Copyright
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 8 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 8 of 30)
Font size
QR-code for this ebook


gine is hollow and of the lide valve type. The cut-off valve is
inside. The change of cut-off is due to the alteration of the
angular advance. The arrangement of the parts which effect the
change of angular advance is shown in Fig. 42. A wheel which



112



THE STEAM ENGINE.



61



contains and supports the various parts of the governor is keyed to
the shaft. Two arms, having weights A A at the ends, are pivoted
to the arms of the wheel at b b. The ends having the weights are
connected to the collar on the loose eccentric C by means of rods B B.
When the weights move to the position indicated by the
dotted lines, the eccentric is
turned on the shaft about a
quarter of a revolution in the
direction in which the engine
runs. That is, the eccentric is
advanced or the angular ad-
vance is increased. Now we
know that if the angular ad-
vance is increased, cut-off occurs
earlier. This is shown by the
table on page 22 of " Valve
Gears." If the engine had a
single plain slide valve the varia-
tion of the angular advance
would produce too great a varia-
tion of lead ; but as this engine
has a separate valve for cut-off,
admission is not altered by the Fig. 41.

cut-off valve.

The springs F F balance the centrifugal force of the weights.
The weights A A are varied to suit the speed ; the tension on the
springs is altered by means of the screws c c. Auxiliary springs
are added in order to obtain the exactness of regulation necessary
for electric lighting. These springs tend to throw the arms out-
ward, but act only during the inner half of this movement.

The Straight-Line Engine Governor. Fig. 43 shows the gov-
ernor of the straight-line engine. It has but one ball, B, which is
' linked to the spring S and to the plate D E, on which is the eccen-
tric C. When the ball flies outward in the direction indicated by
the arrow F, the eccentric is shifted about the pivot O ; the links
moving in the direction of arrow H. The ball is heavy and at
a considerable distance from the center; hence it has a great
centrifugal force, and the spring must be stiff.




62



THE! STEAM ENGINE.



The governor of the Buckeye engine alters the cut-off by
changing the angular advance. The straight-line engine governor
changes the travel of the valve. Shaft governors which alter the
cut-off by changing the valve travel are very commoR.

LUBRICATION.

If two pieces of cast iron, just as they come from the foundry,
are rubbed together, they will not slide over each other easily,
because of little projections. If this same iron is filed or planed,




the pieces will slide much more easily. This is because the rough
places have been smoothed, or filled up with dust. If now we put
some engine oil on the pieces, they will slide very easily. This is
because the more minute depressions have been filled up and the
whole surface is made comparatively smooth. No matter how
carefully we might plane and polish any surface, a microscope
would show that iu was still a little rough.

One cause of loss of powerin the steam engine is friction. In all
engines there are so many moving parts that it is of great impor-
tance that friction should be reduced as much as possible. This is



THE STEAM ENGINE.



done by making the surfaces in contact smooth and of ample size ;
also making them of different metals and using oils or other
lubricants. The effect of the lubricant is to interpose a thin film
between the surfaces. This prevents their coming into actual con-
tact. If the oil is too thin or the pressure too great, the lubricant
is squeezed out and the metal surfaces come in contact.

Thus we see that there are certain qualities which a lubricant
must have. They are as follows :

The lubricant must be sufficiently fluid, so that it will not
itself make the bearing run hard.

It must not be too fluid or it will be squeezed out from be-
tween the bearing surfaces. If this happens, .the bearing will




Fig. 43.

immediately begin to heat and cut. The heating will tighten the
bearing, and will thus increase the pressure and the cutting.

It must not gum or dry when exposed to the air.

It must not be easily decomposed by the heat generated. It'
it should be decomposed, it might form substances which would
be injurious to the bearings.

It must not take fire easily.



115



64 THE STEAM ENGINE.

It must contain no acid, and should form no acid in decom-
posing, as acids corrode the bearings.

Both mineral and animal oils are used as lubricants. For-
merly animal oils were used entirely, but they were likely to
decompose at high temperatures and form acids. It is important
in using high-pressure steam to have " high-test oils," that is, oils
which will not decompose or volatilize at the temperature of the
steam. It was the difficulty of getting such oils which made great
trouble when superheated steam was first used. Mineral oils will
stand these temperatures very readily, and even if they do decom-
pose, they form no acids.

The Liquid Lubricants, whether of animal, vegetable or
mineral origin, may be used for ordinary bearings, but for valves
and pistons heavy mineral oils only are suitable.

Solid Lubricants . G-rapldte is used as a lubricant. It is well
adapted for heavy pressures when mixed with certain oils. It is
especially valuable for heavy pressures and low velocities.

Metalline is a solid compound, containing graphite. It is
made in the form of solid cylinders, which are fitted to holes drilled
into the surface of the bearing. When a bearing is' thus fitted no
other lubricant is necessary.

Soapstone in the form of powder and mixed with oil or fat is
sometimes used as a lubricant. Soap mixed with graphite or soap-
stone is often used where wood is in contact with wood or iron.

A preparation called Filer Graphite is used for self-lubricat-
ing bearings. It is made of finely divided graphite mixed with
fibers of wood. It is pressed in molds and afterward fitted to
bearings.

For great pressure at slow speed, graphite, lard, tallow and
other solid lubricants are suitable. If the pressure is great and
the speed high, castor, sperm and heavy mineral oils are used.

For low pressure and high speed, olive, sperm, rape and
refined petroleum give satisfaction.

In ordinary machinery, heavy mineral and vegetable oils and
lard oil are good. The relative value of various lubricants depends
upon the prevailing conditions. Oil that is suitable for one place
might not flow freely enough for another.

The quality of oil is of great importance. In many branches



116



THE STEAM ENGINE.



65



of industry it is imperative that the machinery run as perfectly as
possible. On this account and because of the high cost of
machinery, only first-class oil should be used. The cylinder oil
especially, should be high grade, because the valves, piston and
piston rods are the most delicate parts of the engine.

Engines are lubricated by means of oil cups and wipers
placed on the bearings wherever required. They are made in
many forms, dependent upon the manufacturers. Commonly the oil
cup is made with a tube extending up through the oil. A piece
of lampwick or worsted leads from the oil in the cup to the tube.
Capillary attraction causes the oil to flow
continuously and drip down the tube. When
not in use, the lampwick should be with-
drawn.

The needle oil cup differs from the cap-
illary oil cup in that a small wire or needle
extends through the tube and oil; one end
rests on the journal to be lubricated. The
needle should fit the tube closely, so that
when the machinery is at rest no oil will flow.
When revolving, the shaft gives the needle
a wabbling motion which makes the oil flow.
To increase the supply, a smaller needle is
used.

The oil cup shown in Fig. 44 is simple
and economical. The opening of the valve is
regulated by an adjustable stop. The oil may
be seen as it flows drop by drop. The cylin-
drical portion is made of glass, so that' the
engineer can see how much oil there is in the cup without open-
ing it.

A form of wiper crank pin oiler is shown in Fig. 45. The
oil cup is attached to a bracket. The oil drops from the cup into
a sheet of wicking or wire cloth and is removed at each revolution
of the crank pin by means of the cup which is attached to the end
of the connecting rod.

Fig. 46 shows a centrifugal oiling device. The oil flows
from the oil cup through the tube to the small hole in the cranfc




Fig. 44.



117



66



THE STEAM ENGINE.




Fig. 45.



pin by centrifugal force. It reaches the bearing surface by means
of another small hole.

In oiling the valve chest
and cylinder the lubricant
must be introduced against
the pressure of the steam.
This can be done in several
ways, in each of which it is
introduced into the steam
before it reaches the valve
chest and is carried to the
surfaces to be lubricated.

The oil can be forced
into the steam pipe by a small
hand pump or in large engines by an attachment from the engine
itself. The supply of oil is, of course, intermittent if the pump is
driven by hand, but continu-
ous and economical if driven
by the engine.

5ight Feed Lubricators.
The most common device for
feeding oil to the cylinder is
that which introduces the oil
drop by drop into the steam
when it is in the steam pipe
or steam chest. The oil be-
comes vaporized and lubri-
cates all the internal surfaces
of the engine.

Fig. 47 shows the sec-
tion of a sight feed lubricator.
The reservoir O is filled Avith
oil. The pipe B, which con-
nects with the steam pipe, is -pig. 46.
partly filled with condensed

steam, which flows down the small curved pipe E to the bottom of
the chamber O. A small portion of the oil is thus displaced and
iiows from the top of the reservoir O down the tube F, by the




118



THE STEAM ENGINE.



regulating valve D, and up through the glass tube S, which is
filled with water. It enters the main steam pipe through the con-
nection A. The gage glass G indicates the height of water in
the chamber O. To fill the lubricator, close the regulating valve
D and the valve in pipe B ; the oil chamber can then be drained
and filled. If the glass S becomes clogged it may be cleaned by
shutting valve D and opening the small valve II. This will
allow steam to blow through the glass. After cleaning close
valve II and allow glass S to become filled with water before
opening the feed valve. The amount of oil fed to the cylinder can
be regulated by opening D
(Fig. 47) the proper amount.
The exact quantity of oil
necessary for the engine is
not easily determined. For
ordinary sizes it is between
one drop in two minutes and
two drops per minute.

Graphite is an excellent
lubricant and can be intro-
duced into the cylinder dry
or mixed with some heavy
grease. It has been used
extensively because of the
trouble which cylinder oil
gives in the exhaust and in
the boilers of condensing
engines.

In slow-speed engines it
is not hard to attend to the




Fig. 47.



oiling ; all the parts are moving slowly and can be readily examined
and oiled. Many high-speed engines run so fast that it is impossible
to examine the various parts, and special means must be provided
for lubricating. It is specially important in high-speed engines
that there should be no heating. High-speed engines are gener-
ally used for electric lighting, and it is absolutely essential that
they l>e kept running at the required speed to avoid flickering
lights. Thus, while there is greater liability to heating in high-



119



THE STEAM ENGINE.



speed engines, there is also much greater loss in case heating
compels the stopping of an engine.

In order to avoid the danger of forgetting to oil a bearing of
a high-speed engine, it is customary to have all the bearings oiled
from one place. All the oil is supplied to one reservoir and from
this reservoir pipes lead to all bearings. If this is not done, large
oil cups are supplied, as a rule, so that oiling need not be attended
to as frequently.

In some high-speed engines the moving parts are enclosed
and the crank runs in a bath of oil. This secures certain oiling
and is very effective. All the bearings may be inside this crank
case, so that all are oiled in this way. It is impossible for a care-
less engineer to overlook one point and so endanger the whole
engine.

STEAH TURBINE.

The very earliest records of the steam engine describe a form
of steam turbine. It consisted of a hollow sphere, as shown in

Fig. 48, mounted on trun-
nions, through which steam
was admitted to the in-
terior. This steam escaped
through pipes bent tangen-
tially to the equator line
of the sphere. The force
of the escaping steam re-
acted upon the sphere,
causing it to revolve on
its trunnions. Many cen-
turies later, in 1629,
Branca, an Italian, invented
a rotary engine (Fig. 49),
in which a jet of steam
struck the vanes of a

\vheel, and thus forced it around in much the same way that a jet
jf water acts on a Pelton water-wheel. These engines were of
little, if any, practical value, and used an immense quantity of
steam.




120



THE STEAM ENGINE.



69



In 1705 the reciprocating engine was introduced, and by
means of Watt's inventions became so efficient that the develop-
ment of the rotary engine was out of the question. It will be
remembered that Watt introduced the expansive use of steam in
the reciprocating engine, which at this time could not be accom-
plished in the rotary engine, and until within the last few years
practically nothing was done to develop the turbine.

Since the days of Watt there has been but one important
thermodynamic improvement in the reciprocating engine ; namely,




Fig. 49.

the introduction of compound expansion. All other improve-
ments have been in the nature of mechanical devices, and it seems
reasonable to suppose that the greatest developments of the
future may possibly be in the production of some type other than
the reciprocating engine.

In 1883 De Laval invented a successful turbine for running
a cream separator, and a short time later Parsons introduced an-
other. Both of these engines employed the expansive force of
steam, but each derived this force in a different way.

Since 1883 the development of the turbine has been very
rapid. The first engine introduced by De Leval, although far
ahead of the earlier forms, was still very wasteful of steam ; but
now such improvements have been made that their steam con-
sumption compares very favorably with the consumption of good
reciprocating engines.



121



70 THE STEAM ENGINE.



A modern turbine is a tremendously high-speed engine. It
does not derive its power from- the static force of steam expanding
behind a piston, as in the reciprocating engine, but in this case
the expanding steam produces kinetic energy of the steam par-
ticles. These particles receive a high velocity by virtue of the
expansion, and, acting upon the vanes of a wheel, force it around
at a high speed of rotation in some such manner as a stream of
water rotates a water-wheel.

In the reciprocating engine the expansion produces a force
which presses on the piston. In the rotary engine the expansion
produces velocity in a jet of steam. This is the fundamental dif-
ference between the two forms.

The essential principles of water turbines are equally true
of steam turbines. The jet must strike the vanes without a sud-
den shock, and must leave them in another direction without any
sharp deflections. For maximum efficiency the De Laval engine
should have a jet velocity equal to twice the linear velocity of a
point. on the wheel-rim; for the Parsons these velocities should
be equal. If the velocity of steam is 8,000 feet per second, it is
easily seen that even one-half of this would cause too great a speed
of rotation for safety. It would be difficult to build a wheel that
would be strong enough to withstand the centrifugal force at this
high speed. It becomes necessary, therefore, to reduce the speed
to the limits of safety, and run under a slightly less efficiency.

At such high speed the shaft and wheel should be perfectly
balanced, in order that its center of gravity may exactly coincide
with the axis of rotation. In practice it has been found impos-
sible to balance the shaft perfectly; and in order that it may
revolve about its center of gravity, various means are adopted to
overcome the rigidity of an ordinary shaft and bearing. This
makes high speed of rotation possible without any apparent
vibration.

De Laval Turbine. The De Laval turbine shown in Fig. 50
consists of a wheel with suitably shaped buckets, against which a
jet of steam is directed. The buckets are on the rim of the
wheel and are surrounded by a casing B, which prevents the
escape of the steam until it has done its work. A piece of this
casing is cut away at A in order to show the buckets. The steam



f&E SfEAM ^tfGItffi. fl

from the nozzle strikes these buckets and is deflected. Thus by
the impact of the jet and the reaction due to its deflection, the
wheel is caused to revolve at a high speed.

There are usually four nozzles that supply steam to the tur-
bine, one of which is shown in section at D. These nozzles are
small at the throat and diverge outward. By making them of
the right length and with the proper amount of divergence, the
steam can be expanded from the pressure of admission to the




Fig. 50.

pressure of the condenser. Complete expansion is obtained in
this diverging nozzle, and the steam leaves it at the exhaust
pressure. The steam then works only by virtue of its high
velocity.

This turbine has a long, flexible shaft C which can deflect
enough to make up for any eccentricity of the center of gravity of
the shaft, and thus allow the shaft to revolve about its center
of gravity and still have rigid bearings at the end.

Admission is regulated by a throttle-valve, controlled by a
fly ball governor.



123



72



THE STEAM ENGINE.




124



THE STEAM ENGINE. 73

Fig. 51 shows a De Laval turbine connected with a generator.
T/te Parsons Turbine. Fig. 52 is a longitudinal section of a
Westinghouse-Parsons turbine. Steam enters the chamber A and
passes through the turbine vanes to the exhaust chamber B. The
vanes are arranged as shown in Fig. 53, and consist of alternate
sets, one stationary, the next movable. The steam strikes one and
is deflected to the next ; thus the action and the reaction occurring
in rapid succession cause the movable sets, which are fixed to the
shaft, to rotate at a high speed. As the steam passes the different
sets of blades, the volume of the passages increases to correspond
with the expansion of the steam. In the De Laval the steam
was expanded entirely before reaching the wheel, but in the
Parsons the expansion is accomplished in the engine itself. As
the steam enters the chamber A (see Fig. 52) it presses on the
turbine vanes and it also presses equally and in the opposite direc-
tion on C, which is really a piston fixed to the shaft. Thus we
see that the pressure to the right is equal to the pressure to the
left, and there is no end pressure on the bearing of the shaft. Cj
and C 2 balance the steam pressure in the chambers E and G. At
H is a bearing which serves to maintain a correct adjustment of
the balance pistons C. There is probably some escape of steam
past these balancing pistons, but it is small. The exhaust steam
at B presses the turbine toward the left, and would cause an end
pressure on the bearing were it not that the pipe K opens a com-
munication between the exhaust chamber B and the back of the
balancing pistons, which makes the pressure equal at both ends.

The bearing consists of a gun-metal sleeve surrounded by
three concentric tubes. There is a small clearance between these
tubes which fills with oil and permits the bearing to run slightly
eccentric to counteract any lack of balance in the shaft. Thus
the shaft may revolve about its center of gravity, and this oil bear-
iDg serves the same purpose as the De Laval flexible shaft.

At P is shown a by-pass valve by means of which live steam
may be admitted to the space E, if desirable. Of course this
reduces one stage of the expansion, with a corresponding loss of
economy, but will increase the power of the turbine. If the con-
denser fails on a condensing turbine it may still be run at full load
by opening the by-pass valve.



125



THE STEAM ErfGltffl.




126



THE STEAM ENGINE.



75



Steam is admitted to this turbine in puffs through a recipro-
cating valve. A fly-ball governor regulates the admission, which
is always at boiler pressure.

For electric generators the turbine has many advantages,
among them high speed and direct connection. They have small
foundations and take up little space ; there is slight loss from fric-



F/xeD BLADES
MOV/NG BLADES

F/XED BLADES

\

MOV/NG BLADES



tion and few parts. Where slow speed is desired a reciprocating
engine is probably tlie best.




127



THE STEAM ENGINE

PART II



ACTION OF HEAT.

There are several types of heat engines, such as the steam
engine, the gas engine, the hot-air engine, etc., each one of which
derives its motive power from the heat contained in steam, gas, oil,
hot air, or some other substance. Heat is imparted to these sub-
stances either by the combustion of fuel in a generator entirely
separate from the engine, or by the combustion of a gaseous sub-
stance in the cylinder of the engine itself. In the case of the hot-
air engine the heat is produced by a fire immediately beneath the
engine.

Steam and hot gases have a tendency to expand, because of
the heat they contain, thus producing a pressure in all directions.
This pressure causes the piston of the engine to move, which allows
the gas to expand. As the gas expands, it gives up heat, which is
converted into useful work.

We shall now discuss the fundamental principles of the action
of heat, and the behavior of gases and vapors, with special refer-
ence to the properties of steam and its action in the cylinder of the
engine.

If a piece of iron or some other substance is placed in a fire,
it becomes hotter than it was before, because heat from the fire
has passed into it. If this hot substance is plunged into cold
water, or is allowed to remain in a cool place, heat will pass from
it, and we say that it becomes cold.

There have been many theories as to what heat is. The
accepted theory of to-day is that heat is the result of motion, or we
may say it is a form of kinetic energy. Heat is produced, not by
the motion of the substance itself (for the hot body may be at
rest), but by a rapid vibration of the minute individual particles
that make up the body. These minute particles are called mol-
ecules. The faster the molecules vibrate, the greater will be their



129



'/S THE STEAM ENGINE.

kinetic energy and the hotter the substance will become. The
hottest bodies are those that have the greatest energy of molecular
vibration. A hot body can transfer its energy of vibration to an-
other, which in turn becomes hotter than it was before, while the
first body loses a part of its energy of vibration and becomes cooler.
The terms hot and cold are only comparative terms ; one body is hot
because it contains a greater degree of heat than another ; the other
is cold because it contains a less degree of heat than the first.
Cold, then, is simply a low degree of heat.

By temperature is meant simply the thermal condition of a
body with reference to its capability of transferring heat to other
bodies. If two bodies are placed in contact and the first gives
more heat to the second than it receives, we say that No. 1 is
hotter than No. 2. If the first receives more heat than it gives,
No. 2 is hotter than No. 1. If both bodies give and receive the
same amount of heat, they are of the same temperature.

According to our theory, it is evident that temperature de-
pends upon the energy of molecular vibration. If the temperature
rises, it means that the molecular vibration, and consequently the
energy, increases ; if the temperature falls, the energy of molecular
vibration decreases. Evidently a point must finally be reached
when this energy of vibration is zero and the molecules are at rest.
At this temperature there is no heat and we call it the absolute
zero. It is evident that this zero is very much below the zero of
the ordinary scale.

In order to determine just how hot a body is, we must com-
pare its temperature with that of some substance whose degree of
heat we know. It would be impossible to keep several bodies at



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 8 of 30)