the center of the disk to the point
round which it revolves is called its eccentricity, and is half the
throw of the eccentric. Thus, if the throw of an eccentric requires
to be 5 inches, the distance between the center of the driving-axle
and the center of the eccentric will be 2½ inches. The movement of an
eccentric is the same as that of a crank of the same stroke, and the
eccentric is preferred merely because it is more convenient for the
purposes to which it is applied than a crank would be.
EARLY APPLICATION OF THE ECCENTRIC.
On the early forms of locomotives, a single eccentric was used to
operate the valve for forward and back motion. The eccentric was
made with a half circular slot, on which it could be turned to the
position needed for forward or back motion. It was held in the required
position by a stop-stud fastened on the axle. Several forms of movable
eccentrics were invented, and received considerable application during
the first decade of railroad operating; but the best of them provided
an extremely defective reversing motion. The first engineer to apply
two fixed eccentrics as a reversible gear was William T. James of New
York, who made a steam carriage in 1829, and worked the engine with
four eccentrics,--two for each side. The eccentrics were connected
with a link, but the merits of that form of connection were not then
recognized here; for it was not applied to locomotives till it became
popular in England, and was re-introduced to this country by Rogers.
The advantage of the double fixed eccentrics seemed, however, to be
recognized from the time James used them; for the plan was adopted by
our first locomotive builders. The first locomotive built by Long, who
started in 1833 what was afterwards known as the Norris Locomotive
Works, Philadelphia, had four fixed eccentrics.
RELATIVE MOTION OF PISTON AND CRANK, SLIDE-VALVE, AND ECCENTRICS.
[Illustration: FIG. 9.]
When a locomotive is running, the wheels turn with something near a
uniform speed; but any part which receives a reciprocating motion
from a crank or eccentric travels at an irregular velocity. Fig. 9
shows the relative motion of the crank-pin and piston during a half
revolution. The points in the path of the crank-pin marked _A_, 1, 2,
_B_, 3, 4, _C_, are at equal distances apart. The vertical lines run
from them to the points _a_, _b_, _c_, _d_, _e_, represent the position
of the piston in relation to the position of the crank-pin. That is,
while the crank-pin traverses the half-circle, _A B C_, to make a half
revolution, the piston, guided by the cross-head, travels a distance
within the cylinder equal to the straight line _A C_. The crank-pin
travels at nearly uniform speed during the whole of its revolution, but
the piston travels with an irregular motion. Thus, while the crank-pin
travels from _A_ to 1, the piston travels a distance equal to the space
between _A_ and _a_. By the space between the lines, it will be seen
that the piston travels slowly at the beginning of the stroke, gets
faster as it moves along, reaches its highest velocity about half
stroke, then slows down towards the end till it stops, and is ready for
the return stroke.
ATTEMPTS TO ABOLISH THE CRANK.
Certain mechanics and inventors have been terribly harassed over this
irregular motion of the piston, and numerous devices have been produced
for the purpose of securing a uniform motion to the power transmitted.
These inventions have usually taken the shape of rotary engines.
Probably the fault these people find with the reciprocating engine is
one of its greatest merits, for the piston stopping at the end of each
stroke permits an element of time for the steam to get in and out of
the cylinder.
VALVE MOVEMENT.
The valve travels in a manner similar to the piston; although its
stroke is much shorter, and its slow movement is towards the limit
of travel. The small circle in the figure shows the orbit of the
eccentric’s center, and the valve-travel is equal to the rectilinear
line across the circle. If the valve opened the steam-ports at the
outside of its travel, the slow movement at that point would be an
objection, since the operation of opening would be slow: but the valve
opens the ports towards the middle of its travel, when its velocity is
greatest; and, the nearer to the mid travel the act of opening is done,
the more promptly it will be performed. This has a good deal to do with
making an engine “smart” in getting away from a station.
EFFECT OF LAP ON THE ECCENTRIC’S POSITION.
[Illustration: FIG. 10.]
With the short valve without lap used on the earliest forms of
locomotives, the eccentric was set at right angles to the crank or
“square” on the dotted line _e_, Fig. 10. The least movement of the
eccentric from its middle position had the effect of opening the
steam-ports. One advantage about an eccentric set in this position,
was that it opened and closed the ports when moving the valve at its
greatest velocity. Lengthening the valve-face by providing lap entails
a change in the location of the eccentric; for, were it left in the
right-angle position, the steam-port would remain covered till the
eccentric had moved the valve a distance equal to the extent of the lap
on one end, and the piston would begin its stroke without steam.
ANGULAR ADVANCE OF ECCENTRICS.
The change made on the eccentric location is to advance it from _e_
to _F_, being a horizontal distance equal to the extent of lap and
lead, and known as the angular advance of the eccentric. The centers
_F_ and _B_ represent the full part, or “belly,” of the forward and
back eccentrics in the position they should occupy, where a rocker is
employed, when the piston is at the beginning of the backward stroke.
It will be perceived that the eccentrics both incline towards the
crank-pin, and the eccentric which is controlling the valve follows the
crank-pin. Thus, when the engine is running forward, _F_ follows the
crank: when she is backing, _B_ follows.
It is a good plan for an engineer to make himself familiar with the
proper position of the eccentrics in relation to the crank, for the
knowledge is likely to save time and trouble when any thing goes
wrong with the valve-motion. With this knowledge properly digested, a
minute’s inspection is always sufficient to decide whether or not any
thing is wrong with the eccentrics.
ANGULARITY OF CONNECTING ROD.
In following out the relative motion of the piston and crank, we
discover a disturbing factor in what is called the angularity of the
connecting rod, which has a curiously distorting effect on the harmony
of the motion. When the piston stands exactly in the mid-travel point,
the true length of the main rod will be measured from the center of the
wrist-pin to the center of the driving-axle. If a tram of this length
be extended between these points, this will be found correct, as every
machinist accustomed to working on rods knows. Now, if the back end
of the tram should be raised or lowered towards the points where the
center of the crank-pin must be when the crank stands on the top or
bottom quarter, it will be found that the tram point will not reach the
crank-pin center, but will fall short a distance in proportion to the
length of the main rod. The dotted lines _a′_ and _b′_ in Fig. 11 show
how far a rod 7½ times the length of the crank falls short. A shorter
rod will magnify this obliquity, while a longer rod will reduce it.
[Illustration: FIG. 11.]
EFFECT ON THE VALVE-MOTION OF CONNECTING-ROD ANGULARITY.
As the opening and closing of the steam-ports by the valves are
regulated by the eccentrics, which are subject to the same motion as
the crank, following it at an unvarying distance, it is evident that
their tendency will be to admit and cut off steam at a certain position
of the crank’s movement. If the motion is planned to cut off at half
stroke, it will be apparent, that, in the backward stroke, the piston
will be past its mid travel before the crank-pin reaches the quarter,
so that end of the cylinder will receive steam during more than half
the stroke. On the forward stroke of the piston, however, the crank-pin
will reach the quarter before the piston has attained half travel;
the consequence being, that in this case steam is cut off too early.
The disturbing effect of the angularity of the connecting rod on the
steam distribution thus tends to make the cut-off later in the backward
stroke than in the forward stroke, resulting in giving the forward end
of the cylinder more steam than what is admitted in the back end. The
link-motion provides a convenient means of correcting the inequality
of valve opening due to the connecting-rod angularity, the details of
which will be explained farther on.
AIDS TO THE STUDY OF VALVE-MOTION.
An engineer or machinist who wishes to study out this peculiarity of
connecting-rod angularity, will find that the use of a tram or long
dividers will help him to comprehend it better than any letter-type
description. All through the study of the valve-motion, there are
numerous difficult problems encountered. The use of a good model will
be found an invaluable aid to the study of the valve-motion, and every
division of engineers or firemen should make a combined effort to
furnish their meeting-room with a model of a locomotive valve-motion.
In no way can the spare time of the men connected with locomotive
running be better employed than in the wide range for study presented
by a well-devised model. Great aid can be obtained in the study of
the valve-motion from good books devoted to the subject, and they
will impart more information than can be obtained by mere contact
with the locomotive. The valve and its movements are surrounded with
so many complicated influences, that an intelligent man may work for
years about a locomotive doing valve setting occasionally, and other
gang boss work, yet, unless he studies the valve-motion by the aid of
the drawing-board, or by models, which admit of changing sizes and
dimensions, he may know less about the cause of certain movements than
the bright lad who has been a couple of years in the drawing-office.
The man who thinks he can study the valve-motion, and understand
its philosophy, by merely running the engine, deceives himself. The
engineer who never looks at a book or a paper in search of information
about his engine, knows very little about any thing not visible to the
eye. Yet many men of this stamp, by looking wise, and by exercising
a judicious use of silence, pass among their fellows as remarkably
profound. But let a fireman, in quest of locomotive knowledge, put a
question to such a man, and he is immediately silenced with a “You
ought to know better” answer.
Where the use of a model can not be obtained, any one beginning
the study of the valve-motion can assist himself by making a cross
section of the valve and its seat, similar to those published, on a
strip of thin wood or thick paper. By slipping the valve on the seat,
its position at different parts of the stroke can be comprehended
more clearly than by a mere description. With a pair of dividers to
represent the motion of the eccentric, and strips of wood to act as
eccentric, and valve rod and rocker, and some tacks to fasten them
together, a helpful model can be improvised on a table or board. By the
time a student gets a rig of this kind going, he will see his way to
contrive other methods of self-help.
EVENTS OF THE PISTON STROKE.
By the aid of Fig. 10, we will trace the relative movements of the
crank and eccentric connections. For the sake of simplicity, the
eccentric is represented as connecting directly with the rocker-arm.
The crank-pin being at the point _A_, or the forward center, the piston
must be in the front of the cylinder, or at the beginning of the
backward stroke. Owing to the angular advance already referred to, the
eccentric center is at _F_; and, being a certain distance ahead of the
middle position, it has pushed the lower arm of the rocker from _a_ to
_b_, drawing back the top arm, which, in its turn, has moved the valve
so that it is just beginning to admit steam at the forward port, _i_.
As the crank-pin goes round, the eccentric follows it, opening the
steam-port wider till the eccentric reaches the point of its travel
nearest _A_, the limit of the throw. When the eccentric is at this
point of its throw, the valve must be at the outside of its travel;
and therefore the steam-port is wide open. By this time the crank-pin
is getting close up towards the quarter. After passing this point,
the forward eccentric begins to draw the bottom rocker-pin towards
the axle, and to push the valve ahead, this being the point where the
valve changes its direction of motion, just as the piston returns
when the crank-pin passes the center. When _F_ reaches the point _B_,
the valve is in the same position it occupied at the beginning of the
stroke; but, as it is traveling in the opposite direction, a very small
movement more closes the port, cutting off steam. When this happens,
the crank-pin has reached the point _x_. When _F_ gets to _g_, it is
on the central point of its throw; so the valve must then be on the
middle point of its travel, with the exhaust cavity just covering the
outside edges of the bridges, the forward edge being ready to put
the steam-port, _i_, in communication with the exhaust cavity. This
releases the steam from the forward end of the cylinder; and at the
same moment the inside edge of the valve covers the back port, _k_,
causing the piston-head to compress any steam left in the back part
of the cylinder. When the piston reaches the beginning of the forward
stroke, the eccentric _F_ has got to the point _f_, and the valve is
beginning to admit steam for the return stroke, the events of which are
similar to those described.
In actual practice, the steam distribution is a little different from
the manner that has been followed; for the link-motion provides the
means of equalizing the cut-off, making it uniform for both strokes.
This changes the events of the stroke a little; but the student who
engraves in his mind the movements as they are represented in the
diagram, will not be far astray.
WHAT HAPPENS INSIDE THE CYLINDERS WHEN AN ENGINE IS REVERSED.
Many men who have a fair understanding of the action of steam in
an engine’s cylinders during ordinary working, have no idea of the
operations performed in the cylinders when a locomotive is running
in reverse motion. All men who have had any thing to do with train
service, know, that, when an engine is reversed, the action works to
stop the train, even if the locomotive should have no steam on the
boiler; but just in what way this result comes round they can not
clearly perceive. In hopes of throwing light upon this subject for
those who have not studied it out, we will follow the events of a
stroke in reversed motion, as we did in the ordinary working.
EVENTS OF THE STROKE IN REVERSED MOTION.
Supposing an engine to be running ahead, and the necessity arises
for stopping suddenly, and the reverse-lever is pulled into the back
notch. When the crank-pin is on the forward center, and therefore
the piston at the forward end of the cylinder, about to begin its
backward stroke, the valve has the forward port open a distance equal
to the amount of lead, as in Fig. 10. But, as the back-up eccentric
has control of the valve, the latter is being pushed forward; and it
closes the forward port just as the piston begins to move back. This
shuts off all communication with the forward end of the cylinder; and
the receding piston creates a vacuum behind it, just as a pump-plunger
does under similar circumstances. At this time the back end of the
cylinder is open to the exhaust, and the piston pushes out the air
freely to the atmosphere. By the time the piston travels about two
inches, the valve gets to its middle position; and, immediately after
passing that point, it opens the forward end of the cylinder to the
exhaust, and closes the back port. When this event happens, the vacuum
in the forward end of the cylinder gets filled with hot gases, that
rush in from the smoke-box; and the receding piston keeps drawing air
into the cylinder in this way during the remainder of the stroke, and
air from that quarter seldom gets in without bringing a sprinkling of
cinders. The back steam-port is closed only during about two inches of
the stroke, while the lap of the valve is traveling over it. About the
time the piston reaches four inches of its travel, the back steam-port
is open to the steam-chest, and the piston forces the air through the
steam-pipes into the boiler during the remainder of the stroke. The
forward stroke is merely a repetition of the backward stroke described.
When it is necessary to reverse a locomotive, it is a better plan to
hook the lever clear back than to have it a notch or two past the
center, as some men persist in doing, under the mistaken belief that
they are in some way saving their engine from harsh usage. When the
link is reversed full, the cylinders are merely turned into air-pumps.
When the links are put near the center, the travel of the valve is
reduced; and the periods when the piston is creating a vacuum in
one end of the cylinder, and compressing the air in the other, are
prolonged. The result is, that, when the exhaust is opened in the
first case, the gases rush in violently from the smoke-box, carrying
a heavy load of cinders: in the other case, the piston compresses the
air in the cylinder so high that it jerks the valve away from its seat
in trying to find outlet. This causes the clattering noise in the
steam-chest, so well known in cases where engines are run without steam
while the reverse-lever is near the center.
A locomotive with the piston-packing in bad order will not hold well
running in reverse-motion. Some kinds of piston-packing do not seem
to act properly when the engine is reversed, especially at low speed.
Where a valve has much inside lap, there will be a vacuum in one end of
the cylinder, and compressed air in the other end. With piston packing
that requires pressure to expand it, the void at one end of the
cylinder may neutralize the pressure at the other by drawing the air
through the piston. This would be most liable to happen where the lever
was kept near the center.
PURPOSE OF RELIEF-VALVE ON DRY PIPE.
Should the throttle-valve close so tight that the compressed air from
the cylinders can not pass into the boiler, there is danger of bursting
the steam-chest or some part of the steam-pipes. The compressed air
will lift most of the throttle-valves far enough to prevent any great
danger from this source. In some engines a relief-valve is secured
in the dry pipe, which provides a passage for this compressed air.
When the cylinder-cocks of an engine are opened when the motion is
reversed, they form an outlet to the compressed air, and also admit
air to the sucking end without letting the piston draw air so freely
through the nozzles. Many cylinder-cocks are now made so that they
will open automatically to permit the piston to draw air through them.
The reversed engine will stop nearly as well with the cylinder-cocks
opened as when they are closed, and it is much more easily handled with
the cocks opened. Where the cocks are kept closed, the rush of hot
air from the smoke-box laps every trace of oil from the valve-seat,
and a heavy pressure--frequently above that of the boiler--is present
in the steam-chest. When the engine stops under these circumstances,
its tendency is to fly back; and an engineer has some difficulty in
controlling it with the reverse-lever till a few turns empty the chest
and pipes.
USING REVERSE-MOTION AS A BRAKE.
Numerous attempts have been made to utilize the reversed engine as a
brake for stopping the train, and even by this means to save some of
the power lost in stopping. Chatelier, a French engineer, experimented
for many years on this mechanical problem. He injected a jet of
water into the exhaust-pipe, which supplied low-tension steam to the
cylinder, instead of hot gas or air coming through the smoke-box. This
was pumped back into the boiler on the return stroke. Thus the act of
stopping a train was used to compress a quantity of steam, converting
the work of stopping into heat, which was forced into the boiler and
retained to aid in getting the train into speed again. Modifications of
this idea produce the car-starters that pass so frequently through our
Patent Office.
As a means of conserving mechanical energy, the Chatelier brake was not
a success; but, in the absence of better power brakes, it met with some
applications in Europe. Some of our mountain railroads use it, under
the name of the water-brake, as an auxiliary to the automatic brake.