THE POPULAR EDUCATOR. 



MECHANICS. XIX. 



ELEMENTS OF MACHINERY PBIME MOVEBS ANIMAI, 

 FOECE, WATEE, WIND, STEAM. 



As it is our object to make these lessons as practical as pos- 

 sible, it will be well to look at a few of the simpler modes of 

 altering and transmitting power. Sometimes this is advanced 

 to the rank of a separate science, and called kinematics, or the 

 science of motion, but it should be referred to here as a part of 

 practical mechanics. 



We seldom have our power available for use in the exact way 

 we desire. Sometimes we have an alternate motion, like that of 

 the piston-rod of an engine, and we want to derive from it a rota- 

 tory or progressive motion ; or we want to transmit it along a 

 direction making some angle with its course, or to make many 

 other alterations in its mode of action. 



In large factories there is frequently a long shaft running 

 long the building, and driven by an engine. From this it is 

 required to drive all the machines in the place. This is accom- 

 plished by fixing wheels on the shaft, and letting endless straps 

 pass over these and then over the driving pulleys of the machines. 

 The motion may often be greatly altered in this way. The strap 

 itself merely transmits the power, and whether there is a gain or 

 loss in speed or power depends on the comparative size of the 

 sheaves. Frequently there are several of these wheels of 

 different sizes fixed on the axle and on the machine, and thus 

 the speed may be altered at pleasure. If the strap passes over 

 a large one on the shaft and a small one on 

 the machine, there will be an increase of 

 speed, and if we reverse the condition there 

 will be a loss. A common illustration of 

 a similar arrangement is seen in a watch. 

 The spring when fully wound up exerts a 

 __ [? much greater power than it does when the 

 watch has run nearly down. Now this 

 would make it go irregularly, and therefore 

 the fusee is introduced. When the force 

 of the spring is greatest the chain acts 

 on the smallest part of the fusee, and there- 

 fore has only a short leverage, but as it 

 unwinds and loses its force the chain acts at a greater lever- 

 age, and a uniform rate of motion is thus maintained. 



Sometimes toothed wheels are used instead of straps, espe- 

 cially when the distance through which the power has to be 

 transmitted is small. The advantage is that they do not slip, 

 as straps are liable to do ; the friction with them is, however, 

 greater. If we want to transmit motion from a shaft to another 

 placed at an angle with it (Fig. 91), we employ what are known 

 as bevelled wheels. The action of these will be clear from the 

 figure, without any explanation. 



Often it is required to change a rotating motion into a pro- 

 gressive one, and we can accomplish this by means of a rack and 

 pinion. A number of notches are 

 cut in a bar of metal (Fig. 92a), 

 of such a size and at such dis- 

 tances that the teeth of the 

 wheel exactly fit into them, and 

 .as the wheel is turned the rack 

 is moved onwards. This is very 

 frequently employed when a 

 slow and regular motion is re- 

 quired, as in the adjustment of 

 the tube of a microscope. In- 

 stead of a rack a chain is some- 



times used (Fig. 92 b), the links being made of a peculiar shape, 

 so that the teeth of the wheel may catch in them. 



The crank (Fig. 93) is, perhaps, one of the most common of these 

 elements of machinery. The piston-rod of an engine is usually 

 jointed, and the end of the jointed part fixed to an arm pro- 

 jecting from the axle to be turned, and called a crank. Some- 

 times it is fixed to a pin in one of the spokes of the wheel, but 

 the action is just the same. The force, however, with which it 

 drives the wheel is continually varying. When the piston is 

 at the bottom of the cylinder the crank and piston-rod are in 

 one straight line, and therefore all the power presses on the 

 bearing of the axle, and is lost. As the wheel turns the power 

 acts with a leverage which increases till the wheel has made 

 nearly one-fourth of a revolution j it is then at its maximum, 



Fig. 91. 



Fig. 92. 



and diminishes till the piston-rod reaches its highest point, 



when it is all again lost. Now it is clear that unless we have 



some means of regulating the speed, 



the machine will work very unevenly, 



and at times stop altogether. To 



obviate this, a large and heavy wheel, 



called the fly-wheel, is fixed on the Fig. 93. 



axle of the crank. This, when once 



started, acquires an amount of momentum or moving force 



which carries it over the dead points when the power is lost. 



On account of the weight of the wheel, its motion is but slightly 



accelerated when the piston acts at its greatest leverage, but 



the additional force is stored up in it, and thus ensures a steady 



motion. The heavy wheel of a foot-lathe serves precisely the 



same purpose. The power is here applied during rather less 



than one-half of the revolution, but the momentum then acquired 



carries it through the remaining part. 



PRIME MOVERS. 



Having seen a few of the principal ways of transmitting and 

 modifying motion, we will now notice the most important of 

 the prime movers or causes of motion, and then pass on to 

 Dynamics. 



As already stated, no machine can create force, there must be 

 some original source whence it proceeds ; and on examination we 

 shall find that nearly all sources of power may be divided into 

 these four classes: muscular action, whether of men or animals; 

 the force of water ; the power of the wind ; and the expansive 

 power of gases or vapours. There are a few other prime movers, 

 as electricity, heat, and chemical action; these, however s are not 

 used at all in practice, but merely for the sake of experi- 

 ment, their cost being at present too great to allow of their 

 employment. 



Muscular action, the first of our four kinds, is the one earliest 

 employed and most frequently used, when no very great exertion 

 is required. The reason of this is, that it can always be em- 

 ployed without previous arrangement, and can readily be 

 applied in almost any way that may be needed ; it is, how- 

 ever, one of the most uncertain of the prime movers, as it is 

 both limited in its power and irregular in its action. The two 

 divisions of it are, the force of men and that of animals. But 

 before noticing these we must decide what unit of work we 

 are to adopt. In our second lesson we saw that the unit of 

 force is that which is required to cause a round ball, equal in 

 weight to a cubic inch of water, to move through one foot in 

 one second, and that this unit is equal to 7'85 grains. This is, 

 however, far too small for practical purposes, and the unit of 

 work which has been fixed on and universally adopted in this 

 country in calculations like those we are about to make, is the 

 force required to raise a weight of 1 pound through a space of 1 

 foot. We call this the unit of work, and not of force, as time 

 is not taken into account. The same amount of work is done in 

 raising 100 pounds to a height of 50 feet, whether a minute or 

 an hour be occupied ; the force required would, however, be much 

 greater in the former than in the latter case. This unit of work 

 is called afoot-pound. In the example just taken, 50 X 100, that 

 is, 5,000 units or foot-pounds, are required to raise the weight. 

 The same force would also raise 5,000 pounds 1 foot, or 5 pounds 

 1,000 feet, the product of the numbers which represent the pounds 

 raised and the feet passed over being the same in each case. 

 Thus we can find the work done in any machine, and we have 

 another way of putting the principle of virtual velocities, the 

 work done by the power being always equal to that done by the 

 weight. By reducing to this unit the work done by the same 

 force applied in different modes, we can discover which is the 

 most advantageous, and what is their comparative efficacy. 



We will now inquire into the different ways of applying human 

 power. In spade labour there is a very great loss. When 

 merely used for turning up the ground, the spade is a lever of 

 the first kind, and the power acts at the longer arm ; but when 

 the earth is lifted or thrown to any height, the spade becomes a 

 lever of the first or third kind, according to which hand we con- 

 sider the fulcrum and which the power; but either way, the 

 weight acts at the longer arm, and thus causes a great waste 

 of power. In turning a winch, though a larger portion of the 

 force employed is utilised, there is still great loss and irregularity. 

 When the handle is being pulled upwards and towards the person 



