Motors Part 16
Examine the diagram, Fig. 107, and note the movement of the pin A which moves along the path B. The crank C in its turning movement around the circle D, moves the pin A into the different positions 1, 2, 3, etc., which correspond with the positions on the circle D.
[Ill.u.s.tration: Fig. 107. Ill.u.s.trating Crank-pin Movement.]
The Dead Centers.--There is also another advantage which the rack possesses. Where reciprocating motion is converted into circular motion, as in the case of the ordinary steam engine, there are two points in the travel of a crank where the thrust of the piston is not effective, and that is at what is called the _dead centers_.
In the diagram, Fig. 108, the ineffectiveness of the thrust is shown at those points.
Let A represent the piston pushing in the direction of the arrow B against the crank C. When in this position the thrust is the most effective, and through the arc running from D to E, and from H to G, the cylinder does fully four-fifths of the work of the engine.
[Ill.u.s.tration: _Fig. 108. The Dead Center._]
While the crank is turning from G to D, or from I to J, and from K to L, no work is done which is of any value as power.
If, therefore, a mangle bar should be used instead of the crank it would add greatly to the effectiveness of the steam used in the cylinder.
[Ill.u.s.tration: _Fig. 109. Crank Motion Subst.i.tute._]
Crank Motion Subst.i.tute.--In Fig. 109 the pinion A is mounted so that its shaft is in a vertical slot B in a frame C. The mangle rack D, in this case, has teeth on its outer edge, and is made in an elongated form. The pinion shaft moves up and down the slot and thus guides the pinion around the ends of the rack.
[Ill.u.s.tration: Fig. 110. Mangle Wheel.]
Mangle Wheels.--The form which is the most universal in its application is what is called the _mangle wheel_. In Fig. 110 is shown a type wherein the motion in both directions is uniform.
Mangle wheels take their names from the ironing machines called _mangles_. In apparatus of this kind the movement back and forth is a slow one, and the particular form of wheels was made in order to facilitate the operation of such machines. In some mangles the work between the rollers is uniform back and forth. In others the work is done in one direction only, requiring a quick return.
In still other machines arrangements are made to provide for short strokes, and for different speeds in the opposite directions, under certain conditions, so that this requirement has called forth the production of many forms of wheels, some of them very ingenious.
[Ill.u.s.tration: _Fig. 111. Quick Return Motion._]
The figure referred to has a wheel A, on one side of which is a peculiarly-formed continuous slot B, somewhat heart-shaped in general outline, one portion of the slot being concentric with the shaft C.
Within the convolutions of the groove is a set of teeth D, concentric with the shaft C. The pinion E, which meshes with the teeth D, has the end of its shaft F resting in the groove B, and it is also guided within a vertical slotted bar G.
The pinion E, therefore, travels over the same teeth in both directions, and gives a regular to and fro motion.
Quick Return Motion.--In contradistinction to this is a wheel A, Fig.
111, which has a pair of curved parallel slots, with teeth surrounding the slots. When the wheel turns nearly the entire revolution, with the pinion in contact with the outer set of teeth, the movement transmitted to the mangle wheel is a slow one.
[Ill.u.s.tration: _Fig. 112. Accelerated Circular Motion._]
When the pinion arrives at the turn in the groove and is carried around so the inner teeth are in engagement with the pinion, a quick return is imparted to the wheel.
Accelerated Motion.--Aside from the rack and mangle type of movement, are those which are strictly gears, one of them being a volute form, shown in Fig. 112. This gear is a face plate A, which has teeth B on one face, which are spirally-formed around the plate. These mesh with a pinion C, carried on a horizontal shaft D. This shaft is feathered, as shown at E, so that it will carry the gear along from end to end.
[Ill.u.s.tration: _Fig. 113. Quick Return Gearing._]
The gear has cheek-pieces F to guide it along the track of teeth. As the teeth approach the center of the wheel A, the latter impart a motion to the gear which is more than twice the speed that it receives at the starting point, the speed being a gradually increasing one.
Quick Return Gearing.--Another much more simple type of gearing, which gives a slow forward speed and a quick return action, is ill.u.s.trated in Fig. 113. A is a gear with internal teeth through one half of its circ.u.mference, and its hub B has teeth on its half which is opposite the teeth of the rim.
A pinion C on a shaft D is so journaled that during one half of the rotation of the wheel A, it engages with the rim teeth, and during the other half with the hub teeth. As the hub B and gear C are the same diameter, one half turn of the pinion C will give a half turn to the wheel A.
[Ill.u.s.tration: _Fig. 114. Scroll Gearing._]
As the rim teeth of the wheel A are three times the diameter of the pinion C, the latter must turn once and a half around to make a half revolution of the wheel A.
Scroll Gearing.--This is a type of gearing whereby at the close of each revolution the speed may be greater or less than at the beginning. It comprises two similarly-constructed gears A, B, each with its perimeter scroll-shaped, as shown.
The diagram shows their positions at the beginning of the rotation, the short radial limb of one gear being in line with the long limb of the other gear, hence, when the gears rotate, their speeds relative to each other change, being constantly accelerated in one or decreased in the other.
CHAPTER XIII
SPECIAL TYPES OF ENGINES
In describing various special types of motors, attention is first directed to that cla.s.s which depend on the development of heat in various gases, and this also necessitates some explanation of ice-making machinery, and the principles underlying refrigeration.
It is not an anomaly to say that to make ice requires heat. Ice and boiling water represent merely the opposites of a certain scale in the condition of matter, just as we speak of light and darkness, up and down, and like expressions.
We are apt to think zero weather is very cold. Freezing weather is a temperature of 32 degrees. At the poles 70 degrees below have been recorded. In interstellar s.p.a.ce,--that is, the region between the planets, it is a.s.sumed that the temperature is about 513 degrees Fahrenheit, below zero, called absolute zero.
The highest heat which we are able to produce artificially, is about 10,000 degrees by means of the electric arc. We thus have a range of over 10,500 degrees of heat, but it is well known that heat extends over a much higher range.
a.s.suming, however, that the figures given represent the limit, it will be seen that the difference between ice and boiling water, namely, 180 degrees, is a very small range compared with the temperatures referred to.
In order to effect this change power is necessary, and power requires a motor of some kind. Hence it is, that to make a lower temperature, a higher degree of heat is necessary, and in the transit between a high and a low temperature, there is considerable loss in this respect, as in every other phase of power mechanism, as has been pointed out in a previous chapter.
In order that we may clearly understand the phenomena of heat and cold, let us take a receiver which holds a cubic foot of gas or liquid, and exhaust all the air from it so the vacuum will be equivalent to the atmospheric pressure, namely, 14.7 pounds per square inch.
Alongside is a small vessel containing one cubic inch of water, which is heated so that it is converted into steam, and is permitted to exhaust into the receiver. When all the water is converted into steam and fills the receiver we shall have the same pressure inside the receiver as on the outside.
It will be a.s.sumed, of course, that there has been no loss by condensation, and that the cubic inch of water has been expanded 1700 times by its conversion into steam.
In a short time the steam will condense into water, and we now have, again, a partial vacuum in the receiver, due, of course, to the change in bulk from steam to water. Each time the liquid is heated it produces a pressure, and the pressure indicates the presence of heat; and whenever it cools a loss of pressure is indicated, and that represents cold, or the opposite of heat.
Now, putting these two things together, we get the starting point necessary in the development of power. Let us carry the experiment a step further. Liquids are not compressible. Gases are. The first step then is to take a gas and compress it, which gives it an increase of heat temperature, dependent on the pressure.
If the same receiver is used, and say, two atmospheres are compressed within it, so that it has two temperatures, and the exterior air cools it down to the same temperature of the surrounding atmosphere, we are ready to use the air within to continue the experiment.
Let us convey this compressed gas through pipes, and thus permit it to expand; in doing so the area within the pipes, which is very much greater than that of the receiver, grows colder, due to the rarefied gases within. Now bearing in mind the previous statement, that loss of pressure indicates a lowering of temperature, we can see that first expanding the gas, or air, by heat, and then allowing it to cool, or to produce the heat by compressing it, and afterwards permitting it to exhaust into a s.p.a.ce which rarefies it, will make a lower temperature.
It is this principle which is used in all refrigerating machines, whereby the cool pipes extract the heat from the surrounding atmosphere, or when making ice, from the water itself, and this temperature may be lowered to any extent desired, dependent on the degree of rarefaction produced.
Let us now see how this applies to the generation of power in which we are more particularly interested.