MECHANICAL DEVICES UTILIZED IN POWER
One of the most important things in enginery is the capacity to
determine the power developed. Although the method of ascertaining this
appears to be somewhat complicated, it is really simple, and will be
comprehended the more readily if it is constantly borne in mind that a
certain weight must be lifted a definite distance within a particular
time.
The Unit of Time.--The unit of time is either the second, or the minute,
usually the latter, because it would be exceedingly difficult to make
the calculations, or rather to note the periods as short as a second,
and a very simple piece of mechanism to ascertain this, is to mount a
horizontal shaft A, Fig. 59, in bearings B, B, and affix a crank C at
one end.
It will be assumed that the shaft is in anti-friction bearings so that
for the present we shall not take into account any loss by way of
friction.
A cord, with one end attached to the shaft and the other fixed to a
weight D, the latter weighing, say 550 pounds, is adapted to be wound
on the shaft as it is turned by the crank.
Knowing the length of the cord and the time required to wind it up, it
will be an easy matter to figure out the power exerted to lift the
weight, which means, the power developed in doing it.
[Illustration: _Fig. 59. Illustrating the Unit of Time._]
Suppose the cord is 100 feet long, and it requires one and a half
minutes to raise the weight the full limit of the cord. It is thus
raising 550 pounds 100 feet in 45 seconds.
One horse power means that we must raise 550 pounds one foot in one
second of time, hence we have developed only 1/45th of one horse power.
Instead of using the crank, this shaft may be attached to the engine
shaft so it will turn slowly. Then add sufficient weight so that the
engine will just lift it, and wind the cord on the shaft.
You can then note the time, for, say, one minute, and when the weight is
lifted, make the following calculation: Weight lifted one hundred feet
in one minute of time was 825 pounds. Multiply 100 by 825, which equals
82,500. This represents _foot pounds_.
[Illustration: Fig. 60. The Proney Brake.]
As there are 33,000 foot pounds in a horse power, 82,500 divided by this
figure will show that 2-1/2 horse power were developed.
The Proney Brake.--Such a device is difficult to handle, but it is
illustrated merely to show the simplicity of the calculation. As a
substitute for this mechanism, a device, called the _Proney brake_ has
been devised, which can be used without rewinding of a cord. This is
accomplished by frictional means to indicate the power, and by the use
of weights to determine the lift.
The following is a brief description of its construction: The engine
shaft A, Fig. 60, which is giving out its power, and which we want to
test, has thereon a pulley B, which turns in the direction of the
arrow. Resting on the upper side of the pulley is a block C, which is
attached to a horizontal lever D by means of bolts E, these bolts
passing through the block C and lever D, and having their lower ends
attached to the terminals of a short sprocket chain F.
Block segments G are placed between the chain and pulley B, and when the
bolts E are tightened the pulley is held by frictional contact between
the block C and the segments G.
The free end of the lever has a limited vertical movement between the
stops H, and a swinging receptacle I, on this end of the lever, is
designed to receive weights J.
The first thing to do is to get the dimensions of the pulley, its speed,
and length of the lever. By measurement, the diameter of the pulley is
six inches. To get the circumference multiply this by 3.1416. The
distance around, therefore, is a little over 18.84 inches. The speed of
the pulley being 225 times per minute, this figure, multiplied by 18.84,
gives the perimeter of the pulley 4239 inches.
As we must have the figures in feet, dividing 4239 by 12, we have 353.25
feet.
The length of the lever from the center of the pulley to the suspension
point of the receptacle, is 4 feet, and this divided by the radius of
the pulley (which is 6 inches), gives the leverage. One half of six
inches, is three inches, or 1/4 of one foot, and 4 divided by this
number, is 1' 4", or 1-1/3 feet, which is the _leverage_.
Now, let us suppose the weight J is 1200 pounds. This must be multiplied
by the leverage, 1-1/3 feet, which equals 1800, and this must be
multiplied by the feet of travel in the pulley, namely, 353.25, which is
equal to 635,850. This represents _foot pounds_.
Now, following out the rule, as there are 33,000 foot pounds in a horse
power, the foregoing figure, 635,850, divided by 33,000, equals 19 horse
power within a fraction.
Reversing Mechanism.--A thorough knowledge of the principles underlying
the various mechanical devices, and their construction, is a part of the
education belonging to motors. One of the important structures, although
it is very simple, when understood, requires some study to fully master.
This has reference to reversing mechanism, which is, in substance a
controllable valve motion, whereby the direction of the valve is
regulated at will.
All motions of this character throw the valve to a neutral point which
is intermediate the two extremes, and the approach to the neutral means
a gradual decrease in the travel of the valve until the reciprocating
motion ceases entirely at the neutral position.
[Illustration: _Fig. 61. Double Eccentric Reversing Gear._]
[Illustration: _Fig. 62. Reversing Gear, Neutral._]
Double Eccentric Reversing Gear.--A well known form of gear is shown in
Fig. 61, in which the engine shaft A has two eccentrics B, C, the upper
eccentric B being connected with the upper end of a slotted segment D by
means of a stem E, and the other eccentric C is connected with the lower
end of the segment by the stem F. The eccentrics B, C, are mounted on
the shaft so they project in opposite directions.
The slotted segment carries therewith the pin G of a valve rod H, and
the upper end of the segment has an eye I, to which eye is a rod J
operated by a lever.
[Illustration: _Fig. 63. Reversing Gear, Reversed._]
[Illustration: _Fig. 64. Single Eccentric Reversing Gear._]
By this arrangement the link may be raised or lowered, and as the valve
rod pin has no vertical movement, either the connecting link E or F may
be brought into direct line with the valve rod H.
Fig. 61 shows the first position, in which the valve rod H is in direct
line with the upper connecting rod E, actuated by the cam B.
Fig. 62 shows the neutral position. Here the pin G serves as a fulcrum
for the rocking movement of the segment; whereas in Fig. 63 the valve
rod H is in line with the lower connecting rod F, so that the valve is
pushed to and fro by the eccentric C.
[Illustration: _Fig. 65. Balanced Slide Valve._]
It is more desirable, in many cases, to use a single eccentric on the
engine shaft, which can be done by pivoting the segment L, Fig. 64, to a
stationary support M, and connecting one end of the segment by a link N
with the single eccentric O.
In this construction the valve rod P is shifted vertically by a rod Q,
operated from the reversing lever, thus providing a changeable motion
through one eccentric.
Balanced Slide Valves.--In the chapter pertaining to the steam engine,
a simple form of slide valve was shown, and it was stated therein that
the pressure of the steam bearing on the valve would quickly grind it
down. To prevent this various types of balanced valves have been made, a
sample of which is shown in Fig. 64.
The valve chest A has in its bottom two ports C, D, leading to the
opposite ends of the cylinder, and within is the sliding valve E, which
moves beneath an adjustable plate F connected with the top or cover G of
the valve chest.
[Illustration: _Fig. 66. Valve Chest. Double Port Exhaust._]
This is also modified, as shown in Fig. 66, in which case the slide
valve H bears against the cover I at two points, so that as there is
steam on the upper surface to a slightly greater area than on the lower
side, there is sufficient downward pressure to hold it firmly on its
seat, and at the same time not cause any undue grinding. This valve also
has double exhaust ports J, J.
Balanced Throttle Valve.--Fig. 67 will give a fair idea of the
construction of throttle valves, the illustration showing its connection
with a simple type of governor.
[Illustration: _Fig. 67. Balanced Throttle-Valve._]
Engine Governors.--Probably the oldest and best known governor for
regulating the inlet of steam to an engine, is what is known as the Watt
design. This is shown in Fig. 68.
The pedestal A which supports the mechanism, has an upwardly-projecting
stem B, to the upper end of which is a collar C, to which the
oppositely-projecting pendent arms D are hinged. These arms carry balls
E at their free ends.
[Illustration: Fig. 68. Watt's Governor.]
The lower part of the stem has thereon a sliding collar F, and links G,
with their lower ends hinged to the collar, have their upper ends
attached to the swinging arms D. The collar has an annular groove at its
lower end, to receive therein the forked end of one limb of a bell-crank
lever H, the other limb of this lever being connected up with the engine
throttle, by means of a link L.
Centrifugal motion serves to throw out the balls, as indicated by the
dotted lines J, and this action raises the bell-crank lever, and opens
the throttle valve.
Numerous types of governors have been constructed, some of which operate
by gravity, in connection with centrifugal action. Some are made with
the balls adapted to swing downwardly, and thrown back by the action of
springs. Others have the balls sliding on horizontally-disposed arms,
and thrown back by the action of springs; and gyroscopic governors are
also made which are very effective.
[Illustration: _Fig. 69. The Original Injector._]
Fly wheel governors are not uncommon, which are placed directly on the
engine shaft, or placed within the fly wheel itself, the latter being a
well known form for engines which move slowly.
Injectors.--The Injector is one of the anomalies in mechanism. It
actually forces water into a boiler by the action of the steam itself,
against its own pressure. It is through the agency of condensation that
it is enabled to do this.
The illustration, Fig. 69, which represents the original type of the
device, comprises a shell A, within which is a pair of conically formed
tubes, B, C, in line with each other, the small ends of the tubes being
pointed towards each other, and slightly separated. The large end of the
conical tube C, which points toward the pipe D, which leads to the water
space of the boiler, has therein a check valve E.
The steam inlet pipe F, has a contracted nozzle G, to eject steam into
the large end of the conical tube B, and surrounding the nozzle F is a
chamber which has a pipe H leading out at one side, through which cold
water is drawn into the injector.
Surrounding the conical pipes B, C, is a chamber I, which has a
discharge pipe J. The action of the device is very simple. When steam is
permitted to flow into the conical tube B, from the nozzle G, it passes
out through the drain port J, and this produces a partial vacuum to form
in the space surrounding the nozzle G.
As a result water is drawn up through the pipe H, and meeting with the
steam condenses the latter, thereby causing a still greater vacuum, and
this vacuum finally becomes so great that, with the inrushing steam,
and the rapid movement through the conical tubes, past their separated
ends, a full discharge through the drain J is prevented.
[Illustration: Fig. 70. Injector with Movable Combining Tube.]
As it now has no other place to go the check valve E is unseated, and
the cold water is forced into the boiler through the pipe D, and this
action will continue as long as condensation takes place at the nozzle
G.
Many improvements have been made on the original form, mostly in the
direction of adjusting the steam nozzle, and to provide the proper
proportion of flow between the steam and water, as this must be adjusted
to a nicety to be most effective.
An example of a movable tube which closes the outlet to the overflow,
is shown in Fig. 70. The steam inlet tube A is at one end of the shell,
and the outlet tube B to the boiler, at the other end, and intermediate
the two is a tube C, with its open flaring end adapted to receive the
steam from the tube A. This tube is longitudinally-movable, so that the
controlling lever D may move it to and fro.
A chamber E surrounds the nozzle A, and has a water inlet pipe F, while
the space G between the ends of the pipes B, C, has an outlet H, a
single check valve I being interposed. In operation the tube C may be
adjusted the proper distance from the end of the pipe B, and when the
current is once established through the injector, the pipe C may be
brought into contact with B, and thus entirely cut out the movement of
the water to the overflow.
Feed Water Heater.--An apparatus of this kind is designed to take the
exhaust steam from the engine and condense it, and from the condenser it
is again returned to the boiler. The water thus used over again goes
into the boiler at a temperature of over 180 degrees, and thus utilizes
the heat that would otherwise be required to raise the temperature of
the water from the natural heat, say 70, up to that point.
In Fig. 71 the illustration shows a typical heater, which comprises an
outer shell A, each end having a double head, the inner head B being
designed to receive the ends of a plurality of horizontally disposed
pipes, and the outer heads C, separated from the inner head so as to
provide chambers, one end having one, and the other head being provided
with two horizontal partitions D, so the water may be diverted back and
forth through the three sets of pipes within the shell.
[Illustration: _Fig. 71. Feed Water Heater._]
The three sets of pipes, E, F, and G, are so arranged that they carry
the water back and forth from one head to the other, and for this
purpose the water for cooling the steam enters the port H at one end,
passes through the upper set of pipes E to the other end, then back
through the same set of pipes on the other side of a partition, not
shown, and back and forth through the two lower sets of pipes F, G.
The steam enters at the port I at the top of the shell, and passes down,
as it is condensed, being discharged at the outlet J.