General Science - Bertha May Clark (children's books read aloud TXT) 📗
- Author: Bertha May Clark
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An ordinary man can do one sixth of a horse power. The average locomotive of a railroad has more than 500 H.P., while the engines of an ocean liner may have as high as 70,000 H.P.
169. Waste Work and Efficient Work. In our study of machines we omitted a factor which in practical cases cannot be ignored, namely, friction. No surface can be made perfectly smooth, and when a barrel rolls over an incline, or a rope passes over a pulley, or a cogwheel turns its neighbor, there is rubbing and slipping and sliding. Motion is thus hindered, and the effective value of the acting force is lessened. In order to secure the desired result it is necessary to apply a force in excess of that calculated. This extra force, which must be supplied if friction is to be counteracted, is in reality waste work.
If the force required by a machine is 150 pounds, while that calculated as necessary is 100 pounds, the loss due to friction is 50 pounds, and the machine, instead of being thoroughly efficient, is only two thirds efficient.
Machinists make every effort to eliminate from a machine the waste due to friction, leveling and grinding to the most perfect smoothness and adjustment every part of the machine. When the machine is in use, friction may be further reduced by the use of lubricating oil. Friction can never be totally eliminated, however, and machines of even the finest construction lose by friction some of their efficiency, while poorly constructed ones lose by friction as much as one half of their efficiency.
170. Man's Strength not Sufficient for Machines. A machine, an inert mass of metal and wood, cannot of itself do any work, but can only distribute the energy which is brought to it. Fortunately it is not necessary that this energy should be contributed by man alone, because the store of energy possessed by him is very small in comparison with the energy required to run locomotives, automobiles, sawmills, etc. Perhaps the greatest value of machines lies in the fact that they enable man to perform work by the use of energy other than his own.
Figure 118 shows one way in which a horse's energy can be utilized in lifting heavy loads. Even the fleeting wind has been harnessed by man, and, as in the windmill, made to work for him (Fig. 119). One sees dotted over the country windmills large and small, and in Holland, the country of windmills, the landowner who does not possess a windmill is poor indeed.
For generations running water from rivers, streams, and falls has served man by carrying his logs downstream, by turning the wheels of his mill, etc.; and in our own day running water is used as an indirect source of electric lights for street and house, the energy of the falling water serving to rotate the armature of a dynamo (Section 310).
A more constant source of energy is that available from the burning of fuel, such as coal and oil. The former is the source of energy in locomotives, the latter in most automobiles.
FIG. 119.—The windmill pumps water into the troughs where cattle drink.
In the following Chapter will be given an account of water, wind, and fuel as machine feeders.
CHAPTER XVII THE POWER BEHIND THE ENGINE171. Small boys soon learn the power of running water; swimming or rowing downstream is easy, while swimming or rowing against the current is difficult, and the swifter the water, the easier the one and the more difficult the other; the river assists or opposes us as we go with it or against it. The water of a quiet pool or of a gentle stream cannot do work, but water which is plunging over a precipice or dam, or is flowing down steep slopes, may be made to saw wood, grind our corn, light our streets, run our electric cars, etc. A waterfall, or a rapid stream, is a great asset to any community, and for this reason should be carefully guarded. Water power is as great a source of wealth as a coal bed or a gold mine.
The most tremendous waterfall in our country is Niagara Falls, which every minute hurls millions of gallons of water down a 163-foot precipice. The energy possessed by such an enormous quantity of water flowing at such a tremendous speed is almost beyond everyday comprehension, and would suffice to run the engines of many cities far and near. Numerous attempts to buy from the United States the right to utilize some of this apparently wasted energy have been made by various commercial companies. It is fortunate that these negotiations have been largely fruitless, because much deviation of the water for commercial uses and the installation of machinery in the vicinity of the famous falls would greatly detract from the beauty of this world-known scene, and would rob our country of a natural beauty unequaled elsewhere.
172. Water Wheels. In Figure 120 the water of a small but rapid mountain stream is made to rotate a large wheel, which in turn communicates its motion through belts to a distant sawmill or grinder. In more level regions huge dams are built which hold back the water and keep it at a higher level than the wheel; from the dam the water is conveyed in pipes (flumes) to the paddle wheel which it turns. Cogwheels or belts connect the paddle wheel with the factory machinery, so that motion of the paddle wheel insures the running of the machinery.
One of the most efficient forms of water wheels is that shown in Figure 121, and called the Pelton wheel. Water issues in a narrow jet similar to that of the ordinary garden hose and strikes with great force against the lower part of the wheel, thereby causing rotation of the wheel. Belts transfer this motion to the machinery of factory or mill.
173. Turbines. The most efficient form of water motor is the turbine, a strong metal wheel shaped somewhat like a pin wheel, inclosed in a heavy metal case.
Water is conveyed from a reservoir or dam through a pipe (penstock) to the turbine case, in which is placed the heavy metal turbine wheel (Fig. 122). The force of the water causes rotation of the turbine and of the shaft which is rigidly fastened to it. The water which flows into the turbine case causes rotation of the wheel, escapes from the case through openings, and flows into the tail water.
The power which a turbine can furnish depends upon the quantity of water and the height of the fall, and also upon the turbine wheel itself. One of the largest turbines known has a horse power of about 20,000; that is, it is equivalent, approximately, to 20,000 horses.
174. How much is a Stream Worth? The work which a stream can perform may be easily calculated. Suppose, for example, that 50,000 pounds of water fall over a 22-foot dam every second; the power of such a stream would be 1,100,000 foot pounds per second or 2000 H.P. Naturally, a part of this power would be lost to use by friction within the machinery and by leakage, so that the power of a turbine run by a 2000 H.P. stream would be less than that value.
Of course, the horse power to be obtained from a stream determines the size of the paddle wheel or turbine which can be run by it. It would be possible to construct a turbine so large that the stream would not suffice to turn the wheel; for this reason, the power of a stream is carefully determined before machine construction is begun, and the size of the machinery depends upon the estimates of the water power furnished by expert engineers.
A rough estimate of the volume of a stream may be made by the method described below:—
Suppose we allow a stream of water to flow through a rectangular trough; the speed with which the water flows through the trough can be determined by noting the time required for a chip to float the length of the trough; if the trough is 10 feet long and the time required is 5 seconds, the water has a velocity of 2 feet per second.
The quantity of water which flows through the trough each second depends upon the dimensions of the trough and the velocity of the water. Suppose the trough is 5 feet wide and 3 feet high, or has a cross section of 15 square feet. If the velocity of the water were 1 foot per second, then 15 cubic feet of water would pass any given point each second, but since the velocity of the water is 2 feet per second, 30 cubic feet will represent the amount of water which will flow by a given point in one second.
FIG. 123.—Estimating the quantity of water which flows through the trough each second.
175. Quantity of Water Furnished by a River. Drive stakes in the river at various places and note the time required for a chip to float from one stake to another. If we know the distance between the stakes and the time required for the chip to float from one stake to another, the velocity of the water can be readily determined.
The width of the stream from bank to bank is easily measured, and the depth is obtained in the ordinary way by sounding; it is necessary to take a number of soundings because the bed of the river is by no means level, and soundings taken at only one level would not give an accurate estimate. If the soundings show the following depths: 30, 25, 20, 32, 28, the average depth could be taken as 30 + 25 + 20 + 32 + 28 ÷ 5, or 27 feet. If, as a result of measuring, the river at a given point in its course is found to be 27 feet deep and 60 feet wide, the area of a cross section at that spot would be 1620 square feet, and if the velocity proved to be 6 feet per second, then the quantity of water passing in any one second would be 1620 × 6, or 9720 cubic feet. By experiment it has been found that 1 cu. ft. of water weighs about 62.5 lb. The weight of the water passing each second would therefore be 62.5 × 9720, or 607,500 lb. If this quantity of water plunges over a 10-ft. dam, it does 607,500 × 10, or 6,075,000 foot pounds of work per second, or 11,045 H.P. Such a stream would be very valuable for the running of machinery.
176. Windmills. Those of us who have spent our vacation days in the country know that there is no ready-made water supply there as in the cities, but that as a rule the farmhouses obtain their drinking water from springs and wells. In poorer houses, water is laboriously carried in buckets from the spring or is lifted from the well by the windlass. In more prosperous houses, pumps are installed; this is an improvement over the original methods, but the quantity of water
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