Mallet Articulated Locomotives
Supplement to The Science of Railways
by Marshall M. Kirkman
1914 Cropley Phillips Company
Embodying Descriptions and Instructions for
the Manipulation of Mallet Locomotives
Mallet Articulated Compound Locomotive (American
Locomotive Company).The Mallet articulated compound
locomotive is one having two sets of cylinders, compounded together
and driving independent groups of wheels. The two sets of cylinders
are supplied with steam from a single boiler; which makes it practically
two locomotives combined in one, and having only one boiler. The
rear group of wheels is carried in frames rigidly attached to
the boiler in the usual manner, while the frames which carry the
front group of wheels are not secured to the boiler, but support
it by means of sliding bearings. There is a hinged connection
between the frames of the front engine and those of the rear engine,
about which the former is permitted a limited swing in relation
to the latter. It will be seen that the front group is a truck
which swivels radially about its articulated connection with the
rear group, when the locomotive passes through a curve. It is
from this feature that the articulated type of locomotive derives
Because of the fact that only the rear group of wheels is carried
in rigid frames, the articulated type of locomotive provides a
short rigid wheel base capable of passing through curves of short
radius. At the same time, the total number of wheels is greater
than in the ordinary types of locomotives; and the weight is distributed
over a greater number of axles. Consequently, an enormous weight
with corresponding tractive power may be provided in this type
without an excessive weight per wheel on the rail. In an articulated
compound locomotive having twice as many driving wheels as a given
locomotive of the rigid-frame type, double the tractive power
of the latter is available, with the same weight per driving wheel
on the rail and with no increase in the length of the rigid wheel
base. Or vice versa, with the same tractive power in each case,
the weight per driving wheel on the rail of the articulated compound
locomotive may, by the use of the proper wheel arrangement, be
reduced to one-half of that of a given locomotive of any of the
types in ordinary use.
The work being divided between two sets of pistons, crank pins,
rods, and driving axles, an enormous tractive power is obtained
in the articulated compound locomotive with practically no increase
in the weights of the moving parts over those of a locomotive
of the rigid-frame type, having half the tractive power; or with
the same tractive power in each case the moving parts of the articulated
locomotive may be made much lighter than those of locomotives
of other types.
In addition to the advantages due to its wheel arrangement,
the articulated compound locomotive possesses all those resulting
from compounding the steam. This type of compound locomotive is
what is known as a two-stage compound; that is, the steam is used
successively in two sets of cylinders. Steam from the boiler is
admitted to the first set or high pressure cylinders, which ordinarily
drive the rear group of wheels; and, having done work in those
cylinders, is then used over again in the second set or low pressure
cylinders which are connected to the front group of wheels. From
the low pressure cylinders, the steam is exhausted to the atmosphere.
Between the high and low pressure cylinders and connecting
the two is a large pipe called the receiver, into which the steam
from the high pressure cylinders exhausts when the locomotive
is working compound. The receiver is simply a reservoir in which
the exhaust steam from the high pressure cylinders is stored until
it is required by the low pressure cylinders. From the receiver,
the steam is admitted into the low pressure cylinders by their
valves in the usual manner.
The low pressure cylinders have a larger piston area than the
high pressure cylinders, the ratios between the two being such
that, at the ordinary working cut-off, the steam at the lower
pressure per square inch acting against the larger piston area,
exerts the same force as the higher pressure steam acting on the
smaller area. Consequently, the high and low pressure cylinders
having the same stroke, each set of cylinders ordinarily does
practically the same amount of work.
By using the steam successively in two sets of cylinders, a
greater range of expansion is obtained than in a simple or single
expansion locomotive. In other words, the difference between the
pressure of the steam entering the high pressure cylinders and
the pressure it has when the exhaust from the low pressure cylinders
opens, is greater than in the case of the simple locomotive.
In a simple locomotive, the steam is ordinarily expanded only
four times, while in a two-stage compound six or seven expansions
are obtained. As a result, more work is performed by the same
amount of steam in a compound than in a simple locomotive; and
a considerable saving in coal and water consumption is thereby
Moreover, compounding divides the range of temperature between
the two sets of cylinders; so that the condensation in the cylinders
is reduced, which effects a further saving in fuel and water consumption.
In every compound locomotive some provision must be made for
admitting steam direct from the boiler to the low pressure cylinders
in starting and until the exhaust from the high pressure cylinders
supplies the low pressure cylinders with steam. Also, provision
is usually made by which in case of emergency when additional
hauling capacity is required, the locomotive may be changed from
working compound into simple with an increase in power. In this
articulated compound locomotive, these functions are performed
by a special mechanism called the intercepting valve, which is
located between the receiver and the exhaust passages from the
high pressure cylinders.
Another device used by some locomotive builders, in place of
the intercepting valve, is an arrangement by which, on opening
a valve operated from the cab, communication is established between
the two ends of the high pressure cylinder through a by-pass pipe;
and live steam reduced in pressure by passing through this pipe
is admitted to the receiver and so to the low pressure cylinders.
With the by-pass arrangement, when the locomotive is working
simple, live steam is necessarily admitted to both sides of the
high pressure pistons. Consequently, these pistons are very nearly
balanced. At the same time, the live steam which is admitted to
the low pressure cylinders is reduced in pressure. The result
is that under these conditions, when the locomotive is starting
or working simple, practically all of the work is done by the
low pressure cylinders, and little, if any, increase in power
In the American Locomotive Company's system of compounding,
the intercepting valve is so designed that when the engine is
working simple the exhaust from the high pressure cylinder passes
directly to the atmosphere and the valve cuts off communication
between the receiver and the exhaust side of the high pressure
pistons, thus relieving them of back pressure, except that of
the steam exhausting to the atmosphere. Moreover, the live steam
from the boiler reduced to a pressure of somewhat above the ordinary
pressure in the receiver is admitted to the low pressure cylinder.
Hence, the low pressure pistons are exerting more power than when
working compound. This additional power, added to that secured
in the high pressure cylinders, because of the reduction of the
back pressure, gives a total increase in power when- working simple
of about 20 per cent. The intercepting valve also automatically
regulates the pressure of the live steam entering the receiver
when starting and when working simple, keeping it at such a pressure
that each of the four cylinders does practically the same amount
Among the distinctive features of this articulated compound
locomotive, practically, the only ones which enter into its operation
are the intercepting valve, the power reversing gear, and the
The intercepting valve is identical in principle with that
used on the two-cylinder cross-compound locomotives known as the
Richmond Compound, differing from the latter only in certain modifications
of the design which the use of four cylinders instead of two necessitates.
Engineers, therefore, who have operated the two-cylinder cross-compound
of this build, will be perfectly familiar with the construction
and operation of the intercepting valve as applied to this locomotive.
This valve is located in the saddle of the left high pressure
cylinder, to the left of the vertical and above the horizontal
center line of the cylinders. It consists, in reality, of three
valves, viz., the intercepting valve, the reducing valve or sleeve,
and the emergency or high pressure valve.
The various parts comprising the
whole mechanism are shown in detail in Fig. 2.
Parts 2, 3 and 5 constitute the intercepting valve proper.
This valve shuts off, at the proper time, communication between
the receiver and the high pressure cylinders, to prevent the pressure
in the receiver backing up against the high pressure pistons,
when the locomotive is working with live steam in all four cylinders.
The reducing valve or sleeve, 1, fits on the stem of the intercepting
valve, 2, along which it is free to slide longitudinally. Its
duty is three-fold:
First, to close the intercepting valve in starting and when
the locomotive is changed from compound to simple working;
Second, to let live steam from the boiler into the receiver
and low pressure steam chests in starting and when the locomotive
is working simple;
Third, to regulate the supply of this live steam and keep its
pressure at a predetermined amount.
The emergency or high pressure exhaust valve, 6, which is located
at one of the outer ends of the intercepting valve chamber, is
the device which makes it possible to change the locomotive from
compound to simple working (that is, using live steam in all four
A wrought iron pipe leads from the emergency valve chamber
along the left side of the locomotive to an elbow at the rear
of the main exhaust pipe. This elbow connects wits. a passage
surrounding the main exhaust opening.
When the locomotive is changed into simple working, the emergency
valve, 6, is opened, which allows the exhaust steam from the high
pressure cylinders to pass through the wrought iron pipe to the
exhaust pipe in the smoke box and to the atmosphere.
Opening of the emergency valve
is accomplished by opening the emergency operating valve, which
is indicated by the letter N in Fig. 3. When the emergency operating
valve is closed (when the locomotive is working compound), the
handle of the valve points forward.
To open the emergency operating valve, N, and change the locomotive
into simple, the handle must be turned so as to point backward.
The opening and closing of the emergency valve, 6, is thus
under the control of the engineer.
It is important to bear in mind that the emergency valve, as
its name indicates, should ordinarily be used only when the locomotive
cannot otherwise move the train; and, as soon as a speed of three
to four miles per hour has been attained, the locomotive should
be changed back to compound.
Except for changing the locomotive into simple, the movements
of all the parts of the intercepting valves are automatic.
The illustrations in Figs. 4, 5, 6 and 7 show the entire mechanism
assembled, and the arrangement of the various steam pipes and
passages These illustrations also give the intercepting valve
in its four different positions; namely:
Fig. 4, the moment after the throttle is open when starting
in the ordinary way, the reducing valve, 1, being open and the
intercepting valve, 2, and the emergency valve, 6, closed;
Fig. 5, at the time when the predetermined pressure has
been reached in the receiver pipe, when the reducing valve, 1,
is closed arid the other parts remain in the same position as
in Fig. 4;
Fig. 6, in the compound position, when the intercepting
valve, 2, is open and the reducing valve, 1, and the emergency
valve, 6, are closed;
Fig. 7, in simple position, when the emergency or high
pressure exhaust valve, 6, and the reducing valve, 1, are open,
and the intercepting valve, 2, is closed.
In these illustrations, the course of the steam is indicated
by arrows, and helps to make clear the explanation of the principle
and operation of this system of compounding.
As will be seen from Fig. 7, the reducing valve, 1, is so fitted
on the stem of the intercepting valve, 2, that when the former
opens, it closes the latter, and vice versa. The reducing valve,
however, can be closed without opening the intercepting valve.
OPERATION OF THE INTERCEPTING VALVE.
Referring to Fig. 4, live steam from the boiler is, as indicated
by the arrows, always admitted through the cored passages in the
cylinder casting to the chamber, A, formed in the intercepting
valve chamber head, 4, and surrounding the reducing valve, 1.
Chamber C communicates with the receiver pipe or steam passage
to the low pressure cylinders, and chamber F connects directly
with the exhaust passages from the high pressure cylinders. The
chamber L communicates with chamber M through the emergency or
high pressure exhaust valve, 6. The latter chamber is connected
with the exhaust pipe in the smoke box, as previously explained.
With the intercepting valve in the position shown in Fig. 4,
steam from the boiler, following the course of the arrows, flows
through the passage in the left high pressure cylinder to chamber
A, and acting against the shoulder, E, of the reducing valve,
1, has forced this valve open or inward, closing the intercepting
valve, 2, and uncovering the ports, B. This allows live steam
to pass into the chamber C, and thence into the receiver and to
the low pressure steam chests and cylinders. Live steam, at the
same time, passes through the high pressure valve into the high
pressure cylinders in the ordinary way. The intercepting valve,
2, being closed, communication between the exhaust passage, F,
from the high pressure cylinders and the chamber, C, is cut off.
This thus prevents the pressure in this latter chamber from backing
up against the exhaust side of the high pressure pistons; and,
consequently, these start free from back pressure; while, at the
same time, the low pressure cylinders are being supplied with
steam direct from the boiler. The pressure of this steam is so
regulated by the reducing valve, 1, that it bears the same relation
to the boiler pressure as the high pressure piston areas bear
to the low pressure piston areas, thus making the work in all
four cylinders equal (the high and low pressure cylinders having
the same length of stroke). For instance, if the area of the low
pressure cylinder is two and one-half times the area of the high
pressure cylinder, then the reducing valve, 1, would be so designed
as to reduce the pressure of the live steam admitted by it to
chamber C, to 1 divided by 2.5 or 40 per cent. of the boiler pressure.
From the above, it will be seen that the locomotive automatically
starts with live steam in all four cylinders, or, in other words,
as a single expansion engine.
Piston, 3, and the chamber, H, in the outer end of the intercepting
valve chamber head, 4, constitute simply an air dash-pot, to prevent
slamming of the valves when changing from compound to simple when
Fig. 5 represents the intercepting valve at the moment when
the predetermined maximum pressure in the low pressure steam chests
is reached. In this case, it will be noticed that the positions
of the valves are the same as in Fig. 4, except that the reducing
valve, 1, has been moved out, closing the ports, B, thus cutting
off the supply of live steam to the chamber C, and to the low
pressure steam chests; until by the movement of the low pressure
pistons the pressure in that chamber has been lowered to the required
The reducing valve automatically keeps the pressure in the
chamber C down to the desired amount because of the fact that
the area of the shoulder E is, as previously stated, usually 1
divided by 2.5 or 40 per cent. of the area of the end D of the
valve. Consequently, when the pressure in the chamber C exceeds
40 per cent. of the boiler pressure, it will overcome the force
of the steam at boiler pressure, acting on the shoulder E; and
move the reducing valve, 1, outward, closing ports B.
The intercepting valve automatically assumes the compound position,
Fig. 6, after one or two revolutions of the driving wheels. In
this position, the intercepting valve 2 is opened, allowing the
exhaust steam from the high pressure cylinders to pass into the
chamber C, and so to the receiver and the low pressure cylinders.
The opening of the intercepting valve 2 has closed the reducing
valve, 1, which thus cuts off the supply of live steam to the
chamber C and receiver.
The principle by which these movements are automatically performed
may need some explanation. The exhaust steam from the high pressure
cylinders in the chamber F acting against the inner face of the
intercepting valve, 2, and also against the inner end of the intercepting
valve stem .(being admitted to the chamber L through the holes
in the unbalancing valve, 5), tends to open the intercepting valve
2. This force is resisted by the pressure on the outer face of
the intercepting valve 2, the pressure on the outer and inner
faces of the unbalancing valve, 5, being balanced. The combined
areas of the face of the intercepting valve 2 and the end of its
stem are greater than the area of the outer face of the valve.
Thus steam in the chamber F at a lower pressure acting against
this larger area overcomes the resistance of the higher pressure
steam in chamber C and forces the valve into the position shown.
This principle is the same as in the case of the reducing valve
These areas are usually so proportioned that when the pressure
in the chamber F is 30 per cent. of the boiler pressure, it overcomes
the resistance of the steam in the chamber C at a pressure of
40 per cent. of boiler pressure.
As will be seen from the above, when the locomotive is working
compound the low pressure steam chests receive all of their steam
from the exhaust from the high pressure cylinders through chambers
F and C and the receiver, the ports B having been closed by the
outward movement of the intercepting valve 2. At full stroke,
the pressure on the low pressure pistons would be, approximately,
30 per cent. of the boiler pressure; while, on the high pressure
pistons, would be exerted the pressure which the live steam from
the boiler has, minus the 30 per cent. in the receiver which acts
on their exhaust sides. The pull on the cross-heads of all four
cylinders is practically equal, as the products of the several
piston areas multiplied by their respective pressures are equal
in each case.
Should the maximum power of the locomotive be required in starting
or in ascending a heavy grade, it may be had at any time by simply
turning the emergency operating valve N in the cab so that the
handle points to the rear. The intercepting valve will then assume
the position shown in Fig. 7.
Opening the emergency operating valve admits live steam into
the chamber G, which forces the emergency valve, 6, open against
the resistance of its own spring plus the pressure of the steam
in the chamber L (which is receiver pressure).
On the opening of the emergency exhaust valve, 6, the steam
in the chamber L is immediately released. This unbalances the
intercepting valve, 2, with the result that the reducing valve,
1, is moved inward or opened by the pressure of the steam from
the boiler in chamber A acting against the shoulder E. The reducing
valve, 1, carries the intercepting valve 2 inward with it, closing
the latter, the two valves assuming the position shown in Fig.
7. Communication between the chamber C and the chamber F, into
which the steam from the high pressure cylinders exhausts, is
thus cut off; while live steam from the boiler, at a pressure
reduced to about 40 per cent. of the boiler pressure, is allowed
to pass through the ports B into the chamber C and thence through
the receiver to the low pressure steam chests.
By the use of the intermediate chamber L between the chamber
F and the emergency valve, 6, which is exhausted the instant that
valve is opened, the intercepting valve 2 is closed and the reducing
valve 1 opened before, or at the same moment, that the receiver
is actually exhausted.
Consequently, there is no drop of pressure in the low pressure
steam chests during the change from compound to simple or prior
to the entrance of live steam into the low pressure steam chests.
As the emergency exhaust valve, 6, is kept open by the pressure
of the steam admitted to the outer side of the piston 8 by the
opening of the emergency operating valve in the cab, the exhaust
steam from the high pressure cylinders passes through the chamber
F into the chambers L and M, and so into the high pressure exhaust
pipe and to the atmosphere.
Thus when the intercepting valve is in position shown in Fig.
7, that is, when the locomotive is working simple, the high pressure
pistons are relieved of the back pressure amounting to 30 per
cent. of the boiler pressure, which acts against them when the
locomotive is working compound, with the intercepting valve in
Fig. 6. On the other hand, the low pressure cylinders are receiving
steam direct from the boiler at a pressure of 40 per cent. of
that which it has in the boiler, instead of exhaust steam from
the high pressure cylinders at a pressure of only 30 per cent.
of boiler pressure, as when the locomotive is working compound.
This explains the 20 per cent. increase in the normal maximum
power, which, as already stated, is obtained by changing the locomotive
into simple. The increase would be greater were it not for the
wire-drawing of the steam through the restricted area of the ports
B, which are intentionally reduced for operation under this condition.
As it is, the actual increase in power at speeds of from three
to four miles per hour would not be greater than the amount given
The reducing valve, 1, is so designed that at speeds of more
than three or four miles an hour no increase in power is obtained
by changing the locomotive into simple. This is done in order
that the emergency feature will not be misused, with injurious
effect on the machinery and the sacrifice of economy in fuel consumption.
If the pressure in the chamber C and consequently in the receiver
pipe and the low pressure steam chests rises to more than 40 per
cent. of the boiler pressure when the engine is working simple,
the reducing valve, 1, will be forced outward to the position
it has in Fig. 5; that is, closing the ports B and shutting off
the live steam from the chamber C. The other parts of the valve,
however, will remain in the same position as shown in Fig. 7.
The reducing valve 1 automatically closes under the conditions
Upon the movement of the low pressure pistons, the steam pressure
in the chamber C will be reduced; and the boiler pressure acting
upon the small shoulder E would again force the reducing valve
1 inward to its position in Fig. 7, opening the ports B. Thus
the pressure in the chamber C and low pressure steam chests would
be again raised to the required 40 per cent. of the boiler pressure.
This alternate opening and closing of the reducing valve 1 will
continue as long as the displacement of the low pressure pistons
does not exceed the supply of steam that comes through the ports
B. When this condition occurs, the reducing valve 1 will remain
These facts explain why, if the locomotive starts to slip when
it is changed into simple, it automatically ceases without necessitating
closing the throttle; since, with the rapid movement of the low
pressure pistons, the power of those engines is reduced; and,
with the increased exhaust from the high pressure engines passing
through the comparatively restricted opening of the emergency
valve 6, the back pressure on the high pressure pistons is increased,
reducing the effective power in those cylinders.
It is very important for the engineer to remember that, the
locomotive having been changed into simple working by opening
the emergency operating valve N in the cab, it is necessary to
close this valve (that is, turn it so that the handle points forward),
in order to change the locomotive back to compound or normal
working. With the emergency operating valve closed, the steam
will be exhausted from the chamber G in front of the piston 8.
The tension of the spring, assisted by the steam pressure upon
the inner end of the emergency exhaust valve 6, will then return
that valve to its seat, thus preventing the exliaust steam from
the high pressure cylinders escaping to the stack. A few exhausts
from the high pressure cylinders will, then, soon raise the pressure
in the chamber F and force the intercepting valve 2, and with
it the reducing valve 1, to assume the compound position, as shown
in Fig. 6.
If, upon starting the locomotive, it is desired to prevent
the valves from changing automatically to the compound position,
the emergency valve 6 may be opened in advance by opening the
emergency operating valve N, turning the handle to the rear.
This, as previously explained, will prevent the pressure in
the chamber F from rising sufficiently to force the intercepting
valve 2 open.
In changing from compound to simple when running, the sudden
unbalancing of the intercepting valve 2, tends to close this valve
rapidly, with the result that it would slain, were it not for
the dash-pot, which prevents this. The dash-pot piston 3 at the
outer end of the intercepting valve stem works in the cylinder
H formed in the outer end of the intercepting valve chamber head
4. When the intercepting valve is forced inward under full pressure,
its too rapid motion is prevented by the slow escape of the air
from under the piston 3 through the small port J. This is practically
the only function of the dash-pot. The port K, extending through
the center of the intercepting valve stein half way to the inner
end, permits the escape of any steam that may leak past the small
rings on the intercepting valve stem and reducing valve 1.
All of the ports of the intercepting valve have important duties
to perform, and their location and sizes must not be changed.
From the above description of the intercepting valve, it will
be seen that to start a train with an articulated compound it
is usually only necessary to open the throttle in the ordinary
way with the reverse lever in the position required for the weight
of the train or, ordinarily, in the extreme notch; and with the
cylinder cocks open. The intercepting valve will automatically
assume the position shown in Fig. 4, and the locomotive will work
simple until the pressure in the receiver has raised sufficiently
to force the intercepting valve 2 into position shown in Fig.
6, or compound position.
If the locomotive fails to move the train when started in this
way, or is about to stall on a steep grade, it should be changed
into simple working by turning the handle of the emergency operating
valve in the cab, so that it points to the rear, which
causes the intercepting valve to assume position shown in Fig.
There is no increased tendency for the locomotive to slip when
working simple; and, moreover, when it does slip, the slipping
is automatically arrested after only a few inches of movement
of the piston. If, however, the locomotive starts to slip, it
is advisable to use sand, should the rail conditions be at all
The engineer can easily tell whether the locomotive is working
simple or compound either by the sound of the exhaust or by the
position of the emergency operating valve in the cab. When working
simple there are eight exhausts to each revolution of the wheels,
and only four when working compound. In the former case the exhaust
has more the sound of a continuous blow, the separate exhausts
being less distinct. When working compound, the handle of the
emergency operating valve, as stated, points forward, and
to the rear when working simple.
If the low pressure engine fails to start when the throttle
is open, the trouble may lie in the reducing valve 1 having stuck
in the closed position, due to the fact that it had not been properly
lubricated or some foreign matter had worked into the bore of
the valve. In such an event the admission ports B, Fig. 4, would
be closed and no steam could get to the low pressure cylinder.
Such a difficulty can ordinarily be remedied by giving the
reducing valve a little more feed of oil for a few minutes; or,
if necessary, the cover of the dash-pot H may be removed and with
a piece of bent ¼-inch wire the reducing valve 1 may be
moved in and out a few times, after which it will probably clear
itself when the throttle is open.
The intercepting valve should be given a liberal feed of oil
for a minute before starting and occasionally during long runs
when the throttle is not shut off for a considerable length of
time. Outside of this, one drop of oil every four or five minutes
is ordinarily ample when running.
POWER REVERSING GEAR.
Because of the size and weight of the parts of the valve motion
of this articulated compound locomotive, a power reversing gear
is generally applied to operate the reverse lever. This is an
engine consisting of two cylinders, one an air cylinder and the
other filled with oil. The two cylinders are set one ahead of
the other, and are usually bolted to the underside of the mudring
or some other convenient location.
Fig. 8 shows the arrangement and construction of this mechanism.
In this case the forward one is the air cylinder and the rear
the oil cylinder, although this arrangement is usually reversed
and can be made whichever the circumstances require. In any arrangement
both pistons are mounted on a common piston rod which is connected
to either the main reverse lever, or, as in the illustration,
to an extension of the reverse shaft arm.
Between the two cylinders are the packing boxes for the common
piston rod, and there is also a stuffing box at the air end of
the cylinder for the rod connecting the piston with the reverse
lever or shaft, as the case may be. Both pistons are packed with
leather packing, that in the air cylinder being held out by spring
rings. The valves of both cylinders are conical, that for the
air cylinders having four openings in addition to the exhaust
cavity, while the oil cylinder valve has two crossed passages.
The valves of the air and oil cylinders are operated by an
auxiliary reverse lever R to which they are connected by a rod.
This lever is pivoted on the main lever Q at the point W. It is
provided with a latch with teeth that fit in a quadrant in the
same manner as the main lever. This latch is so interlocked with
the latch of the main reverse lever Q that raising the former
raises the latter, which cannot drop again unless the main reverse
lever Q is in its normal position relative to the auxiliary lever
The levers are so designed that when the two latches are lifted
the auxiliary reverse lever R is allowed sufficient movement abut
its pivot, point W (limited by lugs on the main reverse lever
latch) to give a full opening of the valves of the air and oil
When a change in cut-off is desired the latch of the auxiliary
lever R is released, which also unlatches the main reverse lever
Q. If the main reverse lever Q is to be thrown ahead, the auxiliary
lever is moved forward about its pivot point W, and back, if it
is desired to move the main lever in that direction. The movement
of the auxiliary lever R, forward or back, swings its lower end
Y which operates the valves of the, air and oil cylinders; and
the valve motion is moved in the desired direction.
For instance, when the auxiliary lever is pushed forward, its
lower end Y is drawn back. This turns the valve of the air cylinder
so that the air is admitted through the air inlet 13 to the front
of the piston, and the exhaust port 14 establishes communication
between the rear end of the air cylinder and the exhaust to the
atmosphere. At the same time the crossed passages of the oil cylinder
valve are so turned as to allow the oil in the cylinder to flow
from one end to the other. The air and oil pistons thus move back
and the valve gear is moved forward. The slow flow of the oil
in the oil cylinder prevents the too rapid movement of the reverse
The auxiliary lever R, being pivoted on the main reverse lever
Q, moves with the latter, and when the gear is to be changed must
be kept in motion until the desired notch in the quadrant is reached
and then latched. By stopping the movement of the auxiliary lever,
the gear automatically moves the main reverse lever up to its
normal position relative to the former, when it also latches,
as already stated. This also automatically closes the valves of
both the air and oil cylinders, giving both an oil and a positive
lock to the gear.
Except in case of lack of air pressure or any accident to the
power reversing gear, the valve motion is handled entirely by
the auxiliary reverse lever R. For this reason the practice is
to cut off that portion of the main reverse lever Q which ordinarily
projects above the deck of the cab, thus leaving more room for
the engineer. . A separate handle is provided for the main reverse
lever, which may be easily applied in case of accident to the
power reversing gear.
It is, of course, important that the air and oil valves be
properly set so that the valve openings and cylinder ports match
up properly in the different positions. For example, a quarter
of a turn of the oil valve one way or the other would result in
the valve being blanked instead of open when the auxiliary lever
R was moved about its pivot point. The gear could not, then, be
operated, as the oil could not circulate from one end of the oil
cylinder to the other. If, therefore, the power reversing gear
fails to operate when the valves are supposedly opened and nothing
has happened to the air supply, first examine the valves to see
that they are in their proper position.
The function of the oil cylinder is to prevent the too rapid
movement of the reversing gear when a change of cut-off is made.
It is imperative, therefore, that this cylinder be always kept
full of oil. The frequency with which it should be filled depends
on the condition of its piston and piston rod packing and these
should, therefore, be kept in good condition. If the reversing
gear operates too rapidly, this indicates that there is a lack
of oil in the oil cylinder and this should be refilled and the
In case of any repairs to the power reversing gear, especial
attention should be given to see that it is properly adjusted
when set up so that when the gear is operated the main reverse
lever will automatically be moved to its normal position relative
to the auxiliary lever and properly latch. If the gear is not
properly adjusted, the latch of the main reverse lever will not
engage properly with the teeth of the quadrant. In consequence
the latch of the auxiliary lever will have to hold the gear with
the result that it will be quickly worn.
Another feature which plays a most important part in the successful
operation of the articulated compound locomotive, and so should
be clearly understood by the engineer, is the by-pass valves.
The purpose of these valves is to prevent the injurious effects
which would otherwise result from the pumping action of the large
low pressure pistons when the locomotive is drifting.
These valves are so designed that they automatically establish
communication between the two ends of the cylinder, when the engine
is running with the throttle closed, thus performing several important
First, they prevent alternating vacuum and compression in the
cylinders when the locomotive is drifting, thus insuring the free
movement of the pistons.
Second, by permitting the circulation of the free air drawn
into the cylinders through the vacuum-relief valves, they prevent
this air from being overheated by the churning of the pistons
and thus destroying the lubrication, when the locomotive is drifting
down a long hill.
Third, by destroying the vacuum which, without them, would
be formed by the large piston, they prevent the smoke and gases
from the smoke box being sucked into the cylinder.
Fourth, they prevent excessive fanning of the fire from the
pumping action of the large pistons when drifting
These valves are located in chambers cast in the outside of
each low pressure cylinder. Their construction is shown in Fig.
9. There are two valves to each cylinder. The lower view in the
illustration shows the two valves U with the heads T of the chambers
in which they are located; while the upper view shows the valves
alone without the valve chamber heads.
Fig. 10 illustrates the arrangement of the valves when assembled
in their chamber and their relation to the steam ports in the
In position A of this latter figure, the valves U are in the
position they assume when the throttle is open. In this position
the steam passing from the steam chest ports through the small
ports S in the head T of the valve chamber, as indicated by the
arrows, acts against the outer ends of the valves U and keeps
them against their seats, cutting off communication between the
admission ports of the cylinders.
Position B of Fig. 10 is the one the by-pass valves U automatically
takes when the throttle is closed. In that event the atmospheric
pressure admitted through the small air vent in the valve chamber
forces the valves U open, closing the steam chest ports and establishing
communication between the admission ports at either end of the
cylinders. This permits circulation from one end of the cylinder
to the other when the locomotive is drifting, accomplishing the
necessary results enumerated.
It is strongly recommended, therefore,
that in drifting the reverse lever be kept at ¾-stroke
or more, since when operated in this way the locomotive will drift
As the by-pass valves perform such important duties, it is
essential that they be properly cared for and kept in good condition,
to prevent them from sticking. The engineer can tell at once if
the by-pass valves are stuck open, as, in that case, steam will
blow from the small pipe projecting from under the jacket midway
between the ends of the cylinder. This pipe connects to the air
vent in the center of the chamber containing the valves.
From the above description it will be seen that if the bypass
valves stick open it will cause a severe blow. When the locomotive
is first put into service the by-pass valves should be taken out
and cleaned quite frequently to keep them free of core sand which
will undoubtedly work in. After this has been done a few times
they require only ordinary attention.
If the low pressure engines are heard to thump as if a piston,
crosshead or box were loose and the locomotive does not drift
freely, the trouble probably lies in the bypass valves being stuck
closed by being gummed, and these should be taken out and cleaned
at the first opportunity.
Sticking of the by-pass valves may be caused by smoke box gases
being sucked into the cylinders by the pistons when the locomotive
is drifting with the reverse lever "hooked up." These
gases would be sent circulating through the bypass valves which
are oily from the steam, and the soot may stick to them and form
a gum. This gum hardens gradually and the valves ultimately work
so hard that the comparatively light suction in the steam chest
is not strong enough to open them. On this account periodical
cleaning of these valves should be made.
The possibility of smoke and gases being sucked into the cylinders
will be minimized, if the reverse lever is kept at ¾-stroke
or more when the locomotive is drifting.
VACUUM AND RELIEF VALVES.
In the high pressure steam chests or some other convenient
place which is in communication with the steam chests, are located
vacuum valves. The function of these valves is to admit free air
into the steam chests when the locomotive is drifting so as to
avoid a vacuum and give a moderate flow of air through the cylinders.
The low pressure cylinders are equipped with combined vacuum
and relief valves which in addition to having functions similar
to the vacuum valves of the high pressure cylinders also regulate
the steam pressure in the low pressure steam chests. These relief
valves are set at 45 per cent of the boiler pressure.
As these valves relieve any excessive pressure in the low pressure
cylinders, they should be tested occasionally to see that they
are correctly set.
From the previous description of the intercepting valve it
will be seen that when the locomotive is working compound the
packing rings of the high pressure valves and pistons alone separate
the boiler pressure from the pressure in the receiver and low
pressure cylinders. Consequently if there was a blow in these
packing rings, the pressure in the receiver would be increased,
causing the relief valves in the low pressure steam chests to
blow off. Therefore, if these valves rise from their seats frequently
when the locomotive is working compound, it might be due to the
fact that there was a blow in either the valves or the pistons
of the high pressure cylinders, and these should be tested.
To test for blows simply throw the emergency operating valve
in the cab to the simple position, namely, with the handle pointing
to the rear. Spot the locomotive and test the same as a simple
Another feature peculiar to the articulated compound locomotive
is the flexible ball and slip joint steam pipe connections.
In this articulated compound locomotive there is a ball joint
connection between the receiver pipe and the high pressure cylinders,
a slip joint connection between the receiver pipe and the Y-pipe
by which the steam is carried to the steam passages of the low
pressure cylinders, a ball joint connection between the exhaust
pipe flexible connection and the low pressure cylinders and also
between the former and the exhaust pipe in the smoke box. This
exhaust pipe flexible connection is likewise provided with a slip
joint to allow for the variations in its length when the engine
rounds a curve. The construction of these flexible connections
is shown in Fig. 11.
As will be noticed, the ball joints consist of a ball-bearing,
gland, stuffing box and packing, while in the slip joints the
construction is very much the same, without the ball-bearing.
The packing in both classes of joints consists of a fiber material.
The free nominal size of the packing rings is five-eighth inch
square in section and they are hammered and worked down to one-half
inch square before being applied, which makes them soft and pliable.
In the case of the receiver pipe joint at the high pressure
cylinders, a brass ring of elongated diamond section is inserted
in the center of the packing. As this ring is just the width of
the packing space it seals all the joints in the packing rings
proper and forces them tightly against the inside of the box and
against the ball.
In repacking the joint this brass ring should be removed, all
the packing rings inspected, new rings put in where necessary
and the ring put back in its proper place. In every case the original
arrangement of joint packing should be preserved.
The diagram in Fig. 11 shows the method of arranging the rings.
These must be cut to the correct lengths and the two ends in every
case meet perfectly when in place. They are so arranged that the
seams in any two adjacent rings are at least a quarter of a circle
or 90 degrees apart, and all the seams in the entire set are at
least one-eighth of a circle or 45 degrees from each other. The
reference numbers on the diagram correspond to those on the packing
It is important, therefore, when the packing is renewed that
care should be taken to arrange there in the manner shown, since
if so arranged trouble from leaky joints will be avoided.
It is also essential that in renewing packing the same kind
of packing should be used as that originally applied.
Owing to the fact that in the articulated compound locomotive
a, long receiver pipe is generally employed and the ball joint
is located in the vertical center of the pivot connection between
the front and rear engines, there is very little movement in the
ball joint and there is practically no tendency for this joint
In case there is any leakage it is usually due to the fact
that the gland is not screwed in tight enough, and can be easily
stopped by a turn or two on the gland bolts.
ADJUSTMENT OF THE ALIGNMENT OF THE FRONT ENGINE
In locomotives of the articulated compound type vertical hanger
or "trim" bolts X, Fig. 12, connect the upper rails
of the rear frames with the lower rails of the front frames. These
bolts have ball and socket bearings in the frame rails and sufficient
play is allowed in the bolt holes to provide for the lateral movement
of the front frames when the locomotive is passing through a curve.
They serve to adjust the weights on the front and rear engines
so that each bears its proportionate amount of the total load,
and to keep the front frames in proper alignment. Ordinarily,
therefore, the alignment of the engine may be easily adjusted
if necessary by means of them without any other change in the
In designs of articulated compound locomotives having no front
truck, where two sliding boiler bearings are employed, the front
sliding bearing does not normally carry any load, but is merely
an emergency stop in case of derailment or any unusual change
in the alignment between the two frames. This bearing is so designed
that when the front and rear frames are in proper alignment there
is a clearance (ordinarily ¼ inch) between the upper and
lower bearing and an equal amount between the upper bearing and
the safety straps or clips, as indicated at Z and Z', Fig. 12.
An exception to the above rule should be noted in the case
of class 0880 engines designed for over 16-degree curvature. In
such designs the front bearing may sometimes be used for supporting
a small part of the weight on the front engine, though a considerably
less amount than when a front truck is used.
About ¾-inch total play is also always allowed between
the rear draw casting (or jaw) and the front draw casting (or
hinge) of the articulated connection. The hinge casting should
not touch either the top or bottom of the jaw.
In the case of this articulated compound locomotive of one
of the designs covered in the above rule, if the hinge casting
of the articulated joint bears on the top of the jaw and the safety
strap, or clips, of the front sliding boiler support bears close
against the upper casting of this support, adjustment may be made
by the "trim" bolts, which should be tightened up until
the vertical play is approximately divided in each case.
On the other hand, if the bearing surfaces of the upper and
lower castings of the front sliding support touch and the articulated
hinge bears on the bottom of its jaw, adjustment should be made
by loosening the "trim" bolts.
Should the upper and lower bearings of the front boiler support
touch, while at the same time the articulated hinge bears on its
top surface against the jaw, a liner plate of a thickness of about
one-half the total play should be inserted between the sliding
block of the rear boiler support and its saddle. This should give
a satisfactory adjustment. To insert this liner, the rear bearing
can be raised the required amount by tightening up the vertical
suspension or "trim" bolts X, while the plate is fitted
in, after which the nuts on the bolts should be eased off till
the correct tension is secured and the proper adjustment is sure
to be obtained.
If, on the other hand, the safety straps, or clips, bear on
the upper casting of the front sliding boiler support and the
articulated hinge bears on its bottom surface against its jaw,
the sliding block of the rear sliding boiler support should be
planed off an amount equal to one-half the total play.
When front trucks are used or where both bearings of the front
engine support weight, the "trim" bolts are provided
with a spring under the nut at one end in order to relieve the
excessive load, which is liable to be concentrated on the rear
bearing because of inequalities in the level of the track, or
In case of any breakdown in which one or more of the cylinders
can be disconnected and the locomotive run in with the remaining
cylinders active, simply throw the emergency operating valve N
in the cab into the simple position and proceed as with a simple
locomotive, namely, disconnect and block the disabled cylinder
or cylinders. This is the only rule to follow and the only one
to be remembered, and covers all cases of accidents which do not
entirely disable the locomotive.
Always open the cylinder cocks in starting.
Usually the locomotive will start the train when the throttle
is opened in the ordinary way with the reverse lever in the position
required for the weight of the train or ordinarily in the extreme
notch. If the locomotive fails to start the train when operated
in this way, change it into simple working by turning the handle
of the emergency operating valve in the cab so that it points
to the rear. This same course should be followed if the
engine is about to stall on a heavy grade. If the speed is over
three or four miles an hour, no increase in power will be obtained
by changing the locomotive into simple working.
When drifting, the reverse lever should be kept at ¾-stroke
or more. As before stated, if this is done, the locomotive will
The oil cylinder of the power reversing gear should always
be kept full of oil. The piston and piston rod packing of the
oil cylinder should be kept in good condition so as to prevent
leakage. If the reversing gear operates too rapidly it indicates
that there is not sufficient oil in the oil cylinder and this
should be refilled and the leakages stopped.
If the reversing gear is not adjusted properly so that the
latch of the main reverse lever does not engage with the teeth
of the quadrant, the trouble should be remedied as soon as possible.
If not properly adjusted, the locking of the reverse gear will
be put almost entirely on the latch of the auxiliary lever, which
is not designed for such duty and would, therefore, quickly wear.
The by-pass valves should be taken out and cleaned periodically
to prevent them from being gummed and sticking. When the locomotive
is first put into service, these valves should be cleaned quite
frequently for a few times so as to keep them free from the core
sand which is sure to work into them. Afterwards they will require
only ordinary attention to work properly. When these valves are
properly performing their functions, the locomotive will drift
freely. If they stick open it will cause a severe blow, while
if stuck in the closed position, it will cause a pounding in the
low pressure engines.
The relief valves in the low pressure steam chests should be
tested occasionally to see that they are correctly set at 45 per
cent of the boiler pressure, as these valves relieve any excessive
pressure in the steam chests.
REPAIRS TO FLEXIBLE JOINTS.In renewing the packing
of the flexible joints the same kind of packing should be used
as that originally applied. Also care should be taken to keep
the arrangement of the packing the same as that shown in the diagram
in Fig. 11.
The brass ring of the receiver pipe joint at the high pressure
cylinder may be removed in order to insert new packing, but the
original arrangement of the joint packing should always be preserved.
LUBRICATION.Give the intercepting valve a liberal
feed of oil for a minute before starting and occasionally during
long runs, when the throttle is not shut off for a considerable
length of time. Except for this, one drop of oil to the intercepting
valve every four or five minutes is ample when running.
Besides the intercepting valve, the other parts of the articulated
compound locomotive which should be oiled, which are not found
on the ordinary locomotive are:
Sliding boiler bearings on the front engine.
The ball joint in front of the high pressure cylinder (before
starting on a trip).
The upper or rear ball joint of the exhaust pipe (before
starting on a trip).
The lower or front ball joint of the exhaust pipe (before
starting on a trip).
The bolt connecting the two engines.
The ball bearings of the vertical suspension or "trim"
bolts which connect the upper rails of the rear frames with the
lower rails of the front frames X, Fig. 12.
The ball bearings of the floating columns (if applied).
The piston rod packing of the cylinders of the power reversing
The air cylinder of the power reversing gear, by means of the
plug in the top of the cylinder (about once a week).
BLOWS.To test for blows in the valves or pistons,
throw the emergency operating valve in the cab to the simple position,
namely, with the handle pointing to the rear. Spot the
locomotive and test the same as a simple locomotive.
NOTE.The Author is Indebted to The American Locomotive
Company for the Foregoing Full and Authoritative Description.
Mallet Articulated Compound Locomotive (Baldwin
Locomotive Company).As the distinctive features
of the Mallet locomotive have been described, the articulated
boiler connection which does not form a part of that locomotive
is given here, also the arrangement of the super-heater and reheater.
The flexible boiler connections used on the two following engines
are entirely different in construction, engine 1158 having a double
ball-jointed connection, while engine 1159 has a pleated or bellows
form of connection.
On engine 1158 the connection consists of two cast iron sleeves,
Fig. 2, fitted one within the other and provided with snap rings
to keep the joint tight. Each sleeve forms a ball joint with a
cast iron ring, which is bolted to the shell of the corresponding
boiler section. These rings are made in halves, to facilitate
assembling. The ball joints are kept tight by rings of soft metallic
packing, which can be adjusted by set screws. The two boiler sections
can thus move in any direction relative to one another and full
provision is made for expansion and contraction.
On engine 1159 the joint is composed of sixty rings of high
carbon steel having a thickness of No. 14 wire gauge, Fig. 3.
These rings are ten inches wide and have an outside diameter of
seventy-five and one-half-inches. They are made with. a set, so
that, when placed adjacent to each other, they form a series of
V-shaped joints. The adjacent rings are riveted together at the
inside and bolted at the outside, and the connection is bolted
in place between the front and rear boiler sections. The products
of combustion traverse the flexible connection through a cylindrical
flue forty-four inches in diameter. This flue is riveted to the
rear boiler section and prevents cinders from lodging in the crevices
between the connecting rings.
To assist in holding the boiler sections in alignment, a centering
device is placed on each side on the horizontal center line of
the boiler. This arrangement consists of a pair of helical springs,
which are seated in cast steel brackets riveted to the shells
of the front and rear boiler sections, Fig. 4. The springs are
held in place between washers, carried by a horizontal thrust
bar. When the engine enters a curve, the two boiler sections assume
an angular position with reference to each other and by reason
of the compression of the springs on the outer side the corresponding
thrust bar is thrown into tension, thereby tending to bring the
boiler sections back into alignment.
It is, of course, necessary in these locomotives to place flexible
joints in all pipes which pass the articulated connections in
the frames and boiler. This, however, introduces no objectionable
complication. The steam piping is simplified, as no flexible joints
are required in the exhaust connection between the low pressure
cylinders and the smoke box. There is claimed to be a distinct
advantage in the avoidance of sliding supports under the forward
boiler section and the stability of the locomotive, when on curves,
is not impaired by the lateral displacement of the boiler on the
front frames, which necessarily occurs in the Mallet locomotive
as usually built.
The arrangement of the superheater and reheater is practically
the same on both engines. An open chamber is located in each boiler
section adjacent to the flexible connection and these chambers
contain the heaters, Fig. 5. The superheater is located in the
rear boiler section and the reheater in the front section. These
heaters are of the Jacobs type and each consists of a steel drum
traversed by horizontal fire tubes.
The superheater is exposed to a higher temperature and steam
pressure than the reheater and its tubes are welded at each end,
while in the reheater the tubes are rolled and beaded. The heaters
are fitted with internal baffle plates, so that the steam is compelled
to follow a circuitous course among the tubes.
The throttle valve is connected with the superheater by an
internal dry pipe and the steam enters the superheater at the
top. There are two outlets, placed right and left in a steel casting
on which the superheater drum is seated, and these outlets communicate
directly with suitable passages which are cored in the high pressure
cylinder saddle. The connections between the saddle and steam
chests are effected by short elbow pipes.