Practical Mechanics for Boys Part 13

Fig. 131 shows a cylinder (A), a piston (B) and a steam inlet port (C), in which is indicated how the steam pressure acts equally in all directions. As, however, the piston is the only movable part, the force of the steam is directed to that part, and the motion is then transmitted to the crank, and to the shaft of the engine.

[Ill.u.s.tration: _Fig. 131. Steam Pressure_]

[Ill.u.s.tration: _Fig. 132. Water Pressure_]

This same thing applies to water which, as stated, is dependent on its head. Fig. 132 represents a cylinder (D) with a vertically movable piston (E) and a standpipe (F). a.s.suming that the pipe (F) is of sufficient height to give a pressure of 50 pounds to the square inch, then the piston (E) and the sides and head of the cylinder (D) would have 50 pounds pressure on every square inch of surface.

FUELS.--In the use of fuels, such as the volatile hydrocarbons, the direct expansive power of the fuel gases developed, is used to move the piston back and forth. Engines so driven are called _Internal Combustion Motors_.

POWER FROM WINDS.--Another source of power is from the wind acting against wheels which have blades or vanes disposed at such angles that there is a direct conversion of a rectilinear force into circular motion.

In this case power is derived from the force of the moving air and the calculation of energy developed is made by considering the pressure on each square foot of surface. The following table shows the force exerted at different speeds against a flat surface one foot square, held so that the wind strikes it squarely:

-------------------------------------------------------------------- -----------------+--------------++-------------------+-------------- SPEED OF WIND | PRESSURE || SPEED OF WIND | PRESSURE -----------------+--------------++-------------------+-------------- 5 Miles per hour | 2 oz. || 35 miles per hour | 6 lb. 2 oz.

10 " " | 8 " || 40 " " | 8 "

15 " " | 1 lb. 2 " || 45 " " | 10 " 2 "

20 " " | 2 " || 50 " " | 12 " 2 "

25 " " | 3 " 2 " || 55 " " | 15 " 2 "

30 " " | 4 " 8 " || 60 " " | 18 "

VARYING DEGREES OF PRESSURE.--It is curious to notice how the increase in speed changes the pressure against the blade. Thus, a wind blowing 20 miles an hour shows 2 pounds pressure; whereas a wind twice that velocity, or 40 miles an hour, shows a pressure of 8 pounds, which is four times greater than at 20 miles.

It differs, therefore, from the law with respect to water pressure, which is constant in relation to the height or the head--that is, for every 28 inches height of water a pound pressure is added.

POWER FROM WAVES AND TIDES.--Many attempts have been made to harness the waves and the tide and some of them have been successful. This effort has been directed to the work of converting the oscillations of the waves into a rotary motion, and also to take advantage of the to-and-fro movement of the tidal flow. There is a great field in this direction for the ingenious boy.

A PROFITABLE FIELD.--In no direction of human enterprise is there such a wide and profitable field for work, as in the generation of power. It is constantly growing in prominence, and calls for the exercise of the skill of the engineer and the ingenuity of the mechanic. Efficiency and economy are the two great watchwords, and this is what the world is striving for. Success will come to him who can contribute to it in the smallest degree.

Capital is not looking for men who can cheapen the production of an article 50 per cent., but 1 per cent. The commercial world does not expect an article to be 100 per cent, better. Five per cent. would be an inducement for business.

CHAPTER XII

ON MEASURES

HORSE-POWER.--When work is performed it is designated as horse-power, usually indicated by the letters H. P.; but the unit of work is called a _foot pound_.

If one pound should be lifted 550 feet in one second, or 550 pounds one foot in the same time, it would be designated as one horse-power. For that reason it is called a foot pound. Instead of using the figure to indicate the power exerted during one minute of time, the time is taken for a minute, in all calculations, so that 550 multiplied by the number of seconds, 60, in a minute, equals 33,000 foot pounds.

FOOT POUNDS.--The calculation of horse-power is in a large measure arbitrary. It was determined in this way: Experiments show that the heat expended in vaporizing 34 pounds of water per hour, develops a force equal to 33,000 foot pounds; and since it takes about 4 pounds of coal per hour to vaporize that amount of water, the heat developed by that quant.i.ty of coal develops the same force as that exercised by an average horse exerting his strength at ordinary work.

All power is expressed in foot pounds. Suppose a cannon ball of sufficient weight and speed strikes an object. If the impact should indicate 33,000 pounds it would not mean that the force employed was one horse-power, but that many foot pounds.

If there should be 60 impacts of 550 pounds each within a minute, it might be said that it would be equal to 1 horse-power, but the correct way to express it would be foot pounds.

So in every calculation, where power is to be calculated, first find out how many foot pounds are developed, and then use the unit of measure, 33,000, as the divisor to get the horse-power, if you wish to express it in that way.

It must be understood, therefore, that horse-power is a simple unit of work, whereas a foot pound is a compound unit formed of a foot paired with the weight of a pound.

ENERGY.--Now _work_ and _energy_ are two different things. Work is the overcoming of resistance of any kind, either by causing or changing motion, or maintaining it against the action of some other force.

Energy, on the other hand, is the power of doing work. Falling water possesses energy; so does a stone poised on the edge of a cliff. In the case of water, it is called _kinetic_ energy; in the stone _potential_ energy. A pound of pressure against the stone will cause the latter, in falling, to develop an enormous energy; so it will be seen that this property resides, or is within the thing itself. It will be well to remember these definitions.

HOW TO FIND OUT THE POWER DEVELOPED.--The measure of power produced by an engine, or other source, is so interesting to boys that a sketch is given of a p.r.o.ny Brake, which is the simplest form of the Dynamometer, as these measuring machines are called.

[Ill.u.s.tration: _Fig. 133. p.r.o.ny Brake_]

In the drawing (A) is the shaft, with a pulley (A'), which turns in the direction of the arrow (B). C is a lever which may be of any length.

This has a block (C'), which fits on the pulley, and below the shaft, and surrounding it, are blocks (D) held against the pulley by a chain (E), the ends of the chain being attached to bolts (F) which pa.s.s through the block (C') and lever (C).

Nuts (G) serve to draw the bolts upwardly and thus tighten the blocks against the shaft. The free end of the lever has stops (H) above and below, so as to limit its movement. Weights (I) are suspended from the end of the lever.

[Ill.u.s.tration: _Fig. 134. Speed Indicator_]

THE TEST.--The test is made as follows: The shaft is set in motion, and the nuts are tightened until its full power at the required speed is balanced by the weight put on the platform.

The following calculation can then be made:

For our present purpose we shall a.s.sume that the diameter of the pulley (A') is 4 inches; the length of the lever (C), 3 feet; the speed of the shaft (A) and the pulley, 210 revolutions per minute; and the weight 600 pounds.

Now proceed as follows:

(1) Multiply the diameter of the pulley (A') (4 inches) by 3.1416, and this will give the circ.u.mference 12.5664 inches; or, 1.0472 feet.

(2) Multiply this product (1.0472) by the revolutions per minute. 1.0472 210 = 219.912. This equals the _speed_ of the periphery of the pulley.

(3) The next step is to get the length of the lever (C) from the center of the shaft (A) to the point from which the weights are suspended, and divide this by one-half of the diameter of the pulley (A'). 36" 2" = 18", or 1-1/2 feet. This is the _leverage_.

(4) Then multiply the _weight_ in pounds by the _leverage_. 600 1-1/2 = 900.

(5) Next multiply this product (900) by the _speed_, 900 219.912 = 197,920.8, which means _foot pounds_.

(6) As each horse-power has 33,000 foot pounds, the last product should be divided by this figure, and we have 197,920.8 33,000 = 5.99 H. P.

THE FOOT MEASURE.--How long is a foot, and what is it determined by? It is an arbitrary measure. The human foot is the basis of the measurement.

But what is the length of a man's foot? It varied in different countries from 9 to 21 inches.

In England, in early days, it was defined as a measure of length consisting of 12 inches, or 36 barleycorns laid end to end. But barleycorns differ in length as well as the human foot, so the standard adopted is without any real foundation or reason.

WEIGHT.--To determine weight, however, a scientific standard was adopted. A gallon contains 8.33 pounds avoirdupois weight of distilled water. This gallon is divided up in two ways; one by weight, and the other by measurement.

Each gallon contains 231 cubic inches of distilled water. As it has four quarts, each quart has 57-3/4 cubic inches, and as each quart is comprised of two pints, each pint has nearly 29 cubic inches.

THE GALLON.--The legal gallon in the United States is equal to a cylindrical measure 7 inches in diameter and 6 inches deep.