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American Technical Society.

Cyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) online

. (page 5 of 30)
Online LibraryAmerican Technical SocietyCyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) → online text (page 5 of 30)
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60



THE STEAM ENGINE.



pressure in the cylinder and the position of the piston at that
instant. His assistant, Murdoch, invented the slide valve as a
means of admitting and releasing the steam. Fig. 4 shows Watt's
final engine.

Watt, like other early inventors, sold many of his engines to
miners, who had been using horses to pump out the mines, and
for this reason he rated his engines by the horse-power. Although
this term has an historical derivation it has no real significance,
and no relation whatever to the power of a horse. It is an
established unit for measuring the rate at which work is done. One
horse-power is the amount of work necessary to raise 33,000 pounds
through one foot in one minute ; and we may say that one horse-
power is equal to 33,000 foot pounds per minute.

Watt saw that by using high-pressure steam he could get
more work from it ; but as it was not possible to make very reli-
able boilers he never used a pressure of more than seven pounds
per square inch above the atmosphere. About the year 1800
comparatively high pressures came more into use and the noil-
condensing engine was introduced. In Watt's engine, and all
those preceding his, a vacuum was produced in front of the piston
by condensing the steam, and either the atmosphere or steam at
atmospheric pressure pushed it through the stroke. In the non-
condensing engine, using high-pressure steam, the space in front
of the piston could be opened to the atmosphere at exhaust, and
although the atmospheric pressure resisted its motion the pressure
of the steam behind the piston was still greater than that of the
air. These engines were much more simple than the condensing
engines, as they required no condenser.

About this time what would now be called a compound engine
was introduced by Hornblower and later by Woolf. It had two
cylinders of different size. Steam was admitted into the smaller
cylinder, and then passed over into the larger. The steam ex-
panded a little in the smaller cylinder and much more in the
larger one.

A great many attempts were made to build locomotives, but
they were generally unsuccessful until George Stephenson built
his engine, the "Rocket," in 1829. The principal new feature
of this engine was the improved steam blast for increasing the



61



10



THE STEAM ENGINE.



draft in the furnace and so making possible the use of a smaller
boiler. Later he used the "link motion," which enabled the
engine to be quickly reversed and the amount of expansion varied.




Ths Stephenson link motion may be seen on almost any locomo-
tive. It is simply a device by which either of two eccentrics ma>
be made to move the valve.

About the year 1814, Woolf introduced a compound pump-



THE STEAM ENGINE. li



ing engine in the mines of Cornwall, but a simpler engine was
later introduced and Woolf's engine fell into disuse. This later
engine became known as the Cornish Pumping Engine and was
famous for many years because of its economy. It was the first
engine ever built that could compare at all with modern engines
in tLe matter of steam consumption. It consisted of a single
cylinder placed under one end of a beam from the other end of which
hung a heavy rod which operated a pump at the foot of the shaft.
Steam was admitted on the upper side of the piston for a short por-
tion of the stroke and allowed to expand for the remainder of the
stroke. This forced the piston down, lifted the heavy pump rod
and filled the pumps with water. Then communication was
established between the upper and under side of the piston, ex-
haust occurred, and the heavy pump rod fell, lifting the piston and
forcing the water out of the pumps. The cut-off was about .3
stroke, and the pump made about seven or eight complete strokes
per minute with a short pause at the end of each stroke to allow
the valves to close easily and the pumps to fill with water. The
cylinder was jacketed. These engines needed great care and were
in charge of competent men, to whom prizes were frequently given
for the best efficiency, which doubtless accounts for their wonderful
performance.

PARTS OF THE STEAH ENGINE.

Fig. 5 shows the elevation of a simple form of steam engine.

The Cylinder A (see plan, Fig. G) is that part of the engine in
which the piston moves back and forth. It is made of cast iron and
accurately bored. Great care must be taken in this work, for any
nnevenness will allow steam to leak through between the piston and
cylinder walls or it may even cause the piston to stick or work
hard. In large engines the cylinder consists of two parts, the
outer or cylinder proper and a comparatively thin cast iron liner.
A spaco can be left between them for a steam jacket. Should the
cylinder liner be damaged, it can be replaced without the expense
of a new cylinder.

The Cylinder Heads B cover the ends of the cylinder and are
securely bolted thereto. In the crank-end cover there is an
opening for the piston rod to pass through. This opening is made



THE STEAM ENGINE.



steam tight by a stuffing box which surrounds the piston rocL
Sometimes the piston rod is prolonged beyond the piston and
through the front cover. This extension of the piston rod is to




help steady the piston in a long stroke and is known as the tail
rod. AVhen a tail rod is used another stuffing box must also be
provided for t}ie head -end cover.



THE STEAM ENGINE. 18

The Piston U, in small engines, is usually a thick disc of iron
or steel, as shown in Fig. 6. It is often made conical, as shown
in Fig. 7, to better withstand the steam pressure and to gain space.
The piston must fit the cylinder steam tight and yet move easily.
To accomplish this, one or more grooves in the piston are filled
with packing (usually metallic), or spring rings may be used.

The Piston Rod C (Figs. 5 and 6) is made of steel and con-
nects the crosshead and the piston to which it is rigidly fixed.

The Crosshead D serves to join the piston rod and connecting
rod. At one end it is fastened to- the piston rod, and at the other
end is the wrist pin V on which the connecting rod swings. It
is guided to and fro by the crosshead guides Q.

The Connecting Rod E is a steel forging from three to eight
times the length of the crank, depending upon the type of engine.
One end is jointed to the crosshead by the pin V, called the wrist
pin, while the other encircles the crank pin and revolves with it.
A detail view of one end is shown in Fig. 8 ; the other end is
frequently similar. In some cases the small end is forked, as
shown in Fig. 9.

The Crank Pin F forms the connection between the crank
and connecting rod.

The Crank G, equal in length to one-half the stroke of the
piston, converts the back and forth motion of the connecting rod
into circular motion. It may be simply an arm, as shown in Fig.
10, or a complete disc keyed to one end of the shaft, as shown in
Fig. 11. The disc is more nearly balanced than the crank.

The Shaft H transmits the rotary motion from the crank to
the fly wheel P.

The Frame of the engine S is a heavy casting, which supports
the cylinder and bearings. It should be securely bolted to the
foundation.

The Steam Chest M receives steam directly from the boiler,
and the steam passes thence through the ports W into the cylinder.

The Eccentric I is a disc keyed to the shaft so that its center
and the center .of the shaft do not coincide. The eccentric strap Y
encircles the eccentric and imparts a reciprocating motion to the
valve stem L and the enoentn'c rod J. This action is similar to



14



THE STEAM ENGINE.



that of the crank and connecting rod, but exactly reversed. K is
the valve stem crosshead and R its guides.

The Slide Valve X is the valve for alternately admitting




steam to the cylinder and releasing it. It has cup-shaped cavity
in its face through which the exhaust steam passes. It is situated

in the steam chest and is moved

by the valve gear, that is, the

/<2fr-<Sfl &-* eccentric and the eccentric rod.

The Clearance Z is the space
between the piston and the
cylinder head (when the piston
is at the end of the stroke), to-
Fig. s. gether with the volume of the

steam ports. This volume must
be filled with steam before the piston can start. It is usual to




THE STEAM ENGINE.



15



express the clearance as a certain per cent of the volume swept
through by the piston.

The crank may revolve in either direction. If we stand by
the cylinder, facing the crank shaft, and the crank moves away
from us as it passes over
the shaft, we say that it
is running over. If it
moves away from us as
it passes under the shaft,
we say that the engine
is running under. The
action of steam in the



Fig. 10.






Fig. 9. Fig. 11.

cylinder of an engine is very complicated, and its discussion
will be taken up later in the course.

TYPES OF ENGINES,

Classification. There are so many different types of engines
that it is difficult to classify them all properly. Most engines
belong to several classes at one and the same time. For instance,
there are condensing and non-condensing engines ; there are



07



THE STEAM ENGINE.



simple, compound, triple and quadruple expansion ; there are high
speed and low speed, vertical and horizontal, locomotive, stationary




and marine, and many other classes into which these might be
further subdivided.

Simple Engines. The simplest type of engine is the simple
expansion. It has one cylinder and admits steam for a part of the
stroke, expands it during the remainder and exhausts either into



08



THE STEAM ENGINE. 17

the atmosphere or into a condenser. Simple engines (see Figs. 5
and 12) are now used only for comparatively small powers, say
100 H. P. or less, and although more extravagant of fuel than the
others, may still be the most economical financially if low first cost
is an important item, if they are not run continuously, or if the
load fluctuates widely.

Compound Engines have two cylinders known as the high pres-
sure and low pressure. Steam enters the smaller or high pressure
cylinder and then expands until release,when it is exhausted into the
larger cylinder, where the expansion is finished. The cylinders
should be so proportioned that approximately the same amount
of work can be done in each. The first cylinder is small, because
it has the higher steam pressure, and a given weight of steam
occupies less space when at high pressure. The second must be
large, so that the volume at cut-off can contain all of the steam
exhausted from the high.

Besides being more economical the compound has a distinct
mechanical advantage. The two cranks may be set at right
angles, so that when one is on dead center the other is at a posi-
tion of nearly its greatest effort. This makes a dead center
impossible, and gives a more uniform turning moment. Then the
individual parts may be made lighter, and are thus more easily
handled, but the engine is much more costly, and it is nearly
twice as much work to take care of it.

When the cranks of a compound engine are at 90, the low-
pressure piston is not ready to receive steam when the high pres-
sure exhausts, hence there must be a receiver to hold the steam
until admission occurs in the low. Such engines are called cross
compound. Fig. 13 shows one form. Sometimes instead of
having the cranks at 90 they are placed together or opposite.
Then the strokes begin and end together, and the high can ex-
haust directly into the low without a receiver. Such engines are
called Woolf engines. A tandem compound engine, shown in
Fig. 14, has both pistons on one rod, the high-pressure piston rod
forming th* low-pressure tail rod. Such engines are less expen-
sive because there is but one set of reciprocating parts instead of
two, but like simple engines they have the disadvantage of dead
points.



18



THE STEAM ENGINE.




70



THE STEAM ENGINE. 19

Triple Expansion Engines expand the steam in three stages
instead of two. There are usually three cylinders, the high, inter-
mediate, and low, arranged with cranks 120 apart. This gives a
more uniform turning moment than a compound. Sometimes there
are four cylinders to the triple, one high, one intermediate, and
two low. This arrangement gives better balance and is often
used in marine work.

For triple engines there must be a receiver between each
two cylinders. Fig. 15 shows the essential features of a triple
expansion engine.

Quadruple Engines expand their steam in four stages instead oi
three. Multiple-expansion engines are nearly always condensing.

Cylinder Ratios. There are several considerations to be
remembered when proportioning the cylinders of multiple-expan-
sion engines. The ratio of the cylinders should be such that each
develops nearly the same power ; the drop in pressure between the
cylinders and receivers should be small, and the strains in the
cylinders about equal.

There are many formulas in use, some simple, others involv-
ing mathematical calculation. A common rule for compound
engines is to make the ratio of the cylinders equal to the square
root of the total ratio of expansion. Thus if the steam has a ratio
of expansion of 9, the ratio of the cylinder volumes will be \/~9~
3, or the low-pressure cylinder will have a volume 3 times
as great as the high-pressure cylinder. If the cylinder ratio is 3,
and the length of stroke is the same for both, the diameter of the
low-pressure cylinder will be 1.75 times that of the high-pressure
cylinder.

Another rule is to make the cylinder ratio equal to the total
ratio of expansion multiplied by the fractional part of the stroke
completed when cut-off occurs in the high-pressure cylinder.

Suppose the ratio of expansion is 9, as above, and that cut-off
occurs at ^ of the stroke in the high-pressure cylinder. The ratio
of cylinder volumes will be 9 X J = 3. If cut-off occurs at J
the stroke, the ratio will be 9 X J = 4.5.

For triple expansion engines the low pressure cylinder is
made large enough to develop the whole power if steam at boiler
pressure is used.



71



THE STEAM ENGINE.




78



THE STEAM ENGINE.



21



The intermediate cylinder is made approximately of a mean
size between the high and the low. The area of the intermediate
piston is found by dividing the area of the low by 1.1 times the
square root of the ratio of the low to the high.

We may write the above thus,

Area of higli-pres. cyl. = Area of low-pres. cyl.

Cut-off of high-pres. X ratio of exp.
Area of inter, cyl. = Area of low-pressure cylinder.
1.1 X V ratio ot ' low




Fig 15.

In general the volumes foi- triple expansion are of about the
following ratios, 1 representing the volume of the high pressure
cylinder :

1 : 2.25 to 2.75 : 5 to 8.

For quadruple-expansion engines the ratios are as follows :
1:2 to 2.33: 4 to ,5: 7 to 12.

High Speed Engines. Of late years there has been a demand
for engines of higher speeds than were formerly used. It was found
desirable to run dynamo-electric machines by connecting them
directly to the shaft of the engine rather than by belts as before.



22



THE STEAM ENGINE.



This required engines running from 200 to 1,000 revolutions per
minute instead of from GO to 90 revolutions. Also for engines in




torpedo-boats, speeds as high as 400 or 500 revolutions are
common.

Running at high speed requires various changes. Reciprocating
parts must be made lighter to reduce the vibration, and must be



74



TFIE STEAM ENGINE. 23

more carefully proportioned to maintain balance. Bearings must
be made very much larger to reduce the pressure in order that the
friction may not be excessive and cause heating. Special care is
necessary that bearings should be tight, since the least looseness
will cause knocking and hammering until the bearing is ruined.
Parts must be as simple as possible and so arranged as to need the
least possible attention. In slow-speed engines the engineer can
watch the oil cups and oil any part while it is running. But this
is impossible in high-speed engines, and special means must be
used to insure plenty of oil to the bearings and the cylinder.

These peculiarities of high-speed engines may be easily seen
in any engine for electric lighting, for running fans, etc. It is
also necessary that the speed of engines for running electric
generators should be very steady, as the slightest Change in the
speed make the lights flicker. The fly wheels are, therefore, larger
or heavier than for other types and the governors are made
specially sensitive. Fig. 16 shows a horizontal high-speed engine
with the working parts encased. Fig. 17 shows a vertical high



.For any double-acting engine, that is, for any engine in which
the steam acts first on one side and then on the other side of the
piston, the piston first pushes and then pulls on the connecting rod
and crank. At each half revolution of the crank the direction of
the pressure reverses. It is this change of pressure which causes
the pounding if the bearings are at all loose. This is one of the
greatest troubles with high-speed engines. In order to avoid these
rapid reversals in pressure, single-acting vertical engines are used
to a considerable extent. In such engines the steam is admitted
only to the head end of the cylinder. The other end
stands open. The connecting rod is in compression throughout
the whole revolution instead of being first in compression and then
in tension. Besides insuring that the piston shall always push,
this arrangement simplifies the valves.

Ther,e are several good single-acting, high-speed engines.
One of the earliest made was introduced by Brotherhood. He
used three cylinders, set around the shaft 120 apart. Another
well-known example is the Willans "central valve" engine.
These are both English engines. A well-known American engine



75



TIIM STKAM K\(JINK.



of this type is the Westinghouse high-speed engine, a section of
which is shown in Fig. 18.

Vertical and Horizontal Engines. At the present time the
most common type of engine is the horizontal direct-acting, that




Fig. 17.

is, an engine whose cylinder is horizontal and whose piston acts on
the crank through a piston rod and connecting rod. In small
engines the whole is often on one bed plate. Such engines are
called self contained. The cylinder is either bolted to the back
of the bed plate or rests directly on it.

In marine work vertical engines are used in almost every case.
The reason for this is, of course, the saving of floor space, which
is so important in a vessel. This saving of space, however, is also



76



THE STEAM ENGINE. 25

very important in many cases such as in crowded engine rooms
of cities where land is expensive, and as there are a number of
advantages which vertical engines have over horizontal, they are
coming largely into use in stationary practice.

A second advantage of the vertical over the horizontal
engine is the reduction of cylinder friction and unequal wear in
the cylinder of the latter. In. the horizontal engine the piston is
generally supported l>y resting on the cylinder, which is gradually
worn until it is no longer round. This causes leakage of steam
from one side to the other. Evidently this is entirely avoided in
the vertical engine.

Still another advantage of the vertical engine is the greater
ease of balancing the moving parts so that there shall be no jarring
or slinking. It is impossible to perfectly balance a steam engine
of one or two cylinders. If it is balanced so that there is no
tendency to shake side wise, it will shake endwise; and if it is
balanced endwise it will shake sidewise. The jarring is due to
the back and forth motion of the reciprocating parts and the
centrifugal force of the crank and connecting rod. The crank
can be readily balanced by making it extend as far on one side of
the shaft as it does on the other, but the piston and connecting
rod are more difficult to balance. We can greatly reduce the
effect of jarring if we balance the crank and make the end-
wise throw come in line with the foundation, which should be
heavy enough to absorb the vibration transmitted. In a horizontal
engine this endwise throw not being in line with the foundation
will cause vibration in the engine itself.

In machines that can be anchored down to a massive founda-
tion a state of defective balance only results in straining the parts
and causing needless wear and friction at the crank-shaft bearings
and elsewhere, and in communicating some tremor to the ground.
The problem of balancing is of much more consequence in loco-
motive and marine engines.

To sum up the general advantages of vertical engines : they
have less cylinder wear, they take up less floor space, and they
can be better balanced. In addition to these there are certain
advantages which vertical engines have for certain kinds of work.

The disadvantages of vertical engines are as follows : The



77



26



THE STEAM ENGINE.



pressure on the crank-pin is greater during the down stroke
than during the up stroke because, during the down stroke the
weight of the reciprocating parts is added to the steam pressure,
and during the up stroke this weight is subtracted.




Another difficulty is that in large engines the various parts are
on such different levels that they require considerable climbing.
This requires more attendants and is sometimes the cause for neglect
of the engine.



78



THE STEAM ENGINE. 27

The foundations for vertical engines generally need to be
deeper than those for horizontal engines. At the same time, how-
ever, they need not be as broad.

Marine Engines. The first steam vessels were fitted with
paddle-wheels, and as beam engines were the most common, this
form of engine was used. Its construction, however, was some-
what modified for this service. This arrangement of beam engine
and paddle-wheel was used for many years and was applied to
ocean vessels as well as to small river boats. It is still used,
especially in this country, on river steamers and some coast
steamers. The beam is supported by large A frames on the
deck, and the engines are about on a level with the shaft.

Engines of this type take up rather more room than those
now in common use, partly because of great size, and also be-
cause of the shaft and paddle-wheels. Another disadvantage
is that in heavy weather, when one paddle-wheel is thrown out
of water the other is deeply immersed and takes all the strain,
so that there is a tendency to rack the boat. Then again if the
boat is loaded heavily the paddle-blades are very deeply immersed
while if light they barely touch the water. It is hard to handle
the engines satisfactorily under both conditions.

The introduction of the screw propeller overcame these diffi-
culties very largely and at the same time required a much faster
running engine. At first, the increased speed was supplied by the
use of spur-wheel gearing, but gradually higher speed engines were
built and connected directly to the propeller shaft. It was, of
course, difficult with small width at each side of the shaft to use
horizontal engines, therefore various arrangements of inclined
engines were used before the vertical engine was finally chosen
by all as the standard form for marine work. It is only in recent
years that the vertical engine has become general in naval work
and in merchant steamers.

In merchant ocean steamers the common form has three
cylinders set in line, fore and aft, above the shaft. The cranks are
120 apart, to give a very even turning moment. The three
cylinders are worked triple expansion. The valves are usually
piston valves on the high and intermediate, #nd double ported
slide valves on the low. Sometimes piston valves are used on all



THE STEAM ENGINE.



the cylinders. Plain slide valves are not suitable for high-
pressure work of any kind.

For engines on ocean vessels it is necessary to use surface
condensers in order that the same water may be used over and
over again. If it were necessary to take in sea-water for the
boilers they would soon become clogged with the salt and require
cleaning. Surface condensers for marine work are generally made
up of a large number of brass tubes of from | inch to 1 inch in
diametei. In some cases the cold water is forced to flow through
the tubes while the steam comes in contact with the outside of the
tubes.

In any marine plant there are four special pumps. The first
is the air pump for the condenser. This is usually made large
so that in case there is a leak in the condenser it can take
charge of the water even if it becomes necessary to run as a jet



Online LibraryAmerican Technical SocietyCyclopedia of engineering : a general reference work on steam boilers, pumps, engines, and turbines, gas and oil engines, automobiles, marine and locomotive work, heating and ventilating, compressed air, refrigeration, dynamos motors, electric wiring, electric lighting, elevators, etc. (Volume 2) → online text (page 5 of 30)