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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

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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 7 of 30)
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It is evident that while the piston can push the crank around
during part of the stroke, and pull it during another part, there
are still two places (called dead points) at the ends of the stroke,
where the pressure on the piston, no matter how great, can exert



97



46



THE STEAM ENGINE.




: >.






THE STEAM ENGINE.



47



no turning moment on the shaft. Therefore, if some means is
not provided for making the shaft turn past these points without
the assistance of the piston, it may stop. This means is provided
in the fly wheel, which is merely a heavy wheel placed on the
shaft. On account of the momentum of the fly wheel it cannot
be stopped quickly, and therefore carries the shaft around until
the piston can again either push or pull.

If a long period be considered, the mean effort and the mean
resistance must be equal; but 'during this period there are tem-
porary changes of effort, the excesses causing increase of speed.
To moderate these fluctuations several methods are employed.




Fig. 33.

The turning moment on the shaft of a single cylinder
engine varies, first, because of the change in steam pressure, and
second, on account of the angularity of the connecting rod. Before
the piston reaches mid-stroke the turning moment is a maximum,
as shown by the curve, (Fig. 33). While near the ends of the
stroke (the dead points) the turning moment diminishes and
finally becomes zero. This, of course, tends to cause a corre-
sponding change in the speed of rotation of the shaft. In order
to have this speed as nearly constant as possible, and to give a
greater uniformity of driving power, the engine may be run at high
speed. By this means the inertia of the revolving parts, such
as the connecting rod and crank, causes less variation. When
the work to be done is steady and always in the same direction
(as in most factories), a heavy fly wheel may be used.

The heavier the fly wheel, the steadier will be the motion.
It is, of course, desirable in all factory engines to have steady
motion, but in some it is more important than in others. For



99



48 THE STEAM ENGINE.

instance, in a cotton mill it is absolutely necessary that the ma-
chinery shall move with almost perfect steadiness ; consequently
mill engines always have very large, heavy fly wheels. It is un-
desirable to use larger wheels than are absolutely necessary,
because of the cost of the metal, the weight on the bearings and
the danger from bursting.

If the turning moment which is exerted on the shaft from
the piston could be made more regular, and if dead points could
be avoided, it would be possible to get a steadier motion with a
much smaller fly wheel.

If the engine must be stopped and reversed frequently, it is
better to use two or more cylinders connected to the same shaft.
The cranks are placed at such angles that when one is exerting its
minimum rotative effort, the other is exerting its maximum turn-
ing moment ; or, when one is at a dead center, the other is exerting
its greatest power. These two cylinders may be identically the
same, as is the case with most hoisting engines and with many
locomotives ; or the engine may be compound or triple expansion.

This arrangement is also used on engines, for mines, collieries,
and for hoisting of any sort where ease of stopping, starting
and reversing are necessary. Simple expansion engines with their
cranks at right angles are said to be coupled.

The governor adjusts the power of the engine to any large
variation of the resistance. The fly wheel has a duty to perform
which is similar to that of the governor. It is designed to adjust
the effort of the engine to sudden changes of the load which may
occur during a single stroke. It also equalizes the variation in
rotative effort on the crank pin. The fly wheel absorbs energy
while the turning moment is in excess of the resistance and restores
it while the crank is at or near the dead points. During these
periods the resistance is in excess of the power.

The action of the fly wheel may be represented as in Figs. 33
and 34. It will be noticed that in Fig. 33 the curve of crank
effort runs below the axis toward the end of the stroke. This is
because the compression is greater than the pressure near the end
of expansion, and produces a resultant pressure on the piston.
In Fig. 34 the effect of compression has been neglected. Let us
suppose that the resistance, or load, is uniform. In Fig. 33 the line



100



THE STEAM ENGINE. 49

A B is the length of the semi-circumference of the crank pin, or it
is the distance the crank pin moves daring one stroke. The curve
A M D O B is the curve of turning moment for one stroke. M N
is the mean ordinate, and therefore A E F B represents the constant
resistance. The effort and resistance must be equal if the speed is
uniform ; hence A E F B = A M D O B. Then area A E M +
area O F B = area M DO. At A the rotative effort is zero be-
cause the crank pin is at the dead point ; from A to N the turning
moment is less than the resistance. At N the resistance and the
effort are equal. From N to P the effort is in excess of the resist-
ance. At P the effort and resistance are again equal. From P
to B the resistance is greater than the effort. In other words,
from A to N the work done by the steam is less than the resist-




Fig. 34.

ance. This shows that the work represented by the area A E M
must have been done by the moving parts of the engine. From
N to P the work done by the steam is greater than the resistance,
and the excess of energy is absorbed by or stored in the moving
parts. From P to the end of the stroke, the work represented by
the area O F B is done on the crank pin by the moving parts.

^y ^2

We know from the formula, E , that energy is pro-

u

portional to the square of the velocity. Hence as W and g
remain the same, the velocity must be reduced when the moving
parts are giving out energy, and increased when receiving energy.
Thus we see that the tendency of the crank pin is to move slowly,
then more rapidly. The revolving parts of an engine have not
sufficient weight to store this surplus energy, hence a heavy fly
wheel is used.

In case there are two engines at right angles, two effort curves



101



50 THE STEAM ENGINE.

must be drawn, as shown in Fig. 34. The mean ordinate A E is
equal to the mean or constant resistance. There are two minimum
and two maximum velocities in one stroke. The diagram shows
that the variation is much less than for a single cylinder ; hence
a lighter fly wheel may be used.

The weight of the fly wheel depends upon the character of the
work done by the engine. For pumping engines and ordinary
machine work the effort need not be as constant as for electric
lighting and fine work. In determining the weight of a fly wheel
the diameter of the wheel must be known, or the ratio of the
diameter of the wheel to the length of stroke. If the wheel is too
large, the high linear velocity of the rim will cause too great a
centrifugal force and the wheel is likely to break. In practice,
about 6,000 feet per minute is taken as the maximum linear velocity
of cast-iron wheels. When made of wood and carefully put
together, the velocity may be taken as 7,000 to 7,500 feet per
minute.

We know that linear velocity is expressed in feet per minute
by the formula, V = 2-rr R N, or V = TT D N.

Then if a wheel runs at 100 revolutions per minute, the
allowable diameter would be,

6,000 = 3.1416 X D X 100

6,000 101 , .

= 3^i4ihrLoo = 19 - lfeefc -

If a wheel is 12 feet in diameter the allowable speed is found
to be,

N = Z_

TrD

6,000
= . loy revolutions per minute.

It is usual to make the diameter a little less than the calcu-
lated diameter.

Having determined the diameter, the weight may be calculated
by several methods. There are many formulas to obtain this result
given by various authorities. One formula is given as follows :

W = CX d2 X j
D^ x N2



102



THE STEAM ENGINE. 51

In the above. W= weight of the rim in pounds

d = diameter of the cylinder in inches
b = length of stroke in inches
D = diameter of fly wheel in feet
N number of revolutions per minute

C is a constant which varies for different types and conditions.

Slide-valve engines, ordinary work, C = 350,000
Corliss engines, ordinary work, C = 700,000

Slide-valve engines, electric lighting, C = 700,000
Corliss engines, electric lighting, C = 1,000,000
Automatic highspeed engines, C = 1,000,000

Example. Let us find the weight of a fly-wheel rim for an
automatic high-speed engine used for electric lighting. The cylin-
der is 24 inches in diameter; the stroke is 2 feet. It runs at
300 revolutions per minute, and the fly wheel is to be 6 feet in
diameter.

W = 1,000,000 X C 24 ) 2 X 2
36 X 90,000

W = 4,266 pounds

Another example.. A plain slide-valve engine for electric
lighting is 20" X 24". It runs at 150 revolutions per minute.
The fly wheel is to be 8 feet in diameter. What is the weight of
its rim ?

W = 700,000 X 4 X 24
64 X 22,500

W = 4,666 pounds.

The weight of a fly wheel is considered as being in the rim.
The weight of the hub and arms is simply extra weight. Then,
if we know the weight of the rim and its diameter, we can find the
width of face and thickness of rim. Let us assume the given
diameter to be the mean of the diameter of the inside and outside
of the rim.

Let b = width of face in inches
t = thickness of rim in inches
d = diameter of fly wheel in inches
.2607 = weight of 1 cubic inch of cast iron
Then,

W= .2607 X b X tX ird
= 6 X * X .819 d



103



52 THE STEAM ENGINE.



Suppose the rim of a fly wheel weighs 6,000 pounds, is 9 feet
in diameter, and the width of the face is 24 inches. What is the
thickness of the rim?

W



6,000



,819 X 108 X 24
= 2.83 inches

In this case the rim would probably be made 21| inches thick.
The total weight, including hub and arms, would probably be
about 8,000 pounds.

GOVERNORS.

The load on an engine is never constant, although there are
cases where it is nearly uniform. While the engine is running at
constant speed, the resistance at the fly-wheel rim is equal to the
work done by the steam. If the load on the engine is wholly or
partially removed, and the supply of steam continues undiminished,
the force exerted by the steam will be in excess of the resistance.
Work is equal to force multiplied by distance ; hence, with con-
stant effort, if the resistance is diminished, the distance must be
increased. In other words, the speed of the engine will be in-
creased. The engine will " race," as it is called. Also, if the
load increases and the steam supply remains constant, the engine
will "slow down."

It is evident, then, that if the speed is to be kept constant
some means must be provided so that the steam supply shall at all
times be exactly proportional to the load. This is accomplished
by means of a governor.

Steam-engine governors act in one of two ways : they may
regulate the pressure of steam admitted to the steam chest, or they
may adjust the speed by altering the amount of steam admitted.
Those which act in the first way are called throttling governors,
because they throttle the steam in the main steam pipe. Those of
the latter class are called automatic cut-off governors, since they
automatically regulate the cut-off,

Theoretically, the method of governing by throttling the



104



THE STEAM ENGINE. 53

steam causes a loss in efficiency, but the throttling superheats the
steam, thus reducing cylinder condensation. By the second
method the loss in efficiency is very slight, unless the ratio of
expansion is already great, in which case shortening the cut-off
causes more cylinder condensation. This subject will be taken
up in detail later.

In most governors, centrifugal force, counteracted by some
other force, is employed. A pair of heavy masses (usually iron
balls or weights) are made to revolve about a spindle, which is
driven by the engine. When the speed increases, centrifugal
force increases, and the balls tend to fly outward ; that is, they
revolve in a larger circle. The controlling force, which is usually
gravity or springs, is no longer able to keep the balls in their
former path. When, therefore, the increase is sufficiently great,
the balls in moving outward act on the regulator, which may
throttle the steam or cause cut-off to occur earlier.

With the throttling governor a balanced throttle valve is
placed in the main steam pipe leading to the valve chest. If the
engine runs faster than the desired speed, the balls are forced to
revolve at a higher speed. The increase in centrifugal force will
cause them to revolve in a larger circle and in a higher plane.
By means of some mechanism (levers and gears) the spindle may
be forced downward, thus partially closing the valve. The
engine, therefore, takes steam at a lower pressure, and the power
supplied being less, the speed falls slightly.

Similarly, if the load is increased, the engine slows down,
which causes the balls to drop and open the valve more widely,
steam at higher pressure is then admitted, and the speed is
increased to the regular number of revolutions.

With the Corliss or Wheelock engine the governor of this type
acts differently. Instead of throttling the steam in the steam pipe,
the governor is connected to the releasing gear by rods. An increase
of speed causes the releasing gear to unhook the disengaging link
earlier in the stroke. This causes earlier cut-off, which of course
decreases the power and the speed, since the amount of steam
admitted is less. If for any reason the load increases, the governor
causes the valves to be held open longer. The cut-off, therefore,
occurs later in the stroke.



105



THE STEAM ENGINE.



One of the most common forms of governor is similar to that
invented by James Watt. It is called, from its appearance, the
pendulum governor. It is shown in Fig. 35. To consider the
theory of the pendulum governor, the masses of the balls are
assumed to be concentrated at their centers, and the rods made of
some material having no weight.

When the governor is revolving about its axis at a constant
speed the balls revolve in a circle having a radius r The distance
from this plane to the intersection of the rods, or the rods pro-
duced, is called the height and is equal to h.




Fig. 35.

If the balls revolve faster, the centrifugal force increases, r
becomes greater and li diminishes. We know that centrifugal
force is expressed by the formula,



gr
Then centrifugal force varies inversely as the radius.

While the pendulum is revolving, centrifugal force acts hori-
zontally outward and tends to make the balls fly from the center;
gravity tends to make the balls drop downward. In order that the
balls shall revolve at a certain height, the moments of these two
forces about the center must be equal, or the weight of the balls
multiplied by their distance from the center must equal the
centrifugal force multiplied by the height, or.
W X r = F X A
h W



from which,



or,



from which,



W



gr



gr



106



THE STEAM ENGINE. 55

Now we know that the linear velocity of a point revolving in
the circumference of a circle is expressed as 2 TT r N feet per
second.

Thpr

Since we know the values of g and TT we can write the
formula,

h = 82 ' 16 = - 8146 feet per second, or

4 X 3.1416 2 X N 2 N2

h = ' inches per second.

If N is the number of revolutions per minute, since 00* =

3600 > 2,932.66 ,

h = _ _ feet

, 35,190.7 . ,
h = . 2 inches

From the above formula, we see that the height is independent
of the weight of the balls or the length of the rod ; it depends
upon the number of revolutions. The height varies inversely as
the square of the number of revolutions.

The ordinary pendulum governor is not isochronous ; that is,
it does not revolve at a uniform speed in all positions ; the speed
changes as the angle between the arms and the spindle changes.

The early form consisted of two heavy balls suspended by
links from a pin connection in a vertical spindle, as shown in Figs.
36 and 37. The spindle is caused to revolve by belting or gear-
ing from the main shaft, so that as the speed increases, centrifugal
force causes the balls to revolve in a circle of larger and larger
diameter. The change of position of these balls can be made to
affect the controlling valves so that the admission or the throttling
shall vary with their position. With this governor it is evident
that for a given speed of the engine there is but one position pos-
sible for the governor ; consequently one amount of throttling or
one point of cut-off, as the case may be. If the load varies, the
speed of the engine will change. This causes the position of the
governor balls to be changed slightly, thus altering the pressure



107



56



THE STEAM ENGINE.



But in order that the pressure or cut-off shall remain changed, the
governor balls must stay in their new position. That is to say,
the speed of the engine must be slightly changed. Thus with
the old ball governors there was. a slightly different speed for each
load. This condition has been greatly improved by various
modifications until now such governors give excellent regulation.
While the engine is running with a light load, the valve con-
trolled by the governor will be open just enough to admit steam
at a pressure that will keep the engine running at a given speed.
Now, if the engine is heavily loaded, the throttle valve must be
wide open. The change of opening is obtained by a variation in
the height of the governor, which is caused by a change of speed.
Thus we see that the governor can control the speed only within
certain limits which are not far apart. The difference in the
extreme heights of the governor must be sufficient to open
the throttle its entire range. In most well-designed engines the
speed will not vary more than 4 per cent ; that is, 2 per cent
above or below the mean speed.

From the formula h = ' g ,we can compute the heights
corresponding to given speeds as shown by the following table :



Number of
Revolutions
per Minute.


Height
in
Inches.


Variation of
Height in Inches
4 per cent.


250


.563


.0225


200


.879


.035


175


1.149


.046


150


1.564


.062


125


2.252


.090


100


3.519


.140


75


6.256


.250


50


14.076


.563



In the above table the second column is found from the



formula h =



35,190.7

N 2



The third column is the variation in



height for a speed variation of 4 per cent or 2 per cent either
above or below the mean.



108



THE STEAM ENGINE.



57



From the table we see that for a considerable variation of
speed there is but slight variation in the height of the governor.
Also for high speeds the height of the governor is so small that it
would be difficult to construct it. The slight variation in height
is too small to control the cut-off or throttling mechanism through-
out the entire range.

Other disadvantages of the fly-ball governor are as follows:
it is apparent that the valves must be controlled by the weight of
the governor balls. In large engines this requires very heavy
balls in order to quickly overcome the resistance of the valves.
But these large balls have considerable inertia and will therefore
be reluctant to change their speed with that of the engine. The




Fig. 36.



Fig. 37.



increased weight will also increase the friction in the governor
joints, and the cramping action existing when the balls are driven
oy the spindle will increase this friction still further. All these
things tend to delay the action of the governor, so that in all
large engines the. old-fashioned governor became sluggish. The
balls had to turn slowly because they were so heavy ; this was
especially troublesome in high-speed engines.

To remedy these defects the weighted or Porter governor was
designed. (See Fig. 38.) .It has a greater height for a given
speed, and the variation in height for a given variation of speed is
greater. When a governor has this latter quality, that is, a great
variation in height for a given variation of speed, it is said to be
sensitive. By increasing this variation in height the sensitiveness
is increased. Thus, if a governor running at 50 revolutions has a



109



58 THE STEAM ENGINE.

variation in height of .57 inch, it is not as sensitive as one having
a variation of 1 inch for the same speed.

In the weighted governor, the weight is formed so that the
center of gravity is in the axis. It is placed on the spindle and
is free to revolve. The weight adds to the weight of the balls,
and thus increases the moment of the weight. It does not, how-
ever, add to the centrifugal force, and hence the moment of this
force is unchanged. We may then say the weight adds effect to
the weight but not to the centrifugal force, and as a consequence
the height of the governor, for a given speed, is increased. If we
let W equal the combined weight of the balls as before, and W
equal the added weight, the moments are,
(W + W) X r = F h
(W + W) X r = ^! X A



W X 4 7r2 r 2 N2

(W + W) v <7
-^^~ X 4-



We know that ^ - ' 81




N 2

Hence the height of a weighted governor is equal to the
height of a simple pendulum governor multiplied by



W ) ( W,

For instance, if the height of a simple pendulum is 10 inches,
and the weight of the balls equal to the added weight, the height
will be,



= 2 X 10



no



THE STEAM ENGINE. 69

Thus we see that if a weight equal to the combined weight of
the balls is added, the height of the governor will be doubled.

We know that if the balls fall, the cut-off comes later. If
the belt driving the governor slips off or breaks, the balls will
drop, and, making the cut-off later, will allow the engine to " run
away." To diminish this danger many governors are provided
with some kind of safety stop, which closes the valve when the
governor loses its normal action. Usually a trip is provided
which the governor does not touch in its normal positions, but
which will be released if the balls drop down below a certain
point.

In another arrangement, instead of a weight, a strong spring
is used, and this makes it possible to put the governor in any
position.

Spring Governors. In many cases a spring is used in place
of the weight. This type of governor
is used frequently on throttling engines ;
it consists of a pendulum governor with
springs added to counteract the cen-
trifugal force of the balls. Thus the
height and sensitiveness are increased.
Fig. 39 shows the exterior view of a
Waters governor, and Fig. 40 the same
governor having the safety stop. In
this governor the weights are always in
the same plane, the variation in height
being due to the action of the bell F . 3g

crank levers connecting the balls and

spindle. When the balls move outward the spindle moves down-
ward and tends to close the valve. The governor balls revolve
by means of a belt and bevel gears. The valve and seat are
shown in section in Fig. 41. The valve is a hollow cylinder
with three ports, by means of which steam enters the valve. The
seat is made in four parts, that is, there are four edges that the
steam passes as it enters the valve. The valve being cylindrical
and having steam on both sides is balanced, and because of the
many openings only a small travel is necessary.

Shaft Governors. Usually some form of pendulum governor



ill




60



THE STEAM ENGINE.



is used for throttling engines. For governing an engine by vary-
ing the point of cut-off, shaft governors are generally used ; how-
ever, Corliss engines and some others use pendulum governors for
this purpose. Cut-off governors are called shaft governors because
they are placed on the main shaft ; they are made in many forms,
but the essential features of all are the same. Two pivoted masses
or weights are arranged symmetrically on opposite sides of the
shaft, and their tendency to fly outward when the speed increases
is resisted by springs. The outward motion of the weights closes
the admission valve earlier, and the inward motion closes it later.
Tiiis change is effected by altering the position of the eccentric,
either by changing the eccentricity or the angular advance.





40.



Shaft governors are made in a great variety of ways, no two
types being exactly alike. If the principles of a few types are
understood, it is easy to understand others. The following illus-
trates two common methods of shifting the eccentric.

Buckeye Engine Governor. The valve of the Buckeye en-



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 7 of 30)